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Spatially resolved optical spectroscopy of plasma etching systems
0042-207X/8453.00 + 00
Vacuum/volume 34fnumbers 3-4Ipages 347 to 349/1984 Printed in Great Britain
Spatially resolved optical plasma etching systems D Field, A J Hydes and D F Klemperer, Bristol BS8 1 TS, UK
Pergamon Press Ltd
spectroscopy
of
The School of Chemistry, University of Bristol, Cantock’s Close,
We summarise results from an extensive study of the emission of a CFJO, discharge (0.2 torr, 70 W, 62.5 kHz), in the presence of Si and thermally grown SiO, substrates. We present data relating to spectrally and spatially resolved emission, from which we conclude for etching of Si that F atoms are the most important agent, that energy is imparted to the surface from metastable species of high energy content and that electron spin angular momentum is conserved in gas-surface reactions. A much more detailed account of this work will be presented in a subsequent issue of this journal.
1. Introduction Plasma assisted etching is beginning to be used extensively in the microfabrication industry for the production of VLSI components. Despite extensive work into the nature of plasma etching little is known of its mechanism’. The work summarized here is concerned with the fundamental physical chemistry involved in plasma etching. A much more detailed account will be published in this journal’. We include in the present report only a small fraction of our experimental results and a brief resume of the conclusions to which our results lead us. We have studied etching of Si and thermally grown Si02 in a commercial parallel plate etching apparatus (ET Associates, Nailsea, UK) with CFJOz gas mixtures at pressures of 0.2 torr, discharge power 70 W and frequency 62.5 kHz. Because of their general nature, the majority of our findings should however be applicable to many types of plasma systems. We have used emission spectroscopy of the plasma glow as a diagnostic tool throughout the present work. Other groups have used a similar technique S-6. As a simple working hypothesis we suggest that changes in emission intensity of an observed species in the presence of a substrate are a measure of the extent to which that species is involved in substrate etching. 2. Experimental Our experiments fall into two categories, those involving observation of the bulk emission of the plasma glow and those involving observation of spatially resolved emission. The latter represents a new and fruitful departure in spectroscopic investigations of discharge systems. (i) Bulk emission studies. We have observed the total emission of the glow in the presence of Si (100) and SiOZ substrates (resting on the earthed cathode) at a spectral resolution of -0.3 A over the
wavelength range BOO-7100 8, using a 0.5 m spectrometer (SPEX 1870). A number of important species have been identified in the emission, among which are F, F+, CO, CO:, CF,, C, 0, 0 +, Si and Si + in a variety of electronic states. There are a number of significant differences between spectra obtained in the presence and absence of wafer material and these are briefly described and discussed in sections 3(i) and 4 below.
(ii) Spatially resolved emission studies. We have used a very fine (0.8 mm dia) quartz light probe to investigate the spatial distribution of F, F+, CO and CO;, recording spectrally resolved emission through the probe by situating it at different positions across the cathode. Concentration profiles were recorded in the absence of wafer material and in the presence of Si, Si02 and an inert sapphire wafer. The latter was included in order to take account of any systematic electrical perturbation to the plasma introduced through the presence of a dielectric resting upon the cathode, rather than through the chemical eflects in which we are interested. We find that concentration profiles are subject to a small but significant perturbation, especially for ions’. The probe is physically as small as is practicable and we believe that it introduces no important distortion into the plasma. This assertion is supported by experimental evidence outlined in Field et ai2. The tip of the probe points downwards towards the cathode and it may traverse a total distance of 8 cm parallel to the cathode (and wafer) surface, the diameter of the reactor being 15.5 cm. Light is admitted only through the tip of the probe by coating it with a film of Al paint of high electrical resistance. Spatial resolution is better than 3 mm across the cathode. Typical profiles of the F atom emission at 7037.5 A (2P-2P0) over Si and SiO, and in the absence of a wafer are shown in Figure 1. A great deal more data for F, F+, CO and CO: are presented in Field et al’. Results are discussed in Section 3(ii) and 4 below. 347
D Field et al: Spatially
resolved
optlcal
spectroscopy
of plasma
etching
systems
in the presence and absence of Si material in transitions m\ol\mg triplet (at right) and singlet (at left) states of the atomic ion. Triplet state emission is largely unaffected, whereas singlet emission drops by a factor of _ 10. Electronic state dependent behaviour has also been found for F atoms. 0’ and possibly C atoms. Among molecular species, CO; emission in both its .x and B states falls in intensity by a factor of 4 2 in the presence of Si. CO. CO’ and CF2 emission intensities are unaffected.
