US20120082943A1

US20120082943A1 - Diffractive photo masks and methods of using and fabricating the same

Info

Publication number: US20120082943A1
Application number: US13/250,011
Authority: US
Inventors: Guy Matthew Burrow; Thomas K. Gaylord
Original assignee: Georgia Tech Research Corp
Current assignee: Georgia Tech Research Corp
Priority date: 2010-09-30
Filing date: 2011-09-30
Publication date: 2012-04-05
Also published as: US20120081687A1; US9019468B2

Abstract

An exemplary embodiment of the present invention provides a diffractive photo-mask comprising a body element, a plurality of interference/image-forming hologram gratings, and a zero-order beam blocking element. The plurality of interference/image-forming hologram gratings can be located on or in the body element. The plurality of interference/image-forming hologram gratings are configured to diffract a light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane when the light beam is incident upon the diffractive photo-mask. The zero-order beam blocking element is configured to block zero-order diffracted beams when the light beam is incident upon the diffractive photo-mask

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 61/388,365, filed on 30 Sep. 2010, and U.S. Provisional Application Ser. No. 61/388,367, filed on 30 Sep. 2010, which are incorporated herein by reference in their entireties as if fully set forth below.

FEDERALLY SPONSORED RESEARCH STATEMENT

The invention described in this patent application was made with Government support under Agreement No. ECCS-0925119, awarded by The National Science Foundation. The Government has certain rights in the invention described in this patent application.

TECHNICAL FIELD OF THE INVENTION

The various embodiments of the present application relate generally to diffractive photo-masks and methods of fabricating and using diffractive photo-masks. More particularly, various embodiments of the present invention are directed to diffractive photo-masks that produce a functional-element optical-intensity distribution when beams of light are directed incident to the photo-masks.

