Abstract:
Microelectromechanical RF and microwave frequency power limiter and electrostatic protection devices for use in high-speed circuits are presented. The devices utilize an airbridge or a cantilever arm including a contact pad positioned operatively adjacent to an electrically conductive and substantially planar transmission line. When the power level in the transmission line exceeds a particular threshold, the airbridge or cantilever arm yields due to force between the contact pad and the transmission line, directing undesired power away from active devices. This characteristic can either serve as a method by which to limit the amount of power passing through the transmission line to a determined value or as a method by which to protect devices along the transmission line from damage due to large electrostatic bursts.

Description:
PRIORITY CLAIM 
   This application is a divisional application claiming priority to U.S. patent application Ser. No. 09/431,308, filed Oct. 30, 1999 now U.S. Pat. No. 6,504,447, issued on Jan. 7, 2003, and titled “Microelectromechanical RF and Microwave Frequency Power Regulator.” 

   TECHNICAL FIELD 
   The present invention discloses an effective technique to provide overload and electrostatic discharge (ESD) protection to microwave/millimeter wave monolithic integrated circuits (MMICs) including low noise amplifiers (LNAs) using a microelectromechanical (MEM) device. 
   BACKGROUND OF THE INVENTION 
   In the construction of MMICs, power regulation and, more specifically, power limiting and ESD protection are desirable to prevent device bum-out from high incident RF power. 
   PIN diodes are typically used as power limiters, but these diodes are lossy (˜1.0 dB) at millimeter wave frequencies. Not only does the loss due to an input power limiter reduce the input signal level and thus the required amplification to reach a specified output level, but also reduces the signal-to-noise ratio by increasing the system&#39;s noise figure. Any loss due to a power limiter adds directly to the noise figure of the amplifier. Furthermore, diodes are difficult to use, as they require impedance matching with the circuitry to which they are connected, tending to reduce the available bandwidth. PIN diodes are also not generally available in low-noise, high electron mobility transistor (HEMT) processes and thus cannot be integrated onto the same substrate as the rest of the MMIC. 
   Semiconductor devices are sensitive to excessive input voltages, such as those generated by ESD. High-speed devices are particularly sensitive. MMIC systems that encounter ESD typically suffer from either immediate or latent component failure. In low frequency applications, the most common technique for protecting input, output, and power pins from damage is to include ESD diodes to shunt the undesired input signal away from the active devices and a series resistor to allow for sufficient time for the diodes to turn on. However, ESD diodes tend to have a large capacitance at high frequencies, which limits their use in radio to millimeter frequency applications. Additionally, a series resistor is not acceptable in a MIMIC system due to the incurred loss which, in order to compensate, would require greater input power. The result of these shortcomings in diodes and resistors leave the typical high-speed devices that operate at RF frequencies and above unprotected. 
   The present invention overcomes many of the difficulties involved in the use of diodes as power limiters and the use of diodes as ESD protection devices. These devices utilize the strong electromagnetic field associated with the high power signal or an ESD event to short out harmful signals and to protect the remainder of the MMIC system. These devices are each considered in two preferred aspects; a flexible bridge cantilever anchored at both ends supporting an electrical contact over a transmission line and as a cantilever anchored at one end with at least one contact at or near the opposite end. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a microelectromechanical RF and microwave frequency power regulator that may be tailored to a variety of applications including uses such as power limiting and electrostatic discharge protection for semiconductor devices. The power regulator includes a substrate on which at least one electrically conductive ground contact and a substantially planar transmission line are formed. A substantially elongated, electrically conductive strip is connected to the at least one ground contact and is positioned so that a portion of the substantially elongated strip is adjacent to the transmission line and so that a gap is formed therebetween. The electrically conductive strip may be formed in shapes such as a bridge or a cantilever arm, or may take other forms, as suitable to a particular application. In operation, when an undesirable signal is present on the transmission line, the resultant force created causes the conductive strip to flex toward, and physically contact the transmission line. Thus, the undesirable signal is diverted away from the circuit being protected by passing the signal through the conductive strip to ground. 
