Patent Application
20070188869
The present invention concerns improved designs for diffractive optical elements (DOE's) as well as systems and methods for manufacturing these DOE's. In particular, these improved designs utilize non-rectangular pixels to allow for simplified formation of optical patterns that are not easily laid out on a square grid.
Diffractive Optical Elements (DOE) are a type of optical element with a surface relief profile that changes the phase of the light passing through it. One example of an application for which a DOE may be used is as a beam splitter. The DOE may be designed for a particular array of output beams. The desired pattern of the DOE is calculated based on the theory of diffraction so that constructive interference causes the intensity of the transmitted light to have a desired set of bright spots (i.e. the array of output beams). Other examples of DOE's include: Fresnel lenses, gratings, computer-generated (phase-only) holograms, micro-lens arrays and beam shaping elements.
DOE's achieve their desired diffracted patterns due to the different phase shifts of the incoming beam that occur as the light is transmitted through the various different thicknesses of the DOE patterns based upon the index of refraction of the DOE substrates and the wavelength of the incident beam.
A DOE pattern is usually generated in photoresist using either a grayscale mask lithography process or a multiple binary mask lithography process. Alternatively, DOE patterns may be directly written using a laser writer. The exposed pattern of the photoresist is then developed and either the developed pattern in the photoresist itself can be used to diffract an incoming beam, or the exposed pattern in the photoresist may be transferred into the underlying substrate using an anisotropic etching procedure. If the pattern is transferred into the substrate then the substrate acts as the diffractive structure.
Existing beam splitter DOE's are currently designed and fabricated on a rectangular (often square) orthogonal grid pattern, such as exemplary square pixel DOE pattern 100, shown in
Because the DOE pattern is formed in a rectangular grid, the resulting diffracted pattern produced by the DOE is also arrayed upon rectangular coordinate output grid 200, as illustrated in
This may limit the design possibilities for the diffracted pattern since the desired pattern spacing (bright orders 400, shown in
The present invention involves improved designs utilize non-rectangular pixels to allow for simplified formation of optical patterns that are not easily laid out on a rectangular grid and may allow for unit cells made up of fewer pixels in periodic DOE structures. Thus, large periodic patterns in devices such as ink jet nozzles may be manufactured by a laser machining system with fewer ‘step and repeats’ iterations, without using a larger DOE.
An exemplary embodiment of the present invention is a diffractive optical element (DOE), including a substrate formed of a substantially transparent material having a substrate index of refraction. The substrate includes a first transmission face that is substantially planar and a second transmission face that is substantially parallel to the first transmission face. The second transmission face includes an array of non-rectangular pixels that form a complete tiling over the functional area of this face. For each of the non-rectangular pixels of the array, the phase shift of light transmitted through the substrate between the transmission faces is approximately equal to one of a set of predetermined phase shifts.
Another exemplary embodiment of the present invention is a DOE, including a substrate formed of a substantially transparent material and a diffractive structure formed of photoresist having a photoresist index of refraction. The substrate includes a first surface and a second surface that is substantially parallel to the first surface, and the diffractive structure is formed on the second surface of the substrate. The diffractive structure includes an array of non-rectangular pixels that form a complete tiling over a functional area of the second surface of the substrate. For each of the non-rectangular pixels of the array, the thickness of the diffractive structure is approximately equal to one of a set of predetermined thicknesses.
A further exemplary embodiment of the present invention is a laser writing system with non-orthogonal axes for laser machining a workpiece. The laser writing system includes: a laser source to generate a laser beam; coupling optics to couple laser light to a beam spot on the workpiece; a workpiece holder to hold the workpiece; and positioning means coupled to the workpiece holder to scan the beam spot over the workpiece. The positioning means includes an X translation stage to move the workpiece holder along an X axis and a Y translation stage to move the workpiece holder along a Y axis. The X axis and the Y axis are substantially orthogonal to the direction of propagation of the laser beam at the beam spot. However, the X axis is neither parallel nor perpendicular to the Y axis.
An additional exemplary embodiment of the present invention is a laser writing system with non-orthogonal axes for laser machining a workpiece. The laser writing system includes: a laser source to generate a laser beam; coupling optics to couple laser light to a beam spot on the workpiece; scanning optics to scan the beam spot on the workpiece along the X axis; a workpiece holder to hold the workpiece; and a Y translation stage coupled to the workpiece holder to move the workpiece holder along the Y axis. The positioning means includes an X translation stage to move the workpiece holder along an X axis and a Y translation stage to move the workpiece holder along a Y axis. The X axis and the Y axis are substantially orthogonal to the direction of propagation of the laser beam at the beam spot. However, the X axis is neither parallel nor perpendicular to the Y axis.
