Excimer laser micromachiningfor rapid fabrication of diffractive optical elements

Gregory P. Behrmann and Michael T. Duignan

We demonstrate the fabrication of binary, multilevel, and blazed diffractive structures by a fast andflexible direct-write process by using an excimer-laser-based tabletop micromachining workstation withan integrated optical surface profiler. © 1997 Optical Society of America

Key words: Diffractive optical elements, excimer lasers, laser ablation, micromachining, direct write.

1. Introduction

Although the tremendous potential of diffractive op-tical elements ~DOE’s! is by now widely appreciated,they are generally so difficult to fabricate and socostly to make a prototype of that their promise is stilllargely unrealized. Sophisticated microfabricationtechniques that stem from VLSI semiconductor man-ufacturing processes are highly developed, but stillusually require mass production to be economicallyfeasible.1 Even when unit cost is not a primary con-sideration, the time period from design to realizationof even the simplest custom DOE’s can frequently runto many weeks or even months. The freedom toevaluate prototypes, long an important step in theengineering process, can be greatly restricted be-cause of these high costs and long cycle times. Thechallenge of producing rapid, low-cost, custom DOE’shas resulted in a number of novel approaches.These include the use of desktop publishing softwareand commercial image setters for the production ofbinary-amplitude DOE’s2 and gray-level masks,3direct-write laser exposure of photoresist,4 gray-levelmasks produced by e-beam exposure of glass,5 andsurface ablation through mask pattern projection ofexcimer laser radiation.6,7

In this paper we describe our direct-write approachin which a small, high-repetition-rate pulsed excimerlaser is utilized to ablate a surface relief directly intothe substrate material.8,9 This system can producepractical diffractive elements in minutes to hours. A

The authors are with Potomac Photonics, Inc., 4445 NicoleDrive, Lanham, Maryland 20706.

Received 17 September 1996; revised manuscript received 3 De-cember 1996.

0003-6935y97y204666-09$10.00y0© 1997 Optical Society of America

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user has the ability to design, fabricate, gauge, andrefine the diffractive structure at a single worksta-tion. We have constructed a wide spectrum of dif-fractive optical structures, including lenses, lensletarrays, blazed gratings, and bitmap computer-generated holograms ~CGH’s!, each in a fraction ofthe time required when more conventional microfab-rication techniques are used.

In Section 2, we describe the major system compo-nents required for fabrication. In Section 3, wepresent the results of a material study that examinesthe ablation characteristics of poly~imide! film. InSection 4 we describe techniques for producing two-level binary structures. In Section 5 we addressmethods for fabricating multilevel and blazed struc-tures.

2. Micromachining Workstation with IntegratedSurface Profilometry

The major components of the micromachining work-station are shown in Fig. 1. A Potomac SGX-1000waveguide excimer laser, operating at 248 nm, isused as the laser source. The laser-pulse energy istypically ;40–50 mJypulse, although only a smallfraction of the available energy is generally utilized.The pulse duration is approximately 50 ns, and themaximum repetition rate is 2000 Hz. The outputbeam profile is roughly top hat with specklelike in-tensity modulation of 610%–25% that is largely con-stant shot to shot. The characteristic size of themodulation is roughly 5%–10% of the overall beamdiameter. The laser beam floods a shaped apertureand a typically 103–303 demagnified image is pro-jected onto the workpiece by a 103 or 203 UV trans-mitting microscope objective and appropriate relaylens ~L2 in Fig. 1!. For a given aperture, laser-pulseenergy is controlled by the varying of the laser spotsize or position at the aperture or by use of transmit-

ting attenuators. Laser fluence at the aperture isgenerally ,10 ~mJycm2! pulse21, low enough to avoiddamage to even fragile aperture substrates. Flu-ence at the work surface can reach .10 Jycm2, highenough to ablate almost any material. For most ofthis work, laser fluence at the workpiece is kept below;1 Jycm2.

