Selective and deep plasma etching of SiO2: Comparison between different fluorocarbon gases (CF4, C2F6, CHF3) mixed with CH4 or H2 and influence of the residence time

Abstract

SiO 2 is a well suited material for integrated optic applications and is also attractive for microelectromechanical system and micro-optical electromechanical system fabrication. Such optical components require deep oxide etching (several microns) and subsequent high selectivity with respect to the mask. In this article, we describe the influence of various process parameters (gas mixture, pressure, plasma power, and residence time) on the selective etching of SiO2 with respect to Si in inductively coupled plasma (ICP) fluorocarbon with the aim of finding the best compromise between high selectivity and high oxide etch rate. Oxide etch rate is improved by decreasing pressure or increasing source power within the acceptable process windows, respectively, 3–20 mTorr and 1000–2000 W, but the gain in selectivity is low (×1.5). Adding methane rather than more commonly usual hydrogen resulted in higher selectivity without significant decrease in the oxide etch rate. A relatively good correlation is found between the selectivity and the (C+H)/F ratio of the precursor molecule. However, we show that varying the hydrofluorocarbon mixture does not allow us to improve both oxide etch rate and selectivity. In this regard, the residence time is the most significant parameter: choosing the appropriate amount of methane mixed with C2F6, and decreasing tR leads to an improvement in both the selectivity (×7) and the oxide etch rate (×1.5). Finally, the influence of these parameters on pattern transfer is investigated.