Patent Publication Number: US-6707429-B1

Title: Self-contained sub-millimeter wave rectifying antenna integrated circuit

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) in which the Contractor has elected not to retain title. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is related to power supplies for deployable Microsystems such as TeraHertz sensors, bioengineering nanodevices, micro-robots, nanofabrication and planar antennas. Supplying electrical power to micro-devices from a battery or from wires is often impractical because the weight of the wires or battery may impair the performance of the microdevice being powered. The present invention provides electrical power from electromagnetic radiation incident on a local antenna mounted on the microdevice to be powered. One problem with such an arrangement is that the antenna performance is affected by the electrical characteristics of the microdevice on which it is mounted. Thus, the design of the underlying microdevice is constrained so as to avoid detracting greatly from antenna performance, which is inconvenient. Another problem is that an antenna sufficiently small to fit on a microdevice, such as a micro-miniature dipole antenna, will typically have poor gain in the direction of the radiation because such an antenna will have little directionality. A further problem is that a diode must be employed to rectify the received RF power. The impedance of the diode will not necessarily match the impedance of the antenna, depending upon the frequency of the incident radiation, so that some power will be lost. Yet another problem is to find a radiation frequency at which the ideal antenna size is small compared to the microdevice on which it is to be mounted, but not so small that the frequency reaches the optical range in which a diode rather than an antenna must be used. This would sacrifice the advantage of tunability of an antenna. Further, it would be desirable if the radiation frequency were one that readily penetrated certain materials such as plastic, human skin (for bio-engineering applications) and the like. 
     SUMMARY OF THE DISCLOSURE 
     The invention is embodied in a monolithic semiconductor integrated circuit in which is formed an antenna, such as a slot dipole antenna, connected across a rectifying diode. In the preferred embodiment, the antenna is tuned to received an electromagnetic wave of about 2500 GHz so that the device is on the order of a wavelength in size, or about 200 microns across and 30 microns thick. This size is ideal for mounting on a microdevice such as a microrobot for example. The antenna is endowed with high gain in the direction of the incident radiation by providing a quarter-wavelength (30 microns) thick resonant cavity below the antenna, the cavity being formed as part of the monolithic integrated circuit. Preferably, the integrated circuit consists of a thin silicon membrane overlying the resonant cavity and supporting an epitaxial Gallium Arsenide semiconductor layer. The rectifying diode is a Schottky diode formed in the GaAs semiconductor layer and having an area that is a very small fraction of the wavelength of the 2500 GHz incident radiation. Preferably, the antenna is a pair of half-wavelength dipole slots in the overlying conductor layer that forms respective power output terminals and respective tuning capacitors across the rectifying diode. At the 2500 GHz frequency, the pair of half-wavelength dipoles exhibit an impedance that nearly matches the impedance of the Schottky rectifying diode, a significant advantage. A most significant advantage is provided by the combination in the integrated circuit of the antenna with the quarter wavelength resonant cavity, because the antenna behavior is determined principally by the resonant cavity. The resonant cavity both provides the directional gain of the antenna and isolates the antenna from surrounding structure. In this way, the integrated circuit may be mounted on any structure without appreciably affecting the antenna behavior. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top perspective view of an integrated circuit embodying the present invention. 
     FIG. 2 is a bottom perspective view of the integrated circuit of FIG.  1 . 
     FIG. 3 is a transparent view corresponding to FIG.  1 . 
     FIG. 4 is a partially cut-away perspective view corresponding to FIG.  3 . 
     FIG. 5 is an enlarged cross-sectional side view of a portion of the integrated circuit of FIG. 1 showing the structure of a Schottky diode therein. 
     FIG. 6 is an enlarged perspective view of the Schottky diode of FIG.  5 . 
     FIG. 7 is an enlargement of portions of FIG. 6 showing in greater detail the semiconductor structure. 
     FIG. 8 is an electrical block diagram of an equivalent circuit corresponding to the apparatus of FIG.  1 . 
     FIG. 9 is a diagram illustrating a side view of a 3-dimensional antenna power distribution pattern of the antenna of the apparatus of FIG.  1 . 
     FIG. 10 is a graph showing the predicted electric field angular distribution of the antenna of the apparatus of FIG.  1 . 
     FIG. 11 is a graph showing the predicted magnetic field angular distribution of the antenna of the apparatus of FIG.  1 . 
     FIG. 12 is a Smith chart of the antenna of the apparatus of FIG. 1, showing the occurrence of a nearly purely resistive impedance of the antenna at a selected frequency (2445 GHz) that nearly matches the impedance of the Schottky diode at the same frequency. 
     FIG. 13 illustrates the mounting of the apparatus of FIG. 1 on a known microrobot. 
     FIG. 14 illustrates how the slot antenna of FIG. 1 may be replaced by an equivalent conductor antenna. 
