Dependence of Residual Stress of Diamond-Like Carbon Films on Precursor Gases and Process Parameters of RF PACVD
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DEPENDENCE OF RESIDUAL STRESS OF DIAMOND-LIKE CARBON FILMS ON PRECURSOR GASES AND PROCESS PARAMETERS OF RF PACVD KWANG-RYEOL LEE, YOUNG-JOON BAIK AND KWANG YONG EUN *Ceramics Processing Lab., Korea Institute of Science and Technology, P.O.Box 131, Cheongryang, 130-161, Seoul, Korea
ABSTRACT Residual compressive stress of diamond-like carbon (DLC) films was measured by beam deflection method. DLC films were deposited on thin Si wafers using r.f. plasma decomposition of methane and benzene. Negative bias voltage of the cathode was varied from -100 to -800 V and deposition pressure from 3 to 100 mTorr. When using benzene as precursor gas, the residual stress monotonically increases as increasing Vb/v/-.t. (Here, Vbis the negative bias voltage of cathode and P the deposition pressure.) In case of using methane, however, the residual stress has a maximum value at Vb/.vP- between 70 and 100 V/mTorrl/2. Because of the difference in molecular size between benzene and methane, the mean free path of ions in benzene discharge is 5 times shorter than that in methane discharge. The contrasting behavior of residual stress is discussed in terms of the difference in ion energies at the specimen surface due to the difference in mean free path. On the other hand, total hydrogen concentration decreases as increasing Vb/Vt-P in both cases. This result thus shows that the total hydrogen concentration cannot be a key to understand the behavior of residual stress. INTRODUCTION Diamond-like carbon (DLC) films, also called hydrogenated amorphous carbon films, have attracted considerable interests owing to their unique combination of properties. The unusual combination of high hardness, optical transparency, low coefficient of friction, chemical inertness, and high electrical resistivity has stimulated studies for various applications [1]. However, the applications are limited due to a problem of poor adhesion on some substrates such as steel. Even if DLC film can be used for lubrication and wear-resistance coating, their application for steel bearing is yet limited. Residual stress of DLC film is one of major reasons for the poor adhesion. Typical DLC films have high residual compressive stresses of up to 10 GPa, which result in the delamination of the thick films from the substrates [2, 3, 4]. Even on Si where DLC film has good adhesion, the thick films tend to delaminate from the substrate to relieve their stresses [2]. Therefore, understanding the behavior and the origin of residual stress is one of prerequisites for the application of DLC coatings. It has been well known that energetic particles play an important role in structure and properties of DLC films [5, 6]. Many investigations reported a strong dependence of the residual stress on the ion energy. With increasing ion energy, the residual stress shows a maximum value at an ion energy between 20 and 100 eV [7, 8, 9]. This behavior of residual stress is correlated with the structural change from polymer-like to diamond-like and further to graphite-like one with increasing ion ene
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