The present invention relates to optical thin films and techniques for the fabrication of optical thin films.
Optical thin films can be formed from a layer of semiconductor material, in which optical properties such as the refractive index and the absorption spectrum are defined by the density or concentration of semiconductor charge carriers. A particular application of semiconductor optical thin films is their use as optical metasurfaces, which have features giving a lateral (this, is, in the plane of the film) modulation or variation of optical properties on the nanometre scale. This sub-wavelength patterning or nanostructuring interacts with incident light by resonance at optical wavelengths. The patterning can therefore be used to manipulate light, offering control over phase, polarisation, emission, reflection and absorption, for example. Optical metasurfaces are hence a promising replacement for bulk optical elements. A particular application, demonstrated using an aluminium-doped zinc oxide metasurface, is that of optical solar reflectors which can be used for radiative cooling of spacecraft and satellites [1]. Lateral modulation of the carrier concentration is also of interest in the fabrication of nano-optical circuits [2, 3].
Metal oxide semiconductors are attractive for use in optical and other applications, owing to a combination of high transparency at visible wavelengths and a high carrier density or concentration. To date, nanostructuring in these materials has typically been achieved by forming nanoscale physical structuring of a semiconductor layer, such as by a plasma etch or a lift-off approach to selectively remove unwanted portions of the semiconductor material and leave island-like features, or by controlled local deposition to form such features. The semiconductor carrier density in the layer is therefore modulated according to the physical presence or absence of the semiconductor material at any position. As a result, the optical metasurface has a non-planar surface which makes the addition of further layers or features over the optical metasurface, such antireflection coatings or electrical contacts, difficult. Also, the lack of flatness of the surface can lead to inferior optical performance owing to scattering from the multiple edges.
Group IV and group III-V semiconductor layers have been provided with lateral carrier density modulation by ion implantation or dopant diffusion applied through a mask defining the desired modulation pattern. These techniques can also be used with metal oxide semiconductors, but are substantially less effective because carrier concentration in these materials is substantially determined by defects, self-compensation and possibly hydrogen incorporation. Also, ion implantation is slow, costly and does not offer very precise control in metal oxides.
Furthermore, carrier concentration in metal oxides can be reduced if the metal oxide layer is grown or deposited in the presence of an oxygen plasma. This has been demonstrated in zinc oxide [4, 5]. The oxygen affects the defects and the hydrogen bonding. However, because this technique is carried out during deposition, the resulting layer has a uniform carrier density, lacking any lateral modulation. Conversely, carrier density increase has been shown in zinc oxide by deposition in the presence of a hydrogen plasma [6, 7]. In a similar approach, H2O plasma treatment has been used in the fabrication of layers in thin film transistors [8], and fluorine plasma treatment to fill oxygen vacancies in a semiconductor is also known [9]. Oxygen plasma treatment in the fabrication of zinc oxide thin film transistors has been reported, where metal parts of the transistor structure shielded the zinc oxide layer from the oxygen plasma [10].
Aspects and embodiments are set out in the appended claims.
According to a first aspect of certain embodiments described herein, there is provided a method of forming an optical thin film, comprising: providing an assembly comprising a layer of semiconductor material deposited on a substrate, the semiconductor material comprising a compound of at least one metal and a group VI element; depositing a masking layer onto the layer of semiconductor material, the masking layer being patterned to expose one or more regions of the layer of semiconductor material; applying to the assembly a plasma of the group VI element in order to cause indiffusion of the group VI element into the semiconductor material in the exposed regions while the masking layer blocks indiffusion in unexposed regions, the indiffusion causing a reduction in carrier density in the semiconductor material; and removing the masking layer; thereby forming, from the layer of semiconductor material, an optical thin film having a variation in carrier density and corresponding variation in optical properties matching the patterning of the masking layer in a plane parallel to the substrate.
According to a second aspect of certain embodiments described herein, there is provided an optical thin film formed according to the method of the first aspect.
