Atomic layer deposition of SiO2 at room temperature using NH3-catalyzed sequential surface reactions

Surface Science 447 (2000) 81–90 www.elsevier.nl/locate/susc

Atomic layer deposition of SiO at room temperature using 2 NH -catalyzed sequential surface reactions 3 J.W. Klaus, S.M. George * Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 1 June 1999; accepted for publication 25 October 1999

Abstract Ultrathin SiO films were deposited at 300–338 K with atomic layer control using NH -catalyzed sequential surface 2 3 reactions. The SiO deposition was achieved by splitting the reaction SiCl +2H OSiO +4HCl into two separate 2 4 2 2 SiCl and H O half-reactions. Successive application of the half-reactions in an ABAB… sequence produced atomic 4 2 layer controlled SiO growth. The NH catalyst dramatically lowered the necessary deposition temperature from 2 3 >600 to 300 K and reduced the required reactant fluxes from ~109 to ~105 L. In situ spectroscopic ellipsometry monitored the SiO deposition versus reaction temperature and SiCl , H O and NH partial pressures. The ellipsometric 2 2 3 ˚4 per measurements obtained a maximum SiO growth rate of 2.16 A AB reaction cycle at 303 K. Atomic force 2 microscopy images of the deposited surface topography indicated smooth SiO films with a roughness similar to the 2 starting Si(100) substrate. Catalysis of the sequential surface reactions that yield atomic layer controlled growth may be general and could facilitate the low temperature deposition of other binary materials. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Amorphous thin films; Ellipsometry; Growth; Infrared absorption spectroscopy; Insulating films; Silicon oxides; Surface chemical reaction; Surface structure, morphology, roughness, and topography

1. Introduction The atomic layer control of thin film growth can be obtained using self-limiting surface reactions applied in a binary reaction sequence [1–3]. This technique is known as atomic layer epitaxy (ALE ) or atomic layer deposition (ALD) and was first demonstrated for the deposition of ZnS films [4]. Recently, many studies have extended this basic approach and explored the atomic layer

* Corresponding author. Tel: +1-303-4923398; fax: +1-303-4925894. E-mail address: [email protected] (S.M. George)

deposition of various oxide [5–11], sulfide [12], nitride [13,14] and phosphide [15] materials. Recent work on SiO atomic layer deposition 2 has focused on dividing the SiCl +2H O 4 2 SiO +4HCl reaction into two half-reactions 2 [5,16 ]: (A) Si–OH1+SiCl SiO–Si–Cl1 +HCl, (1) 4 3 (B) Si–Cl1+H OSi–OH1+HCl, (2) 2 where the asterisks designate the surface species. Alternative sequential reaction approaches for SiO atomic layer controlled growth have utilized 2 Si(NCO) /H O [17], and CH OSi(NCO) /H O 4 2 3 3 2 2 [18]. Each half-reaction occurs on the growing

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SiO surface and involves a reaction between a gas 2 phase precursor and a surface functional group. The products of the two half-reactions are a new surface species and a volatile reaction product. Once the surface reaction is complete, no further reaction can occur because the newly deposited surface species are unreactive with additional gas phase precursors. Successive application of the half-reactions in an ABAB binary reaction sequence produces atomic layer controlled SiO 2 growth [5]. The only drawbacks to this deposition technique are the high deposition temperatures >600 K and the large reactant exposures >109 L (1 L=10−6 Torr s) required for the surface halfreactions to reach completion. To perform the SiO atomic layer deposition at 2 room temperature and reduced reactant exposures, the two surface half-reactions may be catalyzed. We have reported earlier that pyridine (C H N ) 5 5 can serve as a catalyst, i.e. an agent that facilitates a chemical reaction without being consumed, for SiO atomic layer deposition [6,19]. Ammonia 2 (NH ) was chosen as the catalyst for this study 3 because, like pyridine, ammonia is a strong Lewis base. NH is expected to interact strongly with the 3 surface functional groups and reactants present during the SiCl and H O half-reactions of the 4 2 binary reaction sequence [20,21]. The atomic layer deposition of SiO is impor2 tant because SiO is the main dielectric material 2 used for silicon device fabrication. SiO film depos2 ition on high aspect ratio trench capacitors is needed for dynamic random access memory (DRAM ) devices [22]. Uniform SiO film depos2 ition on extremely large substrates are also required for flat panel displays [23]. Low temperature SiO deposition techniques may facilitate the 2 fabrication of thin film transistors, light emitting diodes and photodetectors that incorporate semiconducting polymers [24–26 ]. Low temperature deposition techniques may also allow SiO to be 2 used as a protective coating or insulator on polymeric or biological materials.

