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J. Phys. Chem. C 2008, 112, 12754–12759
Carboxylic Acids as Oxygen Sources for the Atomic Layer Deposition of High-K Metal Oxides Erwan Rauwel,† Marc-Georg Willinger,† Fre´de´rique Ducroquet,‡ Protima Rauwel,§ Igor Matko,| Dmitry Kiselev,§ and Nicola Pinna*,† Departments of Chemistry and Ceramics, CICECO, UniVersity of AVeiro, 3810-193 AVeiro, Portugal and IMEP, UMR CNRS 5130 and LMGP, UMR CNRS 5628, INPG-Minatec, BP 257, 38016 Grenoble cedex 1, France ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: June 3, 2008
A nonaqueous approach inspired from sol-gel chemistry and adapted to the deposition of metal oxide thin films by atomic layer deposition (ALD) is presented. The process is based on the reaction of a carboxylic acid with a metal alkoxide. Contrary to classical approaches, no oxygen source that could lead to the oxidation of the silicon substrate is required. Instead, a surface esterification reaction is responsible for the film formation. The growth of metal oxides is achieved at temperatures as low as 50 °C on various supports including carbon nanotubes, organic fibers, and silicon wafers. The as-grown films show an excellent conformality and possess good dielectric properties due to their high purity. Inherent to the chemical approach is the possibility to grow oxides on silicon while minimizing the formation of a low-κ interfacial layer. Introduction Synthesis of nanometric thin films with desired compositions and uniform coverage holds a central position in nanotechnology research. As the deposition of oxide thin films on many different supports appears to be a real challenge, new deposition processes are constantly investigated. Among all of the deposition processes, atomic layer deposition (ALD)1–3 appears to be one of the most promising techniques due to its simplicity, reproducibility, and the high conformality of the obtained films. ALD is based on a reaction between precursor materials that are separated into successive surface reactions. In this manner, the reactants are kept separated until the adsorbed species react at the surface in a self-limiting process,1,4,5 that is, without the presence of a gas phase reaction. ALD is already being used in the fabrication of microelectronics devices on a large scale. The complementary metal oxide semiconductor field effect transistor (CMOSFET) based on silicon technology is presently the most important device in microelectronics for its performance and low power consumption. However, due to downscaling of metal oxide semiconductor (MOS), the International Technology Roadmap for Semiconductors (ITRS) projects that CMOS technology will now require gate dielectric layers with higher dielectric constants than SiO2 or SiON6 in order to reduce the tunneling leakage current.7,8 Many high-κ materials have been investigated for the replacement of SiO2 in high performance CMOS.9,10 In this respect, considerable attention has been given to HfO2 due to its relatively high dielectric constant,11 wide band gap,12 and a good thermodynamic stability with silicon.13 Titanium dioxide also has very attractive dielectric properties despite its lower band gap.10 However, it is more known for its photocatalytic properties and its high refractive index, making it suitable for * To whom correspondence should be addressed. E-mail: pinna@ ua.pt; fax: +351 234370004. † Department of Chemistry, CICECO, University of Aveiro. ‡ IMEP, UMR CNRS 5130, INPG-Minatec. § Department of Ceramics, CICECO, University of Aveiro. | LMGP, UMR CNRS 5628, INPG-Minatec.
optical coatings and as white pigment. Alternatively to Si-based technology, it was recently demonstrated that the deposition of high-κ metal oxides on carbon nanotubes (CNTs) can be used to fabricate new metal oxide field effect transistors.14–16 Usually, the deposition of metal oxides by ALD is achieved by using an oxygen source (e.g., water, oxygen, ozone) with a metal precursor (e.g., halide, alkoxide, amide).17 General limitations of this traditional method mainly reside in the presence of undesired impurities, ranging from unreacted carbon species to halides, and in the relatively high growth temperatures (200-500 °C).18,19 Depending on the targeted application, a tailored postsynthetic treatment is generally required in order to improve the quality of the films. Even though Chabal et al. have shown that H-terminated Si(001) is stable in the presence of water for temperatures up to 300 °C,20–22 metal oxide films grown on silicon by ALD processes employing “traditional” oxygen sources systematically present an oxidized interfacial layer (silica or silicates). In the search of alternative approaches making use of lessoxidizing conditions, nonaqueous sol-gel routes seem to be appropriate because the metal-oxygen-metal (M-O-M) bonds can in certain cases be formed without a hydrolysis step. Instead, the oxygen is provided by other molecular species such as alcohols, ethers, or metal alkoxides.23,24 These routes were extensively applied to the synthesis of nanoparticles because the as-synthesized oxides display high crystallinity and purity.25–27 They were also applied to ALD, but up to now they did not prove to bring significant advantages compared to approaches making use of water or other oxidizing agents.28,29 Recently, we showed that the ALD of metal oxides from carboxylic acids and metal alkoxides takes place via an ester elimination condensation step.30 The reaction mechanism taking place during the deposition was deeply studied and discussed in the same article. In this article a detailed study of titania and hafnia thin films deposited on various supports according to this approach will be presented.
