Abstract:
A memristive radio frequency (RF) switch circuit comprises a first metal electrode and a second metal electrode arranged on an insulating substrate and separated by an air gap, wherein the air gap is fifty nanometers (50 nm) or less, and wherein applying and removing an enabling voltage to the memristive RF switch enables the memristive RF switch to pass RF signals between the first electrode and the second electrode even when the enabling voltage is removed from the memristive switch, and wherein applying and removing a disabling voltage to the memristive switch disables the memristive switch.

Description:
CLAIM OF BENEFIT 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/980,884, filed on Apr. 17, 2014, which is hereby incorporated by reference in its entirety. 
     
    
     GOVERNMENT RIGHTS 
       [0002]    This invention was made with United States Government support under Grant Number N66001-12-1-4217. The United States Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0003]    Radio frequency (RF) systems include RF switches. An RF switch is a routing device that can be used to enable different circuit paths in the RF system. Some requirements of an RF switch are low insertion loss and high isolation. RF switches in use today are solid state switches in which a flow of electrons is turned on or off, such as by application of a gate voltage. Another type of RF switch is a microelectromechanical system (MEMS) RF switch that includes movable parts of sub-millimeter (mm) dimensions. Phase change material (PCM) RF switches may be switches that transition between a crystalline state and an amorphous state, or between a metallic phase and insulator phase through heating. MEMS RF switches and PCM RF switches typically have one or more disadvantages in terms of high power consumption, large size, limited reliability, low usable temperature range, and low speed. The present inventors have recognized a need for improvement in RF switches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example and not by way of limitation, various embodiments discussed in the present document. 
           [0005]      FIG. 1  shows an example of a memristive I-V curve. 
           [0006]      FIG. 2  shows a circuit schematic for a memristive switch circuit. 
           [0007]      FIGS. 3A and 3B  show an example of a memristive RF switch circuit. 
           [0008]      FIGS. 4A and 4B  show a picture of an example of a memristive RF switch circuit in the on state. 
           [0009]      FIG. 5  illustrates portions of an example of a fabrication process for a memristive RF switch circuit. 
           [0010]      FIG. 6  illustrates portions of another example of a fabrication process for a memristive RF switch circuit. 
           [0011]      FIG. 7  shows a picture of another example of a memristive RF switch circuit. 
           [0012]      FIG. 8  shows an example of an RF circuit model for a memristive RF switch circuit including parasitic effects. 
           [0013]      FIG. 9  shows an example of multiple memristive RF switch circuits electrically coupled together. 
           [0014]      FIG. 10  is picture of a monolithic CMOS integrated circuit die. 
           [0015]      FIG. 11  illustrates a block diagram of portions of an example of an RF system. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As explained above, there are tradeoffs involved in RF switches in use today. An approach to improve RF switch performance is to use a memristive switch. A memristive device is a non-linear passive device that has a pinched hysteresis current-voltage (I-V) curve. Memristive devices can be two terminal non-volatile resistance switches whose internal states are determined by the history of one or both of applied voltage and applied current. 
         [0017]      FIG. 1  shows an example of a memristive I-V curve. A memristive device has a large off resistance (Roff) and a low on resistance (Ron). The lobes of the hysteresis curve correspond to an on-switching loop and an off switching loop. The device ‘remembers’ or maintains its previous state in the absence of current or voltage sufficient to change the state. This makes the memristive device a nonvolatile device because it does not need power to maintain its present state. 
         [0018]      FIG. 2  shows a schematic of a circuit model for a memristive switch  205 . Instead of performing a switching function by controlling a flow of electrons, a memristive switch uses a low resistance state as the on state and a high resistance state as the off state. Because a memristive switch is nonvolatile it uses low energy. The challenge for the design of a memristive RF switch is to meet the design requirements for use in an RF system. These requirements include low insertion loss and high isolation. Low insertion loss can be achieved by minimizing Ron and high isolation can be achieved through low capacitance in the ‘off’ state (Coff). Additional requirements for a memristive RF switch are a high Off/On resistance ratio (or high ON/OFF conductance ratio) to reduce performance degradation and a high cutoff frequency (fc=1/(2π*RonCoff)) to accommodate a wide wider badnwidth. A memristive RF switch can have a fast switching speed (e.g., a sub-nanosecond transition time between states and is low power (e.g., sub picojoule). 