2. Experimental The experiments were performed in an apparatus that has been described in detail elsewhere [7].

The apparatus consists of a sample load lock chamber, a central deposition chamber and an ultra high vacuum chamber for surface analysis. Using computer-controlled automated valves, the central deposition chamber can expose the sample to molecular precursors under a wide variety of conditions [7]. The central deposition chamber is equipped with an in situ spectroscopic ellipsometer (J.A. Woolam Co., M-44) that collects ellipsometric data at 44 visible wavelengths simultaneously. Mass spectometric analysis of the gases in the central deposition chamber can be performed using a controlled leak to a UTI-100C quadrupole mass spectrometer residing in the surface analysis chamber. The sample substrate was a 0.75 in×0.75 in Si(100) wafer. The Si(100) sample was p-type, boron-doped with a resistivity of 0.1–0.4 V cm. A ˚ Mo film deposited on the back of the 3000 A substrate was used for resistive heating of the sample. The Si(100) sample was first cleaned with a 48% HF acid solution to remove the native oxide. The sample was then moved into the vacuum chamber and annealed to 875 K for 1 min. This anneal was followed by a high frequency H O plasma discharge at 300 K to hydroxylate the 2 surface and remove carbon contamination. This cleaning procedure leaves a very thin SiO film 2 ˚ ) on the Si(100) substrate. (~5 A An automated SiCl and H O exposure 4 2 sequence was employed to deposit the SiO films. 2 The SiCl half-reaction was performed using the 4 gas pulse sequence shown in Fig. 1. During the SiCl half-reaction, a steady-state NH pressure 4 3 was maintained by continuously flowing NH 3 through the deposition chamber. The NH partial 3 pressure was controlled by varying the conductance from the deposition chamber to a mechanical pump using an in-line butterfly valve. The SiCl 4 precursor was then introduced by opening the SiCl automated valve for 50 ms. This opening 4 produces a small SiCl pressure transient in the 4 chamber. Following each SiCl pulse, the deposition 4 chamber was continually purged with NH to 3 re-establish a steady-state NH pressure prior to 3 the next SiCl pulse. The total SiCl exposure for 4 4 each SiCl half-reaction was controlled by varying 4

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Fig. 1. Gas pulse sequence used during the SiCl half-reaction. 4 A steady state NH pressure was maintained at 75 mTorr during 3 the consecutive 50 ms SiCl pulses. 4

the number of individual SiCl pulses. For a 4 SiCl pressure of 350 Torr backing the automated 4 valve, the resultant SiCl exposure in the depos4 ition chamber was ~4400 L for every 50 ms pulse. This SiCl exposure is only approximate because 4 the pressure transient was measured by a capacitance manometer operating at the lower limit of its recommended range. Using a SiCl pulse sequence and keeping the 4 SiCl pressure to a minimum helps overcome sev4 eral experimental difficulties associated with the catalyzed SiO ALD. The direct reaction of SiCl 2 4 with H O is negligible at low SiCl pressures. In 2 4 addition, the NH purge between individual SiCl 3 4 pulses helps remove the NH Cl salt that is depos4 ited on the SiO surface and the chamber walls. 2 This salt forms as a result of the NH catalyst 3 complexing with the HCl reaction product. The NH Cl salt has a vapor pressure of 4×10−5 Torr 4 at 300 K [27]. The H O half-reaction is not susceptible to the 2