As illustrated in Fig. 1, the laser beam remainsfixed while precision X–Y servodriven stages movethe workpiece under the focusing objective. Thework surface is imaged with the same objectivethrough a visible-transmitting dichroic beam splitterand onto a CCD camera, permitting real-time obser-vation of the process. An appropriate cutting-toolshape and size are chosen and achieved when anappropriate beam aperture and demagnification fac-tor are selected. Apertures are cut from metal foil ora poly~imide! sheet by use of the same micromachin-ing system. This simple approach avoids the ex-pense and the complexity associated with maskgeneration, maintenance, and alignment. The min-imum feature size is determined ultimately by theablation spot size; ;1 mm is a practical minimum forthe current system.

A PC-based motion-control system drives the ser-vostages over a travel range of 100 mm 3 250 mm.The stages are addressable to 0.25 mm and are spec-ified by the manufacturer to be accurate to 61 mmover the full range of travel. The excimer laser isexternally triggered, and firing is coordinated withstage motion such that shots can be fired at specifiedincremental distances down to 0.25 mm, even for arcsor arbitrary curves. The maximum writing speedusually depends on the required shot spacing, limitedby the maximum laser repetition rate of 2000 Hz.Mechanical vibration of the stages, exacerbated byhigh acceleration, may also restrict the stage velocity.Typical write speeds for DOE fabrication are of theorder of ;200–5000 mmys.

Fig. 1. Schematic of the UV laser micromachining workstationused for this work. Beam size and shape are selected under com-puter control from an array of apertures and imaged onto theworkpiece. The UV objective ~Obj! with the CCD camera allowsthrough-the-lens real-time monitoring. The integrated opticalsurface profiler permits rapid characterization and refinement ofthe micromachining process. Diffractive elements can be de-signed and fabricated at a single workstation. L’s, lenses; BS,beam splitter.

A noncontact, optical profilometer ~Wyko RST-Plus! is directly integrated into our DOE micromach-ining system. Based on white-light interferometry,this microscope is used to characterize depth andsmoothness.10 It has an in-plane resolution of ;0.5mm and a depth resolution of a few nanometers overa vertical range of 500 mm. The optical profiler andthe micromachining system share the same motionsystem, making it possible to machine a test piece,move to the profilometer for characterization, andreturn to the micromachining system to make anynecessary adjustments, all without removing the sub-strate from its original fixture.

3. Poly(imide) Laser Material Study

It is well known that high-intensity UV pulsed laserscan uniformly ablate material from the surface of awide range of substrates11,12 through a processsometimes called photoablation. UV photoablationcan be so accurately controlled that it is now used toresculpt human corneas to correct nearsightedness.13

To date, most of our DOE micromachining has beendone with poly~imide!. Poly~imide! films and coat-ings are preferred for several reasons. Poly~imide!absorbs strongly and ablates cleanly at excimerwavelengths. This allows precise depth control anda repeatable three-dimensional ~3D! structure withminimal debris. In addition, poly~imide! is rela-tively durable, nonhygroscopic, chemically inert, andthermally stable. It is transparent through much ofthe visible and near-infrared regions. When spin-coating techniques are used, uniform thicknesses ofless than 400 nm to greater than 25 mm can beachieved on a variety of substrate sizes. There areat least two other attractive properties of poly~imide!films. They can be metal coated to produce reflec-tive structures or electroplated to form masters forreplication.14 They can also be used as resists forreactive ion etching and ion-beam milling patterntransfer to other substrate materials such as fusedsilica. These aspects will be addressed in a futurepublication.

The purpose of the material study was to deter-mine the ablation characteristics of our poly~imide!films so that the direct-write fabrication process couldbe optimized. We measured the etch depth of thepoly~imide! film as a function of laser shot fluence @in~millijoules per centimeters squared! times inversepulses# and total dose. Depressions were machinedinto a 2-mm-thick poly~imide! film ~DuPont PyralinPI-2801! that had been cured with a hot-plate bake to350 °C. A 5 mm 3 5 mm 248-nm beam was imagedonto the substrate, and 10 basins, each 100 mm 3 100mm, were milled into the film by raster scanning thestages. The first basin in each set received a singlepass of pulses on 5-mm centers, evenly covering thearea. The second was treated in the same way ex-cept that the procedure was repeated twice, doublingthe total dose. The third was repeated three times,and so on for the 10 depressions in each set. Themilling was carried out under computer control andtook only a few minutes. Using the same repetition

