The influence of the substrate thermal conductivity on scanning thermochemical lithography Marten Tolk, Oliver Fenwick, Sadi Ahmad, and Franco Cacialli Citation: J. Appl. Phys. 111, 124317 (2012); doi: 10.1063/1.4729809 View online: http://dx.doi.org/10.1063/1.4729809 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i12 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 111, 124317 (2012)
The influence of the substrate thermal conductivity on scanning thermochemical lithography Marten Tolk, Oliver Fenwick, Sadi Ahmad, and Franco Caciallia) Department of Physics and Astronomy, and London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom
(Received 18 January 2012; accepted 19 May 2012; published online 22 June 2012) We present a joint experimental and computational study of the role of the substrate thermal conductivity on scanning thermochemical lithography (SThL) of thin organic films. We aim this study at lithography of the luminescent conjugated polymer poly(p-phenylene vinylene) (PPV) from its soluble precursor poly(p-xylene tetrahydrothiophenium chloride) (PXT), but our results provide relevant insights into the SThL of thermosensitive polymers in general, and into a wide range of nanoscale thermal and thermochemical processes in thin films. As high thermal conductivity substrates we used gold films on silicon, and indium-tin oxide (ITO) films on glass, successfully patterning PPV on both substrates. We find that a higher probe temperature (>300 C instead of 250 C) is necessary for lithography of PXT films on ITO compared to those on fused silica (for the same scanning speed and comparable precursor thickness). Surprisingly, however, our experiments show that minimum feature sizes are nearly independent of the underlying substrate. While a lateral resolution (full width at half maximum, FWHM) of 37 nm was achieved previously on fused silica for a 40 nm thick PXT film, we obtain here a FWHM of 36 nm for a 35 nm thick PXT layer on ITO. We compare our experiments with finite element simulations and gain further insight into the possibilities of thermochemical lithography, the necessary minimum probe temperature and the highest attainable resolutions. The model shows that for high thermal conductivity substrates there should be a region of unconverted polymer near the polymer-substrate interface. Our experiments demonstrate that patterned features are able to adhere to the substrate despite this unconverted layer, thus allowing SThL to work on very high thermal conductivity substrates such as gold. Our model builds on this experimental finding and accounts for the experimental lack of dependence of lateral size with substrate conductivity, i.e. it predicts that the minimum feature size increases only slightly for increasing thermal conductivities of the C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729809] substrates. V
I. INTRODUCTION
Scanning thermochemical lithography (SThL)1–5 is a versatile nanopatterning technique where a hot probe is scanned across a surface to induce a local chemical reaction in a thin film to generate the desired pattern. In general, nanolithography is of crucial importance in today’s science and industry, the most prominent application of nanopatterning techniques being in electronics, for fabrication of nm-sized transistors. In addition, novel and emerging areas of application include the development of nanosensors and of nanoelectromechanical systems (NEMS). To fabricate devices incorporating components with dimensions of only few tens of nm a variety of techniques are available, including “conventional” ones such as optical far-field,6,7 electron-beam or focused-ion beam (FIB) lithography,8,9 and less conventional techniques such as scanning near-field optical,10–13 thermal,14 thermomechanical,15,16 and thermochemical lithography.1–5 SThL is particularly appealing because it does not suffer from the resolution limitations imposed by the Abbe diffraction limit in far-field photolithography, or from the irradiation a)
E-mail:
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0021-8979/2012/111(12)/124317/8/$30.00
