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Reflection absorption infrared spectroscopy during atomic layer deposition of HfO2 films from tetrakis(ethylmethylamido)hafnium and water
Applied Surface Science 256 (2010) 5035–5041
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Reflection absorption infrared spectroscopy during atomic layer deposition of HfO2 films from tetrakis(ethylmethylamido)hafnium and water Brent A. Sperling ∗ , William A. Kimes, James E. Maslar Process Measurements Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8360, United States
a r t i c l e
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Article history: Received 15 January 2010 Received in revised form 24 February 2010 Accepted 5 March 2010 Available online 15 March 2010 Keywords: Atomic layer deposition Hafnium dioxide In situ diagnostics Infrared spectroscopy
a b s t r a c t Tetrakis(ethylmethylamido)hafnium and water are commonly used precursors for atomic layer deposition of HfO2 . Using reflection absorption infrared spectroscopy with a buried-metal-layer substrate, we probe surface species present during typical deposition conditions. We observe evidence for thermal decomposition of alkylamido ligands at 320 ◦ C. Additionally, we find that complete saturation of the SiO2 substrate occurs in the first cycle at ≈100 ◦ C whereas incomplete coverage is apparent even after many cycles at higher temperatures. The use of this technique as an in situ diagnostic useful for process optimization is demonstrated. Published by Elsevier B.V.
1. Introduction Atomic layer deposition (ALD) is a technique capable of producing highly conformal, nanometer-scale thin films in a wellcontrolled manner. During the process, two or more gas-phase precursors are alternately exposed to the substrate [1]. Ideally, ALD proceeds by self-limiting, irreversible ligand-exchange reactions at the surface. However, these surface reactions are not always irreversible. Additionally, it is often found that precursors can condense at low temperatures or decompose at high temperatures [1]. Understanding how specific precursors behave is key to optimizing an ALD process. Alkylamido compounds are commonly employed as the metal source for ALD because of a number of favorable characteristics. For HfO2 films in particular, tetrakis(ethylmethylamido)hafnium (TEMAH), Hf[N(CH3 )(C2 H5 )]4 , has been widely used because it is a liquid at room temperature, which facilitates delivery, and because it has a fairly high vapor pressure [2,3], which permits faster cycling times. However, alkylamido precursors are known to produce C, N, and H impurities if the growth temperature is too high [4,5]. Water vapor is frequently used in conjunction with alkylamido precursors as well as with halide precursors, e.g. hafnium tetrachloride. Ideally, the surface reactions with water leave behind only hydroxyl (OH) groups on the surface, which are consumed during the subsequent metal precursor pulse. However, at low temperatures, HfO2 films can accumulate bulk OH groups. When HfO2 is
∗ Corresponding author. Tel.: +1 301 330 3831; fax: +1 301 869 5924. E-mail address: [email protected] (B.A. Sperling). 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.03.050
used as a gate dielectric for metal-oxide-semiconductor transistors, the bulk OH groups can be problematic since they cause interfacial SiO2 to form between HfO2 and Si during annealing [6]. While increasing the deposition temperature decreases the amount of OH in the films [7], it also is thought to partially dehydroxylate the HfO2 surfaces, which lowers the growth rate per cycle [8]. We report on the use of in situ reflection absorption infrared spectroscopy (RAIRS) with buried-metal-layer substrates [9] (BMLRAIRS) to examine the surface species present during ALD of HfO2 using TEMAH and water. The use of this particular method has a number of benefits. Because it is an optical measurement, it can be made under typical ALD reactor pressures without transferring the sample to an ultra-high vacuum environment. Using a reflection arrangement allows us to deposit on planar substrates without the accommodations that must be made for heating planar [10] or powder [11] samples in transmission setups. A further benefit comes from the large size of the BML substrates, which make high signal-to-noise ratios (SNRs) quickly obtainable. Therefore, the BML-RAIRS method also offers an advantage over attenuated total reflection measurements, which often limit optical throughput and hence data acquisition speeds by focusing the infrared beam onto a narrow facet [12]. 2. Experimental 2.1. Atomic layer deposition (ALD) The experiments are performed in a custom-built flow reactor that has been previously described [13]. Only pertinent details of the reactor will be presented here. The reactor is horizontally ori-
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ented with the substrate wafer chuck oriented vertically. Gases flow in a laminar manner before impinging at the wafer surface. Optical access to the wafer is made through 50 mm diameter BaF2 windows positioned on opposites sides of the reactor with the wafer chuck partially between them. The windows are recessed into the reactor walls to limit perturbations to the flow field that can result from unswept volumes in the reactor. A reactor pressure of 130 Pa (1.0 Torr) is maintained by continuously flowing gas through the reactor and by throttling a rotary vane pump. Two gas lines continuously deliver He to the reactor while two others alternate between delivering TEMAH or water. During the purging step between precursor injections, 75 sccm (standard cm3 per minute, 0 ◦ C and 101.3 kPa) of He flow through each of the four lines. During the TEMAH injection, pneumatic valves momentarily divert the He flow through a bubbler containing TEMAH at 77 ◦ C for typically 3 s. Afterwards, the flow is returned to its original path to purge both the delivery line and the reactor. During the water injection, valves stop the flow of He down one line and open the reactor to a vessel containing water at room temperature. Flow of water vapor into the reactor is restricted to ≈75 sccm by a needle valve. After a period of typically 100 ms, the flow of He is returned to purge the water delivery line and the reactor. The water and TEMAH lines are heated to prevent condensation of the precursors. The reactor walls are heated to 110 ◦ C and are constructed of aluminum to ensure a uniform temperature profile. The wafer chuck is externally heated and monitored with an embedded thermocouple. At the empirically determined optimal wafer temperature of 230 ◦ C, the ALD reactor produces high-quality HfO2 films as characterized ex situ by vacuum-ultraviolet spectroscopic ellipsometry and X-ray photoelectron spectroscopy (not shown). The windows are heated mainly through contact with the walls and process gases and, to a lesser degree, by radiation from the wafer chuck. 