2 - Technologies

p⁺-stop method

Anisotropic etching is a process of preferential directional etching of material. Numerous anisotropic etchants are used for silicon, the most frequently used one is potassium hydroxide. A basic feature of anisotropic etchants is that their etch rates are strongly dependent on crystallographic orientation. More specifically, <111> surfaces etch at a slower rate than all other crystallographic planes. The dissolution rate is a function of the crystal orientation on the silicon wafer. Qualitatively, etching rate is a function of the areal density of atoms, the energy need to remove an atom from the surface, and geometric screening effects. Etch-stop mechanisms, shown on the animations, permit to control when to stop the etching process. The reduction of the etch rate in heavily boron-doped silicon can be used very effectively to control etching as an etch-stop mechanism. Electrochemical etching is an alternative method.

Electrochemical stop method

The etch-stop mechanism is the following: The reverse biased p-n junction allows only a small current to flow from the anode to the p-Si substrate. When the p-Si in the diaphragm region is etched away, the p-n junction disappears and large current begins to flow from the n-Si into the solution. This current causes a thin oxide film to grow on the n-Si diaphragm, this oxide layer terminates the current, and the etching.

In practice, the CMOS technology process flow should be built up connecting the individual process steps in series. The optimum technology is the one that minimizes the number of processing steps, while guaranteeing a specified device performance at the same time. In microsensor structures, metal-oxide-silicon-based field-effect transistors, the so-called "MOSFET" devices are of great importance. The application of individual technology steps in an integrated processing is illustrated through the example of conventional complementary MOS, the CMOS technology, which is often used in microsensor fabrication. In this case, both p-channel (PMOS) and n-channel (NMOS) transistors and the micromachined device are realized on the same substrate It should be stressed that the very large scale integrated circuits need more sophisticated and much more complicated processes at present time, however, the given example is a good demonstration for integrated processing.