Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.

Portable, low cost and real-time gas sensors have a considerable potential in various biomedical and industrial applications. For such applications, nano-photonic gas sensors based on standard silicon fabrication technology offer attractive opportunities. Deposition of high surface area nano-porous coatings on silicon photonic sensors is a means to achieve selective, highly sensitive and multiplexed gas detection on an optical chip. Here we demonstrate selective and reversible ammonia gas detection with functionalized silicon-on-insulator optical micro-ring resonators. The micro-ring resonators are coated with acidic nano-porous aluminosilicate films for specific ammonia sensing, which results in a reversible response to NH 3 with selectivity relative to CO 2 . The ammonia detection limit is estimated at about 5 ppm. The detectors reach a steady response to NH 3 within 30 and return to their base level within 60 to 90 seconds. The work opens perspectives on development of nano-photonic sensors for real-time, non-invasive, low cost and light weight biomedical and industrial sensing applications. ©2012 Optical Society of America

Atomic layer deposition is demonstrated to be greatly suitable to tune the pore size of ink-bottle mesopores at the atomic level. This is confirmed by quantitative electron tomography, which enables a direct and local 3D characterization of an ultrathin HfO2 coating deposited in the mesopores of a thin titania film.

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. ...

By the powerful combination of in situ FTIR and in vacuo XPS, the surface species during ALD of TDMAT with different reactants could be identified.