damage caused by e-beam lithography.17,18 High resolution SThL probes are readily machined in silicon, and are more robust than probes used in scanning near-field optical lithography (SNOL).11,13 Although all scanning probe techniques are typically “serial” in nature, their throughput can be “upscaled” by using an array of probes, as demonstrated by IBM.15 SThL is remarkably versatile and has already been used to crosslink commercially available photoresists,2 to convert a tetrahydropyranyl analogue from hydrophilic to hydrophobic,3 to reduce graphene oxide,4 and to create threedimensional structures within a thin layer of a molecular glass,5 with resolutions down to 15 nm (Ref. 5) and a throughput5 of 5 104 lm2/h or scanning speeds up to 1.4 mm/s.3 Previously, we have used SThL to prepare nanostructures of the prototypical electroluminescent polymer, poly(p-phenylene vinylene), PPV, from its soluble precursor, poly(p-xylene tetrahydrothiophenium chloride), PXT, deposited on fused silica substrates. In this process, we scan our heated probe, a Wollaston wire, across the polymer film selectively converting the precursor PXT to insoluble PPV. A subsequent rinsing step removes the unconverted PXT leaving isolated lines of polymer on the surface. A final thermal annealing ensures complete conversion of the remaining material to PPV. Surprisingly, we could achieve minimum
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resolutions of better than 28 nm,1 even though the radius of curvature of our probes was several orders of magnitude bigger (2.5 lm). This was due to the combined effects of a careful control of scanning speed and probe contact pressure, to the dissolution of partially converted precursor during development (rinsing), and also to the shrinking of the polymer precursor in its conversion to PPV. The question arises as to whether a significant change in the substrate thermal conductivity would influence the details of the conversion process and therefore the minimum resolution attainable. To a first order approximation, if the tip temperature remained constant we would expect a decrease of the feature size for an increase of the substrate thermal conductivity k because the increased rate of heat removal from the bottom of the film would provide a better “vertical” confinement of the thermal fields and avoid their lateral spread. However, a higher thermal conductivity of the substrates would also impose lower temperatures near the film-substrate interface, and thus require a higher tip temperature to ensure film conversion and anchoring at such interface, hence resulting in a lateral heat spread and deteriorated resolution. Making predictions on which of these two antagonistic effects will dominate and by which factor the lateral resolution changes is far from trivial, due to both the complexity of the mechanical contact between the tip and the sample, and also to an incomplete understanding of the effects of development and post-baking on the feature size. Here, we gain further insight into these issues by replacing the fused silica substrate with indium-tin oxide (ITO)-coated glass, one of the most common transparent electrodes/substrates in organic electronics. The thermal conductivity of ITO ranges from 3.1 W m1 K1 for very thin films19 to bulk conductivities20 of 14 W m1 K1, which is significantly higher than that of fused silica21 (k ¼ 1.4 W m1 K1). Surprisingly, we find that the lateral resolution, here defined as the full width at half maximum (FWHM) of the lithographed features, is essentially independent of the thermal conductivity of the layer underneath the precursor (which we will also term the “interlayer” from now on). We corroborate these observations with experiments on gold-coated Si wafers (kgold ¼ 317 W m1 K1).22 We then model the conversion process by using the simulated temperature profile and show how the final resolution is affected by the competition of the two opposing effects discussed above.
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II. METHODS A. Experimental methods
Our thermochemical lithography setup has been described in detail in Ref. 1 and utilizes as the hot probe a so-called Wollaston wire23,24 (Bruker) of the type employed in micro-thermal analysis.25,26 It is a 75 lm diameter silver wire which is etched, exposing a 5 lm diameter platinumrhodium (9:1) core that is bent around to from a probe (schematically illustrated in Fig. 1(a)) that can be mounted onto the head of an atomic force microscope, AFM. The probe was scanned in constant-force contact mode across the surface of a 35 nm thick PXT film which was spin-coated from a water solution onto an oxygen plasmatreated27 ITO-coated glass substrate. The chemical structure of PXT (purchased as a 0.25 wt. % water solution from Aldrich) and PPV is illustrated in Fig. 1(b). The ITO film is 120–160 nm thick as specified by the supplier. Various temperatures, writing speeds, and line densities were used while the force and feedback parameters were constant throughout the experiments. We have previously noted1 that high resolutions with this technique rely on avoiding mechanical deformation of the polymer film surface by the probe, and therefore use the minimum necessary contact force to keep the probe in contact with the surface. This force was found to be 2 lN. After patterning, the samples were rinsed in methanol for 10 s to remove the unconverted precursor and annealed in a vacuum oven at 200 C and 0.
