Photolithographic patterning of cellulose: a versatile dual-tone photoresist for advanced applications
In many areas of science and technology, patterned films and surfaces play a key role in engineering and development of advanced materials. Here, we present a versatile toolbox that provides an easy patterning method for cellulose thin films by means of photolithography and enzymatic digestion. A patterned UV-illumination of trimethylsilyl cellulose thin films containing small amounts of a photo acid generator leads to a desilylation reaction and thus to the formation of cellulose in the irradiated areas. Depending on the conditions of development, either negative and positive type cellulose structures can be obtained, offering lateral resolutions down to the single-digit micro meter range by means of contact photolithography. In order to highlight the potential of this material for advanced patterning techniques, cellulose structures with sub-µm resolution are fabricated by means of two-photon absorption lithography. Moreover, these photochemically structured cellulose thin films are successfully implemented as dielectric layers in prototype organic thin film transistors. Such photopatternable dielectric layers are crucial for the realization of electrical interconnects for demanding organic device architectures.
KeywordsCellulose Photochemistry Photoresist Lithography Organic electronics Organic thin film transistor
Cellulose, as the most abundant biopolymer on earth and major component of green plants features a set of properties which can hardly be found in any other single material (Klemm et al. 2005). It has been used for ages by mankind to maintain information and knowledge (paper), to provide clothing (fibers) and to protect mankind against nature’s forces (wood) (Klemm et al. 2004). In the past two decades, also other cellulose based materials entered into the focus of interest, since progress in preparation and analysis of materials allowed scientists to move from the macro- and microscale to the nanoscale world. A wide variety of cellulose based materials such as nanofibrils, nanocrystals, nanofibers, nanoparticles, aerogels and ultrathin cellulose films have recently been explored, to give just a few examples (Schaub et al. 1993; Kontturi et al. 2003a, b; Eichhorn et al. 2010; Habibi et al. 2010; Moon et al. 2011; Olsson et al. 2010). The properties of these materials can be classified as unusual compared to those of macroscale cellulose materials. Particularly, the isolation of nanocrystalline moieties from bulk cellulose lead to a variety of non-classic applications of cellulose in charge storage for supercapacitors (Thielemans et al. 2009; Liew et al. 2010, 2013), high mechanical strength materials (Cranston et al. 2011), and optics (Cranston and Gray 2006, 2010). Lately, also large efforts have been made to use (nano)paper as a substrate for field effect transistors (Fortunato et al. 2008; Huang et al. 2013; Fujisaki et al. 2014), since paper provides several interesting properties such as low price, ready availability and excellent printability with organic polymers. Most notably, the combination of its mechanical properties, environmental stability and raw material availability makes cellulose an ideal candidate for environmentally sustainable and biocompatible products for a wide range of applications. One of the main disadvantages of cellulose, however, is its poor solubility in common organic solvents and therefore its constrained processability, limiting its applications especially in the growing field of organic electronics. In order to overcome these limitations, various procedures for the regeneration of cellulose from organosoluble cellulose derivatives have been developed, paving the way towards novel biodegradable functional materials (Klemm et al. 2005). A promising cellulose derivative for the preparation of cellulose thin films is trimethylsilyl cellulose (TMSC), which is soluble in several common organic solvents, including eco-friendly solvents such as ethanol and can be regenerated to cellulose by a treatment with vapors or solutions of hydrochloric acid (Rolland 1993; Kontturi et al. 2003b; Kontturi and Lankinen 2010). While such thin films have been widely employed to study and to understand the interaction of a variety of biomolecules with cellulose, micro- and macropatterned cellulose films have been shown to be promising materials for the fabrication of protein microarrays, high protein affinity matrices or for sensitive DNA detection (Löscher et al. 1998; Orelma et al. 2011, 2012; Mohan et al. 2013b, c). Blends on the basis of TMSC and other polymers such as styrene or lignins can lead to the formation of micro- and nanostructures as well due to phase separation (Nyfors et al. 2009; Hoeger et al. 2012). Although such structures may be used for