Structural devices
Structural devices

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Structural devices

8.4.3 Self-limiting etches

In practice, it is often possible to design microsystems in such a way that there is no need to pay great attention to knowing the precise moment when the etching has gone far enough. A good example is the etching of the movable structures in surface-micromachined electromechanical devices.

Figure 41 shows a simplified process sequence by which a movable cantilever is made by surface micromachining. Here, two different etching steps are used to make the device. The first is an anisotropic dry etch using a reactive ion etcher. The end-point of this etch is reached when the silicon has been removed down to the underlying oxide layer. This layer is etched much more slowly by the process, and so forms a barrier to further deepening of the etch hole. All that is required of the process is to etch for a long enough time to be sure that all the polysilicon has gone – there's no need to time it very precisely.

Surface micromachining is the name given to the set of processes by which microsystems are formed on top of a silicon wafer, rather than cut into it. These are typically built up as multilayer sandwiches of thin oxide layers between thicker polycrystalline silicon layers, which eventually form the mechanical structures of the device.

Figure 41
Figure 41 Process sequence for making a surface-micromachined cantilever

To release the moving structures, a second etch step is used to remove the oxide without affecting the polysilicon. This is usually done with HF, either in solution or as a vapour. Once more, there's no need to time the process precisely: the silicon beneath the oxide layer resists attack from the HF very effectively.

In anisotropic wet etching of silicon, the etch stops are composed of the {111} planes of the crystal lattice. Figure 42 shows a KOH etch of a (100) silicon wafer on which a patterned silicon nitride layer will eventually form a suspended cantilever. Once the {111} planes defined by the opening in the nitride layer are fully formed, there is nothing left for the KOH to attack and the etch stops of its own accord.

Figure 42
Figure 42 Stages in the KOH etching of a (100) silicon wafer to form a suspended silicon nitride cantilever

Another important etch barrier is the boron etch stop. Silicon that has been heavily doped with boron resists etching by anisotropic wet etchants. The boron can be introduced into the silicon either by diffusion, or by ion implantation (in which an energetic beam of boron ions crashes into the wafer, finally coming to rest some way beneath the surface, the depth of penetration being dependent on the energy of the ions). Figure 43 shows how a boron etch stop can be used to create a diaphragm of well-defined thickness with KOH etching.

Wet chemical etching depends on the transfer of electrons between the etchant and the target material. This fact makes it possible to control etch rates by applying potential differences between the target material and the etchant. This is an elaborate process in which two differently doped layers in the silicon form a p-n diode junction. By applying the appropriate potential difference between a contact on the n-type silicon on the back of the wafer and the etchant solution, this material is protected from attack. For instance, this enables diaphragms like that in the boron etch stop example to be made but with the use of much lighter doping levels.

Figure 43
Figure 43 Use of the boron etch stop to fabricate a thin diaphragm with KOH etching

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