Low-temperature and low-pressure direct copper-to-copper bonding by highly (111)-oriented nanotwinned Cu

Chih Chen*, Chien Min Liu, Han Wen Lin, Yi Sa Huang, Yi Cheng Chu, Dian Rong Lyu, Kuan-Neng Chen, King-Ning Tu

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference contribution

4 Scopus citations

Abstract

As the size of microbumps continues to shrink, the amount of solder decreases gradually, resulting the brittleness of solder joints due to formation of intermetallic compounds. Low-temperature Cu-to-Cu direct bonding appears to be one of the solutions for fine-pitch microbumps for 3D IC packaging. However, the high bonding temperature and pressure are the main problems of this approach. We achieve low-temperature Cu-to-Cu direct bonding at low pressure and ordinary vacuum. In addition, the cleaning process is simple. To bond the electroplated Cu films, the samples were cut into 3 × 3-mm2 pieces. Then, the pieces were cleaned in acetone for 5 min, dried with a N2 purge, cleaned with a mixed solution of HCl and deionized water (DI) water for 30 sec, rinsed with DI water, and purged with N2 gas again. The bonding temperature can be lowered to 150°C at a compressive stress of 114 psi held for 60 min at 10-3 torr, or at 200°C at for 30 min. The temperature is lower than the reflow temperature of 250°C for most Pb-free solders. We achieve low-temperature bonding using electroplate highly (111)-oriented nanotwinned Cu (nt-Cu) films. Excellent bonding interface can be accomplished by bonding two highly (111)-oriented nt-Cu films. In addition, excellent interface with few voids can be achieved between a highly (111)-oriented nt-Cu film and a randomly-oriented Cu film. Our breakthrough is based on the finding that the (111) orientation of the Cu surface is critical for bonding. The diffusivity of Cu atoms on (111) surfaces is approximately 3-4 orders higher than other major planes. The (111) plane allows for fast surface diffusion, which enables low-temperature creep to occur. The bonded interface between two (111) surfaces forms a twist-type grain boundary (GB). If the GB has a low angle, it has a hexagonal network of screw dislocations. The hexagonal network image was obtained by plan-view transmission electron microscopy. A simple kinetic model of surface creep will be presented. Our breakthrough provide a potential solution with a huge impact to the interconnect materials in 3D IC. The detail results will be presented in the conference.

Original languageEnglish
Title of host publication2016 Pan Pacific Microelectronics Symposium, Pan Pacific 2016
PublisherInstitute of Electrical and Electronics Engineers Inc.
ISBN (Electronic)9780988887398
DOIs
StatePublished - 7 Mar 2016
EventPan Pacific Microelectronics Symposium, Pan Pacific 2016 - Big Island, United States
Duration: 25 Jan 201628 Jan 2016

Publication series

Name2016 Pan Pacific Microelectronics Symposium, Pan Pacific 2016

Conference

ConferencePan Pacific Microelectronics Symposium, Pan Pacific 2016
CountryUnited States
CityBig Island
Period25/01/1628/01/16

Keywords

  • 3D IC
  • Cu-to-Cu direct bonding

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