WO2022043448A1

WO2022043448A1 - Interference filter

Info

Publication number: WO2022043448A1
Application number: PCT/EP2021/073631
Authority: WO
Inventors: Patrik PERTL; Gerhard Eilmsteiner
Original assignee: Ams Ag
Priority date: 2020-08-31
Filing date: 2021-08-26
Publication date: 2022-03-03
Also published as: GB202013643D0

Abstract

An optical filter (100) and method of manufacturing an optical filter comprising a stack of layers, the stack comprising at least one layer comprising amorphous deuterated silicon (104), and at least layer of a dielectric material (106), wherein within a targeted transmission band the refractive index of the dielectric material is less than a refractive index of the amorphous deuterated silicon (104).

Description

INTERFERENCE FILTER

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure is in the field of optical filters, and in particular relates to interference filters and associated methods of manufacture.

BACKGROUND

Optical sensor systems may be used to detect and measure electromagnetic radiation. Such optical sensor systems may comprise a detector, a filter and an electronic circuit to facilitate the data processing. The filter determines the nature of the incoming radiation allowed to reach the detector.

Several forms of filters for use in such optical sensor systems are known in the art. A first category are absorption-based filters. These dye or pigment-based filters remove defined parts of the incoming radiation via absorption and subsequent transformation of the radiation to heat. Another category are classic interference filters, which make use of refraction occurring at the interface between layers with different refractive indices. Interference filters consist of stacked dielectric layers, which are nonabsorbing for the electromagnetic radiation. The filter characteristics are determined by the sequence and thicknesses of the individual layers. A further category are hybrid filters, which make use of absorption as well as refraction. Such hybrid filters consist of multiple layers similar to interference filters, albeit exhibiting radiation-absorbing elements in the structure.

In recent years, infrared (IR) and near-infrared (NIR) based sensing applications, such as face recognition or time-of-flight (TOF), have emerged into the public eye. In particular, applications making use of vertical cavity surface emitting laser (VCSEL) radiation sources have gained large interest. The most common wavelength for the VCSEL applications is -940 nm, which is in the range of a standard silicon photodetector. As these detectors show a high sensitivity in the visible light (VIS) range, associated optical filters are required. In order to achieve sufficient signal- to-noise ratio, a substantial suppression of radiation in the visible light range is required. Furthermore, in some applications it is important that transmission characteristics of the filter are not dependent upon an angle of incidence of radiation, as this may result in a loss of light at the targeted wavelength. Existing solutions for IR/NIR sensing applications offer two basic concepts for IR/NIR sensing and the related suppression of UV/VIS light. The first approach makes use of color coating, a visibly black resist exhibiting transmission in the IR/NIR range. The coating can be structured via lithography or inkjet printing (for a higher integration level) or unstructured (such as a packaging glass coating).

The second solution utilizes hybrid filters with hydrogenated amorphous silicon as a high refractive index material. As this material exhibits a significant absorption in the UV/VIS range, these interference filters offer an intrinsic suppression for this wavelength range. Furthermore, the high refractive index of hydrogenated amorphous silicon enables the opportunity to design filters with a low angular dependency.

Despite the useful properties of hydrogenated amorphous silicon in this field, the material possesses major drawbacks, particularly when being used in optical filters. First, the material is sensitive towards oxygen. As the H-Si bonds in the material tend to degrade slowly forming silicon oxide in presence of ambient conditions, a proper environmental shield is required to ensure long-term stability of the material. Further to this, hydrogenated amorphous silicon is prone to the so-called blistering effect. As hydrogen exhibits very high diffusion constants, it tends to segregate in the material. The emerging hydrogen bubbles in the material damage or destroy the interference filters due to the delamination of the individual layers.

It is therefore desirable to provide an optical filter suitable for use in infrared (IR) and near-infrared (NIR) sensing applications. Furthermore, it is also desirable to provide a method of manufacturing such an optical filter.

