Nanoelectromechanical infrared detector

Abstract

The sensitive detection of infrared (IR) radiation is a essential task in today's modern world. The sensitivity of the state-of-the-art uncooled thermal infrared detectors is still several orders of magnitude above the fundamental photon noise limit. Thermal detectors based on temperature sensitive micro-and nanomechanical resonators are a promising approach to obtain improved thermal IR detectors. Here, we present an uncooled infrared detector based on a 1 mm×1 mm large nanoelectromechanical drum resonator made of 50 nm thick low-stress silicon nitride (SiN). The detector features a thin film absorber with an absorptivity of ∼30% over the entire mid-IR range. The detector drum is driven at its resonance frequency by means of a phase-locked loop. Absorbed IR radiation results in an observable detuning of the drum's oscillation frequency. We measured an Allan deviation of σ A = 5.5 × 10 −7 at room temperature at a noise bandwidth of 25 Hz. With a responsivity of R = 343 W −1 this results in a sensitivity defined as noise equivalent power (NEP) of NEP = 320 pW/rtHz for an IR beam at a wavelength of 9.5 µm. For this measurement, the IR beam focus spot diameter was equal to the drum size. The drum's responsivity improves by a factor of ten for a focal spot size smaller than ∼ 100 µm. For smaller spots the responsivity remains constant. Based on this analysis we predict a sensitivity of ∼ 30 pW/rtHz for an IR spot size smaller than 100 µm. The detector can be improved further by e.g. optimizing the tensile pre-stress to a lower value or by improving the absorptivity.

Frequency fluctuations in nanomechanical silicon nitride string resonators

“…As the size of a mechanical resonator is minimized, the responsivity towards changes in the resonator environment is increased [1,2]. This has led to a tremendous amount of research in the field of nanomechanical resonators, with devices being employed to detect mass [3][4][5], force [6,7], and temperature [8][9][10][11]. Most studies have employed changes in the resonance frequency f 0 as the detection principle.…”

Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators

“…We demonstrate optimized trampoline structures made of SiN as enhanced NEMS-based infrared detectors. Compared to an earlier work on drum based nanomechanical resonators as infrared detectors, 20 which were also measured in this work as a reference, we could improve the NEP by two orders of magnitude with a minimum measured value of 5 pW/ √ Hz. Since the small trampoline structures inevitably lead to a smaller detection area, focusing becomes of great significance and needs further investigations prior to benefit from the enhancement in applications.…”

“…The incident light causes a change in temperature the resonator and a thermal expansion leading to a reduction of the tensile stress. 8,20,26,27 Hence, the responsivity of such a nanoelectromechanical detector with a detector area A is given by the relative frequency change δf = ∆f /f 0 per power of IR light irradiated over the detector area…”

Micromechanical Bolometers for Subterahertz Detection at Room Temperature

“…Considering exclusively the infrared spectral range, several MEMS resonator bolometers have been proposed in recent years, resulting in a wide assortment of geometries and materials. These include GaN plates suspended by two tethers, 28 Si 3 N 4 drums with a thin-film absorber, 29 a Si torsional resonator with a TiN absorber, 30 , 31 graphene–AlN–Pt nanoplates, 32 SiN phononic crystal membranes, 33 and graphene drum/trampolines. 34 To date and limited to our knowledge, only one work has demonstrated a MEMS resonator capable of room-temperature bolometric detection in the THz range.…”