Abstract:
Among other things, a cavity having a cavity dimension is configured so that the cavity dimension changes in response to electrostatic forces applied to the cavity, and at least two electrical structures are configured to apply electrostatic forces in the vicinity of the cavity, the electrical structures being independently controllable.

Description:
[0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/378,143, filed Aug. 20, 1999, which is a continuation of U.S. patent application Ser. No. 08/744,253, filed Nov. 5, 1996, now issued as U.S. Pat. No. 5,986,796; a continuation-in-part of U.S. patent application Ser. No. 09/056,975, filed Apr. 8, 1998; and a continuation-in-part of U.S. patent application Ser. No. 09/974,544, filed Oct. 10, 2001, which is a divisional of U.S. patent application Ser. No. 08/769,947, filed Dec. 19, 1996, now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 08/554,630, filed Nov. 6, 1995, now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 08/238,750, filed May 5, 1994, now issued as U.S. Pat. No. 5,835,255, all incorporated here by reference. 
     
    
     
       BACKGROUND  
         [0002]    This invention relates to controlling micro-electro-mechanical cavities.  
           [0003]    As shown in FIG. 1, a micro-electro-mechanical structure (MEMS)  10  can be formed to have two walls  12 ,  14  connected mechanically  16  to define a cavity  18 . The walls of the cavity can be movable relative to one another to control, for example, interferometric optical properties of the cavity. An electrode  20  can be formed on one of the walls so that, when a sufficient voltage V A  (see FIG. 2) from a voltage source  24  is applied between the electrode and the opposite wall  12 , the activation threshold of the cavity is exceeded and the wall  12  is drawn close to the wall  14  by electrostatic force  26 . Because of a hysteresis effect, the wall  12  will then remain close to wall  14  even if the voltage falls below V A . Only when the voltage falls below a lower value, V B , will the wall  12  return to its original position.  
         SUMMARY  
         [0004]    In general, in one aspect, the invention features apparatus that includes a cavity having a cavity dimension, the cavity being configured so that the cavity dimension changes in response to electrostatic forces applied to the cavity, and at least two electrical structures configured to apply electrostatic forces in the vicinity of the cavity, the electrical structures being independently controllable.  
           [0005]    Implementations of the invention may include the following features. The cavity dimension is determined by a distance between two walls, and the cavity dimension determines optical properties of the cavity. The optical properties include interference or reflectance. The two electrical structures comprise electrodes. The electrical structures lie on a wall of the cavity. The electrical structures lie side by side on the wall of the cavity. The cavity comprises an interference modulator and the cavity dimension determines an optical state of the modulator. Changes in the cavity dimension that occur in response to the electrostatics forces are characterized by hysteresis. There is also a second cavity adjacent to the cavity. The cavity and the second cavity share a common wall.  
           [0006]    There are also stops within the cavity, the stops defining an intermediate cavity dimension between a minimum cavity dimension and a maximum cavity dimension the stops define channels between them portions of a wall of the cavity lie in response to electrostatic forces. One of the electrical structures comprises electrodes embedded within the stops. The stops lie on a movable wall of the cavity. Apertures in a second wall of the cavity are configured to receive the stops. There are also additional cavities having cavity dimensions, each of the cavities being configured so that its cavity dimension changes in response to electrostatic forces applied to the cavity. There are additional electrical structures configured to apply electrostatic forces in the vicinities of the cavities, each of the additional cavities being associated with at least two of the additional electrical structures. The electrical structures with which each of the cavities is associated are independently controllable. At least some of the electrical structures associated with at least some of the respective cavities are coupled together.  
           [0007]    The cavities are organized in groups by coupling together of selected electrical structures. The coupling comprises bus conductors. The coupling comprises bus elements fabricated on multiple levels of the apparatus.  
           [0008]    In general, in another aspect, the invention features apparatus that includes an array of interferometric modulators, actuation electrodes associated with the respective interferometric modulators, and a pattern of conductors connecting the actuation electrodes in groups.  