sensor purposes (e.g. by selective immobilization of Au-nanoparticles), the major drawback of this method is that spatially resolved structures can hardly be realized (Taajamaa et al. 2013). Recently, TMSC has been successfully applied as a precursor for the fabrication of cellulose based high-k dielectric layers in pentacene- and fullerene (C60) based OTFTs, as demonstrated by our group (Petritz et al. 2013). However, for the realization of complex organic circuits, efficient patterning procedures for dielectric materials are of particular importance, in order to enable the fabrication of electrical interconnections. In many areas of research ranging from biosensors to lab-on-a-chip devices or organic electronics, there is a need for high-throughput production methods for microstructured cellulose surfaces. So far, methods to create such cellulose micropatterns are rather rare and include soft lithography and deep UV lithography, using UV-light with wavelengths below 260 nm, both of which have some disadvantages and limitations (Kargl et al. 2013; Tanaka et al. 2004). While the use of soft lithography is too laborious, the high energy input of deep UV lithography limits its usage in a variety of material fabrication processes (e.g. for organic thin film transistors). In addition, the cellulose patterns are created by photodegradation in the illuminated areas, which only allows for the realization of negative tone photoresists. A widely used concept for the fabrication of polymer micro structures is based on a photoinduced alteration of the solubility of polymeric materials. This concept is also applied in chemically amplified photoresists (CARs), which utilize photo acid generators (PAGs) to adjust the solubility by means of UV-light (Ito et al. 1990).
In this contribution, we present the photo-induced conversion of acid labile TMSC to rather insoluble cellulose with the aid of PAGs. Although CARs which exploit desilylation reactions are well known (Cunningham 1987; Cunningham et al. 1987), these methods have, to the best of our knowledge, not yet been used for the fabrication of patterned cellulose thin films from easily accessible TMSC. Moreover, the herein described approach enables the realization of both positive and negative type microstructured cellulose thin films, following the concept of dual-tone photoresists. Going a step beyond conventional lithographic techniques, two-photon absorption (TPA) lithography has been successfully applied to realize feature sizes in the sub-µm range. To demonstrate the versatility of this biopolymer based photoresist towards potential applications in organic electronics, this material has also been investigated as a photo-patternable ultrathin dielectric layer for low-voltage pentacene based OTFTs.
Unless otherwise stated, all chemicals were obtained from commercial sources and were used without further purification. Trimethylsilyl cellulose with a degree of substitution of DSSi = 2.8 was provided by the Thuringian Institute of Textile and Plastics Research (Rudolstadt, Germany). N-hydroxynaphthalimide triflate (electronic grade, ≥99 %) and cellulase from Trichoderma viride were obtained from Sigma Aldrich. Silicon wafers were obtained from Taisil Electronic Materials Corp. and were rinsed with acetone and cleaned with a polymer cleaning solution (First Contact, Photonic Cleaning Technology, LLC) after cutting.
TMSC films were fabricated by spin coating from chloroform solutions with concentrations ranging from 5 to 20 mg ml−1 (v = 2,000 rpm, a = 1,000 rpm s−1) containing varying amounts of photoacid generator onto silicon wafers or CaF2 plates.
UV-irradiation experiments were carried out with a medium pressure Hg-lamp (100 W, Newport, 66990) equipped with a filter transmissive for wavelengths in the range of 350–450 nm. The light intensity (power density) at the sample surface was measured with a UV radiometer (UV Power Puck, EIT, Inc.) and was determined as 7.6 mW cm−2 in the spectral range from 250 to 390 nm (UV-A, UV-B and UV-C). Photolithographic patterning was carried out with a mask aligner (500 W HgXe, SUSS, MJB4) equipped with a filter transmissive for wavelengths in the range of 365 nm with a measured power density of 9.0 mW cm−2.
For all TPA lithography experiments, a commercial lithography setup (Photonic Professional, Nanoscribe GmbH) was used. A laser power of 15 mW and a lateral feed rate of 50 µm s−1 with a 100× oil immersion objective with NA = 1.4 and a tight focusing of the laser beam were chosen.
After photolithographic patterning, a development was performed in chloroform for 10 min at room temperature or via enzymatic digestion using cellulase from T. viride (1 mg ml−1, dissolved in a 100 mM sodium acetate/acetic acid buffer at pH 4.8). The illuminated samples were immersed in 3–5 ml of cellulase solution at 37 °C overnight.
