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Study by electron spectroscopy for chemical analysis of silicon, SiO2 and Si3N4 surfaces treated with various CF4-containing plasmas
Surface Technology, 18 (1983) 189 - 199
189
STUDY BY E L E C T R O N SPECTROSCOPY F O R CHEMICAL ANALYSIS OF SILICON, SiO2 AND Si3N4 S U R F A C E S T R E A T E D WITH V A R I O U S CF4-CONTAINING PLASMAS
A. L I C C I A R D E L L O
and S. P I G N A T A R O
Istituto Dipartimentale di Chimica e Chimica Industriale Universitd di Catania, viale A. Doria 6, 95125 Catania (Italy)
M. P~,NCZI~L Eotvos Lorand University, Department o f General and Inorganic Chemistry, 1445 Budapest (Hungary) (Received October 4, 1982)
Summary Silicon, SiO: and Si3N4 surfaces were treated with three different types of commercial plasma containing CFa. The spectra obtained in electron spectroscopy for chemical analysis of these surfaces show relatively low carbon and fluorine contamination. The latter contaminant was found on SiO2 or Si3Na surfaces but was no longer present on the silicon.like surfaces after treatment. The results are interpreted in terms of known mechanisms of plasma etching.
1. Introduction Silicon, SiO: and Si3N4 thin films are widely used in the semiconductor and integrated circuit industry. The fabrication of a semiconductor device or an integrated circuit requires the silicon, SiO2 and Si3Na surfaces to be etched. Among the etching procedures, dry etching involving the plasma treatment of the surface is a rapidly emerging technique. Mixtures of gases containing fluorocarbons are often used in these cases as etching gases. In this technology it is particularly important to be able to control the etch rate, selectivity and directionality. This can be achieved through the understanding of the chemical and physical processes which occur both in the plasma phase and at the solid-gas interface. A great effort has been made in these directions and our present knowledge is summarized in recent reviews [1]. Although much progress has been made, a number of problems remain and much work is still needed for the full understanding of the plasma etching process. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
190
This is not surprising considering the complex nature of plasma chemistry and plasma-surface interaction phenomena and the very large number of parameters which influence the etching process [1]. The progress made stems from the contribution of a number of diagnostic studies intended to identify and monitor the various species formed both in the gas phase and at the solid-gas interface. Mass spectrometry and emission spectroscopy are the techniques mainly used in these studies to provide information on the gas phase composition, while Auger spectroscopy, electron spectroscopy for chemical analysis (ESCA) (X-ray photoelectron spectroscopy (XPS)) and secondary ion mass spectrometry have been mainly used to monitor the surface conditions. Apart from the problems connected with the understanding of the plasma etching mechanisms, the industrial use of plasma etching cannot disregard the contamination phenomena at the surfaces which may be due to the plasma treatment itself or to the subsequent exposure to air of the treated surface during the production process. This is because the electrical or structural properties of the films may be strongly influenced by surface contamination. The above surface diagnostic tools have also been used [ 2 - 4 ] to characterize the contaminants left on silicon and SiO2 surfaces during dry etching. In this paper we report an ESCA (XPS) study of silicon, SiO2 and Si3N 4 surfaces treated with various commercial plasmas all containing fluorohydrocarbons. No precaution was taken after the plasma treatment to avoid the contact with air of the surface to be analysed. This simulates plant conditions for the surface. The aim of the study was to gain information on the contamination level encountered on the above surfaces in plant conditions and possibly to make a contribution to our knowledge on the mechanisms of plasma etching by fluorohydrocarbon-containing plasmas. The study has also to be considered as a part of a general programme intended to apply the surface analysis tools to fabrication problems in the semiconductor industry [5].
2. Experimental details The samples were (100) p-type silicon wafers. Two different SiO 2 or Si3N 4 films were grown on these samples. The SiO2 was obtained by using the thermal oxidation m e t h o d at 1100 °C and growing the film in wet or dry oxygen. The Si3N 4 was obtained by low pressure chemical vapour deposition (CVD) at 740 °C from an NH3-SiH2CI 2 mixture or CVD at 900 °C from an NH3-SiH4 mixture. The thickness of the films was measured with a Talystep, a step and/or ellipsometry apparatus and the results are reported in the figures below. The plasma etching was carried out in a commercial (LFE Corporation, model PDS/PDE 301) barrel-type apparatus.
191
The etching gases used were DE 101 (plasma A), PDE 100 (plasma B) and DS 300 (plasma C) from Scientific Gas Product Inc. A semiquantitative mass spectrometry analysis showed the etching gas A to contain far less CF4 than B. 02, noble gases (argon, helium) and H2 were also contained in the mixtures. The [H2]/[CF4] ratio was higher for gas A than for B. The gas C is an oxygen-rich stripping gas. The applied power in the plasma apparatus was 150 W for plasmas A and B unless otherwise stated and 400 W for plasma C. ESCA spectra were obtained with a Kratos ES 300 electron spectrometer. The base pressure in the sample chamber was about 10 -8 TorT. Before sputtering, the sample chamber was backfilled with high purity argon to a pressure of 5 × 10 -s Torr. A 2 keV ion beam was used for sputtering the sample. The spectra were taken in the fixed retarding ratio mode. The ESCA peaks were deconvoluted by using a simple gaussian sum program involving linear background subtraction.
