Abstract:
In one embodiment, the invention provides a method for fabricating a microelectromechanical systems device. The method comprises fabricating a first layer comprising a film having a characteristic electromechanical response, and a characteristic optical response, wherein the characteristic optical response is desirable and the characteristic electromechanical response is undesirable; and modifying the characteristic electromechanical response of the first layer by at least reducing charge build up thereon during activation of the microelectromechanical systems device.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to microelectromechanical systems devices. In particular it relates to thin film structures in microelctromechanical systems devices and to electromechanical and optical responses of such thin film structures.  
         BACKGROUND OF THE INVENTION  
         [0002]    Today a wide variety of microelectromechanical systems (MEMS) devices may be fabricated using microfabrication techniques. Examples of these MEMS devices include motors, pumps, valves, switches, sensors, pixels, etc.  
           [0003]    Often these MEMS devices harness principles and phenomena from different domains such as the optical, electrical and mechanical domains. Such principles and phenomena, while seemingly difficult to harness in the macroscopic world, can become extremely useful in the microscopic world of MEMS devices, where such phenomena become magnified. For example, electrostatic forces which are generally considered to be too weak in the macroscopic world to be harnessed, are strong enough in the microscopic world of MEMS devices to activate these devices, often at high speeds and with low power consumption.  
           [0004]    Materials used in MEMS devices are generally selected based on their inherent properties in the optical, electrical, and mechanical domains and the characteristic response to input, such as for example, a driving or actuation voltage.  
           [0005]    One problem affecting the fabrication of MEMS devices is that in some cases, a material having a highly desirable response to input, for example an optical response to incident light, may also have an undesirable response to input, for example, an electromechanical response to an actuation or driving voltage. To overcome, or at least reduce, the undesirable response, new materials have to be found or developed often at great expense.  
           [0006]    Another problem with the fabrication of MEMS devices is that sometimes, a material selected for its characteristic response may become damaged due to exposure to chemical agents used during a particular microfabrication process. This causes the material to demonstrate less of the characteristic response to the input.  
         SUMMARY OF THE INVENTION  
         [0007]    In one embodiment, the invention provides a method for fabricating a microelectromechanical systems device. The method comprises fabricating a first layer comprising a film or structured film having a characteristic electromechanical response, and a characteristic optical response, wherein the characteristic optical response is desirable and the characteristic electromechanical response is undesirable; and modifying the characteristic electromechanical response of the first layer by manipulating charge build up thereon during activation of the microelectromechanical systems device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIGS. 1 and 2 show a block diagram of a MEMS device in an unactuated, and an actuated state respectively;  
         [0009]    [0009]FIG. 3 shows a chart of the actuation and release voltages for the MEMS device of FIGS. 1 and 2;  
         [0010]    [0010]FIG. 4 shows one embodiment of a thin film stack for a MEMS device, in accordance with one embodiment of the invention;  
         [0011]    [0011]FIG. 5 shows a hysteresis curve for a MEMS device including the thin film stack shown in FIG. 4 of the drawings;  
         [0012]    [0012]FIG. 6 shows another embodiment of a thin film stack for a MEMS device;  
         [0013]    [0013]FIG. 7 shows a hysteresis curve for a MEMS device including the thin film stack of FIG. 6 of the drawings;  
         [0014]    [0014]FIG. 8 a  shows a block diagram of an electrostatic fluid flow system within a MEMS device in accordance with one embodiment of the invention;  
         [0015]    [0015]FIG. 8 b  shows a schematic drawing of the fluid flow system of FIG. 8 a  illustrating its principle of operation; and  
         [0016]    [0016]FIG. 9 shows another embodiment of a MEMS device in accordance with the invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]    A particular structure or layer within a microelectromechanical systems (MEMS) device may be desirable for its optical response to input in the form of incident light, but may at the same time have an undesirable electromechanical response to input in the form of an actuation or driving voltage. The present invention discloses techniques to manipulate or control the electromechanical response of the structure or layer, thus at least reducing the undesirable electomechanical response.  
