KR20110070925A - Digital micro-mirror device - Google Patents

Abstract

The digital micromirror device includes an array of micromirror assemblies located on a substrate. Each micromirror assembly includes a mirror spaced apart from the substrate, a stem supporting the mirror, and first and second electrodes located on either side of the stem. The stem is made of a resilient flexible material such that the mirror can be tilted either by the electrostatic force towards the first electrode or towards the second electrode. The digital micromirror device may be used for a data projector and the like.

Description

DIGITAL MICRO-MIRROR DEVICE}

The present invention relates to a digital mirror device (DMD). The present invention was primarily developed to provide an improved device that may be manufactured using simple MEMS manufacturing steps.

Digital micromirror devices (DMDs) are now commonly used in many optical devices such as data projectors. In such a device, an image is produced by microscopically small mirrors arranged in a matrix on a semiconductor chip (DMD). Each mirror represents one or more pixels in the projected image. The number of mirrors corresponds to the resolution of the projected image.

DMD technology was developed by Texas Instruments in the 1980s (see, eg, US Pat. No. 4,956,619, US Pat. No. 4,662,746 and related patents).

The DMD chip has on its surface hundreds, thousands of microscopic mirrors arranged in a rectangular array corresponding to the pixels in the image to be displayed. These mirrors can be rotated ± 10-12 ° individually for either the on state or the off state. In the on state, light from the projector bulb is reflected off the lens causing the pixels to appear bright on the screen. In the off state, the light is directed away (usually above the heatsink), causing the pixels to appear dark.

To generate grayscale, the mirror toggles very quickly on and off and the shade (binary pulse-width) is generated when the ratio of on time to off time is generated. Modulation).

The mirror itself is formed of aluminum and is typically approximately 16 micrometers square. Each mirror is mounted on a yoke through a rigid stem extending from the bottom surface of the mirror. This yoke is supported by a flexible torsion hinge, which enables the movement of the yoke (and thus the mirror) between its on and off positions. Torsional hinges are relatively resistant to fatigue and vibration shocks.

The electrodes control the position of the mirror by electrostatic attraction / repulsive force. On each side of the hinge a pair of electrodes is located, one acting on the yoke and the other acting directly on the aluminum mirror. A bias potential of about 20-30V is applied to the mirror and yoke, while the electrodes are addressed using 5V CMOS. Therefore, if the electrodes on one side with respect to the mirror are driven at + 5V, the mirror is tilted toward the opposite side where the electrodes are at 0V. Inverting the CMOS voltage causes the mirror to tilt in different ways. Thus, the on / off state of each mirror is controllable via CMOS.

For a more detailed description of the DMD as described above, see David Armitage et al, Introduction to Microdisplays, John Wiley and Sons, 2006.

The design of the DMD has been relatively unchanged over the last decade. However, the relatively complex design of the DMD has several moving parts in each mirror assembly and requires a fairly complex MEMS manufacturing process. Such complexity increases manufacturing costs and may have a significant impact on the size at which each mirror assembly can be miniaturized. Thus, it would be desirable to provide a DMD with a relatively simple design compared to known DMDs.

In a first form, a digital micromirror device comprising an array of micromirror assemblies located on a substrate, wherein each micromirror assembly comprises:

A mirror spaced apart from the substrate and having a top reflective surface and a bottom support surface,

A stem that supports the mirror and extends from the substrate to the lower support surface and forms a tilt axis of the mirror,

A first and a second electrode located on either side of the stem, each individually addressable via an electronic circuit in the substrate,

The stem is made of a resiliently flexible material so that the mirror can be tilted to either side towards the first electrode or towards the second electrode by electrostatic force.

Since the mirror is inclined around the flexible stem, the present invention eliminates the need for yoke and torsion hinge arrangements in conventional DMDs. This greatly simplifies not only the overall design of the DMD but also its manufacturing process.

Optionally, the stem consists of a polymer such as polydimethylsiloxane (PDMS). PDMS has a relatively low Young's module (less than 1000 MPa) so that the stem can be bent by the electrostatic force applied through the electrode (electrodes). Moreover, Applicant has already demonstrated the usefulness of PDMS in MEMS devices and its easy incorporation into the MEMS manufacturing process.

Optionally, the entire area of the upper reflecting surface is flat. This is in contrast to conventional DMDs, where there is a central dimple in the upper reflective surface resulting from engagement with the stem. The fully planar upper reflecting surface advantageously improves optical quality compared to conventional devices.

Optionally, said mirror comprises a metal plate, said metal plate forming an upper reflective surface. Optionally, the metal plate is an aluminum plate.

Optionally, the mirror comprises a support platform for the metal plate, the support platform forming the lower support surface. Therefore, the mirror is typically of a structure in which two members are integrated, including an upper metal plate and a lower support platform for the metal plate.

