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CIRP Annals - Manufacturing Technology 64 (2015) 201–204

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Concept for laser-assisted nano removal beyond the diffraction limit usingphotocatalyst nanoparticles

S. Takahashi (2)a,*, Y. Horita b, F. Kaji b, Y. Yamaguchi b, M. Michihata a, K. Takamasu b

a Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153-8904, Japanb Department of Precision Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan

1. Introduction

In the world of micro manufacturing, microstructure formationprocesses such as photolithography and microstereolithography[1] play an important role in ensuring the functionality ofsemiconductors and micro-3D functional devices, as their overallperformance is greatly dependent on the fine microstructure ofphotosensitive materials. This has led to a rapid increase in boththe micronization and complexity of the functional structurescreated as a way of increasing device performance [2–5]; however,the more micronized and complex the microstructure becomes[6,7], the more important it is to minimize the formation of thevarious imperfections that can be generated. For instance, asprocess resolutions approach an order of several tens of nm [6],surplus curing that extends beyond the intended structure hasbecome a major problem (Fig. 1). Thus, in order to improve thestructure quality in next-generation micro manufacturing pro-cesses, there is a need for a material removal process that can beused to remove surplus curing defects from complex micro-3Dstructures with a 10-nm process resolution.

Many material-removal methods already exist for soft sub-stances such as photosensitive resin [5,8], and these can be mainlyclassified as either mechanical-probe-based (Fig. 2(a)) or laser-assisted methods (Fig. 2(b)) based on the physical principleemployed. The former type has a considerable advantage in thatthe process resolution is determined by the physical contact size of

the practical application of this technique is not reliable, as

process resolution gradually deteriorates due to the low wresistance of the fine AFM tip. Furthermore, this method can obe applied to the removal of surplus curing on surfaces thatnearly perfectly flat, and therefore cannot be used on 3D-structuwith a high degree of flexibility. The second option is characteri

A R T I C L E I N F O

Keywords:

Laser micro machining

Material removal

Micro tool

A B S T R A C T

A new concept for the laser-assisted removal of material is proposed for achieving nanoscale correctio

next-generation functional microstructures such as nanostructured photoresist surfaces and micro

objects fabricated using microstereolithography. This proposed method is characterized by

entrapment of TiO2 photocatalyst nanoparticles by a remotely controlled radiation force, which all

not only for remote processing using the inherent properties of light, but also a fine process resolution

goes beyond the limits of diffraction focusing. Both theoretical and experimental analyses are use

verify the basic feasibility of this proposed concept.

� 2015 C

Fig. 1. Typical imperfections (surplus curing defects) present in: (a) micro per

structures produced by photolithography, (b) micro mechanical structures cre

by microstereolithography.

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the mechanical-probe, thus allowing for a high resolution. Indeed,if an atomic force microscope (AFM) cantilever is used, then aresolution of less than 10 nm can be easily achieved [8]. However,

Fig. 2. (a) Mechanical-probe-based and (b) laser-assisted micro/nano material-

removal.

* Corresponding author.

E-mail address: takahashi@nanolab.t.u-tokyo.ac.jp (S. Takahashi).

http://dx.doi.org/10.1016/j.cirp.2015.04.041

0007-8506/� 2015 CIRP.

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S. Takahashi et al. / CIRP Annals - Manufacturing Technology 64 (2015) 201–204202

he use of laser ablation; and by taking advantage of remoteessing using the inherent properties of propagating lightgy, can be applied to both flat surfaces and complex, highlyble 3D-structures. There is, however, a critical problemrding the process resolution, which is largely determined bydiffraction limit of the light source [5].iven the problems associated with using conventional micro

erial-removal methods for the nanoscale correction of surplusng defects in complex micro 3D structures, this study aims tolop an approach that is not only suitable for highly flexible

ctures like a laser-assisted process, but also offers the highess resolution of a mechanical-probe based method.

aser-assisted removal using photocatalyst nanoparticles

ig. 3 shows a schematic diagram for a new material-removalept based on using photocatalyst nanoparticles. This methodplicable to complex micro 3D structures with a high degree ofbility, and has a fine process resolution that goes beyond theaction limit. This is made possible by using laser-trappingnology [9] as a nano processing tool to manipulate photo-lyst TiO2 nanoparticles. When this TiO2 absorbs photons withnergy corresponding to its energy gap an electron is excited

the valence to conduction band, thereby generating antron-hole pair [10]. The resulting oxidation-reduction reaction,ch is shown in Fig. 3(b), is capable of decomposing polymererials in contact with the TiO2 [11–13]. Thus, by illuminating

exciting the TiO2 nanoparticles with UV light, they cantion as a nano removal tool for microstructured resin objects

Similar micro processing methods based on using both laserablation and laser-trapping technology have been reported[14–16], but as their physical principles were still based on laserenergy, their processing direction is directly dependent on thedirection of laser beam propagation. As such, it is difficult to applysuch methods to the correction of surplus curing in complex micro3D structures. The proposed new method, on the other hand, doesnot use laser energy directly, and so omnidirectional processing isindependent of the laser propagation direction. This shouldallow for the nano removal of surplus curing defects even if theyare on side walls.

