Plasma immersion ion implantation of insulating materials

Plasma immersion ion implantation of insulating materials

Surface & Coatings Technology 196 (2005) 162 – 166 www.elsevier.com/locate/surfcoat Plasma immersion ion implantation of insulating materials X.B. Ti...

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Surface & Coatings Technology 196 (2005) 162 – 166 www.elsevier.com/locate/surfcoat

Plasma immersion ion implantation of insulating materials X.B. Tiana,b, K.Y. Fub, P.K. Chub,*, S.Q. Yanga a

State Key Laboratory of Advanced Welding Production Technology, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, China b Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Available online 18 October 2004

Abstract Plasma immersion ion implantation (PIII) was proposed in the mid-1980s and has mostly been applied to conducting materials or semiconductors. There is much interest in extending the technology to the treatment of insulating materials such as polymers, ceramics, rubber, etc. Implantation into these electrical insulating materials can enhance the properties and performance. In this paper, we describe our research work related to the plasma implantation of insulating materials. We have conducted numerical simulation using plasma fluid model and particle-in-cell (PIC) model as well as experimental investigations. During implantation of insulating materials, capacitance effects and surface charging reduce the energy of the incident ions. Severe charging occurs at the initial time stage and the insulating sample on the metal target holder distorts the local plasma sheath leading to complicated implantation dynamics. In order to improve the implantation energy, a metal mesh is used to accelerate ions from the plasma and our results show that it is an effective technique for thick or large insulating objects. The metal mesh not only improves the implantation energy and the incident dose but also reduces the possibility of surface arcing. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Insulating materials; Metal mesh

1. Introduction Plasma immersion ion implantation (PIII) emulates conventional beam-line ion implantation in large-area processing as well as the treatment of objects with an irregular shape while obviating the need for complex manipulation of the target holder [1]. Hence, it has attracted much attention since its inception. There is tremendous interest in extending the technology to the treatment of insulating materials such as polymers, ceramics, rubber, etc. Ion implantation of polymeric materials has been reported to improve the surface properties [2]. Hence, the application of PIII to insulating materials is of interest to both researchers and industry as PIII offers advantages such as low costs, small instrument footprint, large area and conformal processing capability compared to conventional beam-line ion implantation.

* Corresponding author. Tel.: +852 27887724; fax: +852 27889549. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.166

However, PIII of insulating materials is challenging as issues pertaining to surface charging, electrical arcing, plasma sheath distortion, and resulting in non-uniformity of the ion dose are not well understood and must be tackled. Since the early 1990s, PIII of insulating materials has been sporadically been reported [3 4]. For example, Lacoste et al. [5] have attempted to improve the wettablity of polymers and Sakudo et al. [6] have investigated the capability of gas barriers in polymers. So far, there has been limited literature on the fundamental aspects, effectiveness, and applications of PIII to insulators. In this paper we review our theoretically and experimental work pertinent to PIII of insulating materials. Two-dimensional numerical simulation using the particle-incell model and fluid model is conducted to investigate the expansion dynamics of the plasma sheath. Surface charging and the effects on plasma sheath expansion have also been investigated [7]. In order to improve the ion energy and reduce surface charging as well as electrical arcing, a metal mesh to cover the insulating samples is employed in our investigation [8,9].

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2. Numerical investigation Using the Child law, one can qualitatively predict the surface charging, potential reduction, and other phenomena for an infinite planar sample [4]. However, the finite size of typical samples and practical experimental setup require at least two-dimensional numerical simulation. In this work, we use 2-D particle-in-cell (PIC) modeling and the simulation is based on the actual dimensions and hardware configuration of the PIII instrument in the City University of Hong Kong [10–13]. The metal target holder is 150 mm in diameter and 50 mm thick, and the inner diameter of the vacuum chamber is 760 mm. In the simulation, a 100 mm diameter insulating sample is placed on the target holder. Considering the potential reduction induced by the capacitance effects and surface charging, the surface potential on the insulating sample is assumed to be 0/5, 1/5, 3/5, 5/5 of the potential applied to the conducting target holder [14]. The simulation parameters are: Vapplied= 40 kV, hydrogen plasma density n 0=3109 cm 3, electron temperature Te=4 eV, and these parameters are normalized for simplicity similar to previous work [15]. In this way, the potential of the metal target holder is equal to unity and the target dimension is shown in Figs. 1 and 2. Fig. 1 also depicts the potential contour (plasma sheath configuration) for different sample potentials at t=1 As. At the center of the sample, the

