nanoscale+deposition_2014

1SNT5039 Nano Processing (Deposition Technology)

1. Fundamentals of Thin Film Deposition- Adsorption of Materials- Reaction Control and Diffusion Control

2. Chemical Vapor Deposition (CVD)- Introduction to CVD- CVD Reactions and PECVD- Various CVD Films

3. Physical Vapor Deposition (PVD)- Introduction to PVD- Various PVD Techniques- Deposition of Interconnect Materials

Part IV: Nanoscale Deposition


Surface adsorption

FUNDAMENTALS OF THIN FILM DEPOSITION


FUNDAMENTALS OF THIN FILM DEPOSITION


• The flux of reactant speciesfrom the gas phase to wafersurfaces through the boundarylayer, F1 is

where, hG is the mass transfercoefficient (in cm/sec), CG-CS

term is the difference inconcentration of the reactantspecies (in molecules/cm3)

INTRODUCTION (DEPOSITION KINETICS)

Diffusion

F1= hG (CG-CS)


• The flux of reactant consumed bythe reaction at the surface, F2

where the kS is thechemical surface reaction rate (incm/sec) and CS is the concentration ofthe reacting species at the surface (incm-3)

Reaction

F2=kSCS

All of the processes involving surfacechemical reactions and surface diffusionare lumped into this one parameter.



• Assuming steady-state deposition,these two processes that act in seriesmust be equal to each other, thus

F = F1 = F2

• Equating the F1 and F2,

• The growth rate of the film is nowgiven by

where, v is the deposition rate or velocityin cm/sec, and N is the number of atomsincorporated per unit volume in the film,or its density, in cm-3 (5x1022 cm-3 for Si inthis case)

1)1( G

SGS h

kCC

NC

hkhk

NFv G

GS

GS



• Now, we define Y to be the mole fraction of the incorporating speciesin the gas phase

where CT is the concentration of all molecules in the gas phase.

• Example : Si deposition by the reaction of SiCl4 and hydrogen

SiCl4(g)+2H2(g) Si(s) + 4HCl(g)Here, CG is the number of molecules of SiCl4 per cm3 in the gas phase and CT wouldcorrespond to the total number of SiCl4 and H2 molecules (plus any other species) per cm3 inthe gas phase.

• Y is also equal to the partial pressure of the incorporating species, PG,divided by the total pressure in the system

T

G

CCY

.......'

GG

G

T

G

T

G

PPP

PP

CC

Y



• After introducing Y, the deposition velocity is now

• ExampleCalculate the deposition rate for a CVD system in which

hG=1.0 cm/sec, kS=10cm/secPartial pressure of incorporating species=PG=1torrTotal pressure = PT = 1atm = 760 torrTotal concentration in gas phase = CT = 1x1019 cm-3Density of deposition film = N = 5x1022 cm-3

• AnswerUsing equation

we can obtain v=2.4x10-7cm/sec = 0.14 um/min

YNC

hkhk

NFv T

GS

GS

YNC

hkhk

NFv T

GS

GS



• If kS << hG– The deposition rate

– This is the surface reaction controlled case.– Physical reason: the mass transfer through the gas boundary layer

is relatively fast, while the surface reaction is sluggish.– The CS approaches CG.

• If hG << kS– The deposition rate

– This is the mass transfer, or gas phase diffusion, controlled case.– The surface reaction is faster compared to mass transfer.– CS approaches to zero.

YkNCY

NC

khY

NC

hkhk

v STT

SG

T

GS

GS

/1/1

1

YhNCY

NC

khY

NC

hkhkv G

TT

SG

T

GS

GS

/1/1

1

Two limiting cases



• For mass transfer controlled regime– Deposition rate is relatively constant

with temperature– However, as it is controlled by the

mass transfer of species through thegas phase boundary layer, the flow ofgas over the wafers and transport ofthe reactants to the wafer surface arevery important and can place majorrestrictions

• For surface reaction controlled regime– Process is very sensitive to the

temperature– However, the mass transfer through

the boundary layer is not as important,leading to fewer restrictions on thegas flow and wafer placement.



