Cracking and Delamination of Heteroepitaxial Barium Hexaferrite Films
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Mat. Res. Soc. Symp. Proc. Vol. 619 ©2000 Materials Research Society
having thicknesses of from 0.35 - 0.46 mm at a fixed oxygen pressure of 20 mTorr in order to measure the effects of film thickness on the film properties. Here, the substrate curvature was measured by Dektak profilometer traces both before and after the film growth, such that a value for the radius of substrate curvature provided by the film deposition could be evaluated. A second set of films was grown similarly to set 1, except to thicknesses of from 0.8 jm to 3.9 jim at a fixed oxygen pressure of 300 mTorr. Finally, this work also uses results found for films grown to thicknesses of 18 jm at 20 mTorr, which constitute the thickest BaFe, 2019 films grown onto c-plane sapphire. Each of these films was deposited from a pressed and sintered target of BaFe1 2019 at a temperature of either 9001C or 9250 C. All of the films measured by substrate curvature measurements were characterized by x-ray diffraction measurements using a powder diffractometer system, while all films were characterized by magnetometry measurements. RESULTS The effects of the large stresses that are present in barium hexaferrite films on c-plane sapphire are most evident in thick films grown at an oxygen pressure of 20 mTorr. Figure 1 presents a plan-view (left) and cross-sectional micrograph (right) of an 18 jim BaFe, 20 19 film deposited at that pressure. Here, the left figure shows the mosaic cracking pattern of the film
Figure 1. Plan-view (left) and cross-section (right) micrographs are shown for an 18 jim BaFe12O19 film deposited at a pressure of 20 mTorr. Bar lengths: 100 Jim (left), 10 Jim (right). surface, while the cross-section shows that the barium hexaferrite film is dense and has a uniform texture. The surface of this film is relatively smooth, with scattered outgrowths having heights of a few microns extending from the film surface. The deadherence of the film from the substrate does not arise from a simple loss of adhesion at the interface for all portions of the film, but instead spalling within the substrate is the most commonly observed failure mechanism [5]. This can be seen in the plan-view micrograph of Figure 2, which shows a portion of this same film where the barium hexaferrite film has partially delaminated. An examination of Figure 2 will reveal lighter shaded spiral regions where the sapphire substrate has spalled with the overlying film, as well as islands of-- I jm thick barium hexaferrite that have remained adhered to the substrate upon delamination of the body of the film. Thus, these micrographs indicate that there are very large stresses present in films of this thickness, and also that a strong interfacial bonding exists between these materials.
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Figure 2. A plan-view micrograph of a partially delaminated film showing areas of spalling within both the substrate and the film. P = 20 mTorr. Bar length: 10 jm. The substantial stress indicated for thick films in the BaFe1 2019 / A120 3 system is presumed to arise because of the mismatch in
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