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same problems as the SiCl half-reaction. 4 Consequently, the H O half-reaction was con2 ducted using a much simpler exposure scheme. For the H O half-reaction, the central deposition 2 chamber was filled with a mixture of NH and 3 H O. The partial pressure of each species was 2 adjusted using the computer-controlled valves. This NH –H O mixture was allowed to react for 3 2 several minutes and then was removed from the chamber using a liquid N trap backed by a 2 mechanical pump. Subsequently, the chamber was pumped with a diffusion pump to reduce the total pressure to ≤1×10−4 Torr. The H O partial pres2 sure was always <5×10−5 Torr before initiating the next SiCl half-reaction. 4 SiO film thicknesses and NH and H O surface 2 3 2 coverages on Si(100) were measured using the in situ spectroscopic ellipsometer (J.A. Woolam Co., M-44). The change in polarization induced by the SiO film was fit to a model where the film 2 thickness and the angle of incidence are the adjustable parameters. Bulk SiO optical constants were 2 assumed because the refractive index of ultrathin films is difficult to determine. The accuracy of this assumption was verified by collecting ex situ data ˚ thick at multiple angles of incidence for a 150 A SiO film. The measured refractive index of n= 2 1.43±0.03 agrees with the refractive index of a dense thermal oxide within experimental error. The surface topography of the SiO films was 2 measured using a NanoScope III atomic force microscope (AFM ) from Digital Instruments operating in tapping mode. The AFM images were collected within 1–2 h of removing the samples from vacuum to minimize surface contamination by ambient dust particles. Scan lengths of 250 nm– 1.2 mm were performed using a 1.2 mm scanning head. The AFM images were conditioned to remove AFM artifacts using the software provided by Digital Instruments.

3. Results Without the NH catalyst, ellipsometric meas3 urements revealed no observable SiO film depos2 ition after five AB cycles at room temperature for SiCl and H O exposures up to 1010 L. In contrast, 4 2

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Fig. 2. SiO film thickness deposited by five AB cycles versus 2 NH pressure during the SiCl exposure at 303 K. The H O 3 4 2 and NH exposures were sufficient for a complete H O half3 2 reaction.

Fig. 3. NH thickness on the hydroxylated SiO surface at 3 2 303 K versus NH pressure. The solid line is a fit based on the 3 BET model. This fit yields an NH adsorption energy of 3 E=9.4 kcal mol−1 and a desorption pre-exponential of n =1.0×1012 s−1. 0

the addition of a small amount of NH initiated 3 immediate SiO film growth. The effect of the 2 NH catalyst on the SiCl half-reaction is 3 4 illustrated in Fig. 2. The SiO film thickness result2 ing from five AB cycles at 303 K was measured for various NH pressures during the SiCl half3 4 reaction. The SiCl exposure consisted of one 4 50 ms pulse (~4400 L) and was intentionally insufficient for a complete SiCl half-reaction. The 4 H O half-reaction was performed at 0.5 Torr 2 H O and 0.5 Torr NH for 4 min. These exposures 2 3 were sufficient for a complete H O half-reaction. 2 Fig. 2 illustrates that the SiO film thickness 2 deposited by five AB cycles increases with increasing NH pressures up to ~2 Torr. Pressures 3 >2 Torr resulted in no additional SiO deposition. 2 The dependence of the SiO deposition on NH 2 3 pressure may result from higher NH coverages at 3 larger NH pressures. The effect of NH may 3 3 saturate when the NH pressure is sufficient to 3 titrate all the SiOH1 surface species. The error bars represent the 90% confidence limits determined by the ellipsometry data and the solid line is meant only to guide the eye. In situ ellipsometry was used to measure the NH coverage on a fully hydroxylated SiO sur3 2

face. Fig. 3 displays the NH coverage on the 3 silanol surface versus NH pressure at 303 K. For 3 NH pressures ≤2 Torr, the NH thickness 3 3 increases rapidly with pressure. In this monolayer regime, the NH molecules are adsorbing strongly 3 to surface SiOH1 groups. The NH thickness 3 increases much more slowly at NH pressures 3 >2 Torr. At these pressures, the NH molecules 3 are adsorbing less strongly onto themselves and form an NH multilayer. A refractive index of n= 3 1.325 was assumed based on the only previous measurements for liquid ammonia at 289 K [28]. The ellipsometric data were fitted using the Brunauer–Emmett–Teller (BET ) adsorption model [29]. The BET model yielded an NH 3 surface adsorption energy of E=9.4±0.8 kcal mol−1 and a desorption preexponential of n =1.0×1012±0.8 s−1 on the silanol surface. A 0 comparison of Figs. 2 and 3 reveals that the NH 3 pressure dependence of the SiO film thickness 2 deposited after five AB cycles and the NH cover3 age are very similar. The SiO growth saturates at 2 ˚ at NH pressures NH thicknesses of ≥3 A 3 3 >2 Torr. The dependence of SiO deposition on SiCl 2 4 exposure was examined by measuring the SiO 2

