Three-dimensional laser microfabrication of metals ......Femtosecond laser fabrication has become a well established technique for three-dimensional (3D) and surface structuring of

Three-dimensional laser microfabrication of metals,semiconductors, and dielectrics

Saulius Juodkazisa, Koichi Nishimuraa, Hiroki Okunob, Yusuke Tabuchib,Shigeki Matsuob, Satoru Tanakac, and Hiroaki Misawaa

aResearch Institute for Electronic Science, Hokkaido University, N21W10, Sapporo 001-0021, Japanb Department of Ecosystem Engineering, The University of Tokushima, Tokushima 770-8506, Japan

cDepartment of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0385,Japan

ABSTRACT

We demonstrate three - dimensional (3D) structuring of materials by femtosecond laser irradiation. The self-organized ordered formation of ripples by a laser raster-scanning on the surface of 4H-SiC is demonstrated. Thismethod is scalable up to areas with sum-millimeter cross-sections. The role of ripple-like structures in the caseof the surface and in-bulk micro-structuring of different materials is discussed. The field enhancement effects atthe nano-structured surfaces and their role in ripple formation are estimated.

Keywords: laser microfabrication, ablation, ripples, dielectric breakdown, field enhancement, light extraction

1. INTRODUCTION

Femtosecond laser fabrication has become a well established technique for three-dimensional (3D) and surfacestructuring of metals, semiconductors, and dielectrics.1 The 3D fabrication is based on removal of material byablation from the surface of non-transparent sample or by photo-modification inside the sample and subsequentwet processing.2 The later method is used in photo-polymerization of resists and resins and wet etching ofchannels inside glasses and crystals.3–5 Channels in polymers can be recorded in a single step of focal spot scanwhen micro-explosion pushes side-ways the material out of focus.6

Another practical approach for a large scale micro-fabrication is to create 2D and 3D structures via self-organization or the bottom-up approach. Examples of such self-organized structures are the propagating poly-merization via self-growing waveguides in resins and ripples formation on the surfaces and in the bulk of materials.On a conceptual level, self-organization helps to achieve large area/volume patterning with high spatial reso-lution. This is highly required in order to break the well known Tennant’s law7 for nano-fabrication based onlithography:

Feature size[nm] = 23 × Throughput0.2[µm2/h], (1)i.e., an effective fabrication is not compatible with small feature size. The direct laser write can guide a bottom-up approach; for example, a photo-activated processes on a molecular scale could control structuring on a largerscale currently achievable only by top-down methods.

Here, we present results on formation of 3D micro - structures in very different materials: stainless steel,semiconductor (SiC), and crystalline quartz. Templating of large area of SiC surface by self-organization ofripples is demonstrated. Self-organization occurred via control of the polarization and overlap of the irradiationspots in a raster line-by-line scanning.

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

Intl. Conf. on Lasers, Applications, and Technologies 2007: Laser-assisted Micro- and Nanotechnologies, edited by Vladislav Panchenko, Oleg Louchev, Sergei Malyshev, Proc. of SPIE Vol. 6732,

67320B, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.751889

Proc. of SPIE Vol. 6732 67320B-1

Downloaded from SPIE Digital Library on 24 Jun 2011 to 136.186.80.71. Terms of Use: http://spiedl.org/terms

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400 450 500 850 900

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2

3

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(nm)

SI-4H:SiCd = 0.39 mm

(a) (b) (c)

pol.

Figure 1. (a) The absorption spectrum of a d = 0.39-mm-thick semi-insulating (SI) sample of 4H-SiC (the bandgapenergy is 3.28 eV). (b-c) SEM images of the surface of 4H-SiC after femtosecond laser structuring. Pulse duration was150 fs, wavelength 800 nm. The scanning pattern is marked by arrowed lines in (b); polarization was horizontal.