Yet another exemplary embodiment of the present invention is a method for manufacturing a DOE having a predetermined pattern of parallelogram-shaped pixels, using a laser writing system with non-orthogonal X and Y axes. A DOE workpiece is mounted in a workpiece holder of the laser writing system. A laser beam is generated using a laser source of the laser writing system. The laser beam is directed to a beam spot on a surface of the DOE workpiece using optics of the laser writing system. The beam spot is then scanned across a functional area of the surface of the DOE workpiece along the X axis. The workpiece holder of the laser writing system is moved using a Y translation stage of the laser writing system such that the beam spot is stepped along the Y axis. The X axis and the Y axis are substantially orthogonal to a direction of propagation of the laser beam at the beam spot. However, they are neither parallel nor perpendicular to each other. The fluence of the laser beam at the beam spot is modulated as the beam spot is scanned to form the predetermined pattern of parallelogram-shaped pixels of the DOE in the functional area on the surface of the DOE workpiece. The scanning, stepping, and modulating steps are repeated until the beam spot has been scanned over the entire functional area on the surface of the DOE workpiece.
The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
An exemplary embodiment of the present invention is a DOE fabricated with a non-rectangular pattern, such as an oblique parallelogram pattern, a triangular pattern, or a hexagonal pattern, to tile the functional area of the DOE surface. In many designs the non-rectangular pixels of the DOE may be congruent to one another. However, it is contemplated that exemplary DOE's utilizing other pixel patterns that may be used as well, as long as the pixels are selected to tile the surface of the functional area. The use of these non-orthogonal surface relief patterns provides a resulting diffracted pattern that is arrayed on a non-orthogonal grid and, thus, allows for greater design flexibility for the array of output beams.
An exemplary DOE according to the present invention includes a substrate formed of a substantially transparent material, such as glass, fused silica, quartz, silicon, sapphire, acrylic, silicone, polystyrene, polycarbonate, a cyclic olefin polymer, a cyclic olefin copolymer, or a perfluorocyclobutane polymer.
In operation, light of a preselected wavelength is transmitted through an exemplary DOE beam splitter from one face to another and phase differences caused by the pixel pattern lead to the desired output array of beams. One of these faces of the substrate is desirably substantially planar. An anti-reflection coating may be formed on this face of the substrate to reduce loses. The other transmission face is substantially parallel to the first face. This second transmission face includes the array of non-rectangular pixels to generate the desired phase differences. This array of non-rectangular pixels desirably forms a complete tiling over the functional area of this face of the DOE.
These pixels in the functional area of the DOE may be formed in a number of ways. For example, the pixels may be etched into the substrate material, using procedures such as three dimensional photolithographic techniques, laser ablation techniques, or etching techniques, including reactive ion etching and plasma etching. Alternatively, the pixels may be formed of material grown on the substrate surface, using procedures such as three dimensional photolithographic or laser assisted chemical vapor deposition techniques. It is also contemplated that the pixels may be formed by controllably altering the index of refraction of the substrate material within the pixels. Such localized alteration of refractive indices may be accomplished using ultrafast laser machining of the substrate material.
In each of the non-rectangular pixels of the array, the phase shift of the light transmitted through the substrate between the transmission faces is approximately equal to one of a set of predetermined phase shifts. In a simple, binary DOE design this set of phase shifts may have only two values, 0° and 180°, for example. Alternatively, a larger number of potential phase shifts may be desired. The phase shifts may desirably be equally spaced to simplify calculation of the resulting output beam array.
In exemplary DOE's in which the phase shift is due to variations in the thickness of the substrate material, the index of refraction of the substrate material and the wavelength of the light for which the DOE is designed determine the desired heights of the surface relief of the pixels. Similarly, the thickness of the photoresist, the index of refraction of the photoresist material and the design wavelength of the DOE determine the desired heights of the surface relief of photoresist pixels. For pixels formed by altering the refractive index of the substrate material it is the thickness of the altered portion, the change in refractive index and the design wavelength that determine the desired DOE pattern.
