Three-dimensional recording inside dielectrics for ......EXPERIMENTAL DETAILS ... G is the...

Three-dimensional recording inside dielectrics for photonicapplications

Saulius Juodkazis, Vygantas Mizeikis, Toshiaki Kondo,Kenta Kannari, Kock Khuen Seet, Hiroaki Misawa

Research Institute for Electronic Science, Hokkaido University, N21W10, Sapporo 001-0021, Japan

ABSTRACT

Three-dimensional (3D) structuring of polymers by tightly focused femtosecond laser pulses and interferenceis demonstrated. Photo-structuring by the tightly focused sub-1 ps laser pulses realizes a direct laser writinginside dielectric materials without self-focusing. Photo-polymerization of 3D photonic crystal templates withphotonic stop bands in IR-spectral region has been achieved. Holographic photo-polymerization of photoniccrystal templates with counter-propagating beams was numerically modeled. Three-dimensional structures withan axial period smaller than the lateral can be recorded using the counter-propagating beam alignment. Photo-polymerization of large-area polymeric structures with a high surface-to-volume ratio is demonstrated.

Keywords: photo-polymerization, laser microfabrication, dielectric breakdown, ionization, photonic crystals,interference, direct laser writing

1. INTRODUCTION

Three-dimensional (3D) laser micro-structuring of crystals and glasses by short, sub-1 ps, pulses can be realizedsince a very high pulse irradiance (∼ 1 PW/cm2) is obtained without self-focusing inside dielectrics.1, 2 Photo-polymerization of resins and resists by pulses of 100-200 fs requires much lower irradiance (< 1 TW/cm2) andpulses of sub-1 nJ energy focused into a spot size comparable with the wavelength, λ = 0.8 − 1.0 µm createsconditions for efficient cross-linking or its initiation, which is later accomplished by a post-fabrication annealing.

The 3D-structured materials are prospective in various fields of modern science and technology. They canfunction as elastic micro-dumpers, storage tanks in fuel cells, super capacitors, scaffolds for bio-materials andimplants, etc. It was recently demonstrated, that photo-polymerized templates of logpile and spiral photoniccrystals (PhCs) have elastic properties similar to foams.3 Differently form the self-organized foams, where thereare micro-volumes which are not accessible for microfluidic flows and fuel/charge storage, 3D structures recordedholographically can be made fully permeable by implementing phase control of interfering beams.4–6 Circularlypolarized laser beams can be used to accomplish the same task as phase retarder plates, since, a circularlypolarized beam can be represented as a combination of two perpendicularly polarized beams with a phase shiftof π/2.7 In the case of five beam interference this results in formation of a diamond-like logpile instead of abody-centered tetragonal structure.8

Here, we report on mechanical properties of 3D photo-polymerized structures. The 3D structures can beused as photonic crystals or cellular materials for microfluidics, micro-shock absorbers, large-surface-area storagematerials in fuel cells and super capacitors.

2. EXPERIMENTAL DETAILS

3D structures were photo-polymerized in a negative SU-8 resist using 150 fs duration pulses of 800 nm cen-tral wavelength. Both, direct laser writing2, 9 and holographic recording10 were used for photo-polymerization.Structural analysis of fabricated samples was carried out by scanning electron microscopy (SEM).

Further author information: (Send correspondence to S. J or H. M.)S. J.: E-mail: [email protected];H. M.: E-mail: [email protected], Telephone: (+81 11) 706 9358, Fax: (+81 11) 706 9359.

Invited Paper

Advanced Fabrication Technologies for Micro/Nano Optics and Photonics,edited by Thomas J. Suleski, Winston V. Schoenfeld, Jian Jim Wang, Proc. of SPIE

Vol. 6883, 68830I, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.762977

Proc. of SPIE Vol. 6883 68830I-1

Downloaded from SPIE Digital Library on 29 Jun 2011 to 136.186.80.71. Terms of Use: http://spiedl.org/terms

(a) (b)

(c)

Figure 1. SEM images of logpile structure photo-polymerized in SU-8 at different magnifications. Formation of a solidline from micro-volumes photo-polymerized by a single-pulse is evidenced. Pulse energy was approximately 2 pJ, duration150 fs, and wavelength 800 nm.

