Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-12-18
2001-06-05
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S572000
Reexamination Certificate
active
06243194
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of modulation of an incident light beam by the use of an electro-mechanical grating device. More particularly, this invention discloses an electro-mechanical grating device which has a significant improvement to minimize charge trapping by the dielectric materials of the electro-mechanical grating device.
BACKGROUND OF THE INVENTION
Electro-mechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing and printing. Optical beam processing for printing with deformable mirrors has been described by L. J. Hornbeck, see U.S. Pat. No. 4,596,992, “Linear spatial light modulator and printer,” issued on Jun. 24, 1984. A device for optical beam modulation using cantilever mechanical beams has also been disclosed, see U.S. Pat. No. 4,492,435, “Multiple array full width electro-mechanical modulator,” issued on Jan. 8, 1985 to M. E. Banton and U.S. Pat. No. 5,661,593, “Linear electrostatic modulator,” issued on Aug. 26, 1997 to C. D. Engle. Other applications of electro-mechanical gratings include wavelength division multiplexing and spectrometers, see U.S. Pat. No. 5,757,536, “Electrically programmable diffraction grating,” issued on May 26, 1998 to A. J. Ricco et al. Electro-mechanical gratings are well known in the patent literature, see U.S. Pat. No. 4,011,009, “Reflection diffraction grating having a controllable blaze angle,” issued on Mar. 8, 1977 to W. L. Lama et al and U.S. Pat. No. 5,115,344, “Tunable diffraction grating,” issued on May 19, 1992 to J. E. Jaskie. More recently, Bloom et al described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV), see U.S. Pat. No. 5,311,360, “Method and apparatus for modulating a light beam,” issued on May 10, 1994. This device was later described by Bloom et al with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate, 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast, 3) solid supports to fix alternate ribbons, and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces, see U.S. Pat. No. 5,459,610, “Deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate,” issued on Oct. 17, 1995, and U.S. Pat. No. 5,808,797, “Method and apparatus for modulating a light beam,” issued on Sep. 15, 1998. Bloom et al also presented a method for fabricating the device, see U.S. Pat. No. 5,677,783, “Method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate,” issued on Oct. 14, 1997.
Another disclosure in Bloom et al '610 was the use of a patterned ground plane in order to realize two-dimensional arrays. Two embodiments were disclosed: the use of a refractory metal on an insulated substrate and selective doping of a semiconducting substrate to create a p-n junction. The purpose of that invention was to create an array of ground electrodes corresponding to the array of grating elements to enable two-dimensional addressing, as opposed to allowing two different voltage levels to be applied below the ribbon elements. J. G. Bornstein et al also disclosed the use of a patterned ground plane, using a patterned refractory metal on an insulator, in order to address a two-dimensional grating element array in U.S. Pat. No. 5,661,592 entitled “Method of making and an apparatus for a flat diffraction grating light valve,” issued on Aug. 26, 1997.
According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. By modulating the voltage waveform, it is possible to modulate the diffracted optical beam as needed by the specific application. However, a single polarity voltage waveform can lead to device operation difficulties if leakage or injection of charge occurs into the intermediate dielectric layers between the ground plane and the conductor on the ribbons.
One method to alleviate this problem is to provide an alternating voltage to the ribbons. A DC-free waveform produces nearly the same temporal modulation of the diffracted optical beam as the corresponding single polarity waveform while minimizing charge accumulation in the dielectric layers. Stable device operation is thus achieved. However, this complicates the driving circuitry requiring bipolar rather than unipolar driving capability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a electro-mechanical grating device which avoids leakage or injection of charge into dielectric layers of the electro-mechanical grating device. Furthermore, the electro-mechanical grating device has to provide a layer structure, which, according to the application of a unique voltage, produces a DC-free result when using a unipolar oscillating drive voltage.
The object is achieved with an electro-mechanical grating device comprising:
a base having a surface;
a spacer layer provided above the base, said spacer layer defining an upper surface and a longitudinal channel is formed in said spacer layer, said channel having a first and a second opposing side wall and a bottom, said side walls being substantially vertically disposed with respect to the bottom, and said channel having a constant cross section along the entire length of the mechanical grating device;
a bottom conductive layer provided within said base wherein said bottom conductive layer is limited essentially to the cross-section of the channel; and
a plurality of spaced apart deformable ribbon elements disposed parallel to each other and spanning the channel, said deformable ribbon elements are fixed to the upper surface of the spacer layer on each side of the channel and each deformable ribbon element is provided with at least one conductive layer.
An advantage of the electro-mechanical grating device is to provide a bottom conductive layer below the ribbon elements of an electro-mechanical grating device that is isolated electrically from ground planes associated with the substrate. The bottom conductive layer below the ribbon elements is used to apply a unique voltage that along with the actuation voltage applied to the ribbon elements, dictates the actuation of the ribbon elements. The bottom conductive layer can be patterned in order to define separate regions within the length of the electro-mechanical grating device and allows for independent control of the ribbons within each region. The substrate or associated ground planes is at a ground reference voltage. The ground plane is screened from the ribbon elements by the bottom conductive layer, and thus has no effect on the actuation of the ribbon elements. The purpose of the ground plane is to provide a voltage reference for microelectronic driver circuitry that may be integrated onto the substrate.
The advantage of this invention is that it allows the ribbon elements to be driven in a manner that reduces charge injection into the dielectric ribbon material using standard CMOS microelectronics integrated onto the substrate. The voltage that is supplied to the ribbon elements from the CMOS circuitry is unipolar with respect to the ground reference voltage. However, with a proper voltage applied to the bottom conductive layer, a unipolar oscillating drive voltage applied to the ribbon elements reduces the charge injection into the ribbon elements. The average of the oscillating drive voltage function is selected to be the same as the voltage applied to the bottom conductive layer to yie
Brazas, Jr. John C.
Kowarz Marek W.
Kruschwitz Brian E.
Eastman Kodak Company
Epps Georgia
Noval William F.
Seyrafi Saeed
Shaw Stephen H.
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