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Study of moiré grating fabrication on metal samples using nanoimprint lithography Minjin Tang,1 Huimin Xie,1,* Jianguo Zhu,1 Xiaojun Li,2 and Yanjie Li1 1

AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China 2 National Center for Nanoscience and Technology, Beijing 100084, China * [email protected]

Abstract: A moiré grating is a basic optical component used in various moiré methods for deformation measurement. In this study, nanoimprint lithography (NIL) was proposed to produce high frequency moiré gratings on metal samples. A new type of NIL mold and a hot embossing system were developed to overcome the poor flatness and roughness of metal samples. This three-layer mold based on nickel grating was unbreakable, and the self-developed hot embossing system used a bellows cylinder to satisfy the parallelism requirement of grating fabrication on metal samples. In order to generate high quality moiré patterns, the grating profile of the mold was optimized. Then, 1200-3000 lines/mm frequency gratings were successfully fabricated on the different materials such as SiO2, aluminum and stainless steel. In order to evaluate the quality of the replication, the distortion in the fabricated SiO2 grating was analyzed by an inverse moiré method. As an application, the replicated grating on the aluminum sample in combination with the moiré interferometry was used to measure the tensile deformation of the sample. The successful experimental results demonstrate the feasibility and reliability of nanoimprint lithography to produce gratings on metal samples. ©2012 Optical Society of America OCIS codes: (120.4120) Moiré techniques; (050.2770) Gratings; (220.4241) Nanostructure fabrication; (220.3740) Lithography.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

R. Weller and B. M. Shepherd, “Displacement measurement by mechanical interferometry,” Proc. Soc. Exp. Stress Anal. 6(1), 35–38 (1948). D. Post, B. Han, and P. Ifju, High Sensitivity Moiré: Experimental Analysis for Mechanics and Materials (Springer-Verlag, New York, 1994), Chap.4. A. Assa, J. Politch, and A. A. Betser, “Slope and curvature measurement by a double-frequency-grating shearing interferometer,” Exp. Mech. 19(4), 129–137 (1979). S. Kishimoto, M. Egashira, and N. Shinya, “Observation of micro-deformation by moiré method using a scanning electron microscope,” J. Soc. Mat. Sci. 40(452), 637–641 (1991). B. Pan, H. M. Xie, S. Kishimoto, and Y. Xing, “Experimental study of moiré method in laser scanning confocal microscopy,” Rev. Sci. Instrum. 77(4), 043101 (2006). H. Chen, D. Liu, and A. Lee, “Moiré in atomic force microscope,” Exp. Mech. 24(1), 31–32 (2000). E. H. Anderson, C. M. Horwitz, and H. I. Smith, “Holographic lithography with thick photoresist,” Appl. Phys. Lett. 43(9), 874–875 (1983). S. Kishimoto, M. Egashira, and N. Shinya, “Microcreep deformation measurements by a moiré method using electron beam lithography and electron beam scan,” Opt. Eng. 32(3), 522–526 (1993). D. Yan, J. Cheng, and A. Apsel, “Fabrication of SOI-based nano-gratings for Moiré measurement using focused ion beam,” Sens. Actuators A Phys. 115(1), 60–66 (2004). J. McKelvie, D. Pritty, and C. A. Walker, “An automatic fringe analysis interferometer for rapid Moiré stress analysis,” in 4th European Electro-Optics Conference (SPIE, Bellingham, 1979), pp. 175–188. S. Y. Chou, P. R. Krauss, and P. J. Renstrom, “Imprint of sub-25nm vias and trenches in polymers,” Appl. Phys. Lett. 67(21), 3114–3116 (1995). I. Amidror, The Theory of the Moiré Phenomenon (Springer-Verlag, London, 2009), Chap.2. L. S. Kong, S. Cai, Z. X. Li, G. Jin, S. Huang, K. Xu, and T. Wang, “Interpretation of moiré phenomenon in the image domain,” Opt. Express 19(19), 18399–18409 (2011).

