The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.
A microshutter device is a key component being used on the Near Infrared Spectrograph (NIRSpec) instrument on the James Webb Space Telescope (JWST), the Next Generation Space Telescope (NGST). NIRSpec is an instrument that allows simultaneous observation of a large number of objects in space. Microshutter arrays are placed in the telescope optical path at the focal plane of NIRSpec detectors for selective transmission of light. A microshutter array comprises a plurality of individually controllable microshutter cells. Each microshutter cell can be placed in either an open state or a closed state. An open microshutter cell lets light in from desired objects, while a closed cell blocks light from objects not desired. Given an image of an area on the sky, the microshutter array can be programmed to admit light from an ensemble of selected objects, providing a capability of simultaneous observation of a large number of objects. Microshutter arrays also have a great potential in other optical applications, such as laser filtering, eye protection, mass-spectroscopy, etc.
The mechanism for opening and closing, i.e., actuation of, microshutter cells in the array is important to the performance of the microshutter array. In a mechanism, the microshutter cells are actuated by a magnetic field. Before observation, a linear magnet sweeps across the array to place each cell in the open state. Then selected cells are closed by dropping an electrostatic force holding the cells to the open position. To scale up the microshutter arrays, a simplified actuation mechanism that has faster opening/closing operations is needed.
A method of actuating a microshutter array, a method of fabricating the microshutter array, and the microshutter array itself are described herein. In one aspect, a method of actuating a microshutter array is provided. The method includes vibrating a shutter blade of a microshutter cell that is selected to be opened in an alternating electrical field. The method further includes capturing the shutter blade with an electrostatic force. In some embodiments, the alternating electrical field is generated by applying an alternating current (AC) voltage across a pair of actuation electrodes. The shutter blade is disposed between the pair of actuation electrodes. In some embodiments, the electrostatic force is generated by applying direct current (DC) voltages on the shutter blade and a vertical electrode. The vertical electrode is disposed proximate to the open position of the shutter blade.
In another aspect, a microshutter array is provided. The microshutter array comprises a frame of grid and a plurality of microshutter cells each contained in an openings of the frame. Each of the plurality of microshutter cells includes a shutter blade, a torsion bar, a vertical electrode, and a pair of actuation electrodes. The shutter blade includes a blade electrode. The torsion bar connects the shutter blade to the frame. The shutter blade is rotatable around the torsion bar. The vertical electrode is on a wall of the frame, wherein the wall forms an inside wall of the shutter cell on a side next to the torsion bar. The shutter blade is disposed between the pair of actuation electrodes. In some embodiments, each microshutter cell further comprises a light shield that blocks light from leaking through the gap between the shutter blade, the torsion bar, and the frame when the microshutter cell is in the closed state. In some embodiments, each microshutter cell further comprises an anti-stiction coating.
In another aspect, a method of fabricating a microshutter array is provided. The method comprises forming shutter blade and torsion bar patterns on a substrate. The method further comprises forming a frame out of the substrate and releasing the shutter blade from the substrate. The method further comprises forming vertical electrodes on wall of the frame; attaching the frame to a first transparent substrate; and attaching the frame to a second transparent substrate.
The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.
In the following detailed description, reference is made to the accompanying drawings, which from a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.
The present disclosure relates generally to microshutter arrays and more particularly to microshutter arrays actuated by electromechanical resonance and electrostatic force. An alternating electrical field is created by applying an alternating current (AC) voltage across a pair of actuation electrodes. A shutter blade with a blade electrode thereon is disposed between the pair of actuation electrodes and vibrates in the alternating electrical field created. Direct current (DC) voltages are applied to a vertical electrode and the blade electrode. The shutter blade is attracted to the vertical electrode and captured to an open position. Since no magnets are needed in this actuation mechanism, much larger arrays can be achieved. For example, a field of view at least 50 times larger than that of magnetically actuated arrays can be achieved. With the magnets eliminated, the microshutter arrays can be made lighter and less prone to mechanical failure. Advantageously, faster opening/closing operations of shutter cells are enabled. In addition, a standard micromachining technology can be used to simplify the fabrication process.
Now refer to
Shutter blade 202 is connected to the vertical wall of the frame through a hinge and torsion bar 204. Shutter blade 202 is a cantilever suspended from the vertical wall. In the illustrated embodiment, shutter blade 202, when actuated, can rotate up to about 90 degree around torsion bar 204. Shutter blade 202 has a blade electrode thereon. In some embodiments, shutter blade 202 includes a dielectric layer and a conductive layer. The conductive layer is used as the blade electrode and the dielectric layer supports the electrode. In some embodiments, the dielectric layer is made of silicon nitride with a thickness of a few hundred microns. In an embodiment, the silicon nitride layer has a thickness of about 250 microns. In another embodiment, the silicon nitride layer has a thickness of about 500 microns. In some embodiments, the blade electrode is made of a thin metal layer, for example, an aluminum (Al) layer with a thickness of a few hundred nanometers (nm) that provides optical opacity. In an embodiment, the Al layer is 200 nm thick. In another embodiment, the Al layer is 800 nm thick The blade electrode may be of various shapes.