From edge
of chamber
(cmi
Figure 1. The profile of the F atom emission ( x concentration of F(*P )J at 7037.5 is (‘P-‘P ) across the chamber. parallel to the cathode. from close lo the wall to the centre. Profiles are shown in the presence of Si and SO, substrates and in the absence of a wafer. (Profile for sapphire wafer not shown. 1 3. Results (i) Bulk emission studies. In general, major changes of emission intensity do not occur on the introduction of SiO, substrate into the chamber. However. CF, emission (2 ‘B, -,? ‘A,) around 2500 A is reduced in intensity by a factor of _ 2 in the presence of SiO,. CF, reacts exothermically with SiO, to form SiOF, and CO (AC”= - 0.604 eV) and our results support the notion that species CF, are important in etching SiOz’.“. The effect of the introduction of Sl into the chamber contrasts with that ofSi0,. In particular. a number of atomic emission lines are strongly reduced in intensity. Our most striking observation is that different states of the same species behave quite differently. This is shown in Figure 2. m which may be seen the emission of F’
(ii) Spatially resolved emission studies. By integrating the mtensltl of emission over the range 6.0 7.5 cm from the chamber wall (close to the centre of the wafer) in the presence of a sapphire wafer and comparing this integrated intensity with that found in the presence of Sl or SiO,, one may obtain a direct measure of the degree of involvement of F. F +, CO or CO; in the etchmg process. This provides a much more sensitive diagnostic than the bulk emission. since the latter samples preferentially the outer part of the discharge. specifically that part not over the wzafer material. Thus. F atom emission (at 7037.5 A) falls by a factor of 10 over a SI wafer (Figure 1) whereas the fall is through a factor of 3.8 in the bulk emission. Again. CO: falls by a factor of 5 over a Si wafer. whereas the bulk emission, as we have noted above. shows a fall of only a factor of _ 2. F’ in its triplet ground state (3P) does not react significantly with Si (or SiOz) substrates. A description of the role of CO (a “fI) may be found in Field ef al’.
4. Discussion
Figure 2. Emission of F’ m the sin ler transnion at 3202.7 A (ID +‘DJ and the triplet transition at 4025.5 1 (3D-+-3P~ In the absence of a wafer
Our results refer almost exclusively to etching of SI. Plasma emission would appear to give little information on the nature of Si02 etching. since the great majority of fluorescent species are not strongly involved. Our discussion and conclusions are therefore only relevant to Si etching. Our observations show that the chemical nature of the ion is important in etching Si, and, moreover, that different electronic states of any one ion type show very different reactivity to Si substrate. Furthermore, our results form a coherent body ifwe use straightforward thermochemical arguments to explain the reactivity or otherwise of the various species we have observed. These findings suggest that ions are not impacted into the Si surface at high translational energy and that etching of Si does not proceed in the ion-assisted manner that has previously been proposed for plasma etching in genera19,‘0. In the present system the substrate is electrically insulated from the earthed cathode and behaves as a floating substrate introduced into the plasma. There are good theoretical grounds for believing that the sheath potential is onl) 0.5-1.0 V under our conditions’.” and thus that ions impact at energies 5 1 eV; this supports our experimental findings. We may explain many of our observations if we introduce the notion that electron spin is conserved in gas-surface interactions, just as in purely gas phase reactions. As one example, singlet F’ may react with Si, known to have a ground state which is singlet in character”, to give singlet SiF’, whereas the triplet state of F’ cannot react in a spin-allowed manner to give a singlet product. In order to test the idea of spin conservation in gas-surface reactions, we predicted that Si (100) should etch more rapidly than Si (I II ) in a fluorine atom rich plasma as we have here. Figure I shows that F atoms are largely responsible for etching of Si (100). This process may be represented by the spin-allowed reaction.
