- Email: [email protected]
Energy fluxes in mixed microwave-r.f. plasma
Thin Solid Films, 193/194 (1990) 155-163
155
E N E R G Y F L U X E S IN M I X E D M I C R O W A V E - R . F . PLASMA O. M. KUTTEL*, J. E. KLEMBERG-SAPIEHA, L. MARTINU AND M. R. WERTHEIMER
"Groupe des Couches Mhwes" and Department of Engineering Physics, Ecole Polytechnique, Box 6079, Station A. Montreal, Quebec H3C 3A 7 (Canada)
Energy fluxes and ion densities in dual-frequency microwave-r.f, plasma have been measured in argon, 02, N2 and N H 3, in the pressure range from 20 to 100 mTorr, and for electrode bias voltage values Vb ranging from 0 to - 2 5 0 V. The ion flux to the powered electrode was found to be up to 50 times higher in the dualfrequency plasma than in "pure r.f." plasma, depending on the pressure, bias voltage and gas. The measured maximum ion energy agrees well with the potential drop across the sheath in front of the cathode, in N 2 and N H 3 plasma. However, ion energies up to 50 eV above the expected value were observed in argon and 02. While a collisionless model explains the ion current to the cathode in an r.f. discharge, a comprehensive model for the dual-frequency plasma still remains to be developed.
1. INTRODUCTION Thin film deposition is frequently accompanied by bombardment with energetic particles, in order to control film composition, structure and other properties ~. While some techniques operate with an ion source which is independent from that providing the depositing material, r.f. plasmas have been recognized to be autonomous sources of ion bombardment and hence powerful tools in plasma sputtering and plasma-enhanced chemical vapour deposition. Since the substrates are exposed to the plasma, ion and neutral bombardment of the growing film occurs. The energy and flux of the impinging particles are important parameters, allowing thin films to be grown at the grounded electrode at low energies, or at the biased electrode (d.c. or r.f. self-biased) with ion energies (up to several hundred electronvolts) determined by the sheath potential. The actual choice is dictated by several considerations, for example the substrate and thin film materials to be deposited. We have recently begun to employ a dual-frequency approach for deposition of thin films 2. This technique uses a microwave tMW) discharge at 2.45 G H z to produce precursors in the gas phase, while at the same time the substrate electrode is * On leave from the Department of Physics, Universityof Fribourg, CH-1700 Fribourg, Switzerland. 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
o.M. KOTTELet al.
156
capacitively coupled to a 13.56MHz r.f. power source. While other plasma parameters are kept constant, selectively controlled film compositions and characteristics are achieved by varying the r.f. self-bias potential, and hence the energy and flux of the ions impinging on the substrate electrode. The advantages of this technique are the high deposition rate (typically 30 ~ s- 1 for silicon nitride, p-SiN 3 characteristic of M W plasmas, the ability to reduce substrate temperature while maintaining good film quality, and the bias-controlled changes in film properties. In this paper we report measurements of ion energies and fluxes in MW, r.f. and dual-frequency plasma (MW-r.f.) using an electrostatic energy analyser. We show how the different excitation modes affect the plasma-surface interactions in argon, 02, N2 and N H 3 plasmas. We also report measurements of the ion density in the plasma bulk using a Langmuir probe, and we correlate these with the flux measured on the r.f.powered electrode for different pressures and bias potentials. The measurements we present here are meant to elucidate the role of plasma-surface interactions in chemically active plasma, for example NH3-SiH4 used to deposit silicon nitride films. Correlations between ion fluxes and film properties are presented elsewhere at this conference 3. 2.
EXPERIMENTAL DETAILS
The stainless steel plasma reactor contains a 1.5 cm diameter electrode which faces a fused silica window through which the MW power (2.45 GHz, pulsed at 120Hz) is supplied from a slow wave applicator 4's. The substrate electrode is capacitively coupled to an r.f. generator (13.56 MHz); its negative d.c. self-bias voltage I/b is measured with respect to ground with a voltmeter using an r.f. choke. The electrode is located 4.5 cm above the silica window. The electrostatic analyser 6-a is shown in Fig. 1. It consists of two grids, the first at the substrate electrode potential I/b, the second at a variable retarding potential I/R. The mesh hole size, 20 lam x 20 ~tm, is much smaller than the Debye length, which prevents the plasma from penetrating into the probe. The ion collector is at a fixed
l 1 I ion flux
'°°t
v////J
V///"AAgrid linsulofo r -grid ]I
v////,~
VIllA
in s u [o to r
collector
v//////////////~
E vo
vc
_,.:-
Ic
Fig. 1. Electrostaticenergyanalyser(gridmeshholes,20pm connected to the powered electrode.
