Optical Quilt Packaging: A New Chip-to-chip Optical Coupling and Alignment Technique
Detecting trace amounts of molecules, such as early stage disease biomarkers or explosives, requires sensing with very small quantities of sample. Optical measurement is a feasible way to do the sensitive chemical or biological measurement. Specially, the mid-infrared (MIR) region (wavelength, λ=4-12 ) of the electromagnetic spectrum contains unique fingerprints corresponding to vibrational modes of molecules. Measuring the amount of light absorbed by these modes allows for highly sensitive and selective molecular detection and has generated much attention in the fields of chemical and biological sensing and explosive imaging. The recent development of the quantum cascade laser (QCL) provides a compact MIR source, feasible for incorporation into on-chip optical systems as compact and portable platforms for sensing and imaging. However, incorporating a QCL into an on-chip chemical or biological sensor, or in a beam combining platform for imaging, is challenging with the currently available techniques. While on-chip optical systems can be realized monolithically, where all optical components (source, beam combining platform, detection medium, detector etc.) are fabricated on the QCL substrate, this scheme is not economically feasible as QCL material is very expensive (~$10k/sq-in). An alternative modular approach, where the system components are fabricated on separate substrates, requires labor extensive butt coupling, complex and expensive fiber alignment, or grating coupling to transmit light from the laser to the other system components. As a result, an improved semiconductor packaging technique for low-cost, highly efficient optical coupling is highly desirable in order to incorporate QCLs into quasi-monolithic on-chip optical systems. In this dissertation, a new and highly efficient optical coupling technique, Optical Quilt Packaging (OQP), is modeled, fabricated and characterized. OQP aligns waveguides of separate substrates by protruding, lithographically-defined interdigitated copper nodules for low-loss chip-to-chip optical coupling.
In this dissertation, the feasibility of OQP is firstly evaluated by theoretical modeling using the eigenmode expansion technique. Next, simulations are performed to quantify the coupling loss associated with the OQP devices. According to the simulation results, the optical coupling loss between a QCL laser and a low-cost Ge-on-Si waveguide can be no worse than 7 dB, when the inter-chip distance is 4 or lower. Additional waveguide geometries are also modeled to aid fabrication design and reduce the lateral misalignment loss. Next, conventional semiconductor fabrication processes are used to fabricate the first OQP chip, where two separate Ge-on-Si waveguides are aligned with an inter-chip distance of ~4.6 , and with a lateral misalignment of ~1 . Based on the simulation results, the expected coupling loss for the fabricated Ge-on-Si OQP device is ~5 dB. The inter-chip gap is reduced further in a second OQP chip to ~1.4 , where copper alignment nodules are fabricated by sputter deposition. Finally, an MIR quantum cascade laser is used to measure the optical coupling loss of the fabricated OQP sample. The MIR light is focused to the Ge-on-Si waveguide facet and corresponding light transmission through the waveguides is measured. The OQP inter-chip coupling loss was measured to be 9.0±0.1 dB when the inter-chip gap was in air and 4.1±0.3 dB when filled with index-matching As2S3 glass. These results represent lower loss than previously reported chip-to-chip MIR optical coupling via butt-coupling (~10 dB), fiber coupling (as low as ~10 dB), and grating coupling (tens of dB loss).
The initial optical coupling results suggest, OQP is a low-loss chip-to-chip MIR optical coupling technique. In the next phase of the OQP research, QCL chips will be incorporated in this new OQP fabrication process to realize an on-chip modular scheme for explosive imaging.
History
Date Created
2017-03-29Date Modified
2018-10-30Defense Date
2017-03-14Research Director(s)
Scott HowardCommittee Members
Douglas Hall Lei Liu Gary BernsteinDegree
- Doctor of Philosophy
Degree Level
- Doctoral Dissertation
Program Name
- Electrical Engineering