holographic printing

4 | IEEE nanotEchnology magazInE | september 2014 1932-4510/14©2014IEEE

OOVER THE lasT TwO dEcadEs, we have witnessed an unprecedented technological advance in nanotechnology. By engineering at quantum-realm scales, devices and material performances can be optimized within a single characteristic wavelength of an electron wave or opti-cal wave to reach their ultimate potential, which is limited only by the laws of phys-ics. as such, products and services based on nanotechnology, as well as the nano-manufacturing technology that produces them, are becoming increasingly impor-tant to our economy.

at present, a large number of fabri-cation approaches have been used to produce functional nanostructures and devices. They can be loosely clas-sified as top-down and bottom-up approaches [1], [2]. The bottom-up approaches seek to build complex structures through self-assembly [2] using more fundamental nano-components such as nanoparticles. On the other hand, the top-down

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Holographic Printing of Three-Dimensional Photonics Structures

Digital Object Identifier 10.1109/MNANO.2014.2326968Date of publication: 11 July 2014

A very large-scale integration approach.

september 2014 | IEEE nanotEchnology magazInE | 5

approaches, such as deep ultraviolet (UV) or e-beam lithography, sculpt out nano-structures using bulk materials in a more predictable way. Both the top-down and the bottom-up approaches have yielded great successes on planar surfaces for two-dimensional (2-d) nanostructure fabrica-tion. Nanostructures with a feature size of 10–45 nm can be produced commercially at the 15-in wafer scale, such as cutting-edge very large-scale integration (VlsI) computer chips.

However, the fabrication of three-dimensional (3-d) nanostructures remains one of the greatest challenges for the modern manufacturing industry. Three-dimensional nanostructures with a feature size less than a single wavelength of an optical wave have found vast appli-cations in energy, optical communication, and sensing. Engineering 3-d structures at the length scale, that is, a fraction of a wavelength of characteristic quantum waves, has yielded some amazing engi-neering possibilities such as fundamental alternation of blackbody radiation spec-trum [3] for high-efficient thermal and photovoltaic power conversation, nega-tive index metal materials [4], ultrahigh Q optical resonators for lasers [5], and ultrasensitive optical sensors. all of these applications are hinged on the develop-ment of a reliable, low-cost, large-scale, and tunable 3-d nanomanufacturing technique for the low-cost, large-scale production of nanostructures.

currently, 3-d nanofabrication can be addressed by both the top-down and the bottom-up approaches. For example, the construction of 3-d photonic crys-tal structures can be deterministically pro-duced using a top-down, layer-by-layer lithography [1], [3], [6]–[13]. alternative-ly, the nanosphere self-assembly approach, a bottom-up approach [2], [14]–[17], can be used to construct 3-d nano-optical structures with a length scale smaller than a single wavelength of a plasmonic electron wave. However, structures through such construction rely mainly on the intersphere interaction, which cannot be easily recon-figured and optimized.

HOLOGRAPHIC LITHOGRAPHYanother prominent 3-d nanofabrication approach is achieved through multibeam

laser holographic lithography [18], which is considered to be one of the most attractive approaches due to its control-lability, flexibility, and, most importantly, scalability. laser holographic lithography is generally considered to be a top-down fab-rication scheme. Three-dimensional nano-structures are first defined in photoresist, and functional materials, such as silicon, are then produced through various infiltra-tion and inversion processes [2], [17].

In contrast to the top-down, layer-by-layer lithography processes, the laser holographic fabrication has clear advan-tages in terms of low complexity and low manufacturing cost. while the layer-by-layer approach often requires more than

ten laser exposures to lithographically produce two to five unit cell structures, the laser holographic fabrication can pro-duce structures with identical symmetry in one or two laser exposures.

compared with bottom-up self-assembly processing, the laser holo-graphic approach can also be superior in terms of scalability, controllability, and configurability. It can produce 3-d nanostructures with all symmetries defined by the Bravis lattices, inscribe functional defects, and be scaled up to produce 3-d structures over a large area in one or two laser exposures.

