SOME MECHANICAL AND THERMAL-PROPERTIES OF PECVD A-SIXC1-X-H AND A-SIXN1-X-H FILMS PREPARED BY MIXED FREQUENCY PLASMA PROCESSES

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-enhance d chemical vapor deposition (PECVD) were studied over a wide range of composition. The films were prepared in a unique PECVD system that use s a controllable mixture of two frequencies (450 KHz and 13.56 MHz) to excite the plasma. The TCE and BEM were determined by measuring the t emperature dependence of the stress of identically deposited films on two substrates having different expansion coefficients. The TCE data a re well behaved and relatively straightforward to understand. Silicon- rich carbide films and nitride films with x near 0.5 had TCE values ne ar those of crystalline Si and Si3N4 respectively. However, their modu li 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 ar e 865 GPa and 482 GPa respectively. Effects of frequency mixing on the hardness of a series of carbide films were determined, using a nanoin denter system and films deposited with a fixed SiH4:CH4 flow rate rati o 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 wil l 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 th ermal inkjet printheads and, as such, are subjected to intensive therm al stress cycles. Thus one can predict that some carbide layers (havin g certain values of TCE, BEM and thickness) will have an excessive sto red energy and thus delaminate during device operation. In another exa mple, such carbide films could be attractive for coating magnetic stor age media and recording head structures. Carbide films can have hardne sses comparable with that of a-C:H films but could, in principle, be t ailored to a specific slider surface to minimize wear.