Masatsugu Nimura

Solder/adhesive bonding using simple planarization technique for 3D integration

This paper describes a hybrid solder/adhesive bonding method using a simple planarization technique for three-dimensional (3D) integration. With the hybrid bonding method, the chip bonding and encapsulation of underfill resin between chips is completed in one step. The simple planarization technique is used to planarize adhesive on a flat Si substrate coated with a release agent. The planarization technique is a simple and inexpensive operation compared to the conventional processes using Chemical Mechanical Polishing (CMP) or fly cutting. Since the CMP process has advantages for wafer-level fabrication, we also evaluated hybrid bonding using CMP. The results of the simple planarization process show that the spaces around the Cu/Sn bumps were fully filled with the adhesive, and the adhesive residual layer on the Cu/Sn bumps was removed by O2 plasma. A cross-sectional SEM image after the hybrid bonding using the proposed planarization process shows that the Cu/Sn solder had properly wetted the Au and the adhesive had uniformly filled the small gaps between the bonded chips. Solder/adhesive bonding using CMP was also succeeded. In addition, Au/adhesive bonding with 10-μm pitch Au bumps was realized.

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.

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.

Hybrid Au-Adhesive Bonding Using Planar Adhesive Structure for 3-D LSI

In this paper, we describe a hybrid bonding technology of Au microbump and adhesive using a planar adhesive structure for 3-D large-scale integration (LSI). Hybrid bonding means that both the microbump electrode and adhesive are simultaneously bonded. In 3-D LSI, the gaps between bonded chips are <;10 μm because the pitch of the microbumps is decreased. Conventionally, adhesive resin is injected into the gaps by means of capillary force. However, the filling of the gaps is insufficient due to surface conditions. To address this challenge, we evaluated hybrid bonding with a planar adhesive structure fabricated by chemical-mechanical polishing. The bonding results showed that connection between the Au bumps and adhesive filling in the 6-μm gap between bonded Si chips was achieved without readily visible void in the range of 6 mm × 6 mm. All 900 bumps were also electrically connected. The shear strength of the bonded sample was 13 MPa. Therefore, we determined that the proposed hybrid bonding technology is highly effective for 3-D LSI with fine-pitch microbumps.

Study on Hybrid Au–Underfill Resin Bonding Method With Lock-and-Key Structure for 3-D Integration

This paper describes a hybrid Au-underfill resin bonding method with lock-and-key structure for 3-D integration. In 3-D large scale integration (LSI), the gap between stacked chips becomes narrower because the bump dimension and pitch are smaller than those encountered in 2-D LSI. Therefore, the filling of gaps less than 10 μm using capillary forces often becomes insufficient because of the surface condition. To address this challenge, we study a hybrid bonding method in which the metal-metal and resin-resin bonding are carried out simultaneously with a chip resin applied previously only around the bump. To realize hybrid bonding on the entire chip, we fabricate indent and protrusion structures, which are called lock-and-key structures. The key structure is fabricated by a process that can remove the resin on the bumps by O2 plasma irradiation. The lock structure is fabricated by conventional photolithography and dry etching. By means of hybrid bonding with the lock-and-key structure, we have achieved the Au bump bonding and the filling of 4-μm gaps between the stacked chips, concurrently. The cross-sectional transmission electron microscopy image of the bonded sample demonstrated that no significant gap exists at both the Au-Au and resin-resin interfaces. In addition, the shear strength of the sample bonded with resin is 10 times higher than that without the resin. The electrical continuity of the Au bump connections after hybrid bonding has also been determined.

