Electroless Deposition of Cu on Glass and Patterning with

© Copyright 2003 American Chemical Society

AUGUST 19, 2003 VOLUME 19, NUMBER 17

Letters Electroless Deposition of Cu on Glass and Patterning with Microcontact Printing Emmanuel Delamarche,*,† James Vichiconti,‡ Shawn A. Hall,‡ Matthias Geissler,† William Graham,‡ Bruno Michel,† and Ronald Nunes‡ IBM Research, Zurich Research Laboratory, 8803 Ru¨ schlikon, Switzerland, and Advanced Display Technology Laboratory, IBM Research, T. J. Watson Research Center, Yorktown Heights, New York 10598 Received May 2, 2003. In Final Form: June 20, 2003 Electroless-depositing a metal from solution to a substrate and patterning it using microcontact printing is an alternative to the conventional patterning of vacuum-deposited metals using photolithography. Here, we pattern Cu onto 15 × 15 sq-inch glass substrates by (i) self-assembly of a thin layer of amino-derivatized silanes to the glass, (ii) binding Pd/Sn catalytic particles to the silanes, (iii) electroless deposition of ∼120 nm of Cu on the catalytic surface, (iv) microcontact printing hexadecanethiol on the Cu film using an accurate printing tool, and (v) selectively etching the printed Cu using hexadecanethiol as a resist. This method is particularly attractive for the fabrication of metallic gates for thin-film transistor liquid-crystal displays.

Electroless deposition (ELD)1 and microcontact printing (µCP)2-6 are techniques that, combined, could have a tremendous technological impact.7-9 With ELD, one can place metals or alloys on a variety of substrates without * To whom correspondence should be addressed. E-mail: emd@ zurich.ibm.com. † IBM Research, Zurich Research Laboratory. ‡ Advanced Display Technology Laboratory, IBM Research. (1) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (2) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (3) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550575. (4) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (5) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382-3391. (6) Tate, J.; Rogers, J. A.; Jones, C. D. W.; Vyas, B.; Murphy, D. W.; Li, W.; Bao, Z.; Slusher, R. E.; Dodabalapur, A.; Katz, H. E. Langmuir 2000, 16, 6054-6060. (7) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (8) Xia, Y.; Venkateswaran, N.; Qin, D.; Tien, J.; Whitesides, G. M. Langmuir 1998, 14, 363-371.

the need for vacuum deposition systems, and with µCP one can pattern an etch-protective monolayer on a metal. However, combining these techniques is far from trivial, and employing them for a specific application must be technologically relevant without being excessively difficult. Similarly to previous work using µCP to fabricate thinfilm transistors (TFTs),9-14 we are investigating the fabrication of gate layers for thin-film transistor liquidcrystal displays (TFT-LCDs) using ELD and µCP, and we report here the patterning with µCP of Cu electroless(9) Zschieschang, U.; Klauk, H.; Halik, M.; Schmid, G.; Radlik, W.; Weber, W. IEEE Polytronic 2002 Conference; 191-195. (10) Rogers, J. A.; Bao, Z.; Baldwin, K.; Dodabalapur, Crone, B.; Raju, V. R.; Kuck, V.; Katz, H.; Amundson, K.; Ewing, J.; Drzaic, P. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4835-4840. (11) Mach, P.; Rodriguez, S. J.; Nortrup, R.; Wiltzius, P.; Rogers, J. A. Appl. Phys. Lett. 2001, 78, 3592-3594. (12) Kagan, C. R.; Breen, T. L.; Kosbar, L. L. Appl. Phys. Lett. 2001, 79, 3536-3538. (13) Loo, Y.-L.; Someya, T.; Baldwin, K. W.; Bao, Z.; Ho, P.; Dodabalapur, A.; Katz, H. E.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10252-10256. (14) Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562-564.

