Film Thickness Regime Research Articles

Photo-Cross-Linkable PNIPAAm Copolymers. 2. Effects of Constraint on Temperature and pH-Responsive Hydrogel Layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide, 2-(dimethylmaleimido)-N-ethyl-acrylamide as the photosensitive component, and 3-acryloylaminopropionic acid or N-(2-(dimethylamino)ethyl)acrylamide as ionizable comonomers were prepared by free radical polymerization. Aqueous solutions of the linear un-cross-linked co- and terpolymers showed lower critical solution temperature behavior. The phase transition temperature, which was detected by differential scanning calorimetry, ranged from 23.1 to 39.2 °C depending on the pH of the solution and the composition of the polymer. Surface plasmon resonance and optical waveguide spectroscopy were used to obtain information about the swelling behavior of hydrogel films of the photo-cross-linked polymers, giving a measurement of film thickness and refractive index. The transition temperatures of the cross-linked polymer gels showed similar trends to those of the corresponding linear polymers in solution, and the gels were shown to be both temperature- and pH-responsive, with the transition temperature ranging from 25.3 to 44.9 °C for films having a 200 nm dry film thickness. However, the swelling behavior of the cross-linked gels was found to vary as a function of dry film thickness, and three samples were selected for a more detailed study of how film thickness affects the transition temperature and swelling ratio of hydrogel films. Dry film thicknesses ranged from 9 nm to 2.3 μm, and the swelling behavior of the films fell into two distinct regimes separated by a critical thickness, which ranged from 280 to 500 nm. In the thin-film regime, the transition temperature of the films was independent of film thickness, but the refractive index of the films in the collapsed state decreased as film thickness decreased, indicating that these films are not able to fully collapse. In the thick-film regime, the swelling ratio of the films was independent of film thickness, but the transition temperature decreased as much as 2.6 °C as the film thickness increased. This was explained by the constraint imposed on the film by the presence of a fixed substrate, with the length scale of this constraint related to the critical thickness. In these films, the ionizable comonomers were found to have little effect on the swelling ratio, which is determined primarily by cross-linking density in the swollen state and by film thickness in the collapsed state.

In Situ Real-Time Characterization of Thin Films

The application of thin film materials in advanced technologies is increasing as new generations of complex electronic, optical and magnetic devices are developed. The dynamics of the growth of thin and ultra-thin films and their evolving, dimensionally dependent, properties are of crucial importance in understanding their performance in particular devices. Anyone who has been concerned with the fabrication of thin films and their associated properties will understand the need to have knowledge of their many physical properties and how these are related to the growth mechanisms and environment in which films are deposited. This book provides a reasonably comprehensive overview of several in situ characterization techniques that, in some cases, are capable of following the developing properties of thin films and surfaces in real time. Several techniques are described including those involving ion- and electron-beam scattering, photoemission electron microscopy, a variety of photometric and ellipsometric optical methods, x-ray reflectivity and curvature-based methods of real-time stress monitoring. The various chapters in this book have been written by a variety of authors and therefore it is inevitable that each topic reflects the style and particular interests of individuals. Nevertheless, for newcomers to the field of thin film technology, such as graduate scientists and postdoctoral researchers, the contents provide a means of rapidly assimilating the basics of some of the in situ methods that have been used in the past for characterizing thin films. From this point of view the book is successful, and many references are given for further reading for those who wish to pursue a particular topic in greater detail. As is often the case, specific examples illustrating the techniques tend to be limited and are often chosen to demonstrate the effectiveness of the methods. Whilst this is understandable, it is often the case that a potential user wishes to explore some system or film-thickness regime that will respond poorly to such probe methodologies and a greater discussion of the limitations of the techniques would have been useful, particularly for the beginner. In the chapter on ellipsometry for example, only thick films are dealt with and examples confined to systems that work particularly well (e.g. SiO2 on Si). Likewise, various effective medium models are discussed in relation to non-planar (rough) surfaces. However, the limitations of such models are not dealt with, nor are those of ellipsometry for dealing with more problematic systems such as ultra-thin films and multilayers. A disappointing aspect of the book is that no specific chapter is devoted to magnetic materials and their properties, currently of considerable interest to the information storage industry. Despite these minor drawbacks the book is very commendable, generally well written and a worthy addition to any library on thin film growth and characterization. It is particularly valuable because of the very real need to understand the dynamically developing properties of thin and ultra-thin films and surfaces, and because it provides readable descriptions of some of the more important instrumental techniques that are used to study the evolution of thin-film characteristics in real time. R Atkinson