3. Results
3.1. Si3N4 A typical ESCA spectrum of an Si3N 4 sample treated with a plasma which did not etch all the nitride film shows silicon, nitrogen, oxygen, carbon and fluorine signals. Traces of chlorine were found in the sample obtained by using an NH3-SiH2C12 mixture. These signals are due to the presence on the surface of Si3N4 and SiO2 together with carbon, fluorine and chlorine contaminations. The relative intensity of the Si 2p components due to Si3N4 and SiO2 and also the relative intensity of the O l s band change with the sample examined. This is in agreement with the fact that the exposure to air of the Si3N4 film causes the oxidation of the film with SiO2 formation [6]. The oxidation of the film may also occur independently of exposure to air. The sample treated with plasma C, which has the highest 02 content, shows the highest O ls and Si 2p (SiO2 component) relative intensities. In this case therefore the plasma treatment causes a surface oxidation which is stronger than that which would occur by simple air oxidation in simulated plant conditions. The intensity ratio of N ls to Si 2p (Si3N4) and to a lesser extent the intensity ratio of O Is to Si 2p (SiO2) do n o t change appreciably with the plasma treatment. In addition the differences in binding energy (BE) between N ls and Si 2p (Si3N4) and between O l s and Si 2p (SiO2) are constant (295.8 + 0.2 eV and 429.4 + 0.2 eV respectively) within the experimental error for all the samples examined. The BE difference between Si 2p (SiO2) and Si 2p (Si3N4) changes with the sample, reflecting the well-known [7] BE shift of the Si 2p and O Is lines of SiO 2 films with changes in their thickness.
192 -~i (¢1)
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Fig. 1. Si 2p and F ls ESCA signals from Si3N4 surfaces treated with (a) plasma A, (b) plasma B and (c) plasma C. The kinetic energy scale is uncorrected and the etching times are given in parentheses. The appl]ed power was 400 W for plasma C and 150 W for plasmas A and B except for the sample treated with plasma B for 1 rain (300 W). The samples were obtained by depositing a layer of Si3N4 1200 A thick on Si(100).
The carbon and fluorine contamination level is relatively low and only slightly dependent on the treatment time and the kind of plasma used. All this shows that the Si3N4 surfaces remain almost unchanged during the plasma treatment. This is in agreement with the fact that plasmas A, B and C unlike carbon-depositing plasmas are able to remove the Si3N4 films witho u t suppressing the nitride etching. The C l s signals are always structured, showing the presence of carbon atoms in various chemical environments. The BE of the most important component, calculated with respect to the Si 2p (SIO2) band, corresponds to "adventitious" [8] carbon. As to the F Is signal, it is interesting to note that 1 min of Ar ÷ sputtering (2 keV, 5 pA) is sufficient for its removal, while the carbon signal shows a much larger depth profile. The whole situation changes when we consider Si3N4 samples where the plasma treatment removed all the nitride film. The O ls and the Si 2p (SIO2) bands are those typical of silicon surfaces exposed to air, in agreement with our sampling procedure. Most interestingly the F l s signal is no longer present. This is shown in Fig. 1. A 12 min treatment with plasma A is sufficient to remove all the Si3N4 film. Correspondingly no F l s signal was found. The SiO2 c o m p o n e n t is mostly due to air oxidation of the sample which occurred between the
193
plasma treatment and the ESCA measurements. The same holds for the reported cases of samples treated with plasma B.
3.2. Silicon and Si02 The ESCA spectra of the silicon and SiO2 samples treated with plasmas A, B and C show the expected silicon, oxygen and carbon signals. In some
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(b)
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Fig. 2. Si 2p and F l s ESCA signals from Ca), (c), (e) silicon and (b), (d), (f) SiO2 surfaces treated with Ca), (b) plasma A, (c), (d) plasma B and (e), (f) plasma C. The kinetic energy scale is uncorrected. The etching times are given in parentheses. The thickness of the SiO2 grown on sic100) was 6000 A for plasmas A and C and 1200 A for plasma B.
