Patent Application
20140247481
The invention is directed to an optical component for the IR range with strain-compensated coating such as is known generically from DE 101 34 157 A1.
In many applications of optical components, it is increasingly required that these optical components be arranged in an increasingly space-saving manner and that the optical components and the coatings thereof can be produced in an ever more economical manner from few individual components.
Optical components of this type may be used as Fabry-Pérot interferometers. These are basically made up of at least two reflective layers which are spaced apart from one another and separated by an intermediate space referred to as resonant cavity. A controlled variability of the size of the resonant cavity and, therefore, of the optical thickness thereof makes it possible to tune the Fabry-Pérot interferometer.
In U.S. Pat. No. 6,618,199 B2 which is cited by way of example, two mirror structures are provided, and the spacing therebetween defines a resonant cavity of a Fabry-Pérot interferometer. At least one of the mirror structures comprises a movable membrane through which electrostatic forces can act on the mirror structure so that the spacing between the two mirror structures can be adjusted.
EP 1 882 917 A1 describes a tunable dual-band Fabry-Pérot filter which is based on a Fabry-Pérot interferometer and is used in IR metrology and encloses the two atmospheric windows (3 to 5 μm and 8 to 12 μm). The filter basically comprises stacks of layers over a silicon substrate. The layers are alternately low-index (refractive index of 1.2 to 2.5) and high-index (3 to 5.9). Each stack has at least five low-index layers and five high-index layers, respectively. A stack is arranged on a reflector substrate in each instance, these reflector substrates being separated by a resonant cavity whose optical thickness is adjustable, and the Fabry-Pérot filter can be tuned in this way.
The resonant cavity can also be realized by means of one or more optical layers such as is known from U.S. Pat. No. 4,756,602 A. In this case, the optical thickness of the resonant cavity can be selected before the layers are produced but can no longer be varied or even tuned after the filter is made.
The above-mentioned interferometers and filters are often mounted on silicon wafers or germanium wafers using MEMS (microelectromechanical systems) or wafer level packaging technologies. In the case of layers which must be designed to be very thin and in case of thin wafers, the problem arises of designing the coatings so as to be free of strain. Strains must be compensated particularly when requirements respecting the surfaces (planarity) of the optical components and the coatings thereof are very high as has been the case heretofore for applications in the X-ray range and lithography (EUV).
In order to develop dual-band reflectors having predefined and distinct reflectivities with respect to one another in two separated, defined spectral ranges (e.g., mid-wave infrared [MWIR] or LWIR [long-wave infrared]), low-index and high-index dielectric layers are alternately stacked one upon the other in a sequence of layers to build optical components of this type, and the differences in the refractive index selected in the layers must be as large as possible so as to minimize the total thickness of the layer sequence. If there are few layers arranged in a layer sequence, they have correspondingly large individual layer thicknesses.
However, it has been shown in practice that layer sequences with large layer thicknesses, e.g., with layers of germanium and fluorides, are very unstable due to the occurrence of high strains oriented in the same direction, e.g., tensile strains.
Various approaches are known for balancing the compressive or tensile strains proceeding from the layers. For example, layers of materials with opposing strain coefficients can be combined.
In JP 2006-281766 A, for example, two layers are arranged over a substrate, wherein the substrate and first layer have a positive strain coefficient, but the second layer has a negative strain coefficient. This solution is intended to compensate occurring thermal stresses.
For use in the field of EUV lithography, WO 00/19247 likewise discloses the possibility of achieving strain compensation by combining layers having different strain coefficients.
Another method is suggested in DE 101 34 157 A1 which describes combining at least one oxidic optical layer with a layer of aluminum oxide as compensation layer, wherein the aluminum oxide layer is to be applied without ion assist. While the oxidic layers exhibit compressive strains (positive strain coefficients), tensile strains (negative strain coefficient) are present as a result of the aluminum oxide layer. If a plurality of oxidic layers are arranged in a stack, the stack will comprise a six-fold sequence of low-index and high-index layers. The compensation layer can be arranged below, above or between the oxidic layers. The disclosure shows a possibility for flexible compensation of strains in a stack, but does not specify the optical effects exhibited by the at least one compensation layer of aluminum oxide.
