An Integrated Experimental and Theoretical Approach to Evaluate Si Strength Dependent on the Processing History

Abstract

Silicon is the most widely used semiconductor material for MEMS and electronic applications. The strength of silicon has a critical impact on the reliable and accurate function of these components. In packaging assembly, reliability tests as well as in operation life the silicon die can be exposed to high stresses. For a theoretical risk assessment of die cracking due to the mentioned loadings a failure criterion is needed, which is capable to account the processing history. Therefore, the experimental characterization via fracture tests, analysis of different Si surface structures by Laser-Scanning-Microscopy (LSM) and Transmission-Electron-Microscopy (TEM) and a numerical approach using Finite Element Simulation was combined to allow failure prediction. In the fracture tests the influences of the backside grinding and the stress relief processes were investigated. The test dies were fabricated using the Dicing-before-Grinding (DBG) process with different mesh sizes of the grinding wheel. Additionally dry polishing and plasma etching were applied as stress relief processes. Bending tests are commonly used for the strength characterization of brittle materials like ceramics and silicon. The ball-ring-test was conducted to exclude edge effects on the strength, which are still under investigation in an additional part of the study. It was found that the determined fracture stresses follow a three-parametric Weibull distribution. The surface topography and roughness, respectively, was measured via LSM. TEM was applied to determine the structure of the surface-near damage-layer. In addition to the experimental investigations numerical calculations by means of Finite Element Simulations were performed in order to calculate critical fracture stresses from testing including anisotropic and geometric nonlinear effects. Furthermore, a submodeling-technique was applied to understand the effects of experimentally observed surface states on the critical stress, i.e. roughness, grinding marks, flaws, and poly Si-layers. Detailed modeling included local modeling of surface roughness on the critical strength.