BACKGROUND OF THE INVENTION

Microelectronics continues to impact society in a multitude of ways every day. For example, the semiconductor revolution is the engine that drives cell phones, the internet, flat-panel televisions, flash memory chips, global positioning system devices, solar cells, etc. Microelectronics has also had a profound impact on the fields of biomedicine, transportation, communications, entertainment, defense, environmental monitoring, and homeland security.
For more than three decades optical lithography has been the recipe for success to meet the semiconductor industry's steady decrease in device size as described by Moore's law. In 1965, Gordon Moore, the co-founder of Intel, predicted that the number of transistors in a commercial integrated circuit would double every two years. Up until now, this prediction has accurately stood the test of time. However, since the second decade of the twenty-first century there has been serious doubt that conventional optical lithography can continue to provide the necessary decreasing sizes. Efforts to decrease the wavelength of the source (e.g. from 192 nm to 157 nm) have not been successful. Increasing the surrounding refractive index in immersion lithography (e.g. from 1.44 to 1.65) remains in research and development. Thus, new approaches are needed. Techniques being considered include 1) self-assembly approaches; 2) construction-based approaches including immersion lithography, double patterning, double exposure, two-photon lithography, printing, direct writing, source mask optimization, and micromanipulation; and 3) interference lithography.
Modern integrated circuits have very regular layouts with an underlying grid pattern that defines the smallest feature size in the integrated circuit. Multi-beam interference lithography (“MBIL”) can be used to define this underlying grid. MBIL immediately has the advantages of 1) simple optics, 2) large working distances, 3) high-speed processing, 4) low cost, and 5) is extendable to higher resolutions. As such, MBIL could be a cornerstone for future optical lithography systems.
Photonic Crystal (“PC”) technology has many important possible commercial applications. The technology potentially offers lossless control of light propagation at a size scale near the order of the wavelength of light. This technology has the potential to produce the first truly dense integrated photonic circuits and systems (“DIPCS”). Individual components that are being developed include resonators, antennas, sensors, multiplexers, filters, couplers, and switches. The integration of these components would produce DIPCS that would perform functions such as image acquisition, target recognition, image processing, optical interconnections, analog to digital conversion, and sensing. Further, the resulting DIPCS would be very compact in size and highly field-portable. Applications using light at telecommunications wavelengths require structures to be fabricated with nano-sized dimensions. Despite the advantages and benefits of using such a technology in commercial devices, there is a major problem—the practical commercial development of PC structures has been very slow. Unfortunately, past research has not provided methodologies for the large-scale, cost-effective integration of the impressive PC-based devices into manufacturable DIPCS. Accordingly, there has been no rapid and inexpensive systematic fabrication procedure for the reliable and reproducible fabrication of nano-sized PC structures.
In addition to PC technologies, current developments in metamaterial (“MM”) technology potentially have many important commercial applications. This technology offers the control of light propagation at a size scale much smaller than the wavelength of light. Ultra-compact objective lenses, frequency-doubling devices, parametric amplifiers, and parametric oscillators all become possible with MMs. Further, the integration of these components would produce DIPCS that are very compact in size and highly field-portable. The subwavelength-sized magnetic dipoles needed to make an MM can be produced by making microscopic split-ring resonators (“SSRs”). These SSR devices are also known as slotted-tube resonators or loop-gap resonators. If SSR devices are produced in the subwavelength arrays, they behave as magnetic atoms, and thus, MMs become possible. While MMs have been successfully developed at microwave frequencies, conventional systems have been unable to produce these precise nanostructure devices at optical frequencies efficiently and at a low cost.
Conventional technology allows for the implementation of wafer-scale fabrication of arbitrary functional element patterns within a periodic array through the use of electron-beam lithography, focused ion beam milling, or nano-imprint techniques. Unfortunately, each of these techniques requires a large number of hours of intensive manufacturing. For example, while electron-beam lithography may be appropriate for research, the time-burden for carrying out the process makes it unsuitable for manufacturing.
