The authors demonstrate the use of interferometric-spatial-phase-imaging (ISPI) to control a gap distance of the order of nanometers for parallel optical near-field nanolithography. In optical near-field nanolithography, the distance between the optical mask and the substrate needs to be controlled within tens of nanometers or less. The ISPI technique creates interference fringes from checkerboard gratings fabricated on the optical mask, which are used to determine the gap distance between the mask and the substrate surfaces. The sensitive of this gapping technique can reach 0.15 nm. With the use of ISPI and a dynamic feedback control system, the authors can precisely align the mask and the substrate and keep variation of the gap distance below 6 nm to realize parallel nanolithography. (C) 2013 American Vacuum Society

Moon, Euclid E.

Srisungsitthisunti, Pornsak

Traverso, Luis M.

Wen, Xiaolei

Xu, Xianfan

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsBirck and NCN Publications Birck Nanotechnology Center7-2013High precision dynamic alignment and gap controlfor optical near-field nanolithographyXiaolei WenBirck Nanotechnology Center, Purdue University; University of Science & Technology - China, wen24@purdue.eduLuis M. TraversoBirck Nanotechnology Center, Purdue University, ltravers@purdue.eduPornsak SrisungsitthisuntiBirck Nanotechnology Center, Purdue University, psrisung@purdue.eduXianfan XuBirck Nanotechnology Center, Purdue University, xxu@purdue.eduEuclid E. MoonMassachusetts Institute of Technology (MIT)Follow this and additional works at: http://docs.lib.purdue.edu/nanopubPart of the Nanoscience and Nanotechnology CommonsThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Wen, Xiaolei; Traverso, Luis M.; Srisungsitthisunti, Pornsak; Xu, Xianfan; and Moon, Euclid E., "High precision dynamic alignmentand gap control for optical near-field nanolithography" (2013). Birck and NCN Publications. Paper 1432.http://docs.lib.purdue.edu/nanopub/1432High precision dynamic alignment and gap control for optical near-fieldnanolithographyXiaolei Wen, Luis M. Traverso, Pornsak Srisungsitthisunti, Xianfan Xu, and Euclid E. Moon  Citation: Journal of Vacuum Science & Technology B 31, 041601 (2013); doi: 10.1116/1.4809519 View online: http://dx.doi.org/10.1116/1.4809519 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/31/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing                    Advertisement:  Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27High precision dynamic alignment and gap control for optical near-fieldnanolithographyXiaolei WenSchool of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette,Indiana 47906 and Department of Optics and Optical Engineering, the University of Science and Technologyof China, Hefei, Anhui 230026, ChinaLuis M. Traverso, Pornsak Srisungsitthisunti,a) and Xianfan Xub)School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette,Indiana 47906Euclid E. MoonDepartment of Electrical Engineering and Computer Science, Massachusetts Institute of Technology,Cambridge, Massachusetts 02139(Received 11 February 2013; accepted 21 May 2013; published 6 June 2013)The authors demonstrate the use of interferometric-spatial-phase-imaging (ISPI) to control a gapdistance of the order of nanometers for parallel optical near-field nanolithography. In optical near-field nanolithography, the distance between the optical mask and the substrate needs to becontrolled within tens of nanometers or less. The ISPI technique creates interference fringes fromcheckerboard gratings fabricated on the optical mask, which are used to determine the gap distancebetween the mask and the substrate surfaces. The sensitive of this gapping technique can reach0.15 nm. With the use of ISPI and a dynamic feedback control system, the authors can preciselyalign the mask and the substrate and keep variation of the gap distance below 6 nm to realizeparallel nanolithography.VC 2013 American Vacuum Society. [http://dx.doi.org/10.1116/1.4809519]I. INTRODUCTIONNanofabrication technologies play important roles in theprogress of nanoscience and nanotechnology. Among vari-ous nanofabrication methods, optical near-field nanolithogra-phy using near-field optical focusing elements has greatpotentials due to its low-cost and possibility of parallel oper-ation.1,2 However, radiation from the exit-plane of near-fieldoptical elements is only collimated within tens of nano-meters, beyond which it diverges and its intensity decreasessignificantly.2–4 Thus, a precise control over the separationdistance between the near-field optical elements and thephotoresist-coated surface is a great challenge to realize opti-cal near-field nanolithography.1,3,5,6 Srituravanich et al. uti-lized the computer hard disk air-bearing slider technique toobtain a nanoscale gap in thermal lithography.7 Kim et al.reported integrating a solid immersion lens with a metallicsharp-ridged nanoaperture and