It is shown experimentally that in laser interference lithography, by using a reference grating, respective grating layers can be positioned with high relative accuracy. A 0.001 degree angular and a few nanometers lateral resolution have been demonstrated

Hoekstra, Hugo J.W.M.

Lambeck, Paul V.

Ridder, René M. de

Soest, Frank J. van

Wolferen, Henk A.G.M. van

Wörhoff, Kerstin

English

van Soest, Frank J.

van Wolferen, Hendricus A.G.M.

Hoekstra, Hugo

de Ridder, R.M.

Worhoff, Kerstin

Lambeck, Paul

NARCIS 

Laser interference lithography with highly accurate interferometric alignment

It is shown experimentally that in laser interference lithography, by using a reference grating, respective grating layers can be positioned with high relative accuracy. A 0.001 angular and a few nanometers lateral resolution have been demonstrated

Frank J. van Soest

Henk A. G. M. van Wolferen

Hugo J. W. M. Hoekstra

René M. de Ridder

Kerstin Wörhoff

Paul V. Lambeck

Crossref

Laser Interference Lithography with Highly Accurate Interferometric Alignment

University of Twente Research Information

Laser Interference Lithography with Highly Accurate Interferometric AlignmentFrank J. VAN SOEST, Henk A. G. M. VAN WOLFEREN, Hugo J. W. M. HOEKSTRA,René M. DE RIDDER, Kerstin WÖRHOFF and Paul V. LAMBECKUniversity of Twente, Mesaþ Research Institute, Integrated Optical MicroSystems Group (IOMS),P.O. Box 217, 7500 AE Enschede, The Netherlands(Received April 28, 2005; accepted June 15, 2005; published September 8, 2005)It is shown experimentally that in laser interference lithography, by using a reference grating, respective grating layers can bepositioned with high relative accuracy. A 0.001 angular and a few nanometers lateral resolution have been demonstrated.[DOI: 10.1143/JJAP.44.6568]KEYWORDS: laser interference lithography, lithography, photonic structures, photonic crystals, alignment1. IntroductionThe down-scaling of integrated optical (IO) devices is ofimportance for both theoretical and practical reasons and hasattracted much research. A major step forward in this fieldwas the invention of photonic crystals,1) which are periodicstructures in space (refractive index lattices) with a period inthe order of the wavelength of light. These structures, whichmay prohibit the transmission of light in any direction for acertain wavelength range, the so-called photonic bandgap,provide new opportunities for developing ultra compact keycomponents for integrated optics, such as splitters, wave-length filters, delay lines and light sources, thus promising asize reduction of IO circuits by orders of magnitude. Severalrequirements regarding lattice symmetry, refractive indexcontrast and microstructure should be satisfied in order toobtain a full three-dimensional (3-D) photonic bandgap.One promising way to realize a 3-D photonic crystal, is bypiling submicron spheres, which self-organize into an(inverse) opal structure.2) Another promising techniqueconcerns the oblique deposition on a rotating substrate. Thismay yield spiral structures with the potential to have arelatively large bandgap.3) The controlled introduction ofarbitrary lattice defects (which act as functional structureslike waveguides, etc.) in this type of structures is far fromobvious. However, lithographic fabrication of photoniccrystals in a layer by layer fashion provides a natural wayfor defining defects.A photonic crystal (PhC) structure that is especially wellsuited for lithographic fabrication is the 3-D woodpilestructured photonic crystal (WPC), see Fig. 1. This structurehas been proposed for the first time in 1994.4) Since thattime, four-layer structures with a stop band between 1.35and 1.95 mm,5) eight-layer structures, created with the use ofwafer bonding,6) and even 3-D 90 waveguide bends in awoodpile structure,7) have been demonstrated.For the lithographic production of 3-D PhC’s, a suffi-ciently accurate process is required. Conventional masklithography does not provide the required resolution andalignment accuracy because PhC’s with a bandgap around1550 nm typically contain structural details smaller than500 nm in a high-index material like silicon. By usingexpensive equipment for deep-UV or (relatively slow) directe-beam writing lithography this problem can be overcome.Laser interference lithography (LIL) is a very suitabletechnique for