Elsevier

Thin Solid Films

Volume 519, Issue 20, 1 August 2011, Pages 6688-6692
Thin Solid Films

Deposition of amorphous hydrogenated carbon films on Si and PMMA by pulsed direct-current plasma CVD

https://doi.org/10.1016/j.tsf.2011.04.075Get rights and content

Abstract

A pulse-modulated direct-current methane plasma is used to deposit amorphous hydrogenated carbon (a-C:H) films on Si and polymethyl methacrylate (PMMA) substrates. The structure and mechanical properties of the films are examined by applying a negative pulse bias voltage of 0.5 to 3 kV to the substrate at a pulse bias period of 100 to 200 μs. The deposition rate on both Si and PMMA increases with increasing the net input power, independent of the pulse period. The Raman spectra demonstrate that the films on Si are diamond-like carbon (DLC), while those on PMMA are polymer-like or soft amorphous carbon because of higher crystallinity of the sp2 phase and lower nanoscale hardness. The residual compressive stress of the films on PMMA is constantly low ranging from 0 to 2 GPa due exclusively to high flexibility of PMMA, which causes the easy relief of the stress and thus the density decrease in the films.

Introduction

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with a rigid network of sp3 and sp2 bonding. DLC has a number of excellent properties similar to diamond such as high hardness, high wear resistance, high chemical inertness, and high optical transparency [1]. It is also suggested that DLC has no toxicity towards various living cells and no inflammatory response or loss of cell integrity [2], [3], [4] and can be used for biocompatible coatings in human body [5], [6], [7], [8], [9], [10]. Formation of DLC films generally needs strong bombardment of energetic ions onto a substrate surface. The most popular method is radio-frequency (rf, 13.56 MHz) plasma-enhanced chemical vapor deposition (CVD) using hydrocarbon sources gases, which produces polymer-like carbon, amorphous hydrogenated carbon (a-C:H), and DLC depending on the condition. However, rf plasma apparatuses are technically difficult and expensive to be scaled up to industrial dimensions. The higher the rf power becomes, the more difficult the impedance matching is. Also, rf plasmas typically operate at pressures below 1 Pa and the penetration of plasma into holes or edges is not good due to a large plasma sheath thickness.

Direct-current (DC) plasma apparatuses are built and scaled up easier and cheaper than rf ones. DC plasmas typically operate at pressures above 10 Pa, so the penetration of plasma into holes or edges is improved due to the smaller plasma sheath thickness. However, DC biases for accelerating ionic species are not well applied to insulating materials such as a-C:H and DLC. This could be solved by applying dynamic DC biases with pulsed or step function voltages. The properties of the films prepared with pulse-modulated DC plasmas have not fully been examined. In early work by Michler et al. [11], some mechanical properties of DLC films prepared with pulsed DC plasmas were comparable to those with rf plasmas. In recent work by Kawai et al. [12], the films showed photoconductivity but did not show characteristics of DLC in the Raman spectra [1].

The substrate materials used in the great majority of work stated above are hard materials such as Si, quartz, and steel. There is a sharp rise in requirement to deposit DLC on soft materials like polymethyl methacrylate (PMMA). This is because PMMA is increasingly used in optical and biomedical devices such as ophthalmic intraocular lenses and artificial dentures and bones [13], [14]. The properties of the films on such soft materials are often different from those of DLC films; polymer-like a-C:H films on PMMA in the same deposition conditions as DLC films on Si [15]. Pulsed DC biases could be applied effectively even to highly insulating PMMA. However, few comparative reports were given for the properties of the films on PMMA and other hard materials.

In this study, a-C:H films are deposited on Si and PMMA substrates by pulsed DC plasma-enhanced CVD. The maximum voltage and the period of the negative pulsed DC bias to the substrate are varied. The structure and mechanical properties of the films on Si and PMMA are compared to explore a way of depositing DLC films on PMMA.

Section snippets

Experiment

A schematic of the deposition apparatus is shown in Fig. 1. The cylindrical reactor was made of stainless steel with sizes of 360 mm in diameter and 280 mm in height and electrically grounded. Two parallel-plate electrodes with a distance of 100 mm were located inside the reactor. The top electrode was grounded and the bottom one was connected with a DC pulse power generator. The maximum voltage and the period of the negative pulsed DC bias at the bottom electrode were varied from 0.5 to 3 kV and

Results and discussion

After deposition, the films on Si were apparently not delaminated, while the films on PMMA occasionally showed partial delamination in optical microscopy images. Hereafter we only present data for the films which were apparently not delaminated after deposition. The deposition rate was derived from the ratio of the film thickness to the deposition time (60 min) by averaging the thickness measured four times at different locations. The deposition rate on Si and PMMA as functions of the maximum

Conclusion

A comparative study was made on a-C:H films deposited on hard Si and flexible PMMA substrates by pulsed DC plasma-enhanced CVD using methane. The structure, residual stress, and hardness of the films were examined as functions of the maximum pulse voltage and the pulse period. The deposition rate on both Si and PMMA was independent of the pulse period, indicating that the films were continuously formed during the pulse modulation. The Rama spectra demonstrated that the films on Si were

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