Efficient methods of nanoimprint stamp cleaning based on imprint self-cleaning effect

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 22 (2011) 185301 (6pp) doi:10.1088/0957-4484/22/18/185301

Efficient methods of nanoimprint stampcleaning based on imprint self-cleaningeffectFantao Meng1,2, Gang Luo3, Ivan Maximov2, Lars Montelius2,Ye Zhou3, Lars Nilsson4, Patrick Carlberg3, Babak Heidari3,Jinkui Chu1 and H Q Xu2

1 The Key Laboratory for Micro/Nano Technology and System of Liaoning Province,Dalian University of Technology, 116024 Dalian, People’s Republic of China2 Division of Solid State Physics, Lund University, Box 118, S-22100 Lund, Sweden3 Obducat AB, SE-20125 Malmo, Sweden4 Department of Food Technology, Engineering and Nutrition, Lund University, Box 117,S-22100 Lund, Sweden

E-mail: ivan.maximov@ftf.lth.se

Received 16 September 2010, in final form 10 January 2011Published 17 March 2011Online at stacks.iop.org/Nano/22/185301

AbstractNanoimprint lithography (NIL) is a nonconventional lithographic technique that promiseslow-cost, high-throughput patterning of structures with sub-10 nm resolution. Contamination ofnanoimprint stamps is one of the key obstacles to industrialize the NIL technology. Here, wereport two efficient approaches for removal of typical contamination of particles and residualresist from stamps: thermal and ultraviolet (UV) imprinting cleaning—both based on theself-cleaning effect of imprinting process. The contaminated stamps were imprinted ontopolymer substrates and after demolding, they were treated with an organic solvent. The imagesof the stamp before and after the cleaning processes show that the two cleaning approaches caneffectively remove contamination from stamps without destroying the stamp structures. Thecontact angles of the stamp before and after the cleaning processes indicate that the cleaningmethods do not significantly degrade the anti-sticking layer. The cleaning processes reported inthis work could also be used for substrate cleaning.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Nanoimprint lithography (NIL) [1, 2] is one of themost promising next-generation lithographic techniques formanufacturing integrated circuits and is a key technology formass production of nanostructures with potential applicationsin electronics [3], photonics [4], magnetic memory [5], andbiological devices [6, 7]. It has been used for high-resolutionpatterning of wafers up to 8 inches in diameter [8] andrectangular wafers with a diagonal size up to 18 inches [9].However, there are some obstacles in industrial applicationsof the NIL technology and contamination of the nanoimprintstamps is one of them. The contamination on stamps candamage both the imprinted patterns on the wafers and thenanostructures of the stamps, leading to rapid degradation

of the transferred pattern. The imprinting defects resultingfrom particles were investigated in detail by Chen et al [10],where particle-amplified defects on the imprinted patternswere observed. Fabrication of large-area stamps with high-resolution features can be extremely expensive and time-consuming. Thus, it is very important to prolong the lifetime ofNIL stamps by applying efficient methods of stamp cleaning.Until now, there have been few publications related to thestamp cleaning. One of them is a stamp cleaning process calledthe dry deep clean (DDC) process presented by Chen et al[10]. The DDC is a non-destructive process, which can removeparticles from stamps, but it requires a specially formulatedpolymer adhesive. There are some other surface-cleaningmethods, some of which have been developed for siliconsemiconductor technology. For example, the standard cleaning

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Nanotechnology 22 (2011) 185301 F Meng et al

method is a hydrogen peroxide-based cleaning technique,called ‘RCA cleaning’, developed by RCA laboratories [11].The drawback of the RCA cleaning processes is etching ofthe stamp structures and degradation of the anti-sticking layeron the stamps. Another commonly used cleaning method isthe use of acetone, isopropyl alcohol (IPA), and deionized(DI) water, combined with ultrasonic agitation. Althoughorganic solvents can remove some contamination on the stampsurface, they do not provide good efficiency in cleaningthe stamp from inorganic particles, besides, the ultrasonicagitation may damage the high-aspect ratio nanostructures onthe nanoimprint stamps.

