| Stress bimorph mems switches and methods of making same -> Monitor Keywords |
|
|
 
Stress bimorph mems switches and methods of making sameStress bimorph mems switches and methods of making same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060181379, Stress bimorph mems switches and methods of making same. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] 1. Field [0002] The present invention relates to micro-electromechanical systems (MEMS) and, in particular, to a micromachined electromechanical radio frequency (RF) switch that can preferably function over a range of signal frequencies from 0 Hz to approximately 100 GHz. [0003] 2. Description of Related Art [0004] MEMS (micro-electromechanical system) switches have a wide variety of uses in both military and commercial applications. For example, electrostatically actuated micro-electromechanical switches can conduct RF current in applications involving the use of antenna phase shifters, in the tuning of reconfigurable antenna elements, and in the fabrication of tunable filters. [0005] A representative example of a prior art MEMS switch is disclosed in Yao, U.S. Pat. No. 5,578,976, issued Nov. 26, 1996. Typically, this type of MEMS switch is fabricated on a semi-insulating substrate with a suspended micro-beam element as a cantilevered actuator arm. The cantilever arm is coupled to the substrate and extends parallel to the substrate, projecting over a ground line and a gapped signal line formed by metal microstrips on the substrate. A metal contact, preferably comprising a metal that does not easily oxidize, such as platinum, gold, or gold palladium, is formed on the bottom of the cantilever arm remote from the fixed end of the beam and positioned above and facing the gap in the signal line. A portion of the cantilever arm and an arm electrostatic plate located thereon reside above the ground line on the substrate. When a voltage is applied to the arm electrostatic plate, electrostatic forces attract the arm electrostatic plate, and thus the cantilever arm, toward the ground line on the substrate, bringing the metal contact into engagement with the separate portions of the gapped signal line, and thereby bridging the gap in the signal line. [0006] Another example of an RF MEMS switch utilizing a cantilever actuator arm is disclosed in Loo et al., U.S. Pat. No. 6,046,659, issued Apr. 4, 2000. In Loo et al., the cantilever actuator arm comprises a multiple layer structure containing the arm electrostatic plate surrounded by insulating layers. As in Yao, the RF MEMS switch disclosed by Loo et al. provides a metal contact that bridges a gap between two portions of an RF signal line, when the switch is closed. Both Yao and Loo et al. disclose that the cantilever actuator arm is generally disposed parallel to the surface of the substrate when the RF MEMS switch is in the open position. Thus, the distance between the metal contact and the RF signal line when the RF MEMS switch is in the open position is limited to the distance between the cantilever actuator arm and the substrate along nearly the entire length of the cantilever actuator arm. [0007] RF MEMS switches provide several advantages over conventional RF switches which use transistors. These advantages include lower insertion loss, improved electrical isolation over a broad frequency range, and lower power consumption. Since this type of switch is fabricated using existing integrated circuit (IC) processing technologies, production costs are relatively low. Thus, RF MEMS switches manufactured using micromachining techniques have advantages over conventional transistor-based RF switches because the MEMS switches function like macroscopic mechanical switches, but without the associated bulk and relatively high cost. [0008] However, integrated RF MEMS switches are difficult to implement. Due to the proximity of the electrical contact formed on the cantilever arm to the signal line formed on the substrate, these switches tend to exhibit poor electrical isolation at high frequencies. In the RF regime, close proximity of the electrical contact and the signal line allows parasitic capacitive coupling between the contact and signal line when the switch is in the OFF-state, creating an AC leakage path for high frequency signals. These losses, which increase with signal frequency, limit the use of MEMS switches in high frequency applications. [0009] Capacitive coupling may be reduced by increasing the separation distance between the signal line formed on the substrate and the metal contact formed on the cantilever arm. However, in the MEMS switch described above, there is a design tradeoff between the OFF-state capacitance and the switch actuation voltage. This tradeoff can be expressed mathematically. The OFF-state capacitance of the switch is given by the relation: C OFF = .times. .times. 0 .times. A d ( 1 ) where A is the area of overlap between the contact and the signal line, d is the distance between the contact and the signal line, e.sub.0 is the permittivity of free space and e is the dielectric constant of the material between the contact and the signal line. [0010] The actuation voltage of a cantilever beam in a switch as described above can be approximated by: V S .apprxeq. 