FIELD OF THE INVENTION
The present invention pertains to microfabricated electromechanical (MEM) devices which may be fabricated on a substrate.
BACKGROUND
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).
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 N2+1 output lines and N2 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.
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.
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
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.
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
FIG. 1 is a top view of a MEM switch embodiment of a MEM device suitable for the present invention.
FIG. 2 is a lateral cross-sectional view of the MEM device of FIG. 1, open.
FIG. 3 is a lateral cross-sectional view of the MEM device of FIG. 1, closed.
FIG. 4 shows the hysteresis of switch state as a function of applied voltage.
FIG. 5 shows details of the photoresistor area of FIG. 1.
FIG. 6 is a schematic of application and control of bias voltage to the MEM device.
FIG. 7 shows the substrate with first metal layer in place.
FIG. 8 is as FIG. 7 after selective addition of a sacrificial layer.
FIG. 9 shows selective addition of an insulating layer and etching of contact dimple.
FIG. 10 shows addition of cantilever conductor metallization and final insulating layer.
FIG. 11 shows an array of optically controlled MEM switches.
FIG. 12 shows a photovoltaically actuated MEM device with no external bias lines.
FIGS. 13 and 14 depict a MEM device in a lateral cross sectional view, the MEM device being a variable capacitor.
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.
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
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.
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.
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.
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.
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 V2. However, when the applied voltage has exceeded V2 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 V2, but rather will remain in the “on” state until the applied bias voltage drops to “hold-on” voltage V1. The switch then opens abruptly when the applied bias voltage drops just below hold-on voltage V1. The on-off differential, V2−V1, is typically a few volts; for example, in the preferred embodiment which has a snap-down voltage of 60 V, the on-off differential V2−V1 is 5V. 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.
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 60V, and are provided by a bias supply (not shown). Rb is a series bias resistor, preferably about 1 megohm. Rp is a photoresistor, which is preferably simply part of the substrate. If Rp is part of the substrate, then the substrate is preferably semi-insulating GaAs. When light is directed onto Rp, the resistance decreases from about 100 megohms to about 10 megohms. Consequently, the voltage available between PlateA, the armature electrostatic plate, and Plates, the substrate electrostatic plate, varies depending upon the intensity of light directed upon Rp. 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.
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.
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.
Switch Fabrication
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.
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.
In FIG. 8, sacrificial support layer 72, preferably two micron thick SiO2, 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.
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.
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.
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
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.
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.
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
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.
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 18 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 18, 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).
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 18, 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 18, 32, also has the effect of helping the device from undergoing a snap action in response to electrostatic forces operating on plates 14, 40.
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 18 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.
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).
The MEM device of FIGS. 13–17 may also be controlled optically according to the embodiment of FIG. 12, for example.
Alternative Embodiments
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 Rp, 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 Rb, 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 Rb, along with the selected photoresistor Rp, 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 Rp 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.
Patent Grant
7388186
