Patent Grant
7359106
The present invention relates generally to spatial light modulators, and more particularly to diffractive spatial light modulators and methods for manufacturing and operating the same.
Spatial light modulators (SLMs) are devices or arrays of one or more devices that can control or modulate an incident beam of light in a spatial pattern that corresponds to an electrical input to the devices. The incident light beam can be modulated in intensity, phase, polarization or direction.
Some modulation can be accomplished through the use of Micro-Electromechanical System devices or MEMS that use electrical signals to move micromechanical structures to modulate light incident thereon. MEMS-based optical modulator devices generally require that pixels have flat optical surfaces that can be either tilted or displaced (vertical piston for example) in order to modulate light.
In one aspect, the present disclosure is directed to a diffractive modulator or diffractor for modulating light incident thereon. Generally, the diffractive modulator includes a substrate having an upper surface, a membrane disposed above the upper surface of the substrate and in spaced apart relation thereto, the membrane including a continuously deformable light reflective surface formed on its upper side facing away from the upper surface of the substrate, and circuitry for controllably deflecting the deformable surface relative to the upper surface of the substrate. The circuitry for controllably deflecting the deformable surface may include, for example, circuitry for applying an electrostatic force between the substrate and the deformable surface.
In accordance with the present disclosure, light reflected from different points or areas of the deformable surface interfere to modulate light reflected from the diffractive modulator in 0th order applications.
In one embodiment, the membrane further includes a static light reflective surface surrounding the deformable surface. Light reflected from the static surface and the deformable surface can interfere to modulate light reflected from the diffractive modulator in non-0th order applications. In one version of this embodiment, the membrane is supported above the upper surface of the substrate by a wall extending substantially entirely around the deformable surface to define a cavity there beneath, and the static surface is formed on a portion of the membrane abutting a top surface of the wall.
Preferably, the static surface and the deformable surface are sized and shaped to define substantially equal areas. More preferably, the static surface is substantially planar.
In one embodiment, the static surface circumscribes the deformable surface to define a parabolic reflector.
In another embodiment, the deformable surface includes a plurality of ring shaped regions concentric with and separated by a plurality of ring shaped regions of the static surface to form a Fresnel mirror.
In yet another embodiment, the deformable surface includes a plurality of rectangular shaped regions parallel with and separated by a plurality of rectangular shaped regions of the static surface to form a linear array.
Optionally, the static surface includes a raised portion raised above a reference plane of the deformable surface in a quiescent state. The two surfaces are separated by a distance, d, between 0 and n*λ/4, where λ is a wavelength of the light incident on the diffractive modulator, and n is an integer equal to or greater than 1. In one version of this embodiment, d is selected to provide a separation between the raised portion of the static light reflective surface and the plane of the deformable surface in a quiescent state such that the diffractive modulator is in a substantially “OFF” state when the deformable surface is not deflected. Alternatively, d is selected to provide a separation between the raised portion of the static light reflective surface and the plane of the deformable surface in a quiescent state such that the diffractive modulator is in a substantially “ON” state when the deformable surface is not deflected.
In yet another embodiment, the static surface further includes a 0° phase surface between the raised portion of the static light reflective surface and the deformable surface, the 0° phase surface substantially co-planer with the plane of the deformable surface in a quiescent state.
It may be appreciated that the diffractive modulators of the present invention is particularly useful in a spatial light modulator (SLM) including an array of a number of such diffractive modulators, where the SLM further includes a number of pixels, each pixel including at least one diffractive modulators.
In another aspect, the present disclosure is also directed to a method of fabricating diffractive modulator such as that described above. Generally, the method includes the steps of: (i) depositing a sacrificial layer onto an upper surface of an electrically active substrate; (ii) forming a membrane on the sacrificial layer by depositing a layer of elastic material thereon; (iii) forming an etch hole extending through the membrane from an upper surface thereof to the lower surface; (iv) removing a portion of the sacrificial layer through the etch hole to partially release the membrane from the substrate and define a cavity there beneath, thereby forming a continuously deformable portion of the membrane disposed above the upper surface of the substrate and in spaced apart relation thereto; (v) depositing a reflective material on an upper surface of the membrane, the reflective material substantially filling and closing the etch hole; and (vi) planarizing and polishing the reflective material to form a continuously deformable light reflective surface formed on the upper surface of the continuously deformable portion of the membrane facing away from the upper surface of the substrate.
