Abstract:
The present invention is an electromagnetic energy, e.g., visible light, controlled low actuation voltage MEMS switch. Stimulation of photovoltaic diodes causes a switching that controls the flow of a signal. A metal or other suitable conductive pad moves freely up and down within brackets, without the need for deformation, in response to the diodes to either ground a signal or permit it to pass. The low activation voltage of the bracketed pad structure permits the use of a reasonable number of photovoltaic diodes to develop sufficient voltage for actuation of the switch, allowing the realization of the present electromagnetic energy, e.g., visible light, controlled MEMS switch in a minimized chip area. The photovoltaic diodes do not require an independent DC power source to operate the switch of the invention. Use of different wavelengths to excite different sets of diodes allows turning on and off of the switch of the invention.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is related to the subject matter of previous application Ser. No. 09/326,771 to Milton Feng and Shyh-Chiang Shen, filed Jun. 4, 1999, now U.S. Pat. No. 6,143,997, issued Nov. 7, 2000. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No. F33615-99-C-1519 awarded by the United States Defense Advanced Research Projects Agency (DARPA).” The Government has certain fights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally concerns switches. More specifically, the present invention concerns microelectromechanical switches. 
     BACKGROUND OF THE INVENTION 
     Switching operations are a fundamental part of many electrical, mechanical, and electromechanical applications. Microelectromechanical systems (MEMS) for switching applications have drawn much interest, especially within the last few years. Products using MEMS technology are widespread in biomedical, aerospace, and communication systems. Recently, the MEMS applications for radio frequency (RF) communication systems have gained even more attention because of the MEMS&#39;s superior characteristics. RF MEMS have advantages over traditional active-device-based communication systems due to their low insertion loss, high linearity, and broad bandwidth performance. 
     SUMMARY OF THE INVENTION 
     The present invention is an electromagnetic energy, e.g., visible light, controlled low actuation voltage MEMS switch. Stimulation of photovoltaic diodes causes a switching that controls the flow of a signal. A metal or other suitable conductive pad moves freely up and down within brackets, without the need for deformation, in response to the diodes to either ground a signal or permit it to pass. The low activation voltage of the bracketed pad structure permits the use of a reasonable number of photovoltaic diodes to develop sufficient voltage for actuation of the switch, allowing the realization of the present electromagnetic energy, e.g., visible light, controlled MEMS switch in a minimized chip area. The photovoltaic diodes do not require an independent DC power source to operate the switch of the invention. Use of different wavelengths to excite different sets of diodes allows turning on and off of the switch of the invention. 
     In a preferred embodiment, the conductive pad electrically grounds a signal when the pad is located in a relaxed position (contacts closed). The pad is oriented for gravity to hold it in the relaxed position, but a voltage may assist the position and should be used where gravity or another force will not assist the contacts. Electromagnetic energy, e.g., visible light, stimulation through photovoltaic diodes provides a voltage to allow the signal to pass when a voltage serves to locate the pad in a stimulated position (contacts open). Voltage from the photovoltaic diodes are provided to electrodes that move the pad up and down with a low actuation voltage compared to known devices. The pad is not bent by the actuation voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be apparent to those skilled in the art with reference to the detailed description and the drawings, of which: 
     FIG. 1A is a schematic cross-sectional side view of a preferred embodiment of a switch of the present invention in a pad down (contacts closed) position; 
     FIG. 1B is the same side view as FIG. 1A in a pad up (contacts open) position; 
     FIG. 2A is a schematic top view showing hinge brackets of the present invention located on sides of a conductive pad; 
     FIG. 2B is a schematic top view showing hinge brackets of the present invention located on the ends of the conductive pad; 
     FIG. 3 is a schematic top view of an alternate embodiment of the hinge brackets of the present invention; 
     FIGS. 4A and 4B are schematic top views respectively showing one-sided and two-sided hinge structures of the present invention; 
     FIGS. 5A-5K are side views showing a process for manufacturing a preferred embodiment switch of the present invention; 
     FIG. 6A is a table of possible dimensions for the switch of the present invention; 
     FIG. 6B is a schematic top view which identifies the dimensions shown in FIGS. 1,  6 A; and 
     FIG. 7 is a table comparing the capabilities of known switches with the RF MEMS switch of the present invention. 
