HK1097605A - Altering temporal response of micro electromechanical elements - Google Patents

Abstract

An array of movable elements is arranged on a substrate. Each element has a cavity and a movable member to move through the cavity. The pressure resistance of the elements varies, allowing actuation signals to be manipulated to activate elements with different pressure resistance at different levels of the actuation signal.

Description

Modifying the time response of a microelectromechanical element

Technical Field

The present invention relates generally to microelectromechanical elements, and more particularly to modifying the time response of microelectromechanical elements.

Background

Microelectromechanical (MEMS) systems are typically constructed of individual moving elements, each fabricated on a micrometer scale. Elements such as tunable switches, capacitors, reflectors for display and printing applications, and the like are examples of MEMS. For the purposes of this description, a MEMS device has at least one movable element, a cavity into or out of which the element moves, and some sort of actuation signal that causes the element to move.

In some applications, the timing of actuation of an element (where actuation is the movement of an element from one position to the next) is a critical part of the operation of the device. For example, in a MEMS switch, the switching elements may be cascaded and the response time of a first switch may determine the response time of the next switch, and so on. In MEMS displays, the movement of the elements typically modulates light, and the timing of the modulation determines the image content seen by the viewer.

Finer control of these elements through their response times may provide better operation, e.g., higher image quality. For example, controlling the display elements by their response times may provide a higher bit depth for display applications.

Disclosure of Invention

Is free of

Drawings

The invention may best be understood by reading the disclosure with reference to the drawings, in which:

FIG. 1 shows an embodiment of a microelectromechanical device.

Fig. 2a and 2b show graphs of the operating time in relation to the pressure.

FIG. 3 shows one embodiment of an array of display elements.

FIGS. 4a and 4b show cross-sectional views of alternative embodiments of a display element.

FIG. 5 shows an actuation/release response curve for one embodiment of a display element.

FIG. 6 shows an alternative embodiment of an array of display elements.

FIG. 7 shows an actuation/release response curve for an alternative embodiment of a display element.

Fig. 8 shows an embodiment of a pixel consisting of several display elements with different pressure resistance levels.

Fig. 9 shows an alternative embodiment of a pixel consisting of several display elements with different pressure resistance levels.

FIG. 10 shows an alternative embodiment of a pixel having support rails.

Detailed Description

Fig. 1 shows a diagram of a generalized structure of a microelectromechanical device 10. The movable element 10 has a movable member 14 adjacent a cavity 18. Opposite the movable member 14 is an actuator 16 of some sort, such as an address transistor or other component capable of actuating the movable element into the cavity 18 toward the substrate 12. Typically, the element will be suspended above the substrate, but may also be oriented horizontally across the cavity, or the actuator may be a suspended portion of the movable element 10. Likewise, movement through the cavity may be movement of the component 14 into the cavity 18, as would occur if a component traveled from a position similar to the position of component 14a to a position parallel to the position of component 14 b. Alternatively, the movement may be in the opposite direction: starting at position 14b and ending at a position parallel to the position of component 14 a.

The gas trapped in the cavity 18 must be vented as the part moves toward the substrate. Depending on the measures taken for such a discharge, the response time of the element may be influenced. The response (or actuation) time is the period of time required for the movable member to reach its actuated position. The mechanical resistance of the gas can damp the movement of the movable part of the element if there is little room for the gas to escape. This damping due to mechanical resistance will be referred to herein as pressure resistance. The pressure resistance can be used to achieve a finer control of the response of the element.

Air or other gas under pressure may act as if it were a liquid, and the resistance caused by the gas is similar to that of viscous liquid damping. When gas resides in a very small gap, it no longer acts as a liquid, but is instead resisted from moving by the pressure of the gas itself. In the case of small gaps, the pressure will be calculated by the equation pressure x volume constant. In the case of a movable element, the pressure resistance of the gas between the movable element and the substrate or other fixed structure may initially be viscous liquid damping and become the pressure characterized above as the gap closes.

The pressure resistance can be manipulated by varying the pressure resistance in each element, with different elements having different pressure resistances and thus different response times. A different method in which the response time of the movable element is varied by means of a vent is described in U.S. patent application No. 09/966,843 entitled "Interferometric Modulation of radiation", filed on 28.9.2001. In this method, it is desirable to accelerate the response time and all the elements have the same pressure resistance by having the same hole pattern. The movable elements all have a uniform pressure resistance when deflected.