194 cases fluorine peaks were also observed. Carbon and (when observed) fluorine contamination are at low levels on both silicon and SiO 2 surfaces. The most interesting feature which seems to be connected with what we found in the case of the Si3N4 surface is the presence or the absence of fluorine peaks. Figure 2 shows some partial ESCA spectra (Si 2p and F l s bands) chosen among those obtained for the silicon and SiO2 samples treated for various times with plasmas A, B and C. No fluorine was observed on the silicon surfaces treated with both plasma A and plasma B (Figs. 2(a) and 2(c)). The SiO 2 c o m p o n e n t is mostly due to air oxidation of the sample which occurred between the plasma treatment and the ESCA experiment. In contrast the SiO2 etched with plasmas A, B and C shows the presence of fluorine on the surface (Figs. 2(b) and 2(f)) unless the underlying silicon layer was reached. In this case the spectrum does not show any fluorine signal (Fig. 2(d)), resembling that of airexposed elemental silicon. Fluorine signals were also found when silicon was etched with plasma C (Fig. 2(e)). However, the SiO2 signal here was also due to silicon oxide formed during the plasma treatment with this O2-rich gas. This is demonstrated by the increase in the relative intensity of the SiO2 peak with respect to the silicon peak with the time of plasma treatment (the SiO2-to-silicon intensity ratios in Fig. 2(e) on going from 5 to 40 min of plasma treatment should be compared).
4. Discussion The plasma treatment leaves on the surfaces contaminants with chemical structures and relative concentrations which depend slightly on the chemical nature of the surface, the chemical composition of the gas used in the plasma and the treatment time. It is interesting to note that the sign of these variations can also be observed in a sample exposed for a long time to the atmosphere. Apart from this interesting observation, the other important results of the study seem to be (a) the relatively low level of carbon contamination found irrespective of the type of plasma, the chemical nature of the surface and the treatment time used, (b) the presence of fluorine on the surfaces which is related to the presence of Si3N4 or SiO2 species and (c) the absence or negligible presence of fluorine on the silicon-like surfaces. These results contribute to our knowledge of the mechanisms involved in the plasma etching of the surfaces considered. In particular, the effect observed on the SiO2 and Si3N 4 surfaces when plasma A is used can be interpreted by assuming several mechanisms to be operative. According to the models reported in the literature a chemical sputtering regime should surely hold and the sputtering process should proceed via a first step in which species such as CxF~ (or F) are adsorbed on the surfaces. This is followed by a reactive step in which more or less volatile species are formed. These species should have the general formulae CxOyFz (C~NyFz) and Si~OyFz
195
(SixNyFz) with x, y or z which may be zero. The reactive step is then followed by the desorption of these species. This model is supported by the mass spectrometry observation that large amounts of CO, CO2, COF2 and SiF4 are evolved when SiO2 is etched in a pure CF 4 plasma [9, 10] ; i.e. the oxygen in the SiO2 combines with carbon to form CO, CO2, COF2 while part of the adsorbed fluorine is used to form SiF 4. If we take into account with our results the fact that CF3 is one of the most important gas phase species [1] for the etching process, the scheme shown in Fig. 3 can be hypothesized as one of the most significant in the etching of SiO2. This scheme would account for at least part of the fluorine present on the SiO2 surfaces. If it is correct this mechanism would support a previous hypothesis assuming a dissociative adsorption of CF3 [11] and the formation of oxyfluorides [10] as important steps in the etching of silicon oxide. It should be noted that, n o t in disagreement with this hypothesis, the Auger parameter [12] for the fluorine was 87.0 + 0.2 eV. This value has to be compared with that of 87.9 + 0.2 eV found for fluorine in Teflon. The Si3N4 case is less clear b u t the general feeling is that the situation should be very similar to that for SiO2 [1]. Moreover, CxNyFz species have recently been found [13] in the effluent of C2F6-N2 plasmas. The value obtained for the Auger parameter (88.0 eV), while significantly different from that found for the SiO2 surface, cannot be used in support of this
®
® COF 2
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I
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COF 2
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196 hypothesis since the fluorine Auger signal is very weak so that a large error can arise in the determination of the Auger parameter itself. However, this value does not allow us to rule out the possibility that some fluorinated carbon is present on the surface examined. Further work is needed on this point; however, we believe that the observation of the Auger parameters is most important for obtaining information on the nature of the plasma-treated surfaces. Some further comments are needed in connection with the rare gases contained in the plasma. The primary role of these gases should be to confer on the treated SiO 2 and Si3N 4 surfaces a certain a m o u n t of energy. This would occur through the b o m b a r d m e n t of the surface with the noble gas ions (or energetic atoms) formed in the plasma. This energy deposition is thought [14] to weaken the chemical bond of the adsorbed species; it therefore helps the etching process through the stimulation of the desorption step involved in the global etching process. A physical sputtering may also be effective. The main effect of these mechanisms should be to ensure a low CxFy coverage of the surfaces. These phenomena become most important when the etching of the silicon surface is considered. The step involving the formation of fluoropolymers on the surface is avoided by the continuous desorption of Cx Fy species in a dynamic regime. (Alternatively, polymerization inhibition in the gas phase may also be operative.) This would keep the carbon and fluorine concentration on the silicon surface at a low level as observed in the present experiment. Small amounts of SiF4 are formed on the surface anyway. However, unlike the oxyfluoride postulated above, SiF4 is a very volatile product and it is mostly lost before ESCA analysis. It is important to note that the fluorine signal measured on the SiO2 and Si3N4 surfaces was also found in samples which were maintained in contact with the atmosphere for 1 year after preparation. However, the above results show that the etching of SiO2 or Si3N4 films grown on a silicon surface with gas A would leave the underlying silicon surface almost free from carbon and fluorine contamination. Gas B should work in a way similar to that described for gas A. The major differences are connected with its higher CF4 c o n t e n t which causes a higher rate of SiO2 and Si3N4 erosion, a slightly higher carbon and fluorine contamination of silicon surfaces and a consequent slight loss of selectivity for silicon erosion. The behaviour of gas C can be rationalized in terms of an etching which also involves oxidation of the surfaces. The oxides present on the SiO 2 surface that interacted with this plasma seem slightly different from those on an untreated SiO2 sample. The BE difference between Si 2p (SiO2 component) and O l s and also the full width at half-maximum of the O ls peak seem to increase with respect to the values found for thermally grown SiO2. However, the etching mechanism should n o t be much different from that described for gas A. In the plasma treatment using gas C, silicon oxides are also formed on the Si3N 4 surface. Evidence that such oxidation involves the last surface
197
~
,~sX
01s
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-- o
Nls
Sl2p
X3
711
715
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1144K.F."'~-eV () ' u n c o r . "
Fig. 4. O ls, N ls and Si 2p ESCA signals from an Si3N4 sample treated with plasma C. The spectra were taken at two different take-off angles.