Optionally, the support platform is substantially coextensive with the metal plate.

Optionally, the support platform and the stem are made of the same material. Typically, the stem and the support platform are formed together in a single deposition step. For example, the stem and the support platform may be formed together by deposition of PDMS.

Optionally, the first and second electrodes form first and second landing pads relative to the mirror.

Optionally, the mirror has first and second contact points for contacting respective first and second landing pads, the first and second contacts being made of a polymer. Since the contact points are made of a polymer (eg PDMS), the tendency of the mirror to stick on either electrode is minimized.

Optionally, said support platform forms said first and second contacts. Therefore, no additional structure is required to deal with possible stiction problems. The support platform performs a dual function of supporting the upper aluminum reflector and minimizing adhesion between the mirror and the electrodes.

Optionally, the mirror is electrically connected to a biasing potential. The bias potential is typically tilted through the electrodes controlled by the CMOS voltage (usually 5V) by keeping the mirror at high potential.

Since the stem is made of a conductive polymer, the stem provides an electrical connection to the bias potential. For example, the stem may be made of PDMS implanted with metal ions.

Alternatively, a plurality of mirrors may be connected together in several rows, one end of each row electrically connected to a bias potential. Therefore, the bias potential may be applied to the mirror of the entire row through the common contact.

Optionally, the mirrors in each column have a common tilt axis.

Optionally, adjacent mirrors in one row are connected together via a linkage, which is aligned along a common tilt axis.

Optionally, said substrate is a silicon substrate comprising at least one CMOS layer, said CMOS layer comprising electronic circuitry.

In a second aspect, there is provided a projector including the digital mirror device as described above. Projectors and projector systems employing DMD will be well known to those skilled in the art.

In the third aspect, as a method for producing a micromirror assembly,

(a) forming a pair of electrodes spaced on the surface of the substrate and connected to the basic electronic circuitry in the substrate,

(b) depositing a layer of sacrificial material on the electrodes and the substrate,

(c) forming a stem opening in the sacrificial material positioned between the electrodes to form a scaffold,

(d) depositing a layer of resilient flexible material on the scaffold,

(e) depositing a metal layer on the flexible layer,

(f) etching through the metal layer and the flexible layer to form individual micromirrors comprising a metal layer supported on the stem of the flexible material and fused to a support platform, and

(g) there is provided a method of making a micromirror assembly comprising removing the sacrificial material to provide the micromirror assembly.

The method according to the third aspect provides a simple and effective means for producing DMD using a minimum number of manufacturing steps.

Optionally, the flexible flexible material consists of polydimethylsiloxane (PDMS).

Optionally, the sacrificial material is a photoresist.

Optionally, the metal layer is made of aluminum.

Optionally, an array of mirror micros is fabricated on the substrate simultaneously, the array forming a digital micromirror device.

Optionally, said substrate is a silicon substrate comprising at least one CMOS layer, said CMOS layer comprising electronic circuitry.

In a fourth aspect, there is provided a micromirror assembly comprising an inclined mirror supported by a stem, wherein the stem is made of an elastic flexible material.

Optionally, said tiltable mirror comprises a metal layer having an upper reflective surface.

Optionally, the tiltable mirror further comprises a support platform having a metal layer mounted thereon, wherein the support platform is made of an elastic flexible material.

Optionally, the flexible flexible material consists of polydimethylsiloxane (PDMS).

Optionally, the mirror can be tilted by electrostatic force.

Optionally, a pair of electrodes is located on either side of the stem, the electrodes providing at least a portion of the electrostatic force.

Best Mode for Carrying Out the Invention The best mode for carrying out the present invention will now be described with reference to the accompanying drawings.

1 is a schematic cross-sectional view of a DMD according to the present invention.
2 shows the DMD of FIG. 1 in an inclined position;
3 is a plan view of the DMD shown in FIG. 1;
4 is a view showing a state in which electrodes are formed as a first step of manufacturing MEMS.
FIG. 5 shows a state in which a sacrificial scaffold is formed as a second step of MEMS fabrication. FIG.
FIG. 6 is a view illustrating a state in which mirror layers and a stem are deposited as a third step of manufacturing a MEMS. FIG.
FIG. 7 is a view showing a state in which individual micromirrors are formed as a fourth step of manufacturing a MEMS. FIG.
8 illustrates a data projector employing a DMD according to the present invention.

Applicant has already demonstrated the versatility of polydimethylsiloxane (PDMS) in MEMS devices (e.g., filed on June 20, 2008, each of which is incorporated herein by reference. US patent application Ser. No. 12 / 142,779, filed March 12, 2007, see US patent application Ser. No. 11 / 685,084. In particular, by incorporating PDMS into conventional MEMS manufacturing processes, not only improvements in mechanical inkjet devices but also new areas of lab-on-a-chip devices and microanalysis systems have been made.