Therefore, there is a desire for systems and methods that overcome the many disadvantages associated with the prior art that are discussed above. Various embodiments of the present invention address such a desire.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to diffractive photo-masks and methods of using and fabricating diffractive photo-masks. An exemplary embodiment of the present invention provides a diffractive photo-mask comprising a body element, a plurality of interference/image-forming hologram gratings, and a zero-order beam blocking element. The plurality of interference/image-forming hologram gratings can be located on or in the body element. The plurality of interference/image-forming hologram gratings can also be configured to diffract a light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane when the light beam is incident upon the diffractive photo mask. The zero-order beam blocking element can be configured to block zero-order diffracted beams when the light beam is incident upon the diffractive photo-mask.
In an exemplary embodiment of the present invention, the diffractive photo-mask can be located proximate to a surface of a photosensitive material. In another exemplary embodiment of the present invention, the interference/image-forming hologram grating can be located proximate to a top surface of the body element. In yet another exemplary embodiment of the present invention, the diffractive photo-mask further comprises a transparent material configured to place the diffractive photo-mask in physical contact with a photosensitive material at the substrate plane. In still another exemplary embodiment of the present invention, the interference/image-forming hologram grating comprises a high-efficiency volume grating. In some embodiments of the present invention, the efficiency of the high-efficiency volume grating exceeds 75%. In some embodiments of the present invention, the efficiency of the high-efficiency volume grating approaches 100%. In another exemplary embodiment of the present invention, the interference/image-forming hologram grating comprise at least one material having an index of refraction that is different than an index of refraction of the body element.
In addition to diffractive photo-masks, embodiments of the present invention provide methods of using diffractive photo-masks to fabricate a high-spatial frequency periodic structure with incorporated functional elements. In an exemplary embodiment of the present invention, a method of fabricating a high-spatial frequency periodic structure with incorporated functional elements comprises directing a light beam to be incident upon a diffractive photo-mask, such that the diffractive photo-mask diffracts the light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane. In another exemplary embodiment of the present invention, the method further comprises placing the diffractive photo-mask proximate to a surface of a photosensitive material, such that the photosensitive material is located proximate the substrate plane.
In addition to diffractive photo-masks and methods of using diffractive photo-masks, embodiments of the present invention provide methods of fabricating a diffractive photo-mask. In an exemplary embodiment of the present invention, a method of fabricating a diffractive photo-mask comprises transmitting a zero-order reference light beam that interferes with a plurality of interference/image-forming light beams to interferometrically record an interference/image-forming hologram grating in a photo-mask recording material. In another exemplary embodiment of the present invention, the method further comprises monitoring and adjusting an alignment of a functional-element image. In still another exemplary embodiment of the present invention, adjusting the alignment of the functional-element image incorporates automatic feedback control technology. In yet another exemplary embodiment of the present invention, the interference/image-forming hologram grating is formed by a scanning writing method.
These and other aspects of the present invention are described in the Detailed Description of the Invention below and in the accompanying figures. Other aspects and features of embodiments of the present invention will become apparent to those of ordinary skill in the art upon reviewing the following description of specific, exemplary embodiments of the present invention in concert with the figures. While features of the present invention may be discussed relative to certain embodiments and figures, all embodiments of the present invention can include one or more of the features discussed herein. While one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used with the various embodiments of the invention discussed herein. In similar fashion, while exemplary embodiments may be discussed below as system or method embodiments, it is to be understood that such exemplary embodiments can be implemented in various devices, systems, and methods of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following Detailed Description of the Invention is better understood when read in conjunction with the appended drawings. For the purposes of illustration, there is shown in the drawings exemplary embodiments of the present invention, but the subject matter of the present invention is not limited to the specific elements and instrumentalities disclosed.

