AVS2011 Session PS+TF-ThM: Plasma Deposition and Plasma Enhanced ALD

[1] S. Yokoyama et al., Applied Surface Science 112 (1997) 75-81

[2] Y. Osawa et al., Proceedings DPS-2009, 2-P51

In this work plasma-polymerized (PP-) TiO_xC_y films derived from titanium (IV) isopropoxide (TTIP) were deposited onto Si and KBr substrates using remote, room temperature plasma enhanced chemical vapor deposition (PECVD). The composition and morphology of the films was varied by systematically changing the ratio of Ar to O₂ in the carrier gas. Chemical compositions were investigated by FTIR and X-ray photoelectron spectroscopy (XPS). Morphological data derived atomic force microscopy (AFM) and scanning electronic microscopy (SEM) studies showed that the morphology was strongly dependent on the ratio of oxygen to total carrier gas composition. The films grown with Ar as the majority carrier gas have a featureless, smooth, one phase 3-D crosslinking morphology due to the incomplete oxidation of Ti to the most stable Ti⁴⁺ valence state during deposition in an oxygen poor environment. As the mixture of carrier gas became more O₂ rich a second phase evolved that had a columnar structure attributed to TiO₂. This increase in oxidation was also noted in high resolution XPS measurements where a peak corresponding to a carboxyl group in the C 1s spectrum increases with increasing O₂ concentration. Development of the structured second phase was also noted in the optical dispersion obtained by spectroscopic ellipsometry. In order to fit the data, an anisotropic model has to be used that took into account the surface roughness determined from the AFM and SEM studies.

We introduce a new class of composite membranes based on transition metal carbide as economical alternatives to palladium for high temperature purification of H₂. In this talk we describe two membrane concepts that were synthesized using plasma-enhanced chemical vapor deposition (PECVD) and magnetron sputtering. The first is a surface diffusion membrane comprised of nanostructured Mo₂C deposited on porous ceramic supports. Stoichiometric Mo₂C was fabricated using a two step synthesis process. Dense molybdenum oxide films were first deposited by plasma-enhanced chemical vapor deposition (PECVD) using mixtures of MoF₆, H₂, and O₂. Oxide films 100 – 500 nm in thickness were then converted into molybdenum carbide using temperature programmed reaction using mixtures of H₂ and CH₄. Permeation testing of these membranes showed very high flux, but limited selectivity. To address this issue we describe a counterflow PECVD approach that we are developing which is used to both modify the pore size of the original supports as well as to repair pinholes that develop during the carburization.

The second strategy is to produce dense composite membranes comprised of Mo₂C layers sputtered onto BCC metal foils. BCC metals (V, Ta, Nb) and their alloys have extremely high permeability for atomic hydrogen, but negligible catalytic activity for hydrogen dissociation. Platinum group metals have been used as catalysts, particularly palladium, but at elevated temperature they alloy with the underlying metal and rapidly lose their activity. In contrast, the Mo₂C/V membranes described in this work displayed no change in permeability when operated at high temperature for >160 hours, and transmission electron microscopy confirmed that negligible interdiffusion occurs between these materials during testing. Hydrogen dissociation is the primary factor limiting hydrogen transport, as evidenced by the sensitivity of performance to carbide morphology. Sputter parameters were systematically varied to optimize the crystal structure and morphology. These composite membranes are perfectly selective to H₂, with permeability values approach and in fact exceed that of pure palladium. These findings demonstrate the potential of low cost group V metals for H₂ separations with simultaneous carbon capture at temperatures compatible with the processes used for H₂ generation.

[1] T. Kakeya, et al.: Thin Solid Films 506-507 (2006) 288.

[2] Y. Kawashima, et al.: Trans. Mater. Res. Soc. Jpn. 35 (2010) 597.

[3] G. Uchida, et al.: Phys. Status Solidi C, (2011) at press.

[4] G. Uchida, et al.: submitted to Jpn J. Appl. Phys.

The interest in plasma-assisted atomic layer deposition (ALD) has increased rapidly over the last

years, since it has been demonstrated that the presence of a plasma step can improve material

properties and ease processing conditions. Although it is known from other plasma-based techniques that

the photons and ions can play an important role during processing, their presence and influence have not

systematically been addressed so far for the specific case of plasma-assisted ALD. In this contribution,

we present a detailed investigation of the impact that VUV photons and energetic ions can have on the

properties of metal oxide thin films prepared by plasma-assisted ALD. We will demonstrate the

detrimental impact that VUV photons can have on electrical properties and we show that structural

material properties can be controlled by tuning the ion energy through substrate biasing. Optical

emission, retarding field energy analyzer, and Langmuir probe measurements were carried out in three

R&D plasma-assisted ALD reactors. In the O₂ plasmas employed, vacuum ultraviolet (VUV) photons with

energies up to 9.5 eV were detected and these photons were found to be able to generate electronic

defects at thin film interfaces. This was demonstrated by experiments in which Al₂O₃ passivated Si(100)

samples were exposed to O₂ plasmas. By exposing the samples through quartz and MgF₂ windows, the

role of ions was excluded and the specific role of the high energy VUV photons was confirmed

unambiguously. Furthermore, during regular ALD conditions, an ion energy of ~30 eV was measured.

