4868 Chem. Mater. 2009, 21, 4868–4872 DOI:10.1021/cm901687w
Atomic Layer Deposition of Iridium Thin Films by Consecutive Oxidation and Reduction Steps :: :: :: † ‡ † Jani Hamalainen,*,† Esa Puukilainen,† Marianna Kemell, ::† Leila Costelle, Mikko Ritala, and Markku Leskela †
Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, FI-00014, University of Helsinki, Finland, and ‡Division of Materials Physics, Department of Physics, P.O. Box 64, FI-00014, University of Helsinki, Finland Received June 18, 2009. Revised Manuscript Received September 4, 2009
Iridium thin films have been grown by atomic layer deposition (ALD) using Ir(acac)3 (acac = acetylacetonato), ozone, and molecular hydrogen as precursors at low temperatures between 165 and 200 °C. At this temperature range, iridium oxide film results in a process without H2. Therefore H2 had a reducing effect on the film after the oxidizing ozone pulse. On the other hand, methanol was not effective in reducing the oxide film. Ir(acac)3 was sublimed at 155 °C, which sets the lowest deposition temperature limit of 165 °C for the process. Iridium films were successfully deposited on Al2O3 nucleation layers but also directly on bare soda lime glasses and native oxide covered silicon substrates. About 60 nm thick films had resistivities and roughnesses less than 12 μΩ cm and 1.4 nm, respectively. The films contained e 2 atom % hydrogen, e 1 atom % carbon, and 4-7 atom % oxygen as impurities. The Ir films passed the common tape test indicating good adhesion to all tested surfaces. A full Ir coverage over the substrate was obtained with 7 nm thick film. Introduction Atomic layer deposition (ALD)1-5 is considered as a modification of a chemical vapor deposition (CVD) method, where precursors are pulsed separately into a reactor chamber one at a time. In ALD, surface reactions proceed in a saturative, self-limiting manner. Repeatability, conformality, uniformity, and thickness controllability of the deposited films are all advantageous characteristics of ALD. The majority of the known ALD processes produce compounds;2 however, noble metals form a notable exception due to their unique properties.6 *Corresponding author. E-mail:
[email protected]. Fax: þ358 9 191 50198. ::
(1) Ritala, M.; Leskela, M. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, CA, 2001; Vol. 1, pp 103-159. (2) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. (3) Elers, K.-E.; Blomberg, T.; Peussa, M.; Aitchison, B.; Haukka, S.; Marcus, :: S. Chem. Vap. Deposition :: :: ::2006, 12, 13. (4) Leskela, M.; Aaltonen, T.; Hamalainen, J.; Niskanen, A.; Ritala, M. Proc.;Electrochem. Soc. 2005, 2005-09, 545. :: (5) Ritala, M.; Niinisto, J. In Chemical Vapor Deposition: Precursors, Processes and Applications; Jones, A. C., Hitchman, M. L., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2009; pp 158-206. (6) Aaltonen, T. Ph.D. thesis, University of Helsinki, Finland, 2005. Available from http://ethesis.helsinki.fi/en/. :: (7) Aaltonen, T.; Alen, M.; Ritala, M.; Leskela, M. Chem. Vap. Deposition 2003, 9, 45. :: (8) Aaltonen, T.; Ritala, M.; Arstila, K.; Keinonen, J.; Leskela, M. Chem. Vap. Deposition 2004, 10, 215. (9) Biener, J.; Baumann, T. F.; Wang, Y.; Nelson, E. J.; Kucheyev, :: S. O.; Hamza, A. V.; Kemell, M.; Ritala, M.; Leskela, M. Nanotechnology 2007, 18, 055303. :: (10) Aaltonen, T.; Ritala, M.; Leskela, M. Electrochem. Solid-State Lett 2005, 8, C99. :: (11) Aaltonen, T.; Ritala, M.; Sammelselg, V.; Leskela, M. J. Electrochem. Soc. 2004, 151, G489.
