X-ray photoelectron study of the reactive ion etching of Si x Ge 1 x alloys in SF 6 plasmas

Transcription

1 X-ray photoelectron study of the reactive ion etching of Si x Ge 1 x alloys in SF 6 plasmas M. C. Peignon, Ch. Cardinaud, and G. Turban Laboratoire des Plasmas et Couches Minces, Institut des Matériaux de Nantes, UMR 110-CNRS, Nantes, France C. Charles a) and R. W. Boswell Plasma Research Laboratory, Research School of Physical Sciences and Engineering, Australian National University, ACT 0200, Australia Received 8 February 1995; accepted 21 October 1995 Reactive ion etching of Si x Ge 1 x alloys 0 x 100% deposited on silicon wafers using the electron-beam evaporation technique was investigated in a low pressure fluorine-based plasma SF 6. The etch rates of the Si x Ge 1 x alloys increase with the Ge content of the alloy but are constantly lower than the etch rates expected from independent etching mechanisms of Ge and Si atoms in the alloys. Analysis of the reactive layer on the surface of the alloys before and after etching was performed in situ by x-ray photoelectron spectroscopy XPS. The reactive layer on the etched alloys consists of silicon, germanium, fluorine, and sulfur atoms. Fluorinated Ge GeF n and Si SiF n species, known to be precursors in the formation of the GeF 4 and SiF 4 etch products, are identified. A precise analysis of the sulfides of Ge GeS n and of Si SiS n present on the surface revealed a strong difference between sulfur germanium and sulfur silicon interactions by the detection of mostly GeS 2 and SiS species. A sulfur-rich reactive layer is measured on the surface of the germanium-rich alloys, whereas a fluorine-rich layer exists on the surface of the silicon-rich alloys. Still, the sulfur-rich reactive layer exists over a large range of alloy stoichiometry 0 x 80% with an increase of GeS 2 density as a function of Si content in the alloy. The XPS analysis showed no enrichment of Ge or Si in the reactive layer compared to the substrate, suggesting equal etch rates of Ge and Si atoms of the alloy American Vacuum Society. I. INTRODUCTION For the past few years, silicon germanium SiGe alloys have been of increasing interest in various application fields because of their electronic and optical properties. Several novel device structures have already been demonstrated; these include the heterojunction bipolar transistor using a SiGe base layer, 1,2 optoelectronic devices, 3,4 and, recently, solar cells. 5 Dry etching of SiGe is one of the basic steps in the fabrication of SiGe-based devices. Reactive ion etching RIE allows fine line pattern reproduction with a large degree of anisotropy possible. 6 For many applications, e.g., heterojunction bipolar transistors or optical waveguides, selective etching of SiGe over Si and vice versa is also required. Etching of elemental Ge and Si has been previously reported, and showed that Ge is etched faster than Si in SF 6 and CF 4 plasmas. 7,8 However, it has been shown that Si can be etched selectively with respect to Ge in a SF 6 /H 2 plasma by forming an involatile germanium sulfide compound on the Ge substrate. 9,10 Etching of Si x Ge 1 x alloys was recently investigated for x 70%, 8,11,12 showing the importance of the plasma and surface chemistry on the etching mechanism. Still, little is known about the dry etching behavior of SiGe alloys and how it differs from that of Si and Ge. In this article, we report on the reactive ion etching of Si x Ge 1 x 0 x 100% alloys in a SF 6 plasma. The aim of a Permanent address: LPCM-Nantes, France. the present work is to determine the respective roles of fluorine and sulfur atoms during SiGe etching and, in particular, the effect of sulfur on SiGe over Si selectivity. In situ x-ray photoelectron spectroscopy is used to characterize the etching of the Si x Ge 1 x alloys as it allows accurate analysis of the etched surface. II. EXPERIMENTAL APPARATUS The experimental apparatus has been described in previous publications; 13,14 it consists of an ultrahigh vacuum UHV surface analysis chamber connected to a RIE diode reactor. The diameter of the MHz powered aluminum electrode and top electrode is 15 cm. The bottom electrode that supports the samples is water cooled at 18 C. Temperature measurements using a fluoroptic probe have shown that the wafer temperature can reach 100 C under the conditions used in our experiments 15 as a result of the poor thermal contact between the back of the wafer and the water-cooled electrode. The etching experiments were carried out using a SF 6 plasma at 100 mtorr and 30 sccm pressure and flow rate conditions, respectively. The constant applied power of 50 W leads to a self-bias voltage on the sample electrode of about 25 V. After a pre-etching sequence of 10 min with a 4 in. silicon wafer placed on the powered electrode, the SiGe sample of about 6 cm 2 in area is placed on the silicon wafer of about 78 cm 2 in area and etched for 1 min in the SF 6 plasma. The effect of the silicon wafer is to decrease the concentration of 156 J. Vac. Sci. Technol. A 14(1), Jan/Feb /96/14(1)/156/9/$ American Vacuum Society 156

