Abstract:
A radio frequency switch includes a first transmission line, a second transmission line, a first electrode electrically coupled to the first transmission line, a second electrode electrically coupled to the second transmission line, and a phase change material, the first transmission line coupled to a first area of the phase change material and the second transmission line coupled to a second area of the phase change material. When a direct current is sent from the first electrode to the second electrode through the phase change material, the phase change material changes state from a high resistance state to a low resistance state allowing transmission from the first transmission line to the second transmission line. The radio frequency switch is integrated on a substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None 
     TECHNICAL FIELD 
     This disclosure relates to radio frequency (RF) switches, and in particular to RF-PCM switches using phase change material (PCM). 
     BACKGROUND 
     RF switches are key elements used in RF systems including communications and radars. RF switches enable low-loss, low-noise, fast, linear signal routing. They may also be used for impedance tuning and phase shifting. Due to varying system RF power handling requirements, it is important that an RF switch have linear performance from approximately a milli-watt (mW) to a watt level. While micro-electromechanical (MEMS) switches have been demonstrated in the prior art for RF systems with the desired low-loss, low-noise, isolation, linearity, and adequate power handling properties, these prior art RF switches have high switching voltage (30-70 V) requirements, low reliability and packaging issues. Thus, even after the decades of research, RF-MEMS switches are not commonly found in RF systems. 
     Monolithic microwave integrated circuit (MMIC) integration has in general been limited due to the size and high voltage requirements of prior art RF-MEMS switches, and mobile platform applications are very difficult to realize due to the high switching voltage requirements. 
     In the prior art Chua et al., “Low resistance, high dynamic range reconfigurable phase change switch for RF applications”, Applied Physics Letters vol. 97, 183506, (2010) mentions using PCM material for RF switches; however, Chua does not describe an RF switch design using PCM materials. Lo et al., “Three-terminal probe reconfigurable phase-change material switches”, IEEE Transactions on Electron Devices., vol. 57, p. 312, (2010) describes a switch with a three-terminal layout, consisting of an array of sub-vias; however, in Lo the switching is performed using external probes. Wen et al., “A phase-change via-reconfigurable on-chip inductor”, IEDM Tech digest, (2010) describes via structures with GeTe material, with an R on  of 1.1 ohm and an Ron/Roff of 3×10 4 ; however, in Wen the switching is also performed using external probes. 
     The principal of PCM has been known since the 1960s. Rewritable optical DVDs have been developed using Ge2Sb2Te5, and also using (Ag or In)Sb2Te. Lately, phase change materials have been being developed for non-volatile memory, as a future replacement of flash memory. Companies involved in these developments include Micron, Samsung, IBM, STMicroelectronics, and Intel. Following are recent publications on use of PCMs for digital applications: EE Times, November, 2011, “Samsung preps 8-Gbit phase-change memory”, Perniola et al”, “Electrical behavior of phase change memory cells based on GeTe”, IEEE EDL., vol. 31, p. 488, (2010). 
     What is needed are RF switches using phase-change materials that are compatible with conventional semiconductor RF integrated circuit (RFIC) and MMIC processes. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a radio frequency switch comprises a first transmission line, a second transmission line, a first electrode electrically coupled to the first transmission line, a second electrode electrically coupled to the second transmission line, and a phase change material, the first transmission line coupled to a first area of the phase change material and the second transmission line coupled to a second area of the phase change material, wherein when a direct current is sent from the first electrode to the second electrode through the phase change material, the phase change material changes state from a high resistance state to a low resistance state allowing transmission from the first transmission line to the second transmission line, and wherein the radio frequency switch is integrated on a substrate. 
     In another embodiment disclosed herein, a radio frequency switch comprises a first transmission line, a second transmission line, a first electrode, a second electrode, and a phase change material, the first transmission line coupled to a first area of the phase change material, the second transmission line coupled to a second area of the phase change material, the first electrode coupled to a third area of the phase change material, and the second electrode coupled to a fourth area of the phase change material, wherein when a direct current is sent from the first electrode to the second electrode through the phase change material, the phase change material changes state from a high resistance state to a low resistance state allowing transmission from the first transmission line to the second transmission line, and wherein the radio frequency switch is integrated on a substrate. 
