Evolution of atomic-scale roughening on Si(001)-(2 1) surfaces resulting from high temperature oxidation

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1 Evolution of atomic-scale roughening on Si(001)-(2 1) surfaces resulting from high temperature oxidation J. V. Seiple and J. P. Pelz Department of Physics, The Ohio State University, Columbus, Ohio Received 3 October 1994; accepted 30 January 1995 Scanning tunneling microscopy was used to study surface morphology on Si samples following elevated temperature O 2 exposure as a function of pressure P ox Torr, temperature 500 T s 700 C, and dose langmuir L. At low O 2 doses D ox 50 L; T s 600 C and P ox Torr preferred B-type step retraction is observed, but single A-domain formation is prevented due to step pinning by nucleated oxide clusters. These pinning centers roughen the surface via step fingering at low doses, while at higher doses result in the formation of three-dimensional conical islands, similar to oxidation-induced growth features reported previously by Smith and Ghidini J. Electrochem. Soc. 129, at higher temperatures. Surface etching rates were determined by measuring the island heights as a function of exposure, and a sticking coefficient s of was estimated for O 2 reaction on Si 001 at 600 C. For moderate O 2 doses 1000 L surface etching was found to dominate SiO 2 growth up to pressures several orders of magnitude above the commonly accepted oxidation critical line, causing significant atomic-scale roughening under these oxidation conditions American Vacuum Society. I. INTRODUCTION It is well known that exposing Si to O 2 can, depending on substrate temperature T s and oxygen pressure P ox, either etch the surface via the evolution of volatile SiO or lead to the formation of a solid SiO 2 film. A critical curve is often referenced 1,2 which separates the pressure temperature conditions for which etching is the dominant process the socalled active oxidation regime from those which produce SiO 2 growth the passive oxidation regime. At high temperatures 1000 C micrometer-sized growth features were observed at high doses within a narrow transition region between the active and passive regimes. 2 Less is known about how the atomic-scale Si surface morphology depends on oxidation conditions, particularly at lower temperatures 800 C which are increasingly important to modern semiconductor processing. Recently it has been shown that atomic-scale surface roughening results on both Si 001 Ref. 3 and Si 111 Ref. 4 surfaces over a range of low temperature and low pressure conditions in which oxide nucleation and surface etching occur simultaneously. To date, however, relatively little is still known about the boundaries of this roughening regime, or how the surface morphology evolves with temperature, pressure, and O 2 dose, particularly on the technologically important Si 001 surface. Since many Si processing steps involve transient, low-temperature exposure to background levels of O 2 and H 2 O, a knowledge of the conditions which produce surface roughening and a characterization of the roughness thus produced has great practical importance. 5 We have used scanning tunneling microscopy STM to study oxidation-induced roughening of Si 001 surfaces in this low temperature roughening regime. The focus of the present article is a description of how the surface roughness evolves with O 2 dose. The temperature dependence of oxide cluster nucleation and atomic modeling of the nucleation process will be discussed in detail elsewhere. 6 At low O 2 doses D ox 50 langmuir L, where 1 L Torr s we have observed the preferential retraction of B-type surface steps, in agreement with ion-sputtering studies by Bedrossian and Klitsner. 7 However, the preferential retraction is not complete due step pinning by nanometer-sized oxide clusters. 3 At higher doses, these pinning sites lead to the creation of three-dimensional conical island structures as prolonged surface etching removes the surrounding material. These structures may be early stages of the micrometer-sized growth features observed by Smith and Ghidini 2 at higher temperatures, pressures, and doses. At T s 600 C and P ox Torr, the average island height and rms surface roughness were found to scale approximately linearly with dose. The surface etching was directly determined from the dependence of island height versus dose, which indicated a sticking coefficient s for O 2 on Si under these conditions. Finally, we have started to delineate the boundaries in temperature pressure space that result in significant atomic-scale surface roughening, and have found them to extend to pressures several orders of magnitude above the critical line for net SiO 2 growth. II. EXPERIMENT Experimental procedures for these experiments have been discussed in detail elsewhere. 