Abstract:
A drive circuit for a MEMS device, an integrated circuit having a plurality of MEMS devices and drivers, a method of operating the drive circuit and a method of manufacturing the integrated circuit. In one embodiment, the drive circuit includes: (1) an electrode driver and (2) a switching network, coupled to an output of said electrode driver that: (a) in a first configuration, couples said output to a first electrode of an axis of said MEMS device and grounds an opposing second electrode of said axis of said MEMS device and (b) in a second configuration, couples said output to said second electrode and grounds said first electrode.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention is directed, in general, to micro-electromechanical systems (MEMS) devices and, more specifically, to a drive circuit for a MEMS device and method of operating the same to drive a MEMS device.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrostatically actuated micro-electromechanical system (MEMS) devices have been proposed for a variety of applications. One promising use for MEMS devices is in optical switching and steering devices. In such devices, movable micro-machined mirrors are used as a switching element to direct an input optical signal to a desired output. The movement of the micro-machined mirrors is accomplished by electrostatic actuation.  
           [0003]    In a typical MEMS device, an individual mirror is affixed to a movable support structure (i.e., a gimbal) via torsional elements such as springs. The gimbal may be coupled to a frame, also via torsional elements. Typically, two torsional elements positioned on opposing sides of the mirror, couple the mirror to the gimbal, and define an axis for mirror rotation. Similarly, two torsional elements positioned on opposing sides of the gimbal couple the gimbal to the frame, and define an axis for gimbal rotation.  
           [0004]    In a typical situation, electrodes are positioned under the mirror and gimbal. The electrodes are configured to rotate the mirror or gimbal in either direction about its axis. The mirror or gimbal rotates under the electrostatic force between the mirror and gimbal, and is balanced in equilibrium by the restoring force of the torsional elements. The degree of rotation depends upon the amount of voltage applied to the electrodes. Traditionally, a degree of rotation up to about 9 degrees is achievable.  
           [0005]    Prior-art attempts to drive the MEMS mirrors to a given degree of rotation used a digital to analog converter (DAC) and an amplifier, perhaps a high-voltage (HV) amplifier, to apply a voltage to each electrode for each axis. In order to control the mirror, a desired drive voltage was programmed into a first DAC to drive the HV amplifier, which in turn drove a first electrode of a given axis. A second DAC was programmed to zero volts, or ground, or virtual ground, and therefore a second electrode of a given axis was also driven to a zero drive voltage by the second DAC. However, the prior-art attempts required both a DAC and HV for each electrode, i.e, each axis used 2 DACs and 2 amplifiers, perhaps HV amplifiers. This plurality of components can lead to a loss of “real estate” on a chip, higher cost, undesirable thermal characteristics, and so on.  
           [0006]    Accordingly, what is needed in the art is a drive circuit for a MEMS device and method of operating the same that overcomes the deficiencies of the prior art.  
         SUMMARY OF THE INVENTION  
         [0007]    To address the above-discussed deficiencies of the prior art, the present invention provides a drive circuit for a MEMS device, an integrated circuit having a plurality of MEMS devices and drivers, a method of operating the drive circuit and a method of manufacturing the integrated circuit. In one embodiment, the drive circuit includes: (1) an electrode driver and (2) a switching network, coupled to an output of the electrode driver that: (a) in a first configuration, couples the output to a first electrode of an axis of the MEMS device and grounds an opposing second electrode of the axis of the MEMS device and (b) in a second configuration, couples the output to the second electrode and grounds the first electrode.  
           [0008]    The present invention is based on the recognition that prior art MEMS device driver circuits employing two electrode drivers per axis essentially wasted one of the two driver circuits. While one driver circuit was performing the useful task of positioning the MEMS device, the other was producing nothing more than a ground signal to inactivate the opposing electrode. Since electrode drivers cost some amount of money to fabricate, occupy some space (“real estate”), require electricity to power and produce heat during operation, elimination of unnecessary electrode drivers is advantageous. The present invention therefore introduces a switching network that allows a single electrode driver to do the work that previously required two, and inactivates unused opposing electrodes to ground in a simpler and more direct manner.  
           [0009]    In one embodiment of the present invention, the electrode driver includes: (1) a digital-to-analog converter and (2) an amplifier that provides the output. Those skilled in the pertinent art are familiar with the structure and function of conventional electrode drivers. The present invention can employ either conventional or later-discovered electrode drivers.  
           [0010]    In one embodiment of the present invention, the first and second configurations are mutually exclusive. Alternatively, the first and second configurations may coexist, advantageously for only a brief period of time.  