IV. DISCUSSION
2. Factors affecting feature adhesion
We start our discussion from the temporal evolution in Fig. 3(a). As already mentioned in the previous section, this indicates that heat transfer occurs essentially on a ls-timescale. For “pixel exposure times” that are significantly larger than this, e.g., in the ms regime (as in our case), we can then use the simplified form of Eq. (1) reported as the time-independent Eq. (2), and it also follows that the specific heat capacity c and material density q should not influence size and shape of the final lithographed pattern. Therefore, we concentrate solely on the thermal conductivity k of the materials. Furthermore, the fact that point B (at the base of the film) is approximately at room temperature in the case of gold means that a further increase in k will not change the curve significantly. Hence, gold can be considered as a limit case representing k-values from kgold to 1.
Such factors (which we also already mentioned briefly in the results section) are: (a) the interlayer/substrate surface roughness, (b) entanglement between polymer chains in the converted and unconverted region, and (c) electrostatic interaction of the precursor polymers with the interlayer ITO. Surface roughness in the range of a few nm can already give a sizable contribution to the reduction of the “effective dz” because non-conformal coverage of the rough surface (so as to form a flat top surface, as it is commonly observed) would result in a spatially non-uniform film thickness, with local minima a few nm smaller than the thickness assumed in the model. Furthermore, it is conceivable that the precursor polymer chains would more easily get entangled and adsorbed via a range of physico-chemical interactions with the nanocavities offered by the rough surface.
Our experiments show that the lithographed patterns are not rinsed away during development, even for those process parameters for which the model predicts dz > 0, i.e., a finite vertical distance between substrate and conversion boundary (indicating an unconverted region at the base of the film), as a consequence of the relatively low temperature of the interlayers with high thermal conductivity (e.g., for Au TB room temperature). This is surprising and in contrast to a model of SNOL of the PPV precursor, for which reasonable agreement was found when assuming that the UV-dose at point B determines the resolution.13 1. Thermal conductivity of the polymer
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In point (b), we need to consider the effect of polymer chain entanglement at the boundary between the converted and unconverted region. While it might be argued that this should be of the order of the polymer gyration radius (3–4 nm for some soluble PPVs, which should provide a reasonable model for PXT),1 we cannot rule out a value a few nanometers larger than that (up to an extreme value of 10 nm for a fully elongated strand).1 This higher value could in fact be induced by the significant uncoiling and straightening of the chains taking place near and on both sides of the conversion boundary, as a result of the conversion of the single to double bonds. There is concomitant volume reduction associated with the conversion28 (due to elimination of the tetrahydrothiophenium) that (by definition) implies local mass transport either vertically or laterally and may affect the true value of dz. In point (c), we note that we have observed that a monolayer of PXT can be electrostatically bound to the ITO surface which has been treated by an oxygen plasma. The oxygen plasma leads to the formation of a dipole layer on ITO via the oxidation of surface SnIV–OH to surface SnIV– O.27,34 PXT acts as a polyelectrolyte in water where the chains are positively charged and compensated by Cl counterions, and the positively charged chains can adhere to SnIV–O surface groups. As a final point, we note that conversion of PXT to PPV releases HCl,35 which has in the past been proposed to etch metallic or ITO substrates with formation of the relevant salts. Although HCl should be evolved mainly towards the top of the film, at distances of >10 nm or so from the boundary with ITO (or Au), diffusion downwards may enable HCl to reach the interlayer. These salts may locally increase surface roughness or introduce polar interactions that would aid adhesion of the features. None of these effects question the finite value of dz, but instead confirm that the predicted dz > 0 actually captures an important aspect of the physics of this process, and ultimately also explain why we can anchor nanopatterns on gold, despite its very low surface temperature (TB room temperature). With this in mind, we can now look at the implications that the scenario of adhesion in the dz > 0 regime has for the ultimate performance of SThL on a range of substrates. 3. Minimum probe temperature
To do this, we first extract the maximum dz which should ensure the patterns are anchored to the surface (dzmax) by combining the results of this model with our previous experimental results. To this end, we plot dz as a function of Ttip in Fig. 3(d), as predicted by the model. In our previous experiments,1 we found that the lowest tip temperature (Ttipmin) necessary for anchoring was 250 C, on fused silica and for a writing speed of 20 lm/s and a similar film thickness. Putting these parameters into our model, we obtain dzmax ¼ 11.7 nm and plot this as a thick (red) horizontal line in Fig. 3(d). The intersection of this line with the ITO curve gives Ttipmin ¼ 335 C, which is in good agreement with the experimental observation that 300 C was too low a temperature for writing speeds 10 lm/s, whereas we could write very faint structures at 350 C for speeds up to 80 lm/s.
