Al2O3 Atomic Layer Deposition With Trimethylaluminum and Ozone Studied by in Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry-goldstein2008

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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

ReceiV

ed: May 14, 2008; ReV

ised Manuscript ReceiV

ed: September 25, 2008

The atomic layer deposition (ALD) of Al2O3using sequential exposures of Al(CH3)3and O3was studied byin situ transmission Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS).The FTIR spectroscopy investigations of the surface reactions occurring during Al2O3ALD were performedon ZrO2particles for temperatures from 363 to 650 K. The FTIR spectra after Al(CH 3)3and ozone exposuresshowed that the ozone exposure removes surface AlCH3* species. The AlCH3* species were converted toAlOCH3* (methoxy), Al(OCHO)* (formate), Al(OCOOH)* (carbonate), and AlOH* (hydroxyl) species. TheTMA exposure then removes these species and reestablishes the AlCH3* species. Repeating the TMA and O3exposures in a sequential reaction sequence progressively deposited the Al2O3ALD film as monitored by theincrease in absorbance for bulk Al2O3infrared features. The identification of formate species was confirmedby separate formaldehyde adsorption experiments. The formate species were temperature dependent and werenearly absent at temperatures g650 K. QMS analysis of the gas phase species revealed that the TMA reactionproduced CH4. The ozone reaction produced mainly CH4with small amounts of C2H4(ethylene), CO, andCO2. Transmission electron microscopy (TEM) was also used to examine the Al2O3ALD films deposited onthe ZrO2 particles. These TEM images observed conformal Al2O3 ALD films with thicknesses that wereconsistent with an Al2O3ALD growth rate of 1.1 /cycle. The surface species after the O3exposures and themass spectrometry results lead to a very different mechanism for Al2O3 ALD growth using TMA and O3compared with Al2O3ALD using TMA and H2O.

I. Introduction

Atomic layer deposition (ALD) is an ideal technique todeposit ultrathin films with high conformality and precisethickness control.1,2 Traditional methods to deposit Al2O3with

ALD involve sequential surface reactions of Al(CH3)3 (tri-methylaluminum (TMA)) and water.3-6 These sequential reac-tions allow conformal Al2O3film growth with thickness controlonavarietyofsubstratesincludingpolymers, 7 porousmembranes,3,8

and nanopowders.9 The details of the Al2O3ALD reaction havebeen extensively studied by a variety of techniques, includingthe quartz crystal microbalance measurements,10,11 Fouriertransform infrared (FTIR) spectroscopy,3,12 ellipsometry,4,5 andX-ray photoelectron spectroscopy (XPS).13 Al2O3 ALD is amodel system and serves as a reference point for other ALDsystems.

The semiconductor industry is interested in growing Al2O3films with ozone instead of water as the oxygen source. Al2O3

is a high-k dielectric that is used as a dielectric film for bothDRAM and MOS-FETs.14 When ozone is used as the oxidant,the Al2O3 ALD films can have leakage current densities thatare reduced by two orders of magnitude in comparison withAl2O3ALD films deposited with water.15 This improvement andsmaller flat band voltage shifts allow Al2O3ALD films grownusing ozone to make better gate oxides.15,16 There are also otheradvantages when replacing H2O with ozone. Water desorbsslowly from substrates and requires longer purge times.10 Watercan also leave unreacted hydroxyl groups in the Al2O3 ALD

films.3 The unreacted hydroxyl groups in the films may affectthe dielectric and material properties of the Al2O3ALD films.However, no change in equivalent oxide thickness (EOT) ofAl2O3 ALD films was observed when ozone was used as the

oxidant.15

Previous research has been conducted on Al2O3 ALD withTMA and O3. A growth rate of0.8 per cycle at 300-450Chas been measured by several investigations.13,14,17,18 XPSmeasurements revealed Al2O3ALD films that had lower carbonimpurities with ozone compared with water.13 Al2O3films grownwith ozone also had a reduced percentage of Al-Al defectsthat degrade the electrical properties of the Al2O3ALD films.These defects have been characterized using XPS by thepresence of a shoulder on the 72.5 eV Al 2p peak.19 Time-of-flight secondary ion mass spectrometer (TOF-SIMS) analysishas probed the bulk of Al2O3ALD films and revealed differentimpurity levels in Al2O3films grown with ozone compared with

Al2O3 films grown with water.13

Hydrogen impurities werereduced in the ozone grown films.13

To understand the differences between Al2O3ALD with TMAand either H2O or ozone, this study employed in situ transmis-sion FTIR spectroscopy to monitor the surface species formedand removed during the TMA and O3 exposures. The FTIRspectra also revealed the growth of Al2O3bulk vibrational modesversus the number of ALD reaction cycles. Additional experi-ments also monitored the gas phase products during O3 andTMA exposures using a quadrupole mass spectrometer (QMS).The resulting Al2O3ALD films on the ZrO2particles were thenanalyzed with transmission electron microscopy (TEM) to obtainthe Al2O3ALD growth per ALD cycle. These FTIR, QMS, and

* Corresponding author. Department of Chemistry and Biochemistry. Department of Chemical and Biological Engineering.

