Ultrasensitive Kilo-Pixel Imaging Array of Photon Noise-Limited Kinetic Inductance Detectors Over an Octave of Bandwidth for THz Astronomy

J Low Temp Phys https://doi.org/10.1007/s10909-018-1962-8 Ultrasensitive Kilo-Pixel Imaging Array of Photon Noise-Limited Kinetic Inductance Detectors Over an Octave of Bandwidth for THz Astronomy J. Bueno1 · V. Murugesan1 · K. Karatsu2 · D. J. Thoen2 · J. J. A Baselmans1,2 Received: 11 October 2017 / Accepted: 10 May 2018 © The Author(s) 2018 Abstract We present the development of a background-limited kilo-pixel imaging array of ultrawide bandwidth kinetic inductance detectors (KIDs) suitable for spacebased THz astronomy applications. The array consists of 989 KIDs, in which the radiation is coupled to each KID via a leaky lens antenna, covering the frequency range between 1.4 and 2.8 THz. The single pixel performance is fully characterised using a representative small array in terms of sensitivity, optical efficiency, beam pattern and frequency response, matching very well its expected performance. The kilo-pixel array is characterised electrically, finding a yield larger than 90% and an averaged noise-equivalent power lower than 3 × 10−19 W/Hz1/2 . The interaction between the kilo-pixel array and cosmic rays is studied, with an expected dead time lower than 0.6% when operated in an L2 or a similar far-Earth orbit. Keywords Kinetic inductance detectors · Kilo-pixel array · THz astronomy 1 Introduction The next generation of space-based imaging spectrometers for sub-millimetre (sub-mm) wave astronomy requires broad band radiation coupling between 1 and 10 THz [1,2]. These spectrometers will allow measurements of a large number of spectroscopic bands over a wide area of the sky in a very limited time. In order to do so, they will require a large number of pixels to cover the telescope field of view or to sample a given frequency band with a high resolution. Kinetic inductance detec- B J. Bueno 1 SRON Netherlands Institute for Space Research, Utrecht, The Netherlands 2 Terahertz Sensing Group, Delft University of Technology, Delft, The Netherlands 123 J Low Temp Phys tors (KIDs) are superconducting pair-breaking resonators [3] that are a very attractive choice for these applications since thousands of detectors can be read out with a single coaxial line [3,4], enabling simple and cost-effective systems. Since these spectrometers can only be used from space at these high frequencies, the requirements on the detector sensitivity [5] are extremely demanding, typically with an noiseequivalent power (NEP) of ∼ 3 × 10−19 W/Hz1/2 for a non-dispersive spectrometer. Such sensitivities have been achieved with antenna-coupled aluminium (Al) KIDs over a broad band [6] around 1.5 THz with poor beam quality and over a narrow band around 850 GHz [4,7]. In this paper, we extend KID technology to higher frequencies and large bandwidths using a leaky lens antenna-coupled device. This device allows high coupling efficiency over an octave of bandwidth at frequencies higher than 1 THz. 2 Design and Fabrication We have designed, fabricated and measured a small chip of leak-lens antenna-coupled KIDs operating in the 1.4–2.8-THz band [8]. The KID design combines the hybrid NbTiN/Al technology to obtain good noise performance [9] and the all-Al antenna concept [6] to provide a very high sensitivity. A long and detailed discussion about the requirements of the detector system, its fabrication and full characterisation (sensitivity, optical efficiency, beam pattern and frequency response) is presented in our previous work [8]. In summary, the device has a beam pattern and frequency response close to the simulated parameters and has a limiting sensitivity given by a NEPopt = 2.5 × 10−19 W/Hz1/2 . In this paper we focus on the scalability of the single pixel device into a kilopixel array. All the fabrication details are discussed in our previous work [8], and the same process flow is followed in the fabrication of the device presented in this paper. An image of the fabricated kilo-pixel leaky lens antenna-coupled KID array is shown in Fig. 1. The detector array consists of 989 pixel KIDs hexagonally packed, with a pixel spacing of 1.6 mm covering an area of 48 × 48 mm on a 55 × 55 mm chip. The THz radiation is coupled to the leaky slot in the Al ground plane, which launches the radiation into the two very narrow Al CPW lines. The length of the Al lines (∼1.25 mm) is such that all THz radiation is absorbed over the whole octave of bandwidth before the lines become wide. The length of the Al has been chosen to absorb more than 10 dB of power for the highest radiation frequency (2.8 THz) before reaching the NbTiN evaluating the attenuation constant of the line using CST. The Al line absorbs even more at the lowest frequency (1.4 THz). The narrow linewidth (0.8 µm strip with a 1.2 µm gap) is needed to limit radiation loss. The narrow Al line broadens at either end and connects to a wide NbTiN CPW (strip of 12 µm with a gap of 8 µm). The NbTiN central conductor is shorted to the NbTiN ground at the far end of the resonator. At the other end, the NbTiN remains wide is deposited on the bare Si substrate for most of its length. The main challenge of the fabrication is to resolve the narrow aluminium line (1.2–0.8–1.2 µm) close to the antenna with a high yield across the whole wafer. 123 J Low Temp Phys Fig. 1 Image of the kilo-pixel leaky lens antenna-coupled KID array. Left: photograph of the array mounted in its holder. Right: back- and front-illuminated optical image of a single pixel of the leaky lens antennacoupled KID. The light goes through the membrane where both the antenna and the Al section of the KID are fabricated. The centre of the antenna is shown as an inset with an SEM image (Colour figure online) 3 Electrical Characterisation A 3D assembly of the detector chip, spacer wafer and lens array is needed to couple radiation efficiently to the device [8]. It is crucial to reach a vacuum gap between the antenna and the spacer wafer of less than 6 µm, which is very challenging for a 55 × 55 mm chip (like the one presented in this paper). A smaller prototype with 19 pixels has been characterised under radiation-loaded conditions, showing very good sensitivity, optical efficiency, beam pattern quality and broad frequency response [8]. In this work we limit ourselves to a dark measurement of the kilo-pixel array, which is possible using a measurement of the detector chip only, without spacer wafer and lens array. We take advantage of the fact that for NbTiN-Al hybrid KIDs it has been proven that the electrical NEP is a very good approximation for the optical NEP [4,10]. To characterise the performance of the kilo-pixel detector array we mount it in a closed sample holder in a ‘box-in-a-box’ configuration on the cold stage of an adiabatic demagnetisation refrigerator (ADR) [11], where the temperature of the chip is stabilised at 120 mK. We use a commercial vector network analyser to measure the forward scattering parameter S21 of the system as a function of frequency. The results (...truncated)