x20pm; V~ = - 100 V). Grid l iselectrically
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
157
potential Vc = - 100V, and the current Ic is measured with an electrometer. In order to simplify the construction of the probe no repeller electrode for secondary electrons is used. The resulting error is assumed not to disturb the ion current in a s i g n i f i c a n t w a y 7'9. Information available from I~ (VR) dependence includes the maximum ion energy and the ion energy distribution function (IEDF), which can be obtained from the derivative of I~ with respect to the retarding voltage VR. In this work we concentrate on the maximum ion energy. To minimize possible r.f. influence, the analyser is placed at the rear of the electrode, and all electrical connections are shielded. The ion flux was determined from the measured ion current, normalized to the grid transparency and to the analyser area. The Langmuir probe was made of an NiCr wire of 0.31 mm diameter inserted into a Pyrex capillary, and its active length was 6 mm. The probe was placed in the plasma, 28 mm in front of the electrostatic analyser. The probe was kept at a negative potential of - 200 V between the measurements, to avoid its contamination by film deposition. The measurements reported here were carried out in argon, 02, N2 and N H 3 plasmas, in a pressure range from 20 to 100 mTorr, and for bias voltages Vb ranging from 0 to - 250 V, during which the corresponding r.f. power was varied from 0 to 80W. The feed gas flow rate Q = 10 standard cm 3 m i n - l , microwave power PMW = 150 W and substrate temperature Ts = 25 °C were kept constant throughout all experiments. 3. ANALYSIS OF LANGMUIR PROBE DATA Despite the simplicity of collecting probe data, their interpretation is complicated in an r.f. plasma, where the current driven by the probe depends on the applied r.f. field. Several researchers have dealt with the analysis of such measurements ~°-13. Comprehensive numerical results have been presented by Laframboise 14 in a form which is useful to the experimentalist. Although the qualitative description of the electron current region is straightforward, the interpretation of the ion saturation current in an r.f. plasma is more reliable, owing to the ions' lower mobility. Assuming that the ion temperature Ti is smaller than the electron temperature To, which is typically a few electronvolts, the ion current towards a cylindrical probe of radius rp is given as ~4
12= e2n2A2(k Te/2~ml)i 2
(1)
where n~ is the ion density, A the probe surface, m~ the ion mass, and il is a dimensionless current which depends on rv/2 D and on the potential Xdefined as
X =- e(Vp - Vs)/k Te
(2)
2D is the Debye length, and Vs and Vp are the plasma and probe potentials respectively. As long as rp/2D ~< 3 and Ti/T , ~, 1, i~ is given by i 2 = 1.27(- X)
(3)
Thus, the square 12 of the ion saturation current is a linear function of the probe
158
o.M. KOTTELet al.
potential Vp, with a slope determined by the ion density: dl~ --
- -
dVp
0.20e3n.2,A2 =
m~
(4)
With a slightly simpler theory, Chen' ' obtained a factor of 0.24 instead of 0.20 in eqn. (4). The difference is mainly due to the ion sheath, which shields the probe only incompletely. The electron temperature may be deduced from the slope of a log I c vs. Vp plot, where the electron current Ic is given by
I~ = eN~A(k TJ2~m~) 1/2 exp X
(5)
As already mentioned, this current is influenced by the r.f. field on account of the higher electron mobility. One must therefore be careful in interpreting electron temperature and plasma potential values obtained from eqn. (5). In this paper we concentrate on the ion density, and use T~ only as a semiquantitative estimate. 4.
RESULTS AND DISCUSSION
All the gases we have investigated exhibit similar behaviours regarding the scaling of ion density and flux with gas pressure p and electrode bias voltage Vb. By way of examples, Figs. 2 and 3 present the results for argon in r.f.- and MW-r.f.excited plasma as functions of p and Vb respectively. While the ion density in r.f. plasma increases slightly with the pressure, the ion flux decreases on account of collisions (Fig. 2(a)). The effect of Vb in an r.f. discharge is shown in Fig. 3(a). In order to increase Vb, more power must be deposited in the plasma, which results in the observed higher ion density and flux. In dual-frequency plasma, the ion density exhibits a similar dependence on pressure and bias voltage as in the case of r.f. plasma. At low pressure the flux is increased by approximately 50 times. However, the ion flux drops sharply at a pressure of 20-40 mTorr. At 100 mTorr the ion flux reaches 5 x 1014 ions c m - 2 s- ~, and varies only slightly with lib; the dependence on bias voltage is shown in Fig. 3(b). Ion densities and fluxes are compared in Table I for different gases and pressures in r.f. and MW-r.f. plasmas for Vb = - 150 V and Puw = 150 W. It should be noted at this point that we did not relate the measured plasma parameters to the total power (MW+r.f.) absorbed in the plasma. Such an approach could be misleading, because in the case of dual-frequency MW-r.f. plasma the power density is not homogenously distributed throughout the discharge zone as judged, for example, on the basis of light emission from the plasma. Since the probe was located near the powered electrode, and hence near the sheath region, its data reflect more the effect of r.f. than that of the MW power. Consequently, a simple summation of the input powers would not appear to be representative of the processes involved. For example, n i per unit power absorbed in "pure" r.f. plasma is higher than in a comparable MW-r.f. discharge. On the contrary, the corresponding tpi value is substantially higher in the MW-r.f. discharge. This underlines the important role of ion flux in MW-r.f. discharges which we can characterize with the help of the
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
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I
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, 60
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, 80
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p [mTorr ] Fig. 2. Ion density and ion flux v s . pressure in an argon plasma: O, A, ion density; O , / X ion flUX;0, O, Vb = - 40 V; A,/X, Vb = - 200 V;(a) r.f. plasma, (b) M W-r.f. plasma.