Generally speaking, the laser holo-graphic lithography approach is achieved through the angles, amplitudes, and phase controls of multiple coherence laser beams to form interference pat-terns. The current technology to per-form holographic lithography is not without shortcomings. Traditionally, multibeam interference was realized by a large number of bulk optical components such as mirrors, beam splitters, and lens-es. These optical setups are among the most difficult to align and are highly

susceptible to thermal and mechanical variations. Furthermore, using these bulk optical elements to produce holographic patterns is incompatible with the photo-lithography process used for most opto-electronic chip fabrication, which often uses one optical element such as a metal mask. This incompatibility brings the major challenge of integration of 3-d photonic structures with other functions in integrated circuit units.

Given these challenges, researchers around the world have made concerted efforts since 2004 to develop a holo-graphic lithography technique that is sim-pler, more robust, more flexible, and, most importantly, VlsI compatible

[19]–[23]. These research efforts have led to the development of holographic lithography techniques using one optical element and one laser exposure.

This article details these exciting advances in laser holographic lithography. In the last decade, various approaches have been reported on the development of laser holographic lithography using a single optical element. Most of these approaches use either a diffractive optical element or a deflective optical element. In this article, two examples are used to illus-trate the essence of these fabrication approaches and to highlight the associated challenges. On the basis of the discussion of the current state of the art, we will present a solution for improving the fabri-cation flexibility and robustness of the laser lithography processes using adaptive optics technology.

HOLOGRAPHIC LITHOGRAPHY USING DIFFRACTIVE OPTICAL ELEMENTSdiffractive optical elements, such as surface relief gratings on a glass plates, have long been used for various optical applications.

Gradient structures can be obtained by designing super-cells with

different gray levels in the SLM.

6 | IEEE nanotEchnology magazInE | september 2014

diffractive optical elements have also been used to produce interference fringes for the purpose of laser fabrication. a prominent example is the phase masks used in fiber Bragg grating fabrication.

Figure 1 shows sEM images of rectan-gular and hexagonal 2-d phase masks fabricated in sU8 photoresists and their optical diffraction patterns. The multiple beams produced by the phase masks can be used to construct 3-d interference patterns. However, the fabrication of 3-d photonic structures using a phase mask, such as those shown in Figure 1, is not straightforward. It is relatively easy to generate multiple diffractive laser beams with the desired angles and amplitude using a diffractive optical element. How-ever, the controls of phases among dif-fractive beams are rather challenging, and phase controls are critical to produce 3-d

photonic structures. Here, we use the 2-d phase mask shown in Figure 1(a) to highlight this challenge.

The 2-d phase mask we used for the 3-d fabrication has a rectangular lattice [Figure 2(a)]. The diffraction angle and diffraction efficiency of four first-order diffraction laser beams labeled (1,0), (–1,0), (0,1), and (0, –1) relative to those of (0, 0) order central beam were mea-sured to be 20° and 10%, respectively. The four second-order diffraction beams had a much lower diffraction efficiency at 1.5%. The self-interference of five beams produced by the 2-d phase mask shown in Figure 2(b) yields face-centered-cubic or FcT structures. The FcT structures generated in photoresist are not intercon-nected and will be destroyed after the photoresist development. Furthermore, the FcT structures do not possess a

sufficient symmetry to produce a large photonic bandgap. a diamondlike lattice structure is more desirable.

Figure 2(a) and (b) illustrates the path for lattice translation from FcT to dia-mondlike structures, which have the larg-est bandgap among all photonic crystal structures. The diamondlike structures can be viewed as the superposition of two FcT structures [20]. This can be achieved by two exposures through the 2-d phase mask on the same photoresist sample with the phase mask displaced along the [201] direction for a quarter of the FcT unit cell diagonal length between exposures. The phase mask offsets xT and zT [shown in Figure 2(a)] between two exposure pat-terns are . a0 5 and . ,c0 25 respectively, where a and c are the lattice constants of the FcT structure in the x and z direc-tion, respectively.

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These displacements were performed with high accuracy by three-axis high- precision motorized linear stages with a resolution of 150 nm. The simulated isointensity surfaces of the first FcT, sec-ond shifted FcT, and final superimposed structures are shown in Figure 2(b).

