Riikka L. Puurunen

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Island growth as a growth mode in atomic layer deposition: A phenomenological model

Atomic layer deposition (ALD) has recently gained world-wide attention because of its suitability for the fabrication of conformal material layers with thickness in the nanometer range. Although the principles of ALD were realized about 40 years ago, the description of many physicochemical processes that occur during ALD growth is still under development. A constant amount of material deposited in an ALD reaction cycle, that is, growth-per-cycle (GPC), has been a paradigm in ALD through decades. The GPC may vary, however, especially in the beginning of the ALD growth. In this work, a division of ALD processes to four classes is proposed, on the basis of how the GPC varies with the number of ALD reaction cycles: linear growth, substrate-enhanced growth, and substrate-inhibited growth of type 1 and type 2. Island growth is identified as a likely origin for type 2 substrate-inhibited growth, where the GPC increases and goes through a maximum before it settles to a constant value characteristic of a steady gr...

Island growth in the atomic layer deposition of zirconium oxide and aluminum oxide on hydrogen-terminated silicon: Growth mode modeling and transmission electron microscopy

Atomic layer deposition (ALD) is used in applications where inorganic material layers with uniform thickness down to the nanometer range are required. For such thicknesses, the growth mode, defining how the material is arranged on the surface during the growth, is of critical importance. In this work, the growth mode of the zirconium tetrachloride∕water and the trimethyl aluminum∕water ALD process on hydrogen-terminated silicon was investigated by combining information on the total amount of material deposited with information on the surface fraction of the material. The total amount of material deposited was measured by Rutherford backscattering, x-ray fluorescence, and inductively coupled plasma–optical emission spectroscopy, and the surface fractions by low-energy ion scattering. Growth mode modeling was made assuming two-dimensional growth or random deposition (RD), with a “shower model” of RD recently developed for ALD. Experimental surface fractions of the ALD-grown zirconium oxide and aluminum oxid...

Atomic layer deposition of hafnium oxide on germanium substrates

Germanium combined with high-κ dielectrics has recently been put forth by the semiconductor industry as potential replacement for planar silicon transistors, which are unlikely to accommodate the severe scaling requirements for sub-45‐nm generations. Therefore, we have studied the atomic layer deposition (ALD) of HfO2 high-κ dielectric layers on HF-cleaned Ge substrates. In this contribution, we describe the HfO2 growth characteristics, HfO2 bulk properties, and Ge interface. Substrate-enhanced HfO2 growth occurs: the growth per cycle is larger in the first reaction cycles than the steady growth per cycle of 0.04nm. The enhanced growth goes together with island growth, indicating that more than a monolayer coverage of HfO2 is required for a closed film. A closed HfO2 layer is achieved after depositing 4–5HfO2 monolayers, corresponding to about 25 ALD reaction cycles. Cross-sectional transmission electron microscopy images show that HfO2 layers thinner than 3nm are amorphous as deposited, while local epita...

Analysis of hydroxyl group controlled atomic layer deposition of hafnium dioxide from hafnium tetrachloride and water

Atomic layer deposition (ALD) has recently gained interest because of its suitability for the fabrication of conformal material layers with thicknesses in the nanometer range. Although the principles of ALD were realized 30 to 40 years ago, the description of many physicochemical processes that occur during ALD growth is still under development. “Substrate-inhibited (SI)” ALD growth is one phenomenon not yet well understood. In SI-ALD, the growth-per-cycle (GPC) increases in the beginning of the growth, goes through a maximum, and levels off to a constant value. The origin of SI growth is investigated in this work with two recent models of ALD: Model A of Puurunen [Chem. Vap. Deposition 9, 249 (2003)] and Model B of Alam and Green [J. Appl. Phys. 94, 3403 (2003)]. The hafnium tetrachloride/water ALD process, of interest for gate dielectric applications, is taken to represent typical SI growth. The possible reaction chemistry is evaluated with two models: Model C of Ylilammi [Thin Solid Films, 279, 124 (19...

