Taiji Sakai

Low temperature Cu-Cu direct bonding for 3D-IC by using fine crystal layer

In this paper, we report a method of low temperature solid diffusion bonding. To investigate bondability of solid diffusion, we examined the effect of bump metals and bump planarization methods. Cu and Au bump were used for bump metals and CMP and ultra-precision cutting were used for bump planarization methods. We found that fine crystal layer could be formed on only cut Cu and Au bumps, and especially cut Cu bumps had a thick fine crystal layer on the surface. The layer on cut Cu bump was found to be easily to recrystallize at low temperature condition of 150 degree C. Moreover, the bonding interface of cut Cu bump disappeared at 200 degree C for 30 min, which means solid diffusion across the interface was realized with the contribution of fine crystal layer. In addition, for Cu-Cu direct bonding, formic acid treatment before bonding is effective because formic acid can react at low temperature without destroying fine crystal layer. That led to achieve high bonding strength between cut Cu bumps.

Electro-migration behavior in low temperature flip chip bonding

In this report, we investigated electro-migration behavior of two types of low temperature bonding. One was Sn-57 Bi using conventional C4 process. The other was Au-In Transient Liquid Phase bonding (TLP). Electron flow to induce the electro-migration was from substrate side (Ni pad) to chip side (Cu post) with current density of 40000A/cm2 at 150 degree C. In the case of Sn-57 Bi conventional C4 process, Bi quickly migrated to accumulate on the anode side (Cu post) and Sn migrated to the cathode side (substrate Ni pad). And the interconnect resistance increased until about 150 hours. Although this temperature was higher than the melting point of Sn57 Bi solder, there was no electrically break failure and the resistance was stabilized at 80% increase of initial resistance for more than 2800 hours, that was 10 times longer life of the Sn3.0wt%Ag0.5wt%Cu (SAC305) solder joint. From the cross-sectional analyses of Sn-57 Bi solder joints after the test, it was found that Bi layer and intermetallic compound (IMC) behaved as the barriers of the Cu atom migration into Sn solder. In the case of Au-In TLP bonding, remarkable change was not observed in metallic structure. And resistance was stabilized at 0.5% increase of initial for more than 1300 hours. Sn57 Bi solder joining and Au-In TLP bonding are promising candidates for the bonding technique of high density Flip Chip packages and 3D packages.

A low temperature Cu-Cu direct bonding method with VUV and HCOOH treatment for 3D integration

In this paper, a low temperature Cu-Cu direct bonding method is demonstrated even at 175 degree C, which is less than solder melting point, by using fine crystalized bump surface and dedicated surface treatments. Our technique includes introducing fine crystal structure on Cu bump surface by ultra-precision cutting for bump planarization, not using conventional CMP method. This fine crystal structure can easily recrystallize at 150 degree C condition, which can provide a metallurgical interdiffusion between Cu-Cu interfaces. In addition, surface treatment is also crucial for direct Cu-Cu bonding. In this study, we investigated formic acid (HCOOH) and vacuum ultraviolet (VUV) irradiation effects on die shear strength. Cut Cu bump with VUV and HCOOH condition at 175 degree C not only showed high bonding strength, but also exhibited solid diffusion at interface while Cut Cu with HCOOH condition showed clear interface.

Design and demonstration of large 2.5D glass interposer for high bandwidth applications

In this paper, a large 2.5D glass interposer is demonstrated with 50 um chip-level interconnect (FLI), 3/3 um line and space (L/S) escape routing, and six metal layers, which are targeted for JEDEC high bandwidth memory (HBM). Our routing design suggests that double sided panel processing with 3/3 um L/S can accommodate required signal lines for HBM. Then, 3/3 um L/S transmission lines on 25mm × 30mm glass interposers with 300 um core thickness can be realized by utilizing semi additive process. Finally, 10mm × 10m dies with daisy chains can be successfully bonded to 25mm × 30mm glass interposer with 6 metal lines using copper microbumps with SnAg solder caps.

Formation of solder cap on Cu pillar bump using formic acid reduction

In this paper, we report a reflow process that uses formic acid to rem ove the native oxide of a so lder on a Cu pillar instead of using flux. To study the effect of the solder on the Cu pillar, we compare the shape and the crystal structure of a wetted solder on a Cup illar afterareflo w process using formic acid and flux. To conduct the experiment, we also had to confirm the effect of the in termetallic compound layer between a solder and a Cu pillar. Therefore, we prepared the sample with a Ni layer between a so lder and a C u pillar to prevent the formation of an intermetallic compound layer. Both samples with/without Ni layer were investigated simultaneously. The results showed that the solder which was fabricated by a reflow process using formic acid crystalized even at a peak temperature of 228 °C, which is ciò se to the solder melting temperature. There was no difference in the reduction abilities between formic acid an d flux in the wettability test o f the solder. However, it was found that the amount of formic acid are proportional to the reflow temperature. Atmosphere of formic acid is main factor of raising peak temperature of reflow.

Cu/adhesive hybrid bonding through a Cu-first bonding approach by using H-containing HCOOH vapor surface treatment

Cu/adhesive hybrid bonding is a promising technology to fabricate 3D integrated microsystems with ultra-fine pitch and short vertical interconnects, low electrical resistance, and high reliability. The main remaining issues of this technology include bonding temperature mismatch between Cu-Cu (350–400 °C) and adhesive (typically ≤250 °C), long thermal-compression time (low throughput), and high thermal stress. In this paper, we present a sub-200 °C Cu/adhesive hybrid bonding method. By using H-containing HCOOH vapor pre-bonding treatment, the bonding temperature is lowered to 180–200 °C and the thermal-compression time is shortened to 600 s, enabling a Cu-first hybrid bonding approach. Cu/adhesive hybrid bonding was successfully demonstrated at bonding temperature of 180 °C. The effects of prebonding treatment temperature and time on Cu-Cu bonding and cyclo-olefin polymer (COP) adhesive bonding are investigated in detail.

Improvement of mechanical properties of In-48mass%Sn solder by Ag and Cu addition

In a lead-free reflow soldering process, it is important to reduce the reflow temperature for low-heat-resistance devices such as optical devices that include organic material. We focused on In-48mass%Sn solder for optical device assembly since the melting point of the solder can be dramatically reduced to 117°C. However, InSn eutectic solder is considered to have poor mechanical properties. Thus, we investigated the effect of adding Ag and Cu to InSn-eutectic solder to improve the mechanical properties. Consequently, it was found that adding Ag and Cu to In-48mass%Sn solder improves both mechanical strength and ductility of the solder and causes AgIn2 to disperse by being partially substituted by Cu in the solder.