A graph of response time versus pressure is shown in fig. 2 a. As can be seen from the 3 different curves, gas pressure is the main factor affecting the response of the device. Fig. 2b shows the response times of two different elements. The top curve is the response time of an element tuned to have a slower response. The bottom curve is the response time of an element tuned to have a faster response. Such pressure resistance variations between devices can be exploited.

The pressure resistance variations may be applied to different movable elements. These movable elements include switches, different types of display elements, tunable capacitors, and the like. For a display element, providing additional space for gas to escape can speed up the response time. In display applications, the MEMS elements are typically arranged in an x-y grid on a substrate. Depending on the size of the elements, the MEMS elements may be further grouped into sub-arrays, with each sub-array forming a pixel (or pixel) in the composite image seen by the viewer. A portion of such an array is shown in fig. 3.

In FIG. 3, a portion of an array of movable elements is shown. The movable elements are grouped into sub-arrays corresponding to pixels, such as sub-array 20. Each element in the array includes a surface with a hole in the center of the surface to allow gas to escape when the movable part of the element is actuated and moved. Although this particular structure is based on interferometric modulators, these apertures can also be used in many different types of structures. To change the pressure resistance of the element, the size of the holes can be changed to provide different response times for the element.

Interferometric modulators (e.g., iMoD)^TM) Relying on interference effects that act on the light within the cavity to modulate the light according to the image data. A cross-sectional view of such a modulator is shown in fig. 4 a. In thatIn this embodiment, the view surface would be the 'bottom' of the image. The modulator array is formed on a transparent substrate 30. An optical stack 36 forms a first optically active surface that can be affected by a second optically active surface-either mechanical or mirror layer 33. A dielectric layer 38 generally protects the optical stack. The mechanical layer 32 is supported by posts (e.g., 32), where the positions of the posts form the individual elements in the array.

When circuitry (not shown) on the substrate is activated in a particular region below the mechanical layer (e.g., the portion of layer 34 that overhangs cavity 40), the mechanical layer is deflected toward the optical stack 36. When it is offset, the mechanical layer causes the portion of the optical stack seen by the viewer to appear black. Thus, by addressing the mechanical layer with image data, the viewer sees an image. This particular embodiment of an interferometric modulator may be referred to herein as a monolithic interferometric modulator.

In an alternative embodiment of an interferometric modulator illustrated in FIG. 4b, the mirror 44, which causes the pixel to appear black when shifted, is separated from the support layer 42. This may be referred to herein as a separable modulator. In both cases, trapping of gas residing inside the array package can be used to change the response time of the movable element. The general principles of this will be discussed below with reference to a monolithic embodiment, and modifications made to the separable modulator will be discussed later.

In an embodiment of the modulator, the layer 34 shown in fig. 4a is visible to a viewer. However, the holes are so small that they do not generally create objectionable artifacts from forming a hole in the center of the actuating portion. In fig. 4b, the holes would be made in the mirror 44, in the support layer 42, or in both. The viewer will not see the holes made in the support layer 42 because they will be shielded by the mirror 44. By adding holes on the surface, the response time will be changed. The response time of such an element is shown in fig. 5. As can be seen, these elements have a response time of about 200 microseconds.

In contrast, a portion of an array of non-porous elements is shown in FIG. 6. Pixel 50, which is made up of several sub-pixels and is referred to herein as a macropixel, will have individual elements (e.g., 52) with no holes in its surface, and edge 51 is a free edge represented by a dashed line. The response time of such a modulator is shown in fig. 7. As can be seen, the response time is approximately 3 milliseconds. In experiments conducted to collect this data, both modulators were made from the same wafer, so other factors that may affect response time (e.g., dielectric charging) would be similar for both. The response time is extended only by trapping of gas beneath the modulator element.

