Maskless Ultraviolet Projection Lithography with a Biorepelling

Article pubs.acs.org/JPCC

Maskless Ultraviolet Projection Lithography with a Biorepelling Monomolecular Resist Y. L. Jeyachandran,†,§ Nikolaus Meyerbröker,†,‡,∥ Andreas Terfort,*,‡ and Michael Zharnikov*,† †

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Institute for Inorganic und Analytical Chemistry, Frankfurt University, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany

‡

ABSTRACT: Here, we describe a universal photolithography setup for the patterning of biorepulsive self-assembled monolayers (SAMs) as well as other monomolecular films. The setup is based on commercial equipment consisting of a computer-controlled digital micromirror device chip combined with a suitable optics and a powerful light-emitting diode (LED) source delivering ultraviolet (UV) light with a wavelength of 375 nm. Digital patterns generated in the computer serve as an input for the chip, which modulates the reflected light accordingly, transferring the pattern to the sample surface. The performance of the setup was demonstrated by UV-induced modification of the nonsubstituted alkanethiolate (NS-AT) SAMs and biorepulsive oligo(ethylene glycol)substituted AT (OEG-AT) monolayers on Au(111), upon homogeneous illumination of the test samples. Further, both nonspecific and specific templates for the protein adsorption were fabricated in the protein-repelling OEG-AT matrix by either direct writing or using an additional irradiation-promoted exchange reaction with a biotin-terminated AT. These templates were used either for nonspecific adsorption of bovine serum albumin (BSA) or for the specific adsorption of avidin, the latter relying on the interaction with the embedded biotin receptors. The density of the adsorbed protein layers across the patterns could be precisely varied by selection of proper irradiation doses.

1. INTRODUCTION Self-assembled monolayers (SAMs), which are densely packed films of rod-like molecules chemically attached to a suitable substrate, are versatile and frequently used as resist or template materials for lithography. The experimental approaches include both SAM-specific methods such as microcontact printing and dip-pen nanolithography as well as standard lithographic tools like electron beam and photolithography (predominately, with ultraviolet light).1−8 The latter techniques have certain advantages since they can rely on quite established fabrication setups and allow both parallel and sequential patterning. A parallel patterning is usually performed in the proximity printing geometry, using a stencil mask.5,9 In contrast, sequential patterning is conducted with a focused beam, as this, e.g., occurs in a secondary electron or transmission electron microscope (SEM and TEM, respectively) equipped with a proper lithographic control unit.5,7,10,11 This patterning mode is of course slower than the parallel writing but offers distinctly higher lateral resolution (defined, in the given case, by the diameter of the primary beam) and full flexibility in terms of the pattern shape. However, whereas this mode can be easily realized in the case of electron lithography, relying on commercial systems (see above), it is quite difficult to achieve for photolithography because of the lack of the analogous setups in the most relevant, ultraviolet (UV) spectral range. At the same time, photolithography has the essential advantages that it can be performed under ambient conditions (as compared to high vacuum required for electron lithography) and on nonconductive substrates. It offers, furthermore, © 2014 American Chemical Society

potentially lower costs of equipment than quite expensive electron microscopy systems with additional lithographic tools. The potential of photolithography with SAMs as resist materials was particularly demonstrated by nanometer-scale and large area patterning, relying on laser-based lithography setups, in combination with scanning near-field optical microscope and diffraction equipment.12−14 Such patterning is, however, almost exclusively performed with short-wavelength UV light (≤254 nm), which requires costly UV laser sources and optical components compatible with UV light in this spectral range.12−14 Examples of SAM-based photolithography at longer wavelengths are rare and require, almost without exception, specially designed photocleavable or photolabile monolayers.15−18 To overcome these limitations, we introduce here a universal photolithography setup, which is exclusively based on reasonably priced commercial equipment and does not rely on any specific SAM design. The key element of the setup is a computer-controlled digital micromirror device (DMD) chip combined with a suitable optics and a powerful light-emitting diode (LED) UV source. Digital patterns generated by any graphical software serve as an input for the chip which projects it onto the sample by reflecting the UV light. This allows full flexibility in the pattern shape in a parallel lithographic regime. Note that analogous setups have already been used to generate Received: October 28, 2014 Revised: December 2, 2014 Published: December 2, 2014 494