It is therefore an aim of at least one embodiment of at least one aspect of the present disclosure to obviate or at least mitigate at least one of the above identified shortcomings of the prior art.

SUMMARY

The present disclosure is in the field of optical interference filters, and in particular interference filters for infrared (IR) and near-infrared (NIR) applications.

According to a first aspect of the disclosure, an interference filter for an optical device is provided, comprising a stack of layers, the stack comprising: at least one layer comprising deuterated amorphous silicon; and at least one layer of a dielectric material, wherein within a targeted transmission band a refractive index of the dielectric material is less than a refractive index of the deuterated amorphous silicon. It will be understood the interference filter may be configured to allow a range of wavelengths through the filter. This range of wavelengths may be referred to as a transmission band or a targeted transmission band. The transmission band may be a given range of, for example, infrared or near infrared wavelengths. The targeted transmission band may be selected such that deuterated amorphous silicon has a relatively high refractive index in comparison to the dielectric material. In this regard, the layer comprising the deuterated amorphous silicon may be considered a high refractive index layer in comparison to the layer comprising the dielectric material. The layer comprising the dielectric material may similarly be considered to be a low refractive index layer. Therefore, henceforth the layer comprising the deuterated amorphous silicon may be described as a high refractive index layer, and the layer comprising the dielectric material may be referred to as a low refractive index layer.

The use of deuterated amorphous silicon in place of hydrogenated amorphous silicon offers several advantages. For example, segregated hydrogen bubbles in the hydrogenated silicon may cause delamination of the individual layers. Deuterium atoms have a lower diffusion constant than hydrogen atoms, which inhibits segregation, and therefore amorphous silicon-deuterium layers are less likely to blister than hydrogenated amorphous silicon layers. In addition, the Si-D bond is more stable than the Si-H bond in the material and therefore less prone to oxidation, enhancing the environmental stability of silicon-deuterium relative to hydrogenated silicon. As a result, the use of a silicon-deuterium layer or layers may enable a reduction or even complete removal of environmental protection of the interference filter, such as encapsulation layers or capping steps.

Advantageously, combining a high refractive index layer such as deuterated amorphous silicon and a low refractive index layer may reduce or remove the angular dependence of the transmission characteristics of the filter. Beneficially, this means that the transmission characteristics of the filter are less impacted by or do not change with different angles of incident light.

Optionally, some of the deuterium atoms in the deuterated amorphous silicon may be replaced with hydrogen atoms. As such the layer comprising the amorphous deuterated silicon may additionally comprise hydrogenated amorphous silicon. A combination of these two materials within a single layer may beneficially retain many of the advantages of a pure deuterated amorphous silicon layer while also significantly reducing manufacturing costs. In some embodiments, the probability of layer delamination is related to the relative proportions of hydrogen and deuterium in the material. In some embodiments, a deuterium concentration may be kept as low as possible to minimize manufacturing costs, but as high as necessary to maintain environmental stability.

Additionally or alternatively, the interface filter may comprise at least one layer comprising hydrogenated amorphous silicon. This layer may further comprise a combination of hydrogenated amorphous silicon and deuterated amorphous silicon with different proportions compared to the other layers of the stack.

In some embodiments, the stack comprises a plurality of alternating layers of the deuterated amorphous silicon and the dielectric material. The first or bottom layer of the stack may comprise a layer of the amorphous silicon-deuterium alloy. Alternatively, the first or bottom layer of the stack may comprise a layer of the dielectric material.

In some embodiments, the dielectric material may comprise of SiOs or AI2O3. The use of these materials as the low refractive index layer may provide a suitable refractive index difference to achieve the desired transmission characteristics.

