Abstract:
The disclosed embodiments combine an electrothermal actuator system with an electrostatic attraction system, in order to orient bistable micromirrors in digital micromirror devices (DMDs). The micromirror, pivotally supported, can switch between two orientations. While typical DMD systems use electrostatic electrodes to orient the micromirror, stiction forces can restrict micromirror motion, affecting optical performance. The disclosed embodiments use an electrothermal actuation system to mechanically assist the electrodes, overcoming stiction without the need for a high-voltage reset pulse.

Description:
FIELD OF THE INVENTION  
       [0001]     The disclosed embodiments herein relate generally to the use of digital micromirror technology to provide image display capability as part of a digital light projection system, and more particularly to an electrothermal actuator mechanism to improve the control characteristics of micromirror orientation.  
       BACKGROUND OF THE INVENTION  
       [0002]     Digital light projection (DLP) technology has been increasingly used for optical display systems, such as those found in upscale home theater systems. This technology has begun replacing traditional cathode ray tube technology, since it can provide high image quality, without the bulk and power requirements associated with the older technology.  
         [0003]     In essence, a DLP system is comprised of a digital micromirror device (DMD), made up of an array of thousands or even millions of bistable mirror elements, interacting with a light source and a projection surface. Each of the mirror elements of the DMD may switch between two positions, corresponding to an open or closed light configuration, based on the angle at which the mirror tilts towards the light source. A micromirror is in an open position when it is oriented to reflect the light source onto the projection surface. A micromirror is in a closed position when it is oriented so that none of the light provided by the light source is projected onto the projection surface. Thus, each micromirror can be oriented in either an open or “on” position, or a closed or “off” position.  
         [0004]     By rapidly turning a particular micromirror “on” and “off”, the appropriate shade of light can be projected for a particular pixel on the projection surface. And color hues may also be added to a DMD projection system by time multiplexing of the white light source through a color wheel. In practice, the micromirrors alternate between open and closed positions so fast that the human eye cannot discern the discreet “on” and “off” positions of each micromirror. Instead, the human eye extrapolates the discreet binary images projected by each mirror element into a wide variety of pixel shades and hues. In this way, DMDs allow for the accurate reproduction of the whole array of necessary shades and hues by taking advantage of the human eye&#39;s averaging of quickly varying brightnesses and colors.  
         [0005]     Typically, each micromirror is oriented in either the open or closed position using electrostatic forces generated by corresponding electrodes. Each micromirror is located atop a hinge mechanism, and an electrode is located on either side of the hinge. These electrodes are typically formed on a semiconductor substrate beneath the micromirrors. Whenever an appropriate voltage is applied to an electrode, it creates an electrostatic force capable of pivoting the micromirror on its hinge. Only one of the two electrodes will be active at any specific moment in time, corresponding to either the open or closed position. By way of example, if a sufficient voltage is applied to the first electrode, then its micromirror would be pulled out of its neutral alignment, so that it angles towards the light source and will reflect light onto the projection surface. This would correspond to an open or “on” position for the micromirror. If a sufficient voltage is applied to the second electrode for the same micromirror (while there is no voltage applied to the first electrode), then the micromirror would pivot to angle away from the light source. In this closed or “off” position, no light would be reflected upon the projection surface. So, each micromirror pivots between open and closed positions based on the electrostatic forces applied on the mirror by the electrodes on either side of the pivot point.  
         [0006]     This conventional DMD approach generally works well, allowing for quite accurate and crisp image reproduction quality. Nevertheless, DMD image quality has historically been impacted by stiction, which is a tendency for each micromirror element to stick when in contact with the electrode contact surface (against which the electrostatic force holds the micromirror in either the open or closed position). This stiction is associated with Van der Waal&#39;s forces, surface contamination, and surface friction, and can cause a delay in the movement of the micromirrors, resulting in possible image degradation.  
