plasma-assisted ald for the conformal deposition of sio2 ...ald process for synthesis of sio 2 at...

Plasma-assisted ALD for the conformal deposition of SiO2 process material and electronic propertiesCitation for published version (APA)Dingemans G Helvoirt van C A A Pierreux D Keuning W amp Kessels W M M (2012) Plasma-assistedALD for the conformal deposition of SiO2 process material and electronic properties Journal of theElectrochemical Society 159(3) H277-H285 httpsdoiorg1011492067203jes

DOI1011492067203jes

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Journal of The Electrochemical Society 159 (3) H277-H285 (2012) H2770013-46512012159(3)H2779$2800 copy The Electrochemical Society

Plasma-Assisted ALD for the Conformal Deposition of SiO2Process Material and Electronic PropertiesG Dingemansa C A A van Helvoirta D Pierreuxb W Keuninga and W M M Kesselsalowastz

aDepartment of Applied Physics Eindhoven University of Technology 5600 MB Eindhoven The NetherlandsbASM Kapeldreef 75 B-3001 Leuven Belgium

Plasma-assisted atomic layer deposition (ALD) was used to deposit SiO2 films in the temperature range of Tdep = 50ndash400C onSi(100) H2Si[N(C2H5)2]2 and an O2 plasma were used as Si precursor and oxidant respectively The ALD growth process andmaterial properties were characterized in detail Ultrashort precursor doses (sim50 ms) were found to be sufficient to reach self-limitingALD growth with a growth-per-cycle of sim12 Aring (Tdep = sim200C) leading to SiO2 films with OSi ratio of sim21 Moreover the plasmaALD process led to a high conformality (95ndash100) for trenches with aspect ratios of sim30 In addition the electronic (interface)properties of ultrathin ALD SiO2 films and ALD SiO2Al2O3 stacks were studied by capacitance-voltage and photoconductancedecay measurements The interface quality associated with SiO2 was improved significantly by using an ultrathin ALD Al2O3capping layer and annealing The interface defect densities decreased from sim1times1012 eVminus1 cmminus2 (at mid gap) for single layer SiO2to lt 1011 eVminus1 cmminus2 for the stacks Correspondingly ultralow surface recombination velocities lt 3 cms were obtained for n-typeSi The density and polarity of the fixed charges associated with the stacks were found to be critically dependent on the SiO2 thickness(1ndash30 nm)copy 2012 The Electrochemical Society [DOI 1011492067203jes] All rights reserved

Manuscript received November 8 2011 Published January 6 2012 This was Paper 1357 presented at the Montreal QC CanadaMeeting of the Society May 1ndash6 2011

The key importance of thermally grown silicon dioxide(SiO2) for applications in silicon-based microelectronics needs nointroduction1 2 Also in silicon photovoltaics SiO2 is a very impor-tant material The low interface defect densities associated with theSiSiO2 interface reduce surface recombination significantly and haveenabled high solar cell efficiencies3ndash6 Temperatures gt 800C are typ-ically required to form SiO2 by thermal oxidation of Si Alternativemethods for the synthesis of SiO2 have been developed to avoid suchhigh temperatures and long processing times They may also enablesingle side deposition and a high level of control of the material prop-erties and film thickness These alternative methods include (wet)chemical oxidation (plasma-enhanced) chemical vapor depositionsputtering and electron beam evaporation

Atomic layer deposition (ALD) is an alternative CVD-like methodthat recently gained considerable attention In ALD the substrate issequentially exposed to two precursors (eg a Si-precursor and anoxidant) which each react with the surface until saturation ALD al-lows for precise thickness control optimal large-area uniformity andthe conformal coating of demanding substrate topologies7ndash9 There-fore the advantages of ALD may expand the range of applications forSiO2 ALD SiO2 films may find application as protective or insulatorcoating or can be used in nanolaminate structures with tailored opti-cal and electronic properties10 11 The films can also be used in spacerdefined double patterning processes for example for dynamic randomaccess memory (DRAM) technology12 In addition the ALD SiO2

process may be useful for engineering the interface between Si andhigh-k materials13 In Si photovoltaics SiO2 can be used in double-layer surface passivation schemes for example comprising Al2O3 ora-SiNxH capping layers with tailored interface properties14ndash16

In the first reports ALD SiO2 was synthesized employing SiCl4

and H2O which required relatively high substrate temperatures(gt300C) and long precursor exposures17 18 In recent years variousalternative Si precursors have been tested in combination with O3

or H2O as the oxidants These processes include the use of pyridine(C5H5N) and Al as catalysts on the surface17 11 19 The approachemploying Al was referred to as rapid ALD as it resulted in depositionrates above the ldquotheoreticalrdquo maximum of one monolayer per ALDcycle In addition more recently a thermal ALD process for low-temperature SiO2 was reported which was free of catalysts or corrosiveby-products20 In this respect the use of precursors with amino ligands

lowast Electrochemical Society Active Memberz E-mail wmmkesselstuenl

has also shown promising results in particular when combined withH2O2 O3 or O2 plasma as the oxidant21ndash24 Very recently also SiH4

has been used as Si precursor during plasma-assisted ALD with CO2

plasma as oxidant25 SiO2 films grown with ALD have been reported toexhibit low carbon content and a high electrical breakdown field10 22

Nevertheless to improve properties such as the chemical etch rateor the interface defect density annealing at a temperature of 1000Cwas shown to be beneficial20 Here we report on a plasma-assistedALD process for synthesis of SiO2 at low substrate temperatures of50ndash400C The ALD process employed H2Si[N(C2H5)2]2 as the Siprecursor (Figure 1) The plasma-assisted ALD process was charac-terized in detail and the results are compared to the well-understoodALD process of Al2O3 using Al(CH3)3 We show that the ALD SiO2

process was compatible with short cycle times and led to an excellentconformality even on structures with an aspect ratio of sim30 The elec-tronic properties of the Si(100)SiO2 interface were investigated usingphotoconductance decay and capacitance-voltage measurements Wedemonstrate that interface defect densities lt 1011 eVminus1 cmminus2 (at midgap) can be reached when using ultrathin Al2O3 capping layers andannealing at moderate temperatures (sim400C)

The paper is organized as follows After the experimental detailsthe characterization of the ALD SiO2 process will be presented Inaddition the material properties and the influence of the substratetemperature will be addressed and data with respect to the confor-mality will be presented Subsequently a brief discussion on thesurface chemistry is included on the basis of in situ measurementsThe final sections discuss the surface passivation properties and theelectronic (interface) properties of the SiO2 films and SiO2Al2O3

stacks

Experimental

The SiO2 films were deposited in the Oxford Instruments OpALreactor This is an open-load system suited for both plasma andthermal ALD and operating at typical pressures of 150 mTorr Aremote O2 plasma was used during the oxidation step in the ALDcycle H2Si[N(C2H5)2]2 (SAM24 Air Liquide) was used as the Siprecursor (Fig 1) This is a liquid (melting point lt minus10C) whichexhibits a high vapor pressure ie sim100 Torr at 100C (Fig 1)The SAM24 was held in a stainless steel bubbler heated to 50Cand the precursor was introduced into the reactor by ultrashort doses(10ndash120 ms) using fast ALD valves A flow of Ar as well as the O2

flow were continuously on during the process The latter was feasible

Downloaded 18 Jan 2012 to 131155151107 Redistribution subject to ECS license or copyright see httpwwwecsdlorgterms_usejsp

H278 Journal of The Electrochemical Society 159 (3) H277-H285 (2012)

Figure 1 Vapor pressure of the H2Si[N(C2H5)2]2 (SAM24) precursor usedfor ALD of SiO2 as a function of the temperature The vapor pressure iscompared to the one of Al(CH3)3 (TMA) This precursor is commonly usedfor ALD of Al2O3 The precurors are schematically represented in the figure

as no evidence was found for reactions between the Si precursor andO2 under the experimental conditions used The substrate temperatureduring deposition Tdep was varied between 50 and 400C Thereactor wall temperature was 180C unless the substrate temperaturewas lower Under these conditions the wall and substrate temperaturewere equal To allow for direct comparison Al2O3 was synthesized inthe same reactor using Al(CH3)3 as the metal precursor and O2 plasmaas the oxidant26ndash28 All films were deposited on Si (100) waferswhich received a short treatment in diluted HF (sim1 in DI-H2O)to remove the native oxide prior to loading the wafers into the ALDreactor