3. RESULTS AND DISCUSSION

In the following sections we discuss the mechanism of photo-polymerization, mechanical properties of a singlespiral, and recording of 3D structures by interference.

3.1. Laser microfabrication of 3D-structured cellular materials

The direct laser writing can be used to photo-polymerize the focal volume of a tightly focused laser beam. We,first, define the focal volume where nonlinear absorption is taking place, then, discuss elastic properties of singlephoto-polymerized spirals and PhCs. By 3D scanning of the focal spot, a photo-polymerization of differentcomplex patterns, cellular materials, can be achieved.

3.1.1. Focal volume and nonlinear photo-polymerization

In direct laser writing, the focal volume of a tightly focused laser beam defines the photo-modified volumedepending on the mechanism of absorption. In the case of two-photon absorption (TPA), the photo-polymerizedvolume is expected to be defined by the Intensity2 of the Gaussian pulse, the intensity point spread function(IPSF2), which can be assumed to be Gaussian. By a 3D integration of a Gaussian distribution one would find11:

VTPA = π3/2ω2xyωz, (1)



where the 1/e-cross sections of the IPSF2 are given by:

ωxy =

{0.320λ√

2NAif NA ≤ 0.7,

0.325λ√2NA0.91 if NA > 0.7.

(2)

These are cross-sections of the focal volume where TPA absorption takes place.

It should be noted, that the amount of TPA is usually not known at the wavelength of laser fabricationsince the most precise method of TPA determination, the z-scan, can not be applied at irradiance of ∼TW/cm2.The nonlinearity of photo-polymerization can be achieved by multi-photon absorption, which is seeding themore efficient electron generation via avalanche, a linear process, at such irradiance. Also, the nonlinear photo-polymerization reactions with a chemical enhancement, present in SU-8, plays an important role.

Figure 1 shows a photo-polymerized line with micro-volumes photo-polymerized by single-pulses which wereoverlapped by more than 70% of the focal spot diameter. Pulse of ∼ 0.5 − 1nJ energy when focused by anobjective lens of numerical aperture of 1.35 delivers irradiance in ∼TW/cm2 for a 150-200 fs pulse at 800 nmwavelength. Such solidified micro-volumes result from photo-polymerization by the nonlinear absorption asdiscussed above. The voxels (volume elements) photo-polymerized by single-pulses are discernable only at theend of suspended lines but not within the region of the main logpile structure. This is most probably due tothe chemical amplification of polymerization during the post-exposure annealing, when diffusion of a photo-generated acid is considerable. This diffusion limits achievable separation between the photo-polymerized logsin the structure. The diffusion affects polymerization on a lesser extent when separation of neighboring featuresbecomes larger as at the ends of logpile structure.

3.1.2. Linear vs. nonlinear ionization

The linear and non-linear absorption of light are the pathways of energy delivery for photo-polymerization andphoto-modification in general by laser pulses. Here, we estimate probabilities of the fundamental transition(the one-photon absorption) and two-photon absorption (TPA) by the one and two-photon photo-effect, corre-spondingly. The probabilities are calculated for electron transition into a free electronic state. The one-photonprobability calculated by Fermi golden rule for the hydrogen-like 1s → p transition is given by12:

w(i)1s→p ≈ 27

3J

�

(eE0a0

J

)2 (J

�ω

)9/2

(3)

where J is the ionization potential, E0 is the amplitude of electric field, �ω is the photon energy, a0 = �2/(me2)

is the Bohr radius, e and m are the electron charge and mass, respectively. The eqn. 3 can be used to calculatetransition from the bound electronic state (1s) into the continuum of states where electron energy is defined bythe momentum p.

The probability of the two-photon photo-effect was calculated by the Fermi golden rule using a 2nd-orderperturbation theory is12:

w(2)i→f = 25 J

�

√2�ω − J

J

(eE0a0

J

)4 (J

2�ω

)8[1 − 8

32�ω − J

�ω+

163

(2�ω − J

�ω

)2]

. (4)

This equation is an exact solution of an electron transfer from a single discrete level inside a potential well intoa free unbound state (zero-energy) position, i.e., the two photon photo-effect.

