thin film processes for microelectronic applications

8/2/2019 Thin Film Processes for Microelectronic Applications

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1390 PROCEEDINGSOF THE IEEE,VOL. 59, NO.10, OCTOBER 197

Thin-Film Processes for Microelectronic ApplicationLAWRENCE V. GREGOR

Abstract-The rapid development f the microelect ronic s indust ryover the last decade has placed exceptional demands on th in -fi lmtechnology since, t o a large extent, t controls the technologicalaceof h at industry. This demand has challenged the hi n-fil m ech-nologist to develop new and improved processes for both thin -filmdevices as well as for the thin-film conductorsnd insulation neededby semiconductor devices. The projected demands of h e com ingdecadewill require advances n he technology comparableo hose ofthe past decade if th e full potential of arge scale integration is toeachieved.

The variety o f materials and rocesses required o mee4 adequatelythe to tal needs of the industr y has necessitated the development o fseveral deposition echnologies.Vacuum evaporation, sputtering,chemicalvapordeposition, di me nt at io n, etc., are all n volumemanufacturing use and the technologies of each of these techniqueshas been significantly improved during the past ten years. A similarincrease in process capab ility and control has been necessary in thearea of pattern definition in order to allow thedevelopment of finelineetchingwhich achieves the equirednarrow inewidths andseparations in todays microelec tron ic assemblies.

The materials of major interest to th endustry as well as the de-position echniques and photoengraving processes usad in heirprocessing are highlighted. The discussion includes the status andlimitations of the technologys itexists todayas we ll as a considera-tion of the advantages and disadvantages of the various processesboth as of today and for the future.

D INTRODUCTIONURING the past decade, advances in semiconductor andthin-film electronics, particularly in the utilization of inte-gratedcircuits,have been limited by theavailable ech-nology of thin films more than by any other factor. For this to havemeaning, the definition ofthin films should be stated. Ratherhanattempt a functional or intrinsic categorization, a simple physicalcriterion will be employed. A thin ilm is a surf- layer 5 p thick,often much less. Thiswill hopefully avoid the ~ t u r a lendency toassociate the termthin film with specific deposition methods, or tonarrow the meaning to include only those filmsused in silicon de-vice and integrated circuit technology.There are many reasons or the widespread use of thin films inmicroelectronics. Perhaps the most important is that only thin filmscanbe processed to yield the small size, low power,and high circuitdensity desired. Secondly,hinfilmsare mportant in other essentialprocesses, e.g., diffusion masking. Also, thin-film deposition tech-niques allow for convenient production of high-purity substancesor materials with losely controlled composition. Finally, some vitalfeatures of present circuitscanonly be achieved by using thin films,such as silicon s u r f a c e passivation with SO2. Many other specificargumentscan be made for the desirability ofhin-film technology,dependingon the material in question r the function to beachieved.Although semiconductor electronics today is largely based onsilicon and the planar diffwion process, other materials and fabrica-tion methods are finding increasing application. In a striking way,the sUCCeSSful employment (or lack of it) of these materials is de-pendent on the s t a tu s of the particular thin film technology asso-ciated with theritical process or functional operationof the device.For example, development of special thin-film diffusion maskingmethods was necessary to enable electroluminescent diode arrays

Manuscript received March 23, 1971; revised May 5 , 1971.The author is with IBM Components Division, East Fishkill Facility,Hopewell Junction, N. . 2533.

to be fabricated; conversely, the unavailability of sufEiciently stabland reproducible hin-filmmaterials and processes has been a severlimitation m attempting to utilize Ge,GaAs,and other semiconductors m integrated circuits, even though the higher electromobility n hese materials appears to offer a performance adv a n t a g e .Presently, most thin-film pplications require either highlyconductive or insulating materials, although resistive and semiconductive films have their uses.These films serve as the basis of passivelements, interconnect active elements,rotect and insulate variouportions of the circuit, connect it to the external world, and evefurnish the active medium of he semiconductor itself. Not only arsuch filmsused onthe semiconductor.s&ace (die or chq levelbut also on the chip carrier or substrate in hybrid or thin-film circuit technology (tobe defined). Since the technology of magnetifilms is a separate subject in its own ight, it will not be considerein thisp a p e r . Thin films are not only used as functional elements othe completed device, but are essential in the processing sequencwhich produces the device. Some feeling forhe variety ofthin-filmaterials used in microelectronics is given by Table I.The terms monolithic circuits, hybrid circuits, and thinfilm circuits are often employed o describe various types of micrelectronic components. These will be used in accordance with thfollowing general definitions:Monolithic Circuits: passive and active elements abricated in oon the semiconductor surface, with thin-film interconnections anterminals.Hybrid Circuits:passive components andnterconnections madfrom thin or thick films with active devicesattached by a separaprocess step.Thin-Film Circuits: all active and passive elements s well as interconnections made from hin films.All are illustrated in Fig. 1. The emphasis n the ensuing discusionwill be on the methods of fabricating and employing thin filmin practical microelectronics, with brief survey of some interestinpotential applications which rely on thin films.

hhXOD.3 OF FILM L I T I O NMany methods are available for forming s u r f a c e films, and moof them are not recent discoveries.For example, sputtering was observed in the mid-19th century, and vacuum evaporation was aannoyance to the early incandescent lamp manufacturers [l 1. It iconvenient to classify thin films by their methods of depositionrealizing that other ordering systems are also possible and usefulIt will be seen that, within each general type of film depositiomethod, here is a wide ranp of materials, properties, and aplications.

Chemical ReactionThis method relies on a chemical reaction between the originsurface and its environment to produce a thin layer of a new sub

s t a n c e on he surface. The basisof the present technologyof silicodevices and the integrated circuits is the formation of a thin film oSiO, on Si by the following reaction:Si,,, + O2 -* SiOZ(r).

The resulting amorphous Si02 ayer serves to passivate the sur


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GREGOR : THIN-FILM PROCESSES FOR MICROELECTRONICS 1391TABLE I

THIN -FILM ATERIALSN M r c ~ o p ~ ~ c m o ~ ~ c shrPo=

Interconnections/nsulation/Terminals Passivationncapsulationesistive Capacitive Semicon ducting Processing Developm entA1A1 alloysCu ,i)iO , SiO, .P,O, TaN , Ta,O, Se InSbibOSiOl,O s SiO, Ta SiO , Si InAs SiO, Si,NrSiO,c u S i ,N , A1203 Cr S i 0 HfO, Te PbSMo-Au NiCr ZrO, S ic PbTe MoTi-Ag BN PbO .B,O, .SiO, SnO, PbTiO,aAs CdS CrCr-Ag S i0 kanthal GaP CdSeolymerCr-CuAuPb-Sn

photopolymer Nb,O,Ge-S+TeZnO

Pt-Au AIN ZnSe

N +

ISOLATION SUBCOLLECTOR TRANSISTORSI LICON SUBSTRA TE

la )

ACTIVE COMPONENTTERM1 CAPACITOR RESISTOR

SUBSTRATE(bl

TH I N - F I LM BISTABLETRANSISTOR RESISTORSOU R C E , GA T E D R A IN I PHOTOSENSOR

SU B ST R A T E

(Cl

Fig. 1 . Cross-section al diagram of the hree t y pes of microelectronic cir-cuits using thin films. (a) Mon olithic siliconntegrated circuit.(b)Hybridsilicon thin-film circuit. (c) Allhin-film integrated circuit.

face of Si [2]. It also functions as a convenient diffusion mask andprovides the electrical insulation between the Si and the intercon-nection lines. The SiO, layer can be formed in a number of dif-ferent oxidizing gases, notablyH,O, N20,and to a lesser extent, inCO, [3]. Primarily, the reaction is controlled by the rate of dif-fusion of oxidant across the SiO, film. hence, the growth rate de-creases with time. The most general expression which relates thefilm thickness to oxidation time is [4]

X + k , x = k2 t + k3where x is the film thickness at time t, and k,, k,, k 3 are constantsdependingon emperature,. oxidizingas,and silicon crystal orienta-tion.No other common semiconductor forms a naturally occurringoxide film which as useful properties. To a great extent, this is whysilicon is predominant in integrated circuit applications.

In thermal oxidation, the oxidant diffuses through he growingfilm to the oxide-silicon interface. Theres another class of reactionsin which the opposite reaction occurs: he silicon (or conductor)cation migrates toward and reacts with the oxidizing species at the

outer surface. It is common o assist the process by making theon-ductor electrically positive with respect to its surroundings. This isthen called anodization nd is widely practicedo yield thin oxidedielectricfilmson Ta, V, Nb, and Al. It canalso be used to producefilms on Si. The film thicknessx is proportional to he voltage dropV across the film ( x = k V ) and hence canbe controlled accurately[ 5 ] . An interesting variation of this process is the conversion of aslowetching film (Si3N4) o SiO, [ 6 ] .In some interesting development programs, a much lower tem-perature is employed o produce an extremely thinfilm - 25 A) sothat the electron tunneling phenomena can be employed to pro-duce useful effects. The best known devicest this time are Joseph-son-effect devices usingSnO or PbO [7] and thevariable V, MNOSFET [SI which employs SiO,. A more conventional MOSFET de-vice has been made y anodizing an AI film over the channel o pro-duce an M2O3gate insulation p].Chemical Vapor Deposition

This is sometimes called the CVD processnd it employs eithera chemical reaction between wo or more species or a chemicaldecomposition to produce the desired film. The latter method issomewhat imprecisely referred to as pyrolysis. In these reactions,the surface does not play n active role in the formationof the thinfilm.All three types of thin films can be formed by this method, asexemplikd by the following reactions [lo]

SiH4 + 0, + SiO,WCI, -b w i-c1,SiCI4 + Si + 2C1,.

