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Arc behaviour during filtered vacuum arc deposition of Sn-O thin films
ELSEVIER
Surface and Coatings Technology 76-77 (1995) 181-189
Arc behaviour during filtered vacuum arc deposition of Sn-O thin films L. Kaplan, Y.N. Zhitomirsky, S. Goldsmith, R.L. Boxman, 1. Rusman ElectricalDischarge and Plasma Laboratory, Tel-Aviv University, POB 39040, Tel-Aviv 69978, Israel
Abstract Sn-O transparent, conductive thin films were produced in a filtered vacuum-arc deposition system, consisting of a 90 mm diameter cathode, 122mm diameter annular anode, and a magnetic quarter torus macroparticle filter. Arc current was in the range 30-200 A and the toroidal magnetic field (BT ) was in the range 0-25 mT. A radial magnetic field (Br ) at the cathode surface was produced by a coil placed inside a cavity in the cathode. Cathode spot motion, plasma jet motion through the toroidal field, and the characteristics of the deposited Sn-O films were studied as a function of arc current, applied magnetic fields and oxygen pressure. Cathode spot motion on the cathode surface was controlled by B r • Random spot motion on the entire cathode surface was obtained in vacuum when B, was less than 2 mT. The spots were forced to move on a circular path with average radius of 25 mm when B, was larger than 2 mT. Cathode surface oxidation occurred when oxygen was introduced into the system, and its rate increased with the gas pressure. The extent of the oxidized area on the cathode surface was affected not only by gas pressure, but also by the magnitude of arc current and the magnetic fields B, and BT • With B, smaller than 2 mT, oxidized surface regions appear on the cathode, and cathode spot activity on them is disturbed. Random cathode spot motion continues undisturbed on non-contaminated regions of the cathode surface. Cathode spots were hardly observed on the oxidized regions, yet visual examination of the cathode surface used in the arc revealed the existence of cathode spots erosion tracks on them. Transparent and conductive tin oxide films were produced at oxygen pressures of 0.8 Pa when arc operation and the plasma jet in the filter were stable. The stability of the arc and the plasma jet were found to depend on the combination of gas pressure, arc current and magnetic fields. When the plasma jet was not stable poorly conductive films were produced, containing the non-conductive oxide SnO. Keywords: Filtered vacuum arc; Cathode spots; Transparent conductive oxides; Tin oxide; STM
1. Introduction Transparent and electrically conducting oxide (TCO) films are widely used in light emitting, light detecting, and light triggered semiconductor devices, displays, solar cells, heat mirrors, smart windows, and anti static coatings. The most popular transparent coatings are fabricated from metal oxides, in particular the oxides of In, Sn and Zn [1]. The TCO's are electron-degenerate semiconductors with a wide band gap. Their properties depend crucially on their deviation from stoichiometry, on the nature and amount of impurities, and on the micro structure. All of these properties depend on the deposition method and parameters, and on postdeposition processing [1]. Recently, filtered vacuum arc deposition (FVAD) of amorphous, and transparent SnO z- x thin films was reported by Kaplan et al. [2] and Ben-Shalom et al. [3]. Film electrical conductivity was larger than 300 Q -1 em -1, but after rapid thermal annealing at 300°C for 30 s, the conductivity increased to 2xl03 a' em-i. 0257-8972/95/$09.50 © 1995 Elsevier Science SA All rights reserved SSDl 0257 -8972 (95) 02592-8
The deposition rate was up to 10 nm s -1. It was also shown there that sustaining the stoichiometry near SnO z is very important for obtaining high-quality thin films, because an excessive deficiency of oxygen leads to the deposition of the non-conductive oxide SnO [2,3]. A very stable plasma flow and a good control of oxygen pressure during the deposition process were required to obtain the proper stoichiometric conditions. A study of plasma behaviour in the FVAD system, equipped with a quarter torus macroparticle magnetic filter, showed that the stray magnetic field at the cathode surface produced by the toroidal coil could cause unstable arc operation, resulting with an unstable plasma flow. Unstable arc operation, such as arc cutoff or strong current fluctuations, occurred because the stray magnetic field forced the spots to move away from the cathode surface [ 4,5]. Proper adjustment of the various magnetic fields in the filtered vacuum arc system is needed to stabilize and maximize the ion current output at the coated substrate. The presence of oxygen in the system could also cause
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L. Kaplan et al./Surface and Coatings Technology 76-77 (1995) 181-189
unstable arc operation. When some regions of the cathode surface became oxidized and had lower electrical conductivity, arc operation became unstable, having significant current fluctuations. As the Sn plasma beam had also a pumping effect on the gas, the oscillations in the arc current caused oxygen pressure fluctuations. Any significant fluctuations in the gas pressure resulted in poorly conducting films. This article presents the results of a study in a filtered vacuum arc system of several interconnected factors that determine the behaviour of the cathode spots on a Sn cathode, the characteristics of the Sn plasma beam, and the properties of the Sn-O film produced by this system. The studied factors are: (1) arc current, (2) magnetic fields, and (3) oxygen gas pressure. A magnetic field configuration that achieves stable arc operation, stable plasma flow, and enhanced deposition rate of highquality SnO z thin films is also described.
