The goal of the CALDER (Cryogenic wide-Area Light Detectors with Excellent Resolution) project is the development of light detectors with large active area and noise energy resolution smaller than 20 eV RMS using phonon-mediated Kinetic Inductance Detectors (KIDs). The detectors are developed to improve the background suppression in large-mass bolometric experiments such as CUORE, via the double read-out of the light and the heat released by particles interacting in the bolometers. In the first part of the project we designed and fabricated KID detectors using aluminum. Detectors are designed by means of state-of-art software for electromagnetic analysis (SONNET). The Al thin films (40 nm) are evaporated on high quality, high resistivity (>10 kΩ·cm) Si(100) substrates using an electron beam evaporator in a HV chamber. Detectors are patterned in direct-write mode, using Electron Beam Lithography (EBL), positive tone resist poly-methyl methacrylate (PMMA) and lift off process. Finally the chip is diced into 20x20 mm2 chip and assembled in a holder OFHC copper (oxygen free high conductivity) holder using PTFE support.
Detector features are designed and simulatedusing SONNET software. Requirements for the chip geometry are:
1.Large active area
2.High resonant frequency and a
3.High fraction of kinetic inductance α
Co-Planar Waveguide (CPW) ground plane must be minimized to reduce the probability of absorbing phonons (those areas
are inactive for detection)
The limit of KIDs is the small active area, to overcome this problem, an indirect scheme of detection is used, where incident photons converted into athermal phonons and those reach the resonator.
To increase the energy resolution of
our detectors we are changing the superconductor to sub-stoichiometric TiN and/or multilayer Ti-TiN, both deposited by means of DC
The typical CALDER detector in its holder (image above), electrically connected to the SMA read out through wire bonding.
dr. Ivan Colantoni, PhD