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Research Papers

Enhanced Bonding by Applied Current in Cu-to-Cu Joints Fabricated Using 20 μm Cu Microbumps

[+] Author and Article Information
Sung Woo Ma

Division of Nanoscale
Semiconductor Engineering,
Hanyang University,
Seoul 04763, South Korea

Chanho Shin

SK Hynix, Inc.,
San 136-1 Ami-ri Bubal-eub,
Icheon-si 467-701,
Kyoungki-do 17327, South Korea

Young-Ho Kim

Division of Nanoscale
Semiconductor Engineering;
Division of Advanced Materials and
Engineering,
Hanyang University,
Seoul 04763, South Korea
e-mail: kimyh@hanyang.ac.kr

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received April 25, 2017; final manuscript received July 26, 2017; published online September 5, 2017. Assoc. Editor: Yi-Shao Lai.

J. Electron. Packag 139(4), 041004 (Sep 05, 2017) (7 pages) Paper No: EP-17-1044; doi: 10.1115/1.4037474 History: Received April 25, 2017; Revised July 26, 2017

The effect of applied current in enhancing bonding was studied in Cu-to-Cu direct bonding using Cu microbumps. A daisy-chain structure of electroplated Cu microbumps (20 μm × 20 μm) was fabricated on Si wafer. Cu-to-Cu bonding was performed in ambient atmosphere at 200–300 °C for 10 min under 260 MPa, during which direct current of 0–10 A (2.5 × 106 A/cm2) was applied. With increasing applied current, the contact resistance decreased and the shear strength in the Cu-to-Cu joints increased. The enhanced bonding imparted by the application of current was ascribed to Joule heating and electromigration effects. Subsequently, the joint temperature was calibrated to isolate the electromigration effects for study. In Cu-to-Cu joints joined at the same adjusted temperature, increasing the current caused unbonded regions to decrease and regions of cohesive failure to increase. The enhanced diffusion across the Cu/Cu interfaces under the applied current was the main mechanism whereby the quality of the Cu-to-Cu joints was improved.

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Figures

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Fig. 1

Schematic diagram showing bonding: (a) before bonding and (b) during bonding

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Fig. 2

Schematic diagram showing the formation of 20 μm Cu microbumps: (a) metal deposition, (b) metal pattering, (c) thick PR coating and via hole formation, (d) Cu electroplating, and (e) PR strip and seed layer etching

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Fig. 3

SEM images of Cu surface: (a) before and (b) after enhancement

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Fig. 4

Back-scattered electron images of (a) 20 μm Cu microbumps on Au/Cu/Ti metal lines and (b) Cu pads

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Fig. 5

(a) Contact resistance and (b) specific contact resistance versus applied currents for various setting temperatures

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Fig. 6

Shear strength versus applied currents for various setting temperatures

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Fig. 7

Cross-sectional SEM images of Cu-to-Cu joints bonded at 300 °C setting temperature: (a) without applied current and (b) with the applied current of 10 A

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Fig. 8

Schematic illustration of bonding temperatures

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Fig. 9

Top surface temperature versus setting temperature for various applied currents

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Fig. 10

Joint temperature versus setting temperature for various applied currents. The dotted rectangle indicates data points collected for the same compensated joint temperature.

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Fig. 11

Contact resistance versus joint temperature for various applied currents. The dotted rectangle indicates data points collected for the same compensated joint temperature.

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Fig. 12

Shear strength versus joint temperature for various applied currents. The dotted rectangle indicates data points collected for the same compensated joint temperature.

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Fig. 13

Cross-sectional SEM images acquired after current-assisted bonding. All joints were bonded at 245 °C; applied currents were (a) 0, (b) 5, and (c) 10 A.

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Fig. 14

Ratios of unbonded regions in joints joined at 245 °C under various levels of applied current

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Fig. 15

Cross-sectional focused ion beam images acquired after current-assisted bonding. The joint was bonded at 245 °C, and applied current was 10 A.

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Fig. 16

Fracture modes observed after shear testing of samples bonded at 245 °C under 0–10 A direct current: (a) mode A1: Cu-to-Cu interfacial failure, (b) mode A2: Cu-to-Cu interfacial failure, and (c) mode B: cohesion failure within 20 μm Cu microbump and Cu-to-Cu interface

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Fig. 17

SEM images of fracture surfaces of Cu pads and Cu bumps joined at 245 °C

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Fig. 18

Fracture mode incidence in joints joined at 245 °C under various levels of applied current

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