2.2. Reflection absorption infrared spectroscopy (RAIRS) A Fourier transform infrared spectrometer (RS-10000, Mattson Instruments) [14] equipped with a high-sensitivity (peak D* = 5.5 × 1010 cm Hz1/2 /W with 1 mm2 area), narrowband HgCdTe detector is employed for the BML-RAIRS measurements. The beam path is sealed and purged with dry nitrogen to remove absorption by atmospheric water vapor and CO2 . Spectra are obtained at 4 cm−1 resolution and a 50 kHz laser frequency with 256 averaged scans each for the background and sample interferograms. The data are collected during the purge steps of the ALD cycle; background interferograms are taken immediately before the precursor pulses while the sample interferograms are taken ≈30 s afterwards to ensure no contribution from gas-phase species. Because a single signal-averaged interferogram requires ≈40 s to complete, the purge times are extended to ≈110 s from the 5 s and 15 s purge times ordinarily used with this reactor for TEMAH and water, respectively [13]. Data reported here are in the form of R/R0 = (R0 − R)/R0 where R is the transformed sample interferogram and R0 is the transformed background interferogram. Positive spectral features therefore result from species added after the pulse, and negative features are from those that have been removed. The enhanced infrared absorption by species on metal surfaces is a well-known effect [15]. However, a metal surface lacks reactive sites to nucleate HfO2 using ALD. Therefore, to enhance the sensitivity of our measurement while providing a chemically active surface, we use BML substrates. These are prepared by coating 5 cm oxidized Si wafers with 10 nm Cr, 200 nm Au, and 20 nm SiO2 by magnetron sputtering. The initial growth surface is thus amorphous SiO2 that has been hydroxylated by atmospheric water vapor. During data acquisition, the infrared beam from the interferometer impinges on the substrate at near-grazing incidence (≈85◦ ) and
passes through a wire-grid polarizer to select p-polarization before reaching the detector. Because the 5 cm substrates do not function as the limiting aperture, which is often the case for RAIRS on small metal crystals [16], higher SNR is possible with less data acquisition time. Enhancement of absorption by the surface species is due to the increased electric field strength in p-polarization near the surface; the spectra we report, therefore, are due to absorption by dipoles normal to the surface. The RAIRS measurements are made while the substrate is still heated to the nominal growth temperatures of 105 ◦ C, 240 ◦ C, and 320 ◦ C. Two complications arise from slight drifts in the substrate temperature. First, the reflectivity of the metal changes depending on the degree of electron scattering by phonons, which results in an upward or downward slope in the baseline of some spectra. Second, the strength of the longitudinal optical (LO) phonon mode of SiO2 at ≈1250 cm−1 is temperature sensitive, so absorption peaks from surface species are superimposed on a very intense, temperature-dependent band, which makes interpretation difficult. Because of the resulting ambiguity, we exclude the spectral region most affected by the LO phonon modes (1100–1300 cm−1 ) from this report. Although the infrared spectra of ALD films deposited on window material has been used as an in situ measurement in previous studies of ALD [17], the focus of our study is on the substrate. We do observe deposition of HfO2 on the windows after an incubation period, and the absorption from the two window surfaces can contribute a significant amount of the measured absorbance (at most ≈1/4) with prolonged deposition lasting over 100 cycles. However, this worst case is not realized here since we limit ourselves to 30 cycles total with one set of windows. Nonetheless, we restrict our discussion to spectral differences we observe with temperature since the windows maintain a nearly constant temperature (≈100 ◦ C). Our results, therefore, are due to the HfO2 on the substrate and not material deposited on the windows. 3. Results and discussion 3.1. First ALD cycle Upon exposing the BML substrates to TEMAH, absorption due to the alkylamido ligands is observed to increase as positive values while absorption due to OH groups is seen to diminish to negative values. Fig. 1 shows the spectra obtained after the initial pulse of TEMAH on the hydroxylated SiO2 surface at 105 ◦ C, 240 ◦ C, and 320 ◦ C. The strong, sharp negative peaks at 3740 cm−1 are due to the O–H stretch mode (O–H) of isolated surface SiOH groups while broad, asymmetric bands extending down to ≈3000 cm−1 are due to (O–H) of hydrogen-bound SiOH groups [18]. Features due to the alkylamido ligands can be observed in the (C–H) region of the spectrum between 2650 cm−1 and 3050 cm−1 and in the deformation modes ı(C–H) below 1500 cm−1 [19]. Modes at 880 cm−1 and 990 cm−1 are apparent as positive features in the spectrum obtained at 105 ◦ C; these alkylamido-related modes correspond to those observed for gas-phase and liquid-phase TEMAH [13,20]. At 320 ◦ C, a broad, negative feature centered at ≈970 cm−1 is apparent. This is likely due to the loss of isolated SiOH groups which have a (Si–OH) mode near this frequency [21]. The complex feature around 980 cm−1 in the spectra obtained at 240 ◦ C is due to a partial exchange of SiOH at 970 cm−1 with the alkylamido ligand at 990 cm−1 . The spectral features discussed thus far are what one expects from the ideal surface reactions during the TEMAH half-cycle, which can be represented as x(–OH)(s) + Hf[N(CH3 )(C2 H5 )]4(g) → (–O–)x Hf[N(CH3 )(C2 H5 )]4−x(s) + x(CH3 )(C2 H5 )NH(g) ,
(1)
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Fig. 1. BML-RAIRS spectra obtained after exposing the initial SiO2 surface to TEMAH. Spectra, which are offset for clarity, are referenced to the substrate before the pulse at a temperature of 105 ◦ C, 240 ◦ C, or 320 ◦ C.
