A new process is presented for the development of a micro, internal-combustion engine fabricated in a process that achieves 900μm deep features via deep reactive ion etching (DRIE). A single-sided 900μm ultra-deep etch process with high mask selectivity is used to generate straight sidewall structures with low sidewall roughness. This research is part of an effort to create an autonomous, portable MEMS-based Rotary Engine Power System (MEMS REPS) capable of producing power on the order of milliwatts with an energy density substantially better than that of a conventional battery. Based on the Wankel-type rotary engine, the design of a 2.4mm silicon prototype features a planar geometry and self-valving operation and has a displacement of 1.18mm3
The engine consists of four basic components: rotor, epitrochoid housing, spur gear plate, and square shaft. The rotor features 25μm wide in-plane cantilever beams, electrodeposited NiFe soft magnetic material used for an integrated electrical generator, and highly accurate annular gear teeth used to control the motion of the rotor within the epitrochoid housing. The rotor and epitrochoid housing are co-fabricated in a highly selective and accurate deep reactive ion etching (DRIE) process which produced 900μm thick MEMS features. Selectivity of this process exceeds 350:1 for low temperature deposited oxide and 150:1 for SPR220 thick resist. This process also features 250μm wide trenches with a variation of only 6μm on each side and low sidewall roughness to prevent leakage around the apex seal. While “black silicon” or grass is developed due to the high process pressures of this etch, the grass does not remain in the areas etched entirely through the wafer. Severe undercutting of the apex seals was found to be the result of aspect ratio dependent etch (ARDE) and poor mask design. A modified process is presented which partially eliminates the effect of ARDE on the apex seal trenches. However, work is still necessary to develop a design which prevents the lateral etching of the apex seal tip.
Vertical, accurate spur gear teeth are produced using a low pressure DRIE process. The low pressure DRIE process creates surfaces with an RMS roughness in excess of 4.4nm as compared to an unetched silicon wafer (3-4nm). The superior roughness of the trench is important for forming a pre-bond between the epitrochoid housing and the spur gear plate for engine assembly. Silicon square shafts were also microfabricated for use in the engine. A square shaft design is advantageous for MEMS fabrication because the shafts can be fabricated in the plane of the wafer which results in more accurate components. Finally, partial assembly of the engine was realized through the use of a Suss Microtec flip chip bonder. Avenues of optimization for the engine fabrication process are discussed.
August 31, 2003
Knobloch, A. J. (2003). Ultra-deep Reactive Ion Etching for Silicon Wankel Internal Combusion Engines. United States: University of California, Berkeley.