defining periodic structures.However, an excellent alignment of these structures isessential, as has been shown for WPCs.8) For thesestructures, the thickness and the width of the bars are veryrelevant as well, but they have significantly less impact thanmisalignment.8) The influence of a deviation of 20% fromthe optimum thickness and width of the bars, results in abandgap reduction of 18 and 15% respectively. On the otherhand, misalignment in one direction by 10% of the latticeperiod, gives already a bandgap reduction of 30%.The above also shows the importance of parallelism for alayer-by-layer fabrication method since each layer must beindividually aligned with respect to the previously fabricatedones. If a relatively large photonic crystal is desired,requiring, e.g., bars of length L, then a small angularmisalignment  may translate into a large, position depend-ent lateral misalignment up to L sin . For example in thecase that L ¼ 1mm and  ¼ 650 nm (typical for a bandgaparound 1.5 mm in a silicon/air structure), the maximumallowed angular misalignment resulting in a maximumlateral error of 0:1, would be around  ¼ 0:0035.2. Laser Interference lithographyLIL is a maskless lithographic technique using theinterference pattern of two obliquely incident beams. Weused a setup based on the Lloyds interferometer,9) schemati-cally depicted in the inset of Fig. 2. One part of a laser beamreaches a photoresist-covered substrate directly, while asecond part of that beam comes in via a mirror, thusproducing a regular interference pattern in the photoresist.Because the second beam emerges from a mirror that isattached to the substrate, the setup is less sensitive tovibrations than a true dual beam interference setup. TheFig. 1. Example of a 3D PhC structure: the woodpile structure.E-mail: h.j.w.m.hoekstra@ewi.utwente.nlJapanese Journal of Applied PhysicsVol. 44, No. 9A, 2005, pp. 6568–6570#2005 The Japan Society of Applied Physics6568period of the interference pattern on the substrate is given by  =ð2 sin yÞ, where  is the wavelength and y is theangle of incidence, as indicated in Fig. 2.Here we have assumed that the angle between substrateand mirror is exactly 90 and we have neglected smallvariations due to the fact that the angles of incidence of thetwo beams vary slightly over the substrate. For a pinhole at adistance of Rw ¼ 2:3m and a grating of a few cm length thelatter leads to a relative error in the grating period oftypically 105. With a 266 nm wavelength light source,periods of 150 to 500 nm can easily be made. Afterillumination and development of the resist, the grating canbe transferred to the substrate by, e.g., reactive ion etching.3. AlignmentAs mentioned before, the geometry of the structure is ofmajor importance in order to produce a photonic crystalhaving a full bandgap. For the stacking of, e.g., WPC’s, thismeans that the lines of every second next layer have to beparallel within a very high degree and the periods of thegratings should be very well matched, in order to satisfy theconditions for a full 3-D bandgap everywhere in the crystal.So, on putting the sample back into the LIL set up fordefining the next layer, accurate alignment is essential. Torealize this, interferometric positioning can be used, asexplained below.The presented interferometric positioning method is basedon analysis of the interference of two almost identicalwavefronts, at two spots, next to the input pinhole (seeFig. 2), which occur when the reference grating is put backin the set-up. The occurrence of these spots next to thepinhole can be understood from geometrical optics consid-erations, taking into account the finite distance betweenpinhole and substrate, leading to a small variation of thegrating period in the x-direction (see Fig. 2). The first(second) spot originates from the interference of the firstorder diffraction of the direct (reflected) beam and the zerothorder diffraction of the reflected (direct) beam.When the substrate with the reference grating is at exactlythe same position as during the production of the grating, thetwo wave fronts at either one of the spots coincide exactly,yielding two relatively homogenous spots on a fluorescentscreen at both sides of the pinhole. Even a very smallmisalignment of the substrate gives rise to a deviation in theorientations of the two wave fronts. From