Self-organized submicron structures in photoresist films by UV-laser irradiation at water-confined conditions

A new kind of self-organized pattern formation process has been found during laser irradiation of polymer films in water confinement just below the laser ablation threshold, resulting in a randomly oriented pattern with a period of about 475 nm. The morphology, orientation, period, and amplitude of these patterns are inconsistent with both laser-induced periodic surface structures that typically consist of linear grooves with periods smaller the laser wavelength and wrinkling patterns that feature a much larger period and appear at layered systems. Excimer laser (λ = 248 nm, tp = 25 ns) exposure of 650 nm thick photoresist films on silicon wafers cause the growth of irregular submicron patterns. The pattern morphology that is examined with imaging techniques is correlated to processing parameters. The amplitude of these laser-induced self-organized (LISE) submicron structures are strengthened with pulse number and laser fluence. The experimental results are discussed together with simulations of laser heating the photoresist film in water confinement. The proposed pattern formation mechanism of such laser-induced self-organized submicron structures at temporal excitation of a confined polymer surface comprises the formation of an oriented roughness based on LIPSS that are developed to wrinkled structures due to the transient formation of a soft subsurface area that provides conditions for wrinkling of the water cooled, stiff polymer surface by laser-induced stress fields. Size, amplitude, and morphology of the LISE pattern provide good properties for applications in such fields as wetting, friction, optics, and bioactivity.


Introduction
Self-organizing structures are well known in nature and various fields of science and technology.Self-organizing processes can take place at molecular level, for instance, at the assembly of block copolymers [1], at material interfaces, for instance forming wrinkles [2], or at irradiation of materials by beams such as ion [3] or laser beams [4].Therefore, self-organization is of particular interest due to the simple process for special fields of micro-and nano-pattern fabrication.Self-organizing by lasers can be related to different effects such as interference of surface scattered waves with the incidence beam resulting in the so called laser-induced periodic surface structures (LIPSS), nano-pattern generation by dewetting effects of laser-irradiated thin films or periodicities in the film growth due to alteration of the reflectivity [5,6].
Regarding the experimental conditions applied here, LIPSS formation is the most closest process that is frequently studied and has been observed for the first time at semiconductor surfaces [6].However, LIPSS can appear at almost any material ranging from organic matter to hard metals.An overview on the status of LIPSS formation has been given recently [4].LIPSS are generated experimentally usually at air under standard lab conditions; hence, the simple fabrication of micron and submicron pattern in almost any material attracts a broad scientific and technical interest.As various surface scattering processes affect the 623 Page 2 of 10 LIPSS formation process, surface roughness has an impact to LIPSS formation due to additional scattering processes as in some cases has been observed [7].However, LIPSS formation at specific experimental conditions were studied with preheating [8], particles filling [9], and lateral confinement [10].
Various studies elaborate the LIPSS formation at polymer surfaces by ns-UV-laser irradiation.In all cases, linear submicron pattern have been found.The period of these LIPSS is influenced by the laser wavelength, the refractive index and the incidence angle; other parameters can have an impact too [11][12][13].The formation of LIPSS in polymer films has been studied also for typical polymers such as poly(ethylene terephthalate) (PET) and polystyrene (PS) by ns-UV laser irradiation.Typically, these LIPSS feature a period that is slightly below the laser wavelength as well as amplitude of approx.60 nm [7,13,14].In the case of ultrashort pulsed lasers in addition to UV wavelengths [15] also, infrared radiation can cause LIPSS generation, however, with larger period [16].
In the case of LIPSS formation in polymer surfaces or layers, in addition to scattering and interference processes of laser radiation [7], temperature field calculations are also used to interpret LIPSS formation [13].Temperature calculations at polymer ablation at water confinement show at laser fluences close to the ablation threshold and maximal temperature of approx.600 °C that drops to the water interface to 200 °C [17].
Laser irradiation of polymers in air can result in various non-periodic patterns that exhibit pattern characteristics that cannot be discussed with standard LIPSS theories.These patterns have a larger period than expected from LIPSS, while the mechanism is not fully explained in all cases [18][19][20].
The exposure of polymers to light, plasmas or heat can cause modifications of the composition or the structure of the polymer [21].These effects occur also at laser irradiation of polymers with a sufficient absorption where UVphotons can induce bond breaking and laser heating and after exceeding the ablation threshold plasmas may develop.The laser-induced change of material properties on the laser ablation process of polymers is summarized as "incubation effects" [22][23][24][25].
Another type of self-organized pattern, so called wrinkles, can be obtained at the laser irradiation of layered systems where the laser causes the formation or relaxation of stress in the system that is usually the origin of wrinkling or buckling.Typical wrinkling structures have periods in the micrometer range [26][27][28].The localization of the wrinkling process by patterning processes and the transition of film or substrate properties were studied in some cases.The guiding of wrinkling processes by laser irradiation has been shown and periods in the range of 100 µm observed [29].
The laser-based material processing assisted or confined by liquids is well known [30].Laser irradiation and laser ablation studies under water with ns-, ps-, and fs-pulse length showed typically differences to laser irradiation at air that are often related to bubbles formation, shock waves generation and confinement effects [30][31][32].In general, water confinement changes the mechanism of laser-based processes that become more complex in general.In the case of UV-laser ablation of typical polymers such as polyimide, PET or PC the confinement by water results in a reduction of the ablation rate and a porous surface that combines exploded bubbles and droplets.Submicron surface pattern have not been observed at these studies [17].LIPSS at freestanding polymer surfaces are also found at conditions where the polymer absorbs the laser radiation [26].Here in addition to the LIPSS some kind of microscopic features, cracks, grooves, etc. that called there wrinkling, have been observed.The height as well as the period of LIPSS or wrinkled surface depend on the number of laser pulses and the laser fluence [26] Surprisingly, we observed during laser irradiation of polymer films in water confinement unexpected self-organized pattern that resemble wrinkling pattern but appear on homogeneous photoresist film and featuring periods much smaller than typical wrinkles but larger than expected LIPSS.A mechanism different to LIPSS formation by scattering phenomena was suggested.This new type of features offering new options for applications but needs to be studied to understand the origin and the options for tuning such patterns.

