Wen-Ting Hsieh

Via-less microwave crossover using microstrip-CPW transitions in slotline propagation mode

This paper presents the design of a microstrip-CPW transition where the CPW line propagates close to slotline mode. This design allows the solution to be determined entirely though analytical techniques. In addition, a planar via-less microwave crossover using this technique is proposed. The experimental results at 5 GHz show that the crossover has a minimum isolation of 32 dB. It also has low in-band insertion loss and return loss of 1.2 dB and 18 dB respectively over more than 44 % of bandwidth.

Fabrication of an Antenna‐Coupled Bolometer for Cosmic Microwave Background Polarimetry

We describe the development of a detector for precise measurements of the cosmic microwave background polarization. The detector employs a waveguide to couple light between a pair of Mo/Au superconducting transition edge sensors (TES) and a feedhorn. Incorporation of an on‐chip ortho‐mode transducer (OMT) results in high isolation. The OMT is micromachined and bonded to the microstrip and TES circuits in a low temperature wafer bonding process. The wafer bonding process incorporates a buried superconducting niobium layer with a single crystal silicon layer which serves as the leg isolated TES membrane and as the microstrip dielectric. We describe the micromachining and wafer bonding process and report measurement results of the microwave circuitry operating in the 29–45 GHz band along with Johnson noise measurements of the TES membrane structures and development of Mo/Au TES operating under 100 mK.

Micro-Spec: an ultracompact, high-sensitivity spectrometer for far-infrared and submillimeter astronomy

High-performance, integrated spectrometers operating in the far-infrared and submillimeter ranges promise to be powerful tools for the exploration of the epochs of reionization and initial galaxy formation. These devices, using high-efficiency superconducting transmission lines, can achieve the performance of a meter-scale grating spectrometer in an instrument implemented on a 4 inch silicon wafer. Such a device, when combined with a cryogenic telescope in space, provides an enabling capability for studies of the early universe. Here, the optical design process for Micro-Spec (μ-Spec) is presented, with particular attention given to its two-dimensional diffractive region, where the light of different wavelengths is focused on the different detectors. The method is based on the stigmatization and minimization of the light path function in this bounded region, which results in an optimized geometrical configuration. A point design with an efficiency of ~90% has been developed for initial demonstration and can serve as the basis for future instruments. Design variations on this implementation are also discussed, which can lead to lower efficiencies due to diffractive losses in the multimode region.

Design techniques for improved noise performance of superconducting Transition Edge Sensor bolometers

We have investigated the noise performance of MoAu-bilayer TES bolometers designed for infrared detectors. A set of devices with variations in geometry were fabricated at the NASA/GSFC detector development facility. These detectors have different bilayer aspect ratios and have varieties of normal metal regions deposited on top of the bilayer to study the effects of geometry on noise. These normal metal regions are oriented either parallel or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. Our detectors with the most favorable design feature negligible excess noise in the in-band region, only slight excess noise in the out-of-band region, and low 1/f noise. The detectors are successfully used in the Submillimeter Broadband Spectrometer FIBRE which is used for astronomical observations at the Caltech Submillimeter Observatory.

Fabrication of MKIDS for the MicroSpec Spectrometer

Microspec is a new class of submillimeter and millimeter (250-700 μm wavelength) spectrometer, in which the wavelength separation and detection of incident light is done on a single substrate. The instrument is designed for space exploration by offering high spectral resolving power over a broad band, while being orders of magnitude smaller in mass and volume than the present state-of-the-art. The key enabling components for Microspec are background-limited microwave kinetic inductance detectors, which operate over the full bandwidth of the spectrometer. Here we present our fabrication strategy for making these sensitive detectors. A microstrip architecture utilizing a 0.45-μm crystalline silicon dielectric with a molybdenum nitride kinetic inductor material has been adopted. We have optimized wafer-scale lithographic patterning, and have developed processes that allow us to minimize surface roughness that may contribute to detector noise. Additionally, we have optimized the low-temperature wafer bonding process; this process allows us to build superconductors on both sides of the silicon dielectric layer. We present a final fabricated device and resonator operation at cryogenic temperatures.

