使用单个蚀刻掩模制造硅MEMS结构的改进的各向异性湿法蚀刻工艺

The feasibility of a desired shape of a microstructure on any  kind of silicon wafer (e.g. (111), (110), or (100)) is determined  by studying the etched profile of various shapes of masking  patterns on the respective wafers. This paper deals with  microstructures on Si(100) wafers. In order to design the  compensating pattern for the realization of convex corners, the  identification of the undercutting planes, their etch rates and  their angles are required.

Round shape masks were designed for the fabrication of  rounded concave corners, whereas several kinds of  compensating geometries at the convex corners were tested,  including a <100> oriented beam, square and triangular  shapes, as illustrated in figure 1. These are used for the  realization of grooves for chip isolation, and mesa structures  that are framed around by V-grooves. For the production of  45º mirrors with highly smooth surfaces for optical MEMS  applications, rectangular mask openings aligned along the  <100> direction were used.  

We evaluated the etching characteristics, such as (1)  the etch rate, (2) the etched surface morphology, and (3)  the amount of undercutting at the sharp convex and  rounded concave corners, in both pure and TMAH + NC- 200. The etch depth was measured using a surface profiler  (DEKTAK 3 ST). The dimensions of the etched profile  were measured using an optical microscope (OLYMPUS  STM6) fitted with an angular and linear dimensions  measurement system. The surface roughness was  measured with a non-contact 3D optical profiler (ZYGO).

The eth rates measured in pure and NC-200 added TMAH  are shown in Fig. 2. When NC-200 is added in TMAH,  the etch rate is reduced to a considerably low level at low  concentration (10 wt%), while at high concentration (25  wt%) it is not affected significantly. In pure TMAH, the  average etched surface roughness of {100} increases with  decreasing concentration of the etchant. The etched  surface morphology is very smooth at high concentration,  whereas it is full of hillocks at low concentration, as  shown in Fig. 3. The incorporation of NC-200 improves  the surface morphology dramatically at low concentration by preventing the formation of hillocks. At high  concentration, it does not affect notably.  

Fig2

If one is required to fabricate square shaped cavities  with smooth surfaces and/or to release the microstructures  made of dielectrics (e.g. SiO2, Si3N4) or metals (e.g. Cr,  Au) by wet anisotropic etching using TMAH, a high  concentration of 20 or 25 wt % is recommended as it  provides negligible undercutting at concave corners and  severe at convex corners. On the other hand, in order to  achieve minimum undercutting by preserving the smooth  surfaces and a reasonable etch rate for the fabrication of  microstructures with rounded and sharp convex corners,  25 wt% TMAH + NC-200 is the most favorable choice.