Nanoimprint lithography for organic electronics

Microelectronic Engineering 61–62 (2002) 25–31 www.elsevier.com / locate / mee

Nanoimprint lithography for organic electronics ˜ a , *, J. Seekamp a , A.P. Kam a , T. Hoffmann a ,1 , S. Zankovych a , C. Clavijo Cedeno C.M. Sotomayor Torres a , C. Menozzi b , M. Cavallini b , M. Murgia b , G. Ruani b , F. Biscarini b , M. Behl c , R. Zentel c , J. Ahopelto d a

Institute of Materials Science and Department of Electronics and Electrical Engineering, University of Wuppertal, Gauss-Str. 20, D-42097 Wuppertal, Germany b Instituto di Spettroscopia Molecolare, CNR, Via P. Gobetti 101, I-40129 Bologna, Italy c Institute for Organic Chemistry, Department of Chemistry and Pharmacy, University of Mainz, Duesbergweg 10 -14, D-55099 Mainz, Germany d VTT Microelectronics, P.O. Box 1101, FIN-02044 VTT, Espoo, Finland

Abstract Thin films made of organic semiconductors (a-sexithiophene, PDAS and PBAS) have been printed and the impact on morphology studied by optical, atomic force and electron microscopy. Surfaces in contact with the stamp during printing undergo a change towards smoother and more ordered material at the macromolecular scale. Interdigitated nanoelectrodes to be used as source and drain in TFTs have been made and printed down to 100 nm. PDAS and PBAS can be printed at room temperature and preserve their printed feature provided they are cross-linked afterwards.  2002 Elsevier Science B.V. All rights reserved. Keywords: Nanoimprint lithography; Oligothiophenes; Triaraylamines

1. Introduction One of the factors which makes organic electronics [1] attractive is low cost, although it is clear that in switching applications organic electronics will most likely not compete with traditional semiconductors. In this context, lithography based on stamps in intimate contact with the film is among the most promising fabrication approaches for organics. Direct printing approaches, such as micro-transfer molding of electroluminescent molecules (e.g. Alq 3 ), are suitable for submicron lengthscales, with 100-nm lateral resolution and vertical resolution of a few nanometers [2]. However, many relevant molecules are poorly soluble and hence it is difficult to process them as ink. Transport * Corresponding author. ˜ E-mail address: [email protected] (C. Clavijo Cedeno). 1 Present address: IMEC, Kapeldreef 75, B-3001 Heverlee, Leuven, Belgium. 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00505-1

 2002 Elsevier Science B.V. All rights reserved.

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phenomena in organics have characteristic lengthscales in the nanometer range and downscaling the size of domains to this range is a promising route in order to enhance the material response. This may not be easy by soft lithography. These aspects motivate the investigation of nanoimprint lithography (NIL) for organic electronics [3], especially in aiming at devices with lateral feature sizes below 100 nm and with tolerance below 10 nm. Cold micropatterning has recently been tested to fabricate organic electronic devices [4]. NIL has been tested for the fabrication of gratings in conjugated materials as a preliminary step towards distributed feedback mirrors and waveguides, with promising results [5,6]. Moreover, patterned organic semiconductors by NIL show no degradation in their electrical [7] or electro-optical [5] properties. In this paper we report NIL applied to three different organic materials: a thiophene oligomer, alpha-sexithiophene (T6) [8], poly(4-diphenyl-aminostyrene) (PDAS) and poly(phenyl-bis-4-aminostyrene) (PBAS) [9,10]. Whereas T6 is an established material in the lead for organic thin film transistors [11], the last two are under development and their properties are just beginning to be studied. Taking the case of thin film transistors, one goal is to fabricate FETs with very small separation between the source and drain, as a strategy to increase the mobility of the device and the switching speed, by bringing electrode separation close to the size of a single domain of the material. The implications are that nanoelectrodes with a separation of around a few tens up to a few hundred nanometers will be needed. For instance, for poly(triarylamines) a mobility increase is expected for a 30-nm lengthscale. At present, metal nanoelectrodes separated by 75 nm have been fabricated. Here we report T6 films patterned with interdigitated structures with channel widths down to 100 nm. Device tests for FET structures are in progress.

