Thermal Analysis of Ultimately-Thinned-and-Transfer-Bonded CMOS on Mechanically Flexible Foils

Thinned CMOS chips transfer-bonded onto a compliant host substrate remain to date the technology of choice for applications requiring both mechanical flexibility and high frequency operation. However, the use of poorly thermally conductive host substrates raises the problem of thermal management of flexible electronics, a topic poorly addressed in literature. In this letter, we report the analysis of flexible SOI-CMOS chips ultimately-thinned-and-transfer-bonded (UTTB) onto polyimide and copper substrates. While the temperature remains limited to ~68°C on the native silicon substrate or after transfer onto a copper host substrate, infrared thermography reveals temperature peaks of up to 118°C on polyimide. The impact of self-heating in flexible SOI-CMOS is correlated with electrical performance for the three types of substrates. Beyond the property of mechanical flexibility it provides, a copper substrate is shown to slightly strengthen electrostatic integrity while maintaining a thermal landscape close to that of silicon.