Patent Publication Number: US-6700172-B2

Title: Method and apparatus for switching high frequency signals

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 09/394,997 filed Sep. 13, 1999 and entitled “Method and Apparatus for Switching High Frequency Signals”, U.S. Pat. No. 6,391,675. 
     This application claims the benefit under 35 USC §119(e) of United States Provisional Application Serial No. 60/109,784, filed Nov. 25, 1998, having a title of Method and Apparatus for Switching High Frequency Signals. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. N66001-96-C-8623. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to electronic devices and more particularly to a method and apparatus for switching high frequency signals. 
     BACKGROUND OF THE INVENTION 
     Microelectromechanical microwave (MEMS) capacitive switches can be used for switching high frequency signals. Examples of microelectromechanical microwave capacitive switches are described in U.S. Pat. No. 5,619,061 entitled, Micromechanical Microwave Switching, which is incorporated herein by reference. Such switches may be used for functions such as beam steering in a phased array radar. MEMS capacitive switches generally are low loss devices because they include no active semiconductor components. The lack of active semiconductor components also makes MEMS capacitive switches relatively inexpensive. 
     A problem with some implementations of microelectromechanical microwave capacitive switches is that they show an inability to remain in a switched on position for more than a few seconds at low frequency bias voltages and show a bipolar response when exposed to high-frequency bias voltages. Bipolar response refers to switching on at both zero and positive bias. 
     SUMMARY OF THE INVENTION 
     Accordingly, a need has arisen for an improved method and apparatus for method and apparatus for dielectric charges reduction in micromechanical microwave capacitive switches that address shortcomings of prior methods and apparatuses. 
     According to one embodiment of the invention, a method of forming a switch includes providing a conductive region, a membrane, and a dielectric material. The method includes disposing a region of the dielectric material between the conductive region and the membrane such that a sufficient voltage applied between the conductive region and the membrane effects a capacitive coupling between the membrane and the conductive region. The dielectric material has a resistivity sufficiently low to inhibit charge accumulation in the dielectric region during application of the voltage. 
     According to another embodiment of the invention, a switch includes a conductive region, a membrane, and a dielectric region. The dielectric region is formed from a dielectric material and is disposed between the membrane and the conductive region. When a sufficient voltage is applied between the conductive region and the membrane, a capacitive coupling between the membrane and the conductive region is effected. The dielectric material has a resistivity sufficiently low to inhibit charging in the dielectric region during operation of the switch. 
     Embodiments of the invention provide numerous technical advantages. For example, in one embodiment of the invention, a switch is provided that does not suffer from bipolar operation in response to high frequency stimulus and does not turn off inadvertently when it should be turned on in response to low frequency stimulus, which are disadvantages associated with some prior devices. Further, according to the invention, a switch is provided that can be repeatedly activated in response to a bias voltage having a fairly constant magnitude. Such switches provide more reliable operation and are desirable. Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding on the present invention and the advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic drawing illustrating a top view of a microelectromechanical microwave capacitive switch according to the teachings of the present invention; 
     FIG. 2 is a side view of the microelectromechanical microwave capacitive switch illustrated in FIG. 1 in an undeflected position; 
     FIG. 3 is a schematic drawing illustrating the microelectromechanical microwave capacitive switch of FIG. 1 in a deflected position; 
     FIG. 4 is a circuit diagram illustrating an effective electrical circuit at high frequency of the microelectromechanical microwave capacitive switch illustrated in FIGS. 1 through 3; 
     FIGS. 5A through 5C are a series of graphs illustrating a desired response and a conventional response for microelectromechanical microwave capacitive switches for high frequency stimulus; 
     FIGS. 6A through 6C are a series of graphs showing a desired response and a conventional response for microelectromechanical microwave capacitive switches in response to a low frequency stimulus; 
     FIGS. 7A through 7B are a series of graphs of switch repetition versus bias voltage illustrating an increasing bias voltage required with switch repetitions; and 
     FIGS. 8A through 8C are a series of schematic drawings illustrating the formation of electric fields between a membrane and an electrode of the microelectromechanical microwave capacitive switch of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention and its advantages are best understood by referring to FIGS. 1 through 8 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIGS. 1 through 3 are schematic drawings illustrating one embodiment of a microelectromechanical microwave (MEMS) capacitive switch  10  according to the teachings of the present invention. FIG. 1 shows a top view of switch  10 . FIG. 2 shows a side view along the lines  2 — 2  of FIG. 1 for an undeflected position. FIG. 3 shows a side view along lines  2 — 2  of FIG. 1 for a deflected position. 
