Evaluation of test methods for silicon die strength

Abstract

The objective of this study is to evaluate the existing test methods of die strength, including widely-accepted three-point and four-point bending tests, and a newly-proposed point-load test by testing silicon die specimens with different surface conditions. It has been reported that there are three factors to influence die strength: the surface conditions of the die (including grinding-mark direction and surface roughness), the edge crack of the die (so-called chipping created during the cutting process), and the weak planes of the crystal lattice of silicon on (1, 1, 0) and (1, −1, 0). In this study, the focus will be on how these factors affect the strength data from these three test methods. Apart from performing these three tests, the roughness of the die surfaces (including the ground, polished and untreated surfaces) measured by atomic force microscopy, and edge chipping conditions by optical microscopy were correlated with strength data obtained from the tests. It was found that the four-point bending test gives the lowest strength data and its independence on the surface condition, due to much domination by the edge chipping. On the other hand, the three-point bending test provides intermediately high strength data, with slight difference between different surface conditions, and some major control by the edge chipping. It was also observed that both bending tests suffer the effect of the grinding-mark direction. By contrast, the point-load test associated with the applied force-maximum stress equation which is proved to be valid for Hertzian contact and, sometimes, required to take into account geometrically nonlinear effect, gives the highest values of strength among these methods. This test not only provides the data which are merely dependent on the surface roughness and free from the edge chipping and grinding-mark direction effects, but also it gives a bi-stress field similar to the temperature loading and provides direct test for dummy or real IC chips. As a result, the point-load test is one of the most adequate test methods for determining the die strength exclusively due to surface roughness effect.