Abstract:
A circuit for guiding electromagnetic waves includes a substrate for supporting components of the circuit. The circuit includes a control device which includes a first conductive element on the substrate for connection to a first component of the circuit and a second conductive element on the substrate for connection to a second component. The control device is made up of a variable impedance switching material on the substrate which exhibits a bi-stable phase behavior. The compound has a variable impedance between a first impedance state value and a second impedance state value which can be varied by application of energy thereto to thereby affect the amplitude or phase delay of electromagnetic waves through the circuit.

Description:
CROSS REFERENCE TO REATED APPLICATIONS  
       [0001]     This application is a continuation in part of application Ser. No. 09/851,619 entitled Phase Change Switches and Circuits Coupling to Electromagnetic Waves Containing Phase Change Switches, which was filed on May 9, 2001, and claims priority to the filing date thereof, the disclosure of which is expressly incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to phase change switches and other control elements or devices, and more particularly, to phase change switches or control devices having a dynamic range of impedance, and circuits and components employing such switches or control devices. More specifically, the invention relates to such switches which can be employed in circuits such as on frequency selective surface arrays, for controlling current flow throughout the array, through the use of the switches. By controlling such current flow, the properties of the frequency selective surface array can be actively controlled. In addition, the invention also relates to implementation of such switches and other control devices in circuits, and the circuits themselves, that use conductive structures and dielectrics to guide electromagnetic (EM) waves.  
         [0004]     2. Background of the Invention  
         [0005]     Mechanical on/off switches have been used in circuits designed to interact with electromagnetic waves, and in particular, circuits designed to handle guided electromagnetic (EM) waves. Another set of such applications includes two-dimensional periodic arrays of patch or aperture elements known as frequency selective surfaces (FSS), the capabilities of which have been extended by addition of active devices, such as switches, and which are generally known as active grid arrays.  
         [0006]     The mechanical process in these on/off switches involves the physical motion of a conductor (the “bridge”) between two positions, i.e., one where the bridge touches another conductor and completes the direct current (DC) conducting path of the circuit (“closed”) or moves close enough to it that the capacitive impedance is low enough to complete the path for alternating current (AC) flow, and the other where it has moved away from the contact (“open”) to break the DC conducting circuit path or to raise the capacitive impedance to block AC flow. Such mechanical switches have been made at micrometer size scale in so-called MEMS—Micro-Electro Mechanical Systems. MEMS switch technology to date has shown poor lifetimes and packaging costs.  
         [0007]     A key goal in the use of MEMS switches with guided EM waves in the so-called radio frequency (RF) bands is to provide controllable phase delays in a circuit. This is done by using a set of switches to introduce combinations of fixed length phase delay branches into a circuit path. The degree of phase delay control is related to how many separate branches (and switches to control them) are added to the circuit. The switching in or out of a given fixed delay branch provides a step change in the net circuit phase delay. In this approach, if finer steps are desired to cover the same range of total phase delay, then more branches and switches are required.  
         [0008]     Alternatively, transistor and transistor-like semiconductor switching devices have been used in circuits designed to interact with electromagnetic waves and in particular, in circuits and components thereof that guide EM waves. Such devices which include PIN diodes and field effect transistors (FETs) form the basis of a collection of solid-state circuits operating on guided EM waves of up to gigahertz (e.g., GHz, 1 GHz=10 9  Hz) for use in microwave and communication systems. However, for the specific applications herein, the semiconductor switching devices typically have shortcomings in several areas, i.e., GHz and above. Such shortcomings may include high switching power required or high insertion losses.  
