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MEMS Mechanical Sensors - Stephen Beeby Part 3

Tham khảo tài liệu 'mems mechanical sensors - stephen beeby part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 30 Materials and Fabrication Techniques because it was developed at the RCA Company . Two wafers can be joined together at room temperature resulting in an immediate weak bond due to van der Waals forces. The bond is then strengthened by heat treating in a furnace or by RF or microwave heating at temperatures above 800 C 31 . At this temperature an hermetic seal is formed between the silicon wafers making it possible to fabricate sealed cavities. The exact nature of the bonding chemistry is not yet fully understood but it is believed that Si-O-Si bonds and water molecules form at the interface as the temperature is increased. The water molecules subsequently break and diffuse into the silicon. Silicon fusion bonding is also used to bond silicon to oxidized silicon wafers as in the manufacture of SOI wafers. The oxide thickness in this process can be as much as 4 um and typically temperatures of 1 100 C are used to obtain a permanent chemical bond. The formation of silicon dioxide to silicon dioxide fusion bonds at 1 100 C has also recently been reported 32 . Silicon-to-silicon bonding can also be done with a thin intermediate layer of sputtered evaporated or spun on glass. The wafers are clamped together and heated to the melting point of the glass typically between 415 C and 1 150 C depending upon the glass. The assembly is then cooled and the glass solidifies and the process can be used to form a hermetic bond. The relatively thin layer of glass minimizes the residual stresses in the assembly. 2.3.5.2 Anodic Bonding Otherwise known as electrostatic bonding this process is used to bond silicon to glass. The method uses electrostatic attraction between the glass and the silicon to facilitate bonding enabling this to be done at much lower temperatures than would otherwise be possible 33 . Circular glass substrates with a thermal expansion coefficient closely matched to that of silicon are readily available. For example Pyrex which has a coefficient of thermal .