In situ spectroscopic ellipsometry (SE) measurements were usedfor optimizing the ALD process The growth-per-cycle GPC andrefractive index were determined by using a Cauchy optical modelto fit the ellipsometry data Rutherford backscattering spectroscopy(RBS) and elastic recoil detection (ERD) employing sim2 MeVHe2+-ions from the singletron at the Eindhoven University of Tech-nology (Acctec BV) and transmission Fourier transform infraredabsorption (FTIR) measurements were used to analyze the film com-position The surface morphology was investigated by atomic forcemicroscopy (AFM) measurements in semi-contact mode and byhigh-resolution transmission electron microscopy (TEM) Scanningelectron microscopy (SEM) was employed to visualize the conformaldeposition of SiO2 in high aspect ratio trenches The deposition pro-cess itself was studied in real time by quadrupole mass spectrometry(QMS) probing the gas in the exhaust line and by optical emissionspectroscopy (OES) through a view port located on top of the ALDreactor

The interface quality and surface passivation properties of theALD SiO2 films was evaluated from the effective lifetime τeff of theminority carriers in double-side coated floatzone (FZ) n-type Si wafers(sim35 cm) τeff was determined with photoconductance decay in thetransient mode and quasi-steady-state-mode (for τeff lt 100 μs) usinga Sinton lifetime tester (WCT 100) The upper level for the surfacerecombination velocity Seffmax was extracted at an injection level of5times1014 cmminus3 by the expression

Seff max = W

2 middot τeff

[1]

with W the thickness of the silicon wafer (sim280 μm) In the deriva-tion of this expression it is assumed that all recombination takesplace at the surface Capacitance-voltage measurements were per-formed on metal-oxide-semiconductor structures to extract the inter-face defect density (Dit) and the fixed charge density Qf29 EvaporatedAl was used for the metal contacts which were applied after post-deposition annealing of the samples The latter was performed in arapid thermal annealing furnace (ramp up gt 30Cs) in N2H2 or N2

environment

0 40 80 120 1600

5

10

15

20

0 10 20 30 40 50 600

2

4

6

8

Tdep

= 50oC

Tdep

= 200oC

Tdep

= 250oCSiO

2 th

ickn

ess

(nm

)

Number of ALD cycles

TEM SE

(b)(a)

Figure 2 SiO2 film thickness as a function of the number of ALD cycles(a) Substrate temperatures of 50C 200C and 250C The film thickness wasmeasured by in situ spectroscopic ellipsometry (SE) (b) SiO2 film thicknessextracted from the transmission electron microscopy (TEM) image of Fig 3compared with values determined by in situ SE on the same sample Thesubstrate temperature Tdep was 200C

ALD Process and Material Characterization

The ALD process was monitored by in situ spectroscopic ellip-sometry (SE) by taking data points after a certain number of cyclesThe SiO2 film thickness is plotted as a function of the number of ALDcycles in Fig 2a for substrate temperatures of 50C 200C and 250CFor the latter two temperatures the SiO2 thickness increased linearlywith the number of cycles A slight non-linearity was observed for Tdep

= 50C which we attribute to parasitic CVD reactions as will be dis-cussed below In addition to SE thickness information was extractedfrom a cross-sectional TEM micrograph of a sample which consistedof alternating layers of Al2O3 and SiO2 with each a varying thicknessas set by the number of cycles per layer (Fig 3) The TEM and SEdata proved to be in excellent agreement (Fig 2b) The growth-per-cycle (GPC) was determined from the slope of the curves in Fig 2with values of 12 Aringcycle and 11 Aringcycle obtained for substrate tem-peratures of 200C and 250 oC respectively These GPC values aresimilar to those obtained for plasma ALD of Al2O326 27 Moreoverno indications for a significant growth delay on H-terminated Si(100)substrates were found for the SiO2 ALD process which is also similarto what has been observed for Al2O326

Figure 4 shows the effect of the duration of the successive stepsin the ALD recipe ie the precursor dosing precursor purge stepplasma exposure and plasma exposure purge step In the correspond-ing experiments one process parameter in the ALD recipe was variedwhereas the duration of the other steps was taken sufficiently long to

Figure 3 High-resolution cross-sectional TEM image of a stack with alter-nating layers of ALD Al2O3 and ALD SiO2 deposited using 10ndash40 cycleseach The substrate temperature was 200C The extracted layer thicknessesare shown in Fig 2b


Journal of The Electrochemical Society 159 (3) H277-H285 (2012) H279

0 20 40 60 80 100 12000

05

10

15

20ALD SiO

2

Dose time (ms)

140

144

148

152

n(a)

(b)

0 1 2 3 4 5

Purge time (s)0 1 2 3 4 5

Plasma time (s)

(c)

(d)

0 1 2 3

Plasma purge (s)

0 20 40 60 80 100 12000

05

10

15

20

ALD Al2O

3

Dose time (ms)

(e)

0 1 2 3 4 5

(f)


(g)

Plasma time (s)0 1 2 3

(h)

Plasma purge (s)

Figure 4 (a)ndash(d) Saturation curves for the growth-per-cycle GPC and refractive index n of the SiO2 films as measured by in situ spectroscopic ellipsometry asa function of the 4 process parameters in the ALD recipe (e)ndash(h) Saturation curves for the GPC of Al2O3 films The substrate temperature was 250C for both theSiO2 and Al2O3 process

guarantee saturated ALD conditions for the non-varied process pa-rameters The substrate temperature was set to 250C The growthprocess of SiO2 is compared to plasma-assisted ALD of Al2O3 at250C (Figures 4endash4h) From the figure it is evident that ultrashortprecursor dosing times (sim50 ms) were already sufficient to reach self-limiting growth with a GPC of sim11 Aring These short dosing times wereonly slightly higher compared to those used for the Al(CH3)3 precur-sor for ALD of Al2O3 The fact that short dosing times are sufficientis in agreement with the expectations based on the relatively highvapor pressure of the SAM24 precursor Many processes for otheroxides using different precursors require much longer dosing timesFor instance ALD of TiO2 and Ta2O5 require dosing times gt1s usingalkylamide precursors in a similar remote plasma and thermal ALDreactor30 31 The duration of the purge step after precursor dosingwas required to be gt 2 s For shorter purge times residual precursorremaining in the reactor volume can react in the plasma causing para-sitic (PE) CVD-like growth and a higher GPC value Regarding the O2

plasma step a plasma exposure time gt 1 s was found to be sufficientto reach a saturated GPC indicating the rapid removal of the precursorligands For very short plasma exposure times a drop in the refractiveindex below sim146 was observed This decrease can be attributed tothe incorporation of impurities in the SiO2 film that originate fromthe Si precursor The plasma exposure time required for ALD SiO2 isslightly shorter than for plasma ALD of Al2O3 which requires plasmatimes of sim2 s to reach saturated growth However with in situ spec-troscopic ellipsometry only the center of the Si wafer is probed and toensure saturation over the full wafer surface a plasma exposure timeof 4 s was employed in all subsequent experiments Interestingly thepurge after the plasma step had a significant impact on the GPC Thisis in contrast to the ALD process for Al2O3 where the purge stepafter O2 plasma exposure was found to have little influence on theGPC and could be reduced well below 05 s We attribute the higherGPC for shorter purges (lt 2 s) to reactions between residual H2Oformed during the plasma process32 with the Si precursor injected inthe subsequent step Although it is known that the H2Si[N(C2H5)2]2

precursor reacts with H2O it is relevant to mention here that we wereunable to develop a thermal ALD process for SiO2 using SAM24as precursor and H2O as the oxidant No film growth was observedInstead even with the shortest possible H2O doses applied powderformation occurred in the reactor as was noticeable by the naked eye

The thickness uniformity of the SiO2 films deposited by plasma-assisted ALD at 250C was evaluated by mapping the thickness byspectroscopic ellipsometry A precursor dose time of 50 ms was usedwhile all other steps in the ALD cycle (Fig 4) were set to 3s For a 8inch (sim200 mm) wafer the nonuniformity defined by the differencebetween the maximum and minimum thicknesses divided by twicethe average thickness of all data points measured26 was lt 35 Thethickness nonuniformity achieved on 4 inch (100 mm) wafers wassim1