FIG. 1 illustrates a diffractive photo-mask in accordance with an exemplary embodiment of the present invention.

FIG. 2 illustrates a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIG. 3 illustrates a schematic view of a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIG. 4 illustrates a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIG. 5 illustrates a section of a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIG. 6 illustrates a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIG. 7 illustrates a multi-beam interference/projection lithography system in accordance with an exemplary embodiment of the present invention.

FIGS. 8( a)-(h) illustrate examples of photonic crystal device patterns that can be prepared in accordance with exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

To facilitate an understanding of the principles and features of the various embodiments of the invention, various illustrative embodiments are explained below. Although exemplary embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the exemplary embodiments, specific terminology will be resorted to for the sake of clarity.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. For example, reference to a component is intended also to include composition of a plurality of components. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named.
Also, in describing the exemplary embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value.
By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.
It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components other than those expressly identified.
The components described hereinafter as making up various elements of the invention are intended to be illustrative and not restrictive. Many suitable components or steps that would perform the same or similar functions as the components or steps described herein are intended to be embraced within the scope of the invention. Such other components or steps not described herein can include, but are not limited to, for example, similar components or steps that are developed after development of the invention.
In particular, the present inventive diffractive photo-mask can be applied to direct photolithographic exposure of micro- and nano-electronic integrated circuits, photonic devices, metamaterial devices, biomedical structures, and subwavelength elements. The present inventive diffractive photo-mask and methods for using and methods for preparing same can be described by referring now to FIGS. 1-7, where exemplary embodiments of the present invention are illustrated.
As shown in FIG. 1, an exemplary embodiment of the present invention provides a diffractive photo-mask 101 in the diffractive photo-mask system 100. The diffractive photo-mask 101 can have an interference/image-forming hologram grating 102 located on or in a body element 104. The diffractive photo-mask 101 can also have a zero-order beam-blocking element 103 that can eliminate or block the zero-order diffracted beam from the reference light beam incident to the diffractive photo-mask. The diffractive photo-mask 101 can diffract a reference light beam incident upon the diffractive photo-mask to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution 121 on a substrate 120 that is located at a substrate plane 110. A single interference/image-forming hologram grating can be described, in part, as an interference grating, an image-forming hologram grating, a diffractive grating, and/or a MBIL-beam grating. The substrate 120 can be any substrate known to one of ordinary skill in the art to be a photosensitive material.
The diffractive photo-mask when appropriately illuminated by, for example, a collimated ultraviolet reference light, can produce by diffraction and interference one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at the substrate plane. This can be done in a single-exposure step. The optical-intensity distribution can be described in a number of different ways. In one embodiment, the optical-intensity distribution can be described as a periodic lattice of points distributed on a substrate with one or more lattice points removed, eliminated, or altered to produce one or more functional elements. In another embodiment, the periodic lattice can be an overall high-spatial-frequency periodic background composed of a lattice points with an overall low-spatial frequency-series of lattice points removed or altered. In another embodiment, the optical-intensity distribution can be described by a first portion having a periodic lattice motif and a second portion having the absence of that periodic lattice motif. In another embodiment, the optical-intensity distribution can be described as a periodic fringe pattern where one or more fringes are removed, eliminated or altered to produce one or more functional elements. In another embodiment, the optical-intensity distribution can be described as a three-dimensional periodic lattice where selected portions of the three-dimensional lattice are removed or altered to produce one or more functional elements.
The functional-element optical-intensity distribution can be described by way of examples. In an embodiment, the functional-element optical-intensity distribution can be a photonic crystal device pattern, including for example, the structures shown in FIG. 8, including a) a waveguide, b) a ninety-degree waveguide bend, c) a directional coupler, d) a Mach-Zehnder interferometer, e) a ring resonator, f) a wavelength selective grating, g) a one-to-three splitter, and g) a resonant filter. In each of these structures the optical-intensity distributions can be seen in the body of the device, e.g. an all-surrounding periodic lattice with functional elements defined by the absence of the periodic lattice distribution.
While not wanting to be bound by theory or limited to a generalization, one way to understand the diffractive photo-mask is to conceive of two components of the single interference/image-forming grating diffractive element. First, a periodic component that produces the high-spatial-frequency periodic optical-intensity distribution on the substrate. Second, a pattern-mask component that produces a negative image of the non-periodic functional elements. These two components are combined into a single interference/image-forming grating 102. The single interference/image-forming grating 102 is preferably a high-efficiency volume hologram grating. However, it may also be a thin grating or a surface-relief grating as well.
