Al2O3 Atomic Layer Deposition with ... - TheEEStory

19530 

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 

onavarietyofsubstratesincludingpolymers, 7porousmembranes, 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. 

J. Phys. Chem. C 2008, 112, 19530–19539 

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

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 

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 × 3cm 2 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 m 2 . 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 

4cm -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 Goldstein et al. 

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 m 2 /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 m 3 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


hydroxyl surface species. In addition, negative absorbance 

features are observed at 2920-2980 cm -1 corresponding to 

C-H stretching vibrations and at 1212 cm -1 corresponding to 

the Al-CH3 deformation mode. These negative absorbance 

features are both from the removal of AlCH3* surface species. 

Figure 3b shows the FTIR difference spectra for the next TMA 

exposure of 0.85 Torr · s at 450 K. This spectrum indicates that 

TMA 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 Al2O3 ALD with TMA 

and H2O. 3,12 

The surface species change dramatically after the first ozone 

exposure. Figure 3c shows a markedly different spectrum with 

new positive absorbance features visible between 1200 and 1700 

cm -1 . The most prominent new features were observed at 1388, 

1404, and 1597 cm -1 . There were also smaller new features 

observed at 1320 and 1475 cm -1 and a shoulder at 1720 cm -1 . 

Not all of the C-H features were eliminated after long ozone 

exposures. There were small absorbance features at 2923 and 

3016 cm -1 for C-H stretching vibrations that partially result 

from slight frequency shifts. In addition, the absorbance for the 

O-H stretching vibrations at 3734-3778 cm -1 was reduced in 

intensity compared with the intensity observed in Figure 3a after 

H2O exposures. In addition, Figure 3d shows that the new 

absorbance features added by the O3 exposure were completely 

removed by the next TMA exposure. TMA exposures reform 

the absorbances from the C-H stretching vibrations at 

2820-2970 cm -1 and the methyl deformation at 1212 cm -1 . 

The absorbance from the O-H stretching vibrations at 

3734-3778 cm -1 was also removed by TMA doses. 

The new absorbance features appearing after the ozone 

exposure are very characteristic of formate and carbonate groups 

on the Al2O3 ALD surface. We first reported these new formate 

features at the AVS Topical Conference for Atomic Layer 

Deposition in 2006 (ALD2006). 24 Formate groups are very 

common on metal oxide surfaces and can be formed by exposing 

metal oxides to a variety of reagents including carbon monoxide, 

methanol, and formaldehyde. 25-27 The absorption features at 

1388 and 1597 cm -1 correspond to the symmetric and antisymmetric 

OCO modes of bound formate species. In addition, 

the 1404 cm -1 shoulder is attributed to the CH bend of formate 

species. In the C-H stretching region, the two peaks at 2923 

and 3016 cm -1 are identified as the Fermi resonance of the 

antisymmetric OCO mode mixing with the lone C-H stretching 

vibration in the formate surface group. 26 All of these values 

match literature values for formate on aluminum oxide. 26,28,29 

Formate features during Al2O3 ALD with ozone have also 

been observed recently by FTIR studies on planar surfaces that 

were complemented by DFT calculations. 30 These studies 

observed the same methoxy modes at 1388 and 1475 cm -1 that 

were monitored in this study on the ZrO2 nanoparticles. In 

addition, the ratios of the absorbances for the methoxy and 

primary formate vibrational features were similar on the planar 

surfaces and the ZrO2 nanoparticles. This agreement also rules 

out the possibility that ozone decomposition may have prevented 

the ozone from reaching the interior surfaces of the ZrO2 

nanoparticle sample. 

The surface coordination of the formate features is described 

by the frequency difference between the symmetric and asymmetric 

OCO stretching vibrations. Our observed experimental 

difference of 212 cm -1 is greater than that of the free formate 

ion. Consequently, the formate species are doubly coordinated 

to aluminum sites on the surface. 28 These doubly coordinated 

surface species formed after ozone exposure may contribute to 

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 exposures 

were conducted at 450 K. F ) formate. 

the oxygen-rich stoichiometry of Al2O3 films deposited using 

TMA and ozone. 31 The remaining shoulders at 1320 and 1720 

cm -1 are the symmetric and asymmetric OCO stretching 

vibrations of carbonate groups bound to alumina. Some of the 

hydroxyl features observed in the difference spectra could also 

result from the C-OH group atop surface carbonate species. 