where x is the number of ligands removed as N-methylethanamine (MEA), (CH3 )(C2 H5 )NH. The similarity of the alkylamido-related features to those of molecular TEMAH indicates that the unreacted ligands remain intact on the surface. Previous infrared studies of adsorbed alkylamido precursors adsorbed onto surfaces at low temperatures have also found this to be the case. When silica powders were exposed to tetrakis(diethylamido)titanium (TDEAT) at room temperature, features similar to molecular TDEAT were observed from the powders [22]. A thermally grown SiO2 surface after exposure to tetrakis(diethylamido)hafnium at room temperature yields a (C–H) region that is also similar in most respects [12]. Dosing a NHx -covered TiN surface with tetrakis(dimethylamido)titanium (TDMAT) at ≈150 ◦ C [23] also produced a (C–H) region similar to molecular TDMAT [24]. In contrast, the (C–H) region indicates that TDMAT dissociates on Si(1 0 0) at room temperature [25]. Interestingly, our spectra are somewhat different from those found in previous in situ infrared studies of TEMAH adsorption on SiO2 . When SiO2 -passivated Si wafers were exposed to TEMAH, the (C–H) region in Refs. [20,26] appeared quite different from what we observe. The relative intensities of the asymmetric (C–H) modes of CH3 (≈2960 cm−1 ) and CH2 (≈2930 cm−1 ) in the 105 ◦ C spectrum of the present study are inverted compared to a previous study at 100 ◦ C [26]. Furthermore, we observe much stronger absorption in the symmetric (C–H) modes in the range of 2650–2900 cm−1 compared to that study. In another study, only weak absorptions at 2855 cm−1 and 2930 cm−1 were noted when the Ti and Zr analogues of TEMAH were used [20]. Further differences between previous studies are found in the (O–H) modes. In Ref. [26], the SiO2 surface was initially deuterated, but no evidence for the loss of hydrogen-bound SiOD groups was observed. In Ref. [20], no loss of any (O–H) modes was noted. While the differences in sampling geometries might partially explain the variations between studies, there appear to be fundamental discrepancies in the surface chemistry. We speculate that procedural or timing differences may explain this. In addition to the spectral features in Fig. 1 that are expected from Eq. (1), there is evidence for some additional reactions. Accompanying the negative peaks attributable to (O–H) are negative features at 2270 cm−1 . These are likely due to the (O3 Si–H) mode of surface hydride groups [27] indicating substoichiometry, which is not unexpected from the sputtered SiO2 films [28]. A small
peak at 2040 cm−1 is apparent in the spectrum obtained at 320 ◦ C, and it attributed to a decomposition product as will be discussed in Section 3.3. Finally, there is a positive absorption feature at ≈1600 cm−1 in the spectrum obtained at 105 ◦ C and to a lesser degree in the higher temperature spectra. The origin of the feature at ≈1600 cm−1 cannot be assigned unambiguously, but previous work has found similar peaks. When the Ti and Zr analogues of TEMAH were exposed to SiO2 at around 200 ◦ C, a peak at 1587 cm−1 was observed and attributed to the (C N) mode of an imine, R2 C NR [20]. Similarly, TDMAT- and TDEAT-treated silica surfaces showed a slow decomposition at 60 ◦ C, which produced a peak at 1608 cm−1 likewise attributed to the (C N) mode of imines [29]. Previous work examining HfO2 layers deposited by chemical vapor deposition also found a similarly positioned peak [30]. As suggested in the latter work, carbonates, carboxylates, acetates, and hafnium hydrides all absorb in the region around ≈1600 cm−1 . Another possible explanation for the peak in our spectra is alkylammonium ions, which could form from the protonation of an amine like MEA by surface acid sites: (–OH)(s) + (CH3 )(C2 H5 )NH(g) → (–O)− [(CH3 )(C2 H5 )NH2 ]+ (s) .
(2)
Spectra of alkylammonium ions formed on hydroxylated Al2 O3 from a secondary amine have a characteristic scissors mode ı(NH2 ) at ≈1600 cm−1 that is absent in the spectra of the unreacted amine [31]. It should be noted that a reaction analogous to Eq. (2) has been found on hydroxylated amorphous SiO2 [32]. The effect of the water pulse is to exchange the alkylamido groups added to the surface according to Eq. (1) with OH groups. This forms new Hf–O bonds and prepares the surface for the next half-cycle (Eq. (1)). The surface reaction during the water pulse should ideally proceed as (–O–)x Hf[N(CH3 )(C2 H5 )]4−x(s) + (4 − x)H2 O(g) → (–O–)x Hf(OH)4−x(s) + (4 − x)(CH3 )(C2 H5 )NH(g) ,
(3)
where MEA is again produced as a byproduct. The spectra we obtain after the water pulse indicates that this is indeed what occurs. Fig. 2 shows that the modes associated with the alkylamido ligands appear as negative features while the (O–H) modes are positive. Differences in the spectra with temperature indicate that growth proceeds somewhat differently. In all cases, two (O–H) modes appear at 3660 cm−1 and 3780 cm−1 . These correspond to
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Fig. 2. BML-RAIRS spectra obtained after exposing the surfaces from Fig. 1 to water vapor at the indicated temperatures.
bridging groups (Hf2 OH or Hf3 OH) and terminal (HfOH) groups, respectively [33,34]. At 240 ◦ C and 320 ◦ C, however, the isolated SiOH peak at 3740 cm−1 accompanies the Hf-bound OH groups. Two possibilities exist for the formation of isolated SiOH during the water pulse. First, the water may hydrate siloxane bridges on the surface: Si–O–Si(s) + H2 O(g) → 2Si–OH(s) .
(4)
The second possibility is that Hf–O–Si linkages formed during the initial TEMAH pulse are being broken as has been previously suggested for TiO2 ALD on SiO2 [35]: Hf –O–Si(s) + H2 O(g) → Hf –OH(s) + Si–OH(s) .
(5)
As will be discussed in the next section, the isolated SiOH peak is consumed with the Hf-bound OH groups during TEMAH pulses, which is not expected for OH groups located at an interface. The reappearance of the peak at 3740 cm−1 , therefore, is attributed to incomplete coverage of the initial SiO2 surface. We also note that in addition to reestablishing the isolated SiOH peak, the water pulse at 240 ◦ C and 320 ◦ C also causes absorption due to (O3 Si–H) to increase. As was the case with the TEMAH pulse, a peak at ≈1600 cm−1 appears in the spectra at 105 ◦ C after the water pulse. Since MEA is common to both half-cycles, the attribution of the peak to an alkylammonium ion is reasonable. In the spectrum obtained at 240 ◦ C, we note a peak at 1650 cm−1 in addition to a broad, low-intensity feature at ≈1600 cm−1 . These features are largely absent in the 320 ◦ C sample. The peak at 1650 cm−1 in the 240 ◦ C spectrum is possibly due to the scissors mode of adsorbed water ı(OH2 ), but its absence in the 110 ◦ C spectrum is not consistent with this conjecture. Further experiments are clearly necessary to explain the features around 1600 cm−1 . A negative peak at 2040 cm−1 is noted in the spectra obtained at 320 ◦ C. As will be discussed in Section 3.3, subsequent experiments have found that the 2040 cm−1 peak decays within minutes after a TEMAH pulse, which indicates that the species either desorbs or decomposes further. The appearance as a negative peak in the post-water pulse spectra in Fig. 2 is at least partially due to time-dependent decay and does not necessarily indicate a chemical reaction with water.