On leaving the toroidal filter the plasma jet passed into a 160 mm diameter cylindrical chamber in which the substrate was placed. Three Helmholtz coils HI, H2 and H3 produced an axial magnetic field of 3.6 mT A -1 in the center of the coils, directing the plasma jet to the substrate. A 99% pure oxygen flow was directed to the substrate by means of an inlet tube, and was controlled by a computer connected to the pressure sensor. Oxygen pressure was varied in the range of 0.8-1.6 Pa in the reported experiments. The coating system was pumped down to an initial pressure of 6.6xl0- 3 Pa using a diffusion pump. Before arc ignition, oxygen pressure was kept constant and in the range of 3.3-4 Pa. After arc ignition, oxygen pressure was brought down to the required value (0.8-1.6 Pa) within 5 s by use of a computerized pressure controller. The pressure controller kept the pressure stable during the whole coating process. The deposition process could last up to 500 s. The SnO z films were deposited on 50x75mm glass substrates, without any external heating during the deposition process. Substrate temperature was monitored during film deposition by a thermocouple attached to the substrate, and it did not exceed 70°C. Cathode spot motion on cathode surface was monitored with a TV camera, and recorded using a VCR [ 4]. The exposure time of a video frame was 40 ms. Ion probe current (Ip) was measured by a 130 mm diameter disc probe, usually negatively biased by 40 V with respect to the cathode (ground), and placed at the center of the first chamber 100 mm from the torus exit (see Fig. 1). I p reached saturation when probe bias was negative in respect to the cathode by 20 V. When the bias relative to the cathode was zero, I p reached about 80% of the saturation value. The electrical conductivity of the coated films was measured by a four-point probe. The surface morphology of the film and local electronic properties were
2. Experimental set-up and procedure A schematic diagram of the experimental filtered vacuum arc deposition apparatus is shown in Fig. 1. The deposition system contained a vacuum arc plasma gun with a 91 mm diameter, 99.9% pure, tin cathode and a 122 mm i.d. annular anode. A small coil was placed in a water cooled cavity in the cathode, and produced a magnetic field with a radial component B, of 0.2 mT A -1 [4]. A 30-200 A d.c. discharge was ignited by a mechanical trigger electrode. The plasma emitted by the cathode passed through the annular anode into a quarter torus macroparticle (MP) filter with a 240 mm major radius and a 80 mm minor radius (toroidal duct diameter, 160 mm), The toroidal magnetic field (B T , 6 mT A -1) was operated up to a maximum value of 25 mT. BT was produced by five coils connected in series.
Vacuum Gauges To Control and Recording Sys. Sub trate
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\
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18
Toroidal coils
HI
1
- Substrate bias
R
Gate Valve Arc Current
Vacuum System
~~ I I I
: Oxygen Inlet Fig. 1. A schematic diagram of the filtered vacuum arc deposition apparatus.
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L. Kaplan et al.TSurface and Coatings Technology 76-77 (1995) 181-189
studied by a scanning tunneling microscopy /spectroscopy (STM/STS) system. A review of the STM method is presented in Ref. [6]. STM images and I-V characteristics were obtained by use of an air STM equipped with etched tungsten tips. Constant current topographic (CCT) images were subjected to two-dimensional Fourier-transform filtering. Spectroscopic measurements were performed by the current vs. voltage (I-V) method at fixed tip-sample separation.
3. Experimental results
3.1. Arc and ion current characteristics In Fig. 2 we present the dependence of the arc voltage (Vare ) and probe ion current (lp) on arc current. Arc current (lare) was varied in the range of 30 to 220 A, Vare was measured for BT = 0, and I p was measured by a probe with a zero bias relative to the cathode, i.e. the value of I p was 80% of ionic saturation current. The magnetic fields BT and B, were set to 12 mT and 3 mT, respectively. The dependence of I p on arc current is closely linear. Threshold arc current for stable arc operation in this configuration was 30 A. Increasing the current from 30 to 220 A leads to an increase in the number of cathode spots that exist simultaneously on the cathode surface, from 1 to 6-7 (Fig. 3 (a)-(d)). The increase in the number of the spots results from spot splitting and not from spot motion. Arc voltage increased with the current from 13.5 V (at currents up to 100 A) to 16 V at current of 170 A. The arc became more stable with the growth of current, but at currents larger than 170 A the cathode spots had the tendency to move to the cathode boundary (Fig. 3 (d)). In Fig. 4 we show the dependence of Vare on BT in
vacuum and when oxygen is introduced into the system. In vacuum, Yare increases almost linearly with BT , i.e. from Vare= 13 V when BT=O to Vare=28 V when BT= 21 mT. Three modes of arc operation were observed at oxygen pressure of 1 Pa . (1) Cathode spots residing on the surface of the cathode; here Vare was in the range of 25 to 26 V. (2) Cathode spots residing on the boundary of the cathode; here Vare was 32 to 35 V. (3) Arc burning between the cathode and the spacer ring with Vare as low as 13 - 15 V. In Fig. 5 the typical dependence of Yare and I p on time is shown for an unstable arc in oxygen at a pressure of 1.3 Pa. (lare = 170 A, Br=O.4 mT, BT = 15 mT). All three arc modes mentioned above were present in this case. Most of the time Yare was equal to 34 V (point 2), however, in some cases (point 1) it dropped to 20 V, and in one case it dropped to 15 V (point 3). The events marked by (1) are correlated with an increase in I p , while event (3) is correlated with a marked decrease, a minimum, in ion current. Patches of oxidized Sn are seen in Fig. 6 (a) on the cathode surface after the arc was run in 1 Pa oxygen pressure, and in magnetic fields where B, = 0.4 mT and BT = 14 mT. The spots were not observed to move on the oxide surfaces, however, spots tracks are seen on the blackened oxidized regions on the cathode when examined after dismantling the arc. Surface oxidation of the cathode led to significant fluctuations in Yare and I p • A circular track of the cathode spots is seen in Fig. 6 (b) showing a photograph of the surface of a cathode that was used in an arc run at a 1 Pa oxygen pressure and in a magnetic field configuration where B, = 3 mT and BT = 14 mT. In this case of larger Bn the spots were observed to move during the whole arcing period on the circular track. The surface of the track was found clean from oxidation, and its resistivity was found to be equal to that of common tin. The arc was very stable in
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184
L. Kaplan et al.jSurfaceand Coatings Technology 76-77 (1995) 181-189
Fig. 3. Video images of cathode surface during arcing for different arc currents. (a) 50 A, (b) 100 A, (c) 170 A, (d) 210 A.
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Fig. 4. Arc voltage (Vare ) as function of the toroidal magnetic field in vacuum (solid line), and in 1 Pa oxygen pressure (points): 1, cathode spots reside on cathode surface; 2, cathode spots moved to the boundary of the cathode; 3, arc burning between the edge of the cathode and the spacer.
B, = 0.4 m'T, BT
this case, Yare was 24-26 V, and both Yare and i, showed minimum fluctuations. The deposited Sn0 2 films were transparent and conducting. The metal on both sides of the track was covered by poorly conducting and blackened oxidized tin, but also showed the signs of cathode spots tracks. In Fig. 7 we present the dependence of I p on B, for
two values of BT (12 and 15 mT). The arc was run in a 1 Pa pressure of oxygen and probe bias was -40 V. I p increases with B, until it reaches a maximum value that is in the range of 2 to 3 mT, depending on the values of BT • Another study of the dependence of I p on B, is presented in Fig. 8. Here, BT is fixed at 15 mT, Vbia s = -40 V, and the oxygen pressure, P, is changed. Three
Fig. 5. Arc voltage and Ion current wave forms for 1"e~ 170 A, = 8.6 mT, and P = 1.3 Pa; 1, cathode spots reside on cathode surface; 2, cathode spots reside on the boundary of the cathode; 3, arc momentary interruption.