The method of the first aspect may further comprise the deposition or other formation of one or more additional uniform or patterned layers of material over the optical thin film in order to produce an optical element or optical device. For example, the masking layer may be patterned in order to form an optical thin film with a first plasmonic resonant frequency, and the one or more additional layers may comprise a patterned layer of metallic material having a second plasmonic resonant frequency different from the first plasmonic resonant frequency, to produce an optical element with dual plasmonic resonance. In other examples, the one or more additional layers may comprise an antireflection coating. In such cases, a third aspect of certain embodiment described herein is directed to an optical element or an optical device formed according to these examples, or comprising an optical thin film according to the second aspect.
According to a fourth aspect of certain embodiments described herein, there is provided an optical solar reflector comprising an optical thin film formed according to the method of the first aspect.
These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, optical thin films and fabrication techniques therefor may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.
For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:
Aspects and features of certain examples and embodiments are discussed/described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed/described in detail in the interests of brevity. It will thus be appreciated that aspects and features of optical thin films and methods discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.
Optical thin films can be formed from semiconductor materials which are compounds of metals and elements from group VI of the periodic table. The group VI elements include oxygen, sulphur, selenium and tellerium. While the group as a whole is referred to as the chalcogens, oxygen is often discussed separately. Hence, the semiconductor compounds may be designated as metal oxides when comprising a compound of metal and oxygen, and as metal chalcogenides when comprising a compound of metal and one of sulphur, selenium and tellerium. However, the term metal chalcogenide can be considered to include metal oxides. The terms may be used interchangeably herein to indicate compounds of metals and group VI elements, unless it is clear from the context that only oxides or only non-oxides are being referred to.
Additionally, these semiconductor materials may comprises one or more dopants. Examples of useful dopants include aluminium, boron, gallium, indium, titanium, zirconium and hafnium, although other elements are not excluded. Herein, the terms metal oxide and metal chalcogenide may be used to include both doped and undoped semiconductor materials. If the context requires, the presence or absence of dopants is specifically indicated. Undoped semiconductor compounds comprising a single metal and a group VI element can also be termed binaries.
Many metal chalcogenides, and in particular metal oxides, are particularly useful materials for optical thin films owing to being transparent at visible optical wavelengths. Semiconductors of this type can be categorised as transparent conducting oxides (TOO). Examples having dopants are indium tin oxide (ITO) and aluminium-doped zinc oxide (AZO). The concentration or density of charge carriers in the semiconductor material defines the optical properties of the material, such as refractive index, and metal chalcogenides can have high carrier concentrations, which adds to their usefulness. In order to provide a thin film which is able to operate as an optical metasurface, the carrier density (and correspondingly, the optical properties) can be patterned to provide plasmonic resonance at optical wavelengths. The modulation is in the plane of the thin film, so can be considered to be a lateral modulation or variation. The plane of the thin film occupies dimensions which are substantially orthogonal or perpendicular to the substrate or other underlying stratum or component of a device on which the thin film is laid or deposited. The size or lateral dimension of the patterned features is selected to achieve a desired frequency for the plasmonic resonance. Typically, features can be smaller than 100 nm for applications in the visible and ultraviolet parts of the spectrum, and conversely as large as 1 cm for radio-frequency applications. For applications where the thin film is intended to operate at near-infrared wavelengths, feature size could be in the range of 200 nm to 5000 nm. Patterning or modulation of the carrier density on other scales may also be useful, for other applications of thin films.
In metal oxides, the patterning is commonly carried out by plasma etching or lift-off techniques through a mask defining the desired pattern. This removes portions of the semiconductor material which are not protected by the mask. A converse approach is that of controlled local deposition of the semiconductor material into the desired pattern. All these techniques create physical island-like portions of the semiconductor material, so that in the lateral direction, the carrier density is modulated by being high where semiconductor is present, and zero where there is no semiconductor. The resulting surface of the semiconductor layer is not flat (non-planar), which has some disadvantages. Optical properties may be inferior owing to from the multiple edges and surfaces. Also, additional process steps that may be required to incorporate the semiconductor layer into a more complex optical element or device may be hindered or prevented altogether.
