The flexible package and applications of ultra-thin sensor chip
In this paper, we intend to fabricate an ultra-thin silicon stress sensor chip which contains a CMOS stress sensor array with 256 cells and two decoder circuits on flexible substrate with flexible packaging technology. The stress sensor chip is fabricated in UMC 0.18μm, and each cell (55μm × 55μm) in the sensor array can measure both the in-plane shear stress and the difference of in-plane normal stresses at the same point simultaneously. Firstly, individual chips are thinned in chip scale to around 35μm. To protect chips from micro cracks formed in the thinning process after temporarily bonding the diced chips on a 100mm carrier wafer, two methods have been provided. Then, the stress sensor chip is transferred to a plastic polyimide film. After that, we use wire bonding process to achieve electrical connection and flexible package. We have successfully achieved wire-bonding for ultra-thin chips on flexible substrate and concluded suitable wire bonding features like bonding force, bonding time and ultrasonic. To enhance the signal-noise ratio (SNR) and linearity of the sensor, a proper current bias about 30μA is applied to settle the sensor to work in the saturation region. Stable and high linearity output signals can be obtained. To verify the function of the ultra-thin stress sensor chip, pulse measurement has already been conducted.