These carbonate species can be formed by further oxidation of 

the formate species. Carbonate species can be prepared by 

reacting CO2 with alumina surfaces. The vibrational frequencies 

observed in this study match closely with literature values for 

carbonate. 25 

Control experiments confirm the identity of the formate 

species formed during ozone exposures. For these control 

experiments, a fresh Al2O3 ALD film was grown at 450 K and 

then exposed to 1.0 Torr · s of formalin solution. Formalin is a 

solution of 37% formaldehyde, 10% methanol, and 53% water. 

Exposing aluminum oxide to formaldehyde will produce surface 

formate groups. 27,32 For reference, Figure 4a shows the FTIR 

difference spectrum after an ozone exposure on the Al2O3 ALD 

surface. This spectrum is identical to the spectrum shown in 

Figure 3c. Figure 4b displays the FTIR difference spectrum after 

a formalin exposure on an Al2O3 ALD surface at 450 K. The 

formaldehyde leads to the same major absorbance features at 

1388, 1404, and 1597 cm -1 observed in Figure 4a. The 

differences 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 likely 

absorbance from C-O stretching vibrations of surface methoxy 

groups. 25,26 

The formate species on the Al2O3 ALD surface formed by 

the formaldehyde exposure was then exposed to TMA at 450 

K. The FTIR difference spectrum after the TMA exposure is 

shown in Figure 4c. The negative absorbances at 1404 and 1597 

cm -1 indicate that TMA removes the formate species from the 

Al2O3 surface. In addition, the TMA removes additional AlOH* 

species as shown by the negative infrared absorbance features 

at 3730 and 3770 cm -1 . The positive absorbance features from 

2920 to 2980 cm -1 in the C-H stretching region and at 1212 

cm -1 for the Al-CH3 deformation mode indicate that TMA 

has repopulated the Al2O3 surface with AlCH3* species. 

The surface chemistry during Al2O3 ALD with TMA and 

ozone may depend on the substrate temperature. Figure 5 shows


Figure 5. FTIR difference spectra after (a) third TMA exposure, (b) 

third ozone exposure, (c) fourth TMA exposure, and (d) fourth ozone 

exposure. All exposures were performed at 550 K. F ) formate. 

FTIR difference spectra after the third and fourth sequential 

TMA and ozone exposures at 550 K. The reference spectrum 

for each difference spectrum is the FTIR spectrum after the 

previous exposure. The absorbance features observed after the 

TMA and ozone exposures are very similar for the third and 

fourth cycle and also correspond closely to the absorbance 

features monitored at 450 K. This close correspondence suggests 

that the Al2O3 ALD reaction mechanism is similar at 450 and 

550 K. 

The FTIR difference spectra in Figure 5 also consistently 

show the disappearance and appearance of a small absorbance 

feature at 2280 cm -1 with TMA and O3 exposures, respectively. 

This feature is attributed to weakly bound CO coordinating to 

Al 3+ centers on an alumina surface. CO can be formed whenever 

the formate species (OCHO) decompose on the alumina 

surface. 25 This absorbance feature becomes more prominent as 

more formate species undergo decomposition. To check this 

hypothesis, formate decomposition was examined during the 

formaldehyde control experiment. The formate features were 

observed to disappear slowly at 550 K. The intensity of the 

formate features was reduced by one-third in 22 h. In contrast, 

when the temperature was reduced to 473 K, the formate 

remained constant over 8hofscanning. This behavior may 

explain why little CO was observed at 450 K. Not enough 

formate is decomposing at 450 K to produce CO on the alumina 

surface. 

Figure 5 also shows the decrease and increase of absorbance 

for the bulk Al2O3 absorbance at ∼900-1000 cm -1 for TMA 

and ozone exposures, respectively. 33 The increase of this 

absorbance during the O3 exposures is always larger than the 

decrease of absorbance during the TMA exposures. As a result, 

the absorbance of this feature increases gradually versus the 

number of AB cycles. This behavior is consistent with the 

growth of the Al2O3 ALD film. 

