Investigation of Photosensitive Polyimide with Photolithography Process and Mechanical Behavior for Wafer-Level Packaging

With the rapid growth of the 5G, artificial intelligence (AI) and Internet markets, the demand for highly integrated and high-reliability chips has greatly expanded, but the development of packaging structures and the enhancement of chip computing power have brought about changes in materials, structures and processes. A thermal mismatch between encapsulation materials can easily lead to defects in the package structure, such as cracks, warpages, and interfacial delamination. Therefore, it is urgent to solve the problem of chip packaging interaction. In wafer-level packaging such as fan-out integrated packaging technology, photosensitive polyimide (PSPI) materials are often used as dielectric layers in RDL due to their excellent thermomechanical and thermodynamic properties, good adhesion and corrosion resistance, which can directly photolithography RDL patterns and simplify the photolithography process. Nevertheless, in general, the CTE of PSPI films in-plane ranges from 40 to 80 ppm/°C, but the CTE of copper metal is much lower about 17 ppm/°C, and the CTE of Silicon is the lowest about 2.8 ppm/°C C. Due to the CTE mismatch among the silicon substrate, the copper metal layer, and the PSPI film, the warpage problem and interface delamination are serious, resulting in the degradation of package reliability. The photolithography process of PSPI material affects its mechanical properties. The mechanical properties will directly affect the design process and the reliability evaluation. Therefore, it is important to optimize the photolithography process, perform a comprehensive mechanical behavior analysis, and establish a reasonable material constitutive model before proceeding with the design and rapid reliability assessments. To solve the above problems, this work mainly explores the photolithography process and mechanical properties of PSPI films. It establishes a finite element simulation model through photolithography tests, thermal expansion properties and viscoelastic mechanical properties experiments, evaluating the reliability in advanced packaging processes. We found that by optimizing the lithography process, the physical properties of PSPI films can be effectively improved, the defects of PSPI films can be reduced, and the thickness uniformity can be improved. In addition, the mechanical properties are greatly affected by temperature, and both stress and modulus decrease rapidly as the temperature increases. And the viscoelastic properties of PSPI films can be well reflected by the prony series. The research results provide effective technical support for solving the reliability analysis of wafer-level packaging and will promote the development of wafer-level packaging.