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This remarkable agreement between experiment and model strengthens our assertion that we do not need to convert to the base of the film to achieve high resolution SThL. Under the same conditions, the model predicts Ttipmin 393 C for gold, and furthermore our modeling shows that increasing the thermal conductivity beyond that of gold has almost no effect on dz. We can therefore conclude that SThL of this material system is never limited by the thermal conductivity of the substrate. At this point, we note that the thermal conductivity of a thin film is usually smaller than that of the same material in the bulk. For 200 nm gold films, k may be reduced by a factor of about 0.5 compared to the bulk value,22 which, according to our model, results in changes of dz of less than 0.1 nm and changes in dr of no more than 0.2 nm for Ttip between 250 and 450 C. B. Minimum lateral feature size 1. Minimum feature size for a fixed probe temperature
In Fig. 3(e), we see that the lateral distance r0 from the edge of the probe-polymer contact to the conversion boundary is 10.4, 12.8, and 18.9 nm for gold, ITO and fused silica, respectively, for a probe temperature of Ttip ¼ 350 C. Hence, a higher k actually leads to a smaller width of converted material if Ttip remains constant. dr, which was defined as FWHM 2 r0, will serve as an indicator for the expected minimum feature size f that can be estimated by f ¼ dr þ w0, where w0 is the experimental probe-polymer contact width. 2. Minimum feature size for the substrate dependent minimum probe temperature
The question now remains if the need for a higher Ttip for substrates with a higher k will overcompensate the improvement of the resolution that we would expect for a constant Ttip. To examine this further we plotted dr as a function of Ttip in Fig. 3(f). The (red) circles mark the intersections between the relevant curves for dr and the smallest necessary tip temperatures Ttipmin for feature adhesion (which we obtained from dzmax ¼ 11.7 nm from Fig. 3(d)), and therefore provide us with the corresponding, substrate dependent drmin. Since drmin increases with k, the model predicts that the resolution will deteriorate for higher thermal conductivity substrates. Quantitatively, f is expected to increase with respect to fused silica by 10.3 nm and 12.6 nm for ITO and gold, respectively, for a 35 nm precursor layer. Experimentally, we found a minimum FWHM of 37 nm on a 40 nm thick precursor film on fused silica and 36 nm on a 35 nm thick precursor film on ITO, but with evident anchoring problems (Fig. 2(c)). If we therefore consider that the FWHM of the smallest well-anchored feature on ITO lies somewhere between the 36 nm from Fig. 2(c) and the 65 nm from Fig. 2(b), we find a reasonable agreement with the model, which predicted 47 nm (37 nm on fused silica þ 10.3 nm upon switching to ITO). We note that our model required only the input of the experimental value of the Ttip necessary to obtain the
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minimum FWHM of adhered, lithographed features on a certain substrate interlayer (which we denote fopt, with the interlayer being silica in our case), to enable determination of the missing parameters dzmax (e.g., from Fig. 3(d)) and the “true” contact width w0 (as fopt 2 dr, determined with the help of Fig. 3(f)) for that particular type of substrate interlayer. These two parameters are then sufficient to predict the minimum feature sizes on arbitrary substrate interlayers (such as ITO and Au, as in the present work). 3. Limitations of the model
We have shown that the lateral resolution achieved in the experiment can be reproduced with the model after accounting for the difference between the modeled contact width (2 r0) and the “real” contact width w0, a parameter that can be extracted from experiments. This adjustment is necessary due to several effects causing a change in feature sizes that have not been introduced in the model yet, most importantly: (a) the writing speed dependent penetration depth and hence contact width that is caused by the viscoelasticity of the polymer, (b) the observed difference between the probe indentation width before development and the postdevelopment feature size (see supplementary information of Ref. 1), and (c) how the shrinking of the polymer (due to the elimination of the tetrahydrothiophene group during conversion) influences the structure during the writing process. The effect of post-baking on the other hand has been investigated for PPV structures1,13 and it was shown1 that whereas the height of the features written by SThL shrinks by about 30%, the width (FWHM) is almost unchanged. In the supplementary