FIELD OF INVENTION

The present disclosure relates to devices comprising a plurality of structures formed on a substrate and defining a metasurface, and associated methods of inspecting a metasurface and inspecting a mold and a resist layer for forming a metasurface.

BACKGROUND TO INVENTION

Metasurfaces are surfaces that may comprise arrangements of structures having dimensions comparable to and/or less than a wavelength of incident radiation to provide desired effects, such a phase shifts, in the radiation.

That is, by selecting particular sizes, orientations, positions and/or spacing of sub-wavelength structures of a metasurface in accordance with a predetermined design criteria, effects the metasurface may have upon electromagnetic radiation propagating through or reflecting from the metasurface may be precisely controlled.

As one example, an optical metasurface, e.g. a metalens, may be configured to introduce a predetermined phase delay into electromagnetic radiation, thereby causing constructive interference of radiation propagating through the metalens at predetermined distances from the metasurface.

In some examples, metasurfaces designed for use with visible and/or infrared radiation may comprise arrangements of large quantities of nanostructures, e.g. hundreds or even thousands of nanostructure. In just one example, such nanostructures may be in the form of columns or pillars, having dimensions and spacing within a range of tens or hundreds of nanometers.

It may be difficult to reliably manufacture metasurfaces comprising such nanostructures at relatively low cost. Existing metasurface manufacturing techniques may include E-beam lithography, nanoimprint and immersion lithography. However, even such established manufacturing techniques may face challenges in consistently producing metasurface arrangements with desired structure sizes, thicknesses, positions and/or spacing, with adequate precision. Many factors may affect an accuracy and precision of a metasurface manufacturing process, such as design artefacts, erosion of molds or templates used in the manufacturing process, debris within the molds, and the like. It is therefore desirable to be able to accurately monitor features of nanostructures during manufacturing of metasurfaces, to ensure characteristics of the nanostructures precisely adhere to design specifications, such as size, shape and orientation, within an allowable tolerance. Furthermore, it is also desirable to monitor features of tool used to implement such nanostructures, and to identify any substantial performance degradation in such tools.

Thus, it is desirable to implement a relatively low-complexity and low cost method of monitoring manufacturing accuracy of metasurfaces in a production environment.

It is therefore an object 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 OF INVENTION

The present disclosure relates to devices, such as metalenses, comprising a plurality of structures, e.g. nanostructures, formed on a substrate and defining a metasurface. The present disclosure also relates to associated methods of inspecting a metasurface and inspecting a mold and resist layer for forming a metasurface.

According to a first aspect of the disclosure, there is provided a device comprising a plurality of structures formed on a substrate and defining a metasurface. The device also comprises a marker structure formed on the substrate amongst the plurality of structures. The marker structure has a different shape and/or orientation to each structure of a subset of the plurality of structures, wherein each structure of the subset neighbors the marker structure.

That is, the marker structure may be substantially different from surrounding structures, such that the marker structure may be readily identified.

Advantageously, implementation of such a marker structure may enable determination of a location within the metasurface during a process of optical inspection and/or Scanning Electron Microscope (SEM) inspection.

That is, the present disclosure provides a metasurface suitable for monitoring of the structures, e.g. nanostructures, in-line and in-situ during a manufacturing process. Such monitoring may refer to performing measurements of critical dimensions of the structures and/or spacing between structures. Without this ability to monitor the actual nanostructures themselves, an assessment of manufacturing quality can only be evaluated during optical tests. However, at that stage it may be extremely difficult to precisely differentiate attributes that may impact on the quality of the device. For example, for a poorly performing metalens, it may be difficult to determine whether the poor performance was due to manufacturing issues, of from a poor metalens design.

The shape and/or orientation may be a shape and/or orientation in a plan view of the substrate. The shape and/or orientation may be a shape and/or orientation in a crosssection in plane parallel to substrate.

Each structure of the subset that neighbors the marker structure may be adjacent the marker structure. Each structure of the subset that neighbors the marker structure may be immediately adjacent the marker structure. Each structure of the subset that neighbors the marker structure may be beside the marker structure. Each structure of the subset that neighbors the marker structure may be beside arranged around, e.g. collectively surrounding, the marker structure.

A cross-sectional area of the marker structure in a plane parallel to the substrate may be substantially the same as a cross-sectional area of each structure in the subset.

Advantageously, by having a substantially same cross-sectional area, a relatively constant fill-factor may be maintained across a local area, which may improve an effectiveness of a nanoimprint process.