2. EXPERIMENTAL

Laser microfabrication setup consists of an oscillator-regenerative amplifier Ti:sapphire femtosecond laser (Spec-tra Physics), an optical microscope (Olympus), and piezo-ceramic 3D-stage. The stage and shutter are controlledby a computer program which records designed patterns onto/into the sample. The focusing is carried out bya dry or oil-immersion high numerical aperture objective lens (NA ≥ 0.9) for formation of ripples and wet-etchable channels. In the case of metal drilling a focusing by a f = 31 mm lens was used with beam delivery byglavano-scanners. The direction of electric field is termed as a polarization direction in the text.

A simple rotary pump was sometimes used to facilitate vapor and debris removal in a metal drilling experi-ments.8 The mean free path of molecules depend on pressure as9:

lf =1√

2πna2=

kT√2πpa2

, (2)

here p is the pressure, T is the absolute temperature, k is the Boltzmann constant, n is the number density anda the size of molecules. Even at low vacuum created by a rotary pump with p = 10−3 Torr and assuming the sizeof air molecules 0.3 nm, the value of lf changes from 60 nm9 at room conditions (with n = 2.69 × 1019 cm−3)to approximately 93 µm, which is comparable with the fabricated feature size in surface processing by ablationin most of cutting and drilling applications. A vacuum enclosure improves the speed of hole drilling and cuttingwhere material removal becomes a limiting factor.

Samples of stainless steel SUS304, 4H-SiC, and quartz were used as obtained from vendors. Post-exposurestructural characterization of the exposed regions was carried out by scanning electron microscopy (SEM).

3. RESULTS AND DISCUSSION

The examples of direct laser writing of photo-modification on the surface or inside different materials follows. Thetightly focused (a diameter of the focal spot was comparable with the wavelength) femtosecond pulses were usedfor surface or in-bulk irradiation. The high pressure and temperature conditions unattainable by irradiation withlonger laser pulses can be created using ultra-short pulses (see, ref.10–13 for popular accounts). These high-p, Tconditions are achieved at small pulse energies limiting the extent of thermal damage facilitating high-precisionmicrofabrication.



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(a) (c)

(b) (d)

(e)

(f)

6 5 4 3 2 1 5 4 3 2 1 4 3 2 1

100 nm 100 nm 100 nm

1 m 1 m 100 nm

Figure 2. (a-b) SEM images of ripples recorded with an overlap od 100 nm between adjacent laser spots (a) and withseparation of 1 µm (b) by 150 fs/800 nm laser pulses on surface of 4H-SiC. Pulse energy was 100 nJ on the sample whichwas at the exact focus. (c-d) Same as previous but at a slightly different axial focal position (out of focus by ∼ 5 µm).(e-f) Same as previous, with pulse energy 200 nJ. Numbers in (b,d,f) marks the sequence of irradiation locations. Thesamples were imaged as fabricated (without cleaning). The arrows in (d) points to the positions of strongest ablationpits.

3.1. Ripple formation on SiC

Figure 1 shows a surface structuring of semi-insulating 4H-SiC by ripples. Laser pulses of 150 fs duration at800 nm central wavelength were focused onto the surface of SiC sample and scanned in such a way that theneighboring horizontal lines were overlapped by approximately 25% of the photo-modification recorded on thesurface by a single pulse. The numerical aperture of the objective lens was NA = 0.9 and typical pulse energywas 100 nJ. The scanning electron microscopy (SEM) images were taken after sample was washed by dippingin diluted aqueous solution of HF. The regions without ripples were formed due to accumulation of the ablationdeposit, which pilled up in front of the laser beam. When ablation pressure become too small to push the debrisfurther ahead of a focal spot, laser pulses were not reaching the surface of the sample resulting in ripple-freeregions. The problem of debris accumulation could be solved by implementing industrial vacuum-chuck forsample fixation together with air blow over the fabrication area. The large sub-mm areas of SiC which has amelting point at over 2730◦C and is chemically inert and resistant to mechanical polishing (the Mohs hardness is9.25 compared to 10 of diamond) were successfully patterned by ripples using focused irradiation by femtosecondlaser pulses.

