Depth resolution limiting factors

Crater Bottom Roughening. Depth resolution is also limited by roughening of the crater bottom under the action of ion bombardment. On polycrystalline samples this can be because of different sputter yields of different crystal orientations, because the sputter yields of single crystals can vary by a factor of two depending on their orientation. Because of this type of roughening, depth resolution deteriorates with increasing sputter depth. 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

For shallow implanted or diffused layers, the semiconductor is angle-bevelled and the probe moved along the bevelled surface. A shallow angle coupled with short probe steps, allows equivalent shallow depth steps. Examples are given in ( ) with depth resolution of 200X. The accuracy is limited by the correction factors in the algorithm used in the data reduction. 

PIXE enables the determination of composition for all elements with Z, atomic number, higher than 5. An examination of PIXE spectra shows that the characteristic X-ray peaks are superimposed on a background, which forms a hmiting factor in sensitivity. It is possible to obtain limited depth profile information using ion induced X-ray emission. The depth resolution is angle and ion energy dependent and is not as good as for other techniques. 

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. 

Although the plasma ions possess a relatively low average energy of <100 eV, a prerequisite for a high depth resolution in profiUng, the achievable resolution is not as good as could be expected. Depth resolutions between 0.05 and 0.5 pm have been reported [162]. Rarely, a lower value is obtained near surface (e.g., 10 nm) [163]. Limiting factors for the depth resolution... 

Ni/Cr multilayers. Factors known to limit the depth resolution include instrumental factors, sample characteristics and radiation-induced effects. 

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