Fig. 2. Incident dose distribution on the top surface with sample surface potential V=3/5.

plasma sheath expands relatively slowly due to a lower bias potential on the insulating samples and the sheath shape is concave in the center. In contrast, the sheath has a convex geometry due to the larger electric field for V=5/5 (without insulating sample on the target holder). It can also be observed that the plasma above the insulating sample is affected by the electric field induced at the other sections of the target holder. The electrons and ions in the plasma

10 V=0

V = 1/5

V = 3/5

V = 5/5

8

Normalized longitudinal distance

6 4 2 0 8 6 4 2 0 0

1

2

3

4

5

0

1

2

3

4

5

6

Normalized radial distance Fig. 1. Potential configuration and plasma sheath for different target potentials at time=1 As. The central bT Q objects in the figures are the metal target holder, where the thin, round insulating sample is located. The potential equipotentials are presented for values from U=0.0 the edge of the sheath, as indicated in the figure) to U=1.0 (the conducting surface along X-axis) in intervals of 0.2.

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respond to these electric fields and there is no electrical neutrality. Consequently, distortion of plasma sheath ensues and the distribution of the incident ion dose is not uniform as shown in Fig. 2. The central zone receives a lower ion dose due to a weak electric field induced by the low potential, whereas the section on the metal target holder adjacent to the insulating sample receives a higher ion dose. As for the insulating regions only, non-uniformity of the ion dose is also observed due to the distortion of the plasma sheath. At a certain potential configuration, some ions flying towards the metal holder do not impact the metal holder but rather impinge into the edge surface of the insulating sample. Hence, the edge of the insulating sample receives more ions originating from the center zone just above the insulating samples.

3. Experimental investigation 3.1. Sheath monitoring In order to verify the distortion of the plasma sheath, experimental investigation of the sheath has been performed [16]. In the experiments, the Langmuir probe is positively biased to detect the electrons in the plasma. The abrupt change in the collected current implies the arrival of the plasma sheath due to the depletion of free electrons in the sheath. The probe is positioned 85 mm away from the target surface and the probe can be moved horizontally during the experiments. The hydrogen gas pressure is 4.010 4 Torr and the plasma is sustained by a radio-frequency source with input power of 1000 W. It is evident that the expansion speed of the plasma sheath at different sites is different as summarized in Fig. 3. The speed near the center of the whole target (including insulating sample and metal target holder) is smaller and the sheath takes a longer time to reach the probe. This is attributed to the weak electric field due to the insulating sample. In contrast, the sheath at 35 mm from the center expands more rapidly. Although the probe is also above the insulating sample, the metal target holder can contribute to the rapid plasma sheath expansion. At the edge site (L=75 mm), the longer time it takes the probe to reach the probe is due to the effect of the spherical sheath [17]. Our experiments also indicate that plasma implantation may be impossible if the insulating samples are too thick and the sheath may retreat even when the high voltage pulse is turned on. 3.2. Surface charging and potential reduction The biggest problem in PIII of insulating materials is surface charging. It stems from the accumulation of incoming ions and emission of secondary electrons. The secondary electron coefficient is generally much higher than unity at high voltage, particularly for ceramics such as Al2O3 and MgO. At the beginning of the applied voltage

Fig. 3. Time for the sheath to reach the probe with the target holder biased to 20 kV.

pulse, charging is more severe [18] and electrical discharge (or arcing) may happen if charging is too severe. The discharge time scale of charged insulating surfaces is determined by the voltage, plasma density, waveform of the applied pulse, and other factors. Owing to surface potential reduction induced by the accumulated charges, the incident ion energy is reduced. A comparative experiment was performed with two types of sample arrangement. A 1515 mm silicon sample was laid on top of a piece of quartz 20 mm wide and long and 1 mm thick and a second silicon sample was put directly on top of the metal target holder. The implantation parameters were: sample voltage= 20 kV, pulse duration=20 As, and pulse frequency=200 Hz. A nitrogen plasma was externally sustained by hot filament glow discharge. During implantation, some electrical arcing was observed on the silicon sample on quartz. XPS analysis shows that the insulating effect induced by the quartz sample reduces the incident ion dose and penetration depth. Compared to the bare silicon sample (second sample), the ion dose of the sample on quartz is reduced by about 21% and the ion range diminishes by 30%. The thickness of the insulating samples affects the implantation dynamics. As for the thin insulating materials, surface arcing occurs but the sample receives a higher incident dose as shown in Fig. 4 [7]. It may be attributed to the effect of the sample thickness on its own capacitance. A smaller thickness gives rise to a larger capacitance and a