The interference/image-forming hologram grating 102 creates an image of the functional element pattern at the substrate plane, effectively blocking or altering the interfering beams at locations corresponding to the functional elements, thus eliminating or altering the desired lattice points and creating the required non-periodic patterns in the overall periodic lattice.
In one embodiment, the interference/image-forming hologram grating 102 can be located at the top of the body element and/or within the body element. In another embodiment, the interference/image-forming hologram grating 102 can be located proximate to a top surface of the body element.
In the diffractive photo-mask there can optionally be no chrome or metallic patterns at the bottom of the mask in contact with the substrate to define the non-periodic functional elements as traditionally required by contact lithography. Not having a chrome or metallic pattern at the bottom of the mask can eliminate a typical problem of the mask degrading upon repeated use due to deterioration of this metallic pattern when it is used in a contact lithography mode of operation. The present inventive diffractive photo-mask can overcome this problem by having all of the mask information contained within the interference/image-forming hologram grating at the top of the photo-mask and/or within the body of the photo-mask.
In another embodiment, the diffractive photo-mask can be a “thick mask” such that the mask extends from its top diffractive elements down to the surface of contact with the substrate. In another embodiment, the diffractive photo-mask can further contain a transparent material that can place the diffractive photo-mask in physical contact with the substrate. In these embodiments, the bottom of the mask is placed in contact, or is proximate to, the substrate to facilitate exposure. This can be equivalent to contact photo-lithography. The “thick mask” format can be particularly suitable for a commercial manufacturing environment. In another embodiment, the absence of elements at the bottom of the mask can create a “thin mask” such that the mask thickness does not extend down to the surface of substrate. In this embodiment, the bottom of the mask might not be in contact with the substrate, but is still proximate to the substrate. This can be equivalent to proximity photo-lithography. The “thin mask” format can be particularly suitable for a research and development environment since it can permit fine adjustment of the mask position for example, for pre-production experimentation purposes.
As illustrated in FIG. 2, the basic production of non-periodic functional elements integrated within the periodic lattices can initially be created in a multi-beam interference/projection lithography system. The multi-beam interference/projection lithography system 200 contains a plurality of light beams 201 which intersect at and illuminate an object plane 210. A pattern mask 211 can be present at the object plane 210. The plurality of light beams are redirected by objective lens 205 to intersect and interfere at the image plane 220 to create a high-spatial-frequency periodic-optical-intensity distribution on or in a photosensitive material at the substrate 221. When a pattern mask 211 is present, the projected image of the mask features effectively block the multiple interfering beams, thereby eliminating or altering selected portions of the periodic-optical-intensity distribution.
As illustrated in the schematic view in FIG. 3, a multi-beam interference/projection lithography system can be modified to form a diffractive photo-mask production system 300. As with the multi-beam interference/projection lithography system in FIG. 2, the system 300 contains a plurality of light beams 201 which intersect at and illuminate an object plane 210 and are redirected by objective lens 205 to intersect and interfere at an image plane 220. In addition, the diffractive photo-mask production system 300 also contains an interfering reference beam 302 that enters beam-splitter 330 and is directed by the objective lens toward photo-mask plane 301 as a collimated reference beam to facilitate the interferometric recording in a single interference/image-forming hologram grating 102 of the multiple interfering beams along with the projected pattern-mask image.
Several aspects of the photo-mask production can be described. Both FIGS. 2 and 3 depict three beams for the plurality of light beams 201. However, two-beam, four-beam and N-Beam systems are also possible. Moreover, the source wavelength, λ, and incidence angle, θ, for the MBIL beams can be selected in order to satisfy design and fabrication requirements. The spacing between the periodic lattice points (points of intensity pattern maxima/minima) is directly proportional to the wavelength of the interfering MBIL beams and inversely proportional to the sine of the incidence angles of the individual MBIL beams with respect to the optical axis. For example, the spacing between two-dimensional photonic crystal lattice of rods in substrate 221 is approximately 1.5λ. Other periodic translational symmetries and lattice constants are possible by adjusting the relative beam configuration, incidence angle, and source wavelength.
The choice of source wavelength can also satisfy the photosensitive recording material requirements at the substrate plane. For example, while many photoresists are designed for operation at 365 nm, some semiconductor photolithography techniques utilize 193 nm wavelength light, with ongoing efforts to utilize progressively shorter wavelength sources to reduce feature size. For the anticipated ultraviolet wavelength used in this apparatus, the beam-splitter/combiner, lenses, and other components will need to be low loss (transparent) at the selected wavelength.
Based on the incidence angles, as well as the intensities and polarization of the interfering beams, all two-dimensional and three-dimensional Bravais lattices and 9 of the 17 plane group symmetries can be produced. Furthermore, for optimum uniform contrast of the resulting interference pattern, the amplitude and polarization of each of the MBIL beams can to be controlled.