     In yet another embodiment disclosed herein, a method of making a radio frequency switch comprises forming an insulator on a substrate, forming a first transmission line on the insulator and coupled to a first area of a phase change material, forming a second transmission line on the insulator and coupled to a second area of the phase change material, forming a first conductor connected to a third area of the phase change material, forming a first electrode connected to the first conductor, forming a second conductor connected to a fourth area of the phase change material, and forming a second electrode connected to the second conductor. 
     In still another embodiment disclosed herein, a reconfigurable circuit comprises a first circuit comprising at least a first radio frequency switch integrated with circuitry comprising GaN based transistors or III-IV bipolar transistors, the first radio frequency switch comprising a first transmission line, a second transmission line, a first electrode, a second electrode, and a phase change material, wherein the first transmission line coupled to a first area of the phase change material, the second transmission line coupled to a second area of the phase change material, the first electrode coupled to a third area of the phase change material, and the second electrode coupled to a fourth area of the phase change material, and wherein when a direct current is sent from the first electrode to the second electrode through the phase change material, the phase change material changes state from a high resistance state to a low resistance state allowing transmission from the first transmission line to the second transmission line. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a four-terminal RF-PCM switch with a vertical arrangement of RF transmission lines in accordance with the present disclosure; 
         FIG. 1B  shows a four-terminal RF-PCM switch with a parallel arrangement of RF transmission lines in accordance with the present disclosure; 
         FIG. 2A  shows an equivalent circuit model for a RF-PCM switch and simulation results for R-SET, when the PCM is at a low resistance, and for R-RESET, when the PCM is at a high resistance, in accordance with the present disclosure; 
         FIG. 2B  shows the simulated R-SET of an RF-PCM switch for different PCM configurations in accordance with the present disclosure; 
         FIG. 3A  shows a three dimensional (3D) arrangement of RF-PCM switches integrated with inductors (Ls) and capacitors (Cs) in accordance with the present disclosure; 
         FIG. 3B  shows a reconfigurable filter with six RF-PCM switches in accordance with the present disclosure; 
         FIG. 3D  shows a filter transfer function of the reconfigurable filter of  FIG. 3B  having pass band center frequencies of 1 GHz and 2.4 GHz, respectively, depending on the RF-PCM switch settings as shown in  FIG. 3C , in accordance with the present disclosure; 
         FIG. 4A  shows an example MMIC layout consisting of RF-PCM switches and GaN LNAs in accordance with the present disclosure; and 
         FIG. 4B  shows an example layout of a GaN MMIC amplifier with a reconfigurable output matching network using RF-PCM switches in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
     In the following RF switches with phase change materials are referred to as RF-PCM switches. Referring now to  FIGS. 1A and 1B , two different four-terminal RF-PCM switches are shown. The RF-PCM switch shown in  FIG. 1A  is designed with a vertical geometry between the RF transmission lines. The RF-PCM switch shown in  FIG. 1B  is designed with a parallel geometry between the RF transmission lines. 
     The RF-PCM switch shown in  FIG. 1A  has a first RF transmission line  12 , which is electrically connected to a first conductor  22 . The first conductor  22  is also electrically connected to a top electrode  16 , which functions as a switch control, and is electrically connected to the PCM  20 . The PCM is formed on an insulator  26 . A second conductor  24  is connected to the PCM  20 , and separated from the first conductor  22  by the PCM  20 . The second conductor  24  is electrically connected to a bottom electrode  18 , which along with the top electrode  16  functions as the switch control. The second conductor  24  is also electrically connected to a second RF transmission line  14 . The RF-PCM switch may be built on a substrate  34 . The RF transmission lines may be also used to transmit signals other than RF signals. 
