Abstract:
An optically controlled mechanical device actuated by electrostatic forces. The device includes electrostatic plates disposed on opposing portions of the device to accumulate charge; conductors to conduct charge to the electrostatic plates from a bias supply; and a photoelectric element having a photoresistive element arranged to affect a quantity of charge reaching the electrostatic plates from the bias supply. The device is caused to actuate to one position when the photoresistive element is exposed to a first level of illumination, and to a another position when the photoresistive element is exposed to a different second level of illumination

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/932,922 filed Sep. 1, 2004, which application is a division of U.S. patent application Ser. No. 10/439,624 filed May 15, 2003, now U.S. Pat. No. 6,803,559, which application is a division of U.S. patent application Ser. No. 09/978,314 filed Oct. 15, 2001, now U.S. Pat. No. 6,639,205, which application is a division of U.S. patent application Ser. No. 09/429,234 filed Oct. 28, 1999, now U.S. Pat. No. 6,310,339, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention pertains to microfabricated electromechanical (MEM) devices which may be fabricated on a substrate.  
       BACKGROUND  
       [0003]     MEM switches in various forms are well-known in the art. U.S. Pat. No. 5,121,089 to Larson, granted in 1992, describes an example of a MEM switch in which the armature rotates symmetrically about a post. Larson also suggested cantilevered beam MEM switches, in “Microactuators for GaAs-based microwave integrated circuits” by L. E. Larson et al., Journal of the Optical Society of America B, 10, 404-407 (1993).  
         [0004]     MEM switches are very useful for controlling very high frequency lines, such as antenna feed lines and switches operating above 1 GHz, due to their relatively low insertion loss and high isolation value at these frequencies. Therefore, they are particularly useful for controlling high frequency antennas, as is taught by U.S. Pat. No. 5,541,614 to Lam et al. (1996). Such use generally requires an array of MEM switches, and an N×N array of MEM switches requires N 2+ 1 output lines and N 2  control circuits for direct electrical control. These control lines may need to be shielded to avoid interfering with the high frequency antenna lines, and accordingly add considerable complexity and cost to the fabrication of these switches.  
         [0005]     MEM capacitors are also very useful for controlling very high frequency phased array antennas and the like. Due to the fact that electrical control lines associated with MEMS capacitors can interfere with the operation of a phased array, shielding those control lines would add considerable complexity and cost to the fabrication of phased array antennas.  
         [0006]     Thus, there exists a need for controlling the MEM devices, both switches and capacitors, in such an array by a means which reduces the difficulties imposed by routing control lines.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention alleviates the above-noted problem of providing control lines for an array MEM devices, and provides other benefits as well. In particular, it provides a mechanism for controlling MEM devices with light, with attendant benefits such as isolation, and indeed remoteness, from a controlling light source.  
         [0008]     The present invention provides optical control of MEM devices. In a preferred embodiment, two DC bias lines are provided to the vicinity of each MEM device. Control of the device is then effected by focusing light on the device substrate. Under illumination, the photo-conductive nature of the semi-insulated substrate causes voltage loss in a series bias resistor to reduce the DC bias voltage applied to the device. The devices may be used in combination to control an antenna array. Another embodiment of the invention employs a photovoltaic device to provide actuating voltage under illumination, thus obviating all bias lines. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a top view of a MEM switch embodiment of a MEM device suitable for the present invention.  
         [0010]      FIG. 2  is a lateral cross-sectional view of the MEM device of  FIG. 1 , open.  
         [0011]      FIG. 3  is a lateral cross-sectional view of the MEM device of  FIG. 1 , closed.  
         [0012]      FIG. 4  shows the hysteresis of switch state as a function of applied voltage.  
         [0013]      FIG. 5  shows details of the photoresistor area of  FIG. 1 .  
         [0014]      FIG. 6  is a schematic of application and control of bias voltage to the MEM device.  
         [0015]      FIG. 7  shows the substrate with first metal layer in place.  
         [0016]      FIG. 8  is as  FIG. 7  after selective addition of a sacrificial layer.  
         [0017]      FIG. 9  shows selective addition of an insulating layer and etching of contact dimple.  
         [0018]      FIG. 10  shows addition of cantilever conductor metallization and final insulating layer.  
         [0019]      FIG. 11  shows an array of optically controlled MEM switches.  
         [0020]      FIG. 12  shows a photovoltaically actuated MEM device with no external bias lines.  