    
    
     TABLE OF ACRONYMS 
     This patent utilizes several acronyms. The following table is provided to aid the reader in understanding the acronyms: 
     C=Centigrade. 
     DC=direct current. 
     MEMS=microelectromechanical system. 
     MMIC=Monolithic Microwave Integrated Circuit. 
     PECVD=Plasma-Enhanced Chemical vapor deposition. 
     RF=radio frequency. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Generally, the present invention is an apparatus and method for controlling the flow of signals through electromagnetic energy, e.g., visible light, activation. More specifically, the method and apparatus is an electromagnetic energy, e.g., visible light, activated MEMS switch which is easy to produce and does not rely on the deformation of at least part of the system to complete an electrical connection of the switch. The switch is activated with a low voltage supplied by photovoltaic diodes. 
     Referring now to the drawings, and particularly FIGS. 1A and 1B, a preferred embodiment switch of the present invention includes a substrate base  10 . Any type of substrate used in semiconductor fabrication can be applied to the present invention such as silicon, GaAs, InP, GaN, sapphire, quartz, glasses, and polymers. Upon the substrate base  10  are waveguides which include one or more ground planes  12  and a signal line  16 . Any form of contacts used in integrated circuits can be used with the present invention, such as coplanar waveguides and microstrip waveguides. For purposes of describing the invention, coplanar waveguides are shown. 
     The ground planes  12  pass signals, for example RF signals, from the signal line  16  to ground when the switch is in a relaxed (contacts closed) position, to produce an off state. While the present invention is described with regard to RF signals, it should be appreciated that other signals can be used, including low frequencies, millimeter-wave frequencies, and sub-millimeter-wave frequencies. The invention can be used for broad-band switching applications. To pass RF signals to ground, a conductive pad  17  is moveably positioned to contact both the signal line  16  and the ground planes  12  when the pad is in the relaxed position (FIG.  1 A). The pad  17  is preferably made of metal, but can be made of any other suitable material. As shown with arrows, the input RF signal enters from an input port  16   a  (shown best in FIGS.  2 - 4 ), flows through the pad  17 , and then flows to ground by the ground planes  12 . Therefore, no RF signal flows through the output port  16   b  and the switch exists in an off state. Thus, unlike known MEMS, an off state occurs when the metal pad  17  is in a relaxed (contacts closed) position. 
     Preferably, a thin dielectric layer  18  is positioned between the signal line  16  and the metal pad  17  to serve as a DC blocking capacitor. A zero dielectric thickness corresponds to a physical short in the switch. A non-zero dielectric thickness corresponds to a capacitively coupled shunt switch, i.e., effectively a low-pass filter or an RF short. Any type of dielectric material can be applied, such as silicon dioxide, silicon nitride, pyralene, polymers, glasses and the like. In addition, bottom electrodes  20  can be inserted between the pad  17  and ground planes  12 , to enhance contact by attracting the pad  17  towards the waveguides. 
     Importantly, the pad  17  moves up and down freely with only the forces of gravity and air resistance to keep the metal pad  17  down. To guide movement of the pad  17 , the pad  17  is slidably positioned with brackets  22 . Preferably, the brackets  22  are placed atop the ground planes  12 , and may be placed on any side of the metal pad  17 . Referring to FIGS. 2A and 2 b,  brackets  22  are placed on sides  24  of the metal pad in FIG. 2A, and at ends  26  of the pad in FIG. 2 b.  As shown, each bracket  22  fits within an access hole  28  formed in the pad  17 , to capture the pad  17  while allowing it to freely slide between its relaxed and excited positions. 
