Selective-Area Atomic Layer Deposition Using Poly(methyl methacrylate) Films as Mask Layers Elina Fa ¨rm,* Marianna Kemell, Mikko Ritala, and Markku Leskela ¨ UniVersity of Helsinki, Department of Chemistry, P.O. Box 55, FI-00014 UniVersity of Helsinki, Finland ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: July 4, 2008 Selective-area atomic layer deposition (ALD) was achieved using poly(methyl methacrylate) (PMMA) films as growth inhibiting mask layers. The PMMA films were prepared from PMMA-toluene solution by spin- coating and patterned by UV lithography through a mechanical mask. The patterned PMMA films were tested in several ALD processes, both noble metals and oxides. The tested noble metal processes were iridium, platinum, and ruthenium, and the oxide processes were Al 2 O 3 and TiO 2 . Al 2 O 3 was deposited using AlCl 3 and H 2 O and trimethylaluminum (TMA) and H 2 O. TiO 2 was deposited using Ti(OMe) 4 and H 2 O. The growth temperatures were 250-350 °C. 1. 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 the surface layer by layer and the film thickness and composition can be controlled by 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 requires that the designated areas of the surface are passivated or protected against ALD precursors, in which case the film is deposited only to desired parts of the surface. Selective-area ALD has been widely studied by using patterned self-assembled monolayers (SAM) 3,4 as growth pre- venting mask layers. The function of the SAM is to passivate the surface against ALD growth so that the film is deposited only on areas without a SAM. SAMs can be formed spontane- ously through adsorption to the solid surface from liquid or gas phase or by microcontact printing. Microcontact printing 5,6 is a fast way to prepare patterned SAMs. Otherwise the preparation of SAMs is time-consuming: it may take up to 48 h 7,8 to prepare a SAM which has no nucleation site for ALD precursors. For selective-area ALD patterned SAMs have been used to prevent the ALD growth of oxides such as ZrO 2 , HfO 2 , TiO 2 , and ZnO. 7,9,10 SAMs have also been used to block the ALD growth of noble metals such as ruthenium, 11 platinum, 12,13 and iridium. 14,15 Another approach to selective-area ALD is to use unreactive and thermally stable polymer films as mask layers. 16 Compared to SAMs, polymer films are fast and simple to prepare by spin- coating. After ALD processing, the polymer film can be removed quite easily. So far, poly(methyl methacrylate) films have been studied as masking layers for selective-area ALD of TiO 2 . 16,17 PMMA films can work in selective-area ALD the same way as SAMs, i.e., the PMMA film can passivate the surface against ALD growth. Alternatively, the film can grow on the PMMA film. The growth on the substrate surface can still be prevented by controlling the thickness of the PMMA film and purge times. Under these conditions, lift-off patterning may be feasible, provided that the film grown on PMMA does not encapsulate it too well. Sinha et al. noted that when TiO 2 is deposited by ALD from TiCl 4 and H 2 O precursors the film grows on a PMMA film 16 but when deposited from Ti[OCH(CH 3 ) 2 ] 4 and H 2 O it does not grow on PMMA. 17 * Tel: +358 9 191 50226, +358 9 191 50198. E-mail: Elina.farm@ helsinki.fi. Figure 1. (a) SEM micrograph of a patterned iridium film deposited at 250 °C. Lighter areas are iridium and darker areas silicon. (b) EDX measured on an Ir dot, on the PMMA film after the Ir process, and on the Si surface after PMMA removal. J. Phys. Chem. C 2008, 112, 15791–15795 15791 10.1021/jp803872s CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008