dx.doi.org/10.1021/jp510809a | J. Phys. Chem. C 2015, 119, 494−501

The Journal of Physical Chemistry C

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complex protein patterns, relying, however, on specially designed photosensitive SAMs and protein thin films and working mostly in the visible light range.15,18,19 In contrast, we show here that maskless projection lithography can also be performed with “standard”, commercially available SAM precursors. The wavelength selected in the given case (375 nm) is the shortest available for the DMD-based lithographic assemblies. This wavelength is, however, large enough to permit the use of additional, reasonably priced optics to modify and to improve the parameters of the device. At the same time, this wavelength still allows a modification of SAMs, even though with a lower efficiency as compared to UV light with shorter wavelengths, usually used for this purpose.20,21 As representative test resists we selected SAMs of nonsubstituted alkanethiolates (NS-ATs) and oligo(ethylene glycol)-substituted alkanethiolates (OEG-ATs) on Au(111). We provide spectroscopic data for both types of SAMs but put emphasis on the protein-repelling OEG-AT monolayers, presenting lithographic data for them as well. On the basis of these monomolecular resists, both nonspecific and specific templates for the protein adsorption were fabricated using the direct writing (DW) method and irradiation-promoted exchange reaction (IPER or UVPER) approach, respectively.12,13,20−22 The DW method relies on quasi-selective damage of the OEG segment in the OEG-ATs SAMs, resulting in controlled deterioration of the biorepulsive properties.12,13,20−22 The IPER approach involves an additional exchange reaction between the irradiation-modified OEG-AT moieties and the molecules bearing a specific receptor for the attachment of the target protein.20,21

Figure 1. Schematic representation of a projection-based, maskless UV lithographic setup. The major elements are labeled. A pattern designed in the computer serves as the input for the DMD chip which projects the preshaped UV beam to the sample, mimicking the original pattern.

Production Bundles and the High-power UV LED Optical Module for DLP DiscoveryTM 0.7 XGA DMD purchased from Texas Instruments Inc. via ViALUX GmbH, Germany. The V4100 Bundles consist of the V4100 board with 0.7″ XGA 2x LVDS (UV) DMD for UV light and the ALP-4.2 “high speed” Controller Suite. The DMD chip contains 1024 × 768 individually controlled micromirrors, with 13.6 μm pitch. The high-power UV LED Optical Module consists of a 10 W UV LED source (375 nm) with the respective illumination and projection optics. The patterns designed in a computer with any commercial graphical software are fed as a picture file to the DMD controller. Each micromirror in the chip can be controlled independently (ON or OFF state). The highspeed controller software enables maximum switching rates of the mirrors under full-array global shutter operation, gray value patterns, precise triggering, and LED control. The flux density at the sample was measured with a multichannel energy meter equipped with a PD300-UV sensor (Ophir Photonics). It was estimated at ∼38 mW/cm 2 homogeneous illumination, with 80% of LED power and the standard adjustment of the optical elements. Spectroscopic Measurements. To monitor the effect of the UV light in the given lithographic setup and at the given wavelength, a homogeneous irradiation of the samples, with all micromirrors in the ON state, was performed. The LED source was operated at 80% power. The resulting samples were characterized without preliminary washing, to preserve all UVmodified species. The characterization was performed by X-ray photoelectron spectroscopy (XPS) using a MAX200 (LeyboldHeraeus) spectrometer equipped with a Mg Kα X-ray source (200 W) and a hemispherical analyzer. The recorded spectra were corrected for the spectrometer transmission, and the binding energy (BE) scale was referenced to the Au 4f7/2 emission at 84.0 eV.24 Spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublets we used two peaks with the same full width at halfmaximum (fwhm), the standard24 spin−orbit splitting of ∼1.18 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). Similar fit parameters were used for identical spectral regions of the different samples. Patterning. The patterning was performed with the EG6 resist. A variety of representative templates for the protein adsorption, including gradient ones, was fabricated. The patterns were designed with Microsof t Paint and fed as JPEG files to the DMD controller. The LED source was operated at 80% power. In addition, several resolution and performance tests were performed using a CMOS monochrome board camera (DMM72BUC02-ML, 5 mega pixel resolution at 2.2

