Glass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, e.g., accelerometers or gyroscopes. The technique utilizes low melting-point glass ("glass solder") and therefore provides various advantages including that viscosity of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching or deposition. A low viscosity promotes hermetically sealed encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon. This results in low stress in the bonded wafer pair. The glass has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures (e.g., metallization layers on chips or ceramic substrates) occurs. The usual temperature of achieving flowing and wetting is between 450 and 550 °C (840 and 1,020 °F). Glass frit bonding can be used for many surface materials, e.g., silicon with hydrophobic and hydrophilic surface, silicon dioxide, silicon nitride, aluminium, titanium or glass, as long as the CTE are in the same range. This bonding procedure also allows the realization of metallic feedthroughs to contact active structures in the hermetically sealed cavity. Glass frit as a dielectric material does not need additional passivation for preventing leakage currents at process temperatures up to 125 °C (257 °F). The process begins with the deposition of glass paste onto the surfaces to be treated. It is then heated to burn out additives and fire it in order to form the glass layer. The bonding process reconfigures the sintered glass into the desired state. Finally, the reconfigured glass is cooled down. Glass frit bonding is used to encapsulate surface micro-machined sensors, i.e. gyroscopes and accelerometers. Other applications are the sealing of absolute pressure sensor cavities, the mounting of optical windows and the capping of thermally active devices.
Procedure
Deposition The glass frit bond procedure is used for the encapsulation and mounting of components. The coating of glass frit layers is applied by spin coating for thickness of 5 to 30 μm or commonly by screen printing for thickness of 10 to 30 μm. Screen printing, as a commonly used deposition method, provides a technique of structuring for the glass frit material. This method has the advantage of material deposition on structured cap wafers without any additional processes, i.e. photolithography. Screen printing enables the possibility of selective bonding. So only in areas where bonding is required the glass frit is deposited. The risk of glass frit flowing into the structures can be prevented by optimization of the screen printing process. Under high positioning precision the sizes of the structures in the range of 190 μm with a minimum spacing of < 100 μm are achievable. The exact positioning of the screen print structures to the cap wafer are required to ensure an accurate bond. The bonded structures are, dependent on the wettability of the printed surface, 10 to 20% wider than the designed screen. To ensure a uniform glass thickness, all structures should have the same width. The printed glass frit high is about 30 μm and provides a gap of 5 to 10 μm between the bonded wafers after bonding (compare to cross sectional SEM images). A bond surface activation is not necessary to promote a higher bonding strength.
Thermal conditioning The printed glass frit structures are heated to form compact glass. The heating process is necessary to drive out the solvents and binder. This results in a subsequent particle fusion of the glass powder. Using mechanical pressure the wafers are bonded at elevated temperatures. Thermal conditioning transforms the glass paste into a glass layer and is important to prevent voids inside the glass frit layer. The conditioning process consists of:
Glazing of organic binder and solvents Melting of glass particle to compact glass Formation of solid connection between glass and wafer surface The initial step comprises drying for 5 to 7 minutes at 100 to 120 °C in order to diffuse solvents out of the interface. This starts the polymerization of the organic binder. The binder molecules are linked to long-chain polymers which solidifies the paste. The organic binder of the glass paste has to be burned with heating up to a specific temperature (325 to 350 °C) where the glass is not fully melted for 10 to 20 minutes. This so-called glazing ensures the outgassing of the organic additives. Further, a pre-melting or sealing step heats the material to the process temperature between 410 and 459 °C for 5 to 10 min. The material fully melts and forms a compact glass without any inclusions. The inorganic fillers are melted down and the properties of the bond glass are fixed. The melting of the glass starts at the silicon-glass interface directed to the glass surface. During the melting process the porosity of the glass eliminates and based on the compression of the intermediate layer the thickness of the glass decreases significantly.
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