Time-of-Flight, Ion-Beam Surface Analysis for In Situ Characterization of Thin-Film Growth Processes
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scattered primary beam and transmits only a narrow portion of the energy distribution of these ions, whereas the TOF detection scheme simultaneously detects the full energy range of both ions and neutrals. Consequently, the required beam dose for TOF detection is 3-4 orders of magnitude smaller than that required by the ESA. A typical analysis beam dose for TOF detection is ~ 1 0 n 1012 ions/cm2, removing or displacing one part in 103-104 of the near-surface atoms and making the TOF scheme essentially nondestructive. However, commercial ISS instruments use electrostatic
£,/£„ = (1 + ay2 X [cos 0, ± (*2 - sin2 6,)1/2 f (1) where Eu is the primary ion kinetic energy and a = M2/M^. Note that Equation 1 has real values only if M2 > Mi, and ISS is not able to detect surface atoms which are lighter than the primary ion beam. The kinetic energy of the backseat tered primary particle may be measured either by using an electrostatic energy analyzer (ESA) or by pulsing the beam and using a time-of-flight (TOF) detection scheme. The ESA only detects the ion fraction (typically 10 2-10 4) of the
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energy analyzers because they represent a relatively inexpensive modification to existing Auger and photoelectron analyzers. One of the unique features of ISS is the ability to determine local surface structure by varying the angle of ion-beam incidence on the sample. A plot of the locus of trajectories followed by each primary ion as a function of impact parameter (Figure la) shows a region behind each target atom (shadow cone) from which the primary beam is excluded. Target atoms lying in this shadow cone are not seen by the primary ions and consequently do not contribute to the ISS signal. As the angle of incidence is increased toward the surface normal, as shown in Figure lb, there is a critical angle i/fc such that the shadowed "B" atom begins to be visible behind the "A" atom which is closer to the surface along the direction of the incident beam. The ISS signal then increases abruptly. The sharpness of the onset of this signal represents a measure of the degree of surface disorder, which is in part due to the presence of point defects, and the temperature dependence of this signal is a consequence of the phonon characteristics of the surface atoms. The value of the critical angle varies from material to material and from one crystal face and beam orientation to another of a given material, and can be quantitatively related to the bond distance and bond angle for specific atomic species within the first few atomic layers as indicated in Figure 1. Equation 1 indicates that a primary ion scattered by a single collision with a surface atom into a specific angle 9j will have a well-defined kinetic energy Eu corresponding to a sharp, symmetric ISS peak. However, if the primary ion suffers a number of large impact parameter collisions, as when interacting with thick layers, the ISS peaks are broad and asymmetric with a long, low-energy tail consisting primarily of neutralized primary ions. Therefore differen
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