SCIENCE CHINA Technological Sciences. Replication of large area nanoimprint stamp with small critical dimension loss

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1 SCIENCE CHINA Technological Sciences RESEARCH PAPER March 2012 Vol.55 No.3: doi: /s Replication of large area nanoimprint stamp with small critical dimension loss MENG FanTao 1, 2, GUAN Le 1, 2, WANG ZhiWen 1, 2, HAN ZhiTao 1, 2 1, 2* & CHU JinKui 1 Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian , China; 2 Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian , China Received April 14, 2011; accepted June 7, 2011; published online December 29, 2011 In this paper, the replication process of large area nanoimprint stamp with small critical dimension (CD) loss was investigated, using the thin residual layer nanoimprint lithography (NIL) technology. The residual layer thickness was optimized by changing the spin-coated resist thickness. The dependences of the residual layer etching rate on gas flow, chamber pressure, and RF power were investigated, and the optimized process conditions were established. By means of the thin residual layer NIL technique and optimized residual layer etching process, large area stamp with small CD loss and multi-orientation patterns was successfully replicated on 2-inch SiO 2 /Si wafer. The CD loss was controlled within 5 nm. The replicated stamp showed high performance in the patterning with thermal NIL. The replication process reported in this work could also be used to fabricate large area nanostructures with small CD loss. nanoimprint stamp, replication, small critical dimension loss, nanoimprint lithography, multi-orientation patterns Citation: Meng F T, Guan L, Wang Z W, et al. Replication of large area nanoimprint stamp with small critical dimension loss. Sci China Tech Sci, 2012, 55: , doi: /s Introduction Nanoimprint lithography (NIL) has been demonstrated as a low-cost, high-throughput patterning process with sub-10 nm resolution [1]. It has great potential for mass production of nanostructures with applications in electronics [2], photonics [3], magnetic devices [4], and biological device [5]. NIL is considered to be an alternative to electron beam lithography and optical projection lithography and as a candidate for next generation lithography. In NIL, one of the essential requirements is to prepare high-quality stamps. Stamps are typically fabricated on silicon, silicon dioxide, nickel, and quartz wafers using interference lithography [6], electron beam lithography (EBL) [7], or scanning probe *Corresponding author ( chujk@dlut.edu.cn) lithography [8], followed by etching or electroplating. For stamps with large size, high density, and multi-orientation patterns, the fabrication can be extremely expensive and time-consuming, which reduces the advantages of NIL technology as a whole. Moreover, because of their stiffness, they tend to break easily when a pressure is applied and when they are separated from the substrate. Replication of stamps from master stamp using NIL may significantly reduce the manufacture cost of NIL and prolong the lifetime of the master stamp, which is beneficial for realization of industrial level manufacturing using NIL. However, the critical dimension (CD) loss, which mainly results from the removal of residual layer caused by NIL process using oxygen reactive ion etching (RIE), is a serious problem in the replication process. It will affect the geometrical parameters of the replicated stamps. Therefore, we have to develop a replication process that has small CD loss. An optimally thin and uniform residual layer and Science China Press and Springer-Verlag Berlin Heidelberg 2011 tech.scichina.com

2 Meng F T, et al. Sci China Tech Sci March (2012) Vol.55 No An optimally thin and uniform residual layer and optimized oxygen RIE process are critical to minimize the lateral trimming and loss of CD control in oxygen RIE process. In this work, we reported on replication of large area nanoimprint stamp with small CD loss and multi-orientation patterns using thin residual layer NIL technique. We focused on how to control the CD loss during the replication process. The residual layer thickness was optimized by changing the spin-coated resist thickness. The dependences of the residual layer etch rate on gas flow, chamber pressure, and RF power were investigated and the optimized process conditions were established. By means of the thin residual layer NIL technique and optimized residual layer etching process, a stamp with 5 nm CD loss was successfully replicated. The quality of the patterns imprinted by replicated stamp is higher than that of the patterns created by master stamp. The proposed replication process provides a simple, convenient, and low cost way to replicate the nanoimprint stamp with small CD loss and high resolution. 