The 3-d template was fabricated in a thick sU8 film sample of 20 μm. Figure 2(c) and (d) shows the sEM top

view of the diamondlike structures recorded in sU8 [23]. The enlarged view of the sur-face feature in Figure 2(d) is consistent with the simulation result of the (001) plane of a diamondlike structure. The cross-linking between two FcT structures formed by the two laser exposures produces a stable 3-d template for further inversion processes to create high-index contrast structures. Fig-ure 2(e) shows the photonic band structure

for / .c a 1 5= for the silicon inverse struc-ture. The filling faction of silicon to achieve such an optimal 27% band gap, as shown in Figure 2(e), is approximately 18.4%.

as we can see from this work, the cum-bersome bulk optical elements for holo-graphic lithography are all but eliminated. Three-dimensional photonic crystal structure templates can now be produced using one diffractive optical element as shown above.

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FIGURE 2 (a) A sketch of the propagation of light through an orthogonal 2-D phase mask. (b) The diamondlike structure constructed by double exposures with one face-centered-tetragonal (FCT) pattern shifted by .x 0 5aT = and . .z 0 25cT = An SEM (c) top view and (d) enlarged view of the fabricated structures. (e) A photonic band diagram for the inverse FCT structure and FCT-based diamondlike structure [3].


However, this phase mask configura-tion still involves two laser exposures and a very precise mechanical movement of the mask to produce the needed phase delay to generate two FcT structures with a proper displacement. This is because phase masks based on surface gratings cannot set phases for different diffractive beams. The need for two laser exposures and a precise mechanic movement is still difficult for VlsI-style production.

HOLOGRAPHIC LITHOGRAPHY USING DEFLECTIVE OPTICAL ELEMENTSanother way to produce interference patterns from one optical element is to use deflective optical elements. One such optical device is a simple prism.

deflective optical elements, such as top-cut prisms, can also be used to pro-duce and combine multiple laser beams for holographic fabrication. we have explored this technique for one laser exposure 3-d fabrication.

The single optical element used to con-struct the five-beam interference pattern is a top-cut, four-sided prism, as shown in Figure 3(a). One laser beam is incident from the bottom side of the prism. after being totally internally reflected at four lat-eral surfaces of the prism, beams 2–5 refract through the top surface of the prism and recombine with beam 1 to form interfer-ence patterns.

The key to producing the diamondlike pattern is to control the phase of one incoming laser beam. To perform the phase modulation, a thin microscope glass cover slide with a uniform thickness is inserted into beam 5. By rotating the glass slide, the phase of beam 5 can be adjusted continu-ously. Figure 3(b) shows the variation of unit cell lattices for the five-beam interfer-ence pattern as the phase change Td evolves from 0 to r in 0.2r increments. The evolu-tion of the phase change Td transforms the interference pattern from face-centered-cubic or FcT structure into interconnected structures. a perfect diamondlike network can be formed with .Td r=

To experimentally validate the phase tuning and structure controlling, 3-d pho-tonic crystal templates were fabricated in photoresist. Figure 4(a) shows an sEM image of the photonic crystal template formed in photoresist with diamondlike structures by one laser exposure. as pre-dicted by the simulation in Figure 4(b), the fine phase tuning transforms the photonic crystal template from the FcT to a dia-mondlike symmetry. The interference pat-tern locked in photoresist clearly shows diamondlike interlaced structures in Figure 4. The surface of the photoresist film shown in Figure 4 is not completely perpendicular to the normal incident laser beam but, instead, is cut through the (001) surface of the diamondlike structure at a small angle. These results in surface

topology represent diamondlike structures at different depths. The comparison between the simulation and the experiment shown in Figure 4 confirms this specula-tion. Figure 4(g)–(j) shows the computed five-beam interference pattern, where one beam has a phase retardation of π relative to the other four beams and its selected cross-section planes along the height (z) direction. The fabricated topographic images shown in Figure 4(c)–(f) match well with those simulation planes.

The results shown in Figures 1–4 demonstrated that the fabrication of highly complex 3-d nanostructures (e.g., diamondlike) can be accom-plished with a single laser beam using a diffractive/deflective optical element. These works promise a truly scalable nanofabrication technique for 3-d struc-ture construction.

However, the nanofabrication tech-niques displayed in the figures are not without disadvantage. Neither a diffractive optical element nor a deflective optical ele-ment can provide full control of the phase, amplitude, and divergence of individual laser beams. This deficiency severely limits the applicability of the diffractive/deflec-tive optical elements in the laser holo-graphic fabrication. Two techniques are being studied to expand the reconfigu-rability and applicability of the one-optical-element hologram approach without losing its intrinsic merits.