2 157 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 157 atomic fluorine in the gas phase and to increase the atomic sulfur concentration as a consequence. Optical emission measurements in the nm range have shown the presence of S 2 molecules, and SF 2,S 2 F, and S 2 F 2 species have been previously detected by mass spectrometry for the plasma conditions used in this article. 16,17 Hence, due to the small Si x Ge 1 x sample area compared to that of the silicon wafer, the chemical composition of the plasma can be considered constant and independent of the alloy composition. After the etching operation, the sample is transferred through a high vacuum buffer chamber to the analysis chamber pressure of a few 10 9 Torr where x-ray photoelectron spectroscopy XPS measurements are performed using a Leybold Heraeus LHS-12 system. Photoemission spectra were obtained using a nonmonochromatized Al K radiation ev and the total resolution was about 1 ev. The time between the end of the etching operation and the start of the XPS data acquisition was less than 10 min and contamination of the sample surface during the vacuum transfer is negligible since no oxygen or carbon atoms were detected on the surface of the etched samples. As shown by Oehrlein in a similar system, 18 the surface layer after etching is the same as that during etching. The Si x Ge 1 x amorphous alloys of about 250 nm in thickness were deposited at room temperature on rotating 4 in. silicon wafers at the Royal Melbourne Institute of Technology RMIT, Australia, by using a dual electron-beam evaporation system Balzers. A series of nine Si x Ge 1 x alloys of variable stoichiometry x varying from 0% to 100% were analyzed by XPS. The etch rate was determined by using a profilometer Alphastep Tencor. The stoichiometry, given by RMIT, was checked by XPS analysis and by x-ray microanalysis before etching. III. EXPERIMENTAL RESULTS AND DISCUSSION A. Etch rate measurements The SiGe samples of area about 6 cm 2 were etched for 1 min using an SF 6 plasma operating at 50 W, 100 mtorr, and 30 sccm rf power, pressure, and flow rate conditions, respectively. The etched thickness was measured ex situ after the etching operation by using a profilometer Alphastep Tencor. The etch rate ER is shown in Fig. 1 as a function of the silicon content x in the Si x Ge 1 x alloy. As the atomic density is very similar for crystalline Ge and Si samples at. cm 3 and at. cm 3, respectively, the etch rate in nm/min is equivalent to the etch rate in atoms/ min. The highest ER is obtained for the pure germanium sample 120 nm/min. The ER decreases monotonically with the silicon content in the alloy, with the values constantly above the ER of pure silicon 75 nm/min, the latter being about half that of pure germanium. A higher ER for germanium Ge compared to silicon Si was measured previously in various plasmas 7,8,11,12,19 CF 4,CF 2 Cl 2,.... However, different ER evolutions of Si x Ge 1 x x 70% alloys as a function of alloy composition have been reported in the literature. 8,11,12,19 FIG. 1. Measured solid line and calculated dotted lines reactive ion etch rates of SiGe alloys in a SF 6 plasma operating at 50 W, 100 mtorr, and 30 sccm rf power, pressure, and flow rate conditions, respectively, as a function of the alloy composition. Curves a, b, and c correspond to the calculated ERs obtained from three distinct etch models developed in Sec. III A Eqs. 1, 2, and 3, respectively. GeF 4 and SiF 4 fluorides are the only stable etch products detected by mass spectrometry when etching Ge and Si in a fluorine plasma, respectively. 20,21 The GeF 4 etch product is less volatile than the SiF 4 etch product b.p. GeF4 37 C and b.p. SiF4 86 C and its formation is less favorable than that of SiF 4 22 H GeF kcal/mol; H SiF kcal/mol. Thus thermodynamical data do not explain the experimental data for pure Si and Ge. Below it will be shown that the nature of various bonds at the alloy surface cannot account for the experimental ER results second model. This suggests that the higher ER for pure Ge compared to that for pure Si is not essentially due to the lower Ge Ge bond energy 2.77 ev compared to that of Si Si bonds 3.38 ev. 22 Hence, a possible explanation in favor of our measured ERs could be that Ge exhibits a lower activation energy for the formation of fluorine containing products compared to Si. Three simple models were developed to obtain some insight into the alloy etching mechanisms and the results are shown in Fig. 1 curves a, b, and c. Curve a shows the expected ER of the alloys assuming independent etching mechanisms for both elements in the alloy ER SiGe x xer Si 1 x ER Ge, where ER Si and ER Ge are the ERs of pure silicon and germanium, respectively. The ERs calculated from this first model are constantly higher than the measured ERs, suggesting that the alloy etching does not result from a simple association of the etching mechanisms corresponding to each element constituting the alloy. We note that different results have been reported in the 1 JVST A - Vacuum, Surfaces, and Films