layers is given b y an ESCA experiment performed by tilting the sample with respect to the electron take-off angle. Figure 4 shows that when a lower take-off angle, for which the surface sensitivity is higher [15], is used the N is signal decreases with respect to the O Is signal. In addition, an inversion of the t w o Si 2p components (SiO2 and Si3N4) is observed. Thus the etching of Si3N4 with plasma C should occur simultaneously with mechanisms similar to those described for the erosion of both SiO2 and Si3N4 by plasma A. The situation on the silicon surface treated with plasma C is similar. Silicon oxides are also formed at the silicon surface treated with plasma C. This is demonstrated by the [SiO2]/[Si] ratio which is larger than that observed for a silicon surface oxidized by simple exposure to air. This ratio increases with the treatment time, so that the a m o u n t of SiO2 in the depth explored by ESCA increases with the time of treatment of the surface with plasma C (Fig. 5). According to this observation the silicon should be mainly etched by mechanisms again similar to that described for the erosion of SiO2 by plasma A. This is supported by the presence on the silicon surface of fluorine indicated by ESCA signals. The similarity of the mechanisms responsible for the erosion of SiO2 surfaces by means of a fluorohydrocarbon-containing plasma and of silicon surfaces by means of the same plasma with added O2 gas has
198
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Si
Si02 I~--
15 °
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1141
1145
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Fig. 5. Si 2p ESCA spectra taken at different take~)ff angles for a silicon sample treated with plasma C for (a) 5 min and (b) 40 rain.
also been demonstrated by previous mass spectrometry experiments. These experiments showed [9] that a silicon target treated with a CF4 plasma contalning 18% 02 gives an effluent with a mass spectrum close to that observed for the effluent gas recovered from a pure CF4 plasma interacting with an SiO2 surface.
Acknowledgment This work was supported by the Consigiio Nazionale delle Ricerche (Progetto Finalizzato Chimica Fine e Secondaria).
References 1 E. Kay, J. Coburn and A. Dilks, Top. Curt. Chem., 94 (1980) 1. J. Coburn, Plasma Chem. Plasma Process., 2 (1) (1982) 1.
199
2 3 4 5 6 7 8 9 10 11 12 13
M. Oschima, Surf. Sci., 86 (1979) 858. J. W. Cohurn, J. Appl. Phys., 50 (1979) 5210. M. M. Millard and E. Kay, J. Electrochem. Soc., 129 (1982) 160. A. Torrisi, G. Nocerino and S. Pignataro, Appl. Surf. Sci., 13 (1982) 389. J. A. Taylor, Appl. Surf. Sci., 7 (1981) 168. G. Hollinger, Appl. Surf. Sci., 8 (1981) 318, and references cited therein. P. Swift, Surf. Interface Anal., 4 (1982) 47. J. W. Coburn and H. F. Winters, J. Vac. Sci. Technol., 18 (1979) 381. B. A. Raby, J. Vac. Sci. Technol., 15 (1978) 205, and references cited therein. H. F. Winters, J. Appl. Phys., 49 (1978) 5165. C. D. Wagner, Faraday Discuss. Chem. Soc., 60 (1975) 291. G. Smolinsky, E. A. Truesdale, D. N. K. Wang and D. Maydan, J. Electrochem. Soc., 129 (1982) 1036. 14 J. W. Coburn and H. F. Winters, J. Appl. Phys., 50 (1979) 3189. 15 C. S. Fadley, Faraday Discuss. Chem. Soc., 60 (1975) 18.
