The present application is a divisional application, filed on 23.2.2006, under the name of 200680005822.2 (international application No. PCT/US2006/006552), entitled "method and apparatus for actuating a display".
Detailed Description
Fig. 1A is an isometric view of a display device 100 according to an exemplary embodiment of the invention. The display device 100 includes a plurality of light modulators, and in particular, a plurality of shutter assemblies 102a-102d (collectively "shutter assemblies 102") arranged in rows and columns. Generally, one shutter assembly 102 has two states, open and closed (although partial opening may be employed to impart gray scale). The shutter assemblies 102a and 102d are in an open state, allowing light to pass through. The shutter members 102b and 102c are in a closed state, and block the passage of light. By selectively setting the state of the shutter assemblies 102a-102d, an image 104 of a projection display or a backlit display may be formed with the display device 100 if illuminated by the lamp 105. In another embodiment, the device 100 may form an image by reflecting ambient light originating from the front of the device. In the display device 100, each shutter assembly 102 corresponds to a pixel 106 in the image 104.
Each shutter assembly 102 includes a shutter 112 and an aperture stop 114. To illuminate a pixel 106 in the image 104, the shutter 112 is arranged such that it allows light to pass through the aperture stop 114 towards a viewer without any significant obstruction. To keep one pixel 106 from emitting light, shutter 112 is arranged to block the passage of light through aperture stop 114. The aperture stop 114 is formed by an open patterning through a reflective or light absorbing material in each shutter assembly 102.
In an alternative embodiment, one display device 100 includes multiple shutter assemblies 102 in each pixel 106. For example, the display device 100 may include a shutter assembly 102 that is proprietary to three colors. The display device 100 can produce a color pixel 106 in the image 104 by selectively opening one or more color-specific shutter assemblies 102 corresponding to a particular pixel 106. In another example, display device 100 includes two or more shutter assemblies 102 per pixel 106 to provide grayscale in one image 104. In still other embodiments, the display device 100 may include other forms of light modulators, such as micromirrors, filters, polarizers, interferometric devices, and other suitable devices, instead of the shutter assembly 102, to modulate light to form an image.
Shutter assembly 102 of display device 100 is formed using standard micromachining techniques known in the art, including photolithography; etching techniques such as wet chemical etching, dry etching, and photoresist removal; thermal oxidation of silicon; electroplating and electroless plating; diffusion processes such as boron, phosphorus, arsenic and antimony diffusion; ion implantation; film deposition, such as evaporation (filament, electron beam, flash and shadow (masking) and step coverage), sputtering, Chemical Vapor Deposition (CVD), plasma enhanced CVD, epitaxy (vapor phase, liquid phase and molecular beam epitaxy), electroplating, screen printing and layering. See generally Jaeger, Introduction to microelectronic fabrication (Addison-Welsley Publishing co., Reading mass.1988); runyan et al, Semiconductor Integrated Circuit Processing Technology (Addision-Wesley Publishing CO., Reading Mass. 1990); proceedings of the IEEE Micro Electro mechanical systems conference 1987-1998; Rai-Choudhury, ed., Handbook of Microlitography, Microchining & Microsimulation (SPIE Optical Engineering Press, Bellingham, Wash. 1997).
More specifically, multiple layers of material (typically alternating between metal and dielectric) are deposited on top of a substrate to form a stack. After one or more layers of material have been added to the stack, a pattern is applied to the topmost layer of the stack, marking material as either to be removed from the stack or to remain on the stack. Various etching techniques, including wet or dry etching or reactive ion etching, are then applied to the patterned stack to remove unwanted material. The etch process may remove material from one or more layers of the stack based on the chemistry of the etch, the layers in the stack, and the amount of time the etch is applied. The manufacturing process may include multiple interactions of tiling, patterning, and etching.
In one embodiment, shutter assembly 102 is fabricated on a transparent glass or plastic substrate. The substrate may be made an integral part of a backlight that is operative before light is emitted from aperture stop 114 to evenly distribute the illumination light emitted from lamps 105. Alternatively and optionally, the transparent substrate can be placed on top of a planar light guide, wherein the array of shutter assemblies 102 function as light modulators in forming an image. In one embodiment, the shutter assembly 102 is fabricated on the same glass or plastic substrate in conjunction with or subsequent to the production of a Thin Film Transistor (TFT) array. The TFT array provides a switch matrix for distributing electrical signals to the shutter assembly.
The process also includes a release step. To provide freedom of movement of the component in the resulting device, the material from which the component is to be formed in the completed device is accessed, and sacrificial material is inserted in the stack. Etching removes most of the sacrificial material and thus frees the part for movement.
After release, one or more surfaces of the shutter assembly may be insulated so that charge is not transferred between moving parts when in contact. This may be done by thermal oxidation and/or by chemical vapour deposition of a conformal insulator, such as Al for example2O3、Ct2O3、TiO2、TiSiO4、HfO2、HfSiO4、V2O5、Nb2O5、Ta2O5、SiO2Or Si3N4Or by depositing similar materials using techniques such as atomic layer deposition and others. The insulated surface is chemically passivated by a chemical conversion process, such as fluorination, silicidation, or hydrogenation of the insulated surface to prevent problems such as stiction between the surfaces when in contact.
The bi-compliant electrode actuator constitutes one type of suitable actuator for driving the shutter 112 in the shutter assembly 102. Generally, a bi-compliant beam electrode actuator is formed with two or more at least partially compliant beams. At least two of the beams function as electrodes (also referred to herein as "beam electrodes"). The beam electrodes are attracted to each other by the resulting electrostatic force in response to a voltage being applied across the beam electrodes. Both beams in a dual compliant beam electrode actuator are at least partially compliant. That is, at least some portion of each beam may flex and/or bend to help bring the beams together. In some embodiments, the compliance is achieved by including a fold or a pin hinge. Some portions of the beam may be substantially rigid or fixed in place. Preferably, at least a substantial portion of the length of the beam is compliant.
The bi-compliant electrode actuator has advantages over other actuators in the art. Electrostatic comb drives are also suitable for actuation over relatively long distances, but can only generate relatively weak forces. Parallel plate or parallel beam actuators can produce relatively large forces but require small gaps between the plates or beams and can therefore only operate over relatively small distances. Legtenberg et al (Journal of microelectromechanical systems, vol 6, 257, 1997) show how the use of a curved electrode actuator can generate relatively large forces and give relatively large displacements. However, the voltage required to initiate actuation in the Legtenberg technique is still quite large. As indicated herein, this voltage may be reduced by enabling the movement or flexing of the two electrodes.
In a dual compliant beam electrode actuator-based shutter assembly, a shutter is coupled to at least one beam of a dual compliant beam electrode actuator. When one of the beams in the actuator is pulled toward the other, the pulled beam also moves the shutter. This is done to move the shutter from a first position to a second position. In one of the positions, the shutter interacts with light in a light path, such as by, but not limited to, blocking, reflecting, absorbing, filtering, polarizing, diffracting, or otherwise altering the characteristics or path of the light. The shutter may be coated with a reflective or light absorbing film to improve its interference characteristics. In the second position, the shutter allows the light to pass therethrough relatively unimpeded.
Fig. 2A and 2B are illustrations of two embodiments of a cantilevered bi-compliant beam electrode actuator-based shutter assembly for use in a display device, such as display device 100. More specifically, fig. 2A illustrates a cantilevered dual compliant beam electrode actuator based shutter assembly 200a ("shutter assembly 200 a"). The shutter assembly 220a modulates the light through an optical path that controllably moves the shutter 202a in and out of the light to form an image. In one embodiment, the light path begins behind a surface 204a to which a shutter 202a is attached. Surface 204a is shown as a dashed boundary line. The dashed line indicates that the surface 204a extends through the space defined by the boundary line. Similar dashed boundary lines are used in other figures to represent the surface. The light passes through an aperture stop 206a in the surface 204a towards a viewer or towards a display screen. In another embodiment, the light path begins in front of surface 204a and reflects off the surface of aperture stop 206a back to the viewer.
The shutter 202a of the shutter assembly 200a is formed from a substantially planar solid body. The shutter 202a can take virtually any shape, whether regular or irregular, such that in a closed position the shutter 202a is sufficient to block the light path through the aperture stop 206a in the face 204 a. In addition, the shutter 202a must have a width that is consistent with the width of the aperture stop so that, in the open position (as shown), enough light can pass through the aperture stop 206a in the surface 204a to illuminate, or contribute to illuminating, a pixel in the display device.
The shutter 202a is connected to one end of one load beam 208 a. At the opposite end of the load beam 208a, a load support point 210a physically connects the load beam 208a to the surface 204a and electrically connects the load beam 208a to the drive circuitry in the surface 204 a. The load beams 208a and load support points 210a together act as a mechanical support for supporting the shutter 202a above the surface 204 a.