Antireflective coatings having only four layers in each instance which are stacked over a substrate are known from U.S. Pat. No. 5,243,458 A. In this case, a layer of zinc sulfide (ZnS) is inserted between layers of materials with tensile strains, e.g., germanium (Ge) or fluorides. The ZnS layer has compressive strains so that the tensile strains in the layer sequence are extensively compensated. Further, the ZnS layer has an adhesion promoting effect between the germanium and the fluorides.
However, the solutions known from the prior art do not eliminate the disadvantageous occurrence of high strains in layers with large layer thicknesses.
It is an object of the invention to provide an optical component with a strain-compensated coating and selected technical characteristics for use in the IR range. Likewise, a method is to be provided for the design of an optical component by means of which strain-compensated coatings of the optical component can be designed while adjusting desired technical characteristics simultaneously.
Accordingly a method for designing an optical component for the IR range has the following steps:
The term “design” as used hereinafter refers to the production of the optical component in virtual form. In this respect, the optical component can be present as dataset during implementation of the method according to the invention and can be represented, for example, in tabular form and/or as a schematic graph.
By “desired technical characteristics” is meant broadly all of the characteristics of the optical component relevant for the function thereof. For example, optical characteristics as well as mechanical and/or chemical properties of the optical component can be technical characteristics.
In an exemplary embodiment of the method according to the invention, the desired technical characteristics are provided in that at least two portions are realized by the optical component over a wavelength range from 0.8 to 16 micrometers, along which two portions the optical component has a determined reflectivity in the range from 50 to 100% in each instance. In a particularly preferred manner, each of the portions in the region of one of the two atmospheric windows, as they are called, lies in the range from 3 to 5 μm (MWIR) and in the range from 8 to 12 μm (LWIR), respectively.
Any reflectivity can be selected for the portions. The design of the optical component by means of the method according to the invention allows the reflectivities of the optical component to be adjusted in accordance with the selection made.
Any manual or computer-assisted method for the design of an optical component with desired technical characteristics can be used for the simulation of the simulated optical component. The simulation is carried out by means of a suitable simulation program such as is known in the art. The further simulation is also carried out using a simulation program. In so doing, the at least one inserted mid-index layer in the layer sequence must be taken into account. A simulation and a further simulation preferably include a substrate of the layer sequence in each instance.
An aspect of the method according to the invention resides in devising a layer sequence (stack) of high-index layers and low-index layers by means of which the desired technical characteristics of the optical component are brought about and the layer sequence is subsequently modified such that strains occurring between and within the layers of the layer sequence are reduced. In order to reduce the very detrimental strains between the high-index layers and low-index layers, mid-index layers are inserted into the devised layer sequence (compensation layer). These mid-index layers have also proved advantages inasmuch as a very desirable adhesion promotion is achieved between high-index layers and low-index layers and between the materials used for the high-index layers and low-index layers, respectively, by means of these mid-index layers.
It is an important feature of the invention that at least one low-index layer is divided into partial layers. In this way, undesirably large layer thicknesses are avoided and are divided up over a plurality of partial layers of an originally simulated layer. As a result of this procedure, at least one mid-index layer is arranged directly between low-index partial layers which are made of the same material.
In further embodiments of the method according to the invention it is also possible to divide a layer into at least three partial layers and to insert a further high-index layer or low-index layer between two of the partial layers in addition to a mid-index layer.
Preferably, the mid-index layers (compensation layers) inserted between the partial layers are selected with layer thicknesses between 20 and 150 nm, preferably between 30 and 100 nm. It is advantageous when the layers are divided such that none of the partial layers has a layer thickness of more than 1500 nm, for example.
Further mid-index layers can be inserted into the layer sequence of the optical component. These further mid-index layers need not be inserted between partial layers.
It is further possible that in step c) at least one high-index layer of the simulated optical component is additionally divided into at least two partial layers and a mid-index layer is inserted between at least two of the partial layers.
In further embodiments of the method according to the invention it is possible that only the layer thickness of the mid-index layer or mid-index layers is adapted in the further simulation in step d). The desired technical characteristics are then adjusted while retaining the simulated layer thicknesses of the low-index layers and high-index layers of the simulated optical component and while changing the layer thicknesses of the mid-index layers.
In a further embodiment of the method according to the invention, the low-index layers and high-index layers or only the low-index layers or the high-index layers, respectively, can also be adapted to the modified simulated optical component in addition to the mid-index layers in the further simulation.
