Abstract
The high depth resolution capability of medium energy ion scattering (MEIS) is becoming increasingly relevant to the characterisation of nanolayers in e.g. microelectronics. In this paper we examine the attainable quantitative accuracy of MEIS depth profiling. Transparent but reliable analytical calculations are used to illustrate what can ultimately be achieved for dilute impurities in a silicon matrix and the significant element-dependence of the depth scale, for instance, is illustrated this way. Furthermore, the signal intensity-to-concentration conversion and its dependence on the depth of scattering is addressed. Notably, deviations from the Rutherford scattering cross section due to screening effects resulting in a non-coulombic interaction potential and the reduction of the yield owing to neutralization of the exiting, backscattered H+ and He+ projectiles are evaluated. The former mainly affects the scattering off heavy target atoms while the latter is most severe for scattering off light target atoms and can be less accurately predicted. However, a pragmatic approach employing an extensive data set of measured ion fractions for both H+ and He+ ions scattered off a range of surfaces, allows its parameterization. This has enabled the combination of both effects, which provides essential information regarding the yield dependence both on the projectile energy and the mass of the scattering atom. Although, absolute quantification, especially when using He+, may not always be achievable, relative quantification in which the sum of all species in a layer adds up to 100%, is generally possible. This conclusion is supported by the provision of some examples of MEIS derived depth profiles of nanolayers. Finally, the relative benefits of either using H+ or He+ ions are briefly considered.
Highlights
The ever shrinking lateral and depth dimensions of microelectronic devices have resulted in the development of viable fabrication technologies of which the functional components cannot be characterized exhaustively using any single analytical technique
The combination of the screening and neutralization effects provides a correction factor to the Rutherford backscattering cross section ratio that enables the reliable conversion of measured ion yields to atomic concentrations to within a few % as is demonstrated in a number of representative examples of depth profiles of nanolayers derived from medium energy ion scattering (MEIS) spectra
In terms of the quantification of atomic composition, the yield ratio of particles scattered off surface atoms and those at greater depth in MEIS has been analyzed which has led to a modification of the Rutherford EÀ2 prediction by the inverse ratio of the energies of the particles arriving at the detector
Summary
The ever shrinking lateral and depth dimensions of microelectronic devices have resulted in the development of viable fabrication technologies of which the functional components cannot be characterized exhaustively using any single analytical technique. The combination of the screening and neutralization effects provides a correction factor to the Rutherford backscattering cross section ratio that enables the reliable conversion of measured ion yields to atomic concentrations to within a few % as is demonstrated in a number of representative examples of depth profiles of nanolayers derived from MEIS spectra. This leads to the conclusion that absolute quantification, especially when using He ions, may not always be achievable, relative quantification in which the sum of all species in a layer add up to 100%, generally is. The relative benefits of either using H+ or He+ ions will be considered
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