This feature can be used to provide more precise control of the movable element. For example, in a monolithic modulator such as that shown in FIG. 8, the edge elements 70a-j may be fabricated as elements having a lower mechanical resistance than the middle of the pixels 72a and b. When an actuation signal is applied at a first level (e.g., the beginning of a ramp signal), the edge element will move first, reducing the mechanical resistance to the attractive force pulling the movable member toward the substrate. As the edge elements move, they trap gas under the elements in the middle of the pixel (72 a and 72b in this example). One Method for making modulators with different mechanical stiffness can be found in U.S. patent No. 6,574,033, entitled "Microelectromechanical System Device and Method for manufacturing Same," issued on 3.6.2003.

The trapped gas provides another opportunity to control the response time of the last two elements of the pixel. When the actuation signal reaches a second level, the intermediate element will then move. In this manner, controlling the voltages enables a system designer to provide pulses with different times or voltages to determine how many elements in a pixel move and affect the resultant pixel seen by a viewer.

Other variations of this non-porous method exist. For example, a first set of elements (e.g., 70a, 70d, 70f, and 70h) located on a free edge of the pixel may be designed to be offset first. A second set of components (e.g., 70c, 70e, 70g, and 70j) located on a second free edge may then be moved after the first set rather than simultaneously as described above. Mechanical resistance as described above may control the timing of the movement. The first edge and the second edge may need to be moved simultaneously in some applications or separately in other applications.

An example of one method of changing the mechanical resistance may include changing the column spacing of the modulator columns. The modulator shown in fig. 8 has wider gaps between the columns of the edge pixels 70a-70j than the columns for the middle element. This can be seen more clearly by comparing element 52 of fig. 6 with element 70c of fig. 8. Other types of mechanical resistance variations are possible.

In an alternative embodiment, holes may be formed in the back side of the movable element, as shown in FIG. 9. The central elements 82a and 82b have a central hole to allow any trapped air to escape. This may allow another level of response time. The edge element with the lower mechanical resistance to the actuation signal may respond first, then the middle element 82a with holes, followed by the element 82b without holes. Again, this allows the actuation signals to be controlled to different degrees so that different numbers of elements are used to form the resulting pixel.

Thus, in the embodiments described above, an array of movable elements is provided. Each element has a movable member and a cavity through which the member moves. The pressure resistance of the elements is varied so that at least one element has a different level of pressure resistance than the other elements in the sub-array or pixel. This difference may be due to air pressing under the element as the adjacent element collapses, or due to the presence or absence of holes patterned into the surface of the element.

Referring back to fig. 4b, these general principles may also be applied to elements that do not have the advantages of monolithic mirrors or mechanical layers such as those described above. The movable element 44 may also have holes formed therein to allow trapped air to escape. Alternatively, the support layer 42 can be made larger than shown here to move and cover the edges of the mirrors as they move, thereby trapping gas under the mirrors. Additionally, channels may be fabricated to restrict or release air between the mirrors.

In another alternative embodiment, the mirror 44 may be formed of two parts or layers. The first layer will be thicker or thinner than the second layer. The second layer will be deposited on the first layer but with a smaller surface area, thereby forming a mirror with a rigid central portion and a flexible outer portion. As the mirror moves, the flexible portion will collapse first and trap air under the edge of the mirror.

In another embodiment, the mirror or moveable element may be supported on all four sides, resting the mirror on a 'track'. This is shown in FIG. 10, where without any posts, each sub-element and each macro-pixel is supported on each side. Any gas trapped under macropixels 90 will be isolated from any gas trapped under macropixels 96. One embodiment would make the rail 98 between the two macropixels so as not to allow any gas movement. In addition to the holes in each sub-pixel, the response time may also be controlled by forming or choosing not to form holes in the tracks around the sub-pixel (e.g., between 92a and 92 b). If holes are formed in the rail, this will allow gas to escape from a moving element more quickly when subjected to pressure and will change the response time.

In yet another embodiment, the substrate may be patterned with structures such as bumps or grooves to facilitate gas movement. This additional aspect will apply to any of the previously described embodiments. The movable element itself may also have bumps thereon to facilitate gas movement. The pattern of bumps and grooves can be varied among different elements of a macropixel to provide the desired variable pressure resistance.

In addition to using alternate types of elements, these two interferometric modulators are merely examples of devices to which the invention may be applied, and the gas trapping properties may also be utilized for the released portion of the cycle rather than the actuated portion. This has been described above with reference to fig. 1. However, given the amount of space on the back side of the mirror and the packaging complexity involved, it may be more feasible to trap gas over the actuation cycle. However, it is not intended that the application of the present invention be limited to only such actuation cycles.