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rate used to mill the sample areas, we measured theaverage laser-pulse energy by using a detector ~StarTech DUV-G110! that had been previously refer-enced to a laser calorimeter ~Scientech MC2500!.The average shot energy was varied by the insertionof calibrated UV neutral-density filters before theobjective. The average single-pulse energies rangedfrom 17 to 97 nJ, with corresponding shot fluences of67 to 389 ~mJycm2! shot21. Although shot-to-shotfluctuations of 610% or more were observed, the av-erage shot energy at constant repetition rate is quitestable, approximately 62% or better over the timeperiod of the experiment. The total dose deliveredto the poly~imide! film covered a range from 67 to2330 mJycm2. At doses greater than 2330 mJycm2,the film was ablated almost all the way to the fused-silica substrate. To be more precise, we have notedthat the KrF laser ablation process leaves behind athin ~;10–30-nm! transparent film on the substrate.The film is presumably a reaction product, but itscomposition is not known. It does not wash awaywith common solvents nor is it preferentially dis-solved in poly~imide! stripper solution. The mate-rial appears to absorb weakly at 248 nm and cannotbe removed with repeated laser pulses.

Ablation depths were measured with the opticalprofilometer. The average depth was determined bycomparison of the average depth of the 104-mm2 ma-chined recess with the average depth of the surround-ing unexposed area. The averaging minimizes thepotentially obscuring effects of beam inhomogeneityand pulse variations.

Figure 2 summarizes the results of the study. Atsingle pulse fluences of $139 ~mJycm2! pulse21, thelinear relation between ablation depth and total doseis striking. In this fluence range, the depth is astraight-line function of total dose alone. For in-stance, at a total dose of approximately 1100 mJycm2,data points from three shots at 389 mJycm2, fiveshots at 227 mJycm2, six shots at 190 mJycm2, andeight shots at 139 mJycm2 closely coincide. Thebest-fit line among all data points indicates that the

Fig. 2. Measured average ablation depth ~in nanometers! versustotal dose ~in millijoules per centimeter squared! at 248 nm forDuPont Pyralin Poly~imide! PI-2801. Symbols correspond to anaverage fluence of individual pulses used to ablate depressions:~■! 389, ~F! 227, ~3! 190, ~E! 139, ~{! 67 ~mJycm2! pulse21.

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simple relation between ablation depth and total doseis ;725 nmy~Jycm2! under these conditions. Thisresult, in qualitative agreement with that of oth-ers,11,12 leads to the conclusion that accuracy andprecision in the micromachining process dependmainly on the ability to maintain precise control ofthe delivered dose as long as the single-shot fluence issufficiently high.

There is a departure from linearity, however, atlow pulse fluence, less than ;100 mJycm2. The 67-~mJycm2! pulse21 data show that the slope of theablation depth versus dose fit drops significantly to;360 nmy~Jycm2!, although the depth is still a linearfunction of dose. The decreasing ablation efficiencyat low fluences can be important. One consequenceis that even small variations in beam homogeneitycan become exaggerated in the ablation profile. Hotspots in the beam will tend to burn in while coolerareas ablate minimally or not at all. Attempts togain fine control of 3D micromachining by use of lowmaterial removal rates ~,100 nmypulse! are frus-trated by this effect. ArF lasers ~193 nm! tend to beless susceptible to this problem,11 but the importantadvantage of a high repetition rate offered by thewaveguide excimer technology would have to be sac-rificed.

4. p-Phase Diffractive Structures

True binary ~two-level or p-phase! diffractive struc-tures are generally the easiest to fabricate by anymethod. Their chief disadvantage is that, at most,40.4% of the incident light is diffracted into the orderof choice.15 However, the relative intensity of thefirst diffracted order at the focus of a p-phase lens willexceed that of all others by orders of magnitude.Off-axis designs can be used to reduce scattering fur-ther from undesired orders.16 Thus reduced effi-ciency may not be a significant drawback for manyapplications, especially when the relative cost bene-fits are considered.