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deposited on 15-in. glass plates. The ELD of Cu on glass builds on the chemistry we set to electroless-deposit on large glass plates a NiB gate layer with good homogeneity, sufficient adhesion, and very few defects such as required for building a TFT array.15 The low specific resistivity of Cu makes it an ideal material for fabricating thin, well-conducting layers on large substrates.16 Moreover, Cu can be microcontactprinted with alkanethiols and then etched selectively,17,18 but Cu is difficult to electroless-deposit onto glass and will pose problems in terms of integrating the various layers of a TFT. Integration problems with Cu as the gate material might be solved by employing a diffusion barrier between the Cu gates and the insulating and semiconducting parts of the TFT.19,20 It is not trivial to translate the ELD process from NiB to Cu. Cu is vastly more difficult than NiB to deposit onto glass with sufficient adhesion because (i) it is less favorable to form Cu-O bonds than Ni-O bonds, particularly at the expense of Si-O bonds from the glass substrate, (ii) the strong evolution of H2 while Cu is being plated21 adds to the stress of the Cu deposit and challenges its adhesion to the glass in the ELD bath, and (iii) the high pH of Cu ELD baths (generally g13) tends to hydrolyze the substrate and/or the layer grafted to the glass that holds the catalyst. It is desirable to keep the glass substrate well transparent and not to roughen it as is routinely done in ELD on polymers. A catalyst is necessary to initiate the ELD. Pd/Sn colloids, which are often employed for this purpose, are immobilized on the substrate first.22 We did so by grafting a homogeneous layer of 3-(2-aminoethylamino)-propyl-trimethoxysilane23,24 (EDA-Si) from a 1% solution in water onto the glass and then using the amino functionalities of the grafted layer to bind Pd/Sn colloids, Figure 1.15,25 Our approach to the general problem regarding the adhesion of Cu on glass was to use a bath that was not too alkaline (pH < 12.5) and slow enough to prevent too strong an evolution of H2 from occurring during plating. We used X-ray photoelectron spectroscopy to verify that diminishing the pH of a Cu ELD bath, from which Cu was omitted,26 lengthened the lifetime of the grafted EDA-Si and also helped the Pd/Sn particles remain on the glass longer (data not shown). However, diminishing the pH too far below 13 can lower the deposition rate of Cu considerably.27 Plating slowly, nearly at room tem(15) Delamarche, E.; Geissler, M.; Vichiconti, J.; Graham, W. S.; Andry, P. A.; Flake, J. C.; Fryer, P. M.; Nunes, R. W.; Michel, B.; O’Sullivan, E. J.; Schmid, H.; Wolf, H.; Wisnieff, R. L. Langmuir, in press. (16) Colgan, E. G.; Alt, P. M.; Wisnieff, R. L.; et al. IBM J. Res. Dev. 1998, 42, 427-444. (17) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (18) Delamarche, E.; Geissler, M.; Magnuson, R. H.; Schmid, H.; Michel, B. Langmuir, in press. (19) Fryer, P. M.; et al. J. Soc. Inf. Disp. 1997, 5, 49-52. (20) Krishnamoorthy, A.; Chanda, K.; Murarka, S. P.; Ramanath, G.; Ryan, J. G. Appl. Phys. Lett. 2001, 78, 2467-2469. (21) Bindra, P.; Roldan, J. J. Electrochem. Soc. 1985, 132, 25812589. (22) Dobisz, E. A.; Bass, R.; Brandow, S. L.; Chen, M.-S.; Dressick, W. J. Appl. Phys. Lett. 2003, 82, 478-480. (23) Chen, M.-S.; Brandow, S. L.; Dulcey, C. S.; Dressick, W. J.; Taylor, G. N.; Bohland, J. F.; Georger, J. H.; Pavelchek, E. K.; Calvert, J. M. J. Electrochem. Soc. 1999, 146, 1421-1430. (24) Chen, M.-S.; Dulcey, C. S.; Brandow, S. L.; Leonard, D. N.; Dressick, W. J.; Calvert, J. M.; Sims, C. W. J. Electrochem. Soc. 2000, 147, 2607-2610. (25) Geissler, M.; Kind, H.; Schmidt-Winkel, P.; Michel, B.; Delamarche, E. Langmuir, in press. (26) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (27) Duffy, J.; Pearson, L.; Paunovic, M. J. Electrochem. Soc. 1983, 130, 876-880.

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Figure 1. Patterning the Cu gate layer of an array of TFTs using ELD and µCP. A large glass substrate is derivatized first with amino-functionalized silanes to immobilize Pd/Sn colloids from solution. These colloids initiate the ELD of Cu, which is then microcontact-printed with alkanethiols and etched selectively.