and in the presence of a Si wafer. Note the very different behavlour s&et and triplet transition mtensltles.
F (2P)+‘Si-+2SiF
No wafer
SI wafer
3202
Wavelength
348
0
4025
/ 8
of the
(1)
D Field et a/: Spatially resolved optical spectroscopy of plasma etching systems
in which Si (100) is represented by ‘Si. Si (111) presents a surface of doublet character to the attacking F atomsI and doublet F atoms cannot therefore react in a spin-allowed manner to give SiF product. Experimentally it was found that Si (100) etched 4.5kO.15 times more rapidly than Si (111). This result lends support to the principle of spin conservation in gas-surface reactions. Following on from this, if singlet F’ may react at the Si surface, then it will impart to the surface some fraction of its internal energy of electronic excitation, 5 30 eV in total. Thus metastables of high internal energy may contribute to an enhancement of surface reactivity in place of high energy ion bombardment. Figure 1 illustrates two further points. The first relates to the gradient of F atoms introduced into the plasma in the presence of the Si substrate. It is clear that in the presence of many wafers there will be competition for the available F atoms and the results of Figure 1 therefore illustrate the so-called ‘loading effect’ (see elsewhere in this issue). The second point is the general lack of uniformity in F atom concentration exemplified on both the large and the small scale. This will be reflected in a lack of etch uniformity, a major problem in the microfabrication industry. Our results suggest how the problem of nonuniformity might be tackled and this is discussed in more detail in Field et al*. 5. Conclusions
In summary, our results suggest that Si etching involves the following mechanisms: (i) chemical attack largely by F atoms; (ii) the rate of attack is strongly crystal plane dependent, being faster for Si (100) than Si (111); (iii) energy is imparted to the
surface from metastables of high internal energy, a process which may lead to the effective evaporation of Si from the surface; (iv) in gas-surface reactions spin angular momentum is conserved in the manner well-known in gas phase reactions. This latter appears to be a new concept and may prove useful in elucidating mechanisms and predicting behaviour in plasma systems in general.
Acknowledgement
We should like to thank E T Associates for supplying equipment and materials and the SERC for a CASE award for one of us (AJH) held with E T Associates.
References
‘ J W Coburn, J Vat Sci Techno/. 21, 557 (1982). 2 D Field, A. J. Hydes and D F Klemperer, Vacuum accepted for publication. 3 W R Harshbarger. R A Porter, T A Miller and P Norton. Appl Specrrosc, 31,201 (1977). 4 B J Curtis and H J Brunner, J Elecrrochem Sot, 125, 829 (1978). ’ C J Mogab, A C Adams and D L Flamm, .I Appl Phys, 49,3796 (1978). 6 R d’Agostino, F Cramarossa, S De Benedictis and G Ferraro, .I Appl Phys. 52, 1259 (1981). ’ D L Flamm, Solid Srare Tech/, 22, 109 (1979). * R A H Heinecke. So/id Srare Electron. 18. 1146 (1975) 9 J W Coburn and H F Winters, J Vat Sci Tech& 16; 391 (1979). *’ G Smolinsky and D L Flamm, J Appl Phys, SO, 4982 (1979). ‘I B N Chapman, Glow Discharge Processes, pp 62-64. Wiley, New York (1980). ‘* A Redondo and W A Goddard, J Vat Sci Technol, 21, 344 (1982). I3 S Ciraci, J P Batra and W A Tiller, Phps Reo, B12, 5812 (1975).