         [0018]    As an illustrative, but a non-limiting example of a MEMS device, consider the interference modulator (IMOD]) device  10  shown in FIG. 1 of the drawings. Referring to FIG. 1, it will be seen that IMOD device  10  has been greatly simplified for illustrative purposes so as not to obscure aspects of the present invention.  
         [0019]    The IMOD device  10  includes a transparent layer  12  and a reflective layer  14  which is spaced from the transparent layer  12  by an air gap  16 . The transparent layer  14  is supported on posts  18  and is electrostatically displaceable towards the transparent layer  12  thereby to close the air gap  16 . An electrode  20  which is connected to a driving mechanism  22  is used to cause the electrostatic displacement of reflective layer  14 . FIG. 1 shows the reflective layer  14  in an undriven or undisplaced condition, whereas FIG. 2 shows the reflective layer  14  in a driven or displaced condition. The reflective layer  14  is generally selected to produce a desired optical response to incident light when it is brought into contact with the transparent layer  12 . In one IMOD design, the transparent layer  12  may comprise SiO 2 . The electrode  20  and the transparent layer  12  are formed on a substrate  24 . The substrate  24 , the electrode  20 , and the transparent layer  12  thereon will be referred to as a “thin film stack.” 
         [0020]    Typically, a plurality of IMOD devices  10  are fabricated in a large array so as to form pixels within a reflective display. Within such a reflective display, each IMOD device  10  essentially defines a pixel which has a characteristic optical response when in the undriven state, and a characteristic optical response when in the driven state. The transparent layer  12  and the size of the air gap  16  may be selected so that an IMOD within the reflective display may reflect red, blue, or green light when in the undriven state and may absorb light when in the driven state.  
         [0021]    It will be appreciated that during operation of the reflective display, the IMOD devices  10  are rapidly energized, and de-energized in order to convey information. When energized, the reflective layer  14  of an IMOD  10  device is electrostatically driven towards the transparent layer  12 , and when the IMOD  10  is de-energized, the reflective layer  14  is allowed to return to its undriven state. In order to keep the reflective layer  14  in its driven condition, a bias voltage is applied to each IMOD device  10 .  
         [0022]    If an actuation voltage, V actuation , defined as a voltage required to electrostatically drive the reflective layer  14  of an IMOD device to its driven condition, as showed in FIG. 2 of the drawings, is equal to a release voltage, V release , defined as the voltage at which the reflective layer  14  returns to its undisplaced condition, as is shown in FIG. 1 of the drawings, then it becomes extremely difficult to select an appropriate bias voltage, V bias , that can be applied to all of the IMOD&#39;s  10  within the reflective display to keep the reflective layers  14  of each respective IMOD device  10  within the reflective display in its driven condition. The reason for this is that each IMOD  10  within the reflective display may have slight variations, for example, variations in a thickness of the layers  12 ,  14 , etc., which, in practice, result in a different release voltage, V release , for each IMOD  10 . Further, due to line resistance, there will be variations in the actual voltage applied to each IMOD  10 , based on its position within the display. This makes it very difficult, if not impossible, to select a value for V bias  that will keep the reflective layer  14  of each respective IMOD  10  within the reflective display in its driven condition. This is explained with reference to FIG. 3 of the drawings, which shows the observed hysteresis behavior of the reflective layer  14  of an IMOD  10 , in which the transparent layer  12  comprised SiO 2 .  
         [0023]    Referring to FIG. 3, a curve,  30  is shown, which plots applied voltage (in volts) on the X-axis, against optical response measured in the volts on the Y-axis for an IMOD  10  comprising a transparent layer of SiO 2 . As can be seen, actuation of the reflective layer  14  occurs at about 12.5 volts, i.e., V actuation  equals 12.5 volts, and the reflective layer  14  returns to its undriven condition when the applied voltage falls to below 12.5 volts, i.e., V release , equals 12.5 volts. Thus, the reflective layer  14  in an IMOD device  10  in which the transparent layer comprises only SiO 2  demonstrates no hysteresis. Therefore, if the reflective display is fabricated using IMOD devices  10 , each comprising a transparent layer  12  having only SiO 2 , it would be impossible to select a value for V bias . For example, if V bias  is selected to be 12.5 volts, because of variations within the IMOD devices  10  in the reflective display, for at least some of the IMOD devices  10 , a V bias  of 12.5 volts would fail to keep the reflective layer  14  of those IMOD devices  10  in the driven condition.  