cathode bias voltage Vb; an important objective of this work is to study the effect of this parameter on film growth and film characteristics ~. The ratio of ion flux in the MW-r.f. and in the r.f. mode at 8 0 m T o r r is shown plotted in Fig. 4 for different gases. It illustrates the important increase in ion flux tpi at low Vb, and a steady state value between 5 and l0 at Vb = - 2 0 0 V . The largest flux increase is observed for the case of N H a, probably because of ionization of molecular fragments in the MW-r.f. plasma. Nevertheless, there appears to be no straightforward relation between the ion density of the bulk plasma and the ion flux to the electrode. The rate at which electron impact reactions take place is proportional to the electron density, the density of reactant molecules and the reaction rate constant
160
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I
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/
0
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._~
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u
i
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Fig. 3..Ion density and ion flux vs. bias voltage Vb in an argon plasma: O, 4k, ion density; O , A, ion flux; 0, O , p = 40 mTorr; 4k, A , p -- 100 mTorr; (a) r.f. plasma; (b) MW-r.f. plasma. TABLE I ION DENSITY AND FLUX FOR DIFFERENT GASES AND PRESSURES Gas
lon density
lollflllX
( x 101 o cm - 3)
( X 1014 c n ' l - 2 $ - 1}
p = 40mTorr
p = lOOmTorr
p = 40mTorr
p = lOOmTorr
R.f .
M W-r.f .
R.f .
M W-r.f .
R.f .
M W-r.f .
R.f .
g W-r.f .
Ar 02
0.95 0.69
1.8 2.1
1.0 0.86
2.2 1.9
1.4 2.1
17.4 21.8
0.9 2.2
5.6 8.7
N2 NH3
0.43 0.31
1.6 0.95
0.52 0.34
1.6 1.2
0.49 1.3
15.3 17
0.28 1.4
4.1 12.4
R.f. self-bias voltage Vb = - 150 V and microwave power P ffi 150 W.
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
50
161
\
NH~
\ ~o
\ \ \
30 o ~
20
\\
\
\
N2..=\\ \ \~, \\ Ar\e~
10 \ O.,.,,,, '
-100 '
'
200
vb Iv] Fig.4. Ion fluxratio (fluxin dual-frequencyplasma to fluxin r.f. plasma)vs. bias voltagefor different gases(p = 80mTorr).
which, in general, increases with increasing electron energies. As shown elsewhere1 s. 16 the MW plasma has a comparatively higher population of energetic electrons. Since the cross-sections for most electron impact reactions have threshold energies, these energetic electrons c.an substantially increase the reaction rate constant. Therefore, the MW or MW-r.f. plasma increases not only the plasma density but also the reaction rate constant ~6. Both effects then lead to the observed high fluxes in the MW-r.f. mode. In a collisionless sheath the maximum ion energy impinging on the electrode is given by the free-fall energy eVsh, where Vsh = Vs- Vb. Measurements in NH 3 and N 2 plasmas are in good agreement with this calculation for pressures up to 100 mTorr in r.f. and MW-r.f. plasmas. Nevertheless, collisions in the sheath give rise to an observable amount of low energy ions, especially in NH 3 plasmas. In oxygen and argon plasma, however, ion energies which are considerably higher than e Vsh are observed for pressures lower than 60mTorr; for example, in MW-r.f. plasma, ion energies up to 50 eV above this value have been observed. In addition, an IEDF with two maxima was found for argon below 20mTorr. Several researchers reported structured IEDFs ~7-2t, which they took to reflect a modulation of the sheath thickness by the r.f. field. Recently, Kuypers and Hopman 22 reported a splitting in the IEDF above the value ofe V~hin argon and oxygen plasma, measured at the powered electrode. An understanding of splitting of the IEDF, especially in the MW-r.f. mode, needs further experimental work. In theory, fluxes to an electrode are expressed by the space-charge-limited
162
o.M. K/2TTELet al.
current j, given by the Child-Langmuir law for the case ofcoll,isionless sheaths, i.e. 4~o{2e~ '/2 V~h 3/2
(6)
or by the mobility-limited version for collisionai sheaths23: 9Co Vsh2 J = --8- P d 3 (7) In these equations eo and e have their usual meanings, dis the sheath thickness, and/a the ion mobility. We found j = 43 laA cm- 2 at 60 mTorr and Vsh = 280 V in r.f. plasma in argon; the collisionless model (eqn. (6)) predictsj = 50 }aAcm- 2 assuming d---0.9cm, in good agreement with the observed value. Moreover, this model predicts reasonably well the ion current density for all gases up to 80 mTorr in r.f. plasmas. At higher pressures, collisions in the sheath reduce the current to a value between those predicted by the collisionless and collisional models (eqns. (6) and (7)) The ratio 2mrp/dof ion-neutral mean free path to the sheath thickness is smaller than unity at lower pressure and reaches values higher than unity at 100mTorr. This confirms our experimental results. The high fluxes in MW-r.f. plasmas can only