Medium-Energy Ion Scattering

Medium-energy ion scattering (MEIS) is a powerful technique in surface science for the determination of structural and compositional properties of surfaces and thin films. Basically, light ions (usually p+ or He+) with an energy of 40-400 keV are incident along a major crystallographic direction in the solid (channeling). Energy and angle resolved detection of backscattered ions provides surface structural and compositional information. The ions are created and accelerated in a 400 keV ion implanter produced by High Voltage Engineering (Amersfoort, The Netherlands).
The layout of the accelerator, beam line, and ultra-high vacuum equipment is shown below. Ions of virtually any type are created at a source and initially accelerated to 10, 20, or 30 keV. After passing a mass selecting magnet, the ions are accelerated to their final energy in the acceleration tube. Another selecting magnet steers the ion beam into the beam line. Several focussing elements lie along the path of the beam, including two quadrupole lenses. Electrostatic steering elements align the beam along the axis of two beam collimating slits. These slits determine the angular divergence and spot size of the beam on the sample. The beam spot size is 1 mm wide and 0.1 mm high.

    On the right you can see a window from the data acquisition program containing raw scattering data for a 98 keV proton beam normally incident on the surface of Fe-9%W(100). The scale perpendicular to the page represents the detected yield of scattered ions. This is shown covering an energy window from 89.5-94.5 keV (vertical direction) and scattering angles from 113-135 degrees (horizontal direction). The regions of high counts indicate energies and angles where ions have scattered off of different elements at the surface. From basic kinematics, ions that scatter from larger masses appear in the detector at higher energies. Two types of spectra are gathered from this raw data: energy spectra, and angular spectra.

    Energy spectra are formed by summing data over a narrow angular range as a function of energy. Figure a shows such a spectrum taken at the center of the scattering angle range of the raw data shown a bove. Surface scattering signals, surface peaks, are seen for the primary alloy component (Fe), and for an impurity that has segregated to the surface (As). Since the ion beam is aligned along a major crystallographic direction of the solid, atoms near the surface deflect the incident ion trajectories, resulting in the formation of a shadow cone (explained in the next paragraph). Thus, although Fe comprises the majority of the bulk of the crystal, we see only scattering of ions from near the surface.

Figure 3

    The area under the As peak directly gives the surface coverage of this element with good accuracy (~5%) and sensitivity (~1% ML or ~1013 atoms/cm2). Due to interaction with the electrons of the target material, the ions gradually lose energy as they traverse the solid. Hence, the detailed shape of these signals in the energy spectrum can be used to get depth profile information. This feature of MEIS is used extensively in the depth profiling of ultrathin SiOxNy and high-K (high dielectric constant) films on Si.

    Angular spectra are obtained by integrating the surface peak area of a given element as a function of the scattering angle. This is shown for Fe and As in figure 2b. Note the sharp minimum in the Fe angular yield, resulting from the blocking effect (figure 3). Ions exiting the crystal towards the detector may be blocked by surface atoms, leading to the non-monotonic dependence of the angular yield. Basic geometrical considerations, and comparison to ion scattering simulation, give detailed structural information from the angular spectra, such as interlayer separations and vibrational amplitudes. In some cases depth profile information may be gathered from these spectra.

Depth profiling and resolution

     Depth profile is based on the energy loss of the ions traveling through the film (stopping power is proportional to dE/dx , which proportional to L). "Near surface" depth resolution ~ 3-5 degrees and it becomes worse for deeper layers due to energy straggling ( ~ L1/2)
     Concentration profiles can be obtained from energy spectra simulations. If the areas under each peak corresponds to the concentration of the element in a 3?slab Peak shapes and positions can be calculated using energy loss, energy straggling and instrumental resolution. Resulting sum of the contributions of the different layers describes the depth profile.