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Fig. 4. SiO film thickness deposited by five AB cycles at 303 K 2 versus the number of 50 ms SiCl pulses during the SiCl half4 4 reaction with a 75 mTorr NH pressure. The H O and NH 3 2 3 exposures were sufficient for a complete H O half-reaction. 2

Fig. 5. SiO film thickness deposited by five AB cycles at 303 K 2 versus H O exposure time during the H O half-reaction. The 2 2 SiCl half-reaction was performed using 40 SiCl pulses (50 ms) 4 4 with a 75 mTorr NH pressure. These conditions were sufficient 3 for a complete SiCl half-reaction. 4

film thickness deposited by five AB cycles at 303 K. The SiCl pulse length was fixed at 50 ms 4 (~4400 L) and the SiCl exposure was controlled 4 by the number of SiCl pulses. The NH pressure 4 3 was set at 75 mTorr during the SiCl pulse 4 sequence. Fig. 2 indicates that this NH pressure 3 was not optimum for the most efficient SiO 2 growth. However, lower NH pressures facilitated 3 a shorter pumping time after the SiCl half-reac4 tion. In contrast, the NH pressure was 0.5 Torr 3 during the H O half-reaction. The H O half-reac2 2 tion was performed at 0.5 Torr H O for 4 min. 2 These conditions were sufficient for a complete H O half-reaction. 2 Fig. 4 demonstrates the self-limiting nature of the SiCl half-reaction at 303 K. Once the SiCl 4 4 half-reaction has reached completion, additional SiCl exposure produces no additional SiO film 4 2 growth. For <30 SiCl pulses, the SiO film thick4 2 ness deposited by five AB cycles is dependent on the number of SiCl pulses. In contrast, the SiCl 4 4 half-reaction reaches completion for ≥30 pulses. The error bars show the 90% confidence limits and the solid line is intended to guide the eye. The dependence of the SiO deposition on 2 H O exposure was examined by measuring the 2 SiO film thickness deposited after five AB cycles 2

at 303 K. The results of these experiments are shown in Fig. 5. The NH and H O pressures were 3 2 fixed at 0.5 Torr. The SiCl half-reaction was per4 formed using 40 SiCl pulses with 75 mTorr 4 NH . Each SiCl pulse was defined by opening the 3 4 automated valve for 50 ms. These conditions were sufficient for a complete SiCl half-reaction. The 4 H O half-reaction exhibits self-limiting behavior. 2 For H O exposure times ≥3 min, the SiO film 2 2 thickness deposited by five AB cycles reaches a maximum. This behavior indicates the completion of the H O half-reaction and further H O exposure 2 2 produces no additional SiO film growth. 2 Fig. 6 shows the total SiO film thickness depos2 ited on the Si(100) wafer versus number of AB cycles. The A and B half-reactions were performed using SiCl and H O exposures that were sufficient 4 2 for complete half-reactions. The growth of the SiO film thickness is extremely linear versus 2 number of AB cycles. The measured growth rate ˚ per AB cycle at 303 K. The constant is 2.16 A growth rate implies that the deposited SiO film is 2 not roughening versus number of AB cycles. The linear growth rate also argues that NH Cl salt 4 formation is not poisioning the SiO surface and 2 degrading the reaction efficiency.

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Fig. 6. Total SiO film thickness deposited versus number of 2 AB cycles at 303 K. The SiCl , H O and NH exposures were 4 2 3 sufficient for complete SiCl and H O half-reactions. The initial 4 2 ˚. SiO layer on Si(100) had a thickness of 5 A 2

The surface topography of the deposited SiO 2 films was studied using an AFM in tapping mode. Fig. 7 shows a 1.2 mm×1.2 mm scan of a SiO 2

film deposited by 100 AB cycles at 303 K. The SiCl and H O exposures were sufficient for com4 2 plete half-reactions. The light-to-dark gray scale ˚ . The AFM image of the spans less than 10 A deposited SiO film yields a root mean square 2 ˚ . This surface (rms) surface roughness of ±3 A topography is comparable with the roughness of ˚ (rms) for the initial thin SiO layer on the ±2 A 2 Si(100) substrate. This smoothness demonstrates that the SiO film grows with negligible 2 roughening. The temperature dependence of the NH -catalyzed SiO film growth is displayed in 3 2 Fig. 8. The SiCl half-reaction employed 40 SiCl 4 4 pulses (50 ms) and 75 mTorr NH . These condi3 tions were sufficient for a complete SiCl half4 reaction only at 303 K. The H O half-reaction was 2 performed using 0.5 Torr H O and 0.5 Torr NH 2 3 for a 4 min exposure time. The SiO film thickness 2 deposited by five AB cycles decreases dramatically as the temperature is increased from 303 to 338 K. The error bars display the 90% confidence limits determined by the ellipsometric data. The solid line is shown only to guide the eye.

