By Applied Microengineering Ltd.
Low melting point glasses have been used in industry for many decades to form hermetic seals. The bonding process is typically carried out in the temperature range 350-650°C and with relatively low contact pressures. A wide range of sealing glasses are available, and the thermal expansion coefficient of the selected glass would usually be between the two values for the wafers being bonded. The glass layer can be applied as a preform, spin-on, screen print, sputtered film etc., and patterned to define the areas to be sealed. The technique has been used for the production of pressure sensors and other MEMS devices, and the process can be carried out under vacuum, for creating sealed, evacuated cavities. The process requires reliable control of temperature profiles and applied forces, and AML wafer bonders are used to achieve high quality, in-situ aligned bonds. In comparison with anodic and direct bonding, the glass frit process relies on glass flow to form a seal and hence suffers poorer dimensional control of micro-machined cavities etc. The process is particularly suited to sealing over surface features. Advantages of glass frit bonding: - Suited to vacuum sealing - Tolerates high topography of the substrates - Can be used to bond a wide variety of materials - Low stress bond Disadvantages: - Intermediate layer - Requires accurate process control, due to frit flow and potential structure contamination - Process temperature 350-600°C - Relatively long cycle time Applications: - MEMS - Microfluidic devices - SAW devices - Vacuum encapsulation
