Heavy Ion Backscattering Research Articles

Comparison of various ions for use in heavy ion induced X-ray emission

The sensitivity and general usability of heavy ion induced X-ray emission for simultaneous elemental analysis with other ion beam methods has been investigated as a function of projectile atomic number. The variation of sensitivity and background radiation as a function of projectile energy was also studied. The ions selected consisted of 7 Li(8–18 MeV), 12 C(10–26 MeV), 16 O (12–32 MeV), 28 Si (16–28 MeV) and 80 Se (24 MeV). The ion energies used are also commonly employed in heavy ion backscattering spectrometry and elastic recoil detection analysis. The HIXE results of simultaneous determination of trace elements with Z = 15, 20, 22, 26 and 82 in thick samples were compared with the standard PIXE analysis by 3 MeV protons. To obtain information on any variations in the sensitivity on sample matrix, commercial standard samples of NBS River Sediment-2704, NBS Bituminous Coal-1632b, NBS Silicon-Aluminium alloy-87a and IAEA Animal Bone H-5 were used for which the elemental concentrations are known.

Evaluation of surface analysis methods for characterization of trace metal surface contaminants found in silicon integrated circuit manufacturing

We report a preliminary investigation of surface analytical methods used to evaluate trace metal contaminants on silicon wafer surfaces. The methods discussed include sputter induced postionization methods, backscattering spectrometry, total reflection x-ray fluorescence, and a sample preparation method known as vapor phase decomposition. In order to initiate the evaluation, a series of silicon wafers was dosed with transition metal contaminants Cu and Fe and then analyzed with total reflection x-ray fluorescence, sputter induced resonant ionization spectroscopy, and heavy ion backscattering spectroscopy. One wafer with low concentrations of Fe was analyzed by time of flight secondary ion mass spectrometry. As a result of this study, we are pursuing improved capability in several areas including standards. Heavy ion backscattering spectrometry is suggested as a means of verifying surface dosed silicon wafer standard.

Ion beam analysis for depth profiling

New techniques in the application of ion beam analysis to depth profiling in solids are briefly surveyed. These include (1) non-Rutherford backscattering analysis using high energy beams, (2) resonant nuclear reaction analysis, (3) time-of-flight elastic recoil detection spectrometry, and (4) heavy ion backscattering spectrometry. The last can be used for very precise depth profiling in one configuration, or as a very sensitive trace contaminant analysis in another configuration.

Screened Rutherford multiple scattering estimates for heavy ion backscattering applications

An analytical expression and an integral representation are presented for the contribution, Y ( n) = ϵ ( n) Δω n , of n-event multiple scattering chains to the observed backscattering spectrum in heavy-ion backscattering (HIBS) measurements. The approximations introduced in deriving the results are chosen such that an upper limit is placed on Y ( n) by the expressions. The Rutherford elastic scattering cross section is used to describe individual collisions between incident projectiles and target atoms. Screening of the Rutherford scattering cross section is included in an approximate fashion which maintains the upper limit estimate. Inelastic energy loss between collision events is assumed proportional to the projectile velocity. Specific application of these expressions is made to HIBS detection of trace amounts of heavy atom impurities on a Si surface by 200–400 keV C + beams. The predicted multiple scattering background for this application is compared with the predicted single scattering signal for 10 10 at./cm 2 of Fe, Cu, Zr, Sn, or Au, as a surface impurity. The comparison shows that the multiple scattering background poses no barrier to extending the sensitivity of HIBS detection of impurities in this mass range to levels as low as 10 8 at./cm 2 for the upper part of the energy range considered. Comparison of calculations with and without screening included show that the screening of the Rutherford cross section by atomic electrons is a significant factor in preventing multiple scattering effects from interfacing with HIBS spectrometry at impurity levels in the 10 10 at./cm 2 range.

RF ion linacs for applied research and industrial applications

The development of the radio-frequency quadrupole (RFQ) structure has made rf ion linacs practical for applications in research and industry requiring energetic ions. Since the RFQ can be designed to operate over a wide range of resonant frequencies, it can be used for a variety of accelerated ions, beam currents and output energies. This compact accelerator is simple to operate and can be used alone or with other structures to achieve even higher energy or lower momentum spread in the ion beam than is available from the RFQ alone. The inherent flexibility of the RFQ, together with its present commercial development worldwide, has resulted in its increased use for applications such as high energy ion implantation, isotope production, and neutron and gamma production for nondestructive examination and testing of materials. Potential applications that have been proposed for rf ion linacs include accelerator mass spectrometry, changed particle activation and heavy ion backscattering spectrometry. This paper will discuss these applications and describe several representative rf ion linac systems.

HEAVY-ION BACKSCATTERING——A NEW METHOD FOR DETERMINATION OF TRACE HEAVY ELEMENTS ON THE SURFACE AND NEAR SURFACE

We describe a new technique for MeV heavy-ion backscattering analysis of trace heavy elements on the surface and near surface. Pulse pileup problems are eliminated by choice of incidence ion mass M1, which is the same as or larger then that of bulk atom M2. Using 3 MeV Si beam, we could measure quantities of arsenic implanted in silicon down to 2×1012 atoms/cm2. Experimental results also show that sensitivity for impurity element on silicon substrate is as high as 2.2 ×109 atoms/cm2 for Au.

