1. JTu4A.56.pdf CLEO:2014 © 2014 OSA
Optical Quilt Packaging: A New Chip-to-Chip Optical
Coupling and Alignment Process for Modular Sensors
Tahsin Ahmed1*
, Aamir A. Khan1
, Genevieve Vigil1
, Jason M. Kulick2
, Gary H. Bernstein1
,
Anthony J. Hoffman1
, and Scott S. Howard1
1
Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
2
Indiana Integrated Circuits, South Bend, IN 46617, USA
* e-mail: tahmed@nd.edu
Abstract: A wide-bandwidth, highly efficient method of inter-chip waveguide coupling suitable
for on-chip, mid-infrared sensing is discussed. Simulations and preliminary fabrication work on
laser-to-waveguide coupling are presented, with losses predicted to be better than 6 dB.
OCIS codes: (140.3325) Laser coupling; (280.3420) Laser sensors
On-chip optical sensors are compact platforms for sensitive, low-cost monitoring of chemical and biological species
in real time by measuring changes in light that interacts with analytes on a chip. An optical on-chip sensor can
comprise either discrete modular optical elements or monolithically integrated elements. Modular sensor systems
use discrete sources, detectors and interaction medium typically coupled through optical fibers or free-space optics
[1, 2]. Monolithic sensors, however, incorporate the optical sources and interaction medium on the same substrate,
which is typically that required to fabricate the source. Mid-infrared (MIR) monolithic or modular sensors are of
great interest because many biologically and chemically relevant molecules exhibit vibrational resonance in the MIR
spectrum (i.e., λ~3-14 µm) [3]. A commonly used MIR source is the quantum cascade laser (QCL). However, since
QCL wafer material is expensive (~$10k/sq-in), monolithic MIR sensors are not economically feasible. What is
needed is a modular approach to incorporating QCLs in quasi-monolithic sensors. In modular sensors, the optical
source and detectors must be coupled with the interaction medium via free-space grating coupling [4], or optical
fiber alignment. Grating coupling, which is more commonly used, suffers losses on the order of tens of dBs. Optical
fiber coupling adds a significant expense to packaging, and is especially difficult for MIR sensors due to the lack of
commercially available fiber for that range of wavelengths. Realization of an improved on-chip modular sensor
scheme requires an efficient and cost-effective inter-chip optical coupling technique.
Optical “Quilt Packaging®
” (OQP) is a novel microelectromechanical system (MEMS) based packaging
technique for low-cost, highly efficient optical coupling between MIR laser sources and interaction platforms.
Waveguides of separate substrates are aligned with sub-micron accuracy by protruding, lithographically defined
interdigitated copper nodules [5] placed on the side of the chip (Fig. 1). These Cu nodules can be soldered to
eliminate the possibility of misalignment due to mechanical vibration. These protruding Cu nodules are formed by
filling deep-etched wells with electroplated copper, and etching the lanes to expose the nodules. The chips are
coupled at their facets to reduce the optical loss that arises from inter-chip gaps. This technique can simultaneously
align an array of waveguides across the gap between separate chips. Optical waveguide chips made with both Si/Ge
and III-IV materials could be combined into a single heterogeneous platform using this technique using standard
fabrication processes [5]. In this paper we investigate 1) the feasibility of OQP by simulation, and 2) initial
fabrication results for Ge-on-Si OQP structure including tolerances.
To investigate the feasibility of OQP, inter-chip coupling loss was calculated using MEEP, a free software
package that performs calculation using the finite-difference time-domain (FDTD) method [6]. We investigated both
inter-chip separation and lateral misalignment effects on coupling loss. In the simulation, transmission between a
typical QCL ridge waveguide and a single-mode Ge-on-Si rib waveguide was calculated for λ = 8 μm [7]. Chip-to-
chip separation is varied from 10 m to 0 (contact) to calculate coupling loss as a function of inter-chip distance.
The coupling loss simulation, shown in Fig. 2, indicates that the coupling loss is better than 6 dB for a gap of less
than 4 µm, which is comparable to conventional off-chip coupling [1], but rapidly improves as the coupling gap is
reduced toward contact. A horn-shaped geometry can be employed to decrease coupling loss [8] due to lateral
misalignment.