2. EXPERIMENTAL METHODS Materials. The SAM precursors, viz., a representative NSAT compound, dodecanethiol (DDT; HS−(CH2)11−CH3), and a representative OEG-AT compound (EG6; HS− (CH2)11(OCH2CH2)6−OH) were purchased from SigmaAldrich and ProChimia Surfaces, respectively, and used as received. A biotin-terminated tri(ethylene glycol) hexadecanethiol (EG3bio; HS−(CH 2 ) 1 5 CONH(CH 2 CH 2 O) 2 − CH2CH2NH−biotin) purchased from Asemblon Inc. was used as the substituent in IPER. Bovine serum albumin (BSA, A7638) and avidin (A9275), used for the preparation of nonspecific and specific protein patterns, respectively, were purchased from Sigma-Aldrich. Substrates. The gold substrates were purchased from Georg Albert PVD-Beschichtungen. They were prepared by thermal evaporation of gold (100 nm thick, 99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. The films are polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. Immediately before use, they were cleaned by ozonation with a low-pressure Hg lamp. SAM Preparation. DDT and EG6 monolayers were prepared using standard procedures,20,23 viz., by immersion of clean gold substrates into 1 mM solutions of the respective precursors in absolute ethanol (Sigma-Aldrich) for 24 h at room temperature. After immersion the samples were thoroughly rinsed with pure ethanol, blown dry with argon, and used immediately for characterization or further treatment. Lithographic Setup. The setup is schematically shown in Figure 1. We used a combination of the V4100-DLP Discovery 495



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μm pixel size) and a commercial photosensitive resist (Positiv20, Kontakt Chemie). The photoresist was handled according to the established procedures.25 Protein Adsorption. Patterned EG6 samples were used as templates for the protein adsorption, which was performed either in nonspecific fashion, directly after irradiation (DW process), or in a specific way, after the additional exchange reaction with the EG3bio substituent (IPER process). The exchange reaction was carried out in 0.1 mM solution of EG3bio in ethanol for 5 min. After immersion, the samples were rinsed thoroughly with pure ethanol, dried under argon flow, and used immediately for characterization or further treatment. BSA and avidin were used for the DW and IPER process, respectively. The protein adsorption was performed from a 0.1 mg/mL solution of the respective protein in a PBS (P4417TAB, pH = 7.4 at 25 °C, Sigma-Aldrich) solution prepared using Millipore water. The incubation time was 30 min. After protein adsorption, the samples were rinsed thoroughly in PBS solution, washed in Millipore water, and dried under argon flow.