Heavy ion backscattering analysis with a time-of-flight detector

We describe a time-of-flight spectrometer for heavy ion RBS. Start and stop signals for the flight time measurement are obtained by detecting the secondary electrons emitted from two thin foils traversed by the ions. MicroChannel plates are used to provide very fast timing signals. The achieved energy resolution of the system is 30 keV (FWHM) at 2 MeV and 75 keV at 10 MeV for 35Cl, which permits a depth resolution of approximately 100 Å for indium profiles in GaAs (normal beam incidence). For 16O the energy resolution is 45 keV at 6 MeV and 110 keV at 12 MeV. We present RBS spectra of thin-layer multielement targets. As possible applications we show indium profiles in In x Ga 1− x As heterostructures, As and Ga profiles in a thin layer GaAs sample and a measurement of the silver isotopic ratio in a sputtering experiment.

Heavy-ion backscattering spectrometry (HIBS) for high-sensitivity surface impurity detection

We describe a medium-energy heavy-ion beam (400 keV C+) technique for backscattering analyses of heavy trace elements on the surface of light substrates. Pulse pileup problems are eliminated by placing a thin (40 μg/cm2), self-supporting C foil in front of the surface barrier detector. In the present example the thickness is selected to range out the ions scattered from a Si substrate, while passing ions scattered from heavier impurities. Using this technique, called heavy-ion backscattering spectrometry (HIBS), we show that sensitivities for impurity elements heavier than Ar on a Si substrate are as high as 1010 atoms/cm2 for Au to 5 × 1011 atoms/cm2 for Fe.

Application of heavy ions to high depth resolution RBS

A detailed study has been made of the use of MeV heavy ions (2 ≤, Z 1 ≤, 10) to improve the limits of depth resolution in surface analysis. While there have been detailed studies of the optimization of the use of He projectiles at glancing angles, heavier ions have received limited attention principally because the energy resolution of surface barrier detectors is significantly degraded for heavy ions and this is a major contributor to limiting the depth resolution of a system near the surface. In this paper the surface barrier detector will be compared with the electrostatic energy analyser (EEA) which does not suffer the same limitations in energy resolution (and hence depth resolution). This work extends a number of limited experimental studies of the application of heavy ions and electrostatic analysers to enhanced depth resolution, though these have rarely been used together. It will be shown that exceptional improvements in depth resolution can be achieved, and subject to the limitations of the theoretical models used in the computer simulation of heavy ion backscattering, it is predicted that depth resolutions of as small as 0.5 nm may be achievable. An algorithm for ascertaining the optimum depth resolution conditions for heavy projectiles is also derived which will identify the best angle of incidence, depth resolution and projectile for a given sample to an accuracy of better than 10%.

Heavy ion rutherford backscattering analysis used to study the alloyed metal/GaAs interface

Heavy ion backscattering analysis of the near surface Pd/GaAs and Au:In/Pd/GaAs chemically prepared systems have been investigated as a function of annealing temperature. We performed RBS analysis using a 7 MeV incident energy nitrogen beam connected with time-of-flight spectrometer detection. Using RBS random spectra, such a method can resolve the composition of the alloyed deposited films (less 5 nm) and can profile As and Ga over 40 nm. The two main results obtained from such analysis are: first, chemical palladium deposition is connected with a lack of arsenic which is preferentially exchanged to gallium atoms. Second, palladium diffusion is enhanced when using Au:In/Pd alloyed contacts.

Observation of Hg diffusion in CdTe by means of 40-MeV O5+ ion backscattering

The diffusion profile of Hg in CdTe crystals was observed by means of heavy ion (40 MeV O5+) backscattering. The near-surface region of CdTe immersed in Hg was investigated up to 1.4 μm from the surface. The observed Hg profile indicated that the concentration of Hg atom at the surface reached 4×1020 cm−3 and the distribution was interpreted by a simple diffusion model. Temperature dependence of the diffusion coefficient was determined to be D=5×103 exp[(−2.0±0.3) eV/kT] (cm2/s). In the case of CdTe immersed in Hg which contained a small amount of Cd, it was found that Hg diffusion did not occur. These experimental facts suggest that the rate of Hg diffusion is controlled by the diffusion of a Cd vacancy which is introduced by the outdiffusion of Cd atoms from CdTe crystals.

Time of flight spectrometry in heavy ion backscattering analysis

Time of flight spectrometry for the backscattering analysis of MeV heavy ions is proposed. The capabilities and limitations of this method are investigated. Depth and mass resolutions obtained in measurements of oxide film thickness as well as in GaAs layers analysis are presented. The importance of minimizing pile-up without significant loss of resolution by the use of an adequate absorber set just in front of the rear detector is underlined.

A high resolution spectrometer used in MeV heavy ion backscattering analysis

A new method in surface analysis by MeV heavy ion backscattering spectrometry is proposed. In order to overcome the difficulty due to limited resolution of solid state detectors we have developed a time-of-flight spectrometer with a resolution up to 150 ps. The capabilities and limitations of this method are investigated. Successful applications are demonstrated by measurements of oxide layers thickness as well as metal diffusion into amorphous material.

Applications of the scattering spectrometry with fast protons (5–8 MeV) and heavy ions (15–25 MeV) to special analysis problems

The combination of proton and heavy ions beams for light element or “deep interface” analysis problems, high mass or depth resolution questions, respectively, provides a versatile extension of the more conventional ion beam analysis methods. Similar to the 4He + -backscattering, computer analysis of heavy ion data is possible with good success using approximation polynomials for the energy loss values tabulated by Northcliffe and Schilling. Applications of the analysis with protons (5–8 MeV) to the Nb+H, Nb 3Sn+Si and Nb 3Au systems and of heavy ion (16–20 MeV) backscattering to CrCuAu-layers, NbSn diffusion couples and sputtered Nb 3Ge layers are presented.