3 158 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 158 literature. 8,11,19 A higher ER of Si x Ge 1 x x 70% alloys compared to those deduced from the first model was measured in various RIE plasmas. To investigate germanium silicon interaction during the etching operation, the second and third models both of which include the presence of Si Ge bonds in the alloy were made. In an amorphous Si x Ge 1 x alloy, the fraction of Si atoms bound to Si is statistically x 2 and that of Si atoms bound to Ge is x(1 x). In the same way, 1 x 2 atoms of Ge are bound to Ge and x(1 x) are bound to Si. In the second model curve b in Fig. 1, the etch rates of the Ge and Si atoms are assumed to be inversely proportional to the bond energy 22 E bsi Si 3.38 ev, E bsi Ge 3.12 ev, and E bge Ge 2.77 ev. For example, the etch rate of the x 2 Si atoms is that of the pure silicon sample ER Si, whereas the ER of the x(1 x) Si atoms is 3.38/3.12 ER Si. Hence the ER of the Si x Ge 1 x alloys is given by ER SiGe x x 2 ER Si x 1 x ER Si 1 x 2 ER Ge x 1 x ER Ge. 2 As shown by curve b in Fig. 1, the values obtained from this second model, though slightly improved compared to those on curve a, are still a lot higher than the measured values. The measurements show that silicon rich alloys x 60% exhibit a constant etch rate equal to that of pure silicon though the second model would have induced an increase of the alloy ER as a result of the large presence of Si Ge bonds of lower energy than Si Si bonds in that range. The difference between the calculated and measured ER suggests that the energy of the various bonds present in the alloy is not the dominant etch parameter. In the third model curve c in Fig. 1, the ER of the SiGe groups is assumed to be limited by that of the silicon. Hence, germanium and silicon atoms bound to silicon are etched at ER Si, whereas germanium atoms bound to germanium are etched at ER Ge. The alloys ERs are obtained as follows: ER SiGe x x 2 x ER Si 1 x 2 ER Ge. 3 Curve c is much closer to the experiment than curve b. However, the values calculated with the third model are still greater than the measured data, in particular for the Ge rich alloys. Hence, the measured ER cannot be explained only by the limitation of the ER of Ge atoms bound to Si. As the discrepancy mainly concerns the Ge rich alloys, it suggests that the etching of the 1 x 2 Ge atoms bound to Ge is also lowered. Thus a complex interaction between silicon and germanium atoms seems to exist during the etching of the alloy. B. Surface analysis Normal XPS surface measurements of the Si x Ge 1 x alloys etched in our SF 6 plasma have shown that sulfur and fluorine atoms are present on the etched surface. Hence, a detailed analysis of the silicon Si 2p, germanium Ge 3d, FIG. 2. Schematic of the bilayer surface structure used for the XPS analysis. fluorine F 1s, and sulfur S 2s distributions was made for the whole range of alloy composition x from 0% to 100%. We assume that the sample surface can be modeled by the presence of a homogeneous reactive layer of thickness z on top of the substrate layer see Fig. 2. For each type of atom A detected on the sample surface, the atomic density N A can be derived from the corresponding XPS distribution intensity I A using the following equations: 23 For atoms A in the reactive layer, I Arl I Arl 1 exp z/ A Cf A N Arl 1 exp z/ A 4 and for atoms in the Si x Ge 1 x substrate, I Asub I Asub exp z/ A Cf A N Asub exp z/ A, 5 where C is a constant related to the apparatus setup, f A is the sensitivity factor corresponding to the photoelectron emitted from atom A, A is the inelastic mean free path for the photoelectron emitted from atom A through the reactive layer, and z is the thickness of the reactive layer. The determination of z is described in Sec. III B 5. The inelastic mean free path for the Al K excited Si 2p and Ge 3d photoelectrons emitted from pure Si and Ge samples, respectively, was obtained by using the data given by Tanuma et al.: 24 Si 2p 2.77 nm and Ge 3d 2.74 nm. The values obtained for both types of photoelectrons are very similar. Moreover, identical values have been calculated for the same photoelectrons in silicon and germanium oxides by using the same data, suggesting that the effect of the reactive layer composition can be neglected on the first approximation for the determination of the mean free path. Hence, a constant mean free path of 2.77 nm through the reactive layer was assumed for both Si 2p and Ge 3d photoelectrons in our Si x Ge 1 x alloys. The results obtained from the XPS analysis and presented in this article correspond to the atomic densities of the various elements in the reactive and substrate layers. 1. S 2s distribution TheS2sdistribution is shown in Fig. 3 for the Si 16 Ge 84 and Si 76 Ge 24 samples. A single component at binding energy about ev is observed, which can be attributed to S Ge or S Si bonds in GeS 25 n or SiS 26 n compounds, respectively. These two types of bonds present similar binding energy and cannot be distinguished. The distributions in Fig. 3 do not reveal any S S bonds corresponding to S n species ev or S F bonds corresponding to SF n species ev. The sulfur atomic density S is reported in Fig. 4 J. Vac. Sci. Technol. A, Vol. 14, No. 1, Jan/Feb 1996

