The present invention relates to a mirror device for an interferometer device, to an interferometer device, and to a method for producing a mirror device.
For spectral filters that are variable (tunable) over a plurality of wavelengths and are transmissive for only specific wavelengths, it is possible to realize miniaturization, for example with Fabry-Perot interferometers (FPI), for example by means of a micro-electromechanical design (MEMS technology). A cavity having two highly reflective mirrors, which are substantially plane-parallel and have a spacing (cavity length) in the order of optical wavelengths, may exhibit strong transmission only for those wavelengths that correspond, in terms of the cavity length, to an integer multiple of half the wavelength. Using for example electrostatic or piezoelectric actuation, the spacing between the mirrors of the interferometer can be modified, as a result of which a spectrally tunable filter element can be obtained.
Fabry-Perot interferometers, which can advantageously cover as large a wavelength range as possible, should be highly reflective, inter alia, over the entire wavelength range that is to be measured. Typically, the mirrors can comprise dielectric layer systems, for example distributed Bragg reflectors (DBR), which can comprise alternating layers of high-index and low-index materials, wherein the optical thickness of these layers ideally includes a quarter of the central wavelength of the wavelength range under consideration. The following relationship gives the wavelength range AA, in which such mirrors can have a high reflectivity. The contrast of the refractive index of the high-index and low-index materials is consequently given by
wherein λ0 denotes the central wavelength, nL denotes the refractive index of the low-index material, and nH denotes the refractive index of the high-index material.
Here, an achievable maximum reflection can, as follows, likewise be higher for the stated wavelength range for a given number of layer pairs with a higher refractive index contrast:
Here, nSUB equals the refractive index of the substrate if the DBR mirror is not exposed. If the DBR mirror is exposed, nSUB=1. To cover the largest possible wavelength range, the refractive index of the low-index material can be as close to 1 as possible, such as in the case of gases or a vacuum. Since plane-parallelism is also important for such mirrors (layers), support structures between the mirror layers are advantageous for keeping the spacing between the individual layers within a mirror of the FPI constant (spacing between the high-index layers). Typically, parts of the upper high-index layer can be formed as support structures. The latter can extend from the upper high-index layer to the bottom one.
In U.S. Pat. No. 7,733,495 B2, a multilayer mirror and a Fabry-Perot interferometer are described. A side wall can extend between the high-index layers.
The present invention provides a mirror device for an interferometer device as claimed in claim 1, an interferometer device as claimed in claim 9, and a method for producing a mirror device as claimed in claim 10.
Preferred developments are the subject of the dependent claims.
The concept on which the present invention is based consists in specifying a mirror device for an interferometer device comprising improved spacing structures between mirror layers in a mirror device. The spacing structures can be used for maintaining a constant spacing between the mirror layers of a mirror device and simultaneously as spacers for the mirror device from another element, such as an electrode, a substrate, or another mirror device.
According to the invention, the mirror device for an interferometer device comprises a first mirror layer and a second mirror layer, which are arranged parallel one above the other with a mirror layer distance between them, wherein the mirror layer distance forms an intermediate space between the first and the second mirror layer, and wherein the intermediate space includes a gas or a vacuum; at least one spacing structure extending at least partially between the first and the second mirror layer, and wherein the spacing structure comprises a material that is the same as or different from the first and/or second mirror layer.
The vertical extent can be tilted perpendicular to the planar plane of extent or can be oblique, for example at an angle of 70° or 80° with respect to the planar plane of extent, that is to say deviating from a vertical direction.
The spacing structure can comprise a material that is the same as or different from the first and/or second mirror layer. In the event that the spacing structure comprises the same material as one or both mirror layers, this can still be detectable in the finished component (mirror device) because the spacing structure and the mirror layers can be producible separately from one another, that is to say not act as one overall component, and can also differ from one another. The spacing structure and the mirror layers can comprise for example silicon (poly-Si), and in each mirror layer and also in the spacing structure, new growth of the poly-Si can thus take place during their production. In the case of separately produced structures, the material structure, for example crystallinity, can be detectably different from a continuous structure made of the same material. For this reason, mirror layers and a spacing structure from the same crystalline material, which were produced separately, can be detectably different in terms of their material structure from a structure that was produced (grown) continuously in one step.
According to a preferred embodiment of the mirror device, the spacing structure comprises side walls that extend vertically from a planar direction of extent of the first and second mirror layers or extend in deviation from a vertical direction by a specific angle.
According to a preferred embodiment of the mirror device, the spacing structure projects at least into one of the two mirror layers.
According to a preferred embodiment of the mirror device, the spacing structure comprises a core between the side walls and a bottom, wherein the side walls and the bottom comprise a different material than the core.
According to a preferred embodiment of the mirror device, the side walls and the bottom comprise an electrically insulating material.
