Demonstration of silver micro-joints for flip-chip interconnect

Abstract

► Silver micro-joints for flip-chip interconnect are formed between silicon chips and copper substrates by solid state bonding. ► No flux is used during bonding. ► Bonding temperature is compatible with typical reflow temperature of lead-free solders used in industries. ► Solid state bonding occurs when silver and copper atoms are brought within atomic distance so as to share electrons. ► Silver joints do not contain intermetallic compound, avoiding reliability issues associated with its growth. We report formation of silver (Ag) micro-joints for flip-chip interconnect. For demonstration, the micro-joints were made between silicon (Si) chips and copper (Cu) substrates. Two different Ag joint sizes, 40 and 15 μm, respectively, were produced. In experiment, arrays of cylindrical cavities were fabricated in thick photoresist on 2-in. Si wafers that were coated with chromium (Cr) and gold (Au) dual layer. The Si wafers are diced into 6 mm × 6 mm chips. The cavities were filled up with pure Ag by electroplating process. The photoresist was then stripped off. The Si chip with Ag columns was bonded to Cu substrate at 260 °C in 80 mTorr vacuum. During bonding, Ag columns deform and their surfaces conform to and mate with the surface of Cu substrate. Solid state bonding can incur when the Ag atoms and the Cu atoms are brought within atomic distance so that they share electrons. The Ag columns were well bonded to Cu. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. The Ag joints do not contain any intermetallic compound (IMC). Accordingly, this interconnect technology avoids all reliability issues associated with IMC growth in solder-based flip-chip joints. Compared to solders, Ag joints have superior electrical and thermal properties. Pull test results show that the strength of 40 μm Ag joints passed MIL-STD-883H by wide margin.