Enhancement of process efficacy using seed plasma in pulsed high-voltage glow-discharge plasma implantation

Transcription

1 Physics Letters A 303 (2002) Enhancement of process efficacy using seed plasma in pulsed high-voltage glow-discharge plasma implantation X.B. Tian, P. Peng, Paul K. Chu Department of Physics & Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Received 25 July 2002; received in revised form 13 August 2002; accepted 22 August 2002 Communicated by F. Porcelli Abstract Pulsed high-voltage glow-discharge plasma implantation is a practical and effective surface modification technique, although the factors influencing the mechanism are still not well understood. In this Letter, we describe the use of seed plasma to alter the discharge characteristics and improve the efficiency of the process. The seed plasma is produced by an ionization gauge that is a common vacuum measurement device in plasma ion implanters. The ignition behavior of the glow discharge in the presence of seed plasma was experimentally investigated. The seed plasma induces early igniting and the impact diminishes with increasing applied voltage or working pressure. Early triggering is favorable with respect to the mitigation of arcing and the enhancement of the processing and electrical efficacy, and the phenomenon may convey useful information pertaining to the mechanism of the complicated pulsed high-voltage glow-discharge process Elsevier Science B.V. All rights reserved. PACS: H; R; 52.65; Keywords: Plasma; Glow discharge; Ion gauge Ion implantation has been proven to be an effective surface modification technique [1,2]. However, the line-of-sight restriction makes the treatment of samples possessing an irregular geometry a challenge. In order to circumvent this limitation, plasma source ion implantation (PSII) was introduced in the late 1980s [3,4]. In PSII that is also called plasma immersion ion implantation (PIII, PI 3 ), plasma-based ion implantation (PBII), and plasma ion implantation (PII), the sample is immersed in plasma produced by ex- * Corresponding author. address: paul.chu@cityu.edu.hk (P.K. Chu). ternal plasma sources and then biased to a high negative potential relative to the chamber wall (earth). Positive ions in the plasma are accelerated via the plasma sheath and implanted into the sample surface. When conducted properly, the entire exposed surface of the specimen is simultaneously implanted without the need of sample manipulation even for samples with a complex shape. Plasma implantation can also be performed in an alternative way without a pre-existing plasma. In pulsed high-voltage glow-discharge plasma implantation, the plasma is produced by the implantation pulse itself [5]. This mode offers at least two advantages. External plasma sources are no longer necessary thereby reducing the hardware cost and /02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 68 X.B. Tian et al. / Physics Letters A 303 (2002) footprint. In addition, unintentional and deleterious plasma processes such as etching and oxidation can be avoided because the plasma only exists for a short time compared to traditional PIII in which the sample and chamber wall are constantly surrounded by plasma. This is particularly advantageous for plasmas with corrosive and etching gases such as chlorine, otherwise the vacuum chamber and RF antennae must be made of special materials such as aluminum to avoid direct interactions with these corrosive gases [6,7]. Generally speaking, high-voltage glow-discharge plasma implantation is similar to but different from pulsed plasma nitriding and pulsed PECVD [8,9]. In this process, the pulse voltage is higher to achieve implantation and the gas pressure is lower to reduce energy loss of the incident particles due to scattering. Experimental results have demonstrated the effectiveness of this technique. In particular, a larger ion projected range and thus thicker modified layer can be attained [10,11]. Nonetheless, this technique has not been widely investigated and there are not much experimental data related to this process in the literature. In order to understand the mechanism, the factors affecting the process must be better understood and the efficacy of the process must also be enhanced to make it more attractive as an industrial tool. Based on our recent experimental findings [12], the ionization gauge affects the ignition behavior of highvoltage glow discharge. This interesting phenomenon was not expected as ion gauges are very common in vacuum instruments as pressure monitoring devices and not supposed to interfere with the formation of the plasma. We have observed that the small seed plasma produced by the ionization gauge has a large impact on the discharge characteristics, namely the delay in the ignition of the glow discharge, and can be used to enhance the efficacy of the pulsed high-voltage glowdischarge process. In this Letter, experimental results describing the relationship between the ionization gauge and igniting properties are presented and the information is expected to assist in disclosing the mechanism of pulsed high-voltage glow discharge. The experiments were performed in a multi-purpose plasma immersion ion implantation facility described in details elsewhere [13]. The vacuum chamber was 1200 mm in height and 1000 mm in diameter. The sample used in this investigation was a stainless steel rod 50 mm in diameter and 400 mm long as Fig. 1. Experimental set up for pulsed high-voltage glow-discharge plasma implantation. exhibited in Fig. 1. The argon plasma was formed by applying a voltage between 10 and 30 kv to the sample without an external plasma source. The highvoltage pulse duration was between 100 and 1000 µs. A Granville-Phillips Series 274 ionization gauge was used in our experiments. Fig. 2 shows the ignition behavior of the glow discharge and influence of the ionization gauge at an applied voltage of 20 kv and gas pressure of 2.5 mtorr. The time delay in the ignition of the glow discharge is indicated by a period of zero implantation current right after the voltage is applied. After a certain time period, the current appears and rapidly reaches the saturation level. Afterwards, the current remains constant until the end of the voltage pulse. The ionization gauge influences this delay time. When the ionization gauge is switched on, the time delay becomes shorter, that is, the glow discharge occurring sooner. It should be pointed out that the ionization gauge only changes the delay time but not the other characteristics of the glow discharge. The slope of the current increase and saturation current are the same regardless of whether the ionization gauge is turned on or off. Two current pulses at the beginning and end of the implantation voltage can also be observed,

