Thin-Film Process Example

TFML interconnections are fabricated using a repetitional sequence of thin film processes to deposit and pattern the conductor and dielectric layers. A variety of individual processes and process sequences, including both additive and subtractive approaches, have been used. The subtractive process sequence shown in Figure 2 has been used at Honeywell for a variety of patterns (8 ) and is offered as an example. 

The second method is chemical vapor deposition (CVD). As suggested by the name, unlike PVD, chemical reactions are involved in CVD. Precursor materials in gas phases are introduced into heated furnaces and react at the substrate surface to deposit the desired thin film. For example, CVD is typically performed in low pressure conditions (< 1 Torr) this technique is called LPCVD and usually requires an inert diluent gas such as nitrogen. CVD processes typically involve high temperatures (above 500°C). This is a very important factor to consider in a designing a fabrication process. For example, no metal except tungsten (W) is allowed into CVD furnaces. LPCVD usually has very slow deposition rate. Plasma-enhanced CVD (PECVD) can deposit dielectric films much faster. It also allows deposition at lower temperatures (<400°C). This is very useful when a substrate has already been metalized. 

Lanthanum hexaboride (LaBe) thin films exposed to air show reasonable work function values that makes them suitable for photoelectron applications that finally may employ LED instead of discharge lamps. Various thin films are investigated and compared with ceramic references. The low work function values associated with a moderate surface reoxidation of the films which, of course sets some limits for thin film processing in MEMS systems. As an example for the apphcations feasible a nano ioniser suitable for ambient conditions is presented, which exceeds the actual physical limits and parameterisation of conventional photo ionisation detectors. 

Spray Pyrolysis. In spray pyrolysis, a chemical solution is sprayed on a hot surface where it is pyrolyzed (decomposed) to give thin films of either elements or, more commonly, compounds (22). Eor example, to deposit CdS, a solution of CdCl plus NH2CSNH2 (thiourea) is sprayed on a hot surface. To deposit Iu202, InCl is dissolved in a solvent and sprayed on a hot surface in air. Materials that can be deposited by spray pyrolysis include electrically conductive tin—oxide and indium/tin oxide (ITO), CdS, Cu—InSe2, and CdSe. Spray pyrolysis is an inexpensive deposition process and can be used on large-area substrates. 

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