Low-Temperature Die Attachment by Pressureless Cu Sintering for Semiconductor Packaging

This study focuses on the process optimization of pressureless Cu sintering for semiconductor die attach application. The die attachment joints were sintered at temperatures of 200-300 degrees C and under sintering times of 10 min, 30 min and 60 min. Both bare Cu substrates and Ag-metallized substrates were studied for all the sintering trials. The bonding quality of sintered Cu joints were evaluated by die shear test and microstructure characterization. The microstructure of as-sintered samples, failure surfaces after shear test and cross-section were characterized by scanning electron microscopy (SEM) and focused ion beam-scanning electron microscopy (FIB-SEM). Results show that similar shear strength can be achieved for both bare Cu substrates and substrates with Ag metallization at the same sintering conditions, but bare Cu substrates are more reliable for Cu sintering than Ag-metallized substrates at high temperatures. Sintering at temperatures below 250 degrees C gives a low shear strength value (below 24 MPa), where lack of sintering is identified in the centre area of the sintered layer; sintering above 250 degrees C helps to improve the diffusion between Cu particles and can obtain joints with a relatively high strength (above 30 MPa). Robust joints (with shear strength of up to 50 MPa and an apparent porosity of similar to 11%) can be obtained by sintering at 260 degrees C for 30 min, bonded on either Ag metallized substrate or bare Cu substrates.