185
L. Kaplan et al.jSurfaceand Coatings Technology 76-77 (1995) 181-189 300r-----:=:::::::="'<::::::::----, p=o
« S.
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Fig. 8. Probe ion current as function of B, in vacuum and at oxygen pressures of 0.92 and 1.32 Pa. The probe was located at the distance of 100 mm from the torus exit.
(a)
(b)
Fig. 6. Oxidation of the cathode surface. The arc was operated with Br=OA mT (a), and Br=3 mT (b).
110
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Fig. 7. Ion current as function of B, at oxygen pressure of 1 Pa, for BT equal to 12 and 15 mT, The probe was located 100 mm from the
The spacer coil (see Fig. 1) produced an axial magnetic field B, in the region between the anode and the entrance to the magnetic torus. Bs improved the plasma transfer into the toroidal filter, however, it also affected the transfer of arc current from the cathode to the anode and the motion of the spots on the cathode surface. When B, was directed opposite to B, and equal to 0.7 mT or less, a stabilization of the arc operation was obtained. Any further increase in B, led to a strong disturbance in arc operation. In Fig. 9 we present the dependence of I p on BT . In this case the magnetic field B, equals 0.7 mT and BT is directed opposite to the field Br • Five plots of I p vs. BT are shown in Fig. 9 with B, as a parameter (Br = 0, 1, 2, 3,4, mT). In all cases I p increases with BT until it reaches a maximum value. Increasing BT beyond this value had only a negative effect on the deposition rate, which is always proportional to I p • The magnitude of the peak increased with Bn suggesting a saturation effect at higher Br . At Br=4 mT I p is closely constant for BT in the range of 14 to 17 mT. It is interesting to note that the value of BT for peak I p increases with Bn as well as the value of peak of I p . Additional increase in I p was obtained by applying to the torus a positive bias voltage of + 17 V relatively to the anode. Biasing the wall increased peak I p by a factor of 3. At higher B, arc
torus exit, and biased by -40 V relative to the cathode.
200-r------------,
cases are presented in Fig. 8: oxygen pressure P = 0 (oxygen was not introduced to the system), P = 0.9 Pa and P = 1.3 Pa . As a function of B" I, has a maximum in all cases. In vacuum, maximum I p equals 300 rnA at B, = 2 mT. Maximum I p decreases with P and is observed at higher Br . At P=1.3 Pa and at Br=3 mT the maximum of I, was equal to 65 rnA . The presence of 1.3 Pa of oxygen in the coating system reduced peak I p by almost a factor of 5, in comparison with its value when oxygen was not flowing into the system.
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10
12
14
16
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Toroidal magnetic field [mT]
Fig. 9. Ion current as function of BT for different values of Br •
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L. Kaplan et al.f Surface and Coatings Technology 76-77 (1995) 181-189
operation without instabilities is possible if higher toroidal field are applied, though peak I p decreases. 3.2. STM studies ofthe films
STM analysis provides a very useful additional information to that obtained by structural, electrical and optical measurements of Sn-O films [2,3]. To investigate the influence of system parameters on the film quality two groups of films were deposited at the same oxygen pressure of 1 Pa but at different Br . The electrical resistivity, p, of the films deposited with B, = 0.4 mT was in the range of 1-6 mO cm, while the resistivity of the films deposited with B, = 3 mT was in the range of 0.9-3 mO em. Samples with p equal to 1 and 3 mO cm, more typical for each respective group, were selected for further study by STM. In Figs. 10 (a) and (b) we present computerized STM images of film surfaces with a 0.1-0.2 nm lateral resolution. A typical sample of surface morphology of a 50 nm (thickness) Sn-O film is shown in Fig. 10 (a), while surface morphology of a 350 nm film is shown in Fig. 10 (b). No correlation between B, and the film
surface morphology was observed. The surface rugged and grainy structure had a lateral periodicity close to 20 nm in both cases. The average height of the crests on the films depended, however, on its thickness, being 5 and 0.6 nm for the 50 nm and 350 nm layers, respectively. Well defined clusters were observed in the case of the thin 50 nm film (Fig. 10 (a)). In Figs. 11 (a) and (b) we present local I-V plots obtained with the STM on samples deposited with B, = 0.4 mT. The I-V plot in Fig. 11 (a) is that of a typical degenerate semiconductor material, while in the I-V plot in Fig. 11 (b) is that of a normal semiconductor having an energy gap of 3 eV, determined by the difference between the biase voltages of the tip at the points where the current to the tip becomes negative or positive. Thus, the surface states of this material is that of a non degenerate semiconductor with an energy band gap of approximately 3 eV. In most tests I-V plots of degenerate semiconductor were obtained, yet a considerable number of tests (20%) resulted with a semiconductor like I-V plot of normal semiconductor. In contrast to these results, STM study of films deposited under the conditions in which B, was equal to 3 mT resulted with I-V
(a)
Cross
5nm
Fig. 10. STM images of Sn02-x films with thickness of 50 nm (a), and 350 nm (b).
L. Kaplan et al.jSurjaceand Coatings Technology 76-77 ( 1995) 181-189
plots of only degenerate semiconductor for all analyzed points on the surface. This result agrees well with the higher conductivity of these films.
