Hannu Sipola

Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate

Abstract A capacitive single-chip silicon microphone with very low-stress polysilicon membrane was fabricated. A mechanism for stress-releasing due to the high stress of the perforated membrane was introduced. With the achieved stress level of 2 MPa, a microphone with the membrane area of 1 mm 2 can be optimally designed, although the measured components did not show the optimal resolution due to excessive acoustic resistance. With a membrane area of 1 mm 2 , the acoustical sensitivity was 4 mV/Pa (at 1 kHz) and the noise equivalent sound level was 33.5 dB (A), which are adequate values for many applications. The packaged components were tested with a thermal cycle between −40°C and +60°C, and due to low packaging-related stresses, no buckling of the membranes was observed.

An Ultra-Low Noise Superconducting Antenna-Coupled Microbolometer With a Room-Temperature Read-Out

In this letter, we report the electrical and optical characteristics of a superconducting vacuum-bridge microbolometer with an electrical noise equivalent power of 26fW radicHz and an effective time constant of 380 ns, when operated at a bath temperature of 4K. We employ a novel room temperature external negative feedback readout architecture, that allows for noise matching to the device without bulky stepup transformers or cooled electronics. Both the detector and the readout lend themselves to be scaled to imaging arrays. The directly measured noise equivalent temperature difference over a 100-1000-GHz bandwidth is 125 mK in a 30-ms integration time

Low-noise readout of superconducting bolometers based on electrothermal feedback

We have developed a simple room-temperature readout for superconducting bolometers and calorimeters. Due to electro-thermal feedback, the bolometer can be noise-matched with the readout amplifier. The constructed amplifier consists of low-noise field-effect transistors, and voltage bias is actively provided by electrical feedback. According to our measurements on superconducting vacuum-bridge bolometers, the total noise is limited by the bolometer phonon and Johnson noise characterized by a critical temperature of 8.5 K. We show that, with proper sensor design, it is possible to reach the photon noise limit in video-rate submillimetre imaging applications.

Antenna-coupled microbolometers for passive THz direct detection imaging arrays

In the recent years, millimetre wave and THz imaging have received a vast amount of interest due to the interesting possibilities and applications that imaging at these frequencies could enable. Many of the applications that have generated substantial interest (such as stand-off concealed weapons detection) often require exquisite sensitivity, while a low system cost is required. In this paper we discuss one potential candidate for affordable imaging arrays: an antenna-coupled superconducting microbolometer. We show that these devices possess capabilities that are hard to meet with other passive detection schemes. While refrigeration to cryogenic temperatures is required for maximum performance, we show that the devices can be operated within a cryogen-free refrigerator that allows for turn-key operation. Comparison with other detectors is presented

Passive Euro-American terahertz camera (PEAT-CAM): passive indoor THz imaging at video rates for security applications

The objective of this program is to demonstrate a system capable of passive indoors detection and identification of concealed threat items hidden underneath the clothing of non-cooperative subjects from a stand-off distance of several meters. To meet this difficult task, we are constructing an imaging system utilising superconducting ultrawideband antenna-coupled microbolometers, coupled to innovative room temperature read-out electronics, and operated within a cryogen-free pulse tube refrigerator. Previously, we have demonstrated that these devices are capable of a Noise Equivalent Temperature Difference (NETD) of 125 mK over a pre-detection bandwidth from 0.2-1 THz using a post-detection integration time of 30 ms. Further improvements on our devices are reducing this number to a few tens of mK. Such an exquisite sensitivity is necessary in order to achieve the undoubtedly stringent requirements for low false positive alarm rate combined with high probability of detection dictated by the application. Our technological approach allows for excellent per frame NETD (objective 0.5 K or below at 30 Hz frame rate), and is also amenable to multispectral (colour) imagery that enhances the discrimination of innocuous objects against real threats. In the paper we present results obtained with an 8-pixel subarray from our linear array of 128 pixels constructed using a modular approach. Two-dimensional imaging will be achieved by the use of conical scanning.

