Patent Description:
The detection of environmental parameters in the ambient atmosphere is becoming increasingly important in the implementation of appropriate sensors within mobile devices, for example, but also in the application in home automation, such as smart home, and, for example, in the automotive sector. However, with the ever more extensive use of sensors, there is also a particular need to be able to produce such sensors as inexpensively as possible and, thus, as cost effectively as possible. However, the resulting reliability and accuracy of the sensors should nevertheless be maintained or should be even increased.

In particular, the field of monitoring the air quality and the gas composition in our environment is receiving more and more attention. A typical optical gas sensor comprises a light source, filter elements for a wavelength selection, a detector and the sample area, where the light between the light source and the detector interacts with the environmental medium. Typically, such sensors are rather expensive to manufacture and/or rather bulky.

On-chip integration of a selective and efficient emitter/absorber currently presents a large challenge to the industrial sector. An implementation possibility on this technical field are quantum cascade structures (both for emission and for absorption), which offer a good performance. However, integrated emitters and detectors, which may be implemented by means of quantum cascade structures (QCLs), require a heterogeneous integration of III-V elements by means of bonding (InGaAs potential wells). Thus, their production involves considerable expenditure and cost, which renders it almost impossible for such structures to be integrated into low-cost, silicon-based CMOS manufacturing processes. Thus, due to the high cost, quantum cascade structures are not suitable for mass production.

An alternative, inexpensive implementation possibility on this technical field has been so far a doped nanowire producing Joule heating and thermal radiation by making contact with a voltage source. Even though such a solution is fully compatible with low-cost, silicon-based CMOS manufacturing processes, it represents a massive restriction to the sensitivity of miniaturized absorption sensors having such an emitter and, therefore, to the detection and resolution limit that may be achieved. The reason for this is the broad-band radiation spectrum that is similar to a Planck radiation. Such a broad spectrum restricts the relative change of the detector response in the presence of the gas to be detected since the gas absorbs only within a very narrow-band range.

<CIT> relates to a fluid sensor and a method for manufacturing a fluid sensor.

<NPL>, relates to a highly selective CMOS-compatible mid-infrared thermal emitter/detector slap design using optical Tamm-states.

<NPL>, relates to mid-infrared rib waveguide absorption sensors based on Si.

Generally, there is a need in the art for an approach to implement an improved optical resonator system and an improved narrowband mid-infrared radiation source, e.g. as respective components of an improved fluid sensor, wherein the components have low fabrication requirements but provide a resulting fluid sensor having an adequate sensitivity for the target fluid to be detected.

Such a need can be solved by the optical resonator system according to claim <NUM>, by the narrowband mid-infrared radiation source according to claim <NUM> and by the fluid sensor according to claim <NUM>.

Specific implementations of the optical resonator system, the narrowband mid-infrared radiation source and the fluid sensor are defined in the dependent claims.

According to an embodiment, the optical resonator system comprises a multi-strip waveguide structure, a STP resonance structure (STP = slab tamm-plasmon-polariton), and an optical coupling structure. The multi-strip waveguide structure comprises a plurality of spaced semiconductor strips for guiding an IR radiation. The STP resonance structure comprises an alternating arrangement of semiconductor strips and interjacent dielectric strips and comprises a metal strip adjacent to the semiconductor strip at a boundary region of the STP resonance structure. The metal strip and the adjacent semiconductor strip are arranged to provide a metal-semiconductor interface at the boundary region of the STP resonance structure, wherein the semiconductor strips of the multi-strip waveguide structure and the semiconductor strips of the STP resonance structure are arranged perpendicular to each other in a common system plane. The optical coupling structure having a semiconductor layer, wherein the semiconductor layer is arranged between the multi-strip waveguide structure and the STP resonance structure for optically coupling the IR radiation between the multi-strip waveguide structure and the STP resonance structure.

According to an embodiment, a narrowband mid-infrared radiation source comprise the optical resonator system (as described above), and an infrared radiation emitter for emitting a broadband infrared radiation to the STP resonance structure, wherein the IR radiation in the resonance wavelength range of the STP resonance structure propagates from the STP resonance structure to the optical coupling structure and is coupled into the multi-strip waveguide structure.

According to an embodiment, a fluid [fluid = gas or liquid] sensor comprises the narrowband mid-infrared radiation source (as described above), and a mid-infrared radiation detector configured to provide a detector output signal based on an intensity of the guided (and filtered) infrared radiation (= infrared light-wave) received from the multi-strip waveguide structure.

Thus, embodiments represent a coupled, selective waveguide absorber system for the middle infrared spectrum which comprises CMOS compatibility and the capability of on-chip integration. Within this context, a guided wave couples with an optical resonator, which results in resonant (and therefore selective) absorption. Selective absorption implies that an external heater element enables selective emission into the guided wave mode (with a reverse sign of the propagation direction). The physical background thereto is Kirchhoff's law of thermal radiation. Thus, the structure may be used both as a resonant absorber and as a resonant emitter.

