EU project discovers easier way to make 3D integrated circuits

January 28, 2008, Brussels, Belgium--According to the European Commission's Research Office, the EU-backed project Waferbonding and Active Passive Integration Technology and Implementation (WAPITI) has discovered an easier way to make complex 3D integrated circuits involving both optical and electronic elements, by using a new wafer bonding technique. The technique was developed by a European consortium, and with the right commercial backing it should help make Europe more competitive in communications and sensor technology.

Photonics is essential for telephone and computer communications because it manages the movement of information along optical fibers. Photonics devices are made on chips in the same way that electronic circuits are, by combining such elements as laser diodes, waveguides, and detectors. A few of these circuits use purely optical technology, but most use both photonic and electronic components.

One problem with photonics technology is that the growing sophistication and complexity of photonics devices is outpacing current manufacturing technology and knowledge. Photonic components are based on semiconductors such as gallium arsenide (GaAs) or indium phosphide (InP), whereas most electronic components use silicon. Therefore it is highly complicated and also expensive for manufacturers to work with two very different materials on the same chip.

The WAPITI project, which ran from June 2004 to September 2007, was coordinated by the Fraunhofer Institute for Telecommunications (Berlin, Germany), along with four other academic partners. The partners built optical elements which are known as active microring resonators. These work as power storage devices and are a vital part of the lasers that allow high-bandwidth communications signals to be transmitted across a range of laser frequencies. They can potentially also be used as wavelength converters for telecommunications and in monitoring applications.

The team created micro-rings with vertical connections that carry light in and out. These vertical connections allow production of smaller micro-rings which allow higher data rates to be produced. Creating accurate alignment of the bonded wafers was one of the most challenging aspects of the project. Wafers consist of slices of semiconductor material which are large enough to hold thousands of chips. Correct alignment is difficult, but even more so when two wafers are produced separately and then bonded together, because different wafer materials have different rates of thermal expansion during temperature changes. The team achieved very good results in wafer bonding by aligning InP and GaAs wafers of 50 mm in diameter.

The WAPITI partners are actively seeking a commercial opportunity to continue this research.