MICROWAVE DEVICE

Abstract

A microwave device includes a microwave cavity, a frame, and a window having an electrically insulating substrate and a structure of metallic wires supported by the substrate. The frame defines a perimeter of an opening in the microwave cavity and the frame is conductive and grounded. The window spans the opening and is arranged to reflect RF radiation back into the cavity and to shield the outside of the microwave cavity from RF radiation. The window is optically transparent. Each metallic wire of the structure is electrically connected to the frame and the width of each metallic wire is between 100 nanometres and 30 micrometres.

Claims

1. A microwave device comprising: a microwave cavity; a frame defining a perimeter of an opening in the microwave cavity, wherein the frame is conductive and grounded; and a window spanning the opening, wherein the window is arranged to reflect RF radiation back into the cavity and to shield the outside of the microwave cavity from RF radiation, wherein the window is optically transparent, the window comprising: an electrically insulating substrate; and a structure of metallic wires supported by the substrate, wherein each metallic wire of the structure is electrically connected to the frame, wherein the width of each metallic wire is between 100 nanometres and 30 micrometres.

2. The microwave device of claim 1, wherein the structure of metallic wires is periodic.

3. The microwave device of claim 1, wherein the period of the periodic structure is less than 500 micrometres.

4. The microwave device of claim 1, wherein the structure of metallic wires is a rectangular grid of intersecting wires.

5. The microwave device of claim 1, wherein each metallic wire of the structure has in-plane curvature.

6. The microwave device of claim 5, wherein the structure of metallic wires comprises a plurality of wire portions, wherein each wire portion is an arc being approximately a quarter of a circle, wherein each connection between adjacent wire portions is a T-junction.

7. The microwave device of claim 1, wherein the width of one or more metallic wire differs along the length of the metallic wire.

8. The microwave device of claim 1, wherein the total metallized area of the structure of metallic wires is less than 20% of the area of the opening.

9. The microwave device of claim 1, wherein the window further comprises: a secondary layer in a plane substantially parallel to the structure of metallic wires, wherein the second layer is arranged to reflect RF radiation back into the cavity and to shield the outside of the microwave cavity from RF radiation.

10. The microwave device of claim 9, wherein the secondary layer comprises a second structure of second metallic wires, wherein each second metallic wire of the second structure is electrically connected to the frame, wherein the width of each second metallic wire is between 100 nanometres and 30 micrometres.

11. The microwave device of claim 9, wherein the secondary layer is separated from the first structure, in a direction perpendicular to the plane, by between 0.08 and 0.42 times the effective wavelength of an operating frequency of the microwave device.

12. The microwave device according to claim 1, wherein the thickness of each metallic wire is between 100 nanometres and 30 micrometres.

13. The microwave device according to claim 1, wherein the window has one or more of the following properties: RF reflectance greater than 99%; RF absorbance of less than 1%; RF reflectance greater than 99% and RF absorbance of less than 1%; RF attenuation greater than 20 dB; RF attenuation greater than 40 dB; DC sheet resistance of the structure of metallic wires less than 2 Ohm per square and RF sheet resistance the structure of metallic wires less than 2 Ohm per square; optical transparency greater than 75%, DC sheet resistance of the structure of metallic wires less than 2 Ohm per square, and RF sheet resistance the structure of metallic wires less than 2 Ohm per square; DC sheet resistance of the structure of metallic wires less than 5 Ohm per square and RF sheet resistance the structure of metallic wires less than 5 Ohm per square; optical transparency greater than 90%, DC sheet resistance of the structure of metallic wires less than 5 Ohm per square, and RF sheet resistance the structure of metallic wires less than 5 Ohm per square; DC sheet resistance of the structure of metallic wires less than 100 Ohm per square and RF sheet resistance the structure of metallic wires less than 100 Ohm per square; optical transparency greater than 98%, DC sheet resistance of the structure of metallic wires less than 100 Ohm per square, and RF sheet resistance the structure of metallic wires less than 100 Ohm per square; transmissive optical haze less than 10%; transmissive optical haze less than 5%; and transmissive optical haze less than 2%.