The third chemical reaction is of particularimportance.Dependingon he surfaceconditions and crystal structure, the resulting Si filmcanbe singlecrystalor polycrystalline n nature. The former processis called epitaxy and is not usually considered a thin-film process,although most epitaxial silicon films are much less than 5 p thick,and fall within the physical classiliation of thinfilms. If the film ispolycrystalline, as t is on SiO,, then this process isdefinitely athin-film rocess. It is widely used o form thegateelectrode in the siliconself-aligned gate FET process [ 11.The major requirements for a practical CVD process are a suit-able compound to cany the desired substances to the surface, aheterogeneous gassolid reaction whose rate is much faster thanany competing process, nd a reactor s y s t em toassure.temperatureuniformity, gas composition homogeneity, and a reasonable batchsize.Temperature control is important since the reaction is usuallythermally activated.The deposition of SiO, is one of the most widely used CVDprocesses. A number of different chemical systems have been em-ployed: SiH4+0, , Si (OR)4, Cot, etc. In addition to pure Si02,processes havebeen developed to incorporate stabilizingsubstances


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GREGOR : THIN-FILM PROCESSESOR 1393nique has been found to produce extremely high component re-liability in field use25]. Unfortunately, the complexglasscomposi-tion and relatively high fusion temperature (575C) do not makethismethod attractive for integrated circuits which employsilicon-aluminum contactsor aresensitive to the traces of alkali ions com-monly found in glass.Another method which has found increasing use is the deposi-tion of a liquid suspension of appropriate materials followed byconversion to a glass-like film from which df is an ts can be supplied to the semiconductor. These are often generically referred toas doped-oxide or paint-on sources [26]. The technique combinesthe ease of pattern formation with the advantages ofhe planar if-fusion process. Some available iffusants are P, B, As, and Au. Onedifficulty with this technique is the inability to get a reproduciblehigh surface concentration ofiffusant. Another is the susceptibilityto deleterious ionic contamination.For extremely thin dielectric films to be used in tunnelling de-vices, a specialized technique, based on the Langmuir monolayeradsorption method, employsa monolayer of a fattyacid salt. Filmsof - 5 A of bariumstearatecanbe superimposed to give insulatingfilms whose thicknessis some multiple of the monolayer thickness[27]. In addition to the fatty acid salts, other materials m a y be pre-pared for study as thin films, uchaschlorophyll-a [281.M i s c e l h o u s

Most of the remaining methods or thin film formation are notclose to everyday use. The formation of polymer dielectric films byirradiation of adsorbed organic molecules on surfaceswas studiesextensively in the pastfor applications in cryoelectronic ircuits andfor capacitor fabrication [29]. One organic polymer method withpotential use is the deposition of poly-pxylylene (called Parylenecommercially). This is a chemical-physical method which relies onthe thermal cracking of a source material, di-pxylene, to producereactive free radicals which repolymerize when impingingon a sur-face [MI.t is claimed that this method has been successful n pro-viding environmental protectiono silicon circuit chips encapsulatedin this manner [31]. Other methods which are useful in preparingthin films of electronic interest are theexploding-wire method[32],and chemical spraying.The latter method isnot free from difficultiessuch as powdery deposits, but hasbeen used to make coherentfilmsofZnO [33].

CHARACTERIZATION OF THIN F WThe control of hin-film properties requires number of methodsof measuring the properties of theilm. In some cases , thismay bedone even duringdeposition,butmostevaluation is performedafterward. The refinement of measurement methods has been re-sponsible for much of the technological advance maden the use of

thin films n the past decade. Theest of the discussion ill deal withmeasurements directly involving the l i l m s themselves.Thickness

A very basic parameter is the thickness of the film. Its value ispredetermined by a knowledge of the depositionr growth rate asfunction of the parameters of the deposition system (power, tem-perature, pressure, etc.). In some cases, the actual thickness canbemeasured in situ during deposition. The optical absorption of atransparent film canbe monitored during evaporationr sputtering.For other films, indirect rate monitoring methods are used whichemploypreviouscalibrationof hesystem.However,when heprocess is complete, t is desirable to be able to measure the actualITlm thickness. Thisisnormally done by physical or optical mt%hods.The simplest physical method employs a sensitive stylus which isallowed to glide over he surface until the edge of the depositedilm

is encountered. Such instruments are often known by their tradenames Dektak, Taly-Surf). The step height thus directly measuredis the film thickness. Of ourse, only the thickness t the edgecanbemeasured. Other more complex methods havebeen developed suchas the absorption of beta-particles of known energy.Finally, if therelative dielectric o n s t a n t of an insulatingfilm s known, as estimateof thicknesscanbe made from the measured capacitance ofhe iilmbetween two suitable electrodes of known area. The latter method,thoughcumbersome, is oftenuseful ncross calibrating othermethods.Most of the methods for transparent or translucent films areoptical techniques, which utilize either absorption or interferenceof light to make the measurement. simple rapid methodor evalua-tion of thicknesses of nonabsorbing films is VAMFO (variableamplitude multiple fringe observation), which is suitable for filmsfrom several hundred o several thousand angstroms, but requires arelatively large area [34]. An even simpler method is color com-parison of the ilm to a calibrated sample with various thicknesses,called a stepgauge. For accuracy,a calibrated sample is requiredfor each material, since the color change isue to nterference whichisa functionof the refractive indix of thematerial. For thicker films,opaque films, etc., thickness measurementsanbe madeby multiple-beam interferometry. Unfortunately, the requirement of a highlyreflecting surface nd the eed for precision optical equipment makethis method more cumbersome.Very thin films of transparent material may be measured bypolarization spectrometry or ellipsometry, in which the phase angleshift of elliptically polarized light being reflected back from thenter-face of he film and substrate is a function of film thickness and re-fractive index. This method is most useful for extremely thin filmsfrom a monolayer p to everal hundred angstraoms. Another mea-surement suitable for very thin films is the Brewster-angle techniquefor accurate measurement of refractive index [35].Structural Properties

The structural details of thin filmsre of importancen governingsuch behavior as resistivity,stress,chemical stability, and othermore subtle properties whichwill be discussed in more detail alongwith reliability. Structural investigations can be simply microscopicobservations of grain size, texture, metallographic structure, stain-ing, etc. However, a large number of thin films are devoid of anyvisible structure and more refined investigatory ools are employed.These include: X-ray and electron diffraction to determine the ex-tent of crystalline structure in the iilm ; canning electron microscopy(SEM) to determine high-resolution surface features; transmissionelectron microscopy (TEM) o detect internal fine structure; prefer-ential chemical etching; and polarized light for internal stress anal-ysis. If the substrate is transparent to infrared or visible radiation,the method of internal reflection spectroscopycanbe used to studythe absorption propertiesof surfacefilms 36].Mechanical Properties

Actually the so-called mechanical properties are really mani-festations of the interaction of physicalnd structural factors. How-ever, hey are conveniently discussed as such, and the most sig-nificant are probably stress, hardness, and the coefficient of thermalexpansion. Stress s particularly important, since the combinationfresidual intrinsic stress and that due to expansion mismatch caneasily exceed the yield strength of the film and/or adhesive forces,resulting in separation or fracture. Both compressive and tensilestress are common. t is generally preferable to have the former toprevent propagating cracks from occurring. Hardness s a measureof the resistance of the material o abrasion and scratching. One ofthe hardest known thin films is silicon nitride [371. The coefficient


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1394TABLE I1

PROCEEDINGS OFTHE IE, OCTOBER 197 1

CWLWATIONOF TYPICAL MICROELECTRONIC THIN-FILM MATERIALBY R~srsrrvrnm OTHER LECTRICALROPERTIES

ResistivityType Example (Q .cm) OtherarameterDielectric Si0 1018Dielectric SrTiO014 tan 6

Resistor Ni Cr 10 TCRResistor Cr SiO,Semiconductor CdS 10-3Semiconductor PbTe P,Conductor Al Al-Si dw rConductor M oConductor Pt k-Si barrier heigh t

&

TABLE 111MBCCHANISMSF ELECTRICALONDUCTIONI DIELECTRICILMS

Thickness CurrentMechanism Range, P Dependence ExampleTunneling ~ 0 . 0 1 I - Vz ex p ( - k / v ) GaSeEmission, Schottky 0.01-0.5 I - T xp ( o @ / T ) Ta,O,Emission, Frenkel-Poole 0.01-0.5 I -T 2 xp (2aJE/T) S i 3 N 4Space charge limitedOhmic~~~ ~ ~~~

-1.0 I - V / X 3I - Vexp ( -b /T j S i 0of thermal expansion is important because of possibly disruptiveforces arising from a grossmismatch between ilms or between filmsand substrate.Electrical Properties

For microelectronics, the electrical properties of thin films areof prime importance.No other single characteristic spans as wide arange aselectrical resistivity, ranging from Q . cm or the mostconductive film to lo* cm for the least (SiO,). In fact, everytype of thin film can be characterized by its resistance and one ormore other electrical properties. This is illustrated schematically inTable 11. Recently, considerableattention has been paid to the con-duction mechanisms by which thin ilms of dielectric materials ex-hibit measurable electrical currents. These currents are non-ohmic,and he more important processes are tunneling, field emission,spacecharge-limited current flow, and internal field emission. Theseare shown in Table111along with some typical aterials. A strikingconfirmation of the theory of electron tunneling has been foundexperimentally for GaSe [38].Resistive films generally obeya relationship called MathiessensruleP = P ( T ) + P G ) + P@ )which equate theum of the temperaturedependent p(V, empera-ture-independent p(i) resistivities plus the geometry dependent term

p@ ) to the observed resistivity p . By controlling p ( T ) , it is possibleto get some control ofhe temperature efficient of esistance (TCR).In order to bring the values of thin-film esistors to predeterminedvalue, it is often necessary to adjust their resistance values. This iscalled trimming. The physical methodsof erosion or removal ofresistor material is usually ot feasible for thin films.A useful tech-nique is the thermal pulse trimming ofesistors. A series of currentpulses are allowed to heat the film, lowering its resistivity in smallincrements until the desired range is obtained [39].A model for a metal-semiconductor contact, whether ohmicor not, s a Schottky barrier in series with the spreadingesistanceof the bulk semiconductor. The metal-semiconductor contact al-

