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Anisotropic wet etching
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Due to different packing/density on different crystalographic orientations, the etch rate is different (it is usually much lower for 111, than for 100), but only in some etchants, like KOH -
If we introduce Si in KOH we can observe that the etching rate varies as follows: 100 > 110 >> 111
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Because of the variation in etch rate, when we etch a 100 wafer, the walls will not be etched as fast as the horizontal surface (see left picture), and the obtained structure will have the shown shape
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the angle between the 100 and 111 directions is around 135o, the resulted structure has the shape shown below in the left picture
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if the window is too small, then the etching can be almost stopped, due to the fact that it will occur only on 111 directions (the resulted structure will have triangular shape, shown in the left part of the left picture)
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For a 110 wafer,the obtained structure will be as in the right picture
 
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Selective etching
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Defect selective etching
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Another factor that influences etching is porosity, for a material less dense, porous, the etch rate increases
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Etch rate depends also on the defect density, thus it can be used as a method to find the defects in the wafer |
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Substrate preparation