Lithography for sub-60 nm resist nanostructures

As the semiconductor community continues to follow the Semiconductor Industry Association Roadmap, resist structures are being printed further into the nanometer domain. However, a persistent issue for successful sub-60 nm resist patterning is mechanical stability at high aspect ratios. The objective of this article is to understand what processing conditions facilitate processing resist nanostructures with useful aspect ratios for the fabrication of sub-60 nm transistors. We have found that, in aqueous based development and rinse, if the resist thickness is reduced, then the aspect ratio is sacrificed for the sake of resolution. The implication is that there is a resolution limit at which resist structures will have aspect ratios that are useful for device fabrication. We have also found that there are development effects that occur in the thick film regime that are not reproducible with thin films. The best resolution structures we have been able to print are lines of 28 nm in width using direct write electron-beam lithography on negative chemically amplified resists NEB-22 and NEB-31 (Sumitomo Chemical Inc.) with an aspect ratio of about 3. To put this result in perspective, this is about 40 molecules wide.

Oil Layer as Source of Hydrocarbon Emissions in SI Engines

The role of lubrication oil film on the cylinder liner as a source of hydrocarbon emissions in spark-ignition engines is assessed. First, the “source strength” is examined via an analytical model of the gasoline vapor absorption/desorption process. The solution shows that depending on engine operating conditions, there are three regimes. The process could be (1) limited by the gas side diffusion process, (2) limited by the liquid phase diffusion process, with the absorbed fuel fully penetrating the oil layer thickness (thin oil film regime), and (3) again limited by the liquid phase diffusion process, but with the absorbed fuel penetration depth small compared to the oil layer thickness (thick oil film regime). In regime (1), the source strength (the integrated absorption or desorption flux over one cycle) is proportional to the inverse of the square root of the rpm, but independent of oil layer parameters. In regimes (2), the strength is proportional to the oil film thickness divided by the Henry’s constant. In regime (3), the strength is independent of the oil film thickness, but is proportional to the fuel penetration depth divided by the Henry’s constant. Then, the oxidation of the desorbed fuel (using iso-octane as fuel) is examined with a one-dimensional reaction/diffusion model. The novel feature of the model is that the desorbed fuel is being exposed to the piston crevice hydrocarbon, which is laid along the liner as the piston descends. At stoichiometric conditions, the oxidation of the crevice HC is reduced by the presence of the desorbed HC from the oil layer.

Microstructural evolution of a-Si:H prepared using hydrogen dilution of silane studied by real time spectroellipsometry

We have applied real time spectroellipsometry to measure the nucleation and growth of hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition from H2-diluted SiH4 on crystalline Si (c-Si) substrates at 200°C. For a H2-dilution ratio R=[H2]/[SiH4] of 10, optimum microstructural evolution is observed during the growth of 0.3 μm a-Si:H film, namely, smoothening during coalescence followed by long-term surface stability. At lower and higher R values, surface roughening in the thick film regime (d>20 nm) is larger, particularly for R≥20 due to crystallite development. Although dilution levels of 20≤R≤30 lead to microcrystallinity in thick films (0.3 μm), the structural evolution and optical properties in thin films (<20 nm) are characteristic of high quality a-Si:H. Thus, real time spectroellipsometry suggests that in the preparation of i-layers for solar cells, R∼10 may be optimum for the bulk i-layer whereas a 10 to 20 nm layer with much larger R may be beneficial at the p/i-interface. These suggestions have been verified in p–i–n cells deposited on specular SnO2-coated glass substrates.

