Jerry Leonard

Integration of deformable mirror devices with optical fibers and waveguides

Deformable Mirror Devices (DMDs) have been under development at Texas Instruments for several years, primarily as spatial light modulators for free-space optical applications such as analog phase modulation and digital projection imaging. A DMD consists of one or more electrostatically deflectable micromechanical aluminum mirror elements, including both micromirrors suspended from thin flexible hinges and membranes. These devices are fabricated using low temperature silicon-compatible semiconductor processing techniques, and thus can be monolithically fabricated over any addressing circuitry. In the last few years DMDs have been integrated into optical fiber switching systems, and efforts are underway to integrate them as routing switches onto optical waveguides. The DMDs used for optical fiber switching are torsion-hinged devices similar to those used for projection imaging. These devices have been integrated with multimode fibers to construct a 4 X 4 multimode optical fiber cross-bar switch with a 19 dB optical (80:1) extinction ratio for all 16 channels. Extinction ratios of 73 dB optical (20 X 106) have been achieved for single point single mode switches. The waveguide switches currently under development are deformable membranes which are monolithically fabricated on silicon wafers with phosphosilicate glass (PSG) waveguide directional couplers to form optical time delay path selection switches. In this paper we describe the fabrication of deformable mirrors, their integration with optical fibers and waveguides, and the resulting system performance.

Phosphosilicate glass waveguides for phased-array radar time delay

The design, fabrication, and performance characteristics of passive waveguides and micromechanical switches for use as time delays for phased-array antennas are discussed. The approach considered here is to fabricate the optical waveguides and switches on silicon substrates using VLSI-compatible technology. This approach permits the use of a single off-chip emitter and detector to perform RF-optical and optical-RF conversion for each delay network. It also allows precise lithographic definition of the shorter delay waveguides. Micromechanical waveguide routing switches of two types designed to perform the path selection functions are considered.

An improved process for manufacturing diffractive optical elements (DOEs) for off-axis illumination systems

We present advancements in the manufacture of high-performance diffractive optical elements (DOEs) used in stepper/scanner off-axis illumination systems. These advancements have been made by employing high resolution lithographic techniques, in combination with precision glass-etching capabilities. Enhanced performance of DOE designs is demonstrated, including higher efficiency with improved uniformity for multi-pole illumination at the pupil plane, while maintaining low on-axis intensity. Theoretical predictions of the performance for several classes of DOE designs will be presented and compared with experimental results. This new process capability results in improved performance of current DOE designs, and enables greater customization including control of the output spatial intensity distribution for future designs. These advancements will facilitate continuous improvements in off-axis illumination optimization required by the end user to obtain larger effective lithographic process windows.

Silica-based optical delay lines and switches for phased-array radar control

We report our progress on integrated phosphorus-doped SiO2 optical waveguide delay lines and membrane optical routing switches for phased-array radar control. We have completed the design and layout of the delay lines for the two shortest bits. We have demonstrated the concept of a microelectromechanical membrane optical routing switch with a Mach-Zehnder interferometer and a fixed aluminum thin film. Channel crosstalk values as low as -12.4 dB and -20.3 dB were measured with and without a 3 mm aluminum film, respectively. We have designed the membrane structure for the switch to have better yield, improved reliability, and lower excitation voltage.

Integrated silica-based optical switch for radar phase control

The development of low-loss, low-cost integrated optical switches is critical for radar phase control applications. We report recent progress in integrated phosphorous-doped SiO2 on Si (PSG) waveguide optical routing switches based on electro-static actuation of an aluminum membrane. We have demonstrated the membrane switching concept and its implementation feasibility with a Mach-Zehnder interferometer and a deposited aluminum thin film. Without the metal film, 95% of the output light was measured in the cross channel, with a channel crosstalk of -13.37 dB. With a 3 mm long aluminum film being deposited on one of the interferometer arms as a phase shifter, 94% of the output light was measured in bar channel. The switch structure is currently being optimized for better performance, and a deformable aluminum membrane switch is being developed.

Performance improvements of binary diffractive structures via optimization of the photolithography and dry etch processes

Increasingly stringent requirements on the performance of diffractive optical elements (DOEs) used in wafer scanner illumination systems are driving continuous improvements in their associated manufacturing processes. Specifically, these processes are designed to improve the output pattern uniformity of off-axis illumination systems to minimize degradation in the ultimate imaging performance of a lithographic tool. In this paper, we discuss performance improvements in both photolithographic patterning and RIE etching of fused silica diffractive optical structures. In summary, optimized photolithographic processes were developed to increase critical dimension uniformity and featuresize linearity across the substrate. The photoresist film thickness was also optimized for integration with an improved etch process. This etch process was itself optimized for pattern transfer fidelity, sidewall profile (wall angle, trench bottom flatness), and across-wafer etch depth uniformity. Improvements observed with these processes on idealized test structures (for ease of analysis) led to their implementation in product flows, with comparable increases in performance and yield on customer designs.