4 159 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 159 FIG. 3. Sulfur S 2s photoemission spectrum obtained for the Si 16 Ge 84 and Si 76 Ge samples etched for 1 min with a silicon cathode same plasma conditions as in Fig. 1. for the series of alloys. S decreases when the Si content increases in the alloy. The sulfur density on the pure Ge sample is three times higher than that on the Si sample. As the silicon sulfur and the germanium sulfur compounds are not volatile in our experimental conditions 22 T subl. SiS C and T subl. GeS2 600 C, this suggests a larger reactivity between sulfur and germanium atoms compared to that between sulfur and silicon atoms. 2. F 1s distribution TheF1sdistributions are shown in Fig. 5 a for the Ge, Si 34 Ge 66, and Si 76 Ge 24 samples. The fluorine distribution of the etched germanium sample x 0% exhibits two distinct components at binding energy about and 687 ev, respectively. The first component corresponds to F Ge bonds FIG. 5. a Fluorine F 1s photoemission spectrum obtained for the Ge, Si 34 Ge 66 and Si 76 Ge samples same etching conditions as in Fig. 3. b Evolution of the GeF n F Ge at ev and SiF n F Si at ev densities obtained from the deconvolution of the fluorine F 1s distribution as a function of the alloy composition. Dotted lines represent the expected GeF n a and SiF n b densities assuming they are proportional to the Ge and Si atomic densities in the alloy, respectively. FIG. 4. Sulfur and fluorine atomic densities in the reactive layer obtained from the S 2s andf1sdistributions as a function of the alloy composition. in GeF n species. 25 As no S F bonds have been previously detected in the S 2s distribution and no oxygen bonds are present on the O 1s distribution, the second peak at energy about 687 ev could be related to F Ge bonds in traces of the GeF 4 etch product. The presence of the WF 6 etch product on etched surfaces has been previously observed when etching tungsten in RIE SF 6 /O 2 plasmas. 27 Still, the F 1s distribution of the pure Si sample does not reveal F Si bonds corresponding to the SiF 4 etch product. This could result from the lower volatility at room temperature of GeF 4 b.p. 37 C compared to SiF 4 b.p. 86 C. For 20 x 80%, the distinction between the F Ge and F Si bonds can be made and the F 1s distribution is decomposed in two components corresponding to F Ge bonds in GeF n species at binding energy of about ev and F Si bonds in SiF n species at binding energy of about ev. 25 For silicon rich alloys x 80%, only one component is JVST A - Vacuum, Surfaces, and Films

5 160 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 160 observed in the F 1s distribution which corresponds to F Si bonds in SiF n species. Although not detected, F Ge bonds could be present at the surface of these etched alloys and be masked by the very intense F Si component as a result of the large silicon content. The total density of fluorine atoms on the etched surface is plotted in Fig. 4 for increasing silicon content x in the alloy. Comparison between the fluorine and sulfur density in the reactive layer shows that the fluorine density is essentially independent of the alloy composition and is much lower than the sulfur density for most of the alloy 0 x 80%. The respective density of the F Ge and F Si bonds obtained from the decomposition of the F 1s distribution is shown in Fig. 5 b as a function of the silicon content x in the Si x Ge 1 x alloy. Lines a and b, respectively, in Fig. 5 b correspond to the densities of GeF n and SiF n species which would be expected on the surface if the alloy fluorine interaction was linearly related to the atomic density of the elements constituting the alloy. Figure 5 b shows that the measured F Ge density strongly decreases when x is increased and is constantly lower than that given by line a. On the contrary, the measured F Si density increases when x is increased and is constantly higher than that given by line b. These results show that the density of the silicon fluorides is higher on the alloy surface compared to that on the pure Si sample respectively to the Si content in the alloy, whereas the density of germanium fluorides is lower on the alloy surface compared to that on the pure germanium sample. FIG. 6. a Silicon Si 2p photoemission spectrum obtained for the Si 76 Ge 24 sample same etching conditions as in Fig. 3. The data were fitted using a Gaussian Lorentzian decomposition procedure with four components at 99.2, 100.3, 101.4, and ev. b Evolution of the density corresponding to the four components Si sub, SiI,SiII, and Si III obtained from the decomposition of the silicon Si 2p distribution as a function of the alloy composition. 