Figure 2 shows the probability of two-photon ionization at the wavelength of 800 nm for the ionizationpotential of J = 3.1 eV. For comparison a single photon transition rate at 400 nm is also plotted (curve (1)).The field strength of approximately 1.4 × 109 V/cm is necessary for the both rates to become comparable. Thecomparison between fundamental and two-photon absorption given above is relevant for ionization of Al atom,TiO2, SiC, and some glasses. The discussion above illustrates that the nonlinear absorption occurs at very highelectric field strength, which is related to the laser irradiance.



Figure 2. The transition rates for the fundamental (eqn. 3 at 400 nm) and two-photon ionization (eqn. 4 around 800 nm)for the ionization potential of J = 3.1 eV.

3.2. Elastic properties of a photo-polymerized single spiral

Photo-polymerized springs and cantilevers (see, Fig. 1) can find number of applications in MEMS and sensorfields. The diameter of “wire”, coil’s radius, axial pitch, and length can be easily controlled by direct laserwriting in the case of spirals. A circular spring of a circular wire will change its height by length, f , under axialload, P , according to13:

f =64PR3n

Gd4

[1 − 3

64

(d

R

)2

+3 + ν

2(1 + ν)tan(α)2

], (5)

where R is the radius of coil measured from the spring axis to center of wire, d is the diameter of circular wire, Pis compressive (or tensile, −P ,) load, n is number of active turns in spring (for compression n is less by 2 whenthe first and end turns are counted), α is the pitch angle of spring, ν is the Poisson’s ratio, G is the shear/rigiditymodulus, G = E/[2(1 + ν)], where E is the Young modulus. The other wire shapes result in slightly differentcoefficients, however, a functional dependence is the same as given by eqn. 5.13 The spring constant of a helicalspring of circular wire is14:

k =πG(d/2)4

2R2L, (6)

where L is the height of spring. For a typical set of parameters: a coil radius R = 750 nm, a wire diameterd = 500 nm (this is an approximation since the polymerized wire had an elliptical cross-section due to axiallyextended focal intensity distribution), a vertical pitch of spiral c = 2.64 µm, number of vertical periods n = 4(the total height of structure L � 10 µm), and E � 4 GPa for a fully cross-linked SU-8, one gets the springconstant k = 1.72 N/m from eqn. 6. The load of P = 1 mN should generate compaction of a l = 10 µm tallstructure by f = 2.3 µm according to eqn. 5. If we consider that the load is equally distributed over the numberof independent springs, each taking the load P , within the footprint of a 30-µm-diameter punching press andν = 0.2 (this is an experimental condition reported in ref.3), the same f/l = 0.23 strain would result.

3.2.1. Compression tests

Structural rigidity of logpile and spiral photo-polymerized structures was carried out. Figure 3 summarizes theload-displacement test on logpile and spiral structure by a flat punch compression over a 30 µm diameter. Inlogpile, the lateral and axial separation between logs and planes was 2.5 and 1.2 µm, respectively. The first testshows that the structure become inelastically altered at approximately ∼ 10 mN load, at which the compression



0.0 0.2 0.4 0.60

20

40

60 Logpile #1 #2

Strain0.0 0.2 0.4 0.6

0

50

100

150

200 Circ. spiral Square siral #1 #2 #3

Stre

ss (M

Pa)

Strain

(a) (b)

Figure 3. The stress-strain curves of the spiral (a) and logpile (b) structures.3 The properties of the structures aresummarized in Table 1. The markers are larger than the error bars beyond the elastic region till approximately strains of0.2.