Implementing ALD layers in MEMS processing

Layers manufactured by the ALD technique have many interesting applications in microelectromechanical systems (MEMS), for example as protective layers for biocompatible coating, highdielectric-constant layers, or low-temperature conformal insulating layers. Before an ALD process can be successfully implemented in MEMS processing, several practical issues have to be solved, starting from patterning the layers and characterizing their behaviour in various chemical and thermal environments. Stress issues may not be forgotten. We have recently implemented two ALD processes, namely the trimethylaluminium/water process to deposit Al2O3 and the titanium tetrachloride/water process to deposit TiO2 in our MEMS processing line and carried out the necessary characterization, details of which are reported here. For us, ALD has been a truly enabling technology in the processing of a three-dimensional micromechanical compass based on the Lorentz force, where Al2O3 acted as a pinhole-free electrical insulation grown at low temperature.

Inductively coupled plasma etching of amorphous Al2O3 and TiO2 mask layers grown by atomic layer deposition

Al2O3 and TiO2 deposited by atomic layer deposition are evaluated as etch masks for dry etch processes in an inductively coupled plasma reactor using the Bosch process. In the inductively coupled plasma chamber during deep silicon etching, because of the chemical nature of the etch process and the inert nature of Al2O3, the result is exceptional selectivity for silicon over as-deposited Al2O3, particularly at relatively low bias and high pressures used for through-wafer etching. TiO2 is less resistant and appears to suffer more from chemical attack. In both cases, etch rate increases slowly with increasing rf bias. However, there is a sharp discontinuity in the etch rate of Al2O3 when the bias power is operated in a pulsed low-frequency mode. This is thought to be due to increased sputtering from heavier ions. Preliminary studies indicate the etching conditions for Al2O3 may be extended into a dielectric etch regime requiring more study.

Hafnium oxide films by atomic layer deposition for high- κ gate dielectric applications: Analysis of the density of nanometer-thin films

The density of hafnium oxide films grown by atomic layer deposition for high-κ gate dielectric applications was investigated for films with thickness in the nanometer range. The density, measured by combining the film thickness from transmission electron microscopy with the amount of hafnium deposited from Rutherford backscattering, decreased with decreasing film thickness. The dielectric constant of hafnium oxide remained constant with decreasing film thickness, however. The main reason for the decrease in the measured density seemed not to be a decrease in the inherent material density. Instead, the relative importance of interface roughness in the density measurement increased with decreasing film thickness.

Chromium(III) supported on aluminum-nitride-surfaced alumina: characteristics and dehydrogenation activity

Abstract Chromium catalysts were prepared by the saturating chemisorption of chromium(III) acetylacetonate (Cr(acac) 3 ) at 200 °C on alumina and on alumina that had been surfaced with aluminum nitride. According to elemental analysis, infrared spectroscopy, and electron spin resonance spectroscopy, chemisorption occurred through ligand exchange reaction of Cr(acac) 3 with surface OH and NH x groups and dissociation of Cr(acac) 3 or Hacac on coordinatively unsaturated sites on the surface. Steric hindrance imposed by the acac ligands defined saturation of the surface with adsorbed species; the same chromium loading (0.64±0.06 atoms per nm 2 ) was obtained on all supports. After removal of the ligands with oxygen, water, or ammonia, the samples were active in isobutane dehydrogenation. For chromium sites with high activity, nearby oxygen ions were needed. The activity of a Cr 3+ ion was mainly determined by its environment and not by whether it had been formed through reduction.

Microscopic silicon-based lateral high-aspect-ratio structures for thin film conformality analysis

Film conformality is one of the major drivers for the interest in atomic layer deposition (ALD) processes. This work presents new silicon-based microscopic lateral high-aspect-ratio (LHAR) test structures for the analysis of the conformality of thin films deposited by ALD and by other chemical vapor deposition means. The microscopic LHAR structures consist of a lateral cavity inside silicon with a roof supported by pillars. The cavity length (e.g., 20–5000 μm) and cavity height (e.g., 200–1000 nm) can be varied, giving aspect ratios of, e.g., 20:1 to 25 000:1. Film conformality can be analyzed with the microscopic LHAR by several means, as demonstrated for the ALD Al2O3 and TiO2 processes from Me3Al/H2O and TiCl4/H2O. The microscopic LHAR test structures introduced in this work expose a new parameter space for thin film conformality investigations expected to prove useful in the development, tuning and modeling of ALD and other chemical vapor deposition processes.