using a gap error signal fromevanescent coupling within the air-gap to realize sub-100 nmnanolithography.8In this work, a gap-control system named interferometric-spatial-phase-imaging (ISPI) is utilized for real-time feed-back in a near-field optical nanolithography system. TheISPI gapping is based on the detection of interference fringesfrom a set of specially designed gratings on an opticalmask.9–12 By analyzing interference signals, the frequencyand phase information of the interference fringes can beobtained and used to derive the value of the gap between themask and the substrate. The ISPI system employs obliquelight incident geometry so that it can be integrated with thelithography system without interfering with the lithographyprocess. In theory, the ISPI gratings can be designed for anarbitrarily gap range.13 For example, it has been used forgaps of tens of micrometers for parallel zone plate arraylithography12,14 and materials growth.15 In this work, wedemonstrate using ISPI to control a gap less than 20 nm fornear-field optical nanolithography with sensitivity in thesubnanometer lever. Moreover, we demonstrate real-timedetection and feedback control of the gap during the nanoli-thography process. Therefore, with the use of ISPI, we canalign the optical mask and the photoresist substrate to a highdegree of parallelism and dynamically control the gap duringthe lithography process, and to realize parallel nanopattern-ing and move toward scalable parallel lithography. Wedemonstrate parallel lithography results using a 55 array ofbowtie aperture antennas as focusing elements, which pro-duce feature line-width below 90 nm.II. EXPERIMENTAL DETAILSThe experimental setup of the near-field optical lithogra-phy system consists of two subsystems as shown in Fig. 1:the lithography system and the ISPI system. In the lithogra-phy system, bowtie aperture antennas are chosen as thefocusing element due to their ability of field localization andenhancement at the nanometer scale.3–5,16 A bowtie apertureantenna is illustrated in Fig. 2(a). When the incident light ispolarized across the gap (along the y-direction), a current isinduced in the antenna and flows toward and concentrates atthe tips at the gap, which produces an optical spot as smallas the gap [Fig. 2(b)]. This near-field optical spot, on theother hand, is subjected to strong divergence. For example, aa)Present address: Production Engineering Department, King Mongkut’sUniversity of Technology North Bangkok, Bangkok 10800, Thailand.b)Electronic mail: xxu@ecn.purdue.edu041601-1 J. Vac. Sci. Technol. B 31(4), Jul/Aug 2013 2166-2746/2013/31(4)/041601/5/$30.00 VC 2013 American Vacuum Society 041601-1 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27spot of 50 nm can double its size at a distance 50 nm fromthe antenna and its intensity also drops rapidly. Therefore,the effective working distance of a bowtie aperture antennaduring a lithography process is well below 50 nm.The ISPI gap detection is based on the analysis of fringesproduced by light diffraction off chirped gratings referred toas transverse chirp gapping.10 As shown in Fig. 3(a), theISPI grating pattern consists of a pair of two dimensionalcheckerboards, whose y-periodicity is uniform and thex-periodicity is chirped oppositely. The y-periodicity isdesigned for oblique angle to avoid interfering with the li-thography processing beam. The x-periodicity creates inter-ference fringes that are sensitive to the mask–substrate gap.Since the x-periodicity is chirped, the transmitted-diffractedangle of the incident beam will vary along the x-direction asshown in Fig. 3(b). After reflecting from the substrate, thetransmitted-diffracted beam will reach the mask some dis-tance away from the incident position and rediffract. All there-diffracted beams will then interfere and display a set ofinterference fringes at the imaging plane of the ISPI micro-scope as shown in Fig. 3(a). The position and number of thefringes are sensitive to the gap distance. When the gap ischanged, the two sets of fringes will translate oppositelyalong the x-axis due to the counter chirped direction of thetwo checkerboard gratings. By analyzing the interferencesignal, we can extract the frequency (i.e., the number of thefringes shown in the x-direction) and the relative phase shiftof the two opposite fringes, and then derive the gap valuefrom such information.Figure 2(c) illustrates the optical mask, which was fabri-cated on a piece of 0.5 in. square optically flat quartz cov-ered by a 70-nm layer of chromium. It contains several sets(six shown) of ISPI patterns and a number of islands (fourshown). The bowtie apertures were milled on top of theisland using focused ion beam milling [Fig. 2(d)]. The use ofthe islands can reduce the contact area between the maskand the photoresist (S1805, from Shipley) surfaces, so as toreduce the probability of contamination from dust particles.An SEM image of the bowtie apertures is shown in Fig. 2(d).To implement the ISPI system, its microscope is mountedat an oblique angle above the lithography system using a 6-axis manually controlled stage so that it can be alternated toFIG. 