Table 1. Summary of flat-punch compression tests.3

Sample Height Punch Crystal Elastic coll- Max Strain at comp- Compres.l area modulus Eeff apse stress load ressive strength distance

[µm] [µm2] [MPa] [MPa] [mN] [%] [µm]Circ. spiral 10.3 ± 0.3 554 ± 17 191 ± 8 15.8 8.76 11.4 1.18

Square spir. #1 8.2 ± 0.2 449 ± 13 258 ± 11 37.3 16.75 17.5 1.43Square spir. #2 9.8 ± 0.3 569 ± 17 528 ± 14 32.8 18.66 7.9 0.77Square spir. #3 9.5 ± 0.3 576 ± 17 494 ± 17 32.5 18.70 7.5 0.71

Logpile #1 3.1 ± 0.1 707 ± 21 170 ± 5 17.9 12.65 12.7 0.40Logpile #2 3.2 ± 0.1 707 ± 21 147 ± 5 17.5 12.38 13.8 0.44

by ∼ 10% occurred. The same test on the cover glass (a borosilicate glass) yielded in the spring constant valueof k = P/∆l � 1.8 × 106 N/m. The estimated Young modulus E = kl/S � 54 GPa (where S is the puncharea and the effective length of a load-affected region is estimated as l = d/

√2, which is a side length of a

right-angle triangle whose diagonal is d) is close to the expected value of Young modulus of borosilicate pyrexglass, 62 GPa.14

It is noteworthy, that laterally unsupported springs are susceptible to buckling when compressed beyond acritical deflection, dc. The values of dc are strongly dependent on the height-to-diameter ratio, L/(2R), e.g., forL/(2R) = 1 buckling will occur at dc/L = 0.72 while for 8 it becomes only 0.17. In our experiments L/(2R) < 7.In some of the tests sliding and detachment of PhC structures occurred when slightly off-axial compression wasapplied. It was presumably caused by buckling.

The ratio of the density of an entire PhC to the density of a cell wall material determines the structuralelastic properties. It should be noted that the structures photo-polymerized by the direct laser writing with aclose-to-Gaussian intensity distribution usually produces a self-similar distribution of polymer cross-linking dueto accumulated exposure and temperature distribution.15 Hence, the elastic modulus is not uniform across thecross-section of polymerized wire, a phenomenon typical for photo-polymerized structures.16, 17

In the studied logpile and spiral (with a circular and square footprint) structures we observed the samebending dominated foam-like behavior. Hence, it can be expected that the structural modulus of the PhCstructures will scale with the relative mass density, ρr = ρ/ρs (ρs is the density of bulk material), following theestablished dependence E ∼ Es × ρ2

r,18–20 here Es is the Young modulus of a fully cross-linked solid polymer

in our case. As a result, by changing the volume fraction of material in PhC both the elastic and the opticalproperties can be controlled.



II:itIi ,ff

'K Ii IIIh \ % ¶11

fl III l\j' \ \

x

(a) (b)

(c)

Figure 4. SEM images of a typical 3D pattern photo-polymerized in SU-8 by interference of five beams (150 fs/800 nm)at different magnification; exposure time tens-of-seconds. The objective lens used to form the interference field hadnumerical aperture NA � 0.75 (n = 1.64 in SU-8).

(a) (b)

Figure 5. Schematics of a pillar pattern collapse due to capillary force via a liquid surface area minimization duringdrying and evaporation.

The observed behavior of the polymerized PhC templates upon loading showed bend-dominated response incontrast to the strut-like structures which exhibit sharp catastrophic collapse to the plateau region right after theelastic limit is surpassed.20 The bend-dominated response is preferable for mechanically tunable PhC crystalapplications at large elastic strains because they provide a larger safe margin of operation before transition intothe plastic and therefore irreversible deformation of the plateau region.

3.3. Holographically formed 3D structures

Holographic recording is realized by interference of several laser beams. A diffractive optical element (DOE)is useful for creation of required number of beams. When used in combination with a standard scaling tele-scope, a fulfilment of a space-time coincidence of ultra-short pulses in the interference volume is inherentlyprovided.10, 21, 22 Typical pattern photo-polymerized in SU-8 by interference of five beams is shown in Fig. 4.The area approximately of ∼ 1 mm in cross-section can be photo-polymerized in an one step exposure of tens-of-seconds. All five femtosecond pulses (150 fs/800 nm) coincide forming intensity distribution, which can becontrolled by the phases of the beams.4–6 After development, such structures have a large surface area to volumeratio. It might be useful in application where such property is required, e.g., fuel cells, super capacitors, or superadhesives. Moreover, such polymer patterns can be coated by noble, catalytic, or magnetic metals using a simplechemical electro-less deposition by dip-coating. Recently, electro-less coating of SU-8 logpile patterns by nickel,which has the relative magnetic permeability only twice lower than that of magnetic iron, was demonstrated.23