Material properties and role of substrate temperaturemdash Figure 5shows the effect of the substrate temperature between 50 and 400Con the growth rate and refractive index of SiO2 The length of thepurge steps in the lower temperature regime was extended (up to 10 s

0 100 200 300 40000

05

10

15

20

n

(b)

Substrate temperature (oC)

14

15 (a)

Figure 5 Influence of the substrate temperature during deposition on (a) therefractive index n and (b) the growth-per-cycle GPC of SiO2 as determinedwith in situ spectroscopic ellipsometry



Table I Data on ALD SiO2 as determined from the RBS and ERD measurements In the calculation of the mass density the film thickness asobtained by SE was used The thickness of the films was in the range of 35ndash45 nm

Substrate temperature (C) Si atoms per cycle (1014 cmminus2) [Si] (atom) [O] (atom) [H] (atom) OSi ratio ρmass (gcm3)

100 28 plusmn 01 291 plusmn 08 613 plusmn 15 96 plusmn 09 21 plusmn 01 20 plusmn 01200 23 plusmn 01 299 plusmn 08 629 plusmn 15 71 plusmn 07 21 plusmn 01 20 plusmn 01300 19 plusmn 01 296 plusmn 08 623 plusmn 15 81 plusmn 08 21 plusmn 01 21 plusmn 01

at 50C) as it is more difficult to remove H2O at lower temperatureswhich could impact the saturation behavior of the process The GPCwas observed to decrease with increasing deposition temperature fromsim17 Aringcycle at 50C to 08 Aringcycle at 400C The refractive indexwas fairly constant with a value of n = 146 plusmn 002 (photon energyof 2 eV) between 100 and 300C Below 100C and above 300C therefractive index was somewhat lower It can therefore not be excludedthat some non-ideal ALD behavior takes place at the lowest andhighest temperatures investigated At the low substrate temperature of50C additional CVD reactions may contribute to the higher GPC andlower n This might also explain the slightly non-linear trend betweenthe film thickness and number of cycles observed at this depositiontemperature as shown in Figure 2 Despite the long purging times(10 s) after the plasma step the accumulation of some residual H2Oin the reactor with which the precursor can react may play a role inthese observations For temperatures reaching 400C thermal stabilityissues of the precursor and its ligands can start to play a role

Table I shows RBS and ERD data obtained for substrate temper-atures of 100 200 and 300C The table shows that the number ofSi atoms deposited per cycle decreases with increasing substrate tem-perature This clearly demonstrates that the decrease of the GPC withincreasing substrate temperature can be attributed to reduced precur-sor adsorption per cycle at higher temperatures Similar results wereobtained for Al2O3 synthesized by plasma-assisted ALD27 31 whichcould be attributed almost fully to the decrease in the number of Alatoms deposited per cycle for increasing substrate temperatures ForAl2O3 the decrease in GPC with increasing temperature is mainly dueto a loss of ndashOH surface groups due to thermally activated dehydrox-ylation reactions8 33 34 The similarity between both ALD processesindicates that the OH surface coverage is also a key parameter con-trolling the growth rate of ALD SiO2 as will be discussed in moredetail later

The fact that good SiO2 material properties were obtained between100 and 300C can be concluded from Table I The Si and O contentcorrespond with an OSi ratio of 21 plusmn 01 and the hydrogen contentof the films is 7 ndash 10 at depending on the substrate temperatureThe carbon and nitrogen content of the films was below the detectionlimit of the RBS measurements of sim5 at Apart from the hydrogencontent the material properties were found to be virtually independentof the substrate temperature in the range of 100ndash300C This also holdsfor the mass density which was found to be 20 plusmn 01 gcm3

Fourier transform infrared absorption spectroscopy was used tocompare the ALD SiO2 films with thermally-grown SiO2 Figure 6shows the FTIR spectra The data reveal a shift of the Si-O-Sistretching and Si-O-Si rocking modes toward lower wavenumbersfor the ALD SiO2 film This is in agreement with the slightly non-stoichiometric nature of the films and the fact that the mass densityof ALD SiO2 is slightly lower compared to typical values for wetthermally-grown SiO2 films (sim22 gcm3) The FTIR data also confirmthe presence of hydrogen in the ALD SiO2 films by the observation ofSiO-H bending (sim920 cmminus1) and SiO-H stretching (2500-3600 cmminus1)signatures in the spectrum

Surface Morphologymdash The surface morphology of the SiO2 filmswas studied by AFM in semi-contact mode A root-mean-square sur-face roughness of 19 and 16 Aring was obtained for films deposited at100C (film thickness 51 nm) and 200C (48 nm) respectively Thefact that these values were not higher than those obtained for uncoated

polished Si wafers demonstrates that the SiO2 films were depositedwith negligible roughening on the Si(100) substrate The latter canalso be appreciated from the interface morphology as visualized inthe TEM image of a SiO2Al2O3 stack displayed in Fig 7 Perfectlysmooth SiSiO2 and SiO2Al2O3 interfaces can be observed From theAFM data it also follows that the aforementioned small (PE)CVDgrowth component at low substrate temperatures is not pronouncedyet at 100C as this would likely have resulted in enhanced surfaceroughness

Conformalitymdash The conformality of the ALD process was ex-amined on a structure with high-aspect ratio trenches using high-resolution SEM The various trenches exhibited aspect ratios (ARie depth of trench divided by average width) between sim10 and sim30and were fabricated by deep reactive ion etching (DRIE) The trenchesexhibited a thermal SiO2 layer of sim 100 nm on the surfaces To ob-tain sufficient optical contrast between the layers an Al2O3 film wasdeposited prior to ALD SiO2 deposition The length of the varioussteps in the ALD SiO2 recipe was increased slightly compared to therequirements for ALD growth on a planar surface (see Fig 4) Therecipe included a dose time of 90 ms a purge time of 5s a plasma timeof 45s and a subsequent purge time of 6s Figure 8 shows the SEMimage of a trench with an aspect ratio of AR = sim30 The SiO2 filmwas deposited using 830 cycles The film thickness at the surface wasdetermined with SEM to be 100 plusmn 5 nm which was in good agree-ment with the 102 plusmn 1 nm obtained by spectroscopic ellipsometry fora planar reference sample At the bottom of the trench a thickness of95 plusmn 5 nm was extracted from the SEM graph This demonstrates avery high conformality (95ndash100) for the plasma-assisted ALD pro-cess These results are important as there is only a limited amount ofliterature available on the conformality of plasma-assisted ALD pro-cesses (see Ref 35 and references therein) For example good resultshave previously been reported for AR up to sim1536

400 600 800 1000 1200 1400

0

1

2Si-O-Si

stretching

1087 cm-1

Si-O-Sirocking

Si-O-Sibending

Nor

mal

ized

abs

orba

nce

(10-3

nm

-1)

Wavenumber (cm-1)

1068 cm-1

SiO-Hbending

2000 3000 4000

SiO-Hstretching

ALD SiO2

Thermal SiO2

Figure 6 FTIR spectra of ALD SiO2 prepared at 200C (48 nm film thickness)and thermal SiO2 grown by wet oxidation at sim900C (295 nm film thickness)The most prominent absorption peaks have been assigned (see Ref 37 andreferences therein) The absorbance is normalized by the film thickness A Siwafer without SiO2 served as a reference to obtain the absorption spectra



Figure 7 High-resolution TEM image of an ALD SiO2 film of 70 plusmn 03 nmthickness deposited on a H-terminated Si(100) wafer The SiO2 was coveredby an Al2O3 film deposited by plasma-assisted ALD

The high conformality that was demonstrated for plasma-assistedALD of SiO2 implies that the recombination of oxygen radicalsin the trench was not significant in affecting the conformalityA recombination-limited growth regime would have resulted in asignificant variation of film thickness along the depth of the trench35

Furthermore it is interesting to note that the deposition of Al2O3 priorto the SiO2 film exhibited a lower conformality for the ALD settingsemployed The thickness of the film at the bottom of the trench wasdetermined to be sim50 nm compared to the sim100 nm on the surfaceFor a trench with AR of sim10 a conformality of 100 was obtainedfor Al2O3 The lower conformality for AR = sim30 could indicate thatlonger plasma exposure times (ie above 45 s) are required to reachsaturation at the bottom of the trench Another factor affecting theconformality could be a higher recombination rate of oxygen radicalson the Al2O3 surface than on SiO235

