WO2006114127A1

WO2006114127A1 - Mems device with mutually perpendicular deflectable reflective and electrode surfaces

Info

Publication number: WO2006114127A1
Application number: PCT/EP2005/010500
Authority: WO
Inventors: Ulric Ljungblad; Peter Enoksson; Martin Bring
Original assignee: Micronic Laser Systems Ab
Priority date: 2005-04-27
Filing date: 2005-09-28
Publication date: 2006-11-02
Also published as: JP2008539455A; EP1875295A1

Abstract

The present invention relates to a MEMS device including at least one pixel element (132); said pixel element (132) including, a support structure (134), at least one deflectable reflective surface (135) supported by said support structure (134), at least one deflectable electrode surface (146, 148), which electrode surface (146, 148) is essentially perpendicular to said deflectable reflective surface (135), said electrode surface (146, 148) is electrostatically attractable by a standing bar (160, 170) arranged in between pixel elements (132), wherein said standing bar is attracting a greater portion of said electrode surface (146, 148) on a first side of the tilting axis of said pixel element (132) compared to a second side of the tilting axis of said pixel element (132).The invention also relates to a method for operating a MEMS device.

Description

MEMS DEVICE WITH MUTUALLY PERPENDICULAR DEFLECTABLE REFLECTIVE AND ELECTRODE SURFACES

TECHNICAL FIELD

[0001] The invention relates to modulation of electromagnetic radiation and more ⁵ particularly to a method of addressing a MEMS device for modulating said radiation.

BACKGROUND OF THE INVENTION

[0002] Micro-electromechanical system (MEMS) may comprise movable/deflectable mirrors or pixel elements fabricated by microelectronic processing techniques on wafer substrates. Electrostatic actuation may most ⁰ commonly be used to deflect micro-mirrors. In order to produce a force, a voltage may be generated between two electrodes, one of which may be stationary arranged beneath the micro-mirror and the other of which may belong to the mirror. [0003] An SLM with an array of actuators (micromirrors or reflecting elements) used in for example a mask writing tool or a chip manufacturing tool may be loaded ⁵ with a specific pattern, where each actuator may be in an addressed state or a non- addressed state before each stamp may be printed, see for instance US patent No. 6 373 619. This pattern may normally be a subset of the pattern to be printed on the mask or chip respectively. Each actuator may be deflected or moved electrostatically by applying voltage between the actuator and an underlying address electrode, after ⁰ which the actuator may be allowed to move into its predetermined deflected state before an electromagnetic radiation source may be triggered to print the stamp. [0004] Loading an SLM in an analog mode is traditionally performed by applying one potential to the mirrors or pixel elements and individually address at least one underlying electrode belonging to each of said mirror, thereby creating a desired ⁵ pattern of the SLM. hi the analog mode, said SLM mirror may be set to a number of different states, for example 64 or 128 states, ranging from totally undeflected to a maximum deflection. Maximum deflection in this case may be defined as maximum extinction of the impinged electromagnetic radiation and min deflection may be defined as full reflection of the impinged electromagnetic radiation. In a digital SLM, ⁰ max deflection may be defined as when the reflected electromagnetic beam may be deflected out of the target plane and min deflection may be defined as full reflection of the impinged electromagnetic radiation. Digital spatial light modulators may operate in a deflection mode and analog spatial light modulators operate in a diffraction mode. The degree of deflecting individual elements varies quite a lot between those two types, where the analog elements are typically only deflected parts of a degree and the digital elements are deflected several degrees. [0005] However, when a thickness of a micro-mirror becomes large a cross talk between mirrors starts to become a problem if the mirrors are individually addressed according to prior art technique.

