40 μm Copper-Silver Composite Flip-Chip Interconnect Technology Using Solid-State Bonding

摘要

Copper-silver (Cu-Ag) composite flip-chip interconnect between silicon (Si) chips and Cu substrates is demonstrated. Array of Cu-Ag columns, each 28 μm in height and 40 μm in diameter, is electroplated on 2-in. Si wafers coated with chromium (Cr)lgold (Au) dual layer. The Si wafers are diced into 6 mm x 6 mm chips, each containing 50 × 50 Cu-Ag columns. The Si chip with Cu-Ag columns is bonded to Cu substrates at 260℃ in 80 mTorr vacuum. A bonding force of only 1.8 kg is applied, corresponding to 0.71 g per Cu-Ag column. During bonding, Ag atoms in Cu-Ag columns deform and their surfaces conform to and mate with the surface of Cu substrate. Solid-state bonding incurs when Ag atoms in Cu-Ag columns and Cu atoms in Cu substrates are brought within atomic distance so that they share conduction electrons. The Cu-Ag columns are indeed bonded to the Cu. No molten phase is involved in the bonding. The joint consists of 60% Cu section and 40% Ag section. The ductile Ag is able to accommodate the thermal expansion mismatch between Si and Cu. The Cu-Ag joints do not contain any intermetallic compound (IMC). This interconnect technology avoids all reliability issues associated with IMC growth in conventional soldering processes. Compared to tin-based lead-free solder joints, Cu-Ag composite joints have superior electrical and thermal properties.