be explained by a collisionless sheath if we assume reduced sheath thicknesses of the order of 1 mm, which have not yet been confirmed experimentally. The transition between collisionless and mobility-limited sheaths occurs when the sheath thicknesses computed from eqns. (6) and (7) are equal. In MW-r,f. plasmas this condition is already satisfied for p = 40mTorr in argon. Thus, it appears that the collisionless model is not the appropriate theory to describe a dual-frequency plasma, even at lower pressure. Perrin et al. 24 reported a drastic increase in deposition rate and ion fluxes in the so called a-7 transition region, where ion-induced secondary electron emission and ionization in the sheath turned out to be the key mechanism. Whether such a transition accounts for the high fluxes in dual-frequency plasma still remains a subject for further investigations. 5. CONCLUSIONS
Dual (MW-r.f.) frequency plasma is characterized by a strong increase in ion flux to the powered electrode in comparison with pure r.f. plasma. The ion flux exhibits a dramatic increase, up to .50times, an effect which is most pronounced at low pressures and at low Vb values. Measurements in argon, 02, N2 and NH 3 display the same qualitative behaviour. The ion flux in r.f. plasma obeys the collisionless Child-Langmuir law up to 80mTorr, while its behaviour in dualfrequency plasma is at present not well understood. Ion energies above the expected free-fall energy have been observed in 02 and argon plasma, together With a structured IEDF at pressures lower than 20 mTorr for the latter gas. The results reported here explain several important features observed earlier in
ENERGY FLUXES IN MICROWAVE--R.F.PLASMA
163
dual-frequency p l a s m a deposition, n a m e l y high d e p o s i t i o n rate a n d bias-voltagei n d u c e d changes in film properties. ACKNOWLEDGMENTS This work was s u p p o r t e d in part by the Swiss N a t i o n a l Science F o u n d a t i o n , the N a t u r a l Sciences a n d E n g i n e e r i n g Research C o u n c i l of C a n a d a (NSERC), a n d the funds, " F o r m a t i o n des Chercheurs et Aide ~i la Recherche" (FCAR) of Quebec. T h e a u t h o r s are grateful to Mr. G. Jalbert for expert technical assistance. REFERENCES I J.M.E. Harper, J. J. Cuomo, R. J. Gambino and H. R. Kaufmann, in O. Auciello and R. Kelly (eds.), Ion Bombardment Modification of Surfaces, Elsevier,Amsterdam, 1984, Chap. 4. 2 L. Martinu, J. E. Klemberg-Sapieha and M. R. Wertheimer, Appl. Phys. Lett., 54 (1989) 2645. 3 J.E. Klemberg-Sapieha,O. M. Kiittel, L. Martinu and M. R. Wertheimer, Thin Solid Films, (1991) in the press. 4 M . R . Wertheimer, J. E. Klemberg-Sapieha and H. P. Schreiber, Thin Solid Films, 115 (1984) 109. 5 M . H . Bernier, J. E. Klemberg-Sapieha, L. Martinu and M. R. Wertheimer, Proc. Int. Symp. on Metallization of Polymers, Montrdal, September 1989, ACS Symposium Series, American Chemical Society, 1990. 6 Y. Catherine and A. Pastol, in P. Koidl and P. Oelhafen (eds.), Proceedings of the European Materials Research Society, Vol. XVII, Les Editions de Physique, Paris, 1987, p. 145. 7 B.Drevi•••n•J.•errin•J.M.Siefert•J.Huc•A.L••ret•G.deR•snyandJ.P.M.Schmitt•App•.Phys. Lett., 42 (1983) 801. 8 A.M. Antoine, B. Drevillon and P. Roca-i-Cabarrocas, J. Appl. Phys., 61 (1987) 2501. 9 R.L. Stenzel, R. Williams, R. Agiiero, K. Kitazaki, A. Ling, T. McDonald and J. Spitzer, Rev. Sci. Instrum., 53 (1982) 1027. l0 1. Langmuir, in G. Suits (ed.), Collected Works oflrving Langmuir, Vol. 4, Macmillan, New York, 1961. I l F.F. Chen, in R. H. Huddleston and S. L. Leonard (eds.), Plasma Diagnostic Techniques, Academic Press, New York, 1965, Chap. 4. 12 R.M. Clements, J. Vac. Sci. Technol., 15 (1978) 193. 13 N. Herschkowitz, in O. Auciello and D. L. Flamm (eds.), Plasma Diagnostics, Vol. 1, Academic Presss, New York, 1989, Chap. 3. 14 J.G. Laframboise, Rep. 100, 1986 (Institute of Aerospace Studies, University of Toronto). 15 M.R. Wertheimer and M. Moisan, J. Vac. Sci. Technol. A, 3 (1985) 2643. 16 M. Moisan, C. Barbeau, R. Claude, C. M. Ferreira, J. Margot-Chaker, J. Paraszczak, A.B.Sa, G. Sauv6 and M. R. Wertheimer, J. Vac. Sci. Technol., (1991) in the press. 17 W.M. Greene, M.A. Hartney, W.G. OldhamandD. W. Hess, J. Appl. Phys.,63(1988) 1367. 18 J.W. Coburn and E. Kay, J. Appl. Phys., 43 (1972) 4965. 19 K. K6hler, J.W. Coburn, D.E. Horne, E. KayandJ. H, Keller, J. AppI. Phys.,57(1985)59. 20 Ch. Wild and P. Koidl, Appl. Phys. Lett., 54 (1989) 505. 21 Y. Okamoto and H. Tamagawa, J. Phys. Soc. Jpn., 29 (1970) 187. 22 A.D. Kuypers and H. J. Hopman, J. Appl. Phys., 63 (1988) 1894. 23 B. Chapman, Glow Discharge Processes, Wiley, New York, 1980. 24 J. Perrin, P. Roea-i-Cabarrocas, B. Allain and J.-M. Friedt, Jpn. J. Appl. Phys., 27 (1988) 2041.
