Abstract:
A micromechanical memory  100  element comprising a deflectable member  102  located between a first member  104  and a second member  106 . The first member  104  is biased at a first member voltage, and the second member  106  is biased at a second member voltage. A bias voltage applied to the deflectable member will drive the deflectable member to either the first member  104  or the second member  106 . A first contact  108  is positioned on the top, or end, of the first member  104 . A second contact  110  is positioned on the top, or end, of the second member  106 . These contacts are biased through resistors  112  and  114  with a first and second contact voltage sufficient to hold the deflectable member in place even after removal of the bias voltage applied to the deflectable member. The state of the micromechanical memory element can be determined by sensing the voltage of the deflectable member  102 . Many of these micromechanical memory elements may be arranged in rows and columns and accessed via pass transistors driven by wordlines to enable a bitline to either read the state of the memory cell, or write data to the memory cell. The preceding abstract is submitted with the understanding that it only will be used to assist in determining, from a cursory inspection, the nature and gist of the technical disclosure as described in 37 C.F.R. §1.72(b). In no case should this abstract be used for interpreting the scope of any patent claims.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of micro-electro-mechanical systems (MEMS), more particularly to micromechanical memory cells.  
         BACKGROUND OF THE INVENTION  
         [0002]    Static random access memory (RAM) is formed using a bistable circuit element, the cross-coupled latch. The cross-coupled latch typically is formed using six transistors. Each element of the static RAM holds one bit of information in the cross-coupled latch. Once the static RAM cell has been written to, the data is held without further action. A read operation from the static RAM is non-destructive, in that it does not deplete the stored data, and the data may be read from the static RAM multiple times.  
           [0003]    A dynamic RAM uses only a single transistor to gate a charge on a capacitor. The state of the capacitor is read back through the same gate transistor. The capacitor can only store the charge, and therefore the information, for a limited time. Reading the state of the stored charge also depletes the charge. A dynamic RAM periodically must be refreshed to reinstate the charge on the capacitor. It is common practice to perform a rewrite operation after each read operation to ensure a sufficient charge remains stored on the capacitor.  
           [0004]    What is needed is a memory element that is bistable like the static RAM, yet composed of very few circuit elements like the dynamic RAM.  
         SUMMARY OF THE INVENTION  
         [0005]    Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for a micromechanical memory element. One embodiment of the claimed invention provides a micromechanical memory element. The micromechanical memory element comprising: a pass transistor; a first member biased at a first member voltage; a second member biased at a second member voltage; a deflectable member between the first and second members, the deflectable member receiving an electrical signal from the pass transistor, the deflectable member operable to deflect toward the first member when the electrical signal is a first state, and toward the second member when the electrical signal is a second state; a first contact positioned to contact the deflectable member when the deflectable member is deflected toward the first member, the first contact providing a first contact voltage operable to hold the deflectable member in contact with the first contact in the absence of a signal from the pass transistor; and a second contact positioned to contact the deflectable member when the deflectable member is deflected toward the first member, the second contact providing a second contact voltage operable to hold the deflectable member in contact with the second contact in the absence of a signal from the pass transistor.  
           [0006]    Another embodiment of the present invention provides a micromechanical memory element. The micromechanical memory element comprising: means for deflecting a deflectable member in one of two positions depending on a state of an input signal line; means for holding the deflectable member in the deflected state; and means for detecting the deflected state.  
           [0007]    Another embodiment of the present invention provides a micromechanical memory element. The micromechanical memory element comprising: a pass transistor; a first member biased at a first member voltage; a deflectable member receiving an electrical signal from the pass transistor, the deflectable member operable to deflect toward the first member when the electrical signal is a first state; and a first contact positioned to contact the deflectable member when the deflectable member is deflected toward the first member, the first contact providing a first contact voltage operable to hold the deflectable member in contact with the first contact in the absence of a signal from the pass transistor.  
           [0008]    Another embodiment of the present invention provides a method of forming a micromechanical memory element. The method comprising: providing a semiconductor substrate; forming addressing circuitry on the substrate; forming a first member and a first contact on the substrate, the first member and the first contact electrically connected to the addressing circuitry; forming a first sacrificial layer on the substrate; and forming a deflectable member on the first sacrificial layer, the deflectable member electrically connected to the substrate.  
