WO2024010452A1

WO2024010452A1 - Method for suspended high-stress films on integrated distributed bragg mirrors

Info

Publication number: WO2024010452A1
Application number: PCT/NL2023/050368
Authority: WO
Inventors: Richard Alexander NORTE; Paolo Maria SBERNA; Andrea CUPERTINO
Original assignee: Technische Universiteit Delft
Priority date: 2022-07-08
Filing date: 2023-07-07
Publication date: 2024-01-11
Also published as: NL2032428B1

Abstract

The invention provides a method (1) for providing a sensor element (2000) for photonic sensing, the method comprising providing a distributed Bragg reflector (100) comprising alternating layers (110, 120) comprising different materials; comprising providing a (sacrificial) silicon layer (200) on at least part of the distributed Bragg reflector (100); comprising depositing a top layer (300) on at least part of the (sacrificial) silicon layer (200); wherein the top layer (300) comprises a high-stress dielectric material (320); comprising providing a mask (310) for a suspended reflector (400) on the top layer (300); comprising etching the top layer (300) through the mask (310) to provide the suspended reflector (400) and etching a cavity (210) in the silicon layer (200).

Description

Method for suspended high-stress films on integrated distributed Bragg mirrors

FIELD OF THE INVENTION

The invention relates to a method to provide a sensor element for optomechanical sensing. Further, the invention relates to such sensor element. The invention also relates to an apparatus for optomechanical sensing comprising the sensor element.

BACKGROUND OF THE INVENTION

Cavity based Optomechanical sensors are known in the art. For instance, EP3292078A1 relates to a method of fabricating a reflector, the reflector being at least partially reflective and at least partially transmissive for at least a wavelength of electromagnetic radiation; the method comprising: forming a first material layer defining a bottom layer; forming a sacrificial layer on the bottom layer; forming a second material layer defining a top layer on the sacrificial layer and a supporting structure connected to the bottom layer; and removing at least part of the sacrificial layer to form a cavity between the bottom layer and the top layer such that the supporting structure supports the top layer relative to the bottom layer and no further supporting structure is provided within the cavity, wherein after the at least part of the sacrificial layer is removed, at least the top layer has residual tensile stress.

Huber Christian et al. “Large-aperture Fabry-Perot filters based on silicon/silicon carbonitride distributed Bragg reflectors for the near-infrared”, 2017 IEEE Sensors, IEEE, 2017-10-29 presents silicon carbonitride as a new low refractive index material for integration together with silicon in suspended distributed Bragg reflector membranes. It states that mechanical stress properties can be used to fabricate inherently flat mirror membranes of up to 5 mm diameter. Due to the large refractive index contrast to silicon such mirror membranes exhibit high reflectance over a wide spectral range. Further, it states that using these mirrors, a proof of concept for a large aperture Fabry -Perot filter with narrow bandwidth that could help miniaturize hyperspectral imaging sensors is demonstrated.

Stephane C: “Nanostructure arrays in free-space: optical properties and applications”, Reports on Progress in Physics, Institute of Physics publishing, Bristol, GB, issue 77, nr. 12, 2014-11-26 reviews the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub -wavelength nanostructure arrays. The role of symmetries and free-space coupling of resonant structures. Different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures are mentioned, as well as the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

US2002176473A1 describes a wavelength selectable, controlled chirp, semiconductor laser system is provided. By coupling a passive cavity, including an external output mirror with a selected reflectivity, to the active cavity of a laser device, chirp is reduced by approximately the ratio of the length of the active cavity to the length of the passive cavity. In such a device, changing the length of the passive cavity by manipulating the position of the output mirror allows for selecting an output wavelength of the laser device.

SUMMARY OF THE INVENTION

Cavity based optomechanical sensors are known in the art and these sensors may work on the principle of coupling mechanical motions with optical resonances. The optomechanical sensors may typically include a cavity (formed between a stressed reflective membrane (or film) and a multi-layered reflector) in which radiation pressure coupling between light and mechanical displacement is exploited to detect mechanical displacements to a high degree of accuracy. The precision offered by these sensors may be used in precision sensing of physical quantities including displacements, masses, temperatures, forces, and accelerations. With the development of micro and nano scale fabrication techniques, it is possible to improve the precision of these sensors compared to other conventional sensors based on Microelectromechanical Systems (MEMS). However, the process or method of providing the optomechanical sensor suffers from many limitations.

Cavity based optomechanical sensors comprise a film under tensile stress suspended over a multi-layered reflector. The film (or “thin-film”, or “high-stress thin-film”) exhibits a resistance to deformation and is stressed in the order of GPa (it is this tensile stress that enables their high sensitivity). Thin-films with a thickness in the order of nanometers comprising silicon nitride have been the subject of recent study on account of the mechanical and optical properties exhibited by these films. These films are conventionally produced by chemical vapor deposition (CVD) (for example using dichlorosilane SiHiCh or silane silicon hydride SiHt and ammonia NH3) by which films of uniform thickness and chemical composition may be obtained. The magnitude of residual stress in the thin-film is in dependence of the temperature at which the thin-film is deposited (on a substrate). Therefore, providing films with a high tensile stress requires chemical vapor deposition at high temperatures. It must be noted that the optomechanical sensors also comprise a reflector. However, it is a challenge to provide a multi-layered reflector in combination with the high-stress silicon nitride films. This is because conventional multi-layered reflectors (such as used in MEMS applications) cannot withstand the high temperatures of the forming process for the high-stress thin-film.

Consequently, suspending a thin-film that has a large residual stress (such as in the order of GPa) over a multi-layered reflector to provide the optomechanical sensor is a challenging task, which requires an additional manufacturing step or process i.e. the alignment of the thin-film and the reflector. Suspending a stressed thin-film a few nanometers in thickness over a reflector, such that the thin-film and the reflector are aligned may involve using additional reflectors and high precision lasers to align the reflector with the thin-film. This may increase the complexity of the manufacturing process and (also) limit the precision offered by the optomechanical sensor. Moreover, the conventional optomechanical sensor (provided by the assembly of the thin-film and the reflector) may be susceptible to manufacturing defects on account of the complexity of the parts involved in its construction. Thus, this may limit its performance on account of its susceptibility to damage and interference from the surrounding environment.

Further, the thin-films are conventionally suspended over the multi-layered reflector in the optomechanical system by first depositing the films over a substrate (such as a silicon wafer) followed by etching a hole for optical access through the silicon substrate. Typically, a wet etchant such as potassium hydroxide (KOH) or tetramethylammoniumhydroxide (TMAH) are used to make the hole in the substrate. The substrate with a thin-film reflector is conventionally aligned with a separate mirror (whose surface is made reflective with a multi-layered coating. The high surface tension of these etchants (wet etchants) results in stiction that can destroy the suspended thin film devices and reduce nanofabrication yield. Here, “stiction” refers to the friction which tends to prevent stationary surfaces from being set in motion.

Hence, it is an aspect of the invention to provide a method to provide a sensor element, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Hence, in a first aspect the invention provides a method for providing a sensor element, especially for optomechanical sensing. In embodiments, the method may comprise providing a distributed Bragg reflector (DBR) comprising alternating layers comprising different materials. Especially, the method may comprise providing a sacrificial silicon layer (or “silicon layer”) on at least part of the distributed Bragg reflector. The method, in embodiments may comprise depositing a top layer on at least part of the (sacrificial) silicon layer. Particularly, the top layer may comprise a (high-stress) dielectric material. Further, in embodiments, the method may comprise providing a mask for a suspended reflector (or “2D membrane” or “thin-film suspended reflector”) on the top layer. Especially, the method may comprise etching the top layer through the mask to provide the suspended reflector and etching a cavity in the (sacrificial) silicon layer. In a specific embodiment, the invention may provide a method for providing a sensor element for optomechanical sensing, the method comprising providing a distributed Bragg reflector comprising alternating layers comprising different materials; comprising providing a sacrificial silicon layer on at least part of the Bragg reflector; comprising depositing a top layer on at least part of the silicon layer; wherein the top layer comprises a (high-stress) dielectric material; comprising providing a mask for a suspended reflector on the top layer; comprising etching the top layer through the mask to provide the suspended reflector and etching a cavity in the (sacrificial) silicon layer. Hence, the invention may provide a method to provide a suspended (high-stress) film on integrated distributed Bragg mirrors.

Further, in embodiments, the invention may provide a method for providing a sensor element for optomechanical sensing, the method comprising providing a distributed Bragg reflector comprising alternating layers comprising different materials; comprising providing a sacrificial silicon layer on at least part of the Bragg reflector; comprising depositing a top layer on at least part of the silicon layer (at a temperature in the range of 500-1100 °C); wherein the top layer comprises a high-stress dielectric material; comprising providing a mask for a suspended reflector on the top layer; comprising etching the top layer through the mask to provide the suspended reflector and etching a cavity in the (sacrificial) silicon layer. Hence, the invention may provide a method to provide a suspended high-stress film on integrated distributed Bragg mirrors. In specific embodiments, the dielectric material may be a high-stress dielectric material. Further, in embodiments, the term “dielectric material” may refer to a high- stress dielectric material.

Herein, the term “high-stress film” or “high-stress dielectric material” may especially refer to a film or material comprising a tensile stress in the range of > 0.1 GPa, especially > 0.5 GPa. Further, the term “tensile stress” is known in the art, and may refer to the resistance of a film or material to a force that attempts to pull apart or stretch said film or material. The tensile stress of a material may be provided by the formula c = F/A, wherein F is the force (in N) along an axis of the material, and A the cross-sectional area (in mm²) of the material perpendicular to said axis. In embodiments, the tensile stress of a thin film may be calculated by depositing said thin film on a wafer substrate such as silicon. The thin film may especially be deposited at a temperature of 500-1100 °C (see below). During depositing, the thin film may conform to the size and/or geometry of the much thicker silicon substrate. After deposition, the substrates with thin films may be allowed to cool to room temperature. Due to a difference in the thermal expansion coefficient of the substrate and the film deposited on top, the film may exert a force on the substrate upon cooling, with a magnitude and direction based on the (positive or negative) difference in thermal expansion coefficients. Said force may create a counter-force and bowing in the substrate, wherein the extent of bowing may be a measure for the tensile stress in the thin film. In embodiments, the tensile stress of the thin film may be measured using a stress tester, which determines the wafer curvature by measuring the angle of deflection of a laser beam off the surface of the substrate. The tensile stress in the film may then be determined by comparing the change in a radius of the curvature of the substrate with and without thin film. In embodiments, a laser may be used to measure the curvature of the wafer. Said laser may be reflected at an angle that corresponds to the radius of curvature of the wafer.

The aforementioned method of providing the optomechanical sensor may be advantageous over conventional methods for forming sensors described in the prior art. In the present method of providing a sensor element, the top layer may e.g. be provided by chemical vapor deposition at high temperatures. This may be desirable for providing thin-films (or membranes) of high tensile stress. The use of the distributed Bragg reflector (or “reflector” or "multi-layered reflector") that comprises alternating layers that are resistant to such high temperatures may provide the benefit of providing the multi-layered reflector along with the suspended reflector at high temperatures i.e. the resistance (of the alternating layers) to high temperatures allows the deposition of the top layer (on the silicon layer) without the risk of damaging the multi-layered reflector during deposition. The integration provided according to the method i.e. providing the reflector and the top layer together as a single unit has several other unique advantages.