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FIGURE 3 (a) The experimental setup of the five-beam interference with one beam modulated by a glass slide. (b) The isosurface of the unit cells of the phase modulated five-beam interference pattern Imod. The phase change Δδ varies from 0 to π in 0.2π increments [4].


PHASE MASKS WITH BUILT-IN PHASE CHANGEThe phase mask approach is an attrac-tive technique to fabricate 3-d photonic structures because of its potential to be complementary metal-oxide-semiconduc-tor (cMOs)-compatible. The diffractive optical elements used for 3-d photonic structure construction can be incorporat-ed into the photolithography amplitude masks used in optoelectronic circuit fab-rications and, thus, enable a full integra-tion of 3-d photonic structures on-chip. However, surface relief phase grating cannot control the relative phase among diffractive laser beams as demonstrated earlier. This deficiency of the phase con-trol has limited the phase mask approach

from the direct fabrication of some of most important 3-d photonic structures such as woodpile or diamondlike pho-tonic crystal structures. The fabrication of interconnected woodpile structures was accomplished by double laser expo-sures. The phase change, which is criti-cal for the formation of interconnected structures, was introduced by a precise displacement (1/4 of the woodpile lat-tice) of the recorded interference pat-tern between two laser exposures. This elaborated fabrication scheme reduces the cMOs compatibility of the phase mask approach.

To address this challenge, we have developed a multilayer diffractive optical element to reduce two laser exposures to

one exposure, which completely removes the need for the phase mask displacement.

The phase mask used in this work is a two-layer phase grating. Two phase grat-ings with desired orientations are separat-ed by a spacer layer. The desired phase changes among different diffractive laser beams can be controlled by the thickness of the spacer layer. By doing so, the need for precise phase mask displacement is eliminated. Interconnected 3-d photonic crystal structures, such as woodpile struc-tures, can be directly fabricated in photo-resist using one laser exposure [24]. This critical improvement enables the fabrica-tion of complex 3-d photonic structures by a single laser exposure through a single optical element.

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FIGURE 4: (a) SEM images for the photoresist templates of the interconnecting diamondlike structure produced by the five-beam interference with a phase retardation. (b) The computed five-beam interference pattern and its selected cross-section planes along the height (z) direction [4].


Figure 5(a) depicts the two-layer phase grating fabrication process in sU8. The polydimethylsiloxane (PdMs) grating mold was first used to imprint grating pat-terns on sU8-2035 photoresist under a flood UV irradiation source. The exposed sU8 was partially polymerized by postbak-ing at 65 °c for 10 min. The PdMs mold is then peeled off from the sU8.

To produce the second grating layer, an sU8 thin film was coated directly on the PdMs mold by a spin-coating pro-cess as shown in Figure 5(a). The spin speed determines the film thickness, which is important to the “built-in” phase delay of the mask. after the second laser exposure on the sU8 film coated on the PdMs mold, both sU8 layers are brought into contact to form a two-layer structure. The two-layer mask was bond-ed at 95 °c for 20 min under 50 kPa

pressure [25]. The mask was further hardened by a hard-baking process at 200 °c for another 20 min. The interme-diate layer between two gratings is about 22 μm thick, which is produced by the spin-coating process at 2,000 r/min. an sEM image of a bound two-layer phase mask is shown in Figure 3(b). a two-layer grating with orthogonal orientations is clearly visible with a spacer layer.

when a single beam goes through the first layer of grating, it produces diffractive beams in the x–z plane as shown in Figure 5(b) labeled (0, 0), (1, 0), and (−1, 0) orders. The (0, 0) beam incurs a different phase from (±1, 0) beams through the intermediate layer due to the propagation path difference. The second layer of grating further diffracts the beams in the y–z plane to form nine diffractive beams labeled (0, 0), (0, ±1), (±1, 0), and

(±1, ±1), respectively. Figure 5(c) shows the diffraction pattern of the grating. Uni-form diffractive beams were found, and the intensity ratios were measured as 50%: 10%: 1.5% for the (0, 0) order: (±1, 0) and (0, ±1) orders: (±1, ±1) orders, respective-ly, which is perfectly consistent with the simulation result.