Torsion bar 204 connects shutter blade 202 to the vertical wall of the frame through a hinge. When there is no external force applied on shutter blade 202, it remains in the horizontal closed position. Shutter blade 202 can rotate around torsion bar 204 up to about 90 degree when external force is applied. The rotation of shutter blade 202 produces a twisting action in torsion bar 202 and causes a torque energy to be stored in torsion bar 202 because the ends of the torsion bar are fixed. As a result, when the external force is removed from shutter blade 202, the torque energy stored in torsion bar 204 causes shutter blade 202 to rotate back to the horizontal closed position. In some embodiments, torsion bar 204 is patterned together with shutter blade 202 and therefore has the same layer structures as shutter blade 202. Torsion bar 204 may be of various shapes.
Shutter blade 202 is disposed between a pair of actuation electrodes, i.e., first actuation electrode 206 and second actuation electrode 208. First actuation electrode 206 and second actuation electrode 208 are made of optically transparent and electrically conductive materials. In some embodiments, first actuation electrode 206 is made of an indium tin oxide (ITO) film patterned on a first glass substrate; second actuation electrode 208 is made of an ITO film patterned on a second glass substrate. It shall be appreciated that the actuation electrodes and the substrates may be made of any suitable materials.
In some embodiments, a spacer (not illustrated in the present figure) is disposed between shutter blade 202 and second actuation electrode 208 so that shutter blade 202 is not in direct contact with second actuation electrode 208. The spacer has an opening under shutter blade 202. In some embodiments, the space is made of a silicon oxide layer or other insulating material layer patterned on the second transparent substrate.
In some embodiments, an light shield (not illustrated in the present figure) is disposed between shutter blade 202 and second actuation electrode 208. The light shield has an opening under shutter blade 202. In some embodiments, the light shield is made of an optically opaque material patterned on the second transparent substrate. The light shield blocks light from leaking through the gaps between shutter blade 202, torsion bar 204, and the frame when microshutter cell 200 is in the closed state.
In some embodiments, shutter blade 202 is coated with an anti-stiction coating (not illustrated in the present figure). In some embodiments, the anti-stiction coating is a oxide/organic material composite layer, the oxide side attaching to surfaces of shutter blade 202, and the organic material side facing out. In some embodiments, the organic material is an hydrophobic monolayer that prevents shutter blade 202 from sticking to either the light shield or vertical electrode 210.
The configurations shown in
In
The rotation of shutter blade 502 produces a twisting action in torsion bar 504 and causes a torque energy to be stored in torsion bar 504 because the ends of the torsion bar are fixed. When the DC voltages are dropped or removed from vertical electrode 510 and the blade electrode, the torque energy stored in torsion bar 504 causes shutter blade 502 to rotate back to the horizontal closed position. In other words, if the electrodes are biased to provide enough electrostatic force to overcome the mechanical restoring force of torsion bar 504, shutter blade 502 remains attached to vertical electrode 510 in its open state. If the bias is insufficient, shutter blade 502 returns to its horizontal closed position.
In some embodiments, the level of the actuation AC voltage applied across first actuation electrode 506 and second actuation electrode 508 is in the range of about 5 Vac to about 35 Vac. In some embodiments, the frequency of the actuation AC voltage is in the range of about 1 KHz to about 4 KHz, depending on the mechanical resonance frequency of shutter blade 502. The level of the capture DC voltages applied on the blade electrode and vertical electrode 510 is in the range of about 20 Vdc to about 40 Vdc.
As noted above, when the frequency of the alternating electrical field matches the frequency of the mechanical resonance of shutter blade 502, the deflection and the vibration of shutter blade 502 peaks. Various methods can be used for obtaining the mechanical resonance frequency of shutter blade 502. In some embodiments, a finite element analysis method is employed to determine the resonance frequency by using micro-electro-mechanical system (MEMS) module structural mechanics-eigenmode analysis. In other embodiments, microshutter actuation is directly observed inside a scanning electron microscope (SEM) when an AC voltage is applied to vibrate the shutter. If the AC frequency is equivalent to one-half of the mechanical resonance frequency of the shutter blade, a maximum deflection of the shutter should be observed. In yet other embodiments, a voltage drop across a resistor placed in series with the microshutter is measured to determine the mechanical resonance frequency of the microshutter. In this method, the microshutter is treated as a capacitor. When it is vibrating its capacitance oscillates at a rate equal to its resonance frequency. By measuring the voltage drop across a resistor placed in series with the microshutter with an oscilloscope, the resonance frequency of the microshutter is obtained.
