Basic Features of Anisotropically Etched Surfaces

The final rectangular cavity bounded by four (111) planes independent of the initial opening shape can be explained with the (111) step etching model illustrated in Fig. 7.41. All of the (111) steps that intercept the etching (100) surface and are exposed within the opening defined by the mask are etched in a direction parallel to the (111) planes. The etching along these planes will continue until perfect (111) planes, which define the walls of the etched cavity, are reached and all of the exposed (111) steps vanish. 

The etched cavities on (100) wafer bounded by the convergent 111 planes have an angle of 54.74° with respect to the surface plane. The dimensions of the resulting square pyramidal cavity, as shown in Fig. 7.44, are given by 

For a (110) wafer the geometries of the etched holes are different because the four sets of 111 planes which form the sidewalls of the cavities are not equivalent. Vertical grooves with a high aspect ratio can be obtained by etching a masked (110) wafer because two sets of (111) planes are vertical to the (110) surface 

FIGURE 7.44. Cross section through anisotropically etched pattern in Si(lOO). The orifice dimension = IFsi 3/2/si, and the underetching U are a function of the etch rate 7J(,n) of the [ 111) planes and etching time t for an accurately aligned pattern and a defect-free Si-Si02 interface. After Bassous. ( 1978 IEEE.) 

FIGURE 7.45. Processing steps to form a silicon pyramid, (a) One Si02 island after HF etching (b) one pyramid after anisotropic etching of the silicon (c) after dissolution of Si02. After Busta et ( 1989 IEEE.) 

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