         [0021]      FIGS. 13 and 14  depict a MEM device in a lateral cross sectional view, the MEM device being a variable capacitor.  
         [0022]      FIG. 15  is a graph of test results showing the capacitance versus electrostatic plate differential voltage for a MEM device having a configuration as shown in  FIGS. 13 and 14 .  
         [0023]      FIGS. 16 and 17  are detailed views of the capacitor plate portion of the device, showing the initial contact between its insulating layer and the opposing conductive plate and the result of the applied voltage continued to rise. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIG. 1  shows a plan view of a preferred embodiment of an optically controlled MEM device, implemented as a switch, according to the present invention. Cantilever beam  10 , preferably 24 microns wide, supports armature structure  12  which includes armature electrostatic plate  14 , which is preferably about 100 microns square, and also switch conductor  16 . A substrate electrostatic plate  40 , not shown in this figure, is approximately the same size as armature electrostatic plate  14 , and is positioned behind armature structure  12  in this top view and visible only as dotted lines. The width of switch conductor  16  depends on usage; shown proportionally to be about 30 microns, it may be narrower and in the preferred embodiment is 69 microns wide for a desirable high frequency impedance. Switch conductor  16  is insulated from armature electrostatic plate  14  by armature insulating region  30 , which in the preferred embodiment is about 30 microns. Switch conductor  16  terminates at each end with contact dimples  18 . Armature electrostatic plate  14  is connected to substrate armature pad  26  through cantilever beam conductor  28  and armature via  24 . Anchor structure  20  attaches cantilever beam  10  to the substrate (not identified in  FIG. 1 ) by means of four anchors, e.g.  22 , plus armature via  24 .  
         [0025]     Signal “A” metallization  32  terminates below a first switch dimple  18  of armature structure  12 , as shown in dashed lines. Signal “B” metallization  34  similarly terminates below a second switch dimple  18  of armature structure  12 . Substrate electrostatic pad connection  36  conducts a common potential to substrate electrostatic pad  40  (designated in  FIG. 2 ) which is disposed on the substrate below armature electrostatic pad  14  and indicated in  FIG. 1  by dashed lines below armature electrostatic plate  14 . When the switch is closed, Signal A is connected to Signal B through the switch dimples  18  and switch conductor  16 .  
         [0026]      FIG. 2  shows a section of the MEM device of  FIG. 1  taken along the indicated section line. In order to clarify the boundaries of substrate electrostatic plate  40 , substrate electrostatic plate connection  36  is not shown where it extends below cantilever  10 . Insulating layers  42  are disposed on the top and bottom of armature assembly  12  and support switch conductor  16 . Lower and upper armature insulators  42  each have approximately equal differential stress with the armature metallization (e.g.  14 ,  28 ), and accordingly the differentials are balanced to minimize bowing of the armature. Plate  14  is connected to substrate armature pad  26  by cantilever beam conductor  28  and armature via  24 . Switch conductor  16  is seen where it merges with dimple  18 , which protrudes through the lower of armature insulations  42 . The termination of Signal “A” connection  32  is seen disposed below switch connection dimple  18 . Substrate  44  underlies all of this structure. Substrate  44  is preferably only about 100 microns thick, partly for purposes of signal line impedance control, but is not represented proportionally.  
         [0027]      FIG. 3  shows the MEM device section of  FIG. 2 , but in closed position. A voltage is applied between armature electrostatic plate  14  and substrate electrostatic plate  40 . Armature structure  12  is drawn down toward substrate  44  by electrostatic force, and counterbalanced by the restoring spring force proportional to the displacement of cantilever beam  10 . (The restoring spring force is provided by elastic resistance to deformation of armature conductor  28  plus upper and lower armature insulators  42 ; the armature structure is supported from substrate  44  by anchor structure  20 ). As the applied voltage continues to increase, the electrostatic force, which is proportional to the bias voltage and inversely proportional to the square of the gap between the two plates, will eventually exceed the restoring spring force of cantilever beam  10 , and the balance cannot be maintained. At this so-called “snapdown” voltage, plate  14  snaps down and firmly rests on plate  40 , such that as little as the lower armature insulation  42  may separate the plates. Insulating region  30  flexes somewhat, providing force so that dimple  18  presses firmly against signal “A” conductor  32 , ensuring repeatable and reliable connection between them.  
