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Patent Issued for Method for Operating a Micromirror Device with Electromechanical Pulse Width Modulation

August 20, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors De Smet, Herbert (Destelbergen, BE); Beernaert, Roel (Gent, BE), filed on October 4, 2011, was published online on August 5, 2014.

The assignee for this patent, patent number 8797629, is IMEC (Leuven, BE).

Reporters obtained the following quote from the background information supplied by the inventors: "The disclosed technology relates to a micromirror device and in particular to a method for operating such a micromirror device.

"Micromirrors are microelectromechanical systems (MEMS) that can be used in several applications, ranging from scanning mirrors (optical scanning, optical switching) to projection displays.

"For example, the digital micromirror device (DMD), described by L. J. Hornbeck in 'Digital Light Processing and MEMS: Timely Convergence for a Bright Future', Proc. SPIE, Vol. 2639, p. 2, 1995, comprises a micromirror array used as a spatial light modulator (SLM) in projection displays. The DMD comprises an array of light switches that use electrostatically controlled MEMS mirrors to modulate light digitally, thereby producing images on a screen.

"The mirrors, with a one-to-one relationship to the pixels of the display, are arranged in a rectangular array. They can rotate between two extreme positions depending on the state of an underlying memory cell, and thus reflect incoming light into a lens (ON state) or not into the lens (OFF state).

"The ON state corresponds to a pixel on the screen that is illuminated ('white' pixel) and the OFF state corresponds to a dark pixel ('black' pixel) on the screen.

"For producing the sensation of grayscale to the observer's eye, binary pulse width modulation (PWM) is used. Video frames are divided into n sub-frames. During every sub-frame, a mirror is either in the ON state (white) or in the OFF state (black). Assuming a light source with constant intensity, the ratio of ON and OFF states within a frame then determines the gray level of the pixel for that frame.

"Using this method, the number and the distribution of gray levels depends on the number of binary sub-frames or bitplanes. With n sub-frames or bitplanes this method gives rise to (n+1) linear gray levels. Digital Pulse width modulation may lead to severe speed requirements (data transfer rates) for the on-chip electronics and for complete elimination of contouring effects.

"In U.S. Pat. No. 6,466,358 an analog pulse width modulation (PWM) method is described that can be used for addressing a digital micromirror array. This method solves some of the problems related to binary pulse width modulation, such as the high cost in terms of data transfer rates and the hardware needed to sample and process the image data.

"In the method described in U.S. Pat. No. 6,466,358, the voltage signal applied to the micromirror addressing electrodes results from a comparison between analog input signals. This comparison is done by means of a transistor circuit in the CMOS layer, i.e. this analog PWM occurs at the electronic level. For each pixel, there is a need for at least six transistors that can withstand large voltages, leading to relatively large chip area consumption.

"Furthermore, as the method is based on a comparison between analog voltages, switching of a micromirror depends on a transistor threshold voltage. It may be difficult to control this threshold voltage accurately, and furthermore the threshold voltage may vary on a chip and thus it may be different from pixel to pixel. This may cause fixed pattern noise.

"U.S. Pat. No. 5,583,688 discloses a digital micromirror device, wherein a mirror is supported by a center support post attached to two torsion hinges by a landing hinge yoke. The ends of the torsion hinges are attached to two support posts which hold the hinges above the substrate and allow the hinges to twist in a torsional fashion.

"US2006/109539 discloses a method for driving an optical deflecting device array, which arranges a plurality of optical deflecting mirrors. In the disclosed method, the mirror position is controlled by several sets of electrodes, each electrode corresponding to one possible mirror position."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "Certain inventive aspects relate to a method for controlling a micromirror device that does not present the drawbacks of prior art methods.

"More particularly, certain inventive aspects relate to a micromirror device comprising at least one micromirror that can be deflected electrostatically, and a method for operating such micromirror device, wherein gray levels can be produced by means of an analog pulse width modulation method without the need for providing an electronic comparator circuit, i.e. without the need for providing additional transistors in the CMOS layer below the micromirror structure.

"It is an advantage of certain inventive aspects that fixed pattern noise resulting from differences in transistor threshold voltages, as may be the case in prior art solutions, can be avoided.

"It is an advantage of certain inventive aspects that the number of transistors needed per pixel and thus the chip area needed per pixel is substantially smaller than in prior art solutions.