We fabricated initial test OQP structures on a bare Si wafer (Fig. 3). The double-trench ridge waveguide
structures on two separate chips were aligned via Cu nodules with a resulting misalignment of ~1 µm; previous
simulations [8] showed that a horn waveguide results in only 0.2 dB more insertion loss at misalignments as large as
2 μm. The fabricated inter-chip waveguide-to-waveguide distance is ~10 m, which could be improved with new
designs, and would provide efficient coupling efficiency when the gap is filled with an index-matching As2S3
chalcogenide spin-on glass [8]. We have fabricated Ge-on-Si waveguide chips (Fig. 4) that, in future work, will be

2. JTu4A.56.pdf CLEO:2014 © 2014 OSA
coupled to QCL chips to make OQP modular on-chip sensors. These Ge-on-Si ridge waveguides were fabricated
using standard photolithography and reactive ion etching (RIE) processes on a Ge-on-Si wafer. The waveguide
facets are prepared by deep reactive ion etching (DRIE), which at the same time releases the extended part of the Cu
nodules used for the alignment. Cu nodules show great stability in the etching process, and were found to be
50.2±0.7 μm in width (designed as 50 μm) and 52.9±0.7 μm spacing between nodules (designed as 54 μm),
demonstrating sub-micron alignment capability.
In summary, simulation and fabrication studies suggest that OQP could be used efficiently in developing
MIR modular on-chip sensors for low-cost, highly efficient optical sensing. Future work includes the measurement
of the MIR optical losses between aligned waveguides on Ge-on-Si and QCLs.
References
[1]. Y.-C. Chang, P. Wägli, V. Paeder, A. Homsy, L. Hvozdara, P. van der Wal, J. Di Francesco, N. F. de Rooij, and H.P. Herzig, “Cocaine
detection by a mid-infrared waveguide integrated with a microfluidic chip,” Lab Chip 12, 17, 3020–3 (2012).
[2]. J. Chen, Z. Liu, C. Gmachl, and D. Sivco, “Silver halide fiber-based evanescent-wave liquid droplet sensing with room temperature mid-
infrared quantum cascade lasers,” Opt. Express 13, 16, 5953–5960 (2005).
[3]. F. K. Tittel, D. Richter, and A. Fried, “Mid-infrared laser applications in spectroscopy,” in Topics in Applied Physics: Solid-State Mid-
Infrared Laser Sources 89, 445-510 (2003).
[4]. J. Dübendorfer, and R. E. Kunz, “Compact integrated optical immunosensor using replicated chirped grating coupler sensor chips,” Appl.
Opt. 37, 10, 1890–1894 (1998).
[5]. G. H. Bernstein, Q. Liu, M. Yan, Z. Sun, W. Porod, G. Snider, and P. Fay, “Quilt packaging: high-density, high-speed interchip
communications,” IEEE T. Adv. Packaging 30, 731–740 (2007).
[6]. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and S. G. Johnson, “Meep: a flexible free-software package for
electromagnetic simulations by the FDTD method,” Comput. Phys. Commun. 181, 3, 687–702 (2010).
[7]. S. S. Howard, Z. L. Z. Liu, D. Wasserman, A. J. Hoffman, T. S. Ko, and C. F. Gmachl, “High-performance quantum cascade lasers:
optimized design through waveguide and thermal modeling,” IEEE J. Sel. Top. Quant. 13, 5, 1054-64 (2007).
[8]. T. Ahmed, T. Butler, A. A. Khan, J. M. Kulick, G. H. Bernstein, A. J. Hoffman, and S. S. Howard, "FDTD modeling of chip-to-chip
waveguide coupling via optical quilt packaging," Proc. SPIE 8844, 88440C (2013).
Fig. 1. OQP technique to couple two separate
waveguide dies.
Fig. 2. Coupling loss variation with inter-chip separation.
Fig. 3. Fabricated OQP structure on a Si wafer. Fig. 4. OQP Ge-on-Si waveguide chip with the SEM of Cu
nodules and a single waveguide.