J. Phys. Chem. B 2001, 105, 3903-3907
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In-Situ FTIR Studies of Reactions at the Silicon/Liquid Interface: Wet Chemical Etching of Ultrathin SiO2 on Si(100)† K. T. Queeney,‡ H. Fukidome, E. E. Chaban, and Y. J. Chabal* Agere Systems, Murray Hill, New Jersey 07974 ReceiVed: September 21, 2000; In Final Form: December 23, 2000
We have developed an experimental setup to facilitate study of the surface reactions of single-crystal silicon with Fourier transform infrared (FTIR) spectroscopy in a variety of aqueous environments. Employing a short optical path length through the silicon sample allows access to the critical low-frequency region of the spectrum (∼850-1500 cm-1) that cannot be probed with traditional multiple internal reflection (MIR) techniques. The utility of this technique is demonstrated in a study of the etching of ultrathin SiO2 on Si(100) in dilute hydrofluoric acid. This approach provides in-situ access to the SiO2/Si(100) interface that is revealed as the overlying oxide is stripped away. We find that this layer is, indeed, structurally distinct from the rest of the SiO2 film, consistent with a marked change in reactivity as etching nears the Si(100) substrate.
Introduction Wet chemical processing of silicon plays an essential role in the manufacture of silicon-based microelectronics. Prior to thermal oxidation of Si(100) to form the SiO2 gate dielectric for transistors, the single-crystal Si(100) wafers must be cleaned and then passivated to protect against further contamination before the high-temperature oxidation process takes place. A variety of solution-based chemistries may be used for these cleaning and passivation steps, but most processes share the common step of hydrofluoric acid (HF) etching to remove native oxide (and any impurities it contains) from the silicon surface. Etching the native oxide off Si(100) in aqueous HF is known to produce a hydrogen-terminated Si surface1 that is sufficiently resistant to further oxidation that this step may be used as the last wet processing step before introduction of the wafers into an oxidation furnace. In fact, recent studies have shown that such “HF-last” processing creates a Si(100) surface that is detectably smoother than those generated by competing processes.2 The smoother Si(100) substrate results in a more homogeneous postoxidation Si/SiO2 interface, which is essential to device performance as transistor (and hence gate oxide) dimensions decrease.3 Because H-termination is generally carried out in dilute HF solutions (as dilute as 25 wt %), understanding the etching process under these conditions is critical in order to optimize the silicon surfaces thus prepared. Recent electrochemical studies have shown that the final layer of SiO2sthat is, the oxide at the Si/SiO2 interfacesexhibits strikingly different etching behavior than does the outer portion of the film.4 Such enhanced resistance may arise from increased density of the film due to the compressive stress that is known to exist between Si and SiO2;5 in addition, it may be affected by chemical differences in the interfacial layer due to oxide substoichiometry.6 This second interpretation is supported by electrochemical measurements that correlate a decrease in oxide etching rate with the onset of anodic dark current that indicates the etching of †
Part of the special issue “John T. Yates, Jr. Festschrift”. Current address: Chemistry Department, Smith College, Northampton, MA 01063. E-mail:
[email protected]. ‡
substoichiometric SiOx (x < 2) species.7 While such electrochemical studies provide welcome insight into kinetics of reactions at the silicon/solution interface, interpretation of these results in terms of specific chemical reactions relies on indirect modeling of the species involved (both dissolved and surfacebound). It is, therefore, highly desirable to couple such studies with the chemical specificity of a technique such as surface infrared spectroscopy. To our knowledge there has been to date only one study utilizing in-situ FTIR to study the etching of an SiO2 film on Si(100) in dilute HF.8 In this work the author used a silicon multiple internal reflection (MIR) element with a relatively long (50 mm) sample axis and was able to perform a fairly detailed analysis of changes in solution composition near the sample interface as etching proceeded. Although such a long sample path carries the advantage of providing enhanced sensitivity to surface-bound species by interrogating the silicon surface multiple times, this sensitivity comes at the expense of spectral range, because the long optical path length of the IR beam through the silicon results in complete absorption of the light by multiphonon bands up to 1500 cm-1. Since the dominant SiO2 vibrational modes fall in the range 900-1300 cm-1, such a technique cannot be used for direct study of the evolution of the SiO2 film