1. (a) Illustration of the experimental setup; (b) photo of the experimen-tal setup.FIG. 2. (a) Sketch of the bowtie aperture, (b) its optical transmission inten-sity at the exit plane, (c) sketch of the mask, and (d) SEM image of a bowtiearray on top of one island (the inset is a zoom-in image of one of the bowtieapertures).041601-2 Wen et al.: High precision dynamic alignment and gap control 041601-2J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27different ISPI grating patterns on the mask during the align-ment process. A fiber laser with a 660 nm wavelength isattached to the microscope. It incidents onto the ISPIpatterns, diffracts through the gap between the mask andsubstrate, and then reflects back to the microscope. Theangle between the incident beam and the reflect beam is setto 4.The lithography process utilizes a frequency-tripleddiode-pumped solid state UV laser (k¼ 355 nm) as the expo-sure source. To ensure the uniformity of the exposure doseon the entire bowtie aperture array for parallel lithography,the laser beam is expanded to cover an area of about 2 cm2.The substrate is held by a piezoelectric stage, which canmove in x-, y-, and z-directions with a 0.4 nm resolution.Above the substrate, the mask is held by another piezoelec-tric stage which can adjust the tip/tilt angles (hx, hy) foralignment. The entire setup is built inside a cleanroom-gradesemiclosed box to minimize contamination from dustparticles.III. RESULTS AND DISCUSSIONA. Gap detection and control using ISPIIn our work, the ISPI pattern along the x-direction wasdesigned to be chirped as a quartic function so that the fre-quency of the fringes would change linearly with the gap asshown in the simulation result [Fig. 4(a), square dots]. Whenan ISPI fringe image is obtained and its frequency and phaseinformation is analyzed, the gap value can then be obtainedusing a calibrated linear equation. Figure 4(b) shows thechange of the experimental data with the height of the piezo-electric stage. Since the position of the mask is fixed, whenthe height of the piezoelectric stage is increased, the gapbetween the mask and the substrate is decreased. From theexperimental data, the frequency signal shows oscillationssuperimposed on the predicted linear trend. These oscilla-tions are due to Fabry–Perot interference within the gap. Onthe other hand, the phase signal does not have any oscilla-tion. Therefore, using the phase signal to determine the gapis more reliable than frequency gapping. From calibration,one phase cycle (2p) corresponds to about 150 nm of gapchange. The detection algorithm in our system is capable todetect <1/1000 of a phase cycle; thus, the sensitivity ofphase gapping is 0.15 nm (150 nm/1000).We also implemented a real-time feedback control systemto maintain the gap distance. Figures 5(a) and 5(b) illustrategapping with and without the feedback control. When thereis no feedback, the gap drifts for hundreds of nanometerseven when the piezoelectric stage is not moving, which canbe caused by the background noises from the vibration of thepiezoelectric stage or thermal expansion. With the feedbackcontrol, the gap can remain at the desired value within about4 nm. When the piezoelectric stage moves, Fig. 5(c) showsvariation of the gap distance is about 40 nm over a scanningdistance of 100 lm without the feedback control, which canbe caused by a small tip–tilt angle from misalignment. WhenFIG. 3. (a) Sketch of ISPI checkerboard gratings and typical fringe imagecaptured by the ISPI camera. (b) Illustration of the working mechanism ofISPI.FIG. 4. (a) Simulated and (b) experimental relation of the frequency andphase of the fringes vs the gap.041601-3 Wen et al.: High precision dynamic alignment and gap control 041601-3JVST B - Microelectronics and Nanometer Structures Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27the feedback control is activated, the variation of the gap dis-tance is limited to 6 nm [Fig. 5(d)].B. ISPI alignmentAs mentioned before, for near-field optical nanolithogra-phy, the alignment between the mask and the substrate iscritical. For parallel lithography using multiple antennas, itis even more important to achieve a high degree of parallel-ism. We obtain gap values on different ISPI patters on themask [Fig. 2(c)] and then adjust the tip/tilt of the mask tomake sure same gap values are obtained on different ISPIpatterns and the mask is parallel to the substrate (Fig. 6).Typically, the process of reading the gap values and adjust-ing the tilt angles of the mask relative to the substrate needsto be repeated several times to achieve a parallelism within0.03 mrad. To demonstrate parallel lithography, we use a55 antenna array, which covers an area of 0.04mm2.Within this area, a tilt of 0.03 mrad corresponds to a heightdifference of 6 nm.C. Parallel nanolithographyAfter aligning the mask parallel to the substrate (within0.03 mrad), the photoresist substrate is brought into contactwith the surface of the mask. The ISPI data are recordedwhen the substrate is moved toward the mask as shown inFig. 