Rutherford Backscattering Spectroscopy

Type of physical science: Condensed matter physics

Field of study: Surfaces

Rutherford backscattering spectroscopy is a method of determining the concentrations of various elements as a function of depth beneath the surface of a sample by measuring the energy distribution of ions reflected backward out of a beam directed at the surface.

Overview

Rutherford backscattering spectroscopy is a technique for determining the concentrations of various elements as a function of depth in a sample. The technique rests on the principle of momentum conservation in "elastic collisions," that is, collisions in which the total kinetic energy is unchanged. If an incoming particle collides elastically with a second particle that is initially at rest, the kinetic energies of both particles after the collision can be calculated using Sir Isaac Newton's laws of motion if the angle of the path of either particle after the collision is measured. For a particle elastically scattered directly backward, conservation of momentum requires that the energy of the backscattered particle is less than that of the incident particle by a factor K given by K = M - m²/M + m², which depends on the mass of the incident particle m and the mass of the target particle M. For scattering through angles other than 180 degrees, or directly backward, an angular factor is also present in the expression for the incident to scattered energy ratio. Thus, if the mass and the initial kinetic energy of the incoming particle are known, a measurement of the final kinetic energy of the particle, coupled with an observation of the angle through which it has been scattered, allows the calculation of the mass of the particle with which it collided.

In fact, Rutherford backscattering measurements determine the mass, rather than the charge, of the nuclei in the target. Thus, "isobars," atoms of different chemical elements having the same mass, such as the isotopes of germanium and selenium, which both have a mass of 76 atomic mass units, will both give rise to backscattered ions of the same energy. This does not generally prove to be a significant complication in the interpretation of Rutherford backscattering data because the most abundant isotope of each element has a unique mass, and the isobaric isotopes are rare.

The basic physical principle underlying the Rutherford backscattering method of chemical analysis is the mechanics of elastic collisions. In the case of Rutherford backscattering, however, the "collisions" do not involve actual physical contact between the two particles.

Rather, the scattering is a result of interactions between the electric charges of the two particles.

In practice, Rutherford backscattering spectroscopy is conducted with a beam of monoenergetic ions, usually protons or α particles from a particle accelerator, which is allowed to strike the sample. Some of the incident ions collide with nuclei in the sample and are scattered through various angles in accordance with the Rutherford scattering formula developed by Ernest Rutherford in 1911.

For an incoming ion beam of fixed energy and mass, such as the beam from a particle accelerator, the energy of each backscattered ion varies with the mass of the nucleus from which it was scattered. The Rutherford backscattering technique is most sensitive if the backscattered ions are detected at exactly 180 degrees, but this geometry is usually impossible, since the ion detector would block the incoming ion beam. In practice, large scattering angles are preferred, so the detector is generally located at about 170 degrees.

The energies of individual backscattered ions are easily measured with the particle detectors used in nuclear physics experiments, such as surface-barrier silicon detectors. These detectors produce a charge proportional to the energy deposited in the detector by the incoming ion. When coupled with sensitive electronic amplifiers, the apparatus will record the energy of each backscattered ion. Once the energy of each backscattered ion is measured, the mass of the nucleus from which it scattered can be determined. If the energies of many backscattered ions are measured, the relative proportions of each element in the sample can be determined.

The principal limitation of the Rutherford backscattering technique is that composition measurements can be made only to a depth equal to half the range of the incident ion beam in the sample, since the ion must travel back through the sample, scatter, and travel through the sample in the opposite direction. This range is determined by a second energy loss mechanism for ions passing through a sample. In addition to colliding with the nuclei of the target material, the incident ions can collide with the electrons in the target material. In fact, collisions with the electrons are much more common than with the nuclei; however, since an electron is one one-thousandth times lighter than a nucleon, the energy lost in a collision with an electron is much smaller than in a collision with a nucleus. Energy loss from collisions with electrons restricts analysis by Rutherford backscattering to samples of about 100 micrometers or less in thickness, making it a particularly useful technique for measurements on small samples or surface layers.