189
STUDY BY E L E C T R O N SPECTROSCOPY F O R CHEMICAL ANALYSIS OF SILICON, SiO2 AND Si3N4 S U R F A C E S T R E A T E D WITH V A R I O U S CF4-CONTAINING PLASMAS
A. L I C C I A R D E L L O
and S. P I G N A T A R O
Istituto Dipartimentale di Chimica e Chimica Industriale Universitd di Catania, viale A. Doria 6, 95125 Catania (Italy)
M. P~,NCZI~L Eotvos Lorand University, Department o f General and Inorganic Chemistry, 1445 Budapest (Hungary) (Received October 4, 1982)
Summary Silicon, SiO: and Si3N4 surfaces were treated with three different types of commercial plasma containing CFa. The spectra obtained in electron spectroscopy for chemical analysis of these surfaces show relatively low carbon and fluorine contamination. The latter contaminant was found on SiO2 or Si3Na surfaces but was no longer present on the silicon.like surfaces after treatment. The results are interpreted in terms of known mechanisms of plasma etching.
1. Introduction Silicon, SiO: and Si3N4 thin films are widely used in the semiconductor and integrated circuit industry. The fabrication of a semiconductor device or an integrated circuit requires the silicon, SiO2 and Si3Na surfaces to be etched. Among the etching procedures, dry etching involving the plasma treatment of the surface is a rapidly emerging technique. Mixtures of gases containing fluorocarbons are often used in these cases as etching gases. In this technology it is particularly important to be able to control the etch rate, selectivity and directionality. This can be achieved through the understanding of the chemical and physical processes which occur both in the plasma phase and at the solid-gas interface. A great effort has been made in these directions and our present knowledge is summarized in recent reviews [1]. Although much progress has been made, a number of problems remain and much work is still needed for the full understanding of the plasma etching process. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
190
This is not surprising considering the complex nature of plasma chemistry and plasma-surface interaction phenomena and the very large number of parameters which influence the etching process [1]. The progress made stems from the contribution of a number of diagnostic studies intended to identify and monitor the various species formed both in the gas phase and at the solid-gas interface. Mass spectrometry and emission spectroscopy are the techniques mainly used in these studies to provide information on the gas phase composition, while Auger spectroscopy, electron spectroscopy for chemical analysis (ESCA) (X-ray photoelectron spectroscopy (XPS)) and secondary ion mass spectrometry have been mainly used to monitor the surface conditions. Apart from the problems connected with the understanding of the plasma etching mechanisms, the industrial use of plasma etching cannot disregard the contamination phenomena at the surfaces which may be due to the plasma treatment itself or to the subsequent exposure to air of the treated surface during the production process. This is because the electrical or structural properties of the films may be strongly influenced by surface contamination. The above surface diagnostic tools have also been used [ 2 - 4 ] to characterize the contaminants left on silicon and SiO2 surfaces during dry etching. In this paper we report an ESCA (XPS) study of silicon, SiO2 and Si3N 4 surfaces treated with various commercial plasmas all containing fluorohydrocarbons. No precaution was taken after the plasma treatment to avoid the contact with air of the surface to be analysed. This simulates plant conditions for the surface. The aim of the study was to gain information on the contamination level encountered on the above surfaces in plant conditions and possibly to make a contribution to our knowledge on the mechanisms of plasma etching by fluorohydrocarbon-containing plasmas. The study has also to be considered as a part of a general programme intended to apply the surface analysis tools to fabrication problems in the semiconductor industry [5].
2. Experimental details The samples were (100) p-type silicon wafers. Two different SiO 2 or Si3N 4 films were grown on these samples. The SiO2 was obtained by using the thermal oxidation m e t h o d at 1100 °C and growing the film in wet or dry oxygen. The Si3N 4 was obtained by low pressure chemical vapour deposition (CVD) at 740 °C from an NH3-SiH2CI 2 mixture or CVD at 900 °C from an NH3-SiH4 mixture. The thickness of the films was measured with a Talystep, a step and/or ellipsometry apparatus and the results are reported in the figures below. The plasma etching was carried out in a commercial (LFE Corporation, model PDS/PDE 301) barrel-type apparatus.
191
The etching gases used were DE 101 (plasma A), PDE 100 (plasma B) and DS 300 (plasma C) from Scientific Gas Product Inc. A semiquantitative mass spectrometry analysis showed the etching gas A to contain far less CF4 than B. 02, noble gases (argon, helium) and H2 were also contained in the mixtures. The [H2]/[CF4] ratio was higher for gas A than for B. The gas C is an oxygen-rich stripping gas. The applied power in the plasma apparatus was 150 W for plasmas A and B unless otherwise stated and 400 W for plasma C. ESCA spectra were obtained with a Kratos ES 300 electron spectrometer. The base pressure in the sample chamber was about 10 -8 TorT. Before sputtering, the sample chamber was backfilled with high purity argon to a pressure of 5 × 10 -s Torr. A 2 keV ion beam was used for sputtering the sample. The spectra were taken in the fixed retarding ratio mode. The ESCA peaks were deconvoluted by using a simple gaussian sum program involving linear background subtraction.