           [0009]    Another embodiment of the present invention provides a method of forming a micromechanical memory element. The method comprising: providing a semiconductor substrate; forming addressing circuitry on the substrate; forming a first member and a first contact on the substrate, the first member and the first contact electrically connected to the addressing circuitry; forming a first sacrificial layer on the substrate; forming a deflectable member on the first sacrificial layer, the deflectable member electrically connected to the substrate; forming a second sacrificial layer on the deflectable member; forming a second member and a second contact on the second sacrificial layer, the second member and the second contact electrically connected to the addressing circuitry; and removing the first and second sacrificial layers.  
           [0010]    The disclosed micromechanical memory element may provide the stability of a conventional static RAM and the low component count of a conventional dynamic RAM.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a side view of a micromechanical memory element according to one embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is a side view of the micromechanical memory element of FIG. 1 showing the deflectable member deflected to touch one of the contacts.  
         [0014]    [0014]FIG. 3 is a top view of a micromechanical memory element similar to the element of FIG. 1 showing the location of a reset electrode.  
         [0015]    [0015]FIG. 4 is a side view of a micromechanical memory element of another embodiment of the present invention.  
         [0016]    [0016]FIG. 5 is a side view of a micromechanical memory element, similar to the one shown in FIG. 4, formed on the surface of a semiconductor wafer.  
         [0017]    [0017]FIG. 6 is an end view of a micromechanical memory element, similar to the one shown in FIGS. 1 and 2, formed on the surface of a semiconductor wafer.  
         [0018]    [0018]FIG. 7 is a schematic view of a portion of a micromechanical memory array.  
         [0019]    [0019]FIG. 8 is a side view of a wafer substrate showing the fabrication of a micromechanical memory device at a point after an initial etch mask has been patterned.  
         [0020]    [0020]FIG. 9 is a side view of the wafer substrate of FIG. 8 showing the partially fabricated micromechanical memory device after the hinge etch mask, hinge support etch mask, and deflectable member etch mask have been completed.  
         [0021]    [0021]FIG. 10 is a side view of the wafer substrate of FIG. 9 after the second member and second contact have been formed on the second planarizing spacer layer.  
         [0022]    [0022]FIG. 11 is a side view of the wafer substrate of FIG. 10 after the first and second sacrificial spacer layers have been removed leaving the cantilevered deflectable member supported between the first member and the second member.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 is a side view of a micromechanical memory element  100  according to one embodiment of the present invention. In FIG. 1, a deflectable member  102  is located between a first member  104  and a second member  106 . The first member  104  is biased at a first member voltage, and the second member  106  is biased at a second member voltage.  
         [0024]    When an electric field is established between the first and second members, the surface charge density is given by εE, where ε is the dielectric permeability of the insulating space between the first and second members and E is the electric field strength at the surface. The pressure acting on a unit area of the either member is the charge density times the field or εE 2 . This force is independent of the sign of the charge and is always an attractive force. The deflectable member  102  placed between the first member  104  and the second member  106  will experience a force attracting it to both the first and second members.  
         [0025]    If the deflectable member is biased at a point evenly between the first and second members, these attractive forces will cancel, otherwise the deflectable member will experience an unbalance force and be deflected. Depending on the voltages involved and the compliance of the deflectable member, the deflectable member may be forced to collapse against the first or second members.  
         [0026]    As an illustrative example of the forces involved at the micromechanical dimensions, if a first member is held at 1.8V and the deflectable member is held at 0V, and the separation between the first and deflectable members is 0.05 μm, the electrostatic pressure on the cantilevered deflectable member is then about 1.2 μg/μm 2 . This pressure is about the same as the mass per unit area of a 0.05 μm thick silicon cantilever.  
         [0027]    [0027]FIG. 1 shows a first contact  108  positioned on the top, or end, of the first member  104 . A second contact  110  is positioned on the top, or end, of the second member  106 . These contacts are biased through resistors  112  and  114 . The purpose of the contacts is to hold the deflectable member in place once the deflectable member is deflected against the contact.  