The shutter assembly 200a includes a pair of drive beams 212a and 214a, either of which is positioned along either side of the load beam 210 a. Drive beams 212a and 214a and load beam 210a together form an actuator. One drive beam 212a functions as a shutter open electrode and the other drive beam 214a functions as a shutter closed electrode. Drive support points 216a and 218a located at the ends of the drive beams 212a and 214a closest to the shutter 202a physically and electrically connect each beam 212a and 214a to the surface 204 a. In this embodiment, the other ends and most of the length of the drive beams 212a and 214a remain unsupported or otherwise free. The free ends of drive beams 212a and 214a are closer to the supported end of load beam 208a than the supported ends of drive beams 212a and 214a are to the shutter end of load beam 208 a.
Load beam 208a and drive beams 212a and 214a are compliant. That is, they are sufficiently flexible and resilient so that they can be bent out of their unstressed ("at rest") position or shape to at least some useful degree without fatigue or breakage. Since the load beam 208a and the drive beams 212a and 214a are supported at only one end, a substantial portion of the length of the beams 208a, 212a and 214a are free to move, bend, flex or otherwise deform in response to an applied force. The operation of the cantilevered bi-compliant beam electrode actuator-based shutter assembly 200a is discussed further below with reference to fig. 3.
Fig. 2B is a second embodiment of a cantilevered dual compliant beam electrode actuator based shutter assembly 200B (shutter assembly 200B). Similar to the shutter assembly 200a, the shutter assembly 200b includes a shutter 202b coupled to a load beam 208b, and two drive beams 212b and 214 b. The shutter 202b is disposed between its fully open position and its fully closed position. Load beam 208b and drive beams 212b and 214b together form an actuator. Drive support points 210b, 216b and 218b attached to each end of the beam connect the beam to a surface 204 b. In contrast to the shutter assembly 200a, the shutter of the shutter assembly 200b comprises several shutter aperture stops 220 in the form of slits. Instead of only one aperture stop, the surface 204b includes one surface aperture stop 206b corresponding to each shutter aperture stop 220. In this open position, the shutter aperture stop 220 is substantially aligned with the aperture stop 206b in the surface 204b, allowing light to pass through the shutter 202 b. In the closed position, the surface aperture stop 206b is blocked by the remainder of the shutter 202b, thereby blocking the passage of light.
Changing the state of a shutter assembly comprising multiple shutter apertures with a corresponding number of surface apertures requires less shutter movement than changing the state of a shutter assembly comprising a solid shutter and a single surface aperture, yet provides the same aperture area. Reducing the required motion corresponds to requiring a lower actuation voltage. More specifically, the required motion is reduced 1/3, reducing the actuation voltage required for the actuator by a factor of about 1/3. Reducing the actuation voltage further corresponds to reducing power consumption. Because the total aperture stop area is substantially the same for either of the two shutter assemblies, each shutter assembly provides a substantially similar brightness.
In other embodiments, the shutter aperture stop and the corresponding surface aperture stop have a shape other than a slit. The aperture stop may be circular, polygonal, or irregular. In alternative embodiments, the shutter may include more shutter apertures than there are surface apertures in the shutter assembly. In such embodiments, one or more of the shutter apertures may function as a filter, such as a color filter. For example, the shutter assembly may have three shutter apertures for each surface aperture stop, each shutter aperture stop including a red, blue or green filter.
Fig. 3A and 3B are graphs illustrating the relationship between the displacement at the end of the load beam and the relative voltage required to move the load beam closer to the drive beam. The displacement that can be achieved at any given voltage depends, at least in part, on the curvature or shape of the drive beam, or more precisely, on the separation, d, and how the bending stresses along the drive and load beams vary as a function of position x along the load beam. One shown in FIG. 3AThe separation function d (x) can be generalized as d ═ axnWhere y is the distance between the beams. For example, if n ═ l, the distance between the drive electrode and the load electrode increases linearly along the length of the load electrode. If n is 2, the distance increases according to the parabola. In general, assuming a constant voltage, the electrostatic force at any point on the beam increases in proportion to 1/d as the distance between the compliant electrodes decreases. At the same time, however, any deformation of the load beam that may reduce the separation distance may also result in a higher stress state in the beam. Below a minimum threshold voltage, a deformation limit is reached at which the electrical energy released by the closer proximity of the electrode is just balanced by the energy that becomes stored in the beam's deformation energy.
As shown in FIG. 3B, for an actuator having a separation function where n is less than or equal to 2, a minimum actuation voltage (V) is applied2) Resulting in a stepped attraction of the load beam to the drive beam without requiring the application of higher voltages. For such actuators, the incremental increase in electrostatic force on the beam caused by the load beam approaching the drive beam is greater than the incremental increase in stress on the beam required to further displace the beam.
For an actuator having a separation function where n is greater than 2, application of a particular voltage results in a distinct local displacement of the load electrode. That is, the incremental increase in electrostatic force on the beam caused by a particular decrease in separation between the beams cannot exceed, at some point, the incremental deformation force required to be applied to the load beam to continuously decrease the separation. Thus, for an actuator having a separation function where n is greater than 2, applying a first level results in a first corresponding displacement of the load electrode. A higher voltage causes a larger corresponding displacement of the load electrode. How the shape and relative compliance of the thin beam electrodes affect the actuation voltage is discussed in more detail in the following references: r. Legtenberg et al Journal of MicroElectromechanical Systems, Vol.6, page 257 (1997) and j.Li et al in Transducers' 03, 12 th International Conference on Solid State Sensors, Actuators, and Microsystems, page 480 (2003).
Referring again to fig. 2A and 2B, a display device including shutter assemblies 202A and 202B is actuated, i.e., the position of the shutter assemblies 202A and 202B is changed, by applying a potential from a controllable voltage source to one of the drive beams 212A, 212B, 214a or 214B through its corresponding drive support point 216a, 216B, 218a or 218B, while the load beam 208a or 208B is electrically grounded, causing a voltage across the beams 208a, 208B, 212A, 212B, 214a, 214B. The controllable voltage source, such as an active matrix array driver, is electrically connected to the load beam 208a or 208b through an active matrix array (see fig. 9 and 10 below). The display device may instead apply a potential to the load beams 208a or 208b via the load support points 210a or 210b of the shutter assemblies 202a or 202b to increase the voltage. A potential difference between the drive beam and the load beam, regardless of sign or ground potential, will create an electrostatic force between the beams.
Referring again to fig. 3, the shutter assembly 200a of fig. 2A has a quadratic separation function (i.e., n-2). Thus, if the voltage or potential difference between the beams 208a and 212a or 214a of the shutter assembly 202a exceeds the minimum actuation voltage (V) at its point of minimum separation2) The deformation of the beams 208a and 212a or 214a is graded along the entire length of the beams 208a and 212a or 214a, pulling the shutter end of the load beam 208a toward the supported end of the drive beam 212a or 214 a. The movement of the load beams 208a displaces the shutter 202a to change its position either from open to closed or vice versa, depending on whether the display device is applying the potential to the drive beams 212a or 214 a. To reverse the change in position, the display device stops applying a potential to the powered drive beam 212a or 214 a. After the display device stops applying the potential, the stored energy in the form of stress on the deformed load beam 208a restores the load beam 208a to its original or rest position. To increase the speed of this recovery and to reduce the surrounding negativeAny oscillation of the rest position of the load beam 208a, the display device can restore the shutter 202a to its previous position by applying a potential to the opposing drive beam 212a or 214 a.
The shutter assemblies 200a and 200b and the shutter assemblies 500 (see fig. 5, 600, 6, 700, 7, and 800 (see fig. 8, below) have an electrical bistable characteristic. In general, this is understood to be possessing, although not limited to, the potential V at which movement between the open and closed states is initiated2Is generally greater than the potential (V) required to maintain the shutter assembly in a stable state1) The apparatus of (1). Once the load beam 208a comes into contact with one of the drive beams, a relatively large amount of power must be applied from the opposing drive beam to move or separate the load beam, which is greater than would be required if the load beam 208 were to begin in a neutral or non-contact position. The bistable devices described herein can operate an array of shutter assemblies, such as 200a, using a passive matrix drive scheme. The stabilization voltage V may be maintained across all shutter assemblies (except those actively driven as a state change) in a passive matrix drive sequence1While one image is retained. With no power at all or substantially no power at all, a potential V is maintained between load beam 208a and drive beam 212a or 214a1It is sufficient to maintain either the shutter assembly in its open state or the shutter assembly in its closed state. To cause a switching event, the voltage between load beam 208a and the previously affected drive beam (e.g., 212a) is allowed to be driven from V1Returns to 0 while bringing the voltage between the load beam 208a and the opposing beam (e.g., 212b) to the switching voltage V2。
In fig. 2B, the actuator has a separation function of three times (i.e., n is 3). Thus, application of a particular potential to one of the drive beams 212b or 214b results in an incremental displacement of the shutter assembly 202 b. The display device utilizes the ability of the incrementally displaced shutter assembly 202b to produce a gray scale image. For example, applying a first potential to one of the drive beams 212b or 214b displaces the shutter 202b to its position shown in the figure, partially blocking light passing through the surface aperture stop 206b, but still allowing some light to pass through the shutter 202 b. Application of other potentials causes other shutter 202b positions, including fully open, fully closed, and other intervening positions between fully open and fully closed. In this way, an analog gray scale image can be achieved using an electrical analog driver circuit.
