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2009, 113, 16307–16310 Published on Web 08/21/2009
Plasma-Enhanced Atomic Layer Deposition of Anatase TiO2 Using TiCl4 Nicholas G. Kubala, Pieter C. Rowlette, and Colin A. Wolden* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: July 29, 2009; ReVised Manuscript ReceiVed: August 12, 2009
Self-limiting deposition of anatase TiO2 was accomplished by plasma-enhanced atomic layer deposition using TiCl4 and O2. Film growth occurred at a rate of ∼1.35 Å/cycle, independent of temperature. The refractive index increased monotonically with temperature, and the presence of the anatase phase was confirmed by FTIR and XRD for T g 110 °C. Films were free of impurities with respect to FTIR and XPS, and conformal coverage was confirmed by cross-section microscopy. These results show that OH groups, the most common surface termination in ALD, is not required for this process. Titanium dioxide (TiO2) thin films are of major technological interest due to their versatile physical and chemical attributes. Titania’s high refractive index makes it a common component in optical filters and coatings.1,2 TiO2 is a leading photocatalyst,3,4 and serves as a critical component in emerging photovoltaic technology.5,6 With its extraordinarily high dielectric constant, TiO2 thin films are a leading candidate for both memory and thin film transistor applications.7,8 Many of the applications described above require conformal coverage on nonplanar topographies. Among the vapor deposition techniques atomic layer deposition (ALD) is uniquely qualified to deliver conformal films with precision control over thickness and composition. Halides are the oldest class of ALD reactants with original investigations dating back to the 1960s.9 ALD of titania using TiCl4 and H2O is a well-established process,4,10 and the reaction mechanism is now quite well understood.11-13 The half reaction with H2O leaves an OH terminated surface to which TiCl4 adsorbs via hydrogen abstraction. However, this chemistry is problematic for low temperature applications since it is difficult to both remove Cl from as-deposited films11,12 and purge water vapor4 from vacuum systems under these conditions. The process releases corrosive HCl, and reactions between metal halides and OH groups have been implicated in particle formation.14 An alternative is plasma-enhanced atomic layer deposition (PEALD), using O radical as the oxidizer. The PEALD studies of TiO2 published to date have employed only organometallic precursors.15-18 Although water is not supplied directly, the reaction of O with the ligands produces combustionlike products with H2O being a major byproduct.19,20 In these systems, there is evidence that plasma oxidation leaves the surface terminated with hydroxyl groups.18,20 To our knowledge, PEALD of TiO2 using TiCl4 has not been explored, which is an intriguing concept since detrimental byproducts such as H2O and HCl are eliminated by design. Another hydrogen-free ALD process for TiO2 was developed by Schuisky et al. using TiI4 and O2.21 However TiI4 is a solid with relatively low vapor pressure and temperatures g255 °C were required to achieve impurity-free films. In this letter, we demonstrate that PEALD * To whom correspondence should be addressed. E-mail: cwolden@ mines.edu.
10.1021/jp907266c CCC: $40.75
Figure 1. Growth rate per cycle as a function of TiCl4 exposure at T ) 200 °C.
is effective for producing conformal, anatase TiO2 using TiCl4 at low temperature. All depositions were performed in an inductively coupled plasma reactor, previously used for pulsed plasma-enhanced chemical vapor deposition (PECVD) of Al2O322 and TiO2.23 Films were deposited on HF-cleaned Si substrates as a function of TiCl4 exposure and temperature, which was varied from 110 to 200 °C using resistive heating of the susceptor. In this remote configuration, continuous plasma operation was found to be a negligible contribution to substrate heating. O2 and Ar were continuously fed at 50 and 15 sccm, respectively. TiCl4 flow rate was controlled by a calibrated leak valve and directed to the chamber by an electronically controlled solenoid valve. The exposure level was controlled by varying the dose time (0.1-10 s). The rf plasma source was operated at 13.56 MHz and 200 W. The results reported here were independent of the duration of both the plasma exposure (5 s) and the purge steps (5 s) employed. Figure 1 shows the growth rate per cycle (GPC) observed as a function of TiCl4 exposure at 200 °C. Film thickness and refractive index were determined using ex situ spectroscopic ellipsometry (SE, Woollam M-44). The ellipsometry data were fit using the Cauchy model over the wavelength range of 2009 American Chemical Society
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J. Phys. Chem. C, Vol. 113, No. 37, 2009
Letters
Figure 2. Growth rate per cycle and index of refraction at 580 nm as a function of substrate temperature.