TABLE IVETCHANTh L U T l O N S FOR COMMON FI LM S

Film Mask EtchSiO,S i 3 N 4S i 0SiA1Au

KTFRSiO,KP RAZ-1350AZ-1350KPRNOS-HF H3PO4-HNOs

M o KPR~~Cr-SiTa

ways has an associated space-charge layer whose behavior is non-linear, but the contact is still ohmic if the space-charge layerimpedance isnegligible in comparison with he bulk semiconductoresistance. It is implicitly assumed that minority carrier injection iseither absent or negligible, and that trapcharging effectsare smallThe fabrication of an ohmic contact thus requires that the m-pedance of he ,Schottky bamer is low. There are two current t r a n s -port mechanisms in Schottky barriers: a) thermionic emission, andb) tunneling. These correspond, respectively, to the two main em-pirical ways of makingontacts: ) choosing a metal which maklow Schottky barrier with the semiconductor,r b) doping the semconductor heavily near the contact so that the barrier will be thinenough to be penetrated easily by tunneling [ a ] .Chemical Properties

The most important propertyfhin is their behaviortowards s p ed c etchants. This determines he usefulness of thefilm,for if high-resolution patterns cannot e generated y chemicalmeans, thefilm is of limiteduse.The etchrate and the edge resolu-tion attainable with a particular etching processs of great concernThe appropriate tching solutions nd procedures for somecommonthin 6lmsare given in Table IV, nd dataonother materials are s u m -marized in the literature [41]. The etch rate is determined by thefilm structure, density, impurity concentration, strain, and otherfactors. A correlation exists between such properties as etch rateand corrosion esistance and observablequantitiessuch as thevisible and infrared absorption spectra [42].Recently, significant advances in instrumental techniques havebeen made which allow muchher structural detail to be disclosedThe importantnew echniques of X-ray fluorescence, electronan-ning chemical analysis ESCA), and ion source spectrometry (ISS)allow, in effect, a quantitative analysis to be performedon he thinfilm. By using X rays, extreme sensitivity may be achieved inetect-ing trace impurities. ESCA allows local analysis of a small area,while ISS allows mass-spectrometric chemical analysis ofmono-layer of the film.Somewhat older but still useful methods employradioisotopes to study film behavior. These can be introduced ex-ternally, or generated in the film by neutron activation of naturalisotopes occurring n the film. Selective etching nd counting of ilmallows a profileo be constructedof the radioactive substance. Themethods have been useful in the study of the distribution of alkaliions and hydrogen in SiO, [43], [MI.An important chemical property of glass films is their abilityto resist diffusion of corrosive substances which can attack inter-connections, the semiconductor surface, or underlying oxide sur-faces. The ability to protect the edges of metallicilms is called edgcoverage, and is vitally important in productionf multi-intercon-nection level integrated circuits [45]. Recently it has been shownthat there are inherent reasons or the difficulty encountered n edgecoverage by evaporated films [MI. Metallic films for use in micro-


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GREGOR : THIN-FTLM PROCESSESOR MICROELECTRONICS 1395

TABLE VPROPERTIES OF METAU EMPLOYEDS THIN FILMS

LimitingCurrentDensityesistivityMetal JI l a n x IO x I O w 6R . a n RemarksSilverCopperGold

4.04.07. 0Aluminum 0. 5Aluminum +Cu.0RhodiumMagnesium

TungstenIndium 20.0Molybdenum 10.0PlatinumTitanium

1.591.672.352.654.454.515. 35. 65.79.855

poor adhesionpoor adhesion; corrosionsiliconeutectic 370C, poorsilicon eutectic 577C, elec-

~~

adhesiontromigration

extremely reactivepoor adhesionpoor adhesionda cu l t e tchingpoor adhesioncorrosion susceptibility

electronics are often judgedby such chemicalcriteriaas the thermo-dynamic stability of their oxide (since this is related to adhesion ofthe metal to underlying SOz) or the physical coherence of theiroxide.Coherent Al,O, acts o passivatealuminum films fromfurther oxidation. Copper films, on the other hand, must be pro-tected from formation of uzO, and olybdenum filmshave as theirmost undesirable feature heir susceptibilityof oxidation corrosion.A summary of some of the more common metalssed as thinhis given in TableV.

PATTERNENERATIONThe fabrication of modem solid-state electronic componentsplaces great emphasison he generation of accurate high-resolutionpatterns in thin films of a wide variety of materials. At this point,it is well to establish the differences between thinh nd the so-called thickh47]. Such differences lie in the physical thick-

nessof theilms and themethods of producing them.hin films werearbitrarily defined as being less than 5 p (50 OO0 A) in thickness,while thick films are greater than 5 p ; in fact, usually many t imesthicker than that.Thickh re generally produced by a direct physical applica-tion, such as silk-screening and reflowing, whereas the films to bediscussed in this paper are produced by more complex processessuch as chemical vapor deposition, vacuum evaporation, puttering,etc. Although thick and thin films are often employed together,the former will not be discussed in his paper since they are treatedelsewhere.There are three general approaches o the task ofpattern genera-tion in thin films. The first methods analogous to the formation of aphotographic image; i.e., a film of material is already present andis exposed to a pattern of radiant energy which defines the cor-responding imagein the film. The image iseither latent or overt; fthe former case, a development step, again analogous to photogra-phy, is required. Thisdirect generation method is only applicableoratherspecialized nstances [MI. The secondmethod,which ismuch more widely applied, is the indirect generation of a film pat-tern. The most common orm is the useof a photosensitive layer onthe film in which a pattern is desired. The photosensitive layer isexposed and developed, and then used as a protective mask whileetching away the exposed underlying material in auitable etchant.A modification of the processutilizes some means of sensitizing asurface in the desired attern, followed by selective growthr deposi-tion of he desired material only n he pattern areas 49]. The finalpattern definition processusesa physical barrier o intercept selectedportions of abeam of material before t strikes and condenses on asurface.This method is often called stencil masking, and is most

suitable for vacuum deposited films. Unlike the first two principalmethods discussed, there is no relianceon he controlled radiationprocess to form thepattern; all that is required is a physical maskStencil Masking

Stencil masking employs a arrier, or mask, interposed betweena particle stream and a surfaceon which the particles can condense.The pattern formed is hat of the openportions of the mask. Severalfeatures of this process are as follows. First, the mean free path ofa particle must be long compared o the mask-surface spacing. ec-ond, the sticking coefficient S,,, of the evaporant material should beclose to unity; this implies an upper limit restriction on he surfacetemperature. Third, the resolution of theattern is governed by theresolution of the mask itself; there are no physical limits (such asdiffraction) but there are obvious mechanical limits. Fourth, thereisa topological constraint inhat a completely connectedwodimen-sional pattern requires more than one mask.These constraints resulted in the use of masking for patterngeneration of materials whichanbe thermally evaporated in a higvacuum system(c torr). The pattern can bedefined to a resolu-tion of k0.0125 mm (12.5 p ) as a practical limit over an area ofmore than 25 cm2. In principle, the use of stencil masking can beextended to a wide variety of materials. In practice, the methodis used primarily for metallic films with some employment for di-electric materials suchas siliconmonoxide. The most complete ap-plication of stencil masking occurs in the fabrication of cryoelec-tronic circuits [50]. In this case, every element of the ircuit may bebuilt from afilm whose shape is defined y stencil masking.Mask Fabrication

The mostcommonmask materialsare metals and alloysbecauseof theease of manufacture and durability. The pattern of openingsin the mask can be made in a number of ways. The most commonmethod is to engrave, drill, or punch out the pattern in a metalsheet. This is usually done by a pantographic or numerically con-trolled system. The x-y coordinate table is moved by digital com-mands and controls an engraving tool which cuts the appropriatelines in a sheet of aluminumor the mask. Photolithographic meth-od s have been used to produce masks with perhaps the bestesolu-tion yet attained for stencils, the pattern is etched in very thin foilswith resulting mechanical fragility. Likewise, glass r ceramic maskshave beenmade by photoetching. he prospect of aser engraving ofmasks is interesting, but little has been reported concerning thispractice.Masks for high-resolution films can also be made by utilizing amesh of closely spaced fine wires 5 11. This technique replaces thesingle stencil mask by a combination of two masks: 1) a wire grillmounted close to the substrate and 2) an interchangeable metalmask mounted as close as possible to the wire grill. The gnll tech-nique is advantageous for depositing very fine atterns, particularlyof a repetitive nature.

PHOTOMASKINGVirtually all formation of high-resolution patterns is done byphotomasking and etching. This process uses a selective patterncalled a photomask o intercept certain portions and transmit otherportions of a beam of collimatedight. The transmitted light causesa chemical change in a photosensitiveayer, called a photoresist

on a surface closely adjacento the photomask.Photoresists

There are many organic compounds whose structure and solu-bility change when exposed to light, pamcularly in the ultravioletregions. The h t aterials found o havethisproperty were naturalproducts such as gelatine, and organic colloids of this type have


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1396 PROCEEDINGSOF THE IEEE,CTOBER 1971L I G H T

I L L U M I N A T E D P HOT DRE S IS l= A R E A S

t*\ UBSTRATENE GAT IV EE S IS T :OS IT IV EE S IS T : R E N D E R E DN S O L U B L EE N D E R E DO L U B L EI I

0.wt

Fig. 3 .