3. Results
3.1. Si3N4 A typical ESCA spectrum of an Si3N 4 sample treated with a plasma which did not etch all the nitride film shows silicon, nitrogen, oxygen, carbon and fluorine signals. Traces of chlorine were found in the sample obtained by using an NH3-SiH2C12 mixture. These signals are due to the presence on the surface of Si3N4 and SiO2 together with carbon, fluorine and chlorine contaminations. The relative intensity of the Si 2p components due to Si3N4 and SiO2 and also the relative intensity of the O l s band change with the sample examined. This is in agreement with the fact that the exposure to air of the Si3N4 film causes the oxidation of the film with SiO2 formation [6]. The oxidation of the film may also occur independently of exposure to air. The sample treated with plasma C, which has the highest 02 content, shows the highest O ls and Si 2p (SiO2 component) relative intensities. In this case therefore the plasma treatment causes a surface oxidation which is stronger than that which would occur by simple air oxidation in simulated plant conditions. The intensity ratio of N ls to Si 2p (Si3N4) and to a lesser extent the intensity ratio of O Is to Si 2p (SiO2) do n o t change appreciably with the plasma treatment. In addition the differences in binding energy (BE) between N ls and Si 2p (Si3N4) and between O l s and Si 2p (SiO2) are constant (295.8 + 0.2 eV and 429.4 + 0.2 eV respectively) within the experimental error for all the samples examined. The BE difference between Si 2p (SiO2) and Si 2p (Si3N4) changes with the sample, reflecting the well-known [7] BE shift of the Si 2p and O Is lines of SiO 2 films with changes in their thickness.
192 -~i (¢1)
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.
.
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i
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Fig. 1. Si 2p and F ls ESCA signals from Si3N4 surfaces treated with (a) plasma A, (b) plasma B and (c) plasma C. The kinetic energy scale is uncorrected and the etching times are given in parentheses. The appl]ed power was 400 W for plasma C and 150 W for plasmas A and B except for the sample treated with plasma B for 1 rain (300 W). The samples were obtained by depositing a layer of Si3N4 1200 A thick on Si(100).
The carbon and fluorine contamination level is relatively low and only slightly dependent on the treatment time and the kind of plasma used. All this shows that the Si3N4 surfaces remain almost unchanged during the plasma treatment. This is in agreement with the fact that plasmas A, B and C unlike carbon-depositing plasmas are able to remove the Si3N4 films witho u t suppressing the nitride etching. The C l s signals are always structured, showing the presence of carbon atoms in various chemical environments. The BE of the most important component, calculated with respect to the Si 2p (SIO2) band, corresponds to "adventitious" [8] carbon. As to the F Is signal, it is interesting to note that 1 min of Ar ÷ sputtering (2 keV, 5 pA) is sufficient for its removal, while the carbon signal shows a much larger depth profile. The whole situation changes when we consider Si3N4 samples where the plasma treatment removed all the nitride film. The O ls and the Si 2p (SIO2) bands are those typical of silicon surfaces exposed to air, in agreement with our sampling procedure. Most interestingly the F l s signal is no longer present. This is shown in Fig. 1. A 12 min treatment with plasma A is sufficient to remove all the Si3N4 film. Correspondingly no F l s signal was found. The SiO2 c o m p o n e n t is mostly due to air oxidation of the sample which occurred between the
193
plasma treatment and the ESCA measurements. The same holds for the reported cases of samples treated with plasma B.
3.2. Silicon and Si02 The ESCA spectra of the silicon and SiO2 samples treated with plasmas A, B and C show the expected silicon, oxygen and carbon signals. In some
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(b)
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3 °,
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(12')~
(1').~
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(e) x3 /
(5') ~ l
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$,
Fig. 2. Si 2p and F l s ESCA signals from Ca), (c), (e) silicon and (b), (d), (f) SiO2 surfaces treated with Ca), (b) plasma A, (c), (d) plasma B and (e), (f) plasma C. The kinetic energy scale is uncorrected. The etching times are given in parentheses. The thickness of the SiO2 grown on sic100) was 6000 A for plasmas A and C and 1200 A for plasma B.
194 cases fluorine peaks were also observed. Carbon and (when observed) fluorine contamination are at low levels on both silicon and SiO 2 surfaces. The most interesting feature which seems to be connected with what we found in the case of the Si3N4 surface is the presence or the absence of fluorine peaks. Figure 2 shows some partial ESCA spectra (Si 2p and F l s bands) chosen among those obtained for the silicon and SiO2 samples treated for various times with plasmas A, B and C. No fluorine was observed on the silicon surfaces treated with both plasma A and plasma B (Figs. 2(a) and 2(c)). The SiO 2 c o m p o n e n t is mostly due to air oxidation of the sample which occurred between the plasma treatment and the ESCA experiment. In contrast the SiO2 etched with plasmas A, B and C shows the presence of fluorine on the surface (Figs. 2(b) and 2(f)) unless the underlying silicon layer was reached. In this case the spectrum does not show any fluorine signal (Fig. 2(d)), resembling that of airexposed elemental silicon. Fluorine signals were also found when silicon was etched with plasma C (Fig. 2(e)). However, the SiO2 signal here was also due to silicon oxide formed during the plasma treatment with this O2-rich gas. This is demonstrated by the increase in the relative intensity of the SiO2 peak with respect to the silicon peak with the time of plasma treatment (the SiO2-to-silicon intensity ratios in Fig. 2(e) on going from 5 to 40 min of plasma treatment should be compared).