3. Si 2p distribution Figures 6 a and 6 b show, respectively, the Si 2p distribution of the etched Si 76 Ge 24 sample and the results obtained from its decomposition. The main peak at energy about 99.2 ev in Fig. 6 a corresponds to silicon atoms bound to silicon or germanium atoms in the Si x Ge 1 x substrate. The position of this peak called Si sub is independent of the alloy composition. Figure 6 a also shows an asymmetry of the Si 2p distribution at high energy which can be attributed to the presence of the reactive layer on top of the substrate layer see Fig. 2 : the presence of SiF n n 4 and SiS n n 2 species resulting from the interaction between the silicon atoms and the SF 6 plasma induces components at energies higher than 99.2 ev. An energy difference of ev has been reported between two consecutive fluorination states of the silicon. 28 A chemical shift of about 1 ev per attached sulfur has been estimated for the Si 2p core level by using the difference in electronegativity between the silicon and the sulfur atoms. A linear evolution of the chemical shift versus the difference in electronegativity was assumed using the values reported in the literature for Si O and Si F bonds. As distinction in energy between the SiF and SiS species cannot be made, we have decomposed the Si 2p spectra using a Gaussian Lorentzian fitting procedure and have fixed a constant binding energy difference of 1.1 ev between all the components. We note that similar results were obtained when using an energy difference of 1 or 1.15 ev. The first component Si I, shifted from the Si sub component by 1.1 ev, corresponds to the SiF and SiS species; the second component Si II, shifted from the Si sub component by 2.2 ev, corresponds to the SiF 2 and SiS 2 species. The third component Si III, shifted from the Si sub component by 3.3 ev, is exclusively attributed to the SiF 3 species as the corresponding sulfuration state of silicon does not exist. Finally, no component corresponding to the SiF 4 species was observed on the Si 2p distribution. The atomic density derived from each component intensity is plotted in Fig. 6 b as a function of the silicon content x in the alloy. The Si sub density increases linearly with the silicon content x in the alloy. A similar evolution in the density is observed for the first and most important Si I component. The density of the Si II and Si III states is very low in all cases, suggesting that the SiF 2, SiS 2, and SiF 3 species are J. Vac. Sci. Technol. A, Vol. 14, No. 1, Jan/Feb 1996

6 161 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 161 not strongly present on the sample surface. Hence, the main species remaining on the surface after the interaction between the plasma and the silicon atoms present in the alloy are the SiS and SiF Si I component in the Si 2p distribution in Fig. 6 a. 4. Ge 3d distribution Figure 7 a shows a typical Ge 3d distribution and its decomposition. As similar energy shift for the Si 2p and Ge 3d photoelectrons have been reported for silicon and germanium atoms in various compounds 23 Na 2 SiF 6 and Na 2 GeF 6, SiO 2 and GeO 2, the Ge 3d distribution was decomposed by using the same constant energy difference of 1.1 ev between the successive fluorination and sulfuration states. The larger component at 29.2 ev corresponds to Ge Ge and Ge Si bonds in the alloy and is noted as Ge sub. The Ge I 30.3 ev, Ge II 31.4 ev, and Ge III 32.5 ev components of the Ge 3d distribution can be attributed to the GeF or GeS species, GeF 2 or GeS 2 species, and GeF 3 species, respectively. The density derived from each component intensity is plotted in Fig. 7 b as a function of the silicon content x in the alloy. The Ge sub density decreases linearly with the silicon content x i.e., increases linearly with the Ge content in the alloy. A decrease in the density is also observed for the Ge I component. The density of the Ge II state is large in all cases and decreases with the Ge content in the alloy. The Ge III component is very small and the Ge IV component is not detected. A similar evolution to that measured for the Ge I component has been previously measured when analyzing thef1sdistribution: Fig. 5 b has shown an equivalent decrease of the density of F Ge bonds with a Ge decrease in the alloy. This would indicate that the Ge I component essentially corresponds to the presence of GeF species on the surface. Also, the total sulfur density previously shown in Fig. 4 presents the same evolution as that of the Ge II component, suggesting that the germanium atoms are bound to the sulfur atoms to form GeS 2 species on the surface of the etched samples. The Ge 2p 3/2 distribution ev range was also decomposed for each sample, showing the presence of four components with similar evolutions as those obtained from the Ge 3d distributions. In summary, the results respectively obtained from the Si 2p and Ge 3d distribution analysis show that the GeS 2 germanium sulfide is largely present on the surface while the SiS 2 silicon sulfide can be neglected. The ratio between the total density of sulfur atoms and the density corresponding to the second germanium component S / Ge II is about 2 for the pure germanium sample. This ratio is essentially constant for x 50%, suggesting