         [0007]     To overcome this stiction problem, conventional DMDs apply large voltages to the electrodes and the micromirrors in sequence, essentially pulsing the electrodes and the micromirrors to reorient the micromirrors into their neutral starting position. This type of reset pulse breaks the stiction, and allows the micromirrors to move freely from one orientation to another. But the use of high-voltage pulses to overcome stiction acts as a limiting factor concerning the size and expense of DMDs, since the transistors operating the electrodes must be capable of handling the high voltage needed to overcome stiction. Likewise, the use of high voltages requires a sufficient gap between each of the micromirrors in order to prevent micromirrors with different voltages from being mutually attracted, and this gap requirement acts as a limitation on the contrast available for the DMD. So, problems associated with the present high-voltage pulse technique for overcoming stiction have led to investigations into alternative techniques for addressing stiction problems.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     Disclosed below are approaches that overcome the stiction problem discussed above, without the negative side effects associated with the conventional, high-voltage pulse fix. Disclosed embodiments use an electrothermal actuator, in conjunction with conventional electrostatic electrodes, to adjust the orientation of micromirrors.  
         [0009]     Each disclosed embodiment comprises a micromirror mounted atop a hinge above a substrate. This hinge serves as a pivot point, allowing the micromirror to tilt from one position to another. Typically, the hinge would be torqued so that it provides a restorative force whenever the micromirror is deflected out of its neutral state. Beneath this hinge lie two layers involved in manipulating the micromirror to control orientation. The first layer includes the electrostatic electrodes. These electrodes provide electrostatic forces for tilting the micromirror. At least one electrode is located beneath the micromirror on each side of the hinge. When voltage is applied to the first electrode, the micromirror tilts about the hinge to face towards the light source (into open position); but when voltage is applied to the second electrode, the micromirror tilts about the hinge to face away from the light source (into closed position). Thus, by switching the voltage from one electrode to the other, the micromirror can be opened and closed as needed.  
         [0010]     The electrostatic electrode arrangement described above is fairly typical for such DMD devices. Disclosed embodiments below, however, further comprise a second layer for electrothermal activation of the micromirror. These electrothermal actuators assist the electrodes in overcoming stiction, so that the micromirrors can be effectively oriented to faithfully reproduce the desired image. The electrothermal actuators provide additional strength to the system, working in conjunction with the electrodes to properly orient the micromirrors despite any stiction forces. And because the electrothermal actuators do not rely on increased electrostatic voltages, stiction can be overcome without the need to use a high-voltage pulse.  
         [0011]     Disclosed electrothermal actuators comprise bimetallic arms, which flex upward upon heating. In this sort of arrangement, there are two bimetallic arms, with each placed to overcome stiction between the micromirror and an electrode stop on one side of the hinge. Thus, when current flows through one of the bimetallic arms, it heats up and flexes upward, breaking the stiction force between micromirror and electrode.  
         [0012]     The presence of the bimetallic actuators allows stiction forces to be overcome mechanically, as the bimetallic arms basically pry the micromirror away from the electrode stop upon heating. This allows the electrodes to effectively orient the micromirror (by controlling which way the micromirror tilts about the hinge) using much lower voltages (since the electrostatic force applied by the electrodes does not have to be strong enough to overcome stiction, but merely to move the micromirror once it is in a neutral position). Using lower voltages allows for miniaturization of DMD devices, since the transistors sending power to each electrode can be smaller. Such lower voltage requirements also allow DMDs to be built less expensively, since smaller, less expensive transistors can handle the lower voltages. The disclosed embodiments also carry the potential for better contrast and image quality, since the lower voltage requirement and the constant voltage applied to the micromirrors allows for a smaller gap between the micromirrors without the concern that micromirrors will be attracted to each other. In addition, the disclosed embodiments enable a faster pixel, by overcoming stiction to allow the micromirrors to switch more quickly between bistable positions. Again, this speed advantage may improve image reproduction quality.  