Figure 3 diagrams our process for rapid fabrication

Fig. 3. Fabrication process for binary ~two-level! DOE’s. Thepolymer layer is spin coated to a thickness t that corresponds to ap-phase shift for the design wavelength l. Selected areas areablated to the transparent fused-silica substrate to create the dif-fractive structure.

of binary elements. A layer of poly~imide! is spincoated onto a fused-silica window and cured by bak-ing. The poly~imide! thickness is chosen to corre-spond to a desired phase shift at the designwavelength. Fused silica is an ideal substrate be-cause it is transparent at 248 nm and has a laserdamage threshold ;100 times greater than that forpoly~imide!. Maintaining a relatively high UV laserfluence results in clean poly~imide! cuts with sharpedges and minimal debris. These substrates can becoated in quantity and stored indefinitely. Ap-phase shift for a 633-nm He–Ne laser correspondsto a poly~imide! ~n > 1.64! thickness of 495 nm. Anearlier inaccurate estimate for the index ofpoly~imide! led us to specify a coating thickness ofonly 405 nm for the samples described here. How-ever, this ;18% phase error will result in only a;10% decrease in the maximum theoretical diffrac-tion efficiency.17 The measured diffraction efficiencyof our two-level lenslets was always within 10% of thetheoretical maximum.

The coated blank is mounted on a tilt stage, whichin turn is mounted on an X–Y servoplatform. Theblank was leveled to ensure flatness of travel over thedimensions of the element to be fabricated. A smalljet of filtered He directed at the work area was helpfulin removing small particulates formed in the ablationprocess.

The motion-control computer interface operatesunder Microsoft Windows, and motion can be directedfrom custom programs or from spreadsheet macrosby calls to compiled libraries. We have written sim-ple programs in VISUAL BASIC as well as MicrosoftEXCEL macros that define the structures and thencommand the motion and laser firing directly. Al-ternatively, structures can be drawn with almost anytwo-dimensional computer-aided-design ~CAD! pack-age, imported and automatically translated to motioncode. Complex formats and long mask-definitionfiles are not required.

The actual machining was carried out in one of twomodes, contour or bitmap, depending on the design ofthe element. In the contour mode, stage motion iscommanded as a series of lines, arcs, or splines in anycombination. It is similar in action to a common penplotter, except that the platen moves instead of thepen. The contour mode is particularly suited to fab-rication of lenses and gratings in which the location ofthe phase transitions lies along continuous paths orcontours. In the bitmap mode, the stages are ras-tered back and forth in a series of parallel lines whilethe desired ablation pattern is clocked out much as ina video display. The bitmap mode is well suited tofabrication of complex phase elements such asCGH’s.18

A. Contour-Mode Machining

The UV laser ablation spot is adjusted to be round, tohave sharp walls, and to remove almost all of thepoly~imide! layer with a single shot. As discussed inSection 3, a thin ~;10–30-nm! transparent film is lefton the substrate surface, even after many shots, and

allowance should be made when the optimalpoly~imide! thickness is calculated. The laser is syn-chronously fired with the motion such that pulses arespaced evenly and overlap sufficiently to producesmooth sidewalls along the cutting path. Typicalcut diameters used are in the range of 2–30 mm. Ifcare is taken, the cutting width and the edge positioncan be controlled to 60.5 mm. We have constructedlinear gratings with a feature size as small as 1 mm.

Once the beam size has been accurately deter-mined, the desired motion program can be generated.To illustrate, we outline the procedure to fabricate aphase-daisy zone plate,19 as shown in Fig. 4.Starting with a spreadsheet macro program, the userenters the overall size of the element, the desiredfocal length, laser kerf, and maximum cut spacing.The program uses these parameters to determine aseries of circular tool paths that will result in twocomplimentary p-phase lenses. The patterns areimported into a CAD program by means of the DXFfile format. The imported circles are trimmed towedged segments and spliced together—simple oper-ations in any CAD program. The tool paths for onequarter of the zone plate are shown in detail in Fig. 5.In this example, the cutting beam is 10 mm in diam-eter, the maximum line spacing is 6 mm, and the lensdiameter is 1 mm. The part was fabricated in fewerthan 6 min. It is a fully functioning p-phase diffrac-tive element. No subsequent processing is required.

