Abstract:
One aspect of the invention provides a method for fabricating a microelectromechanical systems device. The method comprises fabricating an array of first elements, each first element conforming to a first geometry; fabricating at least one array of second elements, each second element conforming to a second geometry; wherein fabricating the arrays comprises selecting a defining aspect of each of the first and second geometries based on a defining characteristic of each of the first and second elements; and normalizing differences in an actuation voltage required to actuate each of the first and second elements, wherein the differences are as a result of the selected defining aspect, the defining characteristic of each of the elements being unchanged.

Description:
FIELD OF THE INVENTION 
     This invention relates to the actuation of microelectromechanical systems devices. In particular, it relates to the actuation or driving of elements in an array in a microelectromechanical systems device. 
     BACKGROUND 
     Microelectromechanical systems (MEMS) devices may include arrays of elements wherein the elements are operable between one or more driven and undriven states by the application of an actuation voltage. Depending on the particular microelectromechanical systems device, the elements may include interferometric modulators (IMODs), switches, Infra Red (IR) detectors, etc. 
     In some microelectromechanical systems devices, it may be necessary to have multiple arrays, wherein each array comprises only elements of a particular type, and wherein each element type requires a different actuation voltage. An example of such a device is the color IMOD-based display described in U.S. Pat. No. 6,040,937, which includes three sets or arrays of IMODs designed to switch between the colors red/black, green/black and blue/black. Each array of IMODS has a different actuation voltage. 
     Driving the elements in these multiple arrays between their undriven and driven states present a challenge because different actuation voltages are required. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a microelectromechanical systems device comprising a plurality of elements each having at least two layers disposed in a stacked relationship with a gap therebetween when the element is in an undriven state, the plurality of elements being of at least two different types, each differing in a height of its gap; and a driving mechanism to drive the plurality of elements to a driven state, wherein one of the layers of each element is electrostatically displaced relative to the other layer, and wherein a minimum voltage required to actuate the driving mechanism is substantially the same for each type of element. 
     According to a second aspect of the invention there is provided a method of fabricating a microelectromechanical systems device comprising constructing an array of elements, each element having a first layer, a second layer spaced from the first layer by a gap when in an undriven state, and an electrode layer to electrostatically drive the second layer to contact the first layer corresponding to a driven state when energized, the elements being of at least two different types which differ in a height of its gap, wherein said constructing includes changing a configuration of each element type to compensate for differences in a voltage required to drive each element to its driven state. 
     According to a further aspect of the invention, there is provided a microelectromechanical systems device comprising a plurality of elements, each having a first layer, a second layer spaced therefrom by a gap when in an undriven state, and an electrode layer to electrostatically drive the second layer to contact the first layer corresponding to a driven state when energized, the elements being of at least two different kinds, each differing in a height of its gap; and an element driving mechanism comprising an integrated drive circuit having multilevel outputs to energize the electrode layer of each element to cause the element to change from its undriven state to its driven state. 
     According to yet a further aspect of the invention there is a provided a method for fabricating a microelectromechanical systems device, the method comprising fabricating an array of first elements, each first element conforming to a first geometry; fabricating at least one array of second elements, each second element conforming to a second geometry; wherein fabricating the arrays comprises selecting a defining aspect of each of the first and second geometries based on a defining characteristic of each of the first and second elements; and normalizing differences in an actuation voltage required to actuate each of the first and second elements, wherein the differences are as a result of the selected defining aspect, the defining characteristics of each of the elements being unchanged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified drawing of a generic MEMs device to which aspects of the present invention apply; 
         FIG. 2  shows an example of how the geometry of the elements in the MEMs device of  FIG. 1  may be changed, according to one embodiment of the invention, to normalize the actuation voltages of the elements; 
         FIG. 3A  shows a different geometry for a driven layer of an element, wherein the driven layer has tabs; 
         FIG. 3B  shows a three dimensional view of the driven layer of  FIG. 3A  supported on supports; 
         FIG. 3C  shows the driven layer of  FIG. 3A  with a different configuration for the tabs; 
         FIG. 4  shows an example of how the configuration of an electrode within each element may be changed in order to achieve voltage normalization in one embodiment of the invention; 
         FIG. 5  shows an example of how the stiffness of the layer which is driven in each element may be varied in order to achieve voltage normalization in accordance with another embodiment of the invention; 
         FIG. 6  shows a simplified drawing of an IMOD-based display array wherein the thickness of the layer which is driven within each IMOD is changed in order to achieve voltage normalization, in accordance with one embodiment of the invention; 
         FIG. 7  shows a schematic end view of an IMOD which includes a dielectric stack; and 
         FIG. 8  shows a block diagram of a driver in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows, in simplified form, a generic structure of a microelectromechanical systems (MEMS) device  100  to which aspects of the present invention relate. Referring to  FIG. 1 , it will be seen that the MEMs device  100  comprises two elements which are designated  102  and  104  respectively. The element  102  and the element  104  each have a common lower or base layer  106 . Element  102  has an upper layer  108  which is spaced from the base layer  106  by a number of supports which are in the form of posts  110 . Likewise, element  104  has an upper layer  112  which is spaced from the base layer  106  by supports in the form of posts  114 . It will be apparent that posts  114  are higher than posts  110  and so the height of a gap  116  between layer  106  and layer  108  is less than that of a gap  118  between layer  112  and layer  106 . Because of the differences in the heights of gaps  116  and  118 , an actuation voltage required to electrostatically drive layers  108  and  112  respectively from an undriven state, corresponding to the state showing  FIG. 1  of the drawings, to a driven state (not shown), in which the layers  106  and  112  contact the base layer  116 , is different. Thus, any driving mechanism must take into account these differences in the actuation voltages. 