4. Discussion 4.1. Sn arc operation
A similar study of the effect of gas pressure and magnetic fields on Ti arcs in the system described here was made by Zhitomirsky et al. [4,5]. The oxidation of the cathode is the new effect observed in the present study of the characteristics of a Sn cathodic arc attached to a magnetic macroparticle filter. Otherwise, the behaviour of both arcs in vacuum is similar in most details. Unstable arc operation that leads to unstable plasma flow is usually correlated with cathode spot displacement to the edge of the cathode. This motion may be caused by the retrograde motion of the spot in the magnetic field produced at the cathode surface by the arc current and the quarter-torus and spacer coils. The spacer coil may also produce an interruption of the arc. However, the operation of the system depends on magnetic fields to obtain proper guiding of the plasma beam from the anode through the filter and into the sample. Hence, a
187
conflict exists between the need to use relatively large guiding fields and their detrimental effect on the arc operation. The observation of a maximum in I p as function of arc current, BT and B, indicates that a balance between opposing effects is achieved, but this does not assure a significant ion current to the substrate [4,5]. It is significant to notice here that samples of Sn-O films deposited with the unstable arc were always of poor quality (i.e. low electrical conductivity). The presence of oxygen in the system is obviously essential for the deposition of oxide films. However, the cathode surface itself may become oxidized. The oxidation of the cathode surface may produce insulating oxide regions, or a poorly conductive layer on the cathode surface. The characteristics of cathode spots residing on oxides were studied by Guile and Juttner [7]. As the average current per spot on an oxide surface is about 1% of its value on clean surfaces, the number of the spots should increase markedly when the spots reside on the oxide regions. The erosion rate which is about 150 ug C -1 for clean Sn surfaces decreases to approximately a fraction of a percent [7]. The experimental data agree well with this prediction. Whenever the spots are forced to rotate on a circle by B, and pass from the clean metal to an oxidized region, they tend to disappear and the ionic current decreases accordingly. There is
188
L. Kaplan et al./Surface and Coatings Technology 76-77 (1995) 181-189
I-V spectrum 20
~
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0
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(ij
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'en
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also a correlated increase in the arc voltage. As the rotating arc moves alternately over clean and oxidized areas, a strong fluctuation is observed in arc and probe currents. Such behavior has been typical to arcs running with Br=OA mT. When B, is much higher, 3 mT, the spot rotation is faster, and the track on which the cathode spots move is actually free of oxides. In this case the higher spot velocity combined with its surface cleaning effect produce an oxide free region along the whole track. The spot returns to its previous location before extensive oxidation could have developed. A stable arc is observed in this case with stable I p and stable v"rc in the region 23-25 V during the whole deposition process. Oxidation of the areas adjacent to the track also occurs in this case (B, = 3 mT), but only few spot tracks are evident (see Fig. 6 (b)).
Arc instability which is caused by the magnetic field or by surface oxidation not only reduces the deposition rate but also affects adversely the quality of the coatings. Oxygen pressure is another factor that determines film quality. Uniform film structure is obtained at oxygen pressure, P, between 0.8-1.3 Pa. The lower pressure limit is imposed by the need to prevent the formation of the electrically insulating SnO phase. An increase in P allows a correlated increase in I p (to keep the proper stoichiometry), but also increases the probability of cathode oxidation and arc instability. Further, an exceedingly high pressure may prevent the plasma from reaching the substrate. Hence, a well defined range of oxygen pressures existed in our system for proper deposition of conducting SnO z. Optimal P was determined to be in the range of 0.9 to 1.3 Pa.
L. Kaplan et al.isurface and Coatings Technology 76-77 (1995)
The procedure adapted for introducing the oxygen into the coating system is also significant. When the system is filled up by a constant gas pressure before arc ignition, the onset of the arc has an effect of introducing an additional gas pump, and the pressure is reduced to a new stationary value. Ben-Shalom et al. [3] showed that a too high initial gas pressure prolonged prohibitively the time interval required for the pressure to reach the proper value. Thus, with this method of gas flow adjustment, the initial gas pressure should agree with arc current so that working pressure level would be reached in as short time as possible. 4.2. STM analysis The STM I-V data suggest the existence ofnanometric size clusters of non-degenerate semiconductor consisting of Sn2+ (instead of Sn H ) ions. We have also seen that when B, was equal to 3 mT, and arc was stable, the density of such sites was markedly reduced. These microobservations agree well with the large scale findings that high quality films were produced by applying B, equal to 3 mT.
5. Conclusions Filtered vacuum arc deposition can be utilized to deposit transparent conductive films of Sn02 at high deposition rate of 10 nm s -1. The deposition of high quality Sn02 films requires a careful adjustment and regulation of the following parameters: magnetic fields, arc current, cathode spot velocity on cathode surface, working arc pressure, and oxygen flow rate. The depos-
I8I~I89
189
ition rate is determined by the ion current density at the sample. The above mentioned parameters can be defined so that arc operation is stable, and a maximum value of I p is reached. Deposition process optimization can be achieved by determining the set of parameters resulting with the maximum of peak I p • Any further optimization may require geometrical changes in the system geometry in addition to the regulation of the parameters listed above.
Acknowledgments The authors are grateful to Uri Kinrot for the gas control computer program, the technical assistance of Hanan Yaloz, the financial support of the Israeli Ministry for the Absorption of Immigration, the Ministry of Science and Arts, and Friends of Tel-Aviv University in France.
References [1] K.L. Chopra, S. Major and D.K. Pandya, Thin Solid Film, 102 (1983) 1. [2] L. Kaplan, A. Ben-Shalom, R.L. Boxman, S. Goldsmith and M. Nathan, Thin Solid Films, 253 (1994) 1. [3] A. Ben-Shalom, L. Kaplan, R.L. Boxman, S. Goldsmith and M. Nathan, Thin Solid Films, 236 (1993) 20. [4] V.N. Zhitomirsky, R.L. Boxman and S. Goldsmith, Surf. Coat. Technol, 68/69 (1994) 146. [5] Y.N. Zhitomirsky, R.L. Boxman and S. Goldsmith, J. Vac. Sci. Technol, A13 (1995) 2233. [6] R.I. Hamers, in D.A. Bonnel1 (ed.), Scanning Tunneling Microscopy and Spectroscopy: Theory, Technics and Applications , Imprint VCR, New York, 1993, pp. 51-103. [7] A.E. Guile and B. Jiittner, IEEE Trans. Plasma sa: PS-8 (1980) 259.