Figure 8 High-resolution SEM images of a high-aspect ratio trench in Sicoated by 830 cycles of ALD SiO2 The SiO2 was deposited on top of thermalSiO2ALD Al2O3 layers for optical contrast The depth and average widthof the trench were 185 and 06 μm respectively resulting in an aspect ratioof sim30

Surface Chemistry

From the data presented and on the basis of literature reportssome aspects of the surface chemistry during ALD SiO2 can be brieflyaddressed As mentioned previously from the decrease of the GPCfor increasing substrate temperatures (Fig 5b) we can conclude thatthe ALD process is governed by surface chemical reactions involvingndashOH groups Note here that the presence of residual ndashOH groupswas demonstrated by the FTIR spectrum showing a clear signatureof SiO-H bonds in the films (Fig 6) During the precursor step itis therefore most likely that the ndashN(C2H5)2 ligands of the precursorreact with the surface ndashOH groups producing volatile HN(C2H5)2 Areaction involving the breaking of the Si-H bond in the precursoris very unlikely24 We propose therefore similar surface chemicalreactions during the first ALD half cycle as reported by Burton et al 19

for the SiH(N(CH3)2)3 precursor which is comparable to the presentprecursor

SiminusOHlowast + H2Si[N(C2H5)2]2 rarr SiOminusSiH2[N(C2H5)2]lowast2minusx

+ xHN(C2H5)2

where surface species are indicated by lowast In this precursor adsorptionreaction only one (x = 1) or both (x = 2) of the ndashN(C2H5)2 ligands mayreact In the second half cycle the surface reactions will be dominatedby O radical species delivered by the plasma32 From similar casesstudied previously (eg Al2O3 from Al(CH3)3 and O2 plasma 32 andTa2O5 from Ta[N(CH3)2]5 and O2 plasma 30) it can be hypothesizedthat combustion-like reactions dominate

SiH2N(C2H5)lowast2 + O rarr SiminusOHlowast + H2O + COx

+ other (Nminuscontaining) species

In the latter expression the species are not balanced as it is unclearwhat reaction products are actually created In the second half cyclealso N-containing species need to be produced for the case that not allprecursor molecules react with the ndashOH covered surface through therelease of both ndashN(C2H5)2 ligands ie when x = 2 for all precursormolecules adsorbing

Evidence for the fact that x = 2 for all precursor molecules wasobtained from quadrupole mass spectrometry (QMS) measurementsFigure 8 shows time-dependent mass spectrometry data for a numberof selected mass-over-charge mz ratios The enhanced signals at mz= 72 (N[C2H5]2

+) and mz = 73 (HN[C2H5]2+) during the first half

cycle are consistent with the removal of the precursor ligands duringprecursor adsorption However it should be noted that these signalscan also originate from the cracking of the precursor molecule inthe mass spectrometer Very small signals at these mz values werealso observed during the second half cycle whereas the signals wereabsent during steps in which the plasma was ignited without precedingprecursor dosing This suggests that after the first half cycle indeeda fraction of the -N(C2H5)2 ligands remain intact on the surface24

The latter can also be concluded from the other species observedduring the second half cycle During this plasma step the prominentmz ratios that were detected included mz = 2 (H2

+) mz = 18(H2O+) mz = 28 (CO+) and mz = 44 (CO2

+) Figure 9 shows thesignals at mz = 18 and mz = 44 The fact that combustion productssuch as CO2 are observed during the plasma step clearly indicatesthat some -N(C2H5)2 ligands remain on the surface after precursoradsorption

The interpretation of mass spectrometry for plasma-assisted ALDprocesses is more complicated than for thermal ALD as the reactionproducts released from the surface can react in the plasma leading tothe creation of other species and fragments The mass spectrometrydata as shown in Figure 9 should therefore be interpreted with care32

However plasma-assisted ALD also allows investigation of the opti-cal emission spectrum during the plasma step38 Figure 10 shows twooptical emission spectra one for a plasma step during ALD (recordedimmediately after plasma ignition) and one for a regular O2 plasmawithout preceding precursor dosing step The presence of OH and Hemission (ie Hα Hβ Hγ of the Balmer series) is clearly observed



0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44

Plasma dosesPrecursor doses ALD cyclesPrecursorPlasma

QM

S s

igna

l (a

u)

Time (s)

Figure 9 Data from quadrupole mass spectrometry (QMS) for selected mass-over-charge ratios ie mz = 18 (H2O+) 44 (CO2

+) 72 (N[C2H5]2+) and

73 (HN[C2H5]2+) During the measurements the substrate temperature was

250C and the wall temperature was 180C In the first part of the figure onlythe precursor is pulsed in the second part only the plasma and in third partboth the precursor and plasma are pulsed (regular ALD cycle with extendedpurge times for clarity)

for the plasma step during ALD These excited fragments are formedin the plasma by (electron-induced) dissociation of volatile speciesoriginating from the reactor surfaces and substrate The inset showsthe transient Hα emission during the plasma step in the ALD cy-cle The increase and subsequent decrease of the signal suggests thatthe reaction products are formed within the first second after plasmaignition This interpretation is consistent with the fast saturation be-havior as displayed in Figure 4c The Hα emission disappears within3ndash4 s after plasma ignition which is similar to the residence time ofthe gas species in our reactor at the operating pressure used (sim150mTorr) This indicates that the surface reactions take place almostinstantly after plasma ignition Interesting is also that no signal due toCN emission is observed during the plasma step This emission wasprominently present in the emission spectra during plasma ALD fromTa2O5 from Ta[N(CH3)2]5 and O2 plasma30

300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104

Time (s) ALD ReferenceH

β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0

Figure 10 Optical emission spectra (OES) for the plasma step during plasma-assisted ALD and for a regular plasma step without preceding precursor dosingThe latter served as a reference The most prominent emission lines have beenassigned The inset shows the transient signal due to Hα emission after theignition of the plasma at 0 s

1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)

Injection level (cm-3)

ALD SiO2

Figure 11 Injection-level-dependent effective lifetime for a single layersim45 nm SiO2 film (annealed 400C N2H2 20 min) and for a sim12 nmSiO2sim30 nm Al2O3 stack (annealed 400C N2 10 min) The inset shows thelong-term stability corresponding to the single layer SiO2 film after annealingAs substrates 35 Ohm cm n-type FZ c-Si wafers were used

Surface Passivation Properties

Surface passivation by single layer SiO2mdash The surface passiva-tion properties were evaluated for n-type Si wafers that were coatedon both sides with SiO2 (20ndash45 nm) using a deposition temperature ofsim200C The films afforded no significant surface passivation in theas-deposited state indicated by a very low effective lifetime of τeff = sim4 μs Similar results were reported for plasma ALD Al2O3 films39 40

The as-deposited Al2O3 films exhibited a very high defect densityat mid gap (Dit sim1013 eVminus1 cmminus2) at least partially related to thevacuum UV (VUV) radiation present in the plasma40 It is expectedthat VUV also plays a role in the poor passivation quality obtained foras-deposited ALD SiO2 Figure 11 shows the injection-level depen-dent effective lifetime after annealing the ALD SiO2 films in forminggas (mixture of sim10 H2 in N2) at a temperature of 400C The pas-sivation properties improved significantly during annealing resultingin τeff values up to 560 μs (n = 1times1014 cmminus3) which correspondto Seff lt 25 cms Nonetheless the level of surface passivation af-ter annealing was not stable in time and gradually deteriorated (insetFig 11) This transient behavior is indicative of progressively higherDit values Issues with the long term stability of the passivation bysilicon oxides have been reported before for chemical oxides41

