Improved high-temperature stability of Si incorporated a-C:H films

Abstract

The temperature stability of silicon incorporated hydrogenated amorphous carbon films (a-C_1−xSi_x:H) is investigated. Elastic recoil hydrogen detection measurements showed that films with x≥0.15 have an improved hydrogen stability when compared with those with smaller silicon contents. This result is consistent with the gas effusion experiments, which showed that silicon incorporation induces a shift of the hydrogen effusion peak to higher temperatures and a strong reduction in the effusion of methane molecules. The Raman spectra of samples annealed at 500 °C clearly evidenced the graphitization process of the low silicon content samples. As x is increased, the graphitization process becomes less clear, and for samples with silicon contents larger than 15 at.%, no evidence of graphitization could be found. Similarly, electron spin resonance (ESR) measurements showed a strong reduction of the ESR linewidth upon annealing in the case of low silicon content samples, indicating the growth of sp² islands with unpaired spins. This effect is also inhibited by silicon incorporation. Based on these observations, the improved thermal stability of silicon incorporated samples is associated with an increased disorder of these films, which inhibits the carbonisation of the material that, consequently, presents a reduced graphitization.

Introduction

Hard a-C:H films have been extensively studied over the last decade because of their interesting properties 1, 2. High hardness, chemical inertness, high electrical resistivity, low friction coefficient, transparency in the infra-red and high wear resistance are some of the properties that make this material of great importance for a large number of applications. In spite of this, thermal degradation of the films is still a major limitation in achieving high-temperature applications. Several studies have been concerned with this problem, and the obtained results depend greatly on the deposition conditions of the films. Limiting our discussion to those films deposited under conditions that lead to hard diamond-like a-C:H films, one generally observes that the films are stable up to about 400 °C. At these temperatures, the graphitization process of the samples starts as revealed by the increasing ratio of the D and G peaks of the Raman spectra [3]. This process can be understood based on the structural model proposed by Robertson for these films [4]. According to this model, these films consist of clusters of sp² carbon atoms embedded in a tetragonally bonded sp³ matrix. When the temperature is raised, the sp² islands grow at the expense of the sp³ tissue leading to an increased sp² character of the material.

Concomitantly to graphitization, the thermal degradation of diamond-like films is also characterized by a strong hydrogen loss. The two processes occur in the same temperature range, so one would expect that they are related to each other [5]. Among the different techniques employed to study hydrogen loss, hydrogen effusion experiments is one that gives very interesting results. Jiang et al. [6]attributed the relatively sharp hydrogen effusion peak observed at about 600 °C to the explosive breakup of voids or small cavities containing hydrogen, whereas a much broader and featureless peak observed at about 800 °C was correlated to the process of hydrogen release by surface desorption from a network interconnected voids. Wild and Koidl [7]obtained similar results and found that hydrogen is released in the form of molecular H₂ and that the effusion process is not diffusion-limited. In addition to hydrogen, the desorption of C_mH_n radicals has also been observed and its presence correlated to a high density of voids in the material 6, 7.

Several attempts have been made to study the incorporation of silicon atoms into hard diamond-like a-C:H films with interesting results. Films with very low friction coefficients, improved adhesion and increased sp³ character have been reported 8, 9. De Martino et al. have shown that a wide range of properties can in fact be obtained by varying the deposition conditions in a reactive sputtering system [10]. In particular, small amounts of silicon added at low substrate temperatures produced the diamond-like structure, whereas silicon added at high substrate temperatures promoted the polymer-like behavior. Films with a very high hardness (40 GPa) could be obtained [11].

Recently, we have reported on a study of the mechanical and structural properties of silicon incorporated hard diamond-like carbon films produced by rf glow discharge and showed that a strong reduction of the residual internal stress with an almost constant mechanical hardness can be obtained [12]. Infra-red absorption and elastic recoil hydrogen detection results showed that silicon is incorporated replacing carbon atoms in the amorphous network with approximately constant bonded hydrogen density and total hydrogen content. Additionally, Raman experiments indicated that silicon incorporation induces a decrease in the sizes of the sp² islands of the material, whereas electron spin resonance measurements show a decrease in the number and size of the sp² graphitic defects. Hydrogen effusion experiments indicated that the observed reduction of residual internal stress may be attributed to a less compact material with an increased density of voids in comparison to pure a-C:H.

In this work, we investigate the high-temperature thermal stability of silicon incorporated hydrogenated amorphous carbon films (a-C_1−xSi_x:H) and show that these films are more stable than pure a-C:H. Combining total hydrogen content measurements obtained by elastic recoil hydrogen detection with gas effusion, Raman and electron spin resonance experiments, we can show that silicon incorporation into the films inhibits the graphitization process and leads to a material with improved thermal stability.