E T C H E D F I L M P A T T E R N S :&c3General principles of po sitive and negative photoresists for

pattern generation [41].been used for some time to produce scales, reticules, and for otherphotoengraving application. In addition to being light-sensitive, apractical photoresist system must also have the ability to form ad-herent uniform coats which are not destroyed by the physical orchemical action of theetching process. Lastly, selective solvents rerequired to develop and finally remove the resist pattern. Gener-ically, there are two types of photoresists. They differ in their re-sponse to light and their solubility. Materials which are renderedinsoluble by illumination yield a negative pattern of the mask andare called negative photoresists. Conversely, positive photoresistsbecome more soluble when subjected to light and yield a positiveimage of the m a s k . Both types of resist are utilized in practice andhave their particular advantages and limitations, Fig. 3.Negative Photoresists: Commercially available negative photo-resists, recommended diluting gents (thinners), and developers areincommon use. Practical information concerning recipes andformulation is readily available and particularly abundant orKodak products [ 5 2 ] . The pMc@ constituents of a photoresistsolution are a polymer, a sensitizer, and the solvent system. Thepolymers are characterized by unsaturated carbon onds apable ofreacting further and forming longer or crosslinked molecules.Thisreaction, however, must be stimulated by energy transferred by thesensitizer. The degree to which reaction and insolubility OCCUT de-pendson he exposureof the resist film.Polymerization of the resistswhile still in solution is neghgibly slow. Hence, resistsa y be storedin brown bottles for long periods of time. Small amounts of anti-oxidant compounds to m e r stabilize hesolutions and sur-factants to enhance wetting of the substratesurface are sometimesadded.Positive Photoresists: So far, there seems to be only a limitedgroup of commercially available positive photoresists, and theseproducts are discussed in the literature [41]. Of the resists listed,AZ-1350 is the most suitable for h e ine etching. Others, e.g.,A Z - 3 4 0 , arechemically similarbut have a higher solidscontent andyield thicker oats asrequired or deep etching and photoengraving.The low viscosity of AZ-1350 ndicates a relatively low molecularweight (21 OOO) of the resin. The solubility of the resin, afterevaporation of the solvent, is strongly dependent on he functionalgroups present in themac romol des. These groupsare responsible

0.03t

0.02I 1 0 . 50.01(96096 296 4 1966 1961( 197097 297 4

YEARFig. 4. Typical metal-6 lm linewidth capability as a functionof time (1961-1971).

for .thenitial insolubility of the resin in the developer which is anaqueous buffered sodiui~ydroxide solution.When he resistlm isilluminated, the sensitizer transfers energy to the functional groupsof the polymer which thereupon change and render the resist filmsoluble. The solubility is KOW~Y confined to the immediate icinityof the absorbing chromophores. Therefore, positive resists yieldgood resolution even in relatively thickoats.For example, 5 p widelinescan be developed in6 p thick layer.Photoresist Pattern Formation

Application and processing of photoresist films is a highly em-pirical art . The procedures vary in detail fromplace to place. Thuswhen a photoresist processing facilitys established, it isnecessary oadjust the conditions and process parameters until a stable pro-cedure s reached.Removal of Photoresist

The f d emoval of cross-linked polymer films is an oneroustask because these compounds are not ruly soluble. The degree ofdifEculty encountered dependson he nature f the photoresistfilm,its thickness, and the underlying substrate. In general, strippingresist films becomes more difEcult the higher the post-baking tem-perature. The most widely used resist removalechniques rely onhotchlorinated hydrocarbons towell the polymer in conjunction withacids to l o o s e n the adhesion of the resist ilm to its base.Oxidizingagents, likehot H,SO,, m ay be used to decompose the organic ma-terial, but film corrosion often prohibits such drastic action. Sub-sequent swabbing or brushing operations are nearly always neces-sary to remove tenaxiously clingingfibers or patches of the resist atthe risk of mechanicallyamaging the film pattern. The jet action ofspray rinses is generally not completely satisfactory, so a secondrinse,a common practice, washes away theoosened resist shredsin


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GREGOR : THIN-FILM PROCESSES FOR MICROELECTROMCS 1397A R T W O R K

1200. 1 )

L I G H T-___)7l T R A N S P A R E N C YI N T E R M E D I A T E( 1 O : l l

- /

M A S T E R1 1 : l ) S U B M A S T E R SO R K I N G MA SK S

i /R E P E A T \

- v 1 S T R E D U C T I O NN DE D U C T I O NO N T A C TR I N T I N G120x1 ( 1 0 x 1

Fig. 5. General method of fabrication of photomasks [41].

a forced stream of waterr solvent. Some successhas been achievedin nonliquid resist removal. Highly reactive media (ozone, 0, reeradicals, and plasmas) can convert the organic photoresist to CO,and H20 [53].Elevated temperatures (- 200C) are required, andthe plasma environments may e detrimental to devices sensitive osurfacecharges,such as MO S circuitry and high-voltage diodearrays.Resolution of PhotoresistsThe question of ultimate resolution attainable with the presentresists is of great concern in integratedircuit technology. The peedand density of these circuits m a y ultimately be limited by size-imposed restrictions in the photoresist technology. The problem ofresolution involves several factors. A single bridging between twoclose-spaced lines caused by a particle of foreign matter in the filmmay be interpreted as a resolution problem, whereas, actually, thetrue resolution capability of the resist has not been approached.It appears that the ultimate resolution of the present resists willnever be achieved in any ractical application. Lines 0.0o01 n wideseparated by O.OOO1-in spaces are possible with present techniques.The resist must be carefully filtered and meticulous cleanliness ob-served. This assumes that a uniform resist film 0.0001-0.0002 inthick canbe applied,and relates only to short ines. The decrease inminimum practicable linewidth for integrated circuits during thepast decade is shown n Fig. 4.The material to be etched away should, in general, not exceedthe resist ilm thickness,if resolution is o be determinedby the film.For deep etching, the amount of undercut by the etchant must betaken into account and compensated for in the dimensions of theimage. Of the wo general photoresisttypescommercially available,the positive substances seem to possess higher resolution and arenot assensitive to fogging by oxygen from the atmosphere. How-ever, negative resists may offer somewhatbetter adhesion to under-lying surfaces.Fabrication of Photomasks

General: The production of photomasks begins with theriginalpattern layout several hundred times larger than the final pattern.A specialcamera semployed to produceasmaller mage ofthe pattern on a photographic plate, usually to 10 t imes the finalsize. This reduced master image is then once again reduced op-

tically and printed out to the final size on a master photomask,Fig. 5. To achieve the maximum accuracynd resolution of thehaphotomask (which earlier was seen to be -0.oooO5 in), great caremust be takeno insure that the riginal layout is as sharpaspossible.Instead of drawing theattern, its cut from a material offering hcontrast and attached to a base. The base material is a polyesteriilm such as Mylar, while a variety of pattern materials have beenused such as Rubylith and Studnite [41]. This master image ( x 10)is then reduced and projected onto special emulsioncoated glassplates which are optically flat surfaces and free of defects either inthe emulsion or the g lass . Since the workingimage s much smallerthan the semiconductor area on which it will be projected, hemethod used produces n arrayof multiple images x 1) of he pat-tern. Using one method, a special camera projects the image ontothe emulsion. Then the emulsion is movedor stepped) a certaindistance, and the projection repeated. This method is thus calledstep-and-repeat.Anothermethod nvolves a specialmultilensprojection system which simultaneously projects separate imagesfrom a single source. This is referred to as the flys-eye method,because of the resemblanceof the multiple lens o that of the com-pound eye of the fly [54].The conventional technology of masks is expensive since thewear occurring when the mask touches the photoresist surface re-quires frequent replacement. Also, the numerous small errors andrandom dimensional changes whichccurduring the complex masmaking process generally cause particular mask to align properlyonly with others of its ownparticular set. Finally, the drafting andcutting operations are slow and difficult to check for error.A num-ber of recent developments have soughto mitigate these problemsBy using a thin film of chromium on a glass blank, the patternof a master emulsion mask can be transferred via conventionalphotomask operations to the much more durable chromium film[ S I . Also, the edge acutance of the chromium pattern is better.Another method for extendingmask life is o coat he emulsionsur-face with a protective layer of transparent material suchs SiO, or apolymer film.

Light Beam Mask Fabrication: The development of automaticpattern generation equipment allows a focused beam of light toexpose the master emulsion selectively. Selectivity is achieved byprogramming the motion of the emulsion plate underhe light beam


9/14

1398 PROCEEDINGSOF THE IEEE,OCTOBER 1971

and a shutter. The program is generated by using the appropriatelanguage to translate a wiring diagram into an interconnection pat-tern by combining logical layout rules with dimensional or geo-metric rules. Error checking and generation of copies is accom-plished moreeadily and quickly, and modificationare rapidly trans-formed into working photomasks [56].Electron beams offer an order of magnitude increase in resolu-

tion. Furthermore, the high energy density and ease of beam de-flection makehe production of patterns by tracing with a deflectablebeam seem potentially apid. However, a beam of charged particlesquires an elaborate ystem for guidance and control.Considerable study of irect exposure of resist ilms by electronbeamshas led to p r e m sage ofthismethod. High resolutioncan be achieved as well as high pattern formation rates. The earliestattempts using direct electron exposure employed fixedbeams,butmore recently sophisticatedbeamdeflection methods have been de-vised [57]. With thismethod, linewidths of p are readily achieved,and spot diameters of 100 A are attainable. A variation of directpattern generationemploys a beamofconsiderableenergy tovolatilize the resist ilm, he remaining ilm functioning as a positiveresist. This method is applicable to forming holes in the resist ilmfor subsequent etching ofia holes, formation of vias,etc.

A rapidexposure process using lectron bombardment hasbeenreported [58]. An mage of the pattern is produced by secondaryelectron emission from the mask image, with n accelerating poten-tial to direct the emitted electrons to the wafer surface. Thelectronbeam is thus analogous to ultraviolet light and the entire s u r f a c e isexposed in a very short time. Since image reduction is ot easy, theprocess requiresascareful mask makingas in conventional photo-lithography. Recently, there haseen considerable interest in holog-raphy due to the rapid development of laser optics.ince no ensesare involved, off-axisaberrations are o problem. Furthermore, de-fects inhe holograms are notransmitted along with the image [59].Maskless Pattern Formation

Contact photomasking has certain difficulties associated withpractical use, such as unwanted mask defects, transfer of materialby contact, wear of the emulsion, and resolution limits due to dif-fraction. Many of thesecan be overcome in principley eliminatingthe mask and directly exposing he resist film. This canbe done byprojecting an image of the desired pattern onto the film using afocused spot of lightor beam of electrons to trace out the pattern, orusing irradiation to promote a chemical reaction and deposit orremove material [a].Projection ofan optical image requires n optical system f suchperfection that thisprocess has yet been widely adapted. Theprin-cipal problems are the difficultyof maintaining a flat focal plane forthe image over large areas, and the alignment oftwo or more pat-terns. The ultimate limit of resolutions imposed by the diffractionof light, hence this method does not offer a large potential for in-creased resolution over contact masking. A direct process lens re-duction scheme is shown in Fig. 6.Subtractive Etching

A wide variety of etchant solutions have been used to subtrac-tively etch metals, dielectrics, and other thin film materials. Thechoice of etchant depends on he nature of the system used o bringthe etchant and film together. In general, the etch olution is either,s t i r r e d or agitated soas to continuously bring fresh reagent incon-tact with thefilm.Alternatively, spray etchingan be used; the pur-pose is to maintain a constant concentration of reagent and hence aconstant rate of etching. Beeause of several necessaryor highly de-sirable restrictions, the actual number ofsuitable etchants is reducedgreatly. In general, theseconditions are as follows.