18 .times. EId 3 5 .times. .times. .times. 0 .times. L 4 .times. w ( 2 ) where E is Young's modulus of the beam material, I is the moment of inertia of the beam cross-section, and L and w are the length and width of the cantilever beam, respectively. For a cantilever beam with a uniform width w, and a thickness t, the moment of inertia is given by: I = t 3 .times. w 12 ( 3 ) and V.sub.S can be simplified to: V S = 3 .times. E .function. ( dt ) 3 10 .times. .times. .times. .times. 0 .times. L 4 ( 4 ) [0011] Combining the above expressions (1) and (4) yields C.sub.OFF.varies.V.sub.S.sup.-2/3 (5) [0012] Thus, in the RF MEMS switches of the type described above, increasing the separation distance between the signal line formed on the substrate and the electrical contact formed on the cantilever arm also increases the voltage required to affect electrostatic actuation of the switch, because the separation distance between the signal line and the contact is also the separation distance between the arm electrostatic plate and the ground line. The energy that must be moved through the switch control in order to activate the switch, and thus the energy dissipated by the switch, is a function of the actuation voltage. Therefore, in order to minimize the energy dissipated by the RF MEMS switch, it is desirable to minimize the actuation voltage of the switch. [0013] Another problem with the conventional cantilever switch described above stems from the methods used to manufacture the switch. A polycrystalline silicon (or polysilicon) cantilever beam can be fabricated by first oxidizing a silicon substrate to provide a sacrificial layer, then depositing and patterning a layer of polysilicon into a long, narrow bar directly over the silicon dioxide. The beam is then separated from the sacrificial silicon dioxide layer by application of a release agent comprising a hydrofluoric acid solution, which dissolves the sacrificial layer and results in a free-standing polysilicon beam spaced apart from the substrate. The substrate is immersed in the release agent for a duration sufficient to result in release of the beam. One problem with the use of this release process for a beam in relatively close proximity to the substrate is that surface tension forces exerted by the release agent tend to pull the beam toward the substrate as the device is immersed in and pulled out of the solutions. This can cause the beam to stick to the substrate during drying, a phenomenon known as stiction. [0014] In view of the foregoing, there is a need for a micro-electromechanical switch having improved electrical isolation and improved manufacturability, without requiring a corresponding increase in actuation voltage. SUMMARY [0015] Embodiments of electromechanical switches according to the present invention minimize the OFF-state capacitance of the electrostatically actuatable micro-electromechanical switch formed on a substrate, without a corresponding increase in the voltage required to actuate the switch. Embodiments of the present invention achieve minimization of the OFF-state capacitance by utilizing an actuator arm bent such that the minimum separation distance between an electrical contact formed on the actuator arm and a transmission line formed on the substrate is equal to or greater than the maximum separation distance between a substrate electrostatic plate formed on the substrate and an arm electrostatic plate formed on the actuator arm. The bilaminar cantilever structure of the preferred embodiments enable a large separation (up to approximately 300 micrometers) to be achieved between the transmission line formed on the substrate and the electrical contact formed on the actuator arm, while maintaining a very low actuation voltage (approximately 20 V). This large separation can be used to reduce the capacitance of the RF MEMS switch in the OFF state, thus providing high isolation at high frequencies. [0016] The desired minimization of the OFF-state capacitance is achieved without a corresponding increase in the actuation voltage by forming the arm electrostatic plate at a point on the actuator arm that allows the distance between the arm electrostatic plate, formed on the actuator arm, and the substrate electrostatic plate, formed on the substrate, to be precisely and repeatably controlled, thus allowing the actuation voltage to be correspondingly controlled. [0017] The tendency of the beam to stick to the substrate during drying is reduced by forming the bend in the actuator arm through the generation of unbalanced residual stresses in either the polycrystalline silicon comprising the actuator arm or the metallic layer formed on the actuator arm, this metallic layer comprising the arm electrostatic plate. The unbalanced residual stresses can be generated by manipulation of deposition process parameters during formation of the actuator arm structure. Due to these residual