In an attempt to improve the passivation properties of ALD SiO2the effect of a high temperature annealing step at 900C (1 min) wasstudied A reduction of the film thickness by sim10 was observedduring this annealing step and also the refractive index decreasedindicating densification of the film However subsequent annealingin forming gas at 400C did not lead to a significantly improved levelof passivation or stability compared to a reference sample which didnot receive the 900C annealing step The results show that the levelof passivation associated with ALD SiO2 was significantly lower thanobtained by thermally-grown SiO2 for which typically Seff values lt10 cms are obtainedSurface passivation by SiO2Al2O3 stacksmdashWe have recently demon-strated that the interface properties of SiO2 films synthesized by high-rate plasma enhanced chemical vapor deposition (PECVD) could beimproved dramatically by the application of an Al2O3 capping filmand subsequent annealing14 For these stacks comprising Al2O3 filmswith a thickness down to sim5 nm very low surface recombinationvelocities with values of Seff lt 5 cms were achieved14 Figure 11shows the effective lifetime for a 12 nm ALD SiO230 nm Al2O3

stack after annealing in N2 (400C 10 min) Note that in contrast tosingle layer ALD SiO2 annealing in N2H2 was not necessary Thehigh effective lifetimes up to sim7 ms correspond to exceptionally lowSeff lt 3 cms for n-type Si15 It is furthermore important to mentionthat the effective lifetime induced by the SiO2Al2O3 stacks remained



virtually constant over the monitored period of several months Pre-liminary results suggested that the passivation performance for p-typeSi was lower than for n-type Si Values of Seff lt 17 cms and lt 50cms were obtained for stacks comprising SiO2 with a thickness ofsim1 and 12 nm respectively using 2 middot cm p-type c-Si substratesThis difference between n- and p-type Si may be related to the pos-itive charges present in the stacks comprising relatively thick SiO2

interlayers as will be discussed below On p-type Si a small positivecharge density may attract the minority carrier electrons toward thesurface increasing the recombination rate

For application in high-efficiency solar cells it is relevant to men-tion that the stacks exhibited a good thermal stability during hightemperature processing14

Capacitance-Voltage Measurements

Capacitance-voltage measurements were carried out to study theelectronic interface properties corresponding to the ALD SiO2 filmsand SiO2Al2O3 stacks The C-V data are shown in Fig 12 for SiO2

thicknesses in the range of 1ndash30 nm The measurements were per-formed using frequencies of 1 10 100 kHz The voltage was sweptfrom minus4 to 4V to go from accumulation to inversion conditions

Dielectric constantmdash The dielectric constant k was determinedfrom the capacitance density Cox of the dielectric (by the Hausermodel) and the thickness information from SE by

k = d times Cox

ε0[2]

with d the film thickness and ε0 the vacuum permittivity Table IIgives the dielectric constants obtained for the SiO2 films and the stacksafter annealing at 400C For the thinnest film (sim5 nm) for examplea dielectric constant of k = 47 plusmn 04 was extracted from the C-Vdata The higher k-values obtained for ALD SiO2 relative to thermally-grown SiO2 (k sim39) can likely be attributed to the presence of residualOH groups in the material (Table I)20 42 In comparison Hiller et alreported a value of k = 61 (as-deposited) and k = 36 for ALD SiO2

films after annealing at 1000C20 For a stack comprising an ALD SiO2

film of 1 nm and an Al2O3 layer of 30 nm a value of k = 8 plusmn 04 wasdetermined This value was similar (within measurement accuracy) tothe value obtained for an Al2O3 reference film deposited directly on H-terminated Si (Table II) These values are in good agreement with thosereported in the literature for single layer Al2O3 (k = sim7ndash9)26 43ndash45

As expected the dielectric constants of the stacks are observed todecrease for increasing SiO2 interlayer thickness due to the lowerk-value associated with SiO2

-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz

Figure 12 Capacitance-voltage (C-V) measurements for (a) single layer ALDSiO2 films after annealing in forming gas (400C N2H2 10 min) and (b) forSiO2Al2O3 stacks after annealing in N2 (400C 10 min) The SiO2 thicknesswas varied and the Al2O3 film thickness was 30 nm The transients weremeasured using frequencies of 1 10 (not shown) and 100 kHz The C-Vmeasurements were carried out several weeks after the annealing treatmentAs substrates 2 cm p-type c-Si wafers were used

Fixed charges in ALD SiO2mdash The extraction of fixed chargesfrom the flatband voltage shift VFB requires an assumption about theposition of these charges In a first approximation it is common toassume that all charges reside at the SiSiO2 interface The effectivefixed charge density Qeff (expressed in cmminus2) is then simply given bythe relation

q Qeff = minus(VF B minus φms) times Cox [3]

with φms the work function difference between the Al contact and thep-type Si substrate and q equal to the elementary charge For singlelayer ALD SiO2 we observed a linear trend between VFB and the SiO2

thickness By extrapolation an y-axis intercept of minus114 plusmn 002 eVwas found which represents the value of φms Moreover this linearrelation implies that the assumption of the presence of the chargesat the interface is reasonable as charge distributed in the bulk of

Table II SiO2 thickness dependent electronic properties of ALD SiO2 films and ALD SiO2Al2O3 stacks As a reference the data for Al2O3 onSi is shown39 The relative dielectric constant k effective fixed charge density Qeff (Eq 3) negative charge at SiO2Al2O3 interface Qneg (Eq 4)and interface defect density at mid gap Dit were extracted from the C-V measurements (Fig 12) The estimated error in the dielectric constantis sim04 the error in Qf for single layer SiO2 and Al2O3 is estimated to be plusmn 2times1011 cmminus2 the error in Dit for single layer SiO2 is sim05times1012

cmminus2 The error in the Dit values for the stacks leads to maximum Dit values of 1011 cmminus2 eVminus1 The SiO2 films were annealed in H2N2 whilethe Al2O3 and SiO2Al2O3 stacks were annealed in N2

Layer(s) k VFB Qeff (cmminus2) Qneg a (cmminus2) Dit (eVminus1 cmminus2)

5 nm SiO2 47 minus 126 +6times1011 ndash 2times1012



30 nm Al2O3 on Si (ref) 80 25 minus58times1012 ndash 9times1010

1 nm SiO230 nm Al2O3 80 083 minus26times1012 minus36times1012 5times1010


125 nm SiO230 nm Al2O3 68 minus 069 minus4times1011 minus19times1012 4times1010

30 nm SiO230 nm Al2O3 58 minus 22 +6times1011 minus4times1011 1times1011

a Using Eq 4 using Qpos = 8times1011 cmminus2

ndash = not applicable



a film would lead to a nonlinear thickness dependence For ALDSiO2 relatively small VFB values were observed with correspondingpositive Qf values in the range of 6ndash8times1011 cmminus2 (Table II) The factthat no hysteresis was observed in the C-V characteristics indicatesthat the VFB shifts were not caused by mobile charges The presenceof positive fixed charges is well-known for SiO2 deposited at lowtemperatures and has for instance been reported for PECVD and ALDSiO2 films14 20 46 This is in contrast to thermally-grown oxides forwhich the charge density is typically much lower unless the films havebeen exposed to significant UV irradiation or electrical stressing1 46

The microscopic origin of the positive charges is not unambiguousas it depends on the nature of the oxide and the post-treatments Onedefect that can be responsible for positive charges is the so-called Ersquocenter (Si atom backbonded by 3 O atoms)1 However for PECVDSiO2 Buchanan et al have provided evidence that another (unknown)type of defect also within tunneling distance (ie lt 5 nm) of the Sisubstrate is more likely to be responsible for the fixed charges46 Ithas also been hypothesized that H may be involved in the generationof the positive charges20 47

Fixed charges in SiO2Al2O3 stacksmdash In contrast to the singlelayers large positive VFB values were observed for the SiO2Al2O3

stacks comprising thin SiO2 interlayers (thickness of 1 and 25 nm) andfor a single layer ALD Al2O3 reference sample deposited directly onH-terminated Si (Table II) These VFB values indicate the presence of asignificant amount of net negative charge When we apply the model ofEq 3 we find a decreasing negative Qeff for increasing SiO2 thickness(Table II) For the thickest SiO2 interlayer of 30 nm a negative VFB

is found corresponding to a net positive Qeff of + 6 times 1011 cmminus2

when assuming that all charges are located at the SiSiO2 interfaceHowever the latter assumption is not realistic for the stacks To furtherinterpret the VFB values in terms of fixed charges we can also considerthe influence of multiple charge contributions It is well known thatthe negative charges associated with Al2O3 are mainly located nearthe SiO2Al2O3 interface48ndash50 The negative charges will shift awayfrom the Si interface when using SiO2 interlayers between Si andAl2O3 In addition positive charges associated with SiO2 need to betaken into account The latter can also be directly appreciated fromthe stack with a 30 nm interlayer Therefore in addition to the relationgiven in (3) an alternative more complex model can be introducedwhich accounts for a positive charge contribution Qpos at the SiSiO2

interface and a negative charge contribution Qneg at the SiO2Al2O3

interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1

q(VF B minus φms) [4]