3. RESULTS AND DISCUSSION 3.1. Effect of UV Light: Spectroscopy. The major effect of UV exposure on thiol-derived SAMs on noble metal substrates under ambient conditions is the photooxidation of the thiolate headgroups.26−30 This process can be monitored by XPS. S 2p and C 1s XP spectra of pristine and homogeneously irradiated DDT SAMs are shown in Figure 2. The S 2p spectrum of the pristine monolayer in Figure 2a exhibits a characteristic S 2p3/2,1/2 doublet at a BE position of ∼162.0 eV (S 2p3/2) with no traces of other sulfur-derived species. The above BE value corresponds to the thiolate species bound to noble metal surfaces,31−33 as can be expected for well-defined thiol-derived SAMs. Upon irradiation, the relative intensity of the thiolate component decreases, and a new doublet at a BE of 167.2 eV (S 2p3/2), associated with photooxidized sulfur species, appears and increases in intensity with progressive UV exposure. This doublet can be tentatively28,34 assigned to the sulfonate (R−SO3¯) which is typically the major photooxidized species in the case of thiol-derived SAMs on noble metal substrates.26−30 This is in particular the case at the comparably small wavelengths, such as 254 nm, whereas at longer wavelengths, corresponding to lower photon energies, other sulfur-derived moieties such as sulfenates (R−SO¯), sulfinates (R−SO2¯), and sulfates (SO4¯) were observed as well.21 The C 1s spectrum of the pristine DDT monolayer in Figure 2b exhibits a single emission at ∼284.8 eV, accompanied by a weak signal at higher BE related to contamination (C−O). The spectrum changes to some extent in the course of the UV exposure. There is a small intensity reduction (∼10% at 274 J/ cm2), accompanied by a shift of the major emission to lower BE, and an increase in the C−O signal intensity. This suggests a partial damage and oxidation of the alkyl chains, which occurs, however, to a much lesser extent than the analogous processes at the SAM−substrate interface. Alternatively, the increase in the C−O signal intensity can be ascribed to the adsorption of airborne adsorbates onto the partly damaged SAM. The progressive and controlled photooxidation of the thiolate headgroup at this interface builds a basis for a variety of lithographic applications in both the framework of conventional lithography and versatile chemical lithography. The latter relies on significant weakening of the adsorbate−

Figure 2. S 2p (a) and C 1s (b) XP spectra of pristine and homogeneously irradiated DDT SAMs (open circles). The UV doses are marked at the respective curves. The S 2p spectra are decomposed into doublets related to the thiolate (red solid lines) and sulfonate (blue solid lines) species. Vertical solid lines in (b) are guides for the eyes.

substrate bond for the sulfonate headgroups as compared to the one of the original thiolate anchors.26−30 Accordingly, oxidized molecules can be easily washed away or exchanged for other species. In the framework of conventional lithography, the primary SAM serves as a resist, transferring the pattern created after the washing step to the underlying substrate upon chemical or plasma-mediated etching.5 In the case of chemical lithography, a chemical pattern can be fabricated after the exchange reaction with a suitable SAM-forming substituent.30 Significantly, the extent of the exchange reaction can be precisely controlled by the irradiation dose, which allows us to vary the content of the substituent, resulting in complex chemical patterns.8,35 Among different types of SAMs, OEG-substituted ones are of special interest because of their protein-repelling properties. This is of particular importance for protein or single-stranded DNA arrays where, usually, predefined sensing spots are embedded into the OEG matrix, which prevents the nonspecific adsorption of biomolecules. This matrix can be prepared by backfilling, following the fabrication of the sensing spots. Alternatively, these spots can be written directly into this matrix by electron or UV irradiation, as discussed in Section 1.7,20,21,36,37 Both these primary tools allow controlled modification of the OEG segments, with a certain damage occurring in parallel to the alkyl segments and the headgroup− substrate interface.12−14,20,21,38 The S 2p and C 1s XP spectra of pristine and homogeneously irradiated EG6 SAMs are presented in Figure 496



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3; the intensities of the individual components are compiled in Figure 4 as functions of UV dose, along with the reference data