Correlation between structure and magnetic anisotropies of Co on Cu(110)

Magnetic anisotropies of molecular-beam-epitaxy-grown fcc Co(110) films on Cu(110) single-crystal substrates have been determined by using Brillouin light scattering and have been correlated with the structural properties determined by low-energy electron diffraction and scanning tunneling microscopy (STM). Three regimes of film growth and associated anisotropy behavior are identified: coherent growth in the Co film thickness regime of up to 13 \AA{}, in-plane anisotropic strain relaxation between 13 and about 50 \AA{} and in-plane isotropic strain relaxation above 50 \AA{}. The structural origin of the transition between anisotropic and isotropic strain relaxation was studied using STM. In the regime of anisotropic strain relaxation long Co stripes with a preferential [1$1\ifmmode\bar\else\textasciimacron\fi{}$0]-orientation are observed, which in the isotropic strain relaxation regime are interrupted in the perpendicular in-plane direction to form isotropic islands. In the Co film thickness regime below 50 \AA{} an unexpected suppression of the magnetocrystalline anisotropy contribution is observed. Symmetry reflections based on a crystal-field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduce the experimentally observed anomalies despite the fact that the thick Co films are quite rough.

Fluid interface fluctuations within the generalized Derjaguin approximation

The fluctuation properties of fluid interfaces bounded by rough surfaces are investigated within a linear generalization of the Derjaguin approximation. In the thick-film regime, the interface roughness amplitude is lower in magnitude from that obtained in the Derjaguin approximation. Nevertheless, for large healing lengths z the power-law asymptotic behavior s w;z 22 , which is observed in the Derjaguin approximation, is still preserved. Moreover, the rms local interface slope r is shown to attain small values for film thicknesses larger than the substrate roughness amplitude and to follow an asymptotic power-law behavior r;z 22 for large z. @S0163-1829~97!07436-5# Wetting phenomena of fluids on solid substrates have been an important topic of applied and fundamental research for more than a century. However, the understanding of the complexity of these phenomena is still incomplete, since wetting is highly sensitive to roughness and chemical contaminants of the solid substrates. 1‐4 In general, these types of surface disorder can have a dramatic influence on interfacial processes which are of experimental and technological interest. Various theoretical treatments 2‐4 of the influence of surface roughness on the wetting properties of liquids have been performed within the so-called Derjaguin approximation. 5 In fact, this approximation accounts for replacing the local disjoining pressure P d by that of a uniform film of thickness h(r)2z(r) @z(r) and h(r) are, respectively, the substrate and liquid-vapor surface/interface profile functions# for small substrate roughness amplitudes, and then linearize the disjoining pressure around the average film thickness « that would exist on a flat surface. 3 The average thickness « is given by the relation Dm5P d(«), with Dm the chemical potential difference between the liquid and vapor phases, and the liquid-vapor interface fluctuations are described by the

The evolution of growth stresses in chemical vapor deposited tungsten films studied by in situ wafer curvature measurements

An in situ study of the evolution of the biaxial state of intrinsic stress during nucleation and growth of polycrystalline tungsten chemical vapor deposition films deposited by the hydrogen reduction of tungsten hexafluoride is presented. The evolution of biaxial stress was determined from in situ wafer curvature measurements. It is shown that the intrinsic stress is a growth stress, i.e., a stress developing in close vicinity to the advancing surface of the film due to metastable film growth processes. The stress developing depends strongly on the thickness of the film. High tensile stress (≊4 GPa) is observed during the initial stage of growth, compressive stress (up to −1 GPa) is observed in an intermediate thickness regime after film closure and tensile stress (0.1–1 GPa) is observed in the thick film regime. The associated stress gradients in the film are preserved during and after growth. The development of growth stress is determined by deposition temperature and growth rate. The tensile stress in the thick film regime is larger at a higher growth rate or a lower deposition temperature, while the compressive stress in the intermediate thickness regime showed the opposite dependency. Film properties as the evolution of grain size, impurity content, and resistivity are found not to vary significantly with the growth conditions. Therefore, the development of growth stress is ascribed to kinetical processes. The development of tensile stress in the thick film regime is described with a (kinetic) grain boundary formation and relaxation model. The compressive stress in the intermediate thickness regime is tentatively ascribed to compressive coherency strains induced by interfacial tensions of the grains in the stage of island growth.