           [0009]    Implementations of the invention may include the following features. The groups comprise rows or columns of the actuation electrodes. The groups comprise pixels of a display. Each of the interferometric modulators is associated with more than one of the electrodes. The pattern of conductors connects different ones of the electrodes associated with each of the interferometric modulators in a configuration that enables them to be energized independently. The electrodes are arranged on walls of cavities of the interferometric modulators.  
           [0010]    In general, in another aspect, the invention features a method that includes energizing one electrical structure to apply an electrostatic force in the vicinity of a cavity, and independently energizing another electrical structure to apply an electrostatic force in the vicinity of a cavity.  
           [0011]    Implementations of the invention may include the following features. The one structure is energized to move an element of the cavity to a first position, and the other electrical structure is energized to maintain the element in the first position. The one structure is de-energized while the other structure remains energized. The energizing of the one electrical structure and the other electrical structure is controlled to effect more than two optical states of the cavity. One electrical structure is energized to apply an electrostatic force in the vicinity of each of multiple other cavities, and another electrical structure is independently energized to apply an electrostatic force in the vicinity of each of the multiple other cavities. The energizing of the electrical structures is controlled to independently control the optical states of groups of one or more of the cavities.  
           [0012]    Other advantages and features will become apparent from the following description and from the claims. 
       
    
    
     DESCRIPTION  
       [0013]    (FIG. 1 is a schematic side view of a MEMS.  
         [0014]    [0014]FIG. 2 shows a hysteresis curve.  
         [0015]    [0015]FIG. 3 is a top view of an iMoD array.  
         [0016]    [0016]FIGS. 4 and 5 are a side view and a top view of an iMoD array.  
         [0017]    [0017]FIG. 6 shows a hysteresis curve.  
         [0018]    [0018]FIGS. 7, 9,  10 ,  11 ,  13 ,  14 ,  15 , and  16  are side sectional views of various interference modulator configurations.  
         [0019]    [0019]FIGS. 17 a ,  17   b , and  17   c  show hysteresis curves.) 
     
    
       [0020]    As shown in FIG. 3, in an array  28  of interferometric modulators (iMoDs)  30 , each of the iMoDs can be controlled by providing a series of column electrodes  32  and row electrodes  34 . A given iMoD in the array can be activated by applying an appropriate voltage between a row electrode  38  and a column electrode  40  associated with the target iMoD. (The row and column electrodes are shown as narrower than they would actually be, for clarity.)  
         [0021]    A wide range of useful behaviors of cavity-type MEMS and arrays of them can be achieved by configuring the electrodes in other ways, for example, by adding one or more electrodes to one or more of the layers of the cavity.  
         [0022]    [0022]FIGS. 4 and 5 illustrate the addition of a latch electrode pattern  50  on the lower wall  52  of an array of iMoDs  54  that includes three iMoDs  44 ,  46 ,  48 . The latch electrode pattern  50  is interleaved with the patterns of column electrodes  62 ,  64 ,  66  for each of the three columns that are shown in the figure (which shows only a fragment of a full display). A row electrode  68  lies on the mechanical membrane  70 , which forms the upper walls of the cavities of the three iMoDs. The availability of the latch electrode permits a useful change in the hysteresis behavior of the cavity as illustrated in FIG. 6.  
         [0023]    In FIG. 6, the solid curve  76  is the response without a latch voltage applied, which reflects behavior similar to the behavior associated with the curve shown in FIG. 2. Actuation of one of the iMoDs occurs when the voltage between the row and column electrodes exceeds an actuation threshold and is maintained as long as a bias voltage of at least V B  (which can be less than the activation voltage) is maintained.  
         [0024]    Unlike the cavity of FIG. 1, however, the activation of the iMoD can be maintained by applying the bias voltage between the membrane and the latch electrode instead of between the membrane and the column electrode. The dashed line  78  of FIG. 6 represents the hysteresis behavior of the device with reference to the voltage between the row and column electrodes when the bias voltage is applied at the latch electrode. Even if the voltage between the row and column electrodes (depicted along the x-axis of FIG. 6) falls to zero, the activation state is maintained. Because the hysteresis loop is effectively widened, it is easier to maintain the activation states of the iMoDs across the array, and the addressing voltages applied to the columns may be chosen with less concern for their impact on the states of the iMoDs.  