It is now known that PDMS may have a much simpler design than the DMDs available on the market, as they have suitable properties for use in DMDs. 1 and 2, a portion of a digital micromirror device according to the present invention is shown. The DMD comprises a plurality of micromirror assemblies 1 arranged in a matrix on the surface of the substrate 2. Typically, each micromirror assembly 1 is spaced less than 5 microns (eg 2 microns) from the adjacent micromirror assembly. The micromirror assembly comprises a mirror 5 spaced apart from the substrate 1. Each mirror is typically square and has a length in the range of about 10-20 microns.

The mirror 5 comprises an aluminum plate 7, which forms the upper reflective surface 8 of the mirror. The mirror 5 further comprises a support platform 10, which forms the lower support surface 11 of the mirror. By the aluminum plate 7 mounted on the support platform 10, the upper reflective surface 8 of the mirror 5 can be formed flat over its entirety. This provides an advantageously good optical definition. In contrast, conventional DMDs typically have an indentation on the reflective surface that couples a support post to the mirror.

Although aluminum is a reflective material typically used in DMDs, it will be appreciated that other metals (eg titanium) may be used instead.

The mirror 5 is supported by an elastic flexible stem 13 that extends from the substrate 2 to the lower support surface 11. Both the stem 13 and the support platform 10 form an integral structure made of the same flexible material. Typically, the stem 13 and the support platform 10 consist of a polymer having a Young's modulus of less than 1000 MPa. Preferred materials for forming the stem 13 are polydimethylsiloxanes having a Young's modulus of about 600 MPa.

The stem 13 forms an oblique axis with respect to the mirror 5. As most clearly seen in FIG. 2, the mirror 5 can be tilted about the tilt axis at an angle of up to about ± 15 degrees, typically ± 7-10 degrees. The flexible flexible stem 13 contrasts with the conventional DMD, whereby the rigid stem is hinged at its base to tilt the mirror.

The stem 13 may be in the form of a support column attached to the center of the mirror 5. Alternatively, the stem 13 may extend at least partially along the tilt axis. Typically, the stem 13 takes the form of a support wall extending along the oblique axis and is coextensive with the mirror 5.

The first electrode 15 and the second electrode 16 are located on either side of the stem 13. The first and second electrodes are individually addressable by electronic circuitry in the silicon substrate 1, which allows the mirror 5 to be tilted by electrostatic attraction. Representative operating relationships of the above DMD will be described in more detail. The electronic circuit is included in the CMOS layer 18 included in the upper portion of the substrate.

As most clearly shown in FIG. 2, the first and second electrodes form landing pads of the mirror 5 when the mirror 5 is tilted. One problem with conventional DMDs is the stiction force between the mirror / yoke and the landing pads. By the fixing force, the mirror can be permanently fixed to one landing pad, resulting in the mirror not working. However, in the micromirror assembly 1, the support platform 10 forms first and second contacts for contacting the landing pads. Since the support platform 10 is advantageously made of PDMS, any fixing force is minimized.

In catching up with a conventional DMD, the DMD of the present invention works most efficiently when the mirror 5 is held at a relatively high potential (for example, 20 to 50 V) by a bias potential. This maximizes the electrostatic force required when either the first electrode or the second electrode is switched on or off by the basic 5V CMOS circuit.

The bias potential may be applied to the aluminum plate 7 via the support stem 13. Although polymeric materials such as PDMS are generally electrically insulated, it is possible to make these materials conductive by implanting metal ions such as titanium ions (eg, Dubois et al, the disclosure of which is incorporated by reference herein). , Sensors and Actuators A, 130-131 (2006), 147-154). Therefore, by the conductive stem 13, the aluminum plate 7 may be maintained at a high bias potential.

Alternatively, the bias potential may be applied to the aluminum plate 7 by connecting the plates together as shown in FIG. 3 and applying a bias potential to the column from a voltage source at one end of the column of the mirror. Adjacent plates 7 are daisy-chained through connecting portions 20 extending along the tilt axis of the mirrors. The connections are located along the tilt axis to minimize its impedance to the mirror tilt.

Although the connections 20 are inevitably subjected to a small torsional force during the mirror tilt, these connections are generally not weakened from this torsional force. This is due to the microscopic scale of the connecting members, which can instantly alleviate any crystal dislocation. Torsional hinges in traditional DMDs do not weaken for the same reason.

Referring now to FIG. 2, the micromirror assembly 1 is shown in an inclined position. To move to the inclined position shown, the first electrode 15 is set to + 5V by the CMOS circuit 18 and the second electrode is set to 0V. Since the aluminum plate is biased at a potential of about +45 V, the mirror 5 is subjected to the electrostatic repulsion force from the first electrode and tilts toward the second electrode. Of course, the mirror 5 will be tilted in the opposite direction by the reaction of the electrode polarities. In order to keep the mirror 5 at its inclined position, both electrodes may be set to + 5V or 0V.