Surface and Coatings Technology 76-77 (1995) 181-189
Arc behaviour during filtered vacuum arc deposition of Sn-O thin films L. Kaplan, Y.N. Zhitomirsky, S. Goldsmith, R.L. Boxman, 1. Rusman ElectricalDischarge and Plasma Laboratory, Tel-Aviv University, POB 39040, Tel-Aviv 69978, Israel
Abstract Sn-O transparent, conductive thin films were produced in a filtered vacuum-arc deposition system, consisting of a 90 mm diameter cathode, 122mm diameter annular anode, and a magnetic quarter torus macroparticle filter. Arc current was in the range 30-200 A and the toroidal magnetic field (BT ) was in the range 0-25 mT. A radial magnetic field (Br ) at the cathode surface was produced by a coil placed inside a cavity in the cathode. Cathode spot motion, plasma jet motion through the toroidal field, and the characteristics of the deposited Sn-O films were studied as a function of arc current, applied magnetic fields and oxygen pressure. Cathode spot motion on the cathode surface was controlled by B r • Random spot motion on the entire cathode surface was obtained in vacuum when B, was less than 2 mT. The spots were forced to move on a circular path with average radius of 25 mm when B, was larger than 2 mT. Cathode surface oxidation occurred when oxygen was introduced into the system, and its rate increased with the gas pressure. The extent of the oxidized area on the cathode surface was affected not only by gas pressure, but also by the magnitude of arc current and the magnetic fields B, and BT • With B, smaller than 2 mT, oxidized surface regions appear on the cathode, and cathode spot activity on them is disturbed. Random cathode spot motion continues undisturbed on non-contaminated regions of the cathode surface. Cathode spots were hardly observed on the oxidized regions, yet visual examination of the cathode surface used in the arc revealed the existence of cathode spots erosion tracks on them. Transparent and conductive tin oxide films were produced at oxygen pressures of 0.8 Pa when arc operation and the plasma jet in the filter were stable. The stability of the arc and the plasma jet were found to depend on the combination of gas pressure, arc current and magnetic fields. When the plasma jet was not stable poorly conductive films were produced, containing the non-conductive oxide SnO. Keywords: Filtered vacuum arc; Cathode spots; Transparent conductive oxides; Tin oxide; STM
1. Introduction Transparent and electrically conducting oxide (TCO) films are widely used in light emitting, light detecting, and light triggered semiconductor devices, displays, solar cells, heat mirrors, smart windows, and anti static coatings. The most popular transparent coatings are fabricated from metal oxides, in particular the oxides of In, Sn and Zn [1]. The TCO's are electron-degenerate semiconductors with a wide band gap. Their properties depend crucially on their deviation from stoichiometry, on the nature and amount of impurities, and on the micro structure. All of these properties depend on the deposition method and parameters, and on postdeposition processing [1]. Recently, filtered vacuum arc deposition (FVAD) of amorphous, and transparent SnO z- x thin films was reported by Kaplan et al. [2] and Ben-Shalom et al. [3]. Film electrical conductivity was larger than 300 Q -1 em -1, but after rapid thermal annealing at 300°C for 30 s, the conductivity increased to 2xl03 a' em-i. 0257-8972/95/$09.50 © 1995 Elsevier Science SA All rights reserved SSDl 0257 -8972 (95) 02592-8
The deposition rate was up to 10 nm s -1. It was also shown there that sustaining the stoichiometry near SnO z is very important for obtaining high-quality thin films, because an excessive deficiency of oxygen leads to the deposition of the non-conductive oxide SnO [2,3]. A very stable plasma flow and a good control of oxygen pressure during the deposition process were required to obtain the proper stoichiometric conditions. A study of plasma behaviour in the FVAD system, equipped with a quarter torus macroparticle magnetic filter, showed that the stray magnetic field at the cathode surface produced by the toroidal coil could cause unstable arc operation, resulting with an unstable plasma flow. Unstable arc operation, such as arc cutoff or strong current fluctuations, occurred because the stray magnetic field forced the spots to move away from the cathode surface [ 4,5]. Proper adjustment of the various magnetic fields in the filtered vacuum arc system is needed to stabilize and maximize the ion current output at the coated substrate. The presence of oxygen in the system could also cause
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L. Kaplan et al./Surface and Coatings Technology 76-77 (1995) 181-189
unstable arc operation. When some regions of the cathode surface became oxidized and had lower electrical conductivity, arc operation became unstable, having significant current fluctuations. As the Sn plasma beam had also a pumping effect on the gas, the oscillations in the arc current caused oxygen pressure fluctuations. Any significant fluctuations in the gas pressure resulted in poorly conducting films. This article presents the results of a study in a filtered vacuum arc system of several interconnected factors that determine the behaviour of the cathode spots on a Sn cathode, the characteristics of the Sn plasma beam, and the properties of the Sn-O film produced by this system. The studied factors are: (1) arc current, (2) magnetic fields, and (3) oxygen gas pressure. A magnetic field configuration that achieves stable arc operation, stable plasma flow, and enhanced deposition rate of highquality SnO z thin films is also described.
On leaving the toroidal filter the plasma jet passed into a 160 mm diameter cylindrical chamber in which the substrate was placed. Three Helmholtz coils HI, H2 and H3 produced an axial magnetic field of 3.6 mT A -1 in the center of the coils, directing the plasma jet to the substrate. A 99% pure oxygen flow was directed to the substrate by means of an inlet tube, and was controlled by a computer connected to the pressure sensor. Oxygen pressure was varied in the range of 0.8-1.6 Pa in the reported experiments. The coating system was pumped down to an initial pressure of 6.6xl0- 3 Pa using a diffusion pump. Before arc ignition, oxygen pressure was kept constant and in the range of 3.3-4 Pa. After arc ignition, oxygen pressure was brought down to the required value (0.8-1.6 Pa) within 5 s by use of a computerized pressure controller. The pressure controller kept the pressure stable during the whole coating process. The deposition process could last up to 500 s. The SnO z films were deposited on 50x75mm glass substrates, without any external heating during the deposition process. Substrate temperature was monitored during film deposition by a thermocouple attached to the substrate, and it did not exceed 70°C. Cathode spot motion on cathode surface was monitored with a TV camera, and recorded using a VCR [ 4]. The exposure time of a video frame was 40 ms. Ion probe current (Ip) was measured by a 130 mm diameter disc probe, usually negatively biased by 40 V with respect to the cathode (ground), and placed at the center of the first chamber 100 mm from the torus exit (see Fig. 1). I p reached saturation when probe bias was negative in respect to the cathode by 20 V. When the bias relative to the cathode was zero, I p reached about 80% of the saturation value. The electrical conductivity of the coated films was measured by a four-point probe. The surface morphology of the film and local electronic properties were
2. Experimental set-up and procedure A schematic diagram of the experimental filtered vacuum arc deposition apparatus is shown in Fig. 1. The deposition system contained a vacuum arc plasma gun with a 91 mm diameter, 99.9% pure, tin cathode and a 122 mm i.d. annular anode. A small coil was placed in a water cooled cavity in the cathode, and produced a magnetic field with a radial component B, of 0.2 mT A -1 [4]. A 30-200 A d.c. discharge was ignited by a mechanical trigger electrode. The plasma emitted by the cathode passed through the annular anode into a quarter torus macroparticle (MP) filter with a 240 mm major radius and a 80 mm minor radius (toroidal duct diameter, 160 mm), The toroidal magnetic field (B T , 6 mT A -1) was operated up to a maximum value of 25 mT. BT was produced by five coils connected in series.
