S. Nakahara

Room temperature interdiffusion studies of Au/Sn thin film couples

Abstract Room temperature interdiffusion studies of Au/Sn thin film couples were made using transmission electron microscopy, X-ray diffraction and Rutherford backscattering techniques. Au/Sn thin film couples were found to interdiffuse at room temperature via both bulk and grain boundary diffusion mechanisms. In the bulk diffusion, gold diffuses interstitially into tin and initially forms AuSn 4 and AuSn intermetallic compounds. Grain boundary diffusion of tin into gold is also observed. The use of various thickness combinations in the thin film couples permitted detection of four intermetallics (ζ phase, AuSn, AuSn 2 and AuSn 4 ) which can form as a result of the interdiffusion. In addition, it was found that interdiffusion accompanied by the formation of hillocks and Kirkendall voids.

Microporosity in thin films

Abstract Because a large number of vacancies and voids can be incorporated into thin films during the deposition processes the films are generally porous. The presence of excess vacancies and voids greatly influences the physical properties. In particular freshly deposited films contain a large number (up to 1%) of excess vacancies which are highly mobile at ambient temperature. These excess vacancies are readily annihilated by diffusing to various sinks available in the thin films. During the annihilation process some film properties, which are sensitive to variations in the vacancy concentration, change continuously with time until all the excess vacancies are eliminated, leading to an aging phenomenon. In this report thin film microporosity is described in terms of the vacancies and voids incorporated during the deposition processes. The origins of these vacancies and voids and their effects on the physical properties of thin films are also discussed.

Growth of rare‐earth single crystals by molecular beam epitaxy: The epitaxial relationship between hcp rare earth and bcc niobium

High‐quality rare‐earth (RE) single‐crystal films of yttrium (Y) and gadolinium (Gd) were successfully grown with the metal molecular beam epitaxy technique on a bcc Nb single‐crystal film which serves as a buffer layer to the sapphire substrates. With reflection high‐energy electron diffraction, the hcp RE (0001) was found to grow epitaxially on the (110) Nb in the Nishiyama–Wasserman orientation. The regrowth of Nb on this RE (0001) surface yielded the (110) orientation with 120° in‐plane domains. These epitaxial relationships suggest the possibility of fabricating an ultrathin, coherent crystalline superlattice in the Nb (110)/ RE (0001) system.

Microporosity induced by nucleation and growth processes in crystalline and non-crystalline films☆

Abstract An electron microscope was used to study the origin of microscopic voids (pores) in crystalline and non-crystalline films prepared by evaporation, sputtering and electrodeposition. Both crystalline and non-crystalline films prepared in this manner were found to contain a large number of small voids. A study on the early stages of nucleation and growth showed that the observed voids are generated at the boundaries between three-dimensional faceted crystallites during their coalescence. The spatial distribution of these voids was found primarily to depend upon the film structure which, in turn, depends upon the nucleation and growth processes. For example, a high density of voids was observed in hard gold electrodeposited films in which continuous nucleation and growth processes take place throughout the thickness. Furthermore, the intrinsically high porosity known in these hard gold films was shown to originate from the observed voids. Several examples of void observations in crystalline and non-crystalline films are described.

Role of Ostwald ripening in islanding processes

We report measurements of the rate of growth of Ga and Sn clusters on clean Si surfaces. The volume rate of growth is linearly dependent on time, consistent with an Ostwald ripening mechanism for island growth. Activation energies for clustering, extracted from the temperature dependence, are shown to be sensitive to the underlying surface structure. This thermodynamical approach to the description of cluster formation is generally useful in determining the operational parameters and ultimate limits to heterostructure growth.

Epitaxial yttrium silicide on (111) silicon by vacuum annealing

Epitaxial YSi2−x films have been fabricated. The smooth 430‐A‐thick silicide films on Si (111) substrates were characterized by a Rutherford backscattering minimum channeling yield χmin =8%, establishing YSi2−x as one of the best known epitaxial silicides. Results of electrical measurements are also presented.

dc magnetron- and diode-sputtered polycrystalline Fe and amorphous Tb(FeCo) films: Morphology and magnetic properties

We have prepared polycrystalline Fe and amorphous Tb(FeCo) films using both dc diode‐ and magnetron‐sputtering techniques. Magnetic properties and aging characteristics of these films were measured by a vibrating sample magnetometer and an automatic torque magnetometer. Film morphologies were studied by transmission electron microscopy. The magnetic and aging characteristics are closely correlated to the film morphology. It was also found that different sputtering methods can be tailored to produce a similar film morphology. Among the sputtering parameters, the Ar pressure during deposition at room or lower temperature strongly influences the film morphology.

Kirkendall void formation in thin‐film diffusion couples

Transmission electron microscope studies of Sn/Au thin‐film couples reveal complexities associated with the Kirkendall effect in thin films. The use of an offset film fabrication technique permitted the identification of two room‐temperature interdiffusion mechanisms in Sn/Au: grain‐boundary diffusion of Sn into Au, which causes film thinning, and bulk diffusion of Au into Sn, which results in the formation of microscopic Kirkendall voids. The ranges of importance of the two mechanisms were explored by varying the Sn and Au film thicknesses and the grain size. It is pointed out that interdiffusion studies in thin films required careful attention to relative film thicknesses, film geometry, and grain sizes in order to determine the fast‐diffusing species and the operative diffusion mechanisms.

Origin of coercivity in a Cr‐Co‐Fe alloy (chromindur)

Lorentz microscopy and magnetization and Mossbauer effect measurements have been performed on a Cr‐Co‐Fe alloy having a two‐phase microstructure. Domains spanning many particles have been seen in samples whose Hc’s vary between 200 and 420 Oe. Walls appear to be ragged and are held back by particles. Magnetization and Mossbauer effect measurements indicate that the two phases constituting the microstructure in each of the three situations examined are ferromagnetic at ambient temperature. Based on these results it is inferred that domain‐wall–particle interaction is responsible for Hc.

The structure of the crack network in amorphous films

Abstract Amorphous carbon and germanium films prepared by evaporation and sputtering are known to contain a network of voids, which are sometimes called a “crack network”. We extended a transmission electron microscope study on the possible existence of a crack network to other amorphous films such as chromate and nickel-phosphorus films obtained by electrodeposition. It was found that both chromate and nickel-phosphorus films also contain a high density of voids, the structure of which is similar to that of a crack network. The presence of these voids was found to contribute significantly to the intrinsic brittleness in these films. A possible mechanism by which voids are generated during film formation is discussed.