J. Phys. Chem. C2008, 112,195301953919530

10.1021/jp804296a CCC: $40.75 2008 American Chemical SocietyPublished on Web 11/13/2008


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TEM studies help to clarify the surface chemistry and thin filmgrowth mechanism during Al2O3ALD with TMA and ozone.

II. Experimental Section

The surface chemistry and thin film growth during Al2O3ALD was studied using sequential exposures of TMA and O3at various temperatures. Al2O3ALD films were grown on ZrO2particles in an ALD reactor designed for in situ FTIR spectros-copy studies.12,20 Figure 1 presents a schematic of the ALDreactor. The reactor was a warm-wall reactor where the chamberwalls were heated to 350 K while the sample could beindependently heated to >900 K. Figure 2 shows a schematic

of all the inlet and outlet connections to the ALD reactor. Twoargon mass flow controllers (MFC) regulated the flow of argon

through the reactor at 220 sccm (110 sccm per MFC). This flowestablished a base pressure of 1.30 Torr. An Alcatel 2012Arotary vane pump removed the argon and reaction byproductsfrom the reactor.

Pneumatic leak valves with conductance metering valvesallowed accurate exposure of the reactants. A Labview measure-ment system controlled the reactant exposures and integratedthe area beneath the pressure transients that occurred duringthe reactant exposures. The reactant exposures were performed

with use of micropulses that were less than the exposuresrequired for the reactions to reach completion. The absolutereactant exposures were determined with no ZrO2nanoparticlesin the reactor after a sufficient number of micropulses for thereaction on the reactor walls to reach completion. Under theseconditions, the reaction products do not interfere with themeasurement of the absolute reactant exposure.

The Al2O3 ALD was coated onto ZrO2 nanopowders sup-ported in a 2 3 cm2 tungsten grid.12,20,21 Each W grid was 50m thick and was photoetched to 100 grid lines per inch.Tantalum foil was spot-welded on the sides of the grid toimprove current transfer through the grid. The entire grid wasthen attached to a copper clamp that was interfaced via an

electrical feedthrough to a Hewlett-Packard 6268B powersupply. Resistive heating was used to heat the sample. A LoveControls 16A3 PID controller interfaced to a type K thermo-couple mounted on the sample grid provided temperature controlof the sample. The feedback loop maintained the sampletemperature at (2 C.

Preparation of the substrate involved pressing ZrO2 nano-particles into the W grid.12,20,21 Each grid was first sonicated indeionized water and methanol and then blown dry with ultrapurenitrogen. The grid was then placed into a stainless steel die andcovered with an excess of nanopowders. Subsequently, a manualpress forced the particles into the W grid until the particles madea dense matrix with very few pinholes in the sample. Excess

nanopowders lying on the top of the grid were easily removedwith a razor blade. The finished sample contained about 22 mgof ZrO2powder. This quantity of ZrO2powder is equivalent toa surface area of 0.44 m2. Finally, a type K thermocouplewas attached to the top of the sample grid with Ceramabond571 Epoxy. This epoxy electrically isolated the thermocoupleand kept the thermocouple firmly attached to the sample duringthe experiment.

An infrared beam from a Nicolet Magna 560 FTIR spec-trometer was externally aligned to pass through the W gridsample. The ZrO2 nanopowder substrates provided a largesurface area and improved the signal-to-noise ratio for infraredabsorption. The entire sample stage could be translated alongthe verticalz-axis direction to move the sample out of the FTIRbeam. This displacement allowed the background referencespectra to be measured frequently over the course of theseexperiments. A liquid nitrogen cooled MCT-B (mercury cad-mium telluride) detector allowed measurement of the infraredspectra from 400 to 4000 cm-1. During the reactant exposures,the gate valves on the CsI windows were closed to preventdeposition on the windows. All FTIR spectra were obtained at4 cm-1 resolution using 100 averaged scans and were referencedto the CsI window background. However, most of the FTIRspectra in this paper are presented as difference spectra.

Mass spectrometry analysis was performed in a rotary reactordesigned for ALD on nanoparticles. The design and operationof this reactor has been discussed in previous publications.9,22

To provide in situ quadrupole mass spectrometry analysis, a200 amu quadrupole mass spectrometer with a pressure reduc-

Figure 1. Schematic of W grid in the ALD reactor.

