Substrate effects from an unbalanced magnetron

Thin Solid Films, 193/194(1990) 127-137

127

SUBSTRATE EFFECTS FROM AN UNBALANCED MAGNETRON R. P. HOWSON AND H. A. J'AFER

Department of Physics. Loughborough University of Technology. Loughborough, Leics. LEI I 3TU (U.K.) A. G. SPENCER

Vacuum Coating Group, Loughborough Consultants Ltd., Loughborough University of Technology, Loughborough. Leics. LEI 1 3TU (U.K.)

The manipulation of the plasma of a d.c. planar magnetron may be achieved easily by adjusting the magnetic field of the magnetron, in conjunction with the placing of the anode. Such an arrangement of an "unbalanced" magnetron has been studied with regard to the bias voltage that appears on the substrate and the resultant ion and electron currents that flow to it. These are compared with the heat load experienced by it, which is related to the energy dissipation of the magnetron system. The magnetron source is considered from the point of view of providing energy bombardment of the substrate, and the growing film, and the efficacy of this bombardment in initiating structural and chemical reactions of the surface, as well as giving a heat load. A typical unbalanced magnetron, made by us, gave an insulated substrate a bias voltage of 25-30 V with an ion current of 3.4 mA cm-2. The heat load was 100 mW cm- 2. Of the energy supplied to the magnetron 82~ went into the cooling water and 3~ into the substrate; the rest was dissipated by the plasma.

1. INTRODUCTION Conventional magnetron sputtering is known to result in low substrate heating, when compared with diode sputtering. In our magnetron system, extending the plasma to the substrate by modifying the magnetic confinement (the "unbalanced" magnetron) gives high density-low energy ion and electron bombardment of it. There is a significant flux of ions diffusing to the vicinity of the substrate, to be accelerated across the sheath region onto the substrate surface ~. Both the energy of the impinging ions and the arrival rate ratio of ions to the condensing atoms are recognized as important parameters in ion-assisted deposition. It is known that energetic particle bombardment during film deposition can strongly modify the structural and chemical properties of the resulting film 2. In a planar magnetron gas discharge, the rate of increase in the cathode voltage .with current is related to the magnitude of the gas density rarefaction which is dependent on the cathode sputter yield 3, and is strongly dependent on the target material 4. It is expected that this dependence will be reflected in substrate effects utilizing the plasma. Our previous investigations have indicated that sputtering atoms play a significant role in the 0040-6090/90/$3.50

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R . P . HOWSON, H. A. J'AFER, A. G. SPENCER

plasma characteristics and the heat load that is delivered to the substrate. Their density is a function of the discharge current and the argon gas pressure. Such an arrangement of an "unbalanced" magnetron has been studied in regard to the bias voltage and ion and electron currents that flow to the substrate; they are compared with the heat flow to it and the efficiency of the power conversion of the magnetron. 2. EXPERIMENTAL DETAILS The unbalanced d.c. magnetron of 80 mm diameter chosen for our studies is shown, with its magnetic configuration, in Fig. i. This has been described elsewhere 5-7. The maximum field strength measured at the cathode surface at a radial distance of 20-30 m m was 240 G; at this radius the most intense glow in the plasma, as well as the highest etching rate of the cathode-target, took place.

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Simple planar probes have been used to make our measurements of substrate effects. We have used two different probes; one of these was circular with a diameter o f 7.6 m m and was used to investigate plasma I - V characteristics, the other was square with an area of 1 cm 2 and was used to measure the ion current and heat load simultaneously. They were used with coaxial shields at the probe edges connected to the probe potential, to reduce the edge effect. The probes were positioned on the centre-line of the target, where the dense beam of plasma is created, at a distance of 7 0 m m from its surface. The square probe was of low thermal capacity, made of stainless steel, and was used to record the initial rate in rise of temperature through a thermocouple welded to its back surface, from which the absorbed power density could be calculated. The vacuum chamber was cylindrical with a length of 600 m m and a diameter of 500 mm. It was pumped by a 1000 min - ~ mechanical p u m p and a 3000s -1 diffusion pump. The working gas was argon of 99.99~ purity. Two platinum resistance temperature detectors were used to measure the heat losses into the cooling water; they were placed in the inlet and outlet water pipes of the magnetron. Four-wire sensor operation was used to obtain m a x i m u m accuracy. Glass substrates were coated and the thickness of the film was measured using the