during etching. In the above-mentioned study, changes in the SiO2 film were therefore deduced indirectly by monitoring the growth of Si-H modes as the surface became hydrogen-terminated. However, this analysis was limited primarily to the observation that oxidized species (e.g. O3Si-H) were replaced by unoxidized Si-H species as oxide was removed from the silicon substrate, with no information available on the structure of the interfacial oxide being removed. Simultaneous observation of Si-O and Si-H vibrational modes during etching of an SiO2 film was achieved in a combined electrochemical/FTIR study of SiO2 on Si(111).9 However, in this case the authors focused on the hydrogenation of the Si(111) surface and therefore did not carry out an analysis of changes in the Si-O modes as a function of oxide etching. Analysis of Si-O vibrational modes in an ultrathin SiO2 film on Si(100) at a silicon/electrolyte interface was used by Chazalviel and co-workers to probe changes in the Si-O
10.1021/jp003409j CCC: $20.00 © 2001 American Chemical Society Published on Web 02/07/2001
3904 J. Phys. Chem. B, Vol. 105, No. 18, 2001 vibrational spectrum as a function of oxide thickness for SiO2 anodic oxides grown under varying conditions.10 This work sought to describe the structure of the Si/SiO2 interface by interpreting changes in the SiO2 vibrational spectrum as a function of thickness, a general strategy that has been employed in a number of FTIR studies. In the present work, though, we focus on thermally oxidized Si(100) to allow direct comparison with the technologically relevant SiO2 gate oxide. The structure of the Si/SiO2 interface is of great interest, not only because of its apparent resistance to etching by HF but also because the formation of this interface during the initial oxidation has a profound effect on the ultimate performance of the gate oxide and thus on the device into which it is incorporated. Chemical etching in combination with infrared spectroscopy has been used by a number of researchers to probe the structure of this interface by removing the overlying oxide to reveal the spectral signature of the Si/SiO2 interface alone.6,11-16 While a recent study using this approach provided strong evidence for a substoichiometric layer contained within the first 6 Å of a 30 Å thermally grown SiO2 film,6 the ex-situ approach used in this (and previous) studies does not allow reliable probing of the true interfacial layer. Specifically, when samples are etched in dilute HF and then removed from the etchant for spectral analysis, the spectra acquired cannot be used to distinguish between changes that occurred in the film during etching and those that may have occurredsparticularly when the oxidation-sensitive Si surface begins to be revealed-upon exposure to ambient conditions. Furthermore, since thermodynamic and kinetic considerations dictate that etching in dilute HF will involve both hydrogenation and hydroxylation reactions at the surface,17 gradual transitions in the dominant chemistry are likely missed due to the stepwise nature of sample acquisition in the ex-situ experiment. In the present work we have combined the sensitivity of the ex-situ experiment to changes in the oxide film as a function of etching with the real-time capabilities of the in-situ approach described earlier. This is accomplished by modification of the in-situ experiment design to allow access to the critical Si-O vibrational modes below 1500 cm-1. The approach used herein draws on the same principles employed for the in-situ electrochemical study mentioned above10 as well as for high-sensitivity studies of Si(100) oxidation at the silicon/vacuum interface.18 Preliminary results of the etching of ultrathin SiO2 are presented to demonstrate the unique ability of this technique to monitor the evolution of silicon surfaces in solution. Our results confirm the electrochemical studies that reported an increased resistance to HF etching of interfacial SiO2; in addition, our approach allows us to correlate this change in chemical reactivity to a specific interfacial oxide layer with a vibrational spectrum distinct from that of the SiO2 film that overlies it. Experimental Section Etching studies were performed on double-side polished, float-zone Si(100) wafers (resistivity ) 1-10 Ω cm) that had been thermally oxidized at 800-850 °C to form an SiO2 layer 30 Å thick. The wafers were subsequently cut into 11 × 15 mm samples with the long edges beveled at 45°. Before each experiment a new sample was degreased in methanol and cleaned briefly in 5:1 H2O2:H2SO4 solution until hydrophilic. After insertion of the sample into the liquid cell (see description below) a reference spectrum of the initial oxide layer was acquired in deionized water. Dilute HF (standard concentration ) 0.05 wt % HF in deionized water) was then introduced into the cell and spectra obtained as a function of time, with
Queeney et al.