Figure 2. Schematic of the inlet and outlet connections to the ALDreactor.

Al2O3Atomic Layer Deposition J. Phys. Chem. C, Vol. 112, No. 49, 2008 19531


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tion system (PPR200, SRS Inc., Sunnyvale, CA) was attachedto the reactor. During reactant exposures, the QMS scanned themass range from 1-75m/zwith 0.1 m/zresolution. A Faradaycup was used as the detector with no electron multiplier. Withthese settings, about 5 s was required to scan the entire massrange.

Micropulses of both TMA and O3 were used to determinethe exposures required for the reactions to reach completionwith 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 abovethe baseline pressure. The O2/O3 mixture was dosed into therotary reactor to a pressure of 0.5 Torr. Each reactant reactedfor 60 s in the chamber and then was purged for 60 s before afinal argon pulse flushed the chamber. The reactor then returnedto base pressure before starting the next set of reactantmicropulses. Approximately 20 micropulses of TMA and 60micropulses of O3 were required for each reaction to reachcompletion with 1.0 g of ZrO2nanoparticles in the rotary reactor.

The ZrO2 particles were obtained from Nanomaterials Re-search Corporation (Longmont, CO). These ZrO2particles werespherical with an average diameter of50 nm and a surfacearea of 20.2 m2/g. TMA was obtained from Aldrich (Mil-

waukee, WI) and had a purity of 97%. The water was highperformance liquid chromatography (HPLC) grade from FisherScientific (Pittsburgh, PA). Ozone was produced from UHPgrade 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 touse.

Ozone was obtained from O2 by flowing 300 sccm of O2into a DelOzone LC-14 ozone generator (San Luis Obispo, CA).This flow produced a 6 psi pressure in the generating cell. At100% power, the ozone concentration at the outlet was 3.7%with the balance being O2. When the ozone was not goingthrough the ALD reactor, the ozone was sent through a magnesia

ozone destruct unit and the remaining O2was evacuated with aseparate rotary vane pump. In the rotary reactor, the O3 wasgenerated with an Ozonia OZAT CFS-1A ozone generator(Duebendorf, Switzerland). The ozone generator ran with O2at a flow rate of 0.2 m3 h-1 and power of 510 W. Theseconditions produced an O3concentration of 12% by mass.

TEM analysis was performed in the Department of Molecularand Cellular Biology at the University of Colorado at Boulder.The TEM results were obtained with a Philips CX11 high-resolution transmission electron microscope with 80 kV beampotential. A Gatan slow scan charge-coupled device cameracaptured the TEM images. The TEM studies monitored theconformality and thickness of the Al2O3 films on the ZrO2

particles.

III. Results and Discussion

A. Fourier Transform Infrared Spectroscopy. Studyingthe surface chemistry of ALD processes requires a reliablestarting surface. FTIR spectroscopy can determine the initialsurface species on the ZrO2 nanopowders to ensure that theywill be suitable for Al2O3 ALD. A range of absorbances isobserved on the ZrO2 nanoparticles including the following:O-H stretching vibrations at 3670-3780 cm-1; C-H stretchingvibrations at 2850-3050 cm-1; and the bulk ZrO2absorbanceat frequencies


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hydroxyl surface species. In addition, negative absorbancefeatures are observed at 2920-2980 cm-1 corresponding toC-H stretching vibrations and at 1212 cm-1 corresponding tothe Al-CH3 deformation mode. These negative absorbancefeatures are both from the removal of AlCH3* surface species.Figure 3b shows the FTIR difference spectra for the next TMAexposure of 0.85 Torr s at 450 K. This spectrum indicates thatTMA removed the AlOH* surface species and added AlCH3*surface species. The FTIR difference spectra in Figure 3a,b are

consistent with previous FTIR studies of Al2O3ALD with TMAand H2O.3,12

The surface species change dramatically after the first ozoneexposure. Figure 3c shows a markedly different spectrum withnew positive absorbance features visible between 1200 and 1700cm-1. The most prominent new features were observed at 1388,1404, and 1597 cm-1. There were also smaller new featuresobserved at 1320 and 1475 cm-1 and a shoulder at 1720 cm-1.Not all of the C-H features were eliminated after long ozoneexposures. There were small absorbance features at 2923 and3016 cm-1 for C-H stretching vibrations that partially resultfrom slight frequency shifts. In addition, the absorbance for theO-H stretching vibrations at 3734-3778 cm-1 was reduced in

intensity compared with the intensity observed in Figure 3a afterH2O exposures. In addition, Figure 3d shows that the newabsorbance features added by the O3exposure were completelyremoved by the next TMA exposure. TMA exposures reformthe absorbances from the C-H stretching vibrations at2820-2970 cm-1 and the methyl deformation at 1212 cm-1.The absorbance from the O-H stretching vibrations at3734-3778 cm-1 was also removed by TMA doses.