3.2. Further ALD cycles The spectra obtained during multiple ALD cycles indicate that deposition occurs by alternating between the reactions described in Eqs. (1) and (3). We present spectra from the tenth ALD cycle after water and TEMAH pulses at the three investigated temperatures in Fig. 3. The behavior is similar to what occurs during the first ALD cycle, but the (O–H) modes of isolated SiOH peaks in the 240 ◦ C and 320 ◦ C spectra are less intense compared to the HfOH absorptions. Absorption due to (O3 Si–H) is absent except as a negative peak after the TEMAH pulse at 320 ◦ C. The persistence of spectral features from the initial SiO2 surface indicates that complete coverage sometimes does not occur until a large number of ALD cycles have been completed. While the coverage of the initial SiO2 surface seems to be complete after the first ALD cycle when the growth temperature is 105 ◦ C, it remains incomplete when the temperature is increased. This observation agrees with a previous study of ALD which found additional Hf–O–Si bonds did not form past the first TEMAH pulse at 100 ◦ C [26], but it also indicates that nucleation can be more complicated at higher temperatures. Whether or not the incomplete coverage at higher temperatures is due to the nature of the sputtered SiO2 used in this study is an open question. Strained bonds and substoichiometry differentiate the material used here from thermally grown or native oxides on Si [28]. Refinement of the BML substrate preparation, however, may provide a means to produce SiO2 surfaces that more closely resemble those produced from oxidized Si wafers. Interestingly, the BML-RAIRS method can be used as an in situ diagnostic for the ALD process. After one ALD cycle at ≈100 ◦ C, the SiO2 is completely covered. By varying the injection time of one precursor while maintaining the injection time of the other precursor at a saturating dose known to complete the surface reactions, one can determine how the surface coverage varies with the injection timing of precursor. We have obtained BML-RAIRS data in such a way at a growth temperature of 110 ◦ C. The TEMAH pulse length was varied from 0.1 s to 6.0 s keeping the water pulse length at 1 s, and the water pulse was varied from 20 ms to 150 ms while keeping the TEMAH pulse length at 3 s. To quantify the extent of the surface reactions, R/R0 was integrated in the (C–H) region. The resulting values are presented in Fig. 4. While saturation occurred for all water pulse times (Fig. 4a), approximately 2 s
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Fig. 3. BML-RAIRS spectra obtained during the tenth ALD cycle. The spectra (offset for clarity) are referenced to the substrate before the TEMAH or water pulse (as indicated) at a deposition temperature of 105 ◦ C, 240 ◦ C, or 320 ◦ C.
of TEMAH injection is necessary to saturate the surface (Fig. 4b). The S-shape to the TEMAH curve at short pulse lengths (Fig. 4b) is likely due to consumption of TEMAH on the walls upstream of the substrate. Usually, the type of information contained in Fig. 4 is determined by measuring the growth rate per cycle. Here, each datum was obtained in situ after one ALD cycle as opposed to the hundreds of cycles typically needed to deposit a film with an easily measurable thickness. Thus, BML-RAIRS could prove to be a useful diagnostic tool for optimizing ALD processes.
Fig. 4. The integrated reflectance change in the region of the C–H stretching vibrations (2700–3050 cm−1 ) when (a) the TEMAH pulse length is varied keeping the water pulse length at 1 s and (b) the water pulse length is varied keeping the TEMAH pulse length at 3 s.
3.3. Thermal decomposition of adsorbed TEMAH The peak at 2040 cm−1 , which we attribute to a decomposition product of adsorbed TEMAH, appears in all the spectra obtained during ALD at 320 ◦ C (Figs. 1–3). An additional experiment was conducted to determine what possible role it might have in deposition. After 15 ALD cycles at 105 ◦ C, the temperature was increased to 320 ◦ C. The background interferogram was taken before injecting TEMAH for 3 s after which a series of sample interferograms were made at ≈60 s intervals. The resulting spectra are shown in Fig. 5. The spectrum obtained after 60 s is similar to those obtained during ALD at 320 ◦ C (Fig. 3) with some noticeable differences. First, the 60-s spectrum in Fig. 5 contains no evidence for the (O–H) and (Si–OH) modes of isolated SiOH groups or the (O3 Si–H) of surface hydrides. Only the (O–H) of bridging and terminal Hfbound OH are apparent as negative features. Deposition at 105 ◦ C has apparently covered the SiO2 surface with HfO2 . The second difference is that the region between 1300 cm−1 and 1700 cm−1 is much more complex. It appears that positive features from ı(C–H) modes are superimposed upon both negative and positive features in that range. These features are not observed during ALD (Figs. 1–3) and may represent contamination of the surface while the sample resides in the reactor under purge for longer periods than are typical. Nonetheless, we observe the peak at 2040 cm−1 at approximately the same intensity observed during ALD. Furthermore, we observe that the peak at 2040 cm−1 decays with time. (The inset of Fig. 5 shows the 2040 cm−1 peak in more detail.) The loss of the 2040 cm−1 peak indicates that the species responsible for this absorption either desorbs or decomposes further. (The appearance as a negative peak in the post-water pulse spectra in Figs. 2 and 3 is at least partially due to time-dependent decay and does not necessarily indicate a chemical reaction with water.) Simultaneous with the decay in the 2040 cm−1 peak, the (C–H) modes were also observed to decrease with time suggesting that the 2040 cm−1 peak is associated with the thermal decomposition of alkylamido ligands. The spectrum obtained after 240 s contains a broad, featureless band centered at 2870 cm−1 instead of the distinct modes apparent in the (C–H) band of the intact ligands. The featureless band at 2870 cm−1 is most likely the (C–H) modes of organic material on the surface that is no longer part of
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Fig. 5. BML-RAIRS spectra obtained after exposing a HfO2 film to TEMAH at 320 ◦ C. The spectra (offset for clarity) are referenced to the same background data, which was obtained immediately before the TEMAH pulse. The inset figure shows a close-up of the peak at 2040 cm−1 with the scale bar indicating 2 × 10−4 .
–N(CH3 )(CH2 CH3 ) ligands, which suggests that the 2040 cm−1 peak is associated with carbon and hydrogen incorporation into the film at high temperatures. While the origin of the 2040 cm−1 peak cannot be assigned definitively from the spectroscopic data available, previous work found that gas-phase CO and HC N were produced from tetrakis(dimethylamido)hafnium when a graphite surface was heated above 250 ◦ C [36]. Furthermore, tetrakis(diethylamido)zirconium adsorbed on Si produced an imine (R2 C NR) and acetonitrile (CH3 C N) in temperature-programmed desorption experiments [37]. Since the (C N) mode of an adsorbed imine should occur at a much lower frequency than 2040 cm−1 (i.e. ≈1590 cm−1 ) [20,38], we rule out that group as a possible assignment. The 2040 cm−1 peak is likely either a metal-bound cyano group (C N), which are observed around 2070 cm−1 in terminal configurations, or a metalbound carbonyl group (C O), which occur between 1700 cm−1 and 2170 cm−1 [19]. Our observation of the peak at 2040 cm−1 highlights the usefulness of BML-RAIRS. Since this species is present on the surface only for a few minutes, it is unlikely that it could be observed using a slower method without rapidly quenching the sample. The relatively weak absorption from this peak would likely be lost if the signal averaging were over the course of tens of minutes (e.g. Refs. [10,12]) instead of the tens of seconds in this experiment.
4. Conclusion The use of BML-RAIRS provides a means to study surface chemistry of ALD under realistic conditions in a flow reactor. This has permitted us to observe the spectroscopic evidence of a transient decomposition product of TEMAH. Furthermore, our findings indicate that incomplete coverage of the SiO2 film occurs at 240 ◦ C and 320 ◦ C but not at 105 ◦ C. The use of this technique as a fast, in situ diagnostic tool has also been demonstrated.
