Optical microcircuits with electrical properties
Optical microcircuits that integrate electronic functions may be achievable thanks to a new bonding technique developed by a European consortium.
The Waferbonding and Active Passive Integration Technology and Implementation (WAPITI) scheme, coordinated by the Fraunhofer Institute for Telecommunications in Sankt Augustin, Germany, has demonstrated a technique to adhere gallium arsenide (GaAs) and indium phosphide (InP) substrates to create an active microring resonator. This could then be used to fuse the substrates to silicon wafers.
Photonic devices are often made with a hybrid of photonic and electronic components, but as these systems grow in complexity, it becomes more difficult to house the two components on one chip cost-effectively. Photonic structures are based on GaAs or InP semiconductors, while electronic components use silicon – all of which have varying thermal expansion rates.
Using InP and GaAs wafer substrates, researchers applied a two-layer technique to vertically couple the transparent waveguides that carry light in and out to the microrings. This enables rings with a radii down to 10µm to be created. The wafers are held together with benzocyclobutene (BCB).
‘The main challenge is shrinkage of the bonding layer, which is given inherently by BCB,’ says Dr Helmut Heidrich of Fraunhofer. ‘The consequence is stress and shrinkage of the epitaxial layer stack after removing the InP substrate.’ Using direct beam lithography, the team has overcome this problem and aligned the wafers accurately. Differing thermal expansion rates are minimised by decreasing the maximum bonding temperature from 300ºC to 200ºC.
The team is hoping to use a similar ‘step and repeat’ lithography techniqe to bond the 50mm InP and GaAs wafers to 300mm silicon wafers.
Viorel Dragoi, Chief Scientist at EV Group, a member of WAPITI based in St. Florian, Austria, says his company hopes to use the technology in its own projects, but ‘the challenges faced during the process show the need for new materials development.
‘The WAPITI project has manufactured demonstrator devices, but in order to transfer the process flow to an industrial environment, specific developments are required, such as yield and throughput analysis,’ he adds.