Daniel P. Kelly

InP-based optical waveguide MEMS switches with evanescent coupling mechanism

An optical waveguide MEMS switch fabricated on an indium phosphide (InP) substrate for operation at 1550 nm wavelength is presented. Compared to other MEMS optical switches, which typically use relatively large mirrors or long end-coupled waveguides, our device uses a parallel switching mechanism. The device utilizes evanescent coupling between two closely-spaced waveguides fabricated side by side. Coupling is controlled by changing the gap and the coupling length between the two waveguides via electrostatic pull-in. This enables both optical switching and variable optical coupling at voltages below 10 V. Channel isolation as high as -47 dB and coupling efficiencies of up to 66% were obtained with switching losses of less than 0.5 dB. We also demonstrate voltage-controlled variable optical coupling over a 17.4 dB dynamic range. The devices are compact with 2 /spl mu/m/spl times/2 /spl mu/m core cross section and active area as small as 500 /spl mu/m/spl times/5 /spl mu/m. Due to the small travel range of the waveguides, fast operation is obtained with switching times as short as 4 /spl mu/s. Future devices can be scaled down to less than 1 /spl mu/m/spl times/1 /spl mu/m waveguide cross-sectional area and device length less than 100 /spl mu/m without significant change in device design.

Advances in Small Pixel TES-Based X-Ray Microcalorimeter Arrays for Solar Physics and Astrophysics

We are developing small-pixel transition-edge sensor microcalorimeters for solar physics and astrophysics applications. These large format close-packed arrays are fabricated on solid silicon substrates and are designed to have high energy resolution, and also accommodate count-rates of up to a few hundred counts per second per pixel for X-ray photon energies up to ~ 8 keV. We have fabricated kilo-pixel versions that utilize narrow-line planar and stripline wiring. These arrays have a low superconducting transition temperature, which results in a low heat capacity and low thermal conductance to the heat sink. We present measurements of the performance of pixels with single 65-μm absorbers on a 75-μm pitch. With individual single pixels of this type, we have achieved a full-width at half-maximum energy resolution of 0.9 eV with 1.5 keV Al K X-rays, to our knowledge the first X-ray microcalorimeter with sub-eV energy resolution. We will discuss the properties of these arrays and their application to new solar and astrophysics mission concepts.

The design, implementation, and performance of the Atro-H SXS calorimeter array and anti-coincidence detector

The calorimeter array of the JAXA Astro-H (renamed Hitomi) Soft X-ray Spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS has a square array of 36 microcalorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistorbearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance.

Development of Embedded Heatsinking Layers for Compact Arrays of X-Ray TES Microcalorimeters

Transition-edge sensor microcalorimeter arrays in compact geometries and large formats experience local heating from bias power and x-ray hits that must be dissipated in the frame. For devices on solid, non-perforated silicon substrates, we have introduced an underlying embedded copper heatsinking layer to enhance the ability of the frame to remove this heat. In particular, such a layer can mitigate thermal crosstalk between nearby pixels within the array. Further improvements in array performance, such as decreased magnetic field sensitivity and stray inductance, are possible by turning the heatsinking layer into a superconducting ground plane. In this presentation, we report on the development of heatsinking layers consisting of a 1-2 μm thick high-quality copper layer which is sandwiched between two thin refractory metal-based diffusion barriers. These diffusion barriers are designed to avoid copper migration into the surrounding material over time, especially during our high temperature TES fabrication process which takes place in excess of 400°C . A 0.3-0.5 μm thick PECVD SiO2 cover layer isolates the heatsinking layer from the detector circuit. We present first results on our attempt to tailor the materials forming the diffusion barrier to fabricate both well defined superconducting ground planes and non-superconducting layers with the desired barrier characteristics.

2-D Electrostatic Actuation of Microshutter Arrays

Electrostatically actuated microshutter arrays consisting of rotational microshutters (shutters that rotate about a torsion bar) were designed and fabricated through the use of models and experiments. Design iterations focused on minimizing the torsional stiffness of the microshutters while maintaining their structural integrity. Mechanical and electromechanical test systems were constructed to measure the static and dynamic behavior of the microshutters. The torsional stiffness was reduced by a factor of four over initial designs without sacrificing durability. The analysis of the resonant behavior of the microshutters demonstrates that the first resonant mode is a torsional mode occurring around 3000 Hz. At low vacuum pressures, this resonant mode can be used to significantly reduce the drive voltage necessary for actuation requiring as little as 25 V. The 2-D electrostatic latching and addressing was demonstrated using both a resonant and a pulsed addressing scheme.