SUMMARY OF THE INVENTION

[0006] Accordingly, it is an object of the present invention to provide a method of addressing microelements, which overcomes or at least reduces the above-mentioned problem and thereby facilitate the use of thicker pixel elements in a MEMS device. [0007] This object, among others, is according to a first aspect of the invention attained by a spatial light modulator including at least one pixel element; said pixel element including, a support structure, at least one deflectable reflective surface supported by said support structure, at least one deflectable electrode surface, which electrode surface is essentially perpendicular to said deflectable reflective surface, said electrode surface is electrostatically attractable by a standing bar arranged in between pixel elements, wherein said standing bar is attracting a greater portion of said electrode surface on a first side of the tilting axis of said pixel element compared to a second side of the tilting axis of said pixel element.

[0008] In another embodiment according to the present invention said method of operating a spatial light modulator, comprising at least one pixel element, adapted to create a plurality of modulation states larger than or equal to two, including the actions of, providing a first bias voltage on a first standing bar, providing an address voltage on at least one of said pixel elements, wherein said first standing bar is capable to electrostatically attract a deflectable electrode surface of said pixel element.

[0009] Further characteristics, objects of the invention, and advantages thereof, will be evident from the claims, detailed description of preferred embodiments of the present invention given hereinafter and the accompanying Figs. 1-3, which are given by way of illustration only, and thus are not limitative of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 illustrates schematically a perspective view of an example embodiment of an inventive pixel element in spatial light modulator. [0011] Figure 2 illustrates schematically a top view of an example embodiment of addressing an inventive spatial light modulator.

[0012] Figure 3 illustrates schematically a side view of a state of the art actuator structure or pixel element.

DETAILED DESCRIPTION

The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. [0013] Figure 3 illustrates schematically a side view of a state of the art actuator structure or pixel element 300. Such an actuator structure 300 may for instance be a micro-mirror structure in a spatial light modulator (SLM). The actuator structure, depicted in figure 3, comprises a substrate 313, a first electrode 312 and a second electrode 314, a support structure 311 and a movable element 310. Said substrate may be made of semi-conducting material and may comprise one or a plurality of CMOS circuits. The first and second electrodes are made of an electrically conductive material, such as gold, copper, silver, aluminum or alloys of said and/or other electrically conductive materials. Said electrodes are connectable to steering circuits, such as the above-mentioned CMOS circuit.

[0014] The support structure 311 is preferably manufactured of a relatively stiff material, such as single crystal silicon, but may of course be made of materials not having pronounced high stiffness. The movable element 310 is preferably manufactured of a material having good optical properties, such as aluminum. However, if a material is selected not having the characteristics as desired, said material may be coated with one or a plurality of other material having more favorable characteristics, thereby creating a sandwich structure. [0015] An electrostatic force may deflect the movable element 310. Applying different potentials on the movable element 310 and one of the first 312 and second 314 electrodes creates electrostatic force. In the event of applying a first potential on the movable element 310 and a second potential on said first and second electrodes, where said first and second potentials are equal, creates electrostatic forces, but may not deflect said movable element because of the symmetry of the forces. In case of equal potential on said first and second electrodes said movable element may anyway be deflected due to non equal attracting areas between the movable element and the first electrode and the movable element and the second electrode or non equal distance between the movable element and the first electrode and the movable element and the second electrode. The two equal attractive forces equalize each other. [0016] In figure 3 the actuator structure is illustrated to comprise two electrodes, the first 312 and second 314 electrodes. However, deflecting the movable element requires only one electrode, either the first 312 or second 314 one. There maybe several reasons for having more than one electrode. One such reason is that it takes two electrodes arranged spaced apart from each other to deflect the mirror in two different directions. Other reasons become apparent from the description herein below describing the inventive method.