         [0025]    [0025]FIG. 7 shows a cross-section of dual non-inverted iMoDs  80 ,  82 . The iMoDs are said to be non-inverted because they are viewed through the substrate  84  from locations  85  on the other side of the substrate from the cavities. By contrast, inverted designs are not viewed through the substrate. However, it should be noted that all of the embodiments described in this document can be used in both inverted and non-inverted designs. In each of the iMoDs, optical interference effects occur within an optical cavity  86 ,  88  that lies between the substrate and its associated optical stack  90 ,  91  and a mechanical membrane  92 ,  94  which is defined by the optical stack and the mechanical membrane. Each of the mechanical membranes  92 ,  94 , includes a conductor/reflector layer  97 ,  99  beneath an insulator layer  95 ,  93 . The insulating layers  95 ,  93  could also reside beneath actuation electrodes  96  and  98 .  
         [0026]    In an undriven state, each of the iMoDs may reflect white light. With a voltage applied between an actuation electrode  96 ,  98  (together called a bus) and the membrane  92 ,  94 , the mechanical membrane is drawn upwards towards the actuation electrode bus and the reflected color changes. The bus and the mechanical membranes are supported by a post  100 . The actuation electrodes may either be continuous between the two iMoDs or be connected by a bus jumper  102 .  
         [0027]    The bus structure of an array is shown in more detail in the three-dimensional view of FIG. 8. The figure reveals that the actuation electrode for each iMoD  110  can include more than one electrode component. Each iMoD is served by two electrode components, one component  112  lies in the middle of the iMoD; the other electrode contains into two elements  116  and  118  that lie on either side of the middle component. In FIG. 8, the electrodes are not continuous along each of the rows, but they are connected to form rows of electrodes by bus jumpers  120  (in the case of the center component  112 ) and by bus jumpers  122  and  124  in the case of the side elements  116  and  118 . Buses  122  and  124  are connected by a jumper  126  at the end of the row. (Note that not all of the jumpers are shown for all of the iMoD rows in the figure, for clarity.)  
         [0028]    The bus jumpers and other elements of the buses may be fabricated on multiple levels of a device to permit more complicated wiring schemes than those shown in FIG. 8. Any arbitrary number of electrodes, shapes of electrodes, and interconnections of electrodes may be useful in various implementations.  
         [0029]    In FIG. 8, the mechanical membranes  130  define the columns The bus  122 ,  124  represents a supplementary connection which can be used for a global latch or other actuation voltage for arbitrary groupings of electrodes.  
         [0030]    Although a particular configuration of electrodes and bus jumpers is shown in FIG. 8, a variety of arrangements are possible. For example, each iMoD could be served by any number of electrodes arranged in any of a variety of configurations. The electrodes of adjacent iMoDs could be connected in combinations other than simple rows. The connections could establish any arbitrary groupings of iMoDs. The groups of connected electrodes could be driven by a variety of driving voltages and driving regimes.  
         [0031]    When multiple iMoDs are used for each pixel of a display, the electrode patterns and bus connections can be designed to enable the addressing of entire pixels or fractions of pixels.  
         [0032]    [0032]FIGS. 9, 10, and  11  show two iMoDs in three different states of actuation.  
         [0033]    An iMoD can be designed to achieve three optical states including one that is not interferometric. Simpler iMoD designs, like those described in U.S. Pat. No. 6,055,090, achieve a reflective colored state using interference and a darkened black state via absorption. The dark state is achieved when there is a finite gap between the mechanical membrane and the absorbing film/stack. The dark state is achieved because the peak of the resulting standing wave is positioned optimally within the absorbing film/stack  135  to maximize absorption. When the mechanical membrane is moved into contact with, or much closer to, the absorbing film stack, the standing wave is moved out of position, yielding overall reflectivity of the absorbing film/stack, which reflects somewhat, with that of the mechanical membrane, which reflects a lot. In effect, the reflectivity of the mechanical membrane is degraded by the absorbing film. The result is a reflective white state.  