An alternative approach to tailoring the carrier density in metal oxides has been demonstrated, in which exposure to an oxygen plasma during deposition of a zinc oxide layer produced a reduction in the carrier density [4, 5]. However, the reduction is uniform across the layer since the plasma treatment takes place while the layer is being formed, so the technique is not useful for patterning or nanostructuring in the lateral direction.
In order to address these issues, the current disclosure presents an alternative method for forming optical thin films of metal oxide and metal chalcogenide semiconductors having lateral modulation of the charge carrier density. After formation of a layer of the semiconductor material on a substrate or other supporting structure or element, a masking layer or mask is laid down over the semiconductor material with a pattern corresponding to the desired pattern of carrier density modulation. The semiconductor is then exposed to a plasma of relevant group VI element comprised in the semiconductor compound for a specified period. It has been found that during this exposure, the plasma material diffuses into or penetrates the semiconductor, and this reduces the carrier density (and correspondingly changes the optical properties). The mask protects those portions of the semiconductor layer which lie under it, hence blocking the plasma exposure and indiffusion in these areas. The portions exposed through the mask experience the plasma treatment so that the carrier density is reduced. At the end of the plasma treatment, the mask is removed, to leave a planar semiconductor surface. The film has a variation of carrier concentration in the lateral direction, comprising regions of higher (unmodified) carrier density where the semiconductor has been protected by the mask, and regions of lower (modified) carrier density where the semiconductor has been exposed to the plasma. Hence, a transverse carrier density profile, with a pattern of high and low carrier densities corresponding to the masking layer, can be formed in the semiconductor in a controlled manner.
The semiconductor layer 2 may have a thickness in the range of about 5 nm to 1000 nm, depending on the intended application. For applications described in detail herein, typical thicknesses of between 50 nm and 200 nm may be useful. However, the invention is not limited in this regard, and other thicknesses may be used.
A masking layer or mask 4 is deposited or otherwise formed onto the layer of semiconductor material 2. The masking layer 4 is a discontinuous layer patterned according to a desired distribution in the lateral direction (that is, in the plane of the semiconductor layer, and parallel to the plane of the substrate) of the carrier density in the semiconductor 2. Areas or regions of the semiconductor layer 2 which are to retain the high carrier density (H) are covered by the masking layer 4, so as to be unexposed, while areas or regions of the semiconductor layer 2 in which a lower carrier density (L) which is less than the high carrier density) are not covered by the masking layer 4, so as to be exposed.
In this example, the masking layer 4 is formed as a so-called “hard mask”, the purpose of which is to protect the underlying, unexposed, portions of semiconductor from a plasma to which the assembly is exposed in a subsequent stage. The hard mask 3 is able to block the plasma material from diffusing into the semiconductor 2. The hard mask 3 is able to wholly or substantially prevent or inhibit such diffusion, so that the carrier concentration in the semiconductor 2 is not affected to any significant degree. For example, the mask may block the plasma such that a ratio of the carrier density in the exposed regions and the unexposed regions of the underlying semiconductor material (so, the ratio low to high, or L:H) is achieved which is at least 1:2 (that is, the plasma treatment reduces the carrier density to about a half of its “high” value) and possibly up to or above 1:100 (so that the plasma treatment reduces the carrier density to about one hundredth or less of the original “high” value). The useful variation or difference in absolute carrier concentration or density between the exposed and unexposed underlying semiconductor layer will depend on the intended resonant frequency of the application for which the semiconductor is designed.
An example of a suitable mask material to implement a hard mask is silicon nitride, SiN. Any other material which is able to provide an appropriate barrier to protect the semiconductor from indiffusion of the plasma can be used. For example, various metals can be used if preferred. An example of a suitable metal is gold. Other hard mask materials will be known to the skilled person, and are not precluded.