the absence of interaction between the sulfur and silicon atoms in that range of alloy composition. Still, for x 50%, the enhancement of the S / Ge II ratio when x is increased suggests that part of the sulfur atoms on the surface is bound to silicon atoms forming the SiS species. The Ge II / Ge rl ratio with Ge rl Ge I Ge II Ge III, which corresponds to the coverage rate of germanium in the FIG. 7. a Germanium Ge 3d photoemission spectrum obtained for the Si 16 Ge 84 sample same etching conditions as in Fig. 3. The data were fitted using a Gaussian Lorentzian decomposition procedure with four components at 29.2, 30.3, 31.4, and 32.5 ev. b Evolution of the density corresponding to the four components Ge sub, GeI,GeII, and Ge III obtained from the decomposition of the germanium Ge 3d distribution as a function of the alloy composition. c Variation of the Ge II / Ge rl ratio as a function of the alloy composition. JVST A - Vacuum, Surfaces, and Films

7 162 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 162 FIG. 8. a Relative composition of the reactive layer see Fig. 2 on the etched SiGe samples estimated from the Si 2p, Ge 3d,F 1s,and S 2s distributions Si%, Ge%, S%, and F%. b Evolution of the reactive layer thickness see Fig. 2 as a function of the alloy composition. These values are derived from the decomposition of the Si 2p and Ge 3d distribution and take into account the relative composition of the reactive layer. reactive layer by sulfur atoms, was obtained from the Ge 3d decompositions. As shown in Fig. 7 c, this ratio increases when x is increased, showing that the formation of germanium sulfide is enhanced when silicon atoms are incorporated in the alloy. 5. Stoichiometry and thickness of the reactive layer The composition of the reactive layer was estimated from the analysis of the Si 2p, Ge3d,F1s,andS2sdistributions. The atomic percentage corresponding to each type of atom A is given by N A % N A i A N i, where N A is the atomic density corresponding to the atom A in the reactive layer. The results are shown in Fig. 8 a. For 6 the pure germanium and silicon samples, the respective Ge and Si atomic percentage in the reactive layer is about 50%. The second half of the atoms corresponds to fluorine and sulfur atoms and strongly differs for both pure samples. A majority of sulfur atoms are detected on the germanium sample S / F 2 while a majority of fluorine atoms are detected on the silicon sample F / S 1.5. The higher percentage of sulfur on the germanium sample compared to the silicon is likely related to the presence of GeS 2 species on the surface. The percentage of Ge and Si atoms in the reactive layer linearly varies with the respective Ge and Si contents in the alloy, as shown by Fig. 8 a. We note that the sum of the Si and Ge percentages is constant and equal to 50% for the whole range of alloys. The evolution of the fluorine and sulfur percentages does not vary linearly between the end values corresponding to the pure Ge and Si samples. For x 80%, the percentage of sulfur atoms does not vary much and is always greater than the percentage of fluorine atoms, suggesting that the surface chemistry when etching Si x Ge 1 x alloys is probably dominated by a stronger affinity of the sulfur atoms for the germanium atoms compared to silicon atoms. Estimation of the reactive layer thickness z was made by using the ratio of Eqs. 4 and 5 I Arl /I Asub I Arl /I Asub 1 exp z/ /exp z/, where I Arl and I Asub are the measured intensities corresponding to the reactive and substrate layers respectively I Arl and I Asub are determined from the Si 2p and Ge 3d decompositions described in Sec. III B 3 and B 4, is an average value /I Asub for the mean free path 2.77 nm, and I Arl is obtained from the ratio between the Si and Ge atomic densities in the reactive layer and in the substrate. This ratio is calculated from the data given in Fig. 8 a. Moreover, as the XPS spectra show that F and S atoms are only bound to Ge or to Si, I Arl takes into account all the species present in the reactive layer. The corresponding results on the thickness of the reactive layer are shown in Fig. 8 b as a function of the alloy composition. We note that the same evolution is obtained when assuming similar Si and Ge atomic densities in I Asub the reactive and substrate layers I Arl but the estimated thickness is nm smaller that what obtained in Fig. 8 b. In both cases the reactive layer thickness increases with the silicon content x and this corresponds to a decrease in the ER, as shown previously in Fig. 1. Hence, a higher ER is related to a thinner reactive layer and this is consistent with results reported in the literature. 20,29 The Si / Ge density ratios in the reactive layer Si rl / Ge rl with Si rl Si I Si II Si III and in the substrate layer Si sub / Ge sub were determined from the decomposition of the respective distributions. These two ratios are similar and equal to the ratio determined from the analysis of the XPS spectra obtained before the etching operation Si sub / Ge sub. Hence, no enrichment is observed in the reactive layer. Opposite results were reported in the literature. Although pure germanium is etched faster than pure silicon, 7 J. Vac. Sci. Technol. A, Vol. 14, No. 1, Jan/Feb 1996