straightforwardgeometrical optics considerations it can be shown that forsmall angular deviations, assuming substrate and mirror tobe perfectly flat, two regular, striped patterns will appearnext to the pinhole. The periods in the direction perpendic-ular to the central beam in the horizontal plane, Wh, and inthe vertical direction, Wy, can be expressed as:Wh ¼ =ð2y cos yÞ and Wy ¼ =ð2z sin yÞ;respectively. Here y and z are the misalignment angles.The strong dependence of the fringe pattern on themisalignment is shown in Fig. 3. Here, a grating with aperiod of 266 nm was re-aligned. The accuracy of realign-ment was limited by the resolution of the rotational stages,which is 0.001. The deviations from straight fringes areattributed to curvature of the substrate.In addition to the rotational realignment, also accuratepositioning in lateral direction is of utmost importance. InWPC’s, every second next layer has to be shifted laterally byhalf a period with respect to the preceding layer. Afterangular alignment the intensity of the fringe spots stilldepends on the phase shift between the two wave frontscaused by the lateral shift of the grating. Figure 4 shows ameasurement (after angular alignment) of the intensity of thecenter of the fringe spot versus the lateral shift. This shiftwas realized by a piezo driven translation stage with aresolution of 1 nm. The irregularities in the figure are causedby power fluctuations of the laser beam. Relative measure-Fig. 2. Set-up for LIL-alignment of a new interference pattern with respect to an existing pattern.Jpn. J. Appl. Phys., Vol. 44, No. 9A (2005) F. J. VAN SOEST et al. 6569ment of the spot intensity with respect to the beam powerwill give better results. The high sensitivity of the spotpower as function of the grating translation is, however, veryclear.The above alignment feature can be used to produce alarge number of well positioned grating structures insuccessive layers, whereby reference gratings enable accu-rate alignment.4. ConclusionsFrom the above we conclude that interferometric position-ing offers good possibilities for aligning a structuresufficiently accurate to make 3-D PhC’s. The realizedangular re-alignment accuracy of 0.001 was limited by theresolution of the rotational stages. With better rotation stagesand better samples, an improvement in re-alignment accu-racy with one order of magnitude is anticipated to befeasible. This is significantly better than the 0.003 men-tioned in ref. 10.The LIL-setup used offers a translational accuracy of onlya few nanometers, which, again, is a strong improvementcompared to the 17 nm accuracy, reported in ref. 10, andalso compared to the 100 nm accuracy found for theprocedure presented in ref. 11. It is expected that thisaccuracy can be improved to 1 nm or better, (limited by thetranslation table), by using a feedback loop from the spotintensity to the translation table controller. The achievedalignment results are sufficiently accurate for the construc-tion of, e.g., WPC’s over an area of a few square mm.AcknowledgmentThis work was supported by the Dutch TechnologyFoundation STW (grant TMM.5554).1) E. Yablonovitch: Phys. Rev. Lett. 58 (1987) 2059.2) J. E. G. J. Wijnhoven and W. L. Vos: Science 281 (1998) 802.3) O. Toader and S. John: Science 292 (2001) 1133.4) K. M. Ho, C. T. Chan, C. M. Soukoulis, R. Biswas and M. Sigalas:Solid State Commun. 89 (1994) 413.5) J. G. Fleming and S.-Y. Lin: Opt. Lett. 24 (1999) 49.6) S. Noda: Physica B 279 (2000) 142.7) S. Noda, K. Tomoda, N. Yamamoto and A. Chutinan: Science 289(2000) 604.8) A. Chutinan and S. Noda: J. Opt. Soc. Am. B 16 (1999) 240.9) E. Hecht: Optics (Addison-Wesley, Reading, MA, 2002) 4th ed.,p. 399.10) A. Feigel, Z. Kotler and B. Sfez: Opt. Lett. 27 (2002) 746.11) N. Yamamoto and S. Noda: Jpn. J. Appl. Phys. 37 (1998) 3334.-0.002o-0.001o0.001o-0.003o0.003o0.002o0.000o-0.003o -0.002o -0.001o 0.000o 0.001o 0.002o 0.003o∆θy∆θzFig. 3. Different fringe patterns for several combinations of misalignmentangles y and z as observed when a wafer with an existing grating isrepositioned in the set-up of Fig. 2.0 133 266 399 53200.10.20.30.40.50.60.70.80.91Translation (nm)IntensityFig. 4. Intensity of the center of the fringe pattern after angular alignment,as a function of lateral displacement.6570 Jpn. J. Appl. Phys., Vol. 44, No. 9A (2005) F. J. VAN SOEST et al.