1WO WATT MERCURY BEAM SPLITTER(1.25 DIAMETERF IL M P L ANESILICON WAFER1-- --\_

e_/-- -

I I1 1 1

~ PH ~TO G R A PH I C \ L E N Sj MASK OF ATTERNOSTAPKAR

UOal I II 1I II I

I

CONDENSER I I

VIEWING SCREENFig. 6. Formation of patterns by direct projectionof image onto surface.Shown is a x 10 reductionof pattern onto a wafer.

2) The etchant must selectively attack the thin film and not the3) Formation of gas bubbles is highly undesirable.photoresist or su r f a ces under the thin film.

A list of common tchants for thin films was given previously iTable IV.The most widely sed etchants are bufered aqueous solu-tions of HF, specrfic for SiO, and silicate glassm, hich do noattack or swell photoresist films and generate no troublesome gasbubbles. For metal films, various mineral acid solutions can beemployed if the metal is not reactive. Noble metals suchas PI andAu require complexing-type chemical reagentsor etching.Some substances are difficult to etch by conventional methods,and special techniques have been devised. Silicon nitride is an ex-ample; it is difficult o etch Si,N, with buffered HF because he lowetch rate results in photoresist deterioration before the process iscompleted. However,t hasbeen ound that hot oncentratedH 3 W 4etches Si3N, much faster than SiO,. Hence, a layer of CVD SiO,is depositedon Si,N,, standard photoresist used to develop the pat-tern in the SiO,, and the hot H,W, then etchesout the Si,N, [61].Sputtered Bi,03 is a convenient etch resist film which can be de-posited as low temperatures [62].Reverse Photolithography

Thin filmsof substances iilicult to etch may e patterned by useof reversephotolithography which isoften referred to as liftoff orstud etching [63]. In this procedure, a base film is deposited andformed into a pattern which is the obverse of the one desired. Thenthe desired material is deposited over the base film and surface.Treatment in a medium which attacks the base film but not theoverlying film produces the desired pattern. Among the substancesused for the base film have been Al, KPR, MgC1,. The processworks best when he base film is several times thicker than theoverlying film. A schematic illustration of the process is shown inFig. 7.An extension of his method, using electron bombardment, hasrecently been employed to etch patterns in silicon oxide layers forplanar transistor fabrication. The etch rate of the bombarded filmwas 2-3 times faster than theuntouchedregions [&I. Electronbombardment must be done under conditions which do not formpolymerized pumpoil layer on he surface, for electron-beam poly-merized 6lmscan act asa resist to etching [65] .Sputter Etching

Certainmaterials are difiicult to etch with onventional chemicalreagents. This is due to their extreme inertness (as n the case ofsiliconnitride or cermet films)or becauseof the ariability of etchin1) The etch rate musteairly rapid. behavior (siliconnitride,siliconmonoxide).ecently, the processf


10/14

GREGOR: THIN-FILM PROCESSES FOR MICROELECTRONICS 1399,-,S /

I

N E G A T I V ERE L IE F MASK

SUBSTRATE

DEPOSITEDF I L M

Fig. 7 . Method of ilm etching known as lift off [41].

sputter etching or subtractive removal of material y ion bombard-ment hasbeen developed [66]. The method dependson he fact thatmost materials have lowenergy sputtering yields thatare approx-imately the same. Hence, a photoresist film and another film willboth erode by positive ion bombardment at about the same rate. Ifthe photoresist is hicker, it will still be present after the underlyingfilm is completely removed. The advantages of this etching methodare its generality and independence of material roperties, its faith-ful reproduction of the resist pattern, and its cleanliness. Disad-vantages are the necessity for vacuum and RF equipment, batchprocessing (limited quantity), and straight film pattern edges. Thelatter often causes problems with subsequently depositedilm. Dueto the phenomenon of resputtering, some of the removed film willfind its way back to the surface unless a catcher is usedo retain thesputtered material [67].

h m PPLICATIONSThe fabrication process or most silicon nd other emiconductormicroelectronic devices is based n he production of precisely de-fined areas of controlled dopant concentration in the surface regionof the semiconductor. Modem microcircuits would not be possiblewithout the ability to dope selectively to a resolution whose precisionis measured in microns. This is possible because a arge number ofdopant atoms arelocked from diffusing nto the semiconductor bya thin ilm of SiO,, Si3N,, etc. The unique combination of propertiesof SiO,thin ilmshave made LSI possible. Thesere its simultaneous

ability to electrically passivate silicon to block diffusion of boron,phosphorus, and arsenic, and to be etched selectively by bufferedHF solutions.Thin filmsof SiO, are also capable of shielding under-lying substances from ion penetration, dependingon the energy ofthe ion. Thus the doping of semiconductorscanbe carriedout usingion beams to implant the desired dopants and SiO, to act as themask [57].The source of dopant atoms for diffusion into the surface isusually from a layer deposited from some ambient gas. However,the sourcecanbe a liquid suspension coatedon he surfaceand thenconverted to a solid film at elevated temperatures. This method,called paint-on diffusion, was discussed earlier as an example ofphysical deposition ofhinfilms [26]. This method maybe useful nsimultaneous formation of p nd n-channel FET devices in com-plementary MOS circuits.

Thin filmsof SiO, re often grown n Si with thententof getter-

ing impurities from the Si. In particular, those fast-diffusing im-purities which decreasecarrier lifetime in bulkSican diffuse to andbe trapped in the SiO, layer [68]. The backside of the Si wafer isthe most likely place where gettering occurs.Thin films of organic photoresistare ubiquitous in pattern gen-eration, but other materials are occasionally employedsetch resists.As mentioned earlier, SiO, films are useful as resists for etchingSi3N4 with hot H3P04 [61]. Films of chromium-silver have beenshown to resist concentrated HF solutions, allowing Si3N4 to be

etched rapidly 69]. Occasionally, thin ilms perform bothas processaids and as functional elements of the device. An outstanding ex-ample is the use of polycrystalline silicon in the silicon-gate MOSdevice [I I]. The silicon film is used as thediffusion maskso that thesource-drain spacing is automatically aligned to the gate electrode.Afterwards, the silicon functions as the gate electrode in circuitoperation. Likewise,filmsof Si02are useful in allowing the methodof dielectric isolation to be employed n complementary circuitfabrication.

DEVICE PPLICATIONSMost of the important phenomenahat govern the function of a

microcircuit or device are embodied in thin films. On the devicelevel, the functions of surface passivation, connections, isulation,terminals, and environmental protection are performed by films ofmetal, glass, SOz,etc. From a circuit standpoint, the ntroductionof resistance, capacitance, voltage and current paths, interconnec-tions, and terminal points involvehe use of thin ilms.The stabilityand reliability of the component in use are largely dependent onthe properties of the thinilms n its structure.The passivation of the surfaceof a semiconductor is a complexproblem. In chemical usage, the term passivation conventionallyrefers to the process of rendering a surface unreactive. Examplere :iron in concentrated acid and aluminum in contact with the at-mosphere. Thus the process involves the formation of a layern hesurface whichnhibits further chemical eaction. In the casef semi-conductors, not only chemicalut also electrical stability is requiredfor a passivated surface. Hence semiconductor surface passivationmeans electrical as well as chemical stabilization.It is appropriate todivide the passivation of semiconductorur-faces into two major categories. First, in primary passivation, theobjective is the controland stabilization of the semiconductor sur-face electrical properties. Then there is the secondary passivationwhich m a y be thought of as the protection or stabilization of theprimary passivating medium. The secondary passivation methodalso performs the functions of insulating and protecting the inter-connection and terminal metallurgy as well as providing overallmechanical and chemicalprotection.Secondarypassivationsnormally provided by some form of thin-film overlay, although thearlier practice of bonding the unprotected chip and hermeticallysealing the unit in a container is still widely practiced. For MOScircuits, which are more sensitive to stray charge on the oxide sur-face, deposition of some form of protectiveielectric layer over themetallurgy and the thermal SiO, is highly recommended,s attestedby numerous manufacturers [70]. The most popular methods areCVD or sputtered SiO, for integrated circuits, and fused glass fordiscrete components. Among more recent candidates for this pur-pose are silicate glasses containing phosphorus, aluminum, lead,silicon nitride, or poly-pxylylene. I t should be noted that plasticencapsulation, in which the circuit or component chip s sealed intoa molded polymer package, generallyequires a reliable secondarypassivation method as well as the plastic package.The use of deposited S i 0 2 is neceSSary for MOS integrated cir-cuits for a reason beyond those mentioned.iO, passivation makesthe circuit less sensitive to surface charge migration whichauses n-