If one considers the fabrication of a zone plate witha fixed beam size, the beam size must be smaller thanor equal to the width of the smallest outer zone. Alllarger zones are machined by the overlapping of thebeam until the zone is milled. A more efficient ap-proach, one that we have now incorporated, allowsthe beam size to change during fabrication. First, alinear array of up to 50 apertures of increasing diam-eter is fabricated with our same workstation. Theaperture diameter step size is set to correspond tocutting widths that are in increments of ;0.5–1 mm.The array is mounted on a stepper stage perpendic-ular to the optical axis in the object plane of theobjective ~Fig. 1!. Therefore it is possible to control

Fig. 4. Phase-daisy zone plate,19 1 mm in diameter, micromach-ined in poly~imide! on fused silica. Fabrication time ,6 min.

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the cutting-beam size automatically. When thismethod is used for machining, an aperture size isselected and positioned under computer control sothat each zone is machined with the fewest number ofpasses possible, often a single pass. The time sav-ings are considerable. For example, a 2-mm-diameter, 15-mm focal-length lens would take 37 minto be machined with a fixed beam smaller than its4.8-mm minimum feature size. With the automatedaperture mount, the same lens can be machined in 16min, a time savings of approximately 59%.

B. Bitmap-Mode Machining

Bitmaps provide a flexible format to calculate andspecify diffractive elements. We use square UV la-ser ablated pixels, typically 2.5–15 mm on a side.The laser energy is adjusted to ablate to the fusedsilica with a single shot. We developed software todisplay a bitmap on the computer monitor and trans-late it to appropriate motion and laser-firing instruc-tions. A p-phase bitmap is imported as a text tableof 1’s and 0’s. The software allows simple repetitionof the imported unit cell if desired. The X–Y stagestrace out a serpentine raster pattern as the bitmap isexecuted.

When large pixels are used, writing speed is lim-ited by the maximum stage velocity, ;15 mmys.With small pixels, although the laser can operate to 2kHz, the stage velocity must be restricted further,owing to mechanical vibrations that can be a signif-icant fraction of the pixel size. Although we havedemonstrated write speeds of up to 1500 pixelsys,500–1000 pixelsys are more the norm. Pixel regis-tration can be held to better than 61 mm in both Xand Y under these conditions. At 1000 pixelsys, a

Fig. 5. CAD-generated tool paths for one quadrant of the lens inFig. 4. Design cutting-beam kerf is 10 mm in diameter, and themaximum line spacing within a zone is 6 mm, which ensures suf-ficient overlap of adjacent cuts.

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1024 3 1024 bitmap, covering an area of 1 cm 3 1 cm,can be executed in less than 20 min. Figure 6 showsa detail from a typical two-level bitmap, and Fig. 7shows its reconstruction. Here, a 128 3 128 unit cellis replicated in a 2 3 2 format to produce a 256 3 256pixel array. The pixel size is 10 mm, and the recon-structed hologram is the NASA Worm logo.

The techniques described are simple yet powerful,flexible, and efficient for fabrication of two-level dif-fractive elements. We have employed them to fash-ion a wide variety of DOE’s, including cylindrical,spherical, aspheric, and toroidal lenses, square, rect-angular, and hexagonal lenslet arrays, linear grat-ings, one- and two-dimensional Dammann gratings,as well as laser beam shaping elements and spatialhomogenizers. The CAD interface is particularlypowerful for the construction of arrays. The designof a single element is imported, trimmed to the de-sired unit cell size and shape, then propagated arbi-trarily with simple copy and paste functions.

5. Multilevel and Blazed Structures

Improving the diffraction efficiency beyond the ;40%limit for two-level structures requires blazing of thegrating. Another advantage is the suppression ofghost images from light scattered into other orders.

Fig. 6. Detail from a two-level bitmapped CGH. Material is 400-nm-thick poly~imide! on fused silica. The pixel size is 10 mm.

Fig. 7. CCD image of the two-level CGH in Fig. 6 reconstructedwith 633-nm He–Ne laser light.

