Takanori Shuto

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.

Conductive polymer/metal composites for interconnect of flexible devices

An interconnect of flexible and foldable devices based on advanced electronics requires high electrical conductivity, flexibility, adhesiveness on a plastic substrate, and efficient productivity. In this study, we investigated the applicability of a conductive polymer/metal composite to the interconnect of flexible devices. By combining an inkjet process and a photochemical reaction, micropatterns of a polypyrrole/silver composite were formed on flexible plastic substrates with an average linewidth of approximately 70 µm within 10 min. The conductivity of the composite was improved to 6.0 × 102 Ω−1cm−1. From these results, it is expected that the conducting polymer/metal composite can be applied to the microwiring of flexible electronic devices.

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.

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

We show that room-temperature bonding of LSI chips on a resin film made of poly(ethylene naphthalate) (PEN) can be realized by using mechanical caulking of a cone-shaped bump electrode made of Au. A 20-µm-pitch area array of cone-shaped Au bumps was fabricated on a Si wafer by photolithography and electroplating. The counter electrode with cross-shaped slits 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. Bonding of about 10,000 bump connections with 184 mΩ/bump has been achieved at room temperature.

Microjoining of LSI chips on poly(ethylene naphthalate) using compliant bump

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

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.

Room-temperature vacuum packaging using ultrasonic bonding with Cu compliant rim

In this study, we demonstrate room-temperature vacuum sealing by combining Cu compliant rim with ultrasonic assist.