[0017] Figure 1 illustrates a perspective view of an example embodiment of a pixeLelement 132 according to the present invention. Said pixel element comprising a deflectable reflective surface 135, support structures 134, a cavity 131, a base element 136, a first leg 142 and a second leg 144. The pixel element 132 may have at least one cross section, which may be as thick as the original substrate, which in this particular embodiment may be the distance from the deflectable reflective surface 135 to a bottom surface 145, 147 of the legs 142, 144 and/or a surface 143 of the base element 136. The original substrate maybe an SOI (Silicon On Insulator) substrate, silicon substrate or any other semiconducting substrate. This may give the mirror structure good mechanical properties, such as high stiffness, i.e., the mirror surface is essentially rigid while being in a deflected position. The supports 134 may be thin pillars. The supports may support the deflectable reflective surface 135, the first and the second legs 142 and 144 respectively and at the same time function as a hinge. In the illustrated example embodiment in figure 1 said support 134 is arranged so that the rotational axis is essentially in the middle of the structure. In an alternative example embodiment said rotational axis may be arranged off center, which may be achieved by displacing the supports from a center position. An axis of rotation of the deflectable reflective surface 135 may be parallel to said surface 135. [0018] The base element 136 and the support 134 may be denoted a hidden hinge, hi another embodiment the base element 136 is minimized so that the support 134 only may be denoted the hidden hinge. The cross section of said pillars may be polygonal, e.g., triangular or rectangular. The base element 136 may be attached to the supports 134. A bottom surface 143 of the base element 136 may be attachable to another surface, such as a wafer with steering electronics. The legs 142, 144 may have electrode surfaces 146, 148 essentially perpendicular to the mirror surface 135. The cavity 131 may be formed by means of an isotropic etching process. The mirror structure 132 may be doped. The doping is preferably made prior to defining the cavity 131 and supports 134, i.e., the substrate to be used for defining said mirror structure may be doped, hi this embodiment the electrode surface 148 may be used to rotate the pixel element 132 clockwise. The electrode surface 146 may be used to rotate the pixel element 132 counter clockwise. The surface 143 of the base element 136 may be at another level compared to the bottom surfaces 145, 147 of the legs 144 and 142 respectively. [0019] Said electrode surfaces 146, 148 may be electrostatically attracted by standing bars 160, 170. Said standing bars may have a surface which is in parallel with and facing towards said electrode surfaces. The standing bars 160, 170 may be arranged in between individual pixel elements. [0020] Figure 2 illustrates schematically a top view of an inventive embodiment of addressing a spatial light modulator including standing bars 210. As is evident from figure 2, columns of pixel elements 230 are separated by said standing bars 210. In the embodiment as depicted in figure 2, said pixel elements are deflectable to the right or to the left. It is of course possible to arrange said standing bars so they separate rows of pixel elements instead of columns thereof. In such an embodiment the pixel elements would be deflectable to the top of the figure or to the bottom of the figure. By a suitable hinge construction said standing bars may be arranged both between rows and columns of pixel elements, hi such an embodiment said pixel elements are free to deflect in any of the four possible deflection directions, which are up, down, to the left or to the right. In the embodiment of standing bars between both columns and rows of pixel elements, one type of the standing bars, i.e., either the ones which are separating rows or columns, have to be discontinuous, otherwise all of the standing bars would be set at the same potential. Said discontinuity of said one type of standing bars coincide in an example embodiment with the intersection of the other type of the standing bars, i.e., the ones which have a perpendicular direction of propagation. [0021] Every second of the standing bars may be set to a first bias voltage and the others may be set to second bias voltage. In the example embodiment illustrated in figure 2, said first bias voltage is exemplified to be 10V and said second bias voltage is exemplified to be OV. It may easily be understood that any set of first and second bias voltages may be chosen, which may fulfill the particular purpose in question, hi figure 2 one can see that the pixel elements are addressed in a checkerboard fashion, i.e., every row has every second pixel element addressed to 0-5 V and the other pixel element in said row addressed to 5-10 V. Every second pixel element in each column is set to 0-5 V and the other pixel elements in said columns are set to 5-10 V. This implies that a particular pixel element, for instance pixel element 260, is set to a first address interval, here 5-10 V. Said pixel element 260 has adjoining neighbor pixel element of a second address interval, here 0-5 V. [0022] By setting every second of the standing bars to a first bias voltage, here 10 V, and the other standing bars to a second bias voltage, here 0 V, in combination with the above mention checkerboard addressing of the pixel elements will have the effect that pixel elements in every second row will deflect in a first direction, e.g., clockwise, while the other rows of pixel elements will be deflected in a counter clockwise fashion.