Acknowledgements The authors acknowledge M. Carrier and R. Zangmeister for technical assistance.
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Reflection absorption infrared spectroscopy during atomic layer deposition of HfO2 films from tetrakis(ethylmethylamido)hafnium and water Brent A. Sperling ∗ , William A. Kimes, James E. Maslar Process Measurements Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8360, United States
a r t i c l e
i n f o
Article history: Received 15 January 2010 Received in revised form 24 February 2010 Accepted 5 March 2010 Available online 15 March 2010 Keywords: Atomic layer deposition Hafnium dioxide In situ diagnostics Infrared spectroscopy
a b s t r a c t Tetrakis(ethylmethylamido)hafnium and water are commonly used precursors for atomic layer deposition of HfO2 . Using reflection absorption infrared spectroscopy with a buried-metal-layer substrate, we probe surface species present during typical deposition conditions. We observe evidence for thermal decomposition of alkylamido ligands at 320 ◦ C. Additionally, we find that complete saturation of the SiO2 substrate occurs in the first cycle at ≈100 ◦ C whereas incomplete coverage is apparent even after many cycles at higher temperatures. The use of this technique as an in situ diagnostic useful for process optimization is demonstrated. Published by Elsevier B.V.
1. Introduction Atomic layer deposition (ALD) is a technique capable of producing highly conformal, nanometer-scale thin films in a wellcontrolled manner. During the process, two or more gas-phase precursors are alternately exposed to the substrate [1]. Ideally, ALD proceeds by self-limiting, irreversible ligand-exchange reactions at the surface. However, these surface reactions are not always irreversible. Additionally, it is often found that precursors can condense at low temperatures or decompose at high temperatures [1]. Understanding how specific precursors behave is key to optimizing an ALD process. Alkylamido compounds are commonly employed as the metal source for ALD because of a number of favorable characteristics. For HfO2 films in particular, tetrakis(ethylmethylamido)hafnium (TEMAH), Hf[N(CH3 )(C2 H5 )]4 , has been widely used because it is a liquid at room temperature, which facilitates delivery, and because it has a fairly high vapor pressure [2,3], which permits faster cycling times. However, alkylamido precursors are known to produce C, N, and H impurities if the growth temperature is too high [4,5]. Water vapor is frequently used in conjunction with alkylamido precursors as well as with halide precursors, e.g. hafnium tetrachloride. Ideally, the surface reactions with water leave behind only hydroxyl (OH) groups on the surface, which are consumed during the subsequent metal precursor pulse. However, at low temperatures, HfO2 films can accumulate bulk OH groups. When HfO2 is
∗ Corresponding author. Tel.: +1 301 330 3831; fax: +1 301 869 5924. E-mail address: [email protected] (B.A. Sperling). 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.03.050
used as a gate dielectric for metal-oxide-semiconductor transistors, the bulk OH groups can be problematic since they cause interfacial SiO2 to form between HfO2 and Si during annealing [6]. While increasing the deposition temperature decreases the amount of OH in the films [7], it also is thought to partially dehydroxylate the HfO2 surfaces, which lowers the growth rate per cycle [8]. We report on the use of in situ reflection absorption infrared spectroscopy (RAIRS) with buried-metal-layer substrates [9] (BMLRAIRS) to examine the surface species present during ALD of HfO2 using TEMAH and water. The use of this particular method has a number of benefits. Because it is an optical measurement, it can be made under typical ALD reactor pressures without transferring the sample to an ultra-high vacuum environment. Using a reflection arrangement allows us to deposit on planar substrates without the accommodations that must be made for heating planar [10] or powder [11] samples in transmission setups. A further benefit comes from the large size of the BML substrates, which make high signal-to-noise ratios (SNRs) quickly obtainable. Therefore, the BML-RAIRS method also offers an advantage over attenuated total reflection measurements, which often limit optical throughput and hence data acquisition speeds by focusing the infrared beam onto a narrow facet [12]. 2. Experimental 2.1. Atomic layer deposition (ALD) The experiments are performed in a custom-built flow reactor that has been previously described [13]. Only pertinent details of the reactor will be presented here. The reactor is horizontally ori-
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ented with the substrate wafer chuck oriented vertically. Gases flow in a laminar manner before impinging at the wafer surface. Optical access to the wafer is made through 50 mm diameter BaF2 windows positioned on opposites sides of the reactor with the wafer chuck partially between them. The windows are recessed into the reactor walls to limit perturbations to the flow field that can result from unswept volumes in the reactor. A reactor pressure of 130 Pa (1.0 Torr) is maintained by continuously flowing gas through the reactor and by throttling a rotary vane pump. Two gas lines continuously deliver He to the reactor while two others alternate between delivering TEMAH or water. During the purging step between precursor injections, 75 sccm (standard cm3 per minute, 0 ◦ C and 101.3 kPa) of He flow through each of the four lines. During the TEMAH injection, pneumatic valves momentarily divert the He flow through a bubbler containing TEMAH at 77 ◦ C for typically 3 s. Afterwards, the flow is returned to its original path to purge both the delivery line and the reactor. During the water injection, valves stop the flow of He down one line and open the reactor to a vessel containing water at room temperature. Flow of water vapor into the reactor is restricted to ≈75 sccm by a needle valve. After a period of typically 100 ms, the flow of He is returned to purge the water delivery line and the reactor. The water and TEMAH lines are heated to prevent condensation of the precursors. The reactor walls are heated to 110 ◦ C and are constructed of aluminum to ensure a uniform temperature profile. The wafer chuck is externally heated and monitored with an embedded thermocouple. At the empirically determined optimal wafer temperature of 230 ◦ C, the ALD reactor produces high-quality HfO2 films as characterized ex situ by vacuum-ultraviolet spectroscopic ellipsometry and X-ray photoelectron spectroscopy (not shown). The windows are heated mainly through contact with the walls and process gases and, to a lesser degree, by radiation from the wafer chuck. 2.2. Reflection absorption infrared spectroscopy (RAIRS) A Fourier transform infrared spectrometer (RS-10000, Mattson Instruments) [14] equipped with a high-sensitivity (peak D* = 5.5 × 1010 cm Hz1/2 /W with 1 mm2 area), narrowband HgCdTe detector is employed for the BML-RAIRS measurements. The beam path is sealed and purged with dry nitrogen to remove absorption by atmospheric water vapor and CO2 . Spectra are obtained at 4 cm−1 resolution and a 50 kHz laser frequency with 256 averaged scans each for the background and sample interferograms. The data are collected during the purge steps of the ALD cycle; background interferograms are taken immediately before the precursor pulses while the sample interferograms are taken ≈30 s afterwards to ensure no contribution from gas-phase species. Because a single signal-averaged interferogram requires ≈40 s to complete, the purge times are extended to ≈110 s from the 5 s and 15 s purge times ordinarily used with this reactor for TEMAH and water, respectively [13]. Data reported here are in the form of R/R0 = (R0 − R)/R0 where R is the transformed sample interferogram and R0 is the transformed background interferogram. Positive spectral features therefore result from species added after the pulse, and negative features are from those that have been removed. The enhanced infrared absorption by species on metal surfaces is a well-known effect [15]. However, a metal surface lacks reactive sites to nucleate HfO2 using ALD. Therefore, to enhance the sensitivity of our measurement while providing a chemically active surface, we use BML substrates. These are prepared by coating 5 cm oxidized Si wafers with 10 nm Cr, 200 nm Au, and 20 nm SiO2 by magnetron sputtering. The initial growth surface is thus amorphous SiO2 that has been hydroxylated by atmospheric water vapor. During data acquisition, the infrared beam from the interferometer impinges on the substrate at near-grazing incidence (≈85◦ ) and