Fig. 7. AFM image of a SiO film deposited at 303 K on the initial SiO layer on Si(100) after 100 AB cycles. The SiCl , H O and 2 2 4 2 NH exposures were sufficient for the SiCl and H O half-reactions to reach completion. The vertical full scale is 5 nm and the light3 4 2 ˚. to-dark range is 10 A

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Fig. 8. SiO film thickness deposited by five AB cycles versus 2 substrate temperature. The SiCl and NH exposures were 4 3 sufficient for a complete SiCl half-reaction only at 303 K. 4

4. Discussion Ammonia catalyzes the SiCl and H O half4 2 reactions during SiO atomic layer deposition. 2 NH lowers the SiO deposition temperatures from 3 2 >600 to 300 K and reduces the required SiCl 4 and H O exposures from ~109 to ~105 L. The 2 results for NH catalysis of these half-reactions 3 are similar to the results obtained earlier using pyridine (C H N ) as the catalyst [6,19]. The com5 5 parable effects of ammonia and pyridine demonstrate the generality of Lewis bases for this catalytic process. Fig. 9 displays the proposed mechanism for the reaction of SiCl with SiOH1 species catalyzed by 4 either (A) pyridine or (B) ammonia. Lewis bases such as pyridine and ammonia will hydrogen bond strongly to acidic SiOH1 species. The hydrogenbonding interaction will substantially weaken the SiOMH bond and increase the nucleophilicity of the O atom for nucleophilic attack on the electrondeficient Si in SiCl [20]. This proposed reaction 4 mechanism is also consistent with results from earlier infrared studies [30,31]. Ammonia and pyridine are both effective catalysts because they have similar basic properties and have nearly identical H-bond strengths with

Fig. 9. Proposed mechanism for Lewis base catalysis of SiO 2 atomic layer deposition during the SiCl half-reaction using: 4 (A) pyridine; and (B) NH . 3

surface SiOH1 species [21,32]. Our results show that NH is slightly more active as a catalyst than 3 pyridine. The SiCl half-reaction reached comple4 tion after a 9×105 L SiCl exposure at a pyridine 4 pressure of 200 mTorr [6,19]. This pyridine pressure produced a pyridine coverage of ~0.7 ML [6,19]. In contrast, the SiCl half-reaction reaches 4 completion after a ~1.1×105 L SiCl exposure at 4 an NH pressure of 75 mTorr. This ammonia 3 ˚ or pressure yields an NH thickness of 1.0 A 3 ~0.3 ML. One NH monolayer (1 ML) has an 3 ˚ based on the estimated thickness of r−1/3=3.6 A density of r=0.5952 g cm−3 or number density of r=2.10×1022 molecules cm−3 for liquid ammonia at 303 K [28]. These results suggest that NH is 3 >10 times more effective than pyridine as a catalyst for SiO growth. 2 The increased catalytic activity for NH may 3 result from the presence of exchangeable H atoms that are lacking in the pyridine molecule. Another