349
Vacuum/volume 34fnumbers 3-4Ipages 347 to 349/1984 Printed in Great Britain
Spatially resolved optical plasma etching systems D Field, A J Hydes and D F Klemperer, Bristol BS8 1 TS, UK
Pergamon Press Ltd
spectroscopy
of
The School of Chemistry, University of Bristol, Cantock’s Close,
We summarise results from an extensive study of the emission of a CFJO, discharge (0.2 torr, 70 W, 62.5 kHz), in the presence of Si and thermally grown SiO, substrates. We present data relating to spectrally and spatially resolved emission, from which we conclude for etching of Si that F atoms are the most important agent, that energy is imparted to the surface from metastable species of high energy content and that electron spin angular momentum is conserved in gas-surface reactions. A much more detailed account of this work will be presented in a subsequent issue of this journal.
1. Introduction Plasma assisted etching is beginning to be used extensively in the microfabrication industry for the production of VLSI components. Despite extensive work into the nature of plasma etching little is known of its mechanism’. The work summarized here is concerned with the fundamental physical chemistry involved in plasma etching. A much more detailed account will be published in this journal’. We include in the present report only a small fraction of our experimental results and a brief resume of the conclusions to which our results lead us. We have studied etching of Si and thermally grown Si02 in a commercial parallel plate etching apparatus (ET Associates, Nailsea, UK) with CFJOz gas mixtures at pressures of 0.2 torr, discharge power 70 W and frequency 62.5 kHz. Because of their general nature, the majority of our findings should however be applicable to many types of plasma systems. We have used emission spectroscopy of the plasma glow as a diagnostic tool throughout the present work. Other groups have used a similar technique S-6. As a simple working hypothesis we suggest that changes in emission intensity of an observed species in the presence of a substrate are a measure of the extent to which that species is involved in substrate etching. 2. Experimental Our experiments fall into two categories, those involving observation of the bulk emission of the plasma glow and those involving observation of spatially resolved emission. The latter represents a new and fruitful departure in spectroscopic investigations of discharge systems. (i) Bulk emission studies. We have observed the total emission of the glow in the presence of Si (100) and SiOZ substrates (resting on the earthed cathode) at a spectral resolution of -0.3 A over the
wavelength range BOO-7100 8, using a 0.5 m spectrometer (SPEX 1870). A number of important species have been identified in the emission, among which are F, F+, CO, CO:, CF,, C, 0, 0 +, Si and Si + in a variety of electronic states. There are a number of significant differences between spectra obtained in the presence and absence of wafer material and these are briefly described and discussed in sections 3(i) and 4 below.
(ii) Spatially resolved emission studies. We have used a very fine (0.8 mm dia) quartz light probe to investigate the spatial distribution of F, F+, CO and CO;, recording spectrally resolved emission through the probe by situating it at different positions across the cathode. Concentration profiles were recorded in the absence of wafer material and in the presence of Si, Si02 and an inert sapphire wafer. The latter was included in order to take account of any systematic electrical perturbation to the plasma introduced through the presence of a dielectric resting upon the cathode, rather than through the chemical eflects in which we are interested. We find that concentration profiles are subject to a small but significant perturbation, especially for ions’. The probe is physically as small as is practicable and we believe that it introduces no important distortion into the plasma. This assertion is supported by experimental evidence outlined in Field et ai2. The tip of the probe points downwards towards the cathode and it may traverse a total distance of 8 cm parallel to the cathode (and wafer) surface, the diameter of the reactor being 15.5 cm. Light is admitted only through the tip of the probe by coating it with a film of Al paint of high electrical resistance. Spatial resolution is better than 3 mm across the cathode. Typical profiles of the F atom emission at 7037.5 A (2P-2P0) over Si and SiO, and in the absence of a wafer are shown in Figure 1. A great deal more data for F, F+, CO and CO: are presented in Field et al’. Results are discussed in Section 3(ii) and 4 below. 347
D Field et al: Spatially
resolved
optlcal
spectroscopy
of plasma
etching
systems
in the presence and absence of Si material in transitions m\ol\mg triplet (at right) and singlet (at left) states of the atomic ion. Triplet state emission is largely unaffected, whereas singlet emission drops by a factor of _ 10. Electronic state dependent behaviour has also been found for F atoms. 0’ and possibly C atoms. Among molecular species, CO; emission in both its .x and B states falls in intensity by a factor of 4 2 in the presence of Si. CO. CO’ and CF2 emission intensities are unaffected.