The highest-order beams (±1, ±1) have a much weaker intensity as measured and have negligible effects on interference pat-terns. Therefore, the two-layer phase masks will produce five-beam interference pat-terns by (0, 0), (±1, 0), and (0, ±1) beams. when a plane wave propagates through the top layer of the phase mask, phases for beams (0, 0) and (0, ±1) are ( / )21d r m=

·n·d and /1 1d d= ,cosi respectively, where n and d are the index of refraction and the thickness of the spacer layer, respectively. The (0, 0) order beam was

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further split into (0, 0) and (0, +/–1) beams after propagating through the sec-ond layer of the phase mask. since the peri-ods for both layers of grating are identical, the phase difference between the (0, ±1) and the (0, 0), (±1, 0) beams remains

2 1Td d d= - after the second layer of the phase mask.

If the built-in phase delays Td for five beams are zero or integral multiples of ,r the generated interference pattern has FcT symmetry, as shown in the 3-d sim-ulation in the inset of Figure 6(a). when Td are odd integral multiples of / ,2r 10, the 3-d template will evolve into the interconnected woodpile structures shown in Figure 6(b).

Figure 6(a) and (b) shows sEM imag-es of the 3-d photonic crystal by using the thermal controlled two-layer phase mask. Figure 6(a) reveals FcT structures when the phase delay 2 1Td d d= - was not well-controlled. since FcT structures are not interconnected, only one layer of period structures was left on the glass slide after the photoresist development. The fabricated structures match the simu-lated structure with zero phase delay

.02 1Td d d= - = when the phase

delays are approaching those values of odd multiples of / ,2r thicker and inter-connected 3-d structures start to develop. Figure 6(b) shows a woodpilelike 3-d multilayer structure. The surface mor-phology closely matches the simulation results shown in the inset with a phase delay of ./22 1Td d d r= - =

RECONFIGURABLE PHASE MASKS USING ADAPTIVE OPTICS ELEMENTSIn holographic fabrication, the above-mentioned single diffractive optical element or defection optical element is considered a stat ic mask, which provides a f ixed interference pattern. Using an electronically programma-ble spatial light modulator (slM) as a digitally tunable phase mask, 3-d holo-graphic interference patterns can be changed to yield a truly reconfigurable and scalable 3-d fabrication tool, as shown in Figure 7.

when a laser beam is incident onto the slM, the optical phase of each pixel of a liquid crystal array can be adjusted in real time to arbitrarily sculpt the wave front of the incoming laser beam on the fly.

Through relay Fourier optics, the phase information can be used to control the phase, amplitude, and divergence angles of the entire or part of the laser beam.

Figure 7(a) shows a phase control approach that we have used. as an exam-ple, a hexagon is divided into six equal-area and rotationally symmetric sections with each section having a different gray level as shown in the inset of Figure 7(a). such a hexagon pattern works as a phase mask because different gray levels correspond to different phases. when an incident laser is reflected by such a mask, we can obtain six first-order side beams and one central beam, and other beams are blocked by the filter. These beams can form an interference pattern. after demagnification and laser exposure, we can obtain 3-d structures, as shown in Figure 7(b). we can obtain 3-d struc-tures with small feature size using large demagnification optics. The desired defects can be obtained by incorporating intrinsic defects in the phase pattern dis-played in the slM. In general, the defect in the interference pattern can be obtained by assigning a constant gray into the desired defect area. By

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FIGURE 6 SEM images of fabricated structures in the photoresist through an orthogonal two-layer phase mask with symmetries of (a) FCT and (b) a woodpile structure. The insets are the simulation structures for comparison with parameter setting (a) 0Td = and (b) / .2Td r=


overlapping the zeroth-order diffracted beam with the first-order diffracted beams, 3-d photonic structures and a positive defect line can be formed and fabricated as shown in Figure 7(b).

we can also build up a phase pattern in the slM for 3-d woodpile structures and gradient structures. as shown in Figure 8(a), a checkerboard phase pattern consists of square unit cells with each unit cell divided into four square (8 # 8 mm2) pixels (the pixel size of a commercial slM). when the laser beam is diffracted by the checkerboard pattern, one can get a zeroth-order and four first-order laser beams. Other diffraction orders can be fil-tered out by a Fourier filter. The phases of the diffracted first-order beams can be

digitally tuned by the gray levels in the square unit cell. we can fabricate woodpile structures as shown in Figure 8(b) when the phase of two first-order beams is phase delayed by /2r relative to two other first-order beams by setting up gray levels of 30, 255, 30, and 255 for dark gray, light gray, dark gray, and light gray pixels.