Therefore, the waveguide absorber system presented here enables a high level of efficiency of emission and absorption, wherein the range of the high efficiency level of quantum cascade structures may possibly be approximately achieved. At the same time, a production process operating within the framework of common mass production methods is enabled.

The proposed concept is potentially compatible with low-cost, silicon-based CMOS manufacturing processes. At the same time, it enables narrow-band emission and absorption. Emission/absorption is effected to/from a type of waveguide which exhibits superior properties with regard to sensitivity. Thus, the concept represents a complete emitter-waveguide and/or waveguide-absorber system. The light emitted, or the light which may be absorbed, is heavily polarized and has an elevated degree of coherence.

To summarize, the waveguide absorber system presented here enables manufacturing within the framework of common CMOS methods and materials (i.e. silicon-based materials and metals).

In the following, embodiments of the present disclosure are described in more detail with respect to the figures, in which:.

In the following description, embodiments are discussed in further detail using the figures, wherein in the figures and the specification identical elements and elements having the same functionality and/or the same technical or physical effect are provided with the same reference numbers or are identified with the same name. Thus, the description of these elements and of the functionality thereof as illustrated in the different embodiments are mutually exchangeable or may be applied to one another in the different embodiments.

In the following description, embodiments are discussed in detail, however, it should be appreciated that the embodiments provide many applicable concepts that can be embodied in a wide variety of semiconductor devices. The specific embodiments discussed are merely illustrative of specific ways to make and use the present concept, and do not limit the scope of the embodiments. In the following description of embodiments, the same or similar elements having the same function have associated therewith the same reference signs or the same name, and a description of such elements will not be repeated for every embodiment. Moreover, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

It is understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or intermediate elements may be present. Conversely, when an element is referred to as being "directly" connected to another element, "connected" or "coupled," there are no intermediate elements. Other terms used to describe the relationship between elements should be construed in a similar fashion (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", and "on" versus "directly on", etc.).

For facilitating the description of the different embodiments, some of the figures comprise a Cartesian coordinate system x, y, z, wherein the x-z-plane corresponds, i.e. is parallel, to a first main surface region of a substrate (= a reference plane = x-z-plane), wherein the direction vertically up with respect to the reference plane (x-z-plane) corresponds to the "+y" direction, and wherein the direction vertically down with respect to the reference plane (x-z-plane) corresponds to the "-y" direction. In the following description, the term "lateral" means a direction parallel to the x- and/or z-direction or a direction parallel to (or in) the x-z-plane, wherein the term "vertical" means a direction parallel to the y-direction.

<FIG> show a schematic representation of the optical resonator system <NUM> according to an embodiment. <FIG> shows a schematic plane view and <FIG> shows a schematic cross-sectional view of the optical resonator system <NUM> according to the embodiment.

As shown in <FIG>, the optical resonator system <NUM> comprises a multi-strip waveguide structure <NUM>, a STP resonance structure <NUM> (STP = slab tamm-plasmon-polariton) and an optical coupling structure <NUM>.

The multi-strip waveguide structure <NUM> comprises a plurality of spaced semiconductor strips <NUM>-<NUM>,. , <NUM>-n (<NUM>-#) for guiding an IR radiation RIR. In <FIG>, the multi-strip waveguide structure <NUM> is exemplarily shown to comprise three spaced (complete) semiconductor strips <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, wherein the number "n" of spaced semiconductor strips <NUM>-<NUM>,. , <NUM>-n may be between <NUM> and <NUM>, or between <NUM> and <NUM>, or (about) <NUM>, for example, when considering an exemplary waveguide-width (= n x w, with w = width of a semiconductor strip) between <NUM> and <NUM>, between <NUM> and <NUM>, or of (about) <NUM>. The spaced semiconductor strips <NUM>-<NUM>,. , <NUM>-n may be separated by means of dielectric strips <NUM> (e.g. air gaps or oxide).

The STP resonance structure <NUM> comprises an alternating arrangement of semiconductor strips <NUM>-<NUM>,. , <NUM>-m and interjacent dielectric strips <NUM>-<NUM>,. , <NUM>-p (e.g. air gaps or oxide) and comprises a metal strip <NUM> adjacent to the semiconductor strip <NUM>-m (= <NUM>-<NUM> in <FIG> at a boundary region <NUM>-<NUM> of the STP resonance structure <NUM>.