Fig. 3C-3F show the motion phases of the load beam 208a, the shutter closing electrode 214a, and the shutter 202A of the shutter assembly 200a of fig. 2A. The initial separation between compliant beams 208a and 214a follows a quadratic separation function. Fig. 3C shows the load beam 208a in a neutral position when no voltage is applied. The aperture stop 206a is covered in half by a shutter 212 a.
Fig. 3D shows the initial step of actuation. A small voltage is applied between the load beam 208a and the shutter closing electrode 214 a. The free end of the shutter closing electrode 214a has been moved to make contact with the load beam 208 a.
Fig. 3E shows the point of actuation of the shutter assembly 200a after the shutter 212 begins to move toward the shutter closing electrode 214 a.
Fig. 3F shows the final state of actuation of the shutter assembly 200 a. This voltage has exceeded the threshold of actuation. The shutter assembly 200a is in the closed position. Contact is made between the load beam 208a and the shutter close electrode 214a over its entire length.
Fig. 4A is a first cross-sectional view of a bi-compliant electrode mirror-based light modulator 400 for inclusion in a display device, such as display device 100, in place of or in addition to shutter assembly 102. Mirror-based optical modulator 400 includes a mechanically compliant reflective platform 402. At least a portion of the reflective platform 402 is reflective itself or coated with or connected to a reflective material.
The reflective platform 402 may or may not be electrically conductive. In embodiments where the reflective platform 402 is conductive, the reflective platform functions as a load electrode for the mirror-based light modulator 400. The reflective platform 402 is physically supported on and electrically connected to a substrate 404 by a compliant support member 406. If the reflective platform 402 is formed of a non-conductive material, the reflective platform 402 is coupled to a compliant, conductive load beam or other form of compliant load electrode. A compliant support member 406 physically supports the combined reflective platform 402 and electrode above the substrate 404. The support member 406 also provides an electrical connection from the electrode to the substrate 404.
The mirror-based optical modulator 400 includes a second compliant electrode 408 that functions as a drive electrode 408. The drive electrode 408 is supported between the substrate 404 and the reflective platform 402 by a second, substantially rigid support member 410. The second support member 410 also electrically connects the second compliant electrode 408 to a voltage source for driving the mirror-based optical modulator 400.
Mirror-based light modulator 400 is shown in FIG. 4A in a resting position, where neither electrode 402 nor 408 carries a potential. FIG. 4B shows mirror-based light modulator 400 in an actuated state. When a potential difference is created between the drive electrode 408 and the load electrode 402 (which may be the reflective platform 402 or an attached load beam), the load electrode 402 is pulled toward the drive electrode 408, bending the compliant support beam 406 and angling the reflective portion of the reflective platform 402 to at least partially traverse the substrate 404.
To form an image, light 412 is directed at a particular angle to an array of mirror-based light modulators 400. In its resting state, mirror-based light modulator 400 reflects light 412 away from a viewer or display screen, and in an actuated state the mirror-based light modulator reflects light 412, or reflects in reverse, toward a viewer or display screen.
Fig. 5 is an illustration of another cantilevered dual compliant beam electrode actuator based shutter assembly 500 ("shutter assembly 500"). As with shutter assemblies 200a and 200b, shutter assembly 500 includes a shutter 502 coupled to a compliant load beam 504. The compliant load beam 504 is then physically supported to a surface 506 and electrically grounded at its opposite end via a load support point 508. The shutter assembly 500 includes only one compliant drive beam 510 positioned substantially along the load beam 504. In response to application of an electrical potential from a controllable voltage source, the drive beams 510 pull the shutter 502 from a first position in a plane substantially parallel to the surface (in which the load beams 504 are substantially unstressed) to a second position in which the load beams 504 are stressed. When the potential is removed, the stress stored in the load beam 504 restores the load beam 504 to its original position.
Additionally, the load beams 504 have a width that varies along their length as compared to the shutter assemblies 202a and 202 b. The load beam 504 is wider near its support point 508 than it is near the shutter 502. The load beams 504 typically have an overall greater stiffness than the shutter assemblies 202a and 202b, and due to their custom width. The inclusion of a beam of greater stiffness typically requires a higher actuation voltage, but also enables higher switching speeds. For example, the shutter assemblies 202a and 202b may switch up to about 10kHz, while the stiffer shutter assemblies 500 may switch up to about 100 kHz.
Fig. 6 is an illustration of a shutter assembly 600 that includes two compliant electrode beam actuators 602 ("actuators 602") according to an exemplary embodiment of the invention. The shutter assembly 600 includes a shutter 604. The shutter 604 may be solid, or it may include one or more shutter apertures as described with reference to FIG. 2B. The shutter 604 is connected on one side to the beam actuator 602. And the actuator 602 moves the shutter laterally over a surface in a plane of motion that is substantially parallel to the surface.
Each actuator 602 includes a compliant load member 606 that connects the shutter 604 to a load support point 608. The compliant load members 606 each include a load beam 610 and an L-bracket 612. The load support point 608, together with the compliant load member 606, acts as a mechanical bearing, keeping the shutter 604 suspended close to the surface. The load support points 608 physically connect the compliant load member 606 and the shutter 604 to the surface and electrically ground the load beams 610 of the load member 606. The connection of the shutter 604 from two positions on one side of the shutter 604 to the shutter 604 at the load bearing points 608 in positions on either side of the shutter assembly 600 helps to reduce torsional movement of the shutter 604 about its central axis 614 during movement.
The L-bracket 612 reduces the in-plane stiffness of the load beam 610. That is, by relieving axial stress in the load beam, the L-bracket 612 reduces the resistance of the actuator 602 to movement in a plane parallel to the surface (also referred to as "in-plane movement" 615).
Each actuator 602 also includes a compliant drive beam 616 disposed adjacent each load beam 610. The drive beams 616 are connected at one end to a drive beam support point 618 that is common between the drive beams 616. The other end of each drive beam 616 is free to move. Each drive beam is curved such that it is closest to the load beam 610 near the free end of the drive beam 616 and the supported end of the load beam 610.
In operation, a display device comprising shutter assembly 600 applies an electrical potential to drive beams 616 via drive beam support points 618. As a result of a potential difference between the drive beams 616 and the load beams 610, the free ends of the drive beams 616 are pulled toward the supported ends of the load beams 610 and the shutter ends of the load beams 610 are pulled toward the supported ends of the drive beams 616. Electrostatic forces pull the shutter 604 toward the drive support points 618. The compliant member 606 acts as a spring so that when the potential is removed from the drive beams 616, the load beam compliant member 606 pushes the shutter 604 back to its original position, releasing the stress stored in the load beam 610. The L bracket 612 also acts as a spring, applying a further restoring force to the shutter 604.
In manufacturing the shutter assemblies 200-800 and the shutter assemblies 1300-1800, it is preferable to provide a rectangular shape for the cross-section of the load beams (such as load beam 610) and the drive beams (such as drive beam 616). By providing a beam thickness (in a direction perpendicular to the surface) that is greater than 1.4 times or more in dimension than the beam width (in a direction parallel to the surface), the stiffness of the load beam 610 for out-of-plane motion 617 versus in-plane motion 615 will be increased. Such a dimension, and therefore the stiffness differential, helps ensure that the motion of the shutter 604 initiated by the actuator 602 is limited to motion of the aperture stop along and across the surface, preventing out-of-plane motion 617 that would inefficiently use energy. It is preferred that the cross-sectional view of the load beam (e.g., 610) be rectangular for some applications, rather than curved or elliptical. Maximum actuation force can be achieved if the opposing beam electrodes have flat surfaces that can be brought into proximity and contact with each other with as little separation as possible during actuation.
Figure 7 is an illustration of a second shutter assembly 700 that includes two bi-compliant electric pole beam actuators 702 according to an exemplary embodiment of the invention. The shutter assembly 700 takes the same general form of the shutter assembly 600 except that it includes a return spring 704. As with the shutter assembly 600, in the shutter assembly 700, two actuators 702 are coupled to a first side of a shutter 706 to displace the shutter 706 in a plane parallel to the surface over which the shutter is physically supported. A return spring 704 is connected on the opposite side of the shutter 706. The return spring 704 is also attached to the surface at a spring support point 708, which acts as an additional mechanical support. By physically supporting the shutter 706 above the surface on the opposite side of the shutter 706, the actuator 702 and return spring 704 reduce movement of the shutter 706 out of the plane of target motion during operation. In addition, the return spring 704 includes several bends that reduce the in-plane stiffness of the return spring 704, thereby further facilitating in-plane motion as compared to out-of-plane motion. The return spring 704 provides an additional restoring force to the shutter 706 so that the shutter 706 returns to its original position relatively quickly once an actuation potential is removed. The addition of the return spring 704 only slightly increases the electrical potential required to initiate actuation of the actuator 702.
