Patent Application
20210255543
This specification generally relates to optical colour filters, particularly but not exclusively, to multi-spectrum colour filters having three-dimensional physical structures, and their fabrication methods.
Converting optical information (light) to electronic information (electrons) lies at the heart of every digital image sensor. Complementary metal-oxide-semiconductor (CMOS) image sensors which are cheap, compact and efficient are now considered ubiquitous. CMOS sensors are implemented in a range of applications from digital photography to medical imaging. Typically, the image sensor is composed of millions of individually addressed silicon photodetectors. To detect colour (a specific optical wavelength), spatially variant spectrally distinct optical filters are required to be used in combination with the CMOS sensors. These colour filter arrays (CFAs) possess mosaic-like patterns, with pixel sizes comparable to the individual CMOS sensor dimensions, and which tessellate atop the image sensor.
Colour filter arrays (CFAs) are critical thin-film optical components used extensively for image sensors. Further alternative uses for such CFA or MSFA filters exist, for example, the direct illumination of a target to be imaged. In the known state of the art, CFAs are typically comprised of either pigment-based filters or multi-layer stacks implemented for colour filtering. Both require a variety of materials in various combinations in order to achieve wavelength discrimination within the filter. Both of these known filters also require a relatively thick filter to achieve a desirable efficacy in wavelength discrimination. Furthermore, multiple successive lithographic steps may typically be required in fabrication, dependent on the number of wavelength bands required in the colour filter.
These color filter arrays (CFAs) are deposited in mosaic-like patterns atop the image sensor with a pitch matched to the pixel size The most widespread CFA is the Bayer filter which includes red, green and blue (RGB) filters. However, more complex mosaics incorporating additional spectral filters are commonplace in multi-spectral imaging systems (for example: Lapray et al., Sensors (Switzerland) 2014, 14, 21626-21659). Referred to as multi-spectral filter arrays (MSFAs), these optical elements are generally commonplace in multiple fields of imaging applications ranging from agriculture to medical diagnostics, for example.
For conventional CMOS image sensors, the CFAs/MSFAs are typically composed of either absorptive dyes or pigments, having one dye or pigment for each colour. Alternatively, a filter may be composed of a many-layer one-dimensional Bragg stack, in which a different combination of alternating dielectric materials corresponds to each colour. However, both compositions and methods are cumbersome from a fabrication point of view. For example, for a filter having N wavelengths, N separate lithographic (or N hard mask) steps are required; one for each wavelength. Additionally, for an N wavelength filter with N material compositions, either dyes or varying combinations of alternating dielectrics in the Bragg stack are required. With carefully aligned lithographic steps required for CFA fabrication, the continual shrinking of pixel dimensions for higher resolutions, and more complex mosaic patterns to exploit added wavelength bands, the typically-used methodology is highly problematic. Moreover, due to the established CFA fabrication techniques, there is a sizeable financial cost associated with producing custom MSFAs with tailored optical characteristics.
It is further known in the art that metal-insulator-metal (MIM) geometries may provide the basis for CFAs. The MIM optical filters' material compositions can be deposited in the same processing step. However, typically in the known art, each thickness of each layer is fixed. As a result, MIM filters are typically fabricated through iterative ‘step-and-repeat’ processes, which limits their use in spatially variant MSFA applications.
Conventional cameras, such as in smartphones, capture wideband red, green and blue (RGB) spectral components, replicating human vision. Multispectral imaging (MSI) captures spatial and spectral information beyond our vision but typically requires bulky optical components and is expensive. Snapshot multispectral image sensors have been proposed as a key enabler for a plethora of MSI applications, from diagnostic medical imaging to remote sensing. To achieve low-cost and compact designs MSFAs based on thin-film optical components may be deposited atop image sensors. Conventional MSFAs achieve spectral filtering through either multi-layer stacks or pigment, requiring: complex mixtures of materials; additional lithographic steps for each additional wavelength; and large thicknesses to achieve high transmission efficiency.
Alternative popular methodologies for colour generation exist, which involve ultrathin plasmonic and high-index dielectric nanostructure arrays, whereby electric and magnetic resonance respectively can be excited (though geometry and material selection) which are wavelength and polarisation selective. However, these techniques still suffer from either low transmission efficiencies and/or broad full-width-half-maximums (FWHMs), i.e. poor wavelength selectivity. These features also render these methodologies unsuitable for multi-spectral imaging technologies.
Therefore, there remains a need in the art to provide a cost-effective and efficient method of fabrication of MSFA/CFAs involving only a single lithographic step, and which produces devices with improved optical wavelength selectivity, and improved transmission efficiencies. The following summary and detailed examples describe a single-step grayscale lithographic process that enables wafer-level fabrication of bespoke MSFAs based on Fabry-Perot type resonances of spatially variant metal-insulator-metal (MIM) cavities, where the exposure dose controls insulator (cavity) thickness.
According to one aspect of the present disclosure, there is provided a method for producing an optical filter, the method comprising: depositing a first mirror layer on a substrate; depositing an insulating layer on the first mirror layer; exposing at least some of a plurality of portions of a surface of the insulating layer to a dose of energy; developing the insulating layer in order to remove a volume from the at least some of the plurality of portions of the insulating layer, wherein the volume of the insulating layer removed from each portion is related to the dose of energy exposed to each portion, and wherein a remaining thickness after the removal of the volume from each portion of the insulating layer is related to the dose of energy exposed to each portion; depositing a second mirror layer on the remaining thickness of each of the plurality of portions of the insulating layer such that the remaining thickness of each of the plurality of portions of the insulating layer define a profile of the optical filter.
Advantageously, this method may be used for fabrication at the wafer level, and provides an optical performance and customizability which surpasses conventional nano-photonic methods. In particular, in the above method, ultra-high resolution in-plane patterning is obviated, unlike in nano-photonic counterparts.
It will be understood that each mirror may be partially optically reflective, and may also be deposited in a uniform thickness. The method also comprises exposing at least some of a plurality of portions of a surface of the insulating layer to a dose of energy, where it will be understood that it is possible that all portions of the surface of the insulator may be exposed, and also that only a select few of the portions of the surface of the insulator may be exposed. These portions of the surface of the insulator may also be referred to as pixels in this disclosure. It will further be understood that the dose of energy may be a chemically activating dose of energy, in that it may induce a chemical change in the insulator or resist material. The substrate may be a transparent layer. In other examples, the substrate may be an image sensor itself, onto which the filter may be directly disposed and/or fabricated.
