WO2024130287A1

WO2024130287A1 - Optomechanical thermal infrared detector element

Info

Publication number: WO2024130287A1
Application number: PCT/AT2023/060447
Authority: WO
Inventors: Silvan SCHMID
Original assignee: Invisible-Light Labs Gmbh
Priority date: 2022-12-22
Filing date: 2023-12-21
Publication date: 2024-06-27

Abstract

Infrared detector element (1), comprising - a frame (2) and - a membrane (3) supported by the frame (2), wherein the membrane (3) comprises an infrared absorption area (6), wherein the infrared absorption area (6) is configured to absorb an incident infrared signal (8), wherein the membrane (3) comprises a readout area (9) for an optical measurement of a frequency of a vibration mode of the membrane (3), wherein the readout area (9) at least partly extends over an antinode (11') of the vibration mode of the membrane (3), wherein the vibration mode is an eigenmode of the membrane (3).

Description

Optomechanical thermal infrared detector element

The invention relates to quantifying an infrared signal by means of a thermal optomechanical infrared sensor. In particular, the invention concerns an infrared detector element, comprising:

- a frame and

- a membrane supported by the frame, wherein the membrane comprises an infrared absorption area, wherein the infrared absorption area is configured to absorb an incident infrared signal, wherein the membrane comprises a readout area for an optical measurement of a frequency of a vibration mode of the membrane.

The detection of infrared radiation with maximal sensitivity is an essential task for many applications, as for example for infrared spectroscopy. Basic types of infrared detectors include for example: photon detectors and thermal detectors (cf. Ro- galski, Antoni. Infrared and Terahertz detectors. CRC press, 2019) . The detection principle of photon detectors is based on the creation of electron-hole pairs in a semiconductor material by absorbed photons. In contrast, thermal detectors measure the temperature increase caused by absorbed photons.

Photon detectors are fast and offer a high sensitivity. However, their spectral response is narrow and highly wavelength dependent. Their sensitivity drops rapidly for wavelengths in the mid- IR region of the electromagnetic spectrum, where the infrared photons lack the energy required to pass the detection threshold of a photon detector. To operate into the mid-IR region, cryogenically cooling is usually necessary. Cryogenic cooling, which requires the handling of cryogens such as liquid helium, is expensive, labour-intensive, and complex, rendering its use impractical in a variety of contexts. Besides liquid nitrogen, a common cryogen is liquid helium, which is a scarce and non-re- newable resource with unpredictable prices (cf. Bare, S. R. et al. Responding to the US Research Community's Liquid Helium Crisis. https : // www .aps.org/policy/ r eport s/popa- report s /up load/ He - liumReport.pdf (2016) , accessed on November 24, 2022; Seidler, C. Helium: Forscher warnen vor weltweiter Knappheit - DER SPIEGEL - Wissenschaf t . Spieg. (2019) ) . This limits the application of cryogenic detectors to specialized laboratory environments and hinders the real-world implementation of future IR/THz technologies. In addition, quantum detectors are often made from highly toxic materials, such as HgCdTe (MCT) , which are being banned from use in photon detectors by many governments including the European Union (cf. Directive 2011/65/EU of the European Parliament and of the Council on the restriction of the use of certain hazardous substances in electrical and electronic equipment) . Hence, there is a sharp increase in the demand for high sensitivity infrared sensors, which can be operated at room temperature .

The mid- to far-IR regime of the electromagnetic spectrum exhibits some of the most characteristic and interesting spectrochemical responses. Without cryogenic cooling, this "fingerprint region" can only be reached with thermal detectors. Typical thermal detectors are bolometers, thermopiles, photoacoustic detectors (e.g. the Golay cell) , or pyroelectric detectors. Today, pyroelectric detectors, e.g., made from triglycine sulfates, are the devices with the highest-perf ormance and are therefore typically the detector of choice in Fourier transform IR (FTIR) spectrometers. In thermal detectors, the spectral response is determined by the infrared absorber, which spectral response is typically constant over a large range from near- to far-IR.

All infrared detectors, photon detectors as well as thermal detectors, are subject to performance limits arising from the statistical nature of the infrared radiation being absorbed and emitted. Photon detectors are fundamentally shot-noise limited, which is associated to the statistical rate of arrival of photons from the radiating background to which the detectors are exposed; this is known as the BLIP (background limited infrared photodetector) limit. Thermal detectors are subject to a similar limit, which is given by the statistical fluctuations in the rate of arriving photons from the radiating background. Vice versa, there is also a contribution to the noise due to random fluctuations in the radiation power emitted by the sensor itself. Thus, temperature fluctuations due to such photon noise is the ultimate noise limit when thermal conduction and convection are negligible and heat transfer is dominated by radiation exchange between the detector and its surroundings.

While thermal detectors' sensitivity has been steadily improved, they still operate significantly below the theoretical photon noise detectivity limit. For any thermal detector of area A with an emissivity a that is in equilibrium and coupled to its surroundings via a single side at temperature T, the sensitivity in terms of specific detectivity is given by (cf. Rogalski, Antoni. Infrared and Terahertz detectors. CRC press, 2019)

under the assumption that the signal frequency is slow (m < 1/ i_r} , where T_r is the thermal time constant of the detector. Here, k_B and o_B re the Boltzmann constant and the Stefan-Boltzmann constant, respectively. The specific detectivity is the figure of merit used for photon detectors, whose quantum limited noise scales with the detector size. For thermal detectors that operate above the photon noise limit, this scaling law with detector size does not apply and NEP is the proper figure of merit. In the case that a thermal detector reaches the temperature fluctuation photon noise limit, the same scaling law appears again and

becomes the appropriate figure of merit. Hence, the fundamental sensitivity limit of an ideal thermal detector according to equation (1) at room temperature (T = 290 K) with a perfect absorber (a = 1) is D* = 2 x 10¹⁰ cm Hz^1/2/W.

