Rectifying antenna and method of manufacture

[0001] This application claims priority to Provisional U.S. Patent Application No. 60/514,327, “An Aperture Rectenna for Converting High Frequency Radiation into DC Power”, by the same inventors, filed Oct. 24, 2003.

[0002] Research relating to the present application was supported by NASA/FSEC. Accordingly, the U.S. federal government may have rights in the present invention.

[0003] Light energy is characterized by a dual nature both from a quantum point of view as photons and from a wave point of view as randomly polarized electromagnetic radiation with wavelengths from 400 nm to 700 nm. If the ultraviolet and infrared portion of the spectrum is included, the range of wavelengths is extended at both extremes. Presently all practical solar cell energy collection schemes use the photon nature of light in the explanation of operation. The conversion of solar energy to electrical energy using the photovoltaic effect depends on the interaction of photons with energy equal to or greater than the band-gap of the material. With continued research the maximum amount of energy captured using this mechanism is estimated to be around 30%. This is equivalent to an efficiency of 30%.

[0004] Another mechanism known in the art for converting solar energy to electrical energy is the rectenna. Rectenna technology relies on the electromagnetic nature of radiation and is not limited by the band-gap of the rectifying material. Advantages exist in the rectenna field because the rectenna approach is not fundamentally band-gap limited. Also, at microwave frequencies, 2.45 GHz, this concept has been demonstrated to be approximately 90% efficient.

[0005] Prior art exists in the field of rectenna technology. The idea of direct electromagnetic conversion of solar energy has been explored as early as 1973. The concept envisioned small cone type antennas with rectifiers mounted behind the cones and the cones arranged in an array as illustrated in FIG. 1. The work preceded the revolution in micro and nano scale manufacturing and was never realized. Other more recent researchers have replaced the cones with planar antenna elements such as half-wave dipoles, bow tie, and spiral antennas.

[0006] The previous efforts in researching rectennas for solar energy conversion have concentrated on either building the rectifiers on a supporting substrate and then building the antennas on top of the rectifying layer, or fabricating the antenna structure and the rectifier on a planar surface. The fabrication processes known in the art for rectenna elements rely on hybrid systems that incorporate different technologies for each component of the rectenna to optimize the system performance. While the incorporation of hybrid technologies often allows for a shorter development time, compromises must be made to ensure that each component can be fabricated without damaging components already existing on the substrate.

[0007] By contrast, monolithic device fabrication allows for all components of a system to be constructed on a single substrate utilizing one technology. Monolithic devices are typically more compact than hybrid devices, and more reliable due to the reduction in interconnections required. Additionally, once the fabrication process for a monolithic device has been developed, they can be manufactured more efficiently since less assembly is required. Monolithic technology minimizes relatively long, high-resistivity connections and high loss metal interconnects.

[0008] Accordingly, what is needed in the art is a monolithic rectifying antenna providing high efficiency solar energy conversion with improved reliability and manufacturability.

[0009] However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.

[0010] The longstanding but heretofore unfulfilled need for a monolithic rectifying antenna providing high efficiency solar energy conversion with improved reliability and manufacturability superior to rectifying antennas known in the art is now met by a new, useful, and nonobvious invention.

[0011] The present invention provides an integrated approach, at the nano-scale level, to fabricate a slot or aperture antenna integrated with a metal-insulator-metal (MIM) quantum tunneling diode to form a building block element in an array of rectenna elements for harvesting electromagnetic energy such as that emanating from the sun.

[0012] While many variations on the theme of an antenna coupled with a high frequency rectifier to harvest electromagnetic energy are known in the art, the present invention provides a novel and nonobvious solution to problems associated with prior art devices. In accordance with the present invention, an integrated nano-scale aperture antenna with a rectifier integrated using nano-scale monolithic processing is provided. Additionally, the rectifying antenna in accordance with the present invention lends itself to an inverted structure with the antenna deposited first on a transparent substrate and the rectifier and DC collection metallization fabricated on top of the antenna portion.

[0013] In accordance with the present invention is provided a rectifying antenna, comprising a transparent substrate layer, an aperture antenna fabricated on a surface of the transparent substrate, a rectifier having an anode and a cathode, the anode and the cathode electrically connected to the aperture antenna and separated from each other, the rectifier fabricated on top of the aperture antenna, a dielectric layer deposited to cover the aperture antenna and the rectifier, and a metal ground plane deposited over the dielectric layer, the metal ground plane being in electrical connectivity with the anode and the cathode of the rectifier.

[0014] While many materials may be used for the transparent substrate, in a particular embodiment the transparent substrate layer is glass.

[0015] The aperture antenna is fabricated is fabricated on the transparent substrate by depositing metal onto the surface of the transparent substrate and etching away the metal to form a slot and a dc break in the metal layer to reveal the transparent substrate, the slot and the dc break being contiguous. In a particular embodiment, the metal is gold, however other metals or metal alloys are within the scope of the present invention.