In some examples, the cross-sectional area of the marker structure in the plane parallel to the substrate may be within +1-2 % of the cross-sectional area of each structure in the subset. On other examples, a variation between the cross-sectional area of the marker structure and the cross-sectional area of each structure in the subset may be as much as +/-5 %, or even +/-10 %.

Each structure of the plurality of structures and/or the marker structure may comprise a column, pillar, fin, mesa or protrusion formed on the substrate.

Each structure of the plurality of structures and/or the marker structure may comprise a nanostructure, e.g. a structure having dimension in the nanometer scale, such as tens or hundreds of nanometers in cross-section.

The plurality of structures may be arranged in an array, a grid, concentrically, or in a regularly repeating pattern.

The device may be configured as a metalens for visible and/or infrared radiation.

That is, the plurality of structures may comprise dimensions, e.g. widths, spacing, diameters and/or cross-sectional extents, that are smaller than a wavelength of the visible and/or infrared radiation for which the device is designed.

The device may be configured as an electromagnetic metamaterial for RF radiation.

In some examples, the device may be configured for use as one or more of: an antenna; an electromagnetic-absorber; a polarizer; an electromagnetic filter. The marker structure may comprise a different number of vertices in a crosssection parallel to the substrate to a number of vertices of each structure in the subset.

Advantageously, a different number of vertices may render the marker structure easily distinguishable from other structures of the plurality of structures.

In an example, the marker structure may be a circle or oval in cross-section and the plurality of structures may be polygonal in cross-section.

In a cross-section parallel to the substrate the marker structure may be a polygon and each structure in the subset may be a single-sided shape; or

In an example, the marker structure may polygonal in cross-section and the plurality of structures may be circular or oval-shaped in cross-section.

In a cross-section parallel to the substrate the marker structure may be a singlesided shape and each structure in the subset may be a polygon.

In an example, the marker structure may be a circle or oval in cross-section and the plurality of structures may be polygonal in cross-section.

The marker structure may have a different shape and/or orientation to all structures of the plurality of structures that neighbor the marker structure.

Such a marker structure may be readily distinguishable from surrounding structures, thereby enabling determination of a location within a microlens.

The marker structure may have a different shape and/or orientation to all of the structures of the plurality of structures.

Advantageously, the marker structure and surrounding structures may define a unique pattern on the microlens, therefore enabling a definitive determination of a location within microlens based on identification of the relatively ‘unique’ pattern.

Advantageously, such a marker structure may be readily distinguishable from surrounding structures, thereby enabling simplified and more accurate SEM pattern recognition.

The device may comprise a plurality of marker structures.

Advantageously, a plurality of marker structures may provide multiple locations at which the device, e.g. the metalens, may be tested, and may be patricianly suitable for relatively large devices. In just an example, a marker structure may be placed every 20 or 30 structures. Advantageously, a speed of testing the device may be increased by implementation of multiple markers.

The plurality of marker structures may be aligned in a substantially straight line.

The straight line may be aligned with a direction of movement of a stage upon which the device to be tested relative to a testing system, e.g. a SEM or optical system, may be disposed. Advantageously, having the plurality of marker structures aligned in a straight line may increase a speed of critical dimension measurements because a scan, such as by a SEM, may only be required in a single direction. In a non-limiting example, a scan may be performed, then the stage supporting the device may be moved an amount in a first direction until the next marker structure is identified, than the stage may be moved yet another amount until the next marker structure is identified, and so on.

According to a second aspect of the disclosure, there is provided a mold for nanoimprinting a device according to the first aspect.

The mold may comprise negatives, e.g. cavities or indentations, of the marker structure and plurality of structures. As such, the mold itself may also be suitable for relatively high-speed critical dimension measurements.

The mold may be a master mold. The mold may be an intermediate mold formed from a master.

According to a third aspect of the disclosure, there is provided a method of inspecting a metasurface. The method comprises determining a location on the metasurface by identifying a marker structure amongst a plurality of structures defining the metasurface, wherein the marker structure has a different shape and/or orientation to each structure of a subset of the plurality of structures, wherein each structure of the subset neighbors the marker structure. The method also comprises comparing a measured characteristic of one or more structures of the plurality of structures to a reference characteristic, wherein the location is used to identify the reference characteristic.

Advantageously, the marker structures may enable a precise determination of a location within the metasurface, even when structures of the plurality of structures on the metasurface appear substantially the same, or vary only slightly in shape and/or dimensions and/or spacing.