10.1021/jp8037363 CCC: $40.75 2008 American Chemical Society Published on Web 07/18/2008
Atomic Layer Deposition of High-k Metal Oxides
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12755
Figure 1. (a) GPC of titania thin films deposited on silicon substrate plotted as a function of temperature. At 300 °C the GPC is dependent on the opening time of the alkoxide valve (square 1 s, star 1.5 s, triangle 2 s). (b) GPC as a function of the opening time of the alkoxide valve at 200 °C.
Experimental Methods Thin films were grown by atomic layer deposition in an ALD deposition system working in exposure mode. They were deposited onto p-type Si(001), p-type Si(111), carbon nanotubes, wool fibers, cellulose fibers, and latex. Some of the silicon substrates were etched in hydrofluoric acid (HF 3%) for 2 min prior to the deposition (labeled “HF-last”) in order to remove the native silicon dioxide. Films were deposited using the following metal alkoxides: hafnium tert-butoxide (STREM, 99.9%) and titanium isopropoxide (Aldrich, 99.99%). Acetic acid (Fluka, > 99.8%) was used as oxygen source. Pure nitrogen was used as carrier and purging gas. The substrate temperature was varied from 50 to 350 °C. Metal precursors and acetic acid vapors were generated in external reservoirs preheated at 80 and 40 °C, respectively. They were introduced into the reactor through an ALD valve. All tubes of the circuit were maintained at 100 °C during the deposition process. Depending on the precursors used, the nitrogen gas flow was varied from 5 to 50 sccm and was maintained throughout the deposition process. In a typical experiment, the valves were opened for 0.02 s for carboxylic acid and 1 s for the metal alkoxides. The residence and purging periods were 20 and 15 s, respectively. CNTs from Applied Science were treated with concentrated HNO3 at 100 °C for 2 h before deposition. The thickness of the films was determined by X-ray reflectometry (XRR) measurements using a Philips X’Pert MPD X-ray Diffractometer with a copper radiation (λKR ) 1.54056 Å) and a graphite monochromator for the selection of pure KR radiation. The X-ray tube was operated at 40 kV and 50 mA. A 1 mm slit was used to reduce the scattered X-ray intensity. Reflection geometry was used in the measurement. Instrumental configuration: divergence slit at the incident beam, equal 1/8 in.; step width, 0.01°; acquisition time, 0.6 s. High resolution transmission electron microscopy (HRTEM) on thin films was carried out on a JEOL 2010 transmission electron microscope operating at 200 kV and equipped with LaB6 filament with a point-to-point resolution of 1.7 nm; measurements on nanotubes were performed using a CM200FEG (Philips) microscope operating at 200 kV and equipped with a field emission gun and post column electron energy loss spectrometer (GATAN Tridiem). Scanning electron microscopy (SEM) images were recorded using a FEG-SEM Hitachi SU-
70 microscope operating at 4 kV with a working distance of 2-3 mm. For SEM, samples were prepared without any carbon coating simply by depositing some powder onto a double gluing tape. The electrical properties of the dielectric thin films were studied in metal oxide semiconductor (MOS) structures by evaporation of gold electrodes through a hard mask. C-V curves were recorded with a HP4284A LCR meter using a 40 mV signal at a frequency ranging from 0.1 kHz to 1 MHz. The stairsweep voltage step was 20 mV with a dwell time of 2 to 4 s between the steps. All measurements were performed at room temperature, on as-deposited thin films without any subsequent thermal or chemical treatment. Atomic force microscopy (AFM) measurements were performed at room temperature with a commercial scanning force microscope (PicoPlus, Agilent Technologies). A commercial cantilever (PPP-NCHR Nanosensors) with spring constant of k ) 42 N/m and typical tip radius less 10 nm of the tip was used. Imaging visualization was performed in air with noncontact mode (tapping mode with resonance frequency at 275 kHz) at the scanning speed 1 µm/s. Results