40 $\mu{\rm m}$ Flip-Chip Process Using Ag–In Transient Liquid Phase Reaction

A flip-chip interconnect process at 180
^°
C using the silver-indium (Ag-In) binary system is reported. An array of 50 × 50 flip-chip joints with 100 μm pitch and 40 μm joint diameter was fabricated. Each joint has the column structure of Ag/(Ag)/Ag
₂
In/(Ag) that connects the silicon (Si) chip to the copper (Cu) substrate. The joint height is approximately 50 μm. In this structure, Ag
₂
In is a dominating intermetallic compound in the Ag-In system with melting temperature of 660
^°
C. (Ag) is a solid solution phase of Ag with In composition up to 20 at.%. It has a solidus temperature range of 695 to 962
^°
C depending on In composition. In long-term operation, (Ag)/Ag
₂
In/(Ag) is expected to gradually convert to a single (Ag) phase, which is more reliable. Thus, the flip-chip joints will get better in use. In fabrication, 50 × 50 Ag columns were made on Si wafer coated with chromium (Cr) and gold (Au). The Cu substrate was electroplated with Ag(10 μm)/In(5 μm)/Ag(thin). Si chips with Ag columns were bonded to Cu substrates at 180
^°
C for 5 min. No flux was used. Cross-sectional scanning electron microscopy images show that all 50 Ag columns in one row are well bonded to the Cu substrate without visible voids or cracks. Energy-dispersive X-ray spectroscopy data indicate that the resulting column structure is Ag/(Ag)/Ag
₂
In/(Ag). The process temperature of this new interconnect method is 80
^°
C below the typical reflow temperature of tin-based lead-free solders.