An investigation of stable bonding for Au-Al solid-phase diffusion bondingtechniques

By using a chip that has Au bumps and a substrate that has only a varying A l film thickness, initial bonding strength is made constant in Au-Al solid- phase diffusion bonding. On the other hand, by changing the Au-Al intermeta llic compound formed during bonding, a relationship is obtained between the formed intermetallic compound and bonding reliability. Samples obtained wh en installing ICs under the same conditions on substrates with Al film thic knesses of 350 nm and 1000 nm were left for 1000 hours at 125 degrees C and the bonding strength and connection resistance were measured. Immediately after bonding, there was no meaningful difference between them. However, af ter 1000 hours, reduced bonding strength and increased connection resistanc e were observed in the sample whose Al film thickness was 1000 nm whereas a stable connection was obtained in the sample whose Al film thickness was 3 50 nm. The difference in reliability in a high-temperature environment resu lts from differences in the Au-Al intermetallic compounds formed at the tim e of bonding and by the existence of unreacted Al. In the sample whose Al f ilm thickness was 350 nm, there was no Al at the junction at the time of bo nding, so that the final product was predominantly Au4Al. On the other hand , in the sample whose Al film thickness was 1000 nm, Al existed at the junc tion, so that several Au-Al intermetallic compounds were formed which subse quently degrade by diffusion reactions at high temperature. (C) 1999 Script a Technica.