Since the alternating layers of the distributed Bragg reflector may be resistant to high temperatures, they may be provided in the same process as the suspended reflector i.e. as an integrated unit. Hence, this may avoid the requirement for providing the sensor element by assembling the different elements or components comprised by it, such as the distributed Bragg reflector and the top layer. Further, since the silicon layer, the reflector, and the top layer may be relatively flat, these elements may be provided such that they are parallel to one another. Hence, the sensor element may essentially have the suspended reflector aligned parallel to the distributed Bragg reflector i.e. the sensor element may be self-aligned. This may particularly be advantageous as no further alignment is required. With such a sensor element, an accuracy in the order of femtometers may be achieved. Furthermore, only a thin layer of (sacrificial) silicon may be removed to suspend the top layer. Because of this, a gas-based etchant may be used to remove the silicon. Further, this method does not have surface tension associated with wet-etchants, which may lead to higher fabrication yield.

As mentioned before, the invention provides a method for providing a sensor element. In embodiments, the method may comprise providing a distributed Bragg reflector comprising alternating layers comprising different materials. Distributed Bragg reflectors are known in the art. These reflectors may especially comprise alternating layers of materials of different refractive indices. The differences in the refractive index between two layers may be used to reflect light by means of total internal reflection (TIR). Hence, light may reach the two layers of the distributed Bragg reflector and be reflected by TIR, and hence these layers may be configured such that the layer with lower refractive index is configured downstream from the layer with higher refractive index along the direction of propagation of light. However, two layers may be ineffective in reflecting light since they may (only) reflect a part of the light reaching them. The advantage of using multiple alternating layers is that each subsequent alternating layer further reflects a portion of the light. Note that here the term “radiation” may also be used in place of “light”. Hence, the many reflections from a stack of reflectors (comprising alternating layers) may be combined as a result of constructive interference, and may thus, act as a high-quality reflector.

In embodiments, the distributed Bragg reflector may be provided on a flat surface. Further, in embodiments, the distributed Bragg reflector may be configured on at least part of the flat surface. The flat surface may be configured to support the sensor element, especially the distributed Bragg reflector. Further, in embodiments, the method may comprise providing the Bragg reflector by depositing alternating layers of a first layer and a second layer by means of chemical vapor deposition. In such embodiments, the first of the alternating layers may be deposited on the flat surface. In embodiments, the flat surface may comprise silicon. In specific embodiments, the flat surface may be a silicon wafer.

Further, in embodiments, the method may comprise providing a (sacrificial) silicon layer on at least part of the distributed Bragg reflector. Silicon is a versatile material and is commonly used in the construction of micro-electromechanical devices. A silicon wafer (i.e. a thin layer of silicon) may be polished to a mirror-like surface, hence ensuring a flat profile. Further, silicon wafers may be made virtually free of microparticles or impurities. Moreover, silicon can be provided in specific shapes by etching away silicon material from the silicon wafer. In embodiments, the silicon layer may be provided above the Bragg reflector. The silicon layer may form a base over which other components may be configured. In embodiments, the silicon layer may be provided on the distributed Bragg reflector such that it covers the entire surface of the distributed Bragg reflector. Alternatively, in embodiments, the silicon layer may (also) be provided such that it covers only a part of the distributed Bragg reflector. In other embodiments, the silicon wafer may extend beyond the lateral dimensions of the distributed Bragg reflector.

The method, in embodiments, may comprise depositing a top layer on at least part of the (sacrificial) silicon layer. The top layer, in embodiments, may be provided on the (sacrificial) silicon layer such that the (sacrificial) silicon layer is configured between the top layer and the distributed Bragg reflector. Especially, the top layer may be provided such that it covers the entire surface of the (sacrificial) silicon layer. Or alternatively, in embodiments, the top layer may (also) be provided such that it covers only a part of the (sacrificial) silicon layer.

Further, in embodiments, the top layer may comprise a high-stress dielectric material (or “dielectric material”). Especially, the dielectric material may comprise siliconnitride-based material or silicon-carbide-based material. In embodiments, the top layer may also be referred to as “thin-film” or “thin-layer” or “high-stress thin-layer”. Thin-films comprising silicon nitride are known in the art. These thin-films may typically be provided by chemical vapor deposition (for example of silane silicon hydride SiEU or dichlorosilane SiEECl and ammonia NHs) at high temperatures. Although, conventionally SisN4 is provided as the thin-film, in embodiments, the silicon-nitride-based material may comprise a composition of the form Si_xN_y where the x and j' may be configured depending on the proportions of the two chemical vapors provided in the deposition process. Silicon-nitride-based compounds or materials essentially refers to silicon nitride having the composition Si_xN_y. Silicon carbide thin- film may (also) be provided by chemical vapor deposition using silane SiEU and propane C3H8. In embodiments, the silicon-nitride-based material may comprise a composition of the form Si_xN_y, where the ratio x/y may be selected from the range of 0.75-1.1, such as SisNi, wherein x/y=0.75.

Alternatively, in embodiments, the top layer may comprise silicon-carbide- based material and silicon-dioxide-based material. Especially, ceramic based materials may (also) be used. These materials may provide the benefit of acquiring high tensile stress when deposited at high temperatures. The coefficient of expansion of silicon in the silicon layer, and the top layer may be different. Hence, on cooling, the differential expansion of the two materials can cause the top layer to be under stress, and hence, a top layer with high tensile stress may be provided. Moreover, these materials show high selectivity to plasma etching, which may be advantageous in providing the suspended reflector.

Hence, in embodiments the method may comprise depositing a top layer on at least part of the (sacrificial) silicon layer by chemical vapor deposition, though other methods are herein not excluded. Further, in embodiments the top layer may comprise one or more of SixNy and SiC.

Further, in embodiments, the method may comprise providing a mask for a suspended reflector on the top layer. Especially, this may be beneficial in a subsequent step where the method may comprise etching a thin film suspended design into the top layer. The suspended reflector, in embodiments, may comprise holes, pores, or gratings that may be useful for operating the sensor element. The suspended reflector may in embodiments be stressed i.e. the suspended reflector may comprise a residual stress in the order of GPa (a quantitative measure of the tensile stress in the suspended reflector is described further below). Especially, the suspended reflector may be patterned to be highly reflective. The suspended reflector may provide optical access (for light) to the optomechanical sensor, especially the cavity in the optomechanical sensor.

In embodiments, etching a thin film suspended design into the top layer may increase the tensile stress of the top layer. Especially, in embodiments, the suspended reflector (after etching) may have a higher tensile stress than the top layer (before etching). In embodiments, the suspended reflector may (thus) have a residual tensile stress. Furthermore, in embodiments, the top layer may have a residual tensile stress, especially after etching.

In embodiments, the grating may be provided in dependence of the incident light or light source radiation on the suspended reflector. Hence, the gratings may have a width especially in the range 190-2000 nm, such as 250-1000 nm, more especially 300-800 nm. Especially, the pitch of the gratings may be selected in dependence of the wavelength of light incident on the suspended reflector. In embodiments, the gratings be arranged such that the grating comprises openings arranged in a ID array with the pitch between the grating openings selected in dependence of the wavelength of incident light. Further, in embodiments, the grating may also be a 2D grating where the openings in the grating may be arranged with a first pitch in one direction and a second pitch in another direction. That is, the spacing between the openings in the 2D array of gratings may be selected individually (in dependence of the wavelength of incident light). In embodiments, the sensor element may be used for optomechanical sensing. In such embodiments, the sensor element may especially be used in combination with a light source (e.g. a laser), wherein the light source is configured to provide light source radiation to the sensor element (see below) (optionally further using optics, known to a person skilled in the art). Hence, in embodiment, the light incident on the sensor element may especially be light source radiation, such as laser radiation. Further, in embodiments, the incident light, especially the light source radiation, may have a wavelength (X selected from the range of 190-2000 nm.

In embodiments, the suspended reflector may have a size. Especially, the suspended reflector may have a first equivalent circular diameter (Di). In embodiments, the first equivalent circular diameter (Di) may be selected from the range of > 10 pm. such as > 100 pm, especially > 500 pm. Further, in embodiments, the first equivalent circular diameter (Di) may be selected from the range of < 45 cm, such as < 30 cm, especially < 20 cm, though larger numbers are herein not excluded. In embodiments, the first equivalent circular diameter (Di) may be selected from the range of 10 pm - 45 cm, such as 100 pm - 30 cm, especially 500 pm - 20 cm. In specific embodiments, the first equivalent circular diameter (Di) may be equal to an equivalent circular diameter of the support. Further, in specific embodiments, the shape and size of the top layer may be equal to those of the support. In embodiments, the size of the suspended reflector may depend on the intended application. Hence, the reflector may be smaller (< 500 pm) to minimize spatial requirements. Conversely, the reflector may be larger (> 20 cm) to facilitate incident light having a larger beam size.

Additionally, in embodiments, the suspended reflector may comprise a plurality of openings, wherein the plurality of openings may comprise a plurality of holes (“pores”). In specific embodiments, the plurality of openings may form a grating, especially for the light source radiation.

In embodiments, the plurality of openings may comprise (a plurality of) first openings. The first openings may have a first opening equivalent circular diameter (D2a). In embodiments, the first opening equivalent circular diameter (D2a) may be smaller than the wavelength ( ) of the light source radiation, D2a < X, such as D2a < X-25 nm, especially D2a < Z.-50 nm. Additionally or alternatively, the first opening equivalent circular diameter (D2a) may, in embodiments, be selected from the range of 50-2000 nm, such as 100-1800 nm, especially 150-1650 nm. A first opening with an equivalent circular diameter smaller than the wavelength (X) of the light source radiation may prevent said light from being transmitted through the opening. Further, in embodiments, a suspended reflector comprising such first openings may reflect light source radiation with a wavelength (X) larger than the first opening equivalent circular diameter (D2a). In embodiments, the first opening equivalent circular diameter (D2a) of the first openings may vary across the suspended reflector. For example, first openings closer to the center of the suspended reflector may have a smaller first opening equivalent circular diameter (D2a) than first openings closer to the edge of the suspended reflector. Hence, in embodiments, the first opening equivalent circular diameter (D2a) of each first opening may be individually selected from the range of 50-2000 nm. In embodiments, a subset of the first openings may have a first opening equivalent circular diameter (D2a) larger than the wavelength (X), X < D2a <1.2* , such as < D2a <1.1 * . In embodiments, at most 20%, such as at most 15%, especially at most 10% of the total number of first openings may have a first opening equivalent circular diameter (D2a) selected from the range of < D2a <1.2* . In specific embodiments, the suspended reflector may comprise a plurality of openings, wherein the plurality of openings may comprise first openings, wherein the first openings may have a first opening equivalent circular diameter (D2a), wherein the first opening equivalent circular diameter (D2a) may be one or more of (i) selected from the range of 50-2000 nm, and (ii) smaller than the wavelength (X) of the light source radiation.