According to a preferred embodiment of the mirror device, the spacing structure projects at least through one of the two mirror layers and beyond an outer side of the first and/or second mirror layers by at least one thickness of one of the mirror layers.
According to a preferred embodiment of the mirror device, the latter comprises a plurality of spacing structures that, in a top view of a planar top side of the second mirror layer, form a hexagonal grid.
According to a preferred embodiment of the mirror device, in a region below and/or above the recess, the first and/or second mirror layer projects perpendicularly from the planar direction of extent of the first mirror layer in a direction away from the recess.
According to the invention, the interferometer device comprises a substrate; a first mirror device and a second mirror device, wherein at least one of them comprises a mirror device according to the invention, which are arranged over the substrate and one above the other, spaced apart from one another by a first spacing, wherein at least the first mirror device is arranged movably in relation to the second mirror device; and an actuating device by means of which at least the first and/or second mirror device is movable.
According to the invention, the method for producing a mirror device includes providing a first sacrificial layer and/or a substrate; applying a first mirror layer onto the first sacrificial layer and/or onto the substrate; applying a second sacrificial layer on the first mirror layer; forming a recess at least in the second sacrificial layer that extends at least up to the first mirror layer; introducing a material for a spacing structure into the recess; applying a second mirror layer onto the second sacrificial layer and over the recess; and at least partially removing the first and/or the second sacrificial layer.
The method can advantageously also be characterized by the features mentioned in connection with the mirror device and the advantages thereof, and vice versa.
According to a preferred embodiment of the method, introducing the material for a spacing structure into the recess involves arranging an electrical insulator layer in the recess and on the top side of the second sacrificial layer and then introducing the material for a core of the spacing structure into the recess such that the recess is filled.
According to a preferred embodiment of the method, the material of the recess or at least the material for the core is backthinned before the second mirror layer is applied in order to produce a planar connection with regions that laterally adjoin the recess.
Further features and advantages of embodiments of the invention are evident from the following description with respect to the attached drawings.
The present invention will be explained in more detail below with reference to the exemplary embodiments specified in the schematic figures of the drawing.
In the drawings:
In the figures, identical reference signs denote identical or functionally identical elements.
The spacing structures 4 shown can undergo lateral deformations, for example resulting from the inner tensile stress (mechanical) in the mirror layers. Since the spacing structures advantageously comprise a different material than the mirror layers, these can be mechanically and advantageously electrically adapted to the requirements of the spacing structure, for example in order to be able to better maintain a tensile stress that is advantageously set in the layers (due to the reduced relaxation of the spacing structures), as a result of which the optically usable surface (the planarity of the mirrors with a defined spacing) can also be increased.
Furthermore, the spacing structures can terminate substantially planar with a top side of the mirror layer, which cannot produce any elevation above the mirror layer (produces hardly any or no topography), which may be advantageous both for process control and also for the optical and mechanical properties of any further (mirror) layers that may follow (consequently, little or no bending of the following layers of a further mirror may occur). During filling of the recess, the material for the spacing structure can form a planar surface with a tolerance with a top side of the mirror layer that faces away from the first mirror layer. The tolerance for a planar termination can have a deviation of at most the thickness of the mirror layer.
In
Furthermore, a plurality of spacing structures 4 may also be present, which can form, in a top view of a planar top side 3b of the second mirror layer 3, a hexagonal grid or other geometric shapes (not shown).
According to
According to
The spacing structure can consequently be deposited separately from the mirror layers and form a base for depositing the second mirror layer. The embodiment can also be expanded to include further mirror layers, advantageously using further mirror layers and sacrificial layers.
The gas (mixture) in the intermediate space 5, for example air, or a vacuum can represent (replace) a low-index layer and have a refractive index of approximately one. The mirror layers 2 and 3 can have, for example, silicon as the high-index material having a refractive index of, for example, 3.5. Rather than silicon, germanium or silicon carbide can also be used, or different materials that can be compatible with (resistant to) sacrificial layer etching processes. If air is used as the low-index material, it is possible to achieve a large refractive index difference with respect to the high-index material and to produce a spectrally broadband, highly reflective mirror device.
The spacing structures 4 can stabilize the mirror layers relative to one another in order to ensure, via as large an optical region (aperture area) of the mirror device as possible, a spacing of the mirrors (mirror devices) of one quarter wavelength of the central wavelength (that is to be transmitted or filtered), that is to say that the low-index layer (air) has a thickness of a quarter wavelength.