         [0019]      FIGS. 3A and 3B  show an example of a memristive RF switch circuit  305 . The switch is a nano-scale device.  FIG. 3A  is an illustration of the memristive RF switch circuit and  FIG. 3B  is a picture of a fabricated memristive RF switch circuit taken with a scanning electron microscope (SEM). The switch includes opposing electrodes and is a parallel device with an air gap separation between the electrodes. To enable the memristive switch a voltage (Vset) is applied between the electrodes. A conductive filament forms between the electrodes as a result of the voltage and the resistance of the switch changes to Ron. To turn the memristive switch off, a voltage of opposite polarity (Vreset) is applied between the electrodes to rupture the conductive filament and the resistance of the switch changes to Roff. 
         [0020]      FIG. 4A  shows a picture of an example of a memristive RF switch circuit in the on state. When the disabling voltage is applied, silver ions leave the electrode to form the conductive filament shown.  FIG. 4B  shows a picture of the memristive RF switch circuit in the off state. When the enabling voltage is applied, the conductive filament ruptures. It is desirable to keep the enabling and disabling voltages to a low value to allow the switch to be compatible with other electronic devices and to lower the energy requirement of the switch. 
         [0021]    In some examples, the material comprising the electrodes may be electrochemically symmetric on both electrodes. For instance, both electrodes may be made of silver s s s containing silver. With an air gap of fifty nanometers (50 nm), Ron was measured to be approximately 3.6 ohms (3.6. Measured isolation was thirty decibels (30 dB) at forty gigahertz (40 GHz). 
         [0022]    In the example shown in  FIGS. 3A and 3B , the material of the electrodes is asymmetric. For instance, the first electrode (the right electrode in the example of  FIG. 2 ) includes silver (Ag) and the other electrode includes nickel with gold (Au/Ni) or titanium with gold (Au/Ti). In variations, the electrodes can include one or a combination of silver, gold, platinum (Pt), aluminum (Al), and nickel. The electrodes are formed on a layer of thermal oxide (e.g., silicon dioxide (SiO 2 )) arranged on top of a silicon substrate. The air gap results in a low dielectric constant which lowers the capacitance of the memristive switch and improves isolation. The thermal oxide layer reduces coupling to the silicon substrate. 
         [0023]    A memristive RF switch circuit can be formed using semiconductor device fabrication methods.  FIG. 5  illustrates an example of portions of a fabrication process for a memristive RF switch circuit. Simplified side views of the device are shown in the example. In part A, resist (e.g., polymethyl methacrylate or PMMA) of the gap size (e.g., 50 nm) is deposited onto a layer of SiO 2  formed on a silicon wafer. In the example shown in  FIG. 5 , the height of the resist is 250 nm. In part B, a layer of gold and nickel (Au/Ni) is formed using a shadow evaporation technique. 
         [0024]    In a shadow evaporation technique, an evaporation mask may be suspended above the insulating layer. The shadow image of the evaporation mask is projected onto the insulting layer. By setting different evaporation angles, features of nano-dimension can be created on the insulating layer, such as gap in the layers of gold and nickel that have a dimension of nanometers. In some examples, a layer of gold and nickel is formed by shadow evaporation with an oblique incidence angle of forty degrees (40°). The Au/Ni layer may include 40 nm of gold and 10 nm of Ni. In part C, a layer of silver (Ag) is formed using evaporation techniques with a normal angle of incidence. In part D, the resist is removed along with the portions of the Au/Ni and Ag layers on the resist layer. In some variations, ultrasonic agitation is used in the removal. 
         [0025]    The on resistance Ron and the operation voltages (Vset, Vreset) are mainly related to the gap size. A smaller gap size results in a shorter conductive filament which can have a smaller resistance. A lower voltage is also required to form a filament across a smaller gap size. Device isolation is due to the off state capacitance Coff and is related to both the geometry of the switch and the substrate material. 
         [0026]      FIG. 6  illustrates another example of portions of a fabrication process for a memristive RF switch circuit. The process shown can be used to form a memristive RF switch circuit having an air gap of 10 nm. Reducing the air gap lowers the capacitance of the memristive switch to improve isolation and the thicker layer of thermal oxide reduces substrate coupling even further. Simplified top views and side views are shown in the example of  FIG. 6 . In part A, two electrode layers of gold and nickel are formed on the thermal oxide. The gold/nickel layers can be formed using shadow evaporation. In certain examples, the height of the gold in the layers is 120 nm, the height of the nickel in the layer is 8 nm, and the thickness of the thermal oxide layer is 500 nm. In part B, two layers of resist are formed on top of the gold/nickel layers. In part C, layers of silver are formed on the resist layers and the gold/nickel layers. In certain examples, the silver layers are formed using shadow evaporation with an incidence angle of 75°. In certain examples, the height of the silver layer is 50 nm. The distance between the silver layer shown in the middle and the gold/nickel layer shown to the left is 10 nm. This smaller gap is produced by the dimensions of the resist structures and using the steeper angle in the shadow evaporation technique. In part D, the resist layers and portions of the silver layers are removed. In some variations, acetone solution is used to remove the resist layers and portions of the layers. 