     As stated above,  FIG. 1  is intended to be a simplified drawing of a generic MEMs device to which aspects of the present invention apply. In actual embodiments, the MEMs device  100  may include multiple arrays each array comprising elements such as the elements  102  or  108 . Thus, the elements in each array would require a different actuation voltage. An example of one such MEMs device is provided by the IMOD display described in U.S. Pat. No. 6,040,937. In this example, there are three arrays, each comprising elements in the form of IMODs designed to have a particular optical characteristic which arises from a size of an air gap in each IMOD. Each array comprises only IMODs which have a particular optical characteristic. As a result, different actuation voltages are required to drive the IMODs in each array. 
     Embodiments of the present invention are concerned with the problem of driving MEMs devices such as are described above, wherein different actuation voltages are required by the elements within th e MEMs device. In describing specific embodiments of the invention, reference will be made to a MEMs device such as is described in U.S. Pat. No. 6,040,937. However, it must be borne in mind that the invention is applicable to any MEMs device comprising elements which each require a different actuation voltage to drive or actuate the element resulting in a geometric configuration or state of the element being changed. Such elements may include IMODs, switches, Infra Red (IR) detectors, etc., where the change in the geometric configuration comprises driving one layer of the element to contact another layer. The layer that is driven will be referred to as the driven layer to distinguish it from the undriven layer. 
     According to embodiments of the present invention, the actuation voltage required to actuate each of the elements is normalized. This is achieved by changing a geometry of the elements within each array. Naturally, aspects of the geometry of an element which impart a defining characteristic to the element are not changed. Thus, in the case of the IMOD displays of U.S. Pat. No. 6,040,937, the height of the air gap in each element (IMOD) imparts a defining optical characteristic to the IMOD and so the one aspect of geometry of an IMOD that is not changed is the height of the air gap. 
       FIG. 2  of the drawings shows an example wherein the geometry of the element  102  shown in  FIG. 1  of the drawings has been changed by increasing the number of posts  110  and by decreasing the spacing therebetween. Thus the layer  108  is supported by posts  110  to a greater degree and therefore a greater actuation voltage will be required to drive layer  108  to contact layer  106 . By selecting the number of posts  110  and the spacing therebetween it will be appreciated that the actuation voltages required to drive element  102  and  108  may be normalized. 
     In other embodiments, the geometry of the driven layer may be changed in order to increase or decrease the degree of support provided thereto. This is illustrated in  FIGS. 3A and 3B  of the drawings. Referring to  FIGS. 3A and 3B , a layer  300 , which is similar to layers  108  and  112  of  FIGS. 1 and 2 , is shown. The layer  300  has a different geometry to that of layers  108  and  112  by virtue of tabs  302  which form tethers which themselves are supported on posts  304 . Thus, the thickness and length of the tabs may be varied to change the degree of support to the layer  300 . Assuming that an actuation voltage is required to drive layer  300  into the plane of the drawings it will be appreciated that the tabs  302  in  FIG. 3A  offer a greater degree of support than the tabs  302  shown in  FIG. 3C  of the drawings. Thus, a lesser actuation voltage will be required to drive layer  300  in  FIG. 3C  of the drawings than in  FIG. 3A  of the drawings. Embodiments of the present invention use the principles illustrated in  FIGS. 3A and 3C  of the drawings to normalize the actuation voltage required to actuate elements within a MEMs device wherein an operatively upper layer (driven layer) is to be driven towards an operatively lower layer across a gap. When the gap is large, the geometry of the tabs is varied in accordance with the principles shown in  FIGS. 3A and 3C  to reduce the degree of support provided to the driven layer. On the other hand when the gap is small then the geometry of the supports is varied to provide a greater degree of support to the driven layer. In this way, regardless of the size of the gap through which a layer must be driven, the voltage required to drive the layer can be normalized. 