• When operating CVD reactors in themass transfer limited regime, one hasto ensure that equal fluxes of reactantgases must reach every location foruniformity issue.

• Wafers are placed side by side (orhorizontally stacked), hence lowthroughput.

• This can be improved by going tolower deposition temperature.

• However, the deposition rate will bereduced also.

• Lower deposition pressure canincrease the deposition rate.



• Advantages over LPCVD are good uniformity, good step coverage andless particle contamination

• Operated in the pressure range of 250mTorr to 2Torr and highertemperature of above 550oC.

• Films deposited by LPCVD are mainly poly-Si, oxides, nitrides, W andWSi.

• Usually films are deposited in the reaction-rate limited regime andtemperature control is important as shown in Arrhenius equation ofsurface reaction rate,

because diffusion of reactants is fast enough in the low pressure regime• Disadvantages of LPCVD could be low deposition rate and the use of

high temperature.

kTEaAeR /






Deposition Technology


INTRODUCTION TO CVD

- CVD is a process to form a non-volatile solid film on the substrate byreaction of vapor phase chemicals.

- Energy for reaction is supplied by thermal methods, photons or electrons.

In microelectronics manufacturing,

polycrystalline Si (called Poly-Si), dielectric

materials such as silicon dioxide (SiO2) and

silicon nitride (Si3N4), interconnect / contact

plug such as tungsten (W), silicide

materials such as tungsten silicide (WSi6),

and diffusion barriers (Ti/TiN) are deposited

by CVD techniques.


• Advantages of CVD processes over other

competing techniques

– Wide variety of chemical compositions

– High deposition rates with good step

coverage

– Low manufacturing cost

– Only one stage (LPCVD) or no pumping

(APCVD) processes are possible

INTRODUCTION TO CVD


INTRODUCTION TO CVD


• Desired characteristics of CVD films are– Good thickness uniformity– High purity and density– Controlled composition and stoichiometry– High degree of structural perfection– Good adhesion– Good step coverage

INTRODUCTION TO CVD

aspect ratio (AR) of a feature (AR = height of feature/width of feature = h/w) A parameter that can reflect filling and bottom coverage

The feature could be a metal line or a spacer such as a gap between metal lines.

A deep and narrow contact hole would have a large aspect ratio and would be harder to fill.


1) Transport of reactants by forcedconvection to deposition region.

2) Mass transfer of reactants by diffusionfrom the main gas stream through theboundary layer to the wafer surface.

3) Adsorption of reactants on the wafersurface.

4) Surface processes, including chemical decomposition or reaction, surfacemigration to attachment sites, site incorporation and other surface reactions.

5) Desorption of byproducts from the surface6) Transport of byproducts by diffusion through the boundary layer and back to

the main gas stream7) Transport of byproducts by forced convection away from deposition region.

INTRODUCTION TO CVD


CVD Chemical Reactions(1) Heterogeneous reactions, taking place on the heated wafer surface, are more desirable, because they can produce high quality films(2) Homogeneous reactions, taking place in the gas phase, are undesirable, because they form gas phase clusters that can result in poor adhesion to the substrate and low quality films.(3) The Table lists typical reactions used in the CVD.

CVD REACTIONS

Thermal decomposition) SiH4(g) Si(c)+2H2(g)Thermal decomposition) SiH2Cl2(g) Si(c)+2HCl(g)Thermal decomposition) CH4(g) C+2H2(g)Oxidation SiH4(g) +2O2(g)SiO2(c)+2H2O(g)Nitridation SiH4(g) +4NH3(g)Si3N4(c)+8H2(g)Reduction WF6(g)+3H2(g) W(c) +6HF(g)Displacement Ga(CH3)3(g)+AsH3(g) GaAs(c) +3CH4(g)Displacement ZnCl2(g)+H2S(g) ZnS(c) +2HCl(g)Displacement 2TiCl4(g)+2NH3(g) +H2(g)TiN(c) +8HCl(g)


CVD REACTIONS (REACTION FEASIBILITY BY THERMODYNAMICS)

-Thermodynamics can address many important issues forCVD, eg, feasibility of a given reaction.

