Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-08-20
2001-07-10
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C359S291000
Reexamination Certificate
active
06259550
ABSTRACT:
The present invention relates to very large scale integrated spatial light modulators for light valves, and in particular to phase-modulating structures for the phase-modulation of light incident upon the structures.
Spatial light modulators, which are employed for light valves, have been in use for some time in connection with so-called schlieren imaging systems for applications in projectors or in the direct exposure of semiconductor wafers. Electronic image information is converted to corresponding phase modulations of a ray of light, which are produced by means of the spatial light modulators. The schlieren imaging system then converts the phase modulations of the ray of light into light intensity variations on an observation plane, for example, by blocking the non-modulated light and allowing passage of the light incident on modulated portions of the spatial light modulator. The mode in which the modulated light reaches the observation plane is referred to as positive mode. When the schlieren imaging system is arranged such that only the non-modulated light reaches the observation plane and the modulated light is blocked, the entire exposure arrangement operates in the so-called negative mode.
For better understanding of the field of application of the present invention, a schlieren imaging system, making use of a known spatial light modulator, shall be discussed briefly in the following. This known exposure means is disclosed in WO 91/17483.
A light source, which often is in the form of a laser, transmits light via a ray expansion optical system and a focusing optical system to a bar mirror arrangement reflecting the light through a schlieren lens means onto a spatial light modulator. Depending on whether or not a picture element of the spatial light modulator is addressed, light reflected from the spatial light modulator is either reflected through the schlieren lens means onto the bar mirror arrangement again, or the light passes the same and reaches a projection lens means which then forms an image of this light on an observation plane. In this observation plane there may be disposed, for example, a wafer to be exposed.
As was already noted hereinbefore, the entire schlieren imaging system can form an image of that light on the observation plane that was modulated by the spatial light modulator, i.e. corresponding to the positive mode, or the optical system can form an image of that light on the observation plane that was not modulated by the spatial light modulator, which corresponds to the negative mode, with the modulated light being blocked by the observation plane and reflected back to the light source, for example.
DESCRIPTION OF THE PRIOR ART
The exposure device described in WO 91/17483 makes use of a spatial light modulator comprising a viscoelastic control layer terminating, in the direction towards the schlieren lens means, by a reflecting surface that may be a metal film, for example. The spatial light modulator furthermore comprises a so-called active addressing matrix that may be formed of a monolithic integrated arrangement of MOS transistors, which is also referred to as active CMOS matrix, having associated control electrode pairs. Each picture element or surface portion of the reflecting surface of the light modulator has associated therewith two transistors having one or more electrode pairs, which each form a diffraction grating with one or more grating periods with the viscoelastic layer and its reflecting surface.
As described in the article entitled “Deformation behavior of thin viscoelastic layers used in active-matrix-addressed spatial light modulator, SPIE, vol. 1018, Electro-Optic and Magneto-Optic Materials” (1988), voltages of at least about ±10 volt are necessary for deformation amplitudes of the visco-elastic control layer in the range of 0.1 &mgr;m. The transistors of the active matrix thus have to withstand at least a peak-to-peak voltage of 20 V or more. Many conventional MOS components, however, have a maximum operating voltage of only 12 V. It is therefore not possible to use for such light modulators conventional, inexpensive CMOS components. Instead, known light modulators having a viscoelastic layer require specifically doped transistors for obtaining sufficient breakdown voltages.
U.S. Pat. No. 4,728,185 discloses a spatial light modulator the reflecting surface of which consists of a multiplicity of electrically addressable, micromechanical lever bars.
The document JP-A-4 350 819 discloses a variable-phase plate producing a phase difference with good reproducibility and good controllability. The variable-phase plate comprises an inner dielectric in the form of a layer, which is mounted in a correspondingly sized hole in an outer dielectric by means of an elastic material. Both major surfaces of the inner dielectric are coated with a first and a second electrode, respectively, and the two major surfaces of the outer dielectric are also coated with a first and a second electrode, respectively. The electrodes are light-transmitting conductor layers to which a potential can be applied each. When a specific differential voltage is applied to the electrodes mounted on the surfaces of the inner dielectric, the thickness of the inner dielectric is changed. When no or a different voltage difference is applied to the electrodes mounted on the surfaces of the outer dielectric, the phase of the light passing through the inner dielectric will be different from the phase of the light passing through the outer dielectric. The elastic material connecting the inner dielectric to the outer dielectric has the function of permitting different thicknesses of the inner dielectric and the outer dielectric.
DE-AS 12 91 416, on which the generic clause of patent claim
1
is based, teaches an optical phase modulator comprising a first, reflecting electrode applied to a registration strip. Disposed above the first electrode is a piezoelectric layer having a second, transparent electrode on its surface facing away from the first electrode, with said second electrode in turn being covered by a transparent substrate in the direction towards light incidence. By applying a voltage to different parts of the first electrode by means of the registration strip, the thicknesses of the portions of the piezoelectric layer are altered in corresponding manner. Light passing through a portion of the piezoelectric layer in which a voltage is applied, and reflected from the first electrode and passing back through said portion of the piezoelectric layer, has a phase difference from light passing through a different portion of the piezoelectric layer to which a different voltage is applied, and reflected from the first electrode and passing back through said different portion of the piezoelectric layer, since the thicknesses of the portions of the piezoelectric layer having different voltages applied thereto are different from each other.
U.S. Pat. No. 4,660,938 discloses a light valve having a fine diffraction grating formed on a transparent electrode, with the depth of grooves of the diffraction grating being such that the optical path length of light of a specific wavelength, which passes between the spaces between the individual diffraction grating bars, constitutes an odd-number difference with respect to half the wavelength of the light passing through the diffracting grating. Opposite the transparent electrode, there is disposed an additional transparent electrode, and in the space between the two electrode there are both air and a transparent liquid. When a voltage is applied to the two electrodes, an electric field is generated attracting the liquid into the spaces between the diffraction grating bars, thereby changing the optical path length of light passing through these spaces. Since the initial difference of the optical path length between light passing through the diffraction grating bars and light passing through the spaces therebetween is altered, there is no longer a cancellation of adjacent light rays, so that light can pass through the diffracti
Gottfried-Gottfried Ralf
Kuck Heinz
Kunze Detlef
Epps Georgia
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Glenn Michael A.
Thompson Timothy
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