STATIC SIMS - A STUDY OF DAMAGE USING POLYMERS

As a result of work to establish the surface potential of insulators a ccurately in a quadrupole static SIMS system of high sensitivity, we h ave been able to study the effects of increasing dose-related damage o n the intensities of the mass spectral peaks in the two archetypal bul k polymers PET and PTFE, as well as thin hydrocarbon contamination lay ers, with high accuracy. It is shown that the intensities follow very well-defined functions which give damage cross-sections whose values r eflect the fragmentation behaviour of the polymers. The effects reflec t the number of bonds to be broken to liberate the fragment, the inter nal complexity of that fragment and the typical damage zone of the ion impact. These concepts show that a static SIMS limit of below 3 x 10( 15) ions m(-2) exists for changes of < 10% in the intensities of the m ost easily damaged species but that some larger fragments may require a dose of 2 x 10(17) ions m(-2) to maximize their intensity! This work has three main conclusions. Firstly, by defining a figure of merit fa ctor, F, equal to the ratio of the absolute intensity of a peak to the fractional rate of change of that peak with the ion dose, it is possi ble to define the optimal beam parameters for static SIMS measurements . The higher the value of F, the more the intensity per unit of onset of damage. The highest F values occur at higher beam energies and, in general, xenon gives greater efficiency than argon. Secondly, the deve lopment of damage may be described by simple bond breaking. Thirdly, a study of tile damage process gives quite detailed structural bonding information not directly available from the traditional static SIMS sp ectrum.