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2009, 113, 6906–6909 Published on Web 04/08/2009
Digital Control of SiO2 Film Deposition at Room Temperature Pieter C. Rowlette, Marilou Canon, and Colin A. Wolden* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: March 09, 2009; ReVised Manuscript ReceiVed: March 31, 2009
In this Letter, we propose and demonstrate a robust process for digital control of high quality SiO2 thin films at room temperature using pulsed plasma-enhanced chemical vapor deposition (PECVD). Plasma activation of the SiCl4 precursor is critical, as atomic layer deposition does not occur under these conditions. Subangstrom control over deposition rate was obtained by adjusting the density of SiCl4 present at plasma ignition. The intrinsic refractive index of 1.46 was obtained at rates between 0.8 and 1.6 Å/pulse. No impurities were detected by either X-ray photoelectron spectroscopy (XPS) or Fourier transform infrared (FTIR) in films deposited under optimal conditions. Atomic layer deposition (ALD) is a leading technique for thin film deposition. Growth proceeds through cycles of sequential, self-limiting surface reactions that impart nanoscale control over thickness and composition. Since its inception over three decades ago,1 ALD has been refined and successfully demonstrated for numerous materials, particularly metal oxides.2 However, a number of materials have proven difficult to deposit by ALD. Perhaps the most auspicious example is silicon dioxide, given its role as the most important and widely used dielectric in the integrated circuit industry.3 The outstanding chemical, optical, and electrical properties of vapor-deposited SiO2 make it an important component for photovoltaics,4,5 optics,6 and barrier applications.7,8 Continued miniaturization of device structures and the extension to flexible substrates requires deposition techniques that impart nanoscale control over thickness at low temperature. Despite the critical importance of SiO2 and maturation of ALD technology, a robust, low temperature process for digital control of SiO2 growth remains elusive. A primary challenge to SiO2 ALD is overcoming the limited reactivity of silicon-containing precursors. The George group first demonstrated silica ALD using SiCl4 and H2O as coreagents, with the two separate half-reactions combining to produce the overall reaction SiCl4 + 2H2O f SiO2 + 4HCl.9 However, this group also acknowledged the limitations of this process including high temperature requirements (600-800 K), large reactant exposure (∼109 L, 1 L ) 10-6 Torr · s), and the production of corrosive HCl as a reaction product.10 Low temperature (2%), Cl is detrimental, leading to porosity and increased surface roughness.19 Our group has established pulsed PECVD as an alternative to ALD for self-limiting growth of metal oxide thin films, including Ta2O5,20,21 Al2O3,22,23 TiO2,24 and ZnO.24 The process has been described in detail previously,25,26 and is briefly reviewed here. In pulsed PECVD, O2 and the metal precursor are mixed and delivered simultaneously without purge steps. Growth occurs discretely by modulating the plasma power at low frequency (∼Hz). The nature of self-limiting growth is fundamentally different than ALD. Instead of relying on surface chemistry, growth terminates during each plasma step due to consumption of the precursor. The process is self-limiting in the sense that no deposition occurs with the plasma off, and the amount of growth per cycle (GPC) is independent of the length of plasma exposure. In pulsed PECVD, there can be two contributions to film growth.25,26 The first is an ALD component that relies on precursor adsorption when the plasma is off, and subsequent oxidation when the plasma is on. In addition, there 2009 American Chemical Society
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J. Phys. Chem. C, Vol. 113, No. 17, 2009 6907
Figure 1. Schematic of the pulsed PECVD cycle used in this work.