In <FIG>, STP resonance structure <NUM> is exemplarily shown to comprise two semiconductor strips <NUM>-<NUM>, <NUM>-<NUM>, i.e. the number "m" of spaced semiconductor strips <NUM>-<NUM>,. , <NUM>-m may be <NUM>, for example. In <FIG>, STP resonance structure <NUM> is exemplarily shown to comprise two dielectric strips <NUM>-<NUM>, <NUM>-<NUM>, i.e. the number "p" of spaced dielectric strips <NUM>-<NUM>,. , <NUM>-p may be <NUM>, for example (with m = p). The numbers m, p usually do not vary due to physical reasons, wherein the widths of the strips <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> are (exactly) defined individually, as will be explained below.

The metal strip <NUM> and the adjacent semiconductor strip <NUM>-m (<NUM>-<NUM> in <FIG> are arranged to provide a metal-semiconductor interface <NUM>-A at the boundary region <NUM>-<NUM> of the STP resonance structure <NUM>, wherein the semiconductor strips <NUM>-<NUM>,. , <NUM>-n of the multi-strip waveguide structure <NUM> and the semiconductor strips <NUM>-<NUM>,. , <NUM>-m of the STP resonance structure <NUM> are arranged perpendicular to each other in a common system plane (= x-z-plane).

The optical coupling structure <NUM> comprises a semiconductor layer <NUM>, wherein the semiconductor layer <NUM> is arranged between the multi-strip waveguide structure <NUM> and the STP resonance structure <NUM> for optically coupling the IR radiation RIR between the multi-strip waveguide structure <NUM> and the STP resonance structure <NUM>.

According to an embodiment, the STP resonance structure <NUM> may be implemented as a Bragg mirror structure laterally extending in the system plane. According to an embodiment, the semiconductor material of the multi-strip waveguide structure <NUM>, of the STP resonance structure <NUM> and of the coupling structure <NUM> may comprise a semiconductor material, e.g. a base layer <NUM> having a silicon or polysilicon material, which is applied on a first main surface region <NUM>-A of a substrate or substrate structure <NUM> (see <FIG>), which provides the system plane of the optical resonator system <NUM>.

As shown in <FIG>, the substrate (or substrate structure) <NUM> may comprises a plurality of layers e.g., a first insulating (= dielectric) layer <NUM>, a second insulating (= dielectric) layer <NUM> and (optionally) a semiconductor substrate layer <NUM>. The first dielectric layer <NUM> may comprise nitride material, e.g. SiN, the second dielectric layer <NUM> may comprise an oxide material, e.g. BOX = buried oxide, such as SiO, or air, and the semiconductor substrate layer <NUM> may comprise silicon. Thus, the first main surface region of the first dielectric layer <NUM> forms the top main surface region <NUM>-A of the substrate <NUM>, i.e. the common system plane (= x-z-plane). As shown in <FIG>, the substrate structure <NUM> comprises a cavity <NUM> vertically below the metal-semiconductor interface <NUM>-A at the boundary region <NUM>-<NUM> of the STP resonance structure <NUM>. The formation of the cavity <NUM> in the substrate structure <NUM> below the metal-semiconductor interface <NUM>-A at the boundary region <NUM>-<NUM> of the STP resonance structure <NUM> reduces the heat transfer from the metal-semiconductor interface <NUM>-A into the adjacent material.

The first insulating layer <NUM> may comprise a thickness between <NUM> to <NUM>, between <NUM> and <NUM>, or of about <NUM>. The second insulating layer <NUM> may comprise a thickness between <NUM> to <NUM>, between <NUM> and <NUM>, or of about <NUM>. The semiconductor layer <NUM> may comprise a thickness between <NUM> to <NUM>, between <NUM> and <NUM>, or of about <NUM>.

The principle of the optical resonator system <NUM> is based on exciting so-called Tamm plasmon polaritons, which may occur as optical resonances at the boundary (interface) <NUM>-A between a Bragg mirror <NUM> and a metal <NUM>. Normally, Bragg mirrors <NUM>-# (<NUM>-<NUM>,. , <NUM>-m), <NUM>-# (<NUM>-<NUM>,. , <NUM>-p) and metal <NUM>, i.e. the Tamm plasmon (TP) structure <NUM>, are built with extended one-dimensional layers. If the Bragg mirror <NUM> and the metal <NUM> are tuned and adjusted to each other, perpendicularly incident light of the resonant wavelength λRES (on the side of the Bragg mirror <NUM>) will be fully absorbed by the resonance step-up (quality factor) of the field. The intrinsic losses of the metal are eventually responsible for complete absorption if the dielectrics are assumed to be approximately free from loss. This tuning (or matching) of the Bragg mirror <NUM> and the metal <NUM> refers to the individual amplitude reflection coefficients assigned to the Bragg mirror and the metal, respectively. Providing the employment of tungsten W, for example, this matching is considered in the design, as outlined below. The resonant absorption is also possible (e.g. with a different or lower performance), if other metals are used. Further, the design may be also tuned and optimized for other metals.