8 163 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 163 Oehrlein et al. have measured a germanium enrichment on the surface of Si x Ge 1 x x 80% alloys etched in fluorine plasma. 30,31 More recent studies by the same authors showed that this enrichment is highly dependent on the ion bombardment on the substrate during etching. 32 Hence, the absence of enrichment in our case could result from our very low ion bombardment 33 E e V dc V p with V dc 25 V and V p 20 V. All three model previously developed in Sec. III A see Fig. 1, in order to investigate the ER evolution versus the alloy composition, would induce a silicon enrichment. For an enrichment not to occur, it is necessary that the Ge and Si atoms should be removed at the same rate during the etching. This implies that i the rate of removal must be independent of the atoms to which they are bound. This supports the previous statement second model in Sec. III A that the energy of the bonds at the surface cannot account for the etching results of the alloy. ii The removal rate of Si atoms for the Si x Ge 1 x alloys must be higher than for pure Si, whereas the etch rate of Ge atoms has to be lowered as compared to pure Ge. C. The role of sulfur in the alloy etching mechanism Surface analysis of the pure Ge and Si samples etched in our SF 6 RIE plasma has shown a higher sulfur concentration on Ge compared to Si. A sulfur rich S / F 2 and a fluorine rich F / S 1.5 reactive layer have been respectively observed on the pure Ge and Si samples. Characterization of the sulfur containing species has revealed a similar plasma surface interaction for the pure Ge and Si samples and for the Si x Ge 1 x alloys, with essentially the presence of GeS 2 and SiS species. The thermodynamic data 22 relative to the formation of GeS 2 H GeS kcal/mol and SiS 2 ( H SiS kcal/mol species are very similar and cannot account for the large presence of GeS 2 species and the absence of SiS 2 species on the surface. Hence the strong interaction between sulfur and germanium probably results from a lower activation energy for the sulfur germanium reactions compared to that for the sulfur silicon reactions. The GeS 2 species are not volatile under our experimental conditions and this likely limits the Ge sites available on the surface during etching, and induces a decrease in the germanium etch rate. Hence, the presence of sulfur in the gas phase is a dominant factor in the respective etch mechanism of Ge and Si. The large affinity between the sulfur and germanium atoms was previously reported by Oehrlein et al. 9,10 They showed that adding hydrogen in a SF 6 plasma induces an increase of sulfur atoms on the germanium surface which is correlated to a decrease in the etch rate. The effect of the sulfur increase in the gas phase does not strongly affect the Si etch rate. In our experiments, a silicon cathode was constantly used. It has been shown that the presence of the Si cathode results in a sulfur rich and fluorine deficient plasma. 16,17 SF 2,S 2 F 2,S 2 F, and S 2 species have been detected in previous experiments carried out in our reactor. Hence this is in favor of a large amount of sulfur germanium bonds on the surface. This suggests that a higher germanium ER and a higher Ge over Si selectivity could be obtained by decreasing the sulfur level in the plasma. Surface analysis of the Si x Ge 1 x alloys has revealed a sulfur rich and fluorine poor reactive layer on a large range of alloy stoichiometry 0 x 80%. Still, no enrichment in silicon has been observed in the alloy reactive layer, though the ER of pure Si is nearly half that of pure Ge. This suggests an important role of the sulfur in the alloy etching mechanism. Fig. 7 c shows the variation of the Ge sites occupied by sulfur atoms Ge II / Ge rl as a function of the alloy composition. The favored sulfur germanium interaction compared to sulfur silicon for each stoichiometry induces a decrease in the fluorine germanium interaction, hence a decrease in the Ge atoms ER. As a consequence, the fluorine silicon interaction is probably enhanced, inducing a higher ER of the Si atoms in the alloy compared to that in the pure Si sample. The etch mechanism of the Si x Ge 1 x alloys in our sulfur rich and fluorine deficient plasma is dominated by the higher reactivity between the Ge and S atoms compared to Si and S atoms and results in a particular equilibrium in the plasma surface interaction which allows the simultaneous removal of the Si and Ge atoms from the surface for each alloy composition, so that no enrichment in any element constituting the alloy occurs during etching. The three models developed in Sec. III A cannot account for the experimental ERs and for the absence of enrichment observed at the surface. On the other hand, the XPS characterization of the reactive layer has shown how complex is the surface chemistry during the plasma/alloy interaction. We