Figure 1. Liquid cell used for in-situ FTIR studies. A top-down schematic of the cell (not to scale) is shown in a, with the corresponding experimental throughput (MCT-A detector) displayed in b.
dilute HF flowing through the cell via peristaltic pumping. Nitrogen was bubbled through all solutions at a ∼50-100 cm3/ min to reduce the amount of dissolved oxygen, which is known to have a strong effect on etching rates in dilute HF. Once the spectra were observed to be constant as a function of time, i.e., no further etching was detectable, concentrated HF (50 wt %) was flowed through the cell to create an H-terminated surface. A background spectrum was then obtained by flushing the cell with H2O and acquiring a spectrum of the H-terminated surface in water. All spectra were acquired with a Nicolet Magna 760 FTIR spectrometer using a liquid-nitrogen cooled MCT-A detector. Spectra acquired during etching consisted of 500 scans each, resulting in a collection time of approximately 5 min/spectrum. This parameter was optimized for a given HF concentration to provide sufficient signal-to-noise while capturing important spectral changes occurring during the etching process. Liquid Cell. The liquid cell used for the FTIR studies described herein is illustrated schematically in Figure 1a. The sample is held between two Teflon pieces that are hollowed out to create a sample/liquid interface approximately 6 mm long (along the axis of the IR beam) on both sides of the sample. Liquid is introduced into the cell on one side and flows from one half to the other before emptying out the second half of the cell. The infrared beam is incident on the sample at an angle of 45° from the normal of the beveled edge, resulting in approximately 13 internal reflections before the beam exits the sample at the same 135° angle. The angle at the silicon/solution interface is 57° from the normal, compared with 45° for standard multiple internal reflection (MIR) configurations where the beam
Reactions at the Silicon/Liquid Interface
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3905
Figure 2. Infrared spectra acquired in-situ during etching of a 30 Å SiO2 layer on Si(100) in 0.05 wt % HF, with (a) unpolarized light and (b) s-polarized light obtained by placing a grid polarizer between the source and sample. All spectra are ratioed to an H-terminated background generated in-situ with 50 wt % HF.
enters the sample normal to the beveled edge. This configuration was chosen because the smaller number of internal reflections resulting from the shallower internal reflection angle prevents complete absorption of the infrared light in regions of strong absorption by the solution. As demonstrated by the single-beam spectrum in Figure 1b, the IR throughput from such a configuration is significantly enhanced relative to a standard MIR configuration. Specifically, while the longer samples normally used for MIR result in complete absorption of the IR radiation below 1500 cm-1, in our setup there is detectable signal as low as 900 cm-1. In addition the shorter sample length, coupled with the shallower angle of incidence at the sample/solution interface (and therefore a reduced number of IR/solution interactions), results in detectable R throughput even in the regions of strong water absorption at ∼1640 and ∼3400 cm-1. While acquisition of spectral data between 900 and 1500 cm-1 is critical for studies of the SiO2 film itself, access to this region presents some unique experimental challenges. For instance, Teflon itself absorbs strongly in this region; since Teflon deforms relatively easily, a seal directly between the Teflon cell and the sample results in a silicon/Teflon interface whose area changes over the time scale of spectral acquisition. These changes result in spurious spectral features, corresponding to the vibrational modes of Teflon that overlap with the desired SiO2 features. To circumvent this problem, a thin piece of polished graphite is inserted between the Teflon and the sample, providing a liquid-tight seal that does not interfere in the desired spectral range. Results and Discussion The spectra acquired during etching of a 30 Å SiO2 film on Si(100) in 0.05 wt % HF are presented in Figure 2. These results were obtained from two separate experiments on identical samples and with identical etching conditions, using in one instance unpolarized infrared light (Figure 2a) and in the other instance s-polarized light (Figure 2b). Unpolarized light provides sensitivity to vibrational modes oriented both perpendicular and parallel to the Si substrate, resulting in the appearance in Figure 2a of both the transverse optical (TO) and the longitudinal optical (LO) phonon modes of SiO2, at 1067 and 1244 cm-1, respectively, for the initial oxide layer. In contrast, using s-polarized light (obtained by placing a grid polarizer between
Figure 3. Plot of oxide thickness (calculated via integration of s-polarized intensity) as a function of etching time in 0.05 wt % HF.