passes through a wire-grid polarizer to select p-polarization before reaching the detector. Because the 5 cm substrates do not function as the limiting aperture, which is often the case for RAIRS on small metal crystals [16], higher SNR is possible with less data acquisition time. Enhancement of absorption by the surface species is due to the increased electric field strength in p-polarization near the surface; the spectra we report, therefore, are due to absorption by dipoles normal to the surface. The RAIRS measurements are made while the substrate is still heated to the nominal growth temperatures of 105 ◦ C, 240 ◦ C, and 320 ◦ C. Two complications arise from slight drifts in the substrate temperature. First, the reflectivity of the metal changes depending on the degree of electron scattering by phonons, which results in an upward or downward slope in the baseline of some spectra. Second, the strength of the longitudinal optical (LO) phonon mode of SiO2 at ≈1250 cm−1 is temperature sensitive, so absorption peaks from surface species are superimposed on a very intense, temperature-dependent band, which makes interpretation difficult. Because of the resulting ambiguity, we exclude the spectral region most affected by the LO phonon modes (1100–1300 cm−1 ) from this report. Although the infrared spectra of ALD films deposited on window material has been used as an in situ measurement in previous studies of ALD [17], the focus of our study is on the substrate. We do observe deposition of HfO2 on the windows after an incubation period, and the absorption from the two window surfaces can contribute a significant amount of the measured absorbance (at most ≈1/4) with prolonged deposition lasting over 100 cycles. However, this worst case is not realized here since we limit ourselves to 30 cycles total with one set of windows. Nonetheless, we restrict our discussion to spectral differences we observe with temperature since the windows maintain a nearly constant temperature (≈100 ◦ C). Our results, therefore, are due to the HfO2 on the substrate and not material deposited on the windows. 3. Results and discussion 3.1. First ALD cycle Upon exposing the BML substrates to TEMAH, absorption due to the alkylamido ligands is observed to increase as positive values while absorption due to OH groups is seen to diminish to negative values. Fig. 1 shows the spectra obtained after the initial pulse of TEMAH on the hydroxylated SiO2 surface at 105 ◦ C, 240 ◦ C, and 320 ◦ C. The strong, sharp negative peaks at 3740 cm−1 are due to the O–H stretch mode (O–H) of isolated surface SiOH groups while broad, asymmetric bands extending down to ≈3000 cm−1 are due to (O–H) of hydrogen-bound SiOH groups [18]. Features due to the alkylamido ligands can be observed in the (C–H) region of the spectrum between 2650 cm−1 and 3050 cm−1 and in the deformation modes ı(C–H) below 1500 cm−1 [19]. Modes at 880 cm−1 and 990 cm−1 are apparent as positive features in the spectrum obtained at 105 ◦ C; these alkylamido-related modes correspond to those observed for gas-phase and liquid-phase TEMAH [13,20]. At 320 ◦ C, a broad, negative feature centered at ≈970 cm−1 is apparent. This is likely due to the loss of isolated SiOH groups which have a (Si–OH) mode near this frequency [21]. The complex feature around 980 cm−1 in the spectra obtained at 240 ◦ C is due to a partial exchange of SiOH at 970 cm−1 with the alkylamido ligand at 990 cm−1 . The spectral features discussed thus far are what one expects from the ideal surface reactions during the TEMAH half-cycle, which can be represented as x(–OH)(s) + Hf[N(CH3 )(C2 H5 )]4(g) → (–O–)x Hf[N(CH3 )(C2 H5 )]4−x(s) + x(CH3 )(C2 H5 )NH(g) ,
(1)
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Fig. 1. BML-RAIRS spectra obtained after exposing the initial SiO2 surface to TEMAH. Spectra, which are offset for clarity, are referenced to the substrate before the pulse at a temperature of 105 ◦ C, 240 ◦ C, or 320 ◦ C.
where x is the number of ligands removed as N-methylethanamine (MEA), (CH3 )(C2 H5 )NH. The similarity of the alkylamido-related features to those of molecular TEMAH indicates that the unreacted ligands remain intact on the surface. Previous infrared studies of adsorbed alkylamido precursors adsorbed onto surfaces at low temperatures have also found this to be the case. When silica powders were exposed to tetrakis(diethylamido)titanium (TDEAT) at room temperature, features similar to molecular TDEAT were observed from the powders [22]. A thermally grown SiO2 surface after exposure to tetrakis(diethylamido)hafnium at room temperature yields a (C–H) region that is also similar in most respects [12]. Dosing a NHx -covered TiN surface with tetrakis(dimethylamido)titanium (TDMAT) at ≈150 ◦ C [23] also produced a (C–H) region similar to molecular TDMAT [24]. In contrast, the (C–H) region indicates that TDMAT dissociates on Si(1 0 0) at room temperature [25]. Interestingly, our spectra are somewhat different from those found in previous in situ infrared studies of TEMAH adsorption on SiO2 . When SiO2 -passivated Si wafers were exposed to TEMAH, the (C–H) region in Refs. [20,26] appeared quite different from what we observe. The relative intensities of the asymmetric (C–H) modes of CH3 (≈2960 cm−1 ) and CH2 (≈2930 cm−1 ) in the 105 ◦ C spectrum of the present study are inverted compared to a previous study at 100 ◦ C [26]. Furthermore, we observe much stronger absorption in the symmetric (C–H) modes in the range of 2650–2900 cm−1 compared to that study. In another study, only weak absorptions at 2855 cm−1 and 2930 cm−1 were noted when the Ti and Zr analogues of TEMAH were used [20]. Further differences between previous studies are found in the (O–H) modes. In Ref. [26], the SiO2 surface was initially deuterated, but no evidence for the loss of hydrogen-bound SiOD groups was observed. In Ref. [20], no loss of any (O–H) modes was noted. While the differences in sampling geometries might partially explain the variations between studies, there appear to be fundamental discrepancies in the surface chemistry. We speculate that procedural or timing differences may explain this. In addition to the spectral features in Fig. 1 that are expected from Eq. (1), there is evidence for some additional reactions. Accompanying the negative peaks attributable to (O–H) are negative features at 2270 cm−1 . These are likely due to the (O3 Si–H) mode of surface hydride groups [27] indicating substoichiometry, which is not unexpected from the sputtered SiO2 films [28]. A small
peak at 2040 cm−1 is apparent in the spectrum obtained at 320 ◦ C, and it attributed to a decomposition product as will be discussed in Section 3.3. Finally, there is a positive absorption feature at ≈1600 cm−1 in the spectrum obtained at 105 ◦ C and to a lesser degree in the higher temperature spectra. The origin of the feature at ≈1600 cm−1 cannot be assigned unambiguously, but previous work has found similar peaks. When the Ti and Zr analogues of TEMAH were exposed to SiO2 at around 200 ◦ C, a peak at 1587 cm−1 was observed and attributed to the (C N) mode of an imine, R2 C NR [20]. Similarly, TDMAT- and TDEAT-treated silica surfaces showed a slow decomposition at 60 ◦ C, which produced a peak at 1608 cm−1 likewise attributed to the (C N) mode of imines [29]. Previous work examining HfO2 layers deposited by chemical vapor deposition also found a similarly positioned peak [30]. As suggested in the latter work, carbonates, carboxylates, acetates, and hafnium hydrides all absorb in the region around ≈1600 cm−1 . Another possible explanation for the peak in our spectra is alkylammonium ions, which could form from the protonation of an amine like MEA by surface acid sites: (–OH)(s) + (CH3 )(C2 H5 )NH(g) → (–O)− [(CH3 )(C2 H5 )NH2 ]+ (s) .
