Marc C. Foote

Thermal and Electrical Conductivity Probe (TECP) for Phoenix

Received 29 November 2007; revised 1 August 2008; accepted 30 November 2008; published 25 March 2009. [1] The Thermal and Electrical Conductivity Probe (TECP) is a component of the Microscopy, Electrochemistry and Conductivity Analyzer (MECA) payload on the Phoenix Lander. TECP will measure the temperature, thermal conductivity, and volumetric heat capacity of the regolith. It will also detect and quantify the population of mobile H2O molecules in the regolith, if any, throughout the polar summer, by measuring the electrical conductivity of the regolith as well as the dielectric permittivity. In the vapor phase, TECP is capable of measuring the atmospheric H2O vapor abundance as well as augmenting the wind velocity measurements from the meteorology instrumentation. TECP is mounted near the end of the 2.3 m Robotic Arm and can be placed either in the regolith material or held aloft in the atmosphere. This paper describes the development and calibration of the TECP. In addition, substantial characterization of the instrument has been conducted to identify behavioral characteristics that might affect landed surface operations. The greatest potential issue identified in characterization tests is the extraordinary sensitivity of the TECP to placement. Small gaps alter the contact between the TECP and regolith, complicating data interpretation. Testing with the Phoenix Robotic Arm identified mitigation techniques that will be implemented during flight. A flight model of the instrument was also field tested in the Antarctic Dry Valleys during the 2007–2008 International Polar Year.

Space science applications of thermopile detector arrays

Thermal detectors, while typically less sensitive than quantum detectors, are useful when the combination of long wavelength signals and relatively high temperature operation makes quantum detectors unsuitable. Thermal detectors are also appropriate in applications requiring flat spectral response over a broad wavelength range. JPL produces thermopile detectors and linear arrays to meet space science requirements in these categories. Thermopile detectors and arrays are currently being fabricated for two space applications. The first is the Mars Climate Sounder (MCS) instrument, to fly on the Mars Reconnaissance Orbiter mission, scheduled to launch in 2005. MCS is an atmospheric limb sounder utilizing nine 21-element thermopile arrays. The second application is the Earth Radiation Budget Suite (ERBS), part of the National Polar Orbiting Environmental Satellite System (NPOESS). This instrument measures upwelling radiation from the earth in the spectral range 0.3-100 μm.

Progress toward high-performance thermopile imaging arrays

Thermopiles are uncooled, broadband detectors that require no chopper or temperature stabilizer. Their wide operating- temperature range, lack of temperature stabilization, and radiometric accuracy make thermopiles well suited for some space-based scientific imaging applications. These detectors may also offer advantages over bolometers for night vision. Previous work at JPL has produced thermopile linear arrays with D* values over 109 cmHz1/2/W by combining high- performance thermoelectric materials Bi-Te and Bi-Sb-Te with bulk micromachining processes. To date, however, 2-D thermopile arrays have demonstrated only moderate performance. The purpose of the present work is to improve thermopile 2-D arrays substantially by combining Bi-Te and Bi-Sb-Te thermoelectric materials with a unique pixel structure and low-noise readout circuitry. The initial goal is a 128 X 128 array with a single multiplexed analog output stream, with system D* values (including readout noise) of 109 cmHz1/2/W, and with a focal-plane power dissipation of 20 mW. 100 micrometers square detectors have been demonstrated with D* values of 2 X 108 cmHz1/2/W and response times of 4 ms. Models predict D* values well over 109 cmHz1/2/W for optimized detectors. Modeling of a preliminary readout design shows that, for the expected detector resistance of 100 kΩ, the total noise will be 50% higher than the detector Johnson noise. CMOS test chips containing front-end circuits presently display a noise about 2.5 times higher than modeled and a power dissipation of 0.6 μW per pixel.

High-performance micromachined thermopile linear arrays

Linear thermopile infrared detector arrays have been produced with D* values as high as 2.2 X 109 cmHz1/2W for 83 ms response times. Typical responsivity is 1000 V/W. This result has been achieved with Bi-Te and Bi-Sb-Te thermoelectric materials on micromachined silicon nitride membranes. Results for several device geometries are described and compared to literature values for Schwartz type thermocouple detectors and for thin film thermopile detectors and arrays. Measurements of responsivity as a function of modulation frequency and wavelength are presented.

Epitaxial YBa2Cu3O7 superconducting infrared microbolometers on silicon

Superconducting transition-edge infrared microbolometers have been fabricated by silicon micromachining using an epitaxial YLa.05Ba1.95Cu3O7-x (YBCO) film on a epitaxial yttria-stabilized zirconia buffer layer on silicon. The low thermal conductance of the micromachined structures combined with the sharp resistance change at the superconducting transition results in very sensitive infrared detectors. The broadband response of these thermal detectors makes them particularly useful at wavelengths longer than the typical operating range of semiconductor detectors ((lambda) greater than about 20 micrometers ) at moderately high temperatures (T approximately 70 K and higher). The use of standard silicon processing promises low-cost monolithic integration of the readout electronics for arrays of these devices. Preliminary measurements are reported here on a device 140 micrometers X 105 micrometers in size with a detectivity, D*, of 8 +/- 2 X 109 cm Hz1/2/Watt, and NEP of 1.5 X 10-12 Watts/Hz1/2 at 2 Hz and 80.7 K. This value of D* exceeds the highest previously reported D* for a YBCO transition-edge bolometer, and is comparable to the highest reported D* for a thermal detector operating at greater than about 70 K. The thermal time constant for this device was 105 +/- 20 msec.