The mechanism of ripple formation on surface and inside dielectrics is currently under active investigation.The period and orientation of some of the ripple patterns especially on semiconductors and dielectrics can notbe explained by the model of surface electromagnetic wave (SEW). Figure 2 shows the ripples recorded by linearscanning with different overlap between neighboring pulses. When pulses were separated by 1 µm the ablatedpatterns reveals locations where the strongest ablation occurred. As would be expected, the center of the pulsemakes the deepest protrusion (b,f). However, additionally, the strong removal of surface occurred at the rim of



Figure 3. SEM images of ripples formed on 4H-SiC (a cleaned surface) by raster scanning. Exposure conditions are sameas for the pattern shown in Fig. 1. An approximate width of a single-line scan was equal to the irradiation wavelengthλ = 800 nm.

earlier irradiated spot (marked by arrows in (d)). Repetition rate of laser was 100 Hz and thermal accumulationeffects were negligible. The molten phase is most probably SiO2 since such patterns were not observed aftersurface cleaning in aqueous solution of HF. Hence, the strongest ablation took place at the locations of a nano-structured surface. This is consistent with the self-organization character of ripple formation, which is clearlydemonstrated by translation of ripples in the case of overlapping pulses (see, Figs. 1, 2, and 3). It has beendemonstrated that occurrence of ripples of the finest feature size (considerably smaller than a typical λ/2 period)are dependent on surface roughness and polishing quality.14 However, the formation of coarser ripples was notinfluenced by the surface finishing quality.14, 15 The finest ripples can be explained by surface ablation dueto the field enhancement effects which are substantial at the feature size approximately λ/10 as discussed inSec. 3.5.

The final pattern of ripples shown in Fig. 3 can be subjected to annealing in nitrogen atmosphere at 1500◦Cto smooth the smallest features of 10-20 nm on the rugged surface. For example, after such annealing the rippleson sapphire appeared more orderly and covered with a terrace-like structures (not shown here).

The ripple period was typically 200 ± 20 nm and did not depended explicitly on pulse energy nor overlapof neighboring pulses when it was larger than 25% of a single pulse ablation spot (Fig. 3). The ripples wereperpendicular to the polarization (the E-field) of the laser pulses the, so called, normal ripples. The numericalaperture of an objective lens used in ripple recording was 0.9 which corresponded to the incidence angles withinthe cone of 0−64.2 degrees (0 degrees is a normal incidence). This spread of incidence angles makes complicatedanalysis of the ripples map on the surface of SiC when standard SEW approach is implemented. This phenomenonof ripple formation is similar to Wood’s anomaly observed on corrugated metallic surfaces when the surface waveis launched at certain angle of incidence, θi. Both, the Wood’s anomaly and SEW are of the same origin andthe period Λ can be calculated according to16:

Λ = λ

(√|�1|

|�1| − 1 ± sin(θi))−1

, (3)

where �1 is the real part of the dielectric susceptibility of the medium on which ripples are formed and λ is thevacuum wavelength of the incident light. Once the angle of incidence is 0 < θi < 64 deg there is a competitionbetween formation of ripples with different periods. The intensity of light at different angle of incidence ismodulated (a Gaussian-like envelope) and creates conditions for the ripples at θi = 0 to dominate. Since the



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Figure 4. (a) Setup of femtosecond drilling at low pressure (rotary pump was used to make a ∼ 10−2 Torr pressure).PBS is polarizing beam splitter; DM and HM are the dichroic and half mirror, respectively. The top (b) and side (c)SEM images of a hole drilled using galvano-scanners in stainless steel SUS304 in air. Pulse energy was 102 µJ; repetitionrate 100 Hz; focusing by a lens of focal length f = 31 mm; the diameter of the laser beam was 4 mm; the pulse durationwas 80 fs at central wavelength of 800 nm.

dielectric constant is dependent on a bound and free electron density it reflects an importance of ionization andis related to the irradiance.

The depth of ripples is usually comparable with their period. An attempt to make a polydimethylsilox-ane (PDMS) replica of the rippled surface and to measure their height was unsuccessful allegedly due to thehydrophobicity of PDMS while the surface of SiC is hydrophilic. The PDMS has not entered into the ripplestructure even in depressurized conditions which were used to remove air from PDMS and ripples.