The method may comprise developing the insulating layer in order to remove a volume from said at least some of the plurality of portions of the insulating layer. In other words, only the certain portions of the insulator may be developed. The volume of the insulating layer removed from each portion may be related to the dose of activating energy exposed to each portion (or pixel). Depending on the type of insulator material used, the volume removed may be roughly proportionally or roughly inversely related to the dose of activating energy exposed to each portion. It will further be understood that, corresponding to the removed volume, a remaining thickness of the insulating layer (after the removal of the volume from each portion of the insulating layer) may also be related to the dose of activating energy (or the total energy) exposed to each portion. The dose of activating energy may be a variable dose of energy, wherein the dose may be varied for each of exposed portions (that is, pixels) of insulating layer.
Furthermore, the developing of the insulating layer to remove the volume from the at least some of the plurality of portions of the insulating layer may comprise chemically developing the insulating layer, wherein the volume removed from said at least some of the plurality of portions of the insulating layer becomes chemically dissolved. The chemically dissolved layer may thus be washed away as part of the chemical development.
The method may further comprise depositing a second mirror layer on the remaining thickness of each of the plurality of portions of the insulating layer such that the remaining thickness of each of the plurality of portions of the insulating layer may define a profile of the optical filter. It will be understood that the remaining thickness is, in other words, the remaining surface of the insulator after having been developed and, after volumes have been removed from each of the exposed portions (applicable to positive or negative resist tone) of the insulating layer.
In one example, the remaining thickness after the removal of the volume from each portion of the insulating layer may be achieved by using a single step lithographic process. Therefore, it will be understood that the ability to perform an efficient single-step lithographic step bears many advantages, for example, any one or all of: lower cost, more efficient fabrication, and a very high level of device versatility and customisability. Generally, the dose of energy, for example an electron beam, may be modulated to produce a grayscale profile of arbitrary patterning, thus allowing for an advantageously efficient single-step process to produce an optical filter. The optical filters which result from such single step dose-modulated/grayscale lithographic methods may proceed MSFA filter of arbitrary complexity with the equivalent manufacturing time as simple filters, e.g. conventional Bayer filters.
In another aspect, the method of fabricating the remaining thickness after the removal of the volume from each portion of the insulating layer may be achieved by using a grayscale lithographic process. It will be further understood that, due to the versatility and precision available using the grayscale lithographic process, it may effectively allow for increasingly small and precise pixels to be fabricated in the device, resulting in an advantageously high resolution.
It will be understood that, resulting from for producing an optical filter, the remaining thickness of each portion of the insulating layer may define a two-dimensional profile of optical wavelengths, wherein the two-dimensional profile may be the an in-plane spatially varying colour profile transmitted through the optical filter. That is, the profile of remaining thicknesses of the plurality of portions of the insulating layer may produce, when incident light hits the optical filter, a corresponding profile of colours over a 2D area. Therefore, it will be further understood that the insulating layer may be optically transmissive, optically transparent, or at least optically translucent. The insulator may further be deposited in a uniform thickness. For the sake of clarity, it will be understood that the resist layer, insulator layer and resist/insulator cavity all refer to the same feature of the optical filter. It will also be understood that the term cavity does not refer to an empty space, but rather refers to the insulator/resist, which may be disposed in-between the first and second mirror layers. Advantageously, this versatile approach may require only a single lithographic processing step, and the same materials may be used for each for each wavelength band of the optical filter, making the fabrication process and resultant optical filter highly customizable.
In one example, the remaining thickness of each portion of the insulating layer (in other words, each pixel) may define a spectral (i.e. wavelength) position of the transmission peak. Further, the spectrum of light transmitted through each portion of the insulating layer (that is, transmitted through each pixel) may correspond to the spectral position. In other words, the light ultimately transmitted through each pixel may exhibit a characteristic optical wavelength profile, or range of wavelengths/colours, which in turn may correspond to the thickness of the insulator cavity in that pixel. It should be appreciated that the spectrum of light ultimately transmitted through the filter is not restricted to lying in the visible spectrum, but may extend to the near-infra-red (NIR), infra-red (IR), and ultra-violet (UV) spectrum of light. Similarly, the term optical used to describe the filters is intended to include at least the NIR, IR, and UV spectrum in addition to the visible electromagnetic spectrum of wavelengths.
Generally, it will be understood that the first mirror layer may be partially optically reflective and possesses a first uniform thickness, and also the second mirror layer may be partially optically reflective and may also be disposed in a uniform thickness.
In another example, the thickness of the first mirror layer may be varied. That is, a thicker, or narrower, first mirror layer may be disposed onto the substrate. When the rest of the device is fabricated, in which the first mirror bears the insulator and the second mirror, it will be understood that the thickness of the first mirror layer may define the breadth of the transmitted spectrum of light through each portion of the insulating layer. In other words, a thicker lower (first) mirror lay may result in a narrower, or more specific, spectrum of light being transmitted through that pixel. It will further be appreciated that a narrower spectrum may also be defined as a smaller full-width-half-maximum (FWHM). Therefore, it will also be apparent that, in correspondence with the above, a narrower first mirror layer may result in a broader spectrum of transmitted light at each pixel.
As discussed, the method may comprise exposing the insulator to a chemically activating dose of energy. In one example, the insulating layer may chemically strengthen upon being exposed to the dose of energy. For example, the resist may be an energy sensitive polymer, which may become crosslinked upon exposure to the activating dose of energy. The degree of strengthening, or crosslinking, in the polymer may alter the resultant solubility of the insulator (or cavity). Therefore, when the insulator is exposed to a chemical developer solution, the volume of the insulating layer removed from each portion may be related to the altered solubility of the insulator. In other words, the remaining thickness of insulating layer from each portion may be proportional to the dose of energy exposed at each portion. It will be apparent that this regime comprises a negative-tone resist polymer.
In one example, the insulating layer may chemically weaken upon being exposed to the dose of energy. For example, the resist may be an energy sensitive polymer, which may become chemically degraded upon exposure to the activating dose of energy. In other words, the remaining thickness of insulating layer from each portion may be inversely-proportional to the dose of energy exposed at each portion. It will be understood that this regime may comprise a positive-tone resist polymer.
