Impact of Micromachining Process on Cu-Sn Solid-Liquid Interdiffusion (SLID) Bonds

Abstract

Copper-tin Solid-Liquid Interdiffusion (Cu-Sn SLID) bonding has shown potential for packaging of microelectromechanical system (MEMS) devices such as microbolometers due to its low cost and high-temperature stability. A thin micromachined silicon cap is desired in the packaging of microbolometers to minimize the infrared light absorption. In the preferred fabrication process flow, the Cu-Sn sealing frames are deposited on both the cap and device wafers prior to micromachining the cap wafer. This method greatly simplifies the process compared to when the metal layers are deposited after the cap etching process. However, such an approach might affect the bonding quality due to the Cu-Sn sealing frame being used directly as a mask for the cap etching. The present work addresses this concern using a cap wafer (with and without micromachining process) bonded to a device wafer using the Cu-Sn SLID technique. A dicing yield at or near 100% is achieved for both samples. The interface of Cu-Sn bonds shows a similar Cu/Cu3Sn/Cu structure between the samples without cavity and with a cavity, which indicates that the micromachining process has a minor impact on the Cu-Sn bonds. However, the measured die shear strengths of the samples with and without cavity are relatively low due to fracture at the Cu/Cu3 Sn interface and adhesion fracture at the TiW layer. The bond strength can be further improved by optimizing the Cu/Sn electroplating process and improving the adhesion layer.