From edge
of chamber
(cmi
Figure 1. The profile of the F atom emission ( x concentration of F(*P )J at 7037.5 is (‘P-‘P ) across the chamber. parallel to the cathode. from close lo the wall to the centre. Profiles are shown in the presence of Si and SO, substrates and in the absence of a wafer. (Profile for sapphire wafer not shown. 1 3. Results (i) Bulk emission studies. In general, major changes of emission intensity do not occur on the introduction of SiO, substrate into the chamber. However. CF, emission (2 ‘B, -,? ‘A,) around 2500 A is reduced in intensity by a factor of _ 2 in the presence of SiO,. CF, reacts exothermically with SiO, to form SiOF, and CO (AC”= - 0.604 eV) and our results support the notion that species CF, are important in etching SiOz’.“. The effect of the introduction of Sl into the chamber contrasts with that ofSi0,. In particular. a number of atomic emission lines are strongly reduced in intensity. Our most striking observation is that different states of the same species behave quite differently. This is shown in Figure 2. m which may be seen the emission of F’
(ii) Spatially resolved emission studies. By integrating the mtensltl of emission over the range 6.0 7.5 cm from the chamber wall (close to the centre of the wafer) in the presence of a sapphire wafer and comparing this integrated intensity with that found in the presence of Sl or SiO,, one may obtain a direct measure of the degree of involvement of F. F +, CO or CO; in the etchmg process. This provides a much more sensitive diagnostic than the bulk emission. since the latter samples preferentially the outer part of the discharge. specifically that part not over the wzafer material. Thus. F atom emission (at 7037.5 A) falls by a factor of 10 over a SI wafer (Figure 1) whereas the fall is through a factor of 3.8 in the bulk emission. Again. CO: falls by a factor of 5 over a Si wafer. whereas the bulk emission, as we have noted above. shows a fall of only a factor of _ 2. F’ in its triplet ground state (3P) does not react significantly with Si (or SiOz) substrates. A description of the role of CO (a “fI) may be found in Field ef al’.
4. Discussion
Figure 2. Emission of F’ m the sin ler transnion at 3202.7 A (ID +‘DJ and the triplet transition at 4025.5 1 (3D-+-3P~ In the absence of a wafer
Our results refer almost exclusively to etching of SI. Plasma emission would appear to give little information on the nature of Si02 etching. since the great majority of fluorescent species are not strongly involved. Our discussion and conclusions are therefore only relevant to Si etching. Our observations show that the chemical nature of the ion is important in etching Si, and, moreover, that different electronic states of any one ion type show very different reactivity to Si substrate. Furthermore, our results form a coherent body ifwe use straightforward thermochemical arguments to explain the reactivity or otherwise of the various species we have observed. These findings suggest that ions are not impacted into the Si surface at high translational energy and that etching of Si does not proceed in the ion-assisted manner that has previously been proposed for plasma etching in genera19,‘0. In the present system the substrate is electrically insulated from the earthed cathode and behaves as a floating substrate introduced into the plasma. There are good theoretical grounds for believing that the sheath potential is onl) 0.5-1.0 V under our conditions’.” and thus that ions impact at energies 5 1 eV; this supports our experimental findings. We may explain many of our observations if we introduce the notion that electron spin is conserved in gas-surface interactions, just as in purely gas phase reactions. As one example, singlet F’ may react with Si, known to have a ground state which is singlet in character”, to give singlet SiF’, whereas the triplet state of F’ cannot react in a spin-allowed manner to give a singlet product. In order to test the idea of spin conservation in gas-surface reactions, we predicted that Si (100) should etch more rapidly than Si (I II ) in a fluorine atom rich plasma as we have here. Figure I shows that F atoms are largely responsible for etching of Si (100). This process may be represented by the spin-allowed reaction.
and in the presence of a Si wafer. Note the very different behavlour s&et and triplet transition mtensltles.