         [0024]    In order to select a V bias  that is sufficient to keep the reflective layer  14  of a respective IMOD device  10  within a reflective display in its driven condition, it is necessary for each reflective layer  14  of a respective IMOD device  10  within the reflective display to demonstrate some degree of hysteresis, defined as a non-zero difference between the V actuation  and V release .  
         [0025]    It will be appreciated that the electromechanical response of the reflective layer  14  of each IMOD device  10  is determined by the electromechanical properties of the reflective layer  14  as well as the electrical properties of the transparent layer  12 . In one particular IMOD device design, the transparent layer  12  comprises SiO 2  which gives a desired optical response when the reflective layer  14  is brought into contact therewith. However, the transparent layer  12  comprising SiO 2  has a certain electrical characteristic or property (the SiO 2  traps negative charge) that affects the hysteresis behavior of the reflective layer  14 . Thus, the transparent layer  12  has a desired optical response but an undesirable electromechanical response to a driving or actuation voltage which affects the hysteresis behavior of the reflective layer  14 .  
         [0026]    In accordance with embodiments of the present invention, the electromechanical behavior of the transparent layer  12  is altered by adding a further layer to the thin film stack. This further layer at least minimizes or compensates for the effect of transparent layer  12  on the hysteresis behavior of the reflective layer  14 .  
         [0027]    In one embodiment of the invention, the further layer comprises Al 2 O 2  which is deposited, in accordance with known deposition techniques, over the transparent layer  12 . This results in a thin film stack  40  as shown in FIG. 4 of the drawings, comprising a substrate  42 , an electrode  44 , an SiO 2  reflective layer  46  and an Al 2 O 3  layer  48  which functions as a charge trapping layer.  
         [0028]    [0028]FIG. 5 of the drawings shows a hysteresis curve  50  for an IMOD device  10  comprising the thin film stack  40 . As with the hysteresis curve  30 , the X-axis plots applied voltage in Volts, whereas the Y-axis plots optical response in Volts. The hysteresis curve  50  shows a hysteresis window of 2.8 volts defined as the difference between V actuation  (7.8 volts) and V release  (5.0 volts). When the individual IMOD&#39;s  10  within a reflective display each have a respective reflective layer  14  which demonstrates hysteresis in accordance with the hysteresis curve  50 , it will be seen that it is possible to choose a value for the V bias  between 5 volts and 7.8 volts which will effectively perform the function of keeping the reflective layer  14  of each respective IMOD device  10  within the reflective display in its driven condition. In a further embodiment of the invention, the thin film stack may be further modified to include an Al 2 O 3  layer above, as well as below, the reflective layer  12 . This embodiment is shown in FIG. 6 of the drawings, where it will be seen that the thin film stack  60  includes a substrate  62 , an electrode  64 , a first Al 2 O 3  layer  66 , an SiO 2  transparent layer  68  and a second Al 2 O 3  layer  70 .  
         [0029]    [0029]FIG. 7 of the drawings shows a hysteresis curve  80  for a transparent layer  14  of an IMOD device  10  having the thin film stack  60  shown in FIG. 6 of the drawings. As will be seen, the hysteresis window is now wider, i.e., 4.5 volts, being the difference between V actuation  (9 volts) and V release  (4.5 volts).  
         [0030]    However, other materials that have a high charge trapping density may be used. These materials include AlO x , which is the off-stoichiometric version of Al 2 O 3 , silicon nitride (Si 3 N 4 ), its off-stoichiometric version (SiN x ), and tantalum pentoxide (Ta 2 O 5 ) and its off-stoichiometric version (TaO x ). All of these materials have several orders of magnitude higher charge trapping densities than SiO 2  and tend to trap charge of either polarity. Because these materials have a high charge trapping density, it is relatively easy to get charge into and out of these materials, as compared to SiO 2 , which has a low charge trapping density and has an affinity for trapping negative charge only.  