Figure 4. Intensities of the characteristic XPS signals for the DDT (a) and EG6 (b,c) SAMs as functions of UV dose. (a,b) Normalized intensities of the total sulfur (black circles, black dashed lines), thiolate (red squares, red dashed lines), and sulfonate (blue diamonds, blue dashed lines) signals. (c) C 1s intensity ratio for the signals related to the OEG and alkyl segments of the EG6 moieties (see Figure 3b). Figure 3. S 2p (a) and C 1s (b) XP spectra of pristine and homogeneously irradiated EG6 SAMs (open circles). The UV doses are marked at the respective curves. The S 2p spectra are decomposed into the thiolate (red solid lines) and sulfonate (blue solid lines) related doublets. The C 1s spectra are decomposed into the emissions assigned to the alkyl (olive solid lines) and OEG (purple solid lines) segments. Vertical solid line is a guide for the eyes.

photooxidation as compared to the DDT SAMs is explained by the larger thickness of the EG6 monolayer. This slows the diffusion of reactive oxygen-derived species to the SAM− substrate interface, slowing, thus, the oxidation of the thiolate headgroups. Similar effects were previously observed for both the NS-AT28 and OEG-AT21 SAMs. Preferable damage of the OEG segment accompanied by a partial photooxidation of the thiolate groups provides a basis for chemical lithography with the EG6 resist, as will be shown in the next section. 3.2. Lithography with the Protein-Repelling Template. As mentioned in Section 1, two different approaches to fabricate templates for nonspecific and specific protein adsorption were applied, based on the controlled and siteselective photodegradation and photooxidation of the EG6 matrix. These approaches, viz., DW and IPER, are schematically illustrated in Figure 5, using a combination of several customdesigned, nonspecific protein (BSA) subpatterns in the EG6 matrix. Within the first step, which is identical for both approaches, an OEG-AT monolayer is exposed to UV light in a lithographic fashion (the center subpattern in Figure 5). The fabricated template, consisting of the pristine OEG-AT areas and regions where the OEG segments of the OEG-AT molecules are partly damaged (along with a partial oxidation of the thiol groups), can be directly used for nonspecific adsorption of proteins within the DW procedure (left subpattern in Figure 5). This adsorption relies on the loss of biorepulsive properties in the UV exposed areas, mimicking, thus, the original OEG-AT template. Alternatively, this template can be subjected to an exchange reaction with substituents bearing a receptor for specific protein attachment within the IPER procedure (right subpattern in Figure 5). At a

for the DDT SAMs given for comparison. The S 2p spectrum of the pristine EG6 monolayer in Figure 3a exhibits the characteristic thiolate-related doublet at a BE of ∼162.0 eV (S 2p3/2), while the C 1s spectrum shows two overlapping emissions at ∼284.6 and ∼286.4 eV associated with the alkyl (C−C) and OEG (C−O) segments of the SAM constituents, respectively.20,21 Similar to the DDT case, a new doublet at a BE of 167.2 eV (S 2p3/2), associated with photooxidized sulfur species (sulfonate), appears and increases in intensity with the progressive UV exposure. However, in contrast to the DDT SAMs (Figures 2a and 4a), the intensity of the thiolate component does not decrease but remains approximately constant at the intermediate stages of the irradiation treatment (Figure 4b). The reason for this behavior is a progressing damage of the OEG segment, which results in a lower attenuation of the S 2p signal, so that its total intensity even increases during the UV exposure (Figure 4b). Indeed, the OEG-related emission in the C 1s spectra of the EG6 monolayer decreases in intensity upon the UV exposure which is underlined by Figure 4c, where the intensity ratio of the C 1s emissions associated with the OEG and alkyl segments is shown as a function of the UV dose. In particular, this ratio decreases by 40% for a dose of 137 J/cm2. The respective extent of the photooxidation at the sulfur atom is ∼15% only, so that the damage of the OEG segment is the dominant photoinduced process in the given case. The lower extent of the 497