         [0028]     Hysteresis in the actuation of the switch is important to crisp functioning.  FIG. 4  shows switch state as a function of applied voltage, which demonstrates the hysteresis characteristics of a typical RF MEM switch. As the applied voltage increases, the switch state will follow the path indicated by the arrows having solid-line shafts. Thus, the switch will turn from the “off” state to the “on” state as the applied voltage exceeds snap-down voltage V 2 . However, when the applied voltage has exceeded V 2  and then is decreased, the switch state will follow the path indicated by the arrows having dashed-line shafts. Thus, the switch will not turn back to the “off” state as the applied bias voltage decreases to just below snap-down voltage V 2 , but rather will remain in the “on” state until the applied bias voltage drops to “hold-on” voltage V 1 . The switch then opens abruptly when the applied bias voltage drops just below hold-on voltage V 1 . The on-off differential, V 2 -V 1 , is typically a few volts; for example, in the preferred embodiment which has a snap-down voltage of 60 V, the on-off differential V 2 -V 1  is  5 V. The hysteresis of the switch actuation in response to applied voltage, along with the photo-conductive nature of the MEM switch described herein, are foundations of the present invention.  
         [0029]      FIG. 5  shows details which form the electrical components used in the preferred embodiment of the present invention, and may be more readily understood with reference to the electrical schematic shown in  FIG. 6 . In  FIG. 6 , Bias and Common are applied to exceed the snap-down voltage, preferably about  60  V, and are provided by a bias supply (not shown). R b  is a series bias resistor, preferably about 1 megohm. R p  is a photoresistor, which is preferably simply part of the substrate. If R p  is part of the substrate, then the substrate is preferably semi-insulating GaAs. When light is directed onto R p , the resistance decreases from about 100 megohms to about 10 megohms. Consequently, the voltage available between Plate A , the armature electrostatic plate, and Plate s , the substrate electrostatic plate, varies depending upon the intensity of light directed upon R p . In the preferred embodiment, 60V is applied to the switch when the substrate is dark, exceeding snap-down voltage and closing the MEM switch, while under strong illumination 54 V is applied, which is less than the hold-down voltage and thus opens the switch.  
         [0030]     Returning to  FIG. 5 , bias is supplied to bias connection  48  from elsewhere, being common to all switches in an array. Bias resistor  46  is preferably 40 to 50 squares of sputtered CrSiO in a 6 micron line width, and conducts current from the bias source to armature substrate pad  26  through an appropriate resistance of preferably about 1 megohm. Bias resistor  46  is preferably covered with any non-conductive opaque material to prevent photoresistive effects from reducing its resistance. Current from the bias source is conducted from armature substrate pad  26  to the armature electrostatic pad, not shown, through armature via  24  of anchor structure  20 , and through cantilever beam conductor  28 , without further significant resistance. Bias supply Common ( FIG. 6 ) may be provided to the substrate electrostatic plate, not shown, along substrate electrostatic connection  36 , without significant resistance.  
         [0031]     Semi-insulating GaAs substrate is preferably below all of the structure of  FIG. 5 . Illumination of the substrate reduces its resistance to very roughly 10 megohms per square. Accordingly, when illuminated the substrate in gap  50  between armature substrate pad  26  and substrate electrostatic connection  36  conducts sufficient current to reduce the voltage available between the armature and substrate electrostatic plates so that the switch opens.  
         [0000]     Switch Fabrication  
         [0032]      FIGS. 7-10  show fabrication steps leading to the completed MEM switch shown in  FIG. 2 . Substrate  44  is preferably semi-insulating GaAs about 100 microns thick, and is chosen primarily for compatibility with the circuit in which the resulting MEM switch will be employed. Any semi-insulating substrate which exhibits a resistance varying under illumination by visible or infrared light may be used, which can be achieved using InP or Si, for example. Other substrates which do not inherently have photoconductive properties may also be used, such as ceramics or polyimides, but would require creation of a separate photoresistor. The thickness of the substrate is largely determined by requirements for the circuit, such as obtaining appropriate spacing from a ground plane for control of the transmission line characteristics of traces.  
         [0033]     In  FIG. 7 , metallization has been patterned upon substrate  44  to form armature substrate pad  26 , substrate electrostatic plate  40 , and Signal A conductor  32 . Any technique may be employed to provide the patterned metallization, including for example lithographic resist lift-off or resist definition and metal etch, but also less common techniques. This metallization is preferably begun with about 250-500 Å of Ti to ensure adhesion to the substrate, followed by about 1000 Å of Pt to protect the Ti from diffusion of Au, and about 2000 Å of Au. Any compatible metallization may be employed, but will of course affect the properties of the completed MEM switch.  