It will be appreciated that the method of using the grayscale lithographic process may comprise using a beam of energy. Further, the beam of energy may be varied for the at least some of the plurality of portions. In examples, the beam of energy may comprise a beam of electrons, or the beam may comprise photons, for example a laser. It will nevertheless be appreciated that any other suitable chemically activating beam of energy may be used. For example, other lithographic techniques could be used, such as a mask-less technique including a direct write ultraviolet (UV) laser lithography (e.g. laser write), DMD (digital micro-mirror device) based lithography. In other examples, a mask-based lithography, e.g. photolithography can be used.
In an alternative example, the method may further comprise providing a mask over the insulating layer. The method may also comprise exposing the mask to a dose of chemically activating energy. For example, the dose of energy incident on the mask may be a uniform dose of energy across the surface of the mask. Additionally, the method may further comprise providing a mask over the insulating layer, wherein the dose of energy which exposes at least some of a plurality of portions of a surface of the insulating layer is transmitted through the mask.
Again, the dose of energy may be a chemically activating dose of energy. It will be understood that the mask may comprise multiple portions, where each portion may possess a variable opacity. This variable opacity may attenuate the uniform dose of activating energy to a varying degree, such that a plurality of variably attenuated energy doses may be exposed to the insulating layer. In other words, the mask may comprise a plurality of portions with each having a degree of opacity, wherein each portion of the mask attenuates the uniform dose of chemically activating energy according to said portion's degree of opacity, such that a plurality of attenuated energy doses are exposed to the insulating layer.
The method may further comprise: providing an attenuating mask over the insulating layer, the attenuating mask comprising a plurality of portions which defines an attenuation profile, wherein the dose of energy which exposes the surface of the insulating layer is transmitted through the mask and attenuated according to the attenuation profile.
The plurality of portions of the attenuating mask may possess at least two different levels of opacity, and one of the levels of opacity may be opaque or substantially opaque. That is, the attenuating mask may be a binary mask comprising opaque regions which substantially do not transmit the dose of energy, and transparent portions. The transparent portions may be arranged periodically across the mask, i.e. each separated by a uniform and repeating dimension.
The method may further comprising laterally translating the mask over the insulating layer and exposing the surface of the insulating layer to a second dose of energy, wherein the second dose of energy is transmitted through the mask and attenuated according to the attenuation profile. It will be appreciated that further lateral translations and exposures to further doses of energy may be applied.
Advantageously, in accordance with the above mask method, a grayscale profile may be attained with only a binary mask. For example, a mask comprising only opaque or transparent regions which is translated and subjected to a second dose of energy, may provide an insulating layer with three different levels of exposure, and thus 3 different resultant transmission wavelengths after development.
Further advantageously, the mask-based method of fabricating the optical filter may be performed on a larger scale, and may be produced on the wafer-level of in image sensor, and/or directly in conjunction with commercial CMOS sensors.
The mask-based method, described with reference to detailed examples in the following, has the advantage over known techniques that the present process uses a single (grayscale) lithographic step. Known (i.e. classical) lithography processes typically repeat a lithographic step many times, and possess little to not modularity/flexibility in filter design. By contrast, examples of dose/energy modulated methods described herein provide for an arbitrary range of filter designs due to the ease with which filter designs can be modulated.
It will be readily understood that the opacity of the mask refers to multiple portions (or pixels) of the mask which may each be opaque, or transparent, to varying degrees. That is, the opacity refers to the proportion of incident light that may be transmitted through the mask. Therefore, it will be apparent that the variable opacity of the plurality of portions of the mask may define the remaining thickness of each of the plurality of portions (that is, the remaining thickness of the pixel) of the insulating layer.
The feature of using the mask may be referred to as a photolithography process. This mask-based process generally involves an energy beam comprising photons, though may alternatively comprise an electron beam. The method involving the mask may further comprise chemically developing the insulating layer, in which a variable volume from the at least some of the plurality of portions of the insulating layer may become chemically dissolved and removed from each of the plurality of portions of the insulating layer. It will again be understood that the remaining thickness of the insulator may be the result of this development step, which may involve the exposure to a chemical development solution and/or de-ionized water. Therefore, in general, it will be understood that the three-dimensional optical filter device able to be fabricated may be identical when fabricated using either the grayscale lithography process or using the photolithography and mask process. It will be understood that both methods fundamentally include a single lithographic step.
In an alternative example of the method, we disclose a method which further comprises depositing a further type insulating layer over the first mirror layer. This further type insulating layer may be a more robust or resilient material, for example any variety of glass, such as quartz.
The method may further comprise depositing an insulating/resist layer on the further type insulating layer. The method may comprise exposing the at least some of the plurality of portions of the insulating layer to the dose of energy, and may further involve etching the remaining thickness of each of the plurality of portions of the insulating layer. Following this, the method may comprise developing (chemically developing as described, or other suitable development procedure) the further type insulating layer. Etching the more robust, further type of insulating layer may remove a volume from at least some of the plurality of portions of the further type insulating layer. The etching may be a dry etch and comprise heavy ion bombardment (reactive ion etching), or in other examples may comprise a wet (chemical) etch such as hydrofluoric acid.
However, the reactive ion etching step (bombardment of ionised particles) may act as a combined exposure and development step, in which the bombardment may comprise physically etching the robust insulator surface. As in other examples, the method may comprise depositing the second mirror layer on the further type insulating layer.
Another example of the method of producing an optical filter is disclosed, which comprises providing a stamping block. The method comprises: providing a stamping block; depositing a first insulating layer on the stamping block; exposing at least some of a plurality of portions of a surface of the first insulating layer to a dose of energy; and developing the first insulating layer in order to remove a volume from said at least some of the plurality of portions of the first insulating layer, wherein the volume of the first insulating layer removed from each portion is related to the dose of energy exposed to each portion, and wherein a remaining thickness after the removal of the volume from each portion of the first insulating layer is related to the dose of energy exposed to each portion. The method also comprises etching the remaining thickness of each of the plurality of portions of the first insulating layer; and wherein the step of etching the remaining thickness removes a volume from at least some of the plurality of portions of the stamping block.