Fig. 8. Figure columns illustrate three approaches to micromachining blazed gratings. The first approach requires a beam withdimensions that are small compared with the period of the grating. The beam has a uniform intensity profile, and each pulse removesa small volume of substrate material. This method is flexible and well suited to bitmapping. The second approach uses a triangular-or trapezoid-shaped beam, also of uniform intensity. When a series of adjacent pulses is laid, deeper ablation is seen in areas of greaterspatial overlap, resulting in a pitched depth profile or blaze. The third approach uses a rectangular beam whose intensity profile is shapedto produce a sloping depression from a single shot.

The literature documents many examples of preciselyfabricated diffractive structures with demonstratedefficiencies that approach 100%,20 although the pro-duction of these elements by traditional lithographictechniques is a complex and costly process requiringmultiple exposure and etch cycles. Excimer micro-machining of these blazed gratings represents a for-midable challenge. Accuracy and precision in theplane of the structure depend mainly on the laserspot size and the quality of motion control. Ex-tending the process to the third dimension, depth,adds new factors, including laser fluence, beam in-tensity profile or homogeneity, and shot-to-shot re-producibility, as well as spot registration, overlap, orstitching patterns. Deviation from an ideal depthprofile can result in light scattering into undesiredorders. Smaller-scale surface roughness leads todiffuse scattering. Either implies a loss of diffrac-tion efficiency.

Figure 8 outlines three general approaches todirect-write micromachining of blazed gratings.The first is simply to focus the beam to a size that issmall compared with the grating period. The laserintensity is adjusted such that the depth of materialremoved per shot is small compared with the maxi-mum desired depth. In this way, 3D pixellike vol-ume elements can be removed with each laser pulse.Control of depth can be achieved by the varying of thebeam energy or number of shots per unit area. Allour structures were made with constant-pulse en-ergy. Although the 3D-bitmap method is the mostversatile in that arbitrary 3D surface structures canbe fabricated, it has a number of disadvantages.Chief among them is the relatively slow writingspeed. The small spot size and the low fluence de-

manded mean that most of the available laser-pulseenergy is discarded.

The second approach uses beam shaping to in-crease the writing speed dramatically. A triangularor trapezoidal aperture ~see Fig. 1! is imaged onto theworkpiece. The height of the triangle or the dis-tance between the parallel sides of the trapezoid de-termines the grating period. The stage is translated~or rotated! in a direction parallel to these sides, andthe laser-pulse spacing is set so that there is littleoverlap at the narrow side. The wider side receivesmore overlapped shots, resulting in greater depth.The ratio of the lengths of the parallel sides and theshot spacing determine the blaze angle. A great ad-vantage in speed, perhaps a factor of 20 or more, isrealized because a much larger fraction of the laser-pulse energy is utilized.

A further increase in writing speed can be attainedby the third method, laser intensity-profile shaping.21

Here, the laser beam passes through a graded attenu-ator, resulting in a ramped intensity profile in theimage plane. The blaze angle may be achieved in aslittle as a single shot. We briefly describe resultsobtained with each of the three approaches.

A. Three-Dimensional-Bitmap Machining

Four-level bitmap machining is conducted in muchthe same way as that for two-level bitmaps exceptthat the laser fluence is decreased to obtain finerdepth resolution. Square UV laser ablated pixels,typically 4–10 mm, are used. To obtain a four-levelstructure, the laser fluence is adjusted to ablate to adepth corresponding to an ;py2-phase shift at thedesign wavelength. Shot-to-shot pulse energy fluc-tuations of roughly 610% produce similar deviations

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from the ideal depth. Care was taken to obtain themost uniform ablation profile possible by adjustmentof the focus and the profiling of depressions fromindividual test shots. The results of laser beam in-homogeneity and diffraction effects, including spotedges with exaggerated intensity and gridlike inter-ference patterns across the depression, could clearlybe seen in the profilometer plots.

We are currently working on ways to minimizeroughness and pixel edge overcut. However, the re-sulting depth errors were such that eight-level struc-tures for visible wavelengths were probablyimpractical. We limited our efforts to four levels.The bitmap is imported as a text table of 3’s through0’s, corresponding to the desired ablation depth inmultiples of py2. The motion system then executesthree complete passes over the work area, each withapproximately constant beam energy. The appro-priate pixels thus received three, two, one, or noshots.