The equivalent circular diameter (or ECD) (or “circular equivalent diameter”) of an (irregularly shaped) two-dimensional shape is the diameter of a circle of equivalent area. For instance, the equivalent circular diameter of a square with side a is 2a/SQRT(7t). For a circle, the diameter D is the same as the equivalent circular diameter D. Would a circle in an xy-plane with a diameter D be distorted to any other shape (in the xy-plane), without changing the area size, then the equivalent circular diameter of that shape would be D.

In embodiments, the first openings may have a cross-sectional shape in a plane parallel to a face of the suspended reflector. In embodiments, the first openings may have a cross-sectional shape selected from the group comprising a circle, a square, a regular polygon, a rectangle, a cross, etc. Further, the first openings may have a first length (Li) and a first width (Wi). In embodiments, the first width (Wi) may be smaller than the wavelength (X) of the light source radiation, Wi < , such as Wi < X-25, especially Wi < X-50. Especially, in embodiments, the first width (Wi) may be selected from the range of 50-2000 nm, such as 100-1800 nm, especially 150-1500 nm. Further, in embodiments, the first length (Li) may be equal to the first width (Wi). Hence, in embodiments, the first length (Li) may be selected from the range of 50- 2000 nm, such as 100-1800 nm, especially 150-1500 nm. Yet, in embodiments, the first length (Li) may be larger than the first width (Wi), Li > 2*Wi, such as Li > 5*Wi, especially Li > 10*Wi. Further, in embodiments, the first length (Li) may be selected from the range of 0.1- 20 mm, such as 0.5-10 mm, especially 1-5 mm. Hence, in embodiments, the first openings may have first dimensions (Wi, Li, wherein at least one of said first dimensions may be one or more of (i) selected from the range of 50-2000 nm, and (ii) smaller than the wavelength (X) of the light source radiation. For circular first openings, the first opening equivalent circular diameter (D2a) may be equal to the first width (Wi) and the first length (Li), D2a = Wi = Li, and both dimensions (Wi, Li) may in embodiments be smaller than the wavelength (X) of the light source radiation.

In embodiments, the first openings may have a center-to-center first distance (di). In embodiments, the first distance (di) may be selected from the range of > 5 nm, such as > 10 nm, especially > 15 nm. Further, the first distance (di) may be selected from the range of < 2000 nm, such as < 1800 nm, especially < 1500 nm. In embodiments, the first distance (di) may be smaller than the wavelength (X) of the light source radiation, di < X, such as di < X-25 nm, especially di < X-50 nm. In embodiments, the distance between first openings may vary. For example, the first distance (di) may be smaller in a first direction, and larger in a second direction. Additionally or alternatively, in a single direction, the first distance (di) may vary. For example, the first distance (di) may be smaller near the center of the suspended reflector, and larger near the edge of the suspended reflector. Hence, in embodiment, the first distance (di) between any two first openings may be individually selected from the range of < 2000 nm. Further, in embodiments, both the first distance (di) and the first opening equivalent circular diameter (D2a) of the first openings may vary across the suspended reflector. This may facilitate the formation of a photonic crystal membrane, especially a focusing photonic crystal membrane. Such membranes are described in Guo et al. (2017); “Integrated optical force sensors using focusing photonic crystal arrays”, Opt. Express 25, 9196-9203, which is herein incorporated by reference. In embodiments, a subset of the first openings may have a first distance (di) larger than the wavelength (X), X < di <1.2* , such as X < di <1.1 * . In embodiments, at most 20%, such as at most 15%, especially at most 10% of the total number of first openings may have a first distance (di) selected from the range of X < di <1.2* .

In embodiments, the first openings may be equidistant, i.e. the first distance (di) may be constant. Hence, in embodiments, the first openings may have a first pitch (pi). Especially, the first openings may have a first pitch (pi) in a first direction. Herein, a pitch may refer to the smallest repetitive distance between the center of a first opening comprised by the first openings to the center of a second opening comprised by the first openings. In embodiments, the first pitch (pi) may be selected from the range of > 5 nm, such as > 10 nm, especially > 15 nm. Further, the first pitch (pi) may be selected from the range of < 2000 nm, such as < 1800 nm, especially < 1500 nm. In embodiments, the first pitch (pi) may be smaller than the wavelength (X) of the light source radiation, pi < X, such as pi < -25, especially pi < X-50. In such embodiments, the first openings may form an (optical) grating in the first direction. In specific embodiments, the first openings may have a first pitch (pi), wherein the first pitch (pi) may be one or more of (i) selected from the range of < 2000 nm, and (ii) smaller than a wavelength (X) of the light source radiation. Further, in embodiments, the first pitch (pi) may gradually or abruptly change across the suspended reflector. Such a first pitch may enhance the reflectivity of the suspended reflector, especially to light trapped in the cavity of the sensor element.

Further, in embodiments, the first openings may have a second pitch (p2) in a second direction. In embodiments, the second direction may be perpendicular to the first direction, though this need not be the case. In embodiments, the second pitch (p2) may be equal to the first pitch (pi). Hence, in embodiments, the second pitch (p2) may be selected from the range of < 2000 nm, such as < 1800 nm, especially < 1500 nm. Further, the second pitch (p2) may be smaller than the wavelength (X) of the light source radiation, pi < X, such as pi < X-25 nm, especially pi < X-50 nm. In such embodiments, the first openings may form an (optical) grating in the second direction. In specific embodiments, the first openings may form an (optical) grating in one or more of the first direction and the second direction. Yet, in embodiments, the second pitch (p2) may be larger than the first pitch (pi). In embodiments, the second pitch (p2) may be selected from the range of > 2*pi, such as > 5*pi, especially > 10*pi. Further, in embodiments, the second pitch (p2) may be selected from the range of < 10 cm, such as < 5 cm, especially < 2 cm. In embodiments, the second pitch (p2) may gradually or abruptly change across the suspended reflector. In embodiments, the first openings may form a repetitive pattern in one or more of the first direction and the second direction.

In embodiments, the plurality of openings may comprise (a plurality of) second openings. In embodiments, the second openings may have a second opening equivalent circular diameter (D2b). In embodiments, the second opening equivalent circular diameter (D2b) may be larger than the wavelength (X) of the light source radiation, D2b > such as D2b > 5*X, like D2b > 10*k, especially D2b > 20* . Additionally, in embodiments, the second opening equivalent circular diameter (D2b) may be larger than the first opening equivalent circular diameter (D2a). Especially, in embodiments, D2b > 2*D2a, such as D2b > 5*D2a, especially D2b > 10*D2a. Further, in embodiments, the second opening equivalent circular diameter (D2b) may be selected from the range of 0.2-200 mm, such as 0.5-100 mm, especially 1-50 mm, though larger sizes are herein not excluded. In embodiments, the addition of second openings having a second opening equivalent circular diameter (D2b) of 0.2-200 mm to the suspended reflector may facilitate transmission of the light source radiation into and out of the cavity of the sensor element. Further, such second openings may improve the mechanical properties of said reflector. For instance, the second openings may provide the suspended reflector with a vibration isolation system, which may reduce or suppress unwanted vibrations in the suspended reflector. Hence, in embodiments, the sensitivity and stability of the suspended reflector may be improved through the addition of second openings. Further, the larger equivalent circular diameter of the second openings may facilitate the removal of the gas plasma used during etching of the cavity (see below).

In embodiments, the second openings may have a shape in a plane parallel to a face of the suspended reflector. In embodiments, the second openings may have a shape selected from the group comprising a circle, a square, a regular polygon, a rectangle, a cross, etc. Further, the second openings may have a second length (L2) and a second width (W2). In embodiments, the second length (L2) and second width (W2) may be larger than the wavelength (X) of the light source radiation. Hence, in embodiments, L2 > X, such as L2 > 10* , especially L2 > 20*X. Further, in embodiments, W2 > such as W2 > 10* , especially W2 > 20* . Additionally or alternatively, in embodiments, the second length (L2) and second width (W2) may be individually selected from the range of 2 pm - 200 mm, such as 5 pm - 150 mm, especially 10 pm - 100 mm. Hence, in embodiments, the second openings may have second dimensions W2 and L2, wherein none of said second dimensions may be smaller than the wavelength (X) of the light source radiation.

In embodiments, the second openings may be configured at a second center-to- center distance (d2) in a first direction. Further, the second openings may be configured at a third center-to-center distance (ds) in a second direction. In embodiments, the second center- to-center distance (d2) and/or the third center-to-center distance (ds) may be equal to the second pitch (P2). Yet, in embodiments, the second center-to-center distance (d2) and/or the third center-to-center distance (ds) may be larger than the second pitch (P2). Further, in embodiments, the second center-to-center distance (d2) and/or the third center-to-center distance (ds) may be larger than the first pitch (pi). In embodiments, the second center-to-center distance (d2) and/or the third center-to-center distance (ds) may be selected from the range of > 2*pi, such as > 10*pi, especially > 20*pi. Further, in embodiments, the second center-to-center distance (d2) and/or the third center-to-center distance (ds) may be selected from the range of < 20 cm, such as < 10 cm, especially < 5 cm. Especially, in embodiments, the second center-to-center distance (d2) and/or the third center-to-center distance (ds) may, for each set of two neighboring second openings, be individually selected from the range of < 20 cm, such as < 10 cm, especially < 5 cm. In embodiments, the second openings may form a repetitive pattern in one or more of the first direction and the second direction. Further, in embodiments, the second openings may form a repetitive pattern across the suspended reflector.

Hence, in specific embodiments, one or more may apply of: (a) the suspended reflector may have a first equivalent circular diameter (Di), wherein the first equivalent circular diameter (Di) may be selected from the range of 10 pm - 45 cm; (b) the suspended reflector may comprises a plurality of openings, wherein the plurality of openings may comprise first openings and second openings, wherein (i) the first openings may have first dimensions (Wi, Li), wherein at least one of said first dimensions (Wi, Li) may be smaller than a wavelength (X) of the light source radiation, and (ii) the second openings may have second dimensions (W2, L2), wherein none of said second dimensions (W2, L2) may be smaller than a wavelength (X) of the light source radiation; and (c) the suspended reflector may comprise a plurality of openings, wherein the plurality of openings may comprise first openings, wherein the first openings may have a first pitch (pi), wherein the first pitch (pi) may be selected from the range of < 2000 nm.

The dimensions of the suspended reflector, including the dimensions of the openings therein, as well as the dimensions of the cavity, and also the dimensions of the layers of the Bragg reflector, as well as the chosen materials, may be selected such that a Bragg reflector and a reflective cavity may be provided for radiation (light) have a one or more specific wavelengths, such as e.g. selected from the range of 190-2000 nm, such as 250-1000 nm, more especially 300-800 nm.

Especially, the cavity mentioned here may refer to the space between the suspended reflector and the distributed Bragg reflector, and (also) enclosed at the sides by the silicon layer. Especially, in embodiments, this cavity may be an optical cavity. Here, an “optical cavity” may specifically describe two aligned reflectors with a space in between where light is at least partly trapped. Hence, the Bragg reflector, the suspended reflector, and the silicon side surfaces may be reflective (see further also below).