The method may further comprise: depositing a first mirror layer onto a substrate; depositing a second insulating layer on the first mirror layer; applying the stamping block on the second insulating layer to imprint a pattern of the stamping block on the second insulating layer so that portions with variable thicknesses are formed in the second insulating layer. Finally, the method may comprise depositing a second mirror layer on each of the portions with variable thicknesses formed in the second insulating layer such that the second insulating layer defines a profile of the optical filter.
It will be understood that this stamping block may also be comprised of a robust or resilient material. It will be understood that a robust material may be able to withstand the effects of a chemically activating dose of energy, but may be etched by more heavy-duty methods, for example with bombardment with ionised particles. For example, the activating dose of energy may be an electron beam, which may only have sufficient energy to activate the resist/insulator layer, but not the stamping block layer. A photolithographic technique may alternatively be used, i.e. using a beam of photons and possible including a mask to attenuate the beam of photons.
It will be understood that the volume of the further insulating layer removed from each portion may be related to the dose of activating energy exposed to each portion, and thus the remaining thickness after the removal of the volume from each portion of the further insulating layer may be related to the dose of activating energy exposed to each portion. For the etching, a dry etching procedure may be used which may bombard the portions of the further insulating layer to positively charged Argon (Ar) atoms, or in other examples a wet (chemical) etch may be used. It will be further understood that the bombardment of ions may be delivered as a uniform dose exposure. The method may ultimately comprise developing the stamping block, in which a volume may be removed from at least some of the plurality of portions of the stamping block.
After the fabrication of the stamping block, which possesses a profile of varying thickness etched into its surface, it will be understood that resultant stamping block may form a master stamping dye. Therefore, one example of the method may further comprise applying the (etched) stamping block (also known as the master stamping dye) on the insulating layer. Doing so may imprint a remaining thickness of each of the plurality of portions of the insulating layer, corresponding to the pattern/profile of thicknesses which may be present on the surface of the stamping block after the etching/ion bombardment. It will be further apparent that, in order to effectively imprint a profile of the remaining thickness of each of the plurality of portions of the insulating layer, the stamping block may be applied by using additional pressure and/or heat.
In another example of the device and method, the mirror layers may be comprised of a metal, which is optionally an inert/unreactive metal, and/or a dielectric material. For example, the metal may be Aluminium, or silver (Ag), and may be disposed in a very thin layer (for example, under about 30 nm). In yet further examples of the method and device, one or more of the mirrors may be patterned, or pre-patterned. The patterning may comprise imparting a different nanostructure of the mirror layer, which may in turn impart a further characteristic, for example polarisation dependence, to the transmitted spectrum of light through each portion of the insulating layer.
It should be understood that any of the described aspects of the method may further comprise depositing a capping or encapsulation layer onto the second mirror layer. The capping or encapsulation layer may be added in order to impart additional mechanical and/or chemical stability into the device. Further in the interest of improving the optical properties of the device, any of the aspects of the method may further comprise (where the insulator is a polymer) heating the fabricated filter above a threshold temperature. This temperature may be the glass transition temperature of the polymer. Performing the heating may improve the smoothness of the surface of the polymer, which may advantageously increase the optical properties (e.g. the transmission efficiency) of the filter.
In another example, there is provided an optical filter device comprising: a substrate; a first mirror layer disposed on the substrate; an insulating layer having a plurality of portions, at least some of the portions having a variable thicknesses; a second mirror layer disposed on the insulating layer. The plurality of portions of the insulating layer is manufactured using the method discussed above.
These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:
This specification describes methods of fabricating a multi-spectrum optical filter device, including grayscale and other single-step lithographic techniques, and the structure and properties of the corresponding device. Using approaches described by way of examples in the following description, it is possible to achieve multispectral imaging of several spectrally distinct target using our bespoke MIM-MSFAs fitted to a monochrome CMOS image sensor. The unique framework described provides an attractive and advantageous alternative to conventional MSFA manufacture, and metasurface-based spectral filters, by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability.
In summary, there is presented in the following descriptive examples a unique approach for producing high efficiency, narrowband, highly customizable MSFAs operating across the visible to NIR using a single lithographic processing step (grayscale-to-color), including the possibility for wafer-level fabrication. A grayscale dose matrix is utilized to generate customizable insulator thickness profiles in MIM geometries producing optical filters spanning the UV-visible-NIR electromagnetic spectrum.
Generally referring to
Preferably, the mirror layers 108, 112 may be made of metal (which may be an inert/unreactive, or noble, metal) which is further preferably disposed as an ultrathin (under around 50 nm, for example) layer. In preferred examples, this metal will be silver, and it may be deposited using physical vapour deposition (for example, evaporation, sputtering etc.), or chemical vapour deposition. Each layer of silver will preferably be between about 20 and 30 nm in width, and ideally about 26 or 27 nm in width. However, the mirror layers may alternatively be made of optically stacked layers of dielectric material. In either scenario, the mirror layers 108, 112 will be sufficiently translucent to allow the incident light through and into the resist cavity 110, but sufficiently optically reflective in order to put in to effect the transmission of only certain wavelengths of light.
Preferably, mirror-insulator-mirror structures can be used to excite optical eigenmodes 718 (see
Further advantageously, the further layer 106 acts as a capping layer which imposes a minimal, if not improved, effect on the optical properties of the filter 100. It will be understood by the skilled person, nonetheless, that not all of these layers may necessarily be present in order to achieve a fully and high efficiency operable MSFA structure. Furthermore, additional layers may exist in other alternative fabrication processes (for example methods 1000, 1100 of
As discussed, the resultant thickness 102 of the resist in the cavity 110, after having been developed by the single-step grayscale lithography, ultimately determines the output colour profile of the filter array.
The present disclosure teaches of an improvement to the known fabrication techniques of MIM structures and MSFA devices in general. This improvement comprises, in part, a grayscale lithography process. Grayscale lithography is a single-step lithographic process in which in-plane spatially variant three-dimensional information can be imparted into a photoresist through a variable energy exposure. The exposure controls the local solubility of the resist and therefore, during resist development, the remaining resist thickness depends on the total energy delivered to the volume of the resist. By determining resist sensitivity (remaining resist thickness vs. dose) a particular grayscale energy dose pattern results in a particular 3D resist profile. Advantageously, this single-step lithographic process allows fabrication of particular 3D resist profiles, which are highly versatile and readily customisable.
