Microcontact Printed RuOx Film as an Activation Layer for Selective

Article pubs.acs.org/cm

Microcontact Printed RuOx Film as an Activation Layer for Selective-Area Atomic Layer Deposition of Ruthenium Elina Far̈ m,* Seppo Lindroos, Mikko Ritala, and Markku Leskela ̈ Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland ABSTRACT: Selective-area atomic layer deposition (ALD) was achieved using microcontact printed RuOx films as an activation layer for ruthenium ALD process. Patterned RuOx films were prepared by transferring RuCl3 layer by a PDMS (polydimethylsiloxane) stamp to Si(100) substrate and exposing this layer to ozone. Patterned films had either 1.5 μm or about 500 nm wide RuOx lines. Ruthenium was deposited on the activated areas at 250 °C. At this temperature the ruthenium did grow only on the activated areas but not on silicon so that the features of the stamps were repeated on the substrate.

KEYWORDS: selective-area atomic layer deposition, atomic layer deposition, ruthenium

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the RuOx layer easier than with nonoxidizing surfaces like SiO2.19 Microcontact printing is a fast and inexpensive way for preparing an activation layer when expensive instruments such as EBID are not available. Previously catalytic palladium film has been prepared by μCP on titanium coated Si/SiO2 surface for selective-area electroless deposition of copper.20 In this study RuCl3 ethanol solution was printed on the silicon substrate and after printings the sample was exposed to ozone so that a patterned RuOx film was produced on the substrate. Ruthenium was deposited on patterned RuOx samples at 250 °C because at that temperature ruthenium grows only on activated areas, not on Si or SiO2.

INTRODUCTION Atomic layer deposition (ALD)1,2 is a method to grow thin films through self-limiting surface reactions between alternately supplied gaseous precursors. The film grows on a surface layer by layer, and the film thickness and composition can be controlled with atomic layer accuracy in the growth direction. The ALD technique ensures the deposition of conformal and uniform films over large areas. The film growth can also be controlled on the surface with selective-area ALD. Selective-area ALD is done typically so that the designated areas of the surface are passivated or protected against ALD precursors in which case the film is deposited only on the desired nonpassivated parts of the surface. This can be done using selfassembled monolayers3,4 that passivate the surface against ALD growth5−11 or by using polymer films12−15 that can passivate or protect the surface against ALD growth. Activation of a surface is quite new approach to selective-area ALD. On the contrary to the surface passivation, the ALD growth is now wanted on the activated areas of the surface. This can be done by preparing a patterned catalytic layer that can be, for example, noble metal. The ALD film is then deposited only on the catalytic layer while rest of the substrate surface will remain free of an ALD film. So far selective-area ALD of platinum has been done using patterned platinum, made by electron beam induced deposition (EBID), as a catalytic layer.16 Sputtered palladium film has been used as a nucleation layer for ALD copper films.17 In this case the copper film was deposited only on palladium, not on SiO2. Ruthenium film has been deposited selectively by chemical vapor deposition on copper surface while no growth occurred on dielectric.18 In this paper selective-area ALD has been done by using microcontact printed RuOx films as activation layers for ruthenium ALD from RuCp2 and oxygen. RuOx can be used as an activation layer due to its oxidizing properties because the RuCp2 precursor has a high reactivity to oxygen and thereby reacts and binds with © 2011 American Chemical Society

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EXPERIMENTAL SECTION

RuOx film was used as an activation layer for ALD ruthenium process. In the beginning unpatterned RuOx films21 were prepared using the SILAR process in ambient air with equipment described earlier.22 At first the substrate was immersed in RuCl3 water solution (10 mM) for 30 s. After cation immersion the substrate was rinsed for 5 s in purified water. The cycle was completed by immersing the substrate in H2O at 65 °C for 60 s. Because Ru(III) may be oxidized by air to Ru(IV) we denote the films as RuOx. For the activation of the Ru ALD process the value of x should not be critical as all oxides of ruthenium are oxidizing. Films were grown for 1−200 cycles. Patterned activation layers were prepared by microcontact printing using a PDMS stamp. The PDMS stamp was prepared from 10:1 (weight:weight) mixture of SYLGARD silicone elastomer 184 and SYLGARD silicone elastomer 184 curing agent. The components were mixed and let to set for 30 min. The polymer was poured onto a master to a thickness of about 5 mm. The first hour of the curing was done at room temperature and the second hour at 80 °C. The PDMS stamp was allowed to cool to room temperature before it was removed from the master and cut to the right size. Received: August 19, 2011 Revised: November 7, 2011 Published: December 14, 2011 275

