Koreo Kinosita

Recent developments in the study of mechanical properties of thin films

Abstract Recent activities in the study of film stress are described. After a brief survey of the stress versus film thickness relations, an analysis of the thermal stress is given, and the diffuculties in thermal stress corrections are pointed out. The models for the origin of the intrinsic stress that have been proposed recently are critically reviewed, and future steps to construct better models are discussed. The complicated stress behaviours observed with Ag films that have been deposited on baked or unbaked mica substrates and kept in a vacuum of 10 −8 torr or 10 −5 torr are described; they present novel problems to the existing models. A Vickers type ultra-microhardness tester for thin films is described. With this instrument, any load between 1 and 100 mg-wt can be applied to a diamond triangular pyramid indenter and its penetration depth determined to ±100 A. The results with LiF, MgF 2 , Cr and Ag films are briefly discussed.

Porosity of MgF2 Films—Evaluation Based on Changes in Refractive Index Due to Adsorption of Vapors

The refractive index nf of evaporated MgF2 films changes almost reversibly with the relative vapor pressure p/po of water in the ambient atmosphere. If q is the porosity, or the fractional volume of the pores in the film, and ns and np are the mean indices of the skeleton of the film and the pore space respectively, it could be assumed that nf = ns(1 — q) + npq. If the further assumptions are admitted that (1) all the pores are accessible to vapor molecules, and (2) np = 1.000, if p = 0, and np = 1.333 (the pore space is filled with water) if p = po, then ns and q can be evaluated from measurements of nf at  p = 0 and p = po. The values of q obtained in this way for a number of MgF2 films range from 0.02 to 0.06. Evaluations of q on the same principle have also been made from measurements of nf in a vacuum and in saturated vapor of ethyl alcohol.

Ultra-microhardness of vacuum-deposited films I: Ultra-microhardness tester

Abstract A Vickers type ultra-microhardness tester which is useful for the study of surface films thicker than a few tens of nanometres is described. A diamond triangular cone indenter with a tip radius of 120 nm is employed, and its penetration depth under a given load is observed. The penetration depth can be determined to ±10 nm for any load between 1 and 100 mgf.

Ellipsometric study of polished glass surfaces

Abstract The refractive index n f and thickness d f of the “polish layer” on the surface of various kinds of glass polished under standard conditions were determined by ellipsometry. The method of ellipsometry employed was based on the measurement of the principal angle of incidence and the ellipticity of the light reflected at this angle. It was found that n f n b ( n b is the bulk index of glass) with glasses which are susceptible to weathering, and n f > n b with chemically durable glasses such as silica glass, Vycor and Pyrex. It is presumed that there are two leading factors which affect the value of n f : One is the very high local pressures exerted on the glass surface by particles of the polishing agent, which may increase n f through densification of glass, and the other is the leaching action of the water-based slurry, which will decrease n f . The value of n f relative to n b will be determined by the balance between these two factors. These hypotheses have been qualitatively verified by annealing the polish layer, which, if the increase in n f be due to densification, ought to decrease n f , and by leaching the polished surface in the slurry for some period.

Stress in Vacuum Deposited Films of Silver

Measurements have been made of the tensile stress in silver films evaporated in a vacuum of 1–2×10-5 torr on to mica strips, electron microscopic observations of the film structure being carried out on the other hand. Films having various thicknesses ranging from about 30 A to 1000 A were under investigation. The tension per unit width of the film, S, increases sigmoidally with the film thickness d: In the initial stages of film growth in which the film is composed of isolated islands, S is very small; S increases rapidly during the late coalescence stage and the channel stage; after the completion of a continuous, hole-free film, S does increase with d, but at a much slower rate. The mean stress per unit cross section of the film, σ (=S/d), increases sharply with d to assume a maximum at a thickness around 200 A where the shoulder of the S-d curve is located, and decreases gradually as d increases further.

Mobility of small clusters on the substrate surface

Abstract Experimental studies of cluster mobility are historically and critically reviewed. Data obtained recently in our laboratory are then given. These include the size distribution curves for gold clusters on amorphous carbon substrates and the surface diffusion coefficients of clusters compatible with these size distributions.

Surface Rumpling of MgO(001) Deduced from Changes in RHEED Kikuchi Pattern. I. Experimental

Vacuum-cleaved MgO(001) surface has been studied by reflection high energy electron diffraction (RHEED). With specimens heated to 300°C and gradually cooled anomalous enhancement of Kikuchi pattern is observed in the [100] azimuth at glancing angle 3.5°, which is unobservable with as-cleaved specimens. The appearance of such pattern is attributed to differential relaxation of surface anions and cations, or surface rumpling, that has occurred during the annealing. From the values of glancing angle given above, the magnitude of rumpling is estimated at 6% of the normal atomic spacing.

Ultra-microhardness of vacuum-deposited films II: Results for silver, gold,copper, MgF2, LiF and ZnS

Abstract The penetration depths e of a diamond triangular cone indenter under loads W of 1–100 mgf have been studied for evaporated films and a bulk specimen of each material, making use of the ultra-microhardness tester described in Part I (Nishibori and Kinosita, Thin Solid Films, 48 (1978) 325). The e versus W curves are featured by the presence of a critical load Wc below which e is unobservable. Wc varies with the material and specimen preparation: it is generally of the order of 10 mgf but is smaller by at least one order for the metal films. When W exceeds Wc, e increases as k1-1(W - Wc) 1 2 for film and bulk specimens of the metals and as k2-1(W - Wc) for those of the dielectrics. The constants k1 and k2, which can be regarded as measures of hardness, vary with the material and the specimen preparation.

Stress in Vacuum-Deposited Films of Ag, Au, and Cu

The tension per unit width of the film S has been determined for Ag, Au, and Cu films deposited at 10−5 Torr from a few tens of angstroms up to 2000, 1500, and 1000 A in thickness, respectively. S increases rapidly with the film thickness d until the film attains a thickness dI( = 220/320/160 A for Ag/Au/Cu films deposited at 2 A/sec). At dI the mean stress within the film σ ( = S/d) assumes a maximum. With thicker Ag and Cu films S remains nearly constant until at a thickness dII it starts to increase again. With Au films the rapid increase in S below dI is immediately followed by a slower increase with d. The first increase in S below dI is associated with the coalescence of islands. The second increase in S observed at larger thicknesses (d > dII for Ag and Cu, d > dI for Au) is probably due to residual gases. The last view is supported by experiments with Ag films deposited at 10−8 Torr.

Principal angle-of-incidence ellipsometry

Abstract In principal angle-of-incidence ellipsometry, or ϕP - K ellipsometry, one makes measurements of the principal angle of incidence, ϕP, and the imaginary part of the ellipsometric function ϱ for this angle of incidence, K, in place of Δ and tan ψ. The parameter K has often been referred to as the ellipticity at principal angle incidence. Following a discussion of the sensitivity of ϕP-K ellipsometry, the results obtained in recent years by ϕP-K ellipsometry are reviewed. A return-path type ellipsometer for use in ϕP-K ellipsometry is described, which requires no compensator and can be a powerful tool for spectroscopic ellipsometry.