Wafer bonding techniques can be broadly grouped into direct and intermediate bonding. Direct bonding is based solely on the adhesion of two wafers brought into contact at room temperature and subsequently heated (1). Various techniques (surface activation, electric field application) can be used to improve the results however extreme cleanliness and flatness of the bonding surfaces remain paramount (2). As such the common application of direct bonding is in substrate creation rather than hermetic MEMS sealing. Despite this, groups have demonstrated the technique for sealing (3). Intermediate bonding includes all bonding mechanisms that depend on an intermediate layer to join the wafers (4). Forms of intermediate bonding include eutectic, polymer, solder and thermocompression. Thermocompression involves the application of both pressure and temperature to the intermediate layers to form the bond. Gold is a typical choice for the thermocompression intermediate layer; this is primarily due to its corrosion resistance and malleability at temperature (5,6). Aluminium with stringent cleaning techniques has also been demonstrated for thermocompression bonding (7). The main issue with thermocompression bonding is topography constraints and surface cleanliness (8), in addition for larger wafer diameters the bond force / pressure requirements become excessive. Polymer bonding does not find broad usage in hermetic sealing as polymers are not true hermetic materials and gases can diffuse through the intermediate material itself (9). Glass frit on the other hand is a common bond technology however for mechanical purposes wide seal widths are recommended when compared to metallic bonding (10). It is noteworthy that developing a bonding technique also requires a thorough investigation to ensure that the preparation steps are compatible with the component wafers (i.e. a bonding method which requires heavy ion bombardment for surface
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