Also depicted in FIGS. 2 and 3, the plurality of beams are focused at a rear focal plane of the objective lens, which then produces collimated beams of a fixed diameter as they intersect at the image plane in order to ensure adequate surface area of the intersecting MBIL beams and a substantially uniform periodic optical-intensity distribution.
The pattern mask 211 at object plane 210 in FIG. 2 can serve in the same manner as conventional masks in modern projection photolithography. As an alternative to a conventional mask, the pattern mask can be a photographic plate reticule or a spatial-light modulator capable of feature size resolutions in the micron range. Effectively, the image of the pattern mask functional elements can block the interfering beams at the substrate, and can eliminate, remove, block, or alter the desired lattice points to create the required functional element in the overall periodic optical-intensity distribution as depicted substrate 221.
The objective lens 205 can serve to focus the MBIL beams and integrated mask pattern on the photosensitive material at the substrate plane. The objective lens can be a single lens, multiple lenses, multiple reflective surfaces, or any combination thereof. The objective lens can be a Fourier transform pair of lenses, allowing filtering in the Fourier plane.
FIG. 4 illustrates another embodiment of the present invention. A diffractive photo-mask production system 400 contains a plurality of light beams 201 which intersect at and illuminate an object plane 210 and pass through a beam-splitter/combiner 330 to reach the objective lens 205. The objective lens 205 then redirects the beams to intersect and interfere at an imaging system 450 located at the sample plane after passing through photo-mask plane 301. In addition, the diffractive photo-mask production system 400 also contains a reference beam 302 that enters beam-splitter 330 and is directed toward photo-mask plane 301 as a collimated reference beam.
FIG. 5 illustrates another embodiment of the present invention for a diffractive photo-mask production system 500. As in FIG. 4, a plurality of light beams 201 pass through beam splitter/combiner 330 to reach the objective lens 205. The objective lens 205 then redirects the beams to intersect and interfere at an image plane 220 after passing through photo-mask plane 301, as shown in the upper inset. A reference beam 302 that enters beam-splitter/combiner 330 is directed towards a photo-mask plane 301. The reference beam 302 is combined with the plurality of light beams via the beam-splitter/combiner 330 to allow the individual MBIL beams and the projected pattern mask information to be interferometrically recorded in the diffractive photo-mask 301 to form an interference/image-forming hologram grating, as shown in the lower inset.
In an embodiment of the present, a photo-mask production system can utilize the interference projection exposure system described and set forth in the co-pending U.S. patent application Ser. No. 13/249,841 filed on 30 Sep. 2011, and hereby incorporated by reference in its entirety as if fully set forth herein. As with the multi-beam interference/projection lithography systems described by FIG. 2, the interference projection exposure system described in the copending application can be combined with at least the structural elements reference beam 302 and beam-splitter/combiner 330, as illustrated in FIGS. 3-7 of the present application. This combination can allow the individual MBIL beams and the projected pattern mask information as described in the co-pending application to be interferometrically recorded in the diffractive photo-mask 301 to form an interference/image-forming hologram grating.
The beam-splitter/combiner used to combine the MBIL beams with the interfering reference beam can record interferometrically the MBIL beams and the pattern mask information in the recording material at the photo-mask plane 301 as depicted in FIG. 5 to produce the diffractive photo-mask. The converging reference beam can be normally incident on the beam-splitter/combiner, a portion of which can reflected towards the objective lens where it is focused at the rear focal plane of the objective lens. This produces the collimated reference beam required to record simultaneously the two components of the diffractive element, the MBIL periodic component and the pattern mask component, into a single interference/image-forming hologram grating as depicted in FIG. 1
In an embodiment, the interfering reference beam and the MBIL beams can be mutually coherent. A maximum interference, fringe visibility, and uniformity at the photo-mask plane can be achieved when the MBIL beams and the interfering reference beam are mutually coherent. Therefore, in an embodiment, the MBIL beams and the interfering beam can all be derived from the same light source, for example an ultraviolet laser source. Additionally, the difference in the optical path lengths between all beam pairs can be much smaller than the coherence length of the laser beams.
The positioning and focusing of the beams can be monitored during exposure of the photo-mask as depicted in FIG. 7 for diffractive photo-mask production system 700. This can be accomplished by using the transmitted interfering beam 202 and reflected MBIL beams, both coming from the beam-splitter/combiner 330. The transmitted beams can be seen in the inset in FIG. 6, which illustrates an analogous diffractive photo-mask production system 600. This configuration allows beam monitoring by a beam monitoring system 750 at the beam monitoring plane 720, after the beams have been focused by objective lens 705, having passed through diffractive photo-mask monitoring plane 701.
The pattern mask 211 depicted in FIG. 2 can typically be mounted on the object plane 210 in FIG. 7. The object plane can be a translation/orientation stage that allows the functional elements to be aligned with respect to the lattice formed by the interfering beams. This alignment operation can be accomplished by automatic feedback control or by human adjustment. This alignment may be monitored before photo-mask exposure at the substrate plane with the imaging system 450 depicted in FIG. 4. Alternatively, the alignment may be monitored at the beam-monitoring system 750 via the configuration shown in FIG. 7, so that the integrated pattern can be imaged and aligned with the lattice in parallel with the photo-mask fabrication process. This can be accomplished by using the MBIL beams and projected image reflected by the beam-splitter, 330. This configuration allows the objective lens system to be duplicated and monitored in the substrate monitor plane.