The self-limiting nature of the surface reactions during Al2O3 

ALD with TMA and ozone can be monitored using the 

integrated infrared absorbance for various surface species. If 

an ALD reaction is self-limiting, then the surface coverage will 

not increase after a certain reactant exposure. For these 

experiments, the integrated absorbance was defined for the C-H 

stretching vibrations at 2820-2980 cm -1 , the Al-CH3 deformation 

mode at 1175-1250 cm -1 , the O-H stretching vibra- 

Figure 6. (a) Normalized integrated absorbances during (a) TMA 

exposure and (b) ozone exposure at 550 K showing C-H stretch, O-H 

stretch, asymmetric OCO stretch, symmetric OCO stretch, and CH3 

deformation. 

tions at 3600-3800 cm -1 , and the two major formate vibrations: 

the antisymmetric OCO band at 1575-1625 cm -1 and the 

symmetric OCO band between 1350 and 1425 cm -1 . 

Figure 6a compares the normalized integrated absorbance of 

the surface species versus TMA exposure at 550 K. The 

absorbances of the O-H stretching vibration and the symmetric 

and asymmetric OCO stretching vibrations for the formate 

species decrease versus TMA exposure. Hydroxyls react much 

more rapidly than the formate features since the absorbance for 

the O-H stretching vibrations is reduced before the absorbance 

for the formate features. In close correspondence, the absorbance 

of the C-H stretching vibrations and the Al-CH3 deformation 

mode for the AlCH3* species concurrently increase versus TMA 

exposure. The measurements indicate that TMA exposures of 

0.9 Torr · s are sufficient for the TMA surface reaction to reach 

completion. However, the absorbance for the antisymmetric 

OCO stretch is not completely extinguished even after 2.0 Torr · s 

of TMA exposure. This behavior indicates that some formate 

groups do not react with TMA at 550 K. 

The normalized integrated absorbance of the surface species 

during the ozone exposure is presented in Figure 6b. The 

absorbance for the various surface species again shows the characteristic 

signature for self-limiting surface reactions. The 

absorbances of the O-H stretching vibration and the symmetric 

and asymmetric OCO stretching vibrations for the formate 

species increase versus O3 exposure. In close correspondence, 

the absorbances of the C-H stretching vibrations and the 

Al-CH3 deformation mode for the AlCH3* species concurrently 

decrease versus O3 exposure. Fermi resonances from the formate


Figure 7. FTIR difference spectra in the region from 900 to 1900 

cm -1 recorded after the second ozone exposure at (a) 363, (b) 450, (c) 

550, and (d) 650 K. M ) methoxy, C ) carbonate, F ) formate. 

groups leave residual absorbances in the 2820-2980 cm -1 range 

and prevent the C-H stretching features from being completely 

removed. These measurements indicate that O3 exposures of 

1.0 Torr · s are sufficient for the O3 surface reaction to reach 

completion. 

The formate species are dependent on the surface temperature. 

Figure 7 displays FTIR spectra that were recorded after the 

second ozone exposure at temperatures between 363 and 650 

K. Below 650 K, the peaks associated with formate and 

carbonate groups were visible between 1200 and 1700 cm -1 . 

The formate features were obscured when the temperature was 

raised to 650 K. This disappearance may result from the 

decomposition of the formate and carbonate species into CO 

and CO2. At all temperatures, surface methyl groups were 

removed based on the negative absorbance of the methyl 

deformation feature at 1212 cm -1 . The bulk infrared absorbance 

of Al2O3 was visible in all the spectra at ∼900-1000 cm -1 . 

The strongest bulk infrared absorbance was observed at the 

higher temperatures. 

Transient species that could produce formate species were 

also resolved during the temperature studies. The low-temperature 

experiments revealed new absorbances at 1089 and 1456 

cm -1 that can be attributed to the C-O stretching vibration and 

antisymmetric CH3 deformation of methoxy species. The C-O 

feature was obscured at higher temperatures by the broad Al-O 

bulk absorption mode. In addition, the symmetric CH3 deformation 

of surface methoxy groups was obscured by the symmetric 

deformation of AlCH3* surface species. Methoxy groups are a 

potential intermediate to surface formate groups. 26 In agreement, 

features attributed to methoxy groups decrease and the formate 

features increase above 363 K. 