450-1300 nm, where absorption is negligible. The GPC initially increases before saturating at ∼1.1 Å/cycle for TiCl4 exposure levels g30 000 L. The functionality displayed in Figure 1 is indicative of irreversible saturating adsorption, which is the ideal ALD process.9 Stable species in the effluent from the reactor was monitored by quadrupole mass spectrometry (QMS, Stanford Research Systems). The only byproduct observed upon plasma exposure was molecular chlorine. Figure 2 plots the GPC and refractive index (580 nm) as a function of substrate temperature obtained at an exposure level of 30 000 L. The GPC is essentially constant at a value of 1.35 Å/cycle, independent of temperature over the range of investigation. The discrepancy between the two GPC values reported in Figures 1 and 2 is attributed to differences in film thickness, and the associated phase transition from amorphous to polycrystalline. An interesting feature of TiO2 ALD is that the deposited material is initially amorphous, but upon reaching a critical thickness films are transformed into a polycrystalline structure. This behavior has been noted by numerous groups, and the critical thickness is on the order of ∼20 nm with some variations due to specific processing conditions.4,10,11,17,23 The films in Figure 1 were deposited for 200 cycles, resulting in ∼20 nm films that were amorphous by both XRD and FTIR. In contrast, the films in Figure 2 were deposited for 400 cycles yielding films ∼54 nm in thickness and displaying the anatase crystal features as shown below. Note that further increasing the number of cycles up to 1200 resulted in no further changes in the GPC. The higher GPC value obtained on anatase material has been noted previously.18,23 Since the dose and temperature were fixed, this suggests that adsorption/reaction on the anatase surface occurs more readily than on the amorphous material. A contributing factor may be differences in surface roughness. Atomic force microscopy (AFM, Digital Nanoscope III) images of the 20 nm films were featureless, with an rms roughness of 0.5 nm. In contrast, the 54 nm films display a nanocrystalline morphology and the roughness increased to ∼2.7 nm. This structured surface provides more surface area than a planar substrate and may contribute to the 20% higher GPC values observed. The surface roughness argument is consistent with GPC measurements, as no appreciable changes were observed in surface roughness as the film thickness was increased from 54 and 170 nm. A second salient point is the relatively large GPC values obtained by this PEALD process. For TiO2 ALD, quite modest
Figure 3. FTIR spectra from the as-deposited TiO2 films shown in Figure 2, offset for clarity. (Inset) XRD pattern obtained at T ) 135 °C.
GPC values of ∼0.5 Å/cycle have generally been reported for both the thermal TiCl4/H2O process10-13 as well as most PEALD reports using organometallic ligands.15,17 An exception is a report of 1.9 Å/cycle for a PEALD process employing titanium isopropoxide.16 The GPC obtained in this work is ∼2.7 times greater than thermal ALD using TiCl4. It is generally thought that the GPC is controlled by the adsorption characteristics of the metal precursor.9 In thermal ALD, the surface site for TiCl4 adsorption is hydroxyl groups,11-13 while a different mechanism must take place in this hydrogen free process. No OH groups or other impurities were detected by XPS or FTIR. In the absence of hydroxyl groups TiCl4 is known to dissociatively adsorb at the temperatures of interest on a number of surfaces.24-26 Ritala and co-workers10 suggested that this reaction proceeds at oxide bridge sites as shown in reaction 1, since the analogous reaction has been observed on silica24
This highlights the important role of O atom in completing this ALD cycle. Reactions between molecular O2 and TiCl4 do not become appreciable until temperatures >400 °C.27 However, in the presence of atomic O chlorine-free, anatase films have been formed as low as 110 °C as shown below. The films deposited in this work were all of high quality as assessed by SE, FTIR, XPS, and XRD. Figure 2 plots the refractive indices as a function of substrate temperature. In all cases, the values are quite high, increasing linearly with temperature from 2.43 to 2.52. These are among the highest values reported for this temperature range,12,16 and are consistent with the presence of a dense film structure and the anatase phase. Ex situ Fourier transform infrared (FTIR, Nexus) transmission analysis was performed using a resolution of 1 cm-1. Figure 3 displays the FTIR spectra obtained from the films shown in Figure 2. Contributions of the silicon substrate were background subtracted, and the spectra are offset for clarity. Figure 3
Letters highlights the low wavenumber region, as the remainder of the spectra was free of absorption features. The only significant feature in each spectra is the vibration band centered at ∼436 cm-1 that has been assigned to the Ti-O-Ti transverse optic mode of the anatase phase.28,29 In vapor-deposited TiO2, it is common that a substantial fraction of film is made up of amorphous material,28 and that appears to be the case here as well. The intensity of the phonon peak increases dramatically with temperature, and these films are all nominally equivalent in thickness. The absorbance peak increased linearly from ∼0.06 in the film deposited at 110 °C to ∼0.18 in the film deposited at 200 °C. The increase in the anatase content mirrors changes in the refractive index, showing that there is a significant increase in the crystal content with temperature. X-ray diffraction (XRD, Siemens Kristalloflex 810) was performed using a Cu KR radiation source to determine the crystalline nature of the thin films. The presence of the anatase phase was confirmed by XRD, and a representative pattern is shown in the inset of Figure 3. The dominant peak in the XRD is due to the (101) crystal plane of anatase phase 2θ ) 25.4°. In pulsed PECVD, we found that intensity of the (101) diffraction peak scaled with the phonon absorbance,23 and this is the case here as well. The temperature dependence of the XRD is similar to the FTIR results shown in Figure 3. The achievement of the anatase phase at 110 °C is similar to what we observed for pulsed PECVD23 and is perhaps the lowest value reported to date for any ALD process.4,11 Notably absent from both FTIR and XRD are signals that may be attributed to the rutile phase. Rutile films typically display a strong (110) diffraction peak at 2θ ) 27.5°, as well as a strong absorption band in the IR at ∼500 cm-1.28,29 Finally, the chemical composition was assessed by X-ray photoelectron spectroscopy (XPS) using a Kratos system with Al KR X-ray source. The base pressure of the analysis chamber was
