Noise Measurements W vs. T bath & Thermal Conductance Measurements NEP measurements at T bath = 311 mK for V BIAS = 1  V with predicted noise levels for SQUID, Johnson and thermal noise contributions for the disconnected thermal model with fitted (green) and literature (red)  values and the ideal thermal model (purple). For these we followed the block diagram method 5, assumed a radiative limit for the thermal noise contributions 10 and used the measured responsivity.  We measure at various TES biases.  The disconnected model with fit Σ provides the best fit to the noise measurements. Responsivity Measurements Sensitivity Measurements of a Transition-Edge Hot-Electron Microbolometer for Millimeter-wave Astrophysical Observations E.M. Barrentine a, P.T. Timbie a, T. R. Stevenson b, S. Ali c, J. A. Chervenak b, E. Wollack b, S. H. Moseley b, C. A. Allen b, W.T. Hseih b, T. M. Miller b, D. J. Benford b, A. D. Brown b a University of Wisconsin-Madison, Madison, WI USA b NASA Goddard Space Flight Center, Greenbelt, MD USA c Lawrence Livermore National Laboratory, Livermore, CA USA Abstract Future experiments to probe the Cosmic Microwave Background (CMB) polarization will need arrays of 1000s of sensitive bolometers. We are developing a Transition-Edge Hot- Electron Micro-Bolometer (THM) to fill this need. This small volume bolometer consists of a superconducting bilayer Transition-Edge Sensor (TES) with a thin film absorber. The THM employs the decoupling between electrons and phonons at mK temperatures to provide thermal isolation. The devices are easily integrated with antennas via microstrip transmission lines, and with SQUID readouts. We present the results of noise, responsivity, and thermal conductance measurements in which electrical power is dissipated in the absorber, and confirm a thermal model for a test THM with a Mo/Au TES and Bi/Au absorber. Scientific Motivation: Polarization in the CMB A 0.1uK B-mode polarization anisotropy has been predicted to exist in the CMB and the detection of such a signal would confirm one of the key predictions of inflation, gravitational waves. 1 The most recent power spectrum results from WMAP 3. An array of THMs would have the sensitivity to reach the predicted B-Mode signal level shown in blue. Temperature Temperature/E-mode Correlation E-Mode B-Mode The THM Design The THM consists of a metal absorber overlapping a superconducting bilayer TES. The absorber forms the termination of a superconducting microstripline that carries RF power from an antenna. The THM test device consists of a thin Bi/Au absorber and bilayer Mo/Au TES. For these tests the microstrip line is replaced by Mo leads attached to the absorber. The TES transitions at mK and has a normal resistance of 0.32 Ω. The absorber has a resistance of 17 Ω. Two Thermal Models Thermal Conductances: Weidemann-Franz, G e-e : Thermal energy is carried by electrons (or quasiparticles). 8 Electron-Phonon, G e-p : At low temperatures significant decoupling between electrons and phonons. G proportional to volume, V, and Σ, a material dependent constant. 7 G e-p T bath TcTc TES/ Absorber W + P BIAS G e-pabs T bath T abs Absorber W G e-pTES/abs TcTc TES/ Absorber P BIAS G e-e (a) Diagram of microstrip- coupled THM. (b) SEM image showing the THM test device. The absorber Bi layer is 500 nm under a 190 nm Au layer. The TES is 40 nm Mo under a 150 nm Au layer. (a) Thermal model for ideal THM. W is power incident on the absorber, P BIAS is Joule power due to the voltage bias of the TES, T c is the operating temperature for the TES, and T bath is the cold stage temperature. (b) Thermal model for thermally disconnected THM. The ends of the absorber are at temperature T abs, while the TES and the absorber nearby the TES are at T c. Test Setup The test device was placed within a Nb can and heat sunk to a cold stage cooled to ~ mK using a shielded ADR. The TES was placed in parallel with a shunt resistor, R S =25 mΩ, and read out by a NIST 2-stage SQUID with M~190 pH. NIST SQUID THM I-V Characteristics.  In the transition, Joule power dissipation (P BIAS =IV BIAS ) is constant, which accounts for the negative slope.  For T bath ~T c, the transition current decreases and the I-V curve approaches the normal linear regime.  The I-V curves maps out the bias points in the transition. Voltage biasing the TES provides negative electrothermal feedback 4. I-V curves at different cold stage temperatures. The black line indicates the normal resistance.,.,. Σ Au [W/m 3 K 5 ]Σ Bi [W/m 3 K 5 ]G e-pabs [W/K]G e-pTES [W/K]G e-e * [W/K] Ideal x x x x x Disconnected x x x x10 -9 Literature 8, x x x x10 -9 Σ Bi, Σ Au & G’s (at 310 mK) resulting from fits to W vs. T bath for the disconnected and ideal models and literature values.  We measured the amount of Joule power dissipated in the absorber, W, at T c as a function of T bath.  The disconnected model provides the best fit to W(T bath ).  The disconnected model fits to Σ values only slightly lower than literature values.  G e-p > G e-e *For R abs =17 Ω. T bath [mK]V BIAS [μV]Measured S [A/W] Disconnected Ideal S [A/W] Fit S [A/W]Literature S [A/W] x x x x10 5  The measured responsivities agree to a disconnected thermal model within a factor of 2 for literature  values, and within a factor of 2-5 for fitted  values.  The ideal thermal model overestimates the responsivity by a much greater factor. Measured and predicted responsivity (following the block diagram method 5 ) for both thermal models with fitted and literature Σ values (Table 1) for W < 3.5x W. References 1. W. Hu and S. Dodelson, Annu. Rev. Astron. And Astrophys. 40, 171 (2002). 2. Task Force on CMB Research, Final Report, (2005). 3. Page, L. et al., astro-ph/ , (2006). 4. K. D. Irwin, Appl. Phys. Lett. 66, 1998 (1995). 5. M. Galeazzi and D. McCammon, J. Appl. Phys., 93, 4856 (2003). 6. F. C. Wellstood, C. Urbina, and J. Clarke, Appl. Phys. Lett. 54, 2599 (1989). 7. F. Pobell, Matter and Methods at Low Temperature, 2nd Ed., Springer, Berlin-Heidelberg, 64 (1996). 8. F. Pierre, A. B. Gougam, A. Anthore, H. Pothier, D. Esteve, and N. Birge, PRB, 68, (2003). 9. S. I. Dorozhkin, F. Lell, and W. Schoepe, Solid State Commun., 60, 245, (1986). 10. W.S. Boyle and K.F.Rodgers, J. Opt. Soc. Am.,49, 66 (1959). Acknowledgements We would like to thank Dan McCammon and Mark Lindeman for their helpful discussions and Kent Irwin for providing the SQUID readouts. EB would like to thank the Wisconsin Space Grant Consortium and the NASA-Goddard Graduate Student Research Program for support. Conclusions The results of W vs. T bath, conductance, responsivity & noise measurements for this THM test device are fit well by a thermal model in which G e-e < G e-p. These results also agree with literature values for the e-p conductance of Au and Bi. To reach phonon noise-limited sensitivity levels we plan to decrease the THM area by two orders of magnitude and optimize the relative dimensions of the absorber and TES to operate within the ideal thermal model for this detector. To detect B-mode polarization:  Need arrays of thousands of photon- noise-limited detectors.  The most promising technologies include planar antenna-coupled bolometers with TES thermometers read out by SQUIDs. 2 (b) Disconnected Model: G e-p > G e-e (a) Ideal Model: G e-p < G e-e W vs T bath fit to disconnected (blue) and ideal (red) model. (a) (b)