the source and sample) allows detection of only the parallel TO mode in Figure 2b. The intensity of the TO mode is proportional to oxide thickness; a recent study6 demonstrated that thicknesses determined by integration of the area under the TO peak for ultrathin films of SiO2 are in good agreement with those determined independently via X-ray photoelectron spectroscopy. The data in Figure 2b can therefore be used to examine the etching rate, as shown in Figure 3. There is a clear transition, just after 40 min, from a linear etching regime to an apparent steady state of surface oxide thickness. Measurement of significant TO intensity beyond this point suggests that the SiO2 is not completely removed from the surface, indicative of competition between surface oxidation and oxide removal. Before investigating in further detail the chemical nature of this last, etching-resistant oxide layer, it is worthwhile to compare these in-situ etching results with ex-situ results acquired previously. In the ex-situ experiment separate pieces from the same oxidized Si(100) wafer were immersed in a bath of 0.05 wt % HF and removed sequentially. Transmission infrared spectra were then obtained at both grazing and normal incidence; the grazing incidence results (showing both LO and TO modes) are reproduced here in Figure 4. In the ex-situ experiment as the ∼30 Å SiO2 layer is etched down to a final thickness of ∼6 Å, both the TO and the LO peak frequencies decrease, from 1065 to 1053 cm-1 and from 1252 to 1220 cm-1, respectively. This red shift as a function of thickness is attributed primarily
3906 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 4. External transmission infrared spectra (grazing incidence) acquired ex-situ after etching a 30 Å SiO2 layer on Si(100) for increasing amounts of time in 0.5 wt % HF.
Figure 5. Comparison of infrared spectra of a ∼60 Å SiO2 film on Si(100) acquired in the liquid cell filled with air (solid line) and water (dashed line).
to the effect of a substoichiometric transition layer that is contained within the final 6 Å film.6 By comparison of Figure 2a and Figure 4 it is immediately apparent that spectra acquired via the two methods exhibit some significant differences. Relative sensitivities to the TO and LO modes in the two distinct optical geometriessexternal transmission in air (ex-situ, Figure 4) and multiple internal reflection in aqueous solution (in-situ, Figure 2a)sare expected to differ. Those differences are predicted by calculating the relative sensitivity of each geometry to parallel and perpendicular dipoles, as described in detail elsewhere.19 For instance, sensitivity to the perpendicular-oriented LO mode is enhanced by utilizing grazing-angle (∼22°) incidence of the infrared beam in the external transmission experiment. While such analysis explains why the intensity of the LO mode relative to that of the TO mode is greater in the external transmission than in the MIR experiment, the appearance in Figure 2a of a negative absorbance feature to the high-frequency side of the LO mode is not immediately apparent. This effect leads to an asymmetry in the LO absorbance line shape, with the position of the LO peak maximum correspondingly red-shifted. That this asymmetry is an optical effect is demonstrated in Figure 5, which compares spectra acquired of a 60 Å oxide in the liquid cell with and without water present. (In each case, the spectrum is ratioed to the spectrum of an H-terminated Si(100) surface in the corresponding environment.) Two things
Queeney et al. are notable about this comparison: (1) the asymmetry is only observed in water, not in air, and (2) the LO intensity is greatly enhanced by the presence of water above the SiO2 surface. The latter point can be understood in light of the fact that the refractive index of water is higher than that of air (1.27 compared to 1); evaluation the three-layer model for each of these cases19 reveals that the higher dielectric constant of water should result in approximately a 1.9-fold enhancement of perpendicular modes. This prediction is in good agreement with the observed enhancement factor (approximated from the difference in LO peak heights in Figure 5) of ∼1.8. The negative absorbance feature and accompanying LO peak asymmetry is associated with the index of the liquid, which incorporates a weak but nonzero imaginary component. The precise nature of this