High conductivity composite flip-chip joints and silver-indium bonding to bismuth telluride for high temperature applications

Two projects are reported. First, the barrier layer and silver (Ag)-indium (In) transient liquid phase (TLP) bonding for thermoelectric (TE) modules at high temperature were studied, and followed with a survey of Ag microstructure and grain growth kinetics. Second, the high electrical conductivity joint materials bonded by both Ag-AgIn TLP and solid-state bonding processes for small size flip-chip applications were designed.;In the first project, barrier and Ag-In TLP bonding layer for TE module at high temperature application were studied. Bismuth telluride (Bi2 Te3) and its alloys are used as materials for a TE module. A barrier/bonding composite was developed to satisfy the TE module for high temperature operation. Titanium (Ti)/ gold (Au) was chosen as the barrier layers and an Ag-rich Ag-In joint was chosen as the bonding layer. An electron-beam evaporated Ti layer was selected as the barrier layer.;An Ag-In fluxless TLP bonding process was developed to bond the Bi 2Te3 chips to the alumina substrates for high temperature applications. To prepare for bonding, the Bi2Te3 chips were coated with a Ti/Au barrier layer followed by a Ag layer. The alumina substrates with titanium-tungsten (TiW)/Au were then electroplated with the Ag/In/Ag structure. These Bi2Te3 chips were bonded to alumina substrates at a bonding temperature of 180ºC with a static pressure as low as 100psi. The resulting void-free joint consists of five regions: Ag, (Ag), Ag2In, (Ag), and Ag, where (Ag) is Ag-rich solid solution with In atoms in it and Ag is pure Ag. This joint has a melting temperature higher than 660ºC, and it manages the coefficient of thermal expansion (CTE) mismatch between the Bi2Te3 and alumina substrate. The whole Ti/Au barrier layer and Ag-In bonding composite between Bi 2Te3 and alumina survived after an aging test at 250°C for 200 hours. The Ag-In joint transformed from Ag/(Ag)/Ag2In/(Ag)/Ag to a more reliable (Ag) rich layer after the aging test.;Ag thin films were grown on various substrates and annealed at different conditions. The substrates include alumina metalized with TiW/Au, Bi 2Te3 metalized with palladium (Pd)/Au, silicon (Si) metalized with chromium (Cr)/Au, Si metalized with Pd/Au, 304 stainless steel metalized with nickel (Ni), and copper (Cu). The pre-exponential factor and activation energy of Ag grown on alumina/TiW/Au and Si/Cr/Au at 250-450°C were deduced from experimental results as 1.26cm2/s and 153kJ/mol, and as 0.07cm2/s and 140kJ/mol, respectively.;In the second project, we studied the Ag-AgIn bonding and solid-state bonding. Several joint materials and bonding systems were studied with small dimenstion (10&mgr;m and 40μm) flip-chip designs, which include Ag joint with Ag-In transient liquid phase bonding, and Ag, Ag/Au and Cu/Au interconnections with solid-state bonding.;The 40&mgr;m Ag-AgIn flip-chip interconnect process occurs at 180°C. An array of 50×50 Ag flip-chip joints with a 100&mgr;m pitch and 40&mgr;m joint diameter was created and bonded to the Cu substrate by depositing Ag/In/Ag at 180°C. The joint has a structure of Ag/(Ag)/Ag2In/(Ag) that connects the Si chip to the Cu substrate. The whole joint layer has a minimum melting temperature at 660°C.;Cu/Au, Ag/Au, and Ag flip-chip interconnects were bonded to Cu substrate by solid-state bonding. The Cu substrate here was emulated as Cu electrodes on a package substrate. During the solid-state bonding process, heat and pressure were applied, and the bonding was done in a 100 torr vacuum.;In the Cu/Au and Ag/Au flip-chip solid-state bonding experiments, an array of Cu/Au or Ag/Au interconnects with columns of 40μm × 40μm were created by photolithography and electroplating processes, and then bonded to the Cu substrate using a solid-state bonding process at 200°C with a static pressure of 250-400psi. The corresponding load for each column was set to as low as 0.22-0.35g. Cross section SEM images show that Cu/Au columns were all well bonded to the Cu substrate without any voids or cracks. The measured fracture force of Cu/Au and Ag/Au flip-chips were 4 times and 2.5 times larger than the MIL-STD-883E criterion of the pull-off test, respectively.;In this dissertation, the high electrical conductive interconnections along with the two fluxless bonding technologies, Ag-In TLP bonding and solid-state bonding, provide new and alternative connection methods in electronic packaging industry. The joints formed by these bonding technologies have potentially high electrical and thermal conductivities, high ductility, high aspect ratio, and high operation temperature. (Abstract shortened by UMI.).