Microjoining of LSI Chips on Poly(ethylene naphthalate) Using Compliant Bump

Abstract

We show that microjoining of LSI chips on a film made of poly(ethylene naphthalate) (PEN), whose glass transition temperature is 155 °C, can be realized by using a cone-shaped compliant bump for flexible electronics. A 20-µm-pitch area array of cone-shaped Au bumps was fabricated on a Si wafer by photolithography and electroplating. The counter electrode on the PEN film was composed of a Au (top)/Ni/Al (bottom) layered structure, where Ni and Au layers were deposited by electroless plating on patterned Al. The bonding was carried out at 150 °C. More than 10,000 connections with 108 mΩ/bump were achieved. Mechanical analysis using finite element method indicates that the joining is due to deformation of the cone-shaped Au bumps.

Ultrasonic Bonding of Cone Bump for Integration of Large-Scale Integrated Circuits in Flexible Electronics

“…The resistance for a bump connection is evaluated to be 83.4 m from the slope of the plot. This value is almost the same value as the resistance obtained from thermo-compression bonding at 150 C. 16) When flat-top bumps were used instead of the cone bumps, number of connected bumps decreased and resistance increased because of generation of bonding failure as shown in Fig. 5(d).…”

“…[18][19][20] In the previous work, solid phase bonding of Au-Au was achieved even at 150 C owing to its mechanical compliance. 16) In addition, we have shown that combination of the cone bump and a counter electrode with a crossshaped slit enables the bonding of LSI chips to circuits on a plastic film at room temperature. 17) However, the formation of the slit in the counter electrode requires additional photolithography process which increases the fabrication cost.…”

“…To realize LSI chip bonding on a plastic film, we have been studying bonding using a cone-shaped microbump. 16,17) The cone bump has been developed to realize high-density inter-chip connections and three-dimensional (3D) staking of LSIs. [18][19][20] In the previous work, solid phase bonding of Au-Au was achieved even at 150 C owing to its mechanical compliance.…”

Room-Temperature Microjoining of LSI Chips on Poly(ethylene naphthalate) Film Using Mechanical Caulking of Au Cone Bump

“…When we analyzed the deformation of the PEN film during bonding by using the finite element method (FEM), the deformation of the PEN film under the Au/Ni/Al electrode was only 0.15 m under the application of 1.0 gf/bump. 17) From the simulation, we also found that the deformation of the PEN film is 0.08 and 0.12 m under the application of 0.5 and 0.75 gf/bump, respectively. Since the cross-shaped slit is not formed in the simulation and the thickness of the Ni obtained by the experiment is about 2 m smaller than that in the simulation, the deformation of the PEN film might be slightly larger than the simulation results under the application of a pressing load.…”

“…Recently, we have shown that the use of cone-shaped compliant bumps 15,16) enables the bonding of a Si chip to metal electrodes on a PEN film at 150 C. 17) However, 150 C is close to the glass transition temperature of PEN, 155 C, and therefore softening of the PEN film surface may take place as was suggested by the experimental results described in a previous report. 17) Therefore, further decrease in bonding temperature is required. Our target is bonding at room temperature.…”