     To switch the RF-PCM switch of  FIG. 1A , a current pulse may be applied from the top  16  electrode to the bottom  18  electrode, thereby passing through the PCM  20 . The current pulse may have a pulse width of less than a microsecond. The current pulse changes the PCM  20  from an amorphous high resistance material to a crystalline low resistance state. When in a crystalline low resistance state, the PCM  20  allows an RF signal to be transmitted from the first RF transmission line  12  to conductor  22 , then through the PCM  20  and through the conductor  24  to the second RF transmission line  14 . 
     To prevent the RF signals from transmitting through the top electrode  16  or the bottom electrode  18 , the top electrode  16  is connected to RF blocking inductor  17 , and the bottom electrode  18  is connected to RF blocking inductor  19  to block RF signals. Also, because the first RF transmission line  12  and the top electrode  16  are electrically connected, to block any direct current (DC) on the top electrode  16  from transmission on the first RF transmission line  12 , the first RF transmission line  12  is connected to DC blocking capacitor  30 . Similarly, because the second RF transmission line  14  and the bottom electrode  18  are electrically connected, to block any DC on the bottom electrode  18  from transmission on the second RF transmission line  14 , the second RF transmission line  14  is connected to DC blocking capacitor  32 . 
     The RF-PCM switch may be fabricated on a substrate  34 , which may be silicon (Si), silicon germanium (SiGe), silicon carbide (SiC), sapphire, pyrex, gallium arsenide (GaAs), or III-V compounds such as GaN, InAs, InSb, and InP. The first and second RF transmission lines  12  and  14 , and the top and bottom electrodes  16  and  18  may be formed from any metal such as aluminum (Al), cooper (Cu), or gold (Au). The insulator  26  is preferably a low-k dielectric insulator, such as silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), or benzo-cyclo-butene (BCB), to reduce any parasitic capacitive coupling between the first and second RF transmission lines  12  and  14 . Other appropriate materials for insulator  26  are polyimide, and polymethylglutarimide (PMGI). The first and second conductors  22  and  24  can be titanium nitride (TiN), tungsten (W) or any other metal that doesn&#39;t deform at high temperature and that doesn&#39;t form an alloy with the phase-change material (PCM). 
     The RF-PCM switch shown in  FIG. 1B  is similar to the RF-PCM switch of  FIG. 1A ; however, the RF-PCM switch shown in  FIG. 1A  is designed with a vertical geometry between the RF transmission lines, while the RF-PCM switch shown in  FIG. 1B  is designed with a parallel geometry between the RF transmission lines. 
     The RF-PCM switch of  FIG. 1B  has a first RF transmission line  42 , which is electrically connected to PCM  50 . A second RF transmission line  44  is electrically connected to PCM  50 , but is not electrically connected to the first RF transmission line  42 . A top electrode  46  is connected to conductor  52 , and the conductor  52  is connected to the PCM  50 . The PCM  50  is also electrically connected to conductor  54  to electrically connect the PCM  50  to the bottom electrode  48 . The RF transmission lines  42  and  44  and the PCM  50  may be formed on insulator  56 , which along with bottom electrode  48  may be formed on substrate  64 . 
     To switch the RF-PCM switch of  FIG. 1B , a current pulse may be sent from the top electrode  46  to the bottom electrode  48  electrode thereby passing through the PCM  50 . The current pulse may have a pulse width of less than a microsecond. The current pulse changes the PCM  50  from an amorphous high resistance material to a crystalline low resistance state. When in a crystalline low resistance state, the PCM  50  allows an RF signal to be transmitted from the first RF transmission line  42  to the second RF transmission line  44 . 
     To prevent the RF signals from transmitting via the top electrode  46  or the bottom electrode  48 , the top electrode  46  is connected to an RF blocking inductor  17 . The bottom electrode  48  is also connected to an RF blocking inductor similar, such as RF-blocking inductor  19  shown in  FIG. 1A  to block RF signals. To block any DC on the top electrode  46  from being transmitted on the first RF transmission line  42 , the first RF transmission line  42  is connected to DC blocking capacitor  60 . Similarly, to block any DC on the bottom electrode  48  from being transmitted on the second RF transmission line  44 , the second RF transmission line  44  is connected to DC blocking capacitor  62 . 