[0023] In order to prevent said standing bars from tilting while electrostatically attracting said pixel elements, said standing bars may be reinforced with perpendicular bar supports 220. Said bar supports 220 may support said standing bars both for clockwise and anti clockwise tilting as illustrated in figure 2. As illustrated in figure 2, when said standing bars are separating columns of pixel elements said bar supports 220 is extending in a direction between rows of pixel elements. The example embodiment illustrated in figure 2 may prevent said standing bars from clockwise and anti clockwise tilting, hi an alternative embodiment said bar supports on one side of said standing bar only, which may also prevent said bar from tilting. [0024] Said standing bar 210, 160, 170 may in an example embodiment have a height which may be lower than the height of the pixel element 132. As illustrated in figure 1, said standing bar 160, 170 is about half the height of the pixel element 132. hi order to deflect the deflectable reflective surface 135, an area of the electrode surface 146, 148, electrostatically capable of attracting said standing bar 160, 170, i.e., facing towards said standing bar, may be larger below a tilting axis of said pixel element than above said tilting axis. A position of said tilting axis may vary depending on the shape of the structure, i.e., the shorter a length of the support structure for a given distance between said deflectable reflective surface 135 and said bottom surfaces 145, 147 the lower the position of said tilting axis will be positioned in said pixel element. Equal areas of the electrode surface facing towards said standing bar above and below the tilting axis will result in no rotation, i.e., the electrostatic forces from the areas above and below the tilting axis will equalize each other.

[0025] In an example embodiment with the tilting axis positioned at half the distance between said deflectable reflective surface 135 and said bottom surface 145, 147 said standing bar may be at any height below the deflectable reflective surface. A standing bar having a height just below said deflectable reflective surface 135 may need more applied voltage difference between the standing bar 160, 170 and the electrode surface 146 than a standing bar with half the height of said pixel element 132 for a given deflection. A standing bar 160, 170 with a top surface 162 at the same height as the tilting axis may need least potential difference between said standing bar 160, 170 and said electrode surface for a given deflection. Any electrode height below said tilting axis of the pixel element may reduce the effective electrostatic attraction surface and thereby reduce an electrostatic force for deflecting said pixel element. In the above one has assumed that the standing bar covers the full width of the electrode surface 146, 148. i.e., the shape of the electrode surface is identical with the standing bar. Any difference in shape of the electrode surface and the standing bar may affect the electrostatic attraction force between the electrode surface and the standing bar. [0026] hi figure 1 said standing bars is illustrated to have a top surface 162, 172 which is essentially flat. By providing a shape of said top surface which is not flat said scattered light may be even more reduced. An example of such shape of said top surface of said standing bar may be tapered or pyramid shaped. [0027] Said top surface 162, 172 of said standing bars may have a coating of a material which is non-reflective, an example of such non reflective material is Ti or Ta. The non reflective coating may also be a stacked layer of appropriate materials. [0028] Said top^' surface 162, 172 of the standing bars may be vertically displaced with respect to the at least one deflectable reflective surface 135 by pi/2 times the wavelength of the illuminating electromagnetic radiation, i.e., said top surface of said standing bars may be displaced such that reflected electromagnetic radiation from bars become 180 degrees out of phase with the electromagnetic radiation from the at least one deflectable reflective element.

[0029] In an alternative example embodiment a top surface of every second standing bar may be vertically displaced with respect to the at least one deflectable reflective surface by pi/4 times the wavelength of the illuminating electromagnetic radiation and the rest of the top surfaces of the standing bars may be vertically displaced with respect to the at least one deflectable reflective surface by -pi/4 times the wavelength of the illuminating electromagnetic radiation, i.e., the top surface of every second standing bar may be vertically displaced such that reflected electromagnetic radiation from said bars become 90 degrees out of phase with the electromagnetic radiation reflected from the at least one deflectable reflective surface and where the rest of said top surfaces of said standing bars may be vertically displaced such that reflected electromagnetic radiation from such top surfaces of said standing bars become -90 degrees out of phase with electromagnetic radiation reflected from the at least one deflectable reflective surface. [0030] Said standing bar may be manufactured in a semiconducting material, metal material or any other electrically conducting material. [0031] While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.