11/14

1400 PROCEEDINGS OFTHE EEE. CTOBER 1971- V

-v > v T I F -a IS L A R G E OR x IS SMALLVI

I \

I P+ I I P t IFOR SAME -R

VT LARGE BECAUSE X IS L ARGEFig. 8. Cross section ofsilicon ntegra ted circuit illustrating effectof charge-Q on insulating film surface. Silicon surface inversion caused by -Q(top) is preventedby interposing a sufiiciently thick dielectric layerbetween -Q an d silicon (bottom).version of the underlying high-resistivityilicon, eeFig. 8. AlthoughCVD methods are more popular, it has recently been shown thatsputtered SiO, also can be used for thispurpose [72]. Fortunately,thin layers of SiOl are also excellent insulators, and furnish theinsulation between he semiconductor and the metal filmsused forcontacts, gateelectrodes, and interconnections. By usingSiO,,capacitancecanalso be introdwed on he semiconductor, althoughlimited to small values by the relatively low dielectric constant ofSiOt and the smallelectrode areasafforded.The formation of electrical contacts to the semiconductor isachieved by etchingvia holes at the appropriate places in the oxidelayer and depositing a metallic layer whichmakes a low-resistancecontact at the via hole. Subsequently, an interconnection pattern isetched in the metal film. For complex highdensity bipolar inte-grated circuits, a second or third layer of metallization is usuallynecessary to allow complete wiring of the circuits whichequiresanadditional insulating layer, generallyiO,. The most commonmetalused s aluminum, pure or alloyed with soluble metals. Basedn heusual criteria of low contact resistance, high conductivity, ease ofetching, and chemical stability, aluminum is easily the best choice.However, there are certain disadvantages such as mechanical soft-ness, relatively low Al-Si eutectic temperature (577C), and lowcurrent-density limit, which have stimulated the development ofother metallurgies, such as Cr-Ag, Mo-Au, Pt, and Ti. A com-parison of the important features of these metal films s given inTableN.Although polycrystalline Si would ot normally be considered apossible material for thin-film interconnections, thinfilmsof Sihavefound use in a related area, he gate electrode material for MOSintegrated circuits. Sincet isalso employed s part of the diffusion-masking step, the Sian not be used for contacts toource and drainregions. However, itanbe used in ertain ways to assist in the inter-connection task, and canalso be used as a guard plane to protectagainst surface inversion [73]. Molybdenum films have been em-ployed in similar fabrication schemes [74].Terminals for microelectronicdevices and circuits are often madeby thin-6lm methods. The simplest method merely extends the ter-minus of an interconnection line to a su5cientlyarge area to allowwire bonding by ultrasonic or thennocompression methods. Moresophisticated techniques involve the formation of beam leads,

relativelymassive extensions of the interconnection metallurgysuitable for welding to chip carriers or ead frames. Beam leads areusually formed by electroplating a relatively heavy deposit of ametal, such as gold, on the thin film interconnections. Anothermethod is the solder reflow fabrication of terminal bumps byallowing a fusible thin-film deposit such as Sn-Pb to flow into andform a pad on a previously deposited terminal area, such as Cr-Cu-Au. The solder bump is then hsed to the appropriateconnectionson a substrate [74].Because of their smallsize and sensitivity, certain hin films havefound special applications as photosensitive, heat sensitive, andstrain sensitive devices. The most common are photoconductive

films such as CdS and CdSe [75]. Thin-film thermometers whichmeasuremaximum temperature have been made from Cr-SiO,exploiting theseilms temperature coefficient of resistance76]. Theuseof ead-only memories fabricated from MOS circuit arrays hasbeen stimulated by the development ofprogrammable ROM arrays(PROM). Unlike the earlier custom devices made by specially pro-grammed interconnection patterns applied to a standard array,helatter consist of arrays in which speclfied address locations can bepermanentaly denoted by disconnecting a linkat this point. This isusualIy done by applying a currentpulse to open a fusible thin-filmmetallic connection.

FUTUREDEVELOPMEKISThe futurecourse of thin-film technology developmentappearsto be inthreemajor directions: themprovement and expansion ofpresent capabilities, the exploitationof novel thin-film phenomenafor electronicuse,and the establishment of pure thin-filni electronicThese are not completely independent and overlap to some extent.

Also, the past decade has shown that technology forecasting s not acompletely accurate science. In the area of thin films, the consider-able potential growth predicted for cryoelectronics failed to ma-terialize, while the capabilities of MOS LSI were scarcely dreameof. Bearing his n mind, a brief discussion of theseareas of development will be given.Expansion and Improvemen1

The understanding of some ofhe basichin ilm deposition meth-ods is well-founded (vacuum evaporation, anodization) but othermethods are just eginning to become unitied in erms of models tounderstand the present and predict future behavior. In particular,sputtering and chemical vapor deposition seem to have significantuntapped areas as far as better control of film properties, moreflexible deposition rates, and upscaling of production capabilityare concerned. It now appears possible to produce a rich variety ofchemical compounds and mixtures of thin films by either method,with sputtering being the more diversified, since it isot restricted tomaterials resulting fromhermodynamicallyfeasible reactions. Likwise, by using argets of mixed composition, a vast number of filmscan be produced whose properties can be altered subtly by varyingthe method of deposition.The extension of surface passivation by thin films from siliconto othersemiconductors is another areawhere improvement can beexpected. Inparticular, he reliability and lifetimeofelectro-luminescent diodes fabricated from GaAsP and other 111-V semi-conductors should be increased through better understanding ofsurface passivation. High erformance devices may also e possiblein these high-mobility materials, if planar methods of fabricationcan be extended successfully from silicon.In silicon, better understanding of the natureof thin oxide filmscan be expected, along with improvements in metal films, photo-processing, etc., to push the size of integrated circuits o a ensity of4-8 times greater than achieved at present. In particular, the nter-


12/14

GREGOR THM -FILM PROCESSES FOR MICROELECTRONICS 1 4 0 1

action of the various thin-film processes with each other will besufficientlyuntangled to allowdynamic and regenerativemulti-phase circuit designs to be implemented in nchannel MOS tech-nology. Likewise, the further development of high-speed Schottky-barrier MOSFET devices can be expected to progress as rapidlyas metal-silicon barrier technology and projection photoprocessingadvance [78].The demands for metallurgy capableof handling highercurrentdensities than aluminum will probably continue, but t is difficult oforecast any breakthroughs inhis area, since almost allf the likelymetals and alloys have already received sufficient ttention tobringout their merits and weaknesses. For bipolar integrated circuits,thin filmsas Cr, Ag, nd Ti ill probably find increased usage.Recent developments in the technology of substratesuitable forheteroepitaxial deposition of Si films may open up an era of SOS(silicon-on-substrate) ntegrated circuits. Both sapphire and spinelsubstrates appear suitable, with the latter material becoming morepopular. The excellent isolation and minimized unction capacitanceof SOS bipolar circuits, aswell ascompatibility of the MOS devicetechnology and the transparency of the substrates, may allow theformation ofhighlycomplexmixedarraysofsiliconswitching,storage, and driver circuits with various kinds of optoelectronicfunctions.

With increased understanding and control of thin-film process-ing, it will be feasible o combine themore advantageous features ofseveral device technologies into complex circuits and subsystems.For instance, the combination ofMO S and bipolar transistors on amonolithic silicon die combines the packing density and high im-pedance of the former with he high-performance surge protectionof the atter. Another example is theseof Schottky-barrier diodesin FET circuits to increase speed; the ET itself mayusea Schottky-barrier gate [77].Novel Phenomena

A number of effects involving thin films undernvestigation holdsome promise for future applications. The variable-threshold FETdevice is n outgrowth of the workn Si,N, films for passivation ofoxidized Si and other whiconductor urfaces. It was observed thatwhenSi3N,-Si02sandwichesweresubjected to relativelyhighvoltages in MIS structures, the flatband voltage could be shiftedconsiderably.Theseobservations ed to themetal-nitride-oxide-silicon (MNOS) FET, whose threshold voltage can be reversiblyshifted by applying a voltageulse. The potential applicationsare inhigh density storage1 bit/device), electronically set ROM, and on-volatile storage. At present, the most efficient form of the devicerequires that the SiOz thickness be thin enoughor efficient tunnel-ing of electrons, -21A. Considerable fundamental and applieddevelopment are still needed to understand the process of chargeretention and to establish values for reproducibility and reliability.Another class of device receiving much notice is the variable-resistance or bistable thin-film diode. This deviceconsists of a thinfilm of a material between two electrode films with a high dc im-pedance whichdrops rapidly at some voltage and remains lowntilthe voltage polarity is reversed. The most popular materials havebeen amorphous semiconducting chalcogenide l ass films preparedby vacuum evaporation 78]. Considerable workhas been done withNb,O, filmsproduced by anodization [79]. Relatively high switch-ing voltages are required and much is still unknown about the de-tailed mechanism of the transition or its consequences to devicereliability and lifetime. Adifferent type of bistable behavior is mani-fested by certain thin organic polymer ilms. In this case, the devicecan be read at low voltages to determine its previous history [SO].A new semiconductor device called a surfacecharge transistoris a circuit element with three electrodes-a source, a transfer gate,

and a receiver electrode. It uses a novel concept for controlling thetransfer of electrical charges across the surface of a semiconducto[81 1 . Fabrication of the device begins with a silicon surface covwith a layer of insulating film10oO A thick. The source and receiverelectrodes-separated by a narrow slit-are formed by depositinga layer of refractory metal, such s molybdenum, over thensulatingfilm. A secondnsulatingfilm is deposited over theselectrodes, andthe third narrow electrode-the transfer gate-is then deposited sothat it overlaps the thin slit between the source and the receiverelectrodes. The transfer gate controls the transfer of charges be-tween the source (higher level)nd receiver (lower level) lectrodes.Only a small mount of charge on thegate is required to control thetransfer of a much larger charge across the gap between the elec-trodes. As a result, the device has both charge and voltage gains.The Josephson effect refers to the tunneling across a thin dielectric film of paired electrons from one superconducting film to an-other [82]. Most of the experimentaltudies have been performed onPb filmsonwhich a film ofbO about 20 A is grown. The device isnextremely sensitive detectorndhashighly nonlinear characteristics.Many possible applications have been discussed, and undoubtedlyothers will be developed as the experimental techniques are im-proved. Since the films superconducting, a cryogenic environment(liquid helium) is required. A tunneling cryoton basedn a Joseph-son junction has een studied or use as a possible switching elemenfor logic applications [83].A simple devicefor storing information in a circulating fashionhas recently been nvestigated. The principleof operation is the or-mation of a depletion layer in high-resistivityiliconat the surface byapplying a voltage o a thin film on he oxide surface[84]. By usingappropriately shaped electrodepatterns and/or multiphase voltagepulses, the depletion layer charge can be shifted serially. Sensing ofthe capacitance of theMO S dot determines if the charge is r is notpresent. The device hasas its major advantage he simplicity ofcon-struction. Since the depletion layer can be altered by photo-gene-rated electron-hole pairs, the device has some possible applicationsin photodetection. Other potential ses are in shift registers.