Electrostatic microshutter arrays

Based on the Microshutter Array (MSA) subsystems developed at NASA Goddard Space Flight Center (GSFC) for the James Webb Space Telescope (JWST), Next Generation Microshutter Array (NGMSA) has been developed to be used as multi-object selectors for future telescopes in space applications. Microshutter arrays function as transmission devices. Selected shutters fully open 90 degrees permitting incoming light to go through, while the rest of shutters remain closed. The programmable microshutters open and close making the device perform as a multi object selector that can be used on space telescopes. Utilizing a multi object selector, the telescope efficiency can be increased to 100 times or more. Like JWST MSAs, NGMSA features torsion hinges, light shields, front and back electrodes for shutter actuation, latch, and closing. The difference is that JWST MSA utilized magnetic actuation while NGMSA uses electrostatic actuation.

James Webb Space Telescope microshutter arrays and beyond

Abstract. Microshutter array (MSA) subsystems were developed at NASA Goddard Space Flight Center as multiobject selectors for the Near-Infrared Spectrograph (NIRSpec) instrument on the James Webb Space Telescope (JWST). The subsystem will enable NIRSpec to simultaneously obtain spectra from >100 targets, which, in turn, increases instrument efficiency 100-fold. This system represents one of the three major innovations on the JWST that is scheduled to be launched in 2018 as the successor to the Hubble Space Telescope. Featuring torsion hinges, light shields, magnetic actuation, and electrostatic latching and addressing, microshutters are designed for the selective transmission of light with high efficiency and contrast. Complete MSA assemblies consisting of 365×171 microshutters were successfully fabricated and tested, and passed a series of critical reviews for programmable 2-D addressing, life tests, and optical contrast tests. At the final stage of the JWST MSA fabrication, we began to develop the next generation microshutter arrays (NGMSA) for future telescopes. These telescopes will require a much larger field of view than JWSTs. We discussed strategies for fabrication of a proof-of-concept NGMSA that will be modular in design and electrostatically actuated. The details of NGMSA development will be discussed in a follow-up paper.

Superconducting Effects in Optimization of Magnetic Penetration Thermometers for X-Ray Microcalorimeters

We have made high-resolution X-ray microcalorimeters using superconducting MoAu bilayers and Nb meander coils. The temperature sensor is a magnetic penetration thermometer. Operation is similar to metallic magnetic calorimeters, but instead of the magnetic susceptibility of a paramagnetic alloy, we use the diamagnetic response of the superconducting MoAu to sense temperature changes in an X-ray absorber. Flux-temperature responsivity can be large for small sensor heat capacity, with enough dynamic range for applications. We find that models of observed flux-temperature curves require several effects to explain flux penetration or expulsion in the microscopic devices. The superconductor is nonlocal, with large coherence length and weak pinning of flux. At the lowest temperatures, behavior is dominated by screening currents that vary as a result of the temperature dependence of the magnetic penetration depth, modified by the effect of the nonuniformity of the applied field occurring on a scale comparable to the coherence length. In the temperature regime where responsivity is greatest, spatial variations in the order parameter become important: both local variations as flux enters/leaves the film and an intermediate state is formed, and globally as changing stability of the electrical circuit creates a Meissner transition and flux is expelled/penetrates to minimize free energy.

Thermal-Stress Control of Microshutter Arrays in Cryogenic Applications for the James Webb Space Telescope

We report on methods to minimize thermally-induced deformation in a MEMS-based reconfigurable aperture. The device is an enabling component of the Near-Infrared Spectrometer, a principle instrument on NASA’s James Webb Space Telescope. The Microshutter Array consists of 384x175 individually addressable shutters which can be magnetically rotated 90° into the plane of the array and electrostatically latched open. Each shutter is a 100x200 μm rectangular membrane suspended by a small neck region and torsion flexure. The primary materials in the shutter are a 5000Å Si3N4 layer for mechanical rigidity, 2000Å Al for opacity and electrostatic latching, and 2200Å Co90Fe10 for magnetic actuation. This multi-layer stack presents a challenge due to the operating temperatures required for the device: both room temperature (300K) and cryogenic temperature (30K). Thermal expansion of the materials causes the shutters to bow out of plane excessively, which can prevent actuation of the shutters, cause damage to portions of the array, and allow light leakage around closed shutters. Here we present our investigation of several methods to prevent microshutter bowing including deposition of additional materials on the shutters to create a symmetrical layer stack and replacing the current stack with low-coefficient of thermal expansion materials. Using shutter-size suspended cantilever beams as a rapid-development test bed, we have reduced out-of-plane bowing between 300K and 30K to 10% or better. We are currently applying these results to microshutter arrays to develop shutters that remain flat from room temperature to cryogenic temperature while retaining the required mechanical, optical, and magnetic properties.