stresses in the actuator arm structure, the actuator arm is in a stressed condition prior to release from the sacrificial layer and will tend to bend away from the substrate when released. This counters the tendency of the arm to deflect toward the substrate in response to surface tension forces exerted by the release solution. [0018] A general embodiment of the electromechanical switches according to the present invention has a cantilevered actuator arm which has an electrostatic plate disposed above an electrostatic plate positioned on a substrate. The switch is open and closed by the electrostatic attraction between the plates. In the open position, the cantilevered arm curves away from the substrate. Switching is provided by a gapped transmission line positioned on the substrate at one end of the cantilevered arm. The arm carries an electrical contact that bridges the gap when the switch is in the closed position. The electrical contact may simply be a region of metal or other electrically conducting material attached to the arm. The electrical contact may also comprise electrically conducting material that projects through the arm to contact the gapped transmission line when the switch is closed. The electrical contact may also be electrically isolated from the arm by a layer of insulating material disposed at the end of the arm. The arm may also be electrically isolated from the electrostatic plate on the substrate when the switch is closed by mechanical stops disposed next to the electrostatic plate that prevent the arm from contacting the plate. [0019] Embodiments of the switches according to the present invention may be fabricated by well-known integrated circuit fabrication processes. Generally, The processes also involve applying one or more layers of sacrificial material. These layers of sacrificial material support the fabrication of the desired structures for the switch. Other processes involve applying one or more layers of electrically conductive material to form the electrically conductive elements, such as the electrostatic plates and electrical contact. As briefly noted above, it is desired that the actuating arm of the cantilever structure according to the present invention be fabricated such that it curls or curves upwards when the switch is open. Processes used to obtain this result are described below. [0020] An aspect of the present invention comprises: a substrate; a first electrical contact formed on the substrate; a substrate electrostatic plate formed on the substrate; a cantilever actuator arm anchored to the substrate at a first end of the actuator arm; a second electrical contact disposed at a second end of the actuator arm, the second electrical contact being in electrically contact with the first electrical contact when the switch is in a closed position; the substrate electrostatic plate disposed beneath the actuator arm and between the first end and the second end of the actuator arm; and an arm electrostatic plate formed on the actuator arm and positioned above the substrate electrostatic plate when the switch is in a closed position, wherein when the switch is in an open position, the actuator arm curves away from the substrate. [0021] Another embodiment of the present invention also provides a method for switching electrical energy comprising providing an electrostatically actuated cantilevered arm on a substrate; applying a voltage to attract the electrostatically actuated cantilevered arm towards the substrate, the cantilevered arm having an electrical contact that electrically connects the input to the output when the voltage is applied; and removing the voltage or applying a second voltage to cause the cantilevered arm to move away from the substrate, the electrical contact no longer electrically connecting the input to the output when the voltage is removed or the second voltage is applied, such that the cantilevered arm curves away from the substrate when the voltage is removed or the second voltage is applied. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading about Stress bimorph mems switches and methods of making same... Full patent description for Stress bimorph mems switches and methods of making same Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Stress bimorph mems switches and methods of making same patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Stress bimorph mems switches and methods of making same or other areas of interest. ### Previous Patent Application: Microswitching element Next Patent Application: Switch pad and micro-switch having the same Industry Class: Electricity: magnetically operated switches, magnets, and electromagnets ### FreshPatents.com Support Thank you for viewing the Stress bimorph mems switches and methods of making same patent info. IP-related news and info Results in 0.31006 seconds Other interesting Feshpatents.com categories: Canon USA , Celera Genomics , Cephalon, Inc. , Cingular Wireless , Clorox , Colgate-Palmolive , Corning , Cymer , 174 |
 * Protect your Inventions * US Patent Office filing 
PATENT INFO |
|