For simplicity we consider dSiO2 to be the nominal ALD SiO2 thick-ness as determined by SE In order to obtain an estimate of Qneg usingEq 4 a value for Qpos of +8times1011 cmminus2 may be appropriate ie thevalue obtained for thick SiO2 films without capping layer In additionin contrast to Eq 3 we cannot use the experimentally determinedvalues for Cox in Eq 4 Therefore we consider kSiO2 and kAl2O3 to beconstant with values of 55 and 8 respectively Using these valuesit is found from the analysis that Qneg decreases with increasing SiO2

interlayer thickness from sim6times1012 cmminus2 (no ALD SiO2) to sim4times1011

cmminus2 (30 nm SiO2) Note that the latter value (and its polarity) shouldbe interpreted with care as this value significantly depends on the ac-curacy of the substituted value for Qpos However as expected models(3) and (4) lead to the same main conclusion that the negative chargedensity associated with Al2O3 is strongly reduced for thicker SiO2 in-terlayers Moreover for sufficiently thick SiO2 films the fixed positivecharges play a more dominant role

In general these observations are in good agreement with a recentstudy by Wolf et al using CV measurements52 In addition the resultsare in line with a recent study using optical second-harmonic gener-ation (SHG) experiments15 The SHG measurements are sensitive to

electric fields induced in the space charge region below the Si surfacecaused by the fixed charges present in the passivation scheme15 48 Itwas found that the induced electric field strongly decreased for in-creasing SiO2 interlayers and vanished for a SiO2 interlayer thicknessof sim5 nm However a small net positive charge density was deter-mined by SHG for an interlayer thickness of 125 nm while the C-Vresults suggested a small but negative Qeff for this stack These ob-servations may be related to the fact that SHG is a fundamentally dif-ferent measurement technique For example SHG is a non-contactingand non-intrusive technique while the C-V measurements obviouslyrequire the application of metal contacts The latter may influencedirectly or indirectly the fixed charges in the stack For example theexact value for φms may slightly depend on the dielectric below (ieSiO2 or Al2O3)47

The strong dependency of Qneg on the SiO2 thickness may point toa role of charge injection from the Si substrate in defect states at theremote SiO2Al2O3 interface during annealing15 The SiO2 interlayercould serve as a barrier for charge injection and affect the formation ofnegative charges accordingly As discussed in Ref 15 the changes innet fixed charge density and associated field-effect passivation had asignificant impact on the level of passivation but also on the injectionlevel dependence of the effective lifetime

Interface defect densitymdash The interface defect density was deter-mined from the frequency dispersion in the C-V data29 The differencein the high and low frequency C-V characteristics corresponding to theALD SiO2 films is indicative of the presence of a significant amountof electronically active interface defects (Fig 12a) Relatively high Dit

values of sim1012 cmminus2 eVminus1 were obtained for single layer ALD SiO2

films after forming gas annealing (Table II) This is consistent withthe relatively high Seff values that were measured for similar samplesespecially after aging (inset Fig 11) In strong contrast ultralow Dit

values were obtained for the SiO2Al2O3 stacks with Dit lt 1011 cmminus2

eVminus1 In addition the data did not reveal a clear relation between Dit

and the SiO2 film thickness (1ndash30 nm) Therefore the stacks induceda high level of chemical passivation regardless of SiO2 thickness Thelow defect densities can be related to the effective hydrogenation ofthe interface under influence of hydrogen from the Al2O3 cappingfilm53 During annealing the prominent Pb-type defects (Si danglingbond) at the SiSiO2 interface are effectively passivated by hydrogenFrom the data in Table II it can be concluded that the use of an Al2O3

capping layer leads to a significantly higher interface quality thanobtained by annealing the ALD SiO2 films in forming gas

Conclusions

A plasma-assisted ALD process for SiO2 has been developed em-ploying H2Si[N(C2H5)2]2 as Si precursor in combination with an O2

plasma as the oxidant It was demonstrated that the ALD processis suited for the low-temperature synthesis of high-quality SiO2 filmswith the material properties being relatively insensitive to the substratetemperature in the range of 100 ndash 300C The ALD process combinedcomparatively high growth rates (08 ndash 17 Aringcycle) with relativelyshort dosing and purge times Moreover the plasma-assisted ALDprocess led to good conformality for trenches with an aspect ratio ofsim30 even for relatively short plasma exposure times of 45s

Regarding the electronic properties the interface defect densitywas relatively high (Dit sim1012 eVminus1 cmminus2 after aging) for single layerALD SiO2 The application of an Al2O3 capping layer in conjunctionwith a low-temperature anneal led to a dramatic improvement in theinterface quality with Dit lt 1011 eVminus1 cmminus2 Corresponding ultralowsurface recombination velocities lt 3 cms were measured for thesestacks The thickness of the SiO2 interlayer was found to controlthe fixed charge density associated with the SiO2Al2O3 stacks Atransition from a high negative net charge density to a relatively lowpositive net charge density was observed

These results may have implications for a wide range of ex-isting and future applications in which the thickness control and



conformality of the ALD SiO2 films are critical factors Moreoverthe ALD SiO2 nanolayers can be applied in effective surface passiva-tion schemes for future high-efficiency silicon solar cells

Acknowledgments

Dr MA Verheijen (TUe) is acknowledged for the TEM analysisCh Lachaud and Dr A Madec (Air liquide) are acknowledged fordonating the SAM24 precursor This work is supported by Q-Cellsand the German Ministry for the Environment Nature Conservationand Nuclear Safety (BMU) under contract number 0325150 (ldquoAL-ADINrdquo) and by the Dutch Technology Foundation STW (Thin FilmNanomanufacturing (TFN) program)

References

1 M L Green E P Gusev R Degraeve and E L Garfunkel J Appl Phys 90 2057(2001)

2 C R Helms and E H Poindexter Rep Prog Phys 57 791 (1994)3 A G Aberle Prog Photovoltaics 8 473 (2000)4 M J Kerr and A Cuevas Semicond Sci Technol 17 35 (2002)5 J Zhao A Wang M A Green and F Ferrazza Appl Phys Lett 73 1991 (1998)6 O Schultz A Mette M Hermle and S W Glunz Prog Photovolt Res Appl 16

317 (2008)7 S M George Chem Rev 110 111 (2010)8 R L Puurunen J Appl Phys 97 121301 (2005)9 H B Profijt S E Potts M C M van de Sanden and W M M Kessels J Vac Sci

Technol A 29 050801 (2011)10 J W Klaus O Sneh and S M George Science 278 1934 (1997)11 D Hausmann J Becker S Wang and R G Gordon Science 298 402 (2002)12 J Beynet P Wong A Miller S Locorotondo D Vangoidsenhoven T-H Yoon M

Demand H-S Park T Vandeweyer H Sprey Y-M Yoo and M Maenhoudt ProcSPIE 7520 75201J (2009)

13 C C Fulton G Lucovsky and R J Nemanich Appl Phys Lett 85 580 (2004)14 G Dingemans M C M van de Sanden and W M M Kessels Phys Status Solidi

RRL 5 22 (2011)15 G Dingemans N M Terlinden M A Verheijen M C M van de Sanden and W

M M Kessels to be published in J Appl Phys 110 093715 (2011)16 G Dingemans M M Mandoc S Bordihn M C M van de Sanden and W M M

Kessels Appl Phys Lett 98 222102 (2011)17 J W Klaus O W Ott J M Johnson and S M George Appl Phys Lett 70 1092

(1997)18 S M George O Sneh A C Dillon M K Wise A W Ott L A Okada and J D

Way Appl Surf Science 82 460 (1994)19 B B Burton M P Boleslawski A T Desombre and S M George Chem Mater

20 7031 (2008)20 D Hiller R Zierold J Bachmann M Alexe Y Yang J W Gerlach A Stesmans

M Jivanescu U Muller J Vogt H Hilmer P Loper M Kunle F Munnik KNielsch and M Zacharias J Appl Phys 107 064314 (2010)