The measured characteristic may correspond to an area, shape, size, spacing and/or geometry of the one or more structures.

For example, the measured characteristic may be a cross-sectional area, wherein the cross-sectional area may correspond to a region of area, such as a center or periphery, of the device.

According to a fourth aspect of the disclosure, there is provided a method of inspecting a mold for nanoimprinting a device according to the first aspect. The method comprises determining a location on the mold by identifying a marker structure amongst a plurality of structures defining a metasurface to be formed by the mold, wherein the marker structure has a different shape and/or orientation to each structure of a subset of the plurality of structures, wherein each structure of the subset neighbors the marker structure. The method comprises comparing a measured characteristic of one or more structures of the plurality of structures to a reference characteristic, wherein the location is used to identify the reference characteristic.

It will be appreciated that the term structure, in particular with reference to a master mold for forming one or more intermediated molds for nanoimprinting, may refer to a column, pillar, fin, mesa or protrusion. Similarly, and especially in the instance of an intermediate mold for nanoimprinting, the term structure may refer to a hole, channel, groove, indentation or the like. That is, in the instance of a mold, the structure may be effectively the negative of a structure to be formed as a feature of a metasurface.

According to a fifth aspect of the disclosure, there is provided a method of inspecting a resist layer for forming a metasurface on a substrate. The method comprises determining a location on the resist layer by identifying a marker structure in the resist layer amongst a plurality of structures in the resist layer. The marker structure has a different shape and/or orientation to each structure of a subset of the plurality of structures, wherein each structure of the subset neighbors the marker structure. The method comprises comparing a measured characteristic of one or more structures of the plurality of structures to a reference characteristic, wherein the location is used to identify the reference characteristic.

For example, the determined location may be associated with structures having certain cross-sectional areas. That is, a particular marker structure may be associated with a location known to have structures having the certain cross-sectional area. As such, once a location is identified by locating the marker structure, a cross-sectional area of surrounding structures may be measured and compared to an expected cross-sectional area. A substantial deviation from the expected cross-sectional area may be indicative of a manufacturing error, or issue such as degradation of a mold.

The resist layer may be a layer formed during a process of manufacturing a metasurface. As such, the resist layer may correspond to an intermediate processing steps in a method of manufacturing a metasurface.

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 1a depicts a plan view of an example of a prior art metalens;

Figure 1 b depicts a magnified portion of the example prior art metalens of Figure

1a;

Figure 1c depicts a cross section of a portion of the example prior art metalens of Figure 1a;

Figure 2a depicts a plan view of an example of a metalens, according to an embodiment of the disclosure;

Figure 2b depicts a magnified portion of the metalens of Figure 2a;

Figure 2c depicts a further magnified portion of Figure 2b;

Figure 3 depicts a plan view of an example of a mold, according to an embodiment of the disclosure; and

Figure 4 depicts a method of inspecting a metasurface, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1a depicts a plan view of an example of a prior art metalens 100. For purposes of example only, the metalens 100 is substantially circular. The metalens 100 comprises many thousands of structures, which are not readily visible in the example of Figure 1a due to their relatively small size. As such, a portion 105 of Figure 1a is depicted in magnified view in Figure 1b.

In the portion 105 depicted in Figure 1 b, it can be seen that hundreds of structures that are substantially circular in cross-section are arranged to form a metasurface. While there is some variation between the magnitude of a cross-sectional area of the structures, it can be seen that a vast majority of the structures are immediately adjacent structures having a very similar cross-sectional area.

For example, a first structure 110 is immediately adjacent a second structure 115 and a third structure 120. While there is a slight variation in cross-sectional area between the first, second and third structures 110, 115, 120, they are visibly very similar. That is, from inspection of a structure in the portion 105 of the prior art metalens 100, it may be extremely difficult to determine whether a particular structure being inspected is the first, second or third structure 110, 115, 120, due to their relatively similarity. To further illustrate this, Figure 1c depicts a cross-section 125 of a small portion of the example prior art metalens 100 of Figure 1a.

It can be seen that a plurality of structures 130a-m are formed on a substrate 140. The structures 130a-m are formed on the substrate 140 to define a metasurface.

For purposes of example, the structures 130a-m are substantially rectangular in cross-section, although it will be understood that other shapes, such as triangles, domes, etc. may be implemented.

In the depicted example cross-section 125, it can be seen that each structure 130a-m is very similar to an immediately adjacent structure 130a-m. For example, a first structure 130a appears visibly similar in dimensions to a second structure 130b. However, a first width D1 of first structure 130a is slightly greater than a second width D2 of second structure 130b.