pubs.acs.org/cm
The most common ALD noble metal processes use oxygen as a reactant.6-14 In these processes molecular oxygen is dissociatively chemisorbed on the noble metal surface as atomic oxygen, and some oxygen atoms also diffuse into the subsurface region.6,15 During the noble metal precursor pulse a reaction takes place between the metal precursor and the adsorbed oxygen atoms producing a metallic film. Each process requires a certain threshold temperature to proceed, above which the reaction between the adsorbed oxygen atoms and the metal precursor seems to be so fast that essentially all oxygen becomes consumed, forming a metallic film instead of an oxide. However, when ozone as the more reactive oxygen compound and lower deposition temperatures are used, noble metal oxide films can be obtained.16-18 ALD Ir processes have been explored for several applications. To name a few, Ir has been applied in the development of X-ray optics, that is, for microchannel :: (12) Aaltonen, T.; Ritala, M.; Sajavaara, T.; Keinonen, J.; Leskela, M. Chem. Mater. 2003, 15, 1924. (13) Aaltonen, T.; Ritala, M.; Tung, Y.-L.; Chi, Y.; Arstila, K.; :: Meinander, K.; Leskela, M. J. Mater. Res. 2004, 19, 3353. :: (14) Aaltonen, T.; Ritala, M.; Leskela, M. Atomic Layer Deposition of Noble Metals. In Advanced Metallization Conference 2004 (AMC 2004), San Diego, CA, U.S.A., Oct. 19-21, and Tokyo, Japan, Sept. 28-29, 2004; Erb, D., Ramm, P., Masu, K., Osaki, A., Eds.; Materials Research Society: Warrendale, PA, 2005; pp 663-667. :: (15) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskela, M. Electrochem. Solid-State Lett. 2003, 6, C130. :: :: :: (16) Hamalainen, J.; Kemell, M.; Munnik, F.; Kreissig, U.; Ritala, M.; :: Leskel :: :: a, :: M. Chem. Mater. 2008, 20, 2903. :: (17) Hamalainen, J.; Munnik, F.; Ritala, M.; Leskela, M. Chem. Mater. 2008, 20, 6840. :: :: :: :: (18) Hamalainen, J.; Munnik, F.; Ritala, M.; Leskela, M. J. Electrochem. Soc. 2009, 156, D418.
Published on Web 09/30/2009
r 2009 American Chemical Society
Article
Chem. Mater., Vol. 21, No. 20, 2009
plates and Fresnel zone plates.19,20 Another application in the field of modern optics is an inductive grid filter for infrared rejection.19,21 Selective area Ir ALD has been realized by patterned self-assembled monolayers (SAMs) for three-dimensional nanostructures.22,23 For catalytic applications, Ir has been deposited on Al2O3 and TiO2 coated cellulose.24 Additionally, ALD Ir has been examined as a diffusion barrier and a seed layer for copper,25 and as a starting surface for HfO2 deposition.26,27 It has been shown earlier that Ir films can be deposited using Ir(acac)3 (acac = acetylacetonato) and oxygen at 225 °C and above by ALD.11 When oxygen is replaced with a more oxidizing agent, namely, ozone, IrO2 films can be obtained at a lower deposition temperature (165 to 200 °C), while metallic Ir is formed already at 210 °C.16 As the noble metals in general are known for their chemical inertness, it is expected that IrO2 can be reduced quite easily and metallic Ir obtained below 200 °C as well. The layer-wise deposition in ALD is inherently an ideal concept to manipulate the film growth by including successive oxidation and reduction to a single deposition cycle. In this study we will show that the Ir deposition temperature range can be lowered to 165 °C, that is, as low as that in the IrO2 process, by employing consecutive oxidation and reduction steps using ozone and hydrogen. This approach allows depositing noble metals by ALD at lower deposition temperatures than by an oxygen based process and should therefore be more easily employable on heat-sensitive substrates. Also, methanol was tested as a reducing agent in the ALD process cycle but with no success. Experimental Section Iridium thin films were deposited between 165 and 200 °C in a commercial hot-wall flow-type F-120 ALD reactor (ASM Microchemistry Ltd., Finland) operated under a nitrogen pressure of about 10 mbar. Nitrogen (99.9995%) was produced with a