Noboru Asahi

Development of high-k inorganic/organic composite material for embedded capacitors

The particle size distribution control and the dispersion agent were investigated, and a higher than 79 vol.% BaTiO/sub 3/ filler loading to epoxy resin was implemented by suppressing the porosity increment. The dielectric constant of a 10 /spl mu/m thick film at 1 MHz was 115, which corresponded to a capacitance density of larger than 10 nF/cm/sup 2/. The dielectric constant was 100 at 10 GHz when evaluated from 45 MHz -10 GHz. The chemical state evaluation of the surface of the filler, which was performed by FT-IR spectroscopic analysis, revealed that the water surface absorption of the filler increased the dielectric loss. After the dispersant was added and the composition of the resin and cure condition was optimized, the dielectric loss of the composite was 0.02 at 1 MHz. The temperature dependence of the dielectric constant was evaluated and was shown to be small. A laser via formation process using the composite material was also examined. It was confirmed that a 100 /spl mu/m diameter via hole into a 10 /spl mu/m thick layer could be formed. The inplane CTE of the composite material was 17 ppm//spl deg/C, obtained by stress measurement of the film. This value is coincident with the CTE of Cu. The elastic modulus was measured and found to be 18 GPa at 75 vol.% of the filler.

High throughput thermal compression NCF bonding

High through put thermal compression NCF bonding was studied and the new process consisting of dividing pre and main bonding, and the multi die gang main bonding has been developed. The dividing could change the process from serial to parallel and enabled to use the constant heated bonder head, which eliminated the time consuming head cooling process of the conventional serial thermal compression bonding. The die of 7.3 × 7.3 × 0.1 mm size with bumps of 38 × 38 μm2 square Cu pillar covered by Sn-Ag cap, which had the pitches of 80 μm at peripheral and 300 μm at corer area, and the organic laminated substrate with Cu/OSP trace were used as the test vehicle in this study. Firstly, the dividing of pre and main bo ndi ng process in the case of si ngle die was investigated. The prebonding was the die placement to the NCF on the substrate, which was carried out at 150°C for 0.5 second. The substrate was kept at 80°C during the process. After the pre bonding the test vehicle was removed out from the equipment and cooled down to room temperature. And then it was mounted back to the equipment again and main bonding was carried out at 240°C for 20 seconds. The same substrate temperature as the pre bonding process was kept. Solder joint formation and NCF curing was made at the process. The assembled test vehicle was evaluated. The cross sectional observation results showed that the bump solder wetted the Cu trace on the substrate and no void was detected in the NCF by C-SAM observation. Secondly, the multi die main gang bonding was studied. The equipment was newly designed and built. 15 dies were pre bonded on the substrate with the same condition as that of the single die experiment. After the pre bonding was finished, the substrate was moved to the main gang bonder. During the transportation the substrate was cooled down to room temperature. The 15 dies were bonded at one time at 240°C for 10 seconds. The substrate was heated at 240°C during the process. The evaluation of the assembled dies revealed that the solder wettability of the joints and void detection in the NCF was almost the same as those of the single die pre and main divided bonding. This main bonding process time corresponded to 2700 UPH.

Low temperature touch down and suppressing filler trapping bonding process with a wafer level pre-applied underfilling film adhesive

Flip chip bonding process of the chip touch down at 40°C and suppressing the material trapping at the joint area with the wafer level NCF (Non conductive film), which is pre applied underfilling film adhesive, has been investigated. The test vehicle wafer has 25 μm diameter and 50 μm height bumps which are 10 μm height Cu pillar and 40 μm height Sn-Ag solder cap. The bump pitch was 200 μm. The 55 μm thickness 50 wt% filler loaded NCF was laminated on the wafer and then the surface was planarized with the bump solder layer exposing by the bit cutting technique. Such prepared chip was bonded as the top chip to the bottom chip which has the 25 μm diameter pad of 3 μm Cu bottom, 2 μm Ni middle and 0.1 μm Au top. To insert the sticking step in the bonding process, which melts and flows down the NCF underneath the top chip to the bottom chip partially, the chips were held well the aligned position during the successive processes. The gang bonding possibility was also proved with the four chips together bonding. PCT (pressure cooker test, 121°C and 100%Rh for 168 hours) was performed to the gang bonded samples. By shortening the joint formation step time form 25 to 5 seconds Cu diffusion into the solder bulk area was suppressed and the durable joint to the PCT was formed. It was confirmed by the cross sectional observations.