Experimental
The experimental setup is sketched in Fig. 1.In particular, an Excimer laser with a wavelength of 248 nm and a pulse duration of 25 ns has been applied for irradiation of the photoresist films.The films were prepared by spinning photoresist maP 1205 (microresist technology) at 2000 rpm Fig. 1 Experimental setup applied in the experiments on a 4-inch silicon wafer that was modified before with an adhesion promoter.The main component of the photoresist film is novolak that is a phenolic resin.Subsequently the film was dried at 90 °C for 2 min at a hot plate.For laser irradiation, the films were immerged to clean water with a height of 5 mm which was kept fixed within the experiments.The imaging systems for mask projection with a doublet of 100 mm focal length were slightly adapted to this water level to enable optimal imaging conditions.Further, at this water level height, laser-induced surface waves and possible breakup of the water surface due to the expanding laser-induced cavitation bubble can be avoided.The laser irradiation was performed in a laser workstation that enables sample movement, laser beam attenuation and pulse number selection for parameter studies and areal irradiation.Experiments with two different spot size of approx.100 × 100 µm 2 and 1 × 1 mm 2 has been accomplished for basic studies and pattern generation at extended areas.
The laser-irradiated surfaces were imaged by optical (OM) and scanning electron microscopy (SEM, Zeiss Ultra Gemini 55).Prior SEM imaging an approximately 10 nm thick gold coating was applied.Selected samples were analyzed by atomic force microscopy (AFM, Dimension ICON, Bruker).The images and data were evaluated by an image processing software (SPIP 6.7.5, Imaging metrology) to extract the period, the amplitude as well as the roughness of the achieved pattern.

Calculation of the temperature distribution
The temperatures near the water-photoresist interface were calculated for a heat-conducting interface of a transparent (water) to an absorbing (photoresist) material.Analytical solutions of the heat equation with constant material parameters, that are provided in Table 1, were used for the calculations of the temperature according to Refs.[22,33,34].
Despite a photoresist film is experimentally utilized, the calculation was performed for bulk material interface.This does not substantially influence the calculated temperatures as the optical penetration depth L O = −1 , with the absorption coefficient, as well as the thermal diffusion length with k Th the thermal conductivity, ρ the density, and c p the specific heat capacity, is much less the film thickness.