8 In short, Si 001 samples mm, n type, 0.25 miscut toward 110 were resistively flashed to 1250 C to remove any oxide present. Clean surfaces exhibited evenly spaced A-type steps upper terrace dimer rows parallel to step and B-type steps upper terrace dimer rows perpendicular to step. 9 Figure 1 a shows a typical large-area scan 300 nm 300 nm of the clean surface. Although this scan area is too large to show the atomic rows, one can easily distinguish the smooth A-type steps from the more ragged B-type steps. Once atomic- 772 J. Vac. Sci. Technol. A 13(3), May/Jun /95/13(3)/772/5/$ American Vacuum Society 772

2 773 J. V. Seiple and J. P. Pelz: Evolution of atomic-scale roughening 773 FIG. 1. a Top-view gray scale STM topograph of nm 2 area of the clean Si starting surface, miscut toward 110. Smooth A-type and rougher B-type steps are marked. b nm 2 closeup showing typical defect density. A- and B-type terraces are labeled. c nm 2 area after 50 L O 2 dose at Torr and 600 C, showing reduction in area of B-type terraces. c Ratio of A- tob-type terrace areas vs O 2 dose see the text. resolution STM scans confirmed that the clean surface had a low defect density 2%, as shown in the close-up image in Fig. 1 b, the sample was heated to the desired oxidation temperature and then exposed to O 2. The oxidation pressure was determined by the ion pump current which had been previously calibrated against a standard ion gauge. All hot filaments were off during an actual dose. Samples were quenched to room temperature about 6 s after the O 2 was turned off and then scanned. Samples were reflashed before each O 2 exposure. III. RESULTS For low oxygen pressures and doses at 600 C, we found that oxidation-induced etching preferentially etches B-type steps over A-type. Figure 1 a shows a clean starting surface with well defined, evenly spaced A- and B-type steps, while Fig. 1 c shows a different area of the surface after exposure to 50 L of O 2 at Torr and 600 C. As discussed elsewhere, 3 oxidation under these conditions primarily results in surface etching through the formation of surface vacancies via desorption of SiO. At these temperatures the vacancies are mobile 7 and diffuse to step edges causing them to retract. Bedrossian and Klitsner 7 have shown that diffusing vacancies created by ion sputtering at T s 450 C preferentially terminate at B-type steps, causing them to preferentially retract to the point where a single A-domain surface may be formed. Figure 1 c shows that oxidation-induced etching at Torr and 600 C also leads to clear, preferential reaction of B-type steps. In contrast to sputter etching, however, the formation of long step fingers primarily on B-type steps prevents the formation of a single A-domain surface. We have shown elsewhere 3 that these step fingers are caused by nanometer-sized structures probably small oxide clusters which pin the steps during etching. As the dose becomes higher, etching becomes prevalent at B-type kinks on A-type steps, causing the local step orientation to randomize. At this point steps have a large degree of mixed A- and B-type character, and hence both types of terraces become equally etched. To quantify this preferential etching behavior, we have measured the relative areas of A-type terraces above an A-type step and B-type terraces above a B-type step. A terrace is assigned to be A type B type if its dimer rows run perpendicular parallel to the original 110 sample miscut direction. The ratio of A-terrace areas to B-terrace areas as a function of dose is shown in Fig. 1 d. The A:B ratio is observed to increase from a value near one with increasing dose indicating preferential retraction of B-type steps up to doses in the range of L. Thereafter, the A:B ratio decreased for higher doses as the step directions randomized. We note here that Wurm et al. 10 have used low energy electron microscopy LEEM to study oxidation-induced etching under similar conditions, but on Si 001 samples with much lower miscut angle. On large terraces, they observed preferential etching in a direction parallel to the dimer rows, behavior consistent with the preferential retraction of B-type steps reported here. One difference, however, is that their LEEM measurements did not indicate substantial step pinning at 600 C even at