interaction of the two indices (water and SiO2) under internal reflection at the solid/liquid interface is currently under study but is believed to account for the slight red shift of the LO peak frequency of the initial oxide in the in-situ experiment (1246 cm-1) relative to that observed in external transmission (1252 cm-1). A second difference between spectra acquired in the in-situ (Figure 2) and ex-situ (Figure 4) experiments lies in the magnitude of the TO and LO shift. In the ex-situ experiment, those modes red shift by 12 and 32 cm-1, respectively, between 30 and 6 Å. In the in-situ experiment, the LO mode appears to red shift by only ∼20 cm-1 and then to increase in frequency as the final oxide layer is approached. At the same time the TO mode appears to undergo at most a negligible red shift before increasing in frequency to slightly above its initial value (note dashed lines in Figure 2a). As outlined below, the behavior of the LO peak frequency is due to a combination of the asymmetry discussed above and the appearance of a new absorption feature for the thinnest oxide layers. To sort out the specifics of the TO/LO behavior, it is useful to compare directly spectra of the same film taken with unpolarized and s-polarized light. In Figure 6a two such spectra are compared for the final surface, i.e., once the etching reaction has slowed to a virtual steady state. This comparison highlights the fact that the high-frequency tail on the unpolarized spectrum does, in fact, still have LO character even though it is distinct in shape from the initial, sharp LO peak (e.g. Figure 2a). While there is a higher-frequency tail to the TO mode (Figure 6a, solid line), also observed in transmission spectra,6 this does not account fully for the final spectrum obtained with unpolarized light. In fact, as demonstrated in Figure 6b, this high-frequency, perpendicularly polarized component to the spectrum is responsible for the difference in the behavior of the LO peak frequency as compared with the ex-situ, external transmission experiment. Specifically, the fact that the LO mode in the in-situ spectra appears to stop decreasing in frequency at an intermediate thickness, with a subsequent shift to higher frequencies, arises from a transition from the true LO modes of SiO2 to the highfrequency tail identified in Figure 6a. This is illustrated in Figure 6b by the downward trend in LO frequency (marked by dashed lines) that disappears as the high-frequency tail emerges from under the LO. The magnitude of the LO red shift between the initial oxide and the last surface for which a distinct LO peak can be detected is the same as that detected in the ex-situ experiment, approximately 30 cm-1 (Figure 2a). Importantly, there is sufficient intensity out to ∼1260 cm-1 in all the spectra that this high-frequency tail (which extends out to a peak at ∼1250 cm-1) need not represent a new surface species, but rather the signature of the final layer that is uncovered as the outer layers are removed. This region was not
Reactions at the Silicon/Liquid Interface
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3907 evolution of surface reactions in a liquid environment. Details of the SiO2 etching process in dilute HF that were not apparent in earlier, ex-situ studies are revealed by this method, particularly the nature of the interfacial Si/SiO2 layer that is known to exhibit markedly different etching chemistry than does stoichiometric SiO2. The formation of a hydroxylated SiOx surface that remains relatively etch-resistant due to equilibrium between hydrogenation and hydroxylation reactions is evidenced by the appearance of novel, solvent-coupled silanol modes previously undetected in this system. Further studies are aimed at elucidating the exact structural origins of the signature vibrational modes of this final surface, as well as their dependence on etching parameters such as HF concentration. Finally, we note that this experimental approach holds great promise for combining time-dependent spectroscopic and electrochemical analysis of this system, as well as the possibility of exploring the nature of the documented influence of photoinduced chemistry on this reaction. In addition this approach can be extended to study a number of the solution-based surface reactions that form an integral part of semiconductor processing for microelectronic and optoelectronic applications.