In embodiments, the suspended reflector, or the distributed Bragg reflector, may be reflective for visible, ultraviolet, or infrared light. Hence, especially, the suspended reflector or the distributed Bragg reflector may be reflective for light in the wavelength range 190- 2000nm, such as 250-1000 nm, especially 300-800 nm.

Particularly, light may be configured to resonate in the cavity of an optomechanical sensor. Light may enter the cavity via the openings in the suspended reflector. The deformation of the suspended reflector (such as when subjected to external stresses or forces) alters the thickness of the cavity. This change in the thickness of the cavity may result in a change in the resonance frequency of light in the cavity. Further, this change in the resonance frequency may be detected from the light escaping the sensor element. Thus, providing a means of detecting the extent of deformation by measuring the shift in the resonance frequency.

In embodiments, the mask may comprise the resulting pattern. The pattern here may, in embodiments, be in a form or shape comprising openings, gratings, etc which may in embodiments be used to etch said openings or gratings on the top layer to provide the suspended reflector. Hence, in embodiments, the method may comprise providing the mask on the top layer.

In embodiments, the method, may comprise etching the top layer through the mask to provide the suspended reflector. The mask may provide the advantage of transferring a specific pattern onto the surface of the suspended reflector. Especially, the mask may be patterned, wherein the mask may comprise a reflective grating. In embodiments, the suspended reflector may be reflective, such as 30%, such as 50%, especially 70% reflective for incident light. Further, in embodiments, the mask may help transfer a photonic crystal to make the suspended reflector reflective or alternatively to improve its reflectiveness.

Methods of transferring a photonic crystal to a deposited layer are known in the art. For example, the photonic crystal may be transferred using one or more of a photoresist and electron-beam lithography.

Further, in embodiments, the method may comprise etching a cavity in the (sacrificial) silicon layer. The mask, in embodiments, may be adhered to the surface of the top layer and may comprise a protective material. Further, the mask may selectively expose parts of the top layer which may then be removed. Thus, in embodiments (after the mask has been provided), the method may comprise etching the top layer i.e. the (high-stress) dielectric material is etched away to reveal the (sacrificial) layer of silicon configured beneath the top layer. Especially, the top layer may be etched away in a pattern determined by the mask to provide the suspended reflector.

The top layer and the suspended reflector may (thus) comprise the same material i.e. the suspended reflector is the top layer after parts of the top layer are etched away (from regions of the top layer not protected by the mask).

Further, in embodiments, the method may comprise cleaning the suspended reflector chemically to remove the mask and to expose the (high-stress) dielectric material comprised by the suspended reflector. Note that the high residual stress achieved by the suspended reflector is a result of depositing the Si_xN_y or SiC over the substrate at high temperatures. In embodiments, the top layer may be grown on the surface of the silicon substrate by means of chemical vapor deposition. The mask may then be provided on the top layer, hence facilitating selective etching of the top layer. The top layer may then be etched selectively by means of a reactive ion etch, such as SFe plasma etch.

The openings comprised by the suspended reflector may provide access to the (sacrificial) silicon layer configured beneath it. Hence, in embodiments, the method may comprise etching a cavity in the (sacrificial) silicon layer. Furthermore, the (sacrificial) silicon layer may be accessed via the openings in the suspended reflector. Especially, the cavity may be provided such that the distributed Bragg reflector configured beneath the (sacrificial) layer is exposed. Subsequently, the exposed silicone surface may be etched using the reactive ion etch to remove the silicon layer. Hence, a part of the silicon substrate is sacrificed to provide a cavity.

In embodiments, the gas plasma etch may be used to provide the cavity in the silicon layer. Using the gas plasma etch may provide the benefit that minimal to no chemical residues are left i.e. the sensor element may not require any further cleaning. Alternatively, in embodiments, a wet-etchant may (also) be used to provide the cavity in the silicon layer. In such embodiment, an additional cleaning step may be required to remove chemical residues left by the wet-etchant. The etch residues may, in embodiments, be cleaned using a UV-ozone surface cleaning system.

Thus, in embodiments, the suspended reflector comprising (high-stress) dielectric material may be suspended over the cavity in the (sacrificial) silicon layer. Especially, the suspended reflector may be provided as a patterned free-standing membrane.

Furthermore, the suspended reflector may be suspended over the distributed Bragg reflectors. Particularly, in embodiments, the method may comprise providing a suspended reflector suspended over the distributed Bragg reflector such that the distributed Bragg reflector is optically coupled with the suspended reflector. Optically coupled, here refers to formation of the optical cavity wherein light reflected from the distributed Bragg reflector may interact with the suspended reflector i.e. light may be reflected back into the optical cavity by the suspended reflector (such that it may resonate). Especially, light may resonate within the optical cavity.

In embodiments, the alternating layers (comprised by the distributed Bragg reflector) may comprise first layers selected from the group comprising Si-N, SiC, and BaO. Further, in embodiments, the alternating layers may comprise second layers selected from the group comprising SiCE and HfCE. Especially, the distributed Bragg reflector may comprise the alternating layers further comprising first layers and second layers. Here, alternating refers to a first layer configured above a preceding second layer, a second layer configured over the preceding first layer, and so on. These alternating layers may form the distributed Bragg reflector. That is, in embodiments, the distributed Bragg reflector may comprise a stack comprising alternating first and second layers. Hence, in embodiments, the alternating layers may comprise first layers selected from the group comprising Si-N, SiC and BaO, and second layers selected from the group comprising SiCE and HfCE.

In embodiments, the herein described method may also comprise providing these alternating layers. Especially, in embodiments the method may (thus) also comprise providing the alternating layers by depositing these layers by means of chemical vapor deposition.

It will be apparent to a skilled person that chemical vapor deposition (CVD) is a process where precursor vapors of chemicals react (in a reaction chamber) to form the first layer or the second layer compound, which may then be deposited on a surface atom by atom to provide a layer. Further, the properties of the layer may be controlled by controlling the deposition time, the deposition temperature, the pressure in the reaction chamber, the rate at which precursor chemicals are provided to the reaction chamber and the temperature of the surface (or substrate) on which the layers are deposited. The chemical vapor deposition process may be carried out at sub-atmospheric pressures, also known as low pressure chemical vapor deposition (LPCVD).

Hence, in embodiments, chemical vapor deposition may be used to provide the alternating layers of the first layer and the second layer. More especially, the method may comprise providing the distributed Bragg reflector by depositing alternating layers of the first layer and the second layer by means of chemical vapor deposition of the each of the first layer or the second layer on the preceding first layer or the second layer.

Specifically, the method may comprise providing the distributed Bragg reflector by depositing alternating layers of the first layer and the second layer by means of chemical vapor deposition of the each of the first layer or the second layer on the preceding second layer or the preceding first layer, respectively.

The choice of compounds for the first layer and the second layer may be dependent on thermodynamic properties. In embodiments, compounds which may be thermodynamically stable at very high temperatures such as 700-1000 °C are selected. Thermodynamically stable refers to compounds that do not melt, vaporize or change their physical state at these temperatures. In embodiments, the suspended reflector may be provided by means of chemical vapor deposition at temperatures in the range 700-1000 °C, such as 750- 950 °C, especially 800-900 °C. Hence, the thermodynamically stable compounds at these temperatures may allow the deposition of the top layer at these temperatures without disintegrating, fracturing, or changing in chemical composition. Especially, the method may comprise selecting the first layer and the second layer from chemical compounds that retain their chemical and structural composition at a temperature in the range 700-1000°C. In embodiments, the method may comprise depositing the top layer on at least part of the (sacrificial) silicon layer at a temperature in the range of 700-1000°C. Especially, a gas mixture of dichlorosilane SiH2Ch and ammonia NH3 at temperatures in the 700-1000°C, at a pressure below 10'³ bar (or typically l/760^th of standard atmospheric pressure) may be reacted to form the (high-stress) silicon nitride. An analogous process may be used to provide (high-stress) silicon carbide from silane (SiHi) and propane (C3H8).

In embodiments, the method may comprise depositing the top layer at a lower temperature, such as > 500 °C, especially > 600 °C, more especially > 650 °C. Hence, in embodiments, the method may comprise depositing the top layer on at least part of the (sacrificial) silicon layer at a temperature in the range of 500-1000°C. Yet, in embodiments, the top layer may be deposited at a higher temperature, such as < 1100 °C, especially < 1050 °C. Hence, in embodiments, the method may comprise depositing the top layer on at least part of the (sacrificial) silicon layer at a temperature in the range of 500-1100°C, such as 500- 1000°C.

Low pressure chemical vapor deposition, in embodiments, may be used to produce films of uniform thickness and composition. The (high-stress) dielectric material comprised by the top layer may have a different thermal coefficient of expansion than the silicon layer on which the top layer is provided. As a result, the (high-stress) dielectric material on cooling may shrink (or reduce in volume) at a different rate than the silicon layer on which it is deposited. Hence, this disproportionate contraction of the two compounds (i.e. the top layer and the silicon layer) may lead to development of residual tensile stresses in the top layer. In embodiments, the top layer may have a tensile strength in the range of 5-20 GPa. The tensile strength of high-stress dielectric material layers provided by LPCVD may be in the range of 5- 20 GPa, such as 9-16 GPa, such as 11-14 GPa. Hence, in embodiments, a top layer comprising silicon nitride or silicon carbide having a residual tensile stress of 5-20 GPa may be provided on the silicon layer. Further, in embodiments, the top layer, especially the high-stress dielectric material, may have a tensile stress in the range of 0.3-20 GPa, such as 0.5-20 GPa, especially 1-20 GPa. Additionally, in embodiments, the top layer may have a tensile stress in the range of 0.3-10 GPa, such as 0.5-10 GPa, especially 1-10 GPa. As indicated above, in embodiment, etching of the top layer may provide a suspended reflector, wherein the (residual) tensile stress of the suspended reflector may be increased compared to the (non-etched) top layer. Hence, in embodiment, the suspended reflector may have a residual tensile stress in the range of 0.5-20 GPa, such as 1-20 GPa, especially 2-20 GPa. Further, in embodiments, the suspended reflector may have a residual tensile stress in the range of 0.5-10 GPa, such as 1-10 GPa, especially 2- 10 GPa. In specific embodiments, the top layer may have a tensile stress in the range of 0.5-20 GPa, wherein etching the top layer may provide the suspended reflector, wherein the suspended reflector may have a residual tensile stress in the range of 1-20 GPa. Further, in specific embodiments, the sensor element may comprise a top layer, wherein the top layer may have a tensile stress in the range of 0.5-10 GPa, wherein the suspended reflector may have a residual tensile stress in the range of 1-10 GPa.

In embodiments, the method may comprise etching the cavity in the (sacrificial) silicon layer by means of gas plasma etching. Especially, the method may comprise etching the silicon layer to form the cavity. Gas plasma etching is known in the art. Gas plasma etching refers to treating or exposing a surface (or substrate) to gas plasma. Gas plasma etching provides the advantage of high selectivity as compared to other forms of etching (for example chemical etching). The gas may be chosen from options as will be further described below in dependence of the material to be etched (or substrate), the mask material and a stop material (which in embodiments may be the distributed Bragg reflector). Hence, the gas plasma may be used to etch the substrate while certain parts of the substrate may be protected by the mask. Hence, in embodiments, the suspended reflector may be provided by first providing a mask over the top layer and then etching the top layer.