   This invention has been reduced to practice in the form of a power limiter and as an electrostatic device protection unit, and has various other applications that will be evident to those skilled in the art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a shunt bridge aspect of the device of the present invention; 
       FIG. 2  is a side view of a shunt bridge aspect of the device of the present invention, demonstrating the airbridge in the “open” position; 
       FIG. 3  is a side view of a shunt bridge aspect of the device of the present invention, demonstrating the airbridge in the “closed” configuration; 
       FIG. 4  is a top view of a shunt cantilever aspect the device of the present invention; 
       FIG. 5  is a side view of a shunt cantilever aspect the device of the present invention in the “open” position; 
       FIG. 6  is a side view of a shunt cantilever aspect the device of the present invention in the “closed” position; and 
       FIG. 7  shows a typical implementation of devices in accordance with the present invention as used in a MMIC. 
       FIG. 8  shows a side view of the series cantilever aspect used as an ESD protection switch in the “open” position. 
   

   DETAILED DESCRIPTION 
   The power regulator of the present invention is useful to regulate power in microwave and millimeter wave circuits, and may be tailored to a variety of applications. The proposed power regulator has been reduced to practice in the context of two specific applications, a power limiter and an electrostatic discharge (ESD) protection unit. In both applications, the device has been utilized in both a flexible cantilever and as a bridge, as described in greater detail in the paragraphs that follow. This description will first detail the cantilever and bridge as examples of aspects of the present invention and will then proceed to detail specific applications of the present invention. These examples of aspects are presented for illustration of this invention, and are not to be considered limitations to its scope. 
   The present invention relates to power regulators such as power limiters and ESD protection units, as well as to apparatus incorporating them therein. The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications to the preferred aspect, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other aspects. Thus, the present invention is not intended to be limited to the aspects shown, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
   A top view of a bridge aspect of the device of the present invention is shown in FIG.  1 . This aspect includes a substrate  9  with ground contacts  1  and  3  formed thereon. An example of a typical substrate material is semi-insulating GaAs with Au as a contact metal, although other material families may be appropriate depending on the particular application. The ground contacts  1  and  3  are electrically connected, through via holes  5  and  7 , respectively, to a metallization layer  15  (see  FIGS. 2 and 3 ) formed on the bottom side of a substrate  9 . The electrically connected via holes  5  and  7  are created by selectively etching holes through the substrate to the top metal layers,  1  and  3 . The sidewalls of the holes are then plated, making contact with the metallization layer  15 . A substantially elongated strip of electrically conductive material in the form of a bridge  11  is designed such that it traverses an electrically conductive transmission line  13  forming an air gap  16  (see  FIG. 2 ) between the bridge  11  and the electrically conductive transmission line  13 . On top of the metal conductive bridge is a spring material such as silicon nitride, which causes the bridge to return to its normally “open” position after an ESD event or the high power signal has subsided. 
     FIGS. 2 and 3  demonstrate the bridge power regulator during operation in the “open” and “closed” positions, respectively, with parts  1 ,  3 ,  5 ,  7 ,  9 ,  11 , and  13  corresponding to the same in FIG.  1 . In  FIG. 2 , there exists a gap  16  between the bridge  11  and the electrically conductive transmission line  13 . This state occurs during normal operation when there are no signals of sufficient power to activate the power regulator. 
     FIG. 3  shows the power regulator&#39;s response to an undesired signal passing along the planar transmission line  13 . The air bridge  11 , in this case, will flex to cause an electrical connection with the transmission line  13 , thereby directing the unwanted signal through the ground contacts  1  and  3  and the via holes  5  and  7  to the metallization layer  15 . Flexing of the bridge is caused by an attractive force developed between the bridge and the transmission line due to charges induced by the signal on the bridge  11 . When the signal is of sufficient strength to induce sufficient charges on the bridge  11  to cause a force sufficient to overcome its mechanical tension, the bridge  11  collapses thereby making contact to the transmission line  13 . A DC bias may be applied to metallization layer  15  in order to change the signal required on the transmission line  13  to activate the device. This provides a means for threshold adjustment. Rather than, or in addition to, a DC bias, a material such as an electret may be used to build-in some static charge on the metallization layer  15  also reducing the required signal on the transmission line  13  for activation. Care must be takes so as to prevent excessive built-in charge to ensure the device will return to the “open” position once the undesired signal has subsided. 
