Angel Yanguas-Gil

Highly conformal film growth by chemical vapor deposition. I. A conformal zone diagram based on kinetics

The authors present a unified description of conformal film growth in recessed features by low pressure chemical vapor deposition. Experimental data on step coverage and growth rate are interpreted in terms of a kinetic model for the elementary surface processes. This model is combined with the continuity equation for gas transport and consumption to predict the values of the precursor pressure and substrate temperature (p,T) that are necessary for conformal coverage to occur for a given precursor. They introduce a conformal zone diagram that considers the gas phase limitations (maximum precursor pressure and onset of secondary reactions), the surface limitations (minimum reaction temperature, precursor adsorption and desorption rates, and film growth rate), and the aspect ratio of the feature to be coated; the allowed intersection between the resulting boundaries defines the (p,T) zone for conformal growth. Sufficient precursor pressure is identified as a major requirement for conformal film growth, one that is not met by many commercially available molecules. Finally, they derive an approximate analytical solution to the inverse problem: what are the experimental conditions necessary to afford a desired degree of step coverage on a given aspect ratio feature at a desired growth rate. The solution depends on a set of rate constants in the kinetic model that can be extracted from measurements of the film growth rate as a function of precursor pressure and substrate temperature on trench or planar substrates.The authors present a unified description of conformal film growth in recessed features by low pressure chemical vapor deposition. Experimental data on step coverage and growth rate are interpreted in terms of a kinetic model for the elementary surface processes. This model is combined with the continuity equation for gas transport and consumption to predict the values of the precursor pressure and substrate temperature (p,T) that are necessary for conformal coverage to occur for a given precursor. They introduce a conformal zone diagram that considers the gas phase limitations (maximum precursor pressure and onset of secondary reactions), the surface limitations (minimum reaction temperature, precursor adsorption and desorption rates, and film growth rate), and the aspect ratio of the feature to be coated; the allowed intersection between the resulting boundaries defines the (p,T) zone for conformal growth. Sufficient precursor pressure is identified as a major requirement for conformal film growth, one ...

Simple model for atomic layer deposition precursor reaction and transport in a viscous-flow tubular reactor

Precursor reaction and transport are both critical in determining the thickness uniformity and conformality of atomic layer deposition (ALD) thin films. However, it is sometimes difficult to predict how changes in conditions, such as mass flow rate or precursor reactivity, will affect the outcome of an ALD experiment. To provide some insight and guidance, we have developed a simple 1D model to describe precursor transport and reaction in a tubular viscous flow ALD reactor. After making some simplifying assumptions, we show that the transport problem depends only on three independent parameters, the Peclet number, the Damkoeler number, and the excess number, which can be easily calculated for most ALD processes. Despite its simplicity, we obtain very good agreement with experimental results for the thickness profiles of ALD Al2O3 films deposited using trimethyl aluminum and H2O. The authors have applied the model to study the impact of precursor properties and experimental conditions on the growth profiles...

Highly conformal film growth by chemical vapor deposition. II. Conformality enhancement through growth inhibition

The authors present a novel strategy for enhancing conformality in chemical vapor deposition (CVD) based on the concept of growth inhibition. In Part I, they showed how surface site blocking was responsible for the increase in conformality observed at higher pressures in high vapor pressure precursors. In this work, they apply this concept to enhance conformality by considering a secondary species that acts as an inhibitor of the precursor. The experimental results obtained for the growth of TiB2 from Ti(BH4)3dme (dme=dimethoxyethane), with dme acting as the growth inhibitor, agree well with the models, based on a first order adsorption/desorption kinetics, and show how this strategy greatly enhances the conformality of nonconformal precursors. Finally, they show how multiple surface mechanisms including surface site blocking, associative desorption, long surface residence time of by-products, or coadsorption of two reactants can induce growth inhibition. Consequently, this strategy can be potentially app...

Growth inhibition to enhance conformal coverage in thin film chemical vapor deposition.

We introduce the use of a growth inhibitor to enhance thin film conformality in low temperature chemical vapor deposition. Films of TiB(2) grown from the single source precursor Ti(BH(4))(3)(dme) are much more highly conformal when grown in the presence of one of the film growth byproducts, 1,2-dimethoxyethane (dme). This effect can be explained in terms of two alternative inhibitory mechanisms: one involving blocking of surface reactive sites, which is equivalent to reducing the rate of the forward reaction leading to film growth, the other analogous to Le Chateliers principle, in which the addition of a reaction product increases the rate of the back reaction. The reduction in growth rate corresponds to a reduction in the sticking probability of the precursor, which enhances conformality by enabling the precursor to diffuse deeper into a recessed feature before it reacts.

