Design and tests of high sensitivity NTD Ge thermometers for the Planck-High Frequency Instrument.

The High Frequency Instrument of Planck needs high sensitivity semi-conductors at low temperature to monitor the temperature of the bolometer plate. We have modeled such thermometers by using a semi-analytical approach of Anderson insulators, taking into account both the electrical field and the electron/phonon decoupling effects. The optimized design uses convenient NTD Ge material and has larger dimension than the initial design. The first measurements of these optimized thermometers showed a significant thermal de-coupling effect due to Kapitza resistance with its mechanical support. Nevertheless, a sensitivity of about 8nK.Hz[sup -0.5], not far from the predicted one, was obtained. The noise spectrum of the thermometer was flat down to 1Hz, dominated at lower frequency by the thermal fluctuations. [ABSTRACT FROM AUTHOR]