Transition Edge YBa 2 Cu 3 O 7-x Microbolometers for Infrared Staring Arrays

High-temperature superconducting staring arrays are potentially important for both space and terrestrial applications which require the combination of high sensitivity over a broad wavelength range and relatively high temperature operation. In many such array applications sensitivity is more important than speed of response. Thus, it is desirable to design low- thermal-mass pixels that are thermally isolated from the substrate. To this end, Johnson, et al. at Honeywell have fabricated meander lines of YBa2Cu3O7-x (YBCO) sandwiched between layers of silicon nitride on silicon substrates. The silicon was etched out from under each YBCO meander line to form low-thermal-mass, thermally isolated microbolometers. These 125 micrometers X 125 micrometers devices are estimated to have a noise equivalent power of 1.1 X 10-12 W/Hz1/2 near 5 Hz with a 5 (mu) A bias (neglecting contact noise). A drawback of this original Honeywell design is that the YBCO is grown on an amorphous silicon nitride underlayer, which precludes the possibility of epitaxial YBCO growth. The YBCO therefore has a broad resistive transition, which limits the bolometer response, and the grain boundaries lead to excess noise. We discuss the potential performance improvement that could be achieved by using epitaxial YBCO films grown on epitaxial yttria-stabilized zirconia buffer layers on silicon. This analysis shows a significant signal to noise improvement at all frequencies in devices incorporating epitaxial YBCO films. Progress toward fabricating such devices is discussed.

Temperature Stabilization Requirements for Unchopped Thermal Detectors

The temperature stabilization requirements of unchopped thermistor bolometers and thermopile detectors are analyzed. The detector temperature, on which the bolometer output signal depends, is quite sensitive to changes in instrument temperature but relatively insensitive to changes in scene temperature. In contrast, the difference in temperature between detector and substrate, on which the thermopile signal depends, is equally sensitive to changes in instrument and scene temperature. Expressions for these dependencies are derived based on a simplified instrument model. It is shown that for a typical uncooled thermal imager, the temperature stabilization requirements for a bolometer are tow orders of magnitude more stringent than those for a thermopile detector.

The AFTA coronagraph instrument

The Astrophysics Focused Telescope Assets (AFTA) study in 2012-2013 included a high-contrast stellar coronagraph to complement the wide-field infrared survey (WFIRST) instrument. The idea of flying a coronagraph on this telescope was met with some skepticism because the AFTA pupil has a large central obscuration with six secondary mirror struts that impact the coronagraph sensitivity. However, several promising coronagraph concepts have emerged, and a corresponding initial instrument design has been completed. Requirements on the design include observations centered 0.6 deg off-axis, on-orbit robotic serviceability, operation in a geosynchronous orbit, and room-temperature operation (driven by the coronagraph’s deformable mirrors). We describe the instrument performance requirements, the optical design, an observational scenario, and integration times for typical detection and characterization observations.

Texture-specific elemental analysis of rocks and soils with PIXL: The Planetary Instrument for X-ray Lithochemistry on Mars 2020

PIXL (Planetary Instrument for X-ray Lithochemistry) is a micro-focus X-ray fluorescence instrument for examining fine scale chemical variations in rocks and soils on planetary surfaces. Selected for flight on the science payload for the proposed Mars 2020 rover, PIXL can measure elemental chemistry of tiny features observed in rocks, such as individual sand grains, veinlets, cements, concretions and crystals, using a 100 μm-diameter, high-flux X-ray beam that can be scanned across target surfaces.

Infrared instrument support for HyspIRI-TIR

The Jet Propulsion Laboratory is currently developing an end-to-end instrument which will provide a proof of concept prototype vehicle for a high data rate, multi-channel, thermal instrument in support of the Hyperspectral Infrared Imager (HyspIRI)–Thermal Infrared (TIR) space mission. HyspIRI mission was recommended by the National Research Council Decadal Survey (DS). The HyspIRI mission includes a visible shortwave infrared (SWIR) pushboom spectrometer and a multispectral whiskbroom thermal infrared (TIR) imager. The prototype testbed instrument addressed in this effort will only support the TIR. Data from the HyspIRI mission will be used to address key science questions related to the Solid Earth and Carbon Cycle and Ecosystems focus areas of the NASA Science Mission Directorate. Current designs for the HyspIRI-TIR space borne imager utilize eight spectral bands delineated with filters. The system will have 60m ground resolution, 200mK NEDT, 0.5C absolute temperature resolution with a 5-day repeat from LEO orbit. The prototype instrument will use mercury cadmium telluride (MCT) technology at the focal plane array in time delay integration mode. A custom read out integrated circuit (ROIC) will provide the high speed readout hence high data rates needed for the 5 day repeat. The current HyspIRI requirements dictate a ground knowledge measurement of 30m, so the prototype instrument will tackle this problem with a newly developed interferometeric metrology system. This will provide an absolute measurement of the scanning mirror to an order of magnitude better than conventional optical encoders. This will minimize the reliance on ground control points hence minimizing post-processing (e.g. geo-rectification computations).