21 S Kamiyama T Miura and Y Nara Thin Solid Films 515 1517 (2006)

22 S-J Won S Suh M Soo Huh and H Joon Kim IEEE Electron Device Lett 31857 (2010)

23 R Katamreddy B Feist and C Takoudis J Electrochem Soc 155 G163 (2008)24 B B Burton S W Rang S W Rhee and S M George J Phys Chem C 113

8249 (2009)25 S W King J Vac Sci Technol A 29 041501 (2011)26 J L van Hemmen S B S Heil J H Klootwijk F Roozeboom C J Hodson M C

M van de Sanden and W M M Kessels J Electrochem Soc 154 G165 (2007)27 G Dingemans M C M van de Sanden and W M M Kessels Electrochem Solid

State Lett 13 H76 (2010)28 G Dingemans R Seguin P Engelhart M C M van de Sanden and W M M

Kessels Phys Status Solidi RRL 4 10 (2010)29 R Castagne and A Vapaille Surf Sci 28 157 (1971)30 S B S Heil F Roozeboom M C M van de Sanden and W M M Kessels J Vac

Sci Technol A 26 472 (2008)31 S E Potts W Keuning E Langereis G Dingemans M C M van de Sanden and

W M M Kessels J Electrochem Soc 157 P66 (2010)32 S B S Heil J L van Hemmen M C M van de Sanden and W M M Kessels J

Appl Phys 103 103302 (2008)33 C Dillon A W Ott J D Way and S M George Surf Sci 322 230 (1995)34 E Langereis J Keijmel M C M van de Sanden and W M M Kessels Appl Phys

Lett 92 231904 (2008)35 H C M Knoops E Langereis M C M van de Sanden and W M M Kessels J

Electrochem Soc 157 G241 (2010)36 H Kim C Cabral C Lavoie and S M Rossnagel J Vac Sci Technol B 20 1321

(2002)37 B Hoex F J J Peeters M Creatore M A Blauw W M M Kessels and M C M

van de Sanden J Vac Sci Technol A 24 1823 (2006)38 A J M Mackus S B S Heil E Langereis H C M Knoops M C M van de

Sanden and W M M Kessels J Vac Sci Technol A 28 77 (2010)39 B Hoex S B S Heil E Langereis M C M van de Sanden and W M M Kessels

Appl Phys Lett 89 042112 (2006)40 G Dingemans N M Terlinden D Pierreux H B Profijt M C M van de Sanden

and W M M Kessels Electrochem Solid State Lett 14 H1 (2011)41 N E Grant and K R McIntosh IEEE Electron Device Lett 31 1002 (2010)42 B Fowler and E OrsquoBrian J Vac Sci Technol B 12 441 (1994)43 M D Groner J W Elam F H Fabreguette and S M George Thin Solid Films

413 186 (2002)44 J W Lim and S J Yun Electrochem Solid State Lett 7 F45 (2004)45 S K Kim S W Lee C S Hwang Y S Min J Y Won and J Jeong J Electrochem

Soc 154 F69 (2006)46 D A Buchanan J H Stathis and P R Wagner Appl Phys Lett 56 1037 (1990)47 M Houssa V V Afanasrsquoev A Stesmans and M M Heyns Appl Phys Lett 77

1885 (2000)48 N M Terlinden G Dingemans M C M van de Sanden and W M M Kessels

Appl Phys Lett 96 112101 (2010)49 R S Johnson G Luckovsky and I Baumvol J Vac Sci Technol A 19 1353

(2001)50 D Hoogeland K B Jinesh F Roozeboom W F A Besling M C M van de

Sanden and W M M Kessels J Appl Phys 106 114107 (2009)51 J A Aboaf D R Kerr and A Bassous J Electrochem Soc 120 1103 (1973)52 S Mack A Wolf C Brosinsky S Schmeisser A Kimmerle P Saint-Cast M

Hofmann and D Biro IEEE J Photovoltaics 1 135 (2011)53 G Dingemans W Beyer M C M van de Sanden and W M M Kessels Appl

Phys Lett 97 152106 (2010)


Journal of The Electrochemical Society 159 (3) H277-H285 (2012) H2770013-46512012159(3)H2779$2800 copy The Electrochemical Society

Plasma-Assisted ALD for the Conformal Deposition of SiO2Process Material and Electronic PropertiesG Dingemansa C A A van Helvoirta D Pierreuxb W Keuninga and W M M Kesselsalowastz

aDepartment of Applied Physics Eindhoven University of Technology 5600 MB Eindhoven The NetherlandsbASM Kapeldreef 75 B-3001 Leuven Belgium

Plasma-assisted atomic layer deposition (ALD) was used to deposit SiO2 films in the temperature range of Tdep = 50ndash400C onSi(100) H2Si[N(C2H5)2]2 and an O2 plasma were used as Si precursor and oxidant respectively The ALD growth process andmaterial properties were characterized in detail Ultrashort precursor doses (sim50 ms) were found to be sufficient to reach self-limitingALD growth with a growth-per-cycle of sim12 Aring (Tdep = sim200C) leading to SiO2 films with OSi ratio of sim21 Moreover the plasmaALD process led to a high conformality (95ndash100) for trenches with aspect ratios of sim30 In addition the electronic (interface)properties of ultrathin ALD SiO2 films and ALD SiO2Al2O3 stacks were studied by capacitance-voltage and photoconductancedecay measurements The interface quality associated with SiO2 was improved significantly by using an ultrathin ALD Al2O3capping layer and annealing The interface defect densities decreased from sim1times1012 eVminus1 cmminus2 (at mid gap) for single layer SiO2to lt 1011 eVminus1 cmminus2 for the stacks Correspondingly ultralow surface recombination velocities lt 3 cms were obtained for n-typeSi The density and polarity of the fixed charges associated with the stacks were found to be critically dependent on the SiO2 thickness(1ndash30 nm)copy 2012 The Electrochemical Society [DOI 1011492067203jes] All rights reserved

Manuscript received November 8 2011 Published January 6 2012 This was Paper 1357 presented at the Montreal QC CanadaMeeting of the Society May 1ndash6 2011

The key importance of thermally grown silicon dioxide(SiO2) for applications in silicon-based microelectronics needs nointroduction1 2 Also in silicon photovoltaics SiO2 is a very impor-tant material The low interface defect densities associated with theSiSiO2 interface reduce surface recombination significantly and haveenabled high solar cell efficiencies3ndash6 Temperatures gt 800C are typ-ically required to form SiO2 by thermal oxidation of Si Alternativemethods for the synthesis of SiO2 have been developed to avoid suchhigh temperatures and long processing times They may also enablesingle side deposition and a high level of control of the material prop-erties and film thickness These alternative methods include (wet)chemical oxidation (plasma-enhanced) chemical vapor depositionsputtering and electron beam evaporation

Atomic layer deposition (ALD) is an alternative CVD-like methodthat recently gained considerable attention In ALD the substrate issequentially exposed to two precursors (eg a Si-precursor and anoxidant) which each react with the surface until saturation ALD al-lows for precise thickness control optimal large-area uniformity andthe conformal coating of demanding substrate topologies7ndash9 There-fore the advantages of ALD may expand the range of applications forSiO2 ALD SiO2 films may find application as protective or insulatorcoating or can be used in nanolaminate structures with tailored opti-cal and electronic properties10 11 The films can also be used in spacerdefined double patterning processes for example for dynamic randomaccess memory (DRAM) technology12 In addition the ALD SiO2

process may be useful for engineering the interface between Si andhigh-k materials13 In Si photovoltaics SiO2 can be used in double-layer surface passivation schemes for example comprising Al2O3 ora-SiNxH capping layers with tailored interface properties14ndash16

In the first reports ALD SiO2 was synthesized employing SiCl4

and H2O which required relatively high substrate temperatures(gt300C) and long precursor exposures17 18 In recent years variousalternative Si precursors have been tested in combination with O3

or H2O as the oxidants These processes include the use of pyridine(C5H5N) and Al as catalysts on the surface17 11 19 The approachemploying Al was referred to as rapid ALD as it resulted in depositionrates above the ldquotheoreticalrdquo maximum of one monolayer per ALDcycle In addition more recently a thermal ALD process for low-temperature SiO2 was reported which was free of catalysts or corrosiveby-products20 In this respect the use of precursors with amino ligands

lowast Electrochemical Society Active Memberz E-mail wmmkesselstuenl

has also shown promising results in particular when combined withH2O2 O3 or O2 plasma as the oxidant21ndash24 Very recently also SiH4

has been used as Si precursor during plasma-assisted ALD with CO2

plasma as oxidant25 SiO2 films grown with ALD have been reported toexhibit low carbon content and a high electrical breakdown field10 22