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distinction between NH and pyridine may be in 3 the transition state architecture. For the pyridinecatalyzed reaction, Fig. 9 shows that a highly strained four-membered ring is proposed in the transition state [6,19]. In contrast, a more energetically favorable six-membered ring may be possible in the transition state for the NH -catalyzed 3 reaction. The H O half-reaction is also catalyzed by 2 pyridine or ammonia. A Lewis base could accelerate this half-reaction by hydrogen-bonding interactions with the H O reactant. This interaction 2 would make the O atom in the H O molecule 2 more nucleophilic for attack on the electron-deficient surface Si atoms in the SiMCl1 species [6,19]. Another possible catalytic pathway is the direct interaction of the N lone pair electrons on the Lewis base with the electron-deficient surface Si atoms. This interaction could weaken the SiMCl bond and facilitate the displacement of Cl by OH. A model has been developed for SiO growth 2 during ALD. This model assumes that the maximum number of Si atoms and SiO units that can 2 be added during each AB cycle is equal to the coverage of SiOH1 hydroxyl groups [5,16 ]. The maximum surface hydroxyl coverage on SiO has 2 been measured to be 4.6±0.4×1014 cm−2 at 300 K [33]. Consequently, the maximum number of Si atoms and SiO units that can be added per AB 2 cycle is predicted to be 4.6±0.4×1014 cm−2 at 300 K. The measured refractive index of n=1.43±0.03 for the SiO films is consistent with the formation 2 of vitreous silica [34]. Silica has a density of r=2.2 g cm−3or a number density of r=2.2× 1022 SiO units cm−3 [27]. The thickness of a 2 SiO monolayer can then be calculated as 2 ˚ . Likewise, r2/3 represents a SiO r−1/3=3.5 A 2 monolayer coverage of 7.9×1014 SiO 2 units cm−2. Using this SiO monolayer thickness and SiO 2 2 monolayer coverage, a surface hydroxyl coverage of 4.6±0.4×1014 cm−2 should produce a SiO 2 coverage of 4.6×1014 cm−2 or a SiO film thick2 ˚ per AB cycle. This prediction comness of 2.0 A pares favorably with the measured SiO deposition ˚ per AB cycle shown in 2Fig. 6. The rate of 2.16 A agreement between the predicted and experimental

SiO growth rates indicates that NH -catalyzed 2 3 SiO film growth is occuring via reaction of SiCl 2 4 with the SiOH1 species. Fig. 2 illustrates that NH facilitates SiO ALD 3 2 with increasing NH pressures up to ~2 Torr. 3 NH pressures >2 Torr resulted in no further 3 enhancement. A comparison of Figs. 2 and 3 reveals the correlation between SiO ALD growth 2 and NH coverage. At NH pressures <2 Torr in 3 3 the monolayer coverage regime, the NH molecules 3 are believed to be adsorbing strongly to surface SiOH1 groups. At NH pressures >2 Torr and 3 NH coverages ≥1 ML, the effect of NH saturates 3 3 when the NH titrates all the SiOH1 surface species. 3 Based on the number density of r=2.10× 1022 molecules cm−3 for liquid ammonia at 303 K [28], the estimated NH monolayer coverage is 3 r2/3=7.6×1014 molecules cm−3. This NH mono3 layer coverage exceeds the maximum surface hydroxyl coverage on SiO of 4.6×1014 cm−2 [33]. 2 The SiCl half-reaction was the rate-limiting 4 step for pyridine-catalyzed SiO ALD [6,19]. 2 Based on the surface reaction mechanism illustrated in Fig. 9B, a possible rate equation can be proposed for NH -catalyzed SiO ALD during 3 2 the SiCl half-reaction: 4 dh1/dt=kh h (1−h1). (3) SiCl4 NH3 In this expression, k is the reaction rate constant, k=n exp(−E /RT ), where n is the pre-exponen0 r 0 tial factor and E is the activation energy. h1 is the r coverage of reacted sites and h and h are SiCl4 NH3 the coverages of the SiCl reactant and NH 4 3 catalyst, respectively. The (1−h1) term is needed because the SiCl half-reaction requires a hydroxyl 4 group to interact with NH and react with the 3 SiCl reactant. A similar rate expression was used 4 to model SiO ALD catalyzed by pyridine [6,19]. 2 The SiCl pulse sequence illustrated in Fig. 1 4 complicates the fitting of the proposed rate equation to the experimental data. However, the rate equation can be used to understand qualitatively the temperature dependence of the SiO growth 2 displayed in Fig. 8. The decrease in the SiO depos2 ition rate cannot be attributed to the loss of reactive hydroxyl groups by dehydroxylation (2SiOH1SiMOMSi+H O) at the higher temper2 atures. The hydroxyl coverage on SiO decreases 2