are currently developing a more realistic etch model of the alloy which includes the role of the sulfur atoms that accounts for the same etch rate of the Si and Ge atoms during the etching of the Si x Ge 1 x alloy. IV. CONCLUSION For the reactive ion etching conditions used in this study, the results obtained from the XPS analysis are consistent with the measured etch rates of the alloys. Sulfur and fluorine atoms were detected on the surface of the etched samples and analysis of this reactive layer showed the presence of F Si SiF n and F Ge GeF n bonds, known as precursors for the formation of the SiF 4 and GeF 4 volatile etch products. The sulfur content at the surface increases with the Ge content in the alloys. A detailed analysis of the XPS spectra was made by deconvoluting the various detected peaks. The results show that bonding between sulfur and germanium atoms corresponds to the GeS 2 species while SiS species are responsible for the interaction between the sulfur and the silicon atoms. Moreover, the deconvolution of the Ge 3d distribution showed that the ratio between the germanium coverage rate by sulfur atoms increases with the Si content in the Si x Ge 1 x alloys. No silicon enrichment was observed at the surface though the Si etch rate is lower than the Ge etch rate and the density of Si F bonds was found to increase linearly with the Si content. Hence, the nonlinear evolution of the alloys etch rates was correlated to a limitation of the JVST A - Vacuum, Surfaces, and Films

9 164 Peignon et al.: XPS study of RIE of Si x Ge 1 x alloys 164 Ge etching in the alloy due to the formation of GeS 2 at the surface during the etching process, and to a simultaneous removal of the Si and Ge atoms from the surface of the alloy. ACKNOWLEDGMENTS This research was partially funded by and carried out on behalf of the Harry Tribugoff AM Research Syndicate. The visits of Dr. M. C. Peignon in Australia and of Dr. C. Charles in France were financially supported by the French Australian PICS No. 110 program Programme International de Coopération Scientifique between the CNRS Centre National de la Recherche Scientifique in France and DITAC Department of Industry, Technology, and Commerce in Australia and by the Harry Tribugoff AM Research Syndicate. The authors would like to thank Dr. A. Campo for very helpful discussions. 1 G. L. Patton et al., IEEE Trans. Electron Device Lett. 11, S. Galdin, P. Dolfus, and P. Hesto, J. Appl. Phys. 75, H. Temkin, T. P. Pearsall, J. C. Bean, R. A. Logan, and S. Luryi, Appl. Phys. Lett. 48, K. L. Wang and R. P. G. Karunasiri, J. Vac. Sci. Technol. B 11, Z. Xu, X. Zou, X. Zhou, B. Zhao, C. Wang, and Y. Hamakawa, J. Appl. Phys. 75, J. G. Couillard and H. G. Craighead, J. Vac. Sci. Technol. B 11, A. A. Bright, S. S. Iyer, S. W. Robey, and S. L. Delage, Appl. Phys. Lett. 53, G. S. Oehrlein, Y. Zhang, G. M. W. Kroesen, E. de Frésart, and T. D. Bestwick, Appl. Phys. Lett. 58, G. S. Oehrlein, T. D. Bestwick, P. L. Jones, and J. W. Corbett, Appl. Phys. Lett. 56, G. S. Oehrlein, T. D. Bestwick, P. L. Jones, M. A. Jaso, and J. L. Lindström, J. Electrochem. Soc. 138, R. Cheung, T. Zijlstra, E. van der Drift, L. J. Geerligs, A. H. Verbruggen, K. Werner, and S. Radelaar, J. Vac. Sci. Technol. B 11, H. H. Richter, A. Wolff, B. Tillack, and T. Skaloud, Mater. Sci. Eng. B 27, G. Turban, B. Grolleau, P. Launay, and P. Briaud, Rev. Phys. Appl. 20, M. C. Peignon, Ch. Cardinaud, and G. Turban, J. Electrochem. Soc. 140, Ch. Cardinaud, M. C. Peignon, and G. Turban, in Proceedings of the 11th International Symposium on Plasma Chemistry 1993, Vol. 4, edited by J. Harry, p N. Sadeghi, H. Debontride, G. Turban, and M. C. Peignon, Plasma Chem. Plasma Process. 10, R. J. M. M. Snijkers, J. F. Coulon, and G. Turban, J. Phys. D 24, G. S. Oehrlein, J. Vac. Sci. Technol. A 11, Y. Zhang, G. S. Oehrlein, and E. de Frésart, J. Appl. Phys. 71, A. Campo, Ch. Cardinaud, and G. Turban, J. Vac. Sci. Technol. B 13, A. Campo, Ch. Cardinaud, and G. Turban, Plasma Sources Sci. Technol. 4, Handbook of Chemistry and Physics, 71th ed., edited by R. C. Weast Chemical Rubber, Boca Raton, FL, C. D. Wagner, Practical Surface Analysis by Auger and XPS, edited by E. Briggs and M. P. Seah Wiley, New York, S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 17, ; 17, J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy Perkin-Elmer, Eden Prairie, MN, J. Royer, G. Turban, and B. Grolleau, Nucl. Instrum. Methods B 72, M. C. Peignon, Ch. Cardinaud, and G. Turban, J. Appl. Phys. 70, F. R. McFeely, J. F. Morar, N. D. Shinn, G. Landgren, and F. J. Himpsel, Phys. Rev. B 30, G. S. Oehrlein, S. W. Robey, and J. L. Lindström, Appl. Phys. Lett. 52, S. W. Robey, A. A. Bright, G. S. Oehrlein, S. S. Iyer, and S. L. Delage, J. Vac. Sci. Technol. B 6, G. S. Oehrlein, G. M. W. Kroesen, E. de Frésart, Y. Zhang, and T. D. Bestwick, J. Vac. Sci. Technol. A 9, Y. Zhang, G. S. Oehrlein, and E. de Frésart, J. Vac. Sci. Technol. A 11, P. Briaud, G. Turban, and B. Grolleau, Mater. Res. Soc. Symp. Proc. 68, J. Vac. Sci. Technol. A, Vol. 14, No. 1, Jan/Feb 1996