Through-Silicon Technology, Applications Growing

For years, people have been making contacts that go through the die, though the number of applications has been limited. In recent years, through-silicon contacts have been examined for use in MEMS packaging and for three-dimensional integration of ICs. Researchers at the Association of Super-Advanced Technologies (ASET, Tokyo) have developed a through-silicon contact technology that has found its way into a commercial camera-phone product. They presented their results at the latest IMAPS.

Previous applications for through-silicon contact technology include RF applications. Several years ago, through-silicon contact technology was used to allow RF die to be attached face-up in cases where it would be advantageous. For an increasing number of MEMS applications, it is either less expensive, more reliable, or both, to make the MEMS devices and their electronics on separate wafers, then bond them later. In many cases, the best choice is to make through-contacts next to the micro-machined device.

There have been some applications where through-silicon interconnects were used to integrate an ultrathin die with a larger die, mainly for the purpose of reducing package volume.

Recent interest in 3-D integration of ICs has increased the number of researchers working on through-silicon interconnection. In addition to increasing the interconnection density, through-silicon contacts can reduce power use by reducing overall wire lengths, while also providing thermal vias for heat removal. The majority of researchers in this area are using copper for vias, and a copper-tin intermetallic process for making connections between wafers, though there is a significant body of work on direct copper-to-copper bonding.

The ASET group has developed a copper-plated through-silicon process that produces 10 µm square vias that are 70 µm deep. The insulating sidewalls of the vias are ~200 nm thick. They have demonstrated the ability to make these vias on a 20 µm pitch. Their process also requires handing of ultrathin wafers, which they accomplish with glass handle wafers and UV-curable adhesives.

ASET’s process for forming through-silicon contacts uses copper vias, a copper-tin intermetallic joint, and a small amount of resin adhesive. (Source: ASET)

The backside of the wafers has a silicon nitride passivation layer. On the front side, a 5 µm copper bump is formed on top of the via, topped with 1.5 µm of tin. The tin thickness is chosen to ensure full incorporation of the tin in a Cu₃Sn intermetallic joint.

A resin adhesive is used to bond the wafers, but the amount used is small enough for the tops of the bumps to remain dry during the process, though there is enough to eliminate void areas. Even though both wafers in each bond were silicon, there were significant thermal mismatch concerns caused by the resin, which they were able to address successfully.

Ring oscillator tests on via chains indicate these through-silicon interconnects perform well enough to connect ICs with signal frequencies up to a few gigahertz.

The commercial application in which this technology has been used is a charge-coupled device (CCD) camera phone. Instead of placing the processing circuitry next to each detector pixel, or multiplexing the CCD signals off to nearby circuitry, through-silicon contacts were used to send the raw detector signals to processing circuitry on another die directly beneath. This allows the fill factor for each detector pixel to be high, while reducing the processing time. Since the raw signals are neither sent very far nor multiplexed, image signals are more likely to be accurate representations of what the detectors captured.

In the United States, there is a Defense Advanced Research Projects Agency (DARPA) project to produce sensors of this type. The Vertically Integrated Sensor Arrays (VISA) project has demanding targets for pixel size, dynamic range and frame rates that require an approach like vertical integration. In the case of the camera phone enabled by ASET's technology, the reduction of circuit complexity and of over-package volume are enough to justify using this approach.

Regardless of whether through-silicon technology will be used for 3-D IC integration, it will be useful in commercial applications as well as military and other applications. This work by ASET and the subsequent incorporation of its technology in a commercially available camera phone are evidence of this.