In this work, we report on the development of twosimple, efficient, non-destructive methods to clean the SiO2/Sinanoimprint stamps, and their evaluation by examination ofcontaminations and contact angles of the stamp before andafter the cleaning processes. Both methods are based onthe self-cleaning effect of the imprinting process reportedby Bailey et al [12]. The first method is an imprintingcleaning process based on a thermal NIL process, where thecontaminated NIL stamp is imprinted onto a thermoplasticpolymer film with subsequent demolding and cleaning ofstamps using commercially available remover S-1165 (ShipleyCo.). The second method uses an ultraviolet (UV) imprintingcleaning process based on the UV NIL process. Thecontaminated NIL stamp is imprinted onto a UV-curable film,followed by UV irradiation. After separation of the stamp andUV-curable film, the stamp is cleaned in remover S-1165 toclean the surface from organic contamination. The thermaland UV imprinting cleaning processes can effectively removeany particles and residual resists, and neither destroy the stampstructures, nor degrade the anti-sticking layer on stamps. Thecleaning processes can be repeated many times to achieve thebest results.

2. Experimental details

The nanoimprint stamp was fabricated on a two-inch siliconwafer with 500 nm thick thermally grown oxide using electronbeam lithography (EBL) (Raith 150, Raith GmbH, Germany)and inductively coupled plasma reactive ion etching (ICP-RIE) (Oxford Instruments, Plasmalab-100, Great Britain).The patterns were formed in an area of 0.2 × 1 mm2 andconsisted of grating structures with 200 nm period, 75 nmlinewidth, and 300 nm height. Prior to imprinting, the SiO2/Sinanoimprint stamp was treated with tridecafluoro (1H, 1H,2H, 2H) tetrahydrooctyltrichlorosilane (F13-TCS), which wascovalently bonded to the SiO2 surface to form an anti-stickingmonolayer. The treatment was performed in an oxygen- andwater-free glove box by evaporation of F13-TCS and reaction ofits vapors with the stamp surface. The details of the procedurecan be found elsewhere [13].

Inspection of the nanoimprint stamp contamination wasperformed by an optical microscope (Axio Imager M1m, CarlZeiss GmbH, Germany) and scanning electron microscope(SEM) (FIB/SEM system, FEI Nova Nanolab 600, FEICompany, Holland). Both types of microscopes were usedto investigate the surface of the stamps for detection of

Figure 1. The principles of the developed stamp cleaning processes.

contamination before and after the cleaning processes. Surfaceproperties of NIL stamps before and after the cleaningprocesses were characterized by measuring the contact anglesof a water droplet on the stamp surface using a tensiometer(Tracker, Teclis Sarl, France). In this work, contact anglesafter the cleaning processes have been measured after repeatingcleaning processes five times.

In order to study the efficiency of the cleaning methods,the nanoimprint stamp has been deliberately contaminatedby particles deposited during an anti-sticking treatment in aglove box under ‘dirty’ conditions. Such conditions allowedformation of a large number of inorganic dust particles on thestamp surface, thus creating suitable test samples.

To prepare a contaminated stamp with residual resist, athermal imprint process was performed. A two-inch siliconwafer was used as the imprinting substrate. 80 nm thick resistmr-I 7010E (Micro Resist Technology GmbH, Germany) wasspin-coated onto the wafer and then soft-baked on a hotplateat 140 ◦C for 2 min. The imprinting process was performedat a pressure of 20 bar and temperature of 110 ◦C for 3 minusing a 6 inch Nanoimprinter (Obducat AB, Sweden). Afterthe imprinting process, the stamp and the wafer were separatedat 40 ◦C. Since a stamp with high-aspect ratio nanostructuresand thin resist layer on substrate were used, there was someresidual resist stuck to the edge of the stamp pattern afterdemolding.