The electron energy loss mechanism also provides Rutherford backscattering spectroscopy with "depth perception," the ability to determine the elemental composition as a function of depth in the sample. This depth perception comes about since the backscattering energy of an incoming ion colliding with a target nucleus at the surface of a sample will be a certain energy, but the backscattering energy of the same incoming ion hitting the same target nucleus at some depth in the sample will be lower because of the energy loss resulting from collisions with electrons encountered on the way through the sample. Typical depth resolution is of the order of a few hundred angstroms. Thus, Rutherford backscattering spectroscopy can provide not only the elemental composition of a sample but also the depth distribution of those elements within the sample. This capability has made Rutherford backscattering spectroscopy useful in the analysis of surface contamination layers as well as interfaces between semiconductors and metal films.

Rutherford backscattering spectroscopy can also be used for isotopic measurements.

Two isotopes of the same element will have different masses because of the different number of neutrons in their respective nuclei. As a result, the backscattering energy for ions scattering off each isotope will be different. Thus, the relative proportions of the two isotopes can be determined. This technique is particularly useful for light atoms, such as carbon, where the mass difference between isotopes of the same element can be very large.

A major advantage of the Rutherford backscattering technique is that it is essentially "nondestructive"--that is, the sample is not altered or destroyed in the analysis. Traditional chemical analysis techniques generally involve dissolving the sample in reactive chemicals, while many of the physical techniques result in nuclear reactions or structural damage. While the ion beam used in Rutherford backscattering spectroscopy does cause some heating of local areas of the sample, this effect can be minimized by limiting the intensity of the ion beam and the duration of the exposure. Nondestructive chemical analysis is particularly important in research areas where the sample must be available for other measurements after its chemical composition has been determined.

Applications

By the 1940's, nuclear physicists were using Rutherford backscattering to recognize the presence of contaminants in the targets being irradiated in the ion beams of particle accelerators.

It was not until 1950, however, that the Rutherford backscattering technique was employed for chemical composition determinations. In that year, S. Rubin and V. K. Rasmussen used backscattered protons to analyze the composition of a thin layer of smog particles collected from the air.

Through the 1950's, a series of pioneering experiments established Rutherford backscattering spectroscopy as a useful tool for the analysis of small samples and surface coatings. Researchers reported on the analyses of bore surfaces of gun barrels, diffusion of gold into copper, and trace contaminants in thin and thick samples.

In 1960, S. K. Allison suggested that Rutherford backscattering spectroscopy might be used for remote analysis of surface compositions. Anthony L. Turkevich, a physicist at the University of Chicago, used this idea to develop an experimental package to analyze automatically the chemical composition of the surface of the Moon from an unmanned lunar landing spacecraft. This α-backscattering experiment used the monoenergetic α particles emitted by radioactive decay of an isotope of curium as the incident ion beam. Turkevich's device was flown on three Surveyor spacecraft, which landed on the Moon between September, 1967, and January, 1968. The apparatus provided data on the elemental abundances of carbon, oxygen, sodium, magnesium, aluminum, silicon, calcium, and iron in the lunar soil.

Measurements from Surveyor 5 and Surveyor 6 confirmed the basaltic nature of the lunar mare areas. The data from Surveyor 7, which landed in the lunar highlands on the ejecta blanket of the crater Tycho, were different from the two mare sites, suggesting a less basaltic rock composition at this site. The chemical abundance results obtained with the Rutherford backscattering spectrometers on the Surveyor spacecraft were in agreement with later laboratory analyses of lunar soils and rocks returned to Earth by the Apollo astronauts. Coming more than fifty years after Rutherford introduced the concept of ion scattering by atomic nuclei, the Surveyor experiment was the first use of Rutherford backscattering spectroscopy that attracted widespread attention.

During the 1970's and 1980's, a number of relatively low-energy particle accelerators, which had been superseded by higher-energy accelerators for nuclear physics experiments, became available for other uses. Physicists recognized that these accelerators were suitable for chemical analysis by Rutherford backscattering, and the use of this technique grew rapidly.