ECIO’05: 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS - ABSTRACT -   Figure 1. Example of a woodpile structure. LASER INTERFERENCE LITHOGRAPHY WITH HIGHLY ACCURATE INTERFEROMETRIC ALIGNMENT  Frank J. van Soest, Henk A.G.M. van Wolferen, Hugo J.W.M. Hoekstra,  René M. de Ridder, Kerstin Wörhoff and Paul V. Lambeck University of Twente , Mesa+ Research Institute,  Integrated Optical MicroSystems group (IOMS),  P.O. Box 217, 7500 AE Enschede, the Netherlands h.j.w.m.hoekstra@el.utwente.nl  SUMMARY It is shown experimentally that in laser interference lithography, by using a reference grating, respective grating layers can be positioned with high relative accuracy. A 0.001 degree angular and a few nanometers lateral resolu-tion have been demonstrated. KEYWORD Laser interference lithography, lithography, photonic structures, photonic crystals, alignment.   ABSTRACT   INTRODUCTION The down-scaling of integrated optical (IO) devices is of importance for both theoretical and practical reasons and has attracted much research. A major step forward in this field was the invention of photonic crystals [1], which are periodic structures in space (refractive index lattices) with a period in the order of the wavelength of light. These structures, which may prohibit the propagation of light in any direction for a certain wavelength range, the so-called photonic bandgap, provide new opportunities for developing ultra compact key components for integrated optics, such as splitters, wavelength filters, delay lines and light sources, thus promising a size re-duction of IO circuits by orders of magnitude.  Several requirements regarding lattice symmetry, refractive index contrast and microstructure should be satisfied in order to obtain a full three-dimensional (3-D) photonic band-gap. One promising way to realize a 3-D photonic crystal, is by piling submicron spheres, which self-organize into an (inverse) opal structure [e.g. 2]. Another promising technique concerns the oblique deposition on a rotating sub-strate. This may yield spiral structures with the potential to have a relatively large bandgap [3]. The controlled introduction of arbitrary lattice defects (which act as functional structures like waveguides, etc.) in this type of structures is far from obvious. However, lithographic fabrication of photonic crystals in a layer by layer fashion provides a natural way for defining defects. A photonic crystal (PhC) structure that is especially well suited for lithographic fabrication is the 3-D woodpile structured photonic crystal (WPC), see Fig. 1. This structure has been proposed for the first time in 1994 [4]. Since that time, four-layer structures with a stop band between 1.35 and 1.95 µm [5], eight-layer structures, created with the use of wafer bonding [6], and even 3-D 90-degree waveguide bends in a woodpile structure [7], have been demonstrated. For the lithographic production of 3-D PhC’s, a sufficiently accurate process is required. Conventional mask lithography does not provide the required resolution and alignment accuracy because PhC’s with a bandgap around 1550 nm typically contain structural details smaller than 500 nm in a high-index material like silicon. By using expensive equipment for deep-UV or (relatively slow) direct e-beam writing lithog-raphy this problem can be overcome. Laser Interference Lithography (LIL) is a very suitable technique for defining periodic structures. However, an excellent alignment of these structures is essential, as has been shown for WPC’s [8]. For these structures, the thickness and the width of the bars are very relevant as well, but they have significantly less impact than misalignment [8]. The influence of a deviation of 20% from the optimum thickness and width of the bars, ECIO’05: 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS - ABSTRACT -  results in a bandgap reduction of 18% and 15% respectively. On the other hand, misalignment in one direction by 10% of the lattice period, gives already a bandgap reduction of 30%. The above also shows the importance of parallelism for a layer-by-layer fabrication method since each layer must be individually aligned with respect to the previously fabricated ones. If a relatively large photonic crystal is de-sired, requiring e.g. bars of length L, then a small angular misalignment θ may translate into a large, position de-pendent lateral misalignment up to L sin θ . For example in the case that L = 1mm and Λ = 650 nm (typical for a bandgap around 1.5 μm in a silicon/air structure), the maximum allowed angular misalignment resulting in a maximum lateral error of 0.1 Λ, would be around θ = 0.0035o.    