The phenomenon mentioned previouslys sometimes referred oas an electrostatic bubble because of its partial analogy to thewidely publicized magnetic bubble or domain formation whichoccurs in certain materials [85]. The best known are orthofenitesand garnets. These are fabricated in the form of platelets carefullycut so that the easy xis is perpendicular to the plate. Upon applyinga magnetic field, circular domains 0.0024.005 in in diameter areformed. By using appropriately shaped thin film electrodes, calledt-bars, the domain can be propagated from one spot to another[86]. ensing can e done by use of the Faraday ffect, which renderthe domains visible with polarizedight. Many applications suchasshift registers, circulating storage, logical functionsof binary multi-plication and division, etc., have been demonstrated.Pure Thin-FilmElectronics

For many years, efforts havebeen made to develop an electroniccomponent or circuit based entirely on thin films. The chief benefitsof such an approach would be lowcost fabrication plus certainexotic features for specific technologies. In the past, a number ofdevices were considered, but no serious contender to the semicon-ductor single crystal exists today. Some of these wayside devicesrelisted in Table VI. There do not appear to be any thin-film deviceswhich are likely to supplant the present monolithic technology, butsome interesting developments are possible. The bistable switchingelement described in the previous section is a pure thin-film deviceas made today. Likewise, Josephson-effect junctionsre made fromthin films. These two classes of device, however, exhibit o gain or


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1402

TABLE VITHIN-FILM CTIV EEVICFS

~~

Device RemarksSpace-charge limited triode lack of suitable materialHigh-field triodeHot-electron triode nonreproducibleMetal-base transistorTunnel-emission triode tunnelling difficult to controlnonreproducibleThin-lilm field effect transistor low carrier mobility; instabilitySuperconducting

In-line cryotron low temperature required; complicated in-Crossed-6lm cryotron lowgain and performanceterfaceResonistor complex structure

require an exotic environment. A general-purpose thin-film activedevice does not yet seem to be available. Thin-film transistors ofvarious sortsare still being tudied; recently it was reported that TFTdevices using seleniums he semiconductor have een fabricatedonflexible plastic film substrates by an inexpensive batch fabricationprocess 1871. However, the use of such devices wouldappear to belimited to low-performance applications.The limitingcost baseof electronic functionswill probably turnout in the future to be the siliconwafer itself. When that point isreached, and if the basic understanding of thin films continues tobroaden and deepen, there may once again be intensive activity inthe tudyof hotelectron triodes, metal-base transistors, TFTdevices, etc. Phenomenasuch as electrochromic films, polymertunneling barriers, or high-temperature superconductors maylead to devices as yet only dimly perceived. The potential for ex-tremely low cost nd wide applicability still exists.

REFERENCESL. Holland, Ed., Thin Film Microelectronics: The Preparation andProperties of Componentsand Circuit Arrays. New York: Wiley, 1965.L. V. Gregor, Passivation of semiconductor surfaces, Solid StateTechnol., Apr. 1971, pp. 37-43.P. Balk, C. F. Aliotta, and L. V. Gregor, Microstructural properties ofthermally grown SiO, layers, Trans.Metall . SOC.A I M E , vol.233,1965, p. 563.B. E. Deal and A. S. Grove, General relationship for the thermaloxidation of silicon,f . Appl . Phys. ,vol. 36, 1965, p. 3770.L.Young, Anodic Oxide Films. London, New York: Academic Press,1961.P. F. Schmidt and D. R. Wonsidler, Conversion ofsiliconnitride filmsto anodic SiO,, f . Electrochem. SOC.,ol. 114,1967, p. 603.P.W. Anderson and J. M. Rowell, Probable observation of theJosephson superconducting tunneling effect,Phys. Rev. Let t . ,vol. 10,1963, p. 230.E. C. Ross and J. T. Wallmark, Theory of the switching behavior ofMIS transistors, RCA Reo . , vol. 30, 1969, p. 366.A. Waxman and K. H. Zaininger, Al,O,-silicon insulated gate fieldeffect transistors, Appl . Phys. L et t . , vol. 12, 1968, p. 109.RCA R ev. (Special Issue on Chemical Vapor Deposition of Conducting,Insulating, and Semiconductor Films), vol. 12, Dec. 1970.J. C. Sarace, R. E. Kenvin, D. L. Klein, and A. Edwards, Metal-nitride+midejilicon FET with self-aligned ate,Solid-state Electron.,vol. 11, 1968, p. 653.T. L. Chu, Dielectric materials in semiconductor devices,1. Vac. Sci.E. J. Mehalchick nd M. B. MacInnis, Preparation ofvapordepositedTech., vol. 6, 1970, p. 25.Tungsten at atmospheric pressure,Electrochem. Technol.,vol. 6,1968,p. 66.H. F . Sterling and R. C. G. Swann, Chemical vapor deposition pro-L. Holland, Vacuum Deposition of Thin Films. London, England:moted by RF discharge, Solid-State Electron., vol. 8, 1965,p. 653.Chapman & Hall, 1963.E.H. Snow and A. S. Grove, Radiation study on MOS structures,Contract AF 19(628)-5747, Rep. 1,Air Force Cambridge Res. Labs.,June 1966.grated circuits,Proc. IEEE, vol. 54, Nov. 1966, pp. 1521-1527.L. Braun and D. E.Loo& Precisionthin-film cermet resistors for inte-G. K. Wehner, Advan. Electron. Electron. Phys.,vol. 7, 1955, p. 239.P. Davidse and L. I. Maissel, Dieletric thin film through RF sputter-ing,1. Appl. Phys., vol. 37, 1966, p. 574.

PROCEEDINGS OF THE IEEE. OCTOBER 1971

[20] R. Frank and W. Moberg, Preparation and properties of reactivelysputtered silicon oxynitride, f . Electrochem. SOC., vol. 117, 1970, p.524.[21] L. I. Maissel, R. E. ones, and C. L. Standley, Re-emission of sputteredSiO, during growth and its relation to 6lm quality, IB M J . Res.Deoelop., vol. 14, 1970, p. 176.[22] G. C. Schwartz and R. E. Jones, Argon content of SiO, films de-posited by RF sputtering in argon,IBM J. Res . evelop . , vol. 14,1970,[23] J. S. Logan, Control of R F sputtered6lm properties through substratep. 52.

[24] E. M. Davis, W. E. Harding, R. S. Schwartz, and J. J. Corning, Solidtuning, I B M J . R e s .Deoelop., vol. 14, 1970, p. 172.logic technology: Versatile, high-performance microelectronics,IBMf . Res. Develop., vol. 8, 1964, p. 102.[25] E. F. Platz, Solid logic technology computer circuits-Billion hourreliability data, Microelectron. Reliability, vol. 8, 1969, p. 55.[26] Emulsitone Company, Livingston, N. J., company publication.[27] R. M . Handy and L. C. Scala, Electrical and structural propertiesofLangmuir films, f . Electrochem. SOC.,vol. 113, 1966,p. 109.W. H . Simpson and P. J. Reucroft, Quantum-mechanical tunneling inthin films of chlorophyll-a, Thin Solid Films, vol. 6, 1970, p. 167.L. V. Gregor, Polymer dielectric films, I B M f. Res . Deoelop .,vol. 12,1968, p. 140.of linear poly-p-xylylenes,f . Polymer Sci., vol. 4, 1966, p. 3027.W. F. Gorham, A new general synthetic methodfor the preparationS. M . Lee, J. J. Licari, and I. Litant, Electrical reliability of Parylenelilms for device passivation, Metall. Trans., vol. 1, 1970, p. 702.L. N. Alexandrov, E. I. Dagman, V. I. Zelevinskaya, E. I. Patrosjan,conductor films deposited by electrical explosion, Thin Solid Films,and P. A.Skirpkina, Peculiarities of formation andproperties of semi-vol. 5, 1970, p. 1.J. M . Nobbs and F. C. Gillespie, Properties of thin films ofZnO pre-pared by a chemical spray method, 1. Phys. Chem. Solids, vol. 31,W. A. Pliskin and R. P.Esch, Refractive index of SiO, l m s grown on1970, p. 2353.silicon, f . Appl. Phys., vol. 36, 1965, p. 201 1.A m . , vol. 54, 1964, p. 198.M . Hacskaylo, Refractive index of thin dielectric films,f . O p t . Soc.N. Hamck, nternal Reflection Sp ectroscopy. New York: Interscience,1967.L. V. Gregor, Study of silicon nitride as a dielectricmaterial for micro-electronic applications, Air Force Avionics Lab., Wright-PattersonAFB, Dayton, Ohio, Tech. Rep. AFAL-TR-68-272, Nov. 1968.C. A. Mead, High field current flow processes inhin insulatingfilms,presented at Electrochem.Soc.Meet., Los Angeles, May 11, 1970.heatings, IEEE Trans. Parts, Mater.,P a c h g . (Corresp.), vol. PMP-2,M . H. Monnier, Trimming thin 6lm resistors by direct resistanceMar. 1966, pp. 44-45.A. Y. C. Yu, Electron tunneling and contact res istance of metal-silicon

act barriers, Solid-State Electron., vol. 13, 1970, p. 239.[41] L. I. Maissel and R. Glann, Eds., Handbook of Thin Film Technolozy.New York: McGraw-Hill; 1970, ch. 7.W. A. Pliskin, The evaluation of thin film insulator, Thin Solid Films,vol. 2, 1968, p. 1.E. Yon, W. H. KO, and A. B. Kuper, Sodium distribution in thermaloxide on silicon by radiochemical and MOS analysis, IEEE Trans.Electron Deoices, vol. ED-13, Feb. 1966, pp. 276-280.P. J. Burkhardt, Tracer evaluation of hydrogen in steam-grown SiO,lilms, f . Electrochem. SOC., ol. 114, 1967, p. 196.C. L. Standley, R. E. Jones, and L. I. Maissel, Sputtered SiO, de-posited over a step, Thin SolidFilms, vol. 5, 1970,p. 355.I. A. Blech, Evaporated 6lm profiles over steps in substrates, ThinR. E. Thun, J. A. Ciccio, D. E. Greentham, R. W. Ilgenfritz, M . P.Solid Films, vol. 6, 1970, p. 113.

