The mechanical properties of thin films on substrates are described and studied. It is shown that very large stresses may be present in the thin films that comprise integrated circuits and magnetic disks and that these stresses can cause deformation and fracture to occur. It is argued that the approaches that have proven useful in the study of bulk structural materials can be used to understand the mechanical behavior of thin film materials. Understanding the mechanical properties of thin films on substrates requires an understanding of the stresses in thin film structures as well as a knowledge of the mechanisms by which thin films deform. The fundamentals of these processes are reviewed. For a crystalline film on a nondeformable substrate, a key problem involves the movement of dislocations in the film. An analysis of this problem provides insight into both the formation of misfit dislocations in epitaxial thin films and the high strengths of thin metal films on substrates. It is demonstrated that the kinetics of dislocation motion at high temperatures are expecially important to the understanding of the formation of misfit dislocations in heteroepitaxial structures. The experimental study of mechanical properties of thin films requires the development and use of nontraditional mechanical testing techniques. Some of the techniques that have been developed recently are described. The measurement of substrate curvature by laser scanning is shown to be an effective way of measuring the biaxial stresses in thin films and studying the biaxial deformation properties at elevated temperatures. Submicron indentation testing techniques, which make use of the Nanoindenter, are also reviewed. The mechanical properties that can be studied using this instrument are described, including hardness, elastic modulus, and time-dependent deformation properties. Finally, a new testing technique involving the deflection of microbeam samples of thin film materials made by integrated circuit manufacturing methods is described. It is shown that both elastic and plastic properties of thin film materials can be measured using this technique.

Mechanical properties of thin films

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

A calculation of the critical layer thickness hc for growth of GexSi1−x strained layers on Si substrates is presented for 0≤x≤1.0. The present results are obtained assuming misfit dislocation generation is determined solely by energy balance. This approach differs therefore from previous theories (e.g., Matthews et al.), in which the absence of mechanical equilibrium for grown‐in threading dislocations determines the onset of the generation of interfacial misfit dislocations. It is assumed that interfacial misfit dislocations will be generated when the areal strain energy density of the film exceeds the energy density associated with the formation of a screw dislocation at a distance from the free surface equal to the film thickness h. For films thicker than this critical value, screw (and edge) dislocations will be generated at the film/substrate interface. Values obtained for the critical thickness versus lattice mismatch are in excellent agreement with measurements of hc for GexSi1−x strained layers on...

Calculation of critical layer thickness versus lattice mismatch for GexSi1−x/Si strained‐layer heterostructures

1. Introduction and overview 2. Film stress and substrate curvature 3. Stress in anisotropic and patterned films 4. Delamination and fracture 5. Film buckling, bulging and peeling 6. Dislocation formation in epitaxial systems 7. Dislocation interactions and strain relaxation 8. Equilibrium and stability of surfaces 9. The role of stress in mass transport.

Thin Film Materials: Stress, Defect Formation and Surface Evolution

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.

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

GexSi1−x films are grown on Si by molecular beam epitaxy and analyzed by Nomarski optical interference microscopy, Rutherford ion backscattering and channeling, x‐ray diffraction, and transmission electron microscopy. The full range of alloy compositions will grow smoothly on silicon. GexSi1−x films with x≤0.5 can be grown free of dislocations by means of strained‐layer epitaxy where lattice mismatch is accommodated by tetragonal strain. Critical thickness and composition values are tabulated for strained‐layer growth. Multiple strained layers are combined to form a GexSi1−x/Si strained‐layer superlattice.

GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy

A GaAs metal–oxide–semiconductor field-effect transistor (MOSFET) with thin Al2O3 gate dielectric in nanometer (nm) range grown by atomic layer deposition is demonstrated. The nm-thin oxide layer with significant gate leakage current suppression is one of the key factors in downsizing field-effect transistors. A 1 μm gate-length depletion-mode n-channel GaAs MOSFET with an Al2O3 gate oxide thickness of 8 nm, an equivalent SiO2 thickness of ∼3 nm, shows a broad maximum transconductance of 120 mS/mm and a drain current of more than 400 mA/mm. The device shows a good linearity, low gate leakage current, and negligible hysteresis in drain current in a wide range of bias voltage.

GaAs metal–oxide–semiconductor field-effect transistor with nanometer-thin dielectric grown by atomic layer deposition

Melt‐textured YBa2Cu3O7−δsuperconductor with a long and well‐aligned grain structure is shown to exhibit magnetic hysteresis at 77 K which is the largest ever reported for bulk polycrystalline Y‐Ba‐Cu‐O. The large ΔM resulted in a magnet‐like behavior as well as a strong suspension phenomenon. It is shown, contrary to the previous reports, that the suspension behavior observed in Y‐Ba‐Cu‐O is a generic consequence of large grain size, and not due to the presence of Ag oxide or Ag particles. There appears to be no substantial enhancement in flux pinning and J c by these particles. Comparisons of magnetization behavior in various YBa2Cu3O7−δ samples (polycrystals, silver‐oxide doped, melt‐textured, and single crystals) indicated that for applied fields substantially larger than H c 1 the current flow that gives rise to the observed magnetization is intragranular. The pinning force and hence the critical current is roughly the same [J c (magn)∼104 A/cm2] within a factor of ∼2 regardless of grain size, grain boundary configuration, presence of second phase particles, or increased dislocation and twin densities.

Large magnetic hysteresis in a melt‐textured Y‐Ba‐Cu‐O superconductor

Significantly improved flux pinning has been achieved in bulk YBa2Cu3O7−δ superconductor (‘‘123’’ compound) containing fine‐scale defects (<∼50 A thick). The measured Jc intragrain of ∼105 A/cm2 at 77 K, H=0.9 T is about ten times higher than the typical values for bulk Y‐Ba‐Cu‐O. The improved structure was produced by rapid decomposition at 920 °C of the YBa2Cu4O8 (‘‘124’’) precursor. This new and simple processing route could lead to a commercially viable processing technique for flux‐pinning enhancement in bulk Y‐Ba‐Cu‐O.

Enhanced flux pinning by phase decomposition in Y-Ba-Cu-O

High‐quality rare‐earth (RE) single‐crystal films of yttrium (Y) and gadolinium (Gd) were successfully grown with the metal molecular beam epitaxy technique on a bcc Nb single‐crystal film which serves as a buffer layer to the sapphire substrates. With reflection high‐energy electron diffraction, the hcp RE (0001) was found to grow epitaxially on the (110) Nb in the Nishiyama–Wasserman orientation. The regrowth of Nb on this RE (0001) surface yielded the (110) orientation with 120° in‐plane domains. These epitaxial relationships suggest the possibility of fabricating an ultrathin, coherent crystalline superlattice in the Nb (110)/ RE (0001) system.

S. Nakahara

Papers

GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy

GaAs metal–oxide–semiconductor field-effect transistor with nanometer-thin dielectric grown by atomic layer deposition

Large magnetic hysteresis in a melt‐textured Y‐Ba‐Cu‐O superconductor

Enhanced flux pinning by phase decomposition in Y-Ba-Cu-O

Growth of rare-earth single crystals by molecular beam epitaxy: the epitaxial relationship between hcp rare earth and bcc niobium