2O

15

ilo

1.25 2.5 •1.25

1250X(2)25 -2.5

2.5 a - a• — • —r

1.251 — —

2 -1,25 0 1.25 2.5x (X)

2.5 r

-1.25 0 1.25 2.5X (A)

Objective2

Objective1

Mirror

Mirror

Achromatic lens

Beam splitter

Diffractive-optical element

Focal

length

ofac

hromati

c len

s

Distan

ceA

Dis

tanc

e B

Objective2

Objective1

Mirror

Mirror

Achromatic lens

Beam splitter

Diffractive-optical element

Focal

length

ofac

hromati

c len

s

Distan

ceA

Dis

tanc

e B

(a) (b)

Objective

Objective

Figure 6. (a) Schematic explanation of the beam mutual alignment in counter-propagating four-beam interference setupwith infinity-corrected objective lense. (b) Numerical simulation of the interference of plane waves in geometry presentedin (a) for the objective lenses of numerical aperture NA = 0.75 (θ = 34◦ and n = 1.64 in SU-8); the phases of all beamsα, β, γ, δ are same. The length is normalized to the wavelength λ. The ratio of the lateral period, dxy, is larger than theaxial, dz (dz/dxy = 0.47).

The reflection spectrum of such metallo-dielectric PhC structures can be engineered for IR-applications wherethey act as band-edge filters.23 Conductivity of 3D polymeric structures coated by metals can be further ex-ploited for the electro-less or electrochemical plating by other metals. Once surfaces acquire electric conductivity,they can be used in super capacitor or electrophoretic applications.

3.3.1. Collapse of 2D patterns due to capillary drainage

Distortion of photo-polymerized 3D/2D/1D structures by a wet post-exposure processing is a common problem.We discuss bellow a collapse of 2D structures which are prospective for fabrication of molds and templatessuitable for replication of the patterns in mass production.

The collapse of 2D pillar pattern can be understood as an energy minimization process. The surface energiesof solvent and pillars’ material are determining the radius of meniscus as schematically shown in Fig. 5(a)(corresponds to the wetting or hydrophilic conditions).

Upon drying and evaporation the level of solvent decreases forcing neighboring pillars to approach each other.Since pillars are elastic, the system solvent-pillars can minimize its energy (the area of surface) by switching tothe more energetically favorable conformation shown in Fig. 5(b).

The same energy minimization explains not only one directional (say along x-axis) collapse but also collapsealong the perpendicular direction. Once the pattern collapsed in one direction as shown in Fig. 5(b) the pair ofpillars (as a structure) is more flexible in y- than x-direction. Hence, upon further drying the pattern is prone tocollapse along y-axis. When meniscus is receding, the same curvature of solvent surface (the meniscus) will notcollapse the structure further since the effective height of the structure becomes smaller. So, once the dislocationlines appears, they will not be removed from a 2D PhC structure. After solvent is evaporated, the pillars are keptconnected by the van-der-Waals force. Since the van-der-Waals force scales with separation, l, as FV W ∼ 1/l7

it is difficult to recover the 2D pattern of free-standing pillars by sample post-processing. The same capillary



1.8

1.6

1.

1.

1.

0.

0.

4

2

0

8

6

0. 4

0. 2

0. 0

N P X Z

Figure 7. Reciprocal lattice of the bcc structure (left) and the calculated photonic band diagram. Photonic stop gaphaving width of about 20% is denoted as PBG.

forces are important in development of 3D PhC templates as shown in Fig. 4(c), where a pattern change due tothe capillary drainage is evident.

