Yield sputtering

When the energy of the arriving particle is well above the bond energy in the solid, the collision cascade can be effectively modeled. The sputtering yield was calculated based on classical collision theory by Sigmund [10] to be  

The effect of this threshold on the sputtering yield Y has been modeled by Bohdansky, resulting in the functional form [13] 

P parison of sputter yield data with both the Sigmund theory and Equation f Ration 11,8 fits the data almost exactly over the range shown and for the values 

Target atom mass Mt Incident atom mass Mi 

Much of this theory has been incorporated into computer simulations of sputtering and ion damage processes. Thus, it is rare to actually employ these expressions directly given the avadabdity of programs such as TRIM, a Monte Carlo type simulation of the collision cascade in sohds struck with ions. [15] 

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Sigmund P 1969 Theory of sputtering. I. Sputtering yield of amorphous and polycrystalline targets Phys. Rev. 184 383-416... 

The sputtering yield is proportional to the number of displaced atoms. In the linear cascade regime that is appUcable for medium mass ions (such as argon), the number of displaced atoms, E (E, is proportional to the energy deposited per unit depth as a result of nuclear energy loss. The sputtering yield Y for particles incident normal to the surface can be expressed as foUows (31). 

Fig. 8. Calculated sputtering yield of several materials bombarded with argon ions at various energy levels. The materials Hsted parenthetically also have...

Useful yield provides an overall measure of the extent to which the sputtered material is used for analysis. It is a quantity employed to estimate the sensitivity of the mass spectrometric method. Values of Y (X (A)) for elements typically range from 10 to 10 in TOF SIMS. The number of sputtered particles A per incident primary ion (sputtering yield) can be measured from elemental and multielemental standards under different operational conditions and can, therefore, by judicious interpolation between standards, be estimated with reasonable accuracy for the material being analyzed. 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

Implantation of Primary Ions. The primary ions are implanted in the sample and thereby influence the chemical constitution. For energies in the 20 keV range the implantation depth is approximately 30 nm. The sputter yield, i. e. the ratio of secondary to primary particles (not only ions), is energy-dependent and has a maximum in the 10 keV range. 

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. 

Equation (3.19) is valid for any species X. If, however, a multi-component material emits only atomic SN after attaining sputter equilibrium, X stands for elements and atoms only, and the total sputter yield Yean be written as ... 

Taking atomic sputtering into account the proportion of the particles emitted as molecules is negligible and the partial sputtering yield for element A in sputter equilibrium can be determined by use of ... 

The quantitative effect of the mass, energy, and angle of impact on the sputter yield for impacting deuterium ions is shown in Figs. 12a and b. As the kinetic energy... 

For intermediate temperatures from 400-1000°C (Fig. 11), the volatilization of carbon atoms by energetic plasma ions becomes important. As seen in the upper curve of Fig. 11, helium does not have a chemical erosion component of its sputter yield. In currently operating machines the two major contributors to chemical erosion are the ions of hydrogen and oxygen. The typical chemical species which evolve from the surface, as measured by residual gas analysis [37] and optical emission [38], are hydrocarbons, carbon monoxide, and carbon dioxide. 

Fig. 12. Sputtering yields for graphite as a funetion of (a) temperature and (b) ineident angle.

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