Nevertheless to improve properties such as the chemical etch rateor the interface defect density annealing at a temperature of 1000Cwas shown to be beneficial20 Here we report on a plasma-assistedALD process for synthesis of SiO2 at low substrate temperatures of50ndash400C The ALD process employed H2Si[N(C2H5)2]2 as the Siprecursor (Figure 1) The plasma-assisted ALD process was charac-terized in detail and the results are compared to the well-understoodALD process of Al2O3 using Al(CH3)3 We show that the ALD SiO2

process was compatible with short cycle times and led to an excellentconformality even on structures with an aspect ratio of sim30 The elec-tronic properties of the Si(100)SiO2 interface were investigated usingphotoconductance decay and capacitance-voltage measurements Wedemonstrate that interface defect densities lt 1011 eVminus1 cmminus2 (at midgap) can be reached when using ultrathin Al2O3 capping layers andannealing at moderate temperatures (sim400C)

The paper is organized as follows After the experimental detailsthe characterization of the ALD SiO2 process will be presented Inaddition the material properties and the influence of the substratetemperature will be addressed and data with respect to the confor-mality will be presented Subsequently a brief discussion on thesurface chemistry is included on the basis of in situ measurementsThe final sections discuss the surface passivation properties and theelectronic (interface) properties of the SiO2 films and SiO2Al2O3

stacks

Experimental

The SiO2 films were deposited in the Oxford Instruments OpALreactor This is an open-load system suited for both plasma andthermal ALD and operating at typical pressures of 150 mTorr Aremote O2 plasma was used during the oxidation step in the ALDcycle H2Si[N(C2H5)2]2 (SAM24 Air Liquide) was used as the Siprecursor (Fig 1) This is a liquid (melting point lt minus10C) whichexhibits a high vapor pressure ie sim100 Torr at 100C (Fig 1)The SAM24 was held in a stainless steel bubbler heated to 50Cand the precursor was introduced into the reactor by ultrashort doses(10ndash120 ms) using fast ALD valves A flow of Ar as well as the O2

flow were continuously on during the process The latter was feasible







Seff max = W

2 middot τeff

[1]


environment

0 40 80 120 1600

5

10

15

20

0 10 20 30 40 50 600

2

4

6

8

Tdep

= 50oC

Tdep

= 200oC

Tdep

= 250oCSiO

2 th

ickn

ess

(nm

)


TEM SE

(b)(a)









0 20 40 60 80 100 12000

05

10

15

20ALD SiO

2

Dose time (ms)

140

144

148

152

n(a)

(b)

0 1 2 3 4 5


Plasma time (s)

(c)

(d)

0 1 2 3

Plasma purge (s)

0 20 40 60 80 100 12000

05

10

15

20

ALD Al2O

3

Dose time (ms)

(e)

0 1 2 3 4 5

(f)


(g)


(h)

Plasma purge (s)







0 100 200 300 40000

05

10

15

20

n

(b)


14

15 (a)














400 600 800 1000 1200 1400

0

1

2Si-O-Si

stretching

1087 cm-1

Si-O-Sirocking

Si-O-Sibending

Nor

mal

ized

abs

orba

nce

(10-3

nm

-1)

Wavenumber (cm-1)

1068 cm-1

SiO-Hbending

2000 3000 4000

SiO-Hstretching

ALD SiO2

Thermal SiO2








Surface Chemistry




+ xHN(C2H5)2















0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)







Seff max = W

2 middot τeff

[1]


environment

0 40 80 120 1600

5

10

15

20

0 10 20 30 40 50 600

2

4

6

8

Tdep

= 50oC

Tdep

= 200oC

Tdep

= 250oCSiO

2 th

ickn

ess

(nm

)


TEM SE

(b)(a)









0 20 40 60 80 100 12000

05

10

15

20ALD SiO

2

Dose time (ms)

140

144

148

152

n(a)

(b)

0 1 2 3 4 5


Plasma time (s)

(c)

(d)

0 1 2 3

Plasma purge (s)

0 20 40 60 80 100 12000

05

10

15

20

ALD Al2O

3

Dose time (ms)

(e)

0 1 2 3 4 5

(f)


(g)


(h)

Plasma purge (s)







0 100 200 300 40000

05

10

15

20

n

(b)


14

15 (a)














400 600 800 1000 1200 1400

0

1

2Si-O-Si

stretching

1087 cm-1

Si-O-Sirocking

Si-O-Sibending

Nor

mal

ized

abs

orba

nce

(10-3

nm

-1)

Wavenumber (cm-1)

1068 cm-1

SiO-Hbending

2000 3000 4000

SiO-Hstretching

ALD SiO2

Thermal SiO2








Surface Chemistry




+ xHN(C2H5)2















0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)



0 20 40 60 80 100 12000

05

10

15

20ALD SiO

2

Dose time (ms)

140

144

148

152

n(a)

(b)

0 1 2 3 4 5


Plasma time (s)

(c)

(d)

0 1 2 3

Plasma purge (s)

0 20 40 60 80 100 12000

05

10

15

20

ALD Al2O

3

Dose time (ms)

(e)

0 1 2 3 4 5

(f)


(g)


(h)

Plasma purge (s)







0 100 200 300 40000

05

10

15

20

n

(b)


14

15 (a)














400 600 800 1000 1200 1400

0

1

2Si-O-Si

stretching

1087 cm-1

Si-O-Sirocking

Si-O-Sibending

Nor

mal

ized

abs

orba

nce

(10-3

nm

-1)

Wavenumber (cm-1)

1068 cm-1

SiO-Hbending

2000 3000 4000

SiO-Hstretching

ALD SiO2

Thermal SiO2








Surface Chemistry




+ xHN(C2H5)2















0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)













400 600 800 1000 1200 1400

0

1

2Si-O-Si

stretching

1087 cm-1

Si-O-Sirocking

Si-O-Sibending

Nor

mal

ized

abs

orba

nce

(10-3

nm

-1)

Wavenumber (cm-1)

1068 cm-1

SiO-Hbending

2000 3000 4000

SiO-Hstretching

ALD SiO2

Thermal SiO2








Surface Chemistry




+ xHN(C2H5)2















0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)







Surface Chemistry




+ xHN(C2H5)2















0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)



0 50 100 150 200 250 300 35000

10x10-10

20x10-10

mz = 73 mz = 72

00

10x10-8

20x10-8

30x10-8

40x10-8

mz = 18

mz = 44


QM

S s

igna

l (a

u)

Time (s)


+) 72 (N[C2H5]2+) and




300 400 500 600 700 800 900

Hγ

0 1 2 3 4 50

1x104

2x104

3x104


β

Hα

OE

S s

igna

l (a

u)

Wavelength (nm)

Hα

OH

O

0


1013 1014 1015 101610-6

10-5

10-4

10-3

10-2

0 10 20 30

10-4

10-3

Time (days)

ALD SiO2 Al

2O

3

Effe

ctiv

e lif

etim

e (s

)


ALD SiO2
















k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)










k = d times Cox

ε0[2]





-4 -2 0 2 40

1

2

30

2

4

6

8

10

30 nm

125 nm

C (

fFμ

m2 )

Voltage (V)

1 nm

25 nm

(a)

(b)

100 kHz

Al

C (

fFμ

m2 ) 5 nm

125 nm

30 nm

Si

SiO2

Al

Si

SiO2

Al

Al2O

3

Al

1 kHz




























interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)










interface49 51

Q pos

(dSi O2

ε0kSi O2

)+ (Q pos + Qneg)

(dAl2O3

ε0kAl2O3

)

= minus 1















Conclusions








Acknowledgments


References






































Phys Lett 97 152106 (2010)




Acknowledgments


References






































Phys Lett 97 152106 (2010)


plasma-assisted ald for the conformal deposition of sio2 ...ald process for synthesis of sio 2 at...

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