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The MEMS Handbook (1st Ed) - M. Gad el Hak Part 8

Tham khảo tài liệu 'the mems handbook (1st ed) - m. gad el hak part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | FIGURE 16.59 Comparison of conventional and SFB processes. The SFB process results in a chip which is at least 50 smaller than the conventional chip. From Bryzek J. et al. Silicon Sensors and Microstructures Novasensor Fremont CA 1990. With permission. 16.8.3 Corner Compensation 16.8.3.1 Underetching Underetching of a mask which contains no convex corners i.e. corners turning outside in in principle stems from mask misalignment and or from a finite etching of the 111 planes. Peeters measured the widening of 111 -walled V-grooves in a 100 Si wafer after etching in 7 M KOH at 80 1 over 24 hours as 9 0.5 am Peeters 1994 . The sidewall slopes of the V-groove are a well-defined 54.74 and the actual etch rate Rm is related to the rate of V-groove widening Rv through R111 2sin 54.74 Rv or R111 0.408 Rv 16.41 with R111 the etch rate in nm min and Rv the groove widening also in nm min. The V-groove widening experiment then results in a R111 of 2.55 0.15 nm min. In practice this etch rate implies a mask underetching of only 0.9 am for an etch depth of 360 am. For a 1-mm-long V-groove and a 1 misalignment angle a total underetching of 18 am is theoretically expected with 95 due to misalignment and only 5 due to etching of the 111 sidewalls Peeters 1994 . The total underetching will almost always be determined by misalignment rather than by etching of 111 walls. Mask underetching with masks that do include convex corners is usually much larger than the underetching just described as the etchant tends to circumscribe the mask opening with 111 walled cavities. This is usually called undercutting rather than underetching. It is advisable to avoid mask layouts with convex corners. Often mesa-type structures are essential though and in that case there are two possible ways to reduce the undercutting. One is by chemical additives reducing the undercut at the expense of a reduced anisotropy ratio and the other is by a special mask compensating the undercut at the expense of more lost