Keiichiro Iwanabe

Room-temperature microjoining using ultrasonic bonding of compliant bump

Ultrasonic bonding was applied to cone-shaped microbump made of Au on a Si chip to perform bonding at room temperature. Array of cone-shaped microbump having 10 μm in diameter and 20μm in pitch was formed on Si using photolithography and electroplating. The counter electrode was a planar electrode which was also made by using electroplating of Au. Bonding was carried out at room temperature under conventional air circumstances. It has been found that bump array whose number is over 10,000 can be electrically connected. Die share tests have revealed that bonding strength can be increased to the strength that bonding at 150°C provides.

Room-temperature bonding of heterogeneous materials for near-infrared image sensor

A room-temperature bonding technique using cone-shaped microbumps with the aid of ultrasonic vibration is applied to the fabrication of a near-infrared (NIR) image sensor. The image sensor is fabricated using the chip-on-chip integration of an InGaAs photodiode array on an InP substrate and a Si CMOS readout IC. The pixel pitch is 25 µm to compose quarter-VGA class (320 × 256 pixels) resolution. A high-quality imaging of a heated object is demonstrated. Bonding of the VGA array with 15 µm pitch is attempted to realize a high-resolution image sensor.

Room-temperature hermetic sealing by ultrasonic bonding with Au compliant rim

We newly introduce a compliant rim to realize hermetic sealing of electronic components at low temperature. The compliant rim easily deforms under pressing load owing to its cone-shaped cross section and, therefore, intermetallic bonding can be performed at low temperature. We demonstrate the room-temperature vacuum sealing using the compliant rim made of Au with the aid of ultrasonic vibration of submicron amplitude. A test vehicle fabricated using silicon and glass showed that the air leak rate of the room-temperature sealing was well below 1 ? 10?12 Pa?m3/s, which is sufficiently low for use in vacuum packaging.

Analysis of room-temperature bonded compliant bump with ultrasonic bonding

Room temperature microjoining of Au or Cu bumps in the air ambient has been achieved by bonding cone-shaped microbumps with ultrasonic application. This technology has been applied to fabrication of near infrared (NIR) image sensor of q-VGA (quarter video graphic array) resolution, where InGaAs/InP photosensor array is joined with CMOS read out in the pixel level and, therefore, low temperature bonding is strongly required to address problems caused by mismatch in thermal expansion of the two materials. In this work, we investigate bonding mechanism of the cone shaped microbump using bumps made of Au. Die shear tests shows that shear strength of the bonded chips is proportional to the bonded contact area of the bump and that room temperature bonding gives sufficient bonding strength for applications while bonding at elevated temperature results in higher bonding strength. Analysis of change in bump height shows that “softening” of Au bump takes place under the application of ultrasonic vibration. Transmission electron microscopy shows that crystal grains at the bonded interface transform to small crystallites.

Room-temperature high-density interconnection using ultrasonic bonding of cone bump for heterogeneous integration

We show that room temperature bonding of a 332 × 268 bump array can be realized by using ultrasonic bonding of cone-shaped bump. 25 μm-pitch area array of cone-shaped Au bump was fabricated on a Si wafer by using a photolithography and electroplating. A conventional planar electrode made of electroplated Au was used as the counter electrode. Ultrasonic bonding was carried out at room temperature in ambient air. Electrical connection test shows all bump connections with low resistance have been achieved. Heterogeneous integration of a photodiode array on InP and Si CMOS readout IC is demonstrated.

Bonding dynamics of compliant microbump during ultrasonic bonding investigated by using Si strain gauge

The bonding dynamics of a cone-shaped microbump during ultrasonic bonding are investigated by in situ measurements of the strain generated in a substrate using a piezoresistance strain sensor. The strain sensor is composed of a pair of p- and n-type piezoresistance gauges to extract strain components in the ultrasonic vibration along the plane parallel to the substrate surface and along the direction perpendicular to the surface. Flip-chip bonding is performed at room-temperature. The time evolution of the strain generated in the substrate according to the load-up of pressing force and application of ultrasonic vibration is clearly detected. The softening of the bump metal during the application of ultrasonic vibration is clearly observed. Results of a comparative study between the bonding of a cone-shaped microbump and that of a flat-top microbump suggest mechanical stress concentration near the top end of the cone-shaped microbump, which results in the transformation of the crystal texture of the bump from grains to fine crystallites.

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

This paper reports that room-temperature bonding of LSI chips to metalization on a plastic film made of poly(ethylene naphthalate) (PEN) can be realized by ultrasonic bonding of a cone-shaped microbump made of Au. A 20-µm-pitch area array of cone-shaped Au microbumps 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 Al interconnection. Bonding of 8,800 bump connections with 83.4 mΩ/bump has been achieved at room temperature.

Sensing local dynamic strain and temperature evolution during ultrasonic bonding of microbumps

We have developed a strain gauge sensor to measure two axial dynamic strain and temperature evolution during ultrasonic flip-chip bonding. The sensor detects strain from change in resistance due to the piezoresistance effect of Si and temperature from change in current-voltage characteristics of pn-junction. The spatial resolution of the sensor is 20 μm. Au planar microbumps were used for the measurement. The number of bumps in the test chip was 12100. The measurement results of dynamic strain indicated the generation of a large strain at the initial stage of ultrasonic vibration. On the other hand, rise in temperature up to only a few degrees was observed during the ultrasonic vibration. Therefore, the damage is considered to be generated by mechanical strain at the initial stage of ultrasonic vibration. The sensing method and results will contribute to improving the reliability of three dimensionally integrated electronic devices.

In-situ strain measurement of ultrasonic ball bonding

Dynamic change in distribution of strain generated in Si under a pad electrode was measured during ultrasonic ball bonding by using newly developed Si strain sensor. The sensor was designed to be able to determine strains in the directions normal and parallel to the surface. Bonding of Cu and Au was measured. It was clearly observed that the position of the largest compressive strain moved from the center of the ball to the periphery according to the progress of bonding under the application of the ultrasonic. Bonding of Cu was found to generate larger strain than bonding of Au. Bonding of Cu was also found to induce under-pad damage. SEM observation of bonded balls suggested that the under-pad damage appeared at the positon where the edge of the capillary existed and at the time when the large force of ultrasonic vibration was applied.

Dynamic Strain of Ultrasonic Cu and Au Ball Bonding Measured In-Situ by Using Silicon Piezoresistive Sensor

Dynamic changes in distribution of mechanical strain generated during wire bonding in Si under and near the bonding pad were measured by using a piezoresistive linear array sensor. The sensor was designed to be able to determine strains in the directions normal and parallel to the surface. Bonding dynamics of Cu and Au balls were investigated. We can clearly observe the oscillating strain according to the application of 150 kHz ultrasonic vibration. It was also clearly observed that the position of the largest compressive strain moved from the center of the ball to the periphery according to the progress of bonding under the application of the ultrasonic vibration. Bonding of Cu was found to generate larger strain than bonding of Au. A large oscillating tensile strain generated at the periphery of Cu ball when ultrasonic amplitude is increased is found to cause fracture of Si. The largest residual strain is observed for Cu bonding at the location where the end of capillary tool was present during bonding.