is a PECVD-like growth component resulting from plasma activation of the precursor. The amount of this latter contribution can be adjusted by varying the precursor density in the chamber at plasma ignition. The process has been demonstrated to deliver digital control over film thickness and produce a high degree of conformality at significant rates (>20 nm/min).24,25 The pulsed PECVD cycle used for digital growth of SiO2 is shown schematically in Figure 1. This is a departure from our previous studies, in which both O2 and the precursor were delivered continuously.20-26 Due to its high vapor pressure (235 Torr at 25 °C), it was not possible with existing equipment to control the deposition rate with angstrom resolution using continuous delivery of SiCl4. Instead, both the precursor and plasma were pulsed, while O2 flowed continuously. A fixed dose of SiCl4 was introduced into the chamber by opening an electronic timing valve on a fixed control volume. The GPC was then readily controlled by adjusting the time delay between application of the SiCl4 dose and plasma ignition. The potential of PE-ALD was also investigated by simply increasing the time delay such that there was no overlap between the SiCl4 and plasma exposures. SiO2 thin films were deposited in a capacitively coupled PECVD system that was previously used for pulsed PECVD/ PE-ALD of both Al2O3 and ZnO.24,25,27 Premixed process gases were delivered through a showerhead that also served as the powered electrode. Substrates were clamped to the grounded electrode, and the system was operated as a hot wall reactor (25-180 °C). O2 (200 sccm) and Ar (160 sccm) flowed continuously, while the precursor dose was delivered by opening an electronic timing valve on the SiCl4 source. The timing of the SiCl4 dose and plasma exposure were both controlled by LabVIEW. The plasma power and its exposure time were fixed at 100 W and 3 s, respectively. Note that SiCl4 and O2 are thermally inert at the conditions explored, with no growth resulting from these reactants with the plasma off. The reactor pressure was P ) 420 mTorr, with the exception of minor excursions during SiCl4 dosing. The SiCl4 exposure was estimated by integrating the pressure surge that accompanied each dose. The first important result to report is that there is no evidence of deposition under pure PE-ALD operation. Experiments consisting of 1000 ALD cycles at 180 °C with exposures up to ∼109 L resulted in only native oxide formation (1-2 nm), comparable to the result obtained when exposing Si to O2 plasma under those conditions. This suggests that SiCl4 adsorption is negligible at the conditions explored, which is consistent with previous ALD studies using this precursor.9 However, growth was readily observed under pulsed PECVD operation. The GPC could be tuned to the desired value with sub-angstrom resolution by simply adjusting the delay between the SiCl4 dose and plasma ignition (Figure 1). Film thickness and refractive index were determined using spectroscopic ellipsometry (J. A. Woollam), and interpreted using a standard Cauchy model. Figure 2 plots the refractive index (λ ) 580 nm) as a function of GPC at T )
Figure 2. Refractive index (λ ) 580 nm) as a function of growth per cycle (GPC) at T ) 180 °C. The GPC was adjusted by varying the plasma ignition delay (Figure 1).
Figure 3. Growth per cycle (diamonds, left) and refractive index (squares, right) as a function of temperature at constant delay. The high index was recovered at room temperature when the GPC was lowered to 0.8 Å/pulse (open symbols) by adjusting the delay.
180 °C. The films in Figure 2 were all ∼60 nm in thickness. Plasma activation was found to be very effective for precursor utilization, as the total SiCl4 exposure was just 250 L/pulse for these experiments. The reported indices are all excellent, approaching the value for thermal oxide. The index plateaus at a maximum of ∼1.462 when the GPC is between 1 and 1.6 Å/pulse. This corresponds to a SiCl4 deposition flux of ∼3 × 1014/cm2, which is in excellent agreement with the values reported for monolayer coverage in the surface science literature.28,29 The optimal GPC values are somewhat higher than those obtained from the high temperature ALD process,11 and a factor of 5 greater than PE-ALD with TEOS.15 The index declines slightly at both lower and higher GPC values, presumably due to the development of porosity that results from submonolayer or multilayer coverage, respectively. Figure 3 plots the GPC and refractive index as a function of temperature, with all other parameters fixed. The GPC is essentially constant at 1.6 Å/pulse, with a slightly higher value obtained at room temperature. The index has a stronger dependence on temperature. At T > 100 °C, an index of n ∼1.46 was measured, while a value of n ∼1.42 was obtained for films deposited at lower temperature. The index is a reflection of film density, and the observed changes may possibly be related to
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Figure 4. Relative etch rate of the films shown in Figure 3.
trace amounts of Cl incorporation as discussed below. To improve the index at low temperature, the GPC was varied using the fine control provided by pulsed PECVD. It was found that the optimal index of 1.46 was recovered when the GPC was reduced to 0.8 Å/pulse, as indicated by the open symbols in Figure 3. A key to the high material quality obtained with pulsed PECVD is the effective in situ O atom annealing that occurs with each plasma exposure. This is critical to remove impurities and produce stoichiometric films. As the temperature is reduced, the amount of material that can be fully annealed must be reduced to maintain the highest quality. The use of refractive index as a measure of film quality was confirmed by measurements of film etch rate. Etch rate is a convenient and accurate method commonly used to characterize SiO2 film quality, being well correlated with important parameters such as breakdown field, density, and impurity content.30,31 Etch rate measurements were performed using a 6:1 H2O/HF solution, buffered with NH4F. To permit fair comparisons, we report values normalized with respect to the etch rate obtained from a thermal oxide standard formed in dry O2 at 1000 °C. Figure 4 plots the relative etch rate (RER) obtained from the films shown in Figure 3. The two films deposited at T > 140 °C had RER ratios