In some embodiments, the stack may be configured to have a passband for infrared wavelengths. Optionally, the stack may be configured to have a passband for near-infrared wavelengths. Infrared wavelengths are generally between 700 nanometers and 1 millimeter, while NIR wavelengths are generally between 700 nanometers and 1400 nanometers. For example, the passband may be between 800 nanometers and 1100 nanometers. Alternatively the pass band may be in the range of 750 nanometers to 1000 nanometers.

In one embodiment, within the passband the refractive index of the deuterated amorphous silicon may be greater than 3 and the refractive index of the dielectric material may be less than 3. Optionally, the refractive index of the dielectric material may be less than 2. For example, the refractive index of the deuterated amorphous silicon may approximately 3.4 and the refractive index of the dielectric material may be approximately 1.4. It will be understood that the refractive index of the materials discussed herein is the refractive index of the materials at the relevant wavelengths of light.

Many photodetectors show a high sensitivity to light in the visible spectrum. By filtering out these wavelengths, the interference filter may advantageously increase the signal-to-noise ratio of photodetectors in infrared or NIR devices. In some embodiments, the interference filter may further comprise an overcoat layer.

The overcoat layer may comprise a resist-based material such as an organic coating or an inorganic layer. The overcoat layer may be formed over any part of the stack or the entire stack. The overcoat layer may act as an environmental barrier, further protecting the material stack from degradation in harsh environments.

Optionally, the overcoat layer may be further configured to act as a stop-band filter or a band-pass filter.

The overcoat layer may act as an additional filter to further suppress unwanted wavelengths of light, in particular wavelengths in the visible spectrum. This may allow the overcoat layer to further improve or extend the optical performance of the total optical stack. Additionally or alternatively, the overcoat layer may be index matched to any of the layers of the material stack in order to achieve similar effects.

Alternatively, the overcoat layer may be optically neutral. An optically neutral overcoat layer does not alter the transmission characteristics of the interference filter, allowing the transmission characteristics of the stack to be tailored for a specified purpose.

The overcoat layer may be formed as a conformal coating. The overcoat layer may be deposited as a thin-film. The overcoat layer may be formed by a process of replication. As such, the overcoat layer may be configured to completely encapsulate the stack.

In some embodiments, the stack may be formed on a substrate. Optionally, in some embodiment an adhesion promoting layer may be disposed between the substrate and the stack, e.g. an adhesive layer may be formed on the substrate and the stack may subsequently be formed on the adhesive layer. The substrate may comprise CMOS wafers, glass or any other proper substrate. Forming the material stack on a substrate may improve the structural integrity of the stack.

According to a second aspect of the disclosure, there is provided an radiationsensitive device comprising an interference filter for an optical device is provided, comprising a stack of layers, the stack comprising: at least one layer comprising deuterated amorphous silicon; and at least one layer of a dielectric material, wherein within a targeted transmission band a refractive index of the dielectric material is less than a refractive index of the deuterated amorphous silicon.

In some embodiments, the radiation-sensitive device may be a sensor, such as an infrared proximity sensor or a time-of-flight (TOF) sensor. Such a proximity sensor may be implemented in a smartphone, tablet device or the like. Such a proximity sensor may be implemented in an automotive system. Such a time-of-flight sensor may be implemented in a Light Detection and Ranging (LIDAR) system.

In a further embodiment, the radiation-sensitive device may be a camera, or a component of a camera. The radiation sensitive device may be implemented in an apparatus such as a smartphone, a cellular telephone, a tablet, a laptop, or the like.

According to a third aspect of the disclosure, there is provided a method of forming an interference filter, the method comprising forming a stack of layers on a substrate, wherein: at least one layer is formed of a material comprising deuterated amorphous silicon; and at least one layer is formed of a dielectric material, wherein within a targeted transmission band a refractive index of the dielectric material is less than a refractive index of the deuterated amorphous silicon.