-Thermodynamics does not tell the speed of reactionsand the film growth rates which are determined by bothvapor transport kinetics and vapor-solid reaction.

- Thermodynamics tells if chemical equilibrium has beenattained.

- Thus, CVD is viewed as an empirical technology withthermodynamic guidelines.


CVD REACTIONS (GIBBS FREE ENERGY AND EQULIBRIUM)

Gibbs free energies (DG) and theequilibrium constants (K) are related,DG= - RT ln K.

(Example) Consider the formation of Poly-Si at 600C from the following two reactions,and suggest which reaction is morethermodynamically favorable.(1) SiCl2H2(g) = Si(c) + 2HCl(g)(2) SiH4(g) = Si(c) + 2H2(g)Where

1,)(,)(

4

22

222

2

1 SiSiH

HSi

HSiCl

HClSi awherePPaK

PPaK


CVD REACTIONS (REACTION FEASIBILITY BY THERMODYNAMICS)(eg SELECTIVE DEPOSITION of W)

)(23)()(

23

46 gSiFcWcSiWF

)(23)(

23)()(

23)( 2426 gOgSiFcWcSiOgWF

Using the graph, testify possibility of

selective deposition of W onto Si, but

not onto the surrounding SiO2 at the

temperature of 700K, by determining

the change of Gibbs free energies (DG)

and the equilibrium constants (K) for the

following CVD reactions, where DG= -

RT ln K.

(1)

(2)


GROWTH RATE UNIFORMITY IN LPCVD

(1) From the result (b), explain why thesurface reaction controlled process ispreferred to attain better uniformity inLPCVD.

(1) Further calculation showed that adeposition uniformity of 3% is achievedwhen f < 0.5. Determine a minimumspacing (D) between wafers whenk=0.5cm/s and D=100cm/s, for thefollowing wafer diameters.

(i) 6 inch (ii) 8 inch

(3) Using the equation,

Determine the deposition rate of poly-Si,where hG=D/d (d=0.1cm), CT=1016cm-3,Y=0.5.Dkr D /2 2

0f

YNC

hkhkv T

G

G


Plasma-Enhanced Deposition (PECVD)

• PECVD uses a rf power to generate a glow discharge to transfer energy tothe reactant gases

• Deposition can be achieved at a lower temperature compared to APCVD orLPCVD

• Desired properties, such as good adhesion, low pinhole density, good stepcoverage, adequate electrical properties, have made PECVD films useful inULSI circuits.

• PECVD silicon nitride is commonly used as the final passivation layer

• PECVD silicon oxide can be used as insulators between the metal layers

• PECVD amorphous silicon has been widely used in TFT LCD area

VARIOUS CVD FILMS (PECVD)


• Use of RF-induced plasma to supply energy into reactant gases• Plasmas are highly ionized gases• With additional energy from the plasma to the reactant gases, deposition can

occur at lower temperatures

VARIOUS CVD FILMS (PECVD EQUIPMENT)


– Plasma is generated by the application of an RF field to a lowpressure gas.

– Electrons in the reactor gain sufficient energy from the electricfield and collide with gas molecules.

– Dissociation and Ionization of reactant gases occur.– Energetic species including ions and radicals are adsorbed on

the surface. Radicals have high sticking coefficient becausedangling bonds are available and migrate easily along the surface.This is why PECVD can provide good step coverage.

– Species adsorbed on the surface are subsequently subjected tobombardment by charged species such as ions and electrons,rearranged, reacted with other adsorbed species.

– Thin film grows.

VARIOUS CVD FILMS (PECVD)Plasma generation and deposition


• Deposition Methods and Variables– PECVD requires

• control and optimization of rf power density, frequency and duty cycle

• Gas composition, flow rate, temperature, pressure etc.– PECVD process is a surface-reaction limited process– PECVD silicon oxide

SiH4 + O2 SiO2 + 2H2

SiH4 + 4N2O SiO2 + 4N2 + 2H2O– PECVD silicon nitride

SiH4 + NH3 SiN:H + 3H2OSiH4 + N2 2SiN:H + 3H2

– PECVD amorphous siliconSiH4 Si + 2H2

PLASMA-ENHANCED DEPOSITION (PECVD)


Plasma-Enhanced Deposition (PECVD)


TEMPERATURE DEPENDENCE OF GROWTH IN LPCVD


• Deposition Variables– The temperature dependence of

deposition rate follows the Arrheniusequation: R=Aexp(-Ea/kT)

– For example, if polysilicon deposits at10nm/min at 600oC with an activationenergy of 1.7eV, the rate at 550oC isfound by:

solving for R2 gives 2.5nm/min.– Deposition rate increases with

increasing pressure.

