Imidization-Induced Wrinkling of a Polyimide Thin Film and Its Use for Piezoresistive Sensing Enabled through Direct Laser Writing Carbonization

The issue of imidization-induced tensile residual stress/strain existing in a polyimide thin film supported by a rigid substrate, e.g., silicon wafer, has been well recognized. Herein, we take advantage of this phenomenon, conventionally viewed as a drawback, to amplify and generate the compressive residual stress/strain by using poly(dimethylsiloxane) (PDMS) as a soft substrate to fabricate wrinkled polyimide thin films and investigate their wrinkling behavior. By varying the film thickness from ∼70 to 800 nm and upon quantitative wrinkling analysis along with IR spectroscopic characterization and thermal residual strain calculation, we establish the effect of film thickness on the imidzation degree, elastic modulus, thermal expansion coefficient, and glass transition temperature for the PDMS-supported polyimide thin films. Moreover, by applying the direct laser writing carbonization to carbonize the wrinkled polyimide thin film, we demonstrate a viable approach for fabricating a highly sensitive carbon-based piezoresistive sensor (gauge factor >2000) with excellent durability. The present study highlights the value of engineering the wrinkled polyimide thin film to acquire its mechanical and thermomechanical properties, as well as to develop highly sensitive piezoresistive sensors.