[0016] In a particular embodiment, the rectifying antenna includes an aperture or slot etched in the metal layer that is approximately 3λ/4 in major length and approximately λ/10 in width, where λ is the center wavelength of the radiation to be collected. The radiation to be collected may be emanating from the sun or another source of electromagnetic radiation as is known in the art.

[0017] The rectifying antenna in accordance with the present invention additionally includes, a dc break etched in the metal layer. In a particular embodiment, the dc break is etched to establish a slot through to the transparent substrate that is approximately λ/4 in major length.

[0018] To accommodate the positioning of the rectifier relative to the aperture antenna, the slot fabricated for the antenna further comprises a metal rectifier pad located approximately in the center of the slot and having a width of less than λ/10. This metal rectifier pad may be referred to as a conducting indentation and connectivity to one side of the rectifier as in the case of a metal-insulator-metal rectifier.

[0019] In a particular embodiment, the dielectric layer deposited to cover the antenna and the rectifier is fabricated to maximize the collection of radiation incident on the aperture antenna. In a particular embodiment, the dielectric layer is approximately λ/4 in thickness. In an additional embodiment, the dielectric layer is selected from the group consisting of ceramic, mica, glass, plastic and metal oxides.

[0020] While various rectifier elements are within the scope of the present invention, in accordance with a particular embodiment, the rectifier is a metal-insulator-metal (MIM) rectifier. As such, a first metal having a first potential and a second metal having a second potential are utilized in the fabrication of the MIM rectifier. In a particular embodiment, the first metal is platinum, the second metal is palladium, and the insulator of the MIM rectifier is selected from the group consisting of silicon dioxide, silicon nitride and aluminum oxide.

[0021] In accordance with an additional embodiment, the MIM rectifier may be replaced by a bolometer.

[0022] In an additional embodiment, a matching transmission line may be fabricated between the antenna and the rectifier to provide impedance matching. Additionally, a cooling mechanism may be employed in accordance with the present invention. The implementation of the cooling mechanism including, but not limited, to a liquid cooling channel positioned coincident with the ground plane or a silicon passive heat exchanger module.

[0023] To reduce the harmonics inherent in the rectifying antenna, it is within the scope of the invention to provide at least one notch filter positioned between the antenna and the rectifier in an effort to reduce unwanted harmonics.

[0024] In providing a rectifying antenna in accordance with the present invention, a method may be employed including the steps of, providing a transparent substrate layer, fabricating an aperture antenna on a surface of the transparent substrate, fabricating a rectifier on the surface of the aperture antenna, the rectifier having an anode and a cathode, the anode and the cathode electrically connected to the aperture antenna and separated from each other, depositing a dielectric layer to cover the aperture antenna and the rectifier and depositing a metal ground plane over the dielectric layer, the metal ground plane being in electrical connectivity with the anode and the cathode of the rectifier.

[0025] While various method are known in the art for the fabrication of elements on substrate materials, in a particular embodiment of the present invention the step of fabricating an aperture antenna on a surface of the transparent substrate further includes, depositing a first metal on the surface of the transparent substrate layer, etching away the deposited first metal to form a slot for the aperture antenna and a dc break.

[0026] In a particular embodiment, the rectifier element is a metal-insulator-metal rectifier. The method of fabricating the MIM rectifier further includes, forming a conducting indentation in the center of the antenna slot, the conducting indentation fabricated of a first metal and establishing the anode of the rectifier, depositing a dielectric layer on top of the conducting indentation and fabricating a second metal layer on top of the dielectric layer to form the metal-insulator-metal rectifier across the aperture antenna. In a preferred embodiment, the dielectric layer of the MIM rectifier is between approximately 1.5 nm and 2 nm and the second metal layer is deposited to a thickness of approximately 50 nm. These dimensions are intended to be exemplary and other thicknesses of the dielectric and the second metal layer are considered within the scope of the invention.

[0027] In an additional embodiment, a dielectric layer to cover the aperture antenna and the rectifier further is deposited and planarized to form a dielectric layer of substantially uniform thickness of λ/4, while other thicknesses are within the scope of the invention.

[0028] In accordance with the present invention, a plurality of rectifying antennas formed in accordance with the method as described may be connected to form an array of rectifying antennas.

[0029] As such, the present invention provides a monolithic rectifying antenna exhibiting high efficiency solar energy conversion with improved reliability and manufacturability.

[0030] With the present invention, the dc current travels in the z-direction thereby eliminating many lossy electrical connections due to additional metal interconnects, thereby improving reliability. The manufacturing of the device requires a minimal investment in nanotechnology manufacturing techniques.