Fig. 8 is an illustration of a shutter assembly according to an exemplary embodiment of the present invention, which includes a pair of shutter open actuators 802 and 804 and a pair of shutter close actuators 806 and 808. Each of the four actuators 802, 804, 806 and 808 takes the form of a bi-compliant beam electrode actuator. Each actuator 802, 804, 806 and 808 includes a compliant load member 810 connected at one end to a shutter 812 and at the other end to a load support point 814, each compliant load member 810 including a load beam 816 and an L-bracket 818. Each actuator 802, 804, 806, and 808 further includes a drive beam 820 connected at one end to a drive support point 822. Each pair of actuators 802/804 and 806/808 share a common drive support point 822. The unsupported end of each drive beam 820 is disposed proximate to the supported end of the corresponding compliant load member 810. The supported end of each drive beam 820 is positioned proximate the L-bracket end of the corresponding load beam 816. In the un-energized state, the distance between one load beam 816 and its corresponding drive beam 820 progressively increases from the supported end of the load beam 816 to the L-bracket 818.
In operation, to open the shutter 812, a display device including the shutter assembly 800 applies a potential to the drive support terminals 822 of the shutter release actuators 802 and 804, pulling the shutter 812 toward the open position. To close the shutter 812, the display device applies a potential to the drive support terminals 822 of the shutter close actuators 806 and 808 to pull the shutter 812 toward the closed position. If neither actuator pair 802/804 or 806/808 is actuated, the shutter 812 remains in an intermediate position, somewhere between fully open and fully closed.
The shutter open actuator 802/804 and the shutter close actuator 806/808 are connected to the shutter 812 at opposite ends of the shutter. The shutter opening and closing actuators have their own load members 810 to reduce the actuation voltage of each of the actuators 802, 804, 806, and 808. Because of the electric bistability described with reference to fig. 3, it would be advantageous to find a method of actuation or higher leverage structure for separating the compliant load member 810 from a drive beam 820 that may be in contact therewith. By arranging the opening and closing actuators 802/804 and 806/808 on opposite sides of the shutter 812, the actuation force to be actuated by the actuators is converted into a force to be separated by the shutters. To do so, the actuation force is used on tasks whose leverage ratio separates the higher one point near the shutter (e.g., near the end of the L-bracket of the load beam 816).
Typical shutter widths (in the direction of the slit) for shutter assemblies such as those shown in fig. 8 would be in the range of 20 to 800 microns. The so-called "travel distance", or distance the shutter will travel between open and closed positions, is in the range of 4 to 100 microns. The width of the drive beam and load beam will be in the range of 0.2 to 40 microns. The length of the drive beam and load beam will be in the range of 10 to 600 microns. Such shutter assemblies may be used for displays with resolutions between 30 and 1000 dots per inch.
Each of the shutter assemblies 200a, 200b, 500, 600, 700, and 800 and the mirror-based light modulator 400 described above fall into a class of light modulators referred to herein as "flexible light modulators". The elastic light modulator has a mechanically stable rest state. In the rest state, the light modulator may be on (open or reflective), off (closed or non-reflective), or somewhere in the middle (partially open or partially reflective). If a voltage across a beam is generated in an actuator forcing the light modulator from its quiescent state to a mechanically unstable state, the voltage across the beam must be maintained at a certain level to maintain the light modulator in the unstable state.
Fig. 9 is an illustration of an active matrix array 900 of light modulators 902 for controlling elasticity in a display device. In particular, the active matrix array 900 is suitable for controlling a flexible light modulator 902 that includes only one passive restoring force, such as the mirror-based light modulator 400 or the shutter-based light modulators 500, 600, and 700. That is, the light modulators 902 require the electrically-excited actuator to enter a mechanically unstable state, but then return to the quiescent state using a mechanical mechanism, such as a spring.
The active matrix array is fabricated as either a diffused or thin film deposited circuit on the surface of a substrate on which the flexible light modulator 902 is formed. The active matrix array 900 includes a series of row electrodes 904 and column electrodes 906 which form a grid-like pattern on the substrate, dividing the substrate into a plurality of grid segments 908. The active matrix array 900 includes a set of drivers 910 and an array of non-linear electrical components, or includes diodes or transistors, that selectively apply electrical potentials to the grid segments 908 to control one or more elastic light modulators 902 contained within the grid 908. Techniques for such thin film transistor arrays are described in Willem den Boer (Elsevier, Amsterdam, 2005) "Active Matrix Liquid Crystal display: fundamentals and Applications ".
Each grid segment 908 operates to illuminate one pixel and includes one or more flexible light modulators 902. In a grating segment comprising only a single flexible light modulator 902, the grating segment 908 comprises at least one diode or transistor 912 and optionally a capacitor 914 in addition to the flexible light modulator 902. The capacitor 914 in fig. 9 may be explicitly added as a design element of the circuit, or it will be understood that the capacitor 914 represents the equivalent parallel capacitance or parasitic capacitance of the flexible optical modulator. An emitter 916 of the transistor 912 is electrically connected to either the drive electrode or the load electrode of the elastic light modulator 902. The other electrode of the actuator is connected to ground or a common potential. The base 918 of the transistor 912 is electrically connected to a row electrode 904 that controls a row of grid segments. When the base 918 of the transistor receives an electrical potential via the row electrode 904, current can flow from a corresponding column electrode 906 through the transistor 912 to create an electrical potential in the capacitor 914 and apply an electrical potential to the drive electrode of the elastic light modulator 902, actuating the actuator.
In one embodiment, the active matrix array 900 produces an image by energizing the grid segments 908 of a corresponding row by applying a potential, one at a time, from a driver 910 to a selected row electrode 904. While a particular row is being actuated, the display device applies an electrical potential to the column electrodes corresponding to the grid segments in the actuated row containing the light modulators that need to be switched out from a quiescent state.
When a row is then de-energized, a stored charge will remain on the electrode of actuator 902 (as determined by the equivalent capacitance of the actuator) and, optionally, on a shunt capacitor 914 that may be designed into the circuit, holding the flexible shutter mechanism 902 in its mechanically unstable state. The flexible shutter mechanism 902 remains in this mechanically unstable state until the voltage stored in capacitor 914 dissipates, or until the voltage is deliberately reset to ground in a subsequent row selection step or actuation step.
Fig. 10 shows a diagram of another embodiment of an active matrix array 1000 of light modulators 1002 for controlling elasticity in a display device. In particular, the active matrix array 1000 is suitable for controlling flexible light modulators, such as the shutter-based light modulators 200a, 200b, and 800, that include a set of actuators for forcing the light modulator from a rest state into a mechanically unstable state, and a second set of actuators for driving the light modulator back to the rest state and possibly a second mechanically unstable state. The active matrix 1000 may also be used to drive the inelastic light modulators further described with reference to fig. 12-20.
The active matrix array 1000 includes a respective row electrode 1004 for each row in the active matrix array 1000 and a respective two-column electrode 1006a and 1006b for each column in the active matrix array 1000. For example, for a display device including shutter-based light modulators, one column electrode 1006a of each column corresponds to the shutter-open actuator of the light modulator 1002 in that column. The other column electrode 1006b corresponds to the shutter close actuator of the light modulator 1002 in that column. The active matrix array 1000 partitions the substrate on which it is deposited into grid segments 1008. Each grid segment 1008 includes one or more light modulators 1002 and at least two diodes or transistors 1010a and 1010b and optionally two capacitors 1012a and 1012 b. The base 1014a and 1014b of each transistor 1010a and 1010b is electrically coupled to one of the column electrodes 1006a or 1006 b. The emitters 1016a and 1016b of the transistors 1010a and 1010b are connected to a corresponding capacitor 1012a or 1012b and a drive electrode of the light modulator 1002 in the grid segment 1008.
In operation, a driver applies a potential to a selected row electrode 1004, actuating the row. The active matrix array 1000 selectively applies a potential to one of the two column electrodes 1006a or 1006b of each column in which the state of the light modulator 1002 in the grid segment 1008 needs to be changed. Alternatively, the active matrix array 1000 may also apply a potential to the column electrodes 1006a or 1006b of the grid segment 1008 that was previously in an actuated state to be held in an actuated state.
For both active matrix arrays 900 and 1000, the driver powering the column electrode is selected from a plurality of possible potentials to apply to each column electrode 1006a and 1006b in some embodiments. The light modulators 1002 in these columns may then be opened or closed by different amounts to produce a grayscale image.
Fig. 11 is a cross-sectional view of the shutter assembly 800 of fig. 8 taken along the line marked a-a'. Referring to fig. 8, 10 and 11, shutter assembly 800 is formed on a substrate 1102 that is shared with other shutter assemblies of a display device, such as a display device 100, that includes shutter assembly 800. The voltage signal for actuating the shutter element is transmitted along a conductor in the lower layer of the shutter element. The voltage signal is controlled by an active matrix, such as active matrix 1000. The substrate 1002 can support up to 4,000,000 shutter assemblies arranged in up to about 2000 rows and up to about 2000 columns.