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only very slightly between 300 and 338 K [33]. The decrease in the SiO growth must result pri2 marily from the reduction in the SiCl , H O and 4 2 NH surface coverages at higher temperatures. 3 According to the proposed rate equation, the net SiO deposition rate is controlled by both 2 the SiCl and NH coverages (h and h ) and 4 3 SiCl4 NH3 the reaction rate (k). The SiCl and NH coverages 4 3 should decrease with temperature. In contrast, the reaction rate is expected to increase with temperature. The reduction in the SiO deposition rate at 2 higher temperatures indicates that the NH and 3 SiCl surface coverages must dominate the 4 observed SiO growth. This behavior was also 2 observed for the pyridine-catalyzed SiO ALD 2 [6,19]. The effect of NH on SiO ALD was also 3 2 examined during the SiCl and H O half-reactions 4 2 at 600–800 K. These higher temperatures for SiO ALD are required in the absence of the 2 NH catalyst [5]. The NH catalyst produced no 3 3 change at these higher temperatures. The absence of the catalytic effect is attributed to the short residence time and negligible coverage for NH at 3 these elevated temperatures. The NH catalyst can complex with the HCl 3 reaction product and produce NH Cl salt. The 4 salt formation has important implications for NH -catalyzed SiO film deposition. The NH Cl 3 2 4 salt may deposit on the growing SiO film and 2 block the reactive surface sites. This deposition would poison the surface and lead to NH Cl 4 incorporation into the SiO films. Salt formation 2 may also convert the NH catalyst to a chemically 3 inactive form. The NH Cl salt has a vapor pressure 4 that ranges from 4×10−5 Torr at 300 K to 25 mTorr at 373 K [27]. The temperature of the SiO surface was purposely elevated slightly to 2 303 K to facilitate NH Cl sublimination. 4 Rutherford backscattering (RBS) measurements were conducted previously on SiO films 2 grown using pyridine-catalyzed ALD [6 ]. These RBS investigations revealed no C, N or Cl in the SiO films. RBS studies were also performed on 2 SiO films grown using NH -catalyzed SiO chemi2 3 2 cal vapor deposition [35]. These measurements indicated no evidence of N or Cl contaminants. Although RBS investigations were not conducted

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on the NH -catalyzed SiO ALD films, these 3 2 related observations argue that NH Cl is not incor4 porated in the SiO film [35]. Additional Auger 2 electron spectroscopy and X-ray photoelectron spectroscopy depth-profiling experiments are needed to confirm the absence of N or Cl contaminants. Temporary NH Cl salt formation may have a 4 beneficial effect on the SiO atomic layer depos2 ition. The formation of NH Cl from HCl and 4 NH is exothermic by 42 kcal mol−1 [27]. This 3 exothermicity provides an enhanced thermodynamic driving force for SiO formation. 2 Consequently, NH Cl salt formation may be desir4 able for SiO ALD provided that the salt sublimi2 nation rate is sufficient to prevent NH Cl from 4 accumulating on the SiO surface and poisoning 2 the SiCl and H O half-reactions. 4 2 Fig. 7 displays the surface topography of an SiO film deposited by 100 AB cycles at 303 K. 2 Power spectral density analysis [7] of the surface topography reveals that this SiO surface is statis2 tically indistinguishable from the initial thin SiO 2 film on Si(100). Smooth surface morphologies have also been observed for Al O [7], Si N [13] 2 3 3 4 and SiO [5,6 ] films grown using ALD. This beha2 vior is consistent with complete surface chemical reactions and layer-by-layer thin film growth.

5. Conclusions SiO ALD was achieved at room temperature 2 with an NH catalyst using alternating SiCl and 3 4 H O exposures in an ABAB reaction sequence. In 2 situ spectroscopic ellipsometry investigations mea˚ per AB sured a SiO deposition rate of 2.16 A 2 cycle at 303 K. This SiO deposition rate is consis2 tent with SiO growth controlled by the coverage 2 of SiOH1 hydroxyl species on the SiO surface. 2 Ellipsometric results also suggested that each SiOH1 hydroxyl species is titrated by an NH 3 molecule during the optimum catalyzed reaction conditions. Atomic force micrographs determined that the SiO films were deposited smoothly on the initial 2 thin SiO layer on the Si(100) substrate with 2 negligible surface roughening. These results

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demonstrate that smooth SiO films can be depos2 ited at room temperature using catalyzed sequential surface reactions. The catalysis of surface reactions with gas phase reagents represents a new strategy for the enhancement of deposition reactions. Gas phase catalysts may be generally applicable and may facilitate the growth of other technologically important materials at low temperatures.

Acknowledgements This work was supported by the Air Force Office of Scientific Research. Equipment utilized in this research was also provided by previous support from the Office of Naval Research.

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