Hollis L. Caswell

Effect of Residual Gases on Superconducting Characteristics of Tin Films

A special oil‐free, ultra‐high vacuum system has been used to deposit tin films at pressures less than 10−9 mm Hg onto room temperature substrates. These films were found to possess extremely sharp and reproducible magnetic field transitions as compared to films deposited by more conventional techniques. This resulted from breakup of the penumbra of a film deposited through a mask into electrically discontinuous islands leaving a film of uniform thickness. Specific residual gases were found to decrease the surface mobility of the tin atoms which contributed to continuous film edges and higher critical fields. For example, the critical field extrapolated to 0°K increased from 370 oe for a pure film to 490, 590, and 820 oe as the ratio of oxygen molecules to tin atoms striking the substrate increased from 0 to 3, 6, and 9% respectively. For more highly doped films the bulk characteristics were also altered, indicating the presence of oxygen in the film material. Water vapor and carbon dioxide were found als...

Low‐Temperature Properties of Evaporated Lead Films

Lead films have been deposited onto room‐temperature substrates having low thermal expansion coefficients and the residual mechanical stresses present in these films have been determined as a function of film thickness and film purity. At room temperature no measurable stress (<5×107 dyn/cm2) was present in any film. At 78° and 4.2°K the stress σ was given by σ=α+β/d, where α is a constant dependent on temperature and d is the film thickness. A model involving surface pinning of dislocations predicts an equation of this form, but the experimental values for the stress were found to be appreciably larger than values calculated on the basis of bulk data. The experimental values were substantiated by superconducting critical temperature measurements on films of various thicknesses. Localized crystalline growth was observed in lead films after cooling to 4°K. The film material around these micron‐size crystals exhibiting five‐ and sixfold rotational symmetry was thinned to the point that physical holes were e...

Stress Anisotropy in Silicon Oxide Films

The residual stress in films formed by vacuum sublimation of bulk silicon monoxide has been studied as a function of the angle of incidence of the evaporant. Using a source temperature of 1350–1400°C and a system total pressure of ∼2×10−6 Torr, a pronounced dependence of stress on angle of incidence and a distinct stress anisotropy was observed. The stress in the x direction (i.e., perpendicular to the direction of the evaporant irrespective of substrate orientation) increases from 4×108 dyn/cm2 for normal incidence to 14×108 dyn/cm2 for an incident angle of ∼50° and then decreases to 4×108 dyn/cm2 for an incident angle of 80°. The stress in the y direction (i.e., perpendicular to the x direction) is peaked at ∼30° at a value of 13×108 dyn/cm2 and then decreases to ∼0.5×108 dyn/cm2 at 80°. Films formed at normal incidence are quite stable and not susceptible to the effects of water vapor. However, films formed by depositing at grazing incident angles (>30°) are very unstable and invariably buckle and rupture when exposed to atmospheres of water vapor. Substrate temperature, film thickness, and inert residual gases were found to have little effect on the stress and stability of films deposited at an angle of incidence of 45°. When a lower source temperature (<1200°C) is used and a high partial pressure (10−4 Torr) of water vapor is present during deposition, the resulting highly oxidized films show a compressive stress (∼2×108 dyn/cm2) and very little dependence on the angle of incidence. These films are stable when exposed to higher partial pressures of water vapor.

Influence of Oxygen on the Surface Mobility of Tin Atoms in Thin Films

Tin films less than 800 A in thickness deposited on 78°K substrates were found to be continuous at low temperatures but to agglomerate into discontinuous islands upon warming to room temperature. Any stage in the agglomeration process could be stabilized by admitting oxygen at a pressure of 10−4 Torr and warming the film to room temperature in the presence of oxygen. Electrical resistance data indicate the surface mobility of the tin is reduced by the formation of a surface oxide one to two monolayers in thickness and that there is no appreciable diffusion of oxygen into the bulk of the film material.

Effect of Residual Gases on the Properties of Indium Films

Indium films have been deposited at pressures less than 10−9 mm Hg and at higher pressures of specific residual gases. N2, H2, CH4, A, CO, and CO2 were found to have no measurable effect on the electrical or superconducting properties of these films when present in partial pressures of 10−5 mm Hg. For oxygen, a pronounced effect was observed when K (the ratio of oxygen molecules to indium atoms striking the substrate) exceeded 3%. The room temperature resistivity increased from 9.0 μohm‐cm to 14 μohm‐cm, the critical temperature decreased from (3.402±0.005)°K to (3.389±0.002)°K, the effective magnetic penetration depth increased from 630±100 A to over 1000 A. For 10−3<K<3×10−2, a larger apparent penetration depth was observed for films deposited through a mask due to edge effects. It was shown that indium films could be deposited at 100 A/sec on clean, baked substrates at 10−5 mm Hg with characteristics indistinguishable from those of films deposited in ultrahigh vacuum provided the partial pressures of o...

The Oregon Report Basic Technology

Computer technology will continue to advance, and the capabilities we need will exist in VLSI. But capitalizing on them will require innovative design tools and architectures.

Characteristics of the Annealing Kinetics of Tin Films Deposited at 88°K

The electrical resistance of evaporated, high purity tin films deposited at 88°K decreases rapidly in two temperature regions as determined by isothermal annealing studies. One pronounced annealing peak occurs at 110°K and has an activation energy of 0.34±0.06 ev. The second much less pronounced peak at 180°K has an activation energy of 0.74±0.10 ev.The annealing characteristics of five pure films varying in thickness from 960 A to 4100 A and a film deposited in a high partial pressure of N2 are quite reproducible. The presence of O2 during evaporation, however, drastically altered the annealing kinetics and increased the activation energy at any given temperature. The defect resistivity at the time of evaporation was the same for all five pure films and a film deposited in a high partial pressure of N2, but is about 75% higher for the film deposited in the presence of O2.

Thin-film circuit technology: Part II — Thin-film superconducting devices

Thin-film techniques have contributed significantly to an improved understanding of superconductivity. Cryogenic thin films are particularly suitable for use in switching and storage devices in computer memories

Magnetic Hysteresis in Superconducting Thin Films

Tin films were deposited from the vapor at various substrate temperatures and their superconducting properties determined as a function of film structure. Magnetic hysteresis in the superconducting state to normal state phase transition of these films increased with increasing grain size. Hysteresis could be attributed solely to supercooling only in the smaller‐grained films deposited at low substrate temperatures. For the larger‐grained films, superheating was also observed. The hysteresis observed experimentally was in agreement with the predictions of the Ginzburg‐Landau theory of superconductivity.

Liquid Helium Trap for High Pumping Speed at Low Pressures

A description is given of a liquid helium thimble trap which is situated immediately in the evaporation chamber. A pumping speed of 700 to 1000 l/sec is indicated for the trap at pressures in the 10/sup -9/ mm Hg range. (C.J.G.)