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Received 17 Oct 2011; revised 13 Dec 2011; accepted 23 Dec 2011; published 24 Jan 2012

30 January 2012 / Vol. 20, No. 3 / OPTICS EXPRESS 2942

14. Z. G. Xu, H. K. Taylor, D. S. Boning, S. F. Yoon, and K. Youcef-Toumi, “Large-area and high-resolution distortion measurement based on moiré fringe method for hot embossing process,” Opt. Express 17(21), 18394– 18407 (2009). 15. H. M. Xie, Q. H. Wang, S. Kishimoto, and F. Dai, “Characterization of planar periodic structure using inverse laser scanning confocal microscopy moiré method and its application in the structure of buttery wing,” J. Appl. Phys. 101(10), 103511 (2007).

1. Introduction Moiré methods are important grating techniques that among others are capable to measure full field deformation in experimental mechanics. Since first introduced by Weller and Shepherd [1] in 1948, geometric moiré has been developed into a mature and effective optical testing method for in-plane and out-of-plane deformation measurement. Usually, the geometric moiré method can only be used for measuring large deformations because the frequency of the grating utilized in this method is lower than 100 lines/mm. In the past thirty years, new experimental techniques, grating-fabrication techniques and high resolution electron microscopes have been introduced into moiré method, and they have brought two significant advances. One important advance has been the moiré interferometry method developed by Post et al. [2,3], which is now well recognized as a very powerful and accurate technique for full field strain measurement. Another important development was represented by the scanning electron microscope (SEM) moiré method, which was first developed by Kishimoto et al. [4] in the 1990s. Unlike the traditional geometric moiré method, the electron beam scan is utilized as a virtual reference grating, instead of the actual reference grating to interfere with the specimen grating for generating moiré patterns. A grating up to a frequency of 10 000 lines/mm can be used in SEM moiré to measure the deformation in a micro-zone, which represents a very high sensitivity. The similar principle has been utilized to form a series of other high-resolution moiré patterns, such as laser scanning confocal microscope (LSCM) moiré [5], atomic force microscope (AFM) moiré [6]. The frequency of the gratings used in moiré techniques is shown in Table 1. Table 1. Frequency of the gratings used in moiré techniques Moiré methods

Grating frequency (lines/mm)

Traditional geometric moiré Moiré interferometry SEM moiré AFM moiré

1-40 600-2400 hundreds to 20000 up to tens of thousands

Gratings are a series of straight, parallel, and equispaced grooves. Orthogonal gratings are two-dimension pillar arrays along two perpendicular directions. As a basic optical component, it can be used in various optical technologies, such as traditional moiré method, moiré interferometry and various microscopic moiré methods, to measure the surface deformation on the object. In grating techniques, the measurement sensitivity increases with the grating frequency. To accurately analyze micro-nano scale deformation, the difficulty comes from the fabrication of high frequency specimen gratings. As a conventional grating fabrication method developed in the early 1960s, holography lithography technique [7] is based on the interference of two coherent beams of light and the exposure of photoresist and can usually produce grating with a frequency at 600-2400 lines/mm. However, the complex arrangement of optical components has made it very difficult to put into practice, and restrained its wide applications. In recent years, researchers have investigated a number of microfabrication technologies to fabricate gratings, such as electron beam writing [8] and focus ion beam writing [9]. Although high-resolution alignment can be created, low throughput and high cost make it impractical for commercialization. Furthermore, researchers also developed some convenient replication methods, in which liquid adhesives, such as epoxy [2] or silicone rubber [10], are used to transfer a grating film