information, we give further data on the influence of the modeled contact width (2 r0) on values of dz and dr. (d) Another aspect that has not been considered so far is that the final structures seem to be wider at the bottom than at the top (even after accounting for AFM tip convolution effects), which is in contrast to the shape of the converted volume that follows from the simulated steady state temperature distribution (Fig. 3(g)). A model for this apparent collapse of the converted polymer onto the substrate has previously been fitted to experimental data in the case of SNOL.36 The effect of film thickness has not been investigated here in detail. General trends, however, are that a higher film thickness requires a larger Ttip to obtain the necessary dzmax. This will in turn increases the lateral heat spread and therefore increases the minimum feature size. (e) We also investigated the influence of thermal contact resistances at the various interfaces by modeling a thin, thermally resistive air-layer in between the respective interface. The results are shown in full in the supplementary information, but we summarize here that thermal contact resistance at the glass-interlayer interface and at the polymer-interlayer interface is negligible as long as it does not exceed values equivalent to a 20 nm air gap. The probe-polymer interface is more sensitive to a thermal contact resistance but even then we find that an effective 10 nm thick air-gap changes dz only by up to 1 nm and dr by up to 3 nm. Despite these margins for improvement, our model captures the fundamental physics of SThL and, most importantly,
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provides quantitative explanations for both the almost substrate independent minimum resolution as a result of two counteracting effects, and the ability to retain lithographed patterns on very high k interlayer/substrates, such as gold, for which the temperature at the base of the film is both intuitively expected and quantitatively predicted to be close to room temperature. V. CONCLUSIONS
In summary, we have shown that thermochemical lithography of the precursor of the conjugated polymer PPV is possible on substrates spanning three orders of magnitude in thermal conductivity. We achieve a maximum resolution (FWHM) of 36 nm on a 35 nm thick precursor layer on ITO, which is almost identical to the published FWHM of 37 nm for a 40 nm precursor film achieved when using fused silica substrates.1 Finite element simulations predict a “conversion boundary” that is several nm away from the film base (dz > 0) in the experimental conditions for highresolution features. Taken together with the experiments confirming that the patterns can still stick to the substrate during development, this result provides clear evidence of additional adhesion mechanisms between films and interlayer/substrate surfaces. We propose that adhesion through a thin layer of the “unconverted” precursor polymer is made possible by a combination of non-covalent secondary interactions such as polar interactions of the polycationic chain strands with the substrates, chains entanglement, and enhanced roughness effects. This explains why SThL is also possible on gold, which features such a high thermal conductivity that the temperature near its polymer interface is not high enough to convert the polymer in that region. The model further predicts that although a higher thermal conductivity substrate is, everything else being constant, expected to lead to smaller feature sizes, the achievable lateral resolution is expected to deteriorate slightly upon increasing k due to the higher probe temperatures required. Nevertheless, the difference of the FWHM between fused silica and gold is predicted to be only 12 nm. From a technological point of view, we showed that SThL is an interesting technique that can achieve nanoscale resolutions on a range of substrates and therefore applications.
ACKNOWLEDGMENTS
We thank the RS and the EC for funding of the RTN THREADMILL (EU-Contract: MRTN-CT-2006-036040), of the ITN SUPERIOR (PITN-CT-2009-238177), as well as the EC Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 212311 (ONE-P). 1
O. Fenwick, L. Bozec, D. Credgington, A. Hammiche, G. M. Lazzerini, Y. Silberburg, and F. Cacialli, Nat. Nanotechnol. 4, 664 (2009). 2 A. S. Basu, S. McNamara, and Y. B. Gianchandani, J. Vac. Sci. Technol. B 22, 3217 (2004). 3 R. Szoszkiewicz, T. Okada, S. C. Jones, T. D. Li, W. P. King, S. R. Marder, and E. Riedo, Nano Lett. 7, 1064 (2007). 4 Z. Wei, D. Wang, S. Kim, S.-Y. Kim, Y. Hu, M. K. Yakes, A. R. Laracuente, Z. Dai, S. R. Marder, C. Berger, W. P. King, W. A. de Heer, P. E. Sheehan, and E. Riedo, Science 328, 1373 (2010).
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D. Pires, J. L. Hedrick, A. D. Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Du¨rig, and A. W. Knoll, Science 328, 732 (2010). 6 A. K.-K. Wong, Resolution Enhancement Techniques in Optical Lithography (SPIE—The International Society for Optical Engineering, Washington, 2001). 7 G. Gigli, R. Rinaldi, C. Turco, P. Visconti, R. Cingolani, and F. Cacialli, Appl. Phys. Lett. 73, 3926 (1998). 8 C. Vieu, F. Carcenac, A. Pe´pin, Y. Chen, M. Mejias, A. Lebib, L. ManinFerlazzo, and H. L. L. Couraud, Appl. Surf. Sci. 164, 111 (2000). 9 A. E. Grigorescu and C. W. Hagen, Nanotechnology 20, 292001 (2009). 10 E. Betzig and J. K. Trautman, Science 257, 189 (1992). 11 R. Riehn, A. Charas, J. Morgado, and F. Cacialli, Appl. Phys. Lett. 82(4), 526 (2003). 12 L. P. Ghislain, V. B. Elings, K. B. Crozier, S. R. Manalis, S. C. Minne, K. Wilder, G. S. Kino, and C. F. Quate, Appl. Phys. Lett. 74, 501 (1999). 13 D. Credgington, O. Fenwick, A. Charas, J. Morgado, K. Suhling, and F. Cacialli, Adv. Funct. Mater. 20, 2842 (2010). 14 E. Scha¨ffer, S. Harkema, M. Roerdink, R. Blossey, and U. Steiner, Adv. Mater. 15, 514 (2003). 15 U. Du¨rig, G. Cross, M. Despont, U. Drechsler, W. Ha¨berle, M. I. Lutwyche, H. Rothuizen, R. Stutz, R. Widmer, P. Vettiger, G. K. Binnig, W. P. King, and K. E. Goodson, Tribol. Lett. 9, 25 (2000). 16 B. Gotsmann, U. Du¨rig, J. Frommer, and C. J. Hawker, Adv. Funct. Mater. 16, 1499 (2006). 17 Y. Doi, A. Saeki, Y. Koizumi, S. Seki, K. Okamoto, T. Kozawa, and S. Tagawa, J. Vac. Sci. Technol. B 23, 2051 (2005). 18 A. Tilke, M. Vogel, F. Simmel, A. Kriele, R. H. Blick, H. Lorenz, D. A. Wharam, and J. P. Kotthaus, J. Vac. Sci. Technol. B 17, 1594 (1999). 19 Y. Takashi, T. Kimiaki, S. Yasushi, T. Naoyuki, B. Tetsuya, and S. Yuzo, J. Vac. Sci. Technol. A 23(4), 1180 (2005).
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M. Schlott, W. Dauth, M. Kutzner, B. Gehman, and S. Vahlstrom, U.S. patent 6,187,253 (2001). 21 D. H. Damon, Phys. Rev. B 8, 5860 (1973). 22 G. Chen and P. Hui, Appl. Phys. Lett. 74, 2942 (1999). 23 M. Reading, D. M. Price, D. B. Grandy, R. M. Smith, L. Bozec, M. Conroy, A. Hammiche, and H. M. Pollock, Macromol. Symp. 167, 45 (2001). 24 V. V. Gorbunov, N. Fuchigami, J. L. Hazeland, and V. V. Tsukruk, Langmuir 15, 8340 (1999). 25 E. Gmelin, R. Fischer, and R. Stitzinger, Thermochim. Acta 310, 1 (1998). 26 A. Majumdar, Annu. Rev. Mater. Sci. 29, 505 (1999). 27 J. S. Kim, F. Cacialli, and R. Friend, Thin Solid Films 445(2), 358 (2003). 28 J. Morgado, F. Cacialli, J. Gruner, N. C. Greenham, and R. H. Friend, J. Appl. Phys. 85, 1784 (1999). 29 D. V. Widder, The Heat Equation (Academic, New York, London, 1976). 30 H. V. Shah and G. A. Arbuckle, Macromolecules 32(5), 1413 (1999). 31 J. Jin, M. P. Manoharan, Q. Wang, and M. A. Haque, Appl. Phys. Lett. 95, 033113 (2009). 32 B.-Y. Lu, C.-C. Liu, S. Lu, J.-K. Xu, F.-X. Jiang, Y.-Z. Li, and Z. Zhang, Chin. Phys. Lett. 27, 057201 (2010). 33 Y. Hiroshige, M. Ookawa, and N. Toshimab, Synth. Met. 157, 467 (2007). 34 D. J. Milliron, I. G. Hill, C. Shen, A. Kahn, and J. Schwartz, J. Appl. Phys. 87, 572 (2000). 35 J. Morgado, D. S. Thomas, R. H. Friend, and F. Cacialli, Synth. Met. 111, 549 (2000). 36 D. V. Cotton, C. J. Fell, W. J. Belcher, and P. C. Dastoor, J. Phys. D: Appl. Phys. 41, 195107 (2008). 37 See supplementary material at http://dx.doi.org/10.1063/1.4729809 for further simulation data and further lithography examples.