The deuterium content of the deuterated amorphous silicon may vary and a partial replacement of deuterium atoms by hydrogen, oxygen or nitrogen may occur. Silicon atoms may similarly be partially replaced by germanium. As an example, the layer formed of the deuterated amorphous silicon may also be partially comprised of hydrogenated amorphous silicon. Optionally, the method may include forming additional layers material with different proportions of hydrogenated amorphous silicon and deuterated amorphous silicon, or layers formed materials only comprising one of these materials.

In one embodiment, the method may further comprise forming the deuterated amorphous silicon by adding deuterium to plasma during a sputtering process, e.g. a magnetron sputtering process

Layer deposition may be facilitated via various deposition methods including but not limited to sputtering, evaporation or chemical vapour deposition. Activation of a potential precursor as well as the deuterium atoms can be maintained thermally or via plasma.

In some embodiments, the method may further comprise thermal annealing the stack. The thermal anneal may be incorporated into the method as an optional post deposition treatment to achieve the final optical properties.

In a further embodiment, the method may further comprise forming an overcoat layer over the stack.

The above summary is intended to be merely exemplary and non-limiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilized, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only, and with reference to the accompanying figures, in which:

Figure 1 depicts a schematic diagram of an interference filter according to an embodiment of the present disclosure;

Figure 2 depicts a schematic diagram of an interference filter according to a further embodiment of the present disclosure;

Figures 3a-c depicts a schematic diagram of the interference filter of Fig. 1 with an additional overcoat layer, according to embodiments of the disclosure;

Figure 4 depicts a block diagram of a system incorporating an interference filter according to embodiments of the present disclosure; and

Figures 5a depicts flowcharts of methods of forming an interference filter according to an embodiment of the present disclosure; and

Figures 5b depicts a flowchart of methods of forming an interference filter according to another embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

Deuterium is an isotope of hydrogen with approximately double the atomic mass compared to a primitive hydrogen atom. Despite its lower abundance, it is frequently used for isotope markings or as a hydrogen substitute in nuclear magnetic resonance solvents. As the electronic shell of deuterium is similar to hydrogen, deuterium compounds show similar chemical properties to their hydrogen derivatives. In particular, many of the optical properties of deuterated amorphous silicon, such as the refractive index and the absorption coefficient, are similar to or the same as those of hydrogenated amorphous silicon. Therefore, due to the similarity of deuterium and hydrogen, deuterated amorphous silicon can replace hydrogenated amorphous silicon in many circumstances without any significant impact to optical performance.

In fact, the material properties of deuterated amorphous silicon are superior to those of hydrogenated amorphous silicon in some aspects. For example, and as discussed above, deuterium atoms have a lower diffusion constant than hydrogen atoms and the Si-D bond is more stable than the Si-H bond. These factors may increase the environmental stability of the deuterated amorphous silicon compared to the hydrogenated amorphous silicon, and may also reduce the blistering effect found in hydrogenated amorphous silicon layers.

Fig. 1 shows an interference filter 100 according to an embodiment of the present disclosure. The interference filter 100 is comprised of a stack of alternating high refractive index layers 104 and low refractive index layers 106, where the high refractive index layer 104 is characterised by being formed from a material with a higher refractive index value than the material forming the low refractive index layer 106. The stack may comprise any number of high and low refractive index layers 104 & 106. In some embodiments, the interference filter 100 may only include one of each of the layers 104 & 106.