     The RF-PCM switch may be fabricated on a substrate  64 , which may be silicon (Si), silicon germanium (SiGe), silicon carbide (SiC), sapphire, pyrex, gallium arsenide (GaAs), or III-V compounds such as GaN, InAs, InSb, and InP. The first and second RF transmission lines  42  and  44 , and the top and bottom electrodes  46  and  48  may be formed from any metal such as aluminum (Al), cooper (Cu), or gold (Au). The insulator  56  is preferably a low-k dielectric insulator, such as silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), or benzo-cyclo-butene (BCB), to reduce any parasitic capacitive coupling between the first and second RF transmission lines  42  and  44 . Other appropriate materials for insulator  56  are polyimide, and polymethylglutarimide (PMGI). The first and second conductors  52  and  54  can be titanium nitride (TiN), tungsten (W) or any other metal that doesn&#39;t deform at high temperature and that doesn&#39;t form an alloy with the phase-change material (PCM). 
     The phase-change materials (PCMs)  20  and  50  may be Ge x Te 1-x , Ge x Sb y Te z , or their derivatives. Measurements of sheet resistance of Ge 0.16 Sb 0.24 Te 0.6  and Ge 0.4 Te 0.6  PCM materials have shown a phase change from an amorphous high resistance state to a crystalline low resistance state with a 106:1 resistance ratio between the high resistance and the low resistance. 
     The sheet resistance of the crystalline state of PCM may be 100 Ω/sq for 100 nm thick GeTe and 82 Ω/sq for 200 nm thick GeSbTe. Important for switch applications, it has been shown that GeSbTe digital-PCM cells fabricated with a 190 nm diameter can successfully have 10 million read/write cycles, and be switched with a current pulse 0.5 mA. 
     A RF-PCM switch with PCM cells of approximately 40 μm 2  may be designed to deliver a resistance in the R-SET state of approximately 1Ω. GeTe-based digital PCM cells with PCM cells having approximately a 0.3 μm diameter may have a resistance in the R-SET state of 20Ω. In this configuration the ratio of the resistance in the R-RESET state to the resistance in the R-SET state is approximately 105, which allows RF-PCM switches to be designed with a low on resistance (Ron), a high off resistance (Roff), and a high Ron/Roff ratio. 
     For example, a RF-PCM switch with 2 μm 2  PCM switch may have a resistance in the R-SET state of &lt;1Ω and a RESET/SET resistance ratio of 105:1. 
     The maximum needed voltage and current for switching the PCM from R-SET to R-RESET may be 3 volts and ˜500 mA, respectively. 
       FIG. 2A  shows a schematic of an equivalent circuit  70  for an RF-PCM switch and its simulated RF insertion loss at the SET state and RF isolation at the RESET state. An RF-PCM switch may be simulated with a resistor  72  and a capacitor  74  to model parasitic capacitance. 
     The RF insertion loss and isolation was simulated for an RF-PCM switch with a R-SET resistance of 10 Ω/sq and a contact resistance of 15 Ω·μm between the conductors  22 ,  24 , or  52 ,  54  and the PCM  20  or  50 . An RF insertion loss S21 SET of 0.1-0.2 dB and an RF isolation S21 RESET of 25 dB or better can be achieved up to 100 GHz. The RF isolation result is mainly due to the parasitic capacitive coupling though the substrate. The RF insertion loss and isolation may also be traded off, one for the other, in RF-PCM switch designs.  FIG. 2A  shows the R-RESET for a PCM configuration with 5×10 4  Ω/sq and for a PCM configuration with 10 5  Ω/sq. 
       FIG. 2B  shows the simulated RF S21_SET insertion loss of an RF-PCM switch for different configurations of the PCM and a contact resistance of 15 Ω·μm. The contact resistance is the resistance between a conductor, such as conductor  22 ,  24 ,  52 , or  54 , and the PCM. The PCM configurations shown in  FIG. 2B  include curves for R-SET equal to 1 Ω/sq, 5 Ω/sq, 10 Ω/sq, and 20 Ω/sq from 0 to 100 GHz. 