F (2P)+‘Si-+2SiF
No wafer
SI wafer
3202
Wavelength
348
0
4025
/ 8
of the
(1)
D Field et a/: Spatially resolved optical spectroscopy of plasma etching systems
in which Si (100) is represented by ‘Si. Si (111) presents a surface of doublet character to the attacking F atomsI and doublet F atoms cannot therefore react in a spin-allowed manner to give SiF product. Experimentally it was found that Si (100) etched 4.5kO.15 times more rapidly than Si (111). This result lends support to the principle of spin conservation in gas-surface reactions. Following on from this, if singlet F’ may react at the Si surface, then it will impart to the surface some fraction of its internal energy of electronic excitation, 5 30 eV in total. Thus metastables of high internal energy may contribute to an enhancement of surface reactivity in place of high energy ion bombardment. Figure 1 illustrates two further points. The first relates to the gradient of F atoms introduced into the plasma in the presence of the Si substrate. It is clear that in the presence of many wafers there will be competition for the available F atoms and the results of Figure 1 therefore illustrate the so-called ‘loading effect’ (see elsewhere in this issue). The second point is the general lack of uniformity in F atom concentration exemplified on both the large and the small scale. This will be reflected in a lack of etch uniformity, a major problem in the microfabrication industry. Our results suggest how the problem of nonuniformity might be tackled and this is discussed in more detail in Field et al*. 5. Conclusions
In summary, our results suggest that Si etching involves the following mechanisms: (i) chemical attack largely by F atoms; (ii) the rate of attack is strongly crystal plane dependent, being faster for Si (100) than Si (111); (iii) energy is imparted to the
surface from metastables of high internal energy, a process which may lead to the effective evaporation of Si from the surface; (iv) in gas-surface reactions spin angular momentum is conserved in the manner well-known in gas phase reactions. This latter appears to be a new concept and may prove useful in elucidating mechanisms and predicting behaviour in plasma systems in general.
Acknowledgement
We should like to thank E T Associates for supplying equipment and materials and the SERC for a CASE award for one of us (AJH) held with E T Associates.
References
‘ J W Coburn, J Vat Sci Techno/. 21, 557 (1982). 2 D Field, A. J. Hydes and D F Klemperer, Vacuum accepted for publication. 3 W R Harshbarger. R A Porter, T A Miller and P Norton. Appl Specrrosc, 31,201 (1977). 4 B J Curtis and H J Brunner, J Elecrrochem Sot, 125, 829 (1978). ’ C J Mogab, A C Adams and D L Flamm, .I Appl Phys, 49,3796 (1978). 6 R d’Agostino, F Cramarossa, S De Benedictis and G Ferraro, .I Appl Phys. 52, 1259 (1981). ’ D L Flamm, Solid Srare Tech/, 22, 109 (1979). * R A H Heinecke. So/id Srare Electron. 18. 1146 (1975) 9 J W Coburn and H F Winters, J Vat Sci Tech& 16; 391 (1979). *’ G Smolinsky and D L Flamm, J Appl Phys, SO, 4982 (1979). ‘I B N Chapman, Glow Discharge Processes, pp 62-64. Wiley, New York (1980). ‘* A Redondo and W A Goddard, J Vat Sci Technol, 21, 344 (1982). I3 S Ciraci, J P Batra and W A Tiller, Phps Reo, B12, 5812 (1975).
349