Gradient structures can be obtained by designing super-cells with different gray levels in the slM. as shown in Figure 8(a), there are 3 # 3 unit cells inside the yellow or red squares. The unit cell in the solid yellow square has gray levels of 30, 94, 30, and 94 for dark gray, light gray, dark gray, and light gray pix-els, while the gray levels are 30, 255, 30, and 255 for unit cells inside the red

squares. Figure 8(c) shows the recorded interference pattern using the charge-coupled device camera when the phase pattern is illuminated by a 532-nm laser through a 4f imaging system and a Fou-rier filter [26].

For a comparison, we draw squares around the interference pattern with the same structures as shown in Figure 8(b). The orientation of squares in the inter-ference pattern is rotated by 45° com-pared with the square orientation in the phase pattern. The interference patterns isolated by the yellow, blue, and red squares in Figure 8(c) are determined correspondingly pixel by pixel by the phase pattern surrounded by the yellow, blue, and red squares in Figure 8(a),

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FIGURE 8 (a) A synthesized phase pattern consisting of two types of super cells with each super cell having 3 # 3 unit cells. (b) An atomic force microscope image of fabricated woodpile structures. (c) A charge-coupled device image of the interference pattern formed by the diffracted zeroth-order and four first-order beams from the phase pattern in (a).

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FIGURE 7 (a) The schematics of one of the spatial light modulator (SLM) setups. The phase mask is displayed in the SLM for dynamic wavefront engineering. Interference patterns are formed after the second lens and its demagnification is realized through an objective lens. (b) An SEM image of a fabricated sample using SLM.


respectively. Inside the yellow and red squares in Figure 8(c), the formed struc-tures are similar to the simulated struc-tures in Figure 3(b) (0 and 1p phase shift), respectively. Thus, gradient struc-tures can be obtained using the gradient phase patterns displayed in the slM.

CONCLUSIONThis article describes the transformation-al changes that have occurred in the last few years in the field of laser holograph-ic lithography. as the nanotechnology industry demands a scalable manufactur-ing solution for 3-d nanostructures and devices, researchers around the world are answering the call. The persistent efforts and ingenuities of scientists and engi-neers have transformed laser holographic lithography into a one-optical-element and one-laser-exposure process that is amendable into the existing VlsI fab-rication scheme. leveraged by advances in other fields of optics technology such as adaptive optics, the laser holographic lithography will continue to evolve into a simple, robust, flexible, and scalable manufacturing tool for sophisticated 3-d nanostructures and devices.

ACKNOWLEDGMENTSThis work was supported by the U.s. National science Foundation under grants cMMI-1300273, cMMI-0900564, cMMI-0923006, cMMI-1109971, cMMI-1266251, and dMR-0722754. support was also received from the U.s. air Force.

ABOUT THE AUTHORSDi Xu ([email protected]) earned his B.s. and M.s. degrees in physics from wuhan University, china, in 2001 and 2004, and his Ph.d. degree in electrical and computer engineering from the Uni-versity of Pittsburgh in 2010. after gradu-ation, he worked as a scientist at stanford linear accelerator center National lab-oratory. He is currently a senior optical etrology engineer at Intel corporation.

Zsolt Poole ([email protected]) is currently pursuing his Ph.d. degree at the Univer-sity of Pittsburgh in electrical engineering while conducting research on the fabrica-tion and applications of nanoengineered subwavelength photonic materials.

Yuankun Lin ([email protected]) earned his B.s. and M.s. degrees in physics from Nankai University, china, in 1991 and 1994, respectively, and his Ph.d. degree in physics from the Univer-sity of British columbia in 2000. He is now an associate professor in physics and electrical engineering at the University of North Texas.

Kevin P. Chen ([email protected]) earned his B.s. degrees in physics and control science from Xiamen University, china, in 1994, his M.sc. degree in physics from the University of British columbia in 1998, and his Ph.d. degree in electrical engi-neering from the University of Toronto in 2002. He is an associate professor and the Paul E. lego Faculty Fellow in electri-cal engineering and bioengineering at the University of Pittsburgh.

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