NITROX UHPN 3000 nitrogen generator and used as a carrier and purging gas. In an ALD pulsing sequence, the metal precursor was followed by separate ozone and hydrogen pulses. The films were grown on Al2O3 films deposited in situ from trimethylaluminum (TMA) and water. Dimensions of silicon(111) and soda lime glass substrates were 55 cm2 each. Ir(acac)3 (99.9%, ABCR) was sublimed from an open boat held inside the reactor at 155 °C. Ozone was produced with a Wedeco Ozomatic (19) Pilvi, T. Ph.D. thesis, University of Helsinki, Finland, 2008. Available from http://ethesis.helsinki.fi/en/. (20) Jefimovs, K.; Vila-Comamala, J.; Pilvi, T.; Raabe, J.; Ritala, M.; David, C. Phys. Rev. Lett. 2007, 99, 264801. (21) Jefimovs, K.; Laukkanen, J.; Vallius, T.; Pilvi, T.; Ritala, M.; :: Meilahti, T.; Kaipiainen, M.; Bavdaz, M.; Leskela, M.; Turunen, J. Microelectron. Eng. 2006, 83, 1339. :: :: (22) Farm, E.; Kemell, M.; Ritala, M.; Leskela, M. Chem. Vap. Deposition 2006, 12, 415. :: :: (23) Farm, E.; Kemell, M.; Ritala, M.; Leskela, M. Thin Solid Films 2008, 517, 972. :: (24) Kemell, M.; Pore, V.; Ritala, M.; Leskela, M. Chem. Vap. Deposition 2006, 12, 419. (25) Josell, D.; Bonevich, J. E.; Moffat, T. P.; Aaltonen, T.; Ritala, M.; :: Leskela, M. Electrochem. Solid-State Lett. 2006, 9, C48. (26) Kukli, K.; Ritala, M.; Pilvi, T.; Aaltonen, T.; Aarik, J.; Lautala, :: M.; Leskela, M. Mater. Sci. Eng., B 2005, 118, 112. (27) Kukli, K.; Aaltonen, T.; Aarik, J.; Lu, J.; Ritala, M.; Ferrari, S.; :: Ha˚rsta, A.; Leskela, M. J. Electrochem. Soc. 2005, 152, F75.
4869
Modular 4 HC Lab ozone generator from oxygen (99.999%, Linde Gas) and pulsed into the reactor through a needle valve and a solenoid valve from the main ozone flow line. The estimated ozone concentration output of the generator was about 100 g/(Nm3) with a flow rate of 30 L/h. A flow rate of H2 (99.999%, Aga) was set to 20 sccm by a needle valve and a mass flow meter during continuous flow, and H2 was pulsed into the reactor with a solenoid valve. Methanol (anhydrous, 99.9%, Alfa Aesar) was cooled in an ethanol bath to a constant temperature of -20 °C and was introduced into the reactor through needle and solenoid valves. Crystal structures of the films were identified from X-ray diffraction (XRD) patterns measured with a PANanalytical X0 Pert Pro X-ray diffractometer. Film thicknesses were determined from X-ray reflectivity (XRR) patterns measured with a Bruker AXS D8 Advance diffractometer and from energydispersive X-ray spectroscopy (EDX) data. The EDX spectra were measured using an Oxford INCA 350 microanalysis system connected to a Hitachi S-4800 field emission scanning electron microscope (FESEM). The EDX results were analyzed using a GMR electron probe thin film microanalysis program.28 Surface morphology of the films was examined by the FESEM and atomic force microscope (AFM) using a Veeco Instruments Multimode V with Nanoscope V controller. AFM samples were measured in tapping mode in air using a phosphorus-doped silicon probe (RTESP) delivered by Veeco Instruments to produce simultaneous topographical and phase images. Several scans were performed from different parts of the samples to check the uniformity of the surface. Final AFM images were measured from a scanning area of 2 2 μm2 with a scanning frequency of 0.5 Hz, and no image processing except flattening was made. Roughness values were calculated as rootmean-square values (Rq). Resistivities of the iridium thin films were calculated from sheet resistances measured with a four-point probe technique and from the film thicknesses. Adhesion of the films was tested with a common tape test. Elemental compositions of the films were determined with time-of-flight elastic recoil detection analysis (TOF-ERDA) by a 5 MV tandem accelerator.29 A 48 MeV 79Br9þ ion beam was applied to obtain reasonable counting statistics for impurities, but the downside was a limited depth resolution which makes it difficult to separate oxygen in the oxide layers from that in the Ir film.