         [0009]     In the field of semiconductor memory devices, it has been proposed to use a reversible structural phase change (from amorphous to crystalline phase) thin-film chalcogenide alloy material as a data storage mechanism and memory applications. A small volume of alloy in each memory cell acts as a fast programmable resistor, switching between high and low resistance states. The phase state of the alloy material is switched by application of a current pulse, and switching times are in the nanosecond range. The cell is bi-stable, i.e., it remains (with no application of signal or energy required) in the last state into which it was switched until the next current pulse of sufficient magnitude is applied.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with one aspect of the invention there is provided a switch or control element or device for use in circuits and components that interact with electromagnetic radiation, and more specifically, in circuits or components that guide EM waves. The switch or control element or device includes a substrate for supporting components of the switch. A first conductive element is on the substrate for connection to a first component of the circuit or component (hereafter collectively “circuit”), and a second conductive element is also provided on the substrate for connection to a second component of the circuit. Such switches and circuits involve implementations to guide EM waves in circuits such as parallel wire transmission lines, coaxial cables, waveguides, coplanar waveguides, striplines and microstriplines. Use of such switch devices allows control of energy flow through the circuits with functional properties such as fast switching times, e.g, about 10 nanoseconds to about 1 microsecond; low insertion loss, e.g., about 1 dB or less; high isolation, e.g., about 20 dB or higher; long lifetime, e.g., at least about 10 13  cycles; and low cost. Addressing of the control devices either electrically or optically allows flexibility in how the devices are used.  
         [0011]     A circuit for guiding electromagnetic waves includes a substrate for supporting components of the circuit for guiding the electromagnetic waves and at least one control device. The control device includes at least one conductive element on the substrate for connection to at least one component of the circuit. A second conductive element is provided on the substrate for connection to at least one second component of the circuit and the control device is made up of a variable impedance switching material on the substrate. The switching material connects the at least one first conductive element to the at least one second conductive element. The switching material is made up of a compound which exhibits a bi-stable phase behavior, and is variably switchable to an impedance between the first impedance state value and up to a second impedance state value by application of energy thereto. As a result, the switching affects the amplitude and/or phase delay of electromagnetic waves through the circuit as a result of a change in the impedance value of the compound. Similarly, the path of the guided EM waves can also be affected and/or controlled.  
         [0012]     In more specific aspects, the first and second impedance state values are such that at one value the control device is conductive, and at the other value the control device is less conductive or non-conductive. Preferably an energy source is connected to the control device for causing the change in impedance value. The energy source can be an electrical energy source with leads connected to the switch. Alternatively, the energy source could be a light source which is a laser positioned to direct a laser beam to the switch or control device to cause the change in impedance value. In a more specific aspect, fiber optics or an optical waveguide is associated with the laser and the switch to direct the laser light to the switch.  
         [0013]     The circuit and components can be a circuit or component employing or made up as parallel wire transmission lines, coaxial cables, waveguides, coplanar waveguides, striplines, or microstriplines. The material making up the switch or control device is preferably a chalcogenide alloy, and more preferably at least one of Ge 22 Sb 22 Te 56 , and AgInSbTe.  
         [0014]     In a more preferred aspect, in some applications, the compounds for the control device are used in a range of stable intermediate stage set on a submicron scale or mixtures of amorphous and crystalline phases, but which exhibit (average) intermediate properties under larger scale measurement or functional conditions.  
         [0015]     In an alternative aspect, the invention is directed to a control device for use in circuits which guide electromagnetic waves. The control device is made up as previously described herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Having thus briefly described the invention, the same will become better understood from the following detailed discussion, made with reference to the appended drawings wherein:  
         [0017]      FIG. 1  is a schematic view of the control device between two conductive elements as described herein;  
         [0018]      FIGS. 2 and 3  are schematic views of a frequency selective surface array shown, respectively, in a reflecting state and in a non-reflecting state, depending on the impedance value of control devices disposed throughout the array;  
         [0019]      FIG. 4  shows three views of increasing magnification of an array, with conductive elements and control devices arranged therein, and with a further magnified view of a typical switch control device;  
         [0020]      FIG. 5  is a schematic view of a circuit element similar to that of  FIG. 1 , for use in a switching frequency selective surface array (as in  FIGS. 2, 3 , and  4 ), where the entire element is made of switchable material but configured so that only the connecting elements change state upon application of electrical energy;  
         [0021]      FIGS. 6 and 7  are graphs illustrating measured values of the complex index of refraction of an alloy used in the control device, in the infrared for the crystalline phase, and the amorphous phase;  
         [0022]      FIG. 8  is a graph illustrating how the resistance of the phase change alloy can be continuously varied to provide reflectivity/transmissivity control in a circuit;  
         [0023]      FIGS. 9-12  are graphs illustrating measurement result for the complex relative permittivity component magnitudes for Ge 22 Sb 22 Te 56  (GST-225 or GST) and AgInSbTe (AIST) phase change material over a frequency range of 26-105 GHz;  
         [0024]      FIG. 13  is a top view of conductor layers and phase change material layer on a dielectric substrate of a guided wave device assembled as a coupled stripline;  
         [0025]      FIG. 14  is a top view of conductor layers and phase change material layer on a dielectric substrate of a guided wave device arranged as a coplanar waveguide;  
         [0026]      FIG. 15  is a perspective view of an alternative design for using phase change material to produce variable impedance switching action in a coplanar waveguide structure; and  
         [0027]      FIG. 16  is a perspective view illustrating the use of phase change material to produce variable impedance switching action in a dual stripline arrangement, and further illustrating how a separate energy source might be coupled directly to the control device to effect switching thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 1  schematically illustrates a switch  11  in accordance with one aspect of the invention. The control device includes a substrate  13  having a variable impedance switch material  15  deposited thereon to form a control device element, and connecting a first conductive element  17 , typically a metal strip, to a second conductive element  19 . In this embodiment, the conductive elements  17  and  19  can be, for example, two circuit paths of an array or circuit such as a frequency selective surface array. The entire array can sit on top of a dielectric substrate  13 , such as polyethylene.  