The new absorbance features appearing after the ozoneexposure are very characteristic of formate and carbonate groupson the Al2O3ALD surface. We first reported these new formatefeatures at the AVS Topical Conference for Atomic LayerDeposition in 2006 (ALD2006).24 Formate groups are very

common on metal oxide surfaces and can be formed by exposingmetal oxides to a variety of reagents including carbon monoxide,methanol, and formaldehyde.25-27 The absorption features at1388 and 1597 cm-1 correspond to the symmetric and anti-symmetric OCO modes of bound formate species. In addition,the 1404 cm-1 shoulder is attributed to the CH bend of formatespecies. In the C-H stretching region, the two peaks at 2923and 3016 cm-1 are identified as the Fermi resonance of theantisymmetric OCO mode mixing with the lone C-H stretchingvibration in the formate surface group.26 All of these valuesmatch literature values for formate on aluminum oxide.26,28,29

Formate features during Al2O3 ALD with ozone have alsobeen observed recently by FTIR studies on planar surfaces thatwere complemented by DFT calculations.30 These studiesobserved the same methoxy modes at 1388 and 1475 cm-1 thatwere monitored in this study on the ZrO2 nanoparticles. Inaddition, the ratios of the absorbances for the methoxy andprimary formate vibrational features were similar on the planarsurfaces and the ZrO2nanoparticles. This agreement also rulesout the possibility that ozone decomposition may have preventedthe ozone from reaching the interior surfaces of the ZrO2nanoparticle sample.

The surface coordination of the formate features is describedby the frequency difference between the symmetric and asym-metric OCO stretching vibrations. Our observed experimentaldifference of 212 cm-1 is greater than that of the free formateion. Consequently, the formate species are doubly coordinated

to aluminum sites on the surface.28 These doubly coordinatedsurface species formed after ozone exposure may contribute to

the oxygen-rich stoichiometry of Al2O3 films deposited usingTMA and ozone.31 The remaining shoulders at 1320 and 1720cm-1 are the symmetric and asymmetric OCO stretchingvibrations of carbonate groups bound to alumina. Some of thehydroxyl features observed in the difference spectra could alsoresult from the C-OH group atop surface carbonate species.These carbonate species can be formed by further oxidation ofthe formate species. Carbonate species can be prepared byreacting CO2with alumina surfaces. The vibrational frequenciesobserved in this study match closely with literature values forcarbonate.25

Control experiments confirm the identity of the formatespecies formed during ozone exposures. For these control

experiments, a fresh Al2O3ALD film was grown at 450 K andthen exposed to 1.0 Torr s of formalin solution. Formalin is asolution of 37% formaldehyde, 10% methanol, and 53% water.Exposing aluminum oxide to formaldehyde will produce surfaceformate groups.27,32 For reference, Figure 4a shows the FTIRdifference spectrum after an ozone exposure on the Al2O3ALDsurface. This spectrum is identical to the spectrum shown inFigure 3c. Figure 4b displays the FTIR difference spectrum aftera formalin exposure on an Al2O3 ALD surface at 450 K. Theformaldehyde leads to the same major absorbance features at1388, 1404, and 1597 cm-1 observed in Figure 4a. Thedifferences between these spectra are the peaks at 1098, 1320,and 1720 cm-1. The first two of these peaks are attributed to

carbonate groups bound on the surface. The last feature is likelyabsorbance from C-O stretching vibrations of surface methoxygroups.25,26

The formate species on the Al2O3 ALD surface formed bythe formaldehyde exposure was then exposed to TMA at 450K. The FTIR difference spectrum after the TMA exposure isshown in Figure 4c. The negative absorbances at 1404 and 1597cm-1 indicate that TMA removes the formate species from theAl2O3surface. In addition, the TMA removes additional AlOH*species as shown by the negative infrared absorbance featuresat 3730 and 3770 cm-1. The positive absorbance features from2920 to 2980 cm-1 in the C-H stretching region and at 1212cm-1 for the Al-CH3 deformation mode indicate that TMAhas repopulated the Al2O3surface with AlCH3* species.

The surface chemistry during Al2O3 ALD with TMA andozone may depend on the substrate temperature. Figure 5 shows

Figure 4. FTIR difference spectra after (a) reference ozone exposure,(b) HCOH (formaldehyde) exposure on hydroxyl-terminated surface,and (c) next TMA exposure after formaldehyde exposure. All exposureswere conducted at 450 K. F )formate.