To simplify the figures in this disclosure, refraction and aberration effects at the beam-splitter/combiner are not depicted. In reality, the light rays are refracted at the exterior surfaces of the beam-splitter/combiner. However, including refraction does not change the angular directions of the resulting beams as depicted in the figures. In another embodiment, a thin diffractive pellicle can be used as a beam-splitter combiner to reduce significantly the aberration effects. Complete accurate design of these types of optical systems can be accomplished with readily available optical design software.
The linearly polarized light from the MBIL beams can develop a degree of ellipticity, in general, due to reflection/transmission by the beam-splitter/combiner and transmission through the objective lens. This subject is extensively treated in optical design literature.
The diffractive photo-mask can enable the single-exposure formation of non-periodic functional elements within a periodic lattice, without the need for complicated mask-alignment procedures. As such, the proposed photo-mask represents a highly efficient way of producing the complex patterns needed for microelectronics, photonic crystal circuits, and numerous other structures. The MBIL-defined lattice can determine the high-spatial-frequency periodic elements within the resulting exposed pattern. While the non-periodic functional elements can also be of high spatial frequency, the overall high-spatial-frequency imaging requirements are significantly reduced when compared to traditional projection lithography alone. Conventional photolithographic chromium masks typically contain linear features five to ten times larger than the final image projected onto the focal plane at the substrate. The pattern mask depicted in FIG. 3 can be five times as large as the projected image, matching the reproduction ratio of 5:1. In one embodiment, the reproduction ratio can be from about 1:1 up to about 10:1. Larger reproduction ratios and correspondingly smaller image feature resolutions can be possible; however, the image of the integrated pattern needs to contain adequate high-spatial-frequency components in order to be sufficiently sharp and clear. Therefore, a suitable aberration corrected lens and sufficiently large numerical aperture is required for the optical system to reproduce faithfully the periodic lattice portion of the composite pattern as well as the functional elements. In some cases, it is anticipated that an immersion configuration will be needed. That is, a relatively high refractive index liquid will fill the space between the output surface of the optics and the surface of the photo-mask being exposed. This increases the numerical aperture by a factor of the liquids refractive index. It should also be noted that a degraded image of the integrated pattern can be compensated partially or entirely by using a photosensitive material with a suitable nonlinear response to exposure or a circuit design with sufficient insensitivity to design parameters.
The diffractive photo-mask described above has direct and immediate use in the six primary fields of 1) micro- and nano-electronic integrated circuits, 2) photonic crystal devices, 3) metamaterial devices, 4) biomedical structures, 5) subwavelength structures, and 6) optical trapping. The photo-mask enables wafer-scale-size manufacturing of critically important devices and structures in all six of these fields. The photo-mask is unique in that it produces, by diffraction and interference, an image of the non-periodic functional elements integrated within a periodic lattice in a single exposure, without the need for complex alignment procedures. It may be in a “thick mask” format particularly suitable for manufacturing environments or in a “thin mask” format particularly suitable for research and development environments.
The apparatus and method described above provide an exemplary configuration and procedure for the fabrication of the above diffractive photo-mask. The apparatus presented provides an efficient optical configuration to allow the MBIL diffraction gratings to be recorded by the interference of the individual MBIL beams and the large on-axis interfering reference beam. Simultaneously, the non-periodic functional-element pattern-mask hologram grating is recorded by the interference of the same on-axis large interfering reference beam. The result is a single interference/image-forming hologram grating 102 in the diffraction photo-mask 101. The apparatus and method allow for 1) before photo-mask exposure monitoring, 2) during photo-mask exposure monitoring of beams, and 3) during photo-mask exposure monitoring of the mask pattern alignment.
It is to be understood that the embodiments and claims disclosed herein are not limited in their application to the details of construction and arrangement of the components set forth in the description and illustrated in the drawings. Rather, the description and the drawings provide examples of the embodiments envisioned. The embodiments and claims disclosed herein are further capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purposes of description and should not be regarded as limiting the claims.
Accordingly, those skilled in the art will appreciate that the conception upon which the application and claims are based may be readily utilized as a basis for the design of other structures, methods, and systems for carrying out the several purposes of the embodiments and claims presented in this application. It is important, therefore, that the claims be regarded as including such equivalent constructions.
Furthermore, the purpose of the foregoing Abstract is to enable the United States Patent and Trademark Office and the public generally, and especially including the practitioners in the art who are not familiar with patent and legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the claims of the application, nor is it intended to be limiting to the scope of the claims in any way. It is intended that the application is defined by the claims appended hereto.

