Impact of Thermo-Mechanical Stress Due to Probing and Wire Bonding on CUP Devices

Abstract

Recently, the circuit under pad (CUP) designs are getting very popular because they could save lots of silicon especially for high-density and high-complexity applications. Polysilicon is a material widely used in many electronic device components. When such devices are placed underneath the bond pads, it is highly expected not to show any change in the electrical behavior due to any kind of thermo-mechanical stresses such as probing or wire bonding mainly during the process of packaging. This article presents a detailed investigation of such impacts on a polysilicon resistor placed underneath the bond pads. The standard meander-shaped polysilicon resistor was used as a test vehicle. In this article, different stress-causing factors such as probing insertions, probing temperature, and wire bond materials were investigated in detail. The impact of such thermo-mechanical stresses is simulated using Comsol to understand the stress phenomenon and for qualitative analysis. Then, the wafer-level silicon testing was carried out to quantify the impact of such thermo-mechanical stresses using a standard design of experiments (DOEs). The impact in terms of change in resistance of the polysilicon resistor of two different widths (5 and 0.42 μm) for two different bond pad metal options viz. Middle thickness metal (METMID) (thickness of 880 nm) and thick metal (METTHK) (thickness of 3000 nm) was investigated. The percent change in resistance was observed to be very negligible after multiple probing attempts and probe temperatures. However, after wire bonding, up to ~14% change in resistance was observed when copper was used as the wire bonding material. Nevertheless, this percent change was still well within the reliability specification limit set by Joint Electron Device Engineering Council (JEDEC), which is widely used in the industry. This detailed investigation will help CUP designers choose the correct back end of line (BEOL) metal options and also wire bonding material upfront and accordingly plan their design margins.