The mask layer 43 can be patterned by any suitable or convenient technique; the invention is not limited in this regard. For example, a SiN hard mask might have a pattern which is defined using an ultraviolet or e-beam resist “soft mask” which is fabricated by exposure of a layer of mask material to light or electron beams to remove unwanted portions of the material and thereby pattern the mask. It may also be possible by careful choice of soft mask resist material and exposure parameters to form a soft mask that can itself act directly as the diffusion barrier masking layer 4, so that a hard mask is not required.
The barrier-providing functionality of the mask layer 4 may depend also on its thickness as well as the material from which it is made. The mask layer 4 should have a thickness adequate to provide the required zero or minimal diffusion of the plasma material into the semiconductor material. In order to save material, and reduce deposition time for the mask layer, a minimal mask thickness may be employed. For example, the mask may have a thickness of about 20 nm; this can provide adequate functionality as a diffusion barrier. Thinner masks may also be usable, such as in the range of 10 nm to 20 nm. However, in order to enable good process control (ensuring that the mask is thick enough in all places for proper blocking, for example), a greater thickness may be preferred. For example, the mask may have a thickness in the range of about 60 nm to 80 nm. Other thicknesses are not excluded, however, for example in the range of 50 nm to 70 nm, or 70 nm to 90 nm, or 50 nm to 90 nm, or even significantly thicker, such as overall, in the range of 20 nm to 80 nm, or 10 nm to 500 nm.
It has been found that the group VI element can be introduced by indiffusion into the semiconductor material, if the semiconductor material is exposed to a plasma of the group VI element after the semiconductor layer has been deposited (in contrast to previous techniques of plasma treatment during deposition). This allows lateral spatially selective modification of the carrier concentration if a mask layer is applied over the semiconductor layer in order to allow the plasma to reach certain areas of the semiconductor material while blocking other areas from the plasma exposure.
In the present example, the semiconductor material is AZO, in other words, a metal oxide semiconductor. Therefore, the plasma used is an oxygen plasma. In this example a plasma of molecular oxygen, O2, is used, but it is also possible to use a plasma of ozone, O3, or a plasma of water, H2O. If the semiconductor material comprises another chalcogen, and not oxygen, the plasma should comprise the relevant element, such as in a molecular form or as a dihydrogen monochalcogen. Hence, the plasma may by a plasma of sulphur, selenium or tellerium, to match the group VI element in the semiconductor.
So,
The plasma may be generated and applied using any known technique. As an example, an inductive coupled plasma (ICP) generator can be used. This is an attractive technique because it does not include ion bombardment present which is present in some plasma generation and which can cause physical damage to the semiconductor material.
The plasma exposure is usefully performed at an elevated temperature, that is, a temperature above room temperature. This has been found to aid the diffusion of the plasma material into the semiconductor material. The temperature can be achieved by placing the assembly 20 and the plasma generation system in an oven, for example, or by mounting the assembly 20 on a heating element to provide direct heating of the assembly 20. Temperatures in the range of about 80° C. to 320° C. might be used, for example. More particularly, the temperature may be in the range of about 100° C. to 300° C., such as a temperature of about 120° C. (or in the range of 100° C. to 140° C. or 110° C. to 130° C.) or about 225° C. (or in the range of about 205° C. to 245° C. or 215° C. to 235° C.). Higher temperatures have been found to be useful in some cases, and may, for example, enable a reduced plasma exposure time. The temperature may be at or around 300° C., for example, such as in the range of 280° C. to 320° C., or 290° C. to 310° C. Temperatures in excess of 320° C. are not excluded, however. The temperature may be chosen having regard to the particular combination of semiconductor material, desired plasma exposure time, and mask material properties. For example, a temperature of about 300° C. has been found to be useful for O2 plasma exposure of AZO through a SiN mask, as in the present example, for a mask thickness in the range of about 60 nm to 80 nm, and a plasma treatment time of about 20 minutes. The parameters may be adjusted to achieve the desired reduction in carrier concentration by providing the appropriate amount of plasma material diffusion into the semiconductor material.