         [0028]    For example, assume a first member voltage is applied to the first member  104  and a second member voltage is applied to the second member  106 . The deflectable member  102  is biased at a voltage closer to the second member voltage than the first member voltage, so the net attractive force on the deflectable member is toward the first member. If the attractive force is large enough—if the voltages are high enough—the deflectable member will collapse toward the first member and make contact with the first contact.  
         [0029]    The first contact  108  is positioned to stop the movement of the deflectable member  102  before the deflectable member  102  comes into contact with the first member  104 . Contact between the first contact  108  and the deflectable member  102  establishes an electrical connection between them. Resistor  112  limits the current generated by a difference between the bias voltage of the deflectable member and the first member voltage applied to the first member.  
         [0030]    Once contact is established between the first contact  108  and the deflectable member  102 , removal of the bias voltage applied to the deflectable member  102  allows the deflectable member to assume the first contact voltage applied to the first contact. Assuming the first contact voltage is sufficiently different from the first member voltage, the deflectable member will remain deflected against the first contact. No current will flow through the device after the bias voltage is removed from the deflectable member.  
         [0031]    In a similar manner, if the deflectable member  102  is biased at a voltage closer to the first member voltage, the deflectable member  102  will deflect toward the second member  104  due to the greater electrostatic force. FIG. 2 is a side view of the micromechanical memory element of FIG. 1 showing the deflectable member  102  deflected against the second contact  110 .  
         [0032]    The micromechanical memory elements shown in FIGS. 1 and 2 may be formed from a deposited metal layer, typically a sputtered aluminum alloy, or from silicon—either crvstalline or polycrystalline—through any of the available micromachining techniques known in the art. Various micromachining techniques etch the members from a solid layer, or deposit the members in holes formed in a sacrificial layer. The members shown may be directed out of the plane of the substrate  116 , or supported above the substrate and parallel to the substrate  116 .  
         [0033]    The first and second contacts are isolated from the first and second members by an insulating layer  118 . In alternate embodiments the first and second contacts are not supported by the first and second members but rather are separate from the first and second members.  
         [0034]    Resistors  112  and  114  prevent the deflectable member  102  and the first and second contacts  108 ,  110  from welding upon contact. The resistors may be formed in the supply lines connected to the contacts, or may be a resistive coating on the contact itself. Use of a resistive coating results in the resistor being located between the contact and the deflectable member, not between the contact and a voltage supply as shown in FIG. 1.  
         [0035]    According to one embodiment of the present invention, the first contact and second member receive the same bias voltage. The second contact and the first member receive another common bias voltage. These two common bias voltages may be equal to the two states of the electrical signal  120  supplied to the deflectable member  102 .  
         [0036]    The state of the micromechanical memory element can be read by sensing the voltage of the deflectable member when it is not being driven by an input voltage. When deflected, the deflectable member will assume the voltage of either the first contact  108  or the second contact  110 .  
         [0037]    While it is beneficial for the deflectable member to stay against the contact when the electrical signal  120  supplied to the deflectable member is removed, it is not desirable for the deflectable member  102  to become stuck to either contact. Stiction is common in micromechanical devices and is caused by van der Waals force, as well as cold welding, arcing, and the surface tension of liquids formed on the contacting parts. To overcome stiction, a lubricant or passivation layer may be used. One lubricant used in micromirror devices if perfluorodecanoic acid, or PFDA. PFDA is deposited in a monolayer, typically through a vapor deposition, and forms an extremely low energy surface.  
         [0038]    In spite of the use of a lubricant, it may be difficult to get the deflectable member  102  to reliably release from the first and second contacts when an electrical signal  120  supplied to the deflectable member changes states. This is because the electrostatic field is a square function of the distance between the two members. Therefore, even a large voltage difference between the deflectable member  102  and the first member  104  may not overcome the forces generated by a very small voltage difference between the deflectable member  102  and the second member  106  when the deflectable member  102  is deflected against the second contact  110  as shown in FIG. 2.  