The 2000-4000 cm -1 region after the second ozone exposure 

at a variety of temperatures is shown in Figure 8. The hydroxyl 

region at 3650-3770 cm -1 revealed a greater proportion of 

higher frequency peaks at higher temperatures. The hydroxyl 

vibrations after ozone exposures have two prominent absorbances 

at 3718 and 3778 cm -1 . In comparison, the hydroxyls 

observed during Al2O3 ALD with H2O have a diffuse band 

ranging from 3670 to 3730 cm -1 . 12 This contrast is consistent 

with a difference in the basicity of the hydroxyls on the alumina 

surface. The hydroxyl vibrations are directly correlated to the 

basicity of surface hydroxyl groups. 34,35 A higher ν(OH) 

Figure 8. FTIR difference spectra in the region from 2000 to 4000 

cm -1 recorded after the second ozone exposure at (a) 363, (b) 450, (c) 

550, and (d) 650 K. M ) methoxy. 

vibrational frequency indicates increased basicity of surface 

hydroxyl groups. The FTIR spectra suggest that Al2O3 ALD 

grown at higher temperatures with ozone produces a larger 

proportion of strongly basic hydroxyls than Al2O3 ALD grown 

with water. 

The C-H stretching features are consistent with the removal 

of AlCH3* species and production of new C-H stretching 

vibrations corresponding to methoxy species. The absorbance 

features around 2850 cm -1 are associated with the CH3 

symmetric stretching vibration of methoxy species. These 

methoxy features are lost at higher temperatures. As the 

temperature is raised to 550 K, Figure 8c reveals an absorbance 

at 2280 cm -1 that has already been identified as CO from the 

decomposition of formate. In addition, a peak at 2324 cm -1 

appeared when the temperature was increased to 650 K. This 

peak is attributed to weakly bound CO2 that coordinates to Lewis 

base sites on the alumina surface. 25 A correlation of the 

vibrations of the observed surface species, their referenced 

literature values, and observed experimental values is given in 

Table 1. 

Another control experiment was performed at temperatures 

between 450 and 650 K to distinguish the effects of ozone from 

oxygen on a surface covered with AlCH3* species following 

the TMA reaction. Oxygen was dosed into the reactor chamber 

through the ozone generator with the same exposure as ozone 

for each growth temperature. The control reactions were 

performed one week after the final ozone experiment to ensure 

that no residual ozone was left in the ozone generator. Direct 

comparison of the control reactions in Figure 9 shows that 

oxygen reacts only slightly up to 550 K because very little of 

the absorbance for the A1–CH3 deformation mode at 1212 cm -1 

was removed from the spectrum. At 650 K, oxygen reacts with 

AlCH3* species but does not produce any absorbance for O-H 

stretching vibrations from hydroxyl species. Al2O3 ALD growth 

may also be occurring at 650 K because the bulk Al2O3 

absorbance increases after the oxygen exposure. However, 

multiple sequential TMA and O2 exposures were not performed 

to confirm Al2O3 ALD growth. 

The bulk Al-O absorbance feature can be used to monitor 

the growth of the Al2O3 ALD film on the ZrO2 nanopowders. 

An Al2O3 ALD 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 s 

purge separated the reactants to minimize possible CVD and to


Figure 9. FTIR difference spectra after an O2 exposure equivalent to 

the O2/O3 exposures at (a) 450, (b) 550, and (c) 650 K. (d) FTIR 

difference spectra after an O3 exposure at 650 K. 

TABLE 1: Vibrational Assignments Comparing Literature 

and Observed Experimental Values 

assignment lit. (cm-1 ) exptl (cm-1 ) 

Al-O phonon 900-1000 

ν(s) C-O (m) 110026,29,30,32,38 1089 

Sym. Al-CH3 def 11973,12 1212 

ν (s) OCO (c) 131525 1320 

ν (s) OCO (f) 138026,29,30,32,38 1388 

δCH (f, m) 139526,29,30,32,38 1404 

asym CH3 def (m) 145026,29,30,32,38 1475 

ν (as) OCO (f) 159526,29,30,32,38 1597 

ν (a) OCO (c) 171025 1720 

ν (s) CH3 (m) 284626,29,30,32,38 2850 

ν (as) CH3 (m) 295026,29,30,32,38 2923 

ν (s) OCO + δCH (f) 297026,29,30,32,38 3016 

allow desorbing species to be swept from the nanopowders. 

