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J. Phys. Chem. C 2008, 112, 19530–19539
Al2O3 Atomic Layer Deposition with Trimethylaluminum and Ozone Studied by in Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry David N. Goldstein,† Jarod A. McCormick,‡ and Steven M. George*,†,‡ Department of Chemistry and Biochemistry, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorad, 80309 ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: September 25, 2008
The atomic layer deposition (ALD) of Al2O3 using sequential exposures of Al(CH3)3 and O3 was studied by in situ transmission Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The FTIR spectroscopy investigations of the surface reactions occurring during Al2O3 ALD were performed on ZrO2 particles for temperatures from 363 to 650 K. The FTIR spectra after Al(CH3)3 and ozone exposures showed that the ozone exposure removes surface AlCH3* species. The AlCH3* species were converted to AlOCH3* (methoxy), Al(OCHO)* (formate), Al(OCOOH)* (carbonate), and AlOH* (hydroxyl) species. The TMA exposure then removes these species and reestablishes the AlCH3* species. Repeating the TMA and O3 exposures in a sequential reaction sequence progressively deposited the Al2O3 ALD film as monitored by the increase in absorbance for bulk Al2O3 infrared features. The identification of formate species was confirmed by separate formaldehyde adsorption experiments. The formate species were temperature dependent and were nearly absent at temperatures g650 K. QMS analysis of the gas phase species revealed that the TMA reaction produced CH4. The ozone reaction produced mainly CH4 with small amounts of C2H4 (ethylene), CO, and CO2. Transmission electron microscopy (TEM) was also used to examine the Al2O3 ALD films deposited on the ZrO2 particles. These TEM images observed conformal Al2O3 ALD films with thicknesses that were consistent with an Al2O3 ALD growth rate of 1.1 Å/cycle. The surface species after the O3 exposures and the mass spectrometry results lead to a very different mechanism for Al2O3 ALD growth using TMA and O3 compared with Al2O3 ALD using TMA and H2O. I. Introduction Atomic layer deposition (ALD) is an ideal technique to deposit ultrathin films with high conformality and precise thickness control.1,2 Traditional methods to deposit Al2O3 with ALD involve sequential surface reactions of Al(CH3)3 (trimethylaluminum (TMA)) and water.3-6 These sequential reactions allow conformal Al2O3 film growth with thickness control on a variety of substrates including polymers,7 porous membranes,3,8 and nanopowders.9 The details of the Al2O3 ALD reaction have been extensively studied by a variety of techniques, including the quartz crystal microbalance measurements,10,11 Fourier transform infrared (FTIR) spectroscopy,3,12 ellipsometry,4,5 and X-ray photoelectron spectroscopy (XPS).13 Al2O3 ALD is a model system and serves as a reference point for other ALD systems. The semiconductor industry is interested in growing Al2O3 films with ozone instead of water as the oxygen source. Al2O3 is a high-k dielectric that is used as a dielectric film for both DRAM and MOS-FETs.14 When ozone is used as the oxidant, the Al2O3 ALD films can have leakage current densities that are reduced by two orders of magnitude in comparison with Al2O3 ALD films deposited with water.15 This improvement and smaller flat band voltage shifts allow Al2O3 ALD films grown using ozone to make better gate oxides.15,16 There are also other advantages when replacing H2O with ozone. Water desorbs slowly from substrates and requires longer purge times.10 Water can also leave unreacted hydroxyl groups in the Al2O3 ALD * Corresponding author. † Department of Chemistry and Biochemistry. ‡ Department of Chemical and Biological Engineering.