O 2 pressures as high as Torr. We will return to this point later. As discussed in Ref. 3 and originally proposed in Ref. 11, a pinning site during step retraction will first form a long finger, but as step retraction proceeds the end of the finger will eventually break away to form a monolayer-high island. As subsequent steps pass by the pinning site, the island will progressively increase in height, eventually leading to the formation of a three-dimensional etch structure. We have studied this island evolution with oxygen dose. Figure 2 shows a sequence of large-area STM scans after an O 2 dose D ox ranging from 100 to 800 L, at constant pressure Torr and temperature 600 C. Each scan is shown both as a top-view grey scale image and as a three-dimensional perspective. The grey scale images show that for all D ox the islands are situated on a flat, vicinal base surface. Close-up scans of areas between the islands not shown reveal Si dimer rows and surface steps, indicating that the base surface is not covered with a continuous oxide film even up to D ox 800 L. The perspective views show that the islands grow in size with increasing D ox. Note that the vertical scale Å in the perspective views has been greatly expanded relative to the horizontal scale to enhance island visibility. Figure 3 a shows how the island heights vary with D ox. For each island, a peak height was measured relative to a plane fit through the vicinal base surface, and then an average peak height was determined for all the islands in a given scan. The error bars reflect scan-to-scan variations in the measured average peak island height for a given dose. We see from Fig. 3 a that the average island height scales approximately linearly with dose, particularly for D ox 200 L. As will be discussed elsewhere, 6 few new islands were found to nucleate for D ox 200 L; the existing islands simply grew JVST A - Vacuum, Surfaces, and Films

3 774 J. V. Seiple and J. P. Pelz: Evolution of atomic-scale roughening 774 FIG. 2. Top-view gray scale and three-dimensional perspective views of nm 2 topographs, after an O 2 dose of a 100 L, b 200 L, c 400 L, d 800 L, respectively. The vertical scale in the perspective views extends from 20 to 50 Å. All oxidations performed at T s 600 C and P ox Torr. in size. Consequently, the average island height approached the maximum island height at higher doses, allowing a direct determination of the average vertical etching rate R ML/s for this temperature 600 C and pressure Torr. This in turn implies a sticking coefficient for O 2 reaction on Si 001 under these conditions of s The large uncertainty in s is mainly due to the large uncertainty 50% in our absolute pressure measurements. This value for s is within the range reported by several recent studies, 12,13 but is significantly higher than the value 14 of s reported by Wurm et al. 10 Figure 3 b shows how the measured rms surface roughness varies with dose. At each dose, was measured for several 500 nm 500 nm scans the data shown in Fig. 2 were cut from these larger scans, and the error bars reflect FIG. 3. a Plot of the average island height vs O 2 dose. b Plot of the rms surface roughness vs O 2 dose. measured scan-to-scan variations. For our oxidation conditions, we see that scales roughly linearly 15 with dose, increasing from a value of 0.4 Å at zero dose due mainly to surface steps to a value of 15ÅatD ox 800 L. IV. DISCUSSION Smith and Ghidini 2 have previously reported that micrometer-sized conical structures could form at high temperatures 1000 C and high dose, provided the sample temperature and O 2 pressure were within a narrow transition region between the active and passive oxidation regimes. They postulated these structures to be Si cones covered with a thin SiO 2 layer, formed from a tiny oxide nucleus which grew down the sides of the cone as the surrounding surface was etched away. We propose that the nanometer-sized conical islands observed in our experiments may in fact represent an early stage of the larger structures reported by Smith and Ghidini. 2 The smaller structures in our case are formed in a corresponding transition region between passive and active oxidation regimes, but at much lower temperature and O 2 pressure. In support of this connection, we note that the STM tunnel current often became unstable at low tunnel voltages when the tip was located over an island, but could generally be stabilized by raising the tunnel voltage above a few volts. This is consistent with the expected behavior if a thin layer of SiO 2 covers the island. We do note, however, that the overall tip-to-width aspect ratio of the cones observed here is substantially smaller than those observed by Smith and Ghidini. This may be because the intrinsic shape of the islands is a strong function of oxidation temperature, because they have not grown large enough to reach their limiting shape, or even because the finite size of the STM tip causes the islands to appear wider than they actually are. It is also possible, J. Vac. Sci. Technol. A, Vol. 13, No. 3, May/Jun 1995