155
E N E R G Y F L U X E S IN M I X E D M I C R O W A V E - R . F . PLASMA O. M. KUTTEL*, J. E. KLEMBERG-SAPIEHA, L. MARTINU AND M. R. WERTHEIMER
"Groupe des Couches Mhwes" and Department of Engineering Physics, Ecole Polytechnique, Box 6079, Station A. Montreal, Quebec H3C 3A 7 (Canada)
Energy fluxes and ion densities in dual-frequency microwave-r.f, plasma have been measured in argon, 02, N2 and N H 3, in the pressure range from 20 to 100 mTorr, and for electrode bias voltage values Vb ranging from 0 to - 2 5 0 V. The ion flux to the powered electrode was found to be up to 50 times higher in the dualfrequency plasma than in "pure r.f." plasma, depending on the pressure, bias voltage and gas. The measured maximum ion energy agrees well with the potential drop across the sheath in front of the cathode, in N 2 and N H 3 plasma. However, ion energies up to 50 eV above the expected value were observed in argon and 02. While a collisionless model explains the ion current to the cathode in an r.f. discharge, a comprehensive model for the dual-frequency plasma still remains to be developed.
1. INTRODUCTION Thin film deposition is frequently accompanied by bombardment with energetic particles, in order to control film composition, structure and other properties ~. While some techniques operate with an ion source which is independent from that providing the depositing material, r.f. plasmas have been recognized to be autonomous sources of ion bombardment and hence powerful tools in plasma sputtering and plasma-enhanced chemical vapour deposition. Since the substrates are exposed to the plasma, ion and neutral bombardment of the growing film occurs. The energy and flux of the impinging particles are important parameters, allowing thin films to be grown at the grounded electrode at low energies, or at the biased electrode (d.c. or r.f. self-biased) with ion energies (up to several hundred electronvolts) determined by the sheath potential. The actual choice is dictated by several considerations, for example the substrate and thin film materials to be deposited. We have recently begun to employ a dual-frequency approach for deposition of thin films 2. This technique uses a microwave tMW) discharge at 2.45 G H z to produce precursors in the gas phase, while at the same time the substrate electrode is * On leave from the Department of Physics, Universityof Fribourg, CH-1700 Fribourg, Switzerland. 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
o.M. KOTTELet al.
156
capacitively coupled to a 13.56MHz r.f. power source. While other plasma parameters are kept constant, selectively controlled film compositions and characteristics are achieved by varying the r.f. self-bias potential, and hence the energy and flux of the ions impinging on the substrate electrode. The advantages of this technique are the high deposition rate (typically 30 ~ s- 1 for silicon nitride, p-SiN 3 characteristic of M W plasmas, the ability to reduce substrate temperature while maintaining good film quality, and the bias-controlled changes in film properties. In this paper we report measurements of ion energies and fluxes in MW, r.f. and dual-frequency plasma (MW-r.f.) using an electrostatic energy analyser. We show how the different excitation modes affect the plasma-surface interactions in argon, 02, N2 and N H 3 plasmas. We also report measurements of the ion density in the plasma bulk using a Langmuir probe, and we correlate these with the flux measured on the r.f.powered electrode for different pressures and bias potentials. The measurements we present here are meant to elucidate the role of plasma-surface interactions in chemically active plasma, for example NH3-SiH4 used to deposit silicon nitride films. Correlations between ion fluxes and film properties are presented elsewhere at this conference 3. 2.
EXPERIMENTAL DETAILS
The stainless steel plasma reactor contains a 1.5 cm diameter electrode which faces a fused silica window through which the MW power (2.45 GHz, pulsed at 120Hz) is supplied from a slow wave applicator 4's. The substrate electrode is capacitively coupled to an r.f. generator (13.56 MHz); its negative d.c. self-bias voltage I/b is measured with respect to ground with a voltmeter using an r.f. choke. The electrode is located 4.5 cm above the silica window. The electrostatic analyser 6-a is shown in Fig. 1. It consists of two grids, the first at the substrate electrode potential I/b, the second at a variable retarding potential I/R. The mesh hole size, 20 lam x 20 ~tm, is much smaller than the Debye length, which prevents the plasma from penetrating into the probe. The ion collector is at a fixed
l 1 I ion flux
'°°t
v////J
V///"AAgrid linsulofo r -grid ]I
v////,~
VIllA
in s u [o to r
collector
v//////////////~
E vo
vc
_,.:-
Ic
Fig. 1. Electrostaticenergyanalyser(gridmeshholes,20pm connected to the powered electrode.
x20pm; V~ = - 100 V). Grid l iselectrically
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
157
potential Vc = - 100V, and the current Ic is measured with an electrometer. In order to simplify the construction of the probe no repeller electrode for secondary electrons is used. The resulting error is assumed not to disturb the ion current in a s i g n i f i c a n t w a y 7'9. Information available from I~ (VR) dependence includes the maximum ion energy and the ion energy distribution function (IEDF), which can be obtained from the derivative of I~ with respect to the retarding voltage VR. In this work we concentrate on the maximum ion energy. To minimize possible r.f. influence, the analyser is placed at the rear of the electrode, and all electrical connections are shielded. The ion flux was determined from the measured ion current, normalized to the grid transparency and to the analyser area. The Langmuir probe was made of an NiCr wire of 0.31 mm diameter inserted into a Pyrex capillary, and its active length was 6 mm. The probe was placed in the plasma, 28 mm in front of the electrostatic analyser. The probe was kept at a negative potential of - 200 V between the measurements, to avoid its contamination by film deposition. The measurements reported here were carried out in argon, 02, N2 and N H 3 plasmas, in a pressure range from 20 to 100 mTorr, and for bias voltages Vb ranging from 0 to - 250 V, during which the corresponding r.f. power was varied from 0 to 80W. The feed gas flow rate Q = 10 standard cm 3 m i n - l , microwave power PMW = 150 W and substrate temperature Ts = 25 °C were kept constant throughout all experiments. 3. ANALYSIS OF LANGMUIR PROBE DATA Despite the simplicity of collecting probe data, their interpretation is complicated in an r.f. plasma, where the current driven by the probe depends on the applied r.f. field. Several researchers have dealt with the analysis of such measurements ~°-13. Comprehensive numerical results have been presented by Laframboise 14 in a form which is useful to the experimentalist. Although the qualitative description of the electron current region is straightforward, the interpretation of the ion saturation current in an r.f. plasma is more reliable, owing to the ions' lower mobility. Assuming that the ion temperature Ti is smaller than the electron temperature To, which is typically a few electronvolts, the ion current towards a cylindrical probe of radius rp is given as ~4
12= e2n2A2(k Te/2~ml)i 2
(1)
where n~ is the ion density, A the probe surface, m~ the ion mass, and il is a dimensionless current which depends on rv/2 D and on the potential Xdefined as
X =- e(Vp - Vs)/k Te
(2)
2D is the Debye length, and Vs and Vp are the plasma and probe potentials respectively. As long as rp/2D ~< 3 and Ti/T , ~, 1, i~ is given by i 2 = 1.27(- X)
(3)
Thus, the square 12 of the ion saturation current is a linear function of the probe
158
o.M. KOTTELet al.