     Switch  10  includes membrane posts  12  and  14 . Membrane post  12  and  14  are generally formed from any suitable conductive material; however, membrane posts  12  and  14  may also be insulative if desired. Disposed between membrane posts  12  and  14  is an electrode  16 . Electrode  16  is connected to, or forms a part of, a transmission line  22 . Transmission line  22  carries a high frequency signal. Overlying electrode  16  is a dielectric region  18 . In one embodiment, dielectric region  18  is formed from silicon nitride (Si 3 N 4 ). However, any suitable dielectric may be used. As described in greater detail below, dielectric region  18  is formed from a dielectric material having a sufficiently low resistivity to inhibit charge accumulation within dielectric region  18 . A membrane  20  is disposed between membrane support posts  12  and  14 , as best illustrated in FIG.  2 . Membrane  20  is connected to a reference voltage, such as ground. According to one embodiment, membrane  20  is formed from a conductive material. A gap  19  exist between membrane  20  and dielectric region  18 , as illustrated in FIG.  2 . In one embodiment, membrane posts  12  and  14 , electrode  16 , dielectric region  18 , and membrane  20  are formed overlying a substrate  24 . 
     If a DC bias voltage is applied to electrode  16  and membrane  20  is held at ground, as illustrated in FIG. 3, membrane  20  is deflected downward, due to an electric field created between membrane  20  and electrode  16  by the bias voltage, until it rests on dielectric region  18 . This contact forms a capacitive coupling that effectively shorts high frequency signals between transmission line  22  and ground. Thus, transmission of a high frequency signal along transmission line  22  can be prevented by application of a bias voltage between electrode  16  and membrane  20 . 
     FIG. 4 is a simplified circuit diagram illustrating an effective circuit of microelectromechanical microwave capacitive switch  10 . As illustrated, when microelectromechanical microwave capacitive switch  10  is closed, signals along transmission line  22  are shorted to ground. This closing of microelectromechanical switch  10  corresponds to the position of membrane  20  illustrated in FIG.  3 . As described above, this positioning of membrane  20  is effected by application of a bias voltage between electrode  16  and membrane  20 . 
     As described in greater detail below, the invention recognizes that, during application of a bias voltage, charge tends to be injected from either membrane  20  or electrode  16  into dielectric region  18 . This charge, once injected, occupies trap sites within dielectric region  18  and creates a shielding effect that effectively lowers the electric field between electrode  16  and membrane  20 . When the injected charge reaches sufficient levels, the electrostatic attraction between electrode  16  and membrane  20  is neutralized and membrane  20  returns to its rest, or up position. This results in a spontaneous and undesired release of microelectromechanical microwave capacitive switch  10 . According to the teachings of the invention, reducing the resistivity of the material used for dielectric region  18  inhibits charge accumulation and effects a more desirable microelectromechanical microwave mechanical capacitive switch. 
     FIGS. 5A through 5C are a series of graphs illustrating the position of a microelectromechanical microwave capacitive switch in response to a high frequency stimulus. Curve  26  in FIG. 5A represents a high frequency stimulus for a bias voltage applied between electrode  16  and membrane  20 . Curve  28  in FIG. 5B illustrates a desired response of microelectromechanical microwave capacitive switch  10 . For curve  28 , an “Up” position indicates that membrane  20  is as illustrated in FIG. 2 and a gap  19  is maintained between membrane  20  and dielectric region  18 . In such a position, microelectromechanical microwave capacitive switch  10  allows signals to flow along transmission line  22 . Conversely, a “Dn” position indicates that microelectromechanical microwave capacitive switch  10  is in a down position, being in contact with dielectric region  18 . In such a position, microelectromechanical microwave capacitive switch  10  shorts high frequency signals to ground and therefore halts transmission of high frequency signals along transmission line  22 . 
     Desired response curve  28  shows that microelectromechanical microwave capacitive switch is in a “Dn” position only when an appropriate bias voltage is applied between electrode  16  and membrane  20 , corresponding to closing of the switch. Curve  30  in FIG. 5C illustrates a problem that occurs in some conventional microelectromechanical microwave capacitive switches. With operation as shown by curve  30 , the switch toggles at a rate equal to twice the desired rate. This operation is undesirable. The invention recognizes that the behavior of some conventional microelectromechanical microwave capacitive switches as exhibited by curve  30  occurs due to charge injection and accumulation, which is described in greater detail below in conjunction with FIGS. 8A through 8C. 
     FIGS. 6A through 6C are a series of graphs illustrating the operation of microelectromechanical microwave capacitive switches in response to a low frequency stimulus. Curve  32  in FIG. 6A illustrates an example of low frequency stimulus in which a positive bias voltage applied between electrodes  16  and membrane  20  generates the desired response illustrated by curve  34 , shown in FIG.  6 B. Curve  36  in FIG. 6C illustrates a response resulting from some conventional microelectromechanical microwave capacitive switches in which the membrane of the microelectromechanical switch toggles back to an up position when it should be in a down position. Thus, after some period of time, the membrane returns to an “Up” position even when a bias voltage is maintained. The invention recognizes that the behavior of some conventional microelectromechanical microwave capacitive switches as exhibited by curve  36  occurs due to charge injection and accumulation, which is described in greater detail in conjunction with FIGS. 8A through 8C. 
     FIGS. 7A and 7B are a series of graphs showing switch repetitions versus bias voltage. In curve  38  in FIG. 7A, the voltage required to displace membrane  20  downward to contact dielectric region  18  is illustrated as a function of the number of switch repetitions. As illustrated, the bias voltage required to effect such contact increases as the number of times the switch is opened and closed increases. A more desirable response is illustrated by curve  40  in FIG. 7B in which the bias voltage required to displace membrane  20  to contact dielectric region  18  remains fairly constant after a few switch repetitions. The behavior as illustrated by curve  38  is also attributed to charge injection and accumulation. 