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FTIR difference spectra after the third and fourth sequentialTMA and ozone exposures at 550 K. The reference spectrumfor each difference spectrum is the FTIR spectrum after theprevious exposure. The absorbance features observed after theTMA and ozone exposures are very similar for the third andfourth cycle and also correspond closely to the absorbancefeatures monitored at 450 K. This close correspondence suggeststhat the Al2O3ALD reaction mechanism is similar at 450 and550 K.

The FTIR difference spectra in Figure 5 also consistentlyshow the disappearance and appearance of a small absorbancefeature at 2280 cm-1 with TMA and O3exposures, respectively.This feature is attributed to weakly bound CO coordinating to

Al3+

centers on an alumina surface. CO can be formed wheneverthe formate species (OCHO) decompose on the aluminasurface.25 This absorbance feature becomes more prominent asmore formate species undergo decomposition. To check thishypothesis, formate decomposition was examined during theformaldehyde control experiment. The formate features wereobserved to disappear slowly at 550 K. The intensity of theformate features was reduced by one-third in 22 h. In contrast,when the temperature was reduced to 473 K, the formateremained constant over 8 h of scanning. This behavior mayexplain why little CO was observed at 450 K. Not enoughformate is decomposing at 450 K to produce CO on the aluminasurface.

Figure 5 also shows the decrease and increase of absorbancefor the bulk Al2O3absorbance at 900-1000 cm-1 for TMAand ozone exposures, respectively.33 The increase of thisabsorbance during the O3exposures is always larger than thedecrease of absorbance during the TMA exposures. As a result,the absorbance of this feature increases gradually versus thenumber of AB cycles. This behavior is consistent with thegrowth of the Al2O3ALD film.

The self-limiting nature of the surface reactions during Al2O3ALD with TMA and ozone can be monitored using theintegrated infrared absorbance for various surface species. Ifan ALD reaction is self-limiting, then the surface coverage willnot increase after a certain reactant exposure. For theseexperiments, the integrated absorbance was defined for the C-H

stretching vibrations at 2820-2980 cm-1, the Al-CH3defor-mation mode at 1175-1250 cm-1, the O-H stretching vibra-

tions at 3600-3800 cm-

1, and the two major formate vibrations:the antisymmetric OCO band at 1575-1625 cm-1 and thesymmetric OCO band between 1350 and 1425 cm-1.

Figure 6a compares the normalized integrated absorbance ofthe surface species versus TMA exposure at 550 K. Theabsorbances of the O-H stretching vibration and the symmetricand asymmetric OCO stretching vibrations for the formatespecies decrease versus TMA exposure. Hydroxyls react muchmore rapidly than the formate features since the absorbance forthe O-H stretching vibrations is reduced before the absorbancefor the formate features. In close correspondence, the absorbanceof the C-H stretching vibrations and the Al-CH3deformationmode for the AlCH3* species concurrently increase versus TMA

exposure. The measurements indicate that TMA exposures of0.9 Torr s are sufficient for the TMA surface reaction to reachcompletion. However, the absorbance for the antisymmetricOCO stretch is not completely extinguished even after 2.0 Torr sof TMA exposure. This behavior indicates that some formategroups do not react with TMA at 550 K.

The normalized integrated absorbance of the surface speciesduring the ozone exposure is presented in Figure 6b. Theabsorbance for the various surface species again shows the char-acteristic signature for self-limiting surface reactions. Theabsorbances of the O-H stretching vibration and the symmetricand asymmetric OCO stretching vibrations for the formatespecies increase versus O3exposure. In close correspondence,the absorbances of the C-H stretching vibrations and the

Al-CH3deformation mode for the AlCH3* species concurrentlydecrease versus O3exposure. Fermi resonances from the formate

Figure 5. FTIR difference spectra after (a) third TMA exposure, (b)third ozone exposure, (c) fourth TMA exposure, and (d) fourth ozoneexposure. All exposures were performed at 550 K. F )formate.

Figure 6. (a) Normalized integrated absorbances during (a) TMAexposure and (b) ozone exposure at 550 K showing C-H stretch, O-Hstretch, asymmetric OCO stretch, symmetric OCO stretch, and CH 3deformation.

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groups leave residual absorbances in the 2820-2980 cm-1 rangeand prevent the C-H stretching features from being completelyremoved. These measurements indicate that O3 exposures of1.0 Torr s are sufficient for the O3 surface reaction to reachcompletion.