potential Vp, with a slope determined by the ion density: dl~ --
- -
dVp
0.20e3n.2,A2 =
m~
(4)
With a slightly simpler theory, Chen' ' obtained a factor of 0.24 instead of 0.20 in eqn. (4). The difference is mainly due to the ion sheath, which shields the probe only incompletely. The electron temperature may be deduced from the slope of a log I c vs. Vp plot, where the electron current Ic is given by
I~ = eN~A(k TJ2~m~) 1/2 exp X
(5)
As already mentioned, this current is influenced by the r.f. field on account of the higher electron mobility. One must therefore be careful in interpreting electron temperature and plasma potential values obtained from eqn. (5). In this paper we concentrate on the ion density, and use T~ only as a semiquantitative estimate. 4.
RESULTS AND DISCUSSION
All the gases we have investigated exhibit similar behaviours regarding the scaling of ion density and flux with gas pressure p and electrode bias voltage Vb. By way of examples, Figs. 2 and 3 present the results for argon in r.f.- and MW-r.f.excited plasma as functions of p and Vb respectively. While the ion density in r.f. plasma increases slightly with the pressure, the ion flux decreases on account of collisions (Fig. 2(a)). The effect of Vb in an r.f. discharge is shown in Fig. 3(a). In order to increase Vb, more power must be deposited in the plasma, which results in the observed higher ion density and flux. In dual-frequency plasma, the ion density exhibits a similar dependence on pressure and bias voltage as in the case of r.f. plasma. At low pressure the flux is increased by approximately 50 times. However, the ion flux drops sharply at a pressure of 20-40 mTorr. At 100 mTorr the ion flux reaches 5 x 1014 ions c m - 2 s- ~, and varies only slightly with lib; the dependence on bias voltage is shown in Fig. 3(b). Ion densities and fluxes are compared in Table I for different gases and pressures in r.f. and MW-r.f. plasmas for Vb = - 150 V and Puw = 150 W. It should be noted at this point that we did not relate the measured plasma parameters to the total power (MW+r.f.) absorbed in the plasma. Such an approach could be misleading, because in the case of dual-frequency MW-r.f. plasma the power density is not homogenously distributed throughout the discharge zone as judged, for example, on the basis of light emission from the plasma. Since the probe was located near the powered electrode, and hence near the sheath region, its data reflect more the effect of r.f. than that of the MW power. Consequently, a simple summation of the input powers would not appear to be representative of the processes involved. For example, n i per unit power absorbed in "pure" r.f. plasma is higher than in a comparable MW-r.f. discharge. On the contrary, the corresponding tpi value is substantially higher in the MW-r.f. discharge. This underlines the important role of ion flux in MW-r.f. discharges which we can characterize with the help of the
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
J
I
I
159
I
I
3
3
"'~Z~-2~O0 V
2
" " - " ~z~ ~Ak
Ak~ ~
A----
"-"
I~ "--
2
-200 v "-~
ul -°
--
=
-
=
-X
--e
I
c- 3
2
¢ o =
.0v
I
I
I
z~ \ \
-o
f
~'-
150 x
b) M W / R F -2o .ov.___ &
,~ \ . . . . . ~ = f
100
\ '\ -40 V
1
~, - • •~.
0
, 20
\-200
'~
"X~
- 4.0 V
"~'~'~
, 40
-0
---
. . . . .
50
V ..... '
". . . . . .
, 60
~ . . . . .
, 80
0
, 0 100
p [mTorr ] Fig. 2. Ion density and ion flux v s . pressure in an argon plasma: O, A, ion density; O , / X ion flUX;0, O, Vb = - 40 V; A,/X, Vb = - 200 V;(a) r.f. plasma, (b) M W-r.f. plasma.
cathode bias voltage Vb; an important objective of this work is to study the effect of this parameter on film growth and film characteristics ~. The ratio of ion flux in the MW-r.f. and in the r.f. mode at 8 0 m T o r r is shown plotted in Fig. 4 for different gases. It illustrates the important increase in ion flux tpi at low Vb, and a steady state value between 5 and l0 at Vb = - 2 0 0 V . The largest flux increase is observed for the case of N H a, probably because of ionization of molecular fragments in the MW-r.f. plasma. Nevertheless, there appears to be no straightforward relation between the ion density of the bulk plasma and the ion flux to the electrode. The rate at which electron impact reactions take place is proportional to the electron density, the density of reactant molecules and the reaction rate constant
160
o.M. KOTTELet al. I
I
I
3 o) RF
/
0
3
,,
2
/ mTorr/
/-,.
