(Invited) Silver Sinter Joining and Stress Migration Bonding for WBG Die-Attach

High temperature lead-free solder has been under development though several candidates have been already proposed such as Bi alloys and pure Zn/Zn-Sn alloys as well as the conventional Au alloys. Transient liquid phase bonding is also examined for off-eutectic Au or Ag alloys. Nowadays, wide band gap power semiconductors (WBGs) such as SiC or GaN with their excellent performances have been expected to be used in harsh environments. Among various proposals for die-attach materials and processes, Ag sinter joining is a promising approach for power semiconductors as well as for power LED,  providing excellent heat-resistant of stable joint structures beyond 200 °C. This paper summarizes the present status of the Ag sinter joining and of the new approach with Ag film bonding, both of which have developed by the authors’ group.

Sinter joining with Ag nanoparticles under pressure is an attractive choice of die-attach [1, 2]. Nevertheless, the high pressure beyond 5 MPa required for the joining may become a critical issue for thin and brittle semiconductor dies. In contrast, sinter joining with micron-sized Ag hybrid particle pastes provide a stable bonding structure at 200 ºC without applied high pressure. The presence of oxygen plays a key role in cleaning the surface of Ag at around 200 ºC in air resulting in a successful low-temperature low-pressure Ag sintering joining. Ag can clean its surface around 200 °C in air atmosphere. The Ag hybrid paste lowers its resistivity even below 200 °C in contrast to the Ag nanoparticle paste. At 200 °C, Ag coating on substrates/dies provides high strength due to Ag sintering ability in air. Thus, Ag sintering joining can be benefitted from the presence of oxygen, which is a great advantage as compared with other sintering materials.

Recently, the Ag thin film stress migration bonding method has been developed, providing a perfect bonding without any voids performed in ambient pressure at 250 ºC in air [3-5]. This joining has two key aspects. One is the reduction reaction of Ag oxides or of other Ag compounds on the surface of Ag at around 200 °C as mentioned above. The other is thermo-mechanical stress caused by thermal expansion mismatch between Ag plating layer and substrates, resulting in massive stress migration of Ag atoms from the bottom of the plating to the surface.

This paper briefly describes the current status of Ag sinter joining and Ag film stress migration bonding at our laboratory. Ag has great advantages in both the surface reaction in air benefitting joining quality and the excellent electric/thermal properties. Ag sinter joining has already exhibited a great potential in the market as high temperature interconnection technology. A new synthesis of Ag particles could lead to further low temperature sintering capability based on the reaction between Ag and oxygen. Our Ag film stress-migration bonding is expected to provide an alternative route for low temperature bonding. Similar bonding methods using Cu or other metal materials instead of Ag would be explored as cost-effective interconnections in future.

References

[1]     K. Suganuma, S.-J. Kim, K.-S. Kim, “High-temperature lead-free solders: Properties and possibilities”, JOM, vol.61, 64-71 (2009).

[2]     K. Suganuma, S. Sakamoto, N. Kagami, D. Wakuda, K. -S. Kim, M. Nogi, “Low-temperature low-pressure die attach with hybrid silver particle paste”, Microelectron. Reliab., vol.52, 375-380 (2012).

[3]     C. Oh, S. Nagao, T. Kunimune, K. Suganuma, “Pressureless wafer bonding by turning hillocks into abnormal grain growth in Ag films”, Appl. Phys. Letters, vol.104, 161603 (2014).

[4]     T. Kunimune, M. Kuramoto, S. Ogawa, T. Sugahara, S. Nagao, K. Suganuma,　”Ultra thermal stability of LED die-attach achieved by pressureless Ag stress-migration bonding at low temperature”, Acta Materialia, 89(2015), 133–140.

[5]     C. Oh, S. Nagao, K. Suganuma, “Silver Stress Migration Bonding Driven by Thermomechanical Stress with Various Substrates”, J. Mater. Sci. Mater. Electron, (2015), in press.