150
6 Stresses of Anodic Oxide Films Grown on Metal Electrode
capacitors may be linked to the magnitude and sign (tensile or compressive) of the
film stress.
The compressive or tensile stress is generated in the anodic oxide film, depending
on the factor which is predominant for the stress generation, e.g., if the film growth
is controlled by mobile oxygen ion in the film, the compressive stress is generated
in the film side near the metal/film interface, while if the film growth is controlled
by mobile metallic cation in the film, the tensile stress is generated in the metal side
near the metal/film interface. The growth of anodic oxide films on valve metals obeys
a high field model in which the film growth is controlled by the high electric field
in the film. The high electric field induces the compressive stress in the film due
to electrostriction during film growth. Furthermore, in the case where the internal
residual stress is present in the substrate metal, the relaxation of the residual stress
resulting from the decrease in the substrate thickness due the film growth has to
be taken into consideration to evaluate the stress of the film itself. Therefore, the
analysis of the stress component for the anodic oxide film is complicated because
many factors influencing the film stress are overlapped. It is essentially necessary to
separate the main factors influencing the film stress and to analyze the contributions of
the respective factors to the film stress for understanding of the generation mechanism
of the film stress.
A cathodic polarization of the anodic oxide film induces the changes in film stress.
The compositional changes of the anodic oxide film during cathodic polarization are
closely linked to the changes in film stress. If the compressive stress in the film
exceeds a critical level of elastic deformation, the film is plastically deformed. The
plastic flow of the film may lead to the growth of a porous layer. In the present chapter,
we discuss the main factors influencing the film stress and the contributions of the
respective factors to the stresses of the anodic oxide films on Al and Ti. Moreover,
we discuss the changes in film stress due to the alteration of the anodic oxide film on
Ti during cathodic polarization and the contribution of plastic flow to the formation
of porous layer on Al.
6.2 High Field Model for Growth of Anodic Oxide Film
The stress generation and the alteration of anodic oxide film formed on metal are
directly associated with the mechanism of film growth. Here, let us explain briefly
a high field model [1, 2] for the growth of anodic oxide films on valve metals such
as Al, Ta, and Zr as fundamentals for understanding of many factors influencing the
film stress. The high field model for the oxide growth is based on the ion hopping
mechanism in which the ions placed at regular or interstitial sites in the oxide lattice
can jump to the neighboring vacancy or other interstitial sites.
Figure 6.1 shows schematically the potential energy of mobile ion versus distance
coordinate with and without an applied potential [3]. It is assumed that the ion vibrates
in simple harmonic mode with a frequency ν
s
−1
at the minimum point of potential
energy corresponding to the lattice plane of the oxide film. In the absence of an
6 Stresses of Anodic Oxide Films Grown on Metal Electrode
capacitors may be linked to the magnitude and sign (tensile or compressive) of the
film stress.
The compressive or tensile stress is generated in the anodic oxide film, depending
on the factor which is predominant for the stress generation, e.g., if the film growth
is controlled by mobile oxygen ion in the film, the compressive stress is generated
in the film side near the metal/film interface, while if the film growth is controlled
by mobile metallic cation in the film, the tensile stress is generated in the metal side
near the metal/film interface. The growth of anodic oxide films on valve metals obeys
a high field model in which the film growth is controlled by the high electric field
in the film. The high electric field induces the compressive stress in the film due
to electrostriction during film growth. Furthermore, in the case where the internal
residual stress is present in the substrate metal, the relaxation of the residual stress
resulting from the decrease in the substrate thickness due the film growth has to
be taken into consideration to evaluate the stress of the film itself. Therefore, the
analysis of the stress component for the anodic oxide film is complicated because
many factors influencing the film stress are overlapped. It is essentially necessary to
separate the main factors influencing the film stress and to analyze the contributions of
the respective factors to the film stress for understanding of the generation mechanism
of the film stress.
A cathodic polarization of the anodic oxide film induces the changes in film stress.
The compositional changes of the anodic oxide film during cathodic polarization are
closely linked to the changes in film stress. If the compressive stress in the film
exceeds a critical level of elastic deformation, the film is plastically deformed. The
plastic flow of the film may lead to the growth of a porous layer. In the present chapter,
we discuss the main factors influencing the film stress and the contributions of the
respective factors to the stresses of the anodic oxide films on Al and Ti. Moreover,
we discuss the changes in film stress due to the alteration of the anodic oxide film on
Ti during cathodic polarization and the contribution of plastic flow to the formation
of porous layer on Al.
6.2 High Field Model for Growth of Anodic Oxide Film
The stress generation and the alteration of anodic oxide film formed on metal are
directly associated with the mechanism of film growth. Here, let us explain briefly
a high field model [1, 2] for the growth of anodic oxide films on valve metals such
as Al, Ta, and Zr as fundamentals for understanding of many factors influencing the
film stress. The high field model for the oxide growth is based on the ion hopping
mechanism in which the ions placed at regular or interstitial sites in the oxide lattice
can jump to the neighboring vacancy or other interstitial sites.
Figure 6.1 shows schematically the potential energy of mobile ion versus distance
coordinate with and without an applied potential [3]. It is assumed that the ion vibrates
in simple harmonic mode with a frequency ν
s
−1
at the minimum point of potential
energy corresponding to the lattice plane of the oxide film. In the absence of an