As an alternative thermal detector design, mechanical thermal detectors have been introduced in the late 60s as promising sensor concepts. A mechanical thermal detector provides a completely alternative way to temperature detection which is not limited by electronic noise as most other IR detectors. A patent on a specific mechanical resonator-based detector was granted to the spectroscopy instrument manufacturer Cary Instruments in 1969 under US 3 457 412 A (see also Piller, M., Sadeghi, P., West, R.G., Luhmann, N., Martini, P., Hansen, 0. and Schmid, S., 2020. Thermal radiation dominated heat transfer in nanomechanical silicon nitride drum resonators. Applied Physics Letters, 117 (3) , p.034101) . US 3 457 412 A discloses a fundamental idea of a macroscopic tensioned foil resonator that acts as the sensor element. Even though the disclosure shows the core principle, such macroscopic IR/THz detectors never have been commercialized. Reliable fabrication of the required nanometer thick mechanical foil resonator element could first be implemented with the invention of microsystem technology. It was in the 1990s that the idea of a mechanical resonant detector was picked up again, this time based on piezoelectric micromechanical resonators (see Vig, J. R., Filler, R. L. & Kim, Y. Uncooled IR imaging array based on quartz microresonators. J. Microelectromechanical Syst. 5, 131-137 (1996) ) . However, the micromechanical piezoelectric detectors never reached competitive sensitivities. It was only in 2013, when a new concept based on smaller nanoelectromechanical system (NEMS) paddle resonators was published that interest was renewed in the concept of a mechanical resonator-based detector (see Zhang, X. C., Myers, E. B., Sader, J. E. & Roukes, M. L. Nanomechanical torsional resonators for frequency-shift infrared thermal sensing. Nano Lett. 13, 1528-34 (2013) ) . This concept has been advanced to a new type of NEMS- based silicon nitride (SiN) drum and trampoline resonators which reach an NEP = 7 pW/Hz^1/2 (see Piller, M., Hiesberger, J., Wistrela, E., Martini, P., Luhmann, N. and Schmid, S., 2021. Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators. arXiv preprint arXiv: 2105.03999) . While these SiN NEMS detectors are in the radiation-limited heat transfer regime (cf. Piller, M., Sadeghi, P., West, R.G., Luhmann, N., Martini, P., Hansen, 0. and Schmid, S., 2020. Thermal radiation dominated heat transfer in nanomechanical silicon nitride drum resonators. Applied Physics Letters, 117 (3) , p.034101; Zhang, C., Giroux, M., Nour, T.A. and St-Gelais, R., 2020. Radiative heat transfer in freestanding silicon nitride membranes. Physical Review Applied, 14 (2) , p.024072) , the sensitivity of these NEMS resonators with electronic readout is still significantly below the fundamental limit given in equation (1) . SiN NEMS detectors have also been demonstrated with optical readout but without an effective broadband infrared absorber with nanowatt sensitivity (see Snell, N., Zhang, C., Mu, G. and St-Gelais, R., 2020, May. Nanowatt Thermal Radiation Sensing using Silicon Nitride Nanomechanical Resonators. In 2020 Photonics North (PN) (pp. 1-1) . IEEE) . Similar detector concepts for the visible and near IR region of the electromagnetic spectrum have been introduced with graphene resonators (see Liu,

S., Chen, Y., Lai, H., Zou, M., Xiao, H., Chen, P., Du, B., Xiao, X., He, J. and Wang, Y., 2022. Room-Temperature Fiber Tip Nanoscale Optomechanical Bolometer. ACS Photonics, 9(5) , pp .1586-1593 ; Blaikie, A., Miller, D. and Aleman, B.J., 2019. A fast and sensitive room-temperature graphene nanomechanical bolometer. Nature communications, 10 (1) , pp.1-8) .

A related application is photothermal microscopy and infrared spectroscopy with nanomechanical SiN resonators. See for example:

- Larsen, T., Schmid, S., Villanueva, L.G. and Boisen, A., 2013. Photothermal analysis of individual nanoparticulate samples using micromechanical resonators. Acs Nano, 7 (7) , pp .6188- 6193 ;

- Schmid, S., Wu, K. , Larsen, P.E., Rindzevicius , T. and Boisen, A., 2014. Low-power photothermal probing of single plas- monic nanostructures with nanomechanical string resonators. Nano letters, 14 (5) , pp .2318-2321 ;

- Andersen, A. J., Yamada, S., Pramodkumar, E.K., Andresen,

T.L., Boisen, A. and Schmid, S., 2016. Nanomechanical IR spectroscopy for fast analysis of liquid-dispersed engineered nanomaterials. Sensors and Actuators B: Chemical, 233, pp.667-673;

- Kurek, M., Carnoy, M., Larsen, P.E., Nielsen, L.H., Hansen, 0., Rades, T., Schmid, S. and Boisen, A., 2017. Nanomechanical infrared spectroscopy with vibrating filters for pharmaceutical analysis. Angewandte Chemie, 129(14) , pp .3959-3963.

For NEMS resonators, the relevant noise sources are temperature fluctuation noise and thermomechanical noise. The noise equivalent power ("NEP") with units 'W/Hz^1/2' is then given by

where S_y,_th (co) and Sy,thm (co) are the fractional frequency noise spectral densities due to temperature fluctuation noise and ther- momechanical noise, respectively. To optimize NEP, the responsivity R(a) must be maximized and the noise minimized. The noise due to temperature fluctuations determines the ultimate detector sensitivity, as given by equation (1) for the case that heat transfer is dominated by thermal radiation. However, this requires that the thermomechanical noise is lower than the noise due to thermal fluctuations .

Absorbed electromagnetic radiation causes a temperature increase of the nanomechanical resonator which in turn detunes its resonance frequency. These frequency changes of a specific mode of the nanomechanical resonator are monitored by implementing a frequency tracking scheme. For this task, closed-loop schemes, such as a self-sustaining oscillator or phase-locked loop, are typically used. The fractional frequency noise density for a tracking scheme can be calculated based on the phase noise due to thermomechanical noise and detection noise from the readout

(see Demir, A., 2021. Understanding fundamental trade-offs in nanomechanical resonant sensors. Journal of Applied Physics, 129(4) , p.044503) .

Here, w₀ is the eigenfrequency of the specific mode, and

and are the transfer functions of the specific tracking scheme. These transfer functions take a typical form as

is the resonator time constant with the quality factor Q (see Hajrudin Besic, et al., "Resonance frequency tracking schemes for micro- and nanomechanical resonators", to be published) . H_R (ja) is a first order low-pass filter with a cut-off frequency 1/ T_R. And H_L ja} is a first order low-pass filter with a cut-off frequency 1/ T_L. This latter filter is tracking scheme specific and, e.g., represents the PID controller inside a phase-locked loop or the integration filter in a frequency counter. The detection phase noise, which depends on the specific readout technique used to detect the vibration of the nanomechanical resonator, can be expressed in relation to the fundamental thermomechanical phase noise

with the factor K which is the ratio of the detection noise amplitude to thermomechanical noise peak amplitude.

The fractional frequency fluctuations are best represented by the Allan deviation, which for a specific S_yfthm (m) can be calculated from (cf. Demir, A., 2021. Understanding fundamental trade-offs in nanomechanical resonant sensors. Journal of Applied Physics, 129(4) , p.044503) :

Nanomechanical SiN resonators are famous for their exceptionally high quality factors of several million (see Schmid, S., Jensen, K.D., Nielsen, K.H. and Boisen, A., 2011. Damping mechanisms in high-Q micro and nanomechanical string resonators. Physical Review B, 84 (16) , p.165307) . This results in long time constants T_R that typically are longer than the frequency tracking scheme's time constant T_R » T_L. In this scenario, it has been shown that detection noise can enhance frequency fluctuations, even for readout schemes that resolve the thermomechanical noise peak K < 1 (cf. Demir, A., 2021. Understanding fundamental trade-offs in nanomechanical resonant sensors. Journal of Applied Physics, 129(4) , p.044503; Sadeghi, P., Demir, A., Villanueva, L.G., Kahler, H. and Schmid, S., 2020. Frequency fluctuations in nanomechanical silicon nitride string resonators. Physical Review B, 102 (21) , p.214106) .

