Our ongoing research integrating 1.55-µm III-V ridge waveguide gain elements (i.e., diode lasers and semiconductor optical amplifiers) co-axially aligned with and coupled to silicon oxy-nitride waveguides on silicon substrates has made significant strides in the past year.  We are working towards the goal of co-axially coupling III-V laser diodes and semiconductor optical amplifiers with waveguides on Si wafers; to do so, we use techniques consistent with fabricating waveguides on Si-CMOS wafers and integrating the III-V gain elements after all standard front- and back-end Si processing has been completed.

A novel micro-cleaving technique has been used to produce active ridge waveguide platelets on the order of 6 µm thick and 100 µm wide, with precisely controlled lengths (in the current work 300 ± 1.25 µm) and very high-quality end facets.  Typical ridge guide platelet lasers have thresholds under 30 mA.

Passive micro-cleaved platelets have been integrated within dielectric recesses etched through the oxy-nitride (SiO_xN_y) waveguides on a wafer so that the ridge and SiO_xN_y waveguides are co-axially aligned.  Transmission measurements indicate coupling losses are as low as 5 db with air filling the gaps between the waveguide ends, and measurements made through filled gaps indicate that the coupling losses can be reduced to below 1.5 dB with a high index (n = 2.2) dielectric fill.  Simulations indicate that with further optimization of the mode profile in the III-V waveguide, the loss can be reduced to below 1 dB.

We have also performed extensive device design and optimization for co-axial recess integration and have recently completed a comparison of co-axial coupling with the evanescently coupled III-V/Si hybrid integration approach recently introduced by researchers at UCSB and Intel.  The latter comparison revealed that the approach we have taken, co-axial end-fire coupling, and the UCSB/Intel approach, vertical evanescent coupling, are complementary, with each optimal for certain applications.  At the same time it pointed out a number of distinct advantages for co-axial coupling of recess-integrated platelet lasers including higher operating efficiency, smaller device footprint, greater flexibility in choice of materials, lower cost, higher modularity, and easier integration of different wavelength emitters ^[1] .

1. C. G. Fonstad, J. J. Rumpler, E. R. Barkley, J. M. Perkins, and S. Famenini, “Recess integration of micro-cleaved laser diode platelets with dielectric wave-guides on silicon,” in Proc. Novel In-plane Semiconductor Lasers Conference VII, Photonics West 2008, SPIE Conference Proc. vol. 6909O, pp. 1-8.

Optogenetics is commonly used for precision modulation of the activity of specific neurons within neural circuits ^[1] , but assessing the impact of optogenetic neural modulation on millisecond-timescale local and global circuit neural activity remains difficult.  We have developed a novel strategy for designing and fabricating silicon-based microelectrode arrays with customizable electrode locations, targetable to defined neural substrates distributed in a 3-D pattern throughout a neural network in the mammalian brain, and compatible with simultaneous use of a diversity of existing light delivery devices.  Our design of these 3-D electrode arrays provides for both easy electrical and mechanical assembly, and provides for scaling of arrays to up to 1000 neural recording channels and beyond.

Our approach relies upon a number of innovations at the material, structural, electrical, and data acquisition levels.  First, typical silicon-based electrodes that are arranged in a 1-dimensional linear array, or 2-dimensional comb-like fashion, often use linear or tetrode-style electrode locations along the comb’s fingers, with stereotyped spacing and pad sizes.  Our software-driven approach enables variable spacing and pad sizes, so that electrode geometries can be customized to the cellular properties of the brain circuits under investigation.  Second, to support the assembly of such electrode arrays into a 3-dimensional array, we have developed novel electrical and mechanical connector strategies to make the assembly as automated and reliable as possible.  Third, we have developed strategies for amplifying and acquiring data that simplify the use of these probes in an intact, in vivo, mammalian context.  Fourth, we have implemented hybrid electrodes that contain both a low-impedance metallic pad for recording of spike activity, as well as an indium tin oxide (ITO) pad that can report local field potentials (LFPs) without the photo-electrochemical artifacts common in optogenetics.  Finally, these 3-D probes are designed to be easy to use, from design to surgery.  We have developed a user-friendly interface that enables neuroscientists to specify probe geometries based upon neural target geometries and coordinates, and are developing supporting surgical and behavioral strategies for use of such arrays in vivo.