         [0013]     The above summary generally describes certain of the disclosed embodiments. A more detailed description of the embodiments and alternatives follows, with specific reference to the illustrative figures. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]     For a more detailed understanding of the embodiments, reference is made to the following drawings:  
         [0015]      FIG. 1  is a perspective view of an embodiment of a DMD pixel with electrothermal actuators as a whole, with a cutaway of the micromirror to provide a better view of the various component parts;  
         [0016]      FIG. 2  is a perspective view similar to  FIG. 1 , cutaway and angled to show the various component parts more clearly;  
         [0017]      FIG. 3  is a section plan view of the lower electrodes;  
         [0018]      FIG. 4  is a section plan view showing the lower layer of the bimetallic arms in relation to the lower electrodes;  
         [0019]      FIG. 5  is a section plan view showing the upper layer of the bimetallic arms as well as the hinge platform, in relation to the lower electrodes;  
         [0020]      FIG. 6  is a section plan view showing the upper electrodes and the hinge; and  
         [0021]      FIG. 7  is a cutaway plan view showing the micromirror in relation to the underlying elements of the DMD pixel.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     As shown in  FIG. 1 , one embodiment of DMD pixel device  20  couples electrothermal actuator elements with electrostatic actuator elements, in order to jointly orient a bistable micromirror  102  (which is shown with a cutaway of ¾ of the micromirror  102  with the remainder of the micromirror  102  illustrated by ghosted line features). The disclosed electrostatic actuator uses electrodes on either side of the micromirror pivot to attract the micromirror  102 , pulling it downward into contact with a stop rest  120 . This electrostatic orientation of the micromirror  102  is accomplished by applying a constant voltage to the micromirror  102 , and selectively applying a voltage to the electrodes. The voltage difference results in electrostatic attraction, orienting the micromirror  102  into either an open or closed position.  
         [0023]     While this electrostatic attraction is one element of the described device  20 , electrothermal actuators are further provided to assist in orienting the micromirror  102 . These electrothermal actuators heat up under the influence of electrical current, due to internal resistance. The heat generated by current flow causes the electrothermal actuators to deflect, and this deflection provides a mechanical force that aids in overcoming stiction. In this way, electrothermal actuators assist the electrostatic force in positioning the micromirror  102 .  
         [0024]     There are several possible electrothermal actuator mechanisms that could function effectively in an embodiment such as the one illustrated in  FIG. 1 . For example, a bimetallic arm  108  could form one type of electrothermal actuator. A bimetallic arm  108  is comprised of two separate layers of materials,  108   a  and  108   b,  joined together to form a unified whole. At least one of the layers of the bimetallic arm  108  would be an imperfect conductor, providing a resistive electrical pathway for current flow, and thereby generating heat. The two layers  108   a,    108   b  of the bimetallic arm would also have different coefficients of thermal expansion, causing the deflection of the bimetallic arm  108  upon heating.  
         [0025]     When current flows through such a bimetallic arm  108 , it heats up and deflects. Basically, the heat generated by the current causes both layers  108   a  and  108   b  of the bimetallic arm to expand. Each layer expands to a different degree, however, based upon their respective coefficients of thermal expansion. This differing degree of expansion between the two joined layers of the bimetallic arm  108  causes the bimetallic arm  108  to deflect, so that it may serve as a mechanical pry, overcoming any stiction experienced by the micromirror  102 .  
         [0026]     The greater the difference in the coefficients of thermal expansion between layers  108   a  and  108   b  of the bimetallic arms  108 , the greater the deflection of the bimetallic arms  108  would be for a given temperature. Likewise, the internal electrical resistance of the layer of the bimetallic arm serving as the conductive pathway would affect the heat generated within the bimetallic arms  108  for a given current. Thus, the response characteristics of the bimetallic arms  108  result from the interplay of the material properties of the upper and lower layers  108   a  and  108   b  of the bimetallic arms. This allows a degree of fine tuning of the operation of the bimetallic arms  108 , based upon material selection characteristics.  
         [0027]     For the embodiment illustrated in  FIG. 1 , the circuit is configured such that the electrothermal actuators work in conjunction with the electrostatic actuators. Thus, the bimetallic arm  108  will typically push upward on one side of the micromirror  102 , while the electrostatic force generated by the electrodes pulls down on the other side of the micromirror  102 . This push/pull interaction of mechanical and electrostatic forces allows for effective micromirror  102  orientation, without the need for high voltages.  
         [0028]     Still referring to  FIG. 1 , with further reference to  FIG. 2 , the micromirror  102  is a flat, reflective surface mounted to pivot from one side to the other about a hinge. There are electrodes on either side of the pivot line (represented by the hinge), and bimetallic arms  108  extend out under the micromirror  102  on either side of the hinge. In this specific embodiment, the hinge is further constructed of a hinge bar  106  and a pivot platform  114 .  
         [0029]     The hinge bar  106  is mounted atop the pivot platform  114 , and serves as the pivot line about which the micromirror  102  pivots between its two bistable positions.  
