Void Formation and Intermetallic Growth in Pulse Electrodeposited Cu-Sn Layers for MEMS Packaging

Abstract

Electrodeposited copper (Cu)-tin (Sn) based solid–liquid interdiffusion (SLID) bonding is becoming popular in wafer-scale packaging of inertial Micro-Electro-Mechanical-Systems (MEMS) sensors due to its inherent advantages of lower cost, low processing temperatures and less stringent surface uniformity requirements. However, eliminating micron-size voids within intermetallic compounds (IMCs) and bond interfaces has remained a challenging task. The present study focuses upon IMC growth and void formation at varying temperatures and times. Stacks of varying thickness of Cu and Sn were fabricated by electrodeposition, and the samples were annealed at temperature ranging up to 300°C. Scalloped shaped Cu₆Sn₅ (η-phase) and comparatively uniform Cu₃Sn (ε-phase) intermetallics were observed. Experimental results show that the growth of metastable Cu₆Sn₅ dominates IMC formation at lower temperatures but as temperature increases, Cu₃Sn dominates over the Cu₆Sn₅ growth. This IMC growth transition from Cu₆Sn₅ dominant growth to Cu₃Sn dominant growth depends on the annealing temperature and has a critical time duration. The IMC thicknesses are compared with those obtained by numerical simulation models. For given annealing temperatures, intermittent voids formed in the IMC layers show increasing size and decreasing void fraction trends with increasing annealing times. The results suggest that Cu-Sn SLID bonding performed at 275°C yields reliable bonding since the void growth is minimal. Based on these results, a test vehicle containing a kelvin structure and daisy chains (having large number of Cu-Sn bonded structure), was fabricated, resulting in electrical resistances lower than 30 m-ohms and 6 ohms, respectively.