Figure 6. Details of the spectra of the final surface layers achieved by etching a 30 Å SiO2 film on Si(100) in 0.05 wt % HF. In a the spectra of the final layer acquired with unpolarized (dashed line) and s-polarized (solid line) light are compared directly. The final transition to the last layer is shown in b via a series of offset spectra acquired with unpolarized light.
explored in previous ex-situ experiments, because of the difficulty inherent in obtaining such a surface and maintaining its integrity upon exposure to ambient conditions. However, the interpretation of the ex-situ experiments leads us to assign this high-frequency region to be due, at least in part, to an extended, perpendicular mode of the suboxide interfacial layer. Additional information about the character of the modes observed in the spectrum of the final overlayer is obtained when D2O is substituted for H2O. Specifically, the dominant mode at ∼1070 cm-1 loses significant intensity upon deuteration of the overying solvent (data not shown).We attribute this unexpected effect to the fact that this mode, while certainly arising in part from the Si-O vibrations analogous to those giving rise to the TO in true SiO2, also gains intensity from a set of solventcoupled silanol (Si-OH) modes recently identified in a combined spectral and theoretical analysis of the pH-dependent surface infrared spectrum of silica in aqueous solution.20 Such modes are expected to exhibit a reasonably strong isotopic shift due to involvement of solvent modes, consistent with the disappearance of this mode upon deuteration of the solvent. Therefore, the anomalous behavior of the TO mode in the insitu as compared with the ex-situ experiment may arise in part from overlap of this mode with modes arising from silanol groups present after substantial hydroxylation has occurred. Further studies, both experimental and theoretical, are currently underway to allow more detailed description of the solvent/ SiOx-OH interactions in the final suboxide layer. Conclusions The experiments presented herein demonstrate the utility of an in-situ spectroscopic approach for studying the temporal
Acknowledgment. Y.J.C. is grateful to John T. Yates, Jr. for his stimulating leadership in the development of surface experimental probes. References and Notes (1) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Burrows, V. A. J. Vac. Sci. Technol. A 1989, 7, 2104. (2) Moccio, S. V.; Sorsch, T. W.; Muller, D. A.; Evans-Lutterodt, K.; Timp, G. Characterization of hyperthin oxides utilizing scanning tunneling microscopy, MRS Spring Meeting, 1999, San Francisco. (3) S. I. A. The National Technology Roadmap for Semiconductors; Sematech: Austin, TX, 1997. (4) Okorn-Schmidt, H. F. IBM J. Res. DeV. 1999, 43, 351-385. (5) Jaccodine, R. J.; Schlegel, W. A. J. Appl. Phys. 1966, 37, 24292434. (6) Queeney, K. T.; Weldon, M. K.; Chang, J. P.; Chabal, Y. J.; Gurevich, A. B.; Sapjeta, J.; Opila, R. L. J. Appl. Phys. 2000, 87, 13221330. (7) Matsumura, M.; Morrison, S. R. J. Electroanal. Chem. 1983, 144, 113. (8) Watanabe, S. Surf. Sci. 1995, 341, 304-310. (9) Rappich, J.; Lewerenz, H. J. Electrochim. Acta 1996, 41, 675680. (10) da Fonseca, C.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. 1996, 365, 1-14. (11) Devine, R. A. B. Appl. Phys. Lett. 1996, 68, 3108-3110. (12) Miyazaki, S.; Nishimura, H.; Fukuda, M.; Ley, L.; Ristein, J. Appl. Surf. Sci. 1997, 113/114, 585-589. (13) Ohwaki, T.; Takeda, M.; Takai, Y. Jpn. J. Appl. Phys. 1997, 36, 5507-5513. (14) Galeener, F. L.; Leadbetter, A. J.; Stringfellow, M. W. Phys. ReV. B 1983, 27, 1052-1078. (15) Boyd, I. W.; Wilson, J. I. B. J. Appl. Phys. 1987, 62, 3195-3199. (16) Ishikawa, K.; Ogawa, H.; Fujimura, S. J. Appl. Phys. 1999, 85, 4076-4082. (17) Osseo-Asare, K.; Wei, D.; Mishra, K. M. J. Electrochem. Soc. 1996, 143, 749-751. (18) Weldon, M. K.; Stefanov, B. B.; Raghavachari, K.; Chabal, Y. J. Phys. ReV. Lett. 1997, 79, 2851-2854. (19) Chabal, Y. J.; Hines, M. A.; Feijoo, D. J. Vac. Sci. Technol. B 1995, 13, 1719-1727. (20) Weldon, M. K.; Queeney, K. T.; Raghavachari, K.; Batteas, J. D. J. Am. Chem. Soc., submitted for publication.