The filter resulting from the exposure in structure 206 (in the second manufacturing step 210), and the subsequent development, possesses multiple remaining resist thickness 202, defining pixels, where the pixels are directly adjacent to one another. At a third step 212, the second mirror layer (as in 108 of
It will be understood by the skilled person that in this example, a negative-tone resist is being used which strengthens upon exposure to the dose of energy. In another example, a positive-tone resist may be used which weakens upon exposure to an dose of energy. In this another example, a high (white) energy dose would result in a thin resist thickness 202. For the negative-tone resist,
A further post-fabrication step may be used after having constructed the MIM structure. The completed device, which may be an MSFA or CFA filter for example, can be heated or baked past the glass transition temperature of the resist/insulator. Performing this bake softens the resist, which may create a smoother surface. This smoother surface persists once the device is cooled after having been baked. The smoother surface may subsequently improve the optical performance characteristics of the filter, for example, by increasing the overall transmission efficiency through the layers. This technique, called resist thermal reflow, is described in more detail in later passages.
The advantages of applying the grayscale lithography to produce the MIM structure are highlighted in this disclosure. In particular, it is possible to produce highly efficient MIM CFAs, which may be disposed on any suitable substrate including glass or directly onto image sensors. Therefore, such CFA (or MSFA) filters may be used to image multi-spectral test scenes when used in combination with a conventional CMOS image sensor. The resist thickness produced as a result of the grayscale lithography, which is used as the insulator (cavity) material, is determined by exposure energy. It will be appreciated that this is applicable to the filters atop of any electronic image sensor (CCD-based, CMOS-based, sCMOS-based) either fabricated directly on top of or bonded to. It will be understood that ‘multi-spectral test scenes’ are for imaging in general with an intention of spectrally discriminating the scene's information i.e. from a conventional RGB based filter array/sensor, up to any kind of multispectral array. For example, the end applications could be diverse, e.g. imaging biological tissue, imaging chemical mixtures, and many others as applicable.
Grayscale electron beam lithography (G-EBL) may be used to spatially vary the insulator (or resist) layer, where the insulator is disposed onto a substrate 211 (see
G-EBL is a technique capable of generating three-dimensional (3D) resist profiles through dose-modulated exposure schemes. For example, in
In one example, the material of the insulator may be a negative-tone e-beam resist material such as ma-N 2400 series. This resist material possesses a high resolution capability for use in G-EBL, which effectively allows for increasingly small and precise pixels in the mosaic. Further advantageously, the resist possesses a relatively high sensitivity. It will be understood by the skilled person that a ‘Negative’ resist has the property that it is chemically strengthened upon exposure to a chemically activating dose of energy, such as an electron beam of sufficient intensity. Specifically, the internal chains of the polymer material become cross-linked upon exposure to energy, which makes it more resilient to removal. As such, a variable dose of energy may be exposed to very specific portions of the resist material in order to generate a complex profile of resist heights. Following exposure by the variable doses of energy beam to a plurality of portions of the resist surface, the portions of resist material which have not been sufficiently strengthened by the beam may be removed/dissolved by some development process; for example, dissolved or washed away using a chemical solvent (or any suitable chemical development solution), optionally followed by a further wash with deionized water.
The amount of resist material subsequently remaining corresponds (proportionally, in the case of negative-tone resist) to the dose of energy received at each portion. This correspondence is sometimes called the resist sensitivity, and a remaining resist thickness vs dose profile 308 (or contrast curve) may be predetermined prior to fabrication for each resist material (see
It is possible to use the spatially variant CFAs, or MSFAs, in combination with monochrome CMOS (complementary metal-oxide semiconductor) images sensors, for multi-spectral imaging of a variety of spectrally distinct targets. MIM structures for optical filters bear the advantage that they possess highly efficient filtering characteristics. In other words, they allow multi-spectrum and selective narrow-band filtering of light, whilst allowing a majority of the desired wavelength of incident light to be transmitted. MIM structures also exhibit reduced angular dependency; the described methodologies allow MIM-based MSFAs of less than around 200 nm, resulting in reduced angular dependencies that typical multilayer alternating index fingers. Both of these features make MIM structures good candidates for CFAs and MFSAs. Further alternative uses for such MSFA filters exist for example, the direct illumination of a target to be imaged.
As described, the transmitted wavelength of light is indicative of the thickness of the resist cavity. In between the two mirror layers, the light is reflected such that an eigenmode is excited by a self-interacting wavelength of light being internally reflected between the mirror layers. Subsequently, only this excited wavelength of light, or light of a very similar wavelength, is allowed to pass though the filter. That is, only light centred about a particular spectral position, defined by the self-interacting wavelength, will be transmitted through the filter.
In more detail, the ultrathin mirror layers (which in may be metallic) are preferably partially reflective dispersive mirrors which allow the coupling of energy between the top-and-bottom mirrors. When the mirrors are separated by an insulator, creating a finite optical path length between the two, eigenmodes (harmonic resonances) are excited which correspond to the electric field of incident light tunnelling through the top-mirror layer and becoming highly concentrated in the central region of the insulator cavity. Due to the insulator thickness, transmission filtering at the system eigenmode wavelength occurs. In other words, the insulator thickness corresponds to the spectral position of the transmission peak.
Furthermore, the mirror thicknesses control the coupling efficiency into the system, and affect the transmission linewidth (the transmission FWHM). Hence, depositing a thicker mirror (either or both of the first and second mirror) results in a more selective and narrow spectrum of transmitted light (i.e., a narrower FWHM). However, the thicker mirror may conversely affect the overall transmission, and as a result the overall transmission of the narrower transmitted spectrum may be lower.
Generally, the mask may define an attenuation profile. When the mask is placed in front of a resist material being used to fabricate an MSFA filter and exposed to a dose of energy such as a beam of photons, or a lamp, the mask attenuates the energy dose according to its attenuation profile. Thus, the attenuation profile of a mask may be transferred onto the resist. After a development step to remove portions/volumes of the resist material, the resultant thickness of the resist is representative of the attenuation profile of the mask. Therefore, it will be understood that this method may be a single-step lithographic process (e.g. in 9b).