The critical Young’s modulus for the pillars to withstand capillary drainage is given by24:

Ecr =24γl4z

(2R)3D2. (7)

eqn. 7 demonstrates the possibility of the pattern collapse when the height lz or the gap between the pillarsD increases, or when their diameter 2R decreases. The capillary force determines the smallest spacing of thepattern as a function of the aspect ratio dx ∝ γf2

ar,25 where the aspect ratio is given by far = lz/t with t being

the wall thickness or rod diameter, depending on the pattern. Consequently, it is difficult to decrease the spacingof the high-aspect ratio structures without risk of deformations. The bending stiffness of the structures dependson their shape and size, for example, 3

√E/� and

√E/� for planes and rods, respectively.19 Hence, for the same

height of the structure, Young modulus, E, and mass density, �, it is easier to buckle a plane than a rod. Inphoto-polymerized structures Young modulus is dependent on the exposure dose and post-exposure treatment,which determine the final degree of cross-linking.16, 17

By controlling the evaporation the collapse of the pattern could be controlled. Distortions of patterns can beavoided altogether by applying super-critical drying (SCD) in wet processing.26, 27 In a super-critical liquids thesurface tension becomes negligible (γ → 0) and the capillary force vanishes. However, high pressure (1−10 MPa)and elevated temperatures (40 − 400◦C), are required,28 depending on the solvent. Therefore this method notuniversally applicable for wet processing. For example, if SCD was to be used for isopropanol removal fromdeveloped SU-8 networks, temperature over 235.2◦C and pressure over 4.8 MPa would be required.28 We haveproposed an alternative to use an inherent hydrophobicity of polymers which facilitates reduction of capillaryforce and patterns of high aspect ratio can be recovered after wet processing in SU-8 resit.29

3.3.2. Holographic recording using a counter-propagating beam alignment

The main disadvantage of holographic patterns recorded using the DOE and scaling telescope in PhC applicationsis large difference in axial and lateral periods of the structures. This limitation is imposed by the allowed rangeof angles of incidence for the side beams. Even using oil-immersion objective lenses, the angular range of sidebeams is limited by the numerical aperture and entrance pupil diameter of the objective lens.



In order to solve this problem and reduce the axial period of photo-polymerized structures, we propose to usea counter-propagating geometry, in which two groups of beams arrive to the photosensitive region from oppositesides, as shown in Fig. 6(a). Interference pattern of the counter-propagating beams has the smallest possibleperiod of λ/2. Such configuration was considered earlier in photo-polymerization of nano-sheet materials.22, 30

Fig. 6(a) also explains the mutual arrangement used for recording, in which two counter-propagating pairsof beams converge in horizontal and vertical planes, respectively. Numerical simulation of the light intensitydistribution resulting from such arrangement is shown in Fig. 6 for the incidence angle of θ = 34◦ with respect tothe optical axis of the system. Such angle is technically achievable even with dry objective lens (NA < 1). Theresulting structure has a body-centered tertagonal (bct) lattice symmetry. Figure 7 shows photonic band diagramexpected for such structure. The band diagram reveals a photonic stop gap of about 20% (width-to-center ratio)along the Γ − Z direction, which is coincident with the optical axis of the setup used for recording.

4. CONCLUSIONS AND OUTLOOK

We demonstrate here that photo-polymerized structures can be used as micro-springs, stress-tunable photoniccrystals, and that they behave mechanically as foams with elastic limit up to 10% of strain. Femtosecondlaser microfabrication becomes a useful tool providing 3D structured cellular materials. Such materials canfind applications in microfluidics, photonics, and micro-mechanical applications. Also, photo-polymerized 3Dscaffolds can be used for regeneration of bio-materials and tissues.

We predict future use of such 3D structured materials in the fields where large surface-to-volume ratio isrequired. 3D patterns photo-polymerized by femtosecond laser beams (by the direct laser writing or holograph-ically) can be coated by metals using the electro-less deposition to provide electric conductivity to the surface.Such funcionalized surfaces are prospective in fuel cell, super capacitor applications; for plasmonic applicationssurface could be coated by Cu, Ag, or Au. More details on the 3D femtosecond laser fabrication can be found inearlier1 and recent reviews.31–34

SJ acknowledges financial support provided by a Grant-in-Aid from the Ministry of Education, Science,Sports, and Culture of Japan No.19360322.