The suspended reflector may comprise openings complimentary to the mask. Further, in embodiments, the suspended reflector may be cleaned chemically to remove the mask. In embodiments, the cavity in the (sacrificial) silicon layer may be provided by gas plasma etching. The gas plasma may be highly selective for silicon, hence, the cavity may be etched in the silicon layer without damaging the suspended reflector or the distributed Bragg reflector. Therefore, in embodiments, the cavity may be provided in the silicon layer by etching the silicon layer configured between the suspended reflector and the Bragg reflector. In embodiments, the top layer may be etched using a first gas plasma, and the (sacrificial) silicon layer may be etched using a second gas plasma. The first gas plasma may, in embodiments, especially facilitate directional etching of the top layer, wherein the direction of etching may be perpendicular to a face of the top layer (and the mask). In such embodiments, the top layer may especially be etched using reactive ion etching, e.g. using CF3/O2 gas plasma. Further, the second gas plasma may facilitate non-directional etching of the (sacrificial) silicon layer. The second gas plasma may especially have a relatively higher selectivity for silicon, and a relatively lower selectivity for the top layer and the distributed Bragg reflector. As indicated, in embodiments, the second gas plasma may comprise SFe gas plasma.

In embodiments, the method may comprise providing the top layer with a top layer thickness in the range 5-500 nm, such as in the range 50-400 nm, especially 200-300 nm. Essentially, the top layer may be grown on the silicon layer by chemical vapor deposition, especially low-pressure chemical vapor deposition. The thickness of the top layer may hence be varied by extending the deposition time.

Especially, in embodiments, the suspended reflector may have the same thickness as the top layer.

The suspended reflector may suffer from two types of failure modes that may thus affect the functioning of the sensor element. The suspended reflector may fail intrinsically due to a failure initiated in a highly stressed region in the suspended reflector, or the suspended reflector may fail extrinsically due to extrinsic defects in the freestanding region of the suspended reflector. Intrinsic failures may (already) be mitigated by the residual stress in the suspended reflector.

Further, in embodiments, the mask may be used to pattern the suspended reflector into specific designs, such that the variable stress throughout the suspended reflector may be controlled. Further, in embodiments, a choice of 5-500 nm may be an ideal thickness to mitigate failures due to extrinsic defects in the top layer, and subsequently in the suspended reflector provided.

In embodiments, the method may comprise providing the (sacrificial) silicon layer with a thickness in the range 0.01- 10 pm, such as 0.1-8 pm, especially 1-5 pm. The size (i.e. thickness) of the cavity and consequently, the thickness of the (sacrificial) silicon layer determines the optical path length travelled by light in the cavity. The optical path length is a crucial parameter that determines the resonance frequency of the cavity comprised by the (sacrificial) silicon layer. In embodiments, a thickness in the range of 0.01 pm - 10 pm may provide the advantage of improving the sensitivity (of the sensor element) to displacement of the suspended reflector.

In embodiments, the cavity may have a thickness (or height) equal to the thickness of the (sacrificial) silicon layer. Hence, in embodiments, the cavity may have a thickness selected from the range of 0.01- 10 pm, such as 0.1-8 pm, especially 1-5 pm.

The method (as indicated above) may comprise providing a distributed Bragg reflector. In embodiments, the distributed Bragg reflectors may be grown layer-wise by the deposition of chemical vapors. Especially, the distributed Bragg reflector may be provided by first depositing the first layer, followed by the deposition of the second layer, and so on. Hence, in embodiments, the method may comprise providing (i) the first layer with a first layer thickness in the range 40-400 nm, such as 100-300 nm, especially 150-200 nm and (ii) the second layer with a second layer thickness in the range of 40-400 nm, such as 100-300 nm, especially 150-200 nm.

In embodiments, the first layer thickness and the second layer thickness may be selected in dependence of the wavelength (X of the incident light, especially of the light source radiation, used for the optomechanical sensing application. Further, in embodiments, the first layer thickness and the second layer thickness may depend on the refractive index n of the first layer and the second layer, respectively. Especially, in embodiments, the first layer thickness may be provided by z*X/4m, wherein m is the refractive index of the material comprised by the first layer, and z is an odd integer. In embodiments, z may be <17, such as <15, especially <13. Further, in embodiments, the second layer thickness may be provided by z*X/4n2, wherein is the refractive index of the material comprised by the second layer, and z is an odd integer as defined above. In embodiments, the value of z for the first layer thickness and the second layer thickness may be equal within a set comprising at least one first layer and one second layer. Additionally or alternatively, in embodiments, the value of z may differ throughout the (stack of) alternating layers. For instance, the alternating layers (comprised by the Bragg reflector) may comprise a first set of N first layers and second layers wherein z is 5, a second set of M first layers and second layers wherein z is 3, and a third set of X first layers and second layers wherein z is 1. In embodiments where m , the first layer thickness within such a set of N, M, or X layers may be different from the second layer thickness within the same set.

In embodiments, the method may comprise providing (i) the first layer with a first refractive index in the range 1.8-2.7, such as 2.0-2.5, especially 2.1-2.3, and (ii) the second layer with a second refractive index in the range of 1.2-1.8, such as 1.3-1.7, especially 1.4-1.6. Increasing the difference in refractive index between the two layers may provide the advantage of increasing the reflectivity and bandwidth of the distributed Bragg reflector. In embodiments, the distributed Bragg reflector may start with a high refractive index layer or low refractive layer first.

In embodiments, the first refractive index and second refractive index may differ by > 0.3, such as > 0.5, especially > 1.0. Additionally, in embodiments, the first refractive index and second refractive index may differ by < 2.0, such as < 1.5, especially < 1.25. Hence, in specific embodiments, the first refractive index and second refractive index may differ by 0.3- 2.0, such as 0.5-1.5, especially 1.0-1.25.

In embodiments, the method may comprise providing the distributed Bragg reflector with a number of layers in the range 4-20, such as 8-16, especially 10-12. A larger number of layer pairs comprised by the distributed Bragg reflector may improve the reflectivity of the distributed Bragg reflector. While conventionally distributed Bragg reflector may be made from alternating layers with two distinctive layer thicknesses, in embodiments, the thickness of these layers may vary from the first layer to the subsequent layers below. Especially, the thickness of the layers may increase from the first to second and then third layer before alternating between two different (unchanging) thicknesses.

Thus, the reflectivity of the distributed Bragg reflector is influenced by the number of layers, the contrast between the refractive indices of the first layer and the second layer, and the thickness of the layers (comprised by the distributed Bragg reflector). The reflectivity (i.e. quantitative measure of reflectivity) of the distributed Bragg reflector is known in the art. For instance, Sheppard, C. J. R. (1995). Approximate calculation of the reflection coefficient from a stratified medium. Pure and Applied Optics: Journal of the European Optical Society Part A, 4(5), 665 669. doi: 10.1088/0963-9659/4/5/018 herein incorporated by reference, describes the reflectivity of the distributed Bragg reflector in dependence of the aforementioned parameters.

In embodiments, the method may comprise providing sulfur hexafluoride as the gas plasma used in the gas plasma etching. Typically, wet etching and/or dry etching may be used to remove material from a substrate. Wet etching typically refers to using chemicals such as potassium hydroxide (KOH) or tetramethylammoniumhydroxide (TMAH) to remove material from a substrate. Dry etching refers to using gas plasma to remove material from a substrate. Dry etching may also be known as reactive-ion etching. Typically, radiofrequency electromagnetic fields may be used to initiate plasma. The term “plasma” is well known in the art. It will be apparent to the skilled person that plasma is one of the four fundamental states of matter comprising charged particles, or ions, or both. Plasma may be generated (or initiated) by heating a gas in a strong electromagnetic field. In embodiments, the etching of the (sacrificial) silicon layer (and the top layer) may be carried out by means of gas plasma etching. Gas plasma may be provided to the surface of the (sacrificial) silicon layer through an inlet (in a vacuum chamber) and be removed via an outlet (in a vacuum chamber). Hence, in embodiments sulfur hexafluoride or SFe gas plasma may be used as the etchant. SFe provides the advantage of removing the silicon without imparting stiction forces or contaminants on the suspended reflector, thus improving fabrication yield.

Furthermore, XeF2 gas plasma may be used in embodiments where the top layer comprises silicon dioxide. This may be beneficial on account of high selectivity of SiCE to XeF₂.

In another aspect, the invention may provide a sensor element, comprising a distributed Bragg reflector, a (sacrificial) silicon layer and a top layer. Especially, the distributed Bragg reflector may comprise alternating layers comprising different materials. Further, in embodiments the (sacrificial) silicon layer may be configured on at least part of the distributed Bragg reflector. Moreover, the top layer may be configured on at least part of the (sacrificial) silicon layer. Especially, the top layer may comprise a (high-stress) dielectric material. Further, the top layer may comprise a suspended reflector. In embodiments, the (sacrificial) silicon layer may further comprise a cavity. Especially, the cavity may be configured between the suspended reflector and the distributed Bragg reflector. In a specific embodiment, the invention may thus provide a sensor element, comprising a (distributed) Bragg reflector, a (sacrificial) silicon layer and a top layer; wherein the (distributed) Bragg reflector comprises alternating layers comprising different materials; wherein the (sacrificial) silicon layer is configured on at least part of the (distributed) Bragg reflector; wherein the top layer is configured on at least part of the (sacrificial) silicon layer; wherein the top layer comprises a (high-stress) dielectric material; wherein the top layer comprises a suspended reflector; wherein the (sacrificial) silicon layer further comprises a cavity, wherein the cavity is configured between the suspended reflector and the (distributed) Bragg reflector.

In a further specific embodiment, the invention may thus provide a sensor element, comprising a Bragg reflector, a (sacrificial) silicon layer and a top layer; wherein the Bragg reflector comprises alternating layers comprising different materials; wherein the (sacrificial) silicon layer is configured on at least part of the Bragg reflector; wherein the top layer is configured on at least part of the (sacrificial) silicon layer; wherein the top layer comprises a high-stress dielectric material; wherein the top layer comprises a suspended reflector; wherein the (sacrificial) silicon layer further comprises a cavity, wherein the cavity is configured between the suspended reflector and the Bragg reflector.

In embodiments, the sensor element as described above may especially be obtained by the method as defined herein. In such embodiments, the top layer of the sensor element may especially have a (residual) tensile stress in the range of 0.3-20 GPa, especially 5-20 GPa. Further, in such embodiments, the method (used to obtain the sensor element) may comprise depositing the top layer on at least part of the silicon layer at a temperature in the range of 500-1100 °C, especially 700-1000 °C (to obtain a top layer comprising a high-stress dielectric material). Additionally, in such embodiments, the method (used to obtain the sensor element) may comprise etching the cavity in the silicon layer by means of gas plasma etching. Hence, in specific embodiments, the sensor element as defined herein may be obtainable by the method as defined herein, wherein one or more may apply of (i) the top layer may have a residual tensile stress in the range of 5-20 GPa; (ii) the method may comprise depositing the top layer on at least part of the silicon layer at a temperature in the range of 700-1000 °C; and (iii) the method may comprise etching the cavity in the silicon layer by means of gas plasma etching.

