Cosmic Microwave. 08/07/2017 Benson Bolometers and the CMB

Transcription

1 Bolometers and the CMB Cosmic Microwave 08/07/2017 Benson Bolometers and the CMB 1

2 The CMB Spectrum I (W m -2 Hz -1 sr -1 ) Rayleigh Jeans (RJ) tail K 15 K 30 K 77 K Freq (GHz) CMB is a K blackbody Spectrum peaks at ~150 GHz Conveniently peak of CMB spectrum is near foreground minimum (i.e., dust, synchrotron) and atmospheric windows Design detector bands to observed within atmospheric windows Aim to design instruments where atmospheric loading dominates detector loading 08/07/2017 Benson Bolometers and the CMB 2

3 Power = Power on a Detector Z P ( )d = Z B(,T) f( ) A d B(ν,T) = Blackbody equation = [ W / m 2 sr Hz ] f(ν) = Frequency response of the detector AΩ = Throughput (or etendue) of instrument = [m 2 sr] 08/07/2017 Benson Bolometers and the CMB 3

4 Power = Power on a Detector Z P ( )d = Z B(,T) f( ) A d B(ν,T) = Blackbody equation = [ W / m 2 sr Hz ] f(ν) = Frequency response of the detector AΩ = Throughput (or etendue) of instrument = [m 2 sr] B(,T)= 2h 3 c 2 1 exp(h /kt ) 1 2k B 2 In RJ limit, x = hv/kt << 1 and exp(x) ~ 1 + x, greatly simplifying the black-body equation. c 2 T RJ 08/07/2017 Benson Bolometers and the CMB 4

5 Power = Power on a Detector Z P ( )d = Z B(,T) f( ) A d B(ν,T) = Blackbody equation = [ W / m 2 sr Hz ] f(ν) = Frequency response of the detector AΩ = Throughput (or etendue) of instrument = [m 2 sr] Can approximate frequency response as a band-width (Δν) times an optical efficiency (η), e.g., for a top-hat filter 08/07/2017 Benson Bolometers and the CMB 5

6 Power = Power on a Detector Z P ( )d = Z B(,T) f( ) A d B(ν,T) = Blackbody equation = [ W / m 2 sr Hz ] f(ν) = Frequency response of the detector AΩ = Throughput (or etendue) of instrument = [m 2 sr] For a single spatial mode experiment (i.e., with a diffraction limited beam), AΩ = λ 2 08/07/2017 Benson Bolometers and the CMB 6

7 Power = Power on a Detector Z P ( )d = Z B(,T) f( ) A d B(ν,T) = Blackbody equation = [ W / m 2 sr Hz ] f(ν) = Frequency response of the detector AΩ = Throughput (or etendue) of instrument = [m 2 sr] Therefore in RJ-limit, this equation reduces to: P RJ =2k B 2 c 2 T RJ( 2 )( ) P RJ =2k B T RJ ( ) 08/07/2017 Benson Bolometers and the CMB 7

8 Power on a Detector P RJ =2k B T RJ ( ) For a typical CMB experiment, one might have: TRJ = 20 K Atmosphere opacity and temperature are about 0.05 and 240 K, respectively. Implies 0.05 x 240 K = 12 K of RJ loading CMB is 2.73 K Internal cryostat loading is ~6 K Band-width of 30 GHz Efficiency of ~0.30 Note: For loading, the efficiency is how much of detector beam s power ends up on the sky, which includes loss from spillover on optical elements, loss in optics, detectors, atmosphere, etc. (more later this week) PRJ = 2(1.38e-23)(20 K)(40e9 Hz)(0.3) = 6.5 pw 08/07/2017 Benson Bolometers and the CMB 8

9 The Bolometer Radiation (Popt) Thermal Link (G) Absorber (C) Thermistor (Pelec) Thermal Bath (Tbath) A bolometer converts a thermal signal on the detector to an electrical signal, via the thermistor. Popt = [pico-watts] = The amount of optical / mm-wave power on the detector C = [J/K] = The heat capacity of the bolometer G = [pw/k] = Thermal conductance to the heat sink. A bolometer typically uses electrical feedback, through the thermistor, to stabilize Tbolo 08/07/2017 Benson Bolometers and the CMB 9