This energy is sufficient to contribute to the ALD process by, e.g., the displacement of lattice atoms and

enhancement of the ALD surface reactions, however, it is low enough to prevent substantial damage to

the deposited layers. The impact of the ions was further explored by enhancing the energy of the ions

through the implementation of substrate biasing, either through substrate self-biasing or by RF biasing.

By enhancing the ion energy up to 230 eV, these experiments demonstrated that at 300°C the crystallinity

of TiO₂ films can be changed from the anatase to the rutile crystalline phase. Moreover, at a substrate

temperature of 200°C the rutile phase can be obtained when employing substrate biasing while normally

amorphous TiO₂ is obtained. These results are particularly significant as generally the deposition of rutile

TiO₂ is difficult to achieve by ALD due to substrate temperature limitations imposed by the precursors

used. It is therefore evident that substrate biasing is a promising method to extend the possibilities of ALD.

Lanthanide series oxides are being evaluated as second-generation high-k gate dielectric materials. In addition to improving transistor electrostatics by reducing the equivalent oxide thickness (EOT), using lanthanide series gate oxide capping layers allows the effective metal gate workfunctions to be tuned toward the silicon band edges, providing the correct transistor threshold voltages. However these non-traditional CMOS materials have several integration challenges that must be overcome, including depositing a thermally-stable, high quality film with low fixed charge and high-k, without damage to the underlying layers of the gate stack. In addition, in some gate-first and gate-last approaches, the oxide must be etched from the source/drain regions prior to silicidation.

In this work, plasma-enhanced atomic layer deposition (PE-ALD) of gadolinium oxide is reported for the first time. Using Gd(iPrCp)₃ as the organometallic precursor and a pure O₂ plasma as the oxygen source, Gd₂O₃ growth is observed from 150^oC to 350^oC, though the optical properties of the film improve at higher temperature. True layer-by-layer ALD growth of Gd₂O₃ does not occur under all conditions, in fact only a relatively narrow window of self-limiting ALD growth of 1.4 Å/cycle was observed at 250^oC and below under certain precursor dose conditions. As the temperature increases, high-quality films are deposited, but the growth mechanism appears to become CVD-like. At 250^oC, the refractive index of the film is stable at ~1.80 regardless of other deposition conditions, and the measured dispersion characteristics are comparable to those of bulk Gd₂O₃. The electrical characteristics of the films, such as fixed charge and dielectric constant, are extracted from C-V measurements using TiN metal gate capacitors, and will be reported.

The plasma etching rate of the ALD Gd₂O₃ film in a high-density helicon reactor is very low. Little difference is observed in etching rate between Cl₂ and pure Ar plasmas, suggesting that physical sputtering dominates the etching at high bias power. A threshold bias power exists below which etching does not occur, thus it may be possible to etch a metal gate material and stop easily on the Gd₂O₃ gate dielectric. The threshold bias power is lower in a Cl₂ plasma compared to an Ar plasma, which suggests there is a small ion-enhanced chemical component to the etching as well.

*This work is sponsored by the Department of the Air Force under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government.

A novel low temperature plasma method using organometallic precursors has been tested and optimised to produce various hierarchical nano-hybrid and micro-hybrid materials. Very fast, operating at low or ambient temperature, this original “one pot” physical method is extremely simple, not requiring any pre- or post-treatment. The plasma-based technique can use any kind of electric discharge (direct current, radio or microwave frequency), does operate at low pressure or at the atmosphere, and can be combined with a large choice of plasma gases and organometallic precursors. Examples of the versatility of the method will be shown, including Pt and Ni-decorated carbon nanotubes (CNTs), Ag and Ti-decorated latex beads, and Pd-decorated clay sheets.

One focus of the presentation will be the preparation and full characterisation of bimetallic Pd/Rh - CNT hybrids. The x-ray diffraction (XRD) and TEM (EDX) analyses were used to confirm that the deposited nano-particles are indeed truly Pd/Rh bimetallic, excluding the possibility of a simple physical aggregate/mixture of the two metals ; complementary analytical tools such as x-ray photoelectron spectroscopy (global information) and scanning transmission x-ray microscopy (truly local information) reveal that the particles contain a metal/oxide ratio depending of the processing gas; they testify also of the possibility of a nano-particle core-shell structure and of a reorganisation of its structure depending of the processing gas.