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In this paper we review some of the ways by which important microstructural features of thin films can be controlled by processing variables. Of particular interest to us will be the control of the crystallographic texture of thin films used as magnetic longitudinal recording media. Other important microstructural features such as grain size, film morphology, compositional inhomogeneity and crystallographic defects will not be emphasized.
The process variables in the production of thin magnetic films by rf-sputtering include the temperature of the substrate, the sputtering power, the argon pressure, the amount of substrate bias as well as the possible use of underlayers, intermediate layers, interlayers and overlayers. All of these are important but can not be included in this review. In this paper we will emphasize the role that underlayers and interlayers play in the development of the crystallographic texture and grain size of thin magnetic films and only briefly discuss the role of the other process variables.
Development of Crystallographic Texture
In order to produce a thin film of a cobalt based alloy (hcp, P63/mmc) with its c axes in or near the plane of the film, an underlayer must be used (1-9). This is so because an hcp phase will usually grow with its c axis perpendicular to the film plane, unless it is forced to do otherwise during the initiation of its formation on the substrate. All crystalline materials have preferred growth directions when produced as thin films. These are invariably the directions perpendicular to the closest packed planes of the structure in question. Hence the planes parallel to the film usually will be the closest packed ones. For the hcp structure these are the basal (0001) planes. For the bcc structure (for example Cr, V) the closest packed planes are the {110}; hence the {110} planes tend to be in the plane of film with the bcc structure for growth controlled textures. Table 1 summarizes the growth textures for several common structures. It should be noted that it is the structure which determines the crystallographic growth texture, not the Bravais Lattice (10).
TABLE 1
Growth Textures of Selected Structures
|
| Structure |
Space Group |
Thin Film
Texture |
|
| bcc |
(Im m) |
{110} |
| fcc |
(Fmm) |
{111} |
| hcp |
 |
{0001} |
| B2 |
(Pmm) |
{110} |
| DO3 |
(Fmm) |
{110} |
| B1 |
(Fmm) |
{100} |
| A4 |
(Fdm) |
{110} |
| B3 |
(F 3m) |
{110} |
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