It will be appreciated that during the inclination, the stem 13 is flexed to adjust the inclination of the mirror 5. Therefore, unlike conventional designs, there is no need for any complicated torsional hinge structure to enable the resilient tilting of the mirror.

Referring now to FIGS. 4-7, a simple MEMS manufacturing process for manufacturing the DMD shown in FIG. 1 is shown. 4-7, the CMOS layers 18 are not shown.

In the first step shown in FIG. 4, the electrodes (or landing pads) are formed by depositing and etching a 1 micron layer of aluminum on the CMOS substrate 1, thereby forming separate first and second electrodes. . The aluminum electrodes are connected to the upper metal layer of the basic CMOS so that each electrode is individually controllable.

In the second step shown in FIG. 5, a stem opening 23 is formed by spinning and patterning a photoresist layer 22 on the electrodes. This photoresist layer 22 acts as a sacrificial scaffold for subsequent deposition of PDMS and aluminum.

In the third step shown in Fig. 6, a PDMS layer is deposited on the photoresist layer 22 and then an aluminum layer is deposited. The PDMS layer includes a stem 13 and a support platform 10 of each micromirror assembly. The aluminum layer comprises plates 7 with an upper reflecting surface 8.

In the fourth step shown in Fig. 7, separate mirrors 5 are formed by etching the PDMS layer and the aluminum layer. This etching step uses a suitably patterned photoresist mask (not shown) and may require different chemicals to etch through the other layers.

In the final step, the sacrificial photoresist layer 22 is removed by exposure to an oxide plasma (eg, O ₂ plasma). This final 'ashing' step is to provide the DMD shown in FIG.

8 illustrates a typical data projector 100 (eg, image projector or video projector) employing the DMD as described above. Any data projector incorporating a known DMD may alternatively incorporate a DMD according to the invention. As described in US Pat. No. 6,966,659, the disclosure of which is incorporated herein by reference, the projector includes a printhead for printing an image received from computer system 101. For example, printout may be ejected from the rear of the projector 100 as shown in FIG.

Of course, while the invention has been described by way of example only, it will be appreciated that detailed changes may be made within the spirit of the invention as defined by the appended claims.

Claims (20)

A digital micromirror device comprising an array of micromirror assemblies located on a substrate, each micromirror assembly comprising:
A mirror spaced apart from the substrate and having a top reflective surface and a bottom support surface,
A stem that supports the mirror and extends from the substrate to the lower support surface and forms a tilt axis of the mirror,
A first and a second electrode located on either side of the stem, each individually addressable via an electronic circuit in the substrate,
The stem is made of a resiliently flexible material, such that the mirror can be tilted to either side towards the first electrode or towards the second electrode by electrostatic force. The method of claim 1,
The stem is a digital micromirror device consisting of a polymer. The method of claim 1,
The stem is a digital micromirror device consisting of polydimethylsiloxane (PDMS). The method of claim 1,
And the entirety of the upper reflecting surface is flat. The method of claim 1,
The mirror comprises a metal plate, the metal plate forming an upper reflective surface. The method of claim 5,
The metal plate is an aluminum plate, a digital micromirror device. The method of claim 5,
The mirror further comprises a support platform for the metal plate, wherein the support platform forms the lower support surface. The method of claim 7, wherein
And said support platform is substantially coextensive with said metal plate. The method of claim 7, wherein
And said support platform and said stem are made of the same material. The method of claim 1,
Wherein the first and second electrodes form first and second landing pads relative to the mirror. The method of claim 10,
Said mirror having first and second contact points for contacting respective first and second landing pads, said first and second contacts being made of a polymer. The method of claim 11,
And the support platform forms the first and second contacts. The method of claim 1,
And the mirror is electrically connected to a biasing potential. The method of claim 13,
Said stem is made of a conductive polymer, said stem providing an electrical connection to said bias potential. The method of claim 14,
The stem is a digital micromirror device consisting of polydimethylsiloxane (PDMS) infused with metal ions. The method of claim 13,
A digital micromirror device in which a plurality of mirrors are connected together in rows, each row of which is electrically connected at one end to a bias potential. The method of claim 1,
Each row of mirrors has a common tilt axis. The method of claim 1,
Adjacent mirrors in a row are connected together via a linkage, wherein the linkages are aligned along a common tilt axis. The method of claim 1,
Wherein said substrate is a silicon substrate comprising at least one CMOS layer, said CMOS layer comprising said electronic circuitry. A projector comprising the digital mirror device according to claim 1.

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