)()/ln(21

2121 TT

TTkERR a

VARIOUS CVD FILMS (POLY-SILICON FILM DEPOSITION)


• Deposition Variables

– Nonlinear dependence of conc. of silane on the deposition rate due to a sequence of surface reaction

– The reactions

SiH4(g) SiH4(ad)

SiH4(ad) SiH2(ad) + H2(ad) SiH2(ad) Si + H2(ad)

H2(ad) H2(g)

VARIOUS CVD FILMS (POLY-SILICON FILM DEPOSITION)





Deposition Technology


INTRODUCTION TO PVD

• PVD is a process to form a non-volatile solid film on the substrate by physical methods.

• PVD is mainly used for metallization in Si IC applications.

• Mainly, the following PVD methods are used:– Evaporation (Thermal and e-beam)

• Very seldom used in Si IC, we will not discuss this technique– Sputtering (Glow discharge)– Pulsed laser deposition (laser ablation)

• Very seldom used in Si IC, we will not discuss this technique


INTRODUCTION TO PVD

• PVD is a process to form a non-volatile solid film on the substrate by physical methods.

• PVD is mainly used for metallization in Si IC applications.

• Mainly, three PVD methods exist– Evaporation (Thermal and e-beam)– Sputtering (Glow discharge)– Pulsed laser deposition (laser ablation)

• Very seldom used in Si IC, we will not discuss this technique


• Wafers are loaded into a high vacuum chamber that is commonly pumped already

• The charge or material to be deposited is loaded into a heated container called the crucible. It can be heated by means of an embedded resistance heater and an external power supply.

• As the material in the crucible becomes hot, the charge gives off a vapor.

• Since the pressure in the chamber is very low, the atoms of vapor travel across the chamber in a straight line until they strike a surface where they accumulate as a film.

EVAPORATION (BASIC OPERATION)


Sublimation and Evaporation• Vapor pressure is a very strong

function of temperature• Normally, a partial of about 1-10 mT

or more is required to achieve reasonable deposition rates (on the order of 0.1-1 um per minute), and one can see that about 1100oC is needed for the evaporation of Al. This means that the Al is evaporated from the liquid phase.

• When the material is evaporated from the liquid phase, the vapor pressure can be given as

Sublimation: vapor is from solidEvaporation: vapor is from liquidNormally, evaporation provides higher vapor pressure and hence higher deposition rate

EVAPORATION (PHASE DIAGRAMS)


• The mass loss rate of the crucible can be calculated as

• Two approximations:– If the charge is completely molten it is common to assume that

natural convection and thermal conduction will keep the temperature of the charge nearly constant across the crucible

– It is also assumed that the opening of the crucible has a constant area, A

EVAPORATION (DEPOSITION RATES)


• To find the deposition rate on the surface of a wafer, the fraction of the material leaving the crucible that accumulates on the surface of the wafer must be determined.

• Material ejected from the crucible travels in a straight path to the wafer surface due to low pressure.

• Assuming that all of the material that arrives at the wafer sticks and remains there, the arrival rate then is governed by simple geometry.

• Thus, the constant of proportionality is just the fraction of the total solid angle subtended by the wafer as seen from the substrate.



