Basic structures

Bulk silicon micromachining

Anisotropic etching with KOH can easily form V shaped grooves, or cut pits with tapered walls into silicon (figure 3 and figure 6).

Figure 6.

KOH can also be used to produce mesa structures (figure 7a). When etching mesa structures the corners can become bevelled (figure 7b), rather than right angle corners. This has to be compensated for in some way. Typically the etch mask is designed to include additional structures on the corners. These compensation structures are designed so that they are etched away entirely when the mesa is formed to leave 90 degree corners. One problem with using compensation structures to from right angled mesa corners is that they put a limit on the minimum spacing between the mesas.

Figure 7.

Silicon diaphragms from about 50µm thick upwards can be made by etching through an entire wafer with KOH (figure 8). The thickness is controlled by timing the etch, and so is subject to errors.

Figure 8.

Thinner diaphragms, of up to about 20µm thick, can be produced using boron to stop the KOH etch (figure 9) - Concentration dependent etching. The thickness of the diaphragm is dependent on the depth to which the boron is diffused into the silicon, which can be controlled more accurately than the simple timed KOH etch.

Figure 9.

The silicon diaphragm is the basic structure used in microengineered pressure sensors, for example. It can also be adapted for use as an acceleration sensor.

Concentration dependent etching can also be used to produce narrow bridges, or cantilever beams. Figure 10a shows a bridge, defined by a boron diffusion, spanning a pit that was etched from the front of the wafer in KOH. A cantilever beam (a bridge with one end free) produced by the same method is shown in figure 10b.

Figure 10.

The bridge and beam in figure 10 project across the diagonal of the pit to ensure that they will be etched free by the KOH. More complex structures are possible using this technique, but care must be taken to ensure that they will be etched free by the KOH.

If it is desired to make beams or bridges of a different orientation, the wafer can be etched through from the back in KOH (figure 11). This will ensure that the structure is released from the silicon. During such etching, it is necessary to ensure that the front of the wafer is adequately protected from the long KOH etch. Another alternative could be to produce a diaphragm, and etch the desired bridge or beam shape using a reactive ion etcher (dry etching).

Figure 11.

One of the applications for these beams and bridges is as resonant sensors. The structure can be set vibrating at it's fundamental frequency. Anything causing a change in the mass, length, etc, of the structure will register as a change frequency. Care has to be taken to ensure that only the quantity to be measured causes a significant change in frequency.

A combination of dry etching and isotropic wet etching can be used to form very sharp points. First a column with vertical sides is etched away using an RIE (figure 12a). A wet etch is then used, which undercuts the etch mask leaving a very fine point (figure 12b), the etch mask is then removed.

Figure 12.

Very fine points like this can be fabricated on the end of cantilever beams as probes for use in atomic force microscopy. The technique can also be used to produce sharp, small blades.