Results
Typical experimental results of surface modifications for UV-laser-irradiated photoresist are shown in Fig. 2 at different scales.In Fig. 2a, an overview of the laser-irradiated field varying the laser fluence and the pulse number is sketched.With increasing laser fluence, the transition from a visible surface modification to laser ablation appears at approximately 25 mJ/cm 2 .The macroscopic less visible modified areas hold LISE pattern as their strength increases with the applied pulse number.The microscopic image seen in Fig. 2b shows in the border of the modified photoresist of the highlighted in a featuring a certain roughness and the pristine photoresist at the edges of the image (green stripe, right).The color change is related to a potential alteration of the film thickness, the surface patterns and additional contributions the scatted light.
The achieved surface structures, the LISE pattern seen in Fig. 2c, are characterized mostly by elongated ridges with a width of approx.200 nm and valleys in between.The ridges appear smooth, whereas the valleys show some roughness and holes.High-pass filtered SEM images show clearly that the hole structures are located only in the valley of the LISE structures, although the peaks feature a slightly increased roughness.Some of the holes with a size in the range of 20-50 nm resembling exploded bubbles due to their regular and smooth rim.In the case that several holes coalescence, the edges of these bigger holes have a jagged border.Further, the crater-like structure of single cavities suggests a broken hard shell.The formation of holes structures is a typical phenomenon during the laser irradiation of polymers already close to the ablation threshold [16].
The morphologies of LISE patterns with increasing laser pulse number (5, 100, 300, 1000) at constant laser fluence of 27 mJ/cm 2 are seen in Fig. 3.In addition, the 2D-FFT spectra of the SEM images are shown as insets.Clearly, the change of the morphology with the pulse number is seen that shows the development of irregular structures.Typically, the 2D-FFT spectra show three circular patterns that can be related to pattern periods of approx.1800, 470 and 230 nm, respectively, whereas the longest period patterns are very week.According to the SEM images and the FFT spectra, the morphology does not change significantly with the pulse number except the pattern strength.However, from the FFT spectra, it can be concluded the LISE have slight preferential direction.Results from the AFM evaluation of the LISE surface structures are seen in Fig. 4. The surface topography in (a) shows the submicron morphology of a photoresist surface irradiated with 10 pulses at a laser fluence of 27 mJ/cm 2 .The pattern in the AFM image feature apparently no preferential direction or order.Mostly curved, elongated pattern appear but also elliptic and dot patterns arise.The relation between dot and long pattern changes with the number of applied laser pulses towards dot-like features.
The height of LISE pattern in dependence on the pulse number is depicted in Fig. 4b.The observed LISE pattern feature amplitudes of up to 170 nm that is much higher compared to LIPSS heights in polymers [13,26,35].The greater amplitude is also a clear indication for the differences to traditional linear LIPSS with an amplitude of approx.60-80 nm.The ridges of the LISE pattern in the SEM images appear comparably smooth.The roughness was calculated from the AFM images by high pass filtering the AFM data.The average roughness of the LISE patterns is in the nm-range and, therefore, slightly higher than that of the spin-on photoresist film with ~ 1 nm rms.
The average period of the self-organized pattern has been determined from the FFT data of the SEM images to be in the range of 430-490 nm.A tendency of the period of the LISE patterns from processing parameters has not been seen within the studied parameter range.The marginal change of the period can be related to the altered pattern structure and the limited image size.