Optical interferometry provides the highest-perf ormance readout of nanomechanical resonators providing the lowest detection noise. However, when focusing a readout laser onto a nanomechanical detector, the laser' s intensity fluctuations will translate into additional frequency noise (cf. Sadeghi, P., Demir, A., Vil- lanueva, L.G., Kahler, H. and Schmid, S., 2020. Frequency fluctuations in nanomechanical silicon nitride string resonators. Physical Review B, 102 (21) , p.214106) .

Thermal IR detectors with nanoelectromechanical silicon nitride trampoline resonators are for example known from Piller, M., Hiesberger, J., Wistrela, E., Martini, P., Luhmann, N., & Schmid, S. (2021) . Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators. arXiv preprint arXiv: 2105.03999. The NEMS trampoline detectors feature an ultrathin impedance-matched absorber film. A gold trace is used for reading out the photothermal detuning of the resonators' resonance frequency due to incident light.

A broadband thermal sensor based on a low-stress nanomechanical silicon nitride drum featuring a phononic-band-gap structure is known from Sadeghi, Pedram, et al. "Thermal transport and frequency response of localized modes on low-stress nanomechanical silicon nitride drums featuring a phononic-band-gap structure." Physical Review Applied 14.2 (2020) : 024068. A SiN mechanical resonator is surrounded with a phononic crystal. The thermal detuning of the resonance frequency is measured using a laser-Dop- pler vibrometer, while a secondary diode laser is used for the photothermal tuning of the frequency.

An infrared detector with a mechanical resonator having a frame and a membrane is shown in WO 2020 047 572 A2. The membrane comprises a circular absorption area in its center. In one of the disclosed embodiments the membrane vibration is detected optically with a probing light beam, which is radiating onto the membrane, a membrane edge or the frame.

Another radiation sensor is for example known from DE 10 2015 214 586 Al. The sensor comprises a cantilevered element with an optical absorber, a holding structure, and a substrate .

WO 2015 053 720 Al shows a vibration based mechanical IR detector with a resonating pixel plate and a pixel substrate attached to the plate . Furthermore , the detector comprises anchors , extension arms or cantilevers that are attaching the plate to a substrate .

Another optical sensor is known from EP 4 009 015 Al . The photosensor has an absorber comprising a metal or a dielectric . The absorber absorbs light that is perpendicularly incident on a surface of the absorber and has the same wavelength as a resonance wavelength of the absorber . The surface of the absorber includes a plurality of raised portions and has a periodic structure .

DE 10 2017 203 882 Al shows a micro-mechanical component with a substrate and an optical resonator . A shi ft of the resonance frequency of the optical resonator is related to a temperature change .

EP 3 136 066 Al shows another device for detecting infrared radiation with a substrate , a first electrode device and an absorption device .

It is therefore an obj ect of the invention to alleviate or eliminate at least some of the disadvantages of the prior art . In particular, it is an obj ect of the invention to increase the sensitivity of an optomechanical thermal infrared sensor .

The invention proposes an infrared detector of the kind stated in the outset , wherein the readout area at least partly extends over an antinode of the vibration mode of the membrane , wherein the vibration mode is an eigenmode of the membrane .

The membrane may for example be a silicon nitride membrane . Other materials for producing the membrane are - without limitation - any 2D materials ( e . g . graphene , molybdenum disulphide , et cetera ) , polymers ( e . g . SU- 8 ) , pyrolytic carbon, silicon carbide , aluminum nitride , silicon dioxide , silicon, gallium arsenide or titanium nitride . The outline of the membrane may for instance be generally rectangular or round, in particular circu- lar. The membrane may for example be circular and comprise a diameter of an interval between 10 pm and 5 mm. The membrane may for example be quadratic and comprise a side length in an interval between 10 pm and 5 mm. The membrane may for example comprise a thickness in an interval between 1 nm and 1 pm. The membrane is supported by the frame, i.e., the membrane is connected to the frame such that the membrane is under tension. The intrinsic tensile stress of the membrane may be at or below 1 giga Pascal (GPa) , optionally below 200 mega Pascal (MPa) , optionally below 50 MPa, optionally below 10 MPa. For silicon nitride membranes the tensile stress can be controlled during deposition via the stoichiometry. Alternatively, low tensile stress values may be achieved by suitable oxygen plasma exposure as disclosed for example by N. Luhmann, A. Jachimowicz, J. Schalko, P.

Sadeghi, M. Sauer, A. Foelske-Schmit z , and S. Schmid, "Effect of oxygen plasma on nanomechanical silicon nitride resonators," Appl . Phys. Lett., vol. Ill, p. 63103, 2017.

The membrane comprises an infrared absorption area, onto which an infrared signal can be guided. In the present context, infrared refers to electromagnetic radiation with wavelengths from 0.75 pm to 1 mm. The absorption area may for example be provided in the form of a thin film on top of the membrane, which shows enhanced infrared absorption compared to a surface of the membrane, which does not comprise a thin film. The thin film may comprise platinum, gold, chromium, black carbon, titanium nitride, or bismuth for example.

By absorbing incident infrared signal, heat is transferred via the absorption area into the membrane, thus heating the membrane. The resulting change in the dimension of the membrane and the changing tensile stress leads to a change of the properties, in particular the frequency, of the eigenmodes of the membrane. In general, the vibration of the membrane can be described by a wave equation, whereas the eigenmodes are solutions of the wave equation. In the case of a rectangular membrane, cartesian coordinates can be used to write down and solve the respective wave equation. In the case of a square membrane, the solutions com- prise sinusoidal functions. In case of a circular membrane, polar coordinates can be used write down and solve the respective wave equation. In the case of a circular membrane, the solutions comprise Bessel functions. In general, the eigenmodes of a membrane can be classified or numbered by a first mode number m and a second mode number n. Each eigenmode comprises nodes, i.e., points or lines along which the amplitude of the vibration in the respective eigenmode has an amplitude of zero. Similar, there are antinodes, which are points or lines along which the amplitude of the vibration has a local maximum or a global maximum, for example when evaluated over a full period of oscillation. Consequently, also the acceleration and the speed of the vibrating membrane have a local or global maximum at the antinodes. The absolute amplitude of the vibration depends not only on the eigenmode itself but on the energy in the eigenmode. In other words, the membrane comprises the eigenmodes regardless if the membrane is vibrating or not. The vibration itself may be induced externally, e.g., by external vibrations or shock, by incident infrared signal or for example a separate actuator. Hence, the amplitude of an antinode does in general not bare significant information on the intensity of the incident infrared light. The frequency and/or the relative frequency change is a more sensitive parameter for quantifying the intensity of the incident infrared signal. The change of the geometry of the membrane and/or the frame leads to a change in frequency of the eigenmode. In this context, it is assumed that the change in geometry is on the other hand so small, that the position of the antinodes is essentially constant. The infrared absorption area leads to an increased probability of absorption of incident infrared signal. Therefore, the heat intake in the membrane, the temperature difference between membrane and frame and consequently the frequency shift of the vibration mode is enhanced by the infrared absorption area, thus enhancing the sensitivity of a measurement utilizing the infrared detector element.

If membrane and frame have different thermal expansion coefficients (e.g., they can be made each of a different material with a different coefficient of thermal expansion) , it is possible to induce a net stress change in the membrane by changing the temperature of the entire ensemble (i.e., the infrared detector element) comprised of frame and membrane. Changing the temperature of the membrane and the frame, if they are made of different materials with different thermal expansion coefficients, causes both membrane and frame to expand or shrink according to their individual thermal expansion coefficient. Due to the difference in thermal expansion coefficient, the resulting strain in membrane and frame is different. This difference in strain creates a change in the stress of the membrane.

In general, the readout area and the infrared absorption area (i.e., which is dedicated for infrared absorption) are non-over- lapping. The probability of absorption of incident infrared signal is lower in the readout area compared to the infrared absorption area. Also, the probability of absorption of the optical readout signal is lower in the readout area compared to the infrared absorption area. The readout area may for example be an unobstructed surface area of the bulk of the membrane. Optionally, the unobstructed bulk of the membrane (without any additional thin film on either side of the membrane) is substantially optically reflective. The term "substantially optically reflective" may for example be understood such that at least 50% of the intensity of an incoming optical readout signal is reflected at the readout area. The readout area may be generally rectangular or round, in particular circular. The readout area may be of arbitrary shape. In particular, the readout area may comprise two separate regions, which are not connected to one another .

The readout area at least partly extends over an antinode of the vibration mode of the membrane. Since the antinodes are points or lines of maximum deflection and speed, the optical readout signal reflected at an antinode gains a relatively high phase difference during the vibration of the membrane. The total optical path length of the reflected optical readout signal varies by twice the amplitude of the deflection of the membrane. Therefore, the optical readout is most efficient and yields the highest signal to noise ratio, when the optical readout signal is incident on an antinode . Since the readout area at least partly extends over an antinode of the vibration mode of the membrane , the disclosed infrared detection element enables the measurement of an infrared signal with an enhanced signal to noise ratio and an enhanced sensitivity . The lateral location of an antinode of a speci fic resonance mode can be predicted from the theoretical mechanical model depending on the membrane shape .

The membrane may comprise a first surface and a second surface, which are parallel to the main extension plane of the membrane . The main extension plane is defined such that the membrane has the largest extension in the main extension plane , whereas the extension of the membrane is smaller in planes transverse to the main extension plane . The second surface is opposite of the first surface . Between the first surface and the second surface is the bulk of the membrane . The infrared absorption area is arranged on the first surface . The optical readout area may be arranged on the first surface . Alternatively, the optical readout area may be arranged on the second surface, opposite of the infrared absorption area .

The infrared absorption area may cover a part of the first surface of the membrane . Alternatively, the infrared absorption area may cover essentially the whole first surface of the membrane . The readout area may be small compared to the infrared absorption area . For example , the surface area of the readout area may be below 50% , in particular below 40% , below 30% , below 20% , below 10% or below 5% , of the surface area of the infrared absorption area . This way, the signal to noise ratio and the sensitivity of the measurement can be further enhanced . In general , shaping the incident infrared signal is complex and leads to loss of infrared signal , thus the sensitivity of the measurement might be decreased . Therefore , a relatively large infrared absorption area can lead to a better sensitivity as a relatively large absorption area might substitute for a shaping, e . g . , a focusing, of the incident infrared signal . In contrast , the optical readout signal may comprise a wavelength of the visible or near infrared spectrum such that beam shaping can be done in a relatively simple way by means of of f-the-shel f refractive optics and consequently a relatively small readout area can be targeted . For example , the readout area may comprise a diameter in an interval between 1 pm up to 1 mm .

In order to quanti fy an infrared signal , the infrared detector element can be used and the following steps can be taken :

- guiding the infrared signal onto the infrared absorption area of the infrared detector element ;

- measuring a frequency of a vibration mode of the membrane with optical readout means , wherein the optical readout means emits an optical readout signal ;

- quanti fying the infrared signal , in particular an intensity of the infrared signal , based on the measured frequency of the vibration mode of the membrane ; wherein the optical readout signal is guided such that it is incident onto the readout area of the membrane .

As stated above , the infrared signal incident on the infrared absorption area heats the membrane , thus changing the vibration mode , i . e . , an eigenmode , of the membrane . The readout means may for example comprise a laser . The laser may emit essentially monochromatic light , for example visible light or near infrared light . The emitted light by the laser may be used as an optical readout signal . The optical readout signal is guided onto the readout area of the membrane . The readout area reflects at least a part of the intensity of the incident readout signal , in particular at least 2 % of the intensity may be reflected . The readout area may provide essentially specular reflection . The shape and si ze of a cross section of the readout signal may be substantially equal to the shape and si ze of the readout area . The optical readout signal may cover essentially the entire optical readout area . This way, measurements can be done in a particularly reproducible way, as the exact position of the readout signal with respect to the targeted antinode is well defined . The reflected signal , in particular the intensity and/or the phase of the reflected signal , are used to estimate the frequency of the vibration mode of the membrane . Since the vibration mode of the membrane depends on the incident infrared signal , the incident infrared signal can consequently be quanti fied . In particular the intensity of the infrared signal can be quanti fied . In addition, the time-dependent frequency change of the vibration mode can be used to quanti fy the timedependency, in particular a modulation frequency, of the incident infrared signal .

The optical readout means may for example comprise an interferometer . By means of an interferometer, e . g . , a Michelson interferometer or a Mach-Zehnder interferometer, the frequency of the vibration mode can be measured in a reliable and sensitive way .

For example , the optical readout means is a laser Doppler vibrometer . Laser-Doppler vibrometers are established devices for measuring mechanical oscillations such as vibrations of the membrane . A laser Doppler vibrometer therefore yields a simple way of measuring the frequency of a vibration mode of the membrane .

For example , the readout area may extend over a center of the membrane , wherein an antinode of the vibration mode is located in the center of the membrane . For example , the fundamental eigenmode has an antinode in the center of the membrane . The readout area may alternatively be placed to overlap with any antinode of any higher order eigenmode . In contrast, the infrared absorption area may cover essentially the entire surface of the membrane . The infrared absorption area may be positioned in the center of the membrane to result in a maximal conversion of absorbed heat to frequency detuning, thus enhancing the sensitivity of a measurement . —Measuring at an antinode yields a similar signal to noise ratio regardless . E . g . , this allows it to place the readout area at the antinode close to a corner of a membrane , far away from the center .

The readout area may be in the same plane as the infrared absorption area . The readout area may for example be provided on a common surface , e . g . , the first surface of the membrane . The readout area may be at least partially surrounded by the infrared absorption area .

The readout area is considered surrounded by the infrared absorption area in an angular range in case for each angle within the angular range a straight line can be drawn from any single point within the readout area to an arbitrary point lying within the infrared absorption area . The angular range may range from a minimum 0 ° to a maximum of 360 ° . The straight line may also cross areas di f ferent from the optical readout area and the infrared absorption area . For example , the readout area may be surrounded by the infrared absorption area in an angular range with a span of more than 180 ° , optionally more than 210 ° , optionally more than 240 ° , optionally more than 270 ° .

The disclosure generally also concerns an infrared detector element , comprising a frame and a membrane supported by the frame , wherein the membrane comprises an infrared absorption area, wherein the infrared absorption area is configured to absorb an incident infrared signal , wherein the membrane comprises a readout area for an optical measurement of a frequency of a vibration mode of the membrane , wherein the readout area is at least partially surrounded by the infrared absorption area . Said readout area may optionally be surrounded by the infrared absorption area in an angular range with a span of more than 180 ° , optionally more than 210 ° , optionally more than 240 ° , optionally more than 270 ° .

The infrared absorption area may comprise an opening, wherein the optical readout area is within the opening, such that the optical readout area is fully surrounded by the infrared absorption area . The readout area is surrounded by the infrared absorption area in an angular range with a span of 360 ° . The optical readout area may comprise the same shape and si ze as the opening and may coincide with the opening . In other words , the opening may define the optical readout area . For example , i f the infrared absorption area may be provided by a thin film on top of the membrane , wherein the thin film may comprise a hole which forms the opening . The optical readout area is surrounded by the infrared absorption area . For example, the infrared absorption area comprises a substantially reflective segment, wherein the substantially reflective segment provides the optical readout area. The readout area may for example be provided by means of a reflective thin film on top of an infrared absorbing thin-film. The reflective thin film may cover only a part of the infrared absorption area. "Substantially reflective" means that at least a part, in particular more than 80%, of the intensity of the optical readout signal is reflected by the substantially reflective segment. The readout area may be a part of the infrared absorption area, in which readout area the optical properties of the infrared absorption area, e.g., an infrared absorbing thin-film, differ such that an optical readout signal is substantially reflected. For example, the optical readout signal may comprise visible, near infrared, and/or UV light. The readout area may be substantially reflective for visible, near infrared, and/or UV light.

For example, the membrane may comprise a phononic crystal. Surrounding mechanical resonators such as the membrane with phononic crystals has been shown to reduce acoustic radiation losses into the supporting frame. Patterning the phononic crystal directly into the membrane leads to so-called "soft- clamping" in addition to reduced radiation losses, which results in enhanced damping dilution and may yield quality factors (Qs) approaching one billion at room temperature (cf. Sadeghi, Pedram, et al. "Thermal transport and frequency response of localized modes on low-stress nanomechanical silicon nitride drums featuring a phononic-band-gap structure." Physical Review Applied 14.2 (2020) : 024068.) . Hence, a phononic crystal can further increase the sensitivity of the measurement.

The membrane may for example comprise a phononic defect. Localized modes on defects engineered into phononic crystals display a larger overlap of the mechanical vibration mode to the temperature field compared to a vibration mode of a membrane without a phononic crystal (cf. Sadeghi et al. (2020) .) .

Eigenmodes of membranes with a phononic crystal and a phononic defect are also referred to as "defect modes" . As a result of this overlap and the increased thermal isolation due to the surrounding phononic crystal , the temperature in the phononic defect can be higher than the average temperature of the membrane . This leads to an over-proportional responsivity of the phononic defect in form of a detuning of the frequency of the vibration mode of the membrane . Consequently, this leads to an additionally increased thermal response and an increased sensitivity . The readout area at least partially extends over the phononic crystal . The readout area may not extend over the phononic defect .

For example , the phononic de fect is located in the center of the membrane . This way, the frequency detuning for an absorbed infrared signal is maximal . In addition, the readout area may extend over the center and over the defect mode in the center .

The membrane may be connected to the frame via at least three tethers . The tethers may be a part of the membrane . The membrane may in this case also be referred to as a nanomechanical trampoline . Four narrow tethers of a nanomechanical trampoline ensure high-stress locali zation to the tethers , while the central membrane ( i . e . , the rest of the membrane without the tethers ) displays a much lower stress . Besides the stress engineering, narrow tethers additionally improve the thermal insulation to the frame . Thermal insulation of the membrane with respect to the frame leads to higher temperature di f ferences between membrane and frame and hence to higher frequency shi fts of the vibration mode . The tethers may have a width of for example 300 nm to 300 pm . The tethers may be arranged in the corners of the membrane and may extend essentially along the diagonals of the membrane .

The use of tethers reduces the amount of material making up the membrane , thereby achieving a lighter and more responsive membrane . Also , a membrane comprising tethers can be less rigid than a membrane of the same planar extensions without tethers . As tether we understand e . g . a limb at a border of the membrane . A tether can typically be a narrow, band-shaped portion ("tethered portion" ) of the membrane , without limitation to a rectangular outline of that portion . The tethers may be made of the same material as other portions of the membrane ; in particular, the material of the membrane in the absorption area and the material of any tethered portions of the membrane may be the same material . The use of tethers also facilitates a connection between the membrane and the frame .

The membrane may be perforated . The addition of holes in the membrane reduces its effective thermal conductivity, which results in increased thermal responsivity . Holes ( further ) reduce weight and rigidity and increase responsivity .

The eigenmode has a first mode number m smaller than twenty, in particular smaller than four, and a second mode number n smaller than twenty, in particular smaller than four . The higher the first mode number m and the second mode number n, the more nodes and antinodes the eigenmode comprises . At least in tendency, the antinodes of eigenmodes with lower mode numbers have higher deflection amplitudes for a speci fic driving force than the antinodes of eigenmodes with higher mode numbers .

For example , a mirror may be connected to the frame, wherein the optical readout area of the membrane and a reflective surface of the mirror form an optical cavity, in particular a Fabry-Perot cavity . The path length in the optical cavity formed by the mirror and the readout area is influenced by the vibration of the membrane . Hence , the path length changes periodically with a frequency that is related to the frequency with which the readout area oscillates . A part of the optical readout signal is reflected by the optical cavity . The fraction of intensity that is reflected, depends on the optical path length of the optical readout signal within the optical cavity . Thus , a relatively simple way of measuring the frequency of the vibration mode of the membrane is enabled .

Optionally, the mirror may be a wafer, in particular a Si wafer, optionally a HRFZ-Si wafer, optionally a sapphire wafer . Optionally, the frame may be provided by a layer on top of the wafer, in particular a SiC>2 layer . The frame may be connected to the wafer, e . g . , the frame may be integrally formed on the wafer . The optical cavity may be formed by a reflective surface of the wafer, as well as the optical readout area . The reflective surface and the readout area may be arranged adj acent or on opposite sides of the frame . The frame may comprise a thickness , which thickness relates to the optical path length within the optical cavity . For example , i f the frame is a Si Ch layer integrally formed on top of a HRFZ-Si wafer, the thickness of the SiC>2 relates to the optical path length of the optical cavity . This yields a very stable optical cavity that does not require a path length control . Optionally, the infrared detector element may be fabricated by means of surface micromachining techniques .

For example , measuring a frequency of the vibration mode may be enabled by an optical interference of the optical readout signal in an optical cavity formed by the optical readout area of the membrane and a reflective surface , in particular a reflective surface of a mirror, wherein the reflective surface is parallel to the membrane . The reflective surface may be a part of the infrared detector element . Alternatively, a mirror may be connected to the frame . Alternatively, the reflective surface may be provided separately . For example , the optical readout means may comprise the reflective surface .

The invention further concerns a method for fabricating a detector element .

By way of example , the disclosure is further explained with respect to some selected embodiments shown in the drawings for purpose of illustration . However, these embodiments shall not be considered limiting for the disclosure .

Fig . 1 schematically shows an infrared detector element in form of a nanomechanical trampoline with four tethers and a central opening in the infrared absorption area defining the readout area . Fig . 2 schematically shows another infrared detector element with a substantially reflective segment , which provides the optical readout area .

Fig . 3 schematically shows another infrared detector element comprising a phononic crystal and a central phononic defect .

Fig . 4 schematically shows another infrared detector element with an absorber opening covering an antinode of an eigenmode with first mode number m equal to two and second mode number n equal to two .

Fig . 5 schematically shows another infrared detector element with an absorber opening covering an antinode of an eigenmode with first mode number m equal to three and mode number n equal to three .

Fig . 6 schematically shows an infrared detector with an infrared detector element , a laser Doppler vibrometer and incident infrared signal .

Fig . 7 schematically shows an infrared detector with an infrared detector element comprising a Fabry-Perot cavity .

Fig . 1 shows an infrared detector element 1 with a frame 2 and membrane 3 . The membrane 3 comprises four tethers 4 . The membrane 3 is connected to the frame 2 via the four tethers 4 and is supported by the frame 2 . The frame may comprise SiC>2 for example . The membrane 3 is essentially quadratic and the tethers 4 are arranged in four corners 5 of the essentially quadratic membrane 3 extending diagonally outwards , i . e . , towards the frame 2 . This embodiment can also be referred to as nanomechanical trampoline or trampoline resonator .

The membrane 3 comprises an infrared absorption area 6, which is provided by a thin film 7 on top of the membrane 3 . The infrared absorption area 6 , i . e . , the thin film 7 , is configured to absorb an incident infrared signal 8 . The infrared signal 8 is indicated by a circle , which shows the outline of a cross section of the infrared signal 8 . The diameter of the infrared signal 8 in the depicted cross section of the infrared signal 8 is larger than a side length of the membrane 3 . Most of the infrared signal 8 is incident on the membrane 3 . However, a smaller portion of the infrared signal 8 is not incident on the membrane . Alternatively, the whole infrared signal 8 may be incident in the membrane 3 . The membrane 3 comprises a readout area 9 for an optical measurement of a frequency of a vibration mode of the membrane 3 . The readout area 9 at least extends over an antinode 11 ' of the vibration mode of the membrane 3 , wherein the vibration mode is an eigenmode of the membrane 3 . In the embodiment of fig . 1 , the readout area 9 is within an opening 10 in the thin film 7 , such that the readout area 9 is surrounded by the infrared absorption area 6 , in this case the thin film 7 . The opening 10 extends over the center 11 of the membrane 3 . The fundamental eigenmode with first mode number m equal to zero and second mode number n equal to one comprises an antinode 11 ' , which is located in the center 11 of the membrane 3 .

Fig . 2 shows another embodiment of the infrared detector element 1 . For simplicity, no frame 2 is shown in fig . 2 . The membrane 3 is covered by a thin film 7 , which defines the infrared absorption area 6 . The infrared absorption area 6 comprises a substantially reflective segment 12 , wherein the substantially reflective segment 12 provides the optical readout area 9 . The substantially reflective segment 12 has a circular shape and extends over the center 11 of the membrane 3 , similar to the opening 10 shown in fig . 1 . However, the substantially reflective segment 12 is provided by a reflective thin film 13 , which is deposited on top of the infrared absorptive thin film 7 .

Fig . 3 shows another embodiment of the infrared detector element 1 . For simplicity, no frame 2 is shown in fig . 2 . The membrane 3 comprises a phononic crystal 14 . The infrared absorption area 6 is provided in the form of a thin film 7 on top of the membrane 3 comprising the phononic crystal 14 . The membrane 3 further comprises a phononic defect 15 , which is located in the center 11 of the membrane 3 , and which is embedded in the phononic crystal 14. The optical readout area 9 is within an opening 10 in the thin film 7, which is also in the center 11 of the membrane 3.

Fig. 4 shows another infrared detector element with an absorber opening 10. The readout area 9 is within an opening 10. The readout area 9 covers a part of an antinode 11' of the eigenmode with first mode number m equal to two and second mode number n equal to two. Both the first mode number m and the second mode number n are smaller than twenty and in particular smaller than four. The respective antinode 11' is off the center 11 of the membrane 3.

Fig. 5 shows another infrared detector 1 element with an absorber opening 10 covering an antinode 11' of an eigenmode with first mode number m equal to three and second mode number n equal to three. The respective antinode 11' is off the center 11 of the membrane 3. As an alternative to the shown embodiment, the membrane 3 may be perforated.

Fig. 6 shows an infrared detector 16 with an infrared detector element 1. The infrared detector element 1 comprises a membrane 3 which is supported by a frame 2. An incident infrared signal 8 is guided towards the membrane 3 and onto the infrared absorption area 6 of the infrared detector element 1. In the depicted embodiment, the infrared absorption area 6 is provided by a thin film 7. The readout area 9 is within an opening 10 in the thin film 7 (details see fig. 1) . The thin film 7 is on the left side of the infrared detector element 1, i.e., on the left side of a bulk of the membrane 3. The incident infrared signal 8 propagates from right to left. In the depicted example, the incident infrared signal 8 is collimated. The incident infrared signal 8 may be unfocused or focused, for example by means of a parabolic mirror, to a spot size of the order of the size of the membrane 3 or the infrared absorption area 6. Due to the absorbed infrared signal 8, the temperature of the membrane 3 rises, which leads to an expansion of the membrane 3 with respect to the frame 2. As a consequence of the expansion of the membrane 3, the tensile stress of the membrane 3 changes. Consequently, the frequencies of the eigenmodes of the membrane 3 shi ft .

The infrared detector may for example comprise means for mechanical actuation (not shown) . For example , the frame 2 , may be in mechanical contact with a piezoelectric element , which shakes the frame 2 in a direction perpendicular to the main extension plane of the membrane 3 , resulting in oscillation of the membrane 3 . The mechanical excitation leads to an excitation of the eigenmodes of the membrane 3 .

In order to measure a frequency of a vibration mode , namely an eigenmode , of the membrane 3 , optical readout means 17 are provided, in this case in form of a laser Doppler vibrometer 18 . The optical readout means 17 emits an optical readout signal 19 with a wavelength of 632 . 8 nm. Alternatively, any wavelength in the visible or near-infrared region of the electromagnetic spectrum may be used . The optical readout signal 19 and the incident infrared signal 8 are incident on the membrane 3 from opposite sides with respect to a main extension plane of the membrane 3 . In the illustrated embodiment , the readout signal 19 is guided onto the membrane 3 from left of the membrane 3 , whereas the infrared signal 8 is incident on the membrane 3 from right of the membrane 3 . The optical readout signal 19 is guided such that it is incident onto the readout area 9 of the membrane 3 .

In this example , the membrane 3 has a thickness of 50 nm and comprises an optically transparent material such as LPCVD silicon nitride . The incident infrared signal 8 comprises a wavelength of the order of 10 pm, whereas the optical readout signal 19 comprises a wavelength of 632 . 8 nm. Therefore , the wavelength of the incident infrared signal 8 as well as the wavelength of the optical readout signal 19 are larger than the optical thickness of the membrane 3 . The membrane 3 can be highly transparent for both the optical signal 19 and the infrared signal 8 . Hence , the reflection at the membrane as well as the absorption of both the incident infrared signal 8 and the optical readout signal 19 are not entirely locali zed at , for example , one or the other surface of the membrane . Whether the infrared absorption area 6 and/or the readout area 9 are located on the same surface or on opposite surfaces of the membrane 3 is therefore of minor relevance . Likewise , it is of minor relevance from which side the incident infrared signal 8 and/or the optical readout signal 19 are incident on the membrane with respect to the readout area 9 , in this case the opening 10 , and/or the infrared absorption area 6 , in this case the thin film 7 . It is however beneficial for the sensitivity of the measurement to minimi ze the absorption of the optical readout signal 8 at the membrane 3 . A fraction of the optical readout signal 8 is reflected by the membrane 3 , another fraction is transmitted thorough the membrane 3 and another fraction of the optical readout signal 19 is absorbed . The absorbed fraction of the readout signal 19 may lead to an increase in the temperature of the membrane 3 . The intensity fluctuations of the absorbed fraction of the readout signal 19 may introduce extra noise to the measurement . In addition, the absorption leads to a reduced signal intensity of the reflected fraction of the optical readout signal 19 . Hence , the readout area 9 leads to an increased signal-to-noise ratio , because the reflected fraction of the optical signal 8 is increased while the absorbed fraction is minimi zed, and the noise is decreased . Overall , the optical readout area 9 increases the sensitivity of the measurement .

By means of the laser Doppler vibrometer 18 , a frequency of a vibration mode of the membrane 3 is measured . The readout area 9 covers an antinode 11 ' of an eigenmode of the membrane 3 . Based on the measured frequency of the vibration mode of the membrane 3 and the known correlation of the frequency with the intensity of the incident infrared signal 8 , the infrared signal 8 , in particular the intensity of the infrared signal 8 , is quanti fied .

Fig . 7 shows another infrared detector 16 with an infrared detector element 1 comprising an optical cavity 20 , in this case a Fabry-Perot cavity . In order to form the optical cavity 20 , a mirror 21 is connected to the frame 2 . The mirror 21 may comprise a dielectric material or a metal . Optionally, the mirror 21 may be a wafer, in particular a Si waver, optionally a HRFZ-Si wafer. Optionally, the frame 2 may be a Si02 layer on top of the wafer. The frame 2 may be connected to the wafer, e.g., the frame 2 may be integrally formed on the wafer. The optical cavity 20 is formed by a reflective surface 22 of the mirror 21, e.g., a reflective surface of a wafer, as well as the optical readout area 9. The optical readout area 9 is defined by an opening 10 in the infrared absorption area 6 (see for example fig. 1 for details) . The reflective surface 22 faces towards the membrane 3. The optical readout area 9 and the reflective surface 22 are essentially parallel to one another and are spaced apart. The reflective surface 22 comprises an equal shape and size as the optical readout area 9. The reflective surface

22 and the membrane 3 are adjacent to opposite sides of the frame 2. The frame 2 comprises a thickness, which thickness relates to the optical path length within the optical cavity 20. For example, if the frame 2 is a SiC>2 layer integrally formed on top of a HRFZ-Si wafer, the thickness of the SiC>2 layer relates to the optical path length of the optical cavity 20. This yields a very stable optical cavity 20 that does not require a path length control.

The optical readout means 17 comprises a laser 23, in this case a Nd:YAG laser, emitting an optical readout signal 19 with a central wavelength 632.8 nm. Alternatively, any wavelength in the UV, visible or near infrared spectrum may be utilized. Furthermore, an optical three-way circulator 24 is provided. A signal applied to port A only comes out of port B; a signal applied to port B only comes out of port C; a signal applied to port C only comes out of port A. The circulator 24 ensures that the optical readout signal is not reflected back into the laser

23 but onto a detector 25.

Due to the vibration of the membrane 3 and consequently an oscillation of the distance between the reflective surface 22 and the readout area 9, the reflected intensity of the optical readout signal 19, which is measured by the detector 25, is also oscillating. This signal at the detector 25 is used to estimate the frequency of the vibration mode, i.e., the eigenmode, of the membrane 3 . Consequently, the incident infrared signal 8 , in particular the intensity of the incident infrared signal 8 , can be determined . Measuring the frequency of the vibration mode is therefore enabled by an optical interference of the optical readout signal 19 in the optical cavity 20 , in this case a Fabry-Perot cavity, formed by the optical readout area 9 of the membrane 3 and the reflective surface 22 , wherein the reflective surface 22 is parallel to the membrane 3 . The surface areas of the mirror 21 , over which the reflective surface 22 does not extend, may be essentially transparent for both the readout signal 19 and incident infrared signal 8 .

In order to guide the incident infrared signal 8 onto membrane 3 and the infrared absorption area 6 , a hal f-ball lens 26 is attached to the mirror 21 , such that incident infrared signal 8 is focused towards the optical redout area 9 . The hal f-ball lens 26 may comprise high resistivity float zone silicon (HRFZ-Si ) . As an alternative to the hal f-ball lens 26 , a parabolic mirror may be used to focus the incident infrared signal 8 . Alternatively, the half-ball lens 26 may be left out . The hal fball lens 26 and the mirror 21 are essentially optically transparent for infrared light , such as the incident infrared signal 8 . The reflective surface 22 may be essentially transparent for the incident infrared signal 8 . The reflective surface 22 may be essentially reflective for the optical readout signal 19 .

An infrared detector element 1 can, for example , be fabricated by low-pressure chemical vapor deposition ( LPCVD ) of silicon nitride ( SiN) on both sides of a silicon support wafer . The tensile stress of the SiN can be tailored by choosing the stoichiometry of the resulting SiN thin film, where a silicon- rich SiN provides a lower tensile stress . The fabrication process of a SiN membrane and trampoline infrared detector element are for example described in Piller, M . , Hiesberger, J . , Wistrela, E . , Martini , P . , Luhmann, N . and Schmid, S . , 2021 . Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators . arXiv preprint arXiv : 2105 . 03999 . The fabrication of SiN phononic defect mode detectors without an infrared absorber are for example described in Sadeghi, Pedram, et al. "Thermal transport and frequency response of localized modes on low-stress nanomechanical silicon nitride drums featuring a phononic-band-gap structure." Physical Review Applied 14.2 (2020) : 024068.

A broadband infrared absorber thin film 7 featuring an opening 10 for the readout area 9 can be fabricated on a front side of the SiN membrane 3 by means of a lift-off process. For membrane structures, this lift-off can be performed after the membrane 3 has already been released by etching through the silicon wafer from the backside, e.g., by a wet etch with potassium hydroxide (KOH) or a dry etch with a Bosch process. A negative photoresist is patterned by a photolithography step to leave an island at the location of the opening 10 for the readout area 9. The entire SiN surface is then covered by physical vapor deposition of a thin conductive metal film, e.g., of 5-7 nm of platinum (see for example Piller, M., Hiesberger, J., Wistrela, E., Martini, P., Luhmann, N. and Schmid, S., 2021. Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators. arXiv preprint arXiv: 2105.03999) or 2 nm of gold assisted by an 1-2 nm thin copper oxide adehesion layer (see for example Luhmann, Niklas, Dennis Hoj , Markus Piller, Hendrik Kahler, Miao-Hsuan Chien, Robert G. West, Ulrik Lund Andersen, and Silvan Schmid. "Ultrathin 2 nm gold as impedance-matched absorber for infrared light." Nature communications 11, no. 1 (2020) : 1-7.) The metal layer is then lift-off by dissolving the photoresist patch on the SiN surface.

An alternative method to produce a broadband absorber thin film 7 with an opening 10 for the readout area 9 is via a shadow mask. This method can be done as the last step, e.g., for trampoline detector elements. For this method, an additional freestanding SiN structure is fabricated by the same method as used for the trampoline element itself (see Piller, M., Hiesberger, J., Wistrela, E., Martini, P., Luhmann, N. and Schmid, S., 2021. Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators. arXiv preprint arXiv: 2105.03999) . The freestanding element has the shape of the opening 10 for the readout area 9 . This shape is fixed to the supporting Si frame by two to four thin tethers . This second shadow-mask chip is then placed over the SiN of the infrared detector element 1 during the deposition of the metal absorber film 7 , creating the opening 10 in the deposited metal film 7 . This shadow-mask process also allows the deposition of the metal absorber film 7 on the frontside or backside of a membrane 3 , trampoline , or phononic defect mode .

The fabrication of a readout area 6 with a high reflectivity for the optical readout signal 19 can be achieved also by a shadow mask process . In this case , an additional SiN membrane has to be fabricated that features an opening with the si ze and shape of the ( to be fabricated) substantially reflective segment 12 . This additional chip is then used as a shadow-mask during the deposition of the reflective metal thin film 13 . This process allows the deposition of the reflective metal thin film 13 on front and/or back side of a membrane 3 , trampoline , or phononic defect mode .

of reference numerals :

Reference infrared detector element frame membrane tether corner infrared absorption area thin film incident infrared signal readout area opening center antinode substantially reflective segment reflective thin film phononic crystal phononic defect infrared detector optical readout means laser Doppler vibrometer optical readout signal optical cavity mirror reflective surface of the mirror laser circulator detector hal f-ball lens

Claims

1. Infrared detector element (1) , comprising

- a frame (2) and

- a membrane (3) supported by the frame (2) , wherein the membrane (3) comprises an infrared absorption area (6) , wherein the infrared absorption area (6) is configured to absorb an incident infrared signal (8) , wherein the membrane (3) comprises a readout area (9) for an optical measurement of a frequency of a vibration mode of the membrane (3) , characterized in that the readout area (9) at least partly extends over an antinode (11' ) of the vibration mode of the membrane (3) , wherein the vibration mode is an eigenmode of the membrane (3) .

2. Infrared detector element (1) according to claim 1, characterized in that the readout area (9) extends over the center (11) of the membrane (3) , wherein an antinode (11' ) of the vibration mode is located in the center (11) of the membrane (3) .

3. Infrared detector element (1) according to claim 1 or 2, characterized in that the infrared absorption area (6) comprises an opening (10) , wherein the optical readout area (9) is within the opening (10) , such that the optical readout area (9) is surrounded by the infrared absorption area (6) .

4. Infrared detector element (1) according to claim 1 or 2, characterized in that the infrared absorption area (6) comprises a substantially reflective segment (12) , wherein the substantially reflective segment (12) provides the optical readout area ( 9 ) .

5. Infrared detector element (1) according to any one of claims 1 to 4, characterized in that the membrane (3) comprises a phononic crystal (14) .

6. Infrared detector element (1) according to claim 5, characterized in that the membrane (3) comprises a phononic defect ( 15) .

7. Infrared detector element (1) according to claim 6, characterized in that the phononic defect (15) is located in the center (11) of the membrane (3) .

8. Infrared detector element (1) according to any one of claims 1 to 7, characterized in that the membrane (3) is connected to the frame (2) via at least three tethers (4) .

9. Infrared detector element (1) according to any one claims 1 to 8, characterized in that the membrane (3) is perforated.

10. Infrared detector element (1) according to any one of claims 1 to 9, characterized in that the eigenmode has a first mode number smaller than twenty and a second mode number smaller than twenty .

11. Infrared detector element (1) according to claim 10, characterized in that the eigenmode has a first mode number smaller than four and/or a second mode number smaller than four.

12. Infrared detector element (1) according to any one of claims 1 to 11, characterized by a mirror (21) connected to the frame

(2) , wherein the optical readout area (9) of the membrane (3) and a reflective surface (12) of the mirror (21) form an optical cavity (20) .

13. Infrared detector element (1) according to claim 12, characterized in that the optical cavity (20) is a Fabry-Perot cavity .

14. Method for quantifying an infrared signal (8) with an infrared detector element (1) according to any one of claims 1 to 13, comprising the steps:

- guiding the infrared signal (8) onto the infrared absorption area (9) of the infrared detector element (1) ;

- measuring a frequency of a vibration mode of the membrane

(3) with optical readout means (17) , wherein the optical readout means (17) emits an optical readout signal (19) ;

- quantifying the infrared signal (8) , based on the measured frequency of the vibration mode of the membrane (3) ; wherein the optical readout signal (8) is guided such that it is incident onto the readout area (9) of the membrane (3) .

15. Method for quantifying an infrared signal (8) according to claim 14, characterized in that quantifying the infrared signal comprises quantifying an intensity of the infrared signal (8) .

16. Method for quantifying an infrared signal (8) according to claim 14 or claim 15, characterized in that the optical readout means (17) comprises an interferometer.

17. Method for quantifying an infrared signal according to any one of claims 14 to 164, characterized in that the optical readout means (17) is a laser Doppler vibrometer (18) .

18. Method for quantifying an infrared signal (8) according to any one of claims 14 to 16, characterized in that measuring a frequency of the vibration mode is enabled by an optical interference of the optical readout signal (19) in an optical cavity (20) formed by the optical readout area (9) of the membrane (3) and a reflective surface (22) , wherein the reflective surface (22) is parallel to the membrane (3) .

19. Method for quantifying an infrared signal (8) according to claim 18, characterized in that the reflective surface (22) is a reflective surface of a mirror (21) .

20. Method for fabricating a detector element (1) according to any one of claims 1 to 13.