The principles of the developed stamp cleaning processesare summarized in figure 1. In the thermal imprinting cleaningprocess, a 200 µm thick polycarbonate (PC) substrate wasused. The PC is a high viscosity and low shrinkage polymermaterial with glass transition temperature (Tg) and Young’smodulus of about 150 ◦C and 2–2.4 GPa [14], respectively.The procedure to clean the nanoimprint stamp was performedas follows. The contaminated NIL stamp was imprinted ontoPC polymer film at pressure of 50 bars and temperature of180 ◦C for 30 s and demolded at 120 ◦C. After demolding, thestamp was cleaned in hot remover S-1165 at 140 ◦C for 2 min.Finally, the stamp was rinsed in DI water and blown dry withN2 gas.

In the UV imprinting cleaning process, a UV-curable film(supplied by Obducat AB, Sweden) was used as substrate. TheUV-film includes three layers: a carrier layer, an adhesivelayer (i.e. UV-curable layer), and a protection layer. The

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Figure 2. (a) Optical micrograph of particles on the stamp and (b) SEM image of a typical particle on the stamp. (c) Optical micrograph ofthe stamp after the first cycle of the thermal imprinting cleaning process and (d) optical micrograph of the stamp after the second cycle of thethermal imprinting cleaning process.

thickness of the adhesive layer is about 25 µm. BeforeUV irradiation, the adhesion of the adhesive layer is veryhigh, while after UV irradiation, it drastically decreased. Theprotection layer is removed before imprinting. The procedureto clean the nanoimprint stamp was performed as follows. Thecontaminated NIL stamp was imprinted onto the UV-film atpressure of 30 bars for 130 s at room temperature, and thenwas irradiated by UV light for 20 s. After demolding, the stampwas cleaned in remover S-1165 at 140 ◦C for 2 min. Finally,the stamp was rinsed in DI water and blown dry with N2 gas.

3. Results and discussion

3.1. Particles and thermal imprinting cleaning process

Stamp characterization was performed under an opticalmicroscope, as shown in figure 2(a). It can be seen thatthere were 21 particles on the stamp, marked by circles. Thedetailed images of the particles were obtained using the SEM.Figure 2(b) shows the SEM image of one of the particles whosesize is about 2 µm.

Most of the particles were removed during the first cycleof the thermal imprinting cleaning process, as shown infigure 2(c). It can be seen that there were only eight particles,marked by cycles, left on the stamp. All the particles werecompletely removed during the second cycle of the thermalimprinting cleaning process, as shown in figure 2(d). Themechanism of the thermal imprinting cleaning process isproposed as follows. As the imprinting temperature exceedsthe glass transition temperature (Tg) of the PC polymer film,

the PC polymer film goes into a viscous state. The viscousPC polymer film acts like glue to the particles, and theparticles are adsorbed or wrapped by the viscous PC polymerfilm. When the temperature is decreased below the Tg ofthe PC polymer film, the PC polymer film solidifies. Afterdemolding, the particles were taken away from the stamp,and stayed in the PC polymer film, as shown in figure 3.It can be seen that the particle is embedded into the PCpolymer film and is wrapped by the PC polymer film. Thefunction of the remover S-1165 cleaning is to remove theorganic contamination from the stamp. Since the removerconsists mostly of n-methylpyrrolidinone, it is efficient forremoval of residual polymers, but not inorganic particles.After the cleaning process, the stamp was characterized bySEM. Due to the small shrinkage of the PC polymer film, theimprinting process did not destroy the stamp nanostructures.Figure 4 shows the contact angles before and after the thermalimprinting cleaning process. It can be seen that the contactangle decreases from 111◦ to 108◦ after repeating thermalimprinting cleaning process five times, which means thatthe cleaning process does not significantly degrade the anti-sticking layer, compared to cleaning in Piranha solution or inoxygen plasma.

3.2. Particles and UV imprinting cleaning process

Some stamps, for example, polymer stamps do not toleratehigh temperature. For this reason, we developed the UVimprinting cleaning process. Optical microscope investigationof the stamp prior to cleaning indicates the presence of 17

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Figure 3. (a) Optical micrograph of particles on the imprinted PC film and (b) SEM image of a particle on the imprinted PC film covered by a10 nm thick Pt layer.