The method described by 9a comprises laterally translating the PL mask, which has binary opacities, in order to impart a grayscale photoresist pattern onto the resist precursor. A PL mask with binary opacity values 902, where individual pixels arranged in a 2D array is shown in plan view. In step 904, the areas in the mask 902 which are most opaque (black) at least partially block the light, and the white (transparent) areas allow the light to substantially pass through the mask. The same mask may be laterally shifted in order to expose a greater area of the resist precursor. The magnitude of light which reaches the surface of the precursor, through the PL mask, can be seen in step 906. In step 908 a second exposure may be performed, which may be a different exposure to the one performed in step 904. This step 908 may be repeated for arbitrary designs an arbitrary number of times, whereby each exposure (seen again in step 910) may yield a different final resist thickness. Thus, on the schematic of
The method described in
The method of producing a master stamp comprises pre-fabricating 1002 (according to one of the previously described G-EBL methods 200) a grayscale resist on top of some robust/resilient material which will form the master stamp. The robust material may comprise silicon, and may be quartz. An etching step 1004 may then be performed to etch portions of the robust master-stamp precursor, to varying depths. Reactive-ion etching (RIE), which is a dry etching technique, may be used to etch the stamp material to impart the resist grayscale profile into the master stamp material. In some examples, heavy ions, such as Ar+, may be used in the RIE. The heavy ions are bombarded into the master-stamp material via the overlying grayscale resist. Alternatively, a wet etching technique with a chemical bath, comprising chemicals such as hydrofluoric acid, can be used. A thicker resist area will more substantially attenuate the intensity of the reactive ion species which reaches the robust stamp material. As such, due to the profile of thicknesses in the resist material, a corresponding grayscale resist pattern is imparted into the master stamp material. The resultant master stamp is seen in step 1006.
Step 1008 depicts inverting the stamp and bringing it into contact with another polymer (for example, a heat-sensitive photoresist or other suitable polymer), which is disposed on top of a bottom mirror and a glass substrate. Step 1010 depicts the imprinting or moulding step, comprising stamping into the polymer. It is further possible to incorporate additional pressure and/or heat over a variable amount of time when imprinting the master stamp to the photoresist. The resultant grayscale pattern is imparted into the resist in step 1012, and, after the removal of the master stamp, the top mirror layer is deposited onto the resist surface in step 1014.
The precursor 1102 in the robust MSFA fabrication method 1100 comprises one additional layer, which is a more robust insulator. This more robust insulator is deposited between the bottom mirror layer (e.g. 924) and the previously-described photo-resist layer (e.g. 922).
With a grayscale resist profile atop of this structure, an etching step (RIE or otherwise) can be performed to impart the grayscale profile into this insulator. Preferably, the more robust insulator layer is a substantially transparent material, and may comprise silicon (for example quartz (SiO2) in its crystalline form). A photolithographic technique (G-EBL 200 or mask photolithography 900) is then performed on the upper resist layer to produce a structure 1104 with a 3D resist thickness profile. Method step 1108 depicts the RIE method in which the robust insulator material is anisotropically etched. In other words, the extent of etching into the various portions of the robust layer is determined by the overlying photo-resist thickness. The intensity of the reactive ion bombardment 1106 is uniform across the entire region of the filter, wherein ions in the ion bombardment may comprise Ar+ ions. This RIE step is analogous to the RIE step 1004 in producing the master-stamp. The overlying photoresist serves to attenuate the ion bombardment, such that a thinner resist thickness will result deeper RIE etching into the robust material.
The result of the RIE step 1108 is robust insulator 1110 which has a grayscale resist thickness profile, disposed on top of the bottom mirror layer. A final step 1112 deposits an upper mirror layer in order to achieve the MIM structure, such that an MSFA optical filter is produced. This final filter 1112 comprising the robust insulator is much more mechanically and thermally robust and the standard resist/polymer layer.
In EBL and grayscale EBL, a phenomenon called the proximity effect may arise, and subsequently accounted for in the fabrication of MSFAs. The proximity effect is the unwanted exposure of regions adjacent to the pattern being exposed due to electron scattering events in the resist. In other words, the proximity effect causes the final pixel thickness to be greater in a more densely packed pixel pattern due to the additional exposure from adjacent pixels. The proximity effect can be lessened through the translation of the grayscale MSFA approach to larger batch processing i.e. photolithography. Each filter pixel has its center wavelength defined by a specific exposure dose. As a result of the proximity effect, the total dose applied to a specific region (pixel) is additionally dependent on the dose applied to surrounding pixels. Thus, a pixel's center wavelength may also be defined (to a lesser extent that the dose) by the density with which pixels are arranged.
The proximity effect can be observed by comparing the patterning of isolated pixels (i.e. arrays with non-exposed spacing between pixels) to dense arrays. The dose required to achieve a specific wavelength (resist thickness) may be is lower in dense arrays than it is in isolated regions.
By way of example,
Therefore, an empirical correction may be adopted: to ‘over pattern’ each MSFA, such that the area of interest (image sensor area) is >100 μm from the edge of the MSFA pattern. This approach also demands reducing the dose profile to compensate for increased cumulative exposure in the central region. It is also be possible to perform Monte Carlo electron scattering simulations for each pattern to optimize the dose patterns and avoid this empirical correction.
The method of producing an MSFA may further comprise applying a technique called thermal reflow. This is a fabrication processing technique that involves the thermal treatment of a photoresist (post-development) such that the resist is brought to a temperature roughly equal to, or slightly above, the glass transition temperature of the resist material. By doing so, the resist ‘reflows’ fully or partially depending on the temperature and time, which results in a smoother resist. The technique, for example, can be used to turn staircase-like 3D-patterns to 3D slopes, or to fabricate microlens (i.e., smooth convex shaped) arrays. For example, thermal reflow may be used to smooth the resist surface post-development (but prior to depositing the second mirror layer) in order to flatten/smooth the second mirror surface, narrowing a FWHM of the spectral response of the MSFA and boosting transmission efficiency.
