Depth resolution incidence

Depth sensitivity is an equally important consideration in the analysis of surfaces. Techniques based on the detection of electrons or ions derive their surface sensitivity from the fact that these species cannot travel long distances in soflds without undergoing interactions which cause energy loss. If electrons are used as the basis of an analysis, the depth resolution will be relatively shallow and depend on both the energy of the incident and detected electrons and on characteristics of the material. In contrast, techniques based on high energy photons such as x-rays will sample a much greater depth due... 

The depth (thickness) of the mixing zone, which limits the depth resolution of a SIMS analysis typically to 2—30 nm, is a function of the energy, angle of incidence,... 

Determination of concentration profiles from the raw data can be more complicated when protons are used as the incident particles. The energy loss ( dE/ die) is smaller for protons and straggling effects are more important. The observed profile A (AJ)) is a convolution of the actual concentration profile C x with a depth resolution function (x, Eq), which broadens with increasing x roughly as Jx- Hence, resolution deteriorates with depth. However, near-surface resolution for resonant profiling may be on the order of tens of A. 

As earlier discussed, the dominant factor in the near-surface region is the particle detection system. For a typical silicon surface barrier detector (15-keV FWHM resolution for Fle ions), this translates to a few hundred A for protons and 100— 150 A for Fle in most targets. When y rays induced by incident heavy ions are the detected species (as in FI profiling), resolutions in the near-surface region may be on order of tens of A. The exact value for depth resolution in a particular material depends on the rate of energy loss of incident ions in that material and therefore upon its composition and density. 

The exit angle, /J, and the angle of incidence, a, of the beam (Fig. 3.63) determine the depth resolution and information depth. Small angles increase the depth resolution but reduce the depth probed. To optimize both quantities recoil angles 6 between 30 ° and 45 ° are often used, with a = /J = d/2. 

Straggling thus limits the depth and mass resolution for features buried within the target material. The depth resolution, Az depends on the stopping power, dE/dz, the detector resolution, AEdet and the beam energy spread, AEbeam of the incident particles ... 

RBS is a quantitative analytical tool which provides simultaneously the depth profile and the composition by mass number of the sample. The disadvantage is that a large and expensive particle accelerator is required to produce the incident beam. The probe depth of RBS is typically 1-2 pm with a depth resolution of 20-30 nm. 

Figure 1.15 Comparison of the lateral and depth resolution allowed by different optical and mass spectrometric techniques used for direct solid analysis (A, IG, incident and emitted ions, respectively cT, electrons ho, photons). XPS and AES are included in the graph for comparison.

The measurements of F depth profiles in chipped flints have been realised using the 4.7 keV wide 872 keV 19F(p,a12.3y)160 resonance. This reaction provides a good depth resolution (about 100 nm in Si02). The 4-7.5 MeV y-rays are detected with a BGO (bismuth germanium oxide) detector at 0°C with respect to the incident beam. The flint artefacts can be settled in the vacuum chamber as whole pieces without any sample preparation [35],... 

A number of effects lead to loss of profile depth resolution. The effect of crystallite orientation on sputtering rate is shown in Fig. 4.20, in which 10 keV Kr+ ions at 50° incidence were used to sputter poly crystalline iron [76]. This ion etching may be useful to bring out grain structure but leads to loss of depth resolution... 

Figure 37 High-depth-resolution profiling with a magnetic sector instrument using a 1-keV 00+ primary beam at 56° incidence with oxygen flooding. (From Ref. 126.)...

A profile of a 500-eV arsenic implant by Hitzman and Mount [127] shows the effect of Cs+ primary ion energy and incidence angle on depth resolution (Fig. 4.38). These As- profiles also demonstrate the high sensitivity of the SIMS technique. 

Figure 38 Effect of beam energy and angle of incidence on depth resolution. Profiles of arsenic in silicon using Cs+ primary beam. (From Ref. 127.)...

The electron microprobe is a local elemental analysis technique with a depth resolution which may vary from a few tenths of a micrometre to several micrometres. It is based on spectrometry of characteristic X-rays emitted by a solid sample under the impact of a focused incident electron beam (electron probe) ... 

The use of glancing incidence or emergence for heavy incident ions or produced ions respectively can give typical depth resolutions at the surface of 10 to 100 nm. 

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