The description made so far has referred to the substance being trapped as a gas under the element. The gas is more likely to be air, although different gases may be used. Using a gas with a density less than air can increase the response time even further, since the element will have a lower pressure resistance. The damping force provided by the gas depends on its properties, such as partial pressure, density and viscosity. The geometry of the device and the geometry of the gas molecules may also have an effect.

In this particular example of an interferometric modulator, the elements each have a response time in the nanosecond range when operated under vacuum. When encapsulated with air, the response time of these elements is in the microsecond range. Thus, it appears that a component with a faster response time under vacuum can use a gas other than air to adjust its response time to within the optimum operating range for this type of component.

Accordingly, while there has been described to this point a particular embodiment of a method and apparatus for varying the response time of a MEMS element, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims.

Claims (24)

1. An array of movable elements on a substrate, each element comprising:

a cavity; and

a movable member moving through the cavity;

wherein a pressure resistance of the element is varied.

2. The array of claim 1, the movable member further comprising holes in the member to allow gas to vent as the elements move.

3. The array of claim 2, wherein different elements in the array have different aperture patterns.

4. The array of claim 1, the movable member further comprising a flexible portion and a rigid portion such that when the movable member moves through the cavity, the flexible portion collapses to trap gas between the rigid portion and the substrate.

5. The array of claim 1, said movable member further comprising a movable mirror formed from a support layer.

6. The array of claim 1, the movable member further comprising a movable mirror suspended over the cavity by at least one support post.

7. The array of claim 6, the support posts further comprising support rails.

8. The array of claim 7, the support track having holes to allow gas to vent.

9. The array of claim 1, the support layer blocking edges of the movable mirrors to trap gas between the mirrors and the substrate.

10. The array of claim 1, said pressure resistance varying due to a mechanical stiffness of said movable member.

11. The array of claim 1, further comprising an x-y grid formed by modulator elements, the x-y grid forming a pixel, wherein modulator elements located at outer edges of the pixel are arranged to actuate first, thereby increasing the pressure resistance of elements located at the middle of the pixel.

12. The array of claim 11, wherein the elements at the outer edges of the pixels have a lower mechanical rigidity, thereby trapping gas under the elements at the middle of the pixels.

13. The array of claim 1, wherein the array further comprises an array formed from one of the group consisting of: display elements, switches, tunable capacitors, and interferometric modulator elements.

14. The array of claim 1, further comprising a structure spanning the cavity from the movable element to facilitate gas flow.

15. The array of claim 1, said movable elements having structures formed thereon to facilitate gas flow.

16. A method for fabricating an array of movable elements, the method comprising:

movable members are formed over a cavity that are movable away from a substrate such that a pressure resistance of at least one of the movable members is different from a pressure resistance of the other of the movable members.

17. The method of claim 16, wherein forming a movable member further comprises forming a movable member having holes patterned in a surface of the member.

18. The method of claim 17, wherein the apertures form a first pattern on at least one of the movable members and the apertures form a second pattern on others of the movable members.

19. The method of claim 16, wherein the pressure resistance is varied by: some of the movable components are made to have a low mechanical resistance so that they actuate in a manner that forces gas into the cavities of other movable components.

20. A method to operate an array of light modulator elements arranged as pixels in sub-arrays on a substrate, the method comprising:

providing an actuation signal at a first level to actuate a first set of elements in the pixel to trap gas between other elements in the pixel and the substrate; and

an actuation signal is provided at a second level to cause a second set of elements in the pixel to actuate according to a response time dependent on the pressure resistance.

21. The method of claim 20, actuating a first set of elements further comprises actuating elements on a first edge of the pixel.

22. The method of claim 22, actuating a first set of elements further comprising actuating elements along a second edge simultaneously with the elements along a first edge.

23. The method of claim 20, actuating a first set of elements further comprising actuating elements along a second edge after the elements along the first edge.

24. The method of claim 20, actuating a second set of elements according to their response times further comprising actuating a second set of elements having holes patterned in their surfaces, the holes for varying the pressure positive force of the elements.