LASER INTERFERENCE LITHOGRAPHY Laser Interference Lithography (LIL) is a maskless lithographic technique using the interference pattern of two obliquely incident beams. We used a setup based on the Lloyds interferometer [9], schematically depicted in the inset of figure 2. One part of a laser beam reaches a photoresist-covered substrate directly, while a second part of that beam comes in via a mirror, thus producing a regular interference pattern in the photoresist.  Because the second beam is emerges from a mirror that is attached to the substrate, the setup is less sensitive to vibrations than a true dual beam interference setup. The period of the interference pattern is given by Λ ≈ λ/(2 sinθy) , where λ is the wavelength and θy is the angle of incidence, as indicated in figure 2.    Figure 2. LIL alignment of a new interference pattern with respect to an existing pattern. The main figure is a 3D schematic showing the degrees of freedom. Inset: principle of interference between 0th and 1st order reflections from the existing grating.   Here we have assumed that the angle between substrate and mirror is exactly 90 degrees and we have neglected small variations due to the fact that the angles of incidence of the two beams vary slightly over the substrate. For a pinhole at a distance of Rw = 2.3 m and a grating of a few cm length the latter leads to a relative error in the grating period of typically 10–5. With a 266 nm wavelength light source, periods of 150 nm to 500 nm can easily be made. After illumination and development of the resist, the grating can be transferred to the substrate by, e.g. reactive ion etching.  ALIGNMENT As mentioned before, the geometry of the structure is of major importance in order to produce a photonic crystal having a full bandgap. For the stacking of, e.g. WPC’s, this means that the lines of every second next layer have ECIO’05: 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS - ABSTRACT -  -0.002o-0.001o0.001o-0.003o0.003o0.002o0.000o-0.003o -0.002o -0.001o 0.000o 0.001o 0.002o 0.003oÄèyÄèzFigure 3. Different fringe patterns for several combination of  misalignment angles Δθy and Δθz as observed when a wafer with  an existing grating is repositioned in the set-up of figure 2. 0 133 266 399 53200.10.20.30.40.50.60.70.80.91Translation (nm)Intensity Figure 4. Intensity of the center of thefringe pattern after angular alignment, asa function of lateral displacement.  to be parallel within a very high degree and the periods of the gratings should be very well matched, in order to satisfy the conditions for a full 3-D bandgap everywhere in the crystal. So, on putting the sample back into the LIL set up for defining the next layer, accurate alignment is essential. To realize this, interferometric positioning can be used, as explained below. The presented interferometric positioning method is based on analysis of the interference of two almost identical wavefronts, at two spots, next to the input pinhole (see Figure 2), which occur when the reference grating is put back in the set-up. The occurrence of these spots next to the pinhole can be understood from geometrical optics considerations, taking into account the finite distance between pinhole and substrate, leading to a small variation of the grating period in the x-direction (see figure 2). The first (second) spot originates from the interference of the first order diffraction of the direct (reflected) beam and the zeroth order diffraction of the reflected (direct) beam. When the substrate with the reference grating is at exactly the same position as during the production of the grat-ing, the two wavefronts at either one of the spots, coincide exactly, yielding two relatively homogenous spots on a fluorescent screen at both sides of the pinhole. Even a very small misalignment of the substrate gives rise to a deviation in the orientations of the two wave fronts. From straightforward geometrical optics considerations it can be shown that for small angular deviations, assuming substrate and mirror to be perfectly flat, two regular, striped patterns will appear next to the pinhole. The periods in the direction perpendicular to the central beam in the horizontal plane, Wh, and in the vertical direction, Wy, can be expressed as: /(2 cos )h y yW λ θ θ= Δ  and /(2 sin )y z yW λ θ θ= Δ , respectively. Here Δθy and Δθz are the misalignment angles. The strong dependence of the fringe pattern on the misalignment is shown in Figure 3. Here, a grating with a period of 266 nm was re-aligned. The accuracy of re-alignment was limited by the resolution of the rotational stages, which is 0.001o. The deviations from straight fringes are attributed to curvature of the substrate. In addition to the rotational realignment, also accurate positioning in lateral direction is of utmost importance. In WPC’s, every second next layer has to be shifted laterally by half a period with respect to the preceding layer. After angular alignment the intensity of the fringe spots still depends on the phase shift between the two wave fronts caused by the lateral shift of the grating. Figure 4 shows a measurement (after angular alignment) of the intensity of the center of the fringe spot versus the lateral shift. This shift was realized by a piezo driven transla-tion stage with a resolution of 1 nm. The irregularities in the figure are caused by power fluctuations of the laser beam. Relative measurement of the spot intensity with respect to the beam power will give better results. The high sensitivity of the spot power as function of the grating translation is, however, very clear.  