The dividing of the low-index layer can take place in a virtual manner by means of individual input, e.g., by an operator of the simulation program. The decisions made about the type (e.g., quantity of partial layers, thickness or ranges of thickness of one or more or all of the partial layers) and location (selection of the layers to be divided in the stack) of the divisions can be inputted as dataset into the simulation program. In further embodiments of the method according to the invention, some or all of the decisions about the type and location of divisions can also be stored already beforehand as datasets, for example, in the form of rules. The division of low-index layers—possibly also of high-index layers—and the insertion of mid-index layers can then also be carried out in an automated manner while taking into account the datasets stored beforehand.
The method according to the invention can be used for producing an optical component. To this end, the optical component is designed in the manner described above and is produced by means of suitable known methods on the basis of the results of the further simulation which are obtained and made available in step e).
An optical component for the IR range may also be provided having a substrate and a stack of optical layers with individual layer thickness which are stacked one upon the other on the substrate. The stack has at least one low-index layer whose refractive index lies in a range from 1.35 to 1.7 and a high-index layer whose refractive index lies in a range from 3 to 5. At least one low-index layer is divided into at least two partial layers. There is present between at least two of the partial layers a mid-index layer whose refractive index lies in a range from 1.8 to 2.5 and whose strain coefficient has an opposite sign with respect to the strain coefficient of every low-index layer and of every high-index layer. A sequence of layers of the stack is selected in such a way that the reflectivity of the coating has selected, mutually independent values in a range from 50 to 100% reflectivity in a wavelength range from 0.8 to 16 μm over at least two portions of this wavelength range.
The terms “stack” and “layer sequence” are used synonymously in the description.
The optical component can have further mid-index layers in addition to the mid-index layer present between the two partial layers. These further mid-index layers can be present between further partial layers, between low-index layers, between high-index layers or between low-index layers and high-index layers.
Preferably, at least two portions having mutually independent reflectivities with values of between 50 and 100% are generated by means of the optical component over the wavelength range from 0.8 to 16 μm. The construction of the optical component is preferably selected such that at least one portion with a reflectivity between 50 and 100% is generated over at least one partial portion of each atmospheric window (3 to 5 μm and 8 to 12 μm).
The at least two portions of the wavelength range can also be referred to as spectral wavelength bands or dual bands.
In certain embodiments of the optical component according to the invention, the layer thicknesses of each of the mid-index layers (compensation layers) which are present in the stack and which are inserted between the partial layers are 20 to 150 nm, preferably 30 to 11 nm. It is further preferable that the mid-index layers make up at least 20%, preferably at least 25%, of the total thickness of the stack. The mid-index layers have strain coefficients with signs opposite to those of the strain coefficients of every low-index layer and every high-index layer.
An advantage of the invention consists in that the reflectivity can be selected through the selection of the layer thickness of at least one of the provided mid-index layers. Accordingly, it is possible to adjust the reflectivity of an optical component to be produced according to the invention in accordance with the requirements resulting from the intended use of the optical component while retaining the quantity, sequence, layer thicknesses and materials of the rest of the layers of the stack such that a given reflectivity of the optical component can be realized. By “adjustment” is meant that a stack can be arranged on a substrate, e.g., through deposition by means of PVD or other known methods, and the adjustment of reflectivity is carried out through the corresponding configuration of the at least one mid-index layer in the course of the arrangement. The optical component according to the invention was preferably designed by means of the method according to the invention.
In further embodiments of the optical component, the sequence, layer thicknesses, quantity and materials of the further layers of the stack present in an optical component can also be selected and adjusted in such a way that the desired optical effects are achieved. In particular, the reflectivity of the optical component to be produced is adjusted corresponding to the requirements resulting from the intended use of the optical component.
In this regard, subject to the presence of the partial layers and the mid-index layer provided therebetween, the adjustability of reflectivity is not bound to a determined embodiment, i.e., to a determined sequence of layers, to the layer thicknesses, to the quantity or to the materials of the layers of the stack of the optical component according to the invention.
In a preferred embodiment of the optical component according to the invention, the strain coefficients of the mid-index layers are positive, i.e., compressive strains are introduced into the stack through the material of the mid-index layers.
The material of the high-index layers is preferably selected separately for each of the high-index layers from a group comprising the elements germanium (Ge), silicon (Si) and the compounds lead telluride (PbTe) and cadmium telluride (CdTe).