The formate species are dependent on the surface temperature.Figure 7 displays FTIR spectra that were recorded after thesecond ozone exposure at temperatures between 363 and 650K. Below 650 K, the peaks associated with formate andcarbonate groups were visible between 1200 and 1700 cm-1.The formate features were obscured when the temperature wasraised to 650 K. This disappearance may result from thedecomposition of the formate and carbonate species into CO

and CO2. At all temperatures, surface methyl groups wereremoved based on the negative absorbance of the methyldeformation feature at 1212 cm-1. The bulk infrared absorbanceof Al2O3 was visible in all the spectra at 900-1000 cm-1.The strongest bulk infrared absorbance was observed at thehigher temperatures.

Transient species that could produce formate species werealso resolved during the temperature studies. The low-temper-ature experiments revealed new absorbances at 1089 and 1456cm-1 that can be attributed to the C-O stretching vibration andantisymmetric CH3deformation of methoxy species. The C-Ofeature was obscured at higher temperatures by the broad Al-Obulk absorption mode. In addition, the symmetric CH3deforma-

tion of surface methoxy groups was obscured by the symmetricdeformation of AlCH3* surface species. Methoxy groups are apotential intermediate to surface formate groups.26 In agreement,features attributed to methoxy groups decrease and the formatefeatures increase above 363 K.

The 2000-4000 cm-1 region after the second ozone exposureat a variety of temperatures is shown in Figure 8. The hydroxylregion at 3650-3770 cm-1 revealed a greater proportion ofhigher frequency peaks at higher temperatures. The hydroxylvibrations after ozone exposures have two prominent absor-bances at 3718 and 3778 cm-1. In comparison, the hydroxylsobserved during Al2O3 ALD with H2O have a diffuse bandranging from 3670 to 3730 cm-1.12 This contrast is consistentwith a difference in the basicity of the hydroxyls on the alumina

surface. The hydroxyl vibrations are directly correlated to thebasicity of surface hydroxyl groups.34,35 A higher (OH)

vibrational frequency indicates increased basicity of surfacehydroxyl groups. The FTIR spectra suggest that Al2O3 ALDgrown at higher temperatures with ozone produces a largerproportion of strongly basic hydroxyls than Al2O3ALD grownwith water.

The C-H stretching features are consistent with the removalof AlCH3* species and production of new C-H stretchingvibrations corresponding to methoxy species. The absorbancefeatures around 2850 cm-1 are associated with the CH3symmetric stretching vibration of methoxy species. Thesemethoxy features are lost at higher temperatures. As thetemperature is raised to 550 K, Figure 8c reveals an absorbanceat 2280 cm-1 that has already been identified as CO from thedecomposition of formate. In addition, a peak at 2324 cm-1

appeared when the temperature was increased to 650 K. Thispeak is attributed to weakly bound CO2that coordinates to Lewisbase sites on the alumina surface.25 A correlation of thevibrations of the observed surface species, their referencedliterature values, and observed experimental values is given inTable 1.

Another control experiment was performed at temperaturesbetween 450 and 650 K to distinguish the effects of ozone fromoxygen on a surface covered with AlCH3* species followingthe TMA reaction. Oxygen was dosed into the reactor chamberthrough the ozone generator with the same exposure as ozonefor each growth temperature. The control reactions wereperformed one week after the final ozone experiment to ensure

that no residual ozone was left in the ozone generator. Directcomparison of the control reactions in Figure 9 shows thatoxygen reacts only slightly up to 550 K because very little ofthe absorbance for the A1CH3deformation mode at 1212 cm-1

was removed from the spectrum. At 650 K, oxygen reacts withAlCH3* species but does not produce any absorbance for O-Hstretching vibrations from hydroxyl species. Al2O3ALD growthmay also be occurring at 650 K because the bulk Al2O3absorbance increases after the oxygen exposure. However,multiple sequential TMA and O2exposures were not performedto confirm Al2O3ALD growth.

The bulk Al-O absorbance feature can be used to monitorthe growth of the Al2O3ALD film on the ZrO2 nanopowders.An Al2O3ALD film was grown using 40 sequential exposures

of 0.9 Torr s TMA and 1.0 Torr s of ozone at 550 K. A 90 spurge separated the reactants to minimize possible CVD and to

Figure 7. FTIR difference spectra in the region from 900 to 1900cm-1 recorded after the second ozone exposure at (a) 363, (b) 450, (c)550, and (d) 650 K. M )methoxy, C )carbonate, F )formate.

Figure 8. FTIR difference spectra in the region from 2000 to 4000cm-1 recorded after the second ozone exposure at (a) 363, (b) 450, (c)550, and (d) 650 K. M )methoxy.



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allow desorbing species to be swept from the nanopowders.FTIR scans were recorded at regular intervals to monitor theAl2O3ALD film growth. The absolute FTIR spectra shown inFigure 10 were acquired after the O3 exposures and arereferenced to the CsI windows. There is a continuous increaseof the bulk Al-O absorbance mode versus number of ABcycles.

