A New Micromachining Technology Using

Abstract

tract :new micromachining technology using (1 1 1)-oriented silicon is developed. The technology utilizes reactive ion etching (RIE) for patterning of microstructures to be released from the substrate, followed h y KOH wet etching of bulk silicon under the patterns to release the microstructures. The advantage of technique is that the microstructures are of single crystalline silicon. Furthermore, unlike bulk ranisotropic etching that can fabricate patterns limited by crystallographic directions, this technique can pattern vertical-walled, arbitrarily-shaped patterns. The pattern depth is limited by RIE, but the recent deep RIE processes can fabricate structures from sub-pn to 500pm depth. This compares favorably with polysilicon micromachining which is generally limited to a thickness of < 10pm. The release of microstructure is accomplished by an aqueous alkaline etch, and the gap between the substrate and microstructure is precisely controlled to almost any distance by RIE. The release etch utilize the high etch selectivity of { I 1 I} planes to (100) and {I IO} planes, and therefore, large plates can be released without additional etch holes, and with smooth structure undersurface and smooth substrate support surfaces . To understand the process, consider the two equilateral triangles bounded by {I 1I} planes of (1 11)oriented silicon as shown in Figure 1. Note that the various {I 1I } planes are tilted at f 19.47' angles from the vertical as indicated. Now consider a pattern opening shown in Figure 2. The pattern is micromachined using RIE processes, and partial nitride passivation performed as shown in Figure 3. If this structure is wet etched in an aqueous alkaline etchant, the pattern is released as in Figure 4. Due to the space limitation, a detailed flow sequence is not shown. Figure 5 shows fabricated single crystalline microbridges. Figure 5 (a) shows a released bridge. The dimensions are: length 55pm, width 20pm, and thickness 4 p . The gap to the substrate is 2pm. Figure 5 (b) shows a bridge with dimensions: 260pm length, 50pm width, and ,4pm thickness. The SEM shows that 260pm x 50pm is released, but that the bridge is stuck to the substrate because of the stiction caused by wet etching. This problem can be improved by the use of sublimation or super critical drying techniques, or by simply making the gap larger. Also note that because the micromachining technology relies on RIE for shape patterning and crystallography-dependent anisotropic etching, all shapes are sharply defined and all surfaces are clear. Other shapes including comb drives can be easily fabricated using this technique. This paper developed a new micromachining technology using (1 11)-oriented silicon for the first time. The technology combines the advantages of dry RIE processes and crystallography of silicon to fabricate sharply-defined, arbitrarily-shaped, released, single-crystalline silicon microstructures. This technology offers much potential as an alternative micromachining technology.