Microshutter array fabrication is carried out through semiconductor processing and micro-electromechanical (MEMS) techniques. Photolithography, wet chemical etching, dry reactive ion etching, electron-beam, and sputtering deposition, etc. are employed to fabricate the microshutter arrays.
In an operation 602, shutter blade and torsion bar patterns are formed. In some embodiments, a silicon wafer with a thickness of a few hundred microns is used as the frame material to provide structural support for shutter cells. In other embodiments, the frame is made out of a silicon on oxide (SOI) wafer. In yet other embodiment, the frame may be made out of other materials. As shown in
There are a number of ways for forming the shutter blade and torsion bar patterns. In some embodiments, the pattern of the blade electrode is formed by wet etching the Al layer. In an embodiment, the electrode pattern are in the shape of strips, as shown in
In an operation 604, the frame that provides structural support for the microshutter cells is formed and the shutter blades are released from the substrate. The portion of the silicon wafer and the silicon oxide layer under the patterned silicon nitride layer is removed to free the shutter blades. The remaining portion of the wafer forms the frame. In some embodiments, the wafer is etched by anisotropic thinning followed by a deep reactive ion etching (DRIE). In an embodiment, the wafer is etched to 100 microns thick. In some embodiment, the wafer is flipped over and attached to another transparent wafer for easy handling during the thinning and subsequent processing. In some embodiments, the silicon dioxide layer is etched off by using a buffered hydrofluoric acid (BHF) etching. In some embodiments, a mask set is used for the etching processes. It shall be appreciated that the methods of patterning the wafer and the silicon dioxide layer are given for illustration only, not for limiting. Any suitable method may be employed. In this manner, the frame is formed, the shutter blades are released from the substrate and suspended from the frame via the torsion bars.
In an operation 606, the vertical electrodes are formed. The vertical electrode is formed on the vertical wall of the frame which forms an inside wall of the shutter cell on the side next to the torsion bar. A thin conductive layer is formed on at least a portion of the vertical wall. In some embodiments, the thin conductive layer is an Al film. In an embodiment, the Al film is formed by an angle deposition. In another embodiment, the Al film is formed by an atomic layer deposition (ALD). In yet another embodiment, the Al film is formed by an electron beam (E-beam) deposition. The metal thin layer may be several hundred nm thick. In an embodiment, the metal layer is 200 nm thick. It shall be appreciated that the material, growth method, and the dimension of the vertical electrodes are given here for illustration only, not for limiting. Any suitable material, growth method, and dimension may be employed. In some embodiments, a dielectric layer is formed between the vertical wall of the frame and the thin metal layer. In an embodiment, the dielectric layer is aluminum oxide formed by vapor deposition. Dielectric layer made of other suitable materials by other methods can be employed. In this manner, vertical electrodes are made on the vertical walls of the frame.
In an operation 608, the frame with the microshutter cells is attached to a first optically transparent substrate. The first actuation electrodes are patterned on the first transparent substrate. In some embodiments, the first transparent substrate is a glass substrate. In some embodiments, the first actuation electrodes are made of an ITO film patterned on the first transparent substrate by photolithography. It shall be appreciated that other suitable materials can be used as the first transparent substrate and the first actuation electrode. In some embodiments, alignment features are patterned on both the frame and the first transparent substrate for aligning the first actuation electrodes with the microshutter cells. In this manner, the frame and the first transparent substrate are aligned and bonded together.
In an operation 610, the frame is attached to a second transparent substrate. The second actuation electrodes are patterned on the second transparent substrate. In some embodiments, the second transparent substrate is a glass substrate. In some embodiments, the second actuation electrodes are made of an ITO film patterned on the second transparent substrate by photolithography. In some embodiments, spacers are also patterned on the second transparent substrate so that the shutter blades are not in direct contact with the second actuation electrodes. In some embodiments, the spacers are made of a silicon dioxide layer or other insulating material layer patterned on the second substrate by photolithography. In some embodiments, light shields are patterned on the second transparent substrate for blocking light from leaking through the gaps between the shutter blades, the torsion bars, and the frame when microshutter cells are in the closed state. In some embodiments, the light shields are made of an Al layer patterned on the second transparent substrate by photolithography. In some embodiments, alignment features are patterned on both wafers for aligning the second transparent electrodes with the first transparent electrodes. Photolithography can be used to make the alignment features. In this manner, the frame and the second transparent substrate are aligned and bonded together.
As utilized herein, the terms “approximately,” “about,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.
References herein to the positions of elements (e.g., “on,” “under,” “above,” “below,” “horizontal,” “vertical,” etc.) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure.
While various embodiments of the methods and systems have been described, these embodiments are exemplary and in no way limit the scope of the described methods or systems. Those having skill in the relevant art can effect changes to form and details of the described methods and systems without departing from the broadest scope of the described methods and systems. Thus, the scope of the methods and systems described herein should not be limited by any of the exemplary embodiments and should be defined in accordance with the accompanying claims and their equivalents.
The embodiments described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457), and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or after.