REFERENCES1. V. Mizeikis, S. Juodkazis, A. Marcinkevicius, S. Matsuo, and H. Misawa, “Tailoring and characterization of

photonic crystals,” J. Photochem. Photobiol. C 2(1), pp. 35–69, 2001.2. S. Juodkazis, S. Matsuo, H. Misawa, V. Mizeikis, A. Marcinkevicius, H. B. Sun, Y. Tokuda, M. Takahashi,

T. Yoko, and J. Nishii, “Application of femtosecond laser pulses for microfabrication of transparent media,”Appl. Surf. Sci. 197-198, pp. 705–709, 2002.

3. S. Juodkazis, V. Mizeikis, K. K. Seet, H. Misawa, and U. G. Wegst, “The mechanical performance and opticalproperties of 3D photonic structures photo-polymerized in SU8,” Appl. Phys. Lett. 91, pp. 241904/1–3, 2007.

4. T. Kondo, S. Juodkazis, V. Mizeikis, S. Matsuo, and H. Misawa, “Fabrication of three-dimensional periodicmicrostructures in photoresist SU-8 by phase-controlled holographic lithography,” New J. Phys. 8(10),p. 250, 2006 (doi:10.1088/1367-2630/8/10/250).

5. T. Kondo, S. Juodkazis, V. Mizeikis, H. Misawa, and S. Matsuo, “Holographic lithography of periodic two-and three-dimensional microstructures in photoresist SU-8,” Opt. Express 14(17), pp. 7943–7953, 2006.

6. T. Kondo, S. Matsuo, S. Juodkazis, V. Mizeikis, and H. Misawa, “Three-dimensional recording by femtosec-ond pulses in polymer materials,” J. Photopolym. Sci. Tech. 16(3), pp. 427–432, 2003.

7. H. Misawa and S. Juodkazis, “Light forms tiny 3D structures,” SPIE Newsroom: Micro/Nano Lithography& Fabrication , p. DOI: 10.1117/2.1200603.0181; http://spie.org/x8778.xml, 2006.

8. T. Kondo, S. Juodkazis, V. Mizeikis, and H. Misawa, “Three-dimensional high-aspect-ratio recording inresist,” J. Non-Crystall. Solids , p. doi:10.1016/j.jnoncrysol.2006.11.048, 2007.

9. S. Juodkazis and H. Misawa, “Forming tiny 3D structures for micro- and nanofluidics,” SPIE Newsroom:Micro/Nano Lithography & Fabrication , p. DOI: 10.1117/2.1200612.0510; http://spie.org/x8371.xml, 2007.



10. T. Kondo, S. Matsuo, S. Juodkazis, and H. Misawa, “A novel femtosecond laser interference technique withdiffractive beam splitter for fabrication of three-dimensional photonic crystals,” Appl. Phys. Lett.. 79(6),pp. 725–727, 2001.

11. W. R. Zipfel, R. M. Williams, and W. Webb, “Nonlinear magic: multiphoton microscopy in the bioscience,”Nature biotechn. 21, pp. 1369–1377, 2003.

12. S.A.Akhmanov, V.A.Vyslouch, and A. Chirkin, Optics of femtosecond laser pulses, Nauka, Moscow (inRuss.), 1988.

13. W. C. Young and R. G. Budynas, Roark’s formulas for stress and strain, McGraw-Hill International Edition,Boston, 7th ed., 2002.

14. W. C. Elmore and M. A. Heald, Physics of waves, Dover Publications Inc., New York, 1985.15. K. K. Seet, S. Juodkazis, V. Jarutis, and H. Misawa, “Feature-size reduction of photopolymerized structures

by femtosecond optical curing of SU-8,” Appl. Phys. Lett. 89, p. 024106, 2006.16. M. Miwa, K. Douoka, S. Yoneyama, S. Tuchitani, Y. Kosihmoto, and R. Kaneko, “Young’s module control

of the micro cantilever made by micro-stereolithography,” in MOEMS and Miniaturized Systems V, A. El-Fatatry, ed., pp. 6 – 13; doi: 10.1117/12.589197, SPIE, Bellingham, WA, 2005, 2005.