A 3D contour plot of a four-level CGH is shown inFig. 9. Here the pixel size is 5 mm. The total arraysize is 512 3 512. When a 700-Hz laser repetitionrate and a 3.5-mmys scan velocity are used, the ma-chine time for the entire device is under 20 min. Thereconstruction of the NASA logo as captured by avideo camera is shown in Fig. 10. In addition toCGH’s, we have machined multilevel spherical lensesand linear gratings by this method.

The theoretical maximum first-order diffraction ef-ficiency of the four-level diffractive optical structureis 81%.15 To date, we have measured first-order dif-fraction efficiencies ranging from 45% to 50% of trans-mitted light. Diffraction into any other order is lessthan 10%. Approximately 20%–25% of the light ap-pears to be lost to wide-angle scatter. The surfaceroughness ~average roughness, Ra ; 35–50 nm, scalesize $0.5 mm! within each pixel as well as stitchingerrors likely accounts for the loss in efficiency.

B. Scanning Triangles

The scanning-triangle method has proven to be aneffective technique for the fabrication of structureswith smooth surfaces and thus higher diffraction ef-ficiencies. In general, triangular patterns with

Fig. 9. 3D contour plot of a detail of a four-phase-level bitmappedCGH machined into poly~imide!. The pixel size is 5 mm. Whiteareas are unablated. The plot was measured with the Wyko op-tical surface profilometer. Reconstruction is shown in Fig. 10.

4672 APPLIED OPTICS y Vol. 36, No. 20 y 10 July 1997

greater than a 3 to 1 ratio of base to height are ade-quate for the slopes required by most blazed DOE’s.Smaller base-to-height ratios result in an insufficientdose gradient. The triangle height is used to deter-mine the grating period. To achieve the desiredslope and optimize smoothness, pulse spacing andfluence are adjusted based on measurements with theoptical surface profiler. We have found that low av-erage fluences ranging from 25 to 80 mJycm2 perpulse and many overlapped pulses ~shot spacings of0.5–2 mm! are suitable for the fabrication of blazedgratings with periods of 12–40 mm and depths of 1–2mm. Surface roughness along the sloped walls par-allel to the grating lines shows a typical Ra of 25–35nm. We have measured diffraction efficiencies of.70% for 633-nm light. Figure 11 shows a profile ofa 40-mm-period grating fabricated by the scanning-triangle method.

For blazed gratings, the overall machine time canbe quite short. For example, at a laser repetitionrate of 1500 Hz, a 1-cm2 linear grating with a 24-mmperiod can be fabricated in less than 1 h. A simple,blazed diffractive lens would require a rotation andtranslation stage combination and a customtriangular-aperture set, with a different aperture foreach zone. However, rotation stages with less thana few micrometers of wobble tend to be large and

Fig. 10. CCD image of reconstruction with 633-nm light of thefour-level GGH shown in Fig. 9. The CGH has a 256 3 256 3four-level unit cell that is repeated in a 2 3 2 array. Overall DOEsize is 2.56 mm 3 2.56 mm. Note the effective suppression of them 5 21 order when compared with the two-level version ~Fig. 7!.

Fig. 11. 40-mm-period blazed grating fabricated by the scanning-triangle method.

expensive. Slightly more sophisticated DOE’s, suchas toroidal lenses, require a change in grating pitchwithin a zone and thus are not easily fabricated withthis technique.

C. Laser Intensity-Profile Shaping

We have briefly investigated laser intensity profilingas a method for the production of blazed gratings.We used commercially available graded attenuators,partially silvered mirrors whose transmission de-creased linearly across its aperture. The laser hadto be expanded to pass through a significant region ofthe filter ~20–40 mm! to allow for an adequate inten-sity gradient. The beam was then reduced ;1003to illuminate a rectangular aperture. A ;103 de-magnified image of that aperture was projected ontothe work surface. Alignment and good control of theintensity gradient were found to be problematic.