Thus, the core of the present disclosure is the optical resonator system <NUM> whose starting point is the dielectric layer waveguide <NUM> made of polysilicon, for example. This resonator system <NUM> is depicted in <FIG> (upper area) and 1b (central area). Here, the layers of the TP structures <NUM> are no longer be extended (approximately) infinitely in two dimensions (parallel to the x-y-plane) but are (vertically) restricted by the height h of the (polysilicon) layer (y direction, see <FIG>). The Bragg mirror <NUM> (= region of alternating refractive indices <NUM>-#, <NUM>-#) may be achieved and produced by completely removing, e.g. by etching to the membrane <NUM>, individual rectangular faces or regions (in the longitudinal section) of the poly-Si layer (= base layer <NUM>). The extension of the poly-Si layer <NUM> and its etching-out of the plane may be considered as infinite. Directly next to the Bragg mirror <NUM>, the metal layer <NUM> of about the same height as the poly-Si layer <NUM> is required. The metal layer <NUM> may comprise a stable, temperature-insensitive metal, such as W (tungsten), Ag (silver), Mo (Molybdenum), etc.. The extension of the metal area/layer <NUM> may be selected, in principle, to be of any size. Only the relation to the membrane <NUM> surface must be taken into account for reasons concerning the heat flow. As there is no transmission of light through the metal, the width of the metal strip can be chosen arbitrarily wide, from an optical point-of-view. So it can be chosen to be convenient for fabrication and thermal aspects. The height of the metal-strip may be equal to the polysilicon in a best-case scenario, for example (the height of the metal layer <NUM> = height of the poly-Si layer (base layer) <NUM>", for example).

Thus, the metal-semiconductor interface <NUM>-A, e.g. a tungsten/silicon interface, is achieved/produced at one position. That boundary layer (interface) <NUM>-A is essential for the "slab Tamm plasmon polariton" (STP) structure (or layer Tamm plasmon structure) <NUM>. Instead of extended planar waves, the modeling is now faced with guided layer modes, where inclusion of the light is effected by the high refractive index of the silicon (e.g. poly-Si). The consequence for STP resonance structures <NUM> as compared to one-dimensional TP structures is that, with/at optical resonance, there are radiation losses into space, and that resonance can be effected only for a specific polarization, namely when the electric field is parallel to the substrate surface, i.e. is transverse-electric polarized (TE-polarized) or s-polarized. Thus, the STP resonance structure <NUM> provide a (field) suppression remote from the resonant wavelength and from TE polarization, wherein the field's increase (step-up) in the plasmonic resonance results in an increase of emission/absorption at the resonance wavelength and TE polarization.

So far, only the STP resonance structure <NUM> has been described which is a pure twodimensional layer structure (infinite in its extension out of the plane). So as to complete the waveguide absorber system <NUM>, the STP structure <NUM> is coupled to the periodic strip waveguide (multi-strip waveguide, see lower area of <FIG>) <NUM>. The multi-strip waveguide <NUM> may be manufactured in an inexpensive manner by etching complete, continuous gaps of defined widths into a slab waveguide (planar waveguide) of polysilicon at periodic intervals or into the base layer <NUM>, e.g. a silicon or poly-Si layer (at periodic intervals). Said type of waveguide <NUM> exhibits superior properties with regard to sensitivity (via simulation and experiment). The coupling of the multi-strip waveguide structure <NUM> to the STP resonance structure <NUM> is effected by means of the optical coupling structure <NUM>, e.g. in form of a polysilicon layer (having an optimized lateral extension) between the STP structure <NUM> and the multi-strip waveguide <NUM>. The optimized lateral extension may be derived from a semi-empirical optimization. It was found via simulation (by the inventors) that the optimized lateral extension may approximately corresponds to five times the averaged quarter-wavelength thickness considering the slab-waveguide and the coupled multi-strip-waveguide mode.

The complete waveguide absorber system <NUM> was simulated with a three-dimensional periodic unit cell (= simulation unit cell) by means of the finite element method. The three-dimensional periodic unit cell is exemplarily shown in <FIG>.

The resulting resonance wavelength range (target wavelength) λRES of the STP resonance structure <NUM> of the optical resonator structure <NUM> may be chosen in a range between <NUM> and <NUM>, for example.

For the resulting resonance wavelength range (target wavelength) λRES of the STP resonance structure <NUM> between <NUM> and <NUM>, the semiconductor strips <NUM>-# of the multi-strip waveguide structure <NUM> may have a width w between <NUM> and <NUM>, a height h between <NUM> and <NUM> and a distance (gap) g between <NUM> and <NUM>. According to an embodiment, the semiconductor strips <NUM>-# of the multi-strip waveguide structure <NUM> may have a width w of <NUM>, a height h of <NUM> and a distance g of <NUM>.

Accordingly, the semiconductor strip <NUM>-m (= <NUM>-<NUM> in <FIG> at the boundary region <NUM>-<NUM> of the STP resonance structure <NUM> may have a width d<NUM> between <NUM> and <NUM> and a height h between <NUM> and <NUM>. According to an embodiment, the semiconductor strip <NUM>-m (= <NUM>-<NUM> in <FIG> at the boundary region <NUM>-<NUM> may have a width d<NUM> of <NUM> and a height h of <NUM>. The further semiconductor strips <NUM>-# (= <NUM>-<NUM> in <FIG> of the STP resonance structure <NUM> may have a width d<NUM> between <NUM> and <NUM>, a height h between <NUM> and <NUM> and a distance d<NUM>, d<NUM> (d<NUM>Air, d<NUM>Air) between <NUM> and <NUM>. According to an embodiment, the further semiconductor strips <NUM>-# (= <NUM>-<NUM> in <FIG> of the STP resonance structure <NUM> may have a width d<NUM> of <NUM>, a height h of <NUM> and a distance d<NUM>, d<NUM> (d<NUM>Air, d<NUM>Air) of <NUM>.

According to an embodiment, the semiconductor layer <NUM> of the coupling structure <NUM> may have a length d<NUM> between <NUM> and <NUM> and a height h between <NUM> and <NUM>. According to an embodiment, the semiconductor layer <NUM> of the coupling structure <NUM> may have a length d<NUM> of <NUM> and a height h of <NUM>.

In principle, a scaling of the above dimensions is possible for achieving a different resulting resonance wavelength range (target wavelength) λRES of the STP resonance structure <NUM> of the optical resonator structure <NUM>. However, it is to be kept in mind that the intrinsic absorption of Si gets dominant for wavelengths λ > <NUM>.

According to an embodiment, the multi-strip waveguide structure <NUM> comprises a plurality of slab waveguides <NUM>-<NUM>,. , <NUM>-n for guiding slab modes of the IR radiation RIR.

<FIG> shows a schematic top view of a narrowband mid-infrared radiation source <NUM> according to an embodiment. The narrowband mid-infrared radiation source <NUM> comprises the optical resonator system <NUM> as described above with respect to <FIG>, and an infrared radiation emitter <NUM> for emitting a broadband infrared radiation RIR to the STP resonance structure <NUM>, wherein the IR radiation RIR in the resonance wavelength range λRES of the STP resonance structure <NUM> propagates from the STP resonance structure <NUM> to the optical coupling structure <NUM> and is coupled into the multi-strip waveguide structure <NUM>.

According to an embodiment, the STP resonance structure <NUM> is configured as an optical filter structure to filter the broadband infrared radiation RIR emitted by the infrared radiation emitter <NUM> and to provide the IR radiation R'IR as a filtered infrared radiation having a center wavelength λ<NUM> according to the resonance wavelength range λRES of the STP resonance structure <NUM>.

According to an embodiment, the multi-strip waveguide structure <NUM> is configured to guide the (filtered) IR radiation R'IR having the center wavelength λ<NUM>, wherein the guided IR radiation R'IR (= the filtered IR radiation guided by the multi-strip waveguide structure) comprises an evanescent field component for interacting with the surrounding atmosphere.

According to an embodiment, the coupling structure <NUM> is configured to couple a mode of the IR radiation with the center wavelength λ<NUM> into the multi-strip waveguide structure.

According to an embodiment, the metal strip <NUM> adjacent to the semiconductor strip <NUM>-<NUM> at the boundary region <NUM>-<NUM> of the STP resonance structure <NUM> may be (optionally) arranged to form a heating element, i.e. the infrared radiation emitter <NUM>. Thus, the metal (e. Ag, W, Mo,. ) <NUM> may be contacted so as to generate Joule heating by applying a voltage.

According to a further (optional) embodiment, a free standing (isolated) highly n-doped polysilicon wire <NUM> may be optionally provided as the heating element (thermal radiation emitter) <NUM>, that emits broadband IR radiation RIR proportionally to the Planck's radiation law. The n-doped polysilicon wire may be heated in the vicinity of the Si/W boundary <NUM>-<NUM>, as optionally depicted in <FIG>.

The heating element <NUM> (optionally in form of the metal strip <NUM> or in form of doped polysilicon wire <NUM>) may be configured to have in an actuated condition an operating temperature in a range between <NUM> and <NUM> or between <NUM> and <NUM>. In the actuated condition of the metal strip <NUM>, the metal strip <NUM> is connected to the voltage source VM. In the actuated condition of the polysilicon wire <NUM>, the polysilicon wire 222is connected to the voltage source VSi. According to an embodiment, the infrared radiation emitter is connected to a power source <NUM> for providing the electrical energy to bring the infrared radiation emitter in the actuated condition.

<FIG> shows an exemplary schematic illustration of a fluid sensor <NUM> according to an embodiment. According to an embodiment, a fluid sensor <NUM> comprises the narrowband mid-infrared radiation source <NUM> as described above with respect to <FIG>, and a mid-infrared radiation detector <NUM> configured to provide a detector output signal based on an intensity of the guided (and filtered) infrared radiation (= infrared light-wave) R'IR received from the multi-strip waveguide structure <NUM>.

According to an embodiment, the mid-infrared radiation detector may comprise a thermopile structure, wherein the mid-infrared radiation detector is configured to sense an incoming guided infrared radiation which is a measure of the concentration of a target fluid FT in the surrounding atmosphere based on the evanescent field absorption effected by the target fluid FT. The interaction of the evanescent field component with the target fluid in the surrounding atmosphere results in a reduction of the transmitted IR radiation (guided infrared radiation transmitted by the multi-strip waveguide <NUM> between the emitter <NUM> and the detector <NUM>) due to absorption which is a measure for the target fluid concentration in the surrounding atmosphere.

According to a further embodiment, the mid-infrared radiation detector may comprise at least one of a pyroelectric temperature sensor, a piezoelectric temperature sensor, a pnjunction temperature sensor and a resistive temperature sensor.

The filtered IR radiation guided by the waveguide <NUM> comprises an evanescent field component for interacting with the surrounding atmosphere having the target fluid, wherein the interaction of the evanescent field component with the surrounding atmosphere results in a reduction of the transmitted thermal radiation R'IR due to absorption of the guided radiation R'IR which is a measure for the target fluid concentration in the surrounding atmosphere or medium. Thus, the fluid sensor <NUM> may provide a CMOS MID IR gas and liquid sensor for sensing the target fluid concentration (e.g. CO/ CO2/ O3/ NOx/ methane, for example) in the surrounding atmosphere or medium, based on a formation of a superimposed evanescent field, with narrowband mid-infrared radiation source <NUM> as described above with respect to <FIG>, and a mid-infrared radiation detector <NUM>.

In the present context, the term fluid may relate to a liquid or a gas. In case, the environmental medium relates to environmental air, the target fluid may relate to a target gas or target gas component which is present in the environmental air. The present concept is equally applicable to sensing a target liquid or a target liquid component in the environmental medium.

<FIG> shows a schematic cross-sectional view of a mid-infrared radiation detector <NUM> having a thermopile structure according to an embodiment. <FIG> shows a schematic cross-sectional view of the mid-infrared radiation detector <NUM> having a thermopile structure <NUM>' according to the embodiment.

The mid-infrared radiation detector <NUM> may be arranged on the substrate (or substrate structure) <NUM> having the plurality of layers <NUM>, <NUM> and (optionally) <NUM>. As shown in <FIG>, the substrate structure <NUM> comprises a further cavity <NUM> vertically below the thermopile structure. The formation of the cavity <NUM> in the substrate structure <NUM> reduces the heat transfer from the thermopile structure <NUM>' into the adjacent material. The thermopile structure comprises a plurality of Si strips <NUM>-<NUM>, <NUM>-<NUM> of alternating doping (phosphorus and boron) which are electrically connected by means of metal contacts <NUM> to provide a meander form of the connected metal strips <NUM>-<NUM>, <NUM>-<NUM>.

At the input portion of the thermopile structure <NUM>' (adjacent to the waveguide <NUM>), the radiation detector <NUM> comprises an alternating arrangement of semiconductor strips <NUM> and interjacent dielectric strips <NUM> (e.g. air gaps or oxide) and comprises a metal strip <NUM> adjacent to the semiconductor strip <NUM> at a boundary region <NUM>-<NUM> of the thermopile structure <NUM>'. The alternating arrangement of semiconductor strips <NUM> and interjacent dielectric strips <NUM> is laterally sandwiched between the (semiconductor) region <NUM> and the metal <NUM>.

The mid-infrared radiation detector <NUM> forms a resonant absorber, which provides a wavelength-selective absorption and, subsequently, selective heat generation. As the metal <NUM> is the absorptive element, the center of the heat-generation will be there. This results in a temperature difference between left and right side of the thermopile structure <NUM>'. The purpose of the strip <NUM>, e.g. a silica (SiO<NUM>) strip, is an electrical insulation from the metal contacts <NUM>, while maintaining close thermal contact with the doped Si-strips <NUM>-<NUM>, <NUM>-<NUM>. The purpose of the semiconductor region <NUM> (Si-slab) is a thermal contact with the substrate <NUM>. So in order to maximize the temperature difference between left and right side, the metal <NUM> and the strip <NUM> next to the thermopile structure <NUM>' should be as narrow as possible while the extent of the semiconductor region <NUM> may form an optimization parameter for the thermal design (depending on the membrane size, the thermal conductivity of the doped strips, etc) and may be equal to zero eventually.