2 Experimental The master stamp was fabricated on a two-inch silicon wafer by NIL Technology ApS (NIL T, Demark). There were six grating patterns with different grating orientation on the master stamp, and the area of each grating pattern was 2 mm by 2 mm. The structural parameters of the master stamp obtained from the producer were as follows: 200 nm pitch, 75 nm linewidth, and nm height. Figure 1 shows the scanning electron microscopy (SEM) images of one of the grating patterns on the master stamp. Prior to using the stamp for imprinting, its surface was treated with an anti-sticking monolayer of tridecafluoro (1H, 1H, 2H, 2H) tetrahydrooctyltrichlorosilane (F 13 -TCS) (Sigma-Aldrich, USA) in vapor phase in order to reduce adhesion at the stamp/resist interface during demolding. The details of the anti-sticking treatment could be found elsewhere [9]. The schematic diagram of the replication process is shown in Figure 2. A 100 nm thick mr-i 7010E layer (Micro Resist Technology, Germany) was spin-coated onto two-inch silicon wafers with 500 nm thick thermally grown oxide layer and then pre-baked at 140 C for 2 min on hotplate. The imprinting process was carried out on an Eitre 6 nanoimprinter (Obducat AB, Sweden), and the imprinting parameters are shown in Table 1. The Eitre 6 nanoimprinter uses the soft press technology, and pressure is applied to the stamp and substrate using compressed air to ensure pressure uniformity over the entire imprint area [10]. Soft press technology enables uniform residual layer over large areas, which is critical for high-resolution imprinting and pattern transfer fidelity. After removing the residual layer using oxygen RIE (Trion T2, Trion Technology, USA), 20 nm thick chrome layer was thermally evaporated, followed by lift-off in Figure 1 SEM images of the master stamp. (a) Top-view and (b) tilted-view. Table 1 Figure 2 Schematic diagram of the replication process. Parameters of the imprinting process Value Imprinting temperature 130 C Imprinting pressure Imprinting time 50 bar 180 s Demolding temperature 40 C

3 602 Meng F T, et al. Sci China Tech Sci March (2012) Vol.55 No.3 warm remover 1165 for 5 min and ultrasonic bath for 30 s. Then, the SiO 2 was etched with the chrome (Cr) layer as a hard mask. Finally, the remaining Cr was stripped using Cr etchant. The SiO 2 dry etching was performed in high-density inductively coupled plasma reactive ion etching (ICP-RIE) system (Oxford Plasmalab-100 system, Oxford Instruments, UK), using the parameters gathered in Table 2. The etching time was chosen according to the required stamp depth and the etching rate. In this work, the etching time was 60 s. To investigate the performance of the replicated stamp, thermal NIL was performed on Eitre 6 nanoimprinter using the replicated stamp. The imprinting was performed using the same parameters shown in Table 1 and with the commercial imprinting resist of mr-i 7010E. Prior to using the replicated stamp for imprinting, its surface was treated with an anti-sticking monolayer of F 13 -TCS in vapor phase. After each step of the stamp replication and imprinting process, the patterns were characterized using a FIB/SEM system (FEI Nova Nanolab 600, FEI Company, USA) for top view and cross-section observation. The imprinted resist patterns were sputter-coated with 5 nm Pt to aid in SEM imaging. 3 Results and discussion 3.1 Control of residual layer thickness After demolding, the imprinted pattern always has a residual layer under the trench of the pattern. Since imprinted resist patterns are used as an etching or lift-off mask, the residual layer under the trench has to be removed by Oxygen RIE. Therefore, it is important to make the residual layer as thin as possible to control the line-width of the final pattern. Figure 3 shows the relationship between the residual layer thickness and spin-coated resist thickness. We can obtain the following conclusions from the plot: The thickness of the final residual layer thickness decreases as the spin-coated resist thickness decreases; a very thin residual layer, even near zero nm thickness, cloud be achieved by optimizing the spin-coated resist thickness. When the spin-coated resist thickness is 70 nm that is about one third of the height of the master stamp, the residual Figure 3 The relationship between residual layer thickness and spincoated resist thickness. layer thickness is nearly zero nm. The very thin residual layer benefits from the combination of the very thin spincoated resist thickness and high master stamp height, as presented in our previously results [11]. However, the very thin spin-coated resist maybe result in the master stamp directly contacting to the rigid substrate during the imprint process, which has the risk to damage the master stamp. On the other hand, if the residual layer is too thick, the subsequent Oxygen RIE will seriously degrade the pattern profile. Therefore, it is important to find a proper thickness of the residual layer to protect the stamp from directly contacting to the rigid substrate. In this work, we used the residual layer of 10 nm for our experiment. Figure 4 shows the imprinted resist grating pattern. Despite the metal coating, the SEM images of resist grating were slightly distorted obviously due to a charging effect. As shown in Figure 4, the total thickness of the Pt layer and residual layer is about 15 nm, so the residual layer thickness is about 10 nm. It can be seen in Figure 4 that the imprinted Table 2 Parameters of the imprinting process Pressure RF power ICP power Value 10 mtorr 40 W 1200 W Gas CF 4 /H 2 Gas flow Etching time 40/5 SCCM 60 s Figure 4 SEM image of the imprinted resist pattern.