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to the specimen. However, the grating quality of replication method depends on the skills and experiences of the operator, thus the quality of the replication is uncontrollable. Nanoimprint lithography (NIL) or hot embossing lithography (HEL), initially proposed and developed by Chou’s group in 1995 [11], has emerged as one of the most promising technologies for nanoscale patterning. It can create micro/nano scale patterns in a resist material with rapid-processing and high-throughput, capable of sub-micron alignment over a large area. Nanoimprint lithography based on mechanical deformation of imprint resist can break through the resolution limit of conventional photolithography. Furthermore, the mold can be used repeatedly for at least one hundred imprints, thus the cost is very low and more feasible for mass production. In general, silicon stamp has been used in hot embossing lithography, however, it has a short lifetime due to its brittleness. The gratings fabricated by nanoimprint lithography in this study are used to generate high quality moiré patterns for deformation measurement. Different from some micromachining technologies, the fabrication technique for moiré grating do not pursue excessively small linewidth of microstructure nor focus on the details of micro/nano scale patterns. Because too high grating frequency may lead to too dense moiré fringes in a certain field size and a grating frequency of thousands lines/mm (Table 1) will meet the measurement accuracy in common mechanical properties testing. Furthermore, metal samples (such as aluminum, copper, iron, and stainless steel et al) are widely used in mechanical testing. However, it is more difficult to fabricate high frequency gratings on metal samples than on silicon substrates due to their poor flatness and roughness. This work aims to develop a simple, easy to operate, low cost and high throughput grating fabrication technique on metal substrates to popularize the moiré method. In this paper, in order to fabricate high frequency gratings on metal substrates, a new type of NIL mold and a hot embossing system were developed. This three-layer mold based on nickel stamp and 3M film was unbreakable and can provide uniform compression. The selfdeveloped hot embossing system used a device of bellows cylinder for parallelism, and can satisfy the need of grating fabrication on metal samples. Then, in order to create high quality moiré patterns, the grating profile of the NIL mold was optimized through computer simulations. Through a number of experiments, the process parameters in HEL process were optimized and 1200-3000 lines/mm frequency gratings were successfully fabricated on SiO2, aluminum and stainless steel substrates. In order to check the quality of the fabricated grating, the distortion in the fabricated SiO2 grating was measured by inverse moiré method. As typical application of the replicated grating, the deformation of a tensile aluminum sample with grating was measured by moiré interferometry. The successful experimental results demonstrate the feasibility of the grating fabricated on metal samples by NIL for the moiré measurement. 2. Principle of NIL and experimental devices 2.1. Principle of grating fabrication using nanoimprint lithography As illustrated in Fig. 1, the grating fabrication with HEL process involves an imprint step with physical deformation of a thin film of a thermoplastic polymer cast on a substrate by using a rigid mold under pressure and elevated temperature above the glass transition temperature (Tg), followed by solidification and demolding after the polymer cooled to a

Fig. 1. Schematic diagram of the grating fabrication with HEL process.

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temperature below Tg. After the patterns were transferred to the polymer, a thin metal layer with thickness less than 100 nm was deposited before it used as a moiré grating at normal temperature. As for gratings in high temperature circumstance, post-processing techniques such as reactive ion etching (RIE) and lift-off process should be adopted. Grating fabrication technique using NIL is a simple nanolithography process with the advantages of low cost, high throughput and high resolution. 2.2. Self-developed hot embossing system

Fig. 2. (a) Self-developed hot embossing system; (b) Imprint chamber in hot embossing system.

The self-developed hot embossing system (Fig. 2(a), THU-NIL-01 type) is a desktop thermal NIL system. It consists of an imprint chamber, a control box, a vacuum pump and a compressor. This small-sized system possesses the advantages of low cost and good operation. It can fabricate sub-micrometer structures on 100 mm wafer. Figure 2(b) illustrates the structure of the imprint chamber which includes a bellow cylinder (Festo EB-145-60), an upper pressure plate, a lower pressure plate, samples etc. The vacuum pump was connected to the external space of the bellow cylinder, and kept the inner space under low vacuum condition. While, the air compressor injected high pressure (1-7 bar) air into the internal space of the air bag (i.e. the bellow cylinder). Because of the difference of pressure inside and outside the air bag, the air bag expanded and rose, and then propelled the lower pressure plate close to the upper one. Finally, the samples were compressed by the two pressure plates with a maximum load of 7000 N. The pressure range to the samples is 0-10 MPa. The hot wire in the lower pressure plate can increase temperature quickly from the room temperature to 200°C. Bellow cylinder was used to ensure stamp and substrate parallel to each other, and transfer the pressure to a large area substrate effectively and uniformly. This good parallel adjustment device of bellows cylinder can overcome the poor parallelism between top and bottom surfaces of mental samples. Therefore, the system can satisfy the need of manufacture of high-frequency gratings on metal samples. 2.3. New type of mold in NIL The mold plays an important role in the nanoimprint technique. Silicon mold has been used widely in HEL, however, it is friable and has a short lifetime due to its brittleness. One of the greatest challenges to fabricate gratings directly on the surface of metal substrate is its poor flatness and roughness, which has great influence on the quality of the imprinted gratings and is also harmful to the silicon mold due to the non-uniform pressure. In this work, a new type of mold is developed for nanoimprint on mental samples. As shown in Fig. 3, the mold consists of three different layers: a 0.1 mm thick nickel grating prepared by electroforming (1200-3000 lines/mm), a buffer layer of a 0.13 mm thick 3M adhesive film and a 1 mm thick silicon baseplate.