Vacuum Gauges To Control and Recording Sys. Sub trate
Trigger"
[8J
tV Power Supply
\
IonCurrent
18
Toroidal coils
HI
1
- Substrate bias
R
Gate Valve Arc Current
Vacuum System
~~ I I I
: Oxygen Inlet Fig. 1. A schematic diagram of the filtered vacuum arc deposition apparatus.
183
L. Kaplan et al.TSurface and Coatings Technology 76-77 (1995) 181-189
studied by a scanning tunneling microscopy /spectroscopy (STM/STS) system. A review of the STM method is presented in Ref. [6]. STM images and I-V characteristics were obtained by use of an air STM equipped with etched tungsten tips. Constant current topographic (CCT) images were subjected to two-dimensional Fourier-transform filtering. Spectroscopic measurements were performed by the current vs. voltage (I-V) method at fixed tip-sample separation.
3. Experimental results
3.1. Arc and ion current characteristics In Fig. 2 we present the dependence of the arc voltage (Vare ) and probe ion current (lp) on arc current. Arc current (lare) was varied in the range of 30 to 220 A, Vare was measured for BT = 0, and I p was measured by a probe with a zero bias relative to the cathode, i.e. the value of I p was 80% of ionic saturation current. The magnetic fields BT and B, were set to 12 mT and 3 mT, respectively. The dependence of I p on arc current is closely linear. Threshold arc current for stable arc operation in this configuration was 30 A. Increasing the current from 30 to 220 A leads to an increase in the number of cathode spots that exist simultaneously on the cathode surface, from 1 to 6-7 (Fig. 3 (a)-(d)). The increase in the number of the spots results from spot splitting and not from spot motion. Arc voltage increased with the current from 13.5 V (at currents up to 100 A) to 16 V at current of 170 A. The arc became more stable with the growth of current, but at currents larger than 170 A the cathode spots had the tendency to move to the cathode boundary (Fig. 3 (d)). In Fig. 4 we show the dependence of Vare on BT in
vacuum and when oxygen is introduced into the system. In vacuum, Yare increases almost linearly with BT , i.e. from Vare= 13 V when BT=O to Vare=28 V when BT= 21 mT. Three modes of arc operation were observed at oxygen pressure of 1 Pa . (1) Cathode spots residing on the surface of the cathode; here Vare was in the range of 25 to 26 V. (2) Cathode spots residing on the boundary of the cathode; here Vare was 32 to 35 V. (3) Arc burning between the cathode and the spacer ring with Vare as low as 13 - 15 V. In Fig. 5 the typical dependence of Yare and I p on time is shown for an unstable arc in oxygen at a pressure of 1.3 Pa. (lare = 170 A, Br=O.4 mT, BT = 15 mT). All three arc modes mentioned above were present in this case. Most of the time Yare was equal to 34 V (point 2), however, in some cases (point 1) it dropped to 20 V, and in one case it dropped to 15 V (point 3). The events marked by (1) are correlated with an increase in I p , while event (3) is correlated with a marked decrease, a minimum, in ion current. Patches of oxidized Sn are seen in Fig. 6 (a) on the cathode surface after the arc was run in 1 Pa oxygen pressure, and in magnetic fields where B, = 0.4 mT and BT = 14 mT. The spots were not observed to move on the oxide surfaces, however, spots tracks are seen on the blackened oxidized regions on the cathode when examined after dismantling the arc. Surface oxidation of the cathode led to significant fluctuations in Yare and I p • A circular track of the cathode spots is seen in Fig. 6 (b) showing a photograph of the surface of a cathode that was used in an arc run at a 1 Pa oxygen pressure and in a magnetic field configuration where B, = 3 mT and BT = 14 mT. In this case of larger Bn the spots were observed to move during the whole arcing period on the circular track. The surface of the track was found clean from oxidation, and its resistivity was found to be equal to that of common tin. The arc was very stable in
16 - , - - - - - - - - - - - - - - - - - - - - - - - - - - - - , 500
;;;
-
400 15
-=
~
300 ......
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14
200
~
13
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250
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=
184
L. Kaplan et al.jSurfaceand Coatings Technology 76-77 (1995) 181-189
Fig. 3. Video images of cathode surface during arcing for different arc currents. (a) 50 A, (b) 100 A, (c) 170 A, (d) 210 A.
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~
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. I
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o
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50
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5 10 15 20 25 30 35 Toroidal field [mT]
~
70
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./ 3
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C
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.Q
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10
15 20 Time [s)
25
Fig. 4. Arc voltage (Vare ) as function of the toroidal magnetic field in vacuum (solid line), and in 1 Pa oxygen pressure (points): 1, cathode spots reside on cathode surface; 2, cathode spots moved to the boundary of the cathode; 3, arc burning between the edge of the cathode and the spacer.
B, = 0.4 m'T, BT
this case, Yare was 24-26 V, and both Yare and i, showed minimum fluctuations. The deposited Sn0 2 films were transparent and conducting. The metal on both sides of the track was covered by poorly conducting and blackened oxidized tin, but also showed the signs of cathode spots tracks. In Fig. 7 we present the dependence of I p on B, for
two values of BT (12 and 15 mT). The arc was run in a 1 Pa pressure of oxygen and probe bias was -40 V. I p increases with B, until it reaches a maximum value that is in the range of 2 to 3 mT, depending on the values of BT • Another study of the dependence of I p on B, is presented in Fig. 8. Here, BT is fixed at 15 mT, Vbia s = -40 V, and the oxygen pressure, P, is changed. Three
Fig. 5. Arc voltage and Ion current wave forms for 1"e~ 170 A, = 8.6 mT, and P = 1.3 Pa; 1, cathode spots reside on cathode surface; 2, cathode spots reside on the boundary of the cathode; 3, arc momentary interruption.
