CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 3
diffused and ion implanted materials, (b) in-depth composition of profiles of oxide layers, (c)
analysis of corrosion films, (d) depth profile of embrittled materials, (e) vapour deposited thin
films, and (f) oxidation of different species [78-82]. However, the interpretation of SIMS spectra
is quite difficult, and thus is not used for the unknown specimens without certain degree of prior
information about the elemental composition. Similarly, quantification of SIMS data is also quite
difficult due to matrix effect problem arising as function of ionization potential of the sputtered
species, and due to the presence of electronegative atoms such as oxygen on the surface. Apart
from this, the bombardment of primary ions modifies the surface due to mixing of atoms, and
fragmentation of surface molecules etc [83].
In SIMS an energetic ion beam (1-20 keV) of focused primary ions is directed at the
sample surface in a high or ultra high vacuum environment. The sputtered ions are termed as
secondary ions. The secondary ions are then mass analyzed using double focusing mass
spectrometer or an energy filtered quadruple mass spectrometer [84]. The principle of SIMS is
schematically represented in Fig. 3.6 [84]. The primary ion beam can be of noble gases (Ar + , Xe + ),
oxygen (O - , O 2- ) and cesium (Cs + ). The choice of the ion species depends up on the required
current, required beam size, and the specimen to be analyzed. The transfer of momentum from the
impinging primary ions to the specimen surface causes collision cascade resulting sputtering of
surface atoms and ions since most of the momentum transfer is redirected towards the surface. The
secondary ion current is represented in Eq. 7,
I m = I p Y m m (7)
where
I m is the secondary ion current of the species m.
I p is the primary ion flux.