Discussion
The observed morphological changes of the LISE morphology from line-type pattern to dot-like features is somehow typical for laser-induces processes as known from LIPSS pattern that show first typical near wavelength line structures, so called LSFL (low spatial frequency LIPSS), that transform to dot structures at prolonged laser irradiation [36].The reason is related in parts to hydrodynamic instabilities and surface tension forces that cause the forming of droplet structures during laser-induced ablation and melting processes [37].
In the SEM and AFM images, no debris, droplets or melt structures are seen.Topographical LIPSS pattern often accomplished by droplets/debris due to the ablative character of the LIPSS formation process [38,39].Further, laser ablation is regularly accomplished by a rough surface due to the involved explosive processes.The absence of debris or particles on top of the LISE pattern as well as their low roughness are indications of the non-ablative mechanism of the pattern formation so that a mechanism different to the LIPSS formation process is probably.Since laser-irradiation of absorbing materials results in heating processes, temperature field calculations of laser-irradiated photoresist at air and water interface are performed.The calculation base on constant material properties and does not consider phase transitions of the water or the photoresist.
The results of temperature field calculations for typical experimental parameters are shown in Fig. 5a.Clearly, the difference of the temperature distribution for air and water confinement are seen.The most significant feature of the temperature for water confinement is that the interface temperature is below the maximum temperature that occurs slightly below the surface within the penetration depth (α −1 ) of the laser photons.This subsurface heating cause instabilities at the surface and together with the confinement by the water a delay of the ablation process.Once the ablation threshold is exceeded, the ablation initiated by the subsurface heating cause a rough surface seen in Fig. 2a left hand side.
The temporal temperature profile calculated for different depths in the photoresist is shown in Fig. 5b.From this can be extracted that in a subsurface region, the photoresist is heated and viscos for a certain time, whereas the surface is cooled by the water and stays close to the boiling point of water due to the thermal dissipation or the evaporation of the water even through higher fluences are applied.
After exceeding the ablation threshold (see Fig. 2a), the morphology of the ablated surface suggests a subsurface ablation process which is supported by the temperature profile with subsurface heating.Such a subsurface ablation is somehow known from laser ablation studies of biological tissue [40].The confinement of the topmost material, here photoresist, prevents the instant evaporation of the material at the surface and allows the formation of subsurface bubbles [41] and, therefore, stress.Indications for bubbles formation are seen in Fig. 2c at the bottom of the grooves where holes with 10th on nanometer are indications for exploded bubbles.
On the other hand, while laser heating the polymer undergoes structural modifications that result in softening and melting.The temperature dependence of the viscosity of polymers is well known and usually characterized by the glass transition temperature T G .The glass transition temperature is related to the relaxation time of a polymer that can be described by the Vogel-Fulcher-Tammann relation or even more complex approaches [42].Therefore, a logarithmic dependence of the viscosity on the temperature with a transition temperature in the range of 10 K is usually found [43].In addition to the temperature, shear rate and stress can influence the viscosity.For a laser-heated polymer system in water confinement, this means according to the temperature profile that a highly viscous polymer layer exists below a still rigid surface for a certain time.The stiffness of the photoresist surface is not only related to the temperature but can be influenced by a combined UV photon irradiation and thermal treatment as is shown for photoresist (novolak) hardening in microelectronics [44].In the case of novolak, such a treatment can shift the softening temperature from ~ 150 °C to ~ 300 °C [45].Hence, both the lower temperatures near the water-photoresist interface as well as the photoresist modification due to UV-photons and laser heating can contribute to a stiff surface layer.Further, the heated viscous polymer provides a stress to the system due to the thermal expansion and potentially generated bubbles.This situation of a stress-loaded system consisting of a soft, low viscous material covered by a rigid film is capable of buckling or wrinkling [27,46].The viscosity of polymer (PET) during ablation has been determined to be 10 times large than water [47].Although the laser fluences used here are below the ablation threshold, slightly lower temperatures can result in a low viscous polymer melt below the surface.
For the estimation of the laser fluence range for the generation LISE patterns, we need to consider the limiting processes that are boiling of the water, ablation of the photoresist as well as a viscous photoresist pool beneath the surface.From thermal calculations, we see that boiling point of water (100 °C) is reached at lower laser fluences compared to the ablation temperature of polymers (appr.600 °C for PMMA [48]).The high fluence limit is, therefore, given by the water boiling.The lower limit of the laser fluence is given by a sufficient viscosity of the photoresist enabling the basic requirement for wrinkling.A very crude estimation may be result from the reflow processing of photoresist that is in the range of 150 °C.As the upper limit is clearly defined by ablation, wrinkling can start at app. 60% of the fluence for laser ablation.These estimations confirm the limited range of LISE pattern formation.The 2D-FFT spectra (seen in Fig. 3) show characteristic structures that indicate the size and the orientation of the LISE pattern.The distance from the center is related to the pattern period (1800, 470 and 230 nm, respectively) and the azimuthal intensity represents the orientation of patterns with the particular period.In all FFT spectra, some orientation of the pattern are seen that can usually observed for LIPSS pattern in photoresist films at air confinement [10].Typical LIPSS periods of UV-laser-exposed polymer surfaces are slightly below the laser wavelength and amount ~ 220 nm for KrF-Excimer laser wavelengths (248 nm).Both orientation and spatial period are characteristics of LIPSS at air confinement, which appear typically at higher pulse numbers, not dominating the topography here.This can be discussed in relation to the temperature at the surface that does not enables melting of the photoresist which is needed for LIPSS formation process at the surface.However, the roughness of the LISE pattern can cause scattering that may result in a feedback process and that can favor LIPSS.