In addition to the shutter 812, shutter open actuators 802 and 804, shutter close actuators 806 and 808, load support points 814 and drive support points 822, the shutter assembly 800 includes a row electrode 1104, a shutter open electrode 1106, a shutter close electrode 1108, and three surface apertures 1110. The illustrated shutter assembly has at least three functional layers, which may be referred to as a row conductor layer, a column conductor layer, and a shutter layer. The shutter assembly is preferably fabricated on a transparent substrate such as glass or plastic. Alternatively, the substrate may be made of an opaque material such as silicon, provided that through-holes for transmitting light are provided at the positions of each of the surface apertures 1110. The first metal layer on top of the substrate is the row conductor layer patterned into row conductor electrodes 1104 and reflective surface segments 1105. Except at the surface aperture stop 1110, the reflective surface segments 1105 reflect light passing through the substrate 1102 back through the substrate 1102. The surface aperture stop may in some embodiments comprise or be covered by a red, green or blue filter material.
On top of the row conductor layer 1104, shutter open electrodes 1106 and shutter closed electrodes 1108 are formed in a column conductor layer 1112 deposited on the substrate 1102. The column conductor layer 1112 is separated from the row conductor layer 1104 by one or more intervening layers of dielectric material or metal. The shutter open electrode 1104 and shutter closed electrode 1106 of the shutter assembly 800 are shared with other shutter assemblies in the same column of the display device. The column conductor layer 1112 also acts to reflect light that passes through the ground electrode 1104 and not through the surface aperture stop 1110. The row conductor layer 1104 and the column conductor layer 1112 are about 0.1 to about 2 microns thick. In alternative embodiments, the column conductor layer 1112 can be located below the row conductor layer 1104. In another alternative embodiment, both the column and row conductor layers may be located above the shutter layer.
The shutter 812, shutter open actuators 802 and 804, shutter close actuators 806 and 808, load bearing points 814 and drive bearing points 822 are formed from three functional layers of the shutter assembly 800, referred to as a shutter layer 1114. The actuators 802, 804, 806 and 808 are formed from a deposited metal, such as, but not limited to, Au, Cr or Ni, or from a deposited semiconductor, such as, but not limited to, polysilicon, or amorphous silicon, or single crystal silicon (also known as silicon on insulator) if formed on top of a buried oxide. The beams of the actuators 802, 804, 806, and 808 are patterned to have widths of about 0.2 to 20 microns. The shutter thickness is typically in the range of 0.5 to 10 microns. To facilitate in-plane movement of the shutter (i.e., reduce lateral beam stiffness relative to out-of-plane stiffness), it is preferred to maintain a beam dimension ratio of about at least 1.4: 1 with the beam thickness being greater than the beam width.
A metal or semiconductor bridge electrically connects the row electrode 1104 and the shutter open electrode 1106 and the shutter close electrode 1108 of the column conductor layer 1112 to parts of the shutter layer 1114. Specifically, the bridge 1116 electrically connects the row electrode 1104 to the load support point 814 of the shutter assembly 800, maintaining the compliant load member 810 and the shutter 812 at the row conductor potential for the shutter open actuators 802 and 804 and the shutter close actuators 806 and 808. Another transition electrically connects the shutter release electrode 1106 to the drive beams 820 of the shutter release actuators 802 and 804 via a drive support point 822 shared by the shutter release actuators 802 and 804. Yet another transition electrically connects the shutter close electrode 1108 to the drive beams 820 of the shutter close actuators 806 and 808 via a drive support point 822 shared by the shutter close actuators 806 and 808.
The shutter layer 1114 is separated from the column conductors 1112 by a lubricant, vacuum, or air, to provide freedom of movement for the shutters 812. The motiles in the shutter layer 1114 are mechanically separated from adjacent components (except for their support points 814) in a release step, which may be a chemical etch or ashing process, which removes a sacrificial material from between all moving parts.
The diodes, transistors and/or capacitors (not shown for clarity of illustration) used in the active matrix array may be patterned in the existing structure of three functional layers, or they may be built into separate layers, either disposed between the shutter assembly and the substrate or on top of the shutter layer. The reflective surface segments 1105 may be patterned as epitaxial portions of the row and column conductor electrodes or they may be patterned as separate segments of reflective material or as electrically suspended segments. Alternatively, the reflective surface segment 1105, along with its associated surface aperture stop 1110, may be patterned into a fourth functional layer disposed between the shutter assembly and the substrate and formed either with a deposited metal layer or with a dielectric mirror. Ground conductors may be added separately from the row conductor electrodes in layer 1104. The separate ground conductor may be required when the row is driven by a transistor, such as in the case of an active matrix array. The ground conductor may either run parallel to the row electrode (and be wired together in the drive circuitry), or the ground electrode may be placed in a separate layer between the shutter assembly and the substrate.
In addition to the flexible light modulator, the display device may include a bi-stable light modulator, such as a bi-stable shutter assembly. As explained above, one shutter in one resilient shutter assembly has one mechanically stable position ("rest position") while all other shutter positions are mechanically unstable. On the other hand, for example, the shutter of a bi-stable shutter assembly has two mechanically stable positions, namely an open and a closed position. An advantage of a mechanically bi-stable shutter assembly is that no voltage is required to hold the shutter in either the open position or the closed position. The bi-stable shutter assemblies can be further subdivided into two classes: wherein each stable position is a substantially equal energy shutter assembly, and wherein one stable position is energetically superior to the shutter assembly of another mechanically stable position.
FIG. 12 is a graphical representation 1200 of the potential energy stored in the three types of shutter assemblies relative to the shutter position. Solid line 1202 corresponds to an elastic shutter assembly. The dotted line 1204 corresponds to a bi-stable shutter assembly having equal energy stable states. Dashed line 1206 corresponds to a bi-stable shutter assembly having unequal energy stable states. As shown in the energy diagram 1200, the energy curves 1204 and 1206 for the two types of bi-stable shutter assemblies each include two local minima 1208 corresponding to stable shutter positions, such as a fully open position 1210 and a fully closed position 1211. As shown in the figure, a component must be energized to move its shutter out of a position corresponding to one of the local minima. However, for a bi-stable shutter assembly with mechanically stable shutter positions of unequal energy, the work required to open one shutter 1212 is greater than the work required to close the shutter 1214. On the other hand, for a flexible shutter assembly, opening the shutter requires work 1218, but the shutter spontaneously closes after the control voltage is removed.
Fig. 13A is a top view of a shutter layer 1300 of a bi-stable shutter assembly. The shutter layer 1300 includes a shutter 1302 driven by two bi-compliant electrode actuators 1304 and 1306. The shutter 1302 includes three slotted shutter apertures 1308. A compliant bipolar electrode actuator 1304 functions as a shutter open actuator. The other compliant electrode actuator 1306 acts as a shutter close actuator.
Each of the bi-compliant electrode actuators 1304 and 1306 includes a compliant member 1310 that connects the shutter 1302, at about its linear axis 1312, to two load bearing points 1314 located in the corners of the shutter layer 1300. The compliant members 1310 each include a conductive load beam 1316, which may have an insulator disposed on a portion of its surface or its entire surface. The load beams 1316 act as mechanical supports to physically support the shutter 1302 above a substrate on which the shutter assembly is constructed. The actuators 1304 and 1306 also each include two compliant drive beams 1318 extending from a shared drive support 1320. Each drive support 1320 physically and electrically connects a drive beam 1318 to the substrate. The drive beams 1318 of the actuators 1304 and 1306 bend away from their corresponding drive support points 1320 to points on the load support points 1314 where the load beams 1316 connect to the load support points 1314. These bends in the drive beam 1318 act to reduce the stiffness of the drive beam, thereby helping to reduce the actuation voltage.
Each load beam 1613, for example, is generally curved in an arcuate (or sinusoidal) shape. The degree of curvature of the arc is determined by the relative distance between the load support point 1314 and the length of the load beam 1316. The curved portions of the load beams 1316 provide this bistability for the shutter assembly 1300. Because the load beam 1316 is compliant, it is possible to bend the load beam 1316 either toward the drive support point 1320 or away from the drive support point 1320. The direction of the arc changes depending on the position the shutter 1302 is in. As shown in the figure, the shutter 1302 is in this closed position. The load beam 1316 of the shutter open actuator 1304 flexes away from the drive support point 1320 of the shutter open actuator 1304. The load beams 1316 of the shutter close actuator 1306 bend toward the drive anchor 1320 of the shutter close actuator 1306.
In operation, a display device applies an electrical potential to the drive beams 1318 of the shutter open actuator 1304 in order to change state, for example, from closed to open. The display device can also apply an electrical potential to the load beams 1316 of the shutter open actuator. Any potential difference between the drive beam and the load beam, regardless of the sign of the ground potential, will create an electrostatic force between the beams. The resulting voltage between the drive beam 1318 and the load beam 1316 of the shutter open actuator 1304 causes an electrostatic force that pulls the beams 1316 and 1318 together. If the voltage is strong enough, the load beam 1316 deforms until its curvature substantially reverses, as shown in the shutter close actuator in FIG. 13A.