SUBSTRATE EFFECTS FROM AN UNBALANCED MAGNETRON

129

T a l y s t e p system; hence the heat o f c o n d e n s a t i o n o f the s p u t t e r e d a t o m s c o u l d be calculated. 3. RESULTS T h e aim o f the e x p e r i m e n t s was to d e t e r m i n e the effect on a substrate, a n d g r o w i n g film, o f the p l a s m a from an u n b a l a n c e d m a g n e t r o n a n d to m e a s u r e the ion a n d electron c u r r e n t flow to the s u b s t r a t e a n d the heat l o a d on it. By using a circular p l a n a r p r o b e the effects o f leaked p l a s m a on the s u b s t r a t e was investigated. T h e m a g n e t r o n was o p e r a t e d at 500 m A , with a t i t a n i u m target, 3 m T o r r a r g o n pressure a n d the s u b s t r a t e 7 0 m m from the target surface. T h e I - V characteristics o f the

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130

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substrate probe with such a magnetron are shown in Fig. 2. The properties of this typical curve have been discussed elsewhere in ref. 8. When the probe bias is very negative with respect to the plasma potential, the electric field around the probe will prevent all but the highest energy electrons from reaching the probe, effectively reducing the electron current to zero. The ions encounter only an attracting electric field. The flow is termed the "ion saturation" current Isi and occurs when the voltage is sufficiently negative to repel nearly all the electrons in the plasma, and yet not so negative that ions bombarding the probe produce significant secondary electron emission. These ions are expected to be effective in modifying thin film growth, dissipating their energy close to the substrate 6. As Vis made more positive the number of electrons which are able to overcome the repelling electric field and contribute a negative current increases exponentially. The electron current collected is equal to Isi at the self-bias voltage Vf. Vr is less than the plasma potential Vp, because the electron thermal velocity is greater than that of the ions 9. Because of the greater energies of electrons than those of ions the plasma potential tends to a positive potential, and is independent of discharge power 1o. At a certain potential Vp, the flux of ions" and electrons reaching the probe is totally representative of the random drift of electrons and ions, and no plasma sheath exists between the substrate probe and plasma. Voltages more positive than the plasma potential result in ions being repelled from the probe until finally an electron saturation current develops. Figure 3 shows the current flow to the probe as a function of negative bias, with the magnetron operating at 3 mTorr argon pressure with a target current of 500 mA. Also shown is the heat loading resulting from the bombardment. The initial rate of temperature rise was measured, rather than using the equilibrium temperature, in order to eliminate the complications of energy exchange by thermal radiation 11. At zero net current to the probe the ion current will be approximately that which is measured at - 100V bias, and the electron current will be equal to it, so that from the current shown in Fig. 3 we have estimated the ion bombardment of the substrate and the heat load at self-bias which was - 2 7 V . These were 3.5mAcm -2 and 100 mW cm- 2 respectively. The self-bias voltage is important in thin film deposition because it gives the potential reached by an insulated substrate. The growing film will then be bombarded with ions of energy equivalent to the voltage difference between the self-bias potential and the plasma potential, and by an equal flux of electrons 6. The ion bombardment will cause physical changes in the deposited film, such as a change in the grain size, the degree or direction of orientation, the film density and number of voids, the film stress, and other related properties such as the electrical resistivity, the dielectric constant and the stability of the film 2. As shown in Fig. 3, the measured rate of energy dissipation in the substrate increases with increasing negative bias voltage, and this power is smaller than the calculated power from the I-V curve. This can be attributed to the dynamics of the sputtered particle-gas ion collision near to the substrate surface with dissipation of their energy close to the substrate. It was, however, sufficiently close (75%) for the adoption of a simple model in which the majority of ions are accelerated through the floating potential to collide with the substrate, their number being indicated by the saturated ion current. The heat load was measured as a function of pressure and is

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shown in Fig. 4, for a probe bias of - 8 0 V with respect to anode, which gives approximately the saturation ion current and repels the electrons. Heat load changes could also be measured as a function of different parameters, such as the power the magnetron was operated at and the pressure. The self-bias potential with respect to the anode and the probe current under negative bias, with respect to the anode, were measured as a function of pressure and are shown in Fig. 5. It is a feature of the magnetron discharge that the operating voltage for a given target and gas combination is only slightly dependent on cathode current and gas pressure 7. Thus the energy of the secondary electrons entering the plasma is approximately constant. As the pressure decreases, the probability of ionization in the plasma decreases, as does the recombination and charge exchange. The probe ion current will increase for a constant magnetron current (Fig. 5) so that the heat load and selfbias voltage will increase as well (Figs. 4 and 6). The distribution of heat load and deposition rate for this magnetron was measured as a function of radial distance from the centre of the magnetron, at a distance 70 mm from the target face, as shown in Fig. 7. For this measurement we estimated that the effective area in this unbalanced magnetron is approximately a 40 mm diameter circle at 70 mm distance from the target face. This gives a uniformity of ion current bombardment o f + 10~. The energy dissipation of the target (Fig. 1) into the cooling water, the substrate and the plasma, was measured for a 2.2A discharge current. These temperature detectors were fixed at the inlet and the outlet of the water cooling from

