Fundamentals of Secondary Ion Mass Spectrometry
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tion between the incident primary ion and the target results in the emission, or sputtering, of the sample matrix. This sputtering of the sample is inherent to SIMS and results in the production of the analytical signal, secondary ions. Because of the fundamental importance of sputtering, an initial understanding of the process is useful. The phenomenon of sputtering was first reported in the 1850s in gas-discharge experiments. Since then, the erosion of solids by energetic beams has received considerable attention, and many theories have been put forth in an attempt to understand this phenomenon. However, despite efforts to rationalize the sputtering process, there still is no single model that can account for all the experimental results. The general topic of sputtering can be divided into two categories: physical and chemical sputtering. When particles are emitted from a solid as a result of moment u m transfer from a collision cascade induced by an incident particle, the process is called physical s p u t t e r i n g . An o v e r v i e w of the v a r i o u s m e c h a n i s m s involved in physical sputtering has been presented by Williams. 1 This process is contrasted to chemical sputtering where the incident particle results in the formation of a metastable compound at the target surface. This process is characteristic of molecules such as CH 4 , CF4, etc., whose binding energy to many surfaces is relatively small. Chemical sputtering has been reviewed by Winters. 2 Both physical and chemical sputtering r e q u i r e that the ejected species h a v e e n o u g h e n e r g y to e s c a p e the surface potential barrier. Sputtering can occur over a relatively wide range of incident ion energies, from the lower eV regime for chemical sputtering to the upper keV region for physical ejection. At low primary ion energies, secondary emission results from a relatively small number of near-surface collisions. For a primary ion of mass mi, and kinetic energy E], in collision with a target of mass m2, the energy transferred to the target mass is given by: E, =
4mim 2 (m 1+ m 2 ) 2
Ei sin2
where 0 is the scattering angle. This classical kinematics is dominant for primary ions below 200 eV. The sputtered species are ejected with a typical energy of about 4 eV.
PAGE 40, MRS BULLETIN, AUGUST 16/SEPTEMBER 30, 1987
The average secondary ion energy distributions are seen to increase with increasing primary ion kinetic energy, but asymptotically approach a limit in the 800-1,000 eV range. In the higher energy range, 1 keV10 keV, the number of sputtered atoms/ion is usually found to be a monotonically increasing function of energy. Sputtering is not the only way to remove a target atom with sufficient energy to escape the surface potential. Other mechanisms include sample heating and subsequent evaporation as well as beam-induced desorption p h e n o m e n a resulting from either electron or ion irradiation. Hart and Cooper 3 have reviewed the energetics of the sputtering event. Physical sputtering can be distinguished by th
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