FTIR spectra were recorded on a Perkin Elmer Spectrum One instrument (spectral range of 850–4,000 cm−1, resolution of 1 cm−1) in transmission mode on CaF2 plates.
Atomic force microscopy
Atomic force microscopy (AFM) micrographs were recorded with a Nanosurf FlexAFM instrument, using silicon AFM probes with a resonance frequency of 190 kHz and a force constant of 48 N m−1 (Tap190AL-G, Budgetsensors).
Organic thin film transistors were fabricated in a staggered bottom-gate top-contact architecture. The gate electrode was processed on pre-cleaned glass slides by thermal evaporation of a 40 nm thick aluminum layer through a shadow mask at a rate of 1 nm s−1 under high vacuum conditions. For the negative type photolithographic patterning of TMSC, a metal shadow mask was used. After spin-coating and patterning (UV-irradiation and development) of the dielectric, a 35 nm thick pentacene layer was evaporated. Source- and drain electrodes were deposited by thermal evaporation of gold through a shadow mask in order to form 50 nm thick contacts. After production, all OTFT samples were protected from light and stored under argon atmosphere. OTFTs were fabricated with a channel-length of 70 µm and width of 1.5 mm.
The dielectric properties of the dielectric were determined by frequency dependent capacitance (C-f) and current–voltage (I-V) measurements on metal (30 nm Al)—cellulose films (photochemical regenerated TMSC films, 32 nm)—metal (50 nm Al) sandwich structures with an overlap area of 0.1 cm2 on glass substrates. The frequency dependence of the gate dielectric capacitance was measured by impedance spectroscopy techniques with an LCR meter (Hioki 3532-50 LCR). For the data processing of the OTFT characteristics, the capacitance at 1 kHz was used. Electrical measurements of the OTFTs were carried out under exclusion of light, using a parameter analyzer from MB-Technologies.
Results and discussion
Investigation of the photoreaction
It has to be mentioned that the desilylation reaction also proceeds in the TMSC films after UV-irradiation. This phenomenon is already known from cationic photo-polymerization reactions and is referred to as “dark reaction”. In this case, the photo-generated protons catalyze the desilylation reaction in absence of UV-light. This reaction causes a further decrease of the silyl ether content from 93 to 82 % and from 73 to 58 % in TMSC films containing 1 and 2 wt% NHNA, respectively, after storage under exclusion of light for 24 h as depicted in Fig. 2a. FTIR spectra, recorded after 48 h, did not exhibit any further decrease of the Si–C band.
Photolithographic patterning of TMSC
Negative type development
Due to the fact that the photo-generated cellulose is insoluble in common organic solvents such as chloroform or toluene, a direct photolithographic patterning of TMSC, containing small amounts of PAG is possible, yielding negative type cellulose structures after development. The changes in solubility of TMSC films, caused by the photoinduced desilylation reaction were assessed by means of sol–gel analysis. The insoluble fraction (gel fraction) was determined by FTIR spectroscopy by evaluating the intensity of the C–O–C stretching vibration of the glycosidic bond at 1,150–1,170 cm−1 and comparing the peak height before and after development in chloroform for 10 min. Figure 2b represents the gel fraction of TMSC-films containing 2 wt% NHNA and 5 wt% NHNA as a function of the irradiation dose, revealing maximum gel fractions of 73 and 98 %, respectively, after UV-illumination with an irradiation dose of E ≥0.97 J cm−2. Prolonged exposure does not further influence the gel fraction, which is in good accordance with the kinetic behavior shown in Fig. 2a. A decrease in solubility after an additional exposure (irradiation doses up to 70 J cm−2) could not be observed, which excludes a photodegradation of the cellulose backbone. Compared to commercially available photoresists, the required illumination doses in the range of 1 J cm−2 are rather high for photolithographic patterning. This can be attributed to an insufficient spectral overlap of the UV absorption spectrum of NHNA and the used polychromatic irradiation source (as displayed in the supporting information). It can be assumed that a better matching of the spectral overlap leads to a reasonable photoresist performance. With respect to a possible application of these films as a dielectric material in organic thin film transistors, further photo-patterning experiments were performed with TMSC films, containing 2 wt% NHNA. Although, this concentration leads to an incomplete conversion of the TMSC to cellulose, the changes in solubility are sufficient for a successful photopatterning. Higher PAG contents (and their ionic photocleavage products) may also negatively influence the device stability, therefore we aimed for a compromise between low PAG concentration and a high obtainable gel fraction. A NHNA concentration of 2 wt% was found to be ideal in this respect.