FTIR scans were recorded at regular intervals to monitor the 

Al2O3 ALD film growth. The absolute FTIR spectra shown in 

Figure 10 were acquired after the O3 exposures and are 

referenced to the CsI windows. There is a continuous increase 

of the bulk Al-O absorbance mode versus number of AB 

cycles. 

Figure 10. Absolute FTIR spectra showing the growth of the bulk 

Al-O bulk absorbance feature after various numbers of TMA and O3 

reaction cycles. 

Figure 11. TEM image of ZrO2 particles coated with Al2O3 after 40 

reaction cycles of TMA and ozone. 

After the 40 AB reaction cycles at 550 K, the sample was 

removed from the reactor and TEM was performed on the coated 

ZrO2 nanopowders to determine the film conformality and ALD 

growth rate. The bright field TEM image recorded at 130 000× 

is shown in Figure 11. The ZrO2 nanopowders have a conformal 

coating with a thickness of 65 Å. This thickness is consistent 

with a growth rate of 1.1 Å/cycle for Al2O3 ALD with ozone at 

550 K. This value was determined by subtracting the estimated 

12 Å base layer of Al2O3 ALD grown using TMA and H2O. A 

second deposition experiment at 650 K provided the same 

growth rate and also showed conformality of the Al2O3 ALD 

film. 

B. Quadrupole Mass Spectrometry. Identification of the 

gas phase species formed after O3 exposures can help determine 

how the new surface species were formed on the Al2O3 surface. 

The mass spectrum recorded during the first O3 micropulse at 

550 K is shown in Figure 12a. This spectrum is shown using a 

log intensity scale because the reaction products are very small 

compared with the O2 fragmentation pattern. The large peak at 

m/z 32 is attributed to a mixture of O2 and O3. Hydrocarbons 

evolved in this first ozone micropulse are identified as C2H4 

(ethylene) by the three peaks from m/z 26 to 28 and CH4 

(methane) by the peaks from m/z 12 to 16. Ethylene peaks are 

observed in the QMS scans until the 10th micropulse of ozone 

when their signals reach the noise level of


Figure 12. (a) Mass spectrum during the first O3 micropulse showing 

CH4 and C2H4 at 550 K. (b) Mass spectrum during the 55th O3 

micropulse showing CH4, CO, and CO2 at 550 K. 

TMA micropulse. No peaks from TMA are observed during 

this first micropulse. The only major reaction product is CH4 

with its characteristic peaks between m/z 12 and 16. In addition, 

a slight amount of CO is detected at m/z 28. The appearance of 

CO reaction product suggests that formate and carbonate species 

are still decomposing on the surface at 550 K. 

Figure 13b shows the mass spectrum after the 55th TMA 

micropulse. The TMA reaction has reached completion on the 

surface and the mass spectrum is consistent with unreacted TMA 

with its primary mass cracking fragments centered at m/z 57 

and 42. The peaks between m/z 12 and 16 are mass cracking 

fragments for TMA. In addition, slight amounts of CO and CO2 

are detected at m/z 28 and 44. The appearance of these reaction 

products suggests that formate and carbonate species are still 

decomposing on the surface at 550 K. Gas phase CO and CO2 

are present in the mass spectrum after subtracting the cracking 

patterns from both CH4 and TMA. Analysis of the QMS data 

shows that the quantity of CO and CO2 steadily decreases versus 

time. 

C. Mechanism of Al2O3 ALD with Ozone. The FTIR 

spectra and the QMS data allow a mechanism to be proposed 

for Al2O3 ALD with TMA and ozone. The mechanism during 

the ozone reaction is presented in Figure 14. The initial surface 

is covered with AlCH3* species resulting from the TMA reaction 

on a hydroxylated initial surface. There are two potential 

pathways for ozone to react with AlCH3* species. One reaction 

pathway involves oxygen insertion into the AlC-H bond. The 

second reaction pathway involves oxygen insertion into the 

Al-C bond. 

