Mechanism for Increased Yield with SF₅⁺ Projectiles in Organic SIMS:  The Substrate Effect

Abstract

Experiments have shown that the use of polyatomic projectiles in secondary ