The invention concerns an infrared detector as well as the use of the infrared detector in infrared spectroscopy and a corresponding method for performing infrared spectroscopy.

Infrared (IR) spectroscopy allows for the chemical analysis of a sample by obtaining an infrared spectrum of absorption of a solid, liquid or gas. While IR spectroscopy can be performed by illuminating a sample with monochromatic IR light (dispersive IR spectroscopy) the standard tool to perform IR spectroscopy is by Fourier- transform IR spectroscopy (FT-IR). FT-IR illuminates the sample with a spectrally broadband time- modulated IR light and covers the entire IR range. Broadband light contains the full spectrum of wavelengths to be measured. FT-IR spectrometers are ubiquitous and can be found basically in every chemical analysis lab. FT-IR measurements require detectors that are fast enough to record the interferogram, which depends on the scan speed. The slowest scan speeds of regular FT-IR spectrometers are of the order of a few kHz. Only very specific research spectrometers provide a so-called step scan function which puts no limit on the detector speed. However, this function is not a common feature of regular FT-IR spectrometers.

Infrared (IR) spectroscopy can also allow for the chemical analysis of tiny amounts of sample. The involved detectors that are capable of detecting the lowest intensities of infrared light are slow, for example, silicon nitride (SiN) based nanoelectromechanical (NEMS) drum resonators. Therefore, dispersive IR spectroscopy with monochromatic light (e.g., from a tuneable quantum cascade laser, QCL) has to be used. Tuneable QCLs are expensive and cover only a limited spectral range. In order to transduce (actuate and detect) the vibrational motion of these nanoelectromechanical resonators, integrated metal electrodes have been used, which allow for an inductive transduction. The implementation of metal electrodes on drums resonators are shown in the article “Nanomechanical infrared spectroscopy with vibrating filters for pharmaceutical analysis” (KUREK, Maksymilian, et al., Angewandte Chemie, 2017, 129. Jg., Nr. 14, S. 3959-3963.) and have been further studied in the article “Thermal IR detection with nanoelectromechanical silicon nitride trampoline resonators” (PILLER, Markus, et al., arXiv preprint arXiv:2iO5.03999, 2021.).

US 2016 252 400 Ai shows a vibration based mechanical IR detector having a resonating pixel structure.

US 2013 170 517 Al shows a bolometer with microsystem including a support and a mobile mass hung from beams above the support. The mobile mass forms ab absorber of optical flux.

Another IR detector is known from US 2015 253 196 Al. An uncooled IR detector array based on nanoelectromechanical systems utilizing a high-quality-factor torsional resonator of microscale dimensions is shown. The resonator has a paddle that is supported by two nanoscale torsion rods.

US 2017331450 Al shows nano- and micro-electromechanical resonators with a piezoelectric plate and an interdigital electrode.

WO 2020 047572 A2 shows an infrared detector with a mechanical resonator having a frame and a membrane. The detector further comprises a tuning means for adjusting the tensile stress of the membrane.

For a different detector type, that is, based on a piezoelectric actuator and an optical interferometric read-out, the article “Thermal radiation dominated heat transfer in nanomechanical silicon nitride drum resonators” (PILLER, Markus, et al., Applied Physics Letters, 2020, 117. Jg., Nr. 3, S. 034101.) shows that it is possible to improve the time constant of our nanoelectromechanical detectors with a metal coating.

It is an object of the present invention to decrease the response time for electric readout of a sensitive infrared detector based on a nanoelectromechanical drum resonator.

The present invention proposes an infrared detector comprising: a nanoelectromechanical resonator having a frame and a membrane supported by the frame, wherein the membrane comprises an absorption area, wherein the detector comprises an electrical readout means for detecting a frequency change of the resonance frequency of the membrane, wherein the membrane comprises one or more electrically conductive connections between the absorption area and the frame, and wherein the total cross section of the one or more electrically conductive connections is greater than 10 pm ² (ten square micrometres). The electrically conductive connections maybe metallic connections, but generally maybe made of any electrically conductive material, including, e.g., graphene or other conducting 2D materials, metals, such as Aluminium (Al), Gold (Au), Platinum (Pt) or Copper (Cu), as well as doped semiconductors based e.g., on Silicon (Si) or Germanium (Ge).

The present disclosure suggests an increased minimal cross section of electrically conductive connections between the border of the absorption area and the frame. The electrically conductive connections are responsible for dissipating the heat generated in the absorption area due to the absorbed infrared light by heat conductance to the frame supporting the membrane. By increasing the cross-section of those electrically conductive connections above a certain limit, the heat conductance to the frame can be increased sufficiently to support a response time for the electric readout that works with widespread/common FT-IR spectrometers. The improved heat dissipation has the effect that the thermal relaxation of the membrane is accelerated. The membrane will return faster to an equilibrium state (“cool down”) after absorbing infrared light. It will thus be more sensitive to the temporal distribution of the received infrared light. This makes it possible to record infrared spectrograms at lower infrared light intensities than current fast infrared detectors support.

The total cross section of the one or more electrically conductive connections is the sum of the minimal cross section of each electrically conductive connection between the border of the absorption area of the membrane and the border of the frame supporting the membrane. Within the scope of the present disclosure, the absorption area may correspond to the surface area of the membrane, i.e., the entire membrane may act as absorber. In practice, the absorption area is the part of the surface of the membrane, where the sample is to be placed that shall be irradiated with infrared light. Typically, the absorption area will be centred on the membrane. The size of the absorption area maybe between 100 pm ² (square micrometres) and 5 mm ² (square millimetres). In an FT-IR application the size of the absorption area will be matched with the cross-section of the infrared light beam irradiating the sample. For example, the border of the absorption area may form a circle or ellipse on the surface of the membrane. There need not be any material difference limiting the absorption area. It may be of the same material and/ or structure of the surrounding parts of the membrane. Optionally, the membrane may comprise an absorber coating or a different surface structure inside the absorption area than outside.

Optionally, the membrane may comprise an electrically conductive layer forming the one or more electrically conductive connections. Optionally, the electrically conductive layer may be essentially uniform over the entire area of the membrane. The uniformity (or evenness) reduces stress issues that otherwise can occur between two different materials with different intrinsic stress and different thermal expansion coefficients, in case the layer is deposited on a substrate. The electrically conductive layer maybe a metallic layer. The electrically conductive layer may be continuous. In this case, all of the one or more electrically conductive connections are interconnected. For example, the electrically conductive connections maybe interconnected through the absorption area. The electrically conductive layer may consist of a homogeneous electrically conductive material. For example, the electrically conductive layer maybe a layer deposited on a non-conductive substrate of the membrane (which forms another layer). The non-conductive substrate may consist of silicon nitride. In one embodiment, the electrically conductive layer may be formed by a homogeneous metal deposition (e.g., through physical vapour deposition) on a silicon nitride membrane. The substrate may have a thickness of 10 - 500 nm. Irrespective of the shape and structure of the electrically conductive layer and the membrane as a whole, the thickness or height of the electrically conductive layer may preferably be between 1 nm (nanometres) and 500 nm, in particular between 1 nm and 200 nm. The electrically conductive layer may cover more than 20 %, preferable more than 50 %, in particular more than 90 % of the area of the membrane. Larger areas allow for smaller thickness at the same total cross section of the one or more electrically conductive connections.

In one particular embodiment, the membrane maybe an electrically conductive membrane, more specifically, a purely electrically conductive membrane. In other words, the membrane may consist of the electrically conductive layer. The electrically conductive membrane may have a thickness of 10 - 500 nm. For example, the membrane may be a metallic membrane. It may preferably consist of aluminium (Al) or gold (Au). These two metals have a relatively high reflectivity over the entire infrared spectrum. Other layers, such as a substrate layer, maybe omitted from the membrane. This reduces mass density and heat capacity and at the same time achieves a lighter membrane at any given total cross section of the one or more electrically conductive connections. The electrically conductive layer forming the electrically conductive membrane may extend beyond the membrane itself, e.g., across a frame supporting the membrane. In this case, the membrane maybe the part of the electrically conductive layer that is supported by and inside the frame.

The electrical readout means maybe electrically connected to the membrane via two electrical contacts, wherein the two electrical contacts maybe arranged on an axis orthogonal to a magnetic field, within which the membrane is arranged to oscillate. The orthogonal arrangement of the electrical contacts (or - more precisely - the axis connecting them) and the magnetic field generated, e.g., by rare-earth magnets, achieves the highest inductance under similar situations and thus highest sensitivity. Optionally the electrical readout means may be electrically connected to the membrane via more than two electrical contacts. The electrical contacts may generally be arranged such that the direction of electrical current is orthogonal to the magnetic field. In general, the present disclosure is not limited to an orthogonal arrangement. Overall, the relative arrangement of the membrane, the magnetic field and an electrical connection across the membrane between the electrical contacts should be such that there is a nonzero angle between the magnetic field and the effective direction of motion of charges moving along said electrical connection across the oscillating membrane.

Optionally, the electrically conductive layer comprises at least one recess extending towards a center of the membrane, and preferably over an outer end of the membrane. The Lorentz force acting on the conductive layer and therefore on the membrane to induce a vibration is proportional to the electric current in the conductive layer. In order to enhance the induced vibration in the membrane, the electrical current through the membrane may be guided centrally over the membrane. The more central the Lorentz force acts on the membrane, the more effective is the inductance of a vibration. The recess may extend from an outer end of the membrane towards the center of the membrane, such that an outer area of the membrane is not covered by the conductive layer in order to enhance the induced vibration. The electrical contacts maybe located on opposite sides of the membrane with respect to the center of the membrane so that when an electrical current flows from one electrical contact to the other electrical contact, the electrical current flows (also) across the center of the membrane. Due to the recess, the electrical current flows in more central regions of the membrane, thus enhancing the inductance of vibration. Preferably, the electrically conductive layer comprises at least two recesses extending towards a center of the membrane from opposite sides, optionally in a symmetrical arrangement (for example symmetrical with respect to the center of the membrane and/ or symmetrical with respect to an axis on a line connecting the electrical contacts) and preferably over a respective outer end of the membrane.

Optionally, the membrane maybe pervious in the absorption area. Optionally, if the electrically conductive layer overlaps with the absorption area, the electrically conductive layer maybe pervious in the absorption area. The perviousness maybe achieved by a particular structuring, for example, by a perforation of the membrane. By being pervious, the membrane maybe configured to capture an aerosol within which the membrane is oscillating. The aerosol may partially flow through the absorption area of the membrane, meanwhile depositing small amounts of the aerosol material on the membrane or within the membrane. The perviousness is not necessarily limited to the absorption area. The entire membrane may be pervious and for example perforated.

For example, the conductive layer may comprise an opening, wherein the absorption area is arranged within the opening. The absorption area is not overlapping with the electrically conductive layer due to the opening of the conductive layer. Therefore, the conductive layer does not block the aerosol even in case the conductive layer is not pervious. The electrically conductive connections between the absorption area and the frame do not require that the absorption area itself is electrically conductive. The connections may be arranged between the absorption area and the frame and may terminate at the border of the absorption area.The membrane may comprise five or more, in particular seven or more, individual electrically conductive connections between the absorption area and the frame. As defined earlier, the total cross section of the one or more electrically conductive connections is the sum of the (minimum) cross sections of the individual electrically conductive connections. Optionally, the membrane may comprise or consist of an electrically conductive grid structure forming the membrane or deposited on a non-conductive substrate of the membrane.

In this context, the angle between any two neighbouring electrically conductive connections in the plane of the membrane maybe smaller than 90°, in particular smaller than 8o°, for example between 12 and 45 ⁰ . This configuration ensures that electrically conductive connections are foreseen in all directions, for example all four edges of a rectangular membrane. This arrangement of individual electrically conductive connections achieves a more homogeneous distribution of the electrically conductive connections over the range of directions and thereby shorter paths from the absorption area to the frame and increased heat conductance from the membrane to the frame.