In contrast, a third structure 130m is clearly visibly different in dimensions to the first structure 130a. That is, the first width D1 of first structure 130a is substantially greater than a third width D3 of third structure 130m.

As such, from an optical or SEM inspection of the prior art metalens 100, it may be difficult to determine which exact structure is being inspected, since adjacent structures may be extremely similar in appearance and dimensions. That is, an approximate region of the prior art metalens 100 being inspected may be determined due to a substantial difference in dimensions between structures separated by a substantial distance, e.g. the first and third structures 130a, 130m. However, without performing extremely precise, accurate and potentially time-consuming measurements it may be difficult if not impossible to identify a specific structure that is under inspection.

Figure 2 depicts a plan view of an example of a device 200, according to an embodiment of the disclosure. For purposes of example only, the device 200 is a metalens and may be manufactured in from a material such as TiO2, SiO2 and/or a-Si.

The device 200 comprises a metasurface defined by a plurality of structures formed on a substrate, which are depicted in plan view. In the example, the structures are columns, although it will be understood that other shapes, such as pillars, fins, mesas or other protrusions may be implemented.

Similarly, for purposes of example only, the device 200 is configured as a metalens for visible and/or infrared radiation. That is, the plurality of structures comprise nanostructures having dimensions, e.g. widths, spacing, diameters and/or cross- sectional extents, that are smaller than a wavelength of the visible and/or infrared radiation for which the device 200 is designed.

The device 200 comprises many thousands of structures, which are not readily visible in the example of Figure 2a due to their relatively small sizes. As such, a portion 205 of Figure 2a is depicted in magnified view in Figure 2b. For purposes of illustration a further magnified portion is also depicted in Figure 2c. The structures are formed on a substrate 210. The substrate 210 is depicted as the space between the structures defining a metasurface. For purposes of non-limiting example, it can be seen that the structures are arranged in a regularly repeating pattern, e.g. in arrangements of concentric circles comprising comparably sized structures. In other examples, the structures may be arranged in arrays, grids, patterns, or the like.

While the disclosed invention is described with reference to example embodiments of metalenses, it will be understood that the disclosed invention relates more generally to a device comprising a plurality of structures formed on a substrate and defining a metasurface. As such, it will be understood that other devices, such as devices configured as electromagnetic metamaterials for RF radiation, e.g. antennas, EM- absorbers, polarizers and electromagnetic filters, fall within the scope of this disclosure.

In the portion 205 depicted in Figure 2b, it can be seen that hundreds of structures that are substantially circular in cross-section are arranged to form a metasurface. While there is some variation between the magnitude of the cross-sectional area of the structures, it can be seen that a vast majority of the structures are immediately adjacent structures having a very similar cross-sectional area.

The device 200 also comprises a first marker structure 215 formed on the substrate 210 amongst the plurality of structures.

The first marker structure 215 has a different shape to a subset of the plurality of structures, wherein each structure 220a-h of the subset neighbors the first marker structure 215. In this example, the term ‘neighbors’ means in close proximity to, e.g. next to. In the example, the first marker structure 215 has a cruciform shape. As such, the first marker structure 215 is visibly quite substantially different from the surrounding structures which are circular in cross section, e.g. each structure 220a-h of the subset, such that the first marker structure 215 may be readily identified. Advantageously, implementation of such a first marker structure 215 may enable determination of a location within the metasurface during a process of optical inspection and/or Scanning Electron Microscope (SEM) inspection.

Although an example of a cruciform first marker structure 215 surrounded by substantially circular structures 220a-h is provided, it will be appreciated that other shapes, sizes and/or orientations of structures may be implemented, such that the first marker structure 215 is rendered easily distinguishable from the other structures 220a- h. For example, the first marker structure 215 may be implemented as any shape having a different number of vertices in cross-section to a number of vertices of each neighboring structure 220a-h in the subset. In an example, one of the first marker structure and the other structures 220a-h may be polygonal in cross section and the other of the first marker structure and the other structures 220a-h may be single-sided in cross section.

In embodiments, a cross-sectional area of the first marker structure 215 in a plane parallel to the substrate is substantially the same as a cross-sectional area of each structure 220a-h in the subset. Advantageously, by having a substantially same cross- sectional area, a relatively constant fill-factor may be maintained across a local area, which may improve an effectiveness of a nanoimprint process employed in manufacture of the device 200. There may be an allowable tolerance in matching the cross-sectional area of the first marker structure 215 and each structure 220a-h in the subset, such as +1-2 %, +/-5 %, or even +/-10 %. Such a tolerance may be defined, at least in part, by technical capabilities of the nanoimprint process and/or a material used.