Electrical and optical properties of ion beam sputtered ZnO:Al films as a function of film thickness

Electrical and optical properties of as-deposited, ion beam sputtered, Al-doped ZnO films have been studied as a function of film thickness and carrier concentration. Hall effect measurements reveal that the bulk electrical resistivity of the film generally decreases with increasing film thickness. Additionally, it is observed that the rate of decreasing resistivity depends on the particular film thickness regime. For thinner films (100–200 nm), the resistivity decreases rapidly with increasing film thickness and is due to increases in both carrier concentration and Hall mobility. However, for thicker films, the resistivity decreases more slowly with increasing film thickness and approaches a nearly constant value at a thickness of 1100 nm. In this thickness regime, the slight decrease in resistivity with increasing film thickness is found to be due to an increase in carrier concentration alone. The above observations suggest the presence of at least two scattering mechanisms. It is speculated that grain boundary and ionized impurity are the most likely mechanisms. These issues, together with observed optical properties of the films, are discussed.

Chemical vapour deposition tungsten film growth studied by in situ growth stress measurements

In this paper we present in situ measurements of the wafer curvature during nucleation and growth of W films deposited by chemical vapour deposition (CVD) on Si substrates. Because of the absence of stress relaxation mechanisms in the films, these measurements directly reflect the growth stress in the W films. The growth stress development is measured during self-limiting W film growth by the Si(100) reduction of WF 6 and during continuous W film growth by the H 2, SiH 4, and GeH 4 reduction of WF 6. It is shown that these measurements provide detailed information on the growth kinetics (for the Si reduction of WF 6) as well as the (high) growth stresses themselves. High stress gradients are observed in the growth direction of the films. In general the growth stresses range from highly tensile during the initial stages of growth, to compressive in an intermediate region and tensile again in the thick film regime. The average film stress decreases with increasing deposition temperature. The process-dependent growth stresses reported in this paper can be used to engineer the magnitude of the average film stress in W CVD metallization of integrated circuits.

Pressure dependencies of copper oxidation for low- and high-temperature parabolic laws

Studies of the oxidation kinetics of copper have been conducted in the thin-film range at temperatures of 383–398 K and in the oxygen pressure range of 0.278–21.27 kPa; whereas in the thick-film regime at 1123 K, studies have been conducted in the oxygen pressure range of 2.53–21.27 kPa. Furthermore, the effect of continuously impressed direct current with oxygen pressure variation in Wagner's parabolic range has been studied also in order to have a better understanding of the effective charge on the migrating species. In the low-temperature range, the rate constant, kP ∝\(P_{O_2 }^{1/4} \), suggesting that the migration of neutral vacancies in the growing film predominates. At high temperature, 1123 K, in the Wagnerian regime, the observed approximate pressure dependencies of the parabolic rate constants are the following: $$\begin{gathered} {\text{k}}_{\text{p}} (normal oxidation) \propto \sim {\text{P}}_{{\text{O}}_{\text{2}} }^{{\text{1/7}}} \hfill \\ {\text{k}}_{\text{p}} (sample cathodic) \propto \sim {\text{P}}_{{\text{O}}_{\text{2}} }^{{\text{1/5}}} \hfill \\ \end{gathered} $$ and $${\text{k}}_{\text{p}} (sample anodic) \propto \sim {\text{P}}_{{\text{O}}_{\text{2}} }^{{\text{1/10}}} $$ .