     FIG. 3 shows a device which is similar to the device of FIGS. 1A and 1B, but is one-sided. One or more brackets  22  can be fabricated within one or two access openings  28  formed on one end of the pad  17 . Preferably, when two brackets and openings are used, as in FIG. 3, spacing between access holes is equal to or less than 25 μm. For the hinge type switch of the present invention, two sacrificial layers each having a thickness of around 2 μm are used. To remove the layers successfully, spacing between openings should be less than 15 μm in all directions. It can be appreciated that the brackets  22  are designed with consideration given to a sacrificial layer removal capability and mechanical strength. Thus, the layer should be robust enough to contain the pad  17  while maintaining its physical integrity as the pad moves up and down, yet be easily removed by etching during a masking process described below. 
     Referring now to FIGS. 4A and 4B, bracket structures which secure the conductive pad  17  through a single opening  28  are shown applied to a one sided switch (FIG. 4A) and a two sided switch (FIG.  4 B). 
     Referring again to FIGS. 1A and 1B, the switch system includes top electrodes  30  which sit atop dielectric suspensions  32 . Any suitable type of dielectric material can be used as the dielectric suspensions such as silicon dioxide, silicon nitride, pyralene, polymers, and glasses. Preferably, the dielectric suspensions  32  are positioned on the ground planes  12 . Actuation voltage is applied alternately to the top electrode  30  and bottom electrode  20 , or the top electrode  30  and ground as illustrated schematically, from photovoltaic sources  33  to provide electrostatic force that causes the metal pad to move, preferably in an up and down direction. It should be appreciated, however, that an operation of the switch does not depend on the metal pad moving in the up and down direction. Since the minimum required electrostatic forces produced by the actuation voltage is approximately equal to the sum of the gravitation and the air friction forces on the pad  17 , the applied voltage is much less than that necessary for the cantilever and membrane structures described above. Thus, a small actuation voltage, e.g., less than 3 Volts, for RF MEMS devices is achieved. 
     Such voltage is easily developed by photovoltaic sources  33 . In the figures, excepting FIGS. 5A-5K, the photovoltaic sources are represented schematically with respect to their positions and connections. Artisans will appreciate the particular form and connection scheme may change. Presuming a photovoltaic diode that develops 1 V, a cascaded arrangement of 5 diodes provides 5V. A number of diodes sufficient to power the top electrodes  30  is used to raise the pad  17 , and a separate set is preferably used to hold the pad down. Thus, the switch can be actuated without any wired connections by making it respond to electromagnetic energy, e.g., visible light, signals. Independent sets of photovoltaic diodes are preferably filtered to respond to different light wavelengths, such that the diodes connected to the bottom electrodes  20  will respond to different excitation wavelengths than diodes connected to the top electrodes  30 . 
     The conductive pad  17  is attracted upward when a small voltage, e.g., less than 3 Volts, is applied to top electrodes  30  (FIG. 1B) as a result of excitation of one set of diodes among the photovoltaic source  33 . A clearance between the bottom electrodes  20  and the top electrodes  30  affects the necessary actuation voltage such that a larger clearance necessitates a greater actuation voltage. When the pad  17  is in the excited position (contacts open), RF signals flow unimpeded from the input port  16   a  to the output port  16   b  through signal line  16 , as shown by the arrows, with only a negligible loss to the signal. In a preferred embodiment, this position corresponds to the switch “on” state. Thus, unlike known switches, the present switch is on when electrical contact is disengaged. In addition, since the actuation voltage is small, the present invention operates in either a normally “on” or in a normally “off” mode by applying DC voltage to either side of an actuation pad. The switching operation can be realized by applying two out-of-phase pulses at the top and bottom actuation electrodes through excitation of sets of diodes in the photovoltaic sources  33 . 