4 775 J. V. Seiple and J. P. Pelz: Evolution of atomic-scale roughening 775 FIG. 5. Plot of rms surface roughness vs P ox for a 63 L dose at T s 600 C. The square symbol is initial rms roughness. STM scans used in this data series were originally presented in Ref. 3. FIG. 4. Temperature pressure parameter space showing different oxidation regimes. The dark solid line from Ref. 1 and the dark dashed line from Ref. 2 represent a critical line which separates the passive and active oxidation regimes. The dotted line shows the conditions which produce maximum roughening during a 63 L O 2 dose and is located well above the critical line. however, that the different shape is due to a completely different physical origin. More work is currently being done to explore these issues. The measurements reported here and recent STM measurements reported elsewhere 3,4,8,16 also have interesting implications concerning the critical conditions for SiO 2 growth, 1,2 shown in Fig. 4. Earlier studies done mainly at higher temperatures and with lower spatial resolution 2 indicated a very sharp boundary between the active and passive oxidation regimes, with large-scale surface roughening only occurring over a narrow transition region a few degrees wide. 2 The more recent atomic resolution studies done at lower temperatures 3,4,8,16 all indicate that a great deal of surface etching can occur up to O 2 pressures several orders of magnitude above the commonly accepted critical line for passive oxidation, at the same time that oxide clusters nucleate and grow. 3,4,16 Consequently, the transition region in which significant atomic-scale surface roughening occurs may actually be much broader than would be expected from the prior measurements. In fact, a recent transmission electron microscopy study has identified a roughening regime under similar conditions on Si 111 which at P ox Torr is as wide as 125 C. 17 Although Ross, Gibson, and Tweston 17 did not quantify the roughness, they did attribute its origin to simultaneous oxide nucleation and etching resulting in a micromasking process. As a practical matter for device processing, it would be useful to know the range of conditions which produce significant surface roughening on Si 001 and the degree and type of roughness thus produced. To explore surface roughening within this transition region, we have made measurements of how the rms surface roughness depends on O 2 pressure and temperature. This is done keeping the total O 2 dose constant to ensure that the same amount of oxygen reacts with the surface at each pressure. Figure 5 shows results for a 63 L O 2 dose at 600 C. At zero dose the measured rms roughness is 0.4 Å, due mostly to regular steps on the clean vicinal surface. As the O 2 pressure is increased, we see that first increases, reaches a maximum near P ox Torr, and thereafter decreases. For P ox Torr, STM images invariably show a very smooth, cloudy topography, consistent with a very thin, smooth layer of surface oxide. At these higher pressures, a continuous surface oxide forms so quickly that the surface has little chance to etch and roughen. Preliminary measurements indicate that at 700 C, a 63 L dose produced maximum roughening at or above P ox Torr. These two points can be used to draw an approximate line of maximum roughening for a 63 L dose, as shown in Fig. 4. The position of this line and the peak value of the surface roughness will of course depend on the oxygen dose. The main point here is that substantial atomic-scale surface roughening can occur for conditions well above the accepted critical line. If one is designing a processing sequence for devices which require atomically smooth interfaces, it is important to avoid these roughening conditions during transient processing steps, 5 i.e., as a substrate is heated or a chamber is vented, etc. Finally, we again note that Wurm et al. 10 have recently used LEEM to study oxidation on Si 001 under similar conditions as the present study P ox Torr, T s 600 C, and D ox 100 L, but did not