14. The microwave device according to claim 1, wherein the microwave cavity includes a door, wherein the door comprises the frame and the window.

15. The microwave device according to claim 1, further comprising: a source of RF radiation arranged to emit RF radiation at an operating frequency into the microwave cavity, wherein the window is arranged to reflect RF radiation back into the cavity at the first wavelength and to shield the outside of the microwave cavity from RF radiation at the operating frequency.

16. The microwave device according to claim 1, further comprising: a plurality of frames including the frame, wherein each frame defines a perimeter of a respective opening of the microwave cavity, wherein each frame is conductive and grounded; and a plurality of windows including the window, wherein each window spans the respective opening of a respective frame, wherein each window comprises: an electrically insulating substrate; and a structure of metallic wires supported by the respective substrate, wherein each metallic wire of the structure is electrically connected to the respective frame, wherein the width of each metallic wire is between 100 nanometres and 30 micrometres.

17. The microwave device of claim 16, wherein the plurality of frames collectively covers the majority of the surface area of the microwave cavity.

18. A method of manufacturing a screen for shielding RF radiation, the method comprising: producing a pattern on a photosensitive material; depositing a structure of metallic wires on the photosensitive material according to the pattern, wherein the width of each metallic wire is between 100 nanometres and 30 micrometres; attaching a window to a frame, wherein the frame defines a perimeter of an opening, such that the window spans the opening, wherein the window is optically transparent, wherein the window comprises: an electrically insulating substrate; and the periodic structure of metallic wires supported by the substrate; and electrically connecting each metallic wire to the frame.

19. A screen for shielding RF radiation comprising: a frame defining a perimeter of an opening, wherein the frame is conductive and grounded; and a window spanning the opening, wherein the window is arranged to not transmit RF radiation therethrough, wherein the window is optically transparent, the window comprising: an electrically insulating substrate; and a structure of metallic wires supported by the substrate, wherein each metallic wire of the structure is electrically connected to the frame, wherein the width of each metallic wire is between 100 nanometres and 30 micrometres.

20. (canceled)

21. A multifunctional microwave metamaterial layer arranged to be reflective and attenuating to microwave radiation and simultaneously transparent to optical radiation, comprising: an electrically insulating, optically transparent substrate; and a structured array of metallic wire patterns supported by the substrate, wherein each metallic wire in each pattern of the array is electrically connected to at least one point on the periphery of the layer, wherein the width of each metallic wire is between 100 nanometres and 30 micrometres.

22. The metamaterial layer of claim 21, wherein the DC sheet resistance averaged over any sub-area of the metamaterial layer is less than 2 Ohm per square, and the optical transparency is greater than 75%.

23. The metamaterial layer of claim 21, wherein the DC sheet resistance averaged over any sub-area of the metamaterial layer is less than 5 Ohm per square, and the optical transparency is greater than 90%.

24. The metamaterial layer of claim 21, wherein the DC sheet resistance averaged over any sub-area of the metamaterial layer is less than 100 Ohm per square, and the optical transparency is greater than 98%.

25. The metamaterial layer of claim 21, wherein the layer is arranged to have transmissive optical haze less than either of the 10%, 5%, 2%.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0035] Specific embodiments are now described by way of example and with reference to the accompanying drawings, in which:

[0036] FIG. 1 shows a microwave device;

[0037] FIG. 2 shows a frame and window of a microwave device and a zoomed-in portion of a periodic structure of metallic wires;

[0038] FIG. 3 shows a portion of a periodic structure of metallic wires;

[0039] FIGS. 4a and 4b show graphs of results;

[0040] FIG. 5 shows a portion of a periodic structure of metallic wires;

[0041] FIG. 6 shows a diffraction plot for a periodic structure as shown in FIG. 3;

[0042] FIG. 7 shows a diffraction plot for a periodic structure as shown in FIG. 5;

[0043] FIG. 8 shows two periodic structures of metallic wires;

[0044] FIG. 9 shows an attenuation versus separation plot;

[0045] FIG. 10 shows a method of manufacturing a screen for shielding RF radiation; and

[0046] FIG. 11 shows a graph of transparency versus sheet resistance for various types of RF shielding.