The ion beam used in Rutherford backscattering spectroscopy penetrates the sample only to a depth of tens of micrometers. This penetration depth cannot be increased beyond about 100 micrometers without increasing the energy of the incident ion beam to a level where nuclear reactions between the incoming ions and the lighter elements within the sample begin to occur.

Thus, Rutherford backscattering spectroscopy is more suitable to surface analysis than analysis of bulk materials.

By the 1970's, Rutherford backscattering spectroscopy had found major applications in the detection of impurities or contamination on supposedly clean surfaces. The technique has also been used to study corrosion and oxidation of metallic surfaces. In the 1980's, the technique found widespread application in the semiconductor industry. It has been used to assess the heavy element surface impurities in silicon manufactured for use in semiconductors and to investigate the contamination of silicon during polishing and cleaning operations. Rutherford backscattering spectroscopy has also been employed to quantify the amount of argon implanted into semiconductors resulting from sputter cleaning (a process of surface cleaning by bombardment with low-energy argon ions) of their surfaces.

The depth profiling capability of the Rutherford backscattering technique has also found applications in the semiconductor industry, particularly in investigating interactions at the interface between a thin metal film and the semiconductor substrate. The migration of silicon into conductive films of copper, gold, and platinum has been studied by this technique. Depth profiling has also been employed in metallurgy, where it has been established that the physical properties of certain metals are altered if hydrogen or helium is implanted into the metal matrix.

Rutherford backscattering spectroscopy is employed to determine the concentration and depth distribution of the implanted element. In this manner, the effect of various concentrations and depth distributions of the implanted elements can be correlated to the resulting strength and brittleness of the metal.

Rutherford backscattering spectroscopy is also applied to the analysis of multilayered foils. The thickness of each layer can be measured, and the diffusion of one material into another near the contact surface can be monitored with the technique. It has been used to monitor various processes important in physical chemistry, such as the diffusion of one element into another.

Studies of the rate of diffusion of tin, arsenic, and phosphorus into silicon; copper and gold into cadmium-telluride, another important semiconductor; and zinc into aluminum have been reported in the literature. The technique has also been used to monitor the thickness of thin films and foils.

For example, a chemically pure, monolayer-thick film composed of a single element would produce a backscattering spectrum with only a sharp energy spike at the appropriate backscattering energy for the element in the film. As the film is made thicker, the electron energy loss mechanism causes the peak to broaden because of the energy loss in traversing the film. The width of the backscattering energy peak is a measure of the thickness of the foil. A thickness resolution of about 500 angstroms is obtainable with Rutherford backscattering spectroscopy.

Context

New methods of chemical analysis are always important, particularly if they can perform some new function or lend themselves to a higher degree of automation than older techniques. The Rutherford backscattering spectroscopy technique provides quantitative measurements of the elemental compositions of small or thin samples with a very high sensitivity. It is particularly suitable for analyzing coatings, such as contamination, on the surfaces of samples. Its depth profiling ability has made it a tool of choice in monitoring trace element distributions in semiconductor materials. Because the Rutherford backscattering technique is essentially nondestructive, provided that the beam current is restricted to avoid substantial sample heating, it is frequently preferred over alternative methods, in which the sample is destroyed in the analysis.

The major contribution of Rutherford backscattering spectroscopy has been in the semiconductor industry. Planar technology, introduced in 1960, made possible smaller and faster semiconductor devices. Yet, this new technology was accompanied by a variety of problems in the fabrication and quality control of the thin layers of masking and electrical contact material.

Rutherford backscattering spectroscopy proved to be a useful tool in quantifying the concentrations of "dopants," the impurities that turn pure silicon or germanium into a semiconductor.