Figure 13. (a) Mass spectrum during the first TMA micropulse 

showing CH4 and CO at 550 K. (b) Mass spectrum during the 55th 

TMA micropulse showing TMA at 550 K. 

Figure 14. Proposed mechanism for O3 reaction during Al2O3 ALD 

using TMA and O3. 

Oxygen insertion into the AlC-H bond has been predicted 

by quantum mechanical simulations and is believed to produce 

ethylene as a reaction product. 6,17,37 Oxygen insertion into the 

AlC-H bond produces AlCH2OH* species. The presence of 

these AlCH2OH* species are not ruled out by the FTIR spectra 

because the O-H stretching vibration for AlCH2OH* species 

would be difficult to distinguish from that of AlOH* species. 

These AlCH2OH* species could also be identified by the 

presence of methylene C-H stretching vibrations or CH2 

rocking modes. However, the intense formate vibrations and 

Fermi resonances obscure these vibrations. The transient nature 

of these species may also prevent these species from being 

isolated by using time- or temperature-dependent measurements. 

Adjacent AlCH2OH* species then eliminate ethylene and 

leave behind two AlOH* species as shown in Figure 14. This


reaction pathway is supported by the immediate production of 

hydroxyl groups after small ozone doses in Figure 6b and the 

ethylene observed by the QMS results as shown in Figure 12a. 

The combination of two AlCH2OH* species to produce two 

AlOH* species is important because this decomposition pathway 

regenerates hydroxyl groups that are needed for the subsequent 

TMA reaction. 

Oxygen atom insertion into the Al-C bond produces a 

methoxy group that is only stable below 473 K according to 

the FTIR spectra. Adjacent methoxy groups then combine to 

form formate species, releasing CH4 and H2 into the gas phase. 

This reaction has been well-characterized in the surface 

chemistry literature. 29 This reaction is also the likely source of 

gaseous CH4 observed by the QMS measurements in Figure 

12. Further oxidation of the formate groups yields carbonate 

groups. These carbonate features are in a minority as suggested 

by their weak infrared absorbances. Oxygen insertion into the 

Al-C bond is likely the primary pathway for ozone activation 

based on the large amount of CH4 observed in the QMS 

compared with the small C2H4 signals that were only monitored 

during the initial O3 micropulses. These minority C2H4 reaction 

products may only be observed during the initial O3 micropulses 

because of higher AlCH3* surface coverages. 

As the reaction progresses, the formate and carbonate groups 

decompose into CO and CO2, respectively. The CO and CO2 

gas phase products were both observed by the QMS measurements 

during the later O3 micropulses in Figure 12b and during 

the TMA micropulses in Figure 13. The formate and carbonate 

decomposition leaves behind one hydroxyl group on alumina 

per decomposed moiety. This hydroxyl group may be the 

isolated hydroxyl observed by the FTIR spectra at 3778 cm -1 . 

CH4 could also be produced when these surface hydroxyl groups 

react with nearby AlCH3* species. 

The TMA reaction is believed to be very similar to the 

reported TMA reaction for Al2O3 ALD with TMA and water. 3 

AlOH* species formed during ethylene elimination or formate 

or carbonate decomposition in Figure 14 can easily react with 

TMA. This reaction reforms the AlCH3* species that are 

observed in the FTIR spectra. The efficiency of this TMA 

reaction is illustrated in Figure 6a and requires similar exposures 

of TMA as the TMA reaction during Al2O3 ALD with TMA 

and water. The hydroxyl species are reduced more rapidly than 

the formate species during the TMA reaction. 