Fig. 13B shows the general case of bi-stable actuation, including the evolution of force versus displacement for the case of fig. 13A. Referring to fig. 13A and 13B, the force required to deform a compliant load beam as a whole will increase with displacement. In a bistable mechanism, however, such as that shown in fig. 13A, a point is reached at which further travel results in a reduction in force (point B in fig. 13B). Upon application of sufficient voltage between the load beams 1316 and the drive beams 1318 of the shutter open actuator 1306, a deformation corresponding to point B of fig. 13B is achieved, at which point further application of force results in a large and spontaneous deformation (a "bump") and the deformation comes to rest at point C in fig. 13B. After removing a voltage, the mechanism will relax to a stable point, or point of zero force. Point D is such a point that represents relaxation or stabilization of the open position. To move the shutter 1302 in the opposite direction, a voltage is first applied between the load beams 1316 and the drive beams 1318 of the shutter close actuator 1306. A point is reached where further force causes a large and spontaneous deformation (point E). Further force in the closing direction causes a deformation represented by point F. After the voltage is removed, the mechanism relaxes to the initial and stable closed position, point a.
In fig. 13A, the length of the compliant member is longer than the linear distance between the anchor point and the attachment point of the shutter. Constrained by the support point, the load beam finds a stable shape by taking a curved shape, two such shapes constituting the configuration of local minima in potential energy. Other configurations of the load beam involve deformation with additional deformation energy.
For a load beam fabricated in silicon, a typical design width is about 0.2 μm to about 10 μm. Typical design lengths are about 20 μm to about 1000 μm. Typical design beam thicknesses are about 0.2 μm to about 10 μm. The amount of pre-bending of the load beam is typically greater than three times the design width.
The load beam of fig. 13A can be designed such that one of the two flexed positions is near a global minimum, i.e., has the lowest energy state or relaxed state, typically a state near zero energy stored as deformation or stress in the beam. Such a design configuration may be referred to as "pre-curved," which primarily means that the shape of the compliant member is patterned into the mask such that little or no deformation is required after releasing the shutter assembly from the substrate. The compliant member is designed to have a shape and curved shape that approximates its stable or relaxed state. Such a relaxed state applies to one of the two shutter positions, either the open position or the closed position. Some deformation energy will have to be stored in the deformation of the beam when switching the shutter assembly to another stable state (this may be referred to as a metastable state); the two states will therefore have unequal potential energies; and less electrical energy will be required to move the beam from the metastable state to the stable state than from the stable state to the metastable state.
However, the alternative design configuration of FIG. 13A may be described as a pre-stressed design. The pre-stress design sets the equivalent potential for both stable states. This may be achieved, for example, by patterning the compliant member to substantially and spontaneously change to its stable shape (i.e., the initial state is designed to be unstable) upon release of the shutter assembly. It is preferred that the two stable shapes are similar such that the deformation energy, or deformation energy, stored in the compliant member for each of the stable states will be similar. For a pre-stressed design, the work required to move between the open and closed shutter positions will be similar.
The pre-stressed condition of the shutter assembly may be provided by several means. The condition may be added post-fabrication, for example by mechanically encapsulating the substrate to introduce a substrate curvature and thereby a surface deformation in the system. A pre-stress condition may also be applied as a film stress applied by a film on or around the load beam. These film stresses are caused by the particularities of a deposition process. Deposition parameters that can impart stress to a film include film material recombination during deposition, deposition rate, and ion bombardment rate.
In fig. 13A, the load beam is curved when in each of its local stable states and all points of deformation of the load beam between the stable states are also curved. However, the compliant member may include any number of segments of a straight or rigid load beam, as will be explained in the following figures. Also, a design of a bi-stable shutter assembly in which neither of the two equivalent stable states has, requires, or accumulates any significant deformation or deformation energy will be shown in fig. 18. Stresses are temporarily stored in the system when moving between the stable states.
Figure 14 is a top view of the shutter layer 1400 of a second bi-stable shutter assembly. As explained above with reference to fig. 6, reducing the resistance to in-plane motion tends to reduce the out-of-plane motion of the shutter. The shutter layer 1400 is similar to the shutter layer 1300, except that the shutter layer 1400 includes an in-plane stiffness reduction feature that facilitates in-plane motion, and a deformation facilitator that facilitates proper transition between the states. As with the shutter layer 1300 of FIG. 13A, the shutter layer 1400 of FIG. 14 includes load beams 1402 connecting load support points 1404 to a shutter 1406. To reduce the in-plane stiffness of the shutter assembly and to provide some axial compliance to the load beam 1402, a load support point 1404 is connected to the load beam 1402 by a spring 1404. The spring 1408 may be formed with a fold, an L bracket, or a curved portion of the load beam 1402.
In addition, the width of the load beam 1402 varies along its length. In particular, the beams are narrower along the section where they meet the load support point 1404 and the shutter 1406. The point along the load beam 1402 where the load beam 1402 widens acts as a pivot point 1410 that limits the deformation of the load beam 1402 to the narrower section 1410.
Fig. 15 is a top view of a shutter layer 1500 of a tristable shutter assembly including a bi-compliant electrode actuator according to an exemplary embodiment of the present invention. The shutter layer 1500 includes a shutter open actuator 1502 and a shutter closed actuator 1504. Each actuator 1502 and 1504 includes two substrates that are physically and electrically coupled to a display device via a drive anchor 1508.
The shutter open actuator 1502 is itself an elastic actuator having a mechanically stable state. If not otherwise constrained, the shutter-open actuator 1502 will return to its resting state after actuation. The shutter open actuator 1502 includes two load beams 1510 connected at one end by an L-bracket 1514 to a load support point 1512 and at the other end via an L-bracket 1518 to the shutter 1516. In the rest state of the shutter open actuator 1502, the load beams 1510 are straight. The L-brackets 1514 and 1518 enable the load beams 1510 to deform toward the drive beams 1506 of the shutter open actuator 1502 upon actuation of the shutter open actuator 1502 and away from the drive beams 1506 upon actuation of the shutter close actuator 1504.
The shutter close actuator 1504 is similarly inherently elastic. The shutter close actuator 1504 includes a single load beam 1520 connected at one end to a load support point 1522. When unstressed, i.e., in its resting state, the load beam 1520 is straight. At the opposite end of the load beam 1520 of the shutter close actuator 1504, the load beam 1520 is connected to a stabilizer 1524 formed by two curved compliant beams 1526 connected at their ends and at the center of their length. The beam 1526 of the stabilizer 1524 has two mechanically stable positions: a position bent away from the shutter close actuator 1504 (as shown in the figure) and a position bent toward the shutter close actuator 1504.
In operation, if either the open actuator 1502 or the shutter close actuator 1504 is actuated, the load beams 1520 of the shutter close actuator 1504 are deformed to bend toward the shutter open actuator 1502 or toward the drive beams 1528 of the shutter close actuator 1504, respectively, at the same time as the shutter 1516 is moved to an actuated position. In either case, the length of the shutter close actuator 1504 load beams 1520 is reduced relative to the width of the entire shutter layer 1500, pulling the beams 1526 of the stabilizer 1524 toward the shutter close actuator 1504. After deactivation of the activated actuator, the energy required to deform the beam 1526 of the stabilizer 1524 back to its original position is greater than the energy stored in the load beams 1510 and 1520 of the actuators 1502 and 1504. Additional energy must be added to the system to return the shutter 1516 to its rest position. Thus, the shutter 1516 in this shutter assembly has three mechanically stable positions, an open, semi-open, and closed position.
Figures 16A-16C are illustrations of another embodiment of a bi-stable shutter assembly 1600 showing the state of the shutter assembly 1600 during a change in position of the shutter 1602. The shutter assembly 1600 includes a shutter 1602 physically supported by a pair of compliant support beams 1604. The support beam is connected to the support point 1603 and the shutter 1602 by means of a swivel hinge 1605. These hinges may be understood to include pin hinges, pleats, or thin connector beams. When no stress is applied to the support beam 1604, the support beam 1604 is substantially straight.
Fig. 16A shows the shutter 1602 in an open position, while fig. 16B shows the shutter 1602 in the middle of transitioning to the closed position, while fig. 16C shows the shutter 1602 in a closed position. The shutter assembly 1600 relies on electrostatic comb drive for actuation. The comb drive consists of a rigid open electrode 1608 and a rigid closed electrode 1610. The shutter 1602 also takes a comb shape complementary to the open and closed electrodes. Comb drives such as that shown in fig. 16 can operate over reasonably long travel distances, but at the expense of reduced actuation power. The primary electric field between the electrodes in a comb drive is generally aligned perpendicular to the direction of travel, so the actuation force is generally not along the line of maximum electrical pressure experienced by the inner surface of the comb drive.
Unlike the bi-stable shutter assemblies described above, rather than relying on a particular flexure of one or more beams to provide mechanical stability, the bi-stable actuator 1600 relies on the straight relaxed state of its support beams 1604 to provide mechanical stability. For example, in their two mechanically stable positions shown in fig. 16A and 16C, the compliant support beams 1604 are substantially straight at an angle to the linear axis 1606 of the shutter assembly 1600. As shown in fig. 16B, where the shutter 1602 transitions from one mechanically stable position to another mechanically stable position, the support beams 1604 physically deform or bend to accommodate this motion. The force required to change the position of the shutter 1602 must be sufficient to overcome the resulting stress on the compliant support beams 1604. Any energy difference between the open and closed states of the shutter assembly 1600 is provided by a small amount of elastic energy in the rotary hinge 1605.