• The proportionality constant is given by

• Deposition rate depends on the location and orientation of the wafer in the chamber

• Wafers directly above the crucible will be coated more heavily than wafers off to the side

• Film uniformity is also a concern• One method to obtain good uniformity is to

place the crucible and wafers on the surface of a sphere. Then

• The deposition rate is the mass arrival per unit area divided by the mass density of the film

rR2

coscos f

1st term: depends on material 2nd term: depends on temperature3rd term: depends on geometry of

the chamber



Two methods to improve step coverage:

(1)To rotate the wafer substrates

(2)To heat the wafer substrate

EVAPORATION (STEP COVERAGE)


INTRODUCTION TO PVD (SPUTTERING EQUIPMENT)


• DC sputter deposition is not suitable for insulator deposition

– A problem with applying the necessary DC voltage to theinsulating target to initiate a plasma

– Other major difficulties arise, such as particle issue

• The solution is to use RF, instead of DC. 13.56MHz RF powersource is commonly used.

• RF voltages can be coupled capacitively through the insulatingtarget to the plasma, so that conducting electrodes are notnecessary

• The RF frequency is chosen to be high enough so that acontinuous plasma discharge is maintained.

SPUTTERING (RF SPUTTER DEPOSITION)


• Sometimes sputtering of wafer is desirable for– Precleaning the wafer before actual deposition– Bias sputtering, where deposition and sputtering are done simultaneously

• A negative bias relative to the plasma is applied to the wafer electrode, which is now electrically isolated from the chamber walls.

• Positive Ar ions from the plasma will now be accelerated to the wafers on the substrate and sputter off the atoms.

• Usually an RF bias is used since the wafers often have insulating films on them.

• In sputter etching or cleaning, no deposition is allowed to occur on the wafer by using a shutter to block sputtered material from the target.– During this step, a controlled thickness of surface material is sputtered off

the wafer, removing any contamination or native oxide– A film can then be sputtered immediately afterward without breaking the

vacuum

SPUTTERING (BIAS SPUTTERING)


• Positive ions in the plasma are accelerated to the negatively biased target(hundreds of volts to thousands of volts)

• Energetic ions strike the target and dislodge or sputter the target atoms• These atoms then travel freely through the plasma as vapor and strike the

surface of the substrates, where they condense to form the deposited film• Note: since the targets acts as an electrode in the DC mode of sputter

deposition, the target must be conductive.

SPUTTERING (FUNDAMENTAL)

Substrate

Positive ion (Ar+)

Negative(-)

Positive(+)

Target


• Sputter yield (S) is the ratio of the number oftarget atoms ejected from the target to thenumber of ions incident on the target.

• Sputter yield depends on ion mass, ionenergy, target mass and target crystallinity.

• Sputter yield is proportional to energy ofincident ion (E) but inversely proportional tosurface binding energy (U0)

• For ion energy larger than threshold energy,sputter yield tends to increase as the squareof the energy up to about 100eV, then linearlywith energy. Above ~1000eV, yield increasesonly slightly until the onset of implantation.

02)(

4UE

mmmm

Siont

iont

SPUTTERING (DEPOSITION RATE)


• Heating substrate can improve dramatically the step coverage due to surface diffusion

• A second technique for improving the step coverage is to apply an RF bias to wafers. (will discuss later)

• Sputter has worse step coverage than CVD

SPUTTERING (STEP COVERAGE)


• Compound films can be processed by reactive sputter deposition• Example: TiN, TiO2

– A reactive gas (N2, O2) is introduced into the sputtering chamber inaddition to the Ar plasma.

– The compound is formed by the elements of that gas combining with thesputtered material.

– The reaction usually happens either on the wafer surface or on the targetitself, and not in the plasma.

SPUTTERING (REACTIVE SPUTTER DEPOSITION)


• Various compounds can be deposited by reactive sputtering using a reactive gas with Ar

SPUTTERING (REACTIVE SPUTTER DEPOSITION)


• In bias-sputter deposition, sputter etching of the wafer and deposition on the wafer (by sputtering the target) are allowed to occur concurrently.

• Conditions are chosen so that more deposition occurs than etching.• By allowing some sputtering of the wafer surface to occur during the

deposition, both the topography and the properties of the deposited films can be altered.

– More planarized film

– Better filling to the holes

SPUTTERING (BIAS SPUTTERING)


• In both conventional DC and RF sputtering, efficiency of ionization fromenergetic collision between electrons and gas atoms is rather low

• In magnetron sputtering, magnets are used to increase the percentage ofelectrons that take part in ionization events, and the ionization efficiency isincreased significantly (about 10-100 times higher)

• A magnetic field is applied at right angle to the electric field, usually by placinglarge rectangular magnets behind the target.