         [0034]    The configuration of the array of FIGS. 9, 10, and  11  is similar to the configuration shown in FIGS. 7 and 8 except that the mechanical membrane  92  is thinner (because it does not include the insulator  93 ,  95 ), and the bus electrode is interrupted by insulator stops  130  situated in a series or array of holes in the bus electrode layer that correspond to the locations of the insulator stops.  
         [0035]    In FIG. 9, with no voltage applied, the mechanical membrane rests in its as-manufactured state. By proper selection of materials and gap sizes, the optical cavity of each of the iMoDs produces a white appearance for a viewer looking through the substrate. For example, for the white state, the optical cavity gap  140  in the as-manufactured state would be as close to zero as possible, say  20  angstroms. Intimate contact would give the highest brightness but may not obtainable in an actual device.  
         [0036]    As shown in FIG. 10, applying voltage between the bus electrode layer and the mechanical membrane causes the membrane to move into contact with the bottom faces  134  of the insulator stops. Because these stops are more closely spaced than the post supports, the spring constant in the span between two adjacent stops is effectively increased relative to the spring constant of the span between two adjacent posts. This increase produces an additional hysteresis mode as shown in FIG. 17 a . Variations in the hysteresis behavior can also take the forms shown in FIGS. 17 b , and  17   c . Careful selection of the height  136  of these stops and the size  138  of the actuation cavity would place the optical cavity  140  into a dark reflective state when the mechanical membrane is actuated to the position shown in FIG. 10.  
         [0037]    A further increase in voltage is required to actuate the membrane beyond the mechanical threshold defined by the insulator stops, as shown in FIG. 11. Careful selection of the height of the stops and the size of the actuation cavity would produce a colored state. The color would appear in the areas labeled  141  while the relatively small areas  149  would appear black. For example, as in the implementations discussed in U.S. Pat. No. 6,055,090, a combination of 50 nm of oxide and 435 nm of air would give a reflected green peak. Here, there is no oxide in the optical cavity, so a total spacing of about 500 nm of air would give a green peak.  
         [0038]    The figures that show stops in them are not too scale. The insulating stops are very small compared to the overall area of the membrane. The portion of the mechanical membrane that is pulled all the way to the bus electrode is very large.  
         [0039]    [0039]FIGS. 12 and 13 show three-dimensional and cutaway views of another arrangement. In this case, additional actuation electrodes  140  in the form of inset columns rest within the insulating posts  142  rather than on the plane of the other electrodes. This configuration allows for better control of the actuation voltages required to achieve the desired states by permitting the designer to select the height of the electrodes within the cavity. Because the inset electrodes are mechanically supported by the larger actuation electrode  146 , their possible position and geometry re not significantly affected by the placement of the support posts, thus providing additional design flexibility.  
         [0040]    In another arrangement, shown in FIG. 14, membrane stops  150  are attached to the actuation cavity side of the mechanical membrane  152 . These stops may be made from a dielectric material, a conductor, or a combination of the two in a configuration resembling the inset electrodes of FIGS. 12 and 13. Holes are provided in the actuation electrode through which the stops can pass when the membrane is actuated. In a second state, shown in FIG. 15, when the membrane is actuated, the stops may come to rest before they have fully penetrated the holes in the actuation electrode. In a third state, shown in FIG. 16, the stops have fully penetrated and the membrane touches the underside of the actuation electrode. The deformation that occurs occur in the mechanical membrane of FIG. 10 is avoided. The depth of actuation is controlled by optional mechanical stops (not shown) or by balancing of mechanical restorative and electrostatic forces. This configuration offers additional flexibility in designing actuation voltages and improves the fill factor.  
         [0041]    Other implementations are within the scope of the following claims.