The shutter 1602 is attached by support beams 1604 at two locations on either side of the shutter 1602, to support points 1603 in locations on either side of the shutter assembly 1600, thereby reducing torsional or rotational movement of the shutter 1602 about its central axis. The shutter is also constrained in motion along a linear translational axis using compliant support beams 1604 connected to separate support points on opposite sides of the shutter 1602. In another embodiment, a pair of substantially parallel compliant support beams 1604 may be attached to each side of the shutter 1602. Each of the four support beams is attached to the shutter 1602 at separate and opposing points. This parallelogram-shaped method of supporting the shutter 1602 helps to ensure that there is linear translational movement of the shutter.
Figure 17A illustrates a bi-stable shutter assembly 1700 in which the beams 1702 included in the shutter assembly 1700 are substantially rigid rather than compliant, whether in the stable positions 17A-1 and 17A-3 or in a transitional position 17A-2 of the shutter assembly. The shutter assembly 1700 includes a shutter 1704 driven by a pair of bi-compliant beam electrode actuators 1706. Two compliant members 1710 support shutter 1704 above one surface 1712. Compliant member 1710 is attached to the opposite side of shutter 1704. The other end of compliant member 1710 is connected to a support point 1714, which connects compliant member 1710 to surface 1712. Each compliant member 1710 includes two substantially rigid beams 1716 connected to a flexure or other compliant element 171g, such as a spring or cantilever. Even though the beam 1716 is rigid in the compliant member, the inclusion of the compliant element 1718 enables the compliant member 1710 as a whole to change its shape in a compliant manner to assume two mechanically stable shapes. The compliant element is allowed to relax to its rest state (see 17A-1 and 17A-3) in the position where the shutter assembly is either closed or open, so that both final states have substantially the same potential energy. There is no requirement that sustained beam bending or beam strain establish stability of the two final states, although deformation energy can be stored in the compliant element 1718 during transition between states (see 17A-2).
The shape of compliant element 1718 is such that in-plane translation of shutter 1704 is relatively facilitated while out-of-plane motion of the shutter is limited.
Actuation of the bistable shutter assembly 1700 is accomplished by a pair of resilient, compliant beam electrode actuators 1706 similar to those used in fig. 15. The actuator 1706 is physically separate and distinct from the compliant member 1710 in the shutter assembly 1700. Compliant member 1710 provides a relatively rigid support for shutter 1704 while providing the bi-stability required to maintain the open and closed states. The actuator 1706 provides the driving force required to switch the shutter between the open and closed states.
Each actuator 1706 includes a compliant load member 1720. A compliant load member 1720 has one end attached to the shutter 1704 and the other end free. The compliant load member in the actuator 1706a in the shutter assembly 1700 is not connected to a support point nor is it otherwise connected to the surface 1712. The drive beam 1722 of the actuator 1706 is connected to the support point 1724 and thus to the surface 1712. In this way the voltage of the actuation is reduced.
Fig. 17B is an illustration of a bi-stable shutter assembly 1700B in which the shutter 1702B is designed to rotate upon actuation. The shutter 1702b is supported at four points along its perimeter by 4 compliant support beams 1704b connected to four support points 1706 b. As in fig. 16, the compliant support beam 1704b is substantially straight in its at-rest state. The compliant member will deform as the distance between the support point and the shutter perimeter decreases when rotating shutter 1702 b. There are two low energy steady states in which the compliant support beam 1704b is substantially straight. The advantage of the shutter mechanism in 1700b is that there is no centroid motion at all in shutter 1702 b.
Shutter 1702b in shutter assembly 1700b has a plurality of shutter aperture stops 1708b, each of which has a segmented shape designed to take full advantage of the rotational motion of the shutter. Fig. 18 is an illustration of a bi-stable shutter assembly 1800 that includes thermoelectric actuators 1802 and 1804. The shutter assembly 1800 includes a shutter 1806 having a set of slotted shutter apertures 1808. The thermoelectric actuators 1802 and 1804 are coupled to either side of the shutters 1806 to move the shutters 1806 laterally in a plane substantially parallel to a surface 1808 upon which one shutter 1806 is supported. The shutters 1806 are connected from two locations on either side of the shutters 1806 to load support points 1807 at locations on either side of the shutter assembly 1800, helping to reduce any twisting or rotational movement of the shutters 1806 about their central axes.
Each of the thermoelectric actuators 1802 and 1804 includes three compliant beams 1810, 1812, and 1814. Compliant beams 1810 and 1812 are each thinner than compliant beam 1814. Each beam 1810, 1812, and 1814 bends into an s-like shape, holding the shutter 1806 in place stably.
In operation, to change the position of the shutter from open (as shown in the figures) to closed, current flows through a circuit comprising beams 1810 and 1814. The thin beams 1810 in each actuator 1802 and 1804 heat up faster than, and therefore also expand faster than, the thick beams 1814. The expansive force of beams 1810, 1812 and 1814 from their mechanically stable curved portions causes lateral movement of shutter 1806 toward this closed position. To open shutter 1806, current flows through a circuit including beams 1812 and 1814, causing an asymmetric heating and expansion of a similar beam 1812, which in turn forces shutter 1806 back to the open position.
The bi-stable shutter assembly may be driven using a passive matrix array or an active matrix array. Figure 19 is an illustration of a passive matrix array 1900 for controlling a bi-stable shutter assembly 1902 to produce an image. As with active matrix arrays, such as active matrix arrays 900 and 1000, passive matrix array 1900 is fabricated as either diffused or thin film deposited circuitry on a substrate 1904 of a display device. In general, passive matrix array 1900 requires less circuitry to implement and is easier to manufacture than active matrix arrays 900 and 1000. The passive matrix array 1900 divides the shutter assemblies 1902 on the substrate 1904 of the display device into rows and columns of grid segments 1906 of a grid. Each grid segment 1906 can include one or more bi-stable shutter assemblies 1902. In this display device, all grid segments 1906 in a given grid row share a single row electrode 1908. Each row electrode 1908 electrically connects a controllable voltage source, such as driver 1910, to the load support points of shutter assembly 1902. All shutter assemblies 1902 in a column share two common column electrodes, namely one shutter open electrode 1912 and one shutter closed electrode 1914. The shutter release electrodes 1912 of a given column electrically connect a driver 1910 to the drive electrodes of the shutter release actuators of the shutter assemblies 1902 in that column. The shutter close electrode 1914 of a given column electrically connects a driver 1910 to the drive electrodes of the shutter close actuators of the shutter assemblies 1902 in that column.
The shutter assemblies 1300, 1400, 1500, 1600, 1700a and 1800 are adaptable for use with a passive matrix array because the mechanical bistability characteristic of the actuator enables it to switch between open and closed states if the voltage across the actuator exceeds a minimum threshold voltage. If the drivers 1910 are programmed such that none of them will output a voltage sufficient to switch the shutter assembly between open and closed states by itself, then a given shutter assembly will be switched if its actuator receives a voltage from two opposing drivers 1910. A shutter assembly at the intersection of a particular row and column can be switched if it receives a voltage from its particular row and column whose voltage difference exceeds the minimum threshold voltage.
To change the state of a shutter assembly 1902 from a closed state to an open state, i.e., to open the shutter assembly 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the grid in which the shutter assembly 1902 is located. A second driver 1910 applies a second potential, in some cases of opposite polarity, to the shutter-open electrodes 1912 corresponding to the columns of the grid in which the shutter assembly 1902 is located. To change the state of a shutter element 1902 from an open state to a closed state, i.e., to close the shutter element 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the display device in which the shutter element 1902 is located. A second driver 1910 applies a second potential, in some cases of opposite polarity, to the shutter-close electrodes 1914 corresponding to the column in the display device in which the shutter assembly 1902 is located. In one embodiment, shutter assembly 1902 changes state in response to a difference in electrical potential applied to one of row electrodes 1908 and column electrodes 1912 or 1914 exceeding a predetermined switching threshold.
In one embodiment, to form an image, a display device sequentially sets the state of the shutter assemblies 1902 in the grid one row at a time. For a given row, the display device first closes each shutter assembly 1902 in that row by applying a potential to the corresponding row electrode 1908 and a potential pulse to all shutter-close electrodes 1914. The display device then opens the shutter assemblies 1902 through which light is to pass by applying an electrical potential to the shutter-open electrode 1912 and an electrical potential to the row electrodes 1908 of the row that includes the shutter assemblies 1902 in the row to be opened. In an alternative mode of operation, instead of closing each row of shutter assemblies 1902 in sequence, after all rows in the display device are properly positioned to form an image, the display device globally resets all shutter assemblies 1902 while simultaneously applying a potential to all shutter-close electrodes 1914 and all row electrodes 1908. In an alternative mode of operation, the display device foregoes resetting the shutter assembly 1902 and only changes the state of the shutter assembly 1902 that needs to be changed to display a subsequent image. Several alternative driver control schemes for imaging have been proposed for ferroelectric liquid crystal displays, many of which may be incorporated into the mechanically bi-stable displays described herein. These techniques are described in. "liquid crystal display by Ernst Lieder: drive scheme and electro-optical effect "(Wfley, new york, 2001).