[0031] Additionally, the delicate structures of the device are protected by the glass substrate.

[0032] Other aspects and advantages of the present invention can be seen upon review of the figures, the detailed description, and the claims, which follow.

[0040] The method in accordance with the present invention establishes a fabrication process to provide a smooth, strong surface to build an aperture antenna on and thus allow the construction of the rectifier on top of the antenna to minimize relatively long, high-resistivity and high loss metal interconnects. In a particular embodiment, a 3λ/4 slot antenna concept described by the present invention has been exemplified. Additional slot antenna dimensions are within the scope of the present invention.

[0041] The method in accordance with the present invention allows for the easy adaptation of dissimilar two metal metal-insulator-metal (MIM) rectifiers. In a particular embodiment, the MIM is build utilizing two metals exhibiting large differences in potential such as Pt or Pd combined with Au or W or Al. The structure requires a dielectric substance, such as SiO₂, Silicon Nitride or Aluminum Oxide. Other insulating materials are within the scope of the present invention.

[0042] With reference to FIG. 2, a rectifying antenna and method of manufacturing the same is provided wherein the construction of a slot rectenna includes the steps of, selecting a suitable transparent substrate 10, like glass, on which to build the structure, depositing a metal 15, like gold, and then etching away the slot 25 and dc break 20 as illustrated. The imaginary centerline 30 is also illustrated.

[0043] In a particular embodiment, the slot etched in the first metal (metal #1) layer opens a hole down to the transparent substrate, which is approximately 3λ/4 in major length and approximately λ/10 in width. Where λ is the center wavelength of the radiation to be collected and corrected for dilation due to the permitivity of the substrate. With reference to FIG. 2, the conducting indentation 35 in the center of the slot, will become one side of the MIM rectifier and the thin λ/4 slot extension on the right 20, will become the ac capacitive short/dc break for the structure. The ac capacitive short acts as a filter to separate the ac and dc components of the received signal.

[0044] After the slot 25 is etched in the metal layer, a dielectric layer 40 λ/4 thick is deposited over the entire first metal layer 15. Holes are opened in the dielectric layer to facilitate the second metal (metal #2) deposition. This is illustrated in FIG. 3 where the dielectric layer 40 has been deposited and holes 45, 50, 55 and 60 have been opened up to the metal #1 layer below.

[0045] In the process of opening the dielectric layer 40 certain areas such as the little pad 45 where the rectifier is to be constructed and one side of the ac capacitive short 60 must still have a thin layer of insulation 40. For the rectifier pad 45 this layer should be between 1.5 nm and 2 nm and for the capacitive short 60 the insulation should be thicker than 10 nm.

[0046] At this point in the fabrication process, the second metal layer 65 is deposited. The second metal layer 65 is approximately 50 nm thick. After the second metal layer 65 is deposited it is etched away from areas where it is not wanted, as shown in FIG. 4. As such, the metal-insulator-metal rectifier 75 is formed. Above metal #2 (second metal) the insulation layer is replaced and planarized 90 so it is a uniform layer of thickness λ/4 then the window for the two dc vias, 80 and 85, are opened up.

[0047] A final layer of the same material as metal #1 is then deposited to form a ground plane 95 and provide connection points to the +via 85 and −via 80 as illustrated in FIG. 5.

[0048] If the rectifying antennas were incorporated into an array, the top layer of the system would include interconnection of the individual rectenna elements so energy could be effectively captured on a macroscopic scale.

[0049] With reference to FIG. 6, an edge sectional view taken through the centerline 30 of the slot is shown.

[0050] In accordance with the present invention, the MIM can be constructed of a large variety of conducting metal combinations, the insulation can be inorganic as has been described or organic and can be made of many different types of materials. The shape of the slot and the diode may be varied. Due to impedance matching concerns between the antenna and the rectifier the dimensions of the slot may vary and it may be necessary to incorporate a matching transmission line between the antenna and the rectifier. Optional cooling methods may be employed such as liquid cooling channels next to the metal layer or a silicon passive heat exchanger module.

[0051] The non-linear nature of the rectifier causes the generation of harmonics. To prevent the loss of efficiency associated with the re-radiation at higher harmonics it is within the scope of the present invention to incorporate notch filters between the antenna and the rectifier.

[0052] Additionally, the array of slot rectennas could be replaced with an array of monopole traveling wave antennas manufactured using carbon nano-tube technology. The nano-tube antennas would be several wavelengths long and have a MIM built into their base.

[0053] In another embodiment of the invention, replaces the slot antenna with either a slot parasitic array antenna or a dipole parasitic array antenna. The use of the array concept maintains the high collection density, and handles the polarization problem to reduce the number of MIM structures required for energy conversion.

[0054] It will be seen that the objects set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0055] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Now that the invention has been described,