/
/
o"
/
I00 mTorr/
/ O/~40mTorr tn
'E
b0 to
O
l
x r-
t
o
I
30 x e.-
b) MW/RF
.~I.,,.A 100mTorr A ~~
._~
~O mTerr
20
100 mTorr
u
i
-100
MW
10
0
-200 v b iv1
Fig. 3..Ion density and ion flux vs. bias voltage Vb in an argon plasma: O, 4k, ion density; O , A, ion flux; 0, O , p = 40 mTorr; 4k, A , p -- 100 mTorr; (a) r.f. plasma; (b) MW-r.f. plasma. TABLE I ION DENSITY AND FLUX FOR DIFFERENT GASES AND PRESSURES Gas
lon density
lollflllX
( x 101 o cm - 3)
( X 1014 c n ' l - 2 $ - 1}
p = 40mTorr
p = lOOmTorr
p = 40mTorr
p = lOOmTorr
R.f .
M W-r.f .
R.f .
M W-r.f .
R.f .
M W-r.f .
R.f .
g W-r.f .
Ar 02
0.95 0.69
1.8 2.1
1.0 0.86
2.2 1.9
1.4 2.1
17.4 21.8
0.9 2.2
5.6 8.7
N2 NH3
0.43 0.31
1.6 0.95
0.52 0.34
1.6 1.2
0.49 1.3
15.3 17
0.28 1.4
4.1 12.4
R.f. self-bias voltage Vb = - 150 V and microwave power P ffi 150 W.
ENERGY FLUXES IN MICROWAVE--R.F. PLASMA
50
161
\
NH~
\ ~o
\ \ \
30 o ~
20
\\
\
\
N2..=\\ \ \~, \\ Ar\e~
10 \ O.,.,,,, '
-100 '
'
200
vb Iv] Fig.4. Ion fluxratio (fluxin dual-frequencyplasma to fluxin r.f. plasma)vs. bias voltagefor different gases(p = 80mTorr).
which, in general, increases with increasing electron energies. As shown elsewhere1 s. 16 the MW plasma has a comparatively higher population of energetic electrons. Since the cross-sections for most electron impact reactions have threshold energies, these energetic electrons c.an substantially increase the reaction rate constant. Therefore, the MW or MW-r.f. plasma increases not only the plasma density but also the reaction rate constant ~6. Both effects then lead to the observed high fluxes in the MW-r.f. mode. In a collisionless sheath the maximum ion energy impinging on the electrode is given by the free-fall energy eVsh, where Vsh = Vs- Vb. Measurements in NH 3 and N 2 plasmas are in good agreement with this calculation for pressures up to 100 mTorr in r.f. and MW-r.f. plasmas. Nevertheless, collisions in the sheath give rise to an observable amount of low energy ions, especially in NH 3 plasmas. In oxygen and argon plasma, however, ion energies which are considerably higher than e Vsh are observed for pressures lower than 60mTorr; for example, in MW-r.f. plasma, ion energies up to 50 eV above this value have been observed. In addition, an IEDF with two maxima was found for argon below 20mTorr. Several researchers reported structured IEDFs ~7-2t, which they took to reflect a modulation of the sheath thickness by the r.f. field. Recently, Kuypers and Hopman 22 reported a splitting in the IEDF above the value ofe V~hin argon and oxygen plasma, measured at the powered electrode. An understanding of splitting of the IEDF, especially in the MW-r.f. mode, needs further experimental work. In theory, fluxes to an electrode are expressed by the space-charge-limited
162
o.M. K/2TTELet al.