Analytic expressions for atomic layer deposition: Coverage, throughput, and materials utilization in cross-flow, particle coating, and spatial atomic layer deposition

In this work, the authors present analytic models for atomic layer deposition (ALD) in three common experimental configurations: cross-flow, particle coating, and spatial ALD. These models, based on the plug-flow and well-mixed approximations, allow us to determine the minimum dose times and materials utilization for all three configurations. A comparison between the three models shows that throughput and precursor utilization can each be expressed by universal equations, in which the particularity of the experimental system is contained in a single parameter related to the residence time of the precursor in the reactor. For the case of cross-flow reactors, the authors show how simple analytic expressions for the reactor saturation profiles agree well with experimental results. Consequently, the analytic model can be used to extract information about the ALD surface chemistry (e.g., the reaction probability) by comparing the analytic and experimental saturation profiles, providing a useful tool for characterizing new and existing ALD processes.

Highly conformal magnesium oxide thin films by low-temperature chemical vapor deposition from Mg(H3BNMe2BH3) 2 and water

Pure, dense, and stoichiometric MgO thin films have been deposited at temperatures as low as 225u2009°C by chemical vapor deposition using a recently reported magnesium precursor, magnesium N,N-dimethylaminodiboranate, which has the highest room-temperature vapor pressure among known Mg-containing compounds, with water as a co-reactant. The films are characterized by x-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and spectroscopic ellipsometry. Conformal coating on a trench with 35:1 aspect ratio is achieved at a film growth rate of 2u2009nm/min. The growth rate can be tuned between 2–20u2009nm/min according to the requirement of the structure to be coated.

A modular reactor design for in situ synchrotron x-ray investigation of atomic layer deposition processes

Synchrotron characterization techniques provide some of the most powerful tools for the study of film structure and chemistry. The brilliance and tunability of the Advanced Photon Source allow access to scattering and spectroscopic techniques unavailable with in-house laboratory setups and provide the opportunity to probe various atomic layer deposition (ALD) processes in situ starting at the very first deposition cycle. Here, we present the design and implementation of a portable ALD instrument which possesses a modular reactor scheme that enables simple experimental switchover between various beamlines and characterization techniques. As first examples, we present in situ results for (1) X-ray surface scattering and reflectivity measurements of epitaxial ZnO ALD on sapphire, (2) grazing-incidence small angle scattering of MnO nucleation on silicon, and (3) grazing-incidence X-ray absorption spectroscopy of nucleation-regime Er2O3 ALD on amorphous ALD alumina and single crystalline sapphire.

A Markov chain approach to simulate Atomic Layer Deposition chemistry and transport inside nanostructured substrates

In this work, we present a new theoretical framework to model the transport and surface chemistry under molecular (Knudsen) flow. Our approach is based on casting the transport inside nanostructures as a single-particle discrete Markov chain process. One of the advantages of this approach is that it allows us to decouple the complexity of the surface chemistry from the transport model, thus allowing its application under general surface chemistry conditions, including atomic layer deposition (ALD) and chemical vapor deposition (CVD). Our model also allows us to determine statistical information of the trajectory of individual molecules, such as the average interaction time or the number of wall collisions for molecules entering the nanostructures as well as to track the relative contributions to thin-film growth of different independent reaction pathways at each point of the feature. This offers a straightforward way of incorporating into ALD simulations non-ideal surface processes, such as parasitic CVD or surface recombination. By studying the asymptotic behavior of the Markov chain process, we were also able to establish a direct link between ballistic models, kinetic Monte Carlo simulations, and continuous models based on the use of the diffusion equation under Knudsen conditions. Finally, we show that, under certain approximations, the coverage profile inside a nanostructure under ALD conditions is controlled by the total exposure, and not by the details of the surface flux dependence with time during the exposure, as long as the reaction probabilities are pressure independent.

Remote plasma treatment of Si surfaces: Enhanced nucleation in low-temperature chemical vapor deposition

The nucleation density on Si(100):H is increased by two orders of magnitude after exposing the surface to a remote argon plasma. We study HfB2 growth from Hf(BH4)4 and MgO growth from Mg(DMDBA)2 plus H2O. In the latter case, pretreatment allows the growth of MgO films with an rms roughness below 0.5 nm, whereas in absence of plasma treatment no nucleation is observed. The plasma does not damage the substrate and is compatible with microelectronics technology. We propose that H desorption is the key mechanism leading to nucleation enhancement, and that remote plasma activation is likely to be generally applicable.

New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al2O3 SIS using trimethyl aluminum (TMA) and H2O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10× slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.