In embodiments, the sensor element may be an optomechanical sensor i.e. a sensing device where a property of light or its propagation is modulated by a modulation of a mechanical aspect of the sensor. Here, the sensor element may comprise a cavity configured between the suspended reflector and the distributed Bragg reflector. This cavity may provide a region of space where light may resonate i.e. light may be reflected between the suspended reflector and the distributed Bragg reflector. The resonance frequency of the optical cavity may be dependent on the thickness of the optical cavity. Mechanical deformation of the suspended reflector (suspended over the cavity) may deform the membrane, thus, altering the size of the optical cavity. Consequently, the resonance frequency of the optical cavity may be altered as a result. Hence, by measuring the changes to the optical resonance frequency of light, the extent of mechanical deformation may be inferred. The distributed Bragg reflector may comprise alternating layer pairs, making it a high-quality reflector for light. In embodiments, the normal to the top layer and the normal to the alternating layers comprised by the distributed Bragg reflector may be aligned. This provides the benefit of avoiding separate alignment of the suspended reflector (or thin-film) and the distributed Bragg reflector.

As mentioned before, in embodiments, the (high-stress) dielectric material may comprise silicon-nitride-based material or silicon-carbide-based material. Especially, the silicon-nitride-based material may comprise a composition of the form Si_xN_y where the x and y may be configured depending on the proportions of the two chemical vapors provided in the deposition process. Silicon-nitride-based compounds or materials essentially refers to silicon nitride having the composition Si_xN_y.

Especially, in embodiments, silicon-nitride-based compounds or materials may refer to silicon nitride having the composition Si_xN_y, where the ratio x/y may be selected from the range of 0.75-1.1, such as SisN4, wherein x/y=0.75.

In embodiments, the alternating layers may comprise first layers selected from the group comprising Si-N, SiC, and BaO. Especially, the second layers may be selected from the group comprising SiCE and HfCE. As mentioned before, in embodiments, the distributed Bragg reflector may comprise the alternating layers further comprising first layers and second layers. Here, alternating refers to a first layer configured above a preceding second layer, a second layer configured over the preceding first layer, and so on. These alternating layers may form the distributed Bragg reflector. That is, in embodiments, the distributed Bragg reflector may comprise a stack comprising alternating first and second layers. Hence, in embodiments, the alternating layers may comprise first layers selected from the group comprising Si-N, SiC and BaO, and second layers selected from the group comprising SiO2 and HfO2.

In embodiments, the top layer may have thickness in the range 5-500 nm, such as in the range 50-400 nm, especially 200-300 nm. The residual tensile stress in the top layer may be in dependence of the thickness of the top layer i.e. a thinner top layer may have higher tensile stress.

In embodiments, the silicon layer may have a thickness in the range 0.01 pm - 10 pm, such as 0.1-8 pm, especially 1-5 pm. As mentioned before, the size (i.e. thickness) of the cavity and consequently, the thickness of the (sacrificial) silicon layer determines the optical path length travelled by light in the cavity. In embodiments, a thickness in the range of 0.01 pm - 10 pm may provide the advantage of improving the sensitivity of the sensor element to displacement of the suspended reflector.

In embodiments, the Bragg reflector may have a number of layers in the range 4-20, such as 8-16, especially 10-12. A larger number of layer pairs comprised by the distributed Bragg reflector may improve the reflectivity of the distributed Bragg reflector.

In embodiments, the first layer may have a first layer thickness in the range 40- 400 nm, such as 100-300 nm, especially 150-200 nm. Especially, the second layer may have a second layer thickness in the range of 40-400 nm, such as 100-300 nm, especially 150-200 nm.

Further, in embodiments, the first layer thickness may be provided by z*X/4m, and the second layer thickness may be provided by z*X/4n2. In embodiments, the first layer may have a first refractive index in the range 1.8-2.7, such as 2.0-2.5, especially 2.1-2.3. Especially, the second layer may have a second refractive index in the range of 1.2-1.8, such as 1.3-1.7, especially 1.4-1.6. Increasing the difference in refractive index between the two layers may provide the advantage of increasing the reflectivity and bandwidth of the distributed Bragg reflector. In embodiments, the distributed Bragg reflector may start with a high refractive index layer or low refractive layer first.

Further, in embodiments, the first refractive index and second refractive index may differ by > 0.3, such as > 0.5, especially > 1.0. Additionally, in embodiments, the first refractive index and second refractive index may differ by < 2.0, such as < 1.5, especially < 1.25. Hence, in specific embodiments, the first refractive index and second refractive index may differ by 0.3-2.0, such as 0.5-1.5, especially 1.0-1.25.

In yet another aspect, the invention may provide an apparatus for optomechanical sensing comprising the sensor element, a laser, and a second sensor element. Hence, in embodiments, the apparatus may comprise the sensor element. Especially, the apparatus may comprise the laser. More especially, the apparatus may comprise the second sensor element. In embodiments, the laser may be configured to provide light source radiation to the sensor element. Further, the second sensor element may be configured to detect light source radiation escaping the sensor element. Hence, the apparatus may be used to measure stresses/ strains at very high accuracy such as in the order of femtometers. Mechanical loading applied on the suspended reflector may cause the stressed membrane to deform. The deformation may alter the space between the suspended reflector and the distributed Bragg reflector. In embodiments, the apparatus may comprise the laser. Especially, the laser may provide laser light, that may be shined through the suspended reflector and be reflected in the cavity. Following a series of reflections, the laser light may be outcoupled from the sensor element via the suspended reflector. In embodiments, the apparatus may comprise the second sensor element, which may be configured to detect the laser light. The shift in frequency in the reflected laser light may be used to infer the extent of deformation of the suspended reflector. Further, the measured deformation may be used to measure the extent of stress/strain applied on the suspended reflector. Embodiments of the apparatus comprising the sensor element described herein, may comprise the one or more features or elements comprised by the sensor element. The said features of the sensor element have been discussed in detail (see above).

Hence, in specific embodiments, the invention may provide an apparatus for optomechanical sensing comprising a sensor element, a laser, and a second sensor element, wherein the sensor element may comprise a Bragg reflector, a silicon layer, and a top layer; wherein: (a) the Bragg reflector may comprise alternating layers comprising different materials; (b) the silicon layer may be configured on at least part of the Bragg reflector; (c) the top layer may be configured on at least part of the silicon layer, wherein the top layer may comprise a (high-stress) dielectric material; (d) the top layer may comprise a suspended reflector; (e) the silicon layer may further comprise a cavity, wherein the cavity may be configured between the suspended reflector and the Bragg reflector; (f) the laser may be configured to provide light source radiation to the sensor element; and (g) the second sensor element may be configured to detect light source radiation escaping from the sensor element.

Further, in embodiments, the apparatus may comprise the sensor elements as defined herein, or the sensor element obtainable by the method as defined herein. As such, embodiments of the apparatus comprising the sensor element described herein, may comprise the one or more features or elements comprised by the sensor element as defined herein, and/or the one or more features or elements comprised by the sensor element obtainable by the method as defined herein.

In embodiments, the apparatus as defined herein may be used to measure stresses/ strains on the sensor elements using the Pound-Drever-Hall (PDH) technique. The term “Pound-Drever-Hall technique” is known in the art. In embodiment, the PDH technique may comprise comparing an output frequency of the laser to a resonance frequency of the sensor element (as detected by the second sensor element). Upon deformation of the suspended reflector, an absolute difference between the output frequency of the laser and the resonance frequency of the sensor element is increased. The extent of the difference correlates to the deformation of the suspended reflector, and thus to the stress/strain applied to the sensor element. Due to the nature of the PDH technique, both negative and positive differences between the output frequency of the laser and the resonance frequency of the sensor element may be detected, thereby providing further information regarding the direction of the stress/strain applied to the sensor element.

In embodiments, the laser may especially be configured to generate light source radiation having one or more wavelengths in the UV, visible, or infrared, especially having a wavelength selected from the spectral wavelength range of 200-2000 nm, such as 300-1500 nm. The term “laser” especially refers to a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. Especially, in embodiments the term “laser” may refer to a solid-state laser. In embodiments, the terms “laser” or “solid state laser” or “solid state material laser” may refer to one or more of cesium doped lithium strontium (or calcium) aluminum fluoride (Ce:LiSAF, Ce:LiCAF), chromium doped chrysoberyl (alexandrite) laser, chromium ZnSe (CrZnSe) laser, divalent samarium doped calcium fluoride (Sm:CaF2) laser, Er:YAG laser, erbium doped and erbium-ytterbium codoped glass lasers, F-Center laser, holmium YAG (Ho:YAG) laser, Nd:YAG laser, NdCrYAG laser, neodymium doped yttrium calcium oxoborate Nd:YCa4O(BO3)3 or Nd:YCOB, neodymium doped yttrium orthovanadate (Nd:YVO4) laser, neodymium glass (Nd:glass) laser, neodymium YLF (Nd:YLF) solid-state laser, promethium 147 doped phosphate glass (147Pm³⁺:glass) solid-state laser, ruby laser (AhO3:Cr³⁺), thulium YAG (Tm:YAG) laser, titanium sapphire (Ti:sapphire; AhO3:Ti³⁺) laser, trivalent uranium doped calcium fluoride (U:CaF2) solid-state laser, Ytterbium doped glass laser (rod, plate/chip, and fiber), Ytterbium YAG (Yb:YAG) laser, Yb2O3 (glass or ceramics) laser, etc. In yet further embodiments, the light source light may especially be a (collimated) laser light source light. In yet further embodiments, the laser may comprise one or more of an F center laser, an yttrium orthovanadate (Nd: YVO4 laser, a promethium 147 doped phosphate glass (147Pm³⁺:glass), and a titanium sapphire (Ti:sapphire; A12O3:Ti³⁺) laser. For instance, considering second and third harmonic generation, such light sources may be used to generated blue light. A laser may be combined with an upconverter in order to arrive at shorter (laser) wavelengths. For instance, with some (trivalent) rare earth ions upconversion may be obtained or with non-linear crystals upconversion can be obtained. Alternatively, a laser can be combined with a downconverter, such as a dye laser, to arrive at longer (laser) wavelengths. In specific embodiments, the terms “laser” or “laser light source”, or similar terms, refer to a diode laser. In embodiments, the terms “laser” or “solid state laser” or “solid state material laser” may refer to one or more of a semiconductor diode lasers, such as GaN, InGaN, AlGalnP, AlGaAs, InGaAsP, lead salt, vertical cavity surface emitting laser (VCSEL), quantum cascade laser, hybrid silicon laser, etc. In further embodiments the solid-state light source may be a solid-state LED light source (such as a LED or diode laser). Especially, the solid-state light source may be a superluminescent diode.