After the 40 AB reaction cycles at 550 K, the sample wasremoved from the reactor and TEM was performed on the coatedZrO2nanopowders to determine the film conformality and ALDgrowth rate. The bright field TEM image recorded at 130 000is shown in Figure 11. The ZrO2nanopowders have a conformalcoating with a thickness of 65 . This thickness is consistentwith a growth rate of 1.1 /cycle for Al2O3ALD with ozone at550 K. This value was determined by subtracting the estimated12 base layer of Al2O3ALD grown using TMA and H2O. Asecond deposition experiment at 650 K provided the samegrowth rate and also showed conformality of the Al2O3 ALDfilm.

B. Quadrupole Mass Spectrometry. Identification of thegas phase species formed after O3exposures can help determinehow the new surface species were formed on the Al2O3surface.The mass spectrum recorded during the first O3micropulse at550 K is shown in Figure 12a. This spectrum is shown using alog intensity scale because the reaction products are very smallcompared with the O2fragmentation pattern. The large peak atm/z32 is attributed to a mixture of O 2and O3. Hydrocarbonsevolved in this first ozone micropulse are identified as C 2H4(ethylene) by the three peaks from m/z 26 to 28 and CH4(methane) by the peaks from m/z12 to 16. Ethylene peaks areobserved in the QMS scans until the 10th micropulse of ozonewhen their signals reach the noise level of


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TMA micropulse. No peaks from TMA are observed duringthis first micropulse. The only major reaction product is CH 4with its characteristic peaks betweenm/z12 and 16. In addition,a slight amount of CO is detected at m/z28. The appearance ofCO reaction product suggests that formate and carbonate speciesare still decomposing on the surface at 550 K.

Figure 13b shows the mass spectrum after the 55th TMAmicropulse. The TMA reaction has reached completion on thesurface and the mass spectrum is consistent with unreacted TMAwith its primary mass cracking fragments centered at m/z 57and 42. The peaks between m/z12 and 16 are mass crackingfragments for TMA. In addition, slight amounts of CO and CO2are detected atm/z28 and 44. The appearance of these reaction

products suggests that formate and carbonate species are stilldecomposing on the surface at 550 K. Gas phase CO and CO2are present in the mass spectrum after subtracting the crackingpatterns from both CH4and TMA. Analysis of the QMS datashows that the quantity of CO and CO2steadily decreases versustime.

C. Mechanism of Al2O3 ALD with Ozone. The FTIRspectra and the QMS data allow a mechanism to be proposedfor Al2O3ALD with TMA and ozone. The mechanism duringthe ozone reaction is presented in Figure 14. The initial surfaceis covered with AlCH3* species resulting from the TMA reactionon a hydroxylated initial surface. There are two potentialpathways for ozone to react with AlCH3* species. One reactionpathway involves oxygen insertion into the AlC-H bond. The

second reaction pathway involves oxygen insertion into theAl-C bond.

Oxygen insertion into the AlC-H bond has been predictedby quantum mechanical simulations and is believed to produceethylene as a reaction product.6,17,37 Oxygen insertion into theAlC-H bond produces AlCH2OH* species. The presence ofthese AlCH2OH* species are not ruled out by the FTIR spectrabecause the O-H stretching vibration for AlCH2OH* specieswould be difficult to distinguish from that of AlOH* species.These AlCH2OH* species could also be identified by thepresence of methylene C-H stretching vibrations or CH2rocking modes. However, the intense formate vibrations andFermi resonances obscure these vibrations. The transient natureof these species may also prevent these species from beingisolated by using time- or temperature-dependent measurements.

Adjacent AlCH2OH* species then eliminate ethylene andleave behind two AlOH* species as shown in Figure 14. This

Figure 12. (a) Mass spectrum during the first O3micropulse showingCH4 and C2H4 at 550 K. (b) Mass spectrum during the 55th O3micropulse showing CH4, CO, and CO2at 550 K.

Figure 13. (a) Mass spectrum during the first TMA micropulseshowing CH4 and CO at 550 K. (b) Mass spectrum during the 55thTMA micropulse showing TMA at 550 K.

Figure 14. Proposed mechanism for O3reaction during Al2O3ALDusing TMA and O3.



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reaction pathway is supported by the immediate production ofhydroxyl groups after small ozone doses in Figure 6b and theethylene observed by the QMS results as shown in Figure 12a.The combination of two AlCH2OH* species to produce twoAlOH* species is important because this decomposition pathwayregenerates hydroxyl groups that are needed for the subsequentTMA reaction.