The stack of the high and low refractive index layers 104 & 106 may be formed on a substrate 102. While Fig. 1 shows the first layer formed on the substrate as a high refractive index layer 104, it will be understood that the first layer may instead be a low refractive index layer 106. Each high refractive index layer 104 may be formed from the same material. Alternatively, the high refractive index layers 104 may each comprise multiple materials within a single layer. Additionally or alternatively, each high refractive index layer 104 may be formed from a different material. Each of these possibilities apply equally to the low refractive index layer 106. Moreover, The thickness of each layer may vary. For example, the skilled person will understand that based on e.g. the refractive index for the high and low refractive index materials and the targeted transmission characteristic it is possible to calculate a layer thickness sequence for a given desired design target. The substrate 102 may be formed from one or more of CMOS wafers, glass or any other suitable substrate material. The high refractive index layer 104 may comprise an amorphous silicon-deuterium alloy. The low refractive index layer 106 may comprise a dielectric material such as SiOs and/or AI2O3, or any other material with suitable optical properties. The low refractive index material may be selected to ensure that the ratio of the refractive indices of high and low refractive index layers 104 & 106 is suitable for the targeted transmission characteristic of the interference filter. For example, within the targeted spectrum of wavelengths a ratio of the refractive index values may be equal to or between 1.5:1 and 3:1. In some embodiments, within the targeted spectrum of wavelengths the high refractive index material (e.g. an amorphous silicon-deuterium alloy) may have a refractive index of between 3.4 and 3.6 while the low refractive index material (e.g. a dielectric material) may have a refractive index between 1.3 and 1.5. More generally, the high refractive index material will have a refractive index over 3 while the low refractive index material will have a refractive index below 3. The deuterium content of the deuterated amorphous silicon can vary and a partial replacement of deuterium atoms by either hydrogen, oxygen or nitrogen can occur. Silicon atoms can be partially replaced by germanium. However, it will be understood that the refractive index of the materials will vary with the wavelength of light.

In some embodiments, the layer sequence may be selected in such a way that the transmission characteristic exhibits a stop band in the UV/VIS region while facilitating a pass band in the NIR/IR region. For example a stop band for wavelengths shorter 800 nanometer and a passband longer 810 nanometer, which is often used for NIR detection in proximity sensors. Alternatively, the interference filter may offer a peak-shaped transmission characteristic, having stop bands at shorter and longer wavelengths while maintaining transmission at a certain wavelength band in between. For example, a typical silicon photodiode may be sensitive to radiation between 300 nanometer and 1100 nanometer wavelength. Therefore, the layer stack may be configured in a way to enable transmission above 950 nanometer up to 1100 nanometer, while facilitating blocking below 950 nanometer, or enabling transmission between 935 nanometer and 945 nanometer while facilitating blocking below 935 nanometer and above 945 nanometer. For example, such layer stacks may be implemented in filters used for time-of-flight or LIDAR applications. Alternatively, the layer stack may be configured to enable transmission over larger ranges of wavelengths, for example between 750 nanometers and 1100 nanometer or 800 nanometer and 1100 nanometer.

Fig. 2 shows an interference filter 200 according to a second embodiment of the present disclosure. Many of the features of Fig. 2 correspond to the features of Fig. 1 , and the same reference numerals are used for these features. Relative to the interference filter 100 of Fig. 1 , the interference filter 200 of Fig. 2 is characterised by the inclusion of an adhesion promoting layer 202 formed between the substrate 102 and the layers of the material stack 104 & 106. The adhesion-promoting layer 202 may be, for example, a layer of titanium oxide or silicon oxide. The adhesion-promoting layer 202 may strengthen the bond between the stack of high and low refractive index layers 104, 106 and substrate 102.