10 The Bolometer Radiation (Popt) Thermal Link (G) Absorber (C) Thermistor (Pelec) Thermal Bath (Tbath) Power on the bolometer is the sum of optical and electrical power, that is conducted away through the G-link P = P opt + P elec = Z Tbolo T bath G(T )dt For an input power, bolometer heats up and goes down with a time constant, tau: T = P/G = C/G 08/07/2017 Benson Bolometers and the CMB 10

11 Thermistors: TES, Semiconductors Al/Ti TES dt ~ 10 mk Transition Edge Sensors (TES) Typically a metal bi-layer, superconducting transition tuned by thickness of normal / superconducting layers Typical combinations (e.g., Al/Ti, Mo/Au, Al/Mn, Ti/Au) require ~ nm film thickness to achieve transitions of ~500 mk Resistance (M ) NTD Germanium Temperature (K) Thermistor: TES vs NTD Germanium TES ~1 Ohm, dr/dt > 0 NTD ~ 2-10 MOhm, dr/dt < 0 Sign of dr/dt determines if current or voltage bias provides negative electrothermal feedback (ETF) i.e., a change in optical power, causes a change in temperature and resistance, electrical power changes via Joule heating (Pelec = V 2 /R or I 2 R) 08/07/2017 Benson Bolometers and the CMB 11

12 Bolometer Saturation Power (Psat) Turn around The saturation power (Psat) is a critical TES bolometer parameter: Defined as: the (optical or electrical) power required to drive the TES normal For noise reasons, typically aim for Psat ~twice the expected optical power Characterize bolometer I-V and R-P curves, i.e., decrease the voltage bias on the bolometer and measure electrical behavior: As Vb decreases, TES will go into superconducting transition and exhibit a turnaround in IV curves where loop gain > 1 Below turn-around, TES changes resistance to keep total power constant Niemack /07/2017 Benson Bolometers and the CMB 12

13 Thermal Conductance: G(T) Current (ua) Increasing Tbath To characterize thermal-link, useful to measure Psat as a function of bath temperature P sat = K(T n c T n bath) Voltage (uv) G dp dt (T c)=nktc n 1 Power (pw) For metals, n ~ 3, which shows characteristic inflection in P-T curve G ~ 100 pw/k is typical value (set by desired Tc, Psat) Marriage 2007 Temp (K) 08/07/2017 Benson Bolometers and the CMB 13

14 Thermistors: TES, Semiconductors Al/Ti TES dt ~ 10 mk Electro-thermal Feedback (ETF) acts to keep total power on the bolometer constant via electrical Joule heating (Pelec = V 2 /R): P opt + P elec = d dt Z Tbolo T base P elec = V 2 b R G(T ) dt dp elec dt = P 2 elec R dr dt If dr/dt>0, then dpelec/dt < 0 08/07/2017 Benson Bolometers and the CMB 14

15 Electro-Thermal Feedback Al/Ti TES dt ~ 10 mk Strength of ETF feedback determined by slope of R(T) curve, parameterized by a loop gain, in analogy with electronic circuits: L = P elec P L = P elec(dr/r) G T L = P elec GT with T R dr dt Thermistor NTD Germanium Resistance (Ohms) Electrical Loop Gain ~2-10 M ~1-5 TES ~1 ~ /07/2017 Benson Bolometers and the CMB 15

16 Bolometer Responsivity (di/dp) Al/Ti TES dt ~ 10 mk S I = di/dp opt =[Amps/W atts] dr dp opt = GdT + P elec R d(i = V/R) di = P elec dr V b R Plug into equation for Responsivity (SI): S I = P elec dr/v b R GdT + P elec dr/r S I = P elec V b T G(1 + P elec GT TdR RdT TdR RdT ) S I = I P = 1 V b with L 1+L T R dr dt 08/07/2017 Benson Bolometers and the CMB 16

17 Loop Gain: Responsivity, Time Constant Al/Ti TES S I = I P = 1 V b L 1+L dt ~ 10 mk In limit of Loop Gain >> 1, responsivity goes like -1/Vb: Large loop gain implies a linear detector, i.e., a responsivity independent of loading or depth in the transition Similarly, it can be shown that detector speeds-up with increasing loop gain: = 0 1+L = C/G 1+L As detector time constant decreases, its band-width increases. To be stable, need to feed-back electrical signals faster than detector bandwidth. 08/07/2017 Benson Bolometers and the CMB 17

