An advanced MEMS sensor packaging concept for use in harsh environments

The presented package for harsh environment fulfills the demands to withstand high temperatures of 400°C, aggressive media and pressures up to 50 bar as it uses a steel membrane with an indenter which pushed on the center-boss structure of the sensor. Thus no transmitting media like oil is needed. The SOI sensor chip consists of a beam with an integrated center-boss which was realized using KOH structuring and DRIE (Deep Reactive Ion Etching/Bosch Process). The SOI technology has the distinct advantage that the piezo-resistors are not isolated by a pn junction from the dielectric substrate but by a buried oxide layer. In combination with a high temperature metallization, the SOI-chip is able to withstand the demands of high temperatures. The high temperature metallization consists of a sputtered Ti/TiWN layer that has been exposed to special RTA (rapid thermal annealing) process. Afterwards a TiWN and an Au layer are sputtered followed by an Au electro plating process. The chip has four piezoresistors that are arranged in pairs of longitudinal and transversal resistors which are compressed when pressure is applied. The four resistors are connected via conductors to a Wheatstone bridge. The sensor chip has a beam thickness of 25 μm and the center-boss has an area of lmm × lmm. The beam has a natural frequency of approx. 20 kHz. The sensor chip is mounted using Flip Chip technology to avoid a wire bond technology. It is mounted on a glass feed through on which stud bumps are located and conducted using thermo-compression process. The pressure range is set by varying the steel membrane thickness. This concept has a mayor advantage that the sensor chip can be used for various pressure ranges. The thickness of the steel membrane is chosen adopted to the pressure range, so that the deflection of the sensor beam is 12μm when the maximum pressure is applied.