Nickel–chromium alloy piezoresistive pressure sensor using eutectic bonding

A nickel-chromium (Ni-Cr) piezoresistive pressure sensor is presented, which has the advantages of low-cost and easy fabrication processes. In the designed sensor, Ni-Cr alloy (80:20-wt-), which can be fabricated using simple processes, is used as the strain-detecting material with a smaller but acceptable gauge factor. Eutectic-bonding technology, based on silver-tin (Ag-Sn) alloy, which contains 3.5-wt- Ag and has 20--m thickness, is used as an alternative and easy bonding choice to complete the vacuum package. Normally, Ag-Sn alloy is mostly obtained by the electrochemical deposition method. However, here the Ag-Sn solder film is directly used as a bonding material. It reduces the fabrication difficulty of eutectic bonding of the proposed pressure sensor. By studying the processes of annealing of Ni-Cr fabrication and eutectic bonding with Ag-Sn, the authors complete the alloy piezoresistive atmosphere pressure sensor. Bonding quality is evaluated by inspection through the deflection of a diaphragm of silicon with more than 95- of the area successfully bonded. The pressure-voltage characteristic test results suggest a precision within 0.3- in square fitting. The temperature coefficient offset is 620-ppm/(-C free space optic (FSO)).