Temperature dependence of threshold voltage in thin-film SOI MOSFETs

A first-order model for the temperature dependence of threshold voltage in thin-film silicon-on-insulator (SOI) n-MOSFETs is described. The temperature dependence of the threshold voltage of thin-film SOI n-channel MOSFETs is analyzed. Threshold voltage variation with temperature is significantly smaller in thin-film (fully depleted) devices than in thick-film SOI and bulk devices. The threshold voltage is shown to be dependent on the depletion level of the device, i.e. whether it is fully depleted or not. There exists a critical temperature below which the device is fully depleted, and above which the device operates in the thick-film regime.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lubrication of selected salivary molecules and artificial salivas

The lubrication regime displayed by human salivas (parotid and submandibular-sublingual), purified salivary molecules (the mucins MG1 and MG2 and alpha-amylases), and selected artificial salivas (Oracare D, Saliva Substitute, and Orthana) was assessed in vitro using a friction-testing device. Thin-film (boundary) lubrication was observed for all of the salivary samples and two of the artificial salivas examined. Oracare D, a glycerol-based artificial saliva, was the exception since it lubricated by a thick-film (hydrodynamic) regime. On a molar basis, the best lubricants of the purified salivary molecules were MG1 greater than MG2 approximately nonglycosylated alpha-amylases approximately glycosylated alpha-amylases.

Thickness and polarization dependence of the magnetooptic signal from ultrathin ferromagnetic films.

Longitudinal Kerr-effect measurements using both s- and p-polarized light are presented as a function of thickness for bcc Fe films grown epitaxially on Au(100) in ultrahigh vacuum (UHV). The motivation is to explore the origins of the magnetooptic response at Fe thicknesses comparable to the depth penetration of light. Special efforts were taken to optically compensate for the birefringence of the UHV window. The compensated measurements yield the magnitude of the complex rotation phi/sub m/ = (phi'/sup 2/+phi''/sup 2/)/sup 1/2/, where phi' and phi'' are the real magnetooptic rotation and the ellipticity, respectively. The results show linear initial increase of phi/sub m/ with thickness (up to the optical penetration depth) followed by a shallow peak and leveling off to a saturation value of the thick-film Kerr rotation. A review of the relevant theory includes the Faraday and Kerr contributions to the magnetooptic response. The Faraday contribution arises from metallic reflection from the substrate and passage back through the iron overlayer. We argue that the Faraday effect dominates the response in the ultrathin limit, while the Kerr effect controls the thick-film regime.

NMR study of thick films of methane on graphite

We report the results of an NMR survey of the system methane adsorbed on graphite near the triple point temperature, in the thick-film regime (10–50 layers) where the methane behavior is substantially bulk-like. We study the interaction of the film with the substrate and find that T 1 is affected by a dipolar coupling with paramagnetic spin centers in the substrate. Since the systematics of these effects are now understood, they can be used as diagnostics in their own right. We are able to conclude that solid methane wets the graphite basal plane substrate to a thickness of at least 50 layers in the temperature range 70 K < T < 105 K. On the Grafoil substrate we employ, such thick films ought to be unstable with respect to capillary condensation, but capillary condensation is evidently not nucleated in our experiments.

Complete wetting of helium on graphite.

Using the vibrating-wire microbalance technique we have found that superfluid $^{4}\mathrm{He}$, normal fluid $^{4}\mathrm{He}$, and $^{3}\mathrm{He}$ all wet graphite completely. Incomplete wetting above ${T}_{\ensuremath{\lambda}}$ was simulated by intentionally introducing a temperature gradient in the apparatus. Data in the thick-film regime are found to be consistent with the Frenkel-Halsey-Hill model of adsorption. The graphite fiber surface has been characterized through volumetric adsorption isotherms, revealing additional surface area not previously accounted for by others using this technique.

Reverse dopant redistribution during the initial stages of the oxidation of heavily doped silicon in dry oxygen

The oxidation of heavily phosphorus-doped (100) and (111) silicon in the 800–1000 °C range in dry oxygen was studied in the thin-film regime using in situ ellipsometry. The oxide growth kinetics indicate that, in the initial stages of oxidation, phosphorus piles up at the Si-SiO2 interface, resulting in a decrease in the surface concentration of electrically active phosphorus and an oxidation rate similar to that of lightly doped silicon. As the oxide grows, the surface concentration of electrically active phosphorus reaches and exceeds its bulk value, and the SiO2 growth rate gradually becomes similar to that of heavily doped silicon in the thick-film regime. An empirical model is introduced to account for the influence of this reverse dopant redistribution on SiO2 growth kinetics in the thin-film regime.