Paper MN-WeM11
High-resolution, MEMS-based Calorimeter for Quantitative Studies of Superconducting Phase Transitions in Thin Film Samples

Wednesday, November 9, 2016, 11:20 am, Room 102B

Quantitative calorimetric studies of minute amount of material is a notoriously sophisticated task. It requires a high-resolution calorimeter design that contributes a negligible background signal (addenda). The issue is further complicated when the electronic specific heat of a sample is concerned, e.g. in the case of characterising the superconducting phase transition of a superconductor in thin film form. Over a wide temperature range, the electronic specific heat in most materials is one or two orders of magnitude lower than its lattice counterpart, raising even more stringent requirements on the resolution and accuracy of calorimetric measurements.

In this work, we describe a state-of-the-art, microelectromechanical system (MEMS)-based differential calorimeter operating in the ac steady-state mode that addresses this challenge. The calorimeter cell, consisted of two 150-nm-thick SiN_x windows placed side by side, is fabricated with a batch processing routine utilising silicon bulk micromachining techniques. Active components of the calorimeter include a gold-germanium resistive thermometer, an AC heater for delivering a well-defined alternating power to the sample, and a DC heater for locally heating the sample above the base temperature. They are defined using UV lithography and deposited onto the centre of the membrane window in the form of a stack of thin film layers [1]. The addenda of the calorimeter cell are as low as a few tens of nJ/K at room temperature, and further decrease down to 10 pJ/K at 1 K. Calorimetric measurements are carried out in a sample-in-vacuum ³He cryostat, under an automatic frequency adjusting, true differential-mode using custom-designed FPGA-based advanced electronics [2]. This provides both absolute accuracy and high resolution, where the addenda from the calorimeter cell are largely eliminated.

Thin films of superconducting niobium and tantalum in the range of hundreds of nanometers are used in demonstrating the capability of our calorimeter. They are deposited onto prefabricated silicon nitride windows, sculptured with a focused ion beam, and then transferred onto one of the calorimeter windows with a micromanipulator under an optical microscope. Specific heat jumps at the respective superconducting phase transition are still found to display an excellent signal-to-noise ratio while in-field measurements allow quantitative studies of the suppression of superconducting order parameters by applied magnetic fields.

[1] S. Tagliati, V. M. Krasnov, and A. Rydh, Rev. Sci. Instrum. 83 (2012) 055107.