(2)
Spectra of alkylammonium ions formed on hydroxylated Al2 O3 from a secondary amine have a characteristic scissors mode ı(NH2 ) at ≈1600 cm−1 that is absent in the spectra of the unreacted amine [31]. It should be noted that a reaction analogous to Eq. (2) has been found on hydroxylated amorphous SiO2 [32]. The effect of the water pulse is to exchange the alkylamido groups added to the surface according to Eq. (1) with OH groups. This forms new Hf–O bonds and prepares the surface for the next half-cycle (Eq. (1)). The surface reaction during the water pulse should ideally proceed as (–O–)x Hf[N(CH3 )(C2 H5 )]4−x(s) + (4 − x)H2 O(g) → (–O–)x Hf(OH)4−x(s) + (4 − x)(CH3 )(C2 H5 )NH(g) ,
(3)
where MEA is again produced as a byproduct. The spectra we obtain after the water pulse indicates that this is indeed what occurs. Fig. 2 shows that the modes associated with the alkylamido ligands appear as negative features while the (O–H) modes are positive. Differences in the spectra with temperature indicate that growth proceeds somewhat differently. In all cases, two (O–H) modes appear at 3660 cm−1 and 3780 cm−1 . These correspond to
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Fig. 2. BML-RAIRS spectra obtained after exposing the surfaces from Fig. 1 to water vapor at the indicated temperatures.
bridging groups (Hf2 OH or Hf3 OH) and terminal (HfOH) groups, respectively [33,34]. At 240 ◦ C and 320 ◦ C, however, the isolated SiOH peak at 3740 cm−1 accompanies the Hf-bound OH groups. Two possibilities exist for the formation of isolated SiOH during the water pulse. First, the water may hydrate siloxane bridges on the surface: Si–O–Si(s) + H2 O(g) → 2Si–OH(s) .
(4)
The second possibility is that Hf–O–Si linkages formed during the initial TEMAH pulse are being broken as has been previously suggested for TiO2 ALD on SiO2 [35]: Hf –O–Si(s) + H2 O(g) → Hf –OH(s) + Si–OH(s) .
(5)
As will be discussed in the next section, the isolated SiOH peak is consumed with the Hf-bound OH groups during TEMAH pulses, which is not expected for OH groups located at an interface. The reappearance of the peak at 3740 cm−1 , therefore, is attributed to incomplete coverage of the initial SiO2 surface. We also note that in addition to reestablishing the isolated SiOH peak, the water pulse at 240 ◦ C and 320 ◦ C also causes absorption due to (O3 Si–H) to increase. As was the case with the TEMAH pulse, a peak at ≈1600 cm−1 appears in the spectra at 105 ◦ C after the water pulse. Since MEA is common to both half-cycles, the attribution of the peak to an alkylammonium ion is reasonable. In the spectrum obtained at 240 ◦ C, we note a peak at 1650 cm−1 in addition to a broad, low-intensity feature at ≈1600 cm−1 . These features are largely absent in the 320 ◦ C sample. The peak at 1650 cm−1 in the 240 ◦ C spectrum is possibly due to the scissors mode of adsorbed water ı(OH2 ), but its absence in the 110 ◦ C spectrum is not consistent with this conjecture. Further experiments are clearly necessary to explain the features around 1600 cm−1 . A negative peak at 2040 cm−1 is noted in the spectra obtained at 320 ◦ C. As will be discussed in Section 3.3, subsequent experiments have found that the 2040 cm−1 peak decays within minutes after a TEMAH pulse, which indicates that the species either desorbs or decomposes further. The appearance as a negative peak in the post-water pulse spectra in Fig. 2 is at least partially due to time-dependent decay and does not necessarily indicate a chemical reaction with water.
3.2. Further ALD cycles The spectra obtained during multiple ALD cycles indicate that deposition occurs by alternating between the reactions described in Eqs. (1) and (3). We present spectra from the tenth ALD cycle after water and TEMAH pulses at the three investigated temperatures in Fig. 3. The behavior is similar to what occurs during the first ALD cycle, but the (O–H) modes of isolated SiOH peaks in the 240 ◦ C and 320 ◦ C spectra are less intense compared to the HfOH absorptions. Absorption due to (O3 Si–H) is absent except as a negative peak after the TEMAH pulse at 320 ◦ C. The persistence of spectral features from the initial SiO2 surface indicates that complete coverage sometimes does not occur until a large number of ALD cycles have been completed. While the coverage of the initial SiO2 surface seems to be complete after the first ALD cycle when the growth temperature is 105 ◦ C, it remains incomplete when the temperature is increased. This observation agrees with a previous study of ALD which found additional Hf–O–Si bonds did not form past the first TEMAH pulse at 100 ◦ C [26], but it also indicates that nucleation can be more complicated at higher temperatures. Whether or not the incomplete coverage at higher temperatures is due to the nature of the sputtered SiO2 used in this study is an open question. Strained bonds and substoichiometry differentiate the material used here from thermally grown or native oxides on Si [28]. Refinement of the BML substrate preparation, however, may provide a means to produce SiO2 surfaces that more closely resemble those produced from oxidized Si wafers. Interestingly, the BML-RAIRS method can be used as an in situ diagnostic for the ALD process. After one ALD cycle at ≈100 ◦ C, the SiO2 is completely covered. By varying the injection time of one precursor while maintaining the injection time of the other precursor at a saturating dose known to complete the surface reactions, one can determine how the surface coverage varies with the injection timing of precursor. We have obtained BML-RAIRS data in such a way at a growth temperature of 110 ◦ C. The TEMAH pulse length was varied from 0.1 s to 6.0 s keeping the water pulse length at 1 s, and the water pulse was varied from 20 ms to 150 ms while keeping the TEMAH pulse length at 3 s. To quantify the extent of the surface reactions, R/R0 was integrated in the (C–H) region. The resulting values are presented in Fig. 4. While saturation occurred for all water pulse times (Fig. 4a), approximately 2 s
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Fig. 3. BML-RAIRS spectra obtained during the tenth ALD cycle. The spectra (offset for clarity) are referenced to the substrate before the TEMAH or water pulse (as indicated) at a deposition temperature of 105 ◦ C, 240 ◦ C, or 320 ◦ C.