Claims

L A MEMS device including at least one pixel element (132); said pixel element (132) including,

- a support structure (134),

- at least one deflectable reflective surface (135) supported by said support structure (134),

- at least one deflectable electrode surface (146, 148), which electrode surface (146, 148) is essentially perpendicular to said deflectable reflective surface (135), said electrode surface (146, 148) is electrostatically attractable by a standing bar (160, 170) arranged in between pixel elements (132), wherein said standing bar is attracting a greater portion of said electrode surface (146, 148) on a first side of the tilting axis of said pixel element (132) compared to a second side of the tilting axis of said pixel element (132).

2. The MEMS device according to claim 1, wherein said at least one standing bar having a surface essentially in parallel with said electrode surface.

3. The MEMS device according to any one of claim 1-2, wherein columns and/or rows of pixel elements are divided by said standing bars

4. The MEMS device according to any one of claim 1-3, wherein every second of said standing bars are set to a first bias voltage and where the other standing bars are set to a second bias voltage.

5. The MEMS, device according to any one of claim 1-4, wherein said standing bars have a height equal to the position of the tilting axis.

6. The MEMS device according to any one of claim 1-5, wherein a direction of deflection of said pixel elements are changed by changing the range of applied voltages to said pixel elements while keeping the ⁴ first and second bias voltages of said first and second standing bars

⁵ fixed.

⁶

7. The MEMS device according to any one of claim 1-6, wherein at least one standing

⁷ bar is vertically displaced with respect to another standing bar.

⁸

8. A method of operating a MEMS device, comprising at least one pixel element,

⁹ adapted to create a plurality of modulation states larger than or equal to two, including ⁰ the actions of, ^! - providing a first bias voltage on a first standing bar, ² - providing an address voltage on at least one of said pixel elements, wherein ³ said first standing bar is capable to electrostatically attract a deflectable electrode ⁴ surface of said pixel element. 5 ⁶

9. The method according to claim 8, further including the action of: ⁷ - providing a second bias voltage on at least a second standing bar, ⁸ wherein at least one of said first or second standing bars are capable to ⁹ electrostatically attract a deflectable electrode surface of said pixel element. Q ¹

10. The method according to claim 8 or 9, wherein said first and second standing bars ² having a surface essentially in parallel with said edge of said pixel element.

³

11. The method according to any one of claim 8- 10, wherein at least two adjoining ⁴ columns of said pixel elements are divided by said standing bar.

⁵

12. The method according to any one of claim 8-11, further comprising the action of:

⁶ - setting every second of said standing bars to said first bias voltage, 7 - setting the other standing bars to said second bias voltage.

⁸

13. The method according to any one of claim 8-12, wherein said standing bar is ⁹ attracting a greater portion of said electrode surface (146, 148) on a first side of the ⁰ tilting axis of said pixel element (132) compared to a second side of the tilting axis of ¹ said pixel element (132). 2

14. The method according to any one of claim 8-13, further comprising the action of: - changing a direction of deflection of said pixel element by changing the range of applied voltage to said pixel element while keeping the first and second bias voltages of said first and second standing bars fixed.

15. The method according to any one of claim 8-14, wherein said deflectable electrode surface is essentially perpendicular to a deflectable reflective surface of said pixel element.

16. The method according to claim 8 or 9, wherein said standing bars have a height equal to the position of the tilting axis.

17. The method according to any one of claim 8-16, wherein at least one standing bar is vertically displaced with respect to another standing bar.

18. The MEMS device according to claim 1, wherein said MEMS device is a spatial light modulator (SLM).

19. The method according to claim 8, wherein said MEMS device is a spatial light modulator (SLM).