BOSTON Several reports at the recent fall meeting of the Materials Research Society revealed some promising new approaches to wafer bonding as a way to integrate diverse electronic materials. While wafer bonding is gaining ground commercially for silicon-on-insulator applications, the basic technique could have a much wider application for integrating optoelectronic materials or microelectromechanical systems (MEMS) with standard CMOS circuits.
One example is a laser liftoff and bonding technique devised at the University of California, Berkeley, for incorporating gallium-nitride devices into silicon circuits. The process starts by growing GaN films on sapphire using a standard process. The sapphire wafers are then bonded face down with epoxy onto silicon wafers. The GaN films are then separated by shining a laser through the back of the sapphire wafer. Short, intense pulses from an excimer laser were scanned back and forth across the wafer to separate the GaN from the sapphire. Analysis of the films showed that the process does not introduce any irregularities into the films.
A group at Sandia National Laboratories (Albuquerque, N.M.) is tackling GaN integration by using a technique similar to silicon-on-insulator. While it's possible to grow high-quality GaN films on silicon substrates that have been sliced at the right angle, the specific lattice direction, known as (111), is not industry standard. The Sandia group bonded a (111) wafer to a standard (100) wafer, which had a thin silicon dioxide layer on top. The (111) wafer was first treated with a hydrogen implant, which creates small bubbles below the wafer surface. When the two-wafer system is heated during bonding, the small bubbles expand, breaking off a thin (111) layer of silicon. The result is a silicon-on-insulator system that could include GaN devices in addition to CMOS transistors.
Research at Cornell University (Ithaca, N.Y.) is using wafer bonding to combine MEMS with optoelectronic devices. The technique is able to integrate surface-emitting laser and detector arrays along with optical MEMS devices to create sophisticated chemical-detection devices. A group at Chalmers University of Technology (Goteborg, Sweden) has created a variation of silicon-on-insulator that uses a diamond film as the isolation layer. Polycrystalline diamond films are first deposited on a standard silicon substrate, followed by a polycrystalline silicon layer that is then polished. The wafer is then bonded to a second silicon wafer, which is ground down to create a device-quality silicon layer over the diamond/polysilicon layer. In addition to providing the dielectric isolation of standard SOI systems, the diamond adds high thermal conductivity, which could aid in cooling circuits.