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|Void formation and bond strength investigated for wafer-level Cu-Sn SLID bonding|
|Keywords: Cu-Sn SLID bonding, Void formation, bond strength|
Wafer-level packaging is receiving much attention since it enables reduction of the package size and mass production of packaged MEMS-devices, which significantly reduces cost and eases handling. Solid-liquid interdiffusion (SLID) is one of several successful approaches for wafer-level bonding: A high- (solid) and low- (liquid) melting point material are bonded at a temperature above the lower melting point. The resulting bond has a higher melting temperature than the bonding temperature, thus facilitating further processing of the package. Common metal systems for SLID-bonding include gold-tin (Au-Sn), gold-indium (Au-In) and copper-tin (Cu-Sn). A Cu-Sn SLID bonding frame has successfully been applied for hermetically sealing an infrared bolometer . The present work investigates void formation and bond strength for such frames, and the possible relation between these.
During Cu-Sn SLID bonding, Cu diffuses into liquid Sn to form the intermetallic compound (IMC) Cu6Sn5, followed by solid state interdiffusion and the formation of Cu3Sn [2-4]. The resulting bond preferentially consists of Cu3Sn sandwiched between Cu layers since this is a thermodynamically stable configuration i.e. no change in bond composition is expected as function of ageing. Voids are typically found in the final Cu3Sn IMC. They are attributed to Kirkendall voiding, plating bath impurities or impinging Cu6Sn5 scallops in combination with insufficient liquid Sn at the time of bonding [5-6]. This can be a concern with regard to the strength and reliability of the final bond. Moreover, in case of severe void formation loss of hermeticity can arise due to percolation through voids.
Voids have previously been observed for the Cu-Sn bond studied in this work , primarily located along a central line at the initial contact plane between the two bonding interfaces. In this work, void formation and resulting strength has been investigated for samples with modified process parameters and significantly increased storage time between plating and bonding. The density of voids has been investigated by optical microscopy and Scanning Electron Microscopy (SEM) on cross sectioned bond frames. Polishing by ion-milling enabled high quality SEM images which unveil the intermetallic granular structure and extensive voiding at various locations. Voids have also been non-destructively detected by Scanning Acoustic Microscopy (SAM).
The strength of the Cu-Sn bonds has been tested with shear-testing, and the fracture surfaces have been investigated to study the fracture mechanics. Fracture surfaces of
|Astrid-Sofie B. Vardoy,