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Fig. 3. Three layer mold in NIL.

The nickel grating is duplicated from silicon stamp or PMMA stamp using electroforming technique. In this study, the microstructures of silicon stamp, PMMA stamp and nickel grating are designed in Section 3 in order to generate high quality moiré patterns. The silicon stamp and the PMMA stamp can be fabricated by electron beam writing, holography lithography technique, or even nanoimprint lithography itself. The nickel grating offers some advantages for metal substrates: (1) unbreakable; (2) low thermal expansion coefficient which leads to small distortion; (3) would not adhere to many resists and make demolding easy. (4) 0.1 mm thick nickel stamp exhibits a certain flexibility which would be beneficial to metal surface with poor flatness. The buffer layer in this study is 3M adhesive film (heat resistant temperature 250°C) which is widely used in compression molding. It can improve the contact condition between the mold and the metal substrate and provide uniform compression in the hot embossing process. Thus, the new type of mold is unbreakable and can improve the quality of the fabricated grating on metal substrates. The resists used in this study are mr-I 75k PMMA (micro resist technology, Germany, no adhesion promoter necessary on silicon, SiO2 or aluminum substrates) and BP-212 positive resist (made in Beijing, China). The substrates contain SiO2, aluminum and stainless steel materials. 3. Optimization design of the profiles of grating mold In this study, the grating fabricated by nanoimprint lithography is used in moiré method to generate high quality moiré patterns. It is well known that the quality of the moiré patterns directly affect the measurement accuracy and the moiré fringe profile is determined by the profiles of composite grids. Thus, it is important to optimize the profiles of the mold and the fabricated gratings, especially the profile of the mold in NIL. Some optimization studies based on the theory of the moiré phenomenon have been given by some researchers [12,13]. In this study, some optimization results through computer simulations are presented visually to show the variations of the moiré quality with grating parameters. Several factors of gratings were investigated, including the grating pitch, the opening ratio, and the form of grating. 3.1 The grating pitch The pitch (or period) of a grating is in inverse ratio of the frequency of a grating. The measurement sensitivity increases with the grating frequency in moiré techniques. However, problems will also arise when the frequency is too high. In the moiré interferometry method, a high frequency grating of 600-2400 lines/mm is usually used. The wavelength of the laser λ and the frequency of the grating f satisfy [2]

λ f < 1.

(1)

thus, He-Ne laser (632.8 nm) is suitable for 1200 lines/mm frequency grating; and when the frequency of the grating is more than 2000 lines/mm, blue laser with the wavelength of less than 500 nm should be employed, for example, 441.6 nm wavelength He-Cd laser. At the same time, the short-wavelength laser may be more expensive, less mature (coherence, safety, et al) than He-Ne laser. Therefore, 1200 lines/mm frequency gratings (833 nm pitch) are most commonly used in moiré interferometry. In the microscopic moiré methods, the grating frequency can be higher. However, too high grating frequency may lead to too dense moiré

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fringes or too small view field. Thus, gratings with several hundred nm pitches (Table 1) will meet the measurement accuracy in common mechanical properties testing in microscopic moiré methods. 3.2. The opening ratio A grating is a series of straight, parallel, and equispaced grooves. If represented by a picture (256 gray levels), it is a collection of parallel black bars separated by white bars. One period

Fig. 4. Gratings (1024×1024 pixels, 20 pixels per period) with opening ratios of: (a) 0.5; (b) 0.75; (c) 0.25.

Fig. 5. Moiré patterns (parallel moiré) generated by gratings with different opening ratios:1024 × 1024 pixels, 256 gray levels; (τ/T)s and (τ/T)r are the opening ratios of specimen grating and reference grating, respectively.