Preferably, the elastic material is an electrically insulating material and the reflective material is a conductive material, and the step of depositing the reflective material on the upper surface of the membrane further includes the step of forming a membrane electrode.
More preferably, the sacrificial layer includes polysilicon, and the step of removing a portion of the sacrificial layer through the etch hole uses an etchant gas comprising Xenon Difluoride (XeF2) for a predetermined time to tailor a size of the cavity and the continuously deformable portion of the membrane.
In one embodiment, the step of planarizing and polishing the reflective material further includes the step of forming a static light reflective surface surrounding the continuously deformable light reflective surface. Optionally, the step can further involve forming a static light reflective surface having a raised portion circumscribing the continuously deformable light reflective surface. As noted above, the raised portion is raised above a reference plane which is the plane of the continuously deformable light reflective surface in a quiescent state, and separated therefrom by a distance, d, between 0 and n*λ/4, where λ is a wavelength of the light incident on the diffractive modulator, where n is an integer equal to or greater than 1.
In yet another version of this embodiment, the step of forming a static light reflective surface further includes the step of forming a 0° phase surface between the raised portion of the static light reflective surface and the continuously deformable light reflective surface, the 0° phase surface substantially co-planer with the plane of the continuously deformable light reflective surface in a quiescent state.
These and various other features and advantages of the present invention may be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where:
Basic Theory
The basic theory of a reflective, diffractive modulator having a non-planar area or cell 102 is now described with reference to
In general, each differential element, dA, of this area 102 may be deflected some distance, d, (either static, or through electrostatic deflection) 108 from a planar reference surface 104. Each element dA may propagate an electric field proportional to the square root of the incident intensity and the surface reflectivity.
Assuming uniform illumination and uniform reflectivity, each element dA would propagate the same magnitude of electric field, but the reflected phases (i.e. the phases of light reflected from the non-planar area 102) may differ depending upon the difference in reflected path length. Thus, the electric field 106 from each elemental area 102 may be written as shown in Equation 1 (in the far field, for normal incidence, and for the 0th order reflection).
d{tilde over (E)}=|dE|·ejφ∝√{square root over (I·R)}·dA·ej(4πd/λ) (Equation 1)
The total specular reflection electric field component (0th order diffraction) may be calculated for area A, by integrating over that area. Ignoring constants such as the uniform intensity and reflectivity, the following equation results.
{tilde over (E)}A≈∫ej(4πd/λ)dA (Equation 2)
In other words, the total complex electric field is calculated by integrating the phase factor over the area. The intensity is required in order to determine the efficiency of periodic structure, and the intensity is proportional to the magnitude of the square of the total electric field, as shown below in Equation 3.
IA∝|{tilde over (E)}A|2≈|∫ej(4πd/λ)dA|2 (Equation 3)
Ribbon Light Modulators
One type of SLM is a ribbon light modulator, such as a Grating Light Valve (GLV™) which is commercially available from Silicon Light Machines Corporation, of Sunnyvale, Calif. Referring to
Generally, the light reflective surfaces of each ribbon 204 are planar surfaces of equal area, A. One surface (AS) 204-S is static, typically taken to be located a the reference plane (i.e., 0° phase), while the other “active” surface (AD) 204-D is electrostatically deflected by a variable distance “d” 208 towards the substrate 202 by integrated drive electronics formed in or on the surface of the substrate 202.
The integrals in Equations 2 and 3 become summations for two planar surfaces. Hence, the total specular reflection electric field from the two ribbons becomes as follows.
{tilde over (E)}A≈ES+ED·ej(4πd/λ) (Equation 4)
where ES 206-S represents the amplitude from the static ribbon surface, and ED 206-D represents the amplitude from the deflected ribbon surface.
With equal areas for static and deflected surfaces, equal reflectivity, and uniform illumination, then the amplitudes ES and ED become equal (and may be referred to as simply E). The reflected intensity from the total area of the two ribbons may then be written as:
IA∝2E2+2E2·cos(4πd/λ) (Equation 5)
The operation of the GLV™ may best be understood by considering phasor diagrams, shown in
Referring to
Usually, the device is operated from no deflection, up to a deflection equal to a quarter wavelength of light, ¼. For no deflection 306, all surfaces are in phase, and the electric field components add—this is the “ON” state of a device (operating in 0th order). For ¼ deflection 308, the phase difference becomes 180°, and the two electric field components cancel completely.
