Using thin-film technology with its characteristic low masses and high aspect ratios a new range of possibilities is made available for the use of dynamic thermal measuring principles. Based on this, a micromachined sensor was developed for the measurement of transient thermal signal responses leading to the thermal characterization of fluids at low sample volumes. The achieved resolution allowed the measurement of thermal parameters of the investigated fluids, i.e., thermal conductivity and specific heat, inside microfluidic systems at a high sensitivity, enabling the detection of inter-fluid boundaries, e.g., as found in micromixers and -reactors, making the sensor a useful tool for micro fluidic system characterization. This is achieved via the measurement of the frequency dependent thermal signal response.

Dynamic thermal sensor - principles in MEMS for fluid characterization

We report on the current status of our work on x-ray microcalorimeters for use as high resolution x-ray spectrometers. To
maximize the x-ray collecting area and the signal to noise ratio, the total heat capacity of the device must be minimized. This
is best achieved if the calorimeter is divided into two components, a thermal sensor and an x-ray absorber. The thermal sensor
is a neutron transmutation doped (NTD) germanium resistor made as small as possible to minimize the heat capacity of the
calorimeter. The thermistor can be attached to a thin x-ray absorber with large area and low heat capacity fabricated from
superconducting materials such as niobium. We discuss results from our most recent studies of such superconducting
absorbers and present the x-ray spectra obtained with these composite microcalorimeters at a temperature of 0. 1 K. An energy
resolution of 19 eV FWHM has been measured.

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X-ray microcalorimeters with germanium-resistance thermometers

A kinetic inductance thermometer is applied to x-ray calorimetry, and its operation over a wide range of fre-quencies and geometries is discussed. Three amplifier configurations are described, one using a superconducting quantum int.erference device (SQUID) amplifier, another incorporating an FET amplifier in an amplitude modulated system, and the third, using a tunnel diode frequency modulated oscillator circuit. The predicted performance of each configuration is presented. 1. INTRODUCTIONWe are interested in fabricating bmad band x-ray detectors operating between 0.1 and 10 keY with thehighest achievable energy resolution. In principle, x-ray microcalorimeters operating at cryogenic temperaturesshould be able to cover this energy band with a 1 eV resolution.1'2 To date, 12 eV has been achieved using resis-tive sensors thermally connected to an x-ray absorbing material.3 In these devices, the resistive Johnson noise ofthe sensor is always present and is a limitation to the overall performance. To further improve the resolution of thedevice, the Johnson noise must be reduced. If the Johnson and amplifier noise can be reduced, the temperaturefluctuation noise will dominate the noise spectrum over a much larger range in frequencies. Experiments arecurrently being carried out with calorimeters using dielectric sensors.4 In principle, dielectric calorimeters are freeof Johnson noise, and produce signal strengths far above the remaining amplifier noise. Another way to circumventJohnson noise is through the use of superconducting materials. Below the critical temperature, only the residualresistance from superconducting surfaces need to be considered, and they prove to be negligible.5 As suggested byMcDonald,6 the temperature dependence of the kinetic inductance in a superconducting thin film can be utilized as

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Yolanda C. Wai

Papers

X-ray microcalorimeters with germanium-resistance thermometers

Application of kinetic inductance thermometers to x-ray calorimetry