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Figure 5. Schematic illustration of the DW and IPER processes comprised of several individual BSA subpatterns in the protein-repelling EG6 matrix. All subpatterns were fabricated simultaneously by the DW procedure, as parts of a joint pattern, then cut and combined together in the presented fashion. The UV dose was ∼205 J/cm2. The scale bar is 200 μm. Black spots in the middle of some subpatterns are imaging artifacts (SEM aberrations because of a large field of view). The patterning was performed at a wavelength of 375 nm, but the light with a longer wavelength, up to 390 nm, can be used as well.21

proper selection of the reaction parameters,20,39 the exchange will only occur in the areas exposed to the UV light, with the extent depending on the dose. The resulting specific template then mimics the original nonspecific OEG-AT template, with the density of specific receptors being controlled by UV dose and adjusted according to the requirements of a particular experiment. The schematic in Figure 5 represents a good illustration for the efficiency of the DW procedure. Two further representative examples of nonspecific protein patterns fabricated within this approach are presented in Figures 6a and 6b. These are BSA

the biorepulsivity and, subsequently, the amount of the adsorbed proteins, giving the contrast between a particular part of the coils and the background. Representative examples of specific protein patterns, prepared within the IPER approach and relying on the biotin-bearing EG3bio substituents, are presented in Figures 6c and 6d. These patterns are formed by avidin which has a specific affinity to biotin. One pattern represents a honeycomblike arrangement (Figure 6c) and another a similar array of gradient-like coils as in Figure 6b (Figure 6d). The contrast is less pronounced as compared to the analogous BSA pattern (generated by DW), but the gradient-like contrast variation along the coils, reflecting the amount of the adsorbed avidin, is clearly visible. Note that generally not all damaged OEG-AT molecules are exchanged by receptor-bearing moieties (such as EG3bio) upon the exchange reaction, so that the resulting specific template is prone to nonspecific protein adsorption as well, even though to a small extent.20,21 The exchange reaction is however complete if the irradiation dose is kept below a certain, wavelengthdependent critical value.20,21 3.3. Practical Aspects. The advantages of the described lithographic approaches, in combination with the maskless UV projection lithography setup, are the possibility to use commercial compounds, simplicity, and flexibility in terms of the character and shape of the fabricated patterns. Any pattern designed by computer can be reproduced by proteins as shown by the representative examples in Figures 5 and 6. It is important that these are not just “black-white” but “gray-scale” patterns, with a specific, variable protein coverage within the predefined spots. The flexibility in terms of the pattern shape is further illustrated by Figure 7, where a SEM image of a complex, artistic pattern, fabricated with a photoresist, is presented. The prototype pattern was just downloaded from the Internet and used as the input for the lithography setup. The current drawbacks of the maskless UV projection lithography setup in its specific application to the fabrication of protein patterns on the basis of the OEG-AT monolayers are a relatively high dose required for the patterning (100−200 J/ cm2) and a rather poor lateral resolution. The high UV dose is related to a low efficiency of UV light to modify OEG-AT monolayers at a long wavelength such as the one used in this study.20,21 The lateral resolution is generally defined by the geometrical size of individual micromirrors (13.6 μm; see Section 2). These constraints can, however, be partly resolved upon focusing of the output pattern onto a smaller area using suitable projection optics, similar to the visible light case.18 This

Figure 6. Representative nonspecific (BSA; a,b) and specific (avidin; c,d) protein patterns in the protein-repelling EG6 matrix fabricated within the DW and IPER procedures, respectively. The UV dose was either fixed (a,c: ∼205 J/cm2) or varied over the patterns (b,d: up to 205 J/cm2). The scale bars in all images are 200 μm. Black spots in the middle of the patterns are imaging artifacts (SEM aberrations because of a large field of view). Note that the SAM templates alone do not provide enough contrast for SEM imaging.21

patterns featuring arrays of the “BSA” letters (Figure 6a) and coil-like gradients (Figure 6b). The letters in Figure 6a were written with a fixed UV dose (205 J/cm2), while in Figure 6b, the dose was successively increased at going from the center to the outer end of the coils. Accordingly, the extent of UVinduced damage varied along the coil, governing the extent of 498



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objective and the sample was reduced from 4.2 to 1.1 cm. Since the side of the squares of the “checker board” patterns corresponds to 4 micromirrors (due to the limitations of the camera chip), the minimal size of the “written” features corresponds to 1/8 of the black-white square combination and can be determined from the intensity profiles across the patterns, as shown in Figures 8b and 8d. Accordingly, this size for the standard arrangement of the output optical elements is ∼14 μm, in good agreement with the dimensions of the micromirrors (13.6 μm; see Section 2). In contrast, the size of the “written” features for the modified arrangement of the output elements is ∼7 μm, which corresponds to an improvement by a factor of 2.