4 Meng F T, et al. Sci China Tech Sci March (2012) Vol.55 No grating patterns consisted of two parts. One was the stretched part with decreased line-width that was found in the lower part of the imprinted grating. The other was the part found on the top of the imprinted grating that kept the original shape of the corresponding trenches on the master stamp. The two parts can be created by the following process: The resist in the trenches on the master stamp stuck to the bottom and the sidewall of the trenches, then the resist was stretched mechanically during the demolding process. The top part detached from the master stamp after the lower part was stretched, which formed two parts with different linewidthes in each nanograting. 3.2 Optimization of residual layer etching process Oxygen RIE is a complicated process that involves many process parameters, such as gas flow, chamber pressure, RF power, and environment condition of etching chamber. In this work, we focused on the effects of gas flow, chamber pressure, and RF power on the etching rate. The process parameters investigated are listed in Table 3 and the effects of varying these parameters are discussed in detail in the following. Figure 5 shows the effect of the O 2 gas flow on the etching rate. The O 2 gas flow range is from 15 to 55 SCCM, and the chamber pressure and RF power are 150 mtorr and 50 W, respectively. According to the plot, with increase of the O 2 gas flow, the etching rate initially increased and then dropped at a much higher O 2 gas flow. When the O 2 gas flow is 35 SCCM, the etching rate is maximum. With increase of the O 2 gas flow, the consumed reactive gas can be Table 3 Investigated parameters and their variation ranges O 2 gas flow Chamber pressure RF power Range SCCM mtorr W replenished sufficiently, which can lead to increase of the etching rate. However, a fast O 2 gas flow rate at a constant pressure can lead to short dwelling time of gas molecules in reaction chamber, because the constant pressure is maintained by high pumping speed. If RF power is not increased accordingly, the number of gaseous ions generated in reaction chamber will decrease, which results in reduction of the etching rate. Therefore, the etching rate is directly linked to the supply rate of O 2 gas. Chamber pressure was varied while keeping all other parameters constant: O 2 gas flow 35 SCCM and RF power 50 W. As the chamber pressure increased, the etching rate dropped, as shown in Figure 6. At low pressure, there are three advantages. First, the density of gas molecule is low in the chamber. Electrons have a longer mean free path and can acquire more acceleration energy between two collisions with gas molecules, which can increase ionization probability. Second, a low chamber pressure results in less lateral etching and improvement of anisotropy, which is due to fewer collisions between ions and gas molecules, which results in more directional movement of ions toward the substrate. Third, a low chamber pressure helps quickly move away the volatile products from etched surface, which improves the reaction rate, hence the etching rate. At a low chamber pressure, RIE is dominated by ion sputtering etching, because chemically active gaseous molecules are less and ion energy is higher. At a high chamber pressure, RIE is dominated by chemical etching, because the mean free path of ions is reduced and the number of chemically active gaseous molecules is large, and the etching process is more isotropic. Increasing RF power will increase electron energy, hence increase ionization probability. Therefore, the etching rate generally increases with the increase of RF power, as shown in Figure 7. In addition, the anisotropy of etching can be improved because ion sputtering etching is highly directional and almost has no lateral etching. Based on the above investigation, the optimized conditions for oxygen RIE process were established and the parameters are listed in Table 4. Using the optimized parameters, Figure 5 The effect of O 2 gas flow on etching rate. Figure 6 The effect of chamber pressure on etching rate.