The present disclosure also concerns a Fourier-transform infrared spectrometer comprising the infrared detector according to one of the variations and embodiments described above.

Moreover, the present disclosure extends to the use of the infrared detector according to one of the variations and embodiments described above in infrared spectroscopy, in particular in Fourier-transform infrared spectroscopy. As discussed in the outset, this set of applications benefit from the small response time of the membrane of the detectors described above.

Finally, the scope of the present disclosure also includes a method for performing infrared spectroscopy, the method comprising the steps of: placing a sample in an absorption area of a membrane of an infrared detector; illuminating the sample with a spectrally broadband time-modulated infrared light having a modulation frequency of 1 kHz or higher; and recording an interferogram with the infrared detector by detecting a frequency change of the resonance frequency of the membrane. The interferogram allows to recognize absorption lines of the sample within the spectral range of the light. The infrared detector as described above is particularly suitable for FT-IR due to the relatively high speed (low response time) of the detector, which can be suitable to match the requirements of regular FT-IR set-ups and light sources.

Referring now to the drawings, wherein the figures are for purposes of illustrating the present disclosure and not for purposes of limiting the same,

Figure 1 schematically shows a top view of an embodiment of an infrared detector according to the present disclosure;

Figure 2 schematically shows side view of the infrared detector shown in Figure 1; and

Figure 3 schematically shows an embodiment of a membrane with an electrically conductive layer comprising an opening.

Figures 1 and 2 show an infrared detector 1 of a Fourier- transform infrared spectrometer. In use, the infrared detector 1 is arranged between magnets generating a static magnetic field as indicated by arrows 2 and in a vacuum chamber. The vacuum chamber may create a pressure below 1 Pascal. The infrared detector 1 comprises a nanoelectromechanical resonator 3 and an electrical readout means 4. The vacuum chamber features electrical feedthroughs for the electrical connections between the electrical readout means 4 and the nanoelectromechanical resonator 3. The vacuum chamber also features a window that is transparent to the used probing radiation 15 (e.g., infrared light). The nanoelectromechanical resonator 3 comprises a frame 5 and a membrane 6 supported by the frame 5. The membrane 6 is arranged to oscillate within the magnetic field. The frame 5 is thermally connected to the vacuum chamber. The infrared detector 1 can be temperature controlled with a Peltier element thermally connected to the frame 5, i.e., placed between the frame 5 and the vacuum chamber.

The membrane 6 has an absorption area 7 for placing the spectrometry sample. The absorption area 7 may feature a broadband IR absorber. The membrane 6 may be chemically treated to become hydrophobic at least in the absorption area 7. The Fourier- transform infrared spectrometer further comprises an infrared source and optics (not shown) for conditioning and directing the infrared light to the absorption area 7 of the membrane 6. The membrane area is quadratic and the absorption area 7 is circular area provided in the centre of the membrane area. The membrane 6 is pervious in the absorption area 7 as schematically indicated by a mesh in Fig. 1.

The membrane 6 is a metallic membrane made of a uniform layer of aluminium. It is thus essentially uniform over the entire area and electrically conductive and forms an electrically conductive connection 8 between the absorption area 7 and the frame 5.

The electrical readout means 4 is configured for detecting a frequency change of the resonance frequency of the membrane 6. It comprises a low-noise amplifier 9 connected to a lock-in amplifier 10 controlling the actuation of the membrane 6 in a phase-locked loop (PLL). Any closed-loop circuit maybe used for controlling the actuation, e.g., also a positive feedback loop. The low-noise amplifier 9 of the electrical readout means 4 is electrically connected to the membrane 6 via two electrical contacts 11, 12. The two electrical contacts 11, 12 are arranged on an axis 13 orthogonal to a magnetic field. The electrons within the membrane 6 are moved through the magnetic field by the membrane vibration (perpendicular to the drawing plane of Fig. 1) and the induced voltage is picked up via the electrical contacts n, 12 at opposite sides of the membrane 6. The electrical contacts 11, 12 may be for example wire bonds or spring-loaded pins.

