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Dependence of the Fracture of PowerTrench MOSFET Device on Its Topography in Cu Bonding Process

Dependence of the fracture-resistance of a PowerTrench MOSFET device on its topography in Cu bonding process was investigated. Two different topographies, namely dimple and round, have been tested. A significantly higher cratering rate has been clearly observed on dimple topography. The dimple topography exhibited a cratering rate of 371 k ppm levels compared to 0 ppm in round topographies. Three-dimensional nonlinear finite-element analysis has shown that the largest compressive and shear stresses and their locations were identified, respectively, in borophosphosilicate glass (BPSG)/barrier metal layers of the dimple topography. The round topography had the smallest stress in BPSG/barrier metal layers. The higher compressive stress transferred to silicon in the dimple topography during the bonding process can induce a local crack, consequently causing silicon fracturing during the shearing processes. A significant improvement in the cratering performance was observed when the Al bond pad metal layer was reinforced by adding a barrier layer sandwiched in the Al metal layers. The cratering rate decreased to 1300 ppm levels. Additionally, the change in composition of a BPSG layer caused cratering was briefly discussed and an oxygen rich BPSG film in round topography was confirmed by the energy dispersive spectroscopy (EDS) of a cross-sectional TEM sample. It has been found that the cratering rate on dimple topography significantly increased from 1 k ppm to 100 k ppm levels, when the resulting residual Al pad thickness is less than 0.65 mum for Cu bonding performed with different ultrasonic (US) power and bond forces.