Figure 6a also shows that TMA is unable to remove all the 

formate species from the surface. This observation has been 

noted by other infrared studies of the TMA reaction during 

Al2O3 ALD with ozone. 30 On the basis of our results, the 

complete removal of the formate species is dependent on the 

decomposition of the formate species. If these formate species 

are not removed, then carbon may build up in the Al2O3 ALD 

films. These residual formate species that are not removed by 

TMA may explain the high carbon concentration observed in 

Al2O3 ALD films grown with TMA and ozone at lower 

temperatures. 13 

IV. Conclusions 

In situ transmission FTIR spectroscopy and QMS were used 

to study Al2O3 ALD with sequential exposures of Al(CH3)3 and 

O3. The FTIR spectroscopy studies of the surface reactions 

occurring during Al2O3 ALD were performed at temperatures 

from 363 to 650 K. The FTIR spectra were recorded after 

Al(CH3)3 and ozone exposures. These FTIR spectra showed that 

the ozone exposure removes surface AlCH3* species and 

produces AlOCH3* (methoxy), Al(OCHO)* (formate), Al(O- 

COOH)* (carbonate), and AlOH* (hydroxyl) species. The TMA 

exposure then removes the methoxy, formate, carbonate, and 

hydroxyl species and reestablishes the AlCH3* species. The 

identification of formate species in the FTIR spectra was 

confirmed by separate formaldehyde adsorption experiments on 

Al2O3 ALD surfaces. The formate species were temperature 

dependent and were nearly absent from the FTIR spectra at 

g650 K. 

Repeating the TMA and O3 exposures in a sequential reaction 

sequence progressively deposited an Al2O3 ALD film as 

monitored by the absorbance increase for bulk Al2O3 infrared 

features. TEM was used to examine the Al2O3 ALD films 

deposited on the ZrO2 particles. The Al2O3 ALD films were 

very conformal to the underlying ZrO2 particles. These TEM 

images were consistent with an Al2O3 ALD growth rate of 1.1 

Å/cycle at 550 K. QMS analysis of the gas phase species 

revealed that the TMA reaction produced CH4. The ozone 

reaction then produced mainly CH4 with a small amount of C2H4 

(ethylene), CO, and CO2. 

On the basis of the surface species observed by the FTIR 

studies and the gas phase species monitored by the QMS 

investigations, a very different mechanism is suggested for Al2O3 

ALD growth using TMA and O3 compared with Al2O3 ALD 

using TMA and H2O. During Al2O3 ALD using ozone, both 

Al-C and C-H bond insertion occurs when O3 reacts with 

AlCH3* species to create methoxy and AlCH2OH* species. The 

methoxy species decompose to formate and carbonate species. 

The formate and carbonate species release CO and CO2 to 

produce AlOH* species. Two AlCH2OH* species also eliminate 

C2H4 and produce two AlOH* species. 

Acknowledgment. This work was supported by the National 

Science Foundation (CHE-0715552). Some of the equipment 

used in this research was provided by the Air Force Office of 

Scientific Research. The authors thank Dr. Thomas Giddings 

for assistance with the TEM analysis. 

References and Notes 

(1) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 

13121. 

(2) Ritala, M.; Leskelä, M. In Handbook of Thin Film Materials; Nalwa, 

H. S., Ed.; Academic Press: San Diego, CA, 2001; Vol. 1, p 129. 

(3) Dillon, A. C.; Ott, A. W.; Way, J. D.; George, S. M. Surf. Sci. 

1995, 322, 230. 

(4) Ott, A. W.; Klaus, J. W.; Johnson, J. M.; George, S. M. Thin Solid 

Films 1997, 292, 135. 

(5) Ott, A. W.; McCarley, K. C.; Klaus, J. W.; Way, J. D.; George, 

S. M. Appl. Surf. Sci. 1996, 107, 128. 

(6) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. 

(7) Wilson, C. A.; Grubbs, R. K.; George, S. M. Chem. Mater. 2005, 

17, 5625. 

(8) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. 

Chem. Mater. 2003, 15, 3507. 

(9) McCormick, J. A.; Rice, K. P.; Paul, D. F.; Weimer, A. W.; George, 

S. M. Chem. Vap. Deposition 2007, 13, 491. 

(10) Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. 

Chem. Mater. 2004, 16, 639. 

(11) Elam, J. W.; Groner, M. D.; George, S. M. ReV. Sci. Instrum. 2002, 

73, 2981. 

(12) Ferguson, J. D.; Weimer, A. W.; George, S. M. Thin Solid Films 

2000, 371, 95. 

(13) Kim, S. K.; Lee, S. W.; Hwang, C. S.; Min, Y. S.; Won, J. Y.; 

Jeong, J. J. Electrochem. Soc. 2006, 153, F69. 