SPUTTERING (MAGNETRON SPUTTER DEPOSITION)


• Small range of arrival angles during deposition can cause non-uniform, or poorstep coverage over a step in topography.

• However, sometimes a small range of arrival angles is desirable. For example,if material is required to be deposited into of a deep contact or via, a largearrival angle distribution can cause problems.

• Example– A relatively large arrival angle distribution when little surface diffusion occurs can

result in little deposition at the bottom of a hole due to shadowing effects. Inaddition, overhang formation occurring at the top corners of a deep holeenhances the shadowing effect. Hence the poor coverage at the hole bottom.

– One way to improve this is by having a narrow range of arrival angles, with mostof the depositing atoms arriving at the wafer perpendicular to the surface.

SPUTTERING (COLLIMATED SPUTTER DEPOSITION)


Gate material

(1) Low resistivity(2) Self-aligned formation on n- and p-polysilicon (Logic devices)(3) Easy formation of polycides(4) Minimum dopant depletion and boron penetration for poly-Si (assisted by good gate dielectrics)

Contact metal

(1) Formation on n- and p-silicon and polysilicon or WSi(2) Low/reproducible contact resistance(3) Minimum consumption of silicon(4) No interaction with oxide/nitride sidewall

Metal lines

(1) Low resistivity and stress(2) Compatible with dielectric

Via fill metal

(1) Low contact resistance with underlying and overlayer metals(2) Planarizable(3) Low resistivity

REQUIREMENTS OF INTERCONNECT MATERIALS


- Deposited by sputtering- Used widely as the contact and second level interconnection material dueto low resistivity and its ability to reduce native SiO2. This leads lowercontact resistance and stronger bonding.

- Because of its low melting temperature of 660C, the post processing isnormally limited to below 450C.

- Annealing of Al on Si causes dissolution of silicon by diffusion into themetal and leads to pit formation. To prevent junction shorts caused bythe preferential dissolution of Si into Al, Si is added to Al during thedeposition of the metal film. The amount of the Si required is determinedby the maximum process temperature and the solid solubility of Si in Al.

- Cu is added to Al or Al-Si alloys to increase the electromigrationresistance. Mean- time-to-failure (MTF) tends to decrease with the lengthof the Al runner. However, for a given current through the runner, theelectromigration-induced mass transport is higher in the small runners.This originates from the fact that grain-boundary diffusion becomesinsignificant in the grain structure of small feature lines.

MATERIALS IN INTERCONNECTS (ALUMINUM)


- Resistivity of Cu is about 37% lower than Al.

- Until recently, Cu has not found wide application in Si IC processingdue to high diffusivity, deep levels in silicon, and low heat offormation of copper oxides.

- Deposited by electrochemical deposition and sputtering.

- At room temperature, Cu etching is very difficult because copperoxides are easily formed but halogenated etching products are hardlyformed.

- Chemical mechanical planarization (CMP) of Cu is predominantlyused, instead of the etching process.

MATERIALS IN INTERCONNECTS (COPPER)


- Passive barriers: Chemically inert to both Si and Al, and good diffusion barrier, e.g. TiN between Si and Al

- Usually, thin Ti is deposited on Si of the contact regions before depositing TiN. Subsequently, Ti is annealed to form TiSi2 to form ohmic contacts.

- Diffusion barrier between interconnects and interlayer dielectrics: between Al, Cu and SiO2, low-k dielecric, e.g. TiN for Al, and Ta, TaN for Cu

- Sputtering: Ti, TiN, Ta, TaN, - CVD: Ti, TiN

6TiCl4 + 8NH3 = 6TiN + 24HCl +N2 at 400-700C2TiCl4 + 2NH3 + H2 = 2TiN + 8HCl +N2 at > 700C2TiCl4 + N2 + 4H2 = 2TiN + 8HCl +N2 at 400-700C

MATERIALS IN INTERCONNECTS (DIFFUSION BARRIERS)

nanoscale+deposition_2014

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