ECIO’05: 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS - ABSTRACT -  The above alignment feature can be used to produce a large number of well positioned grating structures in suc-cessive layers, whereby reference gratings enable accurate alignment.    CONCLUSIONS It can be concluded that interferometric positioning offers good possibilities for aligning a structure sufficiently accurate to make 3D PC’s. The realized angular re-alignment accuracy of 0.001o was limited by the resolution of the rotational stages. With better rotation stages and better samples, an improvement in re-alignment accuracy with one order of magnitude is anticipated to be feasible. This is significantly better than the 0.003o mentioned in [10]. The LIL-setup used offers a translational accuracy of only a few nanometers, which, again, is a strong improve-ment compared to the 17 nm accuracy, reported in [10], and also compared to the 100nm accuracy found for the procedure presented in [11]. It is expected that this accuracy can be improved to 1 nm or better, (limited by the translation table), by using a feedback loop from the spot intensity to the translation table controller. The achieved alignment results are sufficiently accurate for the construction of e.g. WPC’s over an area of a few square mm.  ACKNOWLEDGMENT This work was supported by the Dutch Technology Foundation STW (grant TMM.5554).  REFERENCES [1] E. Yablonovitch, "Inhibited spontaneous emission in solid-state physics and electronics," Phys. Rev. Lett., vol. 58, pp. 2059-2062, 1987. [2]  J.E.G.J. Wijnhoven and W.L. Vos, “Preparation of photonic crystals made of air spheres in titania,” Science, vol. 281, pp.802–804, 1998. [3]  O. Toader and S. John, “Proposed square spiral microfabrication architecture for large threedimensional photonic band gap crystals,” Science, vol. 292, pp. 1133–1135, 2001. [4]  K.M. Ho, C.T. Chan, C.M. Soukoulis, R. Biswas, and M. Sigalas, “Photonic band gaps in three dimensions: new layer-by-layer periodic structures,” Solid State Communications, vol. 89, pp. 413–416, 1994. [5]  J.G. Fleming and Shawn-Yu Lin, “Three-dimensional photonic crystal with a stop band from 1.35 to 1.95 µm,” Optics Letters, vol. 24, pp. 49–51, 1999. [6]  S. Noda, “Three-dimensional photonic crystals operating at optical wavelength region,” Physica B, vol. 279, pp. 142–149, 2000. [7] S. Noda, K. Tomoda, N. Yamamoto, and A. Chutinan, “Full Three-Dimensional Photonic Bandgap Crystals at Near-Infrared Wavelengths,” Science, vol. 289, pp. 604-606, 2000.  [8]  A. Chutinan and S. Noda, “Effects of structural fluctuations on the photonic bandgap during fabrication of a photonic crystal,” Journal of the Optical Society of America B, vol. 16, pp. 240–244, 1999. [9]  E. Hecht, Optics, 4th Edition, Addison-Wesley, 2002.  [10]  A. Feigel, Z. Kotler, and B. Sfez, “Scalable interference lithography alignment for fabrication of threedimensional photonic crystals,” Optics Letters, vol. 27, pp.746–748, 2002. [11]  N. Yamamoto and S. Noda, “100-nm-Scale alignment using laser beam diffraction pattern observation techniques and wafer fusion for realizing 3D PC structure”, Jpn. J. Appl. Phys. Vol. 37, pp. 3334-3338, 1998.   

100-nm-Scale alignment using laser beam diffraction pattern observation techniques and wafer fusion for realizing 3D PC structure”,

Effects of structural fluctuations on the photonic bandgap during fabrication of a photonic crystal,”

Full Three-Dimensional Photonic Bandgap Crystals at

Inhibited spontaneous emission in solid-state physics and electronics,&quot;

Optics, 4 th Edition,

Photonic band gaps in three dimensions: new layerby-layer periodic structures,”

Preparation of photonic crystals made of air spheres in titania,”

Proposed square spiral microfabrication architecture for large threedimensional photonic band gap crystals,”

Scalable interference lithography alignment for fabrication of threedimensional photonic crystals,”

Three-dimensional photonic crystal with a stop band from 1.35 to 1.95 µm,”

Three-dimensional photonic crystals operating at optical wavelength region,”

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Laser interference lithography with highly accurate interferometric alignment

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