of TEMAH injection is necessary to saturate the surface (Fig. 4b). The S-shape to the TEMAH curve at short pulse lengths (Fig. 4b) is likely due to consumption of TEMAH on the walls upstream of the substrate. Usually, the type of information contained in Fig. 4 is determined by measuring the growth rate per cycle. Here, each datum was obtained in situ after one ALD cycle as opposed to the hundreds of cycles typically needed to deposit a film with an easily measurable thickness. Thus, BML-RAIRS could prove to be a useful diagnostic tool for optimizing ALD processes.
Fig. 4. The integrated reflectance change in the region of the C–H stretching vibrations (2700–3050 cm−1 ) when (a) the TEMAH pulse length is varied keeping the water pulse length at 1 s and (b) the water pulse length is varied keeping the TEMAH pulse length at 3 s.
3.3. Thermal decomposition of adsorbed TEMAH The peak at 2040 cm−1 , which we attribute to a decomposition product of adsorbed TEMAH, appears in all the spectra obtained during ALD at 320 ◦ C (Figs. 1–3). An additional experiment was conducted to determine what possible role it might have in deposition. After 15 ALD cycles at 105 ◦ C, the temperature was increased to 320 ◦ C. The background interferogram was taken before injecting TEMAH for 3 s after which a series of sample interferograms were made at ≈60 s intervals. The resulting spectra are shown in Fig. 5. The spectrum obtained after 60 s is similar to those obtained during ALD at 320 ◦ C (Fig. 3) with some noticeable differences. First, the 60-s spectrum in Fig. 5 contains no evidence for the (O–H) and (Si–OH) modes of isolated SiOH groups or the (O3 Si–H) of surface hydrides. Only the (O–H) of bridging and terminal Hfbound OH are apparent as negative features. Deposition at 105 ◦ C has apparently covered the SiO2 surface with HfO2 . The second difference is that the region between 1300 cm−1 and 1700 cm−1 is much more complex. It appears that positive features from ı(C–H) modes are superimposed upon both negative and positive features in that range. These features are not observed during ALD (Figs. 1–3) and may represent contamination of the surface while the sample resides in the reactor under purge for longer periods than are typical. Nonetheless, we observe the peak at 2040 cm−1 at approximately the same intensity observed during ALD. Furthermore, we observe that the peak at 2040 cm−1 decays with time. (The inset of Fig. 5 shows the 2040 cm−1 peak in more detail.) The loss of the 2040 cm−1 peak indicates that the species responsible for this absorption either desorbs or decomposes further. (The appearance as a negative peak in the post-water pulse spectra in Figs. 2 and 3 is at least partially due to time-dependent decay and does not necessarily indicate a chemical reaction with water.) Simultaneous with the decay in the 2040 cm−1 peak, the (C–H) modes were also observed to decrease with time suggesting that the 2040 cm−1 peak is associated with the thermal decomposition of alkylamido ligands. The spectrum obtained after 240 s contains a broad, featureless band centered at 2870 cm−1 instead of the distinct modes apparent in the (C–H) band of the intact ligands. The featureless band at 2870 cm−1 is most likely the (C–H) modes of organic material on the surface that is no longer part of
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B.A. Sperling et al. / Applied Surface Science 256 (2010) 5035–5041
Fig. 5. BML-RAIRS spectra obtained after exposing a HfO2 film to TEMAH at 320 ◦ C. The spectra (offset for clarity) are referenced to the same background data, which was obtained immediately before the TEMAH pulse. The inset figure shows a close-up of the peak at 2040 cm−1 with the scale bar indicating 2 × 10−4 .
–N(CH3 )(CH2 CH3 ) ligands, which suggests that the 2040 cm−1 peak is associated with carbon and hydrogen incorporation into the film at high temperatures. While the origin of the 2040 cm−1 peak cannot be assigned definitively from the spectroscopic data available, previous work found that gas-phase CO and HC N were produced from tetrakis(dimethylamido)hafnium when a graphite surface was heated above 250 ◦ C [36]. Furthermore, tetrakis(diethylamido)zirconium adsorbed on Si produced an imine (R2 C NR) and acetonitrile (CH3 C N) in temperature-programmed desorption experiments [37]. Since the (C N) mode of an adsorbed imine should occur at a much lower frequency than 2040 cm−1 (i.e. ≈1590 cm−1 ) [20,38], we rule out that group as a possible assignment. The 2040 cm−1 peak is likely either a metal-bound cyano group (C N), which are observed around 2070 cm−1 in terminal configurations, or a metalbound carbonyl group (C O), which occur between 1700 cm−1 and 2170 cm−1 [19]. Our observation of the peak at 2040 cm−1 highlights the usefulness of BML-RAIRS. Since this species is present on the surface only for a few minutes, it is unlikely that it could be observed using a slower method without rapidly quenching the sample. The relatively weak absorption from this peak would likely be lost if the signal averaging were over the course of tens of minutes (e.g. Refs. [10,12]) instead of the tens of seconds in this experiment.
4. Conclusion The use of BML-RAIRS provides a means to study surface chemistry of ALD under realistic conditions in a flow reactor. This has permitted us to observe the spectroscopic evidence of a transient decomposition product of TEMAH. Furthermore, our findings indicate that incomplete coverage of the SiO2 film occurs at 240 ◦ C and 320 ◦ C but not at 105 ◦ C. The use of this technique as a fast, in situ diagnostic tool has also been demonstrated.
Acknowledgements The authors acknowledge M. Carrier and R. Zangmeister for technical assistance.
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