Supplementary Information Paper title:
The
influence
of
the
substrate
thermal
conductivity
on
scanning
thermochemical lithography Authors:
Marten Tolk, Oliver Fenwick, Sadi Ahmad and Franco Caciallia)
Affiliations:
Department of Physics and Astronomy, and London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom
Email: a)
[email protected]
I. Conversion ratio SUPPLEMENTARY FIG. 1 shows the conversion ratio α , i.e. the ratio of converted to initially unconverted precursor monomers, as a function of temperature T and exposure time t as calculated from the Arrhenius equation (Eq. 1). The pre-exponential factor (A = 1019/min ) and the activation energy (Ea = 128 kJ/mol) for this conversion reaction was taken from the literature1. − Ea ⎛ ⎞ α = 1 − exp ⎜ − Ae RT t ⎟ , ⎜ ⎟ ⎝ ⎠
(1)
1
SUPPLEMENTARY FIG. 1. Visualization of the Arrhenius equation (Eq. 1) applied to the precursor poly(p-xylene tetrahydrothiophenium chloride).
II. Probe motion SUPPLEMENTARY FIG. 2 shows the path of a probe while scanning over a surface at 450°C and 80 µm/s. The hot probe approaches the surface at position A, moves up (trace) and down (retrace) along the fast scan axis, then moves right along the slow scan axis and again up and down along the fast scan axis, etc. At the end of the scanning process, the probe returns to its initial/final position A along the path L. Due to the longer exposure time of the precursor in the proximity of A, a considerable amount of polymer has been converted in that region. The images shows that especially at high writing speeds of 80 µm/s and faster, there can be a gap between trace and retrace of the probe. We see that the trace-retrace gap becomes smaller with further distance from the trajectory along the slow scan axis.
2
SUPPLEMENTARY FIG. 2. AFM image of a patterned ITO substrate at 450°C and 80 µm/s showing the motion of the probe while scanning across a surface. In particular one can see a trace-retrace gap which becomes smaller towards the top of the image.
III. Thermal lithography on gold SUPPLEMENTARY FIG. 3 shows PPV lines written across a SiO2/gold interface. Because the
lines are flat compared to the interface step (200 nm), a gradient image of (a) is shown in (b) to better distinguish the lines from the substrate. One can also see how the PPV line is broken just above the interface, which is a result of the rather large radius of curvature of the probe in direction of the fast scan axis.
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SUPPLEMENTARY FIG. 3. (a) AFM image of PPV lines written across a silicon oxide (SiO2) – gold interface at 400°C at 20 µm/s with an initially 20 nm thick precursor layer. The evaporated gold layer is ≈ 200 nm thick. (b) Gradient image (i.e. an image of the height gradient at each point) of (a) for better visualization of the lines.