3 X.B. Tian et al. / Physics Letters A 303 (2002) Fig. 2. Early igniting of glow discharge induced by ionization gauge at 20 kv and 2.5 mtorr. Fig. 3. Relationship between implantation current and early triggering and working pressure (applied voltage V appl = 20 kv). Fig. 4. Influence of applied voltage on implantation current and early ignition of glow discharge (gas pressure P gas = 2.5 mtorr). and they are mainly induced by the charging and discharging of the capacitance of the co-axial highvoltage cable [14]. The early ignition behavior depends on the implantation voltage and working pressure, as shown in Figs. 3 and 4. The influence of the gauge is more evident at lower pressure and implantation voltage. For example, at 1.5 mtorr, the ignition is advanced by 67 µs, while it is 6 µs at 3.5 mtorr. The effect becomes hardly noticeable at 4.5 mtorr. A similar trend is observed for the applied voltage. Our results show a linear decrease in the early ignition time with increasing discharge voltage as shown in Fig. 4. The variation of the time delay with voltage or pressure is consistent

4 70 X.B. Tian et al. / Physics Letters A 303 (2002) Fig. 5. Influence of filament current on glow-discharge behavior in a nitrogen ambient (applied voltage V appl = 20 kv and pulsing frequency of 25 Hz). with that of the implantation current with voltage and pressure as shown in Figs. 3 and 4. The higher the implantation current, the smaller is the delay time. The ion gauge is the only variable in our experiments, and so we believe that the reduction in the ignition delay time is due to the seed plasma (maybe the electrons in the seed plasma) produced by the ion gauge. We have observed similar trends by varying the current on the hot filament, as shown in Fig. 5. With increasing filament current, the delay time decreases confirming the effects of free electrons. An ion gauge produces charged particles to detect the pressure and they can diffuse out into the vacuum chamber. This provides the seed for the multiplication of charge carriers in the glow-discharge process in spite of the relatively small amount. Under high voltage and low pressure, Townsend theory does not seem to adequately explain the mechanism, as conformed by the simulations at tens of Pa high-voltage glow discharge [15]. The probability of electron neutral collisions is very smallathighvaluesofe/p (high voltage and low pressure). However, the electron effect cannot be ignored, at least during the initiation phase. Three effects should be noted. Firstly, the electric field that is spatially non-linear around cylindrically or spherically anodes compared to small cathode changes slowly towards the wall and so the electrons nearby may get a suitable energy (perhaps tens of V to hundreds of V) increasing the cross section of collisions to enhance the formation of the plasma. Secondly, the electrons are generated in the ion gauge near the vacuum chamber wall, where the acceleration voltage induced by the anode cathode potential is low but enough for ionization. Thirdly, the electrons bombard the wall inducing secondary electrons. These secondary electrons are driven back by the electric field to ignite the plasma.

