3 research outputs found
Reaction Mechanisms of the Atomic Layer Deposition of Tin Oxide Thin Films Using Tributyltin Ethoxide and Ozone
Uniform
and conformal deposition of tin oxide thin films is important
for several applications in electronics, gas sensing, and transparent
conducting electrodes. Thermal atomic layer deposition (ALD) is often
best suited for these applications, but its implementation requires
a mechanistic understanding of the initial nucleation and subsequent
ALD processes. To this end, in situ FTIR and ex situ XPS have been
used to explore the ALD of tin oxide films using tributyltin ethoxide
and ozone on an OH-terminated, SiO<sub>2</sub>-passivated Si(111)
substrate. Direct chemisorption of tributyltin ethoxide on surface
OH groups and clear evidence that subsequent ligand exchange are obtained,
providing mechanistic insight. Upon ozone pulse, the butyl groups
react with ozone, forming surface carbonate and formate. The subsequent
tributyltin ethoxide pulse removes the carbonate and formate features
with the appearance of the bands for CH stretching and bending modes
of the precursor butyl ligands. This ligand-exchange behavior is repeated
for subsequent cycles, as is characteristic of ALD processes, and
is clearly observed for deposition temperatures of 200 and 300 Ā°C.
On the basis of the in situ vibrational data, a reaction mechanism
for the ALD process of tributyltin ethoxide and ozone is presented,
whereby ligands are fully eliminated. Complementary ex situ XPS depth
profiles confirm that the bulk of the films is carbon-free, that is,
formate and carbonate are not incorporated into the film during the
deposition process, and that good-quality SnO<sub><i>x</i></sub> films are produced. Furthermore, the process was scaled up
in a cross-flow reactor at 225 Ā°C, which allowed the determination
of the growth rate (0.62 Ć
/cycle) and confirmed a self-limiting
ALD growth at 225 and 268 Ā°C. An analysis of the temperature-dependence
data reveals that growth rate increases linearly between 200 and 300
Ā°C
Role of Initial Precursor Chemisorption on Incubation Delay for Molybdenum Oxide Atomic Layer Deposition
In
an effort to grow metal oxide films (e.g., MoO<sub>3</sub>)
at low temperatures, a novel molybdenum precursor, SiĀ(CH<sub>3</sub>)<sub>3</sub>CpMoĀ(CO)<sub>2</sub>(Ī·<sup>3</sup>-2-methylallyl)
or MOTSMA, is used with ozone as the coreactant. As is often observed
in atomic layer deposition (ALD) processes, the deposition of molybdenum
trioxide displays an incubation period (ā¼15 cycles at 250 Ā°C).
In situ FTIR spectroscopy reveals that ligand exchange reactions can
be activated at 300 Ā°C, leading to a shorter incubation periods
(e.g., ā¼ 9 cycles). Specifically, the reaction of MOTSMA with
OH-terminated silicon oxide surfaces appears to be the rate limiting
step, requiring a higher temperature activation (350 Ā°C) than
the subsequent ALD process itself, for which 250 Ā°C is adequate.
Therefore, in order to overcome the nucleation delay, the MOTSMA precursor
is initially grafted at 350 Ā°C, with spectroscopic evidence of
surface reaction, and the substrate temperature then lowered to 250
or 300 Ā°C for the rest of the ALD process. After this initial
activation, a standard ligand exchange is observed with formation
of surface SiĀ(CH<sub>3</sub>)<sub>3</sub>CpMoĀ(Ī·<sup>3</sup>-2-methylallyl)
after precursor and its removal after ozone exposures, resulting in
MoĀ(ī»O)<sub>2</sub> formation. Under these conditions, the ALD
process proceeds with no nucleation delay at both temperatures. Postdeposition
X-ray photoelectron spectroscopy spectra confirm that the film composition
is MoO<sub>3</sub>. This work highlights the critical role of precursor
grafting to the substrate as essential to eliminate the nucleation
delay for ultrathin ALD grown film deposition
Selective Atomic Layer Deposition Mechanism for Titanium Dioxide Films with (EtCp)Ti(NMe<sub>2</sub>)<sub>3</sub>: Ozone versus Water
The
need for the conformal deposition of TiO<sub>2</sub> thin films
in device fabrication has motivated a search for thermally robust
titania precursors with noncorrosive byproducts. Alkylamido-cyclopentadienyl
precursors are attractive because they are readily oxidized, yet stable,
and afford environmentally mild byproducts. We have explored the deposition
of TiO<sub>2</sub> films on OH-terminated SiO<sub>2</sub> surfaces
by in situ Fourier transform infrared spectroscopy using a novel titanium
precursor [(EtCp)ĀTiĀ(NMe<sub>2</sub>)<sub>3</sub> (<b>1</b>),
Et = CH<sub>2</sub>CH<sub>3</sub>] with either ozone or water. This
precursor initially reacts with surface hydroxyl groups at ā„150
Ā°C through the loss of its NMe<sub>2</sub> groups. However, once
the precursor is chemisorbed, its subsequent reactivities toward ozone
and water are very different. There is a clear reaction with ozone,
characterized by the formation of monodentate formate and/or chelate
bidentate carbonate surface species; in contrast, there is no detectable
reaction with water. For the ozone-based ALD process, the surface
formate/carbonate species react with the NMe<sub>2</sub> groups during
the subsequent pulse of <b>1</b>, forming TiīøOīøTi
bonds. Ligand exchange is observed within the 250ā300 Ā°C
ALD window. X-ray photoelectron spectroscopy confirms the deposition
of stoichiometric TiO<sub>2</sub> films with no detectable impurities.
For the water-based process, ligand exchange is not observed. Once <b>1</b> is adsorbed, there is no spectroscopic evidence for further
reaction. However, there is still TiO<sub>2</sub> deposition under
typical ALD conditions. Co-adsorption experiments with controlled
vapor pressures of water and <b>1</b> indicate that deposition
arises solely from <b>1</b>/water <i>gas-phase</i> reactions. This striking lack of reactivity between chemisorbed <b>1</b> and water is attributed to the electronic and steric effects
of the EtCp group and facilitates the observation of gas-phase reactions