Thin-Film Technology and Patterning Procedures

Thin films are of interest here only in connection with the techniques that evolved from microelectronics. The preferred material of such techniques is highly purified crystalline silicon. This material is not only a universally applicable semiconductor but is also characterized by very useful mechanical properties. This is the reason why siUcon is preferred also in micromechanics and microfluidics. Even porous layers can be manufactured. 

Thin films of inorganic materials on a substrate are deposited by high vacuum deposition, by sputter deposition or by chemical procedures hke CVD or MOD. 

Sputtering is able to produce layers of elements or alloys which are not volatile. The deposited films have the same composition as the target material. If films of insulating materials must be deposited, DC sputtering will not work. It can be performed, however, by RF sputtering, where a high-frequency field is applied between the poles. The equipment shown schematically in Fig. 2.51 

By chemical vapour deposition (CVD), volatile compounds ofthe material to be deposited are evaporated. The vapour is decomposed thermally, sometimes supported by gas plasma. At the substrate surface, the products react with components of the gas phase, generating a non-volatile film. The process runs at a relatively low temperature. The stoichiometric composition of the resulting amorphous films can be controlled widely. 

Generally, films must be patterned to get useful devices. The simplest case is conducting structiu es which are nested like two combs. Such interdigitated structures are useful to measure conductance or to perform electrochemical reactions. Microelectronics makes use of much more complex structm-es, which meanwhile are utihzed also in highly efficient modern electrochemical sensors. A highly complex example is discussed later in connection with ion-selective field effect transistors (ISFET) in Chap. 7. 

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