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smaller potential reduction. It can be envisioned that implantation is impossible if the sample is too thick since a smaller capacitance leads to a smaller equivalent surface potential. 3.3. Mesh-assisted PIII As aforementioned, PIII of a thick insulator may not be feasible and sheath retraction may be observed even if the voltage pulse is on. That is to say, severe surface potential reduction (due to a small equivalent capacitance) makes the implantation impossible. In this respect, a metal mesh can help. The insulating sample can be covered by a conducting mesh, which is subjected to the same potential as the target holder. In this way, a cage-shaped target (the target holder is also considered) forms and the insulating object inside the cage floats electrically. Consequently, ions do not see the insulating samples before they pass through the metal mesh if the mesh size is relatively small. Therefore, the metal cage can improve the equivalent potential on the insulating sample and increase the incident ion flux and penetration depth. The metal mesh has two positive effects on the plasma implantation dynamics. The first one is to increase the equivalent sample capacitance induced by the mesh compared to the sample. If the top surface of the insulating sample acts as a reference point, the sample capacitance is a composite of the capacitance between the sample surface and the target holder surface and the capacitance between the top surface and the mesh (when used). Addition of the mesh increases the equivalent sample capacitance, thereby retarding the reduction of the surface potential assuming that the same charges are accumulated on the sample surface. Consequently, the implantation efficacy can be improved. Secondly, the internal electric field in the cage automatically builds up once plasma implantation starts. This field substantially suppresses the emission of secondary electrons from the

Fig. 5. Nitrogen depth profiles acquired using SMIS from the samples treated at 40 kV. The density of the fused quartz used in the LSS model is 2.21022 atoms cm 3.

insulator surface. In contrast, in the absence of the cage, the electric field is directly between the chamber wall (anode) and sample (cathode) and secondary electrons fly away from the sample surface contributing significantly to surface charging. On the other hand, the metal mesh (cage) alters the configuration of the electric field. If the metal mesh acts as a reference point, the electric field with opposite directions exists on each side of the mesh. In this way, electrons from the insulating sample do not directly see the chamber wall (anode). The metal mesh produces the effect of a dual anode and secondary electrons rejected from the sample are attracted back to the anode or sample. As a result, reduction of the surface potential is mitigated and the risk of surface arcing becomes smaller. It should be noted that the capability of the secondary electron suppression depends on the mesh dimensions, materials, and the sample. Fig. 5 demonstrates the effect of the metal mesh on the implantation dynamics. The bmesh-assistedQ sample receives a higher incident dose compared to the bare sample [9], event though they are implanted simultaneously using the same processing parameters: negative pulse potential of 40 kV, pulse width of 20 As, and repetition rate of 200 Hz. Unfortunately, the mesh also imposes an adverse effect. Sputtering of the grid produces metallic contamination and the mesh casts a shadow on the sample surface leading to potential dose non-uniformity. The former can be addressed by coating of the grid with a compatible material, whereas the latter effect can be reduced by introducing some sort of sample rotation or changing the distance between the grid and sample surface [19]. More work is being done in this area to investigate these issues.

4. Conclusion Fig. 4. Nitrogen depth profile in soda lime glass for different sample thickness implanted with V 0=40 kV acquired by secondary ion mass spectrometry (SIMS).

Plasma immersion ion implantation of insulating materials is attractive but challenging. Problems associated with

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surface charging, arcing, reduction of surface potential, etc., have to be solved. Our 2-D numerical simulation shows that the plasma sheath may be distorted, and it is verified by our experimental results. This may lead to nonuniform incident dose. Mesh-assisted PIII is shown to improve the equivalent surface potential and better implantation dynamics. A higher ion flux and smaller risk of surface arcing result. This may be attributed to the dual anode effects relative to the metal mesh. All in all, PIII of insulating materials is a relatively complex process and more work is needed.

[2] [3] [4] [5] [6]

[7] [8] [9] [10] [11]

Acknowledgments

[12]

The work was jointly supported by the National Natural Science Foundation of China (No. 10345003), Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grant (CERG) # CityU1137/03E, and City University of Hong Kong Strategic Research Grant (SRG) # 7001642. We are grateful to Dr. Dixon T.K. Kwok for valuable discussions.

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