18 Thermistors: TES, Semiconductors Al/Ti TES dt ~ 10 mk TES Advantages: 1) Fab - TES s can be fabricated on bolometer 2) Linearity - Steepness of R(T) curve determines strength of electrothermal response 3) Microphonics - Low-impedance = lowmicrophonic response Shaking wires will cause changing capacitance to ground, large impedance implies low frequency of RC-filter Resistance (M ) NTD Germanium Thermistor NTD Germanium Resistance (Ohms) Electrical Loop Gain ~2-10 M ~1-5 TES ~1 ~ Temperature (K) 08/07/2017 Benson Bolometers and the CMB 18

19 SuZIE Bolometers ( ) Made by UC-Berkeley (1992) NbTi Wires Nylon Threads JPL (1998) Gold Leads Thermistor Bi on Sapphire Sapphire Substrate 1 cm Hand-made bolometers! Sapphire substrate with 100 nm thick bismuth absorber suspended by nylon threads NTD Germanium thermistor, bonded to gold wires, epoxied to center Gold is indium soldered to NbTi wires for readout Gold wires set heat capacity (C), Nylon threads set thermal conductivity (G) Cooled to 300 mk NEP ~ 150 aw / Hz 1/2, Time constant ~ 150 msec 08/07/2017 Benson Bolometers and the CMB 19

20 Spiderweb Bolometers (~ ) Jet Propulsion Lab / JPL (1998) Absorbing Gold Web Thermistor SuZIE was the first experiment to use a spider-web bolometer! JPL design later used for ACBAR, Boomerang, Planck experiments; UC- Berkeley version used for SPT-SZ 1 cm Planck Satellite (2008) Incorporated micro-fabrication: Silicon-nitride (SiN) substrate, 20 nm thick gold (Au) absorber, on silicon wafer Provides a 20x reduction in heat capacity and cosmic ray cross-section NTD Germanium thermistor indium bump bonded to web Thermistor dominates heat capacity (C), gold leads set thermal conductivity (G) NEP ~ 40 aw / Hz 1/2 Time constant ~ 15 msec 15 cm 08/07/2017 Benson Bolometers and the CMB 20

21 SPT-SZ Detectors ( ) 4 mm Si Al Made at UC-Berkeley by Erik Shirokoff, Jared Mehl, Sherry Cho Copied JPL spider-web absorber design; - suspended 1mm thick Silicon Nitride (SiN) substrate with 12 nm thick Gold (Au) absorber Replaced NTD Germanium with TES bilayer of Aluminum/Titanium (Al/Ti); - Film thickness 40 nm Al, 80 nm Ti, gives a superconducting transition (Tc) of ~ 0.5 K G set by gold finger to TES Cross-section 30 mm Ti Au web (Etched Silicon) Au, G-link SiN 08/07/2017 Benson Bolometers and the CMB 21

22 TES Time Constant and Stability Side TES Bolometer has two time constants: 1) Optical time constant: How fast optical power is distributed across the bolometer 2) Electrical time constant: The electrical time constant / response of the TES Side-TES design was too slow optically: - Optical time constant dominated by thermalization time of the spider-web - Changed from optical time constant from msec to 10 msec by moving TES to center of the web Center TES 08/07/2017 Benson Bolometers and the CMB 22

23 TES Time Constant and Stability Side TES But Side-TES design was too fast electrically, TES stability requires: 1) Bandwidth requirement for bolometer stability: TES bandwidth < 5.8 fmux bandwidth 2) Given fmux filter bandwidth, this implies: 0.2 msec < ttes < 5 msec 3) TES speeds up as loop gain increases [ttes = t0 / (1 + L)], so assuming L ~ 10-30: 6 msec < t0 < 50 msec Side-TES had a t0 < 0.1 msec! became unstable as soon as TES went into its transition. Center TES Gold Ring added for heat capacity to slow down bolometer to t0 ~ 20 msec. 08/07/2017 Benson Bolometers and the CMB 23

24 Thermal Decoupling of TES Current (arb units) R/Rnormal = 0.70 SPT-SZ 2007 focal plane used design with center-tes and gold ring. However, design had additional instability from gold ring decoupling from the TES. Current (arb units) R/Rnormal = 0.55 TES Thermal conductivity (G ) between gold ring and TES is too small Current (arb units) R/Rnormal = 0.50 Gold Ring Time (msec) 08/07/2017 Benson Bolometers and the CMB 24

