4. Technology Collaborations

The previous sections have discussed the mechanisms behind gray-scale technology, and the steps taken to improve and expand upon its capabilities. Yet these abilities and developments are best understood when discussing specific applications where precisely controlled 3-D silicon structures play a crucial role in overall system/device performance. The following sub-sections briefly describe three technology collaborations, where my research was partnered with outside organizations to develop and demonstrate unique 3-D silicon structures for enhanced performance.

1. Micro-compressor (ARL + MIT)

Researchers at the U.S. Army Research Laboratory (ARL) and Massachusetts Institute of Technology (MIT) are currently developing a micro turbine engine device, towards portable power generation for the future soldier [108-110]. The development of such a micro-gas turbine engine requires an efficient micro-compressor design that preferably emulates their high efficiency macro-scale counterparts, which are 3-D in nature [111, 112]. While designing 3-D microstructures would not be realistic using traditional micro-fabrication techniques, with the development of gray-scale technology, the design may be driven by optimum performance goals rather than what is achievable with conventional fabrication techniques.

A micro-compressor based on the capabilities of gray-scale technology was recently designed [111, 113], and is shown in Figure 2.15. This improved design has the tops of the blades defined by the original wafer surface, while the bottom of the flow passage is etched to a variable depth. To complete the flow passage, another wafer would be bonded on top for encapsulation. The design calls for the base of the flow passage to slope from 400pm deep at the inner radius to 200pm deep at the outer radius (a 2mm long, 200pm tall slope), resulting in a mass flow inlet to exit ratio of 2:1 in the vertical dimension.

The first generation design, before the introduction of the Gaussian approximation, had a sloped photoresist height of ~3p,m, so that a selectivity of 67 was needed. When using the base etch I from Table 2.4 a height of 210p,m was achieved, and the final structure is shown in Figure 2.16 [17]. It is apparent that in this first demonstration, the sloped was quite non-linear. By using the Guassian approximation method, a 2^nd design was fabricated with a linear slope, as shown in the optical profilometer scan of Figure 2.17.

The final step in developing this 3-D structure was its integration into an existing MIT compressor wafer process flow. A nested mask approach was developed to create the gray-scale slope and outer flow channels in separate etches, thereby eliminating the need for large single gray levels [114].

An abbreviated schematic of this process flow is shown in Figure 2.18, while Figure 2.19 is an SEM of the final variable height silicon micro-compressor with smooth outer flow channels.

At the time of this writing, the improved performance of variable height micro­compressor structures has not been confirmed in a rotating platform within the micro­engine device, largely due to the many other technical challenges associated with the project. However, our research was able to advance gray-scale technology to the point where such geometries can be achieved with regularity using the design methods and etch development guidelines described earlier.