films.3 The unreacted hydroxyl groups in the films may affect the dielectric and material properties of the Al2O3 ALD films. However, no change in equivalent oxide thickness (EOT) of Al2O3 ALD films was observed when ozone was used as the oxidant.15 Previous research has been conducted on Al2O3 ALD with TMA and O3. A growth rate of ∼0.8 Å per cycle at 300-450 °C has been measured by several investigations.13,14,17,18 XPS measurements revealed Al2O3 ALD films that had lower carbon impurities with ozone compared with water.13 Al2O3 films grown with ozone also had a reduced percentage of Al-Al defects that degrade the electrical properties of the Al2O3 ALD films. These defects have been characterized using XPS by the presence of a shoulder on the 72.5 eV Al 2p peak.19 Time-offlight secondary ion mass spectrometer (TOF-SIMS) analysis has probed the bulk of Al2O3 ALD films and revealed different impurity levels in Al2O3 films grown with ozone compared with Al2O3 films grown with water.13 Hydrogen impurities were reduced in the ozone grown films.13 To understand the differences between Al2O3 ALD with TMA and either H2O or ozone, this study employed in situ transmission FTIR spectroscopy to monitor the surface species formed and removed during the TMA and O3 exposures. The FTIR spectra also revealed the growth of Al2O3 bulk vibrational modes versus the number of ALD reaction cycles. Additional experiments also monitored the gas phase products during O3 and TMA exposures using a quadrupole mass spectrometer (QMS). The resulting Al2O3 ALD films on the ZrO2 particles were then analyzed with transmission electron microscopy (TEM) to obtain the Al2O3 ALD growth per ALD cycle. These FTIR, QMS, and
10.1021/jp804296a CCC: $40.75 2008 American Chemical Society Published on Web 11/13/2008
Al2O3 Atomic Layer Deposition
Figure 1. Schematic of W grid in the ALD reactor.
Figure 2. Schematic of the inlet and outlet connections to the ALD reactor.
TEM studies help to clarify the surface chemistry and thin film growth mechanism during Al2O3 ALD with TMA and ozone. II. Experimental Section The surface chemistry and thin film growth during Al2O3 ALD was studied using sequential exposures of TMA and O3 at various temperatures. Al2O3 ALD films were grown on ZrO2 particles in an ALD reactor designed for in situ FTIR spectroscopy studies.12,20 Figure 1 presents a schematic of the ALD reactor. The reactor was a warm-wall reactor where the chamber walls were heated to 350 K while the sample could be independently heated to >900 K. Figure 2 shows a schematic of all the inlet and outlet connections to the ALD reactor. Two argon mass flow controllers (MFC) regulated the flow of argon
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19531 through the reactor at 220 sccm (110 sccm per MFC). This flow established a base pressure of 1.30 Torr. An Alcatel 2012A rotary vane pump removed the argon and reaction byproducts from the reactor. Pneumatic leak valves with conductance metering valves allowed accurate exposure of the reactants. A Labview measurement system controlled the reactant exposures and integrated the area beneath the pressure transients that occurred during the reactant exposures. The reactant exposures were performed with use of micropulses that were less than the exposures required for the reactions to reach completion. The absolute reactant exposures were determined with no ZrO2 nanoparticles in the reactor after a sufficient number of micropulses for the reaction on the reactor walls to reach completion. Under these conditions, the reaction products do not interfere with the measurement of the absolute reactant exposure. The Al2O3 ALD was coated onto ZrO2 nanopowders supported in a 2 × 3 cm2 tungsten grid.12,20,21 Each W grid was 50 µm thick and was photoetched to 100 grid lines per inch. Tantalum foil was spot-welded on the sides of the grid to improve current transfer through the grid. The entire grid was then attached to a copper clamp that was interfaced via an electrical feedthrough to a Hewlett-Packard 6268B power supply. Resistive heating was used to heat the sample. A Love Controls 16A3 PID controller interfaced to a type K thermocouple mounted on the sample grid provided temperature control of the sample. The feedback loop maintained the sample temperature at (2 °C. Preparation of the substrate involved pressing ZrO2 nanoparticles into the W grid.12,20,21 Each grid was first sonicated in deionized water and methanol and then blown dry with ultrapure nitrogen. The grid was then placed into a stainless steel die and covered with an excess of nanopowders. Subsequently, a manual press forced the particles into the W grid until the particles made a dense matrix with very few pinholes in the sample. Excess nanopowders lying on the top of the grid were easily removed with a razor blade. The finished sample contained