11 Depth Distribution and Migration of Low Ζ Elements in Solids Using Proton Elastic
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Scattering R O B E R T S. B L E W E R Sandia Laboratories, Albuquerque, Ν.M.87115
Rutherford ion backscattering spectrometry (RIBS) is a powerful analytical technique but is insensitive to light atoms in higher Ζ hosts. This can be overcome by using nuclear elastic scattering cross section anomalies which exist for ~ 2.5-MeV protons incident on such low Ζ elements as D, He, He, Be, C , and O. Optimum detection sensi tivity (0.5 at. % He in Cu) is achieved using foil targets mounted on an efficient transmitted-beam trap. Since cross section enhancements for D and He are more than 100 times greater than Rutherford values, thick as well as foil targets can be analyzed. The depth distribution of helium and hydrogen isotopes can be observed nondestructively within the first 10µmof a solid surface with a resolution of better than 750 Å. Tenets of the technique and results from its application to new energy research materials problems are described. 3
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O i n c e the introduction of commercially available solid state particle ^ detectors and the publication of 'Ton Implantation i n Semiconductors" ( I ) i n 1970, surface and near-surface elemental analysis b y Rutherford Ion Backscattering Spectrometry ( R I B S ) has become widely used and has joined a host of other techniques which use particles or electromagnetic radiation to probe the character of surfaces. Using incident helium ions of 2 - 3 M e V energy, several investigators (2, 3, 4) have demonstrated the fractional monolayer detection sensitivity and ^ 100-Â depth resolution that are possible when analyzing high Ζ impurities i n low Ζ lattices. F o r many applications, however, the need arises to test for the presence and to observe the depth distribution of low Ζ atoms i n higher Ζ substances, e.g., hydrogen isotopes i n metals. 262
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Depth Distribution and Migration
Though conventional helium ion backscattering has become a pow erful analytical tool i n revealing the concentration and depth profiles of impurities in the near-surface region of solids, it cannot detect helium and hydrogen isotopes i n materials for kinematic reasons. Proton backscattering could remove the kinematic limitations. However, for a given energy, the Rutherford elastic scattering cross section of elements de creases as the square of decreasing atomic number, and unfavorable cross section considerations have discouraged experimenters from working along these lines. This paper reports on investigations using a modified form of proton backscattering which have been able to detect less than 7 at. % concentrations of such low Ζ elements as D , H e , and H e with depth resolutions below 750 À. Mervine et al. (5) have observed helium in an implanted P d foil using proton backscattering. H e l i u m has also been detected i n metals by forward scattering (6) and nuclear reaction techniques (7), but the experiments reported by the author two years ago (8) are believed to be the first description i n the open literature of the use of an ion backscattering technique to determine helium depth distributions. This capability is important in unravelling the physics of the behavior of gases i n metals, an area which is becoming particularly important with respect to emerging energy technologies. In fact, largely because of these technologies, there has been a surge of interest i n the field. W i t h i n the last two years, several other ion beam techniques have been developed to detect and profile low Ζ elements in materials. Re views of nuclear reaction methods by F . L . Vook and by J . C. Overly and of elastic recoil techniques by B. Terreault appear i n this volume. Backscattering phenomena which occur at lower energies are reviewed by H . Verbeek. 3
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Helium Ion Backscattering If a monoenergetic, mass-analyzed beam of ions impinges normally on a solid surface, most particles penetrate into the surface to depths of the order 1-10/xm for incident 2.5-MeV H e ions or protons and ulti mately come to rest within the target. However, many of the incident particles elastically scatter from target atoms, and a small fraction ( ~ 10~ ) undergo large angle collisions ( > 9 0 ° ) , are scattered i n the backward direction, and escape from the target. The energy Ε of each such ion after scattering is related to its energy before scattering E by the formula: 4
4
Q
E = kE
0
(1)
where k is the kinematic recoil factor and is determined from the laws of conservation of energy and momentum. Specifically:
264
RADIATION E F F E C T S O N SOLID SURFACES
cos θ + [ ( M / m ) - sin θ] 1 + M/m
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2
2
1 / 2
1
2
(2)
where θ represents the scattering angle (laboratory system), and m and M denote the mass of the incident ion and struck target atom, respectively. k equals the fraction of the incident particle energy which is retained after backscattering from a target atom and therefore varies from 0 (for a head-on collision w i t h a target atom of equal mass) to 1.0 (for a head-on collision w i t h a target atom of infinite mass). F o r a fixed type of incident ion at a given incident energy and backscattering angle, there is a unique value of retained energy as a result of a collision between that ion and a given target atom. Thus each backscattered ion which escapes from the target carries information through its remaining energy about the mass of the target atom with which it collided. Moreover, since the probability that a certain incident ion w i l l scatter through a given angle is known (Rutherford scattering cross section), the concentration of each different species of atoms i n the surface of the target can be deduced by counting the fraction of incident ions which are backscattered at the energy which is appropriate for that element. In addition to the discrete kinematic energy loss, incident ions also lose energy continuously by electronic excitation (ionization) while trav eling through the solid to and from the point of collision. The energy loss can be characterized by the relation: (3) where Ν is the target atom density, and c is called the stopping cross section, c is a function of ion energy Ε and, in the energy range of interest in RIBS (1.0-2.5 M e V ) , decreases with increasing energy. The value of c has been measured for many materials, and formulas providing good estimates for c are available for those elements for which experimental data do not exist (9, 10, 11, 12, 13). Because of this additional "drag" energy loss, ions backscattered from target atoms which are beneath the target surface w i l l emerge from the target with a smaller fraction of their original energy than those which rebound from like atoms i n the first monolayer. This energy dispersion of backscattered ions from any given type of target atom is a distinct analysis advantage. Since e can be experimentally measured, it is possible to correlate the energy of the backscattered ion with the depth i n the target at which it was scattered through Equations 1 and 3. F o r a typical experimental run, an incident dose of ~ 10 i o n s / c m (10 monolayers) provides data with adequate statistical accuracy for composition and depth profile analysis of the constituents of the target. Thus the R I B S method is essentially non16
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Depth DistHbution and Migration
destructive. Because of the ability to "see" the depth profiles and iden tify the mass of both host and all "impurity" atoms i n the target, R I B S has been called "mass sensitive depth microscopy" (3). Several factors influence the choice of incident beam. The variation of k with target atom mass is shown i n Figure 1 for four types of inci dent ion. It is necessary to use an ion beam composed of atoms which have lower mass than the lightest target atom species one wishes to detect. It is also useful for detection and analysis purposes to select a probe beam for which the percentage change i n k value between adjacent elements is greatest among the target species of primary interest. F o r instance, protons are optimum for analyzing targets w i t h low Ζ constitu ents ( M = 2-20) while C or 0 ions would provide greater energy separation of backscattered ions for heavy elements ( M > 50). 1 2
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Variation of the kinematic recoil factor k for each of four incident ions as a function of target atom mass
Range and stopping cross section characteristics of the incident ion beam also influence the choice of analysis beam. F o r example, protons penetrate, and therefore sample, much greater depths of a given target than oxygen ions, as shown i n Figure 2 for gold, but because the stopping cross section for oxygen i n solids is much greater than that for protons, the depth resolution for a gold sample w i l l be considerably better for an incident oxygen beam than for a proton beam. The inherent energy reso lution (and thus mass and depth resolution) of the silicon surface barrier detector (energy analyzer) is also a factor, decreasing with increasing mass of the incident beam ions (14). H e ion analysis beams are most 4
+
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RADIATION E F F E C T S O N SOLID SURFACES
0
2
4
6
8
10
12
14
16
PROJECTILE ENERGY (MeV)
Figure 2. Range and stopping cross section variation as a function of incident ion energy in gold for 0 C, He, and * H (9). The vertical line at 2.5 MeV represents a commonly used ion backscattering analysis energy. 16
9
12
4
commonly used because they represent the best trade-off i n factors dis cussed above for analyzing targets containing medium Ζ materials. A schematic of the elastic scattering processes w h i c h occur i n a thin film containing a subsurface impurity layer deposited on a thick substrate is shown i n Figure 3a. Typical spectra for an incident H e ion beam are illustrated at the base of Figure 3b. Details of the accelerator layout and of electronics needed to count detector pulses, are given i n Refs. 15 and 16. F o r a backscattering event which occurs at the surface at an angle φ ( = π — θ) the energy measured b y the detector is given b y E i = fcfiimEo where the value for k m is determined b y Equation 2 or may be taken from Figure 1 for a scattering angle of 164°. Conversely, those incident ions which penetrate to the film substrate interface before scattering suffer ( i n addition to the recoil energy loss) ionization energy loss before collision, as w e l l as on retraversing the film. These ions w i l l reach the detector with energy E . T h e stopping cross section of metals for H e ions varies sufficiently slowly with energy that, when analyzing thin surface layers at energies above 1 M e V , an average value of c may be used for the inbound path and another average value for the outbound path. T h e measured energy difference A E i — E± — E is 4
+
9
m
3
f I
m
3
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Depth Distribution and Migration
thus a direct measure of the film atomic areal density (atoms/cm ) using the equations i n Figure 3. This energy difference is easily converted to film thickness using the appropriate value of the film density. Likewise the depth of the impurity layer can be deduced b y measur ing E and subtracting its value from the energy E ' of a n incident ion assumed to strike an identical impurity atom on the surface: A E — E . Ions which penetrate the thin film and are backscattered i n the substrate experience stopping (energy loss) from c b w h i c h is different from c nm i n magnitude although similar i n energy dependence and must be taken into account as indicated i n Figure 3 i n the expression for E . Strictly speaking, impurities, such as an implanted helium layer, alter the value of € , but most impurities are present at l o w concentrations and, for these cases, the effects are negligible. F o r compound or alloy 2
2
2
i m p u r
2
gU
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ION ENERGY LOSS AFTER LEAVING POINT OF COLLISION
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