The vibration of the membrane 6 can be induced by the lock-in amplifier 10, as in this example, by putting one electrical contact 11 to ground 14 and applying an oscillating (AC) voltage to the other electrical contact 12. In general, the membrane may also be driven by other means, e.g., with a piezoelectric element.

The thickness of the membrane 6 in this example is 30 nm. The edge length L of the membrane in this example is 500 pm. Thereby, the total cross section of the electrically conductive connection formed by the membrane is 60 pm ² , which is greater than 10 pm ² . The time constant r of such a rectangular NEMS drum detector of size L is given by (PILLER 2020, see above): with mass density p, specific heat capacity Cp, and the thermal conductivity K of the membrane 6. For the present exemplary embodiment this results in a time constant r of ~o.2 ms. This is within the requirements of typical FT-IR spectrometers (detector size L > 0.5 mm and speeds 1/r > 2 kHz at 633 nm).

According to a different embodiment, instead of a homogeneous layer, the membrane may be structured as a mesh over its entire area or there may be strings connecting the outer mesh ends at the border of the absorption area to the frame, in which case the membrane has numerous individual and electrically conductive connections between the absorption area and the frame. In yet another embodiment, the membrane is uniform in a central part comprising the absorption area and the central part is suspended by five or more strings or tethers, similar to a trampoline. For example, there may be eight tethers arranged in regular intervals around the absorption area, such that the angle between any two neighbouring electrically conductive connections in the plane of the membrane is 45 °.

In another embodiment, instead of a gold membrane, the membrane may be made of aluminium or graphene. In further embodiments, the membrane may be a SiN substrate coated with a metal film, e.g., of gold or aluminium. While the present disclosure refers to an infrared detector, the detector is not limited to the use with infrared probing radiation. In general, any means of depositing heat energy in the sample and ultimately in the absorption area may be used for probing. The suitability of the detector is not limited by the origin of the heat energy, but only by the amount and dynamics thereof, as it is designed to quickly detect the tiniest amounts and variations of heat energy deposited in the sample. For example, more energetic electromagnetic radiation, such as visible or ultraviolet light, may naturally also be used as probing radiation for illuminating the sample in the absorption area and the corresponding absorption characteristics (e.g., an absorption spectrum) maybe recorded with the disclosed detector.

Figure 3 shows another embodiment of a membrane 6. The membrane 6 comprises an absorption area 7 in the center 16 of the membrane 7. The membrane 6 is pervious in the absorption area 7. The absorption area 7 is partially pervious (indicated by the crosshatching) and partially non-pervious (not hatched in the figure). Furthermore, the membrane 6 comprises an electrically conductive layer 17. The electrically conductive layer 17 comprises an opening 18. The absorption area 7 is arranged within the opening 18 of the electrically conductive layer 17. The non-pervious part of the absorption area 7 is located adjacent to the electrically conductive layer 17. The electrical readout means 4 (see fig. 1) is electrically connected to the membrane 6 via electrical contacts 11, 12. The electrical contacts 11, 12 are arranged on opposite sides of the membrane 6 with respect to the center 16 of the membrane 6, such than an electrical current flowing from one electrical contact 11 to the other electrical contact 12 is guided across a central area of the membrane 6. The electrically conductive layer 17 comprises two recesses 19 extending towards the center 16 of the membrane 6. The recesses 19 each also extend over an outer end 20 of the membrane 6. Electrical current flowing from one electrical contact 11 to the other electrical contact 12 flows across two relatively thin sections 21 of the electrically conductive layer 17. Consequently, the electrical current flows across a central area of the membrane 6 leading to an enhanced induction of vibration of the membrane 6 due to a Lorentz force caused by the electrical current and the magnetic field (for details on the magnetic field see figs. 1 and 2).