A 1.5 nm Ti adhesion layer is thermally evaporated [Edwards E306 Evaporator] (base pressure ˜2×10−6 mbar, deposition at 0.1 nm·s−1), followed by a 26 nm layer of Ag (with relatively fast deposition, 0.2-0.3 nm·s−1, for improved optical performance), followed by a second 1.5 nm Ti layer. The first Ti layer promotes adhesion between the glass and Ag, the second increases the wettability of Ag for resist spin-coating and increases chemical stability by reducing Ag oxidation. The optimal thickness of the Ag is determined through simulations trading transmittance against FWHM. The thickness of the Ti layers is such that resist wettability is increased and adhesion is promoted with minimal effect on optical transmittance. MaN-2405 eB resist is spin-coated on top of the samples at 5,200 rpm for 45 s to form a ˜350 nm layer, then baked at 90° C. for 3 min. High voltage (80 kV), high current (4.2 nA) EBL (nB1, Nanobeam Ltd.) is used for the patterning. The bottom metallic mirror layer additionally acts to dissipate accumulated charge during electron beam exposure. The MSFAs have total area dimensions ˜1.1× greater than the image sensor area (4.85 mm diagonal) to correct for the proximity effect (described above) and ensure all sensor pixels are utilized.
The effect of stitching error is reduced due to the rectangular geometry (edges) of the patterns corresponding to the main-field and sub-field fractures. No sample registration marks are used for the samples shown in this study. The high current, in combination with low critical dose (due to inherent high sensitivity) of the resist, allows for fabrication over relatively large areas (millimetres) in quick time periods. The critical parameters in grayscale-to-color fabrication are the exposure dose and development conditions, which are determined empirically through a variety of dose tests, as demonstrated above and in reference to
SU-8 2000 series negative photoresist [Microchem] is utilized for the wafer-level MSFA processing. It is widely used commercially, has high thermal stability (glass transition temperature>200° C.) and designed to be permanent; typically incorporated into the final processed device. A SUSS Microtec MA/BA6 semi-automated mask aligner, with 365 nm (i-line) exposure and 5× alignment objectives, was operated in hard contact mode. 3 inch double-side polished borosilicate (Borofloat 33) glass wafers [Pi-kem], thickness 500±25 μm are cleaned in successive ultrasonic baths of acetone and IPA for 10 min, rinsed in de-ionized (DI) water, blow-dried with UHP compressed N2 and dehydrated at 200° C. for 10 minutes.
A set of crosshair alignment markers (30×30 μm) are patterned with PL (500 mJ·cm−2 exposure) using MaN-1400 series photoresist (2,500 rpm, 50 s; softbake: 95° C., 2 min) and developed with AZ 726 MIF for 3 minutes. The first metallic mirror, composed of Ti/Ag/SiO2 (1.2/38/12 nm), is deposited on the marker-patterned glass using a Lesker PVD-75 electron beam evaporator (base pressure ˜9×10−6 mbar, deposition rate 0.5 Å·s−1). During the deposition the wafer chuck is rotated at ˜5 rpm in order to increase coverage uniformity. Lift-off is performed in an ultrasonic bath of N-Methyl-2-pyrrolidone (NMP) at 60° C. for 3 minutes, followed by wafer cleaning (acetone, IPA, DI rinse, N2 dry, dehydration bake). The resultant wafers have a continuous bottom metallic mirror with a regular array of transparent alignment markers.
SU-8 2000.5 photoresist is spin-coated on top of the wafers at 5,500 rpm for 50 s to form a ˜350 nm layer, then soft-baked at 95° C. for 3 minutes.
The binary mask 1302 comprises 1 transmissive (i.e. transparent) pixel per unit square 1306, where the remaining 8/9ths of the mask is opaque, or substantially opaque. Initial exposure of a resist, using the binary mask 1302, results in the structure shown in step 1308. The mask is incrementally translated in steps 1310 and 1312 such that different regions of the resist are exposed. However, in steps 1310 and 1312 a higher energy density of exposure is applied. Thus, the multiple exposures (with different dosages of exposures in 1308, 1310, 1312) using the translated binary mask 1302 may result in exactly the same resultant resist profile as that produced by a single dose exposure with the grayscale mask 1306.
The grayscale mask step advantageously only requires a single exposure step. The binary mask method (1308, 1310, 1312) requires 3 separates exposures of different doses of energy. However, it should be understood that no further cleaning or development step is required in between the exposures in the method of
For a 30 μm pixels example, the photomask pixels are separated in order to give a final 3×3 (9-band) mosaic, hence 120×120 μm separation. The mask aligner is operated in constant dose and hard contact mode. The mask (with a 3×3 array of alignment crosshairs) is translated above the substrate (e.g. in between steps 1308, 1310, and 1312) (aligned with each band in the 3×3 array and flood exposed; the dose matrix ranges from 10-120 mJ·cm−2). A post-exposure bake of 65° C. for 2 minutes is then followed by a 2 minute development in 1-methoxy-2-propanol acetate (PGMEA), IPA rinse and N2 blow dry. The resultant structure is a bottom metallic mirror with a 3D thickness profile (cavities) across the entire wafer. The second metallic mirror, composed of Ag/SiO2 (38/38 nm), is deposited using the electron beam evaporator. Custom horizontal and vertical alignment markers are patterned in order to determine the alignment accuracy of the final MSFA pixels.
In the above example, the MSFA was fabricated at the wafer-level. The corresponding resultant transmission spectra of each spectral band spans 460-630 nm, and exhibits excellent optical characteristics, from shortest to longest wavelength: FWHMs of 27, 26, 24, 22, 21, 20, 19, 18, 17 nm (±5 nm) and peak transmission efficiencies of 76, 76, 75, 73, 72, 70, 68, 66, 65% (±6%). The example wafer above also exhibits the narrower 2nd order FP-type resonances (thicker final resist thickness). However, by adjusting the flood exposure dose it is possible to easily incorporate 1st and 2nd order modes, for example by utilizing a different dose matrix for the 3×3 pixel mosaic, which mixes the mode types.
Bespoke wafer-level 9-band MIM-based MSFAs are able to outperform conventional approaches for color filter fabrication, such as plasmonic and high-index dielectric nanostructure arrays/metasurfaces. For example, MSFA transmission bands in the present examples are narrower, have higher transmission efficiencies, exhibit no polarization dependency (up to high angle of incidence chief ray angles). Advantageously, the MSFAs have been fabricated at the wafer level (over large areas), illustrating translation ease to commercial processing.