An embodiment of the optical component according to the invention in which the material of the mid-index layers is selected separately for each of the mid-index layers from a group comprising the compounds zinc sulfide (ZnS), zinc selenide (ZnSe), silicon oxide (SiO) and chalcogenide is likewise preferred.
Further, the material of the low-index layers is preferably selected separately for each of the low-index layers from a group comprising the compounds ytterbium fluoride (YbF3), barium fluoride (BaF2), magnesium fluoride (MgF2) and calcium fluoride (CaF2). The material of the low-index layers can also be selected from oxides with refractive indexes in the range from 1.35 to 1.7, for example, SiO.
The material of the substrate is preferably selected from a group comprising the elements Ge, Si and the compounds chalcogenide glasses, ZnS, ZnSe, sapphire, quartz, fused silica, CaF2 and MgF2.
Surprisingly, it transpired that in addition to a strain compensation and an adjustability of the reflectivity an improved adhesion promotion, particularly between Ge and YbF3, is also achieved by means of the at least one mid-index layer through the use of ZnS.
The specific configuration of the optical component according to the invention required for achieving desired technical characteristics such as a determined optical effect, for example, can be carried out by means of a suitable computer-assisted simulation.
The optical component according to the invention can be a MEMS component. It can likewise be used as narrow-band filter or as a single-band, dual-band or multi-band reflector. When used as single-band filter, only one band is used even if the optical component according to the invention has a plurality of bands.
Embodiments of the optical component are described more fully in the following with reference to embodiment examples and diagrams. The diagrams show:
In a first embodiment of the optical component 1 according to the invention shown in
As is shown schematically in
In a second embodiment according to
As is shown schematically in
In a third embodiment according to
As is shown schematically in
In a fourth embodiment according to
As is shown schematically in
In a fifth embodiment of the optical component 1 according to the invention shown in
As is shown schematically in
In a sixth embodiment of the optical component 1 according to the invention shown in
As is shown schematically in
In a seventh embodiment of the optical component 1 according to the invention shown in
As is shown schematically in
The basic outline of the method according to the invention for designing an optical component 1 for the IR range is described with reference to the first embodiment example according to
First, the desired technical characteristics of the optical component 1 to be designed are determined. The optical component 1 should have reflectivities of at least 90%, respectively, over a portion in the wavelength range from 3.7 to 4.3 mm of the first atmospheric window and over a portion in the wavelength range from 7.5 to 10 μm of the second atmospheric window. No reflectivity is specified between the above-mentioned portions. Further, strains occurring in the optical component are to be minimized and a small total layer thickness is to be obtained. Accordingly, a dual-band reflector with the above-mentioned technical characteristics is to be designed. YbF3 is to be used as material for the low-index layers and Ge is to be used as material for the high-index layers. Both have tensile strains (negative strain coefficients).
The desired technical characteristics are entered into a simulation software as input data and a simulation is carried out. A simulated optical component having a layer sequence of a high-index layer H1 (layer thickness: 698 nm), a low-index layer L1+L2 (702 nm) and a further high-index layer H2+H3 (1190 nm) is obtained as result in a virtual manner. The low-index layer L1+L2 is then divided into two partial layers L1 and L2, and a mid-index layer M2 is inserted as “compensating layer”. The high-index layer H2+H3 is also divided and a mid-index layer M3 is inserted. To promote adhesion between the high-index layer H1 and the low-index (partial) layer L1, a mid-index layer M1 is inserted. All of the mid-index layers are made of ZnS and have compressive strains (positive strain coefficient). The simulated optical component modified in this way is simulated again in a further simulation taking into account all of the modifications carried out. In so doing, the layer thicknesses of the modified simulated optical component are adapted in such a way that their sequence remains unchanged, but the individual layer thicknesses of all of the layers are recalculated. An optical component 1 with the desired technical characteristics is obtained.
In a further embodiment example of the method according to the invention, only the individual layer thicknesses of the mid-index layers are adapted.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 053 807.7 | Sep 2011 | DE | national |
The present application is a U.S. National Stage application of International PCT Application No. PCT/DE2012/100289 filed on Sep. 19, 2012 which claims priority benefit of German Application No. DE 10 2011 053 807.7 filed on Sep. 20, 2011, the contents of each are incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2012/100289 | 9/19/2012 | WO | 00 | 3/20/2014 |
