In Situ IR Spectroscopic Investigation of Alumina ALD on Porous Silica Films: Thermal versus Plasma-Enhanced ALD

Abstract

A novel in situ infrared (IR) approach is demonstrated for investigating and identifying ALD surface reactions during both the steady state and the initial growth regime. The unique combination of reflection−absorption IR spectroscopy in grazing incidence mode with a high surface area reflecting substrate allows for ALD process monitoring with an acceptable acquisition time and a high sensitivity in the entire mid-IR spectral region. Using a mesoporous silica film deposited on a reflecting platinum layer as substrate, the thermal and plasma-enhanced ALD processes of alumina with use of trimethylaluminum (TMA) are compared. Due to the high sensitivity of the method, the relative amount of surface hydroxyl groups added or removed during the process could be determined versus the number of ALD half-cycles. These data reveal substrate-inhibited growth on the silica surface for the thermal process with use of TMA and water, as compared to direct growth for the plasma-based ALD process with use of TMA and O 2 plasma. This different behavior could be linked to the formation of Si−CH 3 surface groups after the first precursor pulse, as evidenced by the raw IR spectra. It is found that the oxygen radicals in the plasma can remove these surface groups during the next few ALD cycles, while the H 2 O molecules cannot, thus explaining the initial slower growth for the thermal process. ■ INTRODUCTIONAtomic layer deposition (ALD) is a thin film deposition technique well-known for its precise thickness control and excellent conformality on high-aspect-ratio structures. The process consists of sequential alternating pulses of precursor gases that react with the substrate surface in a self-limiting manner. Because the start of the film growth depends on the chemical nature of the substrate, knowledge of the initial growth kinetics and surface chemistry of ALD is important, especially when ultrathin films of a few nanometers thickness are needed. Each ALD precursor pulse leaves at most a monolayer of precursor molecules on the surface. As a consequence, very sensitive characterization techniques are needed to perform a detailed investigation of the surface reactions after each pulse.Infrared spectroscopy has proven to be a valuable tool for studying surface chemistry during ALD processes. Chabal et al. published several papers demonstrating a transmission IR technique that allows for monitoring of ALD processes on planar substrates. 1−3 George et al. combined transmission geometry with high surface area powders pressed into a grid to improve signal-to-noise ratio and thus reduce measurement times. 4−6 The high surface area improves the quality of the spectra because more surface groups can be detected compared with a planar substrate. However, uniform heating of samples in transmission geometry can be a challenge. 7 Attenuated total reflection (ATR)-FTIR spectroscopy of ALD processes has been reported, but the need for an ATR crystal and the accurate alignment that this setup requires complicate the experiment. 8,9 Sperling et al. ...

Ab Initio Investigation of Surface Chemistry of Alumina ALD on Hydroxylated γ-Alumina Surface

“…These calculations are consistent with infrared peaks reported at 3800−2600, 2000− 1250 and 700−400, and 1000−500 cm −1 corresponding to OH stretches, vibrations of water, and lattice vibrations of alumina, respectively. 8,24,38,41 Also, as shown in Figure 10a, the infrared peaks for the final hydroxylated surface were calculated at 3835−2830 cm −1 corresponding to OH stretches consistent with reported values. 24 Dashed line was reported after 20 ALD cycles on mesoporous silica substrate.…”

“…24,38 In this way, the consumption of hydroxyls during the TMA pulse and the generation of hydroxyls during the water pulse could be monitored. Infrared spectra of these hydroxylated and methylated surfaces were calculated, and difference spectra were produced for comparison with those reported by experiments, namely, Figure 9.…”

Chemical Analysis of the Interface between Hybrid Organic–Inorganic Perovskite and Atomic Layer Deposited Al2O3

“…49−53 It is interesting to note that the shape of the detected Al–O–Al phonon mode on top of perovskite is different as compared to what has been reported earlier for ALD Al 2 O 3 on top of Si. 54−56 This might be indicative for a different growth behavior of ALD Al 2 O 3 based on the underlying substrate. The peak at 1217 cm –1 pertains to the Al–CH 3 species from unreacted TMA ligands.…”

Atomic Layer Deposition of Localized Boron- and Hydrogen-Doped Aluminum Oxide Using Trimethyl Borate as a Dopant Precursor

“…groups being re-created by combustion of the CH 3 ligands during the O 2 plasma pulse. [58][59][60]63 The same process on the TMB-O 2 PE-ALD films reveals a large interaction with the underlying films in the B-O region. This is further corroborated by the hydroxyl region: during the first cycles, B-OH surface groups are re-instated after the O 2 plasma pulse, while after cycles only Al-OH groups are being formed.…”