In-depth resolution

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

As to expected performance, Figure 8.3 shows some (conservative) values for lateral and in-depth resolutions for various physical methods of analysis. Recent developments already allow better performance, e.g. a lateral resolution of about 0.1 irn with liquid metal ion sources in SIMS and small beams of 150 xm in diameter in [xXPS. 

SSMS can be classified among the milliprobe techniques (Figure 8.3), i.e. it is a unique link between microprobe techniques and macroanalytical methods that are characterised by poor lateral and in-depth resolutions (as in OES), or that have no lateral resolution whatsoever (as in NAA). Also, the achievable precision and accuracy are poor, because of the irreproducible behaviour of the r.f. spark. Whereas analysis of metals, semiconductors and minerals is relatively simple and the procedures have become standardised, the analysis of nonconducting materials is more complex and generally requires addition of a conducting powder (e.g. graphite) to the sample [359]. Detection limits are affected by the dilution, and trace contamination from the added components is possible. These problems can be overcome by the use of lasers [360]. Coupled with isotope dilution, a precision of 5% can be attained for SSMS. 

Method In-depth resolution Lateral resolution Information... 

Fig. 9.17. In-depth profiles obtained by LIBS and definition of in-depth resolution (DR).

As we shall show below, two of the limiting factors in depth resolution and, therefore, the ability to determine interface widths are the temporal and spatial stability of the ion beam. One of the advantages of rastering the ion gun is to improve the spatial uniformity of the erosion rate. The most pronounced temporal variations occur during the first few seconds after the ion beam is turned on. But they are only important if changes in surface composition occur in the same time frame. Long term drifts in ion beam current or position are usually not critical in depth profiles. 

An overall licensing and regulatory basis appears to be lacking. This is an issue which requires in-depth resolution in the medium term. It is anticipated that the utility will undertake to develop in-house the regulatory guidelines. 

Heavy-ion ERD (HI-ERD) The use of heavy ions such as increases the stopping efficiency of the beam, minimizing the thickness of particle filter required. This results in an improvement in depth resolution but limits the depth that can be profiled. The possibility of beam damage must also be considered. The heavy-ion accelerators required for this technique are not as widely available as those used for standard ERD. 

Fig. 4.19 Carbon in GaAs with carbon doping layers of 8 x lO atoms/cm is profiled with two different beam energies and angles of incidence in a Cameca IMS 3f. By switching polarity of the secondary ions from -4.5 to +4.5 kV with a primary beam voltage of 12.5 kV, the impact energy switches from 17 to 8 keV and the angle from 25° to 39° with significant improvement in depth resolution at the lower impact energy...

Table 4.11 compares SIMS and SNMS (cfr. also Table 8.57 of ref. [110a]). Detection limits in the sub-ppm range are accessible under optimised analytical conditions. A lateral resolution of less than 100 nm and an in-depth resolution of a few nm can be achieved. One of the unique features of SNMS is the ease of analysis of insulators. This is at variance to SSMS, GD-MS and SIMS, which are handicapped by electrical charging effects. Laser SNMS is not strictly restricted to elemental analysis, but can also be applied to the characterisation of molecular surfaces. For an optimum yield of intact molecular ions and characteristic fragments it is necessary to optimise laser power density, wavelength, and pulse width [112],... 

Most typically, the analyzed region represents between 4 and 20% of the sputtered area, i.e. should be less than 33% (factor of 3 rule). Pictorial examples of the analyzed to sputtered regions accessed along with the improvements in depth resolution are demonstrated in Figure 5.19(b) and 5.19(c), respectively. Moreover, when nsing a Gaussian beam, the raster size must be greater than four times the beam diameter. 

The ion-sputtering process is statistical in nature in that the. second and subsequent atom layers may be depleted before the first layer is removed entirely. This fact leads to a progressive degradation in depth resolution with sputtering... 

The basic limitation in depth resolution is due to the sputtering process itself ... 

The in-depth resolution of the electron probe is about the same as the lateral resolution (1-3 /im). Normally, the analysis is restricted to regions as large or larger than these dimensions. However, the instrument is employed increasingly in the characterization of films of submicrometer dimensions, (determining both film thickness and composition " ) as well as of small particles. A search of the literature shows that the electron probe has been applied to a wide range of materials. ... 

The limitations in sensitivity and in depth resolution of the electron-probe microanalyzer prompted the development of ion-probe microanalysis. This technique is based on mass spectro-graphic analysis of the secondary ions emitted from a sample under the impact of a focused and accelerated primary ion beam. This type of analysis also offers, in comparison with the electron probe, the possibilities of isotopic analysis and the investigation of elements of low atomic number, including hydrogen, at trace concentrations. 

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