5 X.B. Tian et al. / Physics Letters A 303 (2002) That is to say, these pre-existing electrons can help to ignite the plasma. Although electrons are more likely to be the enhancement medium, whether the multiplication of the charge carriers is due to electrons or ions must be further investigated as different models have been proposed [10,11]. With increasing working pressure or applied voltage, the discharge current increases and the influence of ion gauge becomes smaller. It is understandable as the intrinsic collision frequency (electrons, ions, neutrals, etc.) increases at a higher working pressure. The effects of the residual plasmas or excited neutrals produced by the previous pulse must also be considered. We have observed that a potential can be detected by the Langmuir probe for about 1.5 ms after the highvoltage pulse has been turned off. These pre-existing species before the next pulse diminish the seed plasma effect produced by the ion gauge. Therefore, a higher voltage leads to a higher plasma density and longer decay time, consequently a weaker effect of the seed plasma of the ion gauge, and the same is true for the working-pressure. However, the influence of the seed plasma is not totally shielded by higher voltage or pressure in spite of the relatively smaller number of active particles from the gauge. It is because that the active particles produced by the previous pulse rest in the central zone of the chamber and they have a smaller collision cross section due to the higher local electric field. In contrast, electrons in the vicinity of the vacuum chamber wall have a large collision cross section. Earlier ignition of glow discharge during pulsed plasma implantation is practically important. A smaller delay time translates into better utilization of the implantation pulse and efficiency. That is to say, a smaller voltage duration may suffice if the delay time is shortened, further reducing detrimental effects such as plasma etching. This also reduces the probability of electrical arcing between the sample and vacuum chamber wall under high voltages and enhances the processing and electrical efficiency of the power supply. In fact, a long voltage pulse is known to damage the hard tubes in pulsing power supplies [16]. In conclusion, we have experimentally investigated the influence of seed plasma produced by the ionization gauge on the ignition behavior of high-voltage glow discharge. It should be mentioned that the role of seed plasma in high-voltage glow-discharge processes has been previously investigated, but based on what we have found in the literature, the influence of the ionization gauge on the discharge characteristics has not been observed before. As a standard device in the vacuum system, the ionization gauge can be used conveniently to decrease the delay time of discharge ignition, particularly at low pressure or low voltage. Early igniting potentially decreases the risk of electrical arcing and increases the processing and electrical efficiency of the power modulator. Our data may provide more clues to the mechanism of high-voltage glow discharge and more work is being conducted to enable a better understanding of the phenomenon. Acknowledgements The work described in this Letter was jointly supported by research grants from the Hong Kong Research Grants Council CERG #CityU1052/02E and City University of Hong Kong SRG # References [1] H. Ryssel, I. Ruge, Ion Implantation, Wiley, [2] J.F. Ziegler, Ion Implantation Technology, North-Holland, [3] J.R. Conrad, J.I. Radke, R.A. Dodd, F.J. Warzala, N.C. Tran, J. Appl. Phys. 62 (1987) [4] A. Anders, Handbook of Plasma Immersion Ion Implantation and Deposition, Wiley, New York, [5] M.M. Shamim, K. Sridhoron, R.P. Fetherston, A. Chen, J.R. Conrad, J. Vac. Sci. Technol. B 12 (1994) 843. [6] U. Hornauer, R. Gunzel, H. Reuther, E. Richter, E. Wieser, W. Moller, G. Schumacher, F. Dettenwanger, M. Schutze, Surf. Coat. Technol. 125 (2000) 89. [7] U. Hornauer, R. Gunzel, E. Richter, E. Wieser, W. Moller, G. Schumacher, F. Dettenwanger, M. Schutze, in: Proceedings of 7th International Conference on Plasma Science Engineering, Garmisch-P, September 17 21, [8] E. Guiberteau, G. Bonhomme, R. Hugon, G. Henrion, Surf. Coat. Technol. 97 (1997) 552. [9] T.A. Beer, J. Laimer, H. Stori, Surf. Coat. Technol. 121 (1999) 331. [10] J.N. Mattossian, R. Wei, Surf. Coat. Technol. 85 (1996) 92. [11] V.I. Khvesyuk, P.A. Tsygankov, Surf. Coat. Technol. 96 (1997) 68. [12] X.B. Tian, P.K. Chu, J. Phys. D 34 (2001) 354. [13] P.K. Chu, B.Y. Tang, Y.C. Cheng, P.K. Ko, Rev. Sci. Instrum. 68 (1997) [14] X.B. Tian, B.Y. Tang, P.K. Chu, J. Appl. Phys. 86 (1999) [15] H. Hillmann, F. Muller, H. Wenz, Plasma Source Sci. Technol. 3 (1994) 496. [16] X.B. Tian, X.C. Zeng, P.K. Chu, IEEE Trans. Plasma Sci. 29 (2001) 529.