Glass-glass anodic bonding using hydrogenated amorphous silicon
Veeraraghavan, Vijay V
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This research work investigates the influence of hydrogenated amorphous silicon in glass-to-glass anodic bonding. Anodic bonding is a field assisted, low temperature heterogeneous glass-to-silicon bonding. With a suitable intermediate layer, homogeneous glass-to-glass bonding has been an important microfabrication steps in MEMS applications, especially p-TAS (micro total analysis systems). Devices created on glass substrate enjoy many advantages over silicon substrates. Glass is a good insulator and MEMS applications employing electric fields for separations, need insulating substrates. Etching spiral shaped channels is not only easier in amorphous materials it also produces a smooth, etched feature, unlike some crystalline substrates. These and many other reasons drive the present p-TAS technology towards glass-to-glass anodic bonding. Amorphous silicon deposited by PECVD is a good intermediate layer. The precursor gas used in this deposition method is silane (SLH4) and hence the film has inherent hydrogen atoms present in them in the form of Si-H, Si-H2, (Si-H)x, and other radicals. The film also contains entrapped hydrogen molecules. The hydrogen content, measured in atomic % (at %), can cause stress in the film. This stress could either be tensile or compressive leading to warping or peeling of the film. Also, at bonding temperatures (-350° C), hydrogen evolves from the film because of bond breaking resulting in strain in the network. This strain causes shrinkage of the film leading to poor or partial anodic bonding. Hence, hydrogen content in the PECVD film is an important factor for proper anodic bonding. Our research included controlling hydrogen content in the film by adjusting process parameters, FTIR measurement of hydrogen content, annealing effect, and correlating the quality of anodic bonding (measured in % area bonded) with hydrogen content in the film, annealing temperature, thickness of the film, and stresses. We also report here anodic bonding results with an e beam evaporated hydrogen free amorphous silicon film as an intermediate layer.