Furthermore, by using a fluid with a refractive index matched tothat of the resin object being processed (Fig. 4), the converginglaser beam used for trapping can illuminate the target particlewithout optical disturbance. This allows for the correction ofsurplus curing defects underneath the overhanging parts ofcomplex micro-3D-structures [7,17], and is most significantfeatures of this approach. That is, accessing such particles is notsomething that is possible with other energy-beam-based correc-tion techniques, including electron-beam-machining.

As a step toward the practical realization of this proposedconcept, we herein present theoretical analyses of the TiO2

nanoparticle handling characteristics using laser-trapping. Thedevelopment of a basic nano removal processing system is alsopresented along with the core principle of the proposed concept,i.e., that the process resolution does not depend on the laser spotsize, but on the TiO2 tool size.

3. Theoretical analyses of TiO2 nanotool handling

Fig. 3. Proposed laser-assisted removal process using photocatalyst nanoparticles: (a) conceptual diagram, (b) oxidation-reduction of TiO2 photocatalyst.

. Applicability to surplus curing defects in a complex micro-3D-structure: (a) functional microstructure with a movable part; (b) surplus curing defects under the

ang, causing movement error; and (c) nano correction process for defects under such an overhang using a refractive index matching fluid. Other energy-beam-based

ction techniques cannot be used with such a complex target.

icated by photolithography, microstereolithography, etc., suchose shown in Fig. 3(a). Furthermore, the process resolution ofproposed method is not restricted by the diffraction limite laser-trapping beam or the UV light, but rather by the size ofTiO2 nanoparticles. Being based on the inherent propertiesropagating light gives the process a similar degree of targetbility to the laser-assisted process, while the near-contact ofTiO2 nanoparticles gives a resolution similar to that of thehanical-probe-based method. More importantly though, as itssical principle is based on catalytic reaction, there is nolem of tip wear.

characteristics

A number of theoretical analyses have been conducted on thelaser-trapping characteristics of dielectric micro particles such asSiO2, which has refractive index of around 1.46 [18], but fewreports exist regarding TiO2 micro/nanoparticles that have a muchhigher refractive index of 2.97. Thus, in order to confirm thefeasibility of the proposed concept, it is important to understandhow the handling characteristics of TiO2 micro/nanoparticlesdiffer from that of SiO2 particles. Figs. 6 and 7 show thenumerically calculated radiation forces for a SiO2 and TiO2

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S. Takahashi et al. / CIRP Annals - Manufacturing Technology 64 (2015) 201–204 203

particle, respectively, as defined by the coordinate system shownin Fig. 5 and under the following conditions: trapping beamwavelength, l = 642 nm; numerical aperture of converging lens,N.A. = 1.20; refractive index of surrounding medium (water),n = 1.33; and micro particle diameter, d = 100 nm. Note thatthere is no significant difference between the trapping forces inthe r-direction, Qr, but the trapping force in the z-direction, Qz, ishighly dependent on the refractive index of the particle. Thismeans that although SiO2 particles can be trapped in thez-direction, TiO2 particles of the same size (d = 100 nm) areinstead pushed in the direction of the laser-beam propagation.By dividing the trapping force into a scattering and gradientcomponent, it can be seen that the higher refractive index of theTiO2 particles generates a larger scattering component thatresults in a greater pushing force.

Fig. 8(a) shows the Qz for TiO2 particles with a diameter of50 nm, in which we see that the scattering component rapidly

4. Fundamental experiment for verification of proposedconcept

4.1. Development of a fundamental experimental system

A nano removal system was developed to verify the cprinciples of the proposed concept, and as shown in Fig. 9,

consisted primarily of: a laser-trapping unit for handling the Tnanoparticles, a UV illumination unit for exciting a photocatalreaction, an XYZ-PZT/stepping motor stage unit for sampositioning, and an infinity-corrected optical microscope fositu monitoring. A laser diode with a 642-nm wavelength was uas the trapping laser source, which after passing through a spafilter to improve the beam quality, was tightly converged bwater immersion objective (N.A. = 1.0). To provide a TiO2 phocatalyst excitation source, a relatively large area (several tenmicrometers) of the sample surface was illuminated by a 356-UV-LED through the same objective. Samples were set ispecially designed, liquid-filled glass cell that could be move

Fig. 5. Coordinate system for trapping force simulation.