The patterns of ripples can find application as light extracting structures in blue LEDs grown on SiC sub-strates. The high refractive index of SiC limits the effectiveness of light extraction in solid-state lighting appli-cations. In micro-fluidics the ripples can serve as sieves and can direct flow since the orientation of ripples iscontrolled via polarization of incident light. Ripples increase a surface area and could serve for an effective heatdissipation in high current applications where SiC is widely used.

It is noteworthy, that surface with ripples could be used to modify hydrophobicity/hydrophylicty . Thecontact angle θ is defined by the surface energies between two of solid, liquid, and vapor substances and is givenby the Young - Dupré equation:

γSV = γSL − γLV cos(θ) (4)where S, V, L denotes solid, vapor, and liquid. Modification of surface by nano-micro-structuring can be describedeffectively as an increase of the surface energy by a factor, r, which typically acquires values 1 < r < 4 (Wenzel’smechanism). The exact r value depends on the surface structure.17 The largest values are for the fractal surfaces,while the rippled surfaces usually show moderate increment determined by r = 1 − 2. Hence, the contact angleon such surface modified by ripples becomes cos(θ

′) = r cos(θ), i.e., the initial hydrophobicity/hydrophobicity of

surfaces becomes enhanced.

3.2. Femtosecond laser microfabrication of stainless steel

Femtosecond laser microfabrication of metals is a well established technology. Figure 4 shows typical setup usedin femtosecond drilling using galvano-scanners at low pressure.8 This setup facilitates drilling higher aspectratio holes: the opening diameter was 190 µm and length 820 µm (Fig. 4). Holes with complex tapered axialcross sections can be fabricated without sample scanning (Fig. 4(c)). The focus was scanned in a 30 µm circleover sample’s surface.

The aspect ratio of drilled holes strongly depended on the focal intensity distribution, a point spread function(PSF). For a Gaussian beam the position and size of its waist is defining the PSF for a given wavelelngth and



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t.(a) (b)pol. pol.

c-axis

Figure 5. SEM images of x-cut sample of quartz after a multi-pulse per spot irradiation and wet etching. Irradiationconditions: 800 nm, 150 fs, ∼ 250 nJ/pulse with more that 90% overlap between adjacent pulses (a hand-scan) at 1 kHzrepetition rate.

refractive index. The divergence angle of the beam is θ = λ/(nπw(G)0 ) = w(G)0 /z

(G)R . The waist (radius) of the

focal spot is given by:

w(G)0 =

2π

λF

2R≡ λ

π

(F

R

)≡ 1.27

2λf#, (5)

where F is the focal length, R is the beam’s radius, f# = F/(2R) is the f-number. If approximation θ ≈ sin(θ)holds then the numerical aperture is NA = 1/(2f#). The confocal parameter reads:

z(G)R =

λ

π

(F

R

)2. (6)

Gaussian beam is less divergent than a plane wave. In eqns. 5 and 6 the wavelength is inside material where thefocus is placed. Smaller beam diameter and the longer focal length (a smaller NA) facilitates axially elongatedPSF required for high-aspect-ratio drilling and dicing.

3.3. Etching of patterns pre-recorded by fs-pulses in dielectrics

Femtosecond laser micro-structuring of dielectrics by a dielectric breakdown creates strongly chemically alteredregions. Those regions can be etched out after exposure. The chemical and physical mechanisms of enhancedetching rate are still under debate. In silica glasses, the regions of higher etchability have larger mass density.Similarly, femtosecond breakdown creates wet etchable amorphous sapphire and quartz. It was observed thatthe etching rate is larger perpendicularly to the polarization of the laser pulses. This corresponds to etchingalong the ripple-like nano-pattern formed inside the glass, sapphire, or quartz.

Figure 5 shows a pattern of fiber-like residual after wet etching in aqueous solution of HF after femtosecondirradiation. The line was recorded few micrometers below the surface of quartz sample by laser beam scanningwith strong overlap between adjacent pulses. The wet etching anisotropy in quartz usually causes the etchedpattern to be elongated in direction of c-axis.18 The exact mechanism of formation of fiber-like structures isunder investigation. Usually, the patterns of ripples are enhanced by etching on the ablated surface where thewavevector of ripples, kΛ, or parallel to the polarization of incident pulse. Such fiber-like residue inside channelscan find some specific applications in microfluidics. If wet etching is prolonged the fibril structures are etchedout. The fiber-like structures were not observed on a c-plane in quartz.

The formation of filament-like structures by etching of optically amorphised quartz by multi-pulse exposureis related to: (i) the anisotropic etching of crystalline quartz and (ii) structural modifications created by multi-pulse exposure. When the irradiation spots are overlapping by less that 20% the 3D channels are etched most



fast similarly as in the case of silicate glasses3, 19 and sapphire.20 The exact mechanism of etching anisotropy inthe case of multi-pulse exposure requires further investigation. Currently, we can put forward a conjecture thatformation of poly-crystalline phase was responsible for a strong etching anisotropy of the multi-pulse exposedquartz. Similarly, a decrease of wet etchability in a multi-pulse exposed sapphire was observed due to amorphous-to-poly-crystalline transition occurring under repeated irradiation.20

3.4. Formation of ripples inside materials

Ripples were first discovered on the surfaces of laser irradiated samples at high irradiance. They appear at theboundary of two environments of different dielectric constants. There is no principal restriction for ripples to occurin the bulk of materials, whenever the optical properties are modified by high irradiance and a distinct boundarywith different optical properties is formed. The ripple-like structures formed inside the bulk of transparentmaterials are a result of self-action of the laser pulse: first, material’s optical properties are modified at the focusand, then, the ripples are formed on the boundary between two materials. Such distinct boundary can be mosteasily formed by use of ultra-short sub-1 ps pulses.

3.5. Field enhancement by ripples

Ripples can find applications in light extraction, increasing hydrophobicity of surfaces, molecular docking, etc.,however, there is an additional potential to use ripples as a local field enhancing medium as discussed in thissection.

3.5.1. Local field

Ripples with feature sizes much smaller than the typical λ/2 at the normal incidence were observed on variousmetallic and semiconducting surfaces. When nano-objects have dimensions smaller that the wavelength of light,the light field inside the nano-object is enhanced by a factor L. An electrostatic approach is valid with well knownsolution for a local field inside a nano-sphere whose radius is much smaller than the wavelength of incident lightwave (r � λ):

Eloc(ω) = L(ω)E0 =3

ε(ω) + 2E0, (7)

where L(ω) is the local field factor, E0 is the amplitude of incident light field, and ε is the dielectric functionof material. The resonant condition for the enhancement of local field factor exist in metals where εm(ω) < 0(in contrast to dielectrics where εd(ω) ∝ 1 − 2 > 0) at certain frequencies where εm = −2. The local fieldenhancement factor on the resonant frequency is21

| L(ωres) |= | ε′(ωr) |

ε′′(ωr), (8)

where ε′(ω) and ε

′′(ω) are the real and imaginary parts of dielectric function. In noble metals this factor can

reach 10 – 20 in the vicinity of ωr where absorption losses are small ε′′m(ω) � 1 (the real and imaginary parts of

complex refractive index ñ = n + ik are related to the dielectric function via ε′= n2 − k2 and ε′′ = 2nk).

The field enhancement factor in close proximity outside of a nano-sphere is not uniform: the tangentialcomponent of the electric field (on the equator of sphere) has the same value as inside the nano-sphere definedby the factor L (eqn. 7), while the normal component (on the poles) is additionally modified by a factor ofε, i.e., Lpoles = ε × L (see, Fig. 6).22 This enhancement has been suggested as a mechanism relevant to thesilica breakdown and nano-grating formation22 in under-critical plasma with ε < 1. It can also explain theanomalous light scattering23 with maximum along the polarization of the incident light field in the lateral plane(a 90◦-rotated distribution would be expected from an oscillating dipole) observed earlier in glass.24 Indeed,when ε < 1 a nano-spherical metallic (breakdown plasma) volume would tend to be extended along the equator(Fig. 6). This is consistent with the ripples’ pattern, i.e., the finest ripple whose formation is strongly influencedby the initial surface nano-roughness14 are, most probably, formed as a result of field enhancement and ensuingablation. The normal ripples (with period from ∼ λ/4 to ∼ λ) are the result of SEW mechanism.25 When the