         [0029]     The switch material  15  is typically a reversible phase change thin film material having a dynamic range of resistivity or impedance. An example of a typical switch material for use in accordance with the invention is a chalcogenide alloy, more specifically, Ge 22 Sb 22 Te 56 . Although a specific alloy has been described, it will be readily apparent to those of ordinary skill in the art that other equivalent alloys providing the same functionality may be employed. Other such phase change alloys include the AgInSbTe (AIST), GeInSbTe (GIST), (GeSn)SbTe, GeSb(SeTe), and Te 51 Ge 15 Sb 2 S 2  quaternary systems; the ternaries Ge 2 Sb 2 Te 5 , InSbTe, GaSeTe, SnSb 2 Te 4 , and InSbGe; and the binaries GaSb, InSb, InSe, Sb 2 Te 3 , and GeTe. As already noted, several of these alloys are in commercial use in optical data storage disk products such as CD-RW, DVD-RW, PD, and DVD-RAM. However, there has been no use or suggestion of use of such an alloy as a control element in applications such as described herein. Typically, the alloy is deposited by evaporation or sputtering in a layer that is typically 20-30 nm thick to a tolerance of ±1 nm or less as part of a large volume, conventional, and well known to those of ordinary skill in the art, manufacturing process.  
         [0030]     In this regard, with reference to the specific alloy discussed,  FIGS. 6 and 7  illustrate measured values of the complex index of refraction of Ge 22 Sb 22 Te 56  over a spectral wavelength range that includes 8-12 μm. At the mid-band wavelength of 10 μm, the real index, n, changes by a factor of 2 between the two phases, but the so-called extinction coefficient, k, goes from approximately 4.8 in the crystalline phase to near zero in the amorphous phase.  
         [0031]     Accordingly, the following table shows calculations using this data to find the changes in resistivity (ρ) and dielectric constant (ε) of the material.  
                                                                             Optical and Electrical Properties of the alloy       Ge 22 Sb 22 Te 56  at IR vacuum wavelength of 10 μm.                Phase                    Crystalline   Amorphous                            n       4.2           k   4.8   0.01           f (frequency in Hz)     3 × 10 13     3 × 10 13             ρ ∝ (nkf) −1  (ohm-cm)   7.6 × 10 −4     0.71           ε = n 2  − k 2     44.2   17.6                      
 
         [0032]     As the table shows, the change in k correlates with a change in resistivity of almost three orders of magnitude.  
         [0033]     In order to determine the thermal IR (infrared) performance, the shunt is modeled as a capacitor and a resistor in parallel. The following table shows the calculated values for the capacitive and resistive impedance components with switch dimensions in the expected fabrication range, using the expressions shown in the table.  