The term “visible light” especially relates to light having a wavelength selected from the range of 380-780 nm. Herein, UV (ultraviolet) may especially refer to a wavelength selected from the range of 190-380 nm, though in specific embodiments other wavelengths may also be possible. Herein, IR (infrared) may especially refer to radiation having a wavelength selected from the range of 780-3000 nm, such as 780-2000 nm, e.g. a wavelength up to about 1500 nm, like a wavelength of at least 900 nm, though in specific embodiments other wavelengths may also be possible. Hence, the term IR may herein refer to one or more of near infrared (NIR (or IR-A)) and short- wavelength infrared (SWIR (or IR-B)), especially NIR.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. 1 schematically depicts an embodiment of the sensor element; Fig. 2 schematically depicts an embodiment of the method for providing the sensor element; Fig. 3 schematically depicts an embodiment of the apparatus for optomechanical sensing; Fig. 4a schematically depicts an embodiment of an operational mode of the apparatus for optomechanical sensing; and Fig. 4b schematically depicts some embodiments of the plurality of openings. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 schematically depicts an embodiment of the sensor element 2000. In embodiments, the invention may provide a sensor element 2000, comprising a distributed Bragg reflector 100, a (sacrificial) silicon layer 200 and a top layer 300. In embodiments, the top layer 300 may have thickness in the range 5-500 nm. Especially, the distributed Bragg reflector 100 may comprise alternating layers 110, 120 comprising different materials. In embodiments, the alternating layers 110,120 may comprise first layers 110 selected from the group comprising Si-N, SiC, and BaO, and the second layers 120 may be selected from the group comprising SiCE and HfCE. Especially, the Bragg reflector may have a number of layers in the range 4-20. Further, in embodiments, the first layer 110 may have a first layer thickness in the range 40-400 nm and, the second layer 120 may have a second layer thickness in the range of 40-400 nm. Yet further, in embodiments, the first layer 110 may have a refractive index in the range 1.8-2.7, and the second layer 120 may have a second refractive index in the range of 1.2-1.8.

In embodiments, the (sacrificial) silicon layer 200 may be configured on at least part of the distributed Bragg reflector 100. Especially, the top layer 300 may be configured on at least part of the (sacrificial) silicon layer 200. In embodiments, the (sacrificial) silicon layer (200) may have a thickness in the range 0.01 pm - 10 pm. Further, the top layer 300 may comprise a (high-stress) dielectric material 320. Especially, the (high-stress) dielectric material 320 may comprise a silicon-nitride-based material or a silicon-carbide-based material. In embodiments, the top layer 300 may comprises a suspended reflector 400. Further, in embodiments, the (sacrificial) silicon layer 200 may further comprise a cavity 210, wherein the cavity 200 may be configured between the suspended reflector 400 and the distributed Bragg reflector 100. Especially, the normal to the top layer 300 and the normal to the alternating layers comprised by the distributed Bragg reflector 100 may be aligned.

In embodiments, the suspended reflector 400 may have a first equivalent circular diameter Di. In specific embodiments, the first equivalent circular diameter Di may be selected from the range of 10 pm - 45 cm. Further, in embodiments, the suspended reflector 400 may comprise a plurality of openings 410. In embodiments, the plurality of openings 410 may comprise first openings 910. In embodiments, the first openings 910 may have a first opening equivalent circular diameter D2a. In embodiments, the first opening equivalent circular diameter D2a may be selected from the range of 50-2000 nm. The first openings 910 may further have a first pitch pi. In embodiments, the first pitch pi may be selected from the range of < 2000 nm.

Fig. 2 schematically depicts an embodiment of the method for providing the sensor element 2000. In embodiments, the invention may provide a method for providing a sensor element 2000 for optomechanical sensing. A flat surface 10 (depicted in I) may be used as a base to construct the different components comprised by the sensor element. Especially, the method may comprise providing a distributed Bragg reflector 100 comprising alternating layers 110, 120 comprising different materials (depicted in II). Especially, the method may comprise providing a (sacrificial) silicon layer 200 on at least part of the distributed Bragg reflector 100 (depicted in III). In embodiments, depositing a top layer 300 on at least part of the (sacrificial) silicon layer 200 (depicted in IV). Especially, the top layer 300 may comprise a (high-stress) dielectric material 320. Further, the method may comprise providing a mask 310 for a suspended reflector 400 on the top layer 300 (depicted in V). Yet further, the method may comprise etching the top layer 300 (depicted in VI) through the mask 310 to provide the suspended reflector 400. In embodiments, the mask 310 is cleaned to expose the suspended reflector 400 (depicted in VII). Following which a cavity 210 is etched in the (sacrificial) silicon layer 200 (depicted in VIII).

In embodiments, the alternating layers 110,120 may comprise first layers 110 selected from the group comprising Si-N, SiC and BaO, and second layers 120 selected from the group comprising SiCE and HfCE. Especially, the top layer 300 may have a tensile strength in the range of 5-20 GPa.

Further, the top layer 300 may have a tensile stress in the range of 0.5-20 GPa. In embodiments, etching the top layer 300 may provide the suspended reflector 400. In specific embodiments, etching the top layer 300 may increase the tensile stress in the (subsequently obtained) suspended reflector 400. Hence, in embodiments, the suspended reflector 400 may have a residual tensile stress in the range of 1-20 GPa. Yet, in embodiments, the top layer 300 may have a tensile stress in the range of 0.5-10 GPa. Further, the suspended reflector 400 may have a residual tensile stress in the range of 1-10 GPa.

Further, in embodiments, the method may comprise depositing the top layer 300 on at least part of the (sacrificial) silicon layer 200 at a temperature in the range of 700-1000°C. Especially, the method may comprise etching the cavity 210 in the (sacrificial) silicon layer 200 by means of gas plasma etching.

Further, the method may comprise depositing the top layer 300 on at least part of the (sacrificial) silicon layer 200 at a temperature in the range of 500-1000°C.

In embodiments, the method may comprise providing the distributed Bragg reflector 100 by depositing alternating layers of the first layer 110 and the second layer 120 by means of chemical vapor deposition of the each of the first layer 110 or the second layer 120 on the preceding first layer 110 or the second layer 120.

Further, in embodiments, the method may comprise selecting the first layer 110 and the second layer 120 from chemical compounds that retain their chemical composition at a temperature in the range 700-1000°C. Yet further, in embodiments, the first layer 110 and second layer 120 may be selected from chemical compounds that may be structurally stable in the range 700-1000 °C. Especially, the top layer 300 may have a top layer thickness in the range 5-500 nm. Further, the method may comprise providing the (sacrificial) silicon layer 200 with a thickness in the range 0.01 pm - 10 pm. In embodiments, the method may comprise providing (i) the first layer 110 with a first layer thickness in the range 40-400 nm, and (ii) the second layer 120 with a second layer thickness in the range of 40-400 nm. Further, in embodiments, the method may comprise providing (i) the first layer 110 with a first refractive index in the range 1.8-2.7, and (ii) the second layer 120 with a second refractive index in the range of 1.2- 1.8. Especially, the method may comprise providing the distributed Bragg reflector 100 with a number of layers in the range 4-20.

In embodiments, the method may comprise providing sulfur hexafluoride as the gas plasma used in the gas plasma etching.

Fig. 3 schematically depicts an embodiment of the apparatus 1000 for optomechanical sensing. In embodiments, the invention may provide an apparatus 1000 for optomechanical sensing comprising the (i) sensor element 2000, a laser 700, and a second sensor element 2500. Especially, the laser may be configured to provide light source radiation 101 to the sensor element 2000. Further, the second sensor element 2500 may be configured to detect light source radiation 101 escaping the sensor element 2000. In embodiments, the laser 700 may be configured to provide light source radiation 101 to the sensor element 2000. Further, the second sensor element 2500 may be configured to detect light source radiation 101 escaping the sensor element 2000. Mechanical loading applied on the suspended reflector 400 may cause the stressed membrane to deform. The deformation may alter the space between the suspended reflector 400 and the distributed Bragg reflector 100. Especially, the laser 700 may provide laser light (or light source radiation 101), that may be shined through the suspended reflector 400 and be reflected in the cavity 210. Following a series of reflections, the laser light may be outcoupled from the sensor element 2000 via the suspended reflector 400. In embodiments, the sensor element 2500, which may be configured to detect the light source radiation 101. The shift in frequency in the reflected light source radiation 101 may be used to infer the extent of deformation of the suspended reflector 400. Further, the measured deformation may be used to measure the extent of stress/strain applied on the suspended reflector 400.

Fig. 4a schematically depicts an embodiment of the apparatus 1000 for optomechanical sensing, specifically using the Pound-Drever-Hall technique. In such embodiments, the apparatus 1000 may comprise a first optical element 610, a polarizing beam splitter 601, and a polarization changing element 810. The first optical element 610, polarizing beam splitter 601, and polarization changing element 810 may be configured in an optical path of the light source radiation 101 between the laser 700 and the sensor element 2000. The sensor element 2000 may especially be the sensor element 2000 as defined herein, or the sensor element 2000 obtainable by the method as defined herein. In embodiments, the first optical element 610 may be selected from the group comprising a lens, a collimator, and an isolator. Further, the polarizing beam splitter 601 may be configured to transmit at least part of the incident light having a first polarization, and reflect at least part of the incident light having a second polarization. Additionally, the polarization changing element 810 may especially comprise one or more of a X/4 waveplate and a Faraday rotator. During an operational mode of the apparatus, the laser 700 may be configured to provide light source radiation 101. At least part of the light source radiation 101 may pass through the first optical element 610 and polarizing beam splitter 601 to be incident on the polarization changing element 810. There, for example, starting with linear p-polarized light, it is converted by the polarization changing element 810 into e.g. right-handed circular polarized light, which is subsequently provided to the sensor element 2000. The light source radiation 101 may be reflected within the sensor element 2000 with a specific resonance frequency, as determined by the thickness of the cavity 210 and the stresses/strains on the suspended reflector 400. The light source radiation 101 may exit the cavity 210 (via the plurality of openings 410) to be incident on the polarization changing element 810. Here, the (reflected) light source radiation 101 may be converted into linear s-polarized light. The polarizing beam splitter 601 may especially be configured to reflect linear s-polarized light, thereby guiding the (reflected) light source radiation 101 to the second sensor element 2500, which may especially comprise a photodetector. A control system 500 may be configured to determine a difference between the output frequency of the laser 700 and the resonance frequency of the (reflected) light source radiation 101 incident on the second sensor element 2500. The control system 500 may further be configured to calculate a direction and magnitude of the stresses/strains on the sensor element 2000 based on said difference.