         [0039]    Reset techniques may be used to force a deflectable member to return to a neutral position. FIG. 3 is a top view of a micromechanical memory element similar to the element of FIG. 1 showing the location of a reset electrode  122 . The reset electrode  122  is used to establish a very strong electrostatic field between the deflectable member  102  and the reset electrode  122 . This very strong electrical field pulls the deflectable member away from the first and second contacts and allows the deflectable member  102  to return to a neutral position.  
         [0040]    The reset voltage applied to the reset electrode  122  may be one or more reset pulses. Reset pulses are used to reset micromirror devices. The reset deforms the deflectable member  102  to store energy in the deflectable member  102 . When the pulse is removed, the deformed deflectable member straightens and springs away from the contact.  
         [0041]    [0041]FIG. 4 is a side view of a micromechanical memory element of another embodiment of the present invention. The micromechanical memory element  400  of FIG. 4 uses only the first member and first contact of the embodiment of FIG. 1. In FIG. 4, an electrical signal  402  applied to the deflectable member  102  may assume one of two voltage states. The first member  104  is biased at the first voltage state. When the deflectable member  102  assumes the second voltage state, the deflectable member is deflected to contact the first contact  108 . The first contact is biased to a voltage sufficient to keep the deflectable member  102  against the first contact  108  even when the electrical signal  402  is removed.  
         [0042]    When the undeflected deflectable member  102  receives the first voltage state, no electrical field exists between the deflectable member  102  and the first member—which is also biased at the first voltage state—so the deflectable member  102  is not deflected. The voltage of the deflectable member is once again sensed to determine the state of the memory element. A resistor between the deflectable member and the first voltage state—often ground—may be necessary to assist reading from the memory element when the deflectable member is not deflected.  
         [0043]    [0043]FIG. 5 is a side view of a micromechanical memory element  500  similar to the one shown in FIG. 4 formed on the surface of a semiconductor wafer. In FIG. 5, the deflectable member  502  is supported above the substrate  512  by a deflectable member support  504 . The deflectable member  502  and the deflectable member support  504  to which it is connected are both formed from a metal layer. A thin region of the metal layer forms a hinge  506  which concentrates the deformation of the metal layer. The thin region may be a narrow portion of the layer, in the dimension perpendicular to FIG. 5 (not shown), or may be thin as shown and formed using the buried hinge methods used to form torsion beam micromirror devices.  
         [0044]    A voltage differential between the deflectable member  502  and a first member  508  will pull the deflectable member  502  toward the first member  508 . The first contact  510  is positioned to contact the deflectable member  502  and prevent the deflectable member from reaching the first member  508 .  
         [0045]    A micromechanical memory element similar to the one shown in FIGS. 1 and 2 may also be formed on the surface of a semiconductor wafer. FIG. 6 shows a micromechanical memory element  600  similar to the element of FIG. 5, but with a second member  608  formed above the deflectable member  602 . The view of FIG. 6 is from the end of an element similar to that shown in FIG. 5. The second member  608  is supported above the substrate  610  by supports  604 . The deflectable member  602  is supported above the substrate by a support similar to the support shown in FIG. 5 (not shown in FIG. 6). A first member  606  is formed on the substrate  610 . The first and second contacts are not shown in FIG. 6.  
         [0046]    [0046]FIG. 7 is a schematic view of a very small portion of a micromechanical memory array. The micromechanical elements described herein are written to and read from using the same types of circuitry commonly used to read and write to semiconductor memory arrays. In FIG. 7, a first bitline  702  and a second bitline  704  are shown. Each bitline provides a data bit signal to a column of memory elements.  
         [0047]    When a first wordline  706  is active, pass transistors  708  and  710  are turned on enabling the data on bitlines  702  and  704  to pass through to the micromechanical switches. The voltage signal on bitline  702  is provided to the deflectable member  712  while the voltage signal on bitline  704  is provided to the deflectable member  714 . The voltage signal on bitline  702  forces deflectable member  712  to deflect toward a first member and touch a first contact  716 , while the voltage signal on bitline  704  is forces the deflectable member  714  to deflect toward a second member and touch a second contact  718 .  