about 22 mg of ZrO2 powder. This quantity of ZrO2 powder is equivalent to a surface area of ∼0.44 m2. Finally, a type K thermocouple was attached to the top of the sample grid with Ceramabond 571 Epoxy. This epoxy electrically isolated the thermocouple and kept the thermocouple firmly attached to the sample during the experiment. An infrared beam from a Nicolet Magna 560 FTIR spectrometer was externally aligned to pass through the W grid sample. The ZrO2 nanopowder substrates provided a large surface area and improved the signal-to-noise ratio for infrared absorption. The entire sample stage could be translated along the vertical z-axis direction to move the sample out of the FTIR beam. This displacement allowed the background reference spectra to be measured frequently over the course of these experiments. A liquid nitrogen cooled MCT-B (mercury cadmium telluride) detector allowed measurement of the infrared spectra from 400 to 4000 cm-1. During the reactant exposures, the gate valves on the CsI windows were closed to prevent deposition on the windows. All FTIR spectra were obtained at 4 cm-1 resolution using 100 averaged scans and were referenced to the CsI window background. However, most of the FTIR spectra in this paper are presented as difference spectra. Mass spectrometry analysis was performed in a rotary reactor designed for ALD on nanoparticles. The design and operation of this reactor has been discussed in previous publications.9,22 To provide in situ quadrupole mass spectrometry analysis, a 200 amu quadrupole mass spectrometer with a pressure reduc-
19532 J. Phys. Chem. C, Vol. 112, No. 49, 2008 tion system (PPR200, SRS Inc., Sunnyvale, CA) was attached to the reactor. During reactant exposures, the QMS scanned the mass range from 1-75 m/z with 0.1 m/z resolution. A Faraday cup was used as the detector with no electron multiplier. With these settings, about 5 s was required to scan the entire mass range. Micropulses of both TMA and O3 were used to determine the exposures required for the reactions to reach completion with 1.0 g of ZrO2 nanoparticles in the rotary reactor.9 TMA was dosed into the rotary reactor to a pressure of 0.3 Torr above the baseline pressure. The O2/O3 mixture was dosed into the rotary reactor to a pressure of 0.5 Torr. Each reactant reacted for 60 s in the chamber and then was purged for 60 s before a final argon pulse flushed the chamber. The reactor then returned to base pressure before starting the next set of reactant micropulses. Approximately 20 micropulses of TMA and 60 micropulses of O3 were required for each reaction to reach completion with 1.0 g of ZrO2 nanoparticles in the rotary reactor. The ZrO2 particles were obtained from Nanomaterials Research Corporation (Longmont, CO). These ZrO2 particles were spherical with an average diameter of ∼50 nm and a surface area of ∼20.2 m2/g. TMA was obtained from Aldrich (Milwaukee, WI) and had a purity of 97%. The water was high performance liquid chromatography (HPLC) grade from Fisher Scientific (Pittsburgh, PA). Ozone was produced from UHP grade oxygen (99.9%) obtained from Airgas Ltd. (Cheyenne, WY). All chemicals were used as purchased, except for water, which was subjected to 3 freeze-pump-thaw cycles prior to use. Ozone was obtained from O2 by flowing 300 sccm of O2 into a DelOzone LC-14 ozone generator (San Luis Obispo, CA). This flow produced a 6 psi pressure in the generating cell. At 100% power, the ozone concentration at the outlet was 3.7% with the balance being O2. When the ozone was not going through the ALD reactor, the ozone was sent through a magnesia ozone destruct unit and the remaining O2 was evacuated with a separate rotary vane pump. In the rotary reactor, the O3 was generated with an Ozonia OZAT CFS-1A ozone generator (Duebendorf, Switzerland). The ozone generator ran with O2 at a flow rate of 0.2 m3 h-1 and power of 510 W. These conditions produced an O3 concentration of 12% by mass. TEM analysis was performed in the Department of Molecular and Cellular Biology at the University of Colorado at Boulder. The TEM results were obtained with a Philips CX11 highresolution transmission electron microscope with 80 kV beam potential. A Gatan slow scan charge-coupled device camera captured the TEM images. The TEM studies monitored the conformality and thickness of the Al2O3 films on the ZrO2 particles. III. Results and Discussion A. Fourier Transform Infrared Spectroscopy. Studying the surface chemistry of ALD processes requires a reliable starting surface. FTIR spectroscopy can determine the initial surface species on the ZrO2 nanopowders to ensure that they will be suitable for Al2O3 ALD. A range of absorbances is observed on the ZrO2 nanoparticles including the following: O-H stretching vibrations at 3670-3780 cm-1; C-H stretching vibrations at 2850-3050 cm-1; and the bulk ZrO2 absorbance at frequencies