4. Discussion The plasma treatment leaves on the surfaces contaminants with chemical structures and relative concentrations which depend slightly on the chemical nature of the surface, the chemical composition of the gas used in the plasma and the treatment time. It is interesting to note that the sign of these variations can also be observed in a sample exposed for a long time to the atmosphere. Apart from this interesting observation, the other important results of the study seem to be (a) the relatively low level of carbon contamination found irrespective of the type of plasma, the chemical nature of the surface and the treatment time used, (b) the presence of fluorine on the surfaces which is related to the presence of Si3N4 or SiO2 species and (c) the absence or negligible presence of fluorine on the silicon-like surfaces. These results contribute to our knowledge of the mechanisms involved in the plasma etching of the surfaces considered. In particular, the effect observed on the SiO2 and Si3N 4 surfaces when plasma A is used can be interpreted by assuming several mechanisms to be operative. According to the models reported in the literature a chemical sputtering regime should surely hold and the sputtering process should proceed via a first step in which species such as CxF~ (or F) are adsorbed on the surfaces. This is followed by a reactive step in which more or less volatile species are formed. These species should have the general formulae CxOyFz (C~NyFz) and Si~OyFz
195
(SixNyFz) with x, y or z which may be zero. The reactive step is then followed by the desorption of these species. This model is supported by the mass spectrometry observation that large amounts of CO, CO2, COF2 and SiF4 are evolved when SiO2 is etched in a pure CF 4 plasma [9, 10] ; i.e. the oxygen in the SiO2 combines with carbon to form CO, CO2, COF2 while part of the adsorbed fluorine is used to form SiF 4. If we take into account with our results the fact that CF3 is one of the most important gas phase species [1] for the etching process, the scheme shown in Fig. 3 can be hypothesized as one of the most significant in the etching of SiO2. This scheme would account for at least part of the fluorine present on the SiO2 surfaces. If it is correct this mechanism would support a previous hypothesis assuming a dissociative adsorption of CF3 [11] and the formation of oxyfluorides [10] as important steps in the etching of silicon oxide. It should be noted that, n o t in disagreement with this hypothesis, the Auger parameter [12] for the fluorine was 87.0 + 0.2 eV. This value has to be compared with that of 87.9 + 0.2 eV found for fluorine in Teflon. The Si3N4 case is less clear b u t the general feeling is that the situation should be very similar to that for SiO2 [1]. Moreover, CxNyFz species have recently been found [13] in the effluent of C2F6-N2 plasmas. The value obtained for the Auger parameter (88.0 eV), while significantly different from that found for the SiO2 surface, cannot be used in support of this
®
® COF 2
®
I
+
Fig. 3. Suggested scheme for the etching o f SiO 2.
COF 2
etc.
196 hypothesis since the fluorine Auger signal is very weak so that a large error can arise in the determination of the Auger parameter itself. However, this value does not allow us to rule out the possibility that some fluorinated carbon is present on the surface examined. Further work is needed on this point; however, we believe that the observation of the Auger parameters is most important for obtaining information on the nature of the plasma-treated surfaces. Some further comments are needed in connection with the rare gases contained in the plasma. The primary role of these gases should be to confer on the treated SiO 2 and Si3N 4 surfaces a certain a m o u n t of energy. This would occur through the b o m b a r d m e n t of the surface with the noble gas ions (or energetic atoms) formed in the plasma. This energy deposition is thought [14] to weaken the chemical bond of the adsorbed species; it therefore helps the etching process through the stimulation of the desorption step involved in the global etching process. A physical sputtering may also be effective. The main effect of these mechanisms should be to ensure a low CxFy coverage of the surfaces. These phenomena become most important when the etching of the silicon surface is considered. The step involving the formation of fluoropolymers on the surface is avoided by the continuous desorption of Cx Fy species in a dynamic regime. (Alternatively, polymerization inhibition in the gas phase may also be operative.) This would keep the carbon and fluorine concentration on the silicon surface at a low level as observed in the present experiment. Small amounts of SiF4 are formed on the surface anyway. However, unlike the oxyfluoride postulated above, SiF4 is a very volatile product and it is mostly lost before ESCA analysis. It is important to note that the fluorine signal measured on the SiO2 and Si3N4 surfaces was also found in samples which were maintained in contact with the atmosphere for 1 year after preparation. However, the above results show that the etching of SiO2 or Si3N4 films grown on a silicon surface with gas A would leave the underlying silicon surface almost free from carbon and fluorine contamination. Gas B should work in a way similar to that described for gas A. The major differences are connected with its higher CF4 c o n t e n t which causes a higher rate of SiO2 and Si3N4 erosion, a slightly higher carbon and fluorine contamination of silicon surfaces and a consequent slight loss of selectivity for silicon erosion. The behaviour of gas C can be rationalized in terms of an etching which also involves oxidation of the surfaces. The oxides present on the SiO 2 surface that interacted with this plasma seem slightly different from those on an untreated SiO2 sample. The BE difference between Si 2p (SiO2 component) and O l s and also the full width at half-maximum of the O ls peak seem to increase with respect to the values found for thermally grown SiO2. However, the etching mechanism should n o t be much different from that described for gas A. In the plasma treatment using gas C, silicon oxides are also formed on the Si3N 4 surface. Evidence that such oxidation involves the last surface
197
~
,~sX
01s
- rays
-- o
Nls
Sl2p
X3
711
715
$~,5
$4g
1140
1144K.F."'~-eV () ' u n c o r . "
Fig. 4. O ls, N ls and Si 2p ESCA signals from an Si3N4 sample treated with plasma C. The spectra were taken at two different take-off angles.