         [0034]     In  FIG. 8 , sacrificial support layer  72 , preferably two micron thick SiO 2 , is deposited using any compatible technique, such as plasma enhanced chemical vapor deposition (PECVD), or sputtering. The thickness of sacrificial support layer  72  affects the spacing of the electrostatic plates and the switch opening, which are both important design parameters. A via  74  is also formed through layer  72 , which may be accomplished, for example, by means of lithographic photoresist and etch.  
         [0035]     In  FIG. 9 , the first armature structural layer  82  has been patterned. Structural layer  82  is preferably silicon nitride, but can also be other materials, desirably having a low etch rate compared to sacrificial layer  72 . Via  84  may be formed by any technique, for example lithography and dry etch, but it is desirable that an etch step remove a portion of sacrificial layer  72  below via  84  to form a dimple receptacle extending a controlled depth below first structural layer  82 .  
         [0036]      FIG. 10  shows the result of two further steps. A second metallization pattern has been added to form dimple  18 , switch conductor  16 , armature electrostatic plate  14  and cantilever beam conductor  28 , and it adheres to armature substrate pad  26  to form armature via  24 . This metallization, typically sputter deposited, is preferably 200 Å of Ti followed by 1000 Å of Au (thinner than the metallization mentioned above), but of course alternative metals and thicknesses may be selected.  FIG. 10  also shows second structural layer  92 , added and patterned after the second metallization step. Second structural layer  92  is preferably the same material and thickness as first structural layer  82 , described above with regard to  FIG. 9 , in order to balance the stresses within the armature and thereby minimize bowing of the armature.  
         [0037]     To complete the MEM switch a further fabrication step of wet etching to remove sacrificial layer  72  is performed, which results in the switch as shown in  FIG. 2 . Sputter deposition of the bias resistor may be performed thereafter, as well as a step of opaquely coating the bias resistor if desired. It is also possible to deposit the bias resistor before the step of deposition of sacrificial layer  72 . Indeed, if an opaque material is selected for sacrificial layer  72 , then simply preventing etch of sacrificial layer  72  in the area of the bias resistor will protect the bias resistor from leakage due to illumination.  
       Additional Embodiments  
       [0038]      FIG. 11  shows an array of MEM switches according to the present invention for changing the characteristics of an antenna. The correct bias supply voltage is applied by connection  103  to each optically controlled MEM switch  107 , which also has bias supply common  105  connected thereto. Each MEM switch  107  may be selectively illuminated by directing light at its photoelectric element individually, for example by means of an optical fiber mounted appropriately, such that antenna elements  101  are selectively connected. The antenna array may extend up toward Antenna A, or continue down toward Antenna B. The antenna elements can be varied widely to provide a finely tunable antenna.  
         [0039]      FIG. 12  shows a MEM device fabricated with a photovoltaic device  120  mounted along with MEM device  1  to form a hybrid. Photovoltaic device  120  is a representative integrated circuit having seventy two individual photovoltaic cells, e.g.  125 , connected in series, with the ends of the series of photovoltaic cells connected to bonding pads  123  and  124 . Bond wire  121  connects the first bond pad  123  of photovoltaic device  120  to substrate electrostatic plate connection  36 , and bond wire  122  connects the second bond pad  124  of photovoltaic device  120  to armature electrostatic plate connection  26 . When illuminated, the photovoltaic device produces sufficient voltage to actuate the switch (greater than 60 V in the presently preferred embodiment), and thus no bias lines for MEM switch  1  need be connected to a bias supply or other external drive source, as is required for other embodiments.  
         [0040]     The hybrid fabrication shown in  FIG. 12  is the presently preferred embodiment for both switches and capacitors (to be discussed below), and is compatible with virtually any surface upon which a MEM device may be fabricated, so that the MEM device may be fabricated upon a wide variety of substrate-like surfaces. However, a photovoltaic device may instead be fabricated into a substrate by appropriate processing. For example, Si or GaAs substrates can be processed to produce a photovoltaic device comprising many photovoltaic cells by steps which are well known in the art. MEM device  1  may then be fabricated on the processed substrate as described above with regard to  FIGS. 2 and 7 - 10  (as a switch) or as described below with reference to  FIGS. 13-15  (as a capacitor) to form a completely integrated device. The switch and capacitor embodiments are sufficiently similar that both switch and capacitor MEM devices may be formed on a common substrate, if desired. These devices, when used in an array, may also be selectively actuated by directing light at individual photovoltaic devices, such as through an optical fiber mounted above each photovoltaic device.  