     RF-PCM switches can be integrated with conventional semiconductor RFIC and MMIC processes, enabling reconfigurable RFICs and MMICs. The semiconductor materials used for the substrate  34  and  64  for integration into RFICs and MMICs may include Si, SiGe, and III-V compounds such as GaN, InAs, InSb, and InP. The device technologies that may be integrated include FETs and bipolar transistors. The RF-PCM switches may also be integrated with resistors (R), inductors (L), and capacitors (C). Integrating the RF-PCM switches with other circuit elements allows the circuits of passive elements, such as L, R, C elements, and active circuits, such as FETs or bipolar transistors or other such elements, to be reconfigurable. 
     For example,  FIGS. 3A and 3B  show filter schematics with LC lumped elements and RF-PCM switches integrated together. The reconfigurable filter shown in  FIG. 3C  may have its passband reconfigured to be 1 GHz or 2.4 GHz, as shown in  FIG. 3D , depending on the R-SET and the R-RESET status, as shown in  FIG. 3C , of the RF-PCM switches  88 . 
     Another aspect of the use of RF-PCM switches is shown in  FIG. 3A . The ability to integrate RF-PCM switches with other circuit elements in a RFIC or MMIC allows very compact structures and even three dimensional (3D) circuitry. As shown in  FIG. 3A , RF-PCM switches  80  and other circuitry, such as capacitors, inductors, resistors, and transistors may be integrated on one circuit plane  82 . The circuit plane  80  may be a substrate, a RFIC, a MMIC, or a circuit board with the integrated RF-PCM switches  80  and other circuitry. Other RF-PCM switches  84  and other circuitry, such as capacitors, inductors, resistors, and transistors may be integrated on another circuit plane  86 , which also may be a substrate, a RFIC, a MMIC, or a circuit board. The RF-PCM switches allow the circuitry to be reconfigurable. By stacking circuit planes on one another and connecting the circuitry on circuit plane  82  to the circuitry on circuit plane  86  with conductors  85  between the circuit planes, a very compact three dimensional reconfigurable circuit may be realized, as shown in  FIG. 3A . The conductors  85  between the circuit planes  82  and  86  may be metal vias. 
       FIG. 4A  shows a reconfigurable low-noise amplifier consisting of RF-PCM switches  90 ,  92 ,  94  and  96  and GaN field effect transistors (FETs) in a MMIC layout. The two GaN LNAs, shown in  FIG. 4A  may be configured to improve the third order intercept point (OIP3) to 51 dBm and the spurious signal performance to less than 98 dBc at a Pin of −10 dBm up to 18 GHz, which enables high dynamic range signal detection immune to jamming signals. 
       FIG. 4B  shows an example layout of a GaN MMIC amplifier with a reconfigurable output matching network  100  using RF-PCM switches  102 . 
     The fabrication process flow for an RF-PCM switch may be made to be similar to a tantalum nitride (TaN) MMIC resistor process with some modifications. The process for fabricating a RF-PCM switch is the following. 
     1. Lift-off metal-1 to form a bottom DC electrode and an RF transmission line, 
     2. Deposit a low-loss dielectric layer #1 such as SiO2, 
     3. Pattern an opening #1 in the dielectric layer around to be formed RF-PCM switches, 
     4. Lift-off an adhesion metal pillar (Tungsten (W) or TiW) on phase change material (PCM), 
     5. Deposit a low-loss dielectric layer #2 such as SiO2, 
     6. Pattern an opening #2 in the dielectric layer #2 to the PCM, 
     7. Lift-off an adhesion metal (TiN), 
     8. Lift-off metal-2 for the top DC electrode and RF transmission line. 
     In summary, the disclosed RF-PCM switches based on PCM materials such as Ge x Te 1-x , Ge x Sb y Te z , or their derivatives enable reconfigurable RF functions in RFICs, MMICs, and passive devices such as single-pole-double-throw (SPDT) switches, phase shifters, and filters. The disclosed RF-PCM switches are binary (on or off). If necessary, the RF-PCM switches can be designed with multi-bit switches, especially for phase-shifter, phase-shift-key (PSK), and quadrature-amplitude-modulation (QAM) applications. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . .”