Direct laser writing of microstructures on optically opaque and reflective surfaces

Optics and Lasers in Engineering 53 (2014) 90–97

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Direct laser writing of microstructures on optically opaque and reflective surfaces S. Rekštytė n, T. Jonavičius, M. Malinauskas Vilnius University, Physics Faculty, Department of Quantum Electronics, Laser Research Center, Saulėtekio Ave. 10, LT-10223 Vilnius, Lithuania

art ic l e i nf o

a b s t r a c t

Article history: Received 12 March 2013 Received in revised form 22 August 2013 Accepted 23 August 2013

Direct laser writing (DLW) based on ultra-localized polymerization is an efficient way to produce threedimensional (3D) micro/nano-structures for diverse applications in science and industry. It is attractive for its flexibility to materialize CAD models out of wide spectrum of materials on the desired substrates. In case of direct laser lithography, photo-crosslinking can be achieved by tightly focusing ultrashort laser pulses to a photo- or thermo-polymers. Selectively exposing material to laser radiation allows creating fully 3D structures with submicrometer spatial resolution. In this paper we present DLW results of hybrid organic–inorganic material SZ2080 on optically opaque and reflective surfaces, such as silicon and various metals (Cr, Ti, Au). Our studies prove that one can precisely fabricate 2D and 3D structures with lower than 1 μm spatial resolution even on glossy or rough surfaces (surface roughness rms 0:068–0:670 μm) using sample translation velocities of up to 1 mm/s. Using femtosecond high pulse repetition rate laser, sample translation velocity can reach over 1 mm/s ensuring repeatable submicrometer structuring resolution. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Direct laser writing Hybrid polymers 3D microstructures Opaque surfaces Electro conductive surfaces

1. Introduction Direct laser writing (DLW) based on ultra-localized polymerization is an efficient way to produce 3D micro/nano-structures for diverse applications in microoptics [1–3], photonics [4,5], microfluidics [6,7] and biomedicine [8,9]. Furthermore, it allows integration of several components and combination of a few before mentioned functions [10,11]. The DLW technique is attractive for its flexibility to materialize CAD models out of a wide spectrum of materials on the desired substrates [12,13]. Laser induced photocrosslinking within the volume of transparent material can be achieved by tightly focusing ultrashort pulses to photo- or thermopolymers [14,15]. Selectively point-by-point exposing the material to laser radiation allows rapid prototyping of fully 3D structures with submicrometer spatial resolution. Additionally, beam shaping techniques employing spatial light modulators (SLM) or diffractive optical elements (DOEs) can be used to obtain parallel processing and dramatically increase fabrication throughput [16,17]. A transparent cover glass is widely used for DLW polymerization as substrate because of its weak interaction with laser radiation and easy transmission microscopy imaging. However, biomedical applications could benefit from possibility to fabricate

n

Corresponding author. Tel.: þ 370 618 44842. E-mail addresses: [email protected], [email protected] (S. Rekštytė). 0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.08.017

3D microstructures on opaque surfaces. Since some metals, like Ti and Cr–Co alloys, are vastly used for implants [18–20], a possibility to create porous structures/3D-scaffolds from biocompatible materials on metallic surfaces could be advantageous for improving cell attachment and proliferation [21,22]. It is worth mentioning, that the majority of photopolymers are toxic as they are prior to exposure, but become biocompatible after photo-crosslinking and carefully washing out the monomers [23]. The DLW technique was already employed for a flexible production of metallic structures, but they were of 2D features by nature [24] or dot-arrays on polymer substrates [25]. In this paper we present DLW 3D structuring results of hybrid organic– inorganic polymer material SZ2080 [26] on opaque surfaces such as Si and various metals (Cr, Ti, Au). Our previous studies proved that one can precisely manufacture 2D and 3D structures on glossy or nano-rough metallic substrates [27,28]. Despite the fact that DLW fabrication of polymers on opaque surfaces has been reported previously by other groups, they were out of purely organic materials SCR-500 and again of 2D features [29]. In [27] we reported the possibility to fabricate microstructures on different opaque substrates out of various materials. Here we investigate the dependence of structuring quality on surface roughness and reflectivity using only one material and demonstrate the possibility for a production of micro-objects on the boundary of glassmetal surface. Selective metalization of 3D polymer structures enables creation of integrated 3D optical and electric circuits [30,31]. Fabrication of dielectric materials on electro conductive

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substrates opens a way for novel metamaterial applications based on plasmonic interactions [32,33]. This experimental study report is believed to be an important step in expanding the constantly growing application field of DLW in polymers.

2. Experimental 2.1. Direct laser writing setup The employed experimental setup is shown in Fig. 1. A frequency doubled fs-laser amplified system (Pharos, Light Conversion) emitting 515 nm light with pulse length approximately 300 fs and tunable repetition rate (in all current experiments set at 200 kHz) was used as irradiation source for the selective sample exposure. Two different microscope objectives were used in the setup to focus the beam into the sample: for fabrication of 3D microstructures – 100  , NA ¼1.25 (Zeiss), and for line arrays and 2D structures – 40  , NA ¼0.65 (Nikon). The positioning system consisted of three stacked linear motion stages (ALS130-100 for translation in XY directions and ALS130-50 for translation in Z direction, Aerotech). The laser exposure was controlled using pulse picker (integrated in the amplifier) and optical power attenuation was realized using a λ=2 half wave-plate combined with a

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polarizer. Wide-field reflection imaging was used to monitor the fabrication in real time. This was possible, as optical contrast arises from the different material densities and hence indices of refraction upon photomodification. For this purpose, a microscope was built by assembling its main components: a LED red light, a CMOS camera (mvBlueFOX-M102G, Matrix Vision), a video monitor and beam delivery optics. When fabricating on a transparent substrate the sample was illuminated from the bottom (LED2 in Fig. 1), however when fabricating on opaque substrates LED light could not pass through the substrate so illumination from above had to be used (LED1 in Fig. 1). The ability to image the sample while performing the DLW is an important issue for a successful manufacturing process.

Table 1 Treatment and surface roughness of Ti plates. Plate Treatment

Ti1 Ti2 Ti3 Ti4

Mechanically polished and electrochemically treated Mechanically polished and etched with HF Treated with sandblast and etched with HF Treated with sandblast

Surface roughness rms (μm) 0.068 0.085 0.250 0.670

Fig. 1. (a) Direct laser writing experimental fabrication setup. Femtosecond beam is frequency doubled with SHG crystal and guided through an optical system, reflected by a dichroic mirror and coupled into an objective lens. The sample is fixed on XYZ stages which are computer controlled. LED1 and LED2 provide illumination needed for CMOS camera to monitor the DLW process online. (b)–(d) Focusing conditions using different substrates: (b) laser light is focused above a transparent substrate and passes through it with very little reflection; (c) laser light is focused directly above the reflective substrate, the radiation is reflected back, thus increasing overall light intensity in the beam's path; (d) laser light is focused lower than the reflective substrate – due to reflection the focus point forms above the substrate.

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2.2. Photosensitive material and substrates For our experiments we used hybrid photopolymer SZ2080 (ORMOSIL, IESL-FORTH, Greece) [26]. Organic–inorganic hybrids are Table 2 Transmission and reflection coefficients for glass plate and glass plates covered with various Au layer thicknesses. Surface

Transmittance, T (%)

Reflection, R (%)

Glass Au 10 nm Au 50 nm Au 100 nm

92 59 10 1

8 41 90 99

attractive as lithographic materials due to their optimized properties for DLW fabrication of 3D micro/nano-structures [34]. SZ2080 was photosensitized by adding 2% of 2benzyl 2dimethylamino 4′morpholinobutyrophenone (Sigma Aldrich). Samples for laser structuring were prepared by drop-casting the pre-polymer on a substrate. Then the samples were pre-baked for the evaporation of the solvent (20 min at 100 1C temperature). When fabrication was performed with oil immersion objective, the immersion oil was poured directly on the pre-polymer to prevent the necessity of using an additional cover glass. The resin thickness was not kept constant. Fabrication of 3D structures was performed using bottom up approach – starting from the substrate and moving to the bulk of the resin. After the DLW irradiation the samples were immersed in an organic solvent (4-methyl-2-pentanone) to rinse

Fig. 2. Optical profilometry images of Ti plates' surfaces ((a), (c), (e) and (g)) and SEM images of grids fabricated on them out of SZ2080 photopolymer ((b), (d), (f) and (h)). Surfaces were differently treated before fabrication: (a) and (b) Ti1 – mechanically polished and electrochemically treated (surface roughness rms¼0.068), (c) and (d) Ti2 – mechanically polished and etched with HF (rms¼0.085), (e) and (f) Ti3 – treated with sandblast and etched with HF (rms¼ 0.250), (g) and (h) Ti4 – treated with sandblast (rms¼0.670).

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Fig. 3. Arrays of line pairs fabricated using different sample translation velocities (from left to right – 30, 50, 70 and 90 μm=s) on (a) glass, (b) 10 nm Au, (c) 50 nm Au and (d) 100 nm Au. Ovals mark areas of self-polymerization.

Fig. 4. Line arrays fabricated on: (a) 10 nm Au layer, (b) 100 nm Au layer. Numbers above pictures indicate laser radiation power P (and corresponding I0) used for fabrication.

away the unexposed material, and thus reveal the produced microstructure. Various opaque substrates were used as surfaces on which laser fabrication was performed – Si, Ti, glass plates covered with 200 nm Cr layer and glass plates covered with various thicknesses (10, 50 and 100 nm) of Au layers. Au covered layers were acquired by coating the cover glasses with Au using sputter-coater Quorum Q150R S. The used Ti plates differed in their roughness as they underwent different surface treatment before fabrication. The treatments and measured surface roughness root mean square (rms) values are given in Table 1. Sensofar PLμ 2300 profilometer was used to measure surface topology of the plates. Then roughness rms values were acquired using Gwyddion free software. All the samples after the exposure and development were characterized using Hitachi TM-1000 table top scanning electron

microscope (SEM). For precise measurements of lines' width additional sputtering of conductive layer on the samples was used. For Au covered cover glasses (150 μm thickness) the transmission coefficient was measured for the used laser wavelength with Shimadzu UV – 3101 PC spectrophotometer. The reflection coefficient was calculated using R¼1  T relation, where R is reflection coefficient and T is transmission coefficient. It must be taken into account that due to technical limitations of the used spectrophotometer the calculated reflection coefficient also includes absorption. T and R values are given in Table 2. It is important to take into account different focusing conditions when fabricating on transparent ant opaque surfaces. In the common laser lithography case, the substrate is transparent to the laser wavelength that is used, so the laser radiation has little interaction with it and almost all passes through (Fig. 1(b)) [35].

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However, when the substrate is opaque, the incident light is reflected, scattered and/or absorbed. If the light is being focused above the substrate (Fig. 1(c)), then only the overall intensity above it increases and interference patterns can be obtained [36]. However, if it is focused lower than the substrate (Fig. 1(d)), then due to reflection the focus spot forms above it and one has to be very cautious not to damage the objective.

While structuring arrays of lines, especially using high laser radiation powers and Au covered surfaces with low transmission coefficients, it was noticed that the sides of the lines are not smooth but have periodic modulation (Fig. 4). This modulation was noticed on all Au covered cover glasses and becomes more prominent with increasing Au layer thickness (decreasing transmission). The same phenomenon has been observed by Reinhardt et al., Mills et al. and Kondo et al. [32,36,39] and is explained as an interference of coming and reflected from the substrate laser

3. Results and discussion Fig. 2 shows 2D gratings with 15 μm period fabricated on Ti plates. It can be seen, that although substrates' surface roughness rms differed in the range of 0.068–0.670 good quality structures with straight and solid lines were obtained on all of them. Thus we can conclude, that as long as surface roughness is smaller than the structure itself it does not influence the quality of the structure. To investigate what influence fabrication surface has to structuring resolution, some arrays of lines were fabricated on various substrates. An example is shown in Fig. 3. Lines fabricated with lower velocities due to their thickness are very close to each other and an effect known as self-polymerization (or repolymerization) [37,38] can be observed in the space between them (marked with ovals in Fig. 3).

Fig. 6. Lines', fabricated on glass and different thickness Au layers, width dependence (a) on intensity at constant sample translation velocity (v ¼ 200 μm=s) and (b) on sample translation velocity at constant intensity (I0 ¼0.42 TW/cm2). At least two lines were fabricated with each parameter set then their width's measured in two different places and averaged. The curves are guides for the eye only. Graph (a) does not show the results of measurements on 100 nm Au layer as most of the lines too much deformed making it difficult to measure their width correctly.

Table 3 Intensity (I) and line width (d) increase on various substrates compared to glass (Iglass ¼ 0.45 TW/cm2) using recalculated I values after including reflection from the substrate, when initial intensity (I0) in focus spot is 0.42 TW/cm2.

Fig. 5. Line width dependence (a) on intensity at constant sample translation velocity (v ¼ 200 μm=s) and (b) on sample translation velocity at constant intensity (I0 ¼ 0.3 TW/cm2). The curves are guides for the eye only.

Surface

I, TW/cm2

Increase in I (%)

Increase in d (%)

Au 10 nm Au 50 nm Au 100 nm

0.59 0.80 0.84

30 76 84

13 15 16

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Fig. 7. Structures fabricated on glass and Au junction: (a) arrow (Au layer on the upper side; it can be seen that arrow's parts which are on glass are thinner, than the ones that are on Au), (b) aqueduct (Au layer on the right side; the inset shows damaged column bases), (c) power lines (Au layer in the lower left corner).

radiation. The periodicity of the interference pattern, measured on a fallen line (Fig. 4(a)), was 169 nm which is in good agreement with theoretical interference pattern period d ¼ λ=2n ¼ 171 nm [32]. Interference patterns could be used for controlling cells', growing on polymeric substrate, orientation as it has been shown that substrate's surface topology affects cells' alignment [40,41] and movement [41]. For the determination of the substrate's influence to structuring resolution, arrays of lines on completely opaque surfaces (Si, Cr, Ti) and on transparent glass were fabricated. The measurements of lines' width dependence on sample translation velocity in the interval of 10–1000 μm=s at constant power (P ¼0.25 mW, I0 ¼0.3 TW/cm2) and also lines' width dependence on power in the interval of P¼ 0.2–0.6 mW (I0 ¼0.24–0.72 TW/cm2) at constant sample translation velocity (200 μm=s) were obtained. Such interval of P was chosen because with lower P values the lines did not survive development process and higher P values in most cases burned the material. Here and further in the text all values of laser power P (mW) are given as measured before objective, and intensity I0 (TW/cm2) is calculated at the focus using equations provided in Ref. [42]. The acquired results are shown in Fig. 5.1 To investigate the influence of reflection coefficient to micro/ nano-structuring resolution, again arrays of lines were fabricated on cover glass with different thicknesses (10–100 nm) of Au layers. The measured lines' width dependence on sample translation velocity in the interval of 30–1000 μm=s at constant power (P ¼0.35 mW, I0 ¼0.42 TW/cm2) and on power in the interval of P ¼0.19–1.1 mW (I0 ¼0.23–1.32 TW/cm2) at constant sample translation velocity (200 μm=s) are shown in Fig. 6. Such interval of P 1 Though in practice laser power P (mW) is taken as a laser irradiation parameter (as it is easy to measure its value in the laboratory), it has been shown that laser intensity I0 (TW/cm2) is a more rigorous and universal parameter to be taken into account, as lasers varying in pulse duration and repetition rate as well as different focusing conditions are widely used for lithographic fabrication [43].

was chosen because with lower P values the lines did not survive development process and higher P values deformed the lines too much to take correct measurements. At least two lines were fabricated with each parameter set then their widths were measured in two different places and averaged. For lines fabricated on glass and various Au layers a standard deviation of their width was calculated using formula qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s ¼ ∑ni¼ 1 ðdi dÞ2 =n, where di is line's width, d – its average value, n – number of values measured for each line. An average standard deviation of 25 nm was acquired. For lines fabricated on Si, Ti and Cr standard deviation was not calculated, however we expect it to be similar. Table 3 shows the calculated increase in intensity due to reflection and corresponding increase in line width on various substrates compared to intensity and line width on glass. Increase in intensity was calculated in the following way. The intensity above substrates was recalculated by taking into account the reflection using formula I ¼I0n(1 þR), where I0 is intensity in the laser focus spot as calculated from the laser power measurement and R is substrate's reflection coefficient. Then the recalculated intensity above the glass substrate (Iglass ¼0.45 TW/cm2) was taken as a baseline to calculate the increase in intensity above substrates with gold layer. We must have in mind that these values are not absolutely correct as we assume, that all the laser light that does not pass the substrate is being reflected and disregard light scattering or absorption. Fig. 7 shows structures fabricated on the junction of glass-Au. From Fig. 7(b) it can be seen that such substrate produces additional challenges in selecting correct formation parameters. The aqueduct was fabricated using P¼ 0.15 mW (I 0 ¼ 0:18 TW=cm2 ) and v ¼ 300 μm=s and it can be seen that although its columns that were fabricated on glass are intact, those on Au have damaged bases. However, it is possible to choose such parameters so that a structure would stay intact no matter

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on what surface material it is fabricated – Fig. 7(c) shows power lines successfully fabricated on the junction of glass-Au (using P ¼0.1 mW, I 0 ¼ 0:12 TW=cm2 , v ¼ 800 μm=s). It is possible that the structure's parts which were fabricated on Au stayed standing only because they are attached to the rest of the polymer which is stuck to the glass. However, our group has shown that 3D structures fabricated out of SZ2080 do stick to the Au surface [28]. Also, from Fig. 7(a) it can be seen, that the structuring resolution differs depending on the substrate. The line parts of the arrow fabricated on glass are thinner than those on Au although fabrication parameters are the same (P ¼0.51 mW, I 0 ¼ 0:69 TW=cm2 , v ¼ 100 μm=s).

4. Conclusions We presented DLW structuring results of hybrid organic– inorganic photopolymer SZ2080 on optically opaque and reflective substrates, such as Si and metals (Cr, Ti, Au) of various surface roughnesses. Our studies prove that one can precisely directly fabricate 2D and 3D structures even on glossy (highly reflective) or rough surfaces (sand-blasted and chemically etched, roughness rms up to 1 μm), as well as on their interface. Increased feature size is proportional to the higher reflectivity of the sample and can increase up to 16% depending on the thickness of Au layer. Using femtosecond high pulse repetition rate laser (  200 kHz) and tight focusing (objective's NA Z 0:65), sample translation velocity can be reached over 1 mm/s while keeping the submicrometer fabrication resolution. Structuring of dielectric materials on electro conductive substrates opens a way for novel applications based on plasmonic metamaterial interactions. Additional metalization of polymer microstructures offers manufacturing of fully integrated 3D optical [30] and electric circuits [31].

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Acknowledgments

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This work was financially supported by the research Grant no. VP1-3.1-ŠMM-10-V-02-007 (Development and Utilization of a New Generation Industrial Laser Material Processing Using Ultrashort Pulse Lasers for Industrial Applications) from the European Social Fund Agency. Authors acknowledge “Altechna R&D” for assembling “Aerotech” stages. TJ acknowledges the Research Council of Lithuania for the Student Research Fellowship Award (no. SMT12/028/SMT12R-148).

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