Positive type development
Two-photon absorption (TPA) lithography
Application of photopatterned cellulose films as OTFT gate dielectrics
As a final remark we want to comment on the interface properties of our photopatternable dielectric material with the organic semiconductor in the OTFT, which is directly affecting the device performance. A low interface charge trap density is particularly essential for the fabrication of fast and stable organic electronic circuits. An upper limit of the density of interfacial trap states Nss,max can be calculated from the obtained subthreshold swing according to the method reported by Rolland (1993). For the determined subthreshold swing of 110 mV dec.−1 an upper trap density limit of Nss,max = 6.9 × 1011 cm−2 eV−1 is calculated. The extracted NSS values for cellulose based OTFTs are exceptionally low and are in fact much smaller than the average interface trap density of states observed in amorphous silicon TFTs being in the range of 1012 cm−2 eV−1.
In this contribution, we demonstrate a versatile method for an efficient photopatterning of cellulose thin films, the most abundant biopolymer on earth. Following the concept of dual-tone photoresist, it is possible to obtain either positive or negative type micropatterns depending on the applied development procedure. Although the development by enzymatic digestion is inferior to a development in organic solvents as revealed by sol–gel analysis and AFM, this process, in combination with cellulose based resists, paves the way towards a renewable and sustainable photolithographic procedure. In order to highlight the potential of this material for advanced patterning techniques, cellulose structures with sub-µm resolution were fabricated by means of TPA lithography. A potential application of this resist has been demonstrated by assembling an OTFT with an ultrathin patterned cellulose gate dielectric layer providing good performance (low interface trap density, low operation voltages, no hysteresis, appropriate field effect mobility). Considerably, the photopatterning capability of the gate dielectric promises fast, highly integrated, low-voltage organic electronic circuits, with a clearly simplified fabrication of via holes and therefore also a simplified design of circuits.
This work has been funded by the Austrian Science Fund (FWF) (Project TRP 181-N19). The research leading to these results has received funding from the People Programme (Marie Curie Actions—Career Integration Grants) of the European Union’s Seventh Framework Programme (FP7/2007–2013) under REA Grant Agreement No. 618158 (PhotoPattToCell).
- Cranston ED, Gray DG (2010) Model cellulose I surfaces: a review. In: Roman M (ed) Model cellulosic surfaces, ACS symposium series, vol. 1019. American Chemical Society, Washington, DC, pp 75–93Google Scholar
- Cunningham JWC (1987) Characterization of a new organosilicon photoresist. SPIE, Proc 0771Google Scholar
- Cunningham JWC, McFarland JC, Park C (1987) Characterization of a new organosilicon photoresist. SPIE, Proc 0811Google Scholar
- Eichhorn SJ, Dufresne A, Aranguren M, Marcovich NE, Capadona JR, Rowan SJ, Weder C, Thielemans W, Roman M, Renneckar S, Gindl W, Veigel S, Keckes J, Yano H, Abe K, Nogi M, Nakagaito AN, Mangalam A, Simonsen J, Benight AS, Bismarck A, Berglund LA, Peijs T (2010) Review: current international research into cellulose nanofibres and nanocomposites. J Mater Sci 45(1):1–33CrossRefGoogle Scholar
- Klemm D, Philipp B, Heinze T, Heinze U, Wagenknecht W (2004) Comprehensive cellulose chemistry: fundamentals and analytical methods, vol 1. Wiley-VCH, WeinheimGoogle Scholar
- Liew SY, Walsh DA, Thielemans W (2013) High total-electrode and mass-specific capacitance cellulose nanocrystal–polypyrrole nanocomposites for supercapacitors. R Soc Chem Adv 3(24):9158Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.