   Although  FIGS. 1 ,  2 , and  3  present an aspect utilizing a microstrip transmission line  13  requiring via holes  5  and  7 , other circuit configurations such as those utilizing coplanar transmission lines may not require via holes and their accompanying electrical paths. Thus the present invention is adaptable to a variety of substrates in a variety of configurations. 
   A top view of a cantilever arm aspect of the present invention is presented in FIG.  4 . This aspect includes a cantilever arm  17  constructed as a rectangular lever made of an electrically neutral material such as silicon nitride, with an anchor end  19 , a contact end  21 , and an actuation portion  23 . The contact end  21  faces and directly opposes the transmission line  25  that is embedded in the substrate  27  (see  FIGS. 5 ,  6 ). 
   As demonstrated in  FIG. 5 , the anchor end  19  of the cantilever arm  17  is mechanically attached to the top of an anchor  26 , with the bottom of the anchor  26  being mechanically attached to the substrate  27  and electrically connected to ground  28 , via a ground contact  30 . A contact strip  29  is mechanically attached to the underside contact end  21  of the cantilever arm  17  such that it faces, and is aligned along, the length of the transmission line  25 . The actuator pads  31  and  33  are formed of an electrically conductive material, with the top actuator pad  31  mechanically attached to the underside of the cantilever arm  17  and situated such that it is in mechanical and electrical contact with the anchor  26  and the contact stripe  29 . A very thin layer of insulating material  35  such as silicon nitride lies under the top actuator pad  31  and between the top and bottom actuator pads  31  and  33 , respectively, to prevent electrical contact therebetween. The bottom actuator pad  33  is situated directly beneath the top actuator pad  31  and is mechanically attached to the substrate  27 . When the device is in the “open” position, that is, when there has not been a signal applied to the bottom actuator pad  33 , there exists an air gap between the actuation pads  31  and  33 , and between the contact stripe  29  and the transmission line  25 . A DC bias may be applied to the actuator pad  33  in order to change the signal required on the transmission line  25  to activate the device. This provides a means for threshold adjustment. Rather than, or in addition to, a DC bias, a material such as an electret may be used to build-in some static charge on pad  33  also reducing the required signal on the transmission line  25  for activation. 
     FIG. 6  shows the operation of the device when a signal is applied to the bottom actuation pad  33 . In this scenario, an electrostatic force is created such that the top actuation pad  31  is drawn toward the bottom actuation pad  33 , resulting in contact between the contact stripe  29  and the transmission line  25 . 
     FIG. 7  shows the application of the preferred aspect of the ESD protection device in the context of a simple system. The system  41 , has a microwave input  43  with a microwave output  45  and an active device “connect” signal  47  serving as a system  41 , turn-on signal. In the input protection aspect  49 , the ESD protection device protects the active devices  53  from unwanted signals from the microwave input  43  by shorting the unwanted signals to ground. In the output protection aspect  51  the ESD protection device protects the output active devices in  53 . The control signals for the input and output protection aspects may come from a number of sources, dependent primarily upon design goals. Another aspect of the ESD protection device is its use as a series“on/off” switch for active devices and their outputs. Series on/off switches  55  and  57  are configured to allow the passage of a signal from the microwave input  43  to the active devices  53 , and from the active devices  53  to the microwave output  45 , respectively, upon activation of  47  to the “on” position. Activation of the on/off switches takes place via an activation voltage generator  59  that, in turn, is activated upon receipt of an active device “connect” signal  47  from a source outside the system  41 . 
     FIG. 8  shows the preferred aspect of the series ESD protection switch with elements  17 ,  19 ,  21 ,  23 ,  25 ,  26 ,  27 ,  29 ,  31 ,  33 , and  35  analogous to those of  FIGS. 5 and 6 , except that in this aspect, the activation pad  31  is not connected to the contact  21 . Thus the activation signal is distinct from the microwave transmission lines.