         [0030]     The embodiment shown in  FIGS. 1 and 2  also employs electrode pairs, rather than single electrodes, with two electrodes on each side of the hinge. While this configuration is not necessary, the use of electrode pairs improves the efficiency of operation of the electrostatic force in acting upon the micromirror  102 . Each electrode pair includes an upper  104  and a lower  110  electrode. The upper electrode  104  directly influences the micromirror  102  using the electrostatic force of attraction. The lower electrode  110 , on the other hand, influences the pivot platform  114  upon which the mirror is mounted with the electrostatic force of attraction. By using these dual electrostatic attractors, the electrostatic force for orienting the micromirror  102  can be maximized while minimizing the voltage.  
         [0031]      FIGS. 1 and 2  show each of the bimetallic arms  108  extending out between the upper  104  and lower  110  electrodes of each electrode pair, and interacting with the pivot platform  114  via a connector bar  118  Typically, the connector bar  118  is somewhat flexible, such that it may deflect as the bimetallic arms  108  move. It is by this attachment of the bimetallic arms  108  to the pivot platform  114  that the bimetallic arms  108  of the disclosed embodiment mechanically assist in orienting the micromirror  102 .  
         [0032]     In the embodiment of  FIGS. 1 and 2 , the bimetallic arms  108  have a cantilevered shape, extending outward from two supports to hang between the electrode pairs in proximity to the pivot platform  114 . Each bimetallic arm  108  also provides a continuous current pathway, leading from one cantilever support (that also serve as electrodes for current flow) to the other. This allows the necessary current flow through the bimetallic arms  108  by completing the circuit. While any shape that provides a continuous current pathway would serve effectively, the bimetallic arms  108  shown in  FIGS. 1 and 2  employ two parallel strips extending outward from the supports, linked together at their unsupported ends. Furthermore, this embodiment has the bimetallic arms  108  oriented so that, when in their undeflected state (i.e. without the influence of heat from the current), the bimetallic arms  108  angle slightly downward as they extend outward. While this downward orientation is not necessary for the functioning of the DMD pixel device  20 , it further aids in the mechanical pivot action applied to the micromirror  102  by providing both an upward push on one side of the pivot platform  114  and a downward pull on the other side of the pivot platform  114 .  
         [0033]     The bimetallic arms  108  in the embodiment of  FIGS. 1 and 2  are constructed so that the upper layer  108   a  has a lower coefficient of thermal expansion than the lower layer  108   b.  This means that the bimetallic arms  108  will deflect upward when heated. While several materials could effectively provide this result, in the disclosed embodiment, the lower layer of each bimetallic arm  108   b  is comprised of titanium nitride, while the upper layer  108   a  is comprised of silicon dioxide. Titanium nitride is particularly suited to this application, since its electrical resistance can be controlled by varying its composition. Silicon dioxide, while not conductive, has an appropriate coefficient of thermal expansion when compared to titanium nitride, creating an effective bimetallic arm  108  deflection.  
         [0034]     In operation, the current and voltage of the embodiment of  FIGS. 1 and 2  are synchronized to switch simultaneously, so that the electrothermal actuator and the electrostatic actuator work in conjunction. By coordinating the application of voltage and current respectively, one pair of electrodes ( 104  and  110 ) exerts a downward force, pulling down on one side of the micromirror  102  and on one side of the pivot platform  114  (the same side), while simultaneously, current flows into the corresponding bimetallic arm  108 , heating it so that it pushes upward on the opposite side of the pivot platform  114 , via the connector bar  118 . And in the embodiment of  FIGS. 1 and 2 , in which the unflexed bimetallic arms  108  are angled slightly downward, the remaining bimetallic arm  108  pulls downward on the pivot platform  114 , via the connector bar  118 , on the same side as the electrostatic force from the electrodes ( 104  and  110 ). Thus, the micromirror  102  hinge tilts, under the combined influences of the electrostatic force from the electrodes  104  and  110  and the mechanical force applied by the electrothermal bimetallic actuators  108 . The combined push/pull effect of these forces on the hinge tilts the micromirror  102  into one of its bistable positions, in contact with stop  120 . By combining electrothermal and electrostatic actuators, stiction forces can be overcome without the need for high voltages. In the disclosed embodiment of  FIG. 1 , electrical current would usually only be applied to the bimetallic actuator  108  during the transition period, in order to assist the electrostatic force in overcoming stiction; once the micromirror  102  has been oriented, the current typically would be cut off such that the micromirror  102  would be held in position solely by the electrostatic force of the electrodes  104  and  110 . By applying current to the electrothermal actuators  108  briefly during the transition period, the heat generated by the device may be kept to a manageable level.  
