Akiko N. Itakura

Surface stress, thickness, and mass of the first few layers of polyelectrolyte

The effects of surface stress and mass loading upon the adsorption of polyelectrolytes onto flexible silicon micromechanical cantilever sensors (MCSs) were studied in situ. A self-assembled monolayer of 2-mercaptoethylamine chloride (2-MEA) on gold was used to achieve single-side adsorption on the MCS. Such a preparation gave a positive surface potential, whereas a bare SiOx surface gave a negative surface potential. Wide scan X-ray photoelectron spectroscopy confirmed that the adsorption of polystyrenesulfonate (PSS) and polyallylamine hydrochloride (PAH) followed the general rule expected from the electrostatic interaction between the substrate and the polyelectrolyte, whereas the adsorption polyethyleneimine (PEI) did not. The adsorption of PAH on SiO(x) from a 3 mM water solution containing 1 M NaCl was associated with a deflection of the MCS toward the polyelectrolyte monolayer (tensile surface stress) owing to the hydrogen bonding between neighboring amino groups. Here, a surface stress change of 1.4 +/- 0.1 N/m was estimated. The adsorption of PSS from a 3 mM water solution containing 1 M NaCl on a 2-MEA surface induced a deflection of the MCS away from the polyelectrolyte layer (compressive stress), toward the SiO(x) side. Here, a surface stress change of 3.1 +/- 0.3 N/m was determined. The formation of a PAH layer on top of the PSS layer resulted in a deflection of the MCS toward the PAH layer. This indicated that the adjacent PSS layer was deswelling, corresponding to a surface stress change of 0.5 +/- 0.1 N/m.

Surface stress in thin oxide layer made by plasma oxidation with applying positive bias

Abstract The time evolution of the surface stress during plasma oxidation of Si(100) at a very early stage (oxide thickness

Electron-stimulated surface stress relaxation of Si

We have observed the nonthermal relaxation of surface stress in Si induced by electron irradiation at room temperature. An atomically thin disordered layer was introduced by Ar ion bombardment. The surface stress change during ion bombardment and the following electron irradiation of Si(100) was measured by means of an optical microcantilever technique. We have found that the compressive stress in the Si surface due to disorder induced by ion bombardment was completely relaxed by electron irradiation at low energy. The criterion for complete relaxation is found not to be total energy deposition, but the number of irradiated electrons.

Effects of surface disorder on the surface stress of Si 100 during oxidation

We have studied the effects of disorder on surface stress during oxidation. The surface stress change during ion bombardment and the following plasma oxidation on Si(100) was measured by means of an optical microcantilever technique. We have found compressive stress on Si surface due to disorder induced by ion bombardment and determined it quantitatively in terms of the number of defects. This disorder-induced compressive stress was completely relaxed by the plasma oxidation processes. The initial evolution of the surface stress during oxidation on bombarded surfaces is quite different from that on unbombarded Si(100) surfaces.

Relationship between Ozone Oxidation and Stress Evolution on an H-Terminated Si Surface

We observed the stress evolution of an H-terminated Si (100) surface during ozone oxidation and the ozone oxidation on a partially H-terminated Si surface of Si (100) and Si (111) to study the surface orientation effect on ozone oxidation. The evolution of stress on the H-terminated Si (100) surface was observed in real-time by an optical micro-mechanical cantilever method. The results show that the stress evolution on the H-terminated Si surface was unexpectedly large when considering that H-termination reduced the sticking amount of oxygen. Both the Si (111) and Si (100) surfaces showed that the rate of ozone oxidation was reduced as hydrogen covered the surfaces. However, at high-H coverage the H-terminated Si (111) surface showed a greater increase of sticking rate than the H-terminated Si (100) surface. The relationship between the oxidation-induced stress and oxidation rate is discussed.

Surface stress control using ultraviolet light irradiation of plasma-polymerized thin films

We investigated the surface stress change of plasma-polymerized allylamine films on 2μm thick silicon micromechanical cantilever substrates induced by ultraviolet light (UV) irradiation. Compressive surface stress was generated during the UV irradiation of the plasma-polymerized films in a dry environment, whereas tensile stress was measured in a humid environment. Fourier transform infrared spectroscopic analysis indicated two mechanisms taking place depending on the environmental conditions. These were attributed to crosslinking and oxidation reactions of the plasma polymer. UV irradiation of plasma polymerized allylamine films at defined humidity suggests a feasible method for achieving tensile and compressive surface stress patterning.

Low-energy ion-induced tensile stress of self-assembled alkanethiol monolayers

Monolayers of alkanethiols on gold have been exposed to low energy Ar ion. A micromechanical cantilever sensor technique was used to determine in situ the influence of the ion dose on the surface stress in the monolayers. In contrast to compressive surface stress during self-assembled monolayer growth, a strong tensile surface stress of about −0.7 N/m was found when the sensor is exposed to Ar ions. This value is 3–4 times larger than the compressive surface stress. We attribute this stress to a reaction between the alkyl chains in the molecules of the alkanethiol monolayer.

Surface stress relaxation in SiO2 films by plasma nitridation and nitrogen distribution in the film

We have investigated the surface stress evolution under nitridation process of silicon oxide by plasma exited nitrogen. The compressive surface stress, which had been formed by thin oxide on Si(1 0 0) were relaxed by the nitridation. During the nitridation, more than 40% of the initial compressive stress in 3 nm oxide was relaxed. We measured the stress evolution for plasma oxide films and thermal oxide films with thickness of 2‐5 nm and found the same amount of the relaxation of oxide stress. Less than 3% of nitrogen was incorporated in SiO2, and not located at the SiO2/Si interface but uniformly distributed in the film with forming a N‐Si2O bonding. # 2003 Elsevier Science B.V. All rights reserved.

Effective Young's Modulus Measurement of Thin Film Using Micromechanical Cantilever Sensors

Determination of mechanical properties of thin films, such as the Youngs modulus, is of fundamental importance when the films are used for coating or for materials of microelectromechanical systems (MEMS). We show a simple method to calculate the effective Youngs modulus of thin films by comparing lateral and vertical expansions. The stress of the film due to expansion in the lateral direction was measured using micromechanical cantilever sensor (MCS) techniques which allow for a calculation of the lateral expansion ratio of the film. The vertical expansion was measured using ellipsometry, surface plasmon resonance (SPR) and other film thickness meters. There is no limitation by the method for a measurement of Youngs modulus, even if soft and thin polymer film. We detected the influence of humidity on effective Youngs modulus of a polymer material by the method, as an example.

Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to -1.9 × 10(8) N m(-2) with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 10(7) N m(-2) for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.