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The PDMS stamps were cast in two masters. A stamp with 300 nm wide lines (Figure 1a) was prepared from a master prepared from a

Article

RESULTS AND DISCUSSION Ruthenium films can be deposited by ALD from RuCp2 and oxygen in air at a temperature range of 275−400 °C on Al2O3 and TiO2 covered silicon substrates,23 but when the substrate is covered by an activation layer, lower deposition temperatures can also be used. Therefore the temperature of the ruthenium process for selective-area ALD purposes was selected to be 250 °C so that the ruthenium film would grow only on the RuOx activation layer but not on the native oxide covered silicon. To test the ruthenium process at 250 °C, unpatterned RuOx activation layers were prepared by SILAR (successive ionic layer adsorption and reaction) process22 on silicon substrates. Cycle numbers for RuOx were 1−200. Figure 2 is a scanning

Figure 2. SEM image of a RuOx activation layer deposited by SILAR for 25 cycles.

electron microscopy (SEM) image of a RuOx activation layer deposited for 25 cycles, and in Figure 3 is an energy dispersive

Figure 1. (a) PDMS stamp prepared from DVD master. (b) PDMS stamp with 1.5 μm lines. blank commercial DVD disk, and a stamp with 1.5 μm lines (Figure 1b) from a master that was made lithographically from nickel. The depth of the structure in the DVD master was about 150 nm and in the nickel master 570 nm. The DVD master was used only once, but the nickel master was used for several stamp preparations. Before printings the stamps were held under UV lamp (264/185 nm, 10 W) for 6 h to make them hydrophilic. If the stamp was not used immediately it was stored in deionized water. The stamps were used only for one printing each. Patterned RuOx films were prepared by a two step process. At first a drop of RuCl3 ethanol solution (2.5 mM) was put on the stamp for 60 s. The stamp was dried with compressed air for 5 s and placed in contact with the substrate within 10 s. Printing time was typically 15 s. The second step of the process was to place the sample for 2 h below a UV lamp (264/185 nm, 10 W) that produces ozone. Ruthenium was deposited on the RuOx films by ALD from RuCp2 (Cp = cyclopentadienyl) and air23 at 250 °C for 1000 cycles. The temperature of 250 °C was chosen because ruthenium does not grow at that low temperature on Si surface but only on the activation layer. The ALD films were grown in an F120 reactor (ASM Microchemistry, Ltd., Finland). Nitrogen was used as a carrier and purging gas. The reactor was operated at pressures of about 10 mbar. The patterned films were studied with a Hitachi S-4800 field emission scanning electron microscope and INCA 350 EDX spectrometer. The film thicknesses were calculated from the EDX results using a GMR electron-probe thin film microanalysis program24 and bulk density of Ru.

Figure 3. EDX spectrum measured on RuOx film on Si(100) after 200 cycles. The film was prepared by SILAR process.

X-ray (EDX) spectrum measured on a RuOx film after 200 cycles showing the film contains ruthenium and oxygen. As an activation layer for ruthenium ALD, thinner 1 and 5 cycles RuOx films were used. Ruthenium ALD film was deposited on these activation layers for 1000 cycles. In Figure 4 are EDX spectra measured after the ALD ruthenium process on a sample which had an activation layer prepared by 5 cycles and on bare silicon. EDX measurements showed that the ruthenium film did grow only on RuOx areas, not on silicon. The thickness of the ruthenium films after 1000 cycles was measured to be 17 nm. The same thickness was measured for a ruthenium film on a RuOx activation layer prepared by only 1 SILAR cycle. 276