1. X. Han and E. S. Boyden, “Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution,” PLoS ONE, vol. 2, no. 3, p. e299, Mar. 2007.

The use of magnetic forces to improve fluidic self-assembly of micro-components has been investigated using Maxwell 3D to model the forces between Ni thin films on semiconductor device micro-pills and Sm-Co thin films patterned on target substrates ^[1] .  Orienting and restraining forces on pills far in excess of gravity are predicted, and it is found that the fall-off of these forces with pill-to-substrate separation can be engineered through the proper design of the Sm-Co patterns to retain only properly oriented pills ^{[1] [2]} .

Micro-scale hybrid assembly is a potentially important way of doing heterogeneous integration, i.e., of integrating new materials on silicon integrated circuits to obtain functionality not readily available from silicon device structures alone, and fluidic self-assembly is an attractive way to automate micro-scale assembly.  A serious limitation of fluidic self-assembly, however, is the lack of a good method for holding properly assembled components in place and accurately positioned until all of the components have been assembled and permanently bonded in place.  We have shown, based on our modeling, that suitably patterned magnetic films can be used to provide the forces necessary to retain, and to accurately orient and position, assembled micro-components.

Our motivation for pursuing micro-scale hybrid assembly is our general interest in doing optoelectronic integration, specifically of vertical cavity surface emitting lasers (VCSELS), edge-emitting lasers (EELs), and light emitting diodes (LEDs), with state-of-the-art, commercially processed Si-CMOS integrated circuits.  Our ongoing research integrating these devices on silicon described elsewhere in this report provides the context for this work and illustrates the types of applications we envision for magnetically assisted self-assembly using the results of this study.

Assembly experiments to verify and demonstrate the theoretical predictions are currently in progress using two sizes of 6-µm-thick pills (50 µm by 50 µm and 50 µm by 100 µm) and a variety of magnetic thin film patterns.  Recesses with different dimensions are also being studied ^[2] .

1. D. Cheng, “Theoretical and experimental study of magnetically assisted fluidic self assembly,” M.Eng. Thesis, Massachusetts Institute of Technology, Cambridge, June 2008.
2. D. Cheng, J. J. Rumpler, J. M. Perkins, M. Zahn, C. G. Fonstad, E. S. Cramer, R. W. Zuneska, and F. J. Cadieu, “Use of patterned magnetic films to retain and orient micro-components during fluidic assembly,” Journal of Applied Physics, vol. 105, p. 07C123, 2009.

Optoelectronic devices intimately integrated on silicon integrated circuits have long been sought for optical intercon-nect applications, optical communications modules, and–more recently–neural stimulation and sensing.  Toward this end we have recently demonstrated a new heterogeneous integration technique for integrating vertical cavity surface emitting lasers (VCSELs) and edge-emitting laser diodes (EELs) on silicon CMOS integrated circuits ^{[1] [2]} .

Fully processed and tested oxide-aperture VCSELs emitting at 850 nm have been fabricated as individual “pills” 55 µm in diameter and 8 µm tall with a disk contact on the n-type backside and a ring contact on the p-type, emitting top-side.  Similarly, 1.55-µm emitting micro-cleaved cavity EEL platelets 5 µm thick, 150 µm wide, and 300 µm long have also been fabricated.  Using a custom micro-pipette vacuum pick-up tool, these micro-laser pills and platelets have been placed on contact pads at the bottom of recesses etched though the dielectric over coating on a Si chip, and batch solder-bonded in place using a custom pressurized-diaphragm bonding apparatus.  Back-end processing of the chip then continues with surface planarization, contact via formation, and interconnect metal deposition and patterning.  An example of a completely integrated VCSEL pill appears in Figure 1.

No adverse effects are seen from fabricating laser diodes as freestanding pills and platelets.  Devices integrated in this manner show the same high performance as devices left on their native substrates and in fact have superior thermal characteristics, largely due to the better thermal conductivity of Si over that of GaAs and InP.

The technique demonstrated in this work offers numerous other advantages over alternative heterogeneous integration techniques.  Both the devices to be integrated, and the target circuit wafers, are fabricated under optimal conditions and are pre-tested and screened prior to integration to insure high yield.   Significantly, many different types of devices can be integrated on the same IC wafer, a feature unique to this approach.  Furthermore, the integration process effectively avoids thermal expansion mismatch limitations and wafer diameter mismatch issues, and it is compatible with parallel assembly techniques, such as fluidic self-assembly.