For measuring a selective detection and resonant absorption of the transmitted thermal radiation R'IR, an implementation by means of a thermopile is possible, as depicted in <FIG>. Selective absorption ensures selective heating in the area of the Si/metal boundary layer. With corresponding contacting of Si strips of alternating doping (phosphorus and boron), a (detectable) voltage V is now generated by the Seebeck effect when the resonant wavelength is contacted by the STP resonance structure <NUM>, i.e. transmitted from the STP resonance structure <NUM> over the multi-strip waveguide <NUM> to the mid-infrared radiation detector <NUM>. In <FIG>, it is also possible and feasible to replace the metal (metal contacts) with a very high doped polysilicon (e.g. a phosphorus doping concentration (~ <NUM><NUM> cm-<NUM>) close to the solubility limit).

According to an embodiment, the substrate comprises a cavity vertically below at least one of the infrared radiation emitter and the mid-infrared radiation detector.

The formation of the cavity <NUM>, <NUM> in the substrate structure <NUM> below the thermal radiation emitter <NUM> and optionally below the thermal radiation detector <NUM> reduces the heat transfer from the thermal radiation emitter <NUM> and/or the thermal radiation detector <NUM> into the adjacent material so that the emission efficiency of the thermal radiation emitter as well as the detection efficiency of the thermal radiation detector can be increased.

<FIG> show (a) the field of the incident mode of the multi-strip waveguide (also representing the WG mode (WG = waveguide) for corresponding sensing applications), (b) the field distribution at resonance (top view) of the waveguide absorber system, and (c) the field distribution at resonance in a longitudinal (cross-sectional) view.

<FIG> show a selective absorption for different heights of the polysilicon layer for (a) air under the nitride membrane and (b) for oxide under the nitride membrane according to an embodiment.

<FIG> shows (in form of a table) a summary of results for varying substrate materials (air/oxide) and SI layer thicknesses h ("inclusion factor evanescent field γ, waveguide self-attenuation α, spectral emissivity at/with resonance ε(λr), and Q factor of resonance").

<FIG> just demonstrate the relative field enhancement from the simulation, which, in turn, is a measure for the possible improvement of the detection limit of such a resonant sensor in relation to a sensor featuring a broadband source. The same is true for <FIG> and <FIG>, which highlight the influence of the polysilicon thickness on the performance.

Additional embodiments and aspects are described which may be used alone or in combination with the features and functionalities described herein.

According to an embodiment, the optical resonator system comprises a multi-strip waveguide structure, a STP resonance structure (STP = slab tamm-plasmon-polariton), and an optical coupling structure.

The multi-strip waveguide structure comprises a plurality of spaced semiconductor strips for guiding an IR radiation. The STP resonance structure comprises an alternating arrangement of semiconductor strips and interjacent dielectric strips and comprises a metal strip adjacent to the semiconductor strip at a boundary region of the STP resonance structure. The metal strip and the adjacent semiconductor strip are arranged to provide a metal-semiconductor interface at the boundary region of the STP resonance structure, wherein the semiconductor strips of the multi-strip waveguide structure and the semiconductor strips of the STP resonance structure are arranged perpendicular to each other in a common system plane. The optical coupling structure having a semiconductor layer, wherein the semiconductor layer is arranged between the multi-strip waveguide structure and the STP resonance structure for optically coupling the IR radiation between the multi-strip waveguide structure and the STP resonance structure.

According to an embodiment, the STP resonance structure is implemented as a Bragg mirror structure laterally extending in the system plane.

According to an embodiment, the semiconductor material of the multi-strip waveguide structure, of the STP structure and of the coupling structure may comprise a semiconductor material, e.g. a silicon or polysilicon material, which is applied on a first main surface region of a substrate, which provides the system plane of the optical resonator system.

According to an embodiment, the semiconductor strips of the multi-strip waveguide structure have a width between <NUM> and <NUM>, a height between <NUM> and <NUM> and a distance between <NUM> and <NUM>, and in particular a width of <NUM>, a height of <NUM> and a distance of <NUM>.

According to an embodiment, the semiconductor strip at the boundary region of the STP resonance structure has a width between <NUM> and <NUM> and a height between <NUM> and <NUM>, and in particular a width of <NUM> and a height of <NUM>, wherein the further semiconductor strips of the STP resonance structure have a width between <NUM> and <NUM>, a height between <NUM> and <NUM> and a distance between <NUM> and <NUM>, and in particular a width of <NUM>, a height of <NUM> and a distance of <NUM>.