Oxygen atom insertion into the Al-C bond produces amethoxy group that is only stable below 473 K according to

the FTIR spectra. Adjacent methoxy groups then combine toform formate species, releasing CH4and H2into the gas phase.This reaction has been well-characterized in the surfacechemistry literature.29 This reaction is also the likely source ofgaseous CH4 observed by the QMS measurements in Figure12. Further oxidation of the formate groups yields carbonategroups. These carbonate features are in a minority as suggestedby their weak infrared absorbances. Oxygen insertion into theAl-C bond is likely the primary pathway for ozone activationbased on the large amount of CH4 observed in the QMScompared with the small C2H4signals that were only monitoredduring the initial O3micropulses. These minority C2H4reactionproducts may only be observed during the initial O3micropulsesbecause of higher AlCH

3* surface coverages.

As the reaction progresses, the formate and carbonate groupsdecompose into CO and CO2, respectively. The CO and CO2gas phase products were both observed by the QMS measure-ments during the later O3micropulses in Figure 12b and duringthe TMA micropulses in Figure 13. The formate and carbonatedecomposition leaves behind one hydroxyl group on aluminaper decomposed moiety. This hydroxyl group may be theisolated hydroxyl observed by the FTIR spectra at 3778 cm-1.CH4could also be produced when these surface hydroxyl groupsreact with nearby AlCH3* species.

The TMA reaction is believed to be very similar to thereported TMA reaction for Al2O3ALD with TMA and water.3

AlOH* species formed during ethylene elimination or formate

or carbonate decomposition in Figure 14 can easily react withTMA. This reaction reforms the AlCH3* species that areobserved in the FTIR spectra. The efficiency of this TMAreaction is illustrated in Figure 6a and requires similar exposuresof TMA as the TMA reaction during Al2O3 ALD with TMAand water. The hydroxyl species are reduced more rapidly thanthe formate species during the TMA reaction.

Figure 6a also shows that TMA is unable to remove all theformate species from the surface. This observation has beennoted by other infrared studies of the TMA reaction duringAl2O3 ALD with ozone.30 On the basis of our results, thecomplete removal of the formate species is dependent on thedecomposition of the formate species. If these formate species

are not removed, then carbon may build up in the Al2O3ALDfilms. These residual formate species that are not removed byTMA may explain the high carbon concentration observed inAl2O3 ALD films grown with TMA and ozone at lowertemperatures.13

IV. Conclusions

In situ transmission FTIR spectroscopy and QMS were usedto study Al2O3ALD with sequential exposures of Al(CH3)3andO3. The FTIR spectroscopy studies of the surface reactionsoccurring during Al2O3ALD were performed at temperaturesfrom 363 to 650 K. The FTIR spectra were recorded afterAl(CH3)3and ozone exposures. These FTIR spectra showed that

the ozone exposure removes surface AlCH3* species andproduces AlOCH3* (methoxy), Al(OCHO)* (formate), Al(O-

COOH)* (carbonate), and AlOH* (hydroxyl) species. The TMAexposure then removes the methoxy, formate, carbonate, andhydroxyl species and reestablishes the AlCH3* species. Theidentification of formate species in the FTIR spectra wasconfirmed by separate formaldehyde adsorption experiments onAl2O3 ALD surfaces. The formate species were temperaturedependent and were nearly absent from the FTIR spectra atg650 K.

Repeating the TMA and O3exposures in a sequential reaction

sequence progressively deposited an Al2O3 ALD film asmonitored by the absorbance increase for bulk Al2O3infraredfeatures. TEM was used to examine the Al2O3 ALD filmsdeposited on the ZrO2 particles. The Al2O3 ALD films werevery conformal to the underlying ZrO2 particles. These TEMimages were consistent with an Al2O3ALD growth rate of 1.1/cycle at 550 K. QMS analysis of the gas phase speciesrevealed that the TMA reaction produced CH4. The ozonereaction then produced mainly CH4with a small amount of C2H4(ethylene), CO, and CO2.

On the basis of the surface species observed by the FTIRstudies and the gas phase species monitored by the QMSinvestigations, a very different mechanism is suggested for Al2O3ALD growth using TMA and O

3compared with Al

2O

3ALD

using TMA and H2O. During Al2O3 ALD using ozone, bothAl-C and C-H bond insertion occurs when O3 reacts withAlCH3* species to create methoxy and AlCH2OH* species. Themethoxy species decompose to formate and carbonate species.The formate and carbonate species release CO and CO2 toproduce AlOH* species. Two AlCH2OH* species also eliminateC2H4and produce two AlOH* species.

Acknowledgment. This work was supported by the NationalScience Foundation (CHE-0715552). Some of the equipmentused in this research was provided by the Air Force Office ofScientific Research. The authors thank Dr. Thomas Giddingsfor assistance with the TEM analysis.

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