IV. Influence of thermal contact resistance We investigated the influence of thermal contact resistance at the various interfaces (substrate/interlayer, polymer/interlayer and probe/polymer) on the results of the model by changing the boundary condition of the respective interface from 'continuity' to a 'thin thermally resistive layer' with the thermal conductivity of air (kair) and thickness tair. Note that the geometry does not change and dz and dr keep their original meanings, as the contact resistance effectively originates from an infinitely thin layer of finite thermal contact resistance, leading to a temperature drop across the interface. kair is set to the function given by the Comsol material library: kair = (-2.2758·10-3 + (T/K) · 1.1548·10-4 + (T/K)² · (-7.90253)·10-8 + (T/K)³ · 4.11703·10-11 + (T/K)4 · (-7.4386)·10-15) W m-1 K-1.
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In SUPPLEMENTARY FIG. 4, we plot dz and dr as a function of tair for a tip temperature (Ttip) of 350 °C. We find that an imperfect contact between the interlayer and the underlying glass substrate (see SUPPLEMENTARY FIG. 4(a)) has no influence as long as the effective air-gap is thinner than ~20 nm. Note that fused silica substrates do not feature a substrate/interlayer interface as the material is the same for substrate and interlayer. Nevertheless, for completeness the additional thermal contact resistance was modeled also in this case. A thin air-layer between polymer and interlayer (see SUPPLEMENTARY FIG. 4(b)) has a similar influence on dz as a thin air-layer between glass and interlayer, but its influence on dr is larger. Note, however, that we expect the effective air-gap to be small for all interlayers because first, surface roughnesses are in the nm range and second, the precursor polymer (PXT) solution is liquid during its deposition (via spin-coating) and is hence expected to effectively fill any gaps. Although we also find that a thermal resistance between the tip of the probe and the polymer (see SUPPLEMENTARY FIG. 4(c)), has, by far, the strongest influence on the lateral resolution and on dz., this is still relatively small in absolute terms. I.e. for an air-gap of 10 nm, dz changes by up to 1 nm and dr changes by up to 3 nm. At tair larger than ~ 300 nm, the probe cannot convert material inside the polymer film anymore, so that dz approaches 32 nm, i.e. the polymer film thickness (35 nm) subtracted by the probe penetration depth (3 nm at r = 0). At this point, dr approaches an interlayer-independent value of -39 nm.
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SUPPLEMENTARY FIG. 4. Diagrams showing the influence of the addition of an air-layer between (a) the glass/interlayer interface, (b) the polymer/interlayer interface, and (c) the tip/polymer interface for a tip temperature Ttip of 350°C. Values of dz (black, filled symbols) and values of dr ( red, open symbols) are given as a function of the thickness of the thin thermally resistive air-layer (tair).
V. Influence of contact width Another uncertainty in the model is the contact width between the tip of the probe and the polymer (2 r0). The uncertainty stems from several effects, such as the writing-speed
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dependent penetration depth and hence contact width, the observed reduction in feature size upon development,2 and the lateral shrinking during the post-baking step2. Most of the modeling is done with a penetration depth of 3 nm and hence a contact width of 245 nm. A smaller contact width will reduce the temperature in the polymer at thermal equilibrium and thus increase dz and reduce dr. This is shown in the case of a 350 °C hot probe in 5(a). We also see that at very small contact widths (smaller than the polymer film thickness, here 35 nm), lateral heat diffusion in the polymer starts to dominate and dz becomes almost independent of the type of interlayer. We further show the effect of the tip temperature (Ttip) on dz and dr for different contact widths in case of indium-tin oxide (ITO) as the interlayer in 5(b). We see again that dz grows for smaller contact widths, further supporting the notion that we expect a region of unconverted polymer near the interlayer.
SUPPLEMENTARY FIG. 5. (a) dz and dr as a function of the probe-polymer contact width for a constant tip-temperature (Ttip) of 350 °C. (b) dz and dr as a function of Ttip for different contact widths (30, 60, 120 and 245 nm) in case of indium-tin oxide (ITO) as the interlayer.
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References 1 2
H. V. Shah and G. A. Arbuckle, Macromolecules 32 (5), 1413 (1999). O. Fenwick, L. Bozec, D. Credgington, A. Hammiche, G. M. Lazzerini, Y. Silberburg, and F. Cacialli, Nat. Nanotechnol. 4, 664 (2009).
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