         [0027]    In some examples, one or more shielding layers are added between the memristive RF switch and the substrate. Shielding may be useful to reduce or eliminate bulk mode resonances that may appear as transmission nulls in the frequency response of the RF switches. The shielding layers may include layers of metallization. In certain examples, the memristive RF switch includes an absorbing substrate that includes carbon-loaded epoxy. 
         [0028]      FIG. 7  shows a picture of another example of a memristive RF switch circuit. The memristive RF switch includes contact pads. As shown in the lower left, the contact pads include a base gold/nickel layer with a copper (Cu) layer above the gold/nickel layer. Above the copper layer is a gold layer. In the example shown, the nickel in the gold nickel layer has a height of 8 nm, the gold in the gold/nickel layer has a height of 2 s s height of 20 nm. On either side of the contact pads and memristive RF switch are ground planes. 
         [0029]    The memristive RF switch circuit in  FIG. 7  includes asymmetric electrodes of silver and gold/nickel, a 10 nm air gap, and a thermal oxide thickness of 500 nm on an intrinsic silicon substrate. The measured Ron for the switch was. Resistance for just the conductive filament of the on state of the switch is about 3 measured parameters include a resistance ratio greater than 10 10 , Vset equal to about two volts (2V). Vreset equal to about (−0.4V), Coff of 1.37 femtofarads, an insertion loss of 0.3 dB at 40 GHz, isolation of 30 dB at 40 GHz, and a cutoff frequency (fc) of about thirty-five terahertz (35T). 
         [0030]    The geometry of the conductive filament formed between the electrodes as a result of Vset may depend on the programming process. A higher current may lead to a filament with larger diameter and hence lower resistance, but this may also increase the energy required for programming. For a device fabricated with a 35 nm air gap, a continuous conductive filament was formed for compliance current as low as ten nanoamps (10 nA), but current of twenty milliamps (20 mA) ensured formation of a good quality conductive filament and a small value of Ron. 
         [0031]    A larger size of air gap may lead to higher values of Ron. This larger value of Ron is sometimes due to isolated islands of metal material forming between the electrodes rather than a continuous metal filament. A smaller narrower gap size (e.g., 10 nm to 50 nm) results in a continuous filament with low on resistance. A smaller gap size may also increases the number of cycles of on and off switching before device failure. Additionally, reducing excess heat from the RF memristive switch during resetting or disabling the device improves cycle life. 
         [0032]      FIG. 8  shows an RF circuit model for the memristive RF switch circuit. In addition to the RF memristive switch, the RF circuit model accounts for parasitic effects specific to the particular test structure. The graphs show the agreement between the extracted circuit model and the actual device measurements. 
         [0033]      FIG. 9  shows an example of multiple memristive RF switch circuits electrically coupled together. At the left of  FIG. 9  is a circuit diagram of the interconnection of two memristive RF switches. At the right of  FIG. 9  is a picture of the fabricated switches. The first memristive RF switch circuit is formed between the contact pads. The second memristive RF switch circuit is connected between the rightmost contact pad and a ground plane. 
         [0034]    The memristive RF switch circuit examples described herein have many applications in RF systems.  FIG. 11  shows a block diagram of portions of an example of an RF system  1100 . A memristive RF switch  1105 A can be used to provide electrical conductivity with an RF antenna  1115 . A memristive RF switch circuit can be used to enable RF circuit paths such as switching between a transmit circuit path and a receive circuit path. Multiple memristive RF switch circuits  1105 B,  1105 C can be electrically coupled together to form a multiplexer circuit  1110  to allow multiplexing between several RF circuit paths. This allows an RF system to easily change performance parameters such as by changing frequencies, or changing RF signal filtering options. 
         [0035]      FIG. 10  is picture of a monolithic complementary metal oxide semiconductor (CMOS) integrated circuit (IC) die. The die includes an opening in which to place an RF memristive device. The memristive RF switches are compatible with CMOS fabrication processes. Multiple memristive RF switches can be seen along a row at the bottom of the IC die and along a row at the right of the IC die. The other circuits shown on the IC die are CMOS circuits. The compatibility with CMOS processes allows memristive RF switches to be easily incorporated into electronic systems. For instance, the memristive RF switches can be incorporated into ICs for cellular telephones or other personal electronic devices. This allows a memristive RF switch for an antenna to be fabricated on the same die as the CMOS circuits that perform the desired functions. 
       ADDITIONAL NOTES 
       [0036]    The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
         [0037]    In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
         [0038]    The above description is intended to be illustrative, and not restrictive. The above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.