The wafer with overlaid MSFA in
In more detail, a grayscale dose matrix 1406 as shown in
The solid horizontal lines in the centre of each box plot in correspond to the median values, and the bottom and top edges of the box indicate the 25th and 75th percentiles respectively. For every CFA, several unit cells in the middle of each array were picked randomly and the spectrum of each pixel was recorded. For the fewer band (<4) MSFAS, ˜12 spectra were recorded for each recipe. For the larger band MSFAs, 18-27 spectra were recorded for each recipe.
The box plots shows the optical transmission characteristics of a range of MSFA geometries fabricated across three different chips These chips include 2×3-channel designs (RGB1 and RGB2), RGB+1, and 3× different 3×3 mosaics (each with a varying dose profile range).
It can be observed from the plots that the variation in optical performance characteristics is minimal within each respective array. For example, the respective channel peak wavelength shift is typically ≲5 nm across the arrays and different recipes (
Array 1610 shows a random dose array for 1 μm pixels. Array 1610 is a good illustration of the advantageously versatile fabrication possibilities of the described grayscale lithographic methods. As mentioned previously, any arbitrary optical filter profile may be created merely by modulating the dose profile, and no additional lithographic steps are needed
In some examples, 11 μm×11 μm pixel dimensions are used, primarily due to limitations with the experimental image sensor setup. However, these length scales can be easily reduced using the described methodologies. Arrays may also be fabricated where exposure dose is varied linearly (as in array 1600), with pixel sizes range from: 5.5 μm down to 460 nm (as in 1606, 1608). Further still, 460 nm is not the limit to resolution, and may be reduced even further. Advantageously, the pixel array may still be fabricated to a high degree of uniformity, even at very small <500 nm size scales, as can be seen by the monodisperse pixel dimensions in
Surface morphology is characterized using an Atomic Force Microscope (AFM) [Asylum Research MFP-3D] in conjunction with Al-reflex-coated Si probes [Budget Sensors, Sigma Aldrich] operated primarily in tapping mode. Scan speed, voltage set-point, and drive amplitude are modified dependent on the feature morphology. Gwyddion software is used for the AFM data visualization and analysis. The raw surface data is plane levelled, scars (strokes) and noise minimized, and the resultant data is presented in 3D topography form. The average height (and standard deviation) of each pixel is obtained using the in-built statistical analysis toolbox. A LEO Gemini 1530VP field emission scanning electron microscope (SEM) operating at 1-5 keV is used for imaging the surface of samples (In-lens operation), which are fixed on angled SEM stubs for non-normal incidence imaging. Carl Zeiss software [SmartSEM] is used to control the SEM and obtain images at several magnifications. The optical characterization is performed using a modified Olympus BX-51 polarizing optical microscope (Halogen light source with IR filters removed) attached via a 300 μm core multi-mode optical fiber [Ocean Optics OP400-2-SR MMF] to a UV-visible spectrometer [Ocean Optics HR2000+] and second optical arm to a digital camera [Lumenera Infinity-2 2MP CCD] for surface imaging. The spectra are normalized to transmission through equivalent thickness borosilicate glass (bright state) and no input light (dark state) using Ocean Optics OceanView software.
For the imaging experiments, the test scene is composed of a Macbeth ColorChecker chart (A5 size) along with a Rubik's cube, which is imaged via a series of lenses through the custom MSFAs onto a CMOS image sensor (Supplementary
The image sensor is mounted at the end of a custom optical cage-system using a 3D printed [Ultimaker 2+] mount. An in-house built XYZ translation mount holds the MSFAs, which are fabricated on 10 mm×10 mm borosilicate glass chips. The imaging optics consist of three achromatic AR-coated lenses (Thor/abs LSBO8-A series): the first (a concave lens) de-magnifies the scene, the second collimates this virtual image (placed at the focal point of the first lens) and the third focuses the light onto the image sensor, through the MSFA mounted in front of it. An aperture stop is located after the third lens, limiting the range of ray angles impingement on the MSFA and thus onto the image sensor. The MSFAs, fixed in a custom 3D printed mount, are brought close to the borosilicate cover glass (thickness 0.4 mm) of the image sensor. Using the image sensor manual (Micron MT9P031 manual and Basler AW001305 documents) to determine the physical sensor geometry, the minimum distance of the MSFA from the image sensor die (plane) is estimated at ˜0.525±0.05 mm. The MSFA is translated in XYZ in order to align the pixels of the filter array with the pixels of the image sensor. For the MSFA imaging results, a series of optical bandpass filters (Thor/abs FKB-V/S-10 series; 10 nm FWHM) are utilized in a filter wheel mount, backlit with 50 W white light (4000K) floodlight LED array. The reflected light from the object test scene is imaged through the MSFA onto a monochrome image sensor.
A commercial-grade simulator (Lumerica/FDTD solutions) based on the finite-difference time-domain (FDTD) method was used to perform the calculations. MIM stacks are simulated using a dielectric between two metal layers (z-dimension). Periodic boundary conditions are used (x-y boundaries of the unit cell) and perfectly matched layers (z-boundary) along the direction of propagation. A uniform 2D mesh (Yee-cell) with dimensions nm and broadband-pulse plane-wave (350-1000 nm) injection source at a significant distance (several microns) above the sample are used. For the E-and-H-field intensity plots, an additional finer mesh is included, whereby the smallest cubic mesh size is <0.01 nm (z-direction). Complex dispersive material models are used for Ag (Johnson and Christy model) and SiO2 (material data), whereas a real-only refractive index of 1.65 is used for MaN-2400 series photoresist (Microchem: Material data sheet) and 1.4 for the transparent Mg F2 capping/encapsulation layer. Transmittance and reflectance values are calculated from 1D power monitors positioned above the range of structures and source injection.
The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘overlap’, ‘under’, ‘lateral’, ‘vertical’, etc. are made with reference to conceptual illustrations of a filter, such as those showing standard cross-sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an optical filter device when in an orientation as shown in the accompanying drawings.
Although the invention has been described in terms of examples as set forth above, it should be understood that these examples are illustrative only and that the claims are not limited to those examples. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1809748.5 | Jun 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2019/051645 | 6/13/2019 | WO | 00 |