     FIG. 15 illustrates a modification of the embodiment of FIG. 1 in which the single rectifying diode is replaced by a full-wave diode rectifying bridge. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1,  2 ,  3  and  4 , an integrated circuit  100  converts incident submillimeter wave radiation of a selected frequency to D.C. power. The integrated circuit  100  consists of a thin base layer  110  formed from a wafer such as a silicon wafer. The thickness of the base layer  110  may be about one quarter wavelength of the incident radiation, or about 30 micrometers (microns) if the submillimeter wave radiation frequency is 2500 GHz. Such a 30 micron base layer  110  may be formed by chemical mechanical polishing of a conventional silicon wafer to the desired thickness, for example. A metallic thin film layer such as gold  115  is formed on the bottom face of the base layer  110  and an etch stop layer  120  is formed on the top face of the base layer. The composition of the etch stop layer  120  depends upon the etchant employed to etch the base layer  110  as will be described below, and may be, for example, silicon nitride. A thin membrane  125  is formed over the etch stop layer  120 , the membrane  125  being about 2 or 3 microns thick and being of either silicon or gallium arsenide. 
     A first etchant that is selective to gold is employed to etch a mesh pattern in the bottom gold layer  115  consisting of an array of small openings  115   a . The length and width of each of small openings may be about one tenth of the incident radiation wavelength, or about 12 microns. A second etchant selective to silicon is employed to flow through the openings  115   a  and etch out the interior of the silicon base layer  110  to form a hollow rectangular cavity  135  inside the base layer  110 , the cavity  135  being shown in FIGS. 3 and 4. The cavity  135  may be slightly in excess of one wavelength in length and width, or about 140 microns. The etch stop layer  120  fixes the depth of this etch step, and thereby determines the depth of the cavity  135 . Preferably, this depth is about a quarter wavelength, or about 30 microns. 
     A slot antenna structure is formed by etching a pair of parallel slots  140 ,  145  through the gold layer  200  and through the silicon membrane  125 . The slots  140 ,  145  are each about a half wavelength in length and their center-to-center spacing is also about a half wavelength, or about 60 microns for a 2500 GHz radiation frequency. They are each about 8 microns in width. 
     FIGS. 1-4 indicate a Schottky diode generally at  147 , the structure of this diode being too small for convenient illustration in these figures. The structure of the Schottky diode  147  is illustrated in detail in the exploded views of FIGS. 5-7. In order to form the diode  147 , a very small window (too small to be seen in FIGS. 1-4 but visible in FIGS.  5 - 7 )) is formed in the gold layer  200  in the region of the diode  147  in order to expose the top surface of the mesa  125  in this small region. A gallium arsenide (GaAs) active semiconductor layer  130  is formed over the small portion of the membrane  125  that has been thus exposed. Preferably, the GaAs layer consists of a bottom highly doped n-type (n+) layer  130   b  and a top lightly doped n-type (n) layer  130   a . Referring to FIGS. 5,  6  and  7 , the GaAs layer  130  is etched to form a GaAs mesa  150  and, if desired, an optional second GaAs mesa  155 . 
     An insulating (e.g. silicon dioxide) layer is formed over the entire structure and then etched to define elongate insulating mesas  160 ,  165 . A conductor (gold) layer is deposited and then etched to define a first elongate conductor  175  on the elongate insulating mesa  160  and bridging between the insulating mesa  160 , the GaAs mesa  155  and the GaAs mesa  150 , and a second elongate conductor  170  on the other elongate insulating mesa  165  bridging between the insulating mesa  165  and the GaAs mesa  150 . The elongate gold conductors  170 ,  175  and the underlying elongate insulating mesas  160 ,  165  are generally congruent so that the gold conductors  170 ,  175  are everywhere insulated from the underlying gold layer  200 . The Schottky diode  147  is formed at the contacts made by the two conductors  170 ,  175  to the top surface of the GaAs mesa  150 . 
     The overall configuration of the two conductors  170 ,  175  is visible in FIG. 1, showing conductors  170 ,  175  extending away from the Schottky diode  147  and over respective ones of the slots  140 ,  145 , and forming respective tuning capacitors  180 ,  185  adjacent respective slots  140 ,  145 . The dielectric of the tuning capacitors  180 ,  185  is the silicon dioxide layer forming the insulating mesas  160 ,  165 . The conductors  170 ,  175  are terminated in respective pads  190 ,  195  that are the external connectors of the integrated circuit  100  and supply D.C. electrical power to a component connected across the pads  190 ,  195  such as a microrobot, for example. 
     The conductors  170 ,  175  effectively divide the respective slots  140 ,  145  into two halves in the manner of a dipole, forming the slot antenna pattern in the gold layer  200  equivalent to a dipole antenna. The insulating (silicon dioxide) mesas  160 ,  165  electrically separate the gold conductors  170 ,  175  from the gold film  200  in the manner indicated in FIG.  6 . Moreover, as shown in FIG. 7, the gold film  200  is terminated away from the GaAs mesa  150  so as to not interfere with the Schottky diode  147  (the conductors  170 ,  175  and the insulating layers  160 ,  165 , are omitted from the partial view of FIG. 7 for the sake of clarity). 