The physical layout of the display tends to compromise between the characteristics of resolution, aperture stop area and drive voltage. Small pixel sizes are generally sought to increase the resolution of the display. However, as the pixel size becomes smaller, the space available for the shutter aperture stop decreases proportionally. Designers seek to maximize aperture ratio as this increases the brightness and efficiency of the display. In addition, a small pixel in combination with a large aperture ratio means a large angular deformation in the compliant member supporting the shutter, which tends to increase the required drive voltage and the energy dissipated by the switching circuitry.
Fig. 20A and 20B show two methods of tiling the shutter assemblies into an array of pixels to maximize the aperture ratio in a dense array and minimize the drive voltage.
For example, fig. 20A shows a tiling 2000 of two cantilevered dual beam electrode actuator-based shutter assemblies 2002 and 2004 that are tiled to form a prismatic pixel 2006 from two generally triangular shaped shutter assemblies 2002 and 2004. The shutter assemblies 2002 and 2004 may be controlled independently or collectively. The prismatic tiling of fig. 20A is relatively close to a rectangular tiling arrangement and in fact applies to pixels of a rectangle with an aspect ratio of 2: 1. Because two shutter assemblies can be built into each rectangle, such a 2: 1 rectangular tiling arrangement can be further affixed or built on top of an active matrix array with a square repeating distance between rows and columns. A 1-to-1 correlation between the pixels in the two arrays can be established. Square pixel arrays are most commonly used for displays of text and picture images. The advantage of the layout in fig. 20B is that it is understood that the length of the load beams in each triangular pixel is maximized to reduce the voltage required to switch the shutter between open and closed states.
Fig. 20B is an exemplary tiling of the plurality of bistable bi-compliant beam electrode actuator-based shutter assemblies 1300 of fig. 13A. For example, in comparison to the bistable bi-compliant beam electrode actuator-based shutter assembly 1400 shown in fig. 14, the width of the shutter 1302 of the shutter assembly 1300 is substantially less than the distance between the load bearing points 1314 of the shutter assembly 1300. Although the narrower shutter 1302 allows less light to pass through each shutter assembly 1300, additional space can be utilized to pack the shutter assemblies 1300 more tightly without losing length in the load beams, as shown in fig. 20B. The longer load beam makes it possible to switch the shutter in the array with a reduced voltage. In particular, the narrower shutter 1302 enables the partial actuators 1304 and 1306 of the shutter assembly 1300 to interleave the gaps between the actuators 1302 and 1304 of adjacent shutter assemblies 1300. The interleaved arrangement of fig. 20B may also map to a square arrangement of rows and columns, which is a common pixel configuration for text displays.
The tiling or pixel arrangement of shutter assemblies need not be limited to a square array of constraints. Dense tiling can also be achieved using rectangular, prismatic or hexagonal pixel arrays, all of which can be used, for example, for video and color image displays.
Fig. 21 is a cross-sectional view of a display device 2100 including a dual compliant electrode actuator-based shutter assembly 2102. The shutter assembly 2102 is disposed on a glass substrate 2104. A reflective film 2106 disposed on substrate 2104 defines a plurality of surface aperture stops 2108 positioned below the closed position of shutter 2110 of shutter assembly 2102. The reflective film 2106 reflects light that does not pass through the surface aperture stop 2108 back toward the back side of the display device 2100. An optional diffuser 2112 and an optional brightness enhancement film 2114 separate the substrate 2104 from a backlight 2116. The backlight 2116 is illuminated by one or more light sources 2118. For example, the light source 2118 may be, for example, but not limited to, an incandescent lamp, a fluorescent lamp, a laser, or a light emitting diode. A reflective film 2120 is disposed behind the backlight 2116 to reflect light toward the shutter assembly 2102. Light emitted from the backlight that does not pass through one of the shutter assemblies 2102 will be sent back to the backlight and reflected again from the film 2120. Light that does not leave the display in this way to form an image on the first pass can be recycled and made available for transmission through other open aperture stops in the array of shutter assemblies 2102. Such light recycling has been shown to improve the illumination efficiency of the display. A cover 2122 forms the front side of the display device 2100. The back side of the cover 2122 may be coated with a black matrix 2124 to improve contrast. The cover 2122 is supported a predetermined distance from the shutter assembly 2102 forming a gap 2126. The gap 2126 is maintained by mechanical support and/or by an epoxy seal 2128 that secures the cover 2122 to the substrate 2104. Preferably, the epoxy 2128 should have a cure temperature preferably below about 200 ℃, it should have a coefficient of thermal expansion preferably below about 50ppm per degree celsius, and it should be water resistant. An exemplary Epoxy 2128 is EPO-TEK B9021-1 sold by Epoxy Technology, Inc.
The epoxy seal 2128 is sealed in a working fluid 2130. Working fluid 2130 is designed with a viscosity preferably below about 10 centipoise and has a relative dielectric constant preferably above about 2.0 and a dielectric breakdown strength of about 104V/cm or more. Working fluid 2130 may also function as a lubricant. Its mechanical and electrical characteristics also contribute to reducing the voltage required to move the shutter between open and closed positions. In one embodiment, the working fluid 2130 preferably has a low refractive index, preferably less than about 1.5. In another embodiment, working fluid 2130 has an index of refraction matching the index of refraction of substrate 2104. Suitable working fluids 2130 include, but are not limited to, deionized water, methanol, ethanol, silicone oil, fluorinated silicone oil, dimethyl siloxane, polydimethylsiloxane, hexamethyldisiloxane, and diethylbenzene.
A piece of metal or molded plastic assembly holder 2132 holds cover 2122, shutter assembly 2102, substrate 2104, backlight 2116 and other component parts together around the edges. The assembly bracket 2132 is fastened with screws or dowels to increase the rigidity of the assembled display device 2100. In some embodiments, light source 2188 is molded in place from an epoxy potting compound.
Fig. 22 is a cross-sectional view of one display assembly 2200 that includes a shutter assembly 2202. The shutter assembly 2202 is disposed on a glass substrate 2204.
The display assembly 2200 includes a backlight 2216 that is illuminated by one or more light sources 2218. The light source 2118 may be, for example, but not limited to, an incandescent lamp, a fluorescent lamp, a laser, or a light emitting diode. One reflective film 2220 is disposed behind the backlight 2216, reflecting light toward the shutter assembly 2202.
Substrate 2204 is oriented such that shutter assembly 2202 faces backlight 2216.
An optional diffuser 2212 and an optional brightness enhancement film 2214 interposed between the backlight 2216 and the shutter assembly 2202. Also interposed between backlight 2216 and shutter assembly 2202 is an aperture plate 2222. One reflective film 2224 is disposed on the aperture stop plate 2222 and faces the shutter assembly. The reflective film 2224 defines a plurality of surface aperture stops 2208 located below the closed positions of the shutters 2210 of the shutter assembly 2202. The aperture plate 2222 is supported at a predetermined distance from the shutter assembly 2202, forming a gap 2226. Gap 2226 is maintained by mechanical support and/or by an epoxy seal 2228 that affixes aperture plate 2222 to substrate 2204.
The reflective film 2224 reflects light that does not pass through the surface aperture stop 2208 back toward the back side of the display assembly 2200. Light emitted from the backlight that does not pass through one of the shutter assemblies 2202 will be sent back to the backlight and reflected again from the film 2220. Light that does not leave the display in this way to form an image on the first pass may be recycled and made available for transmission through other open aperture stops in the array of shutter assemblies 2202. Such light recycling has been shown to improve the illumination efficiency of the display.
The substrate 2204 forms the front side of the display assembly 2200. An absorber film 2206 disposed on substrate 2204 defines a plurality of surface aperture stops 2230 located between shutter assembly 2202 and substrate 2204. The film 2206 is designed to absorb ambient light and thus improve the contrast ratio of the display.
Epoxy 2228 should have a cure temperature preferably below about 200 ℃, it should have a coefficient of thermal expansion preferably below about 50ppm per degree celsius and should be water resistant. An exemplary Epoxy 2228 is EPO-TEK B9021-1 sold by Epoxy Technology, Inc.
Epoxy seal 2228 is sealed in a working fluid 2232. The working fluid 2232 is designed with a viscosity preferably below about 10 centipoise and has a relative dielectric constant preferably above about 2.0 and a dielectric breakdown strength of about 104V/cm or more. The working fluid 2232 may also act as a lubricant. Its mechanical and electrical characteristics also contribute to reducing the voltage required to move the shutter between open and closed positions. In one embodiment, the working fluid 2232 preferably has a low refractive index, preferably less than about 1.5. In another embodiment the working fluid 2232 has an index of refraction matching that of the substrate 2204. Suitable working fluids 2232 include, but are not limited to, deionized water, methanol, ethanol, silicone oil, fluorinated silicone oil, dimethyl siloxane, polydimethylsiloxane, hexamethyldisiloxane, and diethylbenzene.
A piece of metal or molded plastic assembly support 2234 holds aperture plate 2222, shutter assembly 2202, substrate 2204, backlight 2216, and other component parts together around the edges. The assembly support 2234 is fastened with screws or dowels to increase the rigidity of the combined display assembly 2200. In some embodiments, light source 2218 is molded in place from an epoxy potting compound.