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Evaporated glass films for silicon-to-silicon anodic bonding in pressure sensors
Abstract - Silicon pressure sensors are based on silicon membranes featured with transducer structures. The different types employ e.g. glass cavity lids to form a reference vacuum on top of the sensing circuitry or glass base plates forming the reference vacuum underneath the silicon membrane. Such base plates may also be featured with through holes in the case of differential pressure sensors.
The advantages of using glass is the possibility to use very reliable and established anodic bonding. But even-though the thermal glass properties are well matched to silicon, glass remains a stiff and brittle material, that may break when higher pressures are applied. The mix of materials may be disadvantageous at higher or changing application temperatures.
Therefore, it would be favourable to design all parts of the pressure sensor in silicon. Direct bonding may be an option but is not as reliable as anodic bonding.
Using an evaporated glass thin film allows to overcome the above drawbacks. Applying a glass film of a few micron thickness allows the anodic bonding of silicon to silicon. Thus etched silicon lids or base plates can be easily integrated by depositing a thin borosilicate glass film onto their surface. The anodic bonding process remains fairly standard (adaptation of bond voltages to very thin dielectric ~60V).