In some embodiments, the device, e.g. the device 200, may comprise a plurality of marker structures. To illustrate this, a second marker structure 225 depicted in Figure 2b.

Although both the first marker structure 215 and the second marker structure 225 are both depicted as cruciform in shape, in other examples the first marker structure 215 and the second marker structure 225 may have different shapes, sizes and/or orientations to each other, thereby enabling the first marker structure 215 and the second marker structure 225 to be readily distinguished from one another by optical and/or SEM inspection.

Although only two marker structures 215, 225 are depicted, in examples a plurality of marker structures may provide multiple locations at which the device, e.g. the device 200, may be tested. For example, a marker structure may be placed every 20 or 30 structures. Advantageously, a speed of testing the device may be increased by implementation of multiple markers. Such a plurality of marker structures may be aligned in a substantially straight line, thereby increasing a speed of critical dimension measurements, as already described above in more detail.

Advantageously, an overall amount of marker structures 210, 225, and in particular an overall density of marker structures, may be selected to ensure that the marker structures have a negligible impact upon a performance of the metasurface. As such, the marker structures may remain present an implemented in production devices.

Advantageously, implementation of the disclosed marker structures on a metasurface may enable implementation of a fully automated method to monitor a size and shape of structures defining the metasurface inline and in-situ during production.

Figure 3 depicts a plan view of an example of a portion of a mold 300, according to an embodiment of the disclosure. The mold 300 may be for nanoimprinting a device as described above, e.g. device 200. As such, the mold 300 comprises a plurality of structures 305, wherein each structure 305 is effectively a negative, e.g. a cavity, recess or indentation, of a structure. The mold 300 also comprises a marker structure 310 corresponding to a negative of a marker structure, e.g. first marker structure 215 or second marker structure 225. The mold 300 may be a master mold. The mold 300 may be an intermediate mold formed from a master.

Figure 4 depicts a method of inspecting a metasurface, according to an embodiment of the present disclosure.

The metasurface may be a metasurface of a metalens, such as device 200 described above.

The metasurface may be a metasurface of a mold for nanoimprinting a device, such as the mold 300 described above. Therefore, it will be understood that the term ‘structure’ may refer to a column, pillar, fin, mesa or protrusion, but may similarly refer to a hole, channel, groove, indentation or the like.

The metasurface may be a metasurface of a resist layer. The resist layer may be a layer formed during a process of manufacturing a metasurface. As such, the resist layer may correspond to an intermediate processing steps in a method of manufacturing a metasurface.

A first step 405 comprises determining a location on the metasurface by identifying a marker structure amongst a plurality of structures defining the metasurface. As exemplified with reference to Figures 2a to 3 above, the marker structure has a different shape and/or orientation to each structure of a subset of the plurality of structures, wherein each structure of the subset neighbors the marker structure. As such, the marker structure may be readily distinguishable from the subset of structures.

A second step 410 comprises comparing a measured characteristic of one or more structures of the plurality of structures to a reference characteristic. The measured characteristic may correspond to an area, shape, size, spacing and/or geometry of the one or more structures. For example, the measured characteristic may be a cross- sectional area, wherein the cross-sectional area may correspond to a region of area, such as a center or periphery, of a structure neighboring a marker structure.

The location determined in the first step 405 may be used to identify the reference characteristic. The reference characteristic may correspond to a known or expected value, e.g. in a look up table or database or the like, or a calculated value.

In an example, the determined location may be associated with structures having certain cross-sectional areas. That is, a particular marker structure may be associated with a location known to have structures having the certain cross-sectional area. As such, once a location is identified by locating the marker structure, a cross-sectional area of surrounding structures may be measured and compared to an expected cross-sectional area, e.g. the reference characteristic. A substantial deviation from the expected cross- sectional area may be indicative of a manufacturing error, or issue such as degradation of a mold.

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 generalization 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.

REFERENCE NUMERALS

100 metalens

105 portion

110 first structure

115 second structure

120 third structure

125 cross-section

130a-m structures

140 substrate

D1 first width

D2 second width

D3 third width

200 device

205 portion

210 substrate

215 first marker structure

220a-h structure

225 second marker structure

300 mold

305 structures

310 marker structure

405 first step

410 second step