     Switches of the invention may be formed by a multi-level process for constructing hinge type RF MEMS switches, as represented in FIGS. 5A-5K, including initial steps in FIGS. 5A through 5D to form a photovoltaic diodes. Preferably, the temperatures for the MEM fabrication process in FIGS. 5E-5K are controlled to be not higher than 300 degrees centigrade (C.), to allow the integration compatibility of the current MMIC process. FIG. 5A shows the preparation of a p-n junction. In FIG. 5B, the p-type region is defined by a mesa etch and a metal contact  33   a  to the diode structure  33  is formed. An n-type deposit is made in FIG.  5 C. Then, the diode structure  33  is isolated by etching away n-type material away from other parts of the substrate, as shown in FIG.  5 D. The diode structure being completed allows interconnect metal to be formed. First, in FIG. 5E coplanar waveguides, i.e., ground planes  12  and signal lines  16 , are defined as first layer of metal  34 , for example gold, is evaporated on the coplanar waveguides. In FIG. 5F a thin dielectric layer  36  is deposited and VIA holes  38  are opened. 
     In FIG. 5G metal contact bumps  39  are formed in VIA holes  38  and a second layer of interconnect metal  40  is formed. A sacrificial layer  41  supports the metal pad  17  formed thereon in FIG. 5H, and a second sacrificial layer  42  is formed on the metal pad  17 . The sacrificial layers  41 ,  42  are patterned with VIA holes  43  in FIG.  5 J. This defines post areas  46  for the top electrodes  30  and for hinge structures that are formed in FIG.  5 K. Sacrificial layers are etched away to release the whole structure of the present switch. Additional details concerning preferred processing parameters and materials are included in U.S. Pat. No. 6,134,997, which is incorporated by reference herein. 
     Referring now to FIGS. 6A and 6B, various parameters are considered in the layout design which lead to the dimensions of the device. Artisans will appreciate that the device is not limited to a rectangular shape, but can be any geometry including a polygon, circle, or ellipse. Since the switch is designed for capacitive coupling operations as well as direct connections, the capacitance should be as large as possible to allow a switch down state. Thus, a contact area of the signal line  16  and metal pad  17  should be as large as possible to gain a wider operation bandwidth and lower impedance at high frequency regime. 
     A width of the metal pad  17  can overlap a width of the signal line  16 . However, large overlap areas cause greater insertion loss in the switch up state. It is noted that coplanar waveguide characteristics with a signal line width of 20 μm, 50 μm, and 100 μm are viable (not shown). A width of the top electrodes  30  was chosen at 100 μM and 150 μm. Combined with the different coplanar waveguide structures, six different impedance sets are available. 
     Bottom electrodes  20  are inserted on the ground planes  12  of coplanar waveguides and are surrounded by the ground planes  12 . A bigger electrode requires a lower actuation voltage. The ground plane  12  should be big enough to sustain 50 Ω impedance over the coplanar waveguides. Typically, a width of the ground plane is about 300 μm. 
     Referring now to FIG. 11, a table shows expectations for the present invention compared to known cantilever and membrane type switches. Of particular interest, note that a required switching voltage can be less than 3 Volts for the present invention, and 28 to 50 Volts for the known switches. This permits a relatively compact photovoltaic source to power a switching operation. Because of the low switching voltage, a large array of photovoltaic diodes is not required for operation of a switch. Accordingly, the electromagnetic energy, e.g., visible light, controlled switch of the invention will occupy a small chip area. The photovoltaic diode source may easily be integrated into a switch, and does not require an independent DC power source to operate. Thus, it should be understood that an improved switch has been shown and described. 
     From the foregoing description, it should be understood that an improved microelectromechanical switch has been shown and described which has many desirable attributes and advantages. It is adapted to switch the flow of a signal based on a relaxed or stimulated position of a metal pad. Unlike known prior art, a signal flow of the present switch is off when the metal pad makes a connection and on when the connection is breached. In addition, the present switch responds to a low actuation voltage of 3 Volts or less. The invention is also easy to manufacture. 
     Other alterations and modifications will be apparent to those skilled in the art. Accordingly, the scope of the invention is not limited to the specific embodiments used to illustrate the principles of the invention. Instead, the scope of the invention is properly determined by reference to the appended claims and any legal equivalents thereof.