Fig. 6. Trapping forces of a 100 nm SiO2 particle: (a) r-axis, (b) z-axis.

Fig. 7. Trapping forces of a 100 nm TiO2 particle: (a) r-axis, (b) z-axis.

Fig. 8. Trapping force of a (a) 50 nm and (b) 5 nm TiO2 particle.

Fig. 9. Fundamental experimental system for validation of proposed concept.

decreases with decreasing diameter (compared with the gradientcomponent). According to the Rayleigh scattering theory thatscattered light energy is proportional to the sixth power of its size[19], this means that a 50-nm TiO2 particle can be trapped in the z-direction by a total trapping force consisting of both scattering andgradient components. Conversely, it is the gradient componentthat becomes dominant in the case of the 5-nm diameter particles(Fig. 8(b)), thereby allowing them to be manipulated in threedimensions using the radiation force of a converging laser beam.This means that the proposed method has the potential to offer a10 nm process resolution.

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S. Takahashi et al. / CIRP Annals - Manufacturing Technology 64 (2015) 201–204204

e dimensions relative to the beam of the nanoparticle trappingr by a XYZ-PZT/stepping motor stage unit (25-nm resolution4-mm travel length). The objective used for focusing theparticle trapping laser was also used for high-magnification

ple observation through the addition of a tube lens with amm focal length. Infinity-corrected optics consisting of thective, tube lens and a notch-filter (642-nm wavelength cut)ided in situ monitoring of the laser-trapped particle andple surface at a magnification of 60�.

Fundamental experiments

o further confirm the validity of the proposed concept, sub-ometer-sized TiO2 particles (Rutile, d = 250 nm, G-1, Showa

nium Co. Ltd.) were used as the laser-trapped micro material-oval tool to allow clear and accurate in situ observation andtion of the processing point. To simulate a micro objecticated by microstereolithography, a photosensitive polymergned for this purpose (KC1162, Free-radical type, JSR Corp.) was employed. This was cured flat to allow quantitativeuation of the removal processing through AFM.ig. 10 shows a laser-trapped TiO2 nanoparticle that wasrved through the aforementioned infinity-corrected magni-

optics of the experimental system. Though TiO2 particles ofsize (>100 nm) cannot be manipulated in three-dimensions,

can be moved in two-dimensions across the surface of thed photosensitive polymer (see Section 3).nce the position of the TiO2 nano removal tool was fixed, the

beam (120 mW/cm2, 5 min) was used to illuminate a �100--diameter area of the sample surface surrounding the TiO2

icle. Fig. 11 provides an AFM image of the processing area, inch a 300-nm diameter, 90-nm deep process mark can be seen.

ite this, the beam spot size is estimated to be over 410 nm. It was also confirmed that no such process mark can beined under the same conditions without UV exposure.

the inherent properties of light energy, but also a fine processresolution similar to that of a mechanical-probe-based methodwithout the usual problem of tip wear. Theoretical analysis verifiedthat TiO2 nanoparticles with a refractive index of �2.97 can bethree-dimensionally handled using a laser trapping technique.Further experimental analysis confirmed the core principles of theproposed concept; i.e., that the process resolution does not dependon the diffraction limit of the energy source, but rather on the TiO2

tool size. This proposed method is therefore considered to have thepotential to facilitate the development of a nano correction processfor 3-D functional microstructures fabricated using microstereo-lithography, etc. Identifying the location of fine defects by makingthe use of the laser trapped TiO2 nanoparticle as nano profilemeasurement probe is one of the most important future tasks.

Acknowledgements

The authors would like to sincerely thank Dr. Yasushi Kuroda ofShowa Titanium Co., Ltd. for preparing the TiO2 nanoparticles, andalso the JSR Corporation for providing the special photosensitiveresin designed for microstereolithography. This work was partiallysupported by the Japan Society for the Promotion of Science (JSPS)under a Grant-in-Aid for Scientific Research (A), ChallengingExploratory Research, and the Mitsutoyo Association for Scienceand Technology.

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rated under UV exposure and measured by AFM.

onclusions

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