• Korean Journal of Metals and Materials
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• v.46 no.9
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• pp.585-592
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• 2008

Flip-chip bonding using Cu-Sn mushroom bumps composed of Cu pillar and Sn cap was accomplished, and the contact resistance and the thermal cycling reliability of the Cu-Sn mushroom bump joints were compared with those of the Sn planar bump joints. With flip-chip process at a same bonding stress, both the Cu-Sn mushroom bump joints and the Sn planar bump joints exhibited an almost identical average contact resistance. With increasing a bonding stress from 32 MPa to 44MPa, the average contact resistances of the Cu-Sn mushroom bump joints and the Sn planar bump joints became reduced from $30m{\Omega}/bump$ to $25m{\Omega}/bump$ due to heavier plastic deformation of the bumps. The Cu-Sn mushroom bump joints exhibited a superior thermal cycling reliability to that of the Sn planar bump joints at a bonding stress of 32 MPa. While the contact resistance characteristics of the Cu-Sn mushroom bump joints were not deteriorated even after 1000 thermal cycles ranging between $-40^{\circ}C$ and $80^{\circ}C$, the contact resistance of the Sn planar bump joints substantially increased with thermal cycling.

• Journal of the Microelectronics and Packaging Society
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• v.15 no.3
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• pp.9-17
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• 2008

Cu mushroom bumps were formed by electrodeposition and flip-chip bonded to Sn substrate pads. Contact resistances of the Cu-mushroom-bump joints were measured and compared with those of the Sn-planar-bump joints. The Cu-mushroom-bump joints, processed at bonding stresses ranging from 19.1 to 95.2 MPa, exhibited contact resistances near $15m\Omega$/bump. Superior contact-resistance characteristics to those of the Sn-planar-bump joints were obtained with the Cu-mushroom-bump joints. Contact resistance of the Cu-mushroom-bump joints was not dependent upon the thickness of the as-elecroplated Sn-capcoating layer ranging from $1{\mu}m$ to $4{\mu}m$. When the Sn-cap-coating layer was reflowed, however, the contact resistance was greatly affected by the thickness and the reflow time of the Sn-cap-coating layer.

• Journal of the Microelectronics and Packaging Society
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• v.10 no.4
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• pp.39-46
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• 2003

Sn-Cu eutectic solder bump was fabricated by electroplating for flip chip and its characteristics were studied. A Si-wafer was used as a substrate and the UBM(Under Bump Metallization) of Al(400 nm)/Cu(300 nm)/Ni(400 nm)/Au(20 nm) was coated sequentially from the substrate to the top by an electron beam evaporator. The experimental results showed that the plating ratio of the Sn-Cu increased from 0.25 to 2.7 $\mu\textrm{m}$/min with the current density of 1 to 8 A/d$\m^2$. In this range of current density the plated Sn-Cu maintains its composition nearly constant level as Sn-0.9∼1.4 wt%/Cu. The solder bump of typical mushroom shape with its stem diameter of 120 $\mu\textrm{m}$ was formed through plating at 5 A/d$\m^2$ for 2 hrs. The mushroom bump changed its shape to the spherical type of 140 $\mu\textrm{m}$ diameter by air reflow at $260^{\circ}C$. The homogeneity of chemical composition for the solder bump was examined, and Sn content in the mushroom bump appears to be uneven. However, the Sn distributed more uniformly through an air reflow.

• Journal of the Microelectronics and Packaging Society
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• v.8 no.1
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• pp.33-38
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• 2001

We investigate the via-size dependance of as-electroplated- and reflow-bump shapes for realizing both high-density and high-aspect ratio of solder bump. The solder bump is fabricated by subsequent processes as follows. After sputtering a TiW/Al electrode on a 5-inch Si-wafer, a thick photoresist for via formation it obtained by multiple-codling method and then vias with various diameters are defined by a conventional photolithography technique using a contact alinger with an I-line source. After via formation the under ball metallurgy (UBM) structure with Ti-adhesion and Cu-seed layers is sputtered on a sample. Cu-layer and Sn/pb-layer with a competition ratio of 6 to 4 are electroplated by a selective electroplating method. The reflow-bump diameters at bottom are unchanged, compared with as-electroplated diameters. As-electroplated- and reflow-bump shapes, however, depend significantly on the via size. The heights of as-electroplated and reflow bumps increase with the larger cia, while the aspect ratio of bump decreases. The nearest bumps may be touched by decreasing the bump pitch in order to obtain high-density bump. The touching between the nearest bumps occurs during the overplating procedure rather than the reflowing procedure because the mushroom diameter formed by overplating is larger than the reflow-bump diameter. The arrangement as zig-zag rows can be effective for realizing the flip-chip-interconnect bump with both high-density and high-aspect ratio.

• Journal of the Microelectronics and Packaging Society
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• v.7 no.4
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• pp.23-29
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• 2000

We demonstrate the fabrication method of high-density and high-quality solder bump solving a copper (Cu) cross-contamination in Si-LSI laboratory. The Cu cross-contamination is solved by separating solder-bump process by two steps. Former is via-formation process excluding Cu/Ti under ball metallurgy (UBM) layer sputtering in Si-LSI laboratory. Latter is electroplating process including Ti-adhesion and Cu-seed layers sputtering out of Si-LSI laboratory. Thick photoresist (PR) is achieved by a multiple coating method. After TiW/Al-electrode sputtering for electroplating and via formation in Si-LSI laboratory, Cu/Ti UBM layer is sputtered on sample. The Cu-seed layer on the PR is etched during Cu-electroplating with low-electroplating rate due to a difference in resistance of UBM layer between via bottom and PR. Therefore Cu-buffer layer can be electroplated selectively at the via bottom. After etching the Ti-adhesion layer on the PR, Sn/Pb solder layer with a composition of 60/40 is electroplated using a tin-lead electroplating bath with a metal stoichiometry of 60/40 (weight percent ratio). Scanning electron microscope image shows that the fabricated solder bump is high-uniformity and high-quality as well as symmetric mushroom shape. The solder bumps with even 40/60 $\mu\textrm{m}$ in diameter/pitch do not touch during electroplating and reflow procedures. The solder-bump process of high-uniformity and high-density with the Cu cross-contamination free in Si-LSI laboratory will be effective for electronic microwave application.