Harold P. Smith

Atomically Clean Surfaces by Pulsed Laser Bombardment

It is shown that high power, Q‐spoiled lasers offer an interesting method for obtaining clean surfaces in vacuum. However, complete removal of oxygen and sulfur contamination from nickel surfaces by laser bombardment at 100 MW/cm2 produced irreparable surface damage that is attributed to filamentizing of the laser beam and/or to nonuniform surface reflectivity. On the other hand, application of the technique at 30 MW/cm2 successfully removed the (7×7) satellite structure from (111) silicon surfaces without surface damage.

Full‐range solution for the measurement of thin‐film surface densities with proton‐excited x rays

A Fredholm‐integral expression for characteristic x‐ray production by proton bombardment is developed relating multicomponent elemental thin‐film concentration profiles to proton‐stopping power, x‐ray‐production cross section, and x‐ray attenuation. The thickness of Al2O3 and Ta2O5 films on their respective metallic substrates were measured with 20‐ to 80‐keV protons with an accuracy of 5% for thick films (1200 A) and 25% for thinner films (30 A). Nonlinear regression analysis of the data indicated that all parameters of interest, including the zero‐voltage anodization potential, can be determined from the raw data. With exception to the oxygen K‐shell x‐ray attenuation coefficient, the parameters were determined with good to reasonable accuracy. A general concentration profile could not be determined by numerical methods as a result of insufficient variation in the kernel of the integral equation.

Competition between Random and Preferential Ejection in High‐Yield Mercury‐Ion Sputtering

A radioactive tracer technique was employed to determine the yield (atoms/ion) and angular distribution (atoms/ion·sr) of atoms sputtered from Cu (100) and Mo (100) surfaces by a 1 to 10 keV Hg+‐ion beam. Values of the yield for Ni (100) surfaces were also determined. Analytical models for normal and oblique ion incidence cases were fitted to the angular distribution data. The functions describing the preferential emission in the closest‐packed direction and the [100] direction were Gaussians, whereas the background distribution was represented by a cosine, which was tilted away from the surface normal for oblique incidence. Integration of the total distribution gave the relative contributions to the sputtering yield of the directed emissions and background. In all cases, the cosine contribution, which is interpreted in terms of emission from a random lattice, accounted for more than ⅔ of the atoms sputtered. However, intense Wehner spots were present even when the cosine distribution accounted for ⅘ of t...

Cesium‐Ion Bombardment of Aluminum Oxide in a Controlled Oxygen Environment

Sputtering of aluminum oxide (sapphire) by 2.5 to 10 keV cesium ions has been investigated using neutron activation analysis. The sputtering yield (aluminum atoms/ion) was measured as a function of ion energy, target temperature, and R, the ratio of background oxygen molecular flux arriving at the target surface to the sputtered aluminum and oxygen flux leaving the surface. Positive‐charge accumulation on the dielectric surface was neutralized by electron flooding of the ion‐beam area while the ion current was measured by a periodic beam deflection technique. In all measurements the ion beam was normal to the target surface and the (001) crystallographic plane. The yield increased monotonically from 0.40 (aluminum atoms/ion) at 2.5 keV to 0.52 at 10 keV and was unaffected for R 10 it approached the saturation value of monocrystalline aluminum. The yield exhibited a temperature dependence decreasing from 0.55 at 77°K to 0.48 at 475°K. The resu...

LEED Studies of Oxygen Adsorption on the (100) Face of Aluminum

Atomically clean aluminum /100/ surface obtained through prolonged heating, noting role of oxygen adsorption

Cesium Ion Sputtering of Aluminum

A neutron activation technique has been developed to investigate cesium‐aluminum sputtering in measured, rarefied atmospheres of oxygen. The sputtering yield was measured as a function of cesium ion energy and as a function of the ratio of oxygen molecular flux arriving at the target surface to sputtered aluminum flux leaving the surface. In all measurements the ion beam was normal to the monocrystalline aluminum target surface and to the (110) crystallographic plane. The yield increased monotonically from 0.74 (atoms/ion) at 1 keV to 2.4 at 10 keV and was unaffected by flux ratios less than 0.2. The yield decreased for higher values of the flux ratio and eventually reached a saturation value where it is conjectured that a surface film of Al2O3 is maintained.

OXYGEN SURFACE-DENSITY MEASUREMENTS BASED ON CHARACTERISTIC X-RAY PRODUCTION BY 100-keV PROTONS.

The oxygen‐K x‐ray yield from known thicknesses of aluminum oxide (1–70 μg O/cm2), bombarded by 100‐keV protons, indicates that oxygen surface density may be measured over the range from 50 to 0.004 μg/cm2. At the lower limit, background is 50% of the signal. In thin oxide layers where proton energy loss and x‐ray absorption are negligible, the x‐ray yield I (x rays/proton) is related to the oxygen surface density t (μg/cm2) by I=(2.9×10−6)t.

Effect of Thin Carbonaceous Films on 500‐keV Helium Ion Sputtering of Copper

A radioactive tracer technique capable of measuring less than 10−10 g of copper has been developed. A 500‐keV magnetically analyzed beam of helium ions is used to observe the sputtering yield as a function of carbonaceous film thickness over a copper target. It is shown that an amorphous film of the order of 2×1017 atoms/cm2 reduces the sputtering yield of copper by a factor of 100. A first collision model of sputtered atom penetration of the film is in good agreement with the data and yields a copper—carbon cross section of 3×10−17 cm2. A clean surface sputtering yield of 2×10−2 atoms/ion is obtained by extrapolation of the data and is consistent with the yield of 800‐keV D+ sputtering of copper reported by Kaminsky [Phys. Rev. 126, 1267 (1962)]. It is postulated that ordering of the amorphous film occurs for areal densities greater than 2×10+17 atoms/cm2 on the basis that the rate of reduction of copper yield with increasing film thickness undergoes a sharp decrease at that point.

Yield and Angular Distribution of Cesium‐Sputtered Molybdenum

A radioactive tracer technique has been adapted to the measurement of the yield (atoms/ion) and angular distribution (atoms/ion/sr) of molybdenum sputtered by a cesium ion beam normal to the (100) crystallographic face. The energy of the ion was varied from 1 to 7.5 keV, and the target temperature was varied from 77° to 473°K. The yield increased with energy and decreased with target temperature. Preferred emission in the 〈100〉 direction was measured, but emission in the 〈111〉 direction could not be resolved. The importance of ion channeling and the effects of thermal annealing on ion‐induced damage are emphasized.

Effect of Collision Focusing on Monocrystalline Sputtering Yield Theory

An improvement is offered to the theoretical model of monocyrstalline sputtering yield proposed by Southern, Willis, and Robinson. The collision focusing effect, not considered in the original model, is included by multiplication of a momentum conductance factor which removes the dependence upon first collision depth and substitutes the effect of momentum propagation along the close‐packed directions from the point of the first collision to the crystal surface. The inclusion of this factor yields better agreement with the data.