7(a). It is seen that the gap decreases linearly when thesubstrate is moving toward the mask. Then there is a suddenturn of the slope of the gap reading, after which the gapFIG. 5. ISPI gapping with and without feedback: (a) and (b) statically, and(c) and (d) during scanning.FIG. 6. Illustration of alignment using the ISPI detection: (a) before align-ment; (b) after alignment.041601-4 Wen et al.: High precision dynamic alignment and gap control 041601-4J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27keeps a constant value. This indicates that the substrate hasmade a contact with the mask at the point when the slopechanges. Therefore, from this contact point, the distancebetween the bowtie aperture antennas and the photoresistsurface can be determined by retracting the substrate awayfrom the mask. This provides a means to operate with asmall gap to both minimize friction and maintain a gap dis-tance within tens of nm for near-field nanolithography.Figure 7(b) shows the lithography result with a gap dis-tance of 10 nm and a scanning speed of 1.5 lm/s, under alaser intensity of 70 mW/cm2. The line-width is measured tobe around 60 nm [Fig. 7(c)]. With the use of ISPI gappingand feedback control, we are able to achieve parallel nanoli-thography. Figure 8 shows the result of lithography using a55 bowtie aperture antenna array. The line-width of thefeatures is about 86 nm. The small variations among differ-ent patterns are due to the difference among the bowtie aper-ture antennas caused by the fabrication process.IV. CONCLUSIONSWe demonstrated the ISPI gapping technique in parallelnanolithography, using bowtie aperture antennas as nearfield optical elements. With the use of ISPI phase gapping,the sensitivity of gap detection can reach 0.15 nm.Combining ISPI and dynamic feedback control, we can pre-cisely align the mask with the substrate with a parallelismbetter than 0.03 mrad, and then control the gap between themask and the substrate during nanolithography with a varia-tion less than 6 nm, which offers a means to realize parallelnanolithography.ACKNOWLEDGMENTSSupport to this work by the Defense Advanced ResearchProjects Agency (Grant No. N66001-08-1-2037) and theNational Science Foundation (Grant No. CMMI-1120577)are gratefully acknowledged. X.W. also acknowledge thesupport from the National Basic Research Program (973Program) of China under Grant No. 2013CBA01703.1S. M. V. Uppuluri, E. C. Kinzel, Y. Li, and X. Xu, Opt. Express 18, 7369(2010).2P. Srisungsitthisunti, O. K. Ersoy, and X. Xu, Appl. Phys. Lett. 98,223106 (2011).3E. X. Jin and X. Xu, Jpn. J. Appl. Phys. 43, 407 (2004).4E. X. Jin and X. Xu, Appl. Phys. Lett. 86, 111106 (2005).5N. Murphy-DuBay, L. Wang, and X. Xu, Appl. Phys. A 93, 881 (2008).6H. Hwang, J.-G. Kim, K. W. Song, K.-S. Park, N.-C. Park, H. Yang, Y.-C.Rhim, and Y.-P. Park, Jpn. J. Appl. Phys. 50, 09MC04 (2011).7W. Srituravanich, L. Pan, Y. Wang, C. Sun, D. B. Bogy, and X. Zhang,Nat. Nanotechnology 3, 733 (2008).8T. Kim et al., Appl. Phys. Lett. 101, 161109 (2012).9E. E. Moon, P. N. Everett, M. W. Meinhold, M. K. Mondol, and H. I.Smith, J. Vac. Sci. Technol. B 17, 2698 (1999).10E. E. Moon and H. I. Smith, J. Vac. Sci. Technol. B 24, 3083 (2006).11E. E. Moon, P. N. Everett, M. W. Meinhold, and H. I. Smith, Proceedingsof Microprocesses and Nanotechnology Conference (IEEE, New York,2000), p. 252.12P. Srisungsitthisunti, E. E. Moon, C. Tansarawiput, H. Zhang, M. Qi, andX. Xu, Proc. SPIE 7767, 776707 (2010).13P. Srisungsitthisunti, Ph.D. dissertation (Purdue University, 2011).14R. Menon, E. E. Moon, M. K. Mondol, F. J. Castano, and H. I. Smith,J. Vac. Sci. Technol. B 22, 3382 (2004).15J. I. Mitchell, S. J. Park, C. A. Watson, P. Srisungsitthisunti, C. Tansarawiput,M. S. Qi, A. Eric, C. Yang, and X. Xu, Opt. Eng. 50, 104301 (2011).16L. Wang, S. M. Uppuluri, E. X. Jin, and X. Xu, Nano Lett. 6, 361 (2006).FIG. 7. (a) Height of the piezoelectric stage vs the gap value detected by theISPI. (b) and (c) AFM results of the line produced with a speed of 1.5lm/sand a gap of 10 nm.FIG. 8. Result of parallel lithography with a line width of 86 nm.041601-5 Wen et al.: High precision dynamic alignment and gap control 041601-5JVST B - Microelectronics and Nanometer Structures Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:  128.46.221.64 On: Thu, 17 Oct 2013 20:16:27

High precision dynamic alignment and gap control for optical near-field nanolithography

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High precision dynamic alignment and gap control for optical near-field nanolithography

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