DETAILED DESCRIPTION

[0047] In overview, microwave devices for processing items using microwave radiation have various uses. Some microwave devices are microwave ovens, such as consumer microwave ovens or commercial kitchen microwave ovens for cooking food, others are used for heating or drying other types of objects such as clothing. Other microwave devices include lab devices for testing samples under microwave radiation. In any of these examples, it is important for microwave radiation not to escape the microwave cavity, since this could cause harm to nearby objects or people. For typical consumer microwave ovens this is done using a metal mesh, wherein the holes and spacing between holes are just less than or close to the wavelength of microwave radiation, i.e. around 1 millimetre to 1 centimetre. This has a similar effect to a conductive sheet (since the holes in the mesh are sub-wavelength) which provides shielding against the microwave radiation, e.g. reflects. The holes provide some visibility into the microwave cavity for a user to gain a limited view of the contents during processing. In contrast, the microwave devices as described herein have higher effective shielding, improved optical properties, or both.

[0048] With reference to FIG. 1, a microwave device 100 comprises a microwave cavity 110, a box comprising six sides enclosing a region in which objects to be processed with microwave radiation is placed. One side of the microwave cavity has a frame 120, in this case a rectangular frame forming a border of the side of the cavity. The frame attaches to the other sides of the cavity, e.g. by a hinge. The frame has an opening in an interior portion through which the contents of the microwave cavity 110 can be seen. In order to shield the viewer from microwave radiation in operation, the opening is covered by a window 130 which is both transparent and shields microwave radiation.

[0049] Apart from the frame and window as disclosed herein, the form and properties of the microwave device and its components may be according to any conventional technology for microwave devices, e.g. turntables, user interfaces, processors for controlling radiation application, etc. A source of microwave radiation (not shown) may be part of the microwave or, alternatively, could be external with the produced radiation produced being directed into the microwave device via waveguides. The microwave cavity walls in general are made from metal and the frame can be made from metal. Alternative materials are possible as well, provided that the frame is conductive. The frame is also grounded. Being grounded means that, the frame is arranged such that, in use, there is a relatively low resistance electrical pathway from the frame to the earth. For example, this may be through the feet of the microwave device, through a plug socket, etc.

[0050] In alternative arrangements, the microwave device may have multiple frames on a single side of the microwave cavity, e.g. defining several openings for viewing, or there may be one or more frames on multiple sides of the microwave cavity 110.

[0051] With reference to FIG. 2, the frame 120 and window 130 will be described in further detail. The window 130 comprises a substrate 132 supporting a structure 134 of metallic wires 136. The width of the metallic wires is below the level of resolution for an unaided human eye at a distance of approximately 1 metre away, which is a typical distance from within which a user might be viewing inside the microwave cavity. Further, the substrate 132 of the window 130 is transparent and so the window as a whole does not inhibit the user's view into the microwave cavity 110.

[0052] As shown in the zoomed-in portion to FIG. 2 (which is not to scale), in an example the structure 134 takes the form of a rectangular grid, with rows and columns of metallic wires 136 intersecting at the grid points.