         [0048]    The memory array is read by enabling the pass transistors using the appropriate wordline, and sensing the voltage provided to the bitlines. For example, driving wordline  706  active turns on pass transistors  708  and  710 . When the pass transistors are turned on, the voltage provided by the first and second contacts, as appropriate, is provided to the bitline. In FIG. 7, the first bitline  702  is driven to a first contact voltage  720  while the second bitline  704  is driven to a second contact voltage  722 .  
         [0049]    A micromechanical memory element is fabricated on a semiconductor, typically silicon, substrate  804 . Electrical control circuitry is typically fabricated in or on the surface of the semiconductor substrate  804  using standard integrated circuit process flows. This circuitry typically includes, but is not limited to, a pass transistor for each element, and a sense amplifier for each bit line of the memory array. The silicon substrate  104  and any necessary metal interconnection layers are isolated from the micromirror superstructure by an insulating layer  806  which is typically a deposited silicon dioxide layer on which the micromechanical memory element structure is formed. Holes  808 , or vias, are opened in the oxide layer  806  to allow electrical connection of the micromirror superstructure with the electronic circuitry formed in the substrate  804 .  
         [0050]    The first layer of the superstructure is a metalization layer  810 . The metalization layer  810  is deposited on the insulating layer  806 . An etch mask is then deposited over the metal layer and patterned to define the first member, first contact, and a support pad for the deflectable member. The etch mask may be photoresist or an oxide hard mask. FIG. 8 shows the etch mask after it has been patterned.  
         [0051]    The metal layer  810  is then etched using the etch mask to define the first member  812 , the first contact  814 , and a support pad  816  for the deflectable member. FIG. 9 shows the micromechanical memory element after a first planarizing photoresist spacer layer  818  has been spun onto the substrate wafer. A via is opened in the first planarizing photoresist spacer layer  818  to provide access to the support pad  816 , and the first planarizing spacer layer is deep UV hardened to protect it during subsequent processing steps. This spacer layer and another spacer layer used later in the fabrication process are often called sacrificial layers since they are used only as forms during the fabrication process and are removed from the device prior to device operation.  
         [0052]    A thin layer  820  of metal is sputtered onto the spacer layer and into the holes. According to the buried hinge fabrication method used to fabricate micromirror devices, an oxide is deposited over the thin metal layer  820  and patterned to form an etch mask  822  over the region of the thin metal layer  820  that will form the hinge. A thicker layer of metal  824 , typically an aluminum alloy, is sputtered over the thin layer  820  and oxide etch mask  822 . Another oxide mask is deposited over the thick metal layer  824  and patterned to define the deflectable member support and deflectable member. FIG. 9 shows the wafer with the hinge etch mask  822 , the hinge support etch mask  830 , and the deflectable member etch mask  828  in place.  
         [0053]    After the etch masks have been formed, the thick and thin metal layers are etch simultaneously to form the deflectable member  836 , thin hinge portion  838 , and deflectable member support  840 . The oxide etch masks are then stripped. The metal layers may be polished at this point to proved a planar surface on which to build the remainder of the device. At this point the fabrication of the single sided memory element of FIG. 5 is completed and the first planarizing spacer layer is removed. Alternatively, a second planarizing photoresist layer  842  is spun-on the wafer and another metal layer sputtered over the second planarizing photoresist layer. The metal layer is patterned to form a second member  832  and a second contact  834  above the deflectable member  836 . The second member  832  and second contact  834  are supported by vias formed out of the plane of FIG. 10 and are not shown in FIG. 10.  
         [0054]    The first and second sacrificial photoresist layers are then ashed away, leaving a cantilevered deflectable member  836  supported between a first member  812  and a second member  832  as shown in FIG. 11.  
         [0055]    Thus, although there has been disclosed to this point a particular embodiment for a micromechanical memory element and method therefore, it is not intended that such specific references be considered as limitations upon the scope of this invention except insofar as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims. In the following claims, only elements denoted by the words “means for” are intended to be interpreted as means plus function claims under 35 U.S.C. §112, paragraph six.