Growth and Structure of Thin Films by High-Intensity Pulsed Ion-Beam Deposition
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h i Fig. 1. Electron micrograph of the films structure: (a) Nb; NP = 1; (b) Cu; Np = 2; (c) Nb; Np = 1; (d) Nb; No = 2; (e) Nb; Np = 25; (f) Mo; Np = 15; (g) Ta; Np = I1;(h) Ta; Np = 1; (i) Ta; Np = 3. 594
pulses (Np) and on the sort of sputtered material. The greater No and the lower the melting point (Tmeit) of the sputtered metal, the larger the average size of the crystalline grains in the film (Fig. 2a and b, respectively). It should be noted that a similar dependence has been found to exist between Tmelt of the sputtered metal and the thickness of the film deposited per pulse (see Fig. 3). The fact that the size of the crystalline grains in a film increases with NP suggests that in I 40 35 Cu vNb the course of deposition, the film 30 is heated up, i.e. the materials undergo study .under E 25 20 -,00: (a) crystallization, with the crystalline S1 5 -1v Mo grains in the upper surface layer 10,a growing at a significantly lower 0 VOL. 11 i I I rate (VC,.g) relative to the film T5
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(Fig. 3). Analysis of the TEM/TED images obtained for the films suggests a mechanism by which growth is likely to occur. From the electron micrographs presented in Fig. 1 c - e it follows that the first pulse results in the formation of a film whose diffraction pattern is a set of Debye rings. The number of discernible rings varies in the from 3 to 8 and depends inversely on Tmeit of the metal. An in N is accompanied by a
regular transformation of the TED images, viz., on the unbroken Debye rings characteristic of the Fig. 2. (a) Average size of the crystalline grains in the film vs one-shot films there appear point reflexes the number of which Np, (b) Tmelt and (c) substrate material. diminishes with increasing Np. It is significant that in the electron micrographs in question, point reflexes are in evidence alongside unbroken rings, which suggests a multilayer film structure. The film layers closest to the substrate preserve a quasi(nano)-crystalline structure, while the next layers added in the course of film growth make up a formation with crystalline grains of relatively larger average size. An important point is that the radial thickness of the Debye rings is considerably greater than the thickness of the point reflexes, the former increasing in some instances with Np (Fig. 1 c, d). This is an evidence for (i) the existence of elastic stresses in the first layer nearest to the substrate; (ii) the relaxation of elastic stresses with growing film thickness in the layers farthest removed from the substrate and (iii) the probable increase of the elastic stresses in the first layer with growing film thickness. A regular dependence has been found to exist between the size of the crystalline grains in the film and the state of the crystal lattice of the substrate. Thus, as is seen from Fig. 2c, with
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