and Discussion The deposition of titania and hafnia according to the present approach shows a growth per cycle (GPC) that increase with temperature (Figure 1a and 2a). For titania, a typical ALD window is observed between 150-200 °C (Figure 1a). In that temperature range, the self-limiting character of the process is evidenced by the saturation of the GPC (0.06 nm) independent of the quantity of metal precursor introduced into the reactor (Figure 1b). Moreover, the observed GPCs are in agreement with those generally observed.31,32 In the case of hafnia, the GPC is larger, although no clear plateau (e.g., ALD window) can be identified. Between 200 and 300 °C the GPCs are in the same range as those obtained in the case of hafnium alkylamides (0.09 nm)33 but are higher than those observed when hafnium halides and water are used.34 The GPC as a function of the valve opening time is plotted for a reaction taking place at 200 °C (Figure 2b). It was found that the GPC saturates and does not further increase for opening times exceeding 0.5 s. This proves the self-limiting character of the process at this temperature.35
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Figure 2. (a) GPC of hafnia thin films deposited on silicon substrate plotted as a function of temperature. At 300 °C the GPC is modulated by the opening time of the alkoxide valve (square 1 s, triangle 1.8 s). (b) GPC as a function of the opening time of the alkoxide valve at 200 °C.
Figure 3. HRTEM images of (a) TiO2 grown at 200 °C on Si(001) substrate with an interfacial layer of ∼0.4 nm, (b) TiO2 grown at 200 °C on Si (111) substrate with no evidence of an interfacial layer, (c) HfO2 grown at 200 °C on Si(001) substrate presenting an interfacial layer thickness of ∼0.7 nm, and (d) HfO2 grown at 80 °C on Si(001) substrate presenting an interfacial thickness of ∼1.15 nm. In the panel (a) the circled numbers denote the silicon wafer (1), the interfacial layer (2), and the metal oxide film deposited (3), respectively.
Depositions taking place at 300 °C (i.e., above the region of self-limiting growth) correspond to a region of diffusioncontrolled growth. It results from the self-decomposition of the metal precursors. In that regime, the GPC can be modulated by the amount of metal alkoxide introduced (Figure 1a and 2a, experimental points at 300 °C) Atomic force microscope (AFM) studies indicate that the films exhibit a smooth surface with a low root-mean-square
(rms) roughness of around 0.2-0.3 nm. An 11 nm thick HfO2 film deposited on silicon, for example, shows a rms roughness of only 0.27 nm, corresponding to a relative thickness of 2.70% (Figure S1, Supporting Information). The high resolution transmission electron microscopy (HRTEM) images recorded from cross section preparations of HFlast Si(001) and Si(111) wafers coated with titania or hafnia are shown in Figure 3. In all the cases a high conformal and
Atomic Layer Deposition of High-k Metal Oxides
Figure 4. Capacitance-voltage curve of a 10 nm thick HfO2 film deposited at 200 °C on (100) Si substrate. The arrows indicate the voltage sweep direction: full symbols are from depletion to accumulation and empty symbols are from accumulation to depletion. In the inset, current-voltage characteristics of MOS structures with a 10 nm thick HfO2 film deposited at 200 °C on (100) Si substrate and (a) 0.7 nm interfacial layer, (b) 1.2 nm interfacial layer, (c) 33 nm thick HfO2 film deposited at 80 °C.
generally homogeneous metal oxide film was detected. In the TEM, the high-κ films appear amorphous, as a matter of fact, they did not show lattice planes that could denote the presence of randomly oriented nanocrystallites or epitaxial layers. The most remarkable findings concern the sharp and thin interface between the silicon substrate and the metal oxide thin films. Especially in the case of TiO2 deposited on Si(111) at 200 °C, no oxide interlayer was detected (Figure 3b). For TiO2 thin films grown on Si(001) under the same conditions, only a very thin low-κ interfacial layer (