Typically, the phase shifts are kept within a range of one period, i.e. 0°-360°. Thus, in exemplary DOE's in which the phase shift is due to variations in the thickness of the substrate material, the difference between the smallest thickness and the largest thickness of the substrate material in the DOE is less than the predetermined wavelength of light divided by the substrate index of refraction minus one (i.e. λ/(ns−1)). Similarly, in exemplary DOE's in which the phase shifts are caused by a photoresist layer, the difference between the smallest thickness and the largest thickness of the photoresist layer is less than the predetermined wavelength of light divided by the index of refraction of the photoresist minus one (i.e. λ/(np−1)).
Depending upon the angles chosen for the parallelogram, there are many possible output patterns. Based upon the desired pattern the optimal angular orientation could be determined using basic geometric mathematics. For example, in exemplary DOE pixel pattern 500, a 60° angle is used for the parallelogram skew. The resulting output grid of points is shown in
For certain applications, having the ability to layout the desired pattern on an oblique grid may allow for fewer design constraints compared to an orthogonal array.
Another advantage of this exemplary embodiment is a reduced total coordinate grid necessary for output beams. Reducing the number of coordinates needed, may also reduce the computer simulation run time used when designing a DOE, which may in turn help with rapid prototyping and reduced design turnaround time for manufacturing design changes.
In certain instances the more efficient layout of output beams on an oblique grid may also allow for a smaller DOE period, potentially allowing more periods to be illuminated for a given input beam size. Having more periods illuminated may allow for better defined diffracted output beams. Alternatively, the use of a more efficient layout pattern may allow for more repetitions of the periodic pattern of output beams to be formed simultaneously. Increasing the repetitions formed simultaneously may allow for increased productivity and/or fewer step-and-repeat operations. The reduction of step-and-repeat operations may be particularly useful for laser machining of repetitive structures due to the associated potential for misalignment with each step.
For example,
As shown in
Designing a DOE pattern for the same inkjet design using the exemplary oblique parallelogram grid of
Fabricating an exemplary parallelogram pixel patterned DOE may by a number of methods. For example, the design may be formed using a grayscale mask that incorporates an oblique parallelogram pixel pattern to expose a photoresist layer. The exposed photoresist may then be developed to form the desired pixel pattern in the photoresist. Alternatively, the developed photoresist, and the DOE substrate, may be anisotropically etched to transfer the pixel pattern onto the substrate material.
Alternatively, a direct writing method such as using a laser writer may be used to form an oblique parallelogram pixel pattern on the DOE. The software of laser writers is typically designed to write patterns using orthogonal X-Y axes. The laser writer software may by modified to write parallelogram patterns using standard orthogonal X-Y motion, however, by building up the larger parallelogram pattern pixels by exposing the smaller individual exposure spots, similar to the manner in which a computer printer creates a diagonal line using X-Y motion.
The exemplary laser writing system of
Laser source 800 may be a continuous wave (CW) or a pulsed laser source. Laser source 800 desirably includes a fluence controller to control the fluence of the laser beam as the beam spot is scanned over the workpiece. Fluence control may be achieved by controlling the average power of laser beam 802 either by directly varying the output power of laser source 800 or by using a variable attenuator coupled along the beam path. Alternatively, the fluence may be controlled by changing the size of the beam spot formed on the surface of the workpiece or by varying the scan speed of the beam spot across the workpiece.
Scanning optics 804 include a scan mirror, or prism, that may pivot as shown by arrows 818 to sweep laser beam 802 though a range of angles to provide the X-axis scan of the beam spot over the surface of the workpiece. Additionally, scanning optics 804 may desirably include a telecentric scan lens to align laser beam 802 to be substantially normally incident to the surface of the workpiece throughout the travel of the scanning mirror. It is noted that laser writing system base 814 may additionally include X translation stage 815 (shown in phantom) coupled to rotation stage 812. Alternative X translation stage 815 may be used to initially align the beam spot on the surface of workpiece 808. It may also be used to step the workpiece in the X direction, thereby allowing the exemplary laser writing system to be used for machining structures with length in the X direction greater than the length of a scan line produced by a single sweep of scanning optics 804.
Coupling optics 806 may desirably focus the laser light at the beam spot on the workpiece and may include additional components to control the polarization of laser beam 802. Focusing of the laser beam at the beam spot may be controlled by moveable lenses or other optical components within coupling optics 806 and/or may be controlled by moving workpiece holder 808 along the Z axis, i.e. substantially parallel to the direction of propagation of laser beam 802 at the beam spot. The workpiece holder may be moved along Z axis using a Z translation stage (not shown) coupled to the workpiece holder.