perature, to minimize thermally induced stress28 in the deposited Cu layer and using a pH of 12.0 permitted the deposition of up to 300 nm of Cu across 15 × 15 sq-inch glass plates before blisters and failures occurred in the adhesion of the Cu to the glass.29,30 Another feature of this method is to graft EDA-Si on glass and deposit the Pd/Sn colloidal catalyst with high homogeneity and cleanliness to prevent having sites that are prone to failure of adhesion and blistering during the ELD process itself.15 Cu is plated on both faces of the glass plates. The unwanted Cu layer will not be microcontact-printed, however, but will be etched away during the selective etch step. The presence of this layer may be beneficial for keeping the stress of the plated glass substrate symmetric and self-compensating.15 We employed Cu layers having a thickness of ∼120 nm to obtain a gate layer which had relatively high conductivity31 and was not too thick for building a TFT array.32 Microcontact printing a pattern having micrometer-sized (28) Parker, K.; Shah, H. Plating 1971, 58, 230-236. (29) We employed a Cu Thru CUP bath from Uyemura, which contained formaldehyde as the reducing agent and had a plating rate of ∼40 nm min-1 at 35 °C. (30) An ELD Cu bath operating at a pH e 9 and a temperature between 45 and 65 °C (see for example: Jagannathan, R.; Krishnan, M. IBM J. Res. Dev. 1993, 37, 117-123) can minimize the hydrolysis of the silanes grafted to the glass substrate during the deposition. Such baths tend to give a larger thermally induced stress and a Cu deposit with a larger surface roughness, however. (31) The as-plated Cu has a specific resistivity of ∼6.4 µΩ cm, and the value was 3.6 µΩ cm when it was annealed after the patterning step for a few hours at 250 °C under N2. (32) The gate layer should preferably be ∼200 nm thick to allow both a good coverage by the next layers forming the TFT and the fabrication of TFT arrays with high yield.

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features is not difficult per se, but doing so with an average long-range (15-in.) overlay accuracy of ∼1 µm constitutes a challenge. We achieved such accuracy (see Supporting Information) by building a high-precision printing tool, using hybrid stamps composed of a thin metallic backplane and a patterned poly(dimethylsiloxane) (PDMS) layer, identifying and minimizing systematic errors in the patterns, and by molding stamps appropriately.33-36 The stamp, which had a ∼10.6 × 8 sq-inch pattern in its center, was inked by spraying ∼5 mL of a 0.4 mM solution of hexadecanethiol (HDT) in ethanol and then mounted onto the microcontact printer. The native oxide forming on the surface of the Cu was removed within 15 s in 4% HCl before printing, and the substrate was then thoroughly rinsed with water and dried. The printing tool had a planar print table and brought the stamp into contact with the substrate from one side (see Supporting Information). Thus, the entire stamp can be left in contact when long printing times are desired and printing with accurate overlaying is possible. At maximum speed, the printing tool could move the stamp at a linear velocity of 1 m s-1, but typically the entire stamp was placed in contact with the substrate within 30 s, left in contact for 3 min, and removed within another 30 s. The overall printing time differs from one side to the other by 25%; the diffusion of HDT on the Cu surface during this time did not compromise the resolution of the patterned monolayer of HDT,37 and the completion of the monolayer was sufficient to etch the microcontact-printed Cu with good selectivity.17 Specifically, the etch bath contained 3-nitrobenzenesulfonic acid as the oxidizer for Cu and polyethyleneimine as the complexing agent for oxidized Cu. The pH of the bath was adjusted to 9.3 by carefully adding 4 M HCl (danger).38 The bath was moderately stirred and had an etch rate of ∼10 nm min-1. The micrographs in Figure 2 show continuous vertical Cu lines (corresponding to the gate lines of a TFT array) and 4-µm-wide horizontal “light shields”. The light shields block light emitted from the back of the display to prevent having light-induced current in the transistors. These images are part of a pattern (33) Rogers, J. A.; Paul, K. E.; Whitesides, G. M. J. Vac. Sci. Technol., B 1998, 16, 88-97. (34) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042-3049. (35) Michel, B.; et al. IBM J. Res. Dev. 2001, 45, 697-719. (36) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318. (37) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334. (38) The etch bath has a pH of ∼11.4 before adding HCl. It is consequently preferable to add HCl slowly and to wear appropriate protection.

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Figure 2. Optical microscope images (in reflection) of a Cu gate layer patterned using the method reported here. The area shown belongs to an array for making 12.1-in. XGA TFT-LCDs.

comprising 1024 × 768 pixels and including structures on the periphery of the array such as probing pads and alignment marks. Further to this demonstration of how to electrolessdeposit and pattern Cu on large glass substrates, much work remains to be done, of course. For example, we intend to characterize in depth the geometric, optical, and electrical characteristics of the Cu gate patterns and, if possible, to prototype TFT arrays. We predict that µCP is applicable to the patterning of large metallic surfaces with local as well as long-range accuracy compatible with the requirements of large-area TFT-LCDs, and we hope that this work will contribute to the creation of important technological applications for microcontact printing. Acknowledgment. We thank our colleagues of the Advanced Display Technology Laboratory, A. Bietsch, H. Wolf, R. Stutz, H. Schmid, and M. Mastro, for their help with stamps, molds, and printing tools and R. Fair for the accuracy measurements. We are grateful to E. J. O’Sullivan for his help with electrochemistry and to P. F. Seidler and R. L. Wisnieff for their continuous support of the work. Supporting Information Available: Photographs of the tool during a printing operation and distortion plot of a printed pattern. This material is available free of charge via the Internet at http://pubs.acs.org. LA034748H