1. J. M. Perkins, and C. G. Fonstad, “ Full recess integration of small diameter low threshold VCSELs within Si-CMOS ICs,” Optics Express, vol. 16, no. 18 pp. 13955-13960, 2008.
2. J. J. Rumpler and C. G. Fonstad, Jr., “Continuous-wave electrically pumped 1.55 µm edge-emitting platelet ridge laser diodes on silicon,” IEEE Photonics Technology Letters, vol. 21, pp. 827-829, 2009.

Professor Ed Boyden uses light to precisely control neural activity.  His lab has invented safe, effective ways to deliver light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively ^[1] .  This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health, and his lab has developed animal models of epilepsy and Parkinson’s disease to explore the use of optical control to develop new therapies.

Professors Boyden and Fonstad have established a collaborative effort to use heterogeneous integration techniques developed in Fonstad’s laboratory to construct miniature linear probes to deliver light to activate and silence neural target regions along their length as desired ^[2] .  The goal is to develop mass-fabricatable multiple light guide microstructures produced using standard microfabrication techniques.  Each probe is a 200- to 250-micron-wide insertable micro-structure comprising many miniature lightguides running in parallel and delivering light to many points along the axis of insertion.  Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage. If 2-D arrays of such probes are built, multiple colors of light can be delivered to 3-dimensional patterns in the brain, at the resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics.  Such devices will allow the substrates that causally contribute to neurological and psychiatric disorders to be systematically analyzed via causal neural control tools.  Given recent efforts to test such reagents in nonhuman primates, these devices may also enable a new generation of optical neural control prosthetics, contributing directly to the alleviation of intractable brain disorders.

The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide (Figure 1), and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns ^[2] .  Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe functions as designed ^[2] .  The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable (Figure 2).  The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule.  This allows for increased modularity and control in initial probe-testing.  Work on the fabrication of visible-emitting platelet laser diodes to be integrated on a similar ferrule mating to the guides is in process.

1. X. Han and E. S. Boyden, “Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution,” PLoS ONE, vol. 2, no. 3, p. e299, Mar. 2007.
2. A. N. Zorzos, E. S. Boyden, and C. G. Fonstad, “A multi-waveguide Implantable probe for light delivery to distributed brain targets,” Applied Optics Letters vol. 35, no. 12, pp. 4133-4135, Dec. 2010.

Collaborators

• E. S. Boyden, MIT
• F. J Cadieu, Queens College of CUNY
• A. Postigo, Instituto de Micro-electronica, Madrid, Spain
• M. Zahn, MIT

Postdoctoral Associate

• J. Scholvin, EECS

Graduate Students

• S. Famenini, Res. Asst., EECS
• A. Zorzos, Graduate Fellow, Media Lab

Undergraduate Student

• K. Dozier, EECS

Support Staff

• M. Pegis, Admin. Asst. II

Publications

A. N. Zorzos, E. S. Boyden, and C. G. Fonstad, “A Multi-Waveguide Implantable Probe for Light Delivery to Distributed Brain Targets,” Applied Optics Letters, vol. 35, no. 12, pp. 4133-4135, Dec. 15, 2010.

A. N. Zorzos, C. G. Fonstad, and E. S. Boyden, “Integrated Laser Diodes and Multi-waveguide Needle Arrays for Opto-genetic Neural Studies,” Workshop on Compound Semiconductor Microwave Materials and Devices (WOCSEMMAD), Savannah, GA, February 20-23, 2011.

A. N. Zorzos, C. G. Fonstad, and E. S. Boyden, “Multi-waveguide Needle Arrays and Integrated Laser Diodes for Opto-genetic Neural Studies,” Technical Digest of the Workshop on Compound Semiconductor Devices and Integrated Circuits (WOCSDICE), Catania, Italy, May 29-June 1, 2011.

M. M. Doroudchi, K. P. Greenberg, A. N. Zorzos, W. W. Hauswirth, C. G. Fonstad, A. Horsager, and E. S. Boyden, “Towards Optogenetic Sensory Replacement,” 33rd Annual International IEEE Engineering in Medicine and Biology Conference, Boston, MA, Aug. 30-Sept. 3, 2011; invited