At the recent IMAPS Device Packaging Conference held in Ft McDowell AZ, Taiji Sakai of Fujitsu presented their new Cu-Cu thermo-compression bonding technology which uses “machined” bumps to achieve a significantly lower monolithic bonding temperature.

In order to achieve tighter pitch for high I/O devices, many device manufacturers are moving to copper pillar bumping where the copper is bonded by a Sn/Ag solder cap. [Ref] 

 
Even tighter pitch and better electrical performance could be obtained if an efficient process existed for direct C pillar – cu pillar bonding due to coppers lower electrical resistivity and better resistance to electromigration and Kirkendall voids under high current conditions. The limiting factor preventing that move is time/temp required to achieve a monolithic Cu-Cu bond. 

 
Sakai concludes that the factors which contribute to the conditions necessary for thermocompression bonding ( time/temp/pressure) include : surface planarity; surface crystal structure and surface purity.
Fujitsu researchers have discovered  that if the Cu bumps are planarized by machining (planing) with a diamond bit, instead of CMP’ing the surface, as is normally done, a surface Ra of 7 nm is obtained and an “amorphous like layer” is produced at the surface. Forming a monolithic interface is possible at 200 – 250 C (30 min) vs the >350 ˚C(30 min)  required for  a CMP’ed surface.  

The machined bumps reveal an amorphous like layer which allows surface recrystallization to produce a monolithic interface at significantly lower temperatures. When test structures were bonded at 200, 250 ˚C (for 30 min under 300 MPa pressure), and then compared to cmp’ed structures  for shear strength and bulk fracture mode  one can see the advantages of the machined surfaces. 

 
Achieving copper - copper thermocompression bonding at 200 C should have significant impact on the adoption of such high performance bonding.