Research On Low Temperature Thermo-compression Bonding Technology Using Nanoporous Copper As Bonding Layer

Three dimensional packaging technology is an advanced interconnection technology for packaging, since it has many huge advantages over other packaging technologies, such as shorter interconnection distance, higher speed signal transmission, higher level integration and less interference. Bonding is the key technology to realize multi-chip stacking. Among many bonding methods, Cu-Cu thermo-compression bonding can realize mechanical, thermal and electrical interconnection between the chips at the same time, thus it becomes the most primary bonding approach for the three dimensional packaging. However, the conventional Cu-Cu thermo-compression bonding needs high temperature(350~400?) and long time, resulting in low efficiency and degrading product reliability, therefore, achieving low temperature thermo-compression bonding is very crucial for three dimensional packaging. It is potential to lower the bonding temperature by using the nanoporous copper as bonding layer due to the fact that nanoporous copper has many unique properties, such as large surface to volume ratio, high surface activity, large diffusion coefficient and size-dependent melting of nanoparticles. At present, dealloying is becoming the main method to fabricate nanoporous metals with its simple process and tunable nanostructures. In this paper, nanoporous copper is fabricated through the dealloying method, then the prepared nanostructures is utilized as the bonding layer to achieve low temperature bonding. The main contents are listed as follows:Firstly, the theoretical foundation of low temperature thermo-compression bonding using nanoporous copper as bonding layer was analyzed. As we know, nanoporous copper has many attractive properties, such as large surface to volume ratio, high surface activity, size-dependent melting of nanoparticles and softer than bulk copper, which are beneficial for lowering the bonding temperature and pressure;Secondly, the key factors affecting the nanoporous structures including annealing temperature, electroplating time of Zn and corrosion duration were studied based on the Cu-Zn films. The results indicated that complete alloying could not be achieved due to low annealing temperature while thermal pressure would be generated owing to high annealing temperature, therefore the appropriate annealing temperature was 150~160 ?.With an increase of electroplating time of Zn, the pore size of nanoporous copper decreased and intended to be stable gradually. The appropriate electroplating time of Zn was 12 min. However, the ligaments became coarsened with increasing corrosion duration. The appropriate corrosion duration was 10 min;Thirdly, the thermal and mechanical properties of nanoporous copper were studied. The test results showed that the nanoporous copper underwent coalescence readily upon 200?, which proved that the threshold temperature of nanoporous copper was much lower than that of bulk copper. The nanoindentation test showed that the nanoporous copper was softer than bulk copper;At last, the effects of different process conditions including bonding temperature, bonding pressure and bonding duration on the bonding quality were studied. Good bonding results were attained upon 250?, with the pressure of 0.5MPa for 120 min. The pulling test indicated that the bonding strength was up to 5.2MPa while that of bulk copper bonding samples was nearly zero, which demonstrated that using nanoporous copper as bonding layer to realize low temperature bonding was feasible.