       Capacitor Embodiments  
       [0041]     In the foregoing disclosure, the MEM device is often implemented as a switch. With minor modification, the MEM device may be instead implemented as a capacitor.  
         [0042]      FIGS. 13 and 14  are similar to  FIGS. 2 and 3  in that they depict a MEM device in a cross sectional view. However, in this embodiment, the conductors  16  and  32 , instead of forming a switch contact, form instead the plates of a capacitor. Indeed, one of the conductors may have a thin (for example a 0.1 μm thick) layer  17  of a dielectric (preferably SiN) formed thereon to insure that the two plates  16 ,  32  of the capacitor do not make electrical contact with one another when the device is actuated (as will be seen the plates can make physical contact but are electrically isolated from one another by layer  17 ).  
         [0043]     MEM capacitors differ from MEM switches in another respect. While a snap action in the closing of the switch may be a desirable feature in a switch, in a capacitor embodiment, the otherwise desirable snap action may be avoided (or reduced) between the two plates  26 ,  32  so that the capacitance between them varies more smoothly as a voltage difference builds up on the electrostatic plates  14 ,  40 . The addition of insulating layer  17 , besides insulating the two plates  26 ,  32 , also has the effect of helping the device from undergoing a snap action in response to electrostatic forces operating on plates  14 ,  40 .  
         [0044]      FIG. 15  is a graph of test results showing the capacitance versus electrostatic plate differential voltage for a MEM device having a configuration as shown in  FIGS. 13 and 14 . As can be seen, the capacitance went from under 0.05 pf to about 0.3 pf when the applied differential voltage on plates  14 ,  40  rose to about 50 volts. At this point, the distal end of the insulating layer  17  made contact with plate  32  (see  FIG. 16 ). As the differential voltage on plates  14 ,  40 , continued to rise, the arm of the MEM device started to bend slightly, allowing more and more of the layer  17  to come into conformal contact with plate  32  (see  FIG. 17 ). As more and more of layer  17  came into conformal contact with plate  32 , the capacitance between plates  26  and  32  continued to rise until it peaked at approximately 1.15 pf with a differential voltage of about 130 volts on plates  14 ,  40 .  
         [0045]      FIGS. 16 and 17  are detailed views of the capacitor plate portion of the device, showing the initial contact between the insulating layer  17  and plate  34  ( FIG. 16 ) and, as the applied voltage continued to rise, showing the conformal contact which occurs in response to the applied differential voltage on plates  14 ,  40  rising above 50 volts to 130 volts (see  FIG. 17 ).  
         [0046]     The MEM device of  FIGS. 13-17  may also be controlled optically according to the embodiment of  FIG. 12 , for example.  
       Alternative Embodiments  
       [0047]     It will be understood by those skilled in the art that the foregoing description is merely exemplary, and that an unlimited number of variations may be employed. In particular, the actuation (closing, in case of a switch embodiment or maximum capacitance in case of a capacitor embodiment) voltage and dropout (opening, in case of a switch embodiment or lower capacitance in case of a capacitor embodiment) voltage of the MEM device will depend upon the armature layer construction, the electrostatic plate sizes, the cantilever material, thickness, length and width, and the spacing between armature and substrate, to mention only a few variables, and thus the actuation voltage(s) will vary widely between embodiments. The substrate photoresistor R p , if utilized, can be varied widely as well. This can be accomplished, for example, by changing the number of illuminated squares of substrate between the armature substrate pad connection and the substrate electrostatic pad connection, by varying impurities to alter the photoresistive effect, and by varying the intensity of the illumination. Moreover, alternative substrates are expected to provide an analogous photoresistive effect, or a different photoresistive material can be disposed on any substrate to provide the photoresistive effect. An unlimited number of different techniques and materials are available to provide a bias resistor R b , if used, of an appropriate value; in addition to the many possible variations of the presently preferred technique of applying a separate material patterned to form a resistor, many substrates can be made into high resistance traces through patterned implantation of impurities. The selected bias resistor R b , along with the selected photoresistor R p , causes the voltage available between the armature and substrate electrostatic plates to vary from above the actuation voltage to below the dropout voltage upon illumination of R p  with a selected light source. Since all of these factors may be varied over a wide range, the invention is defined only by the accompanying claims.