Figure 4. Images of a water droplet on the NIL stamp showing contact angles. (a) Before the thermal imprinting cleaning process (contactangle 111◦) and (b) after repeating the thermal imprinting cleaning process five times (contact angle 108◦).

Figure 5. Optical micrographs of the stamp. (a) Before the UV imprinting cleaning process and (b) after the UV imprinting cleaning process.

particles marked by circles, as shown in figure 5(a). After onecycle of the UV imprinting cleaning process, the particles werecompletely removed from the stamp, as shown in figure 5(b).The following mechanism is proposed: when the nanoimprintstamp is imprinted onto the UV-film, the particles on thestamp attached to the UV-curable layer or sank onto the UV-curable layer. After UV irradiation, the UV-curable layeris polymerized and solidified, and the particles are adsorbedor wrapped by the polymerized UV-curable layer. Afterdemolding, the particles are removed from the stamp andstayed in the UV-curable layer. After the cleaning process, thestamp was characterized with SEM. The results show that thestamp structures were still intact. Figure 6 shows the contactangle before and after repeating the UV imprinting cleaning

process five times. It indicated a decrease of the contact anglefrom 112◦ to 106◦, which is somewhat larger than after thethermal imprinting cleaning process.

3.3. Resist and cleaning process

Residual resist is another main type of contamination, whichcomes from the imprinted resist after demolding. The residualresist contamination on the stamp was characterized using anoptical microscope and SEM before and after the cleaningprocess. It can be seen in figure 7(a) that there is some residualresist at the edge of the stamp pattern. As shown in the high-resolution SEM image of figure 7(b), the grating pattern hasnon-uniform brightness and the line edges are blurred, which

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Figure 6. Images of a water droplet on the NIL stamp showing contact angles. (a) Before the UV imprinting cleaning process (contact angle112◦) and (b) after repeating the UV imprinting cleaning process five times (contact angle 106◦).

Figure 7. (a) Optical micrograph and (b) SEM image of the stamp before cleaning. (c) Optical micrograph and (d) SEM image of the stampafter the thermal imprinting cleaning process. The inset images are the high-magnification SEM images.

means there are some residues in this area. After one cycle ofthe thermal imprinting cleaning process, the stamp was free ofresidual resist contamination, as shown in figures 7(c) and (d).During the imprinting process, particles and part of residualresist were removed from the stamp. Most of the residualresist on the stamp was removed efficiently by hot remover S-1165, which easily dissolves the organic contamination. Theseresults show that the thermal imprinting cleaning process isefficient in removing the residual resist from the stamp. TheUV imprinting cleaning process demonstrated similar cleaningability to remove the residual resist from the stamp as thethermal imprinting cleaning process. To avoid repetition, thecleaning results of residual resists using the UV imprintingprocess are not presented here.

Both thermal and UV cleaning processes of NIL stampsshowed a very small decrease of the contact angles afterrepeating the cleaning processes five times. This clearlyindicates that the above-mentioned processes do not induceany significant damage to the anti-sticking layer on the stampsurface, compared to the conventional cleaning techniques.Certainly, it also showed the potential application of thisapproach in industry, however, more detailed studies withlarger numbers of cleaning cycles are required.

4. Conclusions

In this work, two efficient methods, thermal and UV imprintingcleaning, were developed to remove the typical contamination

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from NIL stamps. The particles on the stamps were removedmainly by imprinting processes, and the residual organiccontamination on the stamps was mainly resolved in hotremover S-1165. The studied results demonstrate that thethermal and UV imprinting cleanings are non-destructiveprocesses, which can effectively remove all the particles andresidual resist from the stamps. These non-destructive cleaningprocesses could also be used as alternative methods forsubstrate cleaning. The proposed stamp cleaning techniquescan be used in industrial applications of NIL.

Acknowledgments

The authors gratefully acknowledge the financial support ofthe National Basic Research Program of China. (GrantNos 2011CB302101 and 2011CB302105.) Technical helpfrom Mariusz Graczyk is appreciated.

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