[0053] With reference to FIG. 3, showing a portion of a structure of metallic wires in the form of a grid, the width of the metallic wires is denoted ‘2a’ (wherein ‘a’ is half-width). The period between two columns or rows of the grid is denoted ‘g’. With these parameters, an analytical model for the transmission of the structure of metallic wires is:

[00001]$\begin{array}{cc}T\cong {\frac{4g^2}{{\lambda }^2}\left[\ln \left(\sin \frac{\pi a}{g}\right)\right]}^2& \left(1\right)\end{array}$

[0054] With reference to FIG. 4, the relationships between width and period with RF attenuation and optical transmittance are plotted. FIG. 4A illustrates RF Attenuation at 3 GHz vs. Optical transmittance for a selection of metallic wire widths and periods. Optical transmission is varied by keeping one wire parameter constant, i.e. width or period, while varying the other parameter to produce a plotted line. The dotted lines (from top to bottom of the graph) are for a width of 0.2 μm, 0.6 μm, 1.0 μm and 2.0 μm. The solid lines (from top to bottom) are for a period of 6 μm, 30 μm, and 150 μm. Optical transmittance can be determined by the fill factor (metallized area divided by the total area of the window), or conversely by aperture ratio (open area divided total area). At any transmittance value, smaller wire width enables higher EMI shielding. For this calculation it is assumed that the wires are thicker than the skin depth (e.g. approximately 1 μm at 3 GHz frequency).

[0055] EMI shielding increases with smaller linewidths for the same fill factor of the structure of metallic wires, while the fill factor determines the transparency of the window. Therefore, using metallic wires with less than 30 μm, and even more so for sub-micron widths, significantly improves the shielding effectiveness compared to conventional microwave shielding.

[0056] FIG. 4B shows the measured shielding effectiveness (microwave attenuation in dB) for two designs of the structure of metallic wires, across a range of frequencies from 5 to 20 GHz. A high shielding (60-70 dB attenuation) is achieved without sacrificing optical transparency (˜90%+ optical transmission).

[0057] With reference to FIG. 5, as an alternative to a rectangular grid, a structure of metallic wires has curvature in the plane of the substrate. FIG. 5 is a zoomed-in birds-eye view of the surface of the window 130. In the example arrangement as shown in FIG. 5, the structure 134 of metallic wires is made up of a number of wire portions 138, each of which are curved approximately in the shape of an arc of a circle, approximately a quarter circle. The ends of each wire portion (except for at the edge of the structure) join an adjacent wire portion at an intermediate position of the adjacent wire. The join is a T-junction, i.e. the end of one wire portion meets the other wire portion perpendicularly. The intermediate position of the adjacent wire portion is at approximately a fifth or a quarter of the length along the wire portion. More generally, the intermediate position may be in a third of the wire portion length towards either end of the wire portion. The reliability of connection between wire portions, and so the metallic wires being connected across the structure is improved by using T-junctions compared to having wire portions cross over each other in an X shape. This is because, for a T-junction, no position of the metallic wire is further than a half-width away from the edge of the metallic wire. By comparison, for wires crossing perpendicularly (in an X shape), the mid-point of the cross is a distance away from the edge of the metallic wire of the square root of 2 (approximately 1.41) times the half-width of the metallic wires. In some circumstances, this results in a break in the continuity of the metallic wires due to the fabrication process which is designed to deposit metal of the normal width of the wire. In turn, breaks in the structure of metallic wires may compromise the high DC conductivity, and low DC resistivity, of the structure. A characteristic dimension of the structure 134 is the distance between the concave sides of wire portions which face each other, denoted as ‘D’. This is approximately half of the period of the structure 134.

[0058] With reference to FIGS. 6 and 7, a structure wherein the metallic wires have in-plane curvature improves the optical performance of the window 130 compared to metallic wires in a grid. FIG. 6 shows a polar plot of the diffraction pattern from a grid of metallic wires and shows a zoomed-in portion A of the central part of the diffraction pattern. The diffraction pattern exhibits a strong signature at the centre and along two directions corresponding to the rows and columns of the grid. FIG. 7 also shows a polar plot of a diffraction pattern, except for a structure of metallic wires having in-plane curvature, e.g. curved wire portions. The diffraction pattern is more even than the diffraction pattern in FIG. 6, and the peak value is lower. This more even diffraction pattern reduces the visual impact of the window, providing better optical properties for viewing the contents of the microwave cavity. Accordingly, by choosing curved metallic wires, the optical properties of the window 130 are further improved.