2. Experimental details

2.1. Materials T6 is grown in ultra-high vacuum (10 210 mbar base pressure) by organic molecular beam deposition (OMBD) at deposition temperatures up to 150 8C and a film thickness typically below 100 nm [12]. Growth in high vacuum results in a variety of morphologies, including island and terraces, lamellar structures and grains, depending on deposition temperature and film thickness. The morphologies of the as-grown films and the dominant growth mechanisms have been studied thoroughly by atomic force microscopy at ambient conditions and optical spectroscopy [12,13]. For this work we used films the thickness of which was varied between 10 and 100 nm, and typically deposition temperature of 300 K. Thus, dominant morphology of the as-grown film is granular, with typical grain sizes on the order of a few hundred nanometers. Thin films are grown on mica for AFM investigations and on silicon dioxide / silicon substrates patterned by nanoimprint lithography to study morphology and eventually their electrical properties. The synthesis of PDAS and PBAS is described elsewhere [8,14]. The monomer is dissolved in cyclohexanone with a photo-initiator. This solution is spun on silicon, silicon with silicon oxide and ITO covered glass in films of | 150-nm thickness. To control the viscosity of the polyarylamine films, they are exposed to UV light to initiate a partial polymerisation of the monomer material.

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Fig. 1. Scanning electron micrograph showing a stamp with eight FET structures of Cr on Si.

2.2. Printing Most of the substrates coated with the polymers were printed in an OBDUCAT NanoImprint NIL-2-OB-l unit at a pressure of 30 bar and 185 8C during 1 min. A few were printed using a home-built press. Typical printing parameters are a separation temperature of 95 8C and a cooling time of | 3 min. Stamps included 2 3 2 cm 2 silicon stamps with fields of 5 3 5 mm 2 containing 200-nm lines with a separation of 800 nm. These stamps were made by UV lithography and were dry etched to a depth of between 200 and 400 nm into silicon. Another type of stamp featured chromium structures on silicon dioxide / silicon substrates (Fig. 1). These structures were written by electron beam lithography onto PMMA films between 60- and 130-nm thick. The metal layer used in the lift off was chromium with thickness between 10 and 30 nm. Patterns consisted of 40 interdigitated lines 11-mm long and 150-nm wide separated by 150 nm. Silicon and mica substrates covered with T6 were imprinted on the simple hydraulic press at a temperature of 200 8C and a pressure of 100 bar. PDAS and PBAS films were imprinted at ambient temperature with a pressure of 100 bar.

3. Results AFM and SEM characterisation of printed samples provide evidence of a topographical contrast with 1-mm and 200-nm wide lines printed on 100-nm T6 thick films. An extensive study on morphological aspects and optical properties will be published elsewhere. One example of these SEM images is shown in Fig. 2. It can be seen that the printing process flattens the T6 layer, in contrast to the rough as-grown layer. This appears to be a case of a smoothing induced by the combination of pressure and temperature during printing [15]. In Fig. 3(a) an AFM image shows the topological

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Fig. 2. Scanning electron micrograph of a printed T6 layer showing the smooth printed lines (bright) compared to the rough as-grown layers (dark). Only the smoother parts were in contact with the stamp during printing.

contrast after printing. In Fig. 3(b) it is seen that the surface of the T6 film is not flattened because the recess on the stamp was higher than the film thickness leaving a ‘rough’ surface and the stamp features were printed through the film. The smooth substrate exhibits no detectable thin film residues. Fig. 4(a) shows a stamp of chromium on silicon with feature height of 10 nm. Fig. 4(b) shows a

Fig. 3. (a) AEM image of 200-nm wide lines with 800-nm spacing and (b) a SEM image of a cross-section of the pattern. The surface of the T6 film is not flattened as the lines on the stamp where higher than the film thickness.

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Fig. 4. (a) AFM scan of an interdigitated Cr structure on silicon with a native oxide. (b) AFM scan of a print from a 10-nm high Cr on Si stamp in a T6 film of a nominal thickness of 10 nm.

print into a nominally 10-nm thick T6 film. The interdigitated finger structures are clearly transferred, although there is a residual roughness along the lines arising from the granularity of the as-grown film. The overlap of the interdigitated fingers is | 2 mm. A print of 200-nm deep silicon lines into a 1-mm thick PBAS film is shown in Fig. 5. The stamp features were transferred with high accuracy. The PBAS has to be cross-linked in order to increase its viscosity, otherwise it could not maintain the printed structure over a few hours. However, after a trial

Fig. 5. SEM image of a print in a 1-mm thick PBAS film with a 200-nm deep silicon stamp. The polymer was cross-linked with a 17-s UV exposure.

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17-s exposure to UV radiation, the shape of the printed wires became rounded. The optimum conditions for cross-linking PBAS while retaining the nanoimprinted pattern remain to be determined. Current–voltage characteristics of a PBAS film on ITO contacted with a Ca–Al top electrode were measured at ambient conditions in an attempt to determine if the polymer was conductive. Preliminary I–V curves superimposed onto each other confirm hysteresis, non-linearities and partial rectification, probably due to non-optimum contacts. However, these I–V curves confirm the conducting nature of PBAS. To the best of our knowledge the I–V characteristics of this kind of polymers have not yet been investigated and work is still in progress to quantify conduction data.

4. Discussion The experiments in NIL of T6 thin films have shown that the morphology of the film can be modified, giving rise to ‘smooth’ and rough patches on the surface following the stamp features. This augurs well for future work of growing T6 thin films onto templated or patterned substrates provided the feature size is comparable with the size of single domains. The contrast could be potentially used for marking out areas of T6 on a substrate, with well-defined optical and electrical properties since they depend on morphology [12]. PDAS and PBAS seem to be promising conducting polymers which, given the rod-like nature of their chains, give grounds to expect that their mobility will improve further with ordering. Much work remains to be done to characterize fully these polymers, in particular the extent to which cross-linking has to be carried out to retain the nanoimprinted features once the stamp is removed. Concerning device configurations, there are clear problems in depositing metal electrodes onto the active polymer film (top-contact approach) because the electrical properties of the thin film may be compromised. On the other hand, depositing the active polymer thin film onto a patterned substrate with source and drain on an insulating layer, appears a better proposition. Nevertheless, problems of depositing T6 molecules on the rounded edges of as well as onto the electrodes may give rise to unwanted contact resistances. One variation may be to use cheaply produced stamp with metal contacts on it. Then the device would be imprinted but stamp and substrate would not be detached and together would form the device. Given the possibility of fabricating metal contacts on transparent materials, these electrodes could protrude from the edges and thus be contacted to an external circuit.

5. Conclusions It has been shown that printing on thin films of T6 results in spatial modulation of roughness of the as-deposited film, where the printed areas exhibit a smoother surface. By carefully choosing the thickness of the T6 film and the depth of the stamp features, one could control the thickness of the residual layer down to tens of nanometers. This may be relevant for applications where single domains are needed. Ideally, one would like to have FETs with nanoelectrodes separated by a distance comparable to a single domain in order to reach high mobility values without the influence of grain boundaries. At present, we are able to print interdigitated gold nanoelectrodes with 75-nm separation and we are currently exploring the growth of T6 films on these patterns.

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PDAS and PBAS have been printed in order to study their prospects as conducting polymers. Preliminary I–V curves of PBAS suggest this is a conducting material.

Acknowledgements This work is supported by the EU Growth project G5RD1-2000-CT00349 MONALISA, the German Research Council (DFG) and the CNR-Progetto Coordinato Nanotecnologie ‘SCRIBA’.

References [1] For a recent review, see, for example C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res. Dev. 45 (2001) 11. [2] M. Cavallini, M. Murgia, F. Biscarini, Nano Lett. 1 (2001) 193. [3] For a recent review, see, for example H.-C. Scheer, H. Schulz, T. Hoffmann, C.M. Sotomayor Torres, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, Vol. 5, Academic Press, San Diego, California, 2002, pp. 1–60. [4] C. Kim, P.E. Burrows, S.R. Forrest, Science 288 (2000) 831. [5] J. Wang, X. Sun, L. Chen, S.Y. Chou, Appl. Phys Lett. 75 (1999) 2767. [6] J. Seekamp, S. Zankovych, T. Maka, R. Slaby, C.M. Sotomayor Torres, C. Boettger, C. Liguda, M. Eich, B. Heidari, L. Montelius, J. Ahopelto (in preparation). ¨ [7] T. Makela, T. Haatainen, J. Ahopelto, H. Isolato, J. Vac. Sci. Technol. B 19 (2) (2001) 487. [8] See, for example D. Fichou (Ed.), Handbook of Oligo- and Polythiophenes, Wiley, VCH, Darmstadt, 1999. [9] E. Hattemer, M. Brehmer, R. Zentel, E. Mecher, D. Muller, K. Meerholz, Polymer Preprints 41 (1) (2000) 785. [10] M. Behl, E. Hattemer, M. Brehmer, R. Zentel, Macro. Chem. Phys. 203 (2002) 503. ¨ A. Dodalabapur, Ch. Kloc, B. Batlogg, Science 290 (2000) 963. [11] J.H. Schon, [12] M. Muccini, M. Murgia, F. Biscarini, C. Taliani, Adv. Mater. 13 (2001) 355. [13] F. Biscarini, P. Samori, O. Greco, R. Zamboni, Phys. Rev. Lett. 78 (1997) 2389. [14] M. Behl, R. Zentel, J. Seekamp, S. Zankovych, C.M. Sotomayor Torres (in preparation). ´ P. Moretti, P. Samori, F. Biscarini, Adv. Mater. 10 (1998) 57. [15] P. Viville, R. Lazzaroni, J.L. Bredas,