Microcantilever suspension using hard masks and a two phase RIE method

• A cost effective RIE method consisting of two phases is used for suspension.
• The two sequential phases of etching were anisotropic and isotropic, respectively.
• Process gases are SF6, CF4, and O2 which were optimized separately for each phase.
• 100 μm × 300 μm microcantilevers were suspended in a total of 90 min.
• Etch progression analysis using contrast differentials in SEM are shown.

This study presents the application of simple and efficient Reactive Ion Etching (RIE) methods to suspend microcantilevers using a two electrode plasma reactor. The process consists of two steps and poses a viable, robust, and cost effective alternative to other methods such as XeF2 vapor etching. Aluminum hard mask was employed for general purpose use in processes where materials other than silicon are used to form the microcantilever. Successive etching steps of silicon substrate with SF6/O2 and CF4/O2 plasmas were applied sequentially in a two phase configuration where the first phase is relatively anisotropic and the second phase is more isotropic. This allowed for suspension of microcantilevers while maintaining well-defined undercut characteristics. Etch characteristics were measured for different process parameters, and the effect of O2 concentration, power, and pressure on etch rate and isotropy was presented. The selectivity to aluminum mask was measured to be larger than 250:1 in the used recipes. It was also shown on the test structures that the amount of undercut is observable as contrast differentials using simple SEM imaging techniques on metal film hard mask. During individual characterization of etch profiles, isotropy parameters varying between approximately 0.4–0.95 were obtained under different etch conditions. The maximum etch rates of SF6 based and CF4 based etch recipes were found to be 30.5 μm/min and 3.2 μm/min, respectively, for varying conditions. Aluminum microcantilevers with dimensions of 100 μm × 300 μm were successfully released after 90 min of processing with 30 μm of travel distance to the underlying notch.

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