Claims

1. A diffractive photo-mask, comprising:

a body element;

a plurality of interference/image-forming hologram gratings located on or in the body element and configured to diffract a light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane, when the light beam is incident upon the diffractive photo-mask; and

a zero-order beam blocking element configured to block zero-order diffracted beams when the light beam is incident upon the diffractive photo-mask.

2. The diffractive photo-mask of claim 1, wherein the diffractive photo-mask is located proximate to a surface of a photosensitive material.

3. The diffractive photo-mask of claim 1, wherein the plurality of interference/image-forming hologram gratings are located proximate to a top surface of the body element.

4. The diffractive photo-mask of claim 1, further comprising a transparent material configured to place the diffractive photo-mask in physical contact with a photosensitive material at the substrate plane.

5. The diffractive photo-mask of claim 1, wherein the plurality of interference/image-forming hologram gratings comprise at least one high-efficiency volume grating.

6. The diffractive photo-mask of claim 1, wherein the plurality of interference/image-forming hologram gratings comprise at least one material having an index of refraction that is different than an index of refraction of the body element.

7. A method of fabricating a high-spatial frequency periodic structure comprising:

directing a light beam to be incident upon a diffractive photo-mask, such that the diffractive photo-mask diffracts the light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane.

8. The method of fabricating a high-spatial frequency periodic structure of claim 7, further comprising placing the diffractive photo-mask proximate a surface of a photosensitive material, such that the photosensitive material is located proximate to the substrate plane.

9. The method of fabricating a high-spatial frequency periodic structure of claim 7, wherein the diffractive photo-mask comprises:

a body element;

a plurality of interference/image-forming hologram gratings located on or in the body element and configured to diffract the light beam to produce one or more functional elements surrounded by a high-spatial-frequency periodic optical-intensity distribution at a substrate plane; and

a zero-order beam blocking element configured to block zero-order diffracted beams.

10. The method of fabricating a high-spatial frequency periodic structure of claim 9, wherein the plurality of interference/image-forming hologram gratings are located proximate to a top surface of the body element.

11. The method of fabricating a high-spatial frequency periodic structure of claim 9, wherein the diffractive photo-mask further comprises a transparent material placing the diffractive photo-mask in physical contact with the photosensitive material at the substrate plane.

12. The method of fabricating a high-spatial frequency periodic structure of claim 9, wherein the plurality of interference/image-forming hologram gratings comprise at least one high-efficiency volume grating.

13. The method of fabricating a high-spatial frequency periodic structure of claim 9, wherein the plurality of interference/image-forming hologram gratings comprise at least one material having an index of refraction that is different than an index of refraction of the body element.

14. A method of fabricating a diffractive photo-mask, the method comprising:

transmitting a zero-order reference light beam that interferes with a plurality of interference/image-forming light beams to interferometrically record a plurality of interference/image-forming hologram gratings in a photo-mask recording material.

15. The method of fabricating a diffractive photo-mask of claim 14, further comprising monitoring and adjusting an alignment of a functional-element image.

16. The method of fabricating a diffractive photo-mask of claim 15, wherein adjusting the alignment of the functional element image incorporates automatic feedback control technology.

17. The method of fabricating a diffractive photo-mask of claim 14, wherein the plurality of interference/image-forming hologram gratings are formed by a scanned writing method.

18. The method of fabricating a diffractive photo-mask of claim 14, wherein the interference/image-forming hologram gratings are configured to diffract a beam of light to produce a functional-element optical-intensity distribution.