Paper PS+MN-WeM4
XeF₂ Vapor Phase Silicon Etch used in the Fabrication of Movable SOI Structures

Wednesday, October 20, 2010, 9:00 am, Room Galisteo

Vapor phase XeF₂ has been used in the fabrication of various types of devices including MEMS, resonators, RF switches, and micro-fluidics, and for wafer level packaging. In this presentation we demonstrate the use of XeF₂ Si etch in conjunction with deep reactive ion etch (DRIE) to release single crystal Si structures on Silicon On Insulator (SOI) wafers. XeF₂ vapor phase etching is conducive to the release of movable SOI structures due to the isotropy of the etch, the high etch selectivity to silicon dioxide (SiO₂) and fluorocarbon (FC) polymer etch masks, and the ability to undercut large structures at high rates. Also, since XeF₂ etching is a vapor phase process, stiction problems often associated with wet chemical release processes are avoided. Monolithic single crystal Si features were fabricated by etching continuous trenches in the device layer of an SOI wafer using a DRIE process optimized to stop on the buried SiO₂. The buried SiO₂ was then etched to handle Si using an anisotropic plasma etch process. The sidewalls of the device Si features were then protected with a conformal passivation layer of either FC polymer or SiO₂. FC polymer was deposited from C₄F₈ gas precursor in an inductively coupled plasma reactor, and SiO₂ was deposited by plasma enhanced chemical vapor deposition (PECVD). A relatively high ion energy, directional reactive ion etch (RIE) plasma was used to remove the passivation film on surfaces normal to the direction of the ions while leaving the sidewall passivation intact. After the bottom of the trench was cleared to the underlying Si handle wafer, XeF₂ was used to isotropically etch the handle Si, thus undercutting and releasing the features patterned in the device Si layer. The released device Si structures were not etched by the XeF₂ due to protection from the top SiO₂ mask, sidewall passivation, and the buried SiO₂ layer. Optimization of the XeF₂ process and the sidewall passivation layers will be discussed. The advantages of releasing SOI devices with XeF₂ include avoiding stiction, maintaining the integrity of the buried SiO₂, and simplifying the fabrication flow for thermally actuated devices. Sandia National Laboratories is a multi program laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.