The material of the spacing structure 4 can be, for example, a semiconductor material and/or the same material as at least one of the mirror layers. The deposition process of the material of the spacing structure can be adapted to the mechanical and electrical properties (conductivity electrical, thermal, vertical electrical insulation of the mirror layers) of the mirror layers and the production process. However, these properties can also be set independently of the requirements regarding the mirror layers. For example, the doping and/or crystallinity can be variable. The spacing structure and the mirror layers can differ in their materials in terms of doping or crystallinity, but can also comprise a different semiconductor material. The spacing structure can be electrically insulating, for example the material of the core. From a mechanical standpoint, this spacing structure can be highly stable and resistant to breakage and hardly permit any deformations of the mirrors (membranes/layers), in particular their separation, for example no or little notch effect under stress.
The spacing structures can be designed as at least partially laterally continuous wall structures and/or as column structures, for example as honeycomb structures.
A predetermined separation between the mirror layers can be maintained due to reduced yielding or no yielding. The spacing structures can be embodied, in a top view, nearly in the shape of points, resulting in minimization of optical losses.
The material in the core 4d can comprise a high-index material (as compared to the intermediate region with gas, gas mixture or vacuum), similar to one of the mirror layers.
In the event of contact between the mirror layer 2 and an underlying structure, the spacers AH (anti-stiction bumps) can reduce the contact area and thus the static friction, which can prevent the mirror layer from irreversibly sticking to an underlying structure. Any overhang of the spacers beyond the mirror layer can preferably be greater than a thickness of the mirror layer (first one) itself. With particular preference, the overhang is greater than a thickness of the second sacrificial layer. The spacers AH can thus be made from an electrically insulating material or surrounded by an electrically insulating layer in order to prevent fusion in the event of contact being made with an underlying structure that is at a different electrical potential.
In a mirror device of this type, reduced deformation of the spacing structures (lateral) and of a mirror region can be attained due to continuous mirror layers that remain substantially planar.
The step of
A further method step can involve, according to
According to the further method, according to
In a further step, according to
After the method step of
In a further method, according to
The recess A or further recesses (smaller ones) can be, in a top view of a planar direction of extent, circular, elliptical or have a different shape, such as elongated.
The elliptical shape can be characterized by better optical properties, in particular by a reduction in optical losses.
Using a third mirror layer and further sacrificial layers and corresponding recesses, the process sequences shown can be modified and multilayer mirror devices having a plurality of low-index layers and high-index layers (mirror layers) can be formed. The spacing structures can then be formed continuously between the plurality of mirror layers.
Furthermore, the first and the second sacrificial layer can be removed, for example by way of a sacrificial layer etching process using etching holes. The etching holes can be distributed (selective etching) in the first and/or second mirror layer (not shown).
The partial steps can relate to the production of a mirror device as shown in
According to
According to
In a further method step, according to
The interferometer device 10 can comprise a substrate S; a first mirror device SP1 and a second mirror device SP2, wherein at least one of these mirror devices can comprise a mirror device according to the invention, as shown in
The mirror devices SP1 and/or SP2 can comprise spacing structures 4 according to the invention with or without an overhanging portion, that is to say the spacers AH, toward the top or the bottom (relative to the substrate). The spacers AH can be placed on the substrate or on different elements. The interferometer device can comprise a peripheral structure RS outside an optical region, wherein the mirror devices SP1 and SP2 may be clamped in the peripheral structure RS and be contacted thereby with a contact K. In the optical region, the mirror devices can be exposed and the light path can be influenced by aperture stops BL and antireflective layers AR on the substrate S. The interferometer device can be designed as a Fabry-Perot interferometer (FPI). The FPI can be produced by depositing a plurality of sacrificial layers, wherein a sacrificial layer can be deposited on the substrate S, then the first mirror device can be formed thereon, then a further sacrificial layer can be deposited on the first mirror device, and a second mirror device can in turn be produced thereon. The thickness of the further sacrificial layer can be used for setting the first distance d12 and be set independently of the actuation gap, with the actuation gap being formed by the actuation electrodes between the substrate S and the first mirror device SP1. An FPI of this type does not need to be advantageously limited to a travel (actuation spacing or first spacing) of a third of the original optical gap (first spacing in the deflected position).
The interferometer device can be formed as a micro-electromechanical device (MEMS), for example as a micro-spectrometer.
The method for producing a mirror device involves providing S1 a first sacrificial layer and/or a substrate; applying S2 a first mirror layer onto the first sacrificial layer and/or onto the substrate; applying S3 a second sacrificial layer on the first mirror layer; forming S4 a recess at least in the second sacrificial layer, which extends at least up to the first mirror layer; introducing S5 a material for a spacing structure into the recess; applying S6 a second mirror layer onto the second sacrificial layer and over the recess; and at least partially removing S7 the first and the second sacrificial layer.
Even though the present invention has been described completely above with reference to the preferred exemplary embodiment, it is not limited thereto, but rather modifiable in multifarious ways.
Number | Date | Country | Kind |
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10 2019 206 758.1 | May 2019 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/061876 | 4/29/2020 | WO | 00 |