In this study, medium energy ion scattering (MEIS) was used to determine the composition of the ALD Hf-aluminate films. MEIS is, in principle, very similar to the common characterization technique, Rutherford backscattering spectrometry (RBS).
MEIS [7, 8] is a powerful technique that provides structural and compositional information with a better depth resolution than that of RBS. In MEIS, a sample is bombarded with light ions (usually H+ or He+) and information is obtained by measuring the energy and direction of the backscattered ions. Compared to conventional RBS, where the incident beam usually has an amount of energy in the MeV range, MEIS uses a lower-energy beam usually in the range of 50-400 keV [7].
The interaction of a beam of incident ions with atoms in a target material can be divided into four categories, depending on the incident ion energy [9]:
• elastic atomic collisions
• inelastic atomic collisions with atomic excitation
• elastic nuclear collisions
• inelastic nuclear collisions with nuclear excitation
MEIS falls within the regime of inelastic atomic collisions and elastic nuclear collisions.
When an ion of energy, Eo, enters a sample, it continuously loses energy due to inelastic atomic collisions until it experiences a scattering event with one of the atomic nuclei (elastic nuclear collisions). If the ion energy prior to nuclear collision is E1, the energy after, E2, is given by the equation 2.2, where E3 is the energy of the ion leaving the sample surface.
In this equation, M1 and M2 are the mass of the incident ion beam and target atom, respectively. The angle, θM, is the angle through which the incident ion is scattered with respect to the incident beam angle. The ratio of incident ion energy after the elastic collision to that before the collision is called the kinematic factor K (tabulations of K exist for the H+ projectile scattered at various angles θM). Those ions not scattered elastically are not detected. Multiple elastic scattering events are too improbable to influence the results.
In MEIS, the detector is placed at a fixed angle and records the number and (2.2)
2 2
2 1
1 M 2 M
2 1 22
1 E
M M
θ cos M θ sin M K M
E =
+ +
= −
backscattered ion energy, the peak corresponding to elastic collisions from a given element of mass M2 in a thin film begins at an energy E1K and continues to a lower energy which depends on the film thickness. The positions and widths of the peaks in an MEIS spectrum thus provide information regarding the components and thickness of a film, respectively. The areas under the peaks correspond to the concentration of elements. Computer simulation program exits that use the tabulated stopping power, can be used to provide the structure and composition information. The stopping power of a material for a particular ion is defined as the energy loss per distance traveled in the material, expressed in eV/Å. The tabulated stopping value for H+ projectile energy at 100 keV in Hf target, for example, is of the order 40 eV/Å [8].
2.2.1 Channeling and Blocking
Channeling is an effect obtained by steering the incident beam into channels between planes of atoms in a substrate (i.e., aligning the beam such that it is parallel to a symmetry direction of the crystal; see Fig. 2.1) [10]. In channeling, as in non-channeling, some ions impinging on the sample scatter elastically from the surface. In channeling, however, those ions entering the sample travel far into the bulk and do not generally make it to the detector. Thus, in the case of channeling, the backscattered ion signal from the bulk is much reduced. In this work, ions were channeled into the Si substrate in order to reduce the substrate contribution to the MEIS spectra. This resulted in improved sensitivity of light elements (such as Al and O) peaks from the film, which often superimposed with the Si peak.
If a backscattering atom is located beneath the surface layer, the backscattered projectile may be blocked by another atom on its way out. This phenomena is known as blocking [10, 11] and is illustrated in Fig. 2.1. The blocking effect causes the Si peak to show a pattern of so-called blocking dips in an angular or blocking spectrum (a plot of ions yield versus a scattering angle). It is worth noting that besides channeling, blocking can also further reduce the background Si signal. In this study, blocking was used for calibration purposes to correlate the angle in the detector with the real scattering angle (since the blocking dip marks angles where backscattered ions are prevented from re- emerging into the vacuum), and to further reduce the Si background signal.
With this simple concept of channeling and blocking in mind, one can imagine that channeling (thus, a blocking dip in the angular spectrum) will certainly occur for a crystalline sample (provided there is proper alignment between the incident beam and target), whereas for an amorphous sample, the normalized backscattered yield essentially equal ~ 1 (no angular dependence). Therefore, observing the angular spectrum gives a
Ion beam (channeling)
Detector
Backscattered ion blocked by an atom
Fig. 2.1 Schematic illustration showing the channeling and blocking phenomena. Note that the incident beam is aligned with a crystal symmetry direction and the ion backscattered from one atom in the sample blocked by another atom results from the position of the detector.
direct indication of the degree of crystalline order in the film. This method is certainly not as sensitive as XRD (section 2.1), but MEIS depth profiling analysis relies on the film being amorphous or polycrystalline. It is important to note here that channeling in the film of interest must be avoided in order to obtain an accurate thickness of the film. The absence of any angular dependence in Hf for instance, verifies this assumption for depth profiling of HfO2, as will be discussed later in section 5.2.
2.2.2 Advantages and Limitations
As mentioned at the beginning of this section, RBS, which uses ion beam energies in the MeV range, is the most commonly used ion scattering technique. This technique is convenient; however, it does not provide the mass resolution, sensitivity, or depth resolution required in some cases. MEIS, with a similar operating principle, utilizes a lower-energy (keV range) ion beam. Whereas high-energy ions can penetrate the order of microns into a solid, medium energy ions are more likely to scatter from the surface and therefore are of considerable use in surface analysis. Also, higher stopping power results from the lower ion energy, and this translates into improved depth resolution. The lower-energy ion beam used for MEIS results in a lower level of target heating as well as a higher scattering cross section, thus permitting use of a higher beam current and lower total analysis doses. However, if the ion energy is too low, then multiple scattering and resonant neutralization effects make quantification difficult.
As with many techniques, the sensitivity of MEIS is limited by background levels.
Thus, MEIS is relatively insensitive to light target constituents such as oxygen in the
detector is 22º. For a ~ 100 keV beam, the energy window is about 2 keV. The total system angular resolution is given as ~ 0.1º, while the total system instrumental energy resolution is approximately 110 eV (estimated from the full width at half maximum of a signal of an element with submonolayer coverage on a flat surface). With this energy resolution, depth resolution for an amorphous thin film is ~ 4 Å.