According to an embodiment, the semiconductor layer of the coupling structure has a length between <NUM> and <NUM> and a height between <NUM> and <NUM>, and in particular a length of <NUM> and a height of <NUM>.

According to an embodiment, the multi-strip waveguide structure comprises a plurality of slab waveguides for guiding slab modes of the IR radiation.

According to an embodiment, a narrowband mid-infrared radiation source comprise the optical resonator system, and an infrared radiation emitter for emitting a broadband infrared radiation to the STP resonance structure, wherein the IR radiation in the resonance wavelength range of the STP resonance structure propagates from the STP resonance structure to the optical coupling structure and is coupled into the multi-strip waveguide structure.

According to an embodiment, the STP structure is configured as an optical filter structure to filter the broadband infrared radiation emitted by the infrared radiation emitter and to provide the IR radiation as a filtered infrared radiation having a center wavelength according to the resonance wavelength range of the STP resonance structure.

According to an embodiment, the multi-strip waveguide structure is configured to guide the (filtered) IR radiation having the center wavelength, wherein the guided IR radiation (the filtered IR radiation guided by the multi-strip waveguide structure) comprises an evanescent field component for interacting with the surrounding atmosphere.

According to an embodiment, the coupling structure is configured to couple a mode of the IR radiation with the center wavelength λ<NUM> into the multi-strip waveguide structure.

According to an embodiment, the metal strip adjacent to the semiconductor strip at the boundary region of the STP resonance structure is arranged to form a heating element and is configured to have in an actuated condition an operating temperature in a range between <NUM> and <NUM> or between <NUM> and <NUM>, and wherein the infrared radiation emitter is connected to a power source for providing the electrical energy to bring the infrared radiation emitter in the actuated condition.

According to an embodiment, a fluid sensor comprises the narrowband mid-infrared radiation source, and a mid-infrared radiation detector configured to provide a detector output signal based on an intensity of the guided (and filtered) infrared radiation (= infrared light-wave) received from the multi-strip waveguide structure.

According to an embodiment, the mid-infrared radiation detector comprises a thermopile structure, wherein the mid-infrared radiation detector is configured to sense an incoming guided infrared radiation which is a measure of the concentration of a target fluid in the surrounding atmosphere based on the evanescent field absorption effected by the target fluid, wherein the interaction of the evanescent field component with the target fluid in the surrounding atmosphere results in a reduction of the transmitted IR radiation (guided infrared radiation transmitted by the multi-strip waveguide) due to absorption which is a measure for the target fluid concentration in the surrounding atmosphere.

Although some aspects have been described as features in the context of an apparatus it is clear that such a description may also be regarded as a description of corresponding features of a method. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be regarded as a description of corresponding features concerning the functionality of an apparatus.

Depending on certain implementation requirements, embodiments of the control circuitry can be implemented in hardware or in software or at least partially in hardware or at least partially in software. Generally, embodiments of the control circuitry can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer.

Claim 1:
Optical resonator system (<NUM>), comprising:
a multi-strip waveguide structure (<NUM>) having a plurality of spaced semiconductor strips (<NUM>-#) for guiding an IR radiation (RIR),
a STP resonance structure (<NUM>) (STP = slab tamm-plasmon-polariton), wherein the STP resonance structure (<NUM>) is implemented as a Bragg mirror structure laterally extending in the system plane, and wherein the STP resonance structure (<NUM>) comprises an alternating arrangement of semiconductor strips (<NUM>-#) and interjacent dielectric strips (<NUM>-#) and comprises a metal strip (<NUM>) adjacent to the semiconductor strip (<NUM>-<NUM>) at a boundary region (<NUM>-<NUM>) of the STP resonance structure (<NUM>),
wherein the metal strip (<NUM>) and the adjacent semiconductor strip (<NUM>-<NUM>) are arranged to provide a metal-semiconductor interface (<NUM>-A) at the boundary region (<NUM>-<NUM>) of the STP resonance structure (<NUM>), and
wherein the semiconductor strips (<NUM>-#) of the multi-strip waveguide structure (<NUM>) and the semiconductor strips (<NUM>-#) of the STP resonance structure (<NUM>) are arranged perpendicular to each other in a common system plane (<NUM>-A), and
an optical coupling structure (<NUM>) having a semiconductor layer (<NUM>), wherein the semiconductor layer (<NUM>) is arranged between the multi-strip waveguide structure (<NUM>) and the STP resonance structure (<NUM>) for optically coupling the IR radiation (RIR) between the multi-strip waveguide structure (<NUM>) and the STP resonance structure (<NUM>).