17. T. Choi, J.-H. Jang, C. K. Ullal, M. C. Lemieux, V. V. Tsukruk, and E. L. Thomas, “The elastic propertiesand plastic behavior of two-dimensional polymer structures fabricated by laser interference lithography,”Adv. Func. Mat. 2006, pp. 1324–1330, 10 2006.

18. L.J.Gibson and M. Ashby, Cellular Solids: Structure and Properties, Cambridge University Press, Cam-bridge, U.K., 2nd ed., 1997.

19. M. F. Ashby, L. J. Gibson, U. Wegst, and R. Olive, “The mechanical properties of natural naterials: I.Material property charts.,” Proc. R. Soc. Lond. A 450, pp. 123–140, 1995.

20. M. F. Ashby, “The properties of foams and lattices,” Phil. Trans. R. Soc. A 364, pp. 15–30, 2006.21. T. Kondo, S. Matsuo, S. Juodkazis, V. Mizeikis, and H. Misawa, “Multiphoton fabrication of periodic

structures by multibeam interference of femtosecond pulses,” Appl. Phys. Lett. 82(17), pp. 2758–2760,2003.

22. T. Kondo, K. Yamasaki, S. Juodkazis, S. Matsuo, V. Mizeikis, and H. Misawa, “Three-dimensional micro-fabrication by femtosecond pulses in dielectrics,” Thin Sol. Films 453-454, pp. 550–556, 2004.

23. V. Mizeikis, S. Juodkazis, R. Tarozaite, J. Juodkazyte, K. Juodkazis, and H. Misawa, “Fabrication andproperties of metalo-dielectric photonic crystal structures for infrared spectral region,” Opt. Express 15,pp. 8454–8464, 2007.

24. T. Tanaka, M. Morigami, and N. Atoda, “Mechanism of resist pattern collapse during development process,”Jpn. J. Appl. Phys. 32, pp. 6059–, 1993.

25. H. Namatsu, K. Kurihara, M. Nagase, K. Iwadate, and K. Murase, “Dimensional limitations of siliconnanolines resulting from pattern distortion due to surface tension of rinse water,” Appl. Phys. Lett. 66(20),pp. 2655–2657, 1995.

26. M. A. McHugh and V. J. Krukonis, Supercritical Fluid Extraction, Butterworth-Heinemann, Boston, 2nd ed.,1994.

27. C. A. Eckert, B. L. Knutson, and P. G. Debendetti, “Supercritical fluids as solvents for chemical andmaterials processing,” Nature 383, pp. 313 – 318, 1996.

28. A. Weissberger, ed., Organic Solvents.Physical properties and methods of purification, John Wiley & Sons,Inc., New York, 4th ed., 1986.

29. T. Kondo, S. Juodkazis, and H. Misawa, “Reduction of capillary force for high-aspect ratio nanofabrication,”Appl. Phys. A 81(8), pp. 1583 – 1586 (DOI: 10.1007/s00339–005–3337–7), 2005.

30. T. Kondo, Photonic crystal fabrication by interference of femtosecond pulses. PhD thesis, Hokkaido Univer-sity, Sapporo, Japan, March 2005.

31. S. Juodkazis, V. Mizeikis, and H. Misawa, “Three-dimensional structuring of resists and resins by directlaser writing and holographic recording,” Adv. Polym. Sci. , pp. doi: 10.1007/12–2007–122, 2007 (publishedon line Oct. 27).

32. S. Juodkazis, V. Mizeikis, S. Matsuo, K. Ueno, and H. Misawa, “Three-dimensional micro- and nano-structuring of materials by tightly focused laser radiation,” Bull. Chem. Soc. Jpn. , 2008 (in press).



33. V. Mizeikis, S. Matsuo, S. Juodkazis, and H. Misawa, Femtosecond laser microfabrication of photonic crys-tals, vol. Three-dimensional laser microfabrication: fundamentals and applications. Willey, 2006.

34. H. Misawa and S. Juodkazis, eds., Three-dimensional laser microfabrication: fundamentals and applications,Wiley, 2006.



Three-dimensional recording inside dielectrics for ......EXPERIMENTAL DETAILS ... G is the...

Documents

Transcript of Three-dimensional recording inside dielectrics for ......EXPERIMENTAL DETAILS ... G is the...