5 604 Meng F T, et al. Sci China Tech Sci March (2012) Vol.55 No.3 Cr etchant (20 g Ammonium Cerium Nitrates, 3.5 Ml CH3COOH, and 100 ml DI water). Figure 10 shows the photograph of the replicated stamp and the SEM images of the stamp patterns with different grating orientation. It can be seen that the replicated stamp with excellent uniformity is the high-fidelity replication of the master stamp. Figure 11 show the SEM image of the cross-section of the replicated grating pattern Figure 10(f). It can be seen that the SiO 2 gratings have the slightly tapered sidewalls which are easier for demolding. The geometric parameters of the replicated stamp are as follows: 200 nm pitch, 80 nm linewidth, and 180 nm height. Figure 7 The effect of RF power on etching rate. Table 4 Optimized parameters for residual layer etching O 2 gas flow Chamber pressure RF power Optimized value 35 SCCM 100 mtorr 90 W the residual layer of the imprinted pattern was completely removed, as shown in Figure 8. As shown in Figures 4 and 8, the linewidth of the imprinted pattern was decreased by 5 nm after removing the residual layer. 3.3 Pattern transfer process After removing the residual layer using the optimized O 2 RIE process, a 20 nm thick Cr layer was thermally evaporated, followed by lift-off in warm remover 1165 for 5 min and ultrasonic bath for 30 s. Figure 9 shows the Cr nanograting pattern created by the lift-off process, and the period and linewidth of the Cr pattern were 200 and 80 nm, respectively. The SiO 2 was etched with the Cr nanograting pattern as a hard mask. Then, the remaining Cr was stripped using Figure 9 SEM image of the Cr pattern created by lift-off process. Figure 8 SEM image of the imprinted resist pattern after residual layer removal. Figure 10 Photograph of the replicated stamp and SEM images of the stamp patterns. (a) Photograph of the replicated stamp; (b) 60-degree- orientation grating patterns; (c) 90-degree-orientation grating patterns; (d) 30-degreeorientation grating patterns; (e) 150-degree-orientation grating patterns; (f) 0- degree-orientation grating patterns; (g) 120-degree-orientation grating patterns.

6 Meng F T, et al. Sci China Tech Sci March (2012) Vol.55 No Figure 12 shows the cross section image of the imprinted resist patterns using the replicated stamp. Large-area, highdensity nangratings array with good shape homogeneity and size uniformity has been successfully created using the replicated stamp. Compared with the resist grating patterns in Figure 4, the shape of the imprinted resist grating is better and the stretched phenomenon is smaller. This could be caused by smaller roughness of the replicated stamp surface and slightly tapered sidewalls of the replicated stamp, which reduce the friction force between the stamp and the imprinted resist patterns during the demolding process. 4 Conclusion In this paper, we demonstrated a replication process for replication of large area nanoimprint stamp with small CD loss and multi-orientation patterns using thin residual layer NIL technique. The residual layer thickness was optimized by changing the spin-coated resist thickness. The parameters of the oxygen RIE process for residual layer removal were investigated, and the optimized conditions were established. Using the optimized imprinting parameters and pattern transfer parameters, large area stamp with smaller CD loss and excellent uniformity was successfully replicated from the master stamp. By means of the thin residual layer NIL technique and optimized oxygen RIE process, the CD loss in this work is about 5 nm. Thermal imprinting result demonstrated that the replicated stamp performs better imprinting result. The replication process paves a way to fabricate large area nanostructures with small CD loss using NIL technique. This work was supported by the National Basic Research Program of China ( 973 Program) (Grant No. 2011CB302105) and the Fundamental Research Funds for the Central Universities (Grant No. DUT10ZD104). Figure 11 SEM images for cross-section of the replicated grating pattern Figure 10(f). Figure 12 SEM image for cross-section of the imprinted grating patterns created using the replicated stamp. 1 Chou S Y, Kruss P R, Zhang W, et al. Sub-10 nm imprint lithography and applications. J Vac Sci Technol B, 1997, 15: Maximov I, Carlberg P, Wallin D, et al. Nanoimprint lithography for fabrication of three-terminal ballistic junctions in In/GaInAs. Nanotech, 2002, 13: Ahn S W, Lee K D, Kim J S. Fabrication of subwavelength aluminum wire grating using nanoimprint lithography and reactive ion etching. Microelectron Eng, 2005, 78-79: Wu W, Cui B, Sun X Y, et al. Large area high density quantized magnetic disks fabricated using nanoimprint lithography. J Vac Sci Technol B, 1998, 16: Falconnet D, Pasqui D, Park S, et al. A novel approach to produce protein nanopatterns by combining nanoimprint lithography and molecular self-assembly. Nano Lett, 2004, 4: Park S, Schift H, Solak H H, et al. Stamps for nanoimprint lithography by extreme ultraviolet interference lithography. J Vac Sci Technol B, 2004, 22: Chu J K, Meng F T, Han Z T, et al. Large area mold fabrication for the nanoimprint lithography using electron beam lithography. Sci China Tech Sci, 2010, 53: Luo G, Xie G Y, Zhang Y Y, et al. Scanning probe lithography for nanoimprinting mould fabrication. Nanotech, 2006, 17: Beck M, Graczyk M, Maximov I, et al. Improving stamps for 10 nm level wafer scale nanoimprint lithography. Microelectron Eng, 2002, 61-62: Meng F T, Luo G, Montelius L, et al. Fabrication of sub-100 nm metal nanowire structure by zero residual nanoimprint lithography. In: The 9th International Conference on Nanoimprint and Nanoprint Technology, Oresund & Copenhagen, October 13-15,