Fig. 4b schematically depicts some embodiments of the suspended reflector 400. In embodiments, the suspended reflector 400 may comprise a plurality of openings 410, wherein the plurality of openings 410 may comprise one or more of holes, pores, and gratings. In embodiments, the plurality of openings 410 may comprise (a plurality of) first openings 910. The first openings 910 may have a first opening equivalent circular diameter D2a. In embodiments, the first opening equivalent circular diameter D2a may be smaller than the wavelength X of the light source radiation 101, D2a < ' . Additionally or alternatively, the first opening equivalent circular diameter D2a may, in embodiments, be selected from the range of 50-2000 nm. Further, the first openings 910 may have a shape in a plane parallel to a face of the suspended reflector 400. In embodiments, the first openings 910 may have a shape selected from the group comprising a circle, a square, a regular polygon, a rectangle, a cross, etc. Further, the first openings 910 may have a first length Li and a first width Wi. In embodiments, the first width Wi may be smaller than the wavelength X of the light source radiation 101, Wi < X. Especially, in embodiments, the first width Wi may be selected from the range of 50-2000 nm. Further, in embodiments, the first length Li may be equal to the first width Wi. Yet, in embodiments, the first length Li may be larger than the first width Wi, such as Li > 2*Wi. Further, in embodiments, the first length Li may be selected from the range of 0.1-20 mm. In embodiments wherein Li > 10*Wi, the first openings 910 may especially comprise a grating, as depicted in Fig. 4b(I). Hence, in embodiments, the first openings 910 may have first dimensions Wi, Li, wherein at least one of said first dimensions Wi, Li may be smaller than the wavelength X of the light source radiation 101.

In embodiments, the first openings 910 may have a first pitch pi in a first direction. In embodiments, the first pitch pi may be selected from the range of > 5 nm. Further, the first pitch pi may be selected from the range of < 2000 nm. In embodiments, the first pitch pi may be smaller than the wavelength X of the light source radiation 101, pi < X. In embodiments, the first openings may be equidistant in the first direction.

Further, in embodiments, the first openings 910 may have a second pitch p2 in a second direction. In embodiments, the second direction may be perpendicular to the first direction, though this need not be the case. In embodiments, the second pitch p2 may be equal to the first pitch pi. Yet, in embodiments, the second pitch p2 may be larger than the first pitch pi. In embodiments, the second pitch p2 may be selected from the range of > 2*pi. Further, in embodiments, the second pitch p2 may be selected from the range of < 20 cm. In embodiments, the first openings 910 may be equidistant in the second direction. Further, in embodiments, the first openings may form a repetitive pattern in one or more of the first direction and the second direction.

In embodiments, the plurality of openings 410 may comprise (a plurality of) second openings 920. In embodiments, the second openings 920 may have a second opening equivalent circular diameter D2b. In embodiments, the second opening equivalent circular diameter D2b may be larger than the wavelength X of the light source radiation 101, D2b > ' . Additionally, in embodiments, the second opening equivalent circular diameter D2b may be selected from the range of > 2*D2a. Further, in embodiments, the second opening equivalent circular diameter D2b may be selected from the range of 0.2-200 mm.

In embodiments, the second openings 920 may have a shape in a plane parallel to a face of the suspended reflector 400. In embodiments, the second openings 920 may have a shape selected from the group comprising a circle, a square, a regular polygon, a rectangle, a cross, etc. Further, the second openings 920 may have a second length L2 and a second width W2. In embodiments, the second length L2 and second width W2 may be larger than the wavelength X of the light source radiation 101, L2 > and/or W2 > . Additionally or alternatively, in embodiments, the second length L2 and second width W2 may be individually selected from the range of 2 pm - 200 mm. Hence, in embodiments, the second openings 920 may have second dimensions W2, L2, wherein none of said second dimensions W2, L2 may be smaller than the wavelength X of the light source radiation 101. In embodiments, the second openings may form a repetitive pattern in one or more of the first direction and the second direction.

The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably.

The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’.

The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term “comprising” may in an embodiment refer to “consisting of’ but may in another embodiment also refer to “containing at least the defined species and optionally one or more other species”.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

CLAIMS:

1. A method for providing a sensor element (2000) for optomechanical sensing, the method comprising: providing a Bragg reflector (100) comprising alternating layers (110, 120) comprising different materials; providing a silicon layer (200) on at least part of the Bragg reflector (100); depositing a top layer (300) on at least part of the silicon layer (200) at a temperature in the range of 500-1100 °C; wherein the top layer (300) comprises a high-stress dielectric material (320); providing a mask (310) for a suspended reflector (400) on the top layer (300); etching the top layer (300) through the mask (310) to provide the suspended reflector (400) and etching a cavity (210) in the silicon layer (200).

2. The method according to any one of the preceding claims, wherein the high- stress dielectric material (320) comprises silicon-nitride-based material or silicon-carbide- based material.

3. The method according to any one of the preceding claims, wherein the alternating layers (110,120) comprise first layers (110) selected from the group comprising Si- N, SiC, and BaO, and second layers (120) selected from the group comprising SiCh and HfCh.

4. The method according to any one of the preceding claims, wherein the top layer (300) has a tensile stress in the range of 0.5-20 GPa, wherein etching the top layer (300) provides the suspended reflector (400), wherein the suspended reflector (400) has a residual tensile stress in the range of 1-20 GPa.

5. The method according to any one of the preceding claims, wherein one or more applies of: (i) the top layer (300) has a residual tensile stress in the range of 5-20 GPa, and (ii) the method comprises depositing the top layer (300) on at least part of the silicon layer (200) at a temperature in the range of 700-1000°C.

6. The method according to any one of the preceding claims, comprising etching the cavity (210) in the silicon layer (200) by means of gas plasma etching.

7. The method according to any one of the preceding claims, comprising providing the Bragg reflector (100) by depositing alternating layers of the first layer (110) and the second layer (120) by means of chemical vapor deposition of the each of the first layer (110) or the second layer (120) on the preceding first layer (110) or the preceding second layer (120).

8. The method according to any one of the preceding claims, comprising one or more of the following: providing the top layer (300) with a top layer thickness in the range 5-500 nm; providing the silicon layer (200) with a thickness in the range 0.01 pm - 10 pm; providing (i) the first layer (110) with a first layer thickness in the range 40-400 nm, and (ii) the second layer (120) with a second layer thickness in the range of 40-400 nm; providing (i) the first layer (110) with a first refractive index in the range 1.8-

2.7, and (ii) the second layer (120) with a second refractive index in the range of 1.2-1.8; and providing the Bragg reflector (100) with a number of layers in the range 4-20.

9. The method according to any one of the preceding claims, comprising providing sulfur hexafluoride as the gas plasma used in the gas plasma etching as defined in claim 6.

10. A sensor element (2000), comprising a Bragg reflector (100), a silicon layer (200) and a top layer (300); wherein: the Bragg reflector (100) comprises alternating layers (110, 120) comprising different materials; the silicon layer (200) is configured on at least part of the Bragg reflector (100); the top layer (300) is configured on at least part of the silicon layer (200); wherein the top layer (300) comprises a high-stress dielectric material (320); the top layer (300) comprises a suspended reflector (400); the silicon layer (200) further comprises a cavity (210), wherein the cavity (200) is configured between the suspended reflector (400) and the Bragg reflector (100).

11. The sensor element (2000) according to claim 10, wherein one or more applies of: the top layer (300) has a thickness in the range of 5-500 nm; the high-stress dielectric material (320) comprises a silicon-nitride-based material or silicon-carbide-based material; the silicon layer (200) has a thickness in the range 0.01 pm - 10 pm; the alternating layers (110,120) comprise first layers (110) selected from the group comprising Si-N, SiC, and BaO, and second layers (120) selected from the group comprising SiCh and HfCh; and

(i) the first layer (110) has a first layer thickness in the range 40-400 nm, and the second layer (120) has a second layer thickness in the range of 40-400 nm; (ii) the first layer (110) has a first refractive index in the range 1.8-2.7, and the second layer (120) has a second refractive index in the range of 1.2-1.8; and (iii) the Bragg reflector (100) has a number of layers in the range 4-20.

12. The sensor element (2000) according to any one of the preceding claims 10-11, wherein the top layer (300) has a tensile stress in the range of 0.5-10 GPa, wherein the suspended reflector (400) has a residual tensile stress in the range of 1-10 GPa.

13. The sensor element (2000) according to any one of the preceding claims 10-12, wherein the sensor element (2000) is obtainable by the method according to any one of the preceding claims 1-9, wherein one or more applies of: the top layer (300) has a residual tensile stress in the range of 5-20 GPa; the method comprises depositing the top layer (300) on at least part of the silicon layer (200) at a temperature in the range of 700-1000°C; and the method comprises etching the cavity (210) in the silicon layer (200) by means of gas plasma etching.

14. An apparatus (1000) for optomechanical sensing comprising a sensor element (2000), a laser (700), and a second sensor element (2500), wherein the sensor element (2000) comprises a Bragg reflector (100), a silicon layer (200) and a top layer (300); wherein: the Bragg reflector (100) comprises alternating layers (110, 120) comprising different materials; the silicon layer (200) is configured on at least part of the Bragg reflector (100); the top layer (300) is configured on at least part of the silicon layer (200); wherein the top layer (300) comprises a high-stress dielectric material (320); the top layer (300) comprises a suspended reflector (400); the silicon layer (200) further comprises a cavity (210), wherein the cavity (200) is configured between the suspended reflector (400) and the Bragg reflector (100); the laser (700) is configured to provide light source radiation (101) to the sensor element (2000); and the second sensor element (2500) is configured to detect light source radiation (101) escaping from the sensor element (2000).

15. The apparatus (1000) according to claim 14, wherein the top layer (300) has a thickness in the range of 5-500 nm.

16. The apparatus (1000) according to any one of the preceding claims 14-15, wherein the high-stress dielectric material (320) comprises a silicon-nitride-based material or silicon-carbide-based material.

17. The apparatus (1000) according to any one of the preceding claims 14-16, wherein the silicon layer (200) has a thickness in the range 0.01 pm - 10 pm.

18. The apparatus (1000) according to any one of the preceding claims 14-17, wherein the alternating layers (110,120) comprise first layers (110) selected from the group comprising Si-N, SiC, and BaO, and second layers (120) selected from the group comprising SiCh and HfCh.

19. The apparatus (1000) according to any one of the preceding claims 14-18, wherein the alternating layers (110,120) comprise a first layer (110) and a second layer (120), wherein: the first layer (110) has a first layer thickness in the range 40-400 nm, and the second layer (120) has a second layer thickness in the range of 40-400 nm; the first layer (110) has a first refractive index in the range 1.8-2.7, and the second layer (120) has a second refractive index in the range of 1.2-1.8; and the Bragg reflector (100) has a number of layers in the range 4-20.

20. The apparatus (1000) according to any one of the preceding claims 14-19, wherein one or more applies of: the suspended reflector (400) has a first equivalent circular diameter (Di), wherein the first equivalent circular diameter (Di) is selected from the range of 10 pm - 45 cm; the suspended reflector (400) comprises a plurality of openings (410), wherein the plurality of openings (410) comprise first openings (910) and second openings (920), wherein (i) the first openings (910) have first dimensions (Wi, Li), wherein at least one of said first dimensions (Wi, Li) is smaller than a wavelength (X) of the light source radiation (101), and (ii) the second openings (920) have second dimensions (W2, L2), wherein none of said second dimensions (W2, L2) is smaller than a wavelength (X) of the light source radiation (101); and - the suspended reflector (400) comprises a plurality of openings (410), wherein the plurality of openings (410) comprise first openings (910), wherein the first openings (910) have a first pitch (pi), wherein the first pitch (pi) is selected from the range of < 2000 nm.