The depth profiling ability of Rutherford backscattering spectroscopy became important in the fabrication of semiconductors when ion implantation became an important semiconductor fabrication tool. Ion implantation allowed accurate control over the quantity of dopants and provided a technique to ensure a uniform surface density of dopants over the whole silicon wafer. With this new fabrication technique came the need to measure accurately both the abundance and depth distribution of the dopant in the silicon wafer, a task for which Rutherford backscattering spectroscopy was ideally suited. Alternative depth profiling techniques, such as Auger spectroscopy and secondary ion mass spectroscopy, analyze the elemental composition of a thin surface layer. This surface layer is then removed from the sample, typically by ion thinning, and the analysis is repeated until the depth profile is constructed. Thus, the surface of the sample is removed to determine a depth profile using either Auger spectroscopy or secondary ion mass spectroscopy, while the Rutherford backscattering analysis leaves an essentially unaltered sample available for subsequent research.

Rutherford backscattering spectroscopy is a particularly valuable tool for chemical analysis and depth profiling because it is quantitative and rapid, making it suitable for large surveys or automated monitoring. The development of automatic multiple-sample holders will allow the technique to be integrated into quality control operations in manufacturing as well as expanding its role in research and development.

Principal terms

ALPHA PARTICLE: the nucleus of a helium atom, consisting of two neutrons and two protons

BACKSCATTERING: the scattering of a particle through an angle of 180 degrees

ELASTIC COLLISION: a collision in which the total kinetic energy is the same before and after the collision

ISOBARS: two atoms of different elements having the same atomic mass

ISOTOPES: two atoms of the same element that have different numbers of neutrons

Bibliography

Chu, Wei-Kan, J. W. Mayer, and M. A. Nicolet. BACKSCATTERING SPECTROMETRY. New York: Academic Press, 1978. Although intended for technical audiences, the first chapter provides a historical account of Rutherford backscattering spectrometry; the second chapter explains the Newtonian mechanics of the backscattering interaction at a level appropriate for general audiences. Well illustrated; includes an extensive bibliography referencing original papers on the various applications of the technique.

French, Bevan M. THE MOON BOOK. New York: Penguin Books, 1977. Describes the contributions of the lunar exploration program to the understanding of the origin and evolution of the Moon. Includes a thorough description and illustration of the α-backscattering soil composition experiment on the Surveyor spacecraft.

Mackintosh, W. D. "Rutherford Scattering." In CHARACTERIZATION OF SOLID SURFACES, edited by P. F. Kane and G. B. Larrabee. New York: Plenum Press, 1974. Describes the technique, apparatus, and samples required for Rutherford backscattering spectroscopy detection of minor elements in or on the surface of solid samples.

Nicolet, M. A., J. W. Mayer, and I. V. Mitchell. "Microanalysis of Materials by Back-scattering Spectrometry." SCIENCE 177 (September 8, 1972): 841-849. An extensive, well-illustrated review of the applications and limitations of the Rutherford backscattering technique. Applications to depth microscopy, crystal structure, and metal corrosion are described in terms suitable for nonspecialists.

Patterson, J. H., A. L. Turkevich, and E. Franzgrote. "Chemical Analysis of Surfaces Using Alpha Particles." JOURNAL OF GEOPHYSICAL RESEARCH 70 (1965): 1311-1327. A detailed introduction to the Rutherford backscattering technique, with an emphasis on its applications for the remote analysis of geological samples by spacecraft instruments.

United States Surveyor Program Office. SURVEYOR PROGRAM RESULTS. NASA SP-184. Washington, D.C.: Government Printing Office, 1969. This document includes a well-illustrated chapter, written by the principal investigator, describing the design, operation, and results from the α-backscattering detectors flown to the Moon on the Surveyor spacecraft. Suitable for nonspecialists.

Ziegler, J. F., J. W. Mayer, B. M. Ullrich, and W. K. Chu. "Material Analysis by Nuclear Backscattering." In NEW USES OF ION ACCELERATORS, edited by J.

F. Ziegler. New York: Plenum Press, 1975. Describes the applications, limitations, and sensitivity of Rutherford backscattering spectroscopy. Intended to explain the technique to a general scientific audience.

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