The DOE workpiece is mounted in a workpiece holder of the laser writing system, step 1000. The DOE workpiece may have a photoresist layer formed on its surface before it is mounted. The DOE workpiece may be mounted such that a predetermined scan line the DOE workpiece is substantially aligned to the X axis of the laser writing system. A rotation stage coupled to the Y translation stage of the laser writing system may be used to orient the Y axis at a predetermined angle relative to the X axis at this point as well.
A laser beam is generated using the laser source of the laser writing system, step 1002. The laser source of the laser writing system may be either a CW laser source or a pulsed laser source. If a pulsed laser source is used then a pulsed laser beam is generated.
The laser beam is directed to a beam spot on a surface of the DOE workpiece using optics of the laser writing system, step 1004. The beam spot may be focused using the optics of the laser writing system such that the beam spot has a predetermined diameter of the surface of the DOE. Additionally, the cross section of the laser beam may be shaped using the optics such that the beam spot has a predetermined shape. For example, a beam spot sized and shaped to match the size and shape of the parallelogram pixels of the DOE may be useful.
The beam spot is scanned across a functional area of the surface of the DOE workpiece along the X axis, step 1006. The workpiece holder of the laser writing system may desirably be moved using an X translation stage of the laser writing system to scan the beam spot across the functional area of the surface of the DOE workpiece along the X axis. Alternatively, the beam spot may be moved using scanning optics of the laser writing system to scan the beam spot across the functional area along the X axis. The scanning of the beam spot across the functional area along the X axis may be continuous or it may be done in steps. Stepping the beam spot may be desirable if a pulsed laser source is used to generate the laser beam. The stepping of the beam spot may be synchronized with the pulsing of the pulsed laser source.
The workpiece holder of the laser writing system is moved, step 1008, using a Y translation stage of the laser writing system such that the beam spot is stepped along the Y axis at the end of each scan along the X axis. The X axis and the Y axis are desirably arranged such that they are substantially orthogonal to the direction of propagation of the laser beam at the beam spot and are neither parallel nor perpendicular to each other.
The fluence of the laser beam is modulated at the beam spot as the beam spot is scanned to form the predetermined pattern of parallelogram-shaped pixels of the DOE in the functional area on the surface of the DOE workpiece, step 1010. If the laser source of the laser writing system is a CW laser source, then the fluence at the beam spot may be modulated by varying the power of the CW laser beam or by varying the scan speed along the X axis. If the laser source of the laser writing system is a pulsed laser source, then the fluence at the beam spot may be modulated by varying the pulse power of the laser beam or by varying the scan speed along the X axis if a continuous scan is used. If the beam spot is stepped along the X axis, then the step time may be varied such that a predetermined number of laser pulses may be incident at each step location. Alternatively, the fluence may be varied by varying the beam spot size, but this method of varying the fluence may be difficult to control.
The desired fluence depends on the laser machining process by which predetermined pattern of parallelogram-shaped pixels of the DOE are to be formed. Exemplary laser machining processes that may be used include: laser ablation of material of the DOE workpiece; deposition of material on the surface of the DOE workpiece using a laser assisted chemical vapor deposition process; exposing a photoresist layer on the surface of the DOE workpiece; and changing the refractive index of material of the DOE workpiece via ultrafast laser irradiation. If the laser writing system is used to expose a pattern of parallelogram-shaped pixels in a photoresist layer rather than being used to perform one of the other laser machining methods, the photoresist layer may be developed to form the predetermined pattern of parallelogram-shaped pixels of the DOE in the photoresist layer. Alternatively, the developed photoresist layer may form a scaled pattern of parallelogram-shaped pixels in the photoresist layer. This scaled pattern may be transferred to the substrate by etching the photoresist layer and material of the DOE workpiece, thus forming the predetermined pattern of parallelogram-shaped pixels of the DOE in the functional area on the surface of the DOE workpiece.
After each scan and step iteration (steps 1006, 1008, and 1010), it is determined if the beam spot has been scanned over the entire functional area on the surface of the DOE workpiece, step 1012. If the entire functional area has been scanned, then the DOE is complete, step 1014 (except for developing and possibly etching the photoresist layer if a photolithographic process is used). If the entire functional area has not yet been scanned, then steps 1006, 1008, and 1010 are repeated until the entire functional area has been scanned.
The present invention includes a number of exemplary embodiments of DOE's having non-rectangular pixel patterns, as well as exemplary methods of manufacturing such DOE's. Additionally, the present invention includes exemplary laser writing systems that may be used with these exemplary methods. Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