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the target, which were inside the magnetron body itself. Vacuum operation prevents any significant losses either by radiation or by conduction, so the energy that went into the cooling water can be calculated. It was 82~ of the energy supplied. By using glass substrates the total sputtered mass was measured; hence the total heat due to condensation of atoms was calculated, and this equalled 20~ of the substrate heat. By using a movable planar probe the total energy delivered to substrate was measured by integrating over the whole area in front of the target. The total energy that went into the substrate, including heat of condensation, was 3 ~ of that supplied. The rest of the energy was assumed to go into heating the plasma. The maximum power at which any magnetron can be operated depends on the efficiency of its cooling system. The substrate temperature will, of course, ultimately depend on the magnetic trap of the cathode and the supply of electrons from it to the substrate, bringing ions with them. An increase in the ion bombardment of the substrate will occur if it is immersed in a denser plasma, which requires more of the plasma created at the planar magnetron cathode to be "leaked" to intercept the substrate. The magnetics of the magnetron design can be helped to do this with a magnetic field created behind the substrate position to concentrate the flux lines to be perpendicular to the surface. This is shown schematically in Fig. 8. The increase in substrate bombardment of ions, and their energy, caused by a coil placed behind the substrate probes is shown in Figs. 9 and 10. The magnetron was operated in the standard conditions of a current of 500 mA at 3 mTorr of argon with different target material. The coil current Ic (A) translates to a centre magnetic field of Ic x 2 x 1 0 - 2 T in the same direction as the field from the centre pole of the magnetron.

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in the deposition rate, indicating that this plasma does not contain a large concentration of ionized sputtered material. The use of uncharacterized "unbalanced" sputtering has been demonstrated to increase the density of metal films and the refractive index o f reactively sputtered titanium dioxide 7. The increase in quality of metal films is evident from their surface appearance and cross-sectional scanning electron microscopy examination. A further advantage of such a system is the increase in reactivity of the process on the substrate surface, giving a requirement for less of the reactive gas to be present and hence a lower operating partial pressure ~3. 4. CONCLUSION The present investigations have demonstrated that an electrostatic probe can be used effectively to measure substrate effects arising from the modification of the plasma used in sputtering. The observed dependence of the substrate parameters and the sputtering variables makes this technique useful in controlling the effects of ion b o m b a r d m e n t during sputter deposition. We have used the probe to investigate the unbalanced magnetron and in particular the substrate effects from the ion currents that flow, including the heat load. The effect of pressure on the current and the self-bias voltage have been measured. A self-bias voltage of from 30 to 60 V could be achieved with ion currents of from 5 to 10 m A c m - 2 with energy dissipation measured as being about 75~o of that calculated, which assumed that all ions had the m a x i m u m energy corresponding to the bias. These values were dependent on the pressure, the power of operation of the source magnetron and the material that was being sputtered. We have found that for our magnetron the effective area, at 70 m m from the target, is equal to a circle of 50 m m diameter. Energy dissipation for the magnetron was measured as 82~o into the cooling water and 3 ~ to the substrate, the rest into the plasma. The b o m b a r d m e n t of the substrate can be increased with a magnetic field generator placed behind it. The control of the ion b o m b a r d m e n t of a growing film to initiate reactive and structure-forming processes, without radiation damage, is, we think, an important additional tool in the preparation of the film material of high quality. O f equal importance is that this can be done with a simple modification of the preferred technique of creating large area c o m p o u n d thin films by a vacuum process: the planar magnetron. REFERENCES

I 2 3 4 5 6 7

B. Window and N. Savvides,J. Vac. Sci. Technol. A, 4 (1986) 453. S.M. Rossnagel, Vacuum, 38 (1988) 73. S.M. Rossnagel,J. Vac. Sci. Technol. A, 6 (1988) 19. S.M. Rossnagel,J. Vac. Sci. Technol. A, 6 (1988) 223. A.G. Spencer, C. A. Bishopand R. P. Howson, Vacuum, 37 (1987) 363. B. Window and N. Savvides,J. Vac. Sci. Technol. A,6(1986) 196. K. Oka, R. P. Howson, R. W. Lewinand A. G. Spencer, Proc. Soc. Photo-Opt. Instrum. Eng., 1019 (I988) 40-48.

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W. Class and R. Hieronymi, SolidState Technol., 55 (1982). B. Lipshult, J. Vac. Sci. Technol. A,4(1986) 1810. N. Savvidesand B. Window, J. Vac. Sci. Technol. A, 4 (1986) 504. D.J. Ball, J. Appl. Phys., 43 (1972) 3047. D.B. Fraser and H. D. Cook, J. Vac. Sci. Technol., 14 (1977) 147. A.G. Spencer, K. Oka, R. W. Lewin and R. P. Howson, Vacuum, 38 (1988) 857. K. Oka and R. P. Howson, Proc. Conf. on Ion Plating and Allied Techniques, Brighton, 1987, 1987, p. 158.