"Certain inventive aspects relate to a micromirror device that can be used as a light switch or a spatial light modulator, e.g. in a projection display.

"Certain inventive aspects relate to a method for operating a micromirror device by pulse width modulation (PWM) providing an amount of grey levels not depending on the number of subframes or bitplanes. The levels can be chosen arbitrarily, allowing less severe speed requirements for the electronic layer below the MEMS, less image processing hardware and memory.

"Certain inventive aspects relate to a method for operating by pulse width modulation a micromirror device comprising the steps of: providing a micromirror device comprising at least one micromirror element being electrostatically deflectable around a rotation axis between at least two positions being a first position and a second position, by applying voltage signals to at least four electrodes controlling the micromirror element, the first and second electrodes being located on one side of the rotation axis, and the third and fourth electrodes on the other side; associating an intermediate value of intensity to the micromirror element during a time frame, the intensity being comprised between a first value and a second value, the first value corresponding to the first position and the second value corresponding to the second position; switching the micromirror element between the first position and the second position and vice-versa so that the micromirror element is either in the first position or in the second position whereby the intermediate value of intensity between the first value and the second value is obtained, the intermediate value of intensity corresponding to the ratio of the periods of time in a time frame in which the micromirror element is either in the first position or in the second position; wherein the switching is obtained by applying fixed voltage signals to the second and third electrodes during the time frame, and periodic voltage signals having a period equal to the length of the time frame to the first and fourth electrodes.

"The intensity or intensity value is the measurable amount of a property, such as brightness, light intensity, gray level or colored level.

"A time frame is the shortest period of time on which an intermediate intensity is defined. This usually corresponds to one individual picture time in motion picture (e.g. 1/50 s in television PAL or SECAM standards, or 1/24 s in film), or eventually corresponds to one individual color picture (i.e. red, green or blue picture in RGB) in case that individual colors are produced sequentially on the same micromirror device ( 1/150 s in PAL or SECAM, 1/72 s in film).

"In one aspect, the method further discloses at least one or a suitable combination of the following features: the periodic signals are characterized by a monotonic variation in a first half of their period, and a monotonic variation in a second half of their period; the periodic voltage signals corresponds to voltage differences that are directly applied between the micromirror element and the first and fourth electrodes while the second and third fixed voltage signals corresponds to voltage differences that are applied between the micromirror element and the second and third electrodes; the first and fourth voltage signals are antiphase signals; the periodic voltage signals are in the form of a triangular waveform, a saw-tooth waveform, gamma corrected triangular waveform or sinusoidal waveform signal; the first value of intensity corresponds to a white pixel while the second value of intensity corresponds to a black pixel with intermediate value of intensity corresponding to gray levels in between; and the first value of intensity corresponds to a colored status while the second value of intensity corresponds to a non colored status with intermediate colored levels in between.

"Another inventive aspect relates to a micromirror device comprising: at least one micromirror, each micromirror being able to rotate along an axis parallel to the micromirror from a first position to a second position; a substrate underneath the micromirror; at least four controlling electrodes for each micromirror, being a first and a second set of two controlling electrodes, each of the set having electrodes located on each sides of the rotation axis of each micromirror, wherein each electrode of the second set of electrodes is connected to a circuit able to keep fixed analog voltage signal during half a time frame.

"In one aspect, the device further discloses at least one or a suitable combination of the following features: the circuit connected to each electrode of the second set of electrodes comprises a storage capacitor able to keep a fixed analog voltage during a time frame; the circuit connected to each electrode of the second set of electrodes comprises a MOSFET switch; the first set of electrodes comprises two subsets of electrodes, each subset comprising one electrode corresponding to each micromirror, the electrodes within each subset being connected to a circuit arranged to provide the same signal to the electrodes; and the electrodes within each subset are connected in parallel, alternatively, they can be connected in series on a low resistive circuit, as far as the signal variation on the serial circuit is acceptable.

"In one aspect, the device is suitable for being operated by the method as described herein.

"In a further aspect, there is a spatial light modulator comprising a micromirror device described herein."

For more information, see this patent: De Smet, Herbert; Beernaert, Roel. Method for Operating a Micromirror Device with Electromechanical Pulse Width Modulation. U.S. Patent Number 8797629, filed October 4, 2011, and published online on August 5, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8797629.PN.&OS=PN/8797629RS=PN/8797629

Keywords for this news article include: IMEC.

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Source: Journal of Engineering


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