Wrinkling and buckling phenomena are of great scientific interest due to their impact to technical processes and the potential utilization also for microfabrication.The standard model for consideration of such phenomena considers a stiff thin film on a compliment substrate.At these well-defined conditions, estimations for the period and the amplitude of wrinkled systems in the steady state has been given according to where λ w , h f , and E denote the wrinkling period, the thickness of the stiff film and E the Young's modulus, respectively [28,49].A modulus ratio of larger 10 can be estimated by considering the experimentally measured wrinkling period and the stiff surface layer thickness (film thickness in the model [28]) from the thermal calculations.This change of the elasticity is related to the temperature profile and can be easily achieved at the calculated temperature differences of more than 100 K. Hence, the basic conditions for wrinkling are likely fulfilled for a certain time.
The constant wrinkling period at constant laser fluence (see Fig. 4b) is consistent with Eq. 1, since the Youngs modulus ratio as well as the film thickness by water cooling h are determined by the laser-induced temperatures given by the laser fluence.The increasing pattern amplitude with pulse number is related to the approach to the equilibrium state described by the standard wrinkling models.
Wrinkling processes of ion-beam irradiated PDMS has been studied and an increase of the wrinkling period at higher ion energies as well as lower annealing temperatures were found; further, the FFT features no preferential (1) direction of the formed wrinkles [50].Similar results were found for UV-irradiation-modified PDMS [51].In both cases, stronger exposure causes the formation of thicker modified surface (SiO x layer) that cause longer periods and higher amplitudes of the wrinkles.The comparison of the ion-beam wrinkling period of 800 nm with the period found in the experiments of approx.475 nm shows that the thickness of the rigid surface modification is similar; the low ion penetration depth suggests a thickness below 50 nm.The stiff top layer thickness of the photoresist can be estimated to be ~ 50 nm from the thermal calculations (see Fig. 5).However, the impact of different ion exposure parameters on the wrinkling period suggests an impact of laser processing parameters to the LISE pattern period.Such dependencies, however, have not been observed yet.
The current models for wrinkled surfaces are developed for defined conditions of the film-substrate system and does not consider may processes that can be expected for laser-irradiated polymer films such as the gradual change of material parameters scuh as viscosity, Young's modulus and absorption coefficient with depth and time (intra pulse and prolonged laser pulse irradiation).Therefore, these models describe the reality in parts and needs to be extended for laser-irradiation scenarios.
The processes that likely govern the mechanism for forming the LISE pattern can be summarized as following: multipulse laser irradiation of the water-confined photoresist results in scattering of the laser beam at surface roughness and in heating the photoresist that can result in a slow development of preliminary LIPSS.However, the linear LIPSS formation probability is reduced due to the lower scattered intensity as a result of the lower refractive index difference of water and photoresist.Nanobubbles, that observed at the surface of LISE pattern, may contribute to the overall scattering intensity similar to the LIPSS formation at air.The much higher number of laser pulses needed for LIPSS formation at air can be the reason for the nonappearance of LIPSS at these experimental conditions [13,52].Despite that, very first corrugations of the surface can be LIPSS related and can be considered as surface instabilities as seed points for the wrinkling process for that the basic requirements-a rigid film on a viscos base-are fulfilled at least for a certain time after the laser pulse.Assuming the seeding LIPSS are instabilities causing the local wrinkle formation, which have an inherent period that is probably larger than the LIPSS period, LISE pattern may appear with an orientation prescribed by LIPSS but showing a manifold, e.g., doubling, of the period.This scenario may explain the partial orientation of the LISE pattern as seen in the FFT of the SEM images (see Fig. 3).
As seen in Fig. 6, wherein LIPSS and LISE pattern formation is compared, the irradiation configuration is almost 623 Page 8 of 10 similar but results in different governing processes although all process happens in both cases.The reason is the different physical processes at the laser-irradiated interface.The pattern formation at surfaces requires movement of material that is driven in the case of LIPSS formation in polymers by the viscous flow of the polymer at the surface.The lower the laser-induced temperatures of the water-confined polymer surface the less viscous flow is assisted, so that in that case, only a little flow can be expected and therefore only a slow development of LIPSS.This weakly developed LIPSS that can be overlay or trigger LISE only little.
Further, LIPSS appearing at water confinement should feature periods in the range of 170 nm to 180 nm which is not in line with the experimentally determined periods of LISE patterns or multiples of that.Therefore, almost no synergetic effects can be expected for the growth of LIPSS and LISE pattern at water confinement.This confirms the experimental result that LIPSS pattern have only a marginal impact to the LISE structures.
However, some physical processes are not considered, such as the evaporation of water at the interface, which may change the scenario due to the formation of bubbles or the buildup of a certain pressure in the system due to thermal expansion or evaporation processes.These and other factors need to be studied in the future.Another key point is the feedback effects that obviously occur when the topography changes during multipulse laser irradiation.Here, the driving force requirements as well as the near-surface mechanics need to be analyzed.These studies must include the dynamics of the driving and relaxation processes of the system.

