Parameter Comparison for Low-Noise Mo/Au TES Bolometers

Transcription

1 Parameter Comparison for Low-Noise Mo/Au TES Bolometers Dominic Benford Harvey Moseley Johannes Staguhn Christine Allen Jay Chervenak Thomas Stevenson Wen-Ting Hsieh NASA / Goddard Space Flight Center LTD 1 Genova, Italia 1

2 Context! Developing bolometers for far-ir astronomy! Building arrays of SQUID-multiplexed bolometers for SOFIA/SAFIRE, ASTRO/SPIFI, GBT/3mmCam! Related presentations: " Staguhn et al. Detector noise analysis talk R5 " Chervenak et al. Detector fabrication description Y18 " Allen et al. Detector fabrication description Y24 " Dicker et al. GBT/3mmCam instrument talk W3 " Moseley et al. 2D (semiconducting) array talk Y6 LTD 1 Genova, Italia 2

3 Bolometer Requirements Optical Power (Absorber)! Parameters: " T bath ~ 3mK (±2%) " Power ~ 1pW (±3x) " NEP ~ 1-17 W/ Hz " τ 1ms " Absorbs at ~1µm-1mm τ = C / G C G T C " T C ~ 45mK (±1%) " R N ~ 4mΩ (±2%) " G(T C ) ~ few x 1-11 W/K (±3x) T bath LTD 1 Genova, Italia 3

4 Mechanical Implementation! 1x8 linear arrays of Pop-Up Bolometers! Folding allows arrays to be close-packed! Thermal isolation high, since flexure hinges are thin! SHARC-II (12x32=384 pixel) semiconducting Pop-Up array (Moseley et al.) LTD 1 Genova, Italia 4

5 History of our TES bilayers! Both Mo/Au and Mo/Cu fabricated! Good performance demonstrated, but not sufficiently repeatable! Always some excess noise, often out-of-band, sometimes 5x-1x thermal noise limit LTD 1 Genova, Italia 5

6 Approach: Boundary Conditions! Current flowing in TES can take many possible paths! Initial trials indicated that normal metal bars on sides could help constrain possible paths, leading to more orderly current flow Lindeman et al. model Superconducting Normal! Other constraints may be more likely to prevent moving chunks of superconducting regions LTD 1 Genova, Italia 6

7 Trial #1: Longitudinal Bars! In order to better understand the effect of boundary condition geometry on detector performance, we produced several nearly identical bolometers with different normal metal bars.! Several designs of longitudinal bars produced! Some designs are wedged to remove isotropy # #1 #2 #3 #4 LTD 1 Genova, Italia 7

8 Trial #2: Transverse Bars! Several designs of transverse bars produced! Some designs are interdigitated; others are continuous # #1 #2 #3 LTD 1 Genova, Italia 8

9 Transition Measurements! 4-wire measurements of resistance vs. temperature, with several bias currents to estimate self-heating and critical currents.! Addition of transverse bars does cause sharpness to degrade, but still achieve α Ch Ch 1 Ch Temperature (K) Ch2 cut Ch3 cut Temperature (K) Longitudinal bars Transverse bars LTD 1 Genova, Italia 9

10 Behavior on Bias (I)! I(V) curves taken on all devices; calibrated to determine resistance and power as a function of base temperature.! Some calibration uncertainties remain C1R CR1 CR2 CR C1R C1R3 C1R2 1 CR3 CR TES Bias Voltage (µv) TES Bias Voltage (µv) Longitudinal bars Transverse bars LTD 1 Genova, Italia 1

11 Behavior on Bias (II)! Resistance vs. bias yields close to expected values.! Some calibration uncertainties remain CR CR2 CR C1R2 C1R3 4 CR3 CR1 35 C1R C1R TES Bias Voltage (µv) Longitudinal bars TES Bias Voltage (µv) Transverse bars LTD 1 Genova, Italia 11

12 Behavior on Bias (III)! Power vs. bias yields similar value for all pixels (as should be the case).! Power on transition quite constant (as should be the case).! Some calibration uncertainties remain CR CR1 CR4 7 6 C1R3 C1R2 1 8 CR CR3 3 2 C1R1 C1R TES Bias Voltage (µv) Longitudinal bars TES Bias Voltage (µv) Transverse bars LTD 1 Genova, Italia 12

13 Pixel Thermal Conductance T = 443 mk C G(T ) =.58 pw/mk C Index ~ 2 P = 84 pw sat! Thermal conductance measured by average power on transition for many bath temperatures! Power law fit: " T C =443mK " G(T C ) = W/K (higher than desired) " Index G=G T N, N~2 (though not tightly constrained) " Psat = 84pW (with T bath =). " Phonon noise: W/ Hz Temperature (mk)! Next device mask will reduce leg width, to reduce P sat and G(T C ) by ~1x. LTD 1 Genova, Italia 13

14 Dynamic Measurements! Measured complex impedance Z(ω) for bolometers, as per Lindeman et al.! Results prove tricky to interpret; devices too fast for reliable high frequency impedance measurements! Time constants of 1/τ e ~5kHz. Imaginary Impedance (mž) 1 ~1 Hz -1-2 Superconducting Z=24.mž -3 Z=32.mž -4 Z=4.mž Z=52.5mž khz -6 1 khz 4.5 khz khz khz Real Impedance (mž) mž 34mž 44mž 55mž 68mž 81mž 96mž 112mž 129mž 147mž 166mž 187mž mž 231mž 255mž 281mž 37mž 335mž 363mž 393mž 4mž 41mž 421mž 421mž LTD 1 Genova, Italia 14

15 Noise Measurements! The subject of Johannes Staguhn s talk; in order not to spoil the surprise: The geometry of the bars does affect the noise Current Noise (pa/ Hz) Frequency (Hz) Frequency (Hz) LTD 1 Genova, Italia 15

16 Optical Measurements! Bismuth absorbing film deposited on bolometers to impedance math to free space.! Dark tests after deposition show same result as tests prior to deposition.! Optically functioning bolometers are demonstrated in the dark; prior tests have verified optical performance. Optical measurement of a blackbody calibrator from an earlier, similar bolometer. LTD 1 Genova, Italia 16

17 Summary! Several nearly identical bolometers produced, with different normal metal bars. Bolometers are optically active detectors.! Tests of quasi-dc parameters (T C, R N, α, P sat, etc.) show good consistency.! Noise performance can be improved by changing bar geometry.! Measurements & analysis ongoing to quantify improvements and differences. Time constant (impedance) analysis to be refined. LTD 1 Genova, Italia 17