Structural devices
Structural devices

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Structural devices

8.5 Review

We can sum up the distinctive features of wet etching with a few key points:

  • Wet etching, in general, is a simple process to operate. Wafers are immersed in a solution for a while before being taken out, rinsed, and dried. However, certain etches require more sophistication. They may need one or more of the following: heating and agitation of the solution; reflux of vapours to maintain concentrations; protection of back side of the wafer; incorporation of the wafer into an electrochemical cell.

  • Anisotropic wet etching is dependent on the existence of slow-etching crystallographic planes in the silicon. It can be applied only to single-crystal materials.

  • There are limitations on the geometries that can be produced by wet anisotropic etching. Any convex corners within the etch mask will result in undercutting of the mask, as fast-etching crystallographic planes are exposed. Figure 42 shows this happening under the silicon nitride cantilever.

  • The etchant must be rinsed off after the process. The mechanical stresses caused by the flowing liquid, and the surface tension effects during drying, can be destructive of movable micromachined structures on devices.

Figure 44
Figure 44 The process steps in the manufacture of a silicon nitride/silicon atomic force microscope probe assembly

A summary for gas-phase etching produces a list as follows:

  • These are dry processes. There is no danger of etchant remaining on the wafers once they have been removed from the process chamber. Any residues present will be in the form of thin coatings of solid reaction or sputtering products.

  • The anisotropy comes essentially from the directional nature of the momentum of the gaseous etchant. The dependence on crystallographic orientation we have seen in the wet etches has gone. This means that there is much greater design freedom, not only in the feature shapes but also in the materials in which they are etched.

  • The anisotropy of gas-phase etching is such that near-vertical side walls are produced. This allows deep-etched features to be packed closely together on the wafer.

  • The investment in equipment is considerably greater than that needed for even the most sophisticated wet etching. However, the problems of danger to the process operators and toxic waste disposal are generally less severe.

When it comes to etching, it is not a contest between wet and dry methods; rather, there is just more than one type of saw in a well-stocked toolbox. It is a matter of choosing the most appropriate for the exact purpose. In any microsystem, you will nearly always find that both wet and dry etches have been used in its fabrication.

SAQ 17

Figure 44 reproduces Figure 14. Bearing in mind the profiles (curved, straight, angled), specify details of each etching step (anisotropic, isotropic, wet, dry) by completing the following description.

The first etching step is a silicon etch (see Figure 44(b)). Judging by the _______ sidewalls, it is a ______________ etch, making use of the crystallographic slow etching planes. It has been stopped by coming up against an oxide layer.

The second etching step (see Figure 44(c)) is the removal of the nitride from the back and the front of the wafer, and the oxide from inside the cavity. The _______ shape of the edges of the remaining oxide indicates that this is an _________ etch.

This could be done either wet or dry, but the fact that the nitride on the back and the front of the wafer have been removed as well suggests that the sensible choice is to do this

The third etch step is the patterning of the nitride layer on the top of the wafer (see Figure 44(d)). This could be done either wet or dry, but the fact that the nitride has been preserved on the back indicates that the best choice here is a ____ etch.

The final etch (see Figure 44(f)) is clearly an _________ etch. The undercut of the masking nitride and the _______ sidewalls are evidence of this. With the information available in Figure 44(f), it is not possible to say whether this was a dry or wet etch.

Answer

The first etching step is a silicon etch (see Figure 44(b)). Judging by the angled sidewalls, it is a wet anisotropic etch, making use of the crystallographic slow etching planes. It has been stopped by coming up against an oxide layer.

The second etching step (see Figure 44(c)) is the removal of the nitride from the back and the front of the wafer, and the oxide from inside the cavity. The curved shape of the edges of the remaining oxide indicates that this is an isotropic etch. This could be done either wet or dry, but the fact that the nitride on the back and the front of the wafer have been removed as well suggests that the sensible choice is to do this wet.

The third etch step is the patterning of the nitride layer on the top of the wafer (see Figure 44(d)). This could be done either wet or dry, but the fact that the nitride has been preserved on the back indicates that the best choice here is a dry etch.

The final etch (see Figure 44(f) is clearly an isotropic etch. The undercut of the masking nitride and the curved sidewalls are evidence of this. With the information available in Figure 44(f), it is not possible to say whether this was a dry or wet etch.

T356_1

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