Depth resolution sample rotation

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. 

Sputter-induced Roughness. Sputtering single crystal semiconductors, in particular II-VI semiconductors, with Cs" ions results in surface roughening. There is currently no satisfactory explanation of this effect. This effect, and the corresponding reduction of the depth resolution, can be avoided by rotating the sample during the measurement. 

Zalar, A. (1985) Improved depth resolution by sample rotation during Auger electron spectroscopy depth profiling. Thin Solid Films, 124, 223-230. 

Bradley, R.M., Cirlin, E. (1996) Theory of improved resolution in depth profiling with sample rotation. Applied Physics Letters, 68,3722-3724. 

This form of surface roughening along with the corresponding loss of depth resolution and increased variations in ion yields can be controlled using sample rotation (Zalar 1985) as will be discussed in Section 5.3.2.4.5. 

Since the major limitation on depth resolution arises from the sputtering process itself and from the sputter-induced microtopography, many attempts have been made to minimize such effects. The main aim is to suppress both the crystal orientation effects and the increase in roughness by rotating the sample during sputtering, as suggested by Zalar. If that is done in the correct way, then the sputter crater can be produced with a flat bottom, in the center of which is the analyzed area (79, 80]. 

There are two mutually exclusive requirements in SIMS there is a need for high sensitivity, which is optimum at normal incidence, and as in AES and XPS, a need for good depth resolution which is optimum at glancing incidence. The best solution seems as before to involve rotation of the sample during sputtering, which produces a depth resolution relatively independent of the incidence angle so that high sensitivity can still be achieved. 

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