Note that the ideal GLV™ device described here is assumed to be “perfect”. That is, the device has an “ON” state which reflects all incident light (high efficiency), and an “OFF” state that completely extinguishes the 0th order (high contrast). Of course, actual devices are not perfect.
Limitations and Drawbacks to Ribbon Light Modulators
Although, an improvement over previous generations of SLMs alone, the above ribbon light modulator or GLV™ is not wholly satisfactory for many applications requiring high or very high resolution, such as leading edge semiconductor processing, for a number of reasons.
Fundamentally, conventional ribbon light modulators inherently provide a low optical efficiency since only a substantially flat, planar portion away from supporting regions can be used to modulate light as described above. Moreover, since even the central portions of the ribbons are not truly planar, contrast achievable using conventional ribbon-type SLMs is compromised or reduced.
Finally and more significantly, the flatness restriction creates significant complexity for the MEMS device design and in particular, its fabrication. Stress must be controlled in suspended films in order that they remain flat, and mechanical actuators must be buried and polished flat. This additional complexity causes decreased yields, increased cost, limited array sizes and potential reliability issues.
Spatial Light Modulator Having Continuously Deformable Optical Surface
The present disclosure is directed to a spatial light modulator (SLM) having a continuously deformable optical surface and a method of manufacturing and operating the same. The continuously deformable optical surface may be advantageously utilized to overcome the above-described limitations and drawbacks.
Spatial light modulators having a continuously deformable surface according to the present invention is now be described with reference to
In accordance with an embodiment of the invention, a non-planar deforming membrane that may be deformed or deflected to form a parabolic revolution is now described in detail with reference to
When undeflected, the deformable surface 405 lies in a reference (zero degrees) plane 408. The means for deflecting the deformable surface 405 may include, for example, circuitry for applying an electrostatic force between the substrate 402 and the circular membrane 404. Light reflected from different points or areas of the deformable surface 405 may interfere to modulate light reflected from the diffractive modulator in 0th order applications.
A functional diagram showing operation of a diffractive modulator according to an embodiment of the present invention is shown in
Calculating the net reflected electric field (the amplitude of reflection 604) from such a structure results in electric field phasors as shown in the phasor diagram 606 of
As the membrane is deflected, a range of phases is produced from various parts of the parabola, and the net sum reduces the magnitude, and changes the net phase. When the center of the parabola is deflected to λ/4, the net phase has changed by 90° (along the negative imaginary axis) and the electric field magnitude is reduced to 64% of maximum. Coincidentally, as the center deflection is increased to λ/2, the amplitude reduces to zero.
A good diffractive modulator (operating in 0th order) requires a state where much of the electric field is reflected in phase with maximum amplitude, and also requires a state where the 0th order light is nulled out, as can be shown by the vector summation. As seen in
An alternative version or embodiment is shown in
It may be appreciated that various properties of the static reflector 704, including separation between the static reflector 704 and the deformable surface 702 may be tailored in order to maximize the performance of the optical modulator.
In one embodiment the static reflector 704 is separated vertically or in the direction of incidence of the light from the deformable membrane 702 by a distance or height of between 0 and λ/4, where λ is the wavelength of light incident on the modulator.
Regarding the null state 904, for each single unit of electric field ED (or area) from the circular membrane 702, then 0.64 units of electric field ES (or area of the static planar surface 704, given same reflectivity per unit area as the membrane 702) is needed at 90° phase shift (i.e. at +λ/8 thickness) at the static surface 704 in order to create the null state 904.
Regarding the ON state 906, the maximum electric field EMAX may be calculated to be 1.187 units of field. This maximum reflective state is achieved when the membrane 702 is undeflected. If a similar-sized reflective area (i.e. the entire square) was entirely in phase, then a 1.64 unit electric field would be produced. Thus, since intensity is proportional to the square of the electric field, the diffraction efficiency in 0th order is calculated to be (1.187/1.64)2=52% of maximum efficiency. Hence, this embodiment has excellent contrast, and approximately half of the theoretical efficiency.