                                                                                                         Resistance (R) and capacitive reactance (X C ) components of the switch       impedance in the crystalline and amorphous states for several representative       values of the switch dimensions shown in  FIG. 1 . The capacitive reactance       values are calculated using ω = 1.9 × 10 14  Hz, which corresponds to f = 30       THz or λ = 10 μm.                Crystalline   Amorphous                        X C  = (ωC) −1  with       X C  = (ωC) −1  with           L   W   t   C = εWt/L   R = ρL/Wt   C = εWt/L   R = ρL/Wt       (μm)   (μm)   (μm)   (ohms)   (ohms)   (ohms)   (ohms)                    1.0   1.0   0.01   1.36K   1K   3.4K    1M       1.0   1.0   0.1   136   100   340   100K       1.0   1.0   0.2   68   50   170    50K       1.0   0.5   0.1   271   200   680   200K                  
 
         [0034]     As further shown in  FIG. 8 , the resistance of the specific alloy discussed herein can therefore be continuously varied to provide reflectivity control.  
         [0035]      FIGS. 2 and 3  thus show the effect on an array of the use of control devices  11 . This is shown, for example, in a frequency selective surface array  31 . In the case of  FIG. 2 , the array includes a plurality of conductors  39  having control devices  41  as described herein interconnected therebetween. In the case of  FIG. 2 , the control devices are in a high impedance state, thereby interrupting the conductive paths such that electromagnetic radiation  33  impinging on the array then becomes reflected radiation  35 . Conversely,  FIG. 3  shows the array with the control devices at a low impedance such that the conductors  39  are continuous, and the impinging radiation  33  passes through the array  31  as transmitted radiation  37 .  
         [0036]      FIG. 4  illustrates in greater detail a typical circuit  51 , which as illustrated in the intermediate magnification  53 , includes a plurality of conductors  39  having the switches shown as dots interconnected therebetween. In order to vary the impedance of the switches, an energy source  57  may be connected to the individual conductors to provide current flow to the control devices  11  to thereby change the impedance of the control devices  11  by the application of energy, in the form of electricity. As further shown in the third magnification  55 , while the conductors  39  themselves can be directly connected to an energy source, it is also possible to selectively establish leads  59  to the switch material  15  to apply energy to the switch material directly and not through the conductors  39  to cause the impedance to vary.  
         [0037]      FIG. 5  shows in detail an additional embodiment  101  of the invention in which conductive elements  103  and the connecting control device  105  are entirely made of the same phase change material to form the control device element as compared to the embodiment of  FIG. 1 . In this embodiment, the control device  105  is purposely made less wide to form a switch element which is narrower than the conductive elements  103  that connect to it on either side, but having a thickness equal to the conductive elements  103 . In this case, the cross section of the control device element is less than the cross section of the conductive elements  103 , causing the electrical resistance per unit length to be greater in the control device element than in the conducting elements. When electrical current is passed through a circuit made up of a series of these constricted switch connections, i.e., control devices  105 , the phase change material in the control devices  105  will dissipate more electrical energy per unit length than the conducting elements because of the higher resistance per unit length. This higher dissipation will cause the control devices  105  to experience a greater temperature rise than the conductive elements  103 . Therefore a correctly sized electrical current pulse will cause the phase change material in the control devices  105  to change state while the phase change material in the conductive elements  103  remains in the low impedance state. As is the case with the earlier described embodiment as shown in  FIG. 4 , the leads  59  (not shown) can also be established to connect to the control devices  105  to apply energy directly to the control device  105 , and not through the conductive elements  103 .  
         [0038]     While in a specific embodiment the impedance of the phase change material of control devices is varied by application of electrical current to change the state of the phase change material, it will be appreciated by those of ordinary skill in the art that given the nature of the material, other energy sources can be employed. For example, selectively targeted laser beams may be directed at the control devices to change the overall circuit current flow configuration, as well as other alternative means of providing energy to change the state and thus vary the impedance can be used. The laser beam can be directed through free space or can be directed through fiber optics or optical waveguide directly onto the control device as, for example, is schematically illustrated in  FIG. 16  for a different embodiment application.  
         [0039]     As already discussed, in its various aspects the invention uses the changing properties of a specific type of metallic alloy. The alloys, as already noted, among others can include the compounds GST-225, GST, or AIST. The amount of energy needed to cause transition in alloy volumes on the order of 1 μm 3  is in the range of about 1 to about 3 nanojoules for known materials depending on the thermal dissipation environment of the alloy volume. The energy can be supplied to the material, as already noted, in various ways including exposure to pulse, focused laser beams or application of a pulse of electrical current. The two phases, crystalline and amorphous, have different electromagnetic properties across a significant part of the electromagnetic spectrum.  