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previous printings did not affect the quality of a print. Also the inexpensive master prepared from DVD was used only for one stamp casting because PDMS traces may remain in the master and change the dimensions of the next stamp. The other master was used for many times. Because the printings were done by hand, the printing force was different in each printing. This affects the width of the printed lines, that is, the RuCl3 lines on the substrate could be wider than the lines in the stamp. This happened mainly with the DVD stamp and was probably due to the shallow structure which was only 150 nm deep. Widths of the produced RuOx lines were 300−500 nm depending on the printing time. Printings with fixed weight were not any better either. There was not a similar problem with the stamp with 1.5 μm lines probably because the structures in this stamp were deeper, 570 nm. After preparation of an activation layer, ALD ruthenium film was deposited on the samples for 1000 cycles at 250 °C. In Figure 6a is a SEM image of a patterned ruthenium film that is

Figure 4. EDX spectra measured after ALD process of ruthenium at 250 °C (a) on silicon substrate with RuOx film prepared by SILAR process for 5 cycles and (b) on bare silicon substrate.

This indicates that 1 SILAR cycle of RuOx is enough to prepare an effective activation layer. The first step in preparing patterned RuOx activation layers was the printing of RuCl3 which was done using a stamp. The printings were done with the solution where RuCl3 was dissolved into ethanol. PDMS stamps absorb easily nonpolar organic solvents but because here the solvent was ethanol, the hydrophobic PDMS stamp was treated to more hydrophilic so that it could absorb the polar precursor solution more easily. To make the stamps more hydrophilic, they were held under UV lamp (264/185 nm, 10 W) for 6 h. The water contact angle measured on a nonpatterned surface of a new PDMS stamp was about 95−105°. After 6 h UV treatment the contact angle decreased to 30−60°. If the water contact angle of a stamp was over 60° before printings, it was noticed that in many cases no features of a stamp were transferred on the sample. This could be because the stamp was not hydrophilic enough and thus could not absorb the RuCl3 ethanol solution. Therefore, if the contact angle was higher than 60° the stamp was not used for printings. The final step in preparing a patterned RuOx activation layer was ozone exposure of a sample that had patterned RuCl3 layer. After the printing step the sample was placed below a UV-lamp that produces ozone. Figure 5 shows a SEM image of patterned

Figure 6. (a) Patterned ruthenium film prepared on a RuOx activation layer made with a stamp with 1.5 μm lines. (b) EDX spectra measured on bright and dark lines of the sample.

grown on a patterned activation layer prepared by the stamp with 1.5 μm lines. EDX measurements proved that the bright areas are ruthenium and dark areas are without film. Ruthenium lines are 1.5 μm wide and so the features of the PDMS stamp (Figure 1b) are repeated on the substrate surface. The thickness of the ruthenium film was 19 nm as measured on nonpatterned area of the sample and it was in accordance with the ruthenium thickness measured on activation layer prepared by one SILAR cycle. The thickness of ruthenium film deposited on a typical starting layer of Al2O3 at the lowest temperature possible (275 °C) is only 13 nm after 1000 cycles.23 This shows how superb RuOx activation layer is in promoting ruthenium nucleation already at 250 °C.

Figure 5. Patterned RuOx film prepared with a DVD stamp.

RuOx film that was prepared by a DVD stamp from 2.5 mM RuCl3 solution. Printing time of the RuCl3 solution was 15 s and ozone exposure time 2 h. RuOx lines can be seen in the image, and the features of a stamp were well repeated on the substrate. The imprint of the stamp could vary, especially with the DVD stamp. Therefore stamps were used only once so that the 277

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ACKNOWLEDGMENTS Financial support from ASM Microchemistry is gratefully acknowledged.

Submicrometer ruthenium lines were deposited on an activation layer prepared by the DVD stamp (Figure 7a).