Results and Discussion It has been previously reported that IrO2 thin films form in the ALD process using Ir(acac)3 and ozone precursors between 165 and 200 °C.16 The films became metallic Ir at deposition temperatures above 200 °C. The growth was found self-limiting when 0.5 s and longer pulse times were applied for both Ir(acac)3 and ozone. In the present study, we added a H2 pulse into the ALD cycle: Ir(acac)3-purge-O3-purge-H2-purge. This process resulted in metallic Ir films at all temperatures. Similarly, Knoops et al.30 have recently demonstrated that addition of H2 exposures, with or without plasma (28) Waldo, R. A. Microbeam Anal. 1988, 23, :: 310. (29) Putkonen, M.; Sajavaara, T.; Niinisto, L.; Keinonen, J. Anal. Bioanal. Chem. 2005, 382, 1791. (30) Knoops, H. C. M.; Mackus, A. J. M.; Donders, M. E.; van de Sanden, M. C. M.; Notten, P. H. L.; Kessels, W. M. M. Electrochem. SolidState Lett. 2009, 12, G34.
4870
Chem. Mater., Vol. 21, No. 20, 2009
H€ am€ al€ ainen et al.
Figure 1. Growth rate and resistivity of Ir on the Al2O3 nucleation layer as a function of H2 pulse length. Substrates were soda lime glass (open symbol) and Si(111) (solid). Pulse lengths were 1 s for Ir(acac)3 and ozone. Purges were 2 s each. 1000 cycles were applied at 185 °C.
Figure 3. FESEM images of the Ir films deposited at 185 °C using (a) 100, (b) 200, (c) 300, and (d) 500 cycles. The growth parameters are identical to those presented in Figure 2.
Figure 2. Thickness of Ir films on Al2O3 nucleation layer as a function of the number of deposition cycles at 185 °C. Pulse lengths and purges were 2 s for each precursor. Each point contains measurements from two samples, that is, films on both Si(111) and soda lime glass substrates. Film thicknesses were measured with XRR.
Figure 4. XRD patterns of the films deposited between 165 and 200 °C. 3000 cycles were applied in each run. Pulse length for H2 was 6 s, while other pulse lengths and all purges were 2 s each. The substrate was soda lime glass with an Al2O3 seed layer.
activation, reduce remote plasma ALD grown platinum oxide to Pt at a low temperature. Figure 1 shows the growth rate of Ir at 185 °C as a function of the H2 pulse length. On the Al2O3 nucleation layer, H2 pulse lengths of 1 s and longer result in a saturated growth rate of about 0.2 A˚/cycle. The resulting 20 nm thick metallic Ir films have a resistivity of about 16 to 17 μΩ cm. The films grew well also without the Al2O3 nucleation layer as similar film thickness was obtained on native oxide covered Si(111); however, the film was about 3 nm thinner on soda lime glass. The increase in H2 dose does not lead to changes in growth rate nor resistivity of the films (Figure 1). This indicates that the reduction reaction is fast. In the Ir(acac)3-O3 ALD process for IrO2, the growth rate was about double and the resistivities of the 40 nm IrO2 films were over 10 times higher16 compared to the present Ir(acac)3-O3-H2 process at the same temperature (Figure 1). Using bulk densities, the growth rates of about 0.2 and 0.4 A˚/cycle for Ir and IrO2 translate into the
corresponding Ir surface coverages of 1.4 and 1.3 atoms/ nm2 cycle. This implies that the adsorption density of Ir(acac)3 is essentially the same on Ir and IrO2 surfaces. The depositions employing anhydrous methanol instead of H2 for reduction resulted in iridium oxide films stained with metallic impurities at 185 °C. As the increase of the methanol pulse from 1 to 6 s did not significantly improve the reduction of the IrO2, it was concluded that methanol does not serve as an efficient reducing agent as H2 does. The Ir film thickness depends linearly on the number of the deposition cycles (Figure 2) as expected for an ALD process. The use of a thin Al2O3 layer as a starting surface results in identical growth rates on both soda lime glass and Si(111) substrates. There seems to be a slight incubation period in the nucleation process as the linear extrapolation does not go through the origin of the graph (Figure 2). This is typical of ALD of noble metals that nucleate in the island growth mode on the starting surface. Indeed, after 100 deposition cycles Ir is scattered on
Article
Chem. Mater., Vol. 21, No. 20, 2009
the Al2O3 surface as a discontinuous layer (Figure 3a), and after 200 cycles the about 5 nm thick film still has tiny holes (Figure 3b). After 300 cycles, the 7 nm thick film seems to be fully continuous according to the FESEM image (Figure 3c); however, the resistivity of the film could not be measured reliably. A resistivity of about 20 μΩ cm was obtained from the 12 nm thick film after 500 cycles (Figure 3d). XRD patterns of the Ir films deposited between 165 and 200 °C are presented in Figure 4. All the reflections originate from metallic Ir with no traces of IrO2 detected.