185
L. Kaplan et al.jSurfaceand Coatings Technology 76-77 (1995) 181-189 300r-----:=:::::::="'<::::::::----, p=o
« S.
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C ~
5c:
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2
3
4
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Fig. 8. Probe ion current as function of B, in vacuum and at oxygen pressures of 0.92 and 1.32 Pa. The probe was located at the distance of 100 mm from the torus exit.
(a)
(b)
Fig. 6. Oxidation of the cathode surface. The arc was operated with Br=OA mT (a), and Br=3 mT (b).
110
«
90
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2
3
4
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Fig. 7. Ion current as function of B, at oxygen pressure of 1 Pa, for BT equal to 12 and 15 mT, The probe was located 100 mm from the
The spacer coil (see Fig. 1) produced an axial magnetic field B, in the region between the anode and the entrance to the magnetic torus. Bs improved the plasma transfer into the toroidal filter, however, it also affected the transfer of arc current from the cathode to the anode and the motion of the spots on the cathode surface. When B, was directed opposite to B, and equal to 0.7 mT or less, a stabilization of the arc operation was obtained. Any further increase in B, led to a strong disturbance in arc operation. In Fig. 9 we present the dependence of I p on BT . In this case the magnetic field B, equals 0.7 mT and BT is directed opposite to the field Br • Five plots of I p vs. BT are shown in Fig. 9 with B, as a parameter (Br = 0, 1, 2, 3,4, mT). In all cases I p increases with BT until it reaches a maximum value. Increasing BT beyond this value had only a negative effect on the deposition rate, which is always proportional to I p • The magnitude of the peak increased with Bn suggesting a saturation effect at higher Br . At Br=4 mT I p is closely constant for BT in the range of 14 to 17 mT. It is interesting to note that the value of BT for peak I p increases with Bn as well as the value of peak of I p . Additional increase in I p was obtained by applying to the torus a positive bias voltage of + 17 V relatively to the anode. Biasing the wall increased peak I p by a factor of 3. At higher B, arc
torus exit, and biased by -40 V relative to the cathode.
200-r------------,
cases are presented in Fig. 8: oxygen pressure P = 0 (oxygen was not introduced to the system), P = 0.9 Pa and P = 1.3 Pa . As a function of B" I, has a maximum in all cases. In vacuum, maximum I p equals 300 rnA at B, = 2 mT. Maximum I p decreases with P and is observed at higher Br . At P=1.3 Pa and at Br=3 mT the maximum of I, was equal to 65 rnA . The presence of 1.3 Pa of oxygen in the coating system reduced peak I p by almost a factor of 5, in comparison with its value when oxygen was not flowing into the system.
Ic
~ 100
;:,
u
c:
.Q
lmT
06
8
10
12
14
16
18
20
Toroidal magnetic field [mT]
Fig. 9. Ion current as function of BT for different values of Br •
186
L. Kaplan et al.f Surface and Coatings Technology 76-77 (1995) 181-189
operation without instabilities is possible if higher toroidal field are applied, though peak I p decreases. 3.2. STM studies ofthe films
STM analysis provides a very useful additional information to that obtained by structural, electrical and optical measurements of Sn-O films [2,3]. To investigate the influence of system parameters on the film quality two groups of films were deposited at the same oxygen pressure of 1 Pa but at different Br . The electrical resistivity, p, of the films deposited with B, = 0.4 mT was in the range of 1-6 mO cm, while the resistivity of the films deposited with B, = 3 mT was in the range of 0.9-3 mO em. Samples with p equal to 1 and 3 mO cm, more typical for each respective group, were selected for further study by STM. In Figs. 10 (a) and (b) we present computerized STM images of film surfaces with a 0.1-0.2 nm lateral resolution. A typical sample of surface morphology of a 50 nm (thickness) Sn-O film is shown in Fig. 10 (a), while surface morphology of a 350 nm film is shown in Fig. 10 (b). No correlation between B, and the film
surface morphology was observed. The surface rugged and grainy structure had a lateral periodicity close to 20 nm in both cases. The average height of the crests on the films depended, however, on its thickness, being 5 and 0.6 nm for the 50 nm and 350 nm layers, respectively. Well defined clusters were observed in the case of the thin 50 nm film (Fig. 10 (a)). In Figs. 11 (a) and (b) we present local I-V plots obtained with the STM on samples deposited with B, = 0.4 mT. The I-V plot in Fig. 11 (a) is that of a typical degenerate semiconductor material, while in the I-V plot in Fig. 11 (b) is that of a normal semiconductor having an energy gap of 3 eV, determined by the difference between the biase voltages of the tip at the points where the current to the tip becomes negative or positive. Thus, the surface states of this material is that of a non degenerate semiconductor with an energy band gap of approximately 3 eV. In most tests I-V plots of degenerate semiconductor were obtained, yet a considerable number of tests (20%) resulted with a semiconductor like I-V plot of normal semiconductor. In contrast to these results, STM study of films deposited under the conditions in which B, was equal to 3 mT resulted with I-V
(a)
Cross
5nm
Fig. 10. STM images of Sn02-x films with thickness of 50 nm (a), and 350 nm (b).
L. Kaplan et al.jSurjaceand Coatings Technology 76-77 ( 1995) 181-189
plots of only degenerate semiconductor for all analyzed points on the surface. This result agrees well with the higher conductivity of these films.
