Initial Stages of Oxide Growth and Pore Initiation in the Porous Anodization of Aluminum

Abstract

Ellipsometry studies of evaporated aluminum films anodized in 0.6M H~PO4 at constant current indicate that the anodic oxide produced is at first homogeneous with a refractive index of 1.62, as found for barrier anodic oxide of thin film and bulk A1. As the thickness increases, the optical properties change signifying the onset of pore formation. This occurs at increasing thicknesses with increasing current density, however, pore formation always occurs well before the peak in voltage is reached at constant current. Ellipsometry indicates that the mode of development of the oxide prior to pore initiation is similar to that of barrier anodic oxides. However, the efficiency of oxide growth and the electric field required to pass a given ionic current, J, are generally lower and approach values for barrier oxides (of thin film and bulk A1) only at high current densities. The curvature of the log J vs. E curve is in the opposite sense to that normally observed for anodic oxides and it is interpreted in terms of cation and anion motion. After pore formation the efficiency of oxide growth increases and becomes near unity when steady-state formation conditions are reached. These results appear consistent with the model that dissolution at the pore bases is an electric field-aided process.Anodization of evaporated aluminum films is becoming increasingly important in the fabrication of monolithic, large-scale integrated circuits utilizing aluminum metal electrical interconnections where anodization provides both improved processing and reliability (1). In order to arrive at optimum processing, it is necessary to understand the anodization properties of aluminum films. In a previous paper, the barrier anodization properties were studied and found to be essentially the same as those of bulk aluminum (2). The present study is concerned with porous anodization. In particular, the initial stages of oxide growth and the onset of pore formation are quantitatively characterized using ellipsometry. This has not been done previously for porous anodization of aluminum and the results should be valuable in arriving at the mechanism of pore formation.Of the vast literature on porous anodization of aluminum, very little deals with initial stages of film formation. That which does is mostly qualitative because of the difficulties involved with measuring very thin films. The evidence that is presented indicates that a barrierlike film initially forms, followed by pore initiation and porous oxide growth. For example, the anodization characteristics, such as the time development of current at constant voltage (3) or of voltage at constant current, are at first similar to those observed in barrier anodizations. Also, direct observations of the surface with electron microscopy indicate that surface roughness occurs after some growth has taken place (3, 4). Thus, the oxide formed in the initial stages of porous oxide growth and that which remains interposed between the pore-containing layer and the metal is often referred to as a barrier layer...

Measurement of Stress Changes during Growth and Dissolution of Anodic Oxide Films on Aluminum

“…Assuming an oxide density of 3.1 g/cm 3 and an electric field of 0.91 V/nm in the barrier film, the film growth rate and oxide formation efficiency are estimated to be 0.90 nm/s and 0.35, respectively. 18,32 The latter value compares favorably with a directly measured efficiency of 0.33 during anodizing at the same conditions. 33 Based on the oxide thickness and measured force per width at 25 V, the apparent average stress in the oxide at the completion of anodizing was −58 MPa, in good agreement with prior measurements.…”

Development and application of a holistic model for the steady state growth of porous anodic alumina films

“…Hence, the number of pores, nucleated towards the end of the first transient stage 0A in Fig. 1 [13,[21][22][23], developed and ordered during the second transient stage AB in Fig. 1 [8,[24][25][26][27][28][29][30][31], rises with both t ca in these stages and the dissolving ability of electrolyte or its ability to solvate ejected Al 3+ .…”

“…1. Three successive stages are apparent: (i) The step 0A of abrupt almost linear rise of V, corresponding to the initial transient stage of flat barrier layer growth in the last range of which the porous structure is nucleated [13,[21][22][23]; it was short enough, of the order of 1 s, and thus it is not easily detected in Fig. 1, however, its detailed separate presentation is unnecessary as there is most interest for the steady state (BC).…”

The origin of nucleation and development of porous nanostructure of anodic alumina films

“…for sulphuric, phosphoric and oxalic acid electrolytes, by a transient stage, where a flat barrier layer is formed on the surface of which pores are later nucleated towards its end [9][10][11][12][13], followed by another transient stage where pores are developed and organized yielding the characteristic structure of these films [5,[11][12][13][14][15][16][17][18][19][20][21] that is finally followed by the steady state one. The steady state structure of films is characterized as a close-packed array of approximately hexagonal, columnar cells, each of which contains an elongated, roughly cylindrical, pore normal to the surface extending between the film's external surface and the Al 2 O 3 |Al interface, where it is sealed by a thin, compact, hemispherical shell shaped barrier type oxide layer with thickness roughly around 1 nm per V of applied voltage [22][23][24].…”