The temperature may be to some extent constrained by the choice of material for the masking layer 4. A hard mask material such as SiN is well able to withstand temperatures up to and above 300° C., while a metallic masks such as gold may require a much lower temperature, such as 100° C. or below, in order to avoid or reduce damage to the mask.
Similarly, the plasma treatment time or exposure time can also be selected in order to obtain a particular reduced carrier concentration in the exposed regions of the semiconductor material. Indeed, if other parameters are fixed, such as by physical characteristics of the system and other fabrication steps, selection of the time can be a convenient and simple way to tailor the carrier concentration. The exposure time may be up to 20 minutes, or up to 30 minutes, or up to 40 minutes, or up to 50 minutes, or up to 60 minutes, for example. Longer or shorter times are not excluded, however. Exposures of around 20 minutes are possible, which may be considered useful as being shorter and hence reducing the overall fabrication time. For example, the exposure may be in the range of 15 minutes to 25 minutes, or 17 minutes to 23 minutes, or 20 minutes. A 20 minute exposure of a AZO semiconductor layer with a SiN mask to an O2 plasma at a temperature of 300° C. has been found to be effective, for example.
These qualities should be contrasted with the non-planar surface produced by the technique described with respect to
Furthermore, the
Hence, the ability of the
In a second step S2, a patterned hard mask is formed over the semiconductor layer, with a pattern corresponding to a desired lateral profile of carrier density in the semiconductor layer.
In a third step S3, the assembly is exposed to a plasma of the group VI element, for example an oxygen plasma when the semiconductor is a metal oxide. This exposure allows diffusion of the group VI element into the semiconductor in the regions not covered by the mask, causing a reduction in carrier concentration in those regions. The regions under the mask are protected from the indiffusion and hence retain a carrier concentration at or close to the original level.
In a fourth step S4, the mask layer is removed to restore the planer surface of the semiconductor layer, which now has lateral variation or modulation of carrier density and correspondingly of optical properties formed within it, owing to the diffusion.
The lower image in
The lower image in
Despite the very different physical structures of the two semiconductor thin film layers shown in
Line 40 in
These results demonstrate how the plasma treatment proposed herein can be used as a substitute for the plasma etching and similar processes which are currently used to form metal-group VI semiconductors into optical metasurfaces and other components. An appropriate amount of indiffusion achieve by exposure to the plasma reduces the carrier concentration below an optically-effective level, having the same effect as the physical removal of the semiconductor material, which corresponds to a carrier concentration of zero.
Optical thin films of semiconductor material patterned using the described plasma exposure technique can be used as they stand, but may also be integrated into more complex devices and components by the addition or inclusion of one or more further layers. The layers may be included under the semiconductor layer, as part of the “substrate” supporting the semiconductor, or may be added over the semiconductor layer after it is patterned, or both.
In summary, the method described herein for forming optical thin films, such as optical metasurfaces, comprises a masked exposure of semiconducting metal chalcogenides (including oxides) to a chalcogenide plasma (including oxygen) to create a lateral carrier density modulation or profile in the semiconductor without any topographical change to the semiconductor layer. The planar surface offers both improved optical and mechanical properties of the film, as well as facilitating any subsequent fabrication steps for integrate the film into optical and optoelectronic devices.
Such devices and thin films have a variety of applications. An example is as optical solar reflectors (OSR) for satellite cooling control; the thin films may be patterned as metasurfaces and applied as coatings. It is expected that metal oxide metasurfaces for OSRs can be fabricated in larger sizes than is possible for conventional quartz OSRs currently used in the satellite industry.
The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.
Number | Date | Country | Kind |
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1913533.4 | Sep 2019 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2020/052197 | 9/11/2020 | WO |