The 35-mm-period blazed grating in Fig. 12 wasmachined by laser intensity shaping. Here a 25-mm-diameter laser beam was passed through a lin-ear transmission filter ~0%–100% over 100 mm!placed between the laser and the first lens in Fig. 1.The beam was reduced with a 300-mm focal-lengthlens and overfilled a ;350 mm 3 350 mm aperture.The aperture was demagnified at the work surface toproduce a 35 mm 3 35 mm image. The intensitygradient was insufficient to blaze to the correct anglewith a single shot, so pulses were overlapped in amanner similar to that of the scanning-trianglemethod. By use of a 800-nm-thick poly~imide! film,the pulse spacing was set at 2.5 mmyshot and the scanspeed set at 500 mmys. The transmission filter po-sition was adjusted to produce an optimal intensityprofile for machining a ramped structure. The sig-nificant shot overlap required indicates that only aweak fluence gradient was achieved. The best mea-sured diffraction efficiency achieved with this tech-nique was ;64% into the first order.

We have not pursued this approach because of dif-ficulties in its implementation. Compared withthose of the scanning-triangle method, the overallperformance and the machine time of gratings aresimilar. However, it has been our experience thatthe scanning-triangle method is easier to set up andoptimize.

Fig. 12. 35-mm-period blazed grating fabricated by laserintensity-profile shaping.

6. Conclusions

We have shown that a tabletop system that incorpo-rates direct-write 248-nm UV laser ablation and non-contact surface profilometry is suitable for the rapidfabrication of a wide range of binary, multilevel, andblazed DOE’s. Spin-coated poly~imide! films, whichstrongly absorb UV radiation, can be machined withcontrollable material remove rates. For laser shotfluences in the range of ;100–400 ~mJycm2! pulse21,the ablation depth is a highly linear function of thetotal dose ~in millijoules per centimeter squared!.

We have investigated two machining methods, con-tour and bitmap. In the contour mode, laser firingand workpiece motion are described as a series oflines, arcs, and splines. This mode is most effectivein the fabrication of two-level DOE’s with phase con-tours that lie along continuous paths. In the bitmapmode, the stages are rastered back and forth in aseries of parallel lines while the desired ablation pat-tern is clocked out a pixel at a time. The bitmapmode is well suited to the fabrication of complexphase elements such as CGH’s.

High-quality two-level structures can be quicklymachined with this technique. Poly~imide! is spincoated onto fused-silica substrates to a thickness thatwill yield a p-phase shift for the design wavelength.Shaped laser pulses remove the poly~imide! coatingcleanly but leave the underlying substrate undam-aged. Coating thickness can be easily controlled to610–30 nm so depth errors can be reduced to insig-nificant levels. The resulting element has excellentsurface smoothness, and wide-angle scattering isminimal. Absolute diffraction efficiencies of $38%in the first order have been measured for two-levelelements.

We have investigated multilevel bitmap, scanning-triangle, and graded-intensity machining methods.To date, we have machined four-level bitmapped el-ements with first-order efficiencies of 45%–50% oftransmitted light. Most losses are due to wide-anglescatter as surface profiles of individual pulses revealthat beam inhomogeneity and diffraction effects pro-duce surface roughness. Overlapping pulses of trap-ezoidal and triangular apertures are shown to be aneffective means for fabricating linear blazed gratingswith first-order diffraction efficiencies greater than70%. However, this technique becomes difficult ifmore complex DOE’s are desired. The same conclu-sions can be drawn for graded-intensity machining.Here, blazed gratings are produced when the laser ispassed through a graded transmission filter before itis imaged on the work surface. We have fabricatedblazed gratings with first-order efficiencies of 64%this way.

In the future we plan to investigate methods that willimprove beam homogeneity and reduce scatter fromroughness to increase diffraction efficiency. We are con-tinuing studies of methods to produce DOE’s from mate-rials other than poly~imide!, including pattern transferinto glass substrates by etching processes. Through im-provements in software, hardware, and technique, we

10 July 1997 y Vol. 36, No. 20 y APPLIED OPTICS 4673

are working to produce further improvements in quality,accessibility, speed, and cost of DOE’s.

This work is supported by a Phase II Small Busi-ness Innovative Research contract with the NationalAeronautics and Space Administration ~ContractNAS8-40572!.

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