report significant step pinning or island nucleation. A possibility is that the lower lateral resolution of the LEEM apparatus 15 nm may have missed the smaller islands we observed for D ox 100 L see Fig. 2 a. Recent STM work 18 by this group at these same conditions did in fact show pinning similar to that observed in this study, but at a much lower density. We also note that Wurm et al. 10 observed a sticking coefficient that was 8 times smaller than ours. 14 A possible cause for this difference in the sticking coefficient may be different amounts of background water vapor which can greatly enhance silicon oxidation 17 or a difference in O 2 ionization levels. Whatever the case, it is important to note that the pinning density should scale as the square of the sticking coefficient at a given temperature, pressure, and dose as indicated in the model from Ref. 3. This follows because the model predicts that the oxide cluster nucleation rate varies quadratically with the rate that oxygen actually reacts with the surface, JVST A - Vacuum, Surfaces, and Films

5 776 J. V. Seiple and J. P. Pelz: Evolution of atomic-scale roughening 776 which is proportional to the product s times P ox. Hence, the lower sticking coefficient in the experiments of Wurm et al. should result in a factor of 64 lower pinning density and a factor of 8 less etching than observed in our study for oxidation at the same temperature, pressure, and dose. V. SUMMARY When vicinal Si 001 surfaces are exposed to O 2 at T s 600 C and P ox Torr, we observe the preferential retraction of B-type surface steps at low dose 50 L, but complete single A-domain formation is prevented by the pinning of steps by nucleated oxide clusters. At higher doses, these pinning sites produce three-dimensional conical etch structures, which may be early stages of the micrometersized structures observed previously 2 at higher temperature, pressure, and dose. By measuring the evolution of these structures with dose, we estimated surface etching rates and a value of the O 2 sticking coefficient s at 600 C. We also point out that oxygen exposure at these lower temperatures can cause significant atomic-scale surface roughening even up to O 2 pressures several orders of magnitude above the commonly accepted critical line for SiO 2 growth. ACKNOWLEDGMENTS The authors would like to acknowledge helpful discussions with G. Rubloff and F. Smith. This work was supported by National Science Foundation Grant No. DMR , PRF Grant No G5, and The Ohio State University Center for Materials Research. 1 J. J. Lander and J. Morrison, J. Appl. Phys. 33, F. W. Smith and G. Ghidini, J. Electrochem. Soc. 129, J. V. Seiple and J. P. Pelz, Phys. Rev. Lett. 73, A. Feltz, U. Memmert, and R. J. Behm, Surf. Sci. 314, M. Offenberg, M. Liehr, and G. W. Rubloff, J. Vac. Sci. Technol. A 9, J. V. Seiple and J. P. Pelz unpublished. 7 P. Bedrossian and T. Klitsner, Phys. Rev. Lett. 68, J. Seiple, J. Pecquet, Z. Meng, and J. P. Pelz, J. Vac. Sci. Technol. A 11, D. J. Chadi, Phys. Rev. Lett. 59, K. Wurm et al., Phys. Rev. B 50, P. B. Price, D. A. Vermilyea, and M. B. Webb, Acta Metall. 6, U. Memmert and M. L. Yu, Surf. Sci. Lett. 245, L J. R. Engstrom, D. J. Bonser, M. M. Nelson, and T. Engel, Surf. Sci. 256, Wurm et al. report a reaction coefficient, r , where r the number of SiO molecules desorbed/the number of O 2 molecules incident, whereas we define s the number of O 2 molecules that etch the surface/ the number of O 2 molecules incident. Thus r 2s, so our sticking coefficient is 8 times greater than theirs see Discussion Sec. IV. 15 We do not fully understand why does not scale as a higher power of the dose, since the average height and base diameter of the islands both clearly increase with dose. Part of the reason is related to the particular number and size of the islands found in our experiments. If one models the surface as a concentration n of conical islands each of height h and base diameter D located on an otherwise flat surface, then 2 z 2 z 2 nh 2 D 2 /24) [1 ( nd 2 /6. This turns out to be approximately independent of D when D D 0 (3/ n) 1/2. In our experiments, n cm 2 for dose 200 L which implies D 0 30 nm, as compared with measured values of nm. It is also possible that subtle changes in measured island shape possibly due to the STM tip may be a factor. 16 F. Donig et al., J. Vac. Sci. Technol. B 11, F. M. Ross, J. M. Gibson, and R. D. Twesten, Surf. Sci. 310, Y. Hong, K. Wurm, Y. Wei, and I. S. T. Tsong private communication. J. Vac. Sci. Technol. A, Vol. 13, No. 3, May/Jun 1995