A high open-loop gain controller

A new controller, especially suited for phase‐locked loops, is discussed. The controller, composed of RC‐circuit electronics, has a gain with a slope of −30 dB/decade over a wide bandwidth. It is shown that in certain systems the present controller (PI3/2 controller) provides a much higher feedback gain than an ordinary proportional‐integrator controller (PI controller). As applications, the superconducting quantum interference device (SQUID) and an electrically adjustable point contact used in surface microscopy or spectroscopy are discussed in some detail. The operation of the controller in a piezo‐driven point contact was studied experimentally.

Performance Comparison of Nb and NbN Antenna-coupled Microbolometers*

We report the experimental results of a comparison between free-standing Nb and NbN microbolometer bridges coupled to equiangular spiral antennas on Si substrates. Because of the difference in material resistivity, bolometer resistance and aspect ratio is varied independently. Room-temperature antenna patterns measured at 650 GHz with a backward-wave oscillator are presented, as are I-V curves at T = 300 K and at T = 4 K. At room temperature, zero-bias resistance and specific responsivity are examined, and at 4 K, normal-state resistance and saturation power are studied. Nb devices display significantly lower saturation powers than NbN devices whose dimensions have been adjusted to provide equal resistance. However, for both materials, the inferred thermal conductances are higher than predicted by the Wiedemann-Franz relation, by approximately a factor of ~2 for Nb and a factor of ~5 for NbN. In general, and especially for the room-temperature responsivity, the substantial spread in device parameters from device to device exceeds any systematic difference in performance between the materials.

Optical and Electrical Characterization of a Large Kinetic Inductance Bolometer Focal Plane Array

Sub-THz imaging techniques are currently emerging with applications especially in security screening requiring higher throughput in mass transit and public areas. In the context of person imagers, the field of view and the spatial resolution set the requirement for the number of image pixels. We perform an experimental feasibility study on a fully staring radiometric camera with one detector per image pixel. The aim is to avoid the shortcomings characteristic of optomechanical scanners with a limited number of detectors. Our approach is based on superconducting kinetic inductance bolometer arrays. We demonstrate successful fabrication and operation of a focal plane array with 2500 nanomembrane-integrated bolometers, and a compatible optical system enabling standoff imaging at the distance of 5 m. We characterize the system in terms of radiometric contrast and spatial resolution.

Bolometric kinetic inductance detector technology for sub-millimeter radiometric imaging

Radiometric sub-millimeter imaging is a candidate technology especially in security screening applications utilizing the property of radiation in the band of 0.2 – 1.0 THz to penetrate through dielectric substances such as clothing. The challenge of the passive technology is the fact that the irradiance corresponding to the blackbody radiation is very weak in this spectral band: about two orders of magnitude below that of the infrared band. Therefore the role of the detector technology is of ultimate importance to achieve sufficient sensitivity. In this paper we present results related to our technology relying on superconducting kinetic inductance detectors operating in a thermal (bolometric) mode. The detector technology is motivated by the fact that it is naturally suitable for scalable multiplexed readout systems, and operates with relatively simple cryogenics. We will review the basic concepts of the detectors, and provide experimental figures of merit. Furthermore, we will discuss the issues related to the scale-up of our detector technology into large 2D focal plane arrays.

NbN Vacuum Bridge Bolometer Arrays With Room Temperature Readout Approaching Photon Noise Limited THz Imaging Applications

We study the applicability of superconducting NbN vacuum bridge bolometer arrays with room temperature readout electronics to passive THz imaging applications. We show that sufficient bandwidth for video-rate mechanical scanning in terms of stability and noise can be reached by exploiting the divergences of the bolometer noise temperature and the differential impedance at the I-V curve minimum. Experimental electrical noise equivalent power is 9 fW/Hz1/2 . This would correspond to ~10 times the photon noise in a bandwidth of 0.5 THz and is comparable to the expected clutter in passive THz images due to atmospheric fluctuations.