         [0040]      FIGS. 9-12  show the measured magnitude of the real and imaginary components (ε′ and ε″ respectively) of the complex (relative, i.e., normalized to ε 0 ) dielectric constant of the alloy GST over a range of RF electromagnetic frequency from about 26 GHz up to about 105 GHz for both phases, and show similar data for the alloy AIST.  
         [0041]     As the figures show, at a frequency of 50 GHz, for example, the real dielectric constant, ε′, changes by a factor of 5 between the two GST phases, and by a factor of approximately 25 between the two IST phases. However, the imaginary dielectric constant magnitude, ε″, which is related to the conductivity of the material goes from approximately 45 (at 50 GHz) in the GST crystalline phase to less than one in the GST amorphous phase. The corresponding change for ε″ of AIST at 50 GHz is from about 350 to about 2.5.  
         [0042]      FIG. 13  shows a schematic depiction of a partial embodiment of the invention in which the phase change material is placed between two metallic conductors  109  as a part of a structure  107 , for example, an electromagnetic (EM) wave guiding structure. In this embodiment, the structure  107  is a dual stripline structure which guides EM waves in a manner well known to those of ordinary skill in this art. Based on the known properties of the phase change material, the change in the lumped impedance of the material can be estimated as the material changes from crystalline to amorphous phase. For the GST material at 50 GHz, the resistive (real) impedance, which scales inversely with ε″, will increase by a factor of over 50 as the material changes from crystalline to amorphous, while the capacitive (imaginary) impedance, which scales inversely with ε′, will increase by a factor of approximately eight (8) at the same time. Similarly, for the AIST material at 50 GHz, the resistive (real) impedance will increase by a factor of approximately 140 as the material changes from crystalline to amorphous, while the capacitive (imaginary) impedance will increase by a factor of about 25 at the same time. Without predicting exact effects in a specific embodiment, it will be readily apparent to those of ordinary skill in the art that this level of change in lumped impedance components is sufficiently large to design devices to produce significant control effects in wave guiding structures. In the case of the dual stripline structure of  FIG. 13 , the components are arranged on a dielectric substrate  113  to guide the electromagnetic waves in desired paths.  
         [0043]     In a more specific embodiment as schematically illustrated in  FIG. 13 , an energy source  115  can be coupled through a direct connection  117  to the control device  111  to effect the change in impedance. The energy source can be an electrical source  115  coupled through a lead or leads  117  to the switch material  111 , or alternatively, can be a laser coupled through a fiber optic fiber to the switch material. As already previously noted, the laser can alternatively also be free standing and the laser beam directed in free space to the control device or switch material to provide the necessary energy to change the state thereof.  
         [0044]      FIG. 14  illustrates yet still another embodiment of an implementation of the invention described herein in which the guided wave device is a coupled stripline  121 . The phase change material  123  is arranged between conductors  125  and  127  of the coupled stripline  121  structure which are respectively connected at each end through conductor layers  133  making up a part of a coplanar waveguide termination  131 .  
         [0045]     In a yet still further embodiment,  FIG. 15  illustrates an implementation of the control device in a guided wave device made up as a coplanar waveguide  141 . The coplanar components  143  are arranged adjacent to each other and include the phase change material  145  arranged between conductor layer  149  on a dielectric substrate  147 .  
         [0046]     In a final embodiment described herein as shown in  FIG. 16 , the guided wave device is a coplanar waveguide structure  151  which includes a metal center conductor  153  with the phase change material or control device  155  arranged as an insert. The device  151  also includes parallel metal ground planes  157  arranged on a dielectric substrate  159 .  
         [0047]     As may be appreciated from the table in  FIG. 8 , in these types of guided wave devices such as shown in  FIGS. 13-16 , the variable impedance carries with it a variation of the phase delay in the guided wave, as will be readily apparent to those of ordinary skill. Thus, the guided wave devices can be employed as variable phase delay devices.  
         [0048]     Having thus described the invention in detail, the same will become better understood from the appended claims in which it is set forth in a non-limiting manner.