Reactive ion etching of piezoelectric materials in CF4/CHF3 plasmas

The reactive ion etching of substrates used in piezoelectric devices (quartz, fused silica, LiNbO3, LiTaO3, and sapphire) has been characterized in CHF3/CF4-based plasmas. For quartz and fused silica, a regime of ion-enhanced chemical etching similar to that established by Steinbruchel [Ch. Steinbruchel, J. Electrochem. Soc. 130, 648 (1983)] for CF4 was indicated over the range of compositions from CF4=1 to CHF3=1. In this regime, the etch rate was dependent on the square root of the rf bias voltage (V1/2). The etch rate of both the quartz and fused silica was at a maximum in a CF4 plasma and decreased continuously with an increase in the ratio of CHF3/CF4 gases in the mixture. In comparison, the etch rates of LiNbO3, LiTaO3, and sapphire were invariant with changes in the ratio of the CHF3/CF4 gases, the flow rate, and the chamber pressure. The constancy of etch rate in these substrates has been attributed to a predominance of etching by a physical process of sputtering.