REFERENCES

(1) Leskelä, M.; Ritala, M. Thin Solid Films 2002, 409, 138. (2) Ritala, M.; Leskelä, M. In Handbook of Thin Film Materials; Academic Press: San Diego, CA, 2002. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Ulman, A. In Introduction to Ultrathin Organic Films, From Langmuir-Blodgett to Self-Assembled Monolayers; Academic Press: San Diego, CA, 1991. (5) Färm, E.; Kemell, M.; Ritala, M.; Leskelä, M. Thin Solid Films 2008, 517, 972. (6) Färm, E.; Kemell, M.; Ritala, M.; Leskelä, M. Chem. Vap. Deposition 2006, 12, 415. (7) Park, K. J.; Doub, J. M.; Gougousi, T.; Parsons, G. N. Appl. Phys. Lett. 2005, 86, 051903. (8) Park, M. H.; Jang, Y. J.; Sung-Suh, H. M.; Sung, M. M. Langmuir 2004, 20, 2257. (9) Yan, M.; Koide, Y.; Babcock, J. R.; Markworth, P. R.; Belot, J. A.; Marks, T. J.; Chang, R. P. H. Appl. Phys. Lett. 2001, 79, 1709. (10) Jiang, X.; Chen, R.; Bent, S. Surf. Coat. Technol. 2007, 201, 8799. (11) Jiang, X.; Bent, S. J. Phys. Chem. C 2009, 113, 17613. (12) Färm, E.; Kemell, M.; Ritala, M.; Leskelä, M. J. Phys. Chem. C 2008, 112, 15791. (13) Färm, E.; Kemell, M.; Santala, E.; Ritala, M.; Leskelä, M. J. Electrochem. Soc. 2010, 157, K10. (14) Sinha, A.; Hess, D.; Henderson, C. J. Vac. Sci. Technol. B 2006, 24, 2523. (15) Sinha, A.; Hess, D.; Henderson, C. Electrochem. Solid-State Lett. 2006, 9, G330. (16) Mackus, A. J. M.; Mulders, J. J. L.; van de Sanden, M. C. M.; Kessels, W. M. M. J. Appl. Phys. 2010, 107, 116102. (17) Gupta, R.; Willis, B. G. Appl. Phys. Lett. 2007, 90, 253102. (18) Yang, C.; McFeely, F. R.; Wang, P.; Chanda, K.; Edelstein, D. C. Electrochem. Solid-State Lett. 2010, 13, D33. (19) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskelä, M. Electrochem. Solid-State Lett. 2003, 6, C130. (20) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367. (21) Patake, V. D.; Lokhande, C. D. Appl. Surf. Sci. 2008, 254, 2820. (22) Kanniainen, T.; Lindroos, S.; Ihanus, J.; Leskelä, M. J. Mater. Chem. 1996, 6, 161. (23) Aaltonen, T.; Alén, P.; Ritala, M.; Leskelä, M. Chem. Vapor. Deposition 2003, 9, 45. (24) Waldo, R. A. Microbeam. Anal. 1988, 310.

Figure 7. (a) Patterned ruthenium film prepared on a RuOx activation layer. The RuOx film was done using a DVD stamp. (b) EDX spectra measured on bright and dark lines of the sample.

The ruthenium lines were about 500 nm wide, and there were about 200 nm wide bare silicon areas between the lines as confirmed by EDX (Figure 7b). The widths of the ruthenium lines are wider than the lines in the stamp. This is probably due to the shallow depth of the structures in the DVD stamp and the printing process which was done by hand. Anyhow, the successful production of the submicrometer lines proved the feasibility of our novel selective-area ALD process. Further improvement can surely be gained by using state-of-the art soft lithography tools.

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CONCLUSIONS RuOx activation layers were studied for selective-area ALD of ruthenium. Ruthenium film was deposited on a RuOx activation layer at 250 °C. Patterned RuOx films were prepared by a two step process where at first RuCl3 layer was microcontact printed to a silicon surface using a PDMS stamp. Then the sample was exposed to ozone. Two types of stamps were used: stamps that produced 1.5 μm and 300−500 nm wide lines. The stamps features were transferred on the substrate, and the ruthenium film did grow only on the activated areas. Ruthenium film thickness on the activated areas was 19 nm after 1000 ruthenium ALD cycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +358 9 191 50226. 278

dx.doi.org/10.1021/cm202468s | Chem. Mater. 2012, 24, 275−278