     The pair of capacitors  180 ,  185  shown in FIG. 1 are sized to provide an optimum antenna impedance match. The quarter wavelength thick resonant cavity  135  in combination with the pair of dipole slots  140 ,  145  form a highly directional beam antenna whose characteristics (e.g., gain, resonance, etc.) are governed by the cavity  135 . A significant advantage of the cavity  135  is that the reactance of nearby structures to which the integrated circuit  100  may be attached (such as various microrobots or bioengineering devices) do not affect antenna performance. Therefore, the integrated circuit  100  may be mounted on any device to which D.C. electrical power is to be supplied. One advantage of the selected submillimeter wave frequency (about 2500 GHz) is that radiation emanating at that frequency from a remote source toward the integrated circuit  100  is capable of penetrating various materials such as plastic, skin or flesh and the like. Thus, the integrated circuit and the microdevice to which it is attached may be buried under a layer of material or under the skin (for bioengineering applications). Another advantage that will be explored in greater detail below is that the antenna has a nearly purely resistive impedance that matches the impedance of the Schottky diode at this frequency. Also, since the selected frequency is clearly below optical frequencies, a RF antenna such as the one disclosed herein may be employed rather than an optically responsive diode. The advantage is that the antenna may be tuned (by selecting the tuning capacitors  180 ,  185 ) across a range of frequencies whereas an optical detecting diode must be designed with a bandgap matching the radiation frequency and therefore cannot be readily tuned. 
     FIG. 8 is a block diagram of the integrated circuit  100  connected to supply D.C. electrical power to a load  800  such as a microrobot. The dipole antenna  805  consists of the pair of slot dipole antennas  140 ,  145  and the resonant cavity  135  of FIG.  1 . The diode  810  is the diode  147  of FIG.  1 . The filter  820  is the pair of filter capacitors  180 ,  185  of FIG.  1 . The connecting pad  830  is the pair of conductive pads  190 ,  195  of FIG.  1 . 
     FIG. 9 illustrates one plane of the 3-dimensional spatial distribution of the gain of the slot antenna structure  135 ,  140 ,  145  of FIG.  4 . The relative gain is plotted as the length of a vector extending from the origin to the curve as a function of angle of incidence A. The plot of FIG. 9 shows that there is a very large forward-to-back gain ratio (in excess of 6 dB or more) and a narrow beam width in the forward direction (65 degrees at 3 dB). The narrow beam width is confirmed by the plots of E-field and H-field attenuation as a function of the angle A of FIGS. 10 and 11 respectively. 
     FIG. 12 is a Smith chart of the impedance of the slot antenna structure  135 ,  140 ,  145 FIG. 4 for impedances normalized to 50 Ohms. FIG. 12 indicates that at a frequency of 2445 GHz the impedance is almost purely resistive at a normalize value of 0.32, which is 16 Ohms. This is the impedance of the Schottky diode  147  at 2445 GHz, so that a nearly perfect impedance match is provided for optimum power transfer efficiency. 
     FIG. 13 illustrates the integrated circuit  100  mounted on a microrobot  400  with 2445 GHz radiation (from a laser for example) illuminating the integrated circuit  100 . 
     FIG. 14 illustrates how the pair of dipole slot antennas  140 ,  145  may be replaced by an equivalent pair of conductor dipole antennas  310 ,  315 . The gold film  200  covering the exposed top of the membrane  125  is eliminated and the dipole antennas are formed integrally with the gold conductors  170 ,  175  in the pattern illustrated in FIG.  14 . The length of each dipole antenna  310 ,  315  is one half wavelength. Each dipole antenna  310 ,  315  is divided into two sections ( 310   a ,  310   b  and  315   a ,  315   b ), the diode  147  being connected across the two sections of each dipole  310 ,  315 . The center-to-center spacing between the two dipole antennas  310 ,  315  is a half wavelength. A pair of tuning capacitors  330 ,  340  connected to opposite sides of the diode  147  may be formed in the gold conductor pattern as shown in FIG.  14 . 
     FIG. 15 illustrates how a full wave rectifier bridge of four matched Schottky diodes  610 ,  620 ,  630 ,  640  may replace the single Schottky diode  147  of FIG.  1 . In FIG. 15, the conductors  170 ,  175  each extend only from the far side of a respective slot  145 ,  140  to a corresponding terminal pair of the rectifier bridge, while a pair of output conductor  650 ,  660  extend from the remaining terminal pair of the rectifier bridge to output pads  670 ,  680 . 
     While the antenna length of the preferred embodiment is a half wavelength, other suitable lengths may be employed such as multiples of ⅛ (e.g., ⅝ wavelength). Moreover, while the cavity length and width have been described as being preferably about one wavelength, they may be multiples of one wavelength. Moreover, the cavity thickness, while having been described as being preferably one quarter wavelength, may be odd multiples of one quarter wavelength. However, it should be noted that it is felt the performance described herein is expected to be most readily attained in the preferred embodiment. 
     While the invention has been described in detail with reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.