     FIGS. 8A through 8C illustrate the generation of an electric field between membrane  20  and electrode  16  for three time periods: time t=0; time t=t 1 , &gt;0; and time t=t 2 &gt;t 1 . The cause of the above-described undesirable behaviors of some microelectromechanical microwave capacitive switches is further described in conjunction with FIG.  8 . 
     For time t=0, an external electric field due to a bias voltage applied between electrode  16  and membrane  20  has the same magnitude as the total electric field between electrode  16  and membrane  20  because there is no internally generated electric field within dielectric region  18 . However, at time t 1 , electrical charges begin to accumulate within dielectric region  18 . These electrical charges are injected into dielectric region  19  due to the applied bias voltage. These electrical charges generate an internal electric field that opposes the externally applied electric field. Thus, the total electric field between membrane  20  and electrode  16  is reduced. At some time t 2  the total electric field between membrane  20  and electrode  16  is reduced to an extent that membrane  20  will return to an “open” position. Thus, the accumulation of a charge that is injected into dielectric region  19  by application of a bias voltage creates an electric field opposing the externally applied electric field generated by application of a bias voltage. This charge accumulation is responsible for the behavior of conventional microelectromechanical microwave mechanical capacitive switches as exhibited by curve  36  in FIG.  6 C. 
     Charge accumulation is also responsible for the behavior of conventional microelectromechanical microwave mechanical capacitive switches in response to low frequency stimulation as exhibited by curve  38  in FIG.  7 A. Each time a bias voltage is applied between membrane  20  and electrode  16 , a little more charge is injected into dielectric region  18 . This additional charge creates a stronger electric field opposing an externally applied electric field resulting from the bias voltage. Therefore, to attain an electric field sufficient to displace membrane  20  to contact dielectric region  18 , a greater bias voltage is required for each successive switch repetition. 
     Charge accumulation is also responsible for the behavior of conventional microelectromechanical microwave mechanical capacitive switches in response to high frequency stimulation as exhibited by curve  30  in FIG.  5 C. This phenomena, which results in switching at twice the desired frequency, occurs due to charge accumulation resulting from charge injection by application of a bias voltage. Upon application of a bias voltage, charge is injected into dielectric region  18 . The bias voltage then returns to zero at a desired time, but the accumulated charge creates a net electric field in dielectric region  18 . This net electric field causes a potential difference between electrode  16  and membrane  20 , which causes membrane  20  to again displace toward electrode  16 . This displacement occurs even though the externally applied bias voltage is zero. Therefore, switching occurs at twice the desired rate. This operation is referred to as bipolar operation. 
     According to the teachings of the present invention, such problems associated with charge injection and accumulation may be addressed by depositing dielectric region  18  in such a way as to make it “leaky.” In other words, dielectric region  18  is deposited with a material having an increased conductivity, or decreased resistivity, to inhibit charge accumulation in dielectric region  18  during operation of switch  10 . As used herein, according to one embodiment, “inhibit charge accumulation” refers to preventing charge accumulation to an extent that microelectromechanical microwave mechanical capacitative switches, during standard operating conditions, generally do not exhibit bipolar response in response to high frequency stimulus or generally do not switch to an “Up” position when they should be in a “Dn” position in response to low frequency stimulus, but not necessarily both. Thus inhibition of charge accumulation occurs if one or more of these two behaviors is generally prevented. 
     Forming dielectric region  18  with decreased resistivity allows migration of the injected charges through dielectric region  18  and avoids charge buildup. According to the invention, increasing the conductivity of dielectric region  18  may be achieved in several ways: dielectric layer  18  may be intentionally doped with an external dopant; the internal stoichiometry may be modified; pre or post processing steps can be introduced or modified; or other suitable techniques that increase the conductivity of dielectric region  18  may be utilized. Although the particular increases from standard conductivities associated with dielectric material used in conventional microelectromechanical microwave capacitive switches varies by application, bias voltage, and magnitude of electric field, increasing the conductivity by a factor of a 10,000 over standard values has been shown to be particularly advantageous and produced the above-described desirable results. 
     According to one embodiment of the invention, silicon nitride (Si 3 N 4 ) is deposited stoichiometrically by plasma enhanced chemical vapor deposition. The conductivity of SiN is sensitive to the Si/N ratio. By increasing the silicon concentration in the film and making the film silicon-rich, the dielectric becomes leaky and prevents charge by accumulation problems. In one embodiment of the invention, the resistivity of the silicon nitride used to form dielectric region  18  was reduced from 1×10 11  to 1×10 7  Ohm-cm. This resistivity occurs in the presence of an electrical field having a magnitude of 200 kilovolts per centimeter. In one embodiment, the dielectric region has a resistivity less than approximately 1×10 11  ohm-cm measured in an electric field of approximately 200 kV/cm. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.