Development of wafer level NCF (non conductive film)

The wafer level non conductive film (WL-NCF) has been developed, which has dynamic temperature dependence of viscosity. The b-stage WL-NCF was laminated onto the wafer without void, which has 870 Au bumps of 15 mum height and 25 mum pitch. The wafer with the WL-NCF on the surface was cut into chips by standard dicing process. The chip which has the NCF on the surface was bonded onto the ITO wired glass substrate by a flip chip bonder and the electrical connection was confirmed.

High productivity thermal compression bonding for 3D-IC

The evaluation result of 4 layer stacked IC which was bonded using thermal compression bonder (TCB) is reported. The throughput can be remarkably improved because chips of multi-layer can be pre bonded by using non-conductive film (NCF) which is pre-applied adhesive and can be thermally pressed at a time. To realize this process, we stacked the 4 chips having through silicon via (TSV) on a Si substrate and evaluated the connectibility. As the evaluation after bonding, wettability of a solder by cross-section observation and a void in NCF layer by constant depth mode scanning acoustic microscope (C-SAM) observation were confirmed. As a result, it was confirmed that the voidless and good solder joints were possible by reducing the temperature difference in a stacking direction. For the evaluation, we used the TEG of 6 mm × 6 mm × 0.05 mm size which has more than 15,000 bumps of 12 μm height and 15 μm diameter. It was also demonstrated that gang bonding for a plurality of pre bonded chips formed on a substrate was possible by using the novel bonding attachment which accepts the thicknesses difference of 5 μm.

Wafer and/or chip bonding adhesives for 3D package

For 3D package application the demanded feature of chip bonding adhesive was discussed and the material was developed, which was wafer level process compatible NCF (Non conductive sheet). It should be high flowability for lamination on a bumped wafer surface without void, diced with wafer without deformation or sticking dust, transparent for alignment mark detection and low coefficient of thermal expansion (CTE) for the package thermal cycle durability. The nano particle dispersed and highly loaded NCF has been developed to satisfy the demanded characteristics. The CTE was 37°C/ppm and 1% weight loss temperature is higher than 350°C. The photosensitive NCF has been also developed. Bump top adhesive can be removed completely by photolithography before bonding process. It can wipe off the anxiety of adhesive residue between bump and pad. This material is also good for lamination and has high heat resistance of 1% weight loss temperature higher than 300°C.

3D-IC thermo-compression collective bonding process using high temperature stage

As one of the methods to stack the 3D-IC fast, the collective bonding process using TCB (thermo-compression bonder) attracts attention [1-3]. In the collective bonding process, we can improve the throughput considerably by postbonding the multilayered pre-bonded chips at a time. However, when the number of the stacked chips increased, we found that the temperature difference between the upper layer and lower layer became large and the good solder connection was not obtained in all the layer in conventional TCB. Therefore, we reported that the collective bonding could be realized by using the heat insulation stage which can prevent an outflow of the heat from the bonding head [1]. By using the heat insulation stage, we could reduce the temperature difference to less than 10oC for four layers, but it was necessary to reduce temperature difference more, depending on the kind of NCF. Besides, it was difficult to reduce difference of temperature when, for example, the number of the stacked chips increased more than eight layers. In this presentation, we report about new collective bonding process that is able to reduce the temperature difference by using high temperature backup stage. We could enable the high temperature process of the stage by using the wafer-handling mechanism which lifts a substrate from the stage every one bonding cycle.

High thermal conductive adhesive film for Cu and Al plate adhesion in power eectronics package

The high thermal conductive of 10 to 15 W/mK and high heat durable adhesive film has been developed. It is composite of the imide base high heat durable thermosetting resin and high thermal conductive particles. The 1% and 5% weight loss temperature were 384 and 494°C, respectively. Breakdown voltage of the 200 μm thick film was larger than 5 kV at 50 Hz. Insulation reliability was evaluated at 85°C and 85%Rh with applying 750 V and no resistance drop was detected for 1000 hours. The material was mainly designed for thermal interface adhesive of heat generator side lead flame Cu and heat sink Al in power electronics packaging field Coefficient of thermal expansions of Cu and Al were 17 and 23 ppm/°C, respectively. That valued of the adhesive was made up to 19 ppm/°C, which lay between them. The numerical stress analysis of the structure by finite element method was performed. It indicated that the modulus of the adhesive at low temperature affected the stress significantly. Cu (50 mm × 60 mm × 7.5 mm) and Al (70 mm × 80 mm × 6.0 mm) thick plates were adhered by the adhesive and 2000 times thermal cycle durability evaluation between -45 to 125°C were implemented. The sample adhered by the adhesive which was designed to be low modulus at low temperature had no delamination after the thermal cycle, which was investigated by scanning acoustic microscope.