In Situ X-ray Fluorescence Measurements During Atomic Layer Deposition: Nucleation and Growth of TiO₂ on Planar Substrates and in Nanoporous Films

Abstract

Atomic layer deposition (ALD) is an attractive film growth technique for a variety of modern technologies. The method relies on sequential self-terminating gasÀsolid reactions separated by evacuation steps, i.e. purging or pumping of the deposition chamber. 1,2 The self-limiting nature of the chemical reactions allows for a layer-by-layer type growth and ensures precise thickness control and excellent conformality on substrates with complex topologies, even at the nanometer scale. So far, ALD has mainly been used in the semiconductor industry, but its ability for conformal deposition of ultrathin films could impact a broad range of additional applications, such as catalysis, fuel cells, batteries, filtration devices, etc.In order to fully exploit the unique advantages of ALD, it is important to optimize the process sequence and to understand the underlying surface reactions. In recent literature, mainly in situ diagnostics have been used to obtain fundamental information about the growth kinetics of ultrathin films deposited by ALD. In situ techniques offer the ability to monitor film properties or reaction products while ALD growth is occurring. Techniques that are often used during ALD are spectroscopic ellipsometry (SE), 3 infrared spectroscopy, 4À8 quadrupole mass spectrometry, 8À12 and quartz crystal microbalance. 9,10,13 They provide information on the thickness and optical properties of the film, the chemical surface groups, the gaseous reaction products and the mass uptake during the different process steps.

Mobile setup for synchrotron based in situ characterization during thermal and plasma-enhanced atomic layer deposition

“…The high sensitivity of the synchrotronbased XRF technique is illustrated by the fact that one single ALD cycle resulted in a clear fluorescence signal, implying submonolayer sensitivity. 13,14 In Figure 5(b), the integrated Ru Lα intensity is plotted against the number of deposition cycles for the thermal ALD process on Si-H and SiO 2 , and for the plasma-enhanced ALD process on SiO 2 . Because the XRF intensity is proportional to the amount of Ru on the surface, the obtained growth curves provide direct information about the difference in deposition rate with process conditions and substrate nature.…”

“…In recent years, several groups have explored extending this concept of using light to study ALD growth towards the x-ray part of the electromagnetic spectrum. 8,[10][11][12][13][14][15][16][17][18][19][20][21][22][23] Indeed, when an ALD reactor is equipped with x-ray transparent windows (e.g., beryllium, graphite, or kapton), then the film can be (intermittently) exposed to x-rays during growth, and a wide range of x-ray based thin film characterization techniques can be used for in situ characterization, as recently reviewed by Devloo-Casier et al 17 Since ALD is typically used for nanocoatings with a thickness of 0.1 to several tens of nanometers, while xrays typically penetrate several micrometers deep into most materials, it is challenging to obtain a sufficient signal to noise ratio for x-ray based characterization techniques, in particular when targeting acquisition rates of 1-60 s in order not to interfere too much with the standard exposure cycle of the ALD process. Therefore, while lab-based x-ray sources are used routinely for ex situ analysis of, e.g., the crystallinity of ALD grown films, the in situ studies typically require the high photon flux that can only be offered by synchrotron based sources.…”

“…In situ XRF has proven an easy-to-implement and valuable tool to monitor ALD growth at the synchrotron. 10,11,13,14,29 It provides direct information about the amount of material deposited per ALD cycle, on planar substrates as well as on more complex 3D surfaces. In situ XAS during ALD allows probing the local atomic environment and provides information such as the level of oxidation, coordination numbers, and bond lengths.…”

In situ synchrotron based x-ray fluorescence and scattering measurements during atomic layer deposition: Initial growth of HfO2 on Si and Ge substrates

“…XRF allows to monitor the atomic composition of the film and thus the amount of material deposited during each ALD cycle. [4][5][6] GISAXS is sensitive to both the surface morphology and the internal structure of thin films and is used to monitor the developing surface roughness. 7 The depositions and measurements were performed in the UHV film growth facility, adapted for thermal ALD, installed at beamline X21 of the National Synchrotron Light Source at Brookhaven National Laboratory.…”

“…Tetrakis͑ethylmethylamino͒hafnium ͑TEMAH͒ and deionized water were used as Hf and O source, respectively. For Si-based devices, the main focus has been on HfCl 4 as Hf source, but promising results have been reported on the use of TEMAH for the growth of HfO 2 on Ge. [8][9][10][11][12] Because of the low vapor pressure of the TEMAH precursor, Ar was used as carrier gas.…”