Microring resonators that are side-by-side with waveguides can be formed by photolithography. The distances and tolerances required for efficient coupling between the waveguides (around 0.3 µm), however, strain the limits of photolithography. Grover's group takes an alternative approach, using vertical stacking of the microrings and waveguides that can be grown during deposition. The method was proposed and demonstrated by Tishinin's group at the University of Southern California (Los Angeles, CA).2 "The advantage of our approach," said Grover, "is that it gives a symmetric refractive index profile, which allows us to make resonators on both sides of the epitaxial layers."
The UM group has also extended the work to InP materials, which can amplify light in the 1.5-µm region, and can be made into active devices. Their work demonstrates that sophisticated ring devices can be fabricated on InP using standard lithographic techniques without recourse to electron beam direct-write.
FABRICATION"We first fabricate rings or waveguides on one layer," says Grover, "bond the wafer to a carrier, flip it over, and fabricate rings and waveguides at the back." The breakthrough for this particular materials system has been the etching procedure, which allows alignment keys to be made through the device.To begin, an etch stop layer of lattice-matched GaInAs is deposited on an InP substrate. This is topped with a lower cladding, and then a pair of higher index GaInAsP core layers (one for the rings and the other for the bus) separated by a 500-nm-thick InP coupling layer. An upper cladding similar to the lower cladding and coupling layers follows (see Fig. 2). Alignment keys are exposed and dry-etched down to the etch stop layer. Then the first core layer is patterned and etched.Figure 2. Fabrication steps to make vertically coupled microring resonators with polymer wafer bonding. First, the researchers deposit two core layers (along with cladding, coupling layer, and etch stop). Then the alignment keys and top core layer are patterned. The structure is attached to a transfer substrate, the original substrate is removed, and the second core layer is patterned. (Photo courtesy of U. of Maryland)
After the patterning, the structure is bonded to a transfer substrate, and the growth substrate is removed. Then the second core layer is patterned and etched, using the keys to align it to the other core layer.
The group created high-Q microring resonator channel-dropping filters with free spectral ranges of 13 and 24 nm, and resonance bandwidths as narrow as 0.6 nm. The researchers expect to be able to optimize the devices for better performance.
Thus far, the group has made only four of the devices, so their manufacturability is still unknown. Grover says, however, "The bonding itself is quite good and works consistently since developed in our group by Philippe Absil."
Yvonne Carts-Powell
REFERENCES
- R. Grover et. al., Opt. Lett 26(8), 506 (April 15, 2001).
- D.V. Tishinin et. al., Photon. Tech. Letters 11, 1003 (1999).
- P. Absil et. al., Photon. Tech. Lett. 13(1), 49 (January 2001).
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.