Figure 7. Representative example of a complex, artistic pattern fabricated with a conventional photoresist. The pattern is an illustration of the design flexibility within the maskless UV projection lithography approach.

4. CONCLUSIONS We describe here a universal photolithography setup for the patterning of biorepulsive SAMs as well as other monomolecular films. The setup is based on reasonably priced commercial equipment consisting of a computer-controlled DMD chip combined with a powerful LED UV source (375 nm) and a suitable optics. Digital patterns generated in the computer serve as an input for the chip, defining the orientation of individual micromirrors. UV light is reflected accordingly, projecting the input pattern onto the sample surface. The chemical changes occurring in the test NS-AT and protein-repelling OEG-AT SAMs, with both precursors commercially available, were monitored in detail in the dedicated experiments relying on the homogeneous illumination of the samples. Further, patterned templates for the nonspecific and specific protein adsorption were formed in the protein-repelling OEG-AT matrix by DW and IPER procedures, respectively. These templates were used for nonspecific adsorption of BSA and specific adsorption of avidin, relying, in the latter case, on the interaction with biotin groups introduced by the IPER process. The density of the adsorbed proteins across the patterns could be varied in a controlled fashion by selection of proper irradiation doses. The advantages of the presented approach are its simplicity, use of commercial products, and full flexibility in terms of the pattern design. Constraints include relatively high UV dose, related to a low efficiency of the long-wave UV light, and a rather low lateral resolution, which is generally defined by the lateral dimensions of individual micromirrors (13.6 μm). We believe, however, that these constraints can be released considerably by the introduction of additional projection optics or/and readjustment of the available optical elements, as was demonstrated in preliminary experiments.

will result in not only a higher UV flux but also in further miniaturization of the “written” features. Interestingly, such a miniaturization can already be achieved to some extent even without additional projection optics, by a relocation of the output elements of the available lithographic setup. An example is shown in Figure 8. This figure presents 4 × 4 micromirror “checker board” patterns acquired with a camera chip located at the position of the sample for the standard and modified adjustments of the projection optics (Figures 8a and 8c, respectively). In the latter case, the projection objective was moved toward the sample stage by several centimeters, while the working distance between this

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AUTHOR INFORMATION

Corresponding Authors

*(M.Z.) Phone: +49 6221 54 4921. Fax: +49 6221 54 6199. Email: [email protected]. *(A.T.) Phone: +49 69 798 29180. Fax: +49 69 798 29188. Email: [email protected]. Present Addresses §

Figure 8. “Checker board” patterns acquired with a camera chip (a,c), along with the respective intensity profiles across the patterns (b, d), as shown by the horizontal red lines in the patterns. The original “checker board” scaling is 4 × 4 micromirrors. 1/8 of the up/down intensity profile defines then the minimal size of the fabricated features. (a,b)Standard adjustment of the projection optics and the sample position. (c,d) Custom adjustment of the projection optics and the sample position as described in the text.

Department of Physics, Bharathiar University, Coimbatore 641046, India. ∥ School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St Andrews KY169ST, United Kingdom. Notes

The authors declare no competing financial interest. 499



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ACKNOWLEDGMENTS We thank Theresa Weber for the assistance regarding the UV flux measurement. This work has been financially supported by a grant from the German Research Society (ZH 63/9-3 and TE 247/9-2).

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