OBSERVATION OF THE PROCESS OF FORMATION OF THIN METAL FILMS USING AN ELECTRON MICROSCOPE

As has been established in recent years, thin metal films deposited on a supporting surface (for example, glass) are not a continuous layer of metal, but an accumulation of particles (granules) separated from one another by spaces free of metal (see, for example,¹). The mechanism of formation of such a film in the process of metal deposition is still largely unclear, and its study is of serious interest both theoretically and practically. In the papers reviewed below¹˒², some essential data have been obtained that shed light on the nature of the course of the process of formation of thin coatings of certain metals.

The authors set themselves the task of observing changes in the structure of a film during its deposition, using an electron microscope for this purpose. To this end they equipped the electron microscope with a special device permitting thermal evaporation of metal onto a substrate located in the field of view of the microscope. Such a device, in addition to the possibility of observation during evaporation, eliminated the distorting influence of extraneous factors, such as atmospheric air, aging of the film, etc., which are unavoidable when transferring films from the evaporation apparatus to the electron microscope. The evaporated metal was placed, as usual, on a V-shaped tungsten wire heated by a current and situated somewhat to the side of the electron beam. A system of diaphragms prevented atoms of the evaporated metal from entering the microscope.

The magnetic field of the current heating the tungsten filament had a deflecting effect on the electron beam; however, it could be compensated for by a slight displacement of the electron gun. In addition, in order to obtain good microphotographs it proved expedient to interrupt the evaporation process during photography. The device made it possible to obtain series of photographs of the same area of the film at different stages of its growth.

For more precise fixation of the photographed region, in the first experiments individual crystals of magnesium oxide were placed on the substrate, and in later measurements—small balls of synthetic latex (or simply disturbances of the integrity of the substrate were produced). As the י

of the underlying surface, a formvar film about 150 Å thick was used. As was shown earlier,^4 the properties of a formvar film as a substrate are identical (for the metals in question) with the properties of glass: films in both cases prove to be the same. The influence of the electron beam on the structure of the layer could also be feared. However, the authors believe that the measures they adopted (weakening the beam intensity, etc.) made it negligibly small.

In the first paper^2 the process of growth of films of silver, gold, and tin was studied. We shall not reproduce the photographs, since in many respects they are similar to those placed in the review. In the case of silver and gold, metal particles are detected on the evaporated surface as soon as their dimensions become large enough for the resolving power of the microscope to distinguish them (at a film thickness of about 40 Å). At this stage they have an almost spherical shape and vary widely in size. As the quantity of evaporated metal increases, the dimensions of the globules grow, but their shape does not undergo substantial changes until the process begins whereby the globules coalesce into particles of irregular shape, increasingly filling the substrate surface. Gradually the gaps between the particles turn into narrow slits and finally disappear altogether—the layer becomes continuous.

Both the shape of the coalesced particles and the thickness of the layer (i.e., the amount of deposited metal) at which their coalescence into a continuous layer occurs depend on the rate at which the evaporation process is conducted. Depending on the vapor density of the metal, the coalescing particles may either have the form of flat plates or form agglomerates that are sufficiently elongated in height and irregular.

No traces of crystalline structure in the form of globules are found, although from X-ray structural and electron-diffraction analysis it is known that, at such film thicknesses, both silver and gold have a clearly pronounced crystalline structure.

An even more distinct feature appears in the case of tin, whose melting point is much lower. Here the particles have an almost regular spherical shape, retained up to very large film thicknesses, when a tendency toward the formation of facets begins to appear. At the same time, the process of growth of large particles at the expense of small ones is clearly observed: as the film thickness increases, the number of particles sharply decreases, and the growth of particles in size does not lead to an increase in the degree to which the substrate surface is covered by metal—it remains exposed to a considerable extent. At the same time, along with a small number of large granules, there is a large number of very small ones, with no particles of intermediate sizes. The last significant result is evidence that the number of particles does not increase as the film thickness increases, but either remains constant or decreases as a result of the coalescence of several particles.

The described picture permits the following conclusions to be drawn. Obviously, the most important factors determining the course of the film-formation process are the forces of surface tension and the migration of atoms. The dependence of the character of the coating on the rate of its formation indicates that the processes of its formation—namely, the action of surface forces, the decay of defects of the crystal lattice, and the assimilation of new atoms, migration, etc.—require a sufficiently long time for their development.

The photographs of cadmium and zinc films^3 (Figs. 1 and 2) have a completely different character. First of all, the particles of these metals have a distinct, regular crystalline form, retained by them

Fig. 1. Various stages of formation of a cadmium film.
Magnification ×30,000.

Fig. 2. Process of formation of a zinc film.
Magnification ×30,000.

and upon coalescence into agglomerates as the film thickness increases. However, the most remarkable thing is that these particles form suddenly, at once in relatively large sizes (up to 200 Å), when the vapor density of the metal reaches a certain critical value. Evidently, at lower vapor densities the metal atoms are reflected from the surface or are desorbed from it[^5]. The process of formation of particles here is similar to the process of crystallization of supersaturated solutions. The existence of such a critical density of metal vapors was predicted by Ya. I. Frenkel[^6].

Here, too, the increase in the thickness of the film is not connected with an increase in the number of particles, but only with the growth of their dimensions. However, if the film is

Fig. 3. Profile of zinc particles forming a film on the edge of a fracture of a Formvar substrate. Magnification ×60,000.

exposed in air, new crystallization centers arise, which, in the authors’ opinion, is connected with oxidation of the surface.

Some of the particles exhibit noticeable transparency. Shadow photographs made by the authors show that in this case the particles have the form of thin plates. In general, the shapes and sizes of the particles in the direction perpendicular to the surface are very diverse. This is illustrated, in particular, by Fig. 3, which shows the profile of particles on the edge of a fracture of a Formvar film.

T. R.

References

1. G. V. Rozenberg, UFN, 47, No. 1, 3 (1952).
2. T. A. McLanchlan, R. S. Sennett and G. D. Scott, Canad. J. Res. 28, No. 5, 370 (1952).
3. R. S. Sennett, T. A. McLanchlan and G. D. Scott, Canad. J. Phys. 30, No. 5, 370 (1952).
4. R. S. Sennett and G. D. Scott, J. Opt. Soc. Am., 40, No. 4, 203 (1950).
5. See, for example, J. Esterman, Rev. Mod. Phys., 18, 300 (1946).
6. Ya. I. Frenkel, Zeits. f. Phys., 26, 117 (1924).