Figure 5. Growth rate and resistivity of Ir on the Al2O3 layer as a function of deposition temperature. Pulse length for H2 was 6 s, other pulses and all the purges were 2 s each. Substrate was Si(111). Film thicknesses were determined with EDX.
4871
However, the films are not highly oriented as the intensity ratios between the (111) and the (200) reflections are 2 or less. In the oxygen based ALD process the (111) to (200) intensity ratios were substantially higher, that is, 4 and 16 at the respective deposition temperatures of 225 and 350 °C.11 The growth rate remains constant (0.2 A˚/cycle) with increasing deposition temperature (Figure 5). It is to be noted that in the studied temperature range there seems to be a classical ALD window29 not usually seen in noble metal ALD. The lowest deposition temperature (165 °C) was set by the sublimation temperature of Ir(acac)3 (155 °C). Therefore, it seems possible that Ir could be deposited at even lower temperatures if a more volatile Ir precursor would be applied. The resistivities of about 60 to 70 nm thick films were between 11 to 12 μΩ cm, showing a slight decrease with increasing deposition
Figure 7. FESEM image of the Ir film deposited at 165 °C on trench patterned Si substrate. 2500 cycles were applied using 5 s pulses and purges for all precursors.
Table 1. Elemental Compositions (TOF-ERDA) and Surface Roughness (AFM) of the Ir Films Deposited between 165 and 200 °C dep. temp. (°C)
thickness (EDX) (nm)
roughness (AFM) (nm)
H (atom %)
C (atom %)
Oa (atom %)
Ir (atom %)
165 175 185 200
64 65 62 64
1.1 1.3 1.4 1.1
1.8 ( 0.3 1.6 ( 0.3 1.9 ( 0.3 1.2 ( 0.2
0.6 ( 0.1 0.4 ( 0.1 0.3 ( 0.1 0.5 ( 0.1
3.6 ( 0.3 7.0 ( 0.3 3.7 ( 0.2 6.3 ( 0.3
94 ( 1 91 ( 1 94 ( 1 92 ( 1
a
Contains also oxygen in SiO2 but not in Al2O3, which was subtracted based on aluminum counts.
Figure 6. AFM topography images of about 60 nm Ir films deposited at (a) 165, (b) 175, (c) 185, and (d) 200 °C. Image (e) represents about 40 nm thick IrO2 film deposited at 185 °C for reference purposes.
4872
Chem. Mater., Vol. 21, No. 20, 2009
temperature. These resistivity values are comparable to the resistivities reported for the oxygen based ALD Ir process.11 Elemental compositions and surface roughnesses of the films are presented in Table 1. The studied films were deposited on an Al2O3 nucleation layer on top of the Si substrates. Hydrogen and carbon impurities in the films were about 2 atom % and less than 1 atom %, respectively, while the films contained between 4 and 7 atom % oxygen. The oxygen impurity levels are relatively high compared to the oxygen based Ir process11 (