current j, given by the Child-Langmuir law for the case ofcoll,isionless sheaths, i.e. 4~o{2e~ '/2 V~h 3/2
(6)
or by the mobility-limited version for collisionai sheaths23: 9Co Vsh2 J = --8- P d 3 (7) In these equations eo and e have their usual meanings, dis the sheath thickness, and/a the ion mobility. We found j = 43 laA cm- 2 at 60 mTorr and Vsh = 280 V in r.f. plasma in argon; the collisionless model (eqn. (6)) predictsj = 50 }aAcm- 2 assuming d---0.9cm, in good agreement with the observed value. Moreover, this model predicts reasonably well the ion current density for all gases up to 80 mTorr in r.f. plasmas. At higher pressures, collisions in the sheath reduce the current to a value between those predicted by the collisionless and collisional models (eqns. (6) and (7)) The ratio 2mrp/dof ion-neutral mean free path to the sheath thickness is smaller than unity at lower pressure and reaches values higher than unity at 100mTorr. This confirms our experimental results. The high fluxes in MW-r.f. plasmas can only be explained by a collisionless sheath if we assume reduced sheath thicknesses of the order of 1 mm, which have not yet been confirmed experimentally. The transition between collisionless and mobility-limited sheaths occurs when the sheath thicknesses computed from eqns. (6) and (7) are equal. In MW-r,f. plasmas this condition is already satisfied for p = 40mTorr in argon. Thus, it appears that the collisionless model is not the appropriate theory to describe a dual-frequency plasma, even at lower pressure. Perrin et al. 24 reported a drastic increase in deposition rate and ion fluxes in the so called a-7 transition region, where ion-induced secondary electron emission and ionization in the sheath turned out to be the key mechanism. Whether such a transition accounts for the high fluxes in dual-frequency plasma still remains a subject for further investigations. 5. CONCLUSIONS
Dual (MW-r.f.) frequency plasma is characterized by a strong increase in ion flux to the powered electrode in comparison with pure r.f. plasma. The ion flux exhibits a dramatic increase, up to .50times, an effect which is most pronounced at low pressures and at low Vb values. Measurements in argon, 02, N2 and NH 3 display the same qualitative behaviour. The ion flux in r.f. plasma obeys the collisionless Child-Langmuir law up to 80mTorr, while its behaviour in dualfrequency plasma is at present not well understood. Ion energies above the expected free-fall energy have been observed in 02 and argon plasma, together With a structured IEDF at pressures lower than 20 mTorr for the latter gas. The results reported here explain several important features observed earlier in
ENERGY FLUXES IN MICROWAVE--R.F.PLASMA
163
dual-frequency p l a s m a deposition, n a m e l y high d e p o s i t i o n rate a n d bias-voltagei n d u c e d changes in film properties. ACKNOWLEDGMENTS This work was s u p p o r t e d in part by the Swiss N a t i o n a l Science F o u n d a t i o n , the N a t u r a l Sciences a n d E n g i n e e r i n g Research C o u n c i l of C a n a d a (NSERC), a n d the funds, " F o r m a t i o n des Chercheurs et Aide ~i la Recherche" (FCAR) of Quebec. T h e a u t h o r s are grateful to Mr. G. Jalbert for expert technical assistance. REFERENCES I J.M.E. Harper, J. J. Cuomo, R. J. Gambino and H. R. Kaufmann, in O. Auciello and R. Kelly (eds.), Ion Bombardment Modification of Surfaces, Elsevier,Amsterdam, 1984, Chap. 4. 2 L. Martinu, J. E. Klemberg-Sapieha and M. R. Wertheimer, Appl. Phys. Lett., 54 (1989) 2645. 3 J.E. Klemberg-Sapieha,O. M. Kiittel, L. Martinu and M. R. Wertheimer, Thin Solid Films, (1991) in the press. 4 M . R . Wertheimer, J. E. Klemberg-Sapieha and H. P. Schreiber, Thin Solid Films, 115 (1984) 109. 5 M . H . Bernier, J. E. Klemberg-Sapieha, L. Martinu and M. R. Wertheimer, Proc. Int. Symp. on Metallization of Polymers, Montrdal, September 1989, ACS Symposium Series, American Chemical Society, 1990. 6 Y. Catherine and A. Pastol, in P. Koidl and P. Oelhafen (eds.), Proceedings of the European Materials Research Society, Vol. XVII, Les Editions de Physique, Paris, 1987, p. 145. 7 B.Drevi•••n•J.•errin•J.M.Siefert•J.Huc•A.L••ret•G.deR•snyandJ.P.M.Schmitt•App•.Phys. Lett., 42 (1983) 801. 8 A.M. Antoine, B. Drevillon and P. Roca-i-Cabarrocas, J. Appl. Phys., 61 (1987) 2501. 9 R.L. Stenzel, R. Williams, R. Agiiero, K. Kitazaki, A. Ling, T. McDonald and J. Spitzer, Rev. Sci. Instrum., 53 (1982) 1027. l0 1. Langmuir, in G. Suits (ed.), Collected Works oflrving Langmuir, Vol. 4, Macmillan, New York, 1961. I l F.F. Chen, in R. H. Huddleston and S. L. Leonard (eds.), Plasma Diagnostic Techniques, Academic Press, New York, 1965, Chap. 4. 12 R.M. Clements, J. Vac. Sci. Technol., 15 (1978) 193. 13 N. Herschkowitz, in O. Auciello and D. L. Flamm (eds.), Plasma Diagnostics, Vol. 1, Academic Presss, New York, 1989, Chap. 3. 14 J.G. Laframboise, Rep. 100, 1986 (Institute of Aerospace Studies, University of Toronto). 15 M.R. Wertheimer and M. Moisan, J. Vac. Sci. Technol. A, 3 (1985) 2643. 16 M. Moisan, C. Barbeau, R. Claude, C. M. Ferreira, J. Margot-Chaker, J. Paraszczak, A.B.Sa, G. Sauv6 and M. R. Wertheimer, J. Vac. Sci. Technol., (1991) in the press. 17 W.M. Greene, M.A. Hartney, W.G. OldhamandD. W. Hess, J. Appl. Phys.,63(1988) 1367. 18 J.W. Coburn and E. Kay, J. Appl. Phys., 43 (1972) 4965. 19 K. K6hler, J.W. Coburn, D.E. Horne, E. KayandJ. H, Keller, J. AppI. Phys.,57(1985)59. 20 Ch. Wild and P. Koidl, Appl. Phys. Lett., 54 (1989) 505. 21 Y. Okamoto and H. Tamagawa, J. Phys. Soc. Jpn., 29 (1970) 187. 22 A.D. Kuypers and H. J. Hopman, J. Appl. Phys., 63 (1988) 1894. 23 B. Chapman, Glow Discharge Processes, Wiley, New York, 1980. 24 J. Perrin, P. Roea-i-Cabarrocas, B. Allain and J.-M. Friedt, Jpn. J. Appl. Phys., 27 (1988) 2041.