microcircuits,InsulationlCircuitry, vol. 16, 1970, p. 250.Lepie, and S. M . Stuhlbarg, Printed and molded circuits; integratedL. N. Kaplan, Photoetching of Pb films with nitromethane, f . Phys.Chem., vol. 68, 1964, p. 94.H. L. Caswell and Y. Budo, Formation of thin film circuits usingpreferential nucleation, Solid-State Electron., vol. 8, 1965, p. 479.J. W. Bremer, Superconductive Devices. New York: McGraw-Hill,1962.P.K.Weimer, The TFT-A new thin-6lm transistor, Proc. IRE,vol.50, June 1962, pp. 1462-1469.Appl icat ions Data for KO& Photosensitive Resists. Eastman KodakCo., Rochester, N. Y., pamphlet, Dec. 1966, p. 91.W. E. Rudge, W. E. Harding, and W. E. Mutter, Flys eye techniqueS. M. Irving, K& Photoresist Seminar Proc., vol. 2, 1968, p. 26.for generating semiconductor device fabricationmasks, IB M f . Res .Dmelop. , vol. 7, 1963, p. 146.A. Rogel, Durable Cr masks for photoresist applications,Reo. Sci.Instrum., vol. 37, 1966, p. 1416.A. E. Brennemann, A. V. Brown,M. atzakis, A. J. Speth, and R. F. M .

-.

Thomley, Two interconnection techniques for large-scale circuit inte-gration, IB M f . Res. Dmelop. , vol. 11, 1967, p. 520.[57l G. R. Brewer, The application of electron/ion beam technology tomicroelectronics,IEEE Spectrum, vol. 8, Jan. 1971, pp. 23-37.


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PROCEEDINGS OF THE IEEE,VOL. 59 , NO . 10, CTOBER 1971 1403[SEI S.M. Angello, Should microelectronic processing be done with elec-[59] M. J. Beesley, H. Foster, and K . G. Hambleton, Holographic projec-[a]D. . Klein and J. W. Raniseski, Photomasks and photolithography,[61] W. VanGelder and V. E.Hauser, J. Electrochem. Soc., vol. 114, 1967,[621 C. S.Murphy, Electron Reliub. Micro miniutw., vol. 2, 1963, p. 235.[63] M .K. tetler, Chrome masks-The ideal master for photoprocessing,[64] T. W.OKeeffe and R. M . Handy, Fabrication of planar siliconSolid State Technol., vol. 9, Mar. 1966, p. 60.transistor without photoresist, Solid-State Electron., vol. 1 1 , 1968,p. 261.[65] R. F.Thornley and T. Sun, Electrical beam exposure of photoresists,J. Electrochem. SOC ., ol. 112, 1965, p. 1151.[66] P.D. Davidse, R F sputter etching-A universal etch, J . Electrochem.Soc . ,vol. 116,1969, p. 1 0 0 .[68] S.W. Ing, Jr., R. F.Momson, L. L. Alt, and R. W. Aldrich, Gettering[67] L. I. Maissel, C. L. Standley, Jr., and L. V. Gregor, to be published.of metallic impurities from planar Si diodes, I .Electrochem., Soe., vol.110, 1962, p. 533.[69] F.Woitsch, Silicon nitride etching, Solid State Technol., vol. 1 1 , no.

1 , 1968, p. 29 .[70] M. M. Schlacter, E. E. Schlegel, R. S.Keen, Jr., R. A. Lathlaen, andG. L. Schnable, Advantages of vapor-plated phosphosilicate films inlarge-scale integrated circuit arrays, IEE E Trans. Electron. Devices,[71] L. V. Gregor, P. Groswald, and R. A. Powlus, Effects of sputteredSi02on MOS integrated circuits, to be published.[72] P. Richman, Suppression of parasitic thick-field conduction mech-

anisms in Si-gateMOS integrated circuits, Electron. Let t ., vol. 7, 1971,[73] D. M . Brown, W. E. Engler, M. Garfinkel, and P. V. Gray, Refractory

tron beams, presented at WESCON, Sept. 1969.tion of microcircuit patterns, Electron. Le t t . , vol. 4, 1968.presented at AlChE Meet., Atlanta, Ga., Feb. IS , 1970.p. 869.

V O ~ .ED-17, Dec. 1970, pp. 1077-1083.

p. 12.

metal silicon device technology, Solid-State Electron., vol. 1 1 , 1968,D. 1105.[74] i. .Miller, Controlled collapse reflow chip joining, IB M I . Res.

[75] F.W. Shallcross, W. S.Pike, P. K. Weimer, and G. M . F ryma n ,Develop. , vol. 13, 1969, p. 239.~~ A photoconductive sensor for card readers, IEEE Trans. Electron[76] P. M . Schaible and L. I. Maissel, Thin film maximum thermometer,Devices (Corresp.), vol. ED-17, Dec. 1970, pp. 10861087 .[77] S. Middlehoek, Metallization processes n fabrication of Schottky-Thin Solid Films, vol. 3, 1969, p. 277.

[78] S. R. Ovshinsky, Reversible electrical phenomena in disorderedbamer FETs, I B M J . R e s .Deoelop., vol. 14, 1970, p. 1 4 8 .[79] T.W. Hickmott, Electroluminescence, bistable switching and dielec-structures, Phys. Rev. Let t . ,ol. 21,1968, p. 1450.tric breakdown of Nb,O, diodes, J . Vol. Sei. Technol., vol. 6 , 1970,p. 828.[EO] L.V. Gregor, Electrical Conductivity of polydivinylbenzene films,Thin Solid Films,vol. 2, 1968,p. 235.[El] W. E.Engeler, J. H. iemann, and R. D. Baertsch, Surface chargetransport in silicon, A&. Phys. Let t . ,vol. 17, 1970,p. 469.[82] B. D. Josephson, Possible new effects in superconductive tunnelling,Phys. Let t . , vol. 1, 1962, p. 251.1831 J. Matisoo, The tunneling cryotron-A superconductive logic elementbased on electron tunneling, Proc. EEE, vol. 55, Feb. 1967, pp.1841 W.S.Boyle and G. E. Smith, Charge coupled semiconductor devices,172-1 80.

[E51 A. H, Bobeck, Properties and devices applications of magneticBell Sysr. Tech. J . , vol. 49, 1970, p. 587.

[86] P. . Bonyhard, I. Danylchuk, E. D. Kish, and J. L. Smith, Applica-domains in orthofemtes, Bell Syst. Tech. J . , vol. 4 6 , 1967.tions of bubble devices, IEEE Trans.Magn., vol. MAG-6, Sept.1970, pp. 4 4 7 4 5 1 .[87] P. Brody andD. Page, Flexible thin-film transistors stretch per-formance, shrink cost, Electronics, vol. 41, 1968, p. 1 0 0 .

Multilayer Metallization forLSIC. . SANTORO AND D. L. TOLLIVER

Abs tract -Rec ent advances n the manufacture of compl ex bipola rintegrated circuitshave led to a variety of techniques for metal inter-connection on the chip.As the need for more and more devices hasincreased chip size, theproblemof andomdefects has becomecatastrophic. Functiona l yields are often seen to drastically decreaseor even vanish with attempts to fabricate very large bipolar parts.

Since the m ajor factor determining dieize is the metal intercon-nect site and spacing, one way to conserve real estate while achiev-ing highly complex c ircuits is to employ more than a single layer ofinterconnectionmetal.Atpresentbothdouble- and triple-layerschemes are being used. Thesemultilayer metallizations, while solvingthe problem of chip defects, are not with ou t serious drawbacks oftheir own. hese problems are iscussed.

The Motorolamul tilaye r systems considered are allaluminumbased; i.e., pure a luminumor ligh tly doped aluminum. Although othermetals are beingexperimented with, aluminum systems make upnearly all of the commerciall y available ICs at this time. In general,these me tal layers are insulated fro m one another by a deposited di-electric, usualty SiO,.

The most prominent yield limiting problemsare discussed. Theseinclude coverage of both meta l dges and oxide steps wit h additionalmetal and/or another layer of oxide. Processing parameters such asprofiles, thickness, temperature,composition, etc., that nflue ncecoverage are discussed aswell as nnovations for improving less-than-desirable results.

Manuscript received February 18, 1971; revised April 14, 1971.

A class of problems related o th e resence of via holes or oxidewindows for interlayer continuity isresented. In addition . the prob-lem of random defects such as pinholes and inclusions in the oxideinsulations layers is mentioned. Finally. these data are related to thetradeoffs that occur in building up additionalayers or increasing diesize.

INTRODUCTION

TE DEMANDS of highly sophisticated electronic systemshave pressured the semiconductor industry nto placing more

and more deviceson a singlechip. This has resulted in a largescale integration (LSI) of previously hybrid designs and le d to theemergence of enormous ie sizes. The process engineer haseen thisnew complexity result in drastically reduced circuit probe yieldsat atime wheness circuits per wafer re available [1 1, [2]. If one assumesthat random statistics apply (usually a worst case), hen circuityields canbe expressedYv = (KY (1)r, = (1 - a@. (2)

Here (1) relates yield to complexity, and & is the yield of thecircuits building block (i.e., ransistors, gates, etc.). N is the level ofintegration; the number of such building blocks integrated into asingle chip. Applying his relationship to a logic array, for example,

thin film processes for microelectronic applications

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