CMOS-MEMS piezoresistive force sensor with scanning signal process circuit for vertical probe card

Abstract

In response to the essential role probe cards play in the semiconductor testing industry, a CMOS-MEMS force sensing devices capable of simultaneously monitoring the probe reacting force and electrical test signals is designed for probe cards. The probe reacting force can assist operators immediately to identify a broken, deformed, or worn-out probe and recognize that the measured electrical signals are erroneous. The debug time and cost of the IC test can therefore be effectively reduced. Array-type CMOS-MEMS force sensors are capable of monitoring the status of vertical probe cards on-line in the case of both die-level and wafer-level applications. The force sensor essentially is a Wheatstone-bridge-based piezoresistive force sensor with a small surface area and can be easily fabricated. It consists of a membrane composed of metal, silicon oxide, and a piezoresistive layer made of polycrystalline silicon. Moreover, as the conventional back-etch process can barely handle the increasingly smaller probe pitch, we use front etching to etch out a cavity under the membrane on the silicon substrate in order to deform the membrane during the post-process. For measuring the output signals of the sensors, on-chip circuits to scan and amplify the output signals of the sensors are integrated in the design process. Furthermore, a finite element method is adopted to analyze the structure of the sensors and to find the optimal piezoresistive sensor design. The TSMC 0.35 μm 2P4M process is used to fabricate the force sensors and the circuits. According to the measurement results, the designed sensor reports a sensitivity of 3.114 mV/N/V while a load-bearing force is 0.0294 N.