(14) Kim, S. K.; Hwang, C. S. J. Appl. Phys. 2004, 96, 2323. 

(15) Kim, J. B.; Kwon, D. R.; Chakrabarti, K.; Lee, C.; Oh, K. Y.; Lee, 

J. H. J. Appl. Phys. 2002, 92, 6739. 

(16) Avice, M.; Grossner, U.; Monakhov, E. V.; Grillenberger, J.; Nilsen, 

O.; Fjellvag, H.; Svensson, B. G. Mater. Sci. Forum 2005, 483-485, 705. 

(17) Elliott, S. D.; Scarel, G.; Wiemer, C.; Fanciulli, M.; Pavia, G. Chem. 

Mater. 2006, 18, 3764. 

(18) Lee, T.-P.; Jang, C.; Haselden, B.; Dong, M. J. Vac. Sci. Technol. 

B 2004, 22, 2295.


(19) Jakschik, S.; Schroeder, U.; Hecht, T.; Krueger, D.; Dollinger, G.; 

Bergmaier, A.; Luhmann, C.; Bartha, J. W. Appl. Surf. Sci. 2003, 211, 352. 

(20) Ferguson, J. D.; Weimer, A. W.; George, S. M. J. Vac. Sci. Technol. 

A 2004, 22, 118. 

(21) Ballinger, T. H.; Wong, J. C. S.; Yates, J. T. Langmuir 1992, 8, 

1676. 

(22) McCormick, J. A.; Cloutier, B. L.; Weimer, A. W.; George, S. M. 

J. Vac. Sci. Technol. A 2007, 25, 67. 

(23) Benfer, S.; Knozinger, E. J. Mater. Chem. 1999, 9, 1203. 

(24) Goldstein, D. N.; George, S. M. 6th International Conference on 

Atomic Layer Deposition, (ALD2006), Seoul, Korea, July 25, 2006. 

(25) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. 

(26) Greenler, R. G. J. Chem. Phys. 1962, 37, 2094. 

(27) Carlos-Cuellar, S.; Li, P.; Christensen, A. P.; Krueger, B. J.; 

Burrichter, C.; Grassian, V. H. J. Phys. Chem. A 2003, 107, 4250. 

(28) Busca, G. Catal. Today 1996, 27, 323. 

(29) McInroy, A. R.; Lundie, D. T.; Winfield, J. M.; Dudman, C. C.; 

Jones, P.; Lennon, D. Langmuir 2005, 21, 11092. 

(30) Kwon, J. H.; Dai, M.; Halls, M. D.; Chabal, Y. J. Chem. Mater. 

2008, 20, 3248. 

(31) Kim, S. K.; Hwang, C. S. J. Appl. Phys. 2004, 96, 2323. 

(32) Busca, G.; Lamotte, J.; Lavalley, J. C.; Lorenzelli, V. J. Am. Chem. 

Soc. 1987, 109, 5197. 

(33) Frank, M.; Wolter, K.; Magg, N.; Heemeier, M.; Kuhnemuth, R.; 

Baumer, M.; Freund, H. J. Surf. Sci. 2001, 492, 270. 

(34) Lundie, D. T.; McInroy, A. R.; Marshall, R.; Winfield, J. M.; Jones, 

P.; Dudman, C. C.; Parker, S. F.; Mitchell, C.; Lennon, D. J. Phys. Chem. 

B 2005, 109, 11592. 

(35) Chesters, M. A.; McCash, E. M. Spectrochim. Acta A 1987, 43, 

1625. 

(36) Juppo, M.; Rahtu, A.; Ritala, M.; Leskela, M. Langmuir 2000, 16, 

4034. 

(37) Prechtl, G.; Kersch, A.; Icking-Konert, G. S.; Jacobs, W.; Hect, 

T.; Boubekeur, H.; Schroder, U. Technical Digest of the IEEE International 

Electron DeVices Meeting (IEDM); IEEE: Piscataway, NJ, 2003; p 9.6.1. 

(38) Gopal, P. G.; Schneider, R. L.; Watters, K. L. J. Catal. 1987, 105, 

366. 

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