     Although not shown in  FIGS. 1  or  2  of the drawings, a driving mechanism to drive layers  108  and  112  comprises electrodes to electrostatically drive layers  108  and  112  towards base layer  106 . The electrodes are disposed on layer  106 . An example of an electrode is indicated generally by reference numeral  400  in  FIG. 4  of the drawings. According to some embodiments of the present invention, in order to normalize the voltage required to drive or actuate elements within an MEMs device, the configuration of electrode  400  may be changed. Changing the configuration of the electrode may include changing the shape of the electrode or providing apertures therein, for example, such as slots  402  shown in electrode  400 . Thus, if a layer is to be driven across a small gap, the electrode may have slots such as slots  402  which serve to reduce the effective electrostatic force created by the electrode. This allows the actuation voltage to be normalized regardless of the height of the gap across which a layer has to be driven. 
     According to other embodiments of the present invention, changing the geometry of the elements in order to normalize the actuation voltage may include changing the stiffness of the driven layer. One way of changing the stiffness of the driven layer includes changing the Young&#39;s Modulus thereof. Thus, the layer which is required to be driven across a small air gap would be made of a material which has a higher Young&#39;s Modulus than a layer which has to be driven across a greater air gap. 
     Another method of changing the stiffness of the driven layer is to form apertures therein to reduce its stiffness. This is shown in  FIG. 5 , of the drawings where a layer  500  which includes, in addition to tabs  502  apertures or slots  504  formed therein. 
     Various aspects of the present invention may be applied in combination, thus in the example shown in  FIG. 5 , it will be seen that while layer  500  has slots formed therein, the layer itself will be supported on tabs  502  which have a thickness which is selected so that it provides a degree of support to the layer  500  to allow an actuation voltage required to actuate layer  500  to be normalized. 
       FIG. 6  of the drawings shows a simplified version  600  of an IMOD based display such as is described in U.S. Pat. No. 6,040,937. The display  600  includes three arrays  602 ,  604  and  606 . Each array is fabricated on a substrate  608  and includes a 2×4 grid of IMODs. Each IMOD includes an upper layer  610  which in use is driven towards a common lower layer  612  across a gap. The layers  610  are self-supporting by virtue of having two downwardly extending limbs  611 . Each IMOD has an electrode  614  which is disposed on layer  612 . It will be seen that the IMODs within array  602  have the highest gap, the IMODs within array  604  have an intermediate size gap and the IMODs within array  606  have the smallest gap. This is because the IMODs in array  602 ,  604  and  606  are fabricated to have the defining characteristic that they each reflect red, green, and blue light, respectively, when in an undriven state. Thus, an actuation voltage required to drive the layers  610  towards the layer  612  will increase as the height of the gap through which the layer must be driven increases. Thus, the IMODs within array  602  will require a greater actuation voltage than the IMODs within array  604  or array  606 . One embodiment of the present invention allows the actuation voltages to be normalized by changing the thickness of the layers  610  in inverse proportion to the size of the gap through which it must be driven. Thus, in  FIG. 6 , the thickness of the layers  610  have been selected to normalize the actuation voltages required by the IMODs within each array. 
     In another embodiment of the invention, the actuation voltages may be normalized by increasing or decreasing the tensile stress of each of the layers  610  as the height of the gap through which the layers must be driven increases or decreases, respectively. This can be accomplished by controlling deposition parameters of the film such as deposition pressure, power, and electric field bias. 
       FIG. 7  of the drawings shows an embodiment of a MEMs device  700  which includes an IMOD comprising a mechanical layer  702  which is supported on posts  704  which are formed on a substrate  706 . Disposed on substrate  706  is an electrode  708  on which is formed a dielectric stack  710 . The space between mechanical layer  702  and dielectric stack  708  defines an air gap. In use, an actuation voltage is applied to drive layer  702  to contact the dielectric stack  710 . The device  700  will typically include three sets of IMODs each differing in the height of its air gap so as to reflect red, blue and green light, respectively, when in an undriven state. In order to normalize the actuation voltages required by each set of IMODs, the dielectric constant of the dielectric stack  710  is varied, in one embodiment of the invention, so that the higher the air gap, the greater the dielectric constant. Alternatively, the thickness of the dielectric stack may be varied so that the thickness of the dielectric stack is increased (or decreased) as the height of the air gap is decreased (or increased). 
     According to another embodiment of the invention, the problem of driving different elements within a MEMs device wherein the elements require different actuation voltages is solved by providing a driving mechanism such as the one shown in  FIG. 8  of the drawings. Referring to  FIG. 8 , the driving mechanism comprises a driver chip  800  which includes an integrated drive circuit which has multi-level outputs  802 ,  804 , and  806 . Each of the outputs  804  to  806  delivers a different voltage and may be used, in one embodiment to drive IMODs with different sized air gaps for example IMODs  808 ,  810 ,  812  which reflect red, green, and blue light, respectively, when in an undriven state. The design and integration of components within driver chip  800  is well-known and is therefore is not further described. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes can be made to these embodiments without departing from the broader spirit of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than in a restrictive sense.