[0059] With reference to FIG. 8, in an arrangement, the window comprises a secondary layer comprising a second structure 234 of second metallic wires 236. The secondary layer is separated from the first structure 134 of first metallic wires 136 by a distance denoted ‘S’. Having a secondary layer also arranged to shield RF radiation increases the overall shielding of the window by further attenuating microwave radiation.

[0060] While the secondary layer shown in FIG. 8 is of a second structure 234 of second metallic wires 236, the secondary layer could be a different time of transparent shielding layer, e.g. using an ITO layer, etc. Likewise, while a structure in the form of a grid is shown in FIG. 8, this could alternatively be a different structure, e.g. having in-plane curvature.

[0061] With reference to FIG. 9, having a secondary layer generally provides greater attenuation (coloured blue in the plot of FIG. 9). However, for certain combinations of separation distance S and frequency, there will be a transmission maximum (coloured yellow or red in the plot of FIG. 9). This is a result of the two RF reflective layers (the first structure and the secondary layer) making a form of Fabry-Perot resonators. Accordingly, where a particular separation S meets the Fabry-Perot condition for transmission maximum, there will be a decrease in RF attenuation. For example, for a separation of 6 mm, there are decreases in RF attenuation at approximately 17 GHz and 33 GHz. For 3 mm, there is a decrease in RF attenuation at approximately 33 GHz only. For 2 mm, there is a slight decrease at low frequencies less than 10 GHz. Accordingly, it is advantageous for the separation S to be less than 3 mm, e.g. approximately 2 mm. This maintains the benefit of an additional RF reflective layer but does not result in higher order Fabry-Perot resonances.

[0062] With reference to FIG. 10, a method 10 for manufacturing a screen for shielding RF radiation comprises producing 11 a periodic pattern on a photosensitive material, depositing 12 a structure of metallic wires on the photosensitive material according to the pattern, attaching 13 a window to a frame, the window comprising an electrically insulating substrate and the periodic structure of metallic wires and electrically connecting 14 each metallic wire to the frame. The producing the pattern may be done by applying a mask to the photosensitive material, removing portions of the mask and etching the patterned mask such that troughs are present where portions of the mask were removed. In an example, the producing and depositing may be performed by Rolling Mask Lithography® as described, for example, in U.S. Pat. No. 9,244,356 to Boris Kobrin, et. al, issued Jan. 26, 2016, the entire contents of which are herein incorporated by reference.

[0063] The connecting each metallic wire to the frame may entail depositing one or more conductive bridge between the frame and the structure of metallic wires. This may be done as part of the depositing 12 the structure of metallic wires, e.g. the pattern extends onto the frame. As another example, the depositing one or more conductive bridge may be performed subsequently to the depositing 12 of the structure, e.g. using typical metal depositing techniques. As another example, the frame may have one or more conductive protrusions which, when attaching 13 the window to the frame, contact the structure of metallic wires thereby electrically connecting the frame and structure. In any example, the electrical connections between the metallic wires and the frame may be at one or more positions of the structure of metallic wires, e.g. at each corner of the window.

[0064] With reference to FIG. 11, a microwave device according to the present disclosure has a window with improved RF shielding and optical properties. A comparison between a substrate with a structure of metallic wires according to the present disclosure, circled points in red, with the other shielding materials is shown in FIG. 11. The present disclosure provides a much lower sheet resistance at lower values (and therefore improved shielding) for a similar level of transparency, or much higher transparency for a similar level of sheet resistance, compared to silver nanowires, ITO, graphene, carbon nanotubes, etc. according to various specifications.

[0065] It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described but can be practised with modification and alteration within the scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. Although various features of the approach of the present disclosure have been presented separately (e.g., in separate figures), the skilled person will understand that, unless they are presented as mutually exclusive, they may each be combined with any other feature or combination of features of the present disclosure.