Enhanced Athermal Phonon Responsivity in a Kinetic Inductance Detector with Integrated Phonon Collectors

Abstract

Cryogenic phonon detectors are adopted in light dark matter searches and coherent elastic neutrino-nucleus scattering experiments as they can achieve low energy thresholds. The phonon mediated sensing of silicon particle absorbers has already been proved with Kinetic Inductance Detectors (KIDs), acting both as sensors and athermal phonon absorbers at the same time. In this work we present the design and the performance of an improved detector, where the KID acts only as sensor and is coupled to dedicated phonon collectors. The separation between the detector and the collectors increases the variation of the quasi-particle density within the device, thereby enhancing its responsivity. The meander of the KID is composed of a 77nm trilayer wire of Aluminum-Titanium-Aluminum, while the phonon collectors are made of a 100nm Aluminum layer and act as quasi-particles funnels. Inside the collectors, the absorbed athermal phonons generate quasi-particles which, after diffusion, are trapped in the lower-gap superconducting trilayer. The performance of this setup is compared to that of a standard phonon-mediated KID, showing an increase in responsivity by around a factor of five.

Figures (6)

• Figure 1: (a) Layout of the FunKID. The funnels in purple intersect the inductive meander indicated with L. The capacity C is adjusted by means of the first two capacitors to tune the resonant frequency. (b) Detail of the funnels. The measures are in $\mathrm{\mu m}$.
• Figure 2: (a) Detection scheme of the FunKID. Athermal phonons (white dots) are generated by an energy deposition in the substrate. A fraction of these phonons is absorbed by the funnels, which have a higher superconducting gap $\Delta_{\mathrm{Al}}$. There, CPs (red paired dots) are broken, producing QPs (single red dots). Finally, the QPs diffuse into the funnel and are absorbed by the KID, which features a lower gap $\Delta_{\mathrm{AlTiAl}}$, where they recombine. (b) Current density simulated with SONNET. Most of the current flows into the KID, while a small leakage is observed. The length reported is measured in $\mathrm{\mu m}$.
• Figure 3: The two pixels on the silicon tile suspended by Teflon supports. On the left is the FunKID, while on the right the standard phonon-mediated KID.
• Figure 4: Transmission function amplitude $|S_{21}|$ around the resonance frequencies $f_r$ of the resonators.
• Figure 5: Relative shift of the resonant frequency with respect to the base temperature resonant frequency, $\delta f_r/ f_r$, for the FunKID in blue and KID in purple. The dashed curves are obtained with the BCS model calculated with our best predictions of $\alpha$ and $\Delta_0$.