0)()( ELELoc

0E

k

0)()()( ELELoc

Figure 6. Local field enhancement in a nanoparticle (the electrostatic approximation, when diameter � λ). The localfield factor L(ω) = 3

ε(ω)+2.

objective lens on high numerical aperture (NA > 0.4) is used for ripples’ recording, it is necessary to consideran angular spread of incidence angles. This can be accomplished via calculating a ripples map25 and explainscomplex patterns of ripples with different periods occurring in actual experiments.

3.5.2. Scattering enhancement

The effective field enhancement factors observed in Raman scattering from molecules on the nano-surfaces areeven larger.26 The intensity of Stokes component of Raman scattering is proportional to the square of dipolemomentum on that frequency21:

I(ωS) ∝ d2(ωS) = α2RL2(ωS)L2(ωL)E20 , (9)

where αR is the Raman polarizability of molecule. Here, two enhancement effects are present: (i) the incidentlaser field is enhanced in the nano-particle according to eqn. 7, and (ii) additionally emission field of oscillatingmolecular dipole is also enhanced by the same nano-particle. Since usually ωL � ωS , the cumulative enhancementfactor becomes L4(ωL) and eqn. 9 reads I(ωS) ∝ α2RL4(ωL)E20 . This demonstrates that nano-particles and nano-structured surfaces act as effective spatial energy redistributors. The Raman scattering can reach single moleculardetection limits by utilizing the surface enhancement.26

3.5.3. Gigantic second harmonic generation

The light frequency up-conversion which is related to the field enhancement occurs upon a second harmonicgeneration (SHG) from nano-structured surfaces.27 In general, the SHG is proportional to the square of nonlinearpolarization I2ω ∝ [PNL(2ω)]2 ∝ (χ(2)E2ω)2, here χ(2) is the second order susceptibility. When surface has anano-pattern, the amplitude of incident field Eω should be changed by the local field given by eqn. 7 yieldingin21:

I2ω ∝ (χ(2)E2loc)2 ∝ (χ(2))2L2(2ω)L4(ω)I2ω. (10)Here, two enhancement effects are present: (i) the incident laser field is enhanced in the nano-particle accordingto eqn. 7, and (ii) additionally SHG field is also enhanced by the same nano-particle. The factors of SHGenhancement from nano-structured Ag (etched in KCl) as high as 105 have been observed as compared fromatomically flat Ag (even a forbidden s-polarization of SHG generated by a s-pol. incident light was observed21).Roughening of Ag surface by one atomic monolayer increased SHG by an order of magnitude. This enhancementof can be utilized facilitating a two-/multi-photon processes.



4. CONCLUSIONS

The femtosecond laser micro-fabrication delivers a sub-wavelength precision of surface and in-bulk structuringof metals, semiconductors, and dielectrics. It can be implemented via approach of direct laser writing discussedhere or, e.g., via a holographic recording using interference of several beams. The latter approach is prospectivein large area templating by photo-polymerization.28–33

The large area nano-structuring by ripples over the surface of 4H-SiC has been demonstrated. By translatingthe line scans with a partial overlap between adjacent lines (approximately 25% overlap of the focal spot in araster scan mode) the self-propagating pattern of ripples was obtained. Such surfaces can serve in light extractionapplications for GaN-based blue-LEDs grown on SiC (the same approach is valid for LEDs grown on sapphiresubstrates). Other applications may include heat exchangers, super-hydrophobic surfaces, and molecular dockingtemplates. The nano-structured surfaces can be used in molecular detection applications due to inherent stronglocal electrical-field enhancement effects.

Discussion on wetting properties of nano-structured surfaces with Dr. H. Mayama is highly acknowledged.This work has been partially supported via a Grant-in-Aid for scientific research Kiban-B(2) No.19360322.

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Three-dimensional laser microfabrication of metals ......Femtosecond laser fabrication has become a well established technique for three-dimensional (3D) and surface structuring of