           [0011]    In one embodiment of the present invention, the switching network includes: (1) a first switch interposing the output and the first electrode, (2) a second switch interposing the output and the second electrode, (3) a third switch interposing the first electrode and an electrical ground and (4) a fourth switch interposing the second electrode and the electrical ground. In a more specific embodiment, the first and fourth switches operate in tandem, the second and third switches operate in tandem and the first and second switches are never simulaneously in an ON state. Of course, as stated above, the first and second switches may be simultaneously in an ON state, but advantageously for only a brief period of time.  
           [0012]    In one embodiment of the present invention, the drive circuit further includes: (1) a second electrode driver and (2) a second switching network, coupled to an output of the second electrode driver that: (a) in a first configuration, couples the output to a third electrode of a second axis of the MEMS device and grounds an opposing fourth electrode of the second axis of the MEMS device and (b) in a second configuration, couples the output to the fourth electrode and grounds the third electrode. Therefore, the present invention can be extended to control multi-axis MEMS devices.  
           [0013]    In one embodiment of the present invention, the electrode driver and the switching network are embodied in an integrated circuit. Those skilled in the pertinent art will understand, however, that the driver circuit of the present invention may be embodied in any appropriate conventional or later-discovered form.  
           [0014]    The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 illustrates one embodiment of a drive circuit and switching network for driving a MEMS device constructed according to the principles of the present invention;  
         [0017]    [0017]FIG. 2 illustrates one embodiment of a multi-axis MEMS device constructed according to the principles of the present invention;  
         [0018]    [0018]FIG. 3 illustrates one detailed embodiment of a drive circuit and switching network embodied in an integrated circuit constructed according to the principles of the present invention; and  
         [0019]    [0019]FIG. 4 illustrates one detailed embodiment of an amplifier switch constructed according to the principles of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]    Referring initially to FIG. 1, illustrated is one embodiment of a MEMS driver  100  employing an electrode driver circuit  110  and a switching network  150  for driving a MEMS device  180  constructed according to the principles of the present invention. The electrode driver circuit  110  and the switching network  150  may be embodied in an integrated circuit  101 .  
         [0021]    The electrode driver circuit  110  has a digital to analog converter (DAC)  120 . The DAC  120  converts an inputted value which represents a desired drive voltage to be applied to a first electrode  190  or a second electrode  193  of the MEMS device  180 . Employment of the first electrode  190  and the second electrode  193  will be described in more detail, below. The analog output voltage of the DAC  120  is then amplified by the amplifier  130 . Perhaps a high-voltage (HV) amplifier  130 . Briefly, the amplifier  130  amplifies a voltage output by the DAC  120 , perhaps as a function of the resistance of a first resistor  145  divided by a second resistor  140 . The amplifier  130  then inputs its voltage signal into the switching network  150 . In the illustrated embodiment of the MEMS driver  100 , advantageously there is only the single electrode driver circuit  110  and a single switching network  150  for the first electrode  190  and second electrode  193  pair on a given axis.  
         [0022]    The switching network  150  employs the single electrode driver circuit  110  to drive the first electrode  190  and the second electrode  193 . In the switching network  150 , a first switch  155  interposes the output of the electrode driver circuit  110  and the first electrode  190 . In the illustrated embodiment, the first switch  155  is open. A second switch is illustrated interposing the first electrode  190  and an electrical ground. In the illustrated embodiment, the second switch  160  is open. A third switch  165  interposes the output of the electrode driver circuit  110  and the second electrode  195 . In the illustrated embodiment, the third switch  165  is closed. Finally, a fourth switch  170  interposes the second electrode  193  and the electrical ground. In the illustrated embodiment, the fourth switch  170  is closed.  
         [0023]    This set of the first switch  155 , the second switch  160 , the third switch  165  and the fourth switch  170  is referred to as a first configuration. If all switches were reversed (i.e., all the on switches were turned off and all the off switches were turned on), this set of switch states is referred to as a second configuration. In one embodiment of the present invention, the first and second configurations are mutually exclusive. Alternatively, the first and second configurations may coexist, advantageously, for only a brief period of time.  
         [0024]    Finally, the MEMS device  180  has a fulcrum  185 , the first electrode  190  and the second electrode  193 , and a mirror  195 . In either the first or second configuration, one of the pair of the first or second electrodes  190 ,  193  is driven by a voltage, while the other of the pair of the first or second electrodes  190 ,  193  is attached to ground. In the opposite configuration, the switches are logically inverted—i.e., on switches become off and off switches become on. In the illustrated embodiment, the second electrode  193  is driven by a drive voltage, and the first electrode  190  is coupled to the electrical ground, by the switching network  150 , thereby controlling the MEMS device  180  with only the single electrode driver circuit  110 . By applying the drive voltage or ground to the first or second electrodes  190 ,  193  the mirror  195  can be made to tilt.  