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of such a grating consists of one black bar and one adjacent white bar. The opening ratio (here is expressed as τ/T) is the ratio of width of the white bar to period. Figure 4 shows three square-wave gratings with different opening ratios, where white means 255, black means 0. Figure 5 is parallel moiré patterns generated by gratings with different opening ratios. (τ/T)s and (τ/T)r are the opening ratios of specimen grating and reference grating, respectively. And pr = 1.1 ps, here, pr and ps respectively are the reference grating pitch and the specimen grating pitch. In order to correspond to the moiré images perceived by CCD, Gaussian blur is applied in computer simulations. Every moiré image in Fig. 5 has a profile section diagram (the horizontal axis shows luminance or gray value) in the right-hand. It can be seen that when the opening ratios of both gratings are equal to 0.5, the intensity difference (the difference between the maximum and the minimum grey values, i.e. twice the amplitude of the periodic fringe pattern) of moiré fringes is the highest, as shown in Fig. 5(a). The moiré fringes with second highest intensity difference are observed when the opening ratios of one grating is equal to 0.5, as shown in Figs. 5(d) and 5(e). The intensity difference of the moiré fringes is the lowest when the opening ratios of neither grating are equal to 0.5, as shown in Fig. 5(b), Fig. 5(c) and Fig. 5(f). Thus, gratings with identical width of the black

Fig. 6. Moiré patterns generated by gratings with different forms.

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and white bars (i.e., the opening ratios is τ/T = 0.5; see Fig. 4(a)) are most preferred to generate high quality moiré patterns. 3.3. The form of grating The forms of grating include sine-wave, square-wave, triangle-wave, and sawtooth-wave et al. Different forms of gratings will create moiré patterns of different qualities. Figures 6(a)6(d) are the four different forms of gratings (original gratings: 1024×1024 pixels, 20 pixels per period, crops of the original images are presented to see the grating structures clearly) and Figs. 6(e)-6(h) are the profiles of corresponding gratings. Figures 6(i)-6(l) show the moiré patterns (parallel moiré, pr = 1.1 ps) generated by gratings with different forms. Seen from the intensity profiles of the moiré (Fig. 7), the intensity difference of the moiré generated by four forms gratings from high to low is: square > sine > triangle > sawtooth. Thus, a square grating would be the best choice. In the intensity distribution, the profiles of the moiré generated by square and sine gratings are approximately triangle-wave and sine-wave, respectively. Because many moiré theories are based on the sinusoidal distribution, sine grating would also be a good choice.

Fig. 7. The intensity profiles of the moiré generated by gratings with different forms (256 gray levels).

In consequence, two types of optimized gratings (Fig. 8) are chosen to be the mold structures in nanoimprint lithography: a sine-wave grating and a square-wave grating, both of them have identical width of the black and white bars. Based on the results in Fig. 8, two nickel stamps were fabricated by electroforming technique. One is a sine-wave grating stamp (Fig. 9(a), 1200 lines/mm, cross type, 50-150 nm

Fig. 8. Two types of optimized grating structures in the NIL molds.

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Fig. 9. Two nickel molds: (a) a sine-wave grid mold, 1200 lines/mm; (a′) AFM image of the sine-wave grid mold; (b) a square-wave grid mold, 2000 lines/mm; (b′) SEM image of the square-wave grid mold.

depth) duplicated from PMMA stamp (fabricated by holography lithography technique), the other is a square-wave grating stamp (Fig. 9(b), 2000 or 3000 lines/mm, 330 nm depth) duplicated from silicon stamp (fabricated by electron beam writing). The AFM or SEM images of the two nickel molds are shown in Figs. 9(a′) and 9(b′). Both of the two nickel stamps will be in the form of three layer molds (Fig. 3) before using as NIL molds. 4. Experimental results on SiO2 substrate and distortion measurement 4.1. Experimental results on SiO2 substrate The experiments on SiO2 substrates are carried out to verify the performance of the selfdeveloped hot embossing system in Fig. 2 and the new type of mold in Fig. 3. In this process, an thickness of 600 nm PMMA (mr-I 75k, micro resist technology, Germany) were spin-coated onto the substrate of a SiO2 wafer (about 20 mm × 20 mm), and was heated to 180°C. A three-layer mold (Fig. 3) which contained a nickel grating of 2000 lines/mm (Fig. 9(b)) was placed on the face of the PMMA under a pressure of 2 MPa. The

Fig. 10. SiO2 grating fabricated by NIL after RIE process, 2000 lines/mm: (a) SiO2 grating; (b) LSCM moiré of the fabricated SiO2 grating in area 1; (c) LSCM moiré of the mold; view field: 513.74 μm × 513.74 μm, 1024 scanning lines, parallel moiré.