25 Alternative Gold Coupling Designs Good Improved TES-Gold Coupling: 1) Centered TES and increased gold connectivity, 2) Al/Ti bi-layer underlies gold, 3) Superconducting Al leads were narrowed, 4) Oxide layer cleaned with etch before gold deposition. (1) Designs that intercepted TES improved thermal coupling to gold, but broadened transition and lowered loop gain Good BAD (2) BAD (3) 08/07/2017 Benson Bolometers and the CMB 25

26 Engineering TES Transition for TES Stability Engineer TES speed and responsivity R(T) curve: Steeper = Faster, more linear Broader = More stable - Palladium-Gold (PdAu) added head capacity to slow detectors (ala SPT-SZ) - Tested Nb stripes and dots on TES to soften R(T) curve and add responsivity high in the transition Stripes PdAu BLING Dots Mo/Au bi-layer TES 08/07/2017 Benson Bolometers and the CMB 26

27 Series Inductor with TES A uh inductor in series with the TES can also be used to reduce response at high-frequency, typically used in time-domain SQUID multiplexing systems (more tomorrow) Niemack /07/2017 Benson Bolometers and the CMB 27

28 Bolometers circa 2015 (SPT-3G) 3 mm Argonne National Lab (2015) Works like your TV antenna; Antenna at center, power sent to 6 superconducting bolometers around perimeter, which measures 3-colors, 2-polarizations per pixel Used for SPT-3G camera, which has 16,000 detectors. Cooled to 0.3 degrees Kelvin above absolute zero.

29 Bolometers circa 2015 (SPT-3G) 3 mm Argonne National Lab (2015) Works like your TV antenna; Antenna at center, power sent to 6 superconducting bolometers around perimeter, which measures 3-colors, 2-polarizations per pixel Used for SPT-3G camera, which has 16,000 detectors. Cooled to 0.3 degrees Kelvin above absolute zero.

30 Useful References Irwin & Hilton 2005, Transition Edge Sensors, link.springer.com/chapter/ / _3 Zmuidznias & Richards 2004, Superconducting Detectors and Mixers for mm and sub-mm Astrophysics, ~jonas/tex/papers/pdf/2004-pieee-zmuidzinas.pdf Richards 1994, Bolometers for Infrared and millimeter waves, Mather 1982, Bolometer Noise: Non-equilibrium Theory, /07/2017 Benson Bolometers and the CMB 30

31 08/07/2017 Benson Bolometers and the CMB 31

32 Polarization Sensitive Bolometers (PSBs) JPL modified spider-web concept to add polarization sensitivity SiN substrate with linear crossed pattern, gold added only along one direction NTD thermistor on edge of absorber, to minimize cross-polar response Design used for QUAD, BICEP, Boomerang2k, and Planck experiments Single-pixel concept needs to be scaled up for ~1000 element focal planes (gold direction) 08/07/2017 Benson Bolometers and the CMB 32

33 SPT-3G detector module assembly at Fermilab

34 Bolometer Thermal Responsivity, SI(w) ttes Gold Decoupling tgold RLC Filter Cutoff Improved gold waffle design: tgold = C/G ~ 20 msec Stable up to loop gain ~ 50 Gold-ring Gold- waffle (Red, Blue) TES Gold Heat Capacity

35 Evolution of Detector Sensitivity CMB science has been driven by advances in detector technology; detector speed has ~doubled every year for 50 years! BLIP CMB Ground BLIP CMB Space Photon ( shot ) noise limit from ground-based observations with 0.25 Kelvin detectors NEP ~ 50 x10-18 W Hz -1/2 Plot from J. Zmuidzinas 08/07/2017 Benson Bolometers and the CMB 35

36 SQUID Bolometer Readout Requirements: Low input impedance Low power dissipation High bandwidth ~100 MHz Low noise. At 4 K: 3 pa/rthz 0.2 nv/rthz Implementation: Use DC SQUID as an ammeter (Superconducting Quantum Interference Device) Current->Flux->Voltage transducer V 0 Lock point I Φ = 1 2 n Φ 0 1 Φ = ( n + ) Φ 2 0 Φ/Φ0 Critical Current V 08/07/2017 Benson Bolometers and the CMB 37