Sensitivity and depth resolution

Reasonable estimates of ultimate sensitivity and depth resolution in ERDA can hardly be given because of the large range of projectiles and energies (from He ions of several MeV up to 200-MeV Au ions), and the use of different detection systems. In addition, stability of the sample under irradiation (which, of course, depends on the target material) is also important in the discussion of sensitivity and detection limits. The sensitivity is mainly determined by the recoil cross-section, the solid an- 

TOF detector systems usually have smaller solid angles and sensitivity than AF - E systems, because of the long TOF system in front of the energy detector and the limited size of the stop detector. They also have worse detection limits for very light elements (hydrogen), because of the low probability of obtaining start and stop signals for particles of very low atomic number [3.172]. 

The depth resolution of ERDA is mainly determined by the energy resolution of the detector system, the scattering geometry, and the type of projectiles and recoils. The depth resolution also depends on the depth analyzed, because of energy straggling and multiple scattering. The relative importance of different contributions to the depth resolution were studied for some specific ERDA arrangements [3.161, 3.163]. 

An example of depth profiling of hydrogen implanted into Si is shown in Eig. 3.64 [3.177]. Measured energy spectra of H recoils are given for impact of 6-MeV C ions. H identification was achieved by the AE-E technique and use of ion-in- 

Surface and Thin Film Analysis Principles, Instrumentation, Applications 

---

The sensitivity and depth resolution of ERDA depend on the type of projectile, on the type of particle, and on energy measurement. Because of the broad range of particles and methods used, general statements about sensitivity and depth resolution are hardly possible. Recent reviews of ERDA techniques are available [3.152-3.154]. 

Sensitivity and depth resolution. The use of a low energy electron beam to excite molecular vibrations physically guarantees a very high surface specificity (101. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

The development of surface analytical techniques such as LA-ICP-MS, GDMS and SIMS focuses on improvements to sensitivity and detection limits in order to obtain precise and accurate analytical data. With respect to surface analytical investigations, an improvement of spatial and depth resolution is required, e.g., by the establishment of a near field effect or the apphcation of fs lasers in LA-ICP-MS. There is a need for the improvement of analytical techniques in the (j,m and nm range, in depth profiling analysis and especially in imaging mass spectrometry techniques to perform surface analyses faster and provide more accurate data on different materials to produce quantitative 3D elemental, isotopic and molecular distribution patterns of increased areas of interest with high spatial and depth resolution over an acceptable analysis time. 

This sophistication is evident in all forms of SIMS as they exist today. Indeed, SIMS is now routinely used to measure isotopic, elemental, and/or molecular distributions (whether existing at the outermost surface or within a substrate) through the application of highly specific instrumentation. Likewise, SIMS, which has long been able to measure atomic ion emissions, can now do so to unprecedented sensitivity, detection limits, dynamic range, mass resolution, spatial resolution, and depth resolution. 

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... 

Reflected Electron Energy-Loss Spectroscopy (REELS) has elemental sensitivities on the order of a few tenths of a percent, phase discrimination at the few-percent level, operator controllable depth resolution from several nm to 0.07 nm, and a lateral resolution as low as 100 nm. 

Figure 8 Quantitaftive high depth resolution profile of O and N in a Ti metal film on Si, using electron-gas SNMS in the direct bombardment mode. Both O and N are measured with reasonably good sensitivity and with good accuracy both at the heavily oxidized surface and at the Ti/Si interface.

---