Conclusions
Experimentally, a new process of forming laser-induced selforganized surface structures has been observed for UV-laserirradiated polymer films on silicon wafers at water confinement.In difference to traditional LIPSS pattern here, the morphology is rather random, the period of appr.450 nm is much higher than the expected LIPSS period in water confinement of 170 nm to 180 nm and the amplitude with max.170 nm is higher than the LIPSS amplitude for the selected laser wavelength.The LISE pattern feature characteristics in between LIPSS and wrinkling patterns such as the period, the amplitude and the orientation.Hence, the exiting mechanism of LIPSS cannot explain the observed surface structures which is believed to be more complex.
The proposed mechanism of forming such laser-induced self-organized (LISE) pattern is wrinkling of a thin top layer of the polymer film on a subsurface viscous polymer due to laser-induced surface hardening as well as cooling of the surface during laser heating by the water confinement.The increase of the wrinkled amplitude of the LISE pattern at prolonged laser irradiation can be related to the temporal subsurface melting of the laser-heated polymer while cooling the surface cause the.This laser-induced mechanism enable pattern that is suitable for surface functionalization for optical, boil-related as well as wetting application.

Fig. 2 Fig. 3
Fig. 2 Images of laser-irradiated photoresist films in water confinement.a Macroscopic image showing the impact of the parameters with naked eye.b Microscopic image of the laserexposed field marked by blue border in a.Right hand side the pristine photoresist film is seen showing a different color.c SEM image of well-developed LISE structures (laser fluence of approx.27 mJ/cm 2 and 150 applied laser pulses)

Fig. 4 623
Fig. 4 Topography LISE structures: a AFM image of a characteristic surface irradiated at 27 mJ/cm 2 with 10 pulses (z max = 75 nm) and b height of the LISE patterns in dependence on the pulse number at a laser fluence of 27 mJ/cm 2

Fig. 5
Fig. 5 Temperature field calculations of laser-heated water-confined photoresist.a Temperature distribution across the water-photoresist interface (at 4 µm) calculated after the pulse for different parameters of laser fluence and absorption coefficient.b Temporal temperature

Table 1
Material parameter used for the calculations of the temperature across the water-photoresist interface ρ: density, k th : thermal conductivity, c p : specific heat capacity, α: absorption coefficient, T B : boiling temperature, T g : glass temperature