Fig. 3a shows an inference filter 300 comprising of a stack of layers 304 and an additional overcoat layer 302. The stack of layers 304 may be formed from e.g. the interference 100 of Fig. 1. While the interference filter 100 of Fig. 1 is shown in this figure, it will be understood that this is merely an example, and the overcoat layer 302 may equally be formed over the interference filter 200 of Fig. 2. Alternatively, as shown in Fig. 3b, the overcoat layer may be formed such that it does not cover the substrate or the substrate may extend horizontally beyond the edge of the material stack such that the overcoat is formed above the substrate, as shown in Fig. 3c. The overcoat layer 302 may consist of a polymer-based or photoresist material. For example, layer 302 may comprise an organic material and/or an inorganic material. Additionally, the overcoat layer 302 may improve or extend the optical performance of the interference filter by adding an additional stop-band suppression or index matching. For example, the overcoat layer 302 may be configured to act as a pass-band filter for wavelengths between 700 to 1400 nanometers or a stop-band filter for wavelengths between 100 and 700 nanometers. Alternative, narrower or wider bands may be selected instead depending on the application of the optical filter 300. However, the overcoat layer 302 may instead be optically neutral. The upper surface of the overcoat layer 302 may be substantially parallel to the upper surface of the stack of layers 304 in order to reduce or minimize the optical interference of the overcoat layer 302 on the stack, for example by providing a defined interface to the incoming light n. Similarly, the overcoat layer 302 may taper towards its upper surface, as shown in Fig. 3a. Beneficially, this may assist in further processing steps, such as bonding or resist coating. Alternatively, the sides of the overcoat layer may be substantially parallel to the sides of the stack of layers 304. Furthermore, the overcoat 302 may act as an environmental barrier, protecting the material stack 100 from degradation in harsh environments. Fig. 3a further shows a beam of incident light 306. Wavelengths of light 308 which are outside of the transmission band are reflected away from the filter. As a result, only the targeted wavelengths from the incident light beam 306 reach the substrate.

Fig. 4 shows a block diagram of an interference filter 404 implemented in a device 402. The device 402 may optionally be integrated into a system or apparatus 400. Interference filter 404 may comprise any of the interference filters described above. The device 402 may for example be an infrared proximity sensor implemented in an apparatus 400 such as a smartphone or a tablet device, or a system 400 such as an automotive system. Alternatively, the device 402 may be a time-of-flight (TOF) sensor implemented in a Light Detection and Ranging (LIDAR) system 400. It will be understood that interference filter 404 may also be implemented in other devices and/or systems, for example device 402 may be a camera or component of a camera, and apparatus 400 may be a camera, smartphone, a cellular telephone, a tablet, a laptop, or the like.

Other applications for the interference filter 404 include any other Infrared or Near Infrared application, for example an infrared gesture control system, a 3D-imaging system or an infrared night vision system. The interference filter 404 may be incorporated into any device 402 suitable for these applications.

Fig. 5a is a flow diagram 500 showing an example of a method of manufacturing an interference filter according to the present disclosure. In step 502 a layer of the high refractive index material (such as an amorphous silicon-deuterium alloy) is formed on the substrate. The layer deposition can be facilitated via various deposition methods including but not limited to sputtering, evaporation or chemical vapour deposition. The activation of a potential precursor as well as the deuterium atoms can be maintained thermally or via plasma. In step 506 a layer of low refractive index material (such as SiOs or AI2O3) is formed above the high refractive index layer. The low refractive index layer may be deposited by the same deposition method as the high refractive index layer or by a different deposition method. Steps 504 and 506 may be repeated until a desired number of layers is reached, with subsequent layers of the high and low refractive index materials being deposited above the previously deposited layers. It will be understood that steps 504 and 506 may be reversed, with the first layer being a low refractive index layer deposited on the substrate.

Fig. 5b is a flow diagram 510 showing a further example of steps for a method of manufacturing the interference filter including optional steps 512, 518 and 520. In the optional step 512 a layer of adhesive promoting material such as an adhesive may be deposited on a substrate. Steps 514 and 516 of Fig. 5b correspond to steps 502 and 504 of Fig. 5a. After the deposition of the high and low refractive index layers in the stack in steps 514 and 516, an optional post deposition treatment may be applied to the stack in step 518. This treatment may be, for example, thermal annealing the stack in order to achieve the final optical properties. In optional step 520, an additional overcoat layer may be formed over the stack of layers and/or over the stack of layers and the substrate. The overcoat layer may be deposited by any of the deposition methods discussed above, as well as any other methods known in the art. As part of the formation of the overcoat layer, various methods may be used to shape the overcoat layer. For example, the overcoat layer may be ground, polished or etched after deposition such that the upper surface of the overcoat layer is substantially parallel to the upper surface of the stack of layers. Additionally or alternatively, the overcoat layer may be formed via methods such as moulding or spin coating in order to achieve the desired shape for overcoat layer. It will be understood that the order of optional steps 518 and 520 may be reversed. Optionally, the interference filter may be patterned during the deposition steps

514 and 516 or after the deposition as part of the post deposition treatment in step 518.