         [0035]     While  FIGS. 1 and 2  illustrate an embodiment of the DMD pixel device  20  in its entirety, showing the interaction between the various elements,  FIGS. 3 through 7  provide additional detail, illustrating the various levels of this embodiment using a series of sectional diagrams. These additional figures provide a series of sectional illustrations, moving from the bottom to the top of the DMD pixel device.  FIG. 3  shows an embodiment of the two lower electrodes  110  of the electrode pairs. One electrode  110  is located on each side of the pivot line, and the two electrodes  110  form mirror images in this embodiment.  FIG. 4  illustrates a sectional plan view one level above the lower electrodes  110 . Thus,  FIG. 4  shows the lower layer of the bimetallic arms  108   b  and the connector bars  118  which will join the bimetallic arms  108  to the pivot platform  114 .  FIG. 5  shows a sectional plan view one level above that of  FIG. 4 , illustrating the upper layer of the bimetallic arms  108   a,  as well as the pivot platform  114 . The upper layer of the bimetallic arms  108   a  lays directly atop the lower layer  108   b,  and the pivot platform  114  is rigidly connected to the lower layer of each bimetallic arm  108   b  by connector bar  118 .  
         [0036]      FIG. 6  shows a sectional plan view yet another level upward. At this level, the two upper electrodes  104  are located, one on each side of the pivot line. Again, these upper electrodes  104  form mirror images in this embodiment. The hinge bar  106  is also shown in  FIG. 6 , located upon the pivot platform  114 , and elevated slightly above the upper electrodes  104  in order to allow the micromirror  102  to pivot.  FIG. 7  then further illustrates the micromirror  102 , resting atop the hinge bar  106 , showing a cutaway plan view that reveals all of the levels of this embodiment of the DMD pixel device interacting as a whole.  
         [0037]     Together, these figures illustrate one embodiment of the present DMD pixel device  20 . It should be understood, however, that there are several alternative embodiments, all of which would be effective. For example, while the embodiment of  FIG. 1  shows electrode pairs on each side of the pivot line, a single electrode located on each side of the pivot line would also operate to apply an electrostatic force upon the micromirror  102  and/or pivot platform. And while the electrodes in the embodiment of  FIG. 1  are located beneath the micromirror, so that the electrostatic force pulls downward on the micromirror, the electrodes could also be located above the micromirror, pulling the micromirror upward using the electrostatic force of attraction. In addition, the particular electrothermal actuator used in the embodiment of  FIG. 2  is simply one alternative. Other electrothermal actuators, such as arms constructed of memory metal, would also function. Likewise, other materials could be used to construct effective bimetallic arms  108 . Other effective embodiments of the DMD pixel device  20  could employ bimetallic arms  108  in which the lower layer  108   b  has the lower coefficient of thermal expansion, and the arms  108  are angled upward when in their neutral, unflexed position. This would mean that the bimetallic arms  108  would work essentially in reverse of the manner set forth above for the embodiment of  FIG. 1 , acting as a mechanical input on the micromirror  102  when undeflected, and deflecting to remove the upward force when current is applied. And the bimetallic arms  108  could also be designed to interact directly with the micromirror  102 , rather than influencing the micromirror  102  through the pivot platform  114 .  
         [0038]     Likewise, the disclosed embodiments are not limited to use with digital micromirrors. A combination of electrostatic and electrothermal actuators could be applied to orient any sort of element. While the above described embodiments discuss flat, reflective surfaces serving as micromirrors, they could be applied to any micromechanical element needing to change orientation. And while the embodiment of  FIG. 1  discusses both the electrode pairs and the bimetallic arms as being mirror images, so that they exert identical but opposite forces upon the micromirror, this need not be the case. Indeed, the electrothermal actuators described above could also be used in conjunction with additional actuator means other than electrostatic. In short, while the DMD pixel device  20  has been described with reference to specific embodiments and uses, this description is purely illustrative and is not intended to be construed in a limiting sense. A host of modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art field. These and all other embodiments are intended to be included within the scope of this DMD pixel device invention, which is more fully described within the claims below.  
         [0039]     While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.  
         [0040]     Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Field of the Invention,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background of the Invention” is not to be construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary of the Invention” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.