Surface and Coatings Technology

Some mechanical and thermal properties of PECVD a-SixC1-x: H and a-SixN1-x: H films prepared by mixed frequency plasma processes

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The thermal coefficient of expansion (TCE) and biaxial elastic modulus (BEM) of a-SixC1-x: H and a-SixN1-x: H films prepared by plasma-enhanced chemical vapor deposition (PECVD) were studied over a wide range of composition. The films were prepared in a unique PECVD system that uses a controllable mixture of two frequencies (450 KHz and 13.56 MHz) to excite the plasma. The TCE and BEM were determined by neasuring the temperature dependence of the stress of identically deposited films on two substrates having different expansion coefficients. The TCE data are well behaved and relatively straightforward to understand. Silicon-rich carbide films and nitride films with x near 0.5 had TCE values near those of crystalline Si and Si3N4 respectively. However, their moduli were much smaller than the bulk values. For x = 0.5, the carbide and nitride films had BEM values of about 125 GPa while the bulk values are 865 GPa and 482 GPa respectively. Effects of frequency mixing on the hardness of a series of carbide films were determined, using a nonoindenter system and films deposited with a fixed SiH4: CH4 flow rate ratio and temperature. Such effects were quite pronounced in that raising the high frequency component from 15% to 85% reduced the hardness from 28 to 12 GPa. These findings and an explanation for their origins will be discussed, together with a few technological examples showing the use of such films as passivants. For example, dual layers of a-SixC1-x: H/a-SixN1-x: H have been used to protect the resistor structure in thermal ink jet printheads and, as such, are subjected to intensive thermal stress cycles. Thus one can predict that some carbide layers (having certain values of TCE, BEM and thickness) will have an excessive stored energy and thus delaminate during device operation. In another example, such carbide films could be attractive for coating magnetic storage media and recording head structures. Carbide films can have hardnesses comparable with that of a-C: H films but could, in principle, be tailored to a specific slider surface to minimize wear. © 1994.