A Method for Independent Measurement of Elastic Modulus and Poisson's Ratio of Diamond-Like Carbon Films
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ABSTRACT A simple method to measure the elastic modulus and Poisson's ratio of diamond-like carbon (DLC) films deposited on Si wafer was suggested. This method involved etching a side of Si substrate using the DLC film as an etching mask. The edge of DLC overhang free from constraint of Si substrate exhibited periodic sinusoidal shape. By measuring the amplitude and the wavelength of the sinusoidal edge, we can determine the strain of the film required to adhere to the substrate. Combined with independent stress measurement by laser reflection method, this method allows calculation of the biaxial elastic modulus, E/(1 - v), where E is the elastic modulus and v Poisson's ratio of the DLC films. By comparing the biaxial elastic modulus with plane-strain modulus, El(1 - 02), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. This method was employed to measure the mechanical properties of DLC films deposited by C6H6 r.f. glow discharge at the deposition pressure 1.33 Pa. The elastic modulus, E, increased from 94 to 128 GPa as the negative bias voltage increased from 400 to 550 V. Poisson's ratio was estimated to be about 0.22 in this bias voltage range. For the negative bias voltages less than 400 V, however, the present method resulted in negative Poisson's ratio. The limitation of the present method was discussed.
INTRODUCTION High ratio of elastic modulus to mass density of diamond-like carbon (DLC) film has attracted much attention for the applications using high acoustic wave velocity. Among the applications are the overcoats for speaker diaphragm and surface acoustic wave devices [1, 2]. Accurate measurement of the elastic properties of the film is the prerequisite for these applications. Elastic modulus of thin film is generally different from that of the bulk materials mainly due to the defects or textures in the thin film structure. Furthermore, the properties of DLC film can be varied in wide range by changing the deposition condition [3]. It is thus desirable to measure the elastic properties of as deposited thin films. The elastic modulus of thin films have been measured by various methods, i.e. nano-indentation [4], Brillouin light scattering measurement [5], ultrasonic surface wave measurement [6], bulge test [7] or vibration membrane method [8]. However, these techniques need sophisticated instruments and analysis technique or delicate micro-machining process. In the present work, we suggest a simple method to measure the elastic modulus and Poisson's ratio of the DLC film deposited on Si wafer. Typical DLC film has high residual compressive stress of up to 10 GPa. Although the residual stress is one of the major reasons for poor adhesion, we could use the residual stress to measure the elastic properties. By using an anisotropic etching technique, the side of Si substrate was etched with controlled etching depth. Since the DLC film is chemically inert, the etching process resulted in unstressed DLC freehang of sinusoidal shape. Measuring th
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