layers is given b y an ESCA experiment performed by tilting the sample with respect to the electron take-off angle. Figure 4 shows that when a lower take-off angle, for which the surface sensitivity is higher [15], is used the N is signal decreases with respect to the O Is signal. In addition, an inversion of the t w o Si 2p components (SiO2 and Si3N4) is observed. Thus the etching of Si3N4 with plasma C should occur simultaneously with mechanisms similar to those described for the erosion of both SiO2 and Si3N4 by plasma A. The situation on the silicon surface treated with plasma C is similar. Silicon oxides are also formed at the silicon surface treated with plasma C. This is demonstrated by the [SiO2]/[Si] ratio which is larger than that observed for a silicon surface oxidized by simple exposure to air. This ratio increases with the treatment time, so that the a m o u n t of SiO2 in the depth explored by ESCA increases with the time of treatment of the surface with plasma C (Fig. 5). According to this observation the silicon should be mainly etched by mechanisms again similar to that described for the erosion of SiO2 by plasma A. This is supported by the presence on the silicon surface of fluorine indicated by ESCA signals. The similarity of the mechanisms responsible for the erosion of SiO2 surfaces by means of a fluorohydrocarbon-containing plasma and of silicon surfaces by means of the same plasma with added O2 gas has
198
SIO~,
Si
Si02 I~--
15 °
X rays
X3
~.~= 60 °
f b
1141
1145
(a)
11~,9
' '
J1140
(b)
11~14
K F:
' 1152 eV (uncor.)
Fig. 5. Si 2p ESCA spectra taken at different take~)ff angles for a silicon sample treated with plasma C for (a) 5 min and (b) 40 rain.
also been demonstrated by previous mass spectrometry experiments. These experiments showed [9] that a silicon target treated with a CF4 plasma contalning 18% 02 gives an effluent with a mass spectrum close to that observed for the effluent gas recovered from a pure CF4 plasma interacting with an SiO2 surface.
Acknowledgment This work was supported by the Consigiio Nazionale delle Ricerche (Progetto Finalizzato Chimica Fine e Secondaria).
References 1 E. Kay, J. Coburn and A. Dilks, Top. Curt. Chem., 94 (1980) 1. J. Coburn, Plasma Chem. Plasma Process., 2 (1) (1982) 1.
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2 3 4 5 6 7 8 9 10 11 12 13
M. Oschima, Surf. Sci., 86 (1979) 858. J. W. Cohurn, J. Appl. Phys., 50 (1979) 5210. M. M. Millard and E. Kay, J. Electrochem. Soc., 129 (1982) 160. A. Torrisi, G. Nocerino and S. Pignataro, Appl. Surf. Sci., 13 (1982) 389. J. A. Taylor, Appl. Surf. Sci., 7 (1981) 168. G. Hollinger, Appl. Surf. Sci., 8 (1981) 318, and references cited therein. P. Swift, Surf. Interface Anal., 4 (1982) 47. J. W. Coburn and H. F. Winters, J. Vac. Sci. Technol., 18 (1979) 381. B. A. Raby, J. Vac. Sci. Technol., 15 (1978) 205, and references cited therein. H. F. Winters, J. Appl. Phys., 49 (1978) 5165. C. D. Wagner, Faraday Discuss. Chem. Soc., 60 (1975) 291. G. Smolinsky, E. A. Truesdale, D. N. K. Wang and D. Maydan, J. Electrochem. Soc., 129 (1982) 1036. 14 J. W. Coburn and H. F. Winters, J. Appl. Phys., 50 (1979) 3189. 15 C. S. Fadley, Faraday Discuss. Chem. Soc., 60 (1975) 18.