4. Discussion 4.1. Sn arc operation
A similar study of the effect of gas pressure and magnetic fields on Ti arcs in the system described here was made by Zhitomirsky et al. [4,5]. The oxidation of the cathode is the new effect observed in the present study of the characteristics of a Sn cathodic arc attached to a magnetic macroparticle filter. Otherwise, the behaviour of both arcs in vacuum is similar in most details. Unstable arc operation that leads to unstable plasma flow is usually correlated with cathode spot displacement to the edge of the cathode. This motion may be caused by the retrograde motion of the spot in the magnetic field produced at the cathode surface by the arc current and the quarter-torus and spacer coils. The spacer coil may also produce an interruption of the arc. However, the operation of the system depends on magnetic fields to obtain proper guiding of the plasma beam from the anode through the filter and into the sample. Hence, a
187
conflict exists between the need to use relatively large guiding fields and their detrimental effect on the arc operation. The observation of a maximum in I p as function of arc current, BT and B, indicates that a balance between opposing effects is achieved, but this does not assure a significant ion current to the substrate [4,5]. It is significant to notice here that samples of Sn-O films deposited with the unstable arc were always of poor quality (i.e. low electrical conductivity). The presence of oxygen in the system is obviously essential for the deposition of oxide films. However, the cathode surface itself may become oxidized. The oxidation of the cathode surface may produce insulating oxide regions, or a poorly conductive layer on the cathode surface. The characteristics of cathode spots residing on oxides were studied by Guile and Juttner [7]. As the average current per spot on an oxide surface is about 1% of its value on clean surfaces, the number of the spots should increase markedly when the spots reside on the oxide regions. The erosion rate which is about 150 ug C -1 for clean Sn surfaces decreases to approximately a fraction of a percent [7]. The experimental data agree well with this prediction. Whenever the spots are forced to rotate on a circle by B, and pass from the clean metal to an oxidized region, they tend to disappear and the ionic current decreases accordingly. There is
188
L. Kaplan et al./Surface and Coatings Technology 76-77 (1995) 181-189
I-V spectrum 20
~
10
Cj) I
0
.....
~
(ij
c::
0
OJ
'en
-10
-20 2
0
-1
-2
Sample Bias (V)
(b) 10
~ Cj) I
0
.....
~
0
(ij
c::
OJ
'en
-10
t -2
0
2
Sample Bias (V) Fig. 11. Typical I-V curves obtained on Sn0 2 - x films: (a) I-V curve showing the typical metallic behaviour of a degenerate semiconductor; (b) I-V curve with a gap typical for a non-degenerate semiconductor.
also a correlated increase in the arc voltage. As the rotating arc moves alternately over clean and oxidized areas, a strong fluctuation is observed in arc and probe currents. Such behavior has been typical to arcs running with Br=OA mT. When B, is much higher, 3 mT, the spot rotation is faster, and the track on which the cathode spots move is actually free of oxides. In this case the higher spot velocity combined with its surface cleaning effect produce an oxide free region along the whole track. The spot returns to its previous location before extensive oxidation could have developed. A stable arc is observed in this case with stable I p and stable v"rc in the region 23-25 V during the whole deposition process. Oxidation of the areas adjacent to the track also occurs in this case (B, = 3 mT), but only few spot tracks are evident (see Fig. 6 (b)).
Arc instability which is caused by the magnetic field or by surface oxidation not only reduces the deposition rate but also affects adversely the quality of the coatings. Oxygen pressure is another factor that determines film quality. Uniform film structure is obtained at oxygen pressure, P, between 0.8-1.3 Pa. The lower pressure limit is imposed by the need to prevent the formation of the electrically insulating SnO phase. An increase in P allows a correlated increase in I p (to keep the proper stoichiometry), but also increases the probability of cathode oxidation and arc instability. Further, an exceedingly high pressure may prevent the plasma from reaching the substrate. Hence, a well defined range of oxygen pressures existed in our system for proper deposition of conducting SnO z. Optimal P was determined to be in the range of 0.9 to 1.3 Pa.
L. Kaplan et al.isurface and Coatings Technology 76-77 (1995)
The procedure adapted for introducing the oxygen into the coating system is also significant. When the system is filled up by a constant gas pressure before arc ignition, the onset of the arc has an effect of introducing an additional gas pump, and the pressure is reduced to a new stationary value. Ben-Shalom et al. [3] showed that a too high initial gas pressure prolonged prohibitively the time interval required for the pressure to reach the proper value. Thus, with this method of gas flow adjustment, the initial gas pressure should agree with arc current so that working pressure level would be reached in as short time as possible. 4.2. STM analysis The STM I-V data suggest the existence ofnanometric size clusters of non-degenerate semiconductor consisting of Sn2+ (instead of Sn H ) ions. We have also seen that when B, was equal to 3 mT, and arc was stable, the density of such sites was markedly reduced. These microobservations agree well with the large scale findings that high quality films were produced by applying B, equal to 3 mT.
5. Conclusions Filtered vacuum arc deposition can be utilized to deposit transparent conductive films of Sn02 at high deposition rate of 10 nm s -1. The deposition of high quality Sn02 films requires a careful adjustment and regulation of the following parameters: magnetic fields, arc current, cathode spot velocity on cathode surface, working arc pressure, and oxygen flow rate. The depos-
I8I~I89
189
ition rate is determined by the ion current density at the sample. The above mentioned parameters can be defined so that arc operation is stable, and a maximum value of I p is reached. Deposition process optimization can be achieved by determining the set of parameters resulting with the maximum of peak I p • Any further optimization may require geometrical changes in the system geometry in addition to the regulation of the parameters listed above.
Acknowledgments The authors are grateful to Uri Kinrot for the gas control computer program, the technical assistance of Hanan Yaloz, the financial support of the Israeli Ministry for the Absorption of Immigration, the Ministry of Science and Arts, and Friends of Tel-Aviv University in France.
References [1] K.L. Chopra, S. Major and D.K. Pandya, Thin Solid Film, 102 (1983) 1. [2] L. Kaplan, A. Ben-Shalom, R.L. Boxman, S. Goldsmith and M. Nathan, Thin Solid Films, 253 (1994) 1. [3] A. Ben-Shalom, L. Kaplan, R.L. Boxman, S. Goldsmith and M. Nathan, Thin Solid Films, 236 (1993) 20. [4] V.N. Zhitomirsky, R.L. Boxman and S. Goldsmith, Surf. Coat. Technol, 68/69 (1994) 146. [5] Y.N. Zhitomirsky, R.L. Boxman and S. Goldsmith, J. Vac. Sci. Technol, A13 (1995) 2233. [6] R.I. Hamers, in D.A. Bonnel1 (ed.), Scanning Tunneling Microscopy and Spectroscopy: Theory, Technics and Applications , Imprint VCR, New York, 1993, pp. 51-103. [7] A.E. Guile and B. Jiittner, IEEE Trans. Plasma sa: PS-8 (1980) 259.