Fabrication of an absorber-coupled MKID detector and readout for sub-millimeter and far-infrared astronomy

We have fabricated absorber-coupled microwave kinetic inductance detector (MKID) arrays for sub-millimeter and farinfrared astronomy. Each detector array is comprised of λ/2 stepped impedance resonators, a 1.5μm thick silicon membrane, and 380μm thick silicon walls. The resonators consist of parallel plate aluminum transmission lines coupled to low impedance Nb microstrip traces of variable length, which set the resonant frequency of each resonator. This allows for multiplexed microwave readout and, consequently, good spatial discrimination between pixels in the array. The Al transmission lines simultaneously act to absorb optical power and are designed to have a surface impedance and filling fraction so as to match the impedance of free space. Our novel fabrication techniques demonstrate high fabrication yield of MKID arrays on large single crystal membranes and sub-micron front-to-back alignment of the microstrip circuit.

Performance of High‐Resolution, Micro‐fabricated, X‐ray Magnetic Calorimeters

We are developing micro‐fabricated x‐ray microcalorimeter arrays that show great promise for use in future x‐ray spectroscopy missions. In each pixel we sense the magnetization change due to the heat input of an absorbed x‐ray using a meander‐shaped superconducting pickup trace that is on a substrate that is separate from the low noise 2‐stage SQUID read‐out. We report on the results from our prototype arrays that have achieved an energy resolution of 3.3 eV at 6 keV, and are progressing future arrays towards the goal of demonstrating sub‐eV energy resolution. The results from studies of the thermalization of gold and gold‐bismuth absorbers on sub‐microsecond timescales will be presented. We will also present the results of comparisons of the magnetization and heat capacity of micro‐fabricated devices to those of larger hand‐constructed devices.

Broadband Planar 5:1 Impedance Transformer

This letter presents a broadband Guanella-type planar impedance transformer that transforms 50 Ω to 10 Ω with a 10 dB bandwidth of 1-14 GHz. The transformer is designed on a flexible 50 μm thick polyimide substrate in microstrip and parallel-plate transmission line topologies, and is inspired by the traditional 4:1 Guanella transformer. Back-to-back transformers were designed and fabricated for characterization in a 50 Ω system. Simulated and measured results are in excellent agreement.

Antenna-coupled transition-edge hot-electron microbolometers

We are developing a new type of detector for observational cosmology and astrophysical research. Incoming radiation from the sky is coupled to a superconducting microstrip transmission line that terminates in a thin film absorber. At sub-Kelvin temperature, the thermal isolation between the electrons and the lattice makes it possible for the electrons in the small absorber (100s of cubic micro-meter) and superconducting bilayer (Transition Edge Sensor) to heat up by the radiation absorbed by the electrons of the normal absorbing layer. We call this detector a Transition-edge Hot-electron Micro-bolometer (THM). THMs can be fabricated by photo lithography, so it is relatively easy to make matched detectors for a large focal plane array telescope. We report on the thermal properties of Mo/Au THMs with Bi/Au absorbers.

Microfabrication of High Resolution X‐ray Magnetic Calorimeters

Metallic magnetic calorimeter (MMC) is one of the most promising x‐ray detector technologies for providing the very high energy resolution needed for future astronomical x‐ray imaging spectroscopy. For this purpose, we have developed micro‐fabricated 5×5 arrays of MMC of which each individual pixel has excellent energy resolution as good as 3.4 eV at 6 keV x‐ray. Here we report on the fabrication techniques developed to achieve good resolution and high efficiency. These include: processing of a thin insulation layer for strong magnetic coupling between the AuEr sensor film and the niobium pick‐up coil; production of overhanging absorbers for enhanced efficiency of x‐ray absorption; fabrication on SiN membranes to minimize the effects on energy resolution from athermal phonon loss. We have also improved the deposition of the magnetic sensor film such that the film magnetization is nearly completely that is expected from the AuEr sputter target bulk material. In addition, we have included a study of a posit...