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Fig. 11. (a) AFM image of SiO2 grating fabricated by NIL; (b) Profile analysis by AFM of SiO2 grating.

samples were cooled to 70°C after 5 minutes, and the load was then removed. The mold was immediately peeled from the substrate. Finally, the PMMA and the SiO2 substrate were etched by RIE with an etching ratio of 1:1 and the grating structure was transferred to the SiO2 substrate. The diffraction image of the SiO2 grating fabricated by NIL was shown in Fig. 10(a). The color region reflected the high-frequency periodical structures on the surface of the SiO2 grating. The AFM image in Fig. 11(a) shows the 3D structures of the grating, and the groove depth of the SiO2 grating analyzed by AFM (Fig. 11(b)) is 96.3 nm. 4.2. Distortion measurement using inverse moiré method for NIL To characterize high frequency grating structure and improve product quality, distortion caused in the hot embossing lithography should be measured [14]. The view field in AFM is so small that it is difficult to obtain the whole structural character of the grating in a large region. In this paper, inverse LSCM moiré method [15] is applied to identify the distortion in the hot embossing process. This method integrates the advantages of moiré method (such as high sensitivity and large field of view), with the advantages of LSCM (high-quality images, open-air environment for operation, et al). In this experiment, a Leica TCS SP5 type LSCM (488 nm blue argon laser) was used. To generate parallel moiré patterns in LSCM, the direction of the scanning lines of the LSCM was finely adjusted to parallel to the SiO2 grating. Figures 10(b) and 10(c) show the parallel moiré pattern obtained with the scan size (denoted by L) of 513.74 μm and the scan line number (denoted by N) of 1024. Thus the pitch of the scanning lines (i.e. reference grating) pr = L/N = 501.7 nm. Based on the principle of inverse moiré method, the fringes contour interval is equal to one pitch of reference grating. The fringe number variation between the LSCM moiré fringes (Figs. 10(b) and 10(c)) of the SiO2 grating (area 1 in Fig. 10(a), i.e., the center region) and the mold is about a fringe. That means the displacement of the specimen in the view field (513.74 μm scan size) is about one pitch (pr). Further, the specimen grating pitch ps can be deduced from Eq. (2) as [15] ps =

pm pr pm ± pr

,

(2)

where pm is the pitch of adjacent moiré fringe; pr is the pitch of the scanning lines (i.e. reference grating), and the positive and negative signs can be determined by experiment. Thus, the distortion in HEL process could be calculated from LSCM moiré and shown in Table 2. As we can see from Table 2, the pitch in SiO2 grating fabricated by HEL is slightly larger than the pitch of the mold. The possible reason is the thermal mismatch between nickel mold and SiO2 substrates. The nickel mold in HEL has thermal expansion due to the high

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Table 2. The distortion in the SiO2 grating fabricated by HEL Gratings

Pitch (nm)

Frequency (lines/mm)

Mold Area 1 in SiO2 grating Area 2 in SiO2 grating Area 3 in SiO2 grating

499.99 500.43 500.40 500.48

2000.0 1998.3 1998.4 1998.1

Error between fabricated grating and mold — 0.088% 0.082% 0.097%

temperature, especially the difference between the room temperature (20°C) to the demolding temperature (70°C). And the flow of the PMMA may also have influence on the distortion of nano scale patterns. The distortions in three areas in SiO2 grating (Fig. 10(a)) are almost equal, and the maximum distortion is less than 0.1%. The distortion in SiO2 grating is small and even, and thus the fabricated grating is suitable for the moiré measurement. 5. Experimental procedure and results on metal substrate 5.1. Parameter optimization The imprint conditions, such as temperature (T), pressure (p), and time (t), are all considered in developing the new lithography technique. The process parameters were optimized through a number of experiments. In this experiment, 1200 lines/mm frequency nickel mold in Fig. 9(a) and mr-I 75k PMMA with 600 nm thickness were used. The aluminum substrates (25 mm square) were polished manually to Ra