         [0025]    Turning briefly to FIG. 2, illustrated is one embodiment of a multix-axis MEMS device  200  (MEMS device  200 ) that is free to be driven inbuilt constructed according to the principles of the present invention. The MEMS device  200  has a mirror  210 . The MEMS device  200  also has two pairs of drive electrodes. A first drive electrode pair  220  has a first drive electrode  220   a  and a second drive electrode  220   b . A second drive electrode pair  225  has a first drive electrode  225   a  and a second drive electrode  225   b.    
         [0026]    The MEMS device  200  may therefore have the first drive electrode pair  220  and the second drive electrode pair  225  may each be set in its own first configuration or second configuration. As each axis of the MEMS device  200  operates independently, any one of the first pair of drive electrodes  220   a ,  220   b  may be set to ground and a first drive voltage applied to the other electrode, and any one of the second pair of drive electrodes  225   a ,  225   b  may be set to ground and a second drive voltage applied to the other electrode. Therefore, in the illustrated embodiment, there will be a second switching network and a second electrode driver circuit, one for each axis.  
         [0027]    Turning now to FIG. 3, illustrated one detailed embodiment of a drive circuit and switching network embodied in an integrated circuit (ic)  300  constructed according to the principles of the present invention. An amplifier  310 , perhaps a HV amplifier, may be employed in an inverting configuration to amplify the output of the previous DAC. The amplifier  310  could also be employed in a non-inverting configuration. The output of the amplifier  310  is then input into a first amplifier switch  320  and a second amplifier switch  325 .  
         [0028]    The first and second amplifier switches  320 ,  325  allows the voltage driver output of the amplifier  310  to amplify and pass through the driver voltage, while the remaining amplifier switch  320 ,  325  output voltage is disabled. A control current into the first or second amplifier switch  320 ,  325  turns the first and second amplifier  320 ,  325  on or off. The amplified value of the output of the first amplifier switch  320  may then be output to a first electrode  327 , or the first electrode  327  may instead be coupled to electrical ground, as to be described in more detail below. Likewise, the amplified value of the output of the second amplifier switch  325  may then be output to a second electrode  329 , or the second electrode  329  may instead be coupled to electrical ground, as to be described in more detail below.  
         [0029]    Two inputs, a first input  303  and a second input  305 , are input into the ic  300  representing a selected member of a pair of drive electrodes (i.e, either the first electrode  327  or the second electrode  329 ) which is to be enabled or disabled. The first input  303  and the second input  305  should be complementary. A first ground switch  330  may then be closed by the first input  303 , or the first ground switch  330  will be open. Likewise, a second ground switch  335  may then be closed by the second input  305 , or the second ground switch  335  will be open. Either way, the first and second ground switches  330 ,  335  should be complements of one another.  
         [0030]    In the illustrated embodiment of the MEMS device  300 , the first ground switch  330  and the second ground switch  335  are both single P-channel transistors. As is well known to those skilled in the art, when transistors are turned on, they may become a short circuit. In the illustrated embodiment, this means that either the first electrode  327  or the second electrode  329  is electrically coupled to a Vswitch  340 . Depending upon the exact system implementation, the Vswitch  340  potential does not need to be exactly 0 Volts. For example, a 5 Volt supply could be used. A first resistor  350  and a second resistor  355  are both employed for current limiting.  
         [0031]    Turning now to FIG. 4, illustrated is one detailed embodiment of an amplifier switch  400  constructed according to the principles of the present invention. In the illustrated embodiment, a single high voltage Nchannel transistor  410  may be used to connect an electrical ground voltage  420  to a drive electrode  430 . A current limiting resistor  440  adds current limiting to the amplifier switch  400 . A control current into a control current node  450  turns the amplifier switch  400  on. If no current is applied, a first resistor  460 , a second resistor  465  and a transistor  470  cooperate to turn the switch off. If a sufficient current is applied, the first resistor  460 , the second resistor  465  and the transistor  470  cooperate to turn the switch on. A first zener diode  480  is employed for gate to source protection of the Nchannel transistor  410 , and a second zener diode  485  is employed for gate to source protection of the transistor  470 .  
         [0032]    Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.