Heat transfer analysis in the thermal compression bonding for CoW process

We report the results of heat transfer analysis of CoW (chip on wafer) process in which IC chips are bonded on a Si wafer by using thermal compression bonding (TCB). The throughput can be remarkably improved because a lot of chips can be pre bonded on a wafer by using non-conductive film (NCF) which is pre-applied adhesive and can be thermally pressed at a time. However, to realize this process, it is necessary to reduce the heat influence on adjacent chips and to suppress the heat conducting to a substrate from the bonding head. In this research, we analyzed the temperature distribution in NCF layer, and the behavior of heat conduction from the bonding head to the adjacent chips. As a result, it was found that thermal influence on the adjacent chips was small in the case of thin substrate but the bonding temperature also decreased, especially in peripheral part of NCF layer. In addition, we proved that it is possible to prevent the reduction of the bonding temperature with heat insulation stage, and to decrease the thermal influence to adjacent chips by using air cooling system.

Evaluation of ultrasonic vibration energy for copper-to-copper bonding by flip-chip bonding technology

The ultrasonic vibration energy for Cu-Cu bonding by flip-chip bonding technology was evaluated in ambient air. Transmissibility of ultrasonic vibration is assumed to be different in bump structure with height or low stiffness. Therefore, we investigated the bonding strength of Cu bump with the different aspect ratio (bump height: 5 μm, 20 μm, and 40 μm). As a result, the Cu bumps with 20 μm height were properly bonded with sufficient bonding strength, compared with the others. In addition, there were no significant voids at Cu-Cu interface of well-bonded Cu bump.

Fine-pitch hybrid bonding with Cu/Sn microbumps and adhesive for high density 3D integration

In this study, we developed single-pitch hybrid bonding technology for high density 3D integration. To realize the single-pitch, smaller than 10-μm pitch, hybrid bonding, 8 μm-pitch Cu/Sn microbumps and nonconductive film (NCF) were used. The planar structure to simultaneously bond was formed by chemical mechanical polishing (CMP). After the planarization, the Cu/Sn bumps and NCF were simultaneously bonded at 250 °C for 60 s. Cross sectional observation of the bonded sample by scanning electron microscope (SEM) indicated that 8 μm-pitch bump bonding was carried out successfully and the NCF filled the 2.5-μm gap between the chip and substrate without significant voids. In addition, a tensile test was performed as a mechanical reliability test. The tensile strength was 3.86 MPa. From SEM observation of the fractured surface after the tensile test, the fractured surface of the bumps was intermetallic compound (IMC) layer between Cu and Sn. These results indicated that hybrid bonding was effective method for single-pitch bonding and underfilling.

Hybrid Au-Au bonding technology using planar adhesive structure for 3D integration

This paper describes the hybrid Au-Au bonding technology in which the Au-Au and adhesive-adhesive bonding are carried out simultaneously using planar adhesive structure for 3D integration. The planar adhesive structure was fabricated by CMP of adhesive, and consists of ultralow-profiled Au bump with flat surface and uncured adhesive. The bonded interface was observed with SAM and SEM. The results indicate that Au bump connection and the adhesive filling of the 6-μm gap between bonded chip and substrate were achieved without significant void. All 900 bumps were also electrically connected. Furthermore, the shear strength of the bonded sample was13 MPa. The adhesive was strongly bonded because the Si substrate of bonded sample was broken to pieces.

Hybrid Au-underfill resin bonding with lock-and-key structure

We developed a novel hybrid bonding technology for Au ultralow-profiled bumps and underfill resin with a modified “lock-and-key structure.” The lock structure interlocks with the key structure. We applied these structures to perform an entire adhesion between the mating surfaces in place of conventional underfilling technique. To fabricate the key structure, we developed a simple process that can remove resin on the bumps. Lock structure was fabricated by photolithography and dry etching. After the bonding was carried out, the bonded interface was observed with a Scanning Electron Microscope (SEM), a transmission electron microscope (TEM) and a Scanning Acoustic Microscope (SAM). The results proved that no significant gap was existed at both Au-Au and resin-resin interface. Furthermore, the shear strength of the bonded sample with resin was ten times stronger than that without resin. The conduction of Au bump connections after hybrid bonding was also confirmed.