It will be understood that the above description is merely provided by way of example, and that the present disclosure may include any feature or combination of features described herein either implicitly or explicitly of any generalisation thereof, without limitation to the scope of any definitions set out above. It will further be understood that various modifications may be made within the scope of the disclosure.

LIST OF REFERENCE NUMERALS:

100 Interference filter

102 Substrate

104 High refractive index layer

106 Low refractive index layer

200 Interference filter

202 Adhesion promoting layer

300 Interference filter

302 Overcoat layer

304 Stack of layers

400 System or apparatus

402 Infrared or Near-Infrared device

404 Interference filter

500 Flow chart of method of manufacturing

502, 504: Manufacturing steps of flow chart 500

510: Flow chart of method of manufacturing

512, 514, 516, 518, 520: Manufacturing steps of flow chart 510

Claims

CLAIMS:

1 . An interference filter (100) for an optical device comprising a stack of layers, the stack comprising: at least one layer comprising deuterated amorphous silicon (104); and at least one layer of a dielectric material (106), wherein within a targeted transmission band a refractive index of the dielectric material (106) is less than a refractive index of the deuterated amorphous silicon (104).

2. The interference filter of claim 1 , wherein the at least one layer comprising deuterated amorphous silicon further comprises hydrogenated amorphous silicon.

3. The interference filter of claim 1 or claim 2, wherein the stack further comprises at least one layer comprising hydrogenated amorphous silicon.

4. The interference filter of any preceding claim, wherein the stack of layers is configured to have a passband for infrared wavelengths, optionally wherein the stack of layers is configured to have a passband for near-infrared wavelengths.

5. The interference filter of claim 4, wherein, for wavelengths within the targeted transmission band at least one of: the refractive index of the deuterated amorphous silicon (104) is greater than 3; and the refractive index of the dielectric material (104) is less than 3.

6. The interference filter of any preceding claim, wherein the at least one layer of dielectric material (104) comprises one or more of SiOs and AI2O3.

7. The interference filter of any preceding claim, further comprising an overcoat layer (302).

8. The interference filter of claim 7, wherein the overcoat layer (302) is configured to act as a stop-band filter or a band-pass filter.

9. The interference filter of claim 7 or 8, wherein the overcoat layer (302) is index matched to at least one layer of the stack of layers.

10. The interference filter of claim 8, wherein the overcoat layer (302) is optically neutral.

11. The interference filter of any preceding claim, wherein the stack of layers is formed on a substrate (102), optionally wherein an adhesion promoting layer (202) is located between the substrate (102) and the stack of layers.

12. A radiation sensitive device (402) comprising the interference filter (100) of any preceding claim, and optionally wherein the radiation sensitive device (402) is a proximity sensor or a time-of-flight (TOF) sensor, optionally wherein the TOF sensor is implemented in a Light Detection and Ranging (LIDAR) system (400).

13. A method (500) of forming an interference filter (100), the method comprising forming a stack of layers on a substrate (102), wherein: at least one layer is formed (502) of a material comprising deuterated amorphous silicon (104); and at least one layer is formed (504) of a dielectric material (106), wherein within a targeted transmission band a refractive index of the dielectric material (106) is less than a refractive index of the deuterated amorphous silicon (104).

14. The method of claim 13, further comprising forming the at least one layer of amorphous deuterated silicon (104) by adding deuterium to plasma during sputtering.

15. The method of claim 13 or 14, further comprising thermal annealing (518) the stack of layers.