The relative size of the square and the circular membrane can also be calculated and predetermined to maximize the performance of the optical modulator. The ratio of areas in this example is preferably about 0.64:1 (static area:deflecting area) so as to generate the “off” state 904. Thus, if the square cell shown in
In another embodiment the static reflector is separated from the deformable membrane by a distance or height of λ/4.
This embodiment is similar to that described above, except the static layer or reflector 704 thickness is now λ/4. In this case, the null state 1004 is designed to match the zero deflection condition. For this embodiment, the required static field is of −1 units (i.e., a thickness of λ/4), as shown in the phasor diagram 1002.
The maximum electric field summation for the “on” state 1006 occurs at ED of ˜(−0.2, −0.4i) units. The maximum electric field EMAX is consequently 1.265 units. This compares with a total potential field (for all surfaces in phase) of 2.0 units. Squaring these numbers gives the theoretical efficiency of (1.265/2.0)=40%.
The ratio of areas in this embodiment is preferably 1:1 (static area:deflecting area) so as to be able to generate the “off” state 1004. If the square cell shown in
In yet another embodiment depicted in
The maximum electric field EMAX is 1.25 units for the undeflected “on” state 1106. For a center deflection of ˜0.35λ, the electric field components are equal and opposite to create the “off” state 1104, giving the required contrast.
The maximum possible electric field for all in phase components is (1.0+0.35)=1.35. Thus the theoretical diffraction efficiency for 0th order is (1.25/1.35)2=86%. The ratio of areas in case 3 must be 0.35:1 (static:deflecting). If the square cell shown in
In still another embodiment, shown in
The previous example, the embodiment described in connection with
A top (plan) view of such a modulator and an array 1302 of these modulators is shown in
A single modulator is shown on the left side of
An array 1302 of these modulators is shown on the right side of
The same magnitude and phase for the static surface may be obtained using the 2-stage static reflector of
One preferred embodiment of a SLM having an array of diffractive modulators according to the present invention is shown in
An embodiment of a method or process for fabricating a diffractive modulator according to the present invention is now described with reference to
As shown in the upper left (#1) of
As shown in the upper right (#2) of
As shown in the lower left (#3) of
As shown in the lower right (#4) of
Optionally, the method may include additional steps to form a 90° phase surface raised above the plane of the continuously deformable light reflective surface, or two stage static reflector.
The advantages of the diffractive modulator of the present invention over previous or conventional approaches include: (i) elimination of film bowing—because there are no free edges, the tensile film of the membrane generally pulls flat when no electrostatic force is present; (ii) simpler manufacturing—no double sacrificial and/or release layers needed; (iii) multiple layers may easily be incorporated, enabling multi-layer dielectric mirrors; (iv) enabling use of enhanced reflectivity layers (dielectric overcoats); (v) static phasing may be used as a transparent overcoat such as SiN, or SiO2. or other “hard” coating; and (vi) thickness bending resistance may be used instead of tension-dominated membranes.
While the above diffractive modulators have been described in detail as having a parabolic shape, it will be understood by those skilled in the art that many other alternative embodiments and configurations are possible without departing from the spirit and scope of the claimed invention. Although the parabolic shape is a good approximation to membrane deformation in many cases, this work is valid beyond this specific example. In general any deflecting surface can be experimentally characterized to extract a locus of reflected electric field vectors. This extracted data can be used to design an efficient optical modulator in the same manner as has been illustrated herein.
For example, the membrane or diaphragm can further include corrugations to lower tensile stress. These corrugations can fit into the dynamic, deformable surface of the membrane, the static reflector or both. Preferably, the corrugations are formed in a lower surface below the reflective surface.
A number of other alternative embodiments for diffractors or SLM according to the present invention are shown in
The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.
The present application claims the benefit of U.S. provisional application No. 60/611,591, entitled “Continuously Deformable Surfaces for Diffractive Light Modulators,” filed Sep. 21, 2004, by inventors David T. Amm, Alexander P. Payne, and James A. Hunter. The disclosure of the aforementioned U.S. provisional application is hereby incorporated by reference in its entirety.
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| Number | Date | Country | |
|---|---|---|---|
| 60611591 | Sep 2004 | US |