         [0031]    Other examples of materials that have a high charge trapping density include the rare earth metal oxides (e.g., hafinium oxide), and polymeric materials. Further, semiconductor materials doped to trap either negative or positive charge may be used to form the further layer above, and optionally below the SiO 2  transparent layer  12 .  
         [0032]    Thus far, a technique for manipulating the electromechanical behavior of a MEMS device has been described, wherein charge buildup within the MEMS device is controlled by the use of a charge trapping layer which has a high charge trapping density. However, it is to be understood that the invention covers the use of any charge trapping layer to alter or control the electromechanical behavior of a MEMS device regardless of the charge trapping density thereof. Naturally, the choice of a charge trapping layer whether it be of a high, low, or medium charge trapping density will be dictated by what electromechanical behavior for a MEMS device is being sought.  
         [0033]    In some embodiments the incorporation of metals, in the form of thin layers or aggregates, provide yet another mechanism for manipulating the charge trapping density of a host film in a MEMS device. Structuring the host film by producing voids or creating a variation or periodicity in its material characteristics may also be used to alter the charge trapping characteristics.  
         [0034]    According to another embodiment of the invention, an IMOD device  10  includes a chemical barrier layer deposited over the reflective layer  12  in order to protect the reflective layer  12  from damage or degradation due to exposure to chemical etchants in the microfabrication process. For example, in one embodiment, the transparent layer  12  which comprises SiO 2 , is protected by an overlying layer comprising Al 2 O 3 , which acts as a chemical barrier to etchants, for example, XeF 2 . In such embodiments, it has been found that when the transparent SiO 2  layer  12  is protected from the etchants, nonuniformities in the SiO2 are eliminated along with attendant nonuniformities in electromechanical behavior, thus causing the transparent layer  14  within each IMOD device  10  to display hysteresis.  
         [0035]    [0035]FIGS. 8 a  and  8   b  show another application within a MEMS device wherein a charged trapping layer is used to control the electromagnetic behavior of a structure within the MEMS device.  
         [0036]    Referring to FIG. 8 a , reference numeral  90  generally indicates a portion of an electrostatic fluid flow system. The electrostatic fluid flow system includes a substrate  92  within which is formed a generally U-shaped channel  94 . The channel  94  includes an inner layer  96  of a first material which is selected, for example, because of its chemical and mechanical properties, for example, the material may be particularly wear-resistant, and may demonstrate little degradation due to the flow a fluid within the channel. The channel  94  also includes an outer layer  98  which is selected for its charge-trapping properties, as will be explained in greater detail below.  
         [0037]    The electrostatic fluid flow system  90  also includes pairs of electrodes  100  and  102  which are selectively energized to cause displacement of charge particles within a fluid in the channel  94  in the direction indicated by the arrow  104  in FIG. 8 b  of the drawings. In one embodiment, the outer layer  98  traps charge in the fluid thereby to provide greater control of fluid flow within the system  101 . In another embodiment, the layer  98  may trap charge in order to eliminate or to reduce hysteresis effects.  
         [0038]    Referring now to FIG. 9 of the drawings, another application of using a charge-trapping layer to alter the electromechanical behavior of a structure within a MEMS device is shown. In FIG. 9, reference numeral  120  generally indicates a motor comprising a rotor  122  which is axially aligned and spaced from a stator of  124 . As can be seen, the stator  124  is formed on a substrate  126  and includes electrodes  128 , which, in use, are energized by a driving mechanism (not shown). The rotor  122  includes a cylindrical portion  130  which is fast with a spindle  132 . The rotor  122  may be of a material that may be selected for its mechanical properties, including resistance to wear, but may have undesirable electrical properties in response to input, such as when the electrodes  128  are energized in order to cause rotation of the rotor  122 . In order to compensate for these undesirable electrical properties, layers  134  and  136  are deposited on the rotor  122  in order to effectively act as a charge trapping layer to alter the electromechanical behavior of the rotor  122 .  
         [0039]    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.