Christopher L. Chua

Pseudomorphically Grown Ultraviolet C Photopumped Lasers on Bulk AlN Substrates

Optically pumped ultraviolet lasers were fabricated on low-defect-density bulk (0001) AlN substrates. AlxGa1-xN/AlyGa1-yN heterostructures were grown by metal–organic vapor phase epitaxy near atmospheric pressure. Time-resolved photoluminescence studies of the multiple quantum well emission show long decay times of 900 ps at room temperature and confirm the high structural quality of the epitaxial layers. Laser resonators with a length of about 1 mm were formed by cleaving the AlN crystal to obtain m-plane mirror facets. Lasing is demonstrated at a wavelength of 267 nm with a threshold power density as low as 126 kW/cm2 at room temperature. The laser emission was transverse electrically polarized.

High Q RF coils on silicon integrated circuits

We report on out-of-plane micro-machined inductors exhibiting record high quality factors (Q) on silicon integrated circuits. The coils are made by three-dimensional self-assembly of stress-engineered structures fabricated with standard semiconductor batch processing techniques. Coils fabricated on low resistance CMOS-compatible silicon exhibit quality factors of over 70 at 1 GHz. BiCMOS test oscillators utilizing these micro-machined coils show significant phase noise reduction over similar oscillators using conventional spiral coils.

Nano-spring arrays for high density interconnect

A new type of compliant interconnect derived from a thin metal film fabricated with a controlled stress profile is being developed for flip- flop interconnects and probing devices. Interconnections have been demonstrated on lateral pitches as tight as 6 microns. The interconnect is highly elastic and can provide up to hundreds of microns of vertical compliance.

Novel nanospring interconnects for high-density applications

A new cantilevered structure, called the nanospring, is being developed to enable a fine-pitch, high density I/O architecture for the next generation chip and probing technology. This technology meets the requirements of the National Technology Roadmap for Semiconductors (NTRS) for 2012 and beyond. Based on its unique structure, a new contact mode of sliding contact with no solder is being tested. To understand the reliability of the package with this novel compliant structure and the typical behavior of sliding contacts, test vehicles with different orientations of the nanospring (21 /spl mu/m pitch) have been fabricated, assembled and subjected to thermal cycling tests. In-situ resistance and temperature measurements have been conducted.

Hybrid integration of GaAs-based VCSEL array with amorphous silicon sensor

We describe an optoelectronic module incorporating a vertical-cavity surface-emitting laser (VCSEL) array with a semitransparent light monitor. The power monitor is a p-i-n amorphous silicon photodetector fabricated on glass. Sets of micromachined springs for electrical contacting are also fabricated in the same process on the same glass substrate. Hybrid packages are formed by pressing the compliant springs against individual contact pads of the GaAs VCSEL array in a flip-chip assembly process. The light sensor is aligned directly on top of the laser elements. Most of the laser light is transmitted through the sensor, yet a large dynamic range is maintained because of the sensors exceedingly low dark current.

Integration of red, infrared, and blue light sources by wafer fusion

In this work, we demonstrate fusion of GaAs-based laser structures to GaN-based light-emitting diode (LED) heterostructures. Successful operation of red and infrared lasers fused to functioning GaN LEDs is achieved. A single heterostructure consisting of AlGaInAs/AlGaAs quantum well (QW) and GaInP/AlGaInP QW laser diode structures was grown by low-pressure organometallic vapor phase epitaxy (OMVPE) on GaAs substrates. The GaN LED structure was grown by OMVPE on an A-face sapphire substrate. The heterostructures were fused at 650 degrees Celsius in an H2 ambient, while under uniaxial pressure. To fabricate the lasers, the GaAs substrate was selectively etched, leaving the red and infrared QW laser stack structure on GaN. Ridge waveguide QW lasers and GaN LEDs were fabricated with the fused epilayers. Infrared, AlGaInAs QW lasers (4 X 500 micrometer), operated with a threshold current (Ith) of 40 mA and external differential quantum efficiency ((eta) d) of 11.5%/facet at about 821 nm. Red, GaInP QW lasers (4 X 500 micrometer), operated with a Ith of 118 mA and (eta) d of 18.7%/facet at about 660 nm. The adjacent InGaN/GaN LED emitted at 446 nm.

Hybrid VCSEL modules with integrated amorphous silicon power monitors

We present a novel optoelectronic package incorporating Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays with built-in power monitors. The power monitor consists of a thin film amorphous silicon p-i-n photodetector that is fabricated on glass. Sets of micro-machined springs for electrical contacting are also fabricated in the same process on the same glass substrate. The springs are made by sputtering, masking, and releasing a stress-engineered conductive thin film. The stress-engineered film is patterned into electrical routing wires whose ends curl up into compliant springs when released from the substrate. Hybrid packages are formed by pressing the micro-machined springs against individual contact pads of the GaAs VCSEL array in a flip-chip assembly process. The power monitor is designed so it lies directly in front of the laser array in the path of the light after module assembly. Although only about 2% of the laser power is absorbed by the sensor, a large signal to noise ratio is retained because of the sensor’s extremely low dark current. Our typical laser output powers of about 1 mW at wavelengths of 811 nm produces power monitor photocurrents of 0.5 to 1 μA, which, for our detectors, correspond to dynamic ranges of over five orders of magnitude.

High-density optoelectronic interconnect using micromachined spring arrays

We report on a novel flip-chip packaging technology capable of interconnecting devices packed at very high density. The process utilizes micro-machined cantilevers for establishing electrical contact, where package assembly is performed at room temperature without solder. The cantilevers, called micro-springs, are fabricated by sputtering, masking, and releasing a stress-engineered conductive thin film on a quartz substrate. The film is patterned into electrical routing wires whose ends are released from the substrate. Upon release, the film stress forces the ends to curl up into compliant springs. Packages are formed by pressing the micro-springs against a set of device contact pads, much like probing pads using tungsten needle probes. The connections between springs and contact pads are anchored by an encapsulating acrylic adhesive. We utilize this packaging technology to interconnect 200-element arrays of independently addressable VCSELs with 4 micrometer-wide pads on 6 micrometer pitch to silicon CMOS driver chips with equally dense output lines. Tests show the technology produces good contacts with excellent robustness.

Densely packed surface-emitting laser arrays for printing applications

We report on our efforts to develop a laser printbar consisting of a very dense array of independently addressable laterally-oxidized top-emitting VCSELs. In order to maintain wafer planarity for easy electrical routing, the buried oxidation layer in our structure is accessed through small via holes instead of a more typical mesa etch. Unlike most VCSELs, our devices utilize transparent indium-tin- oxide top contacts that allow for a more compact device design. The 200-element array we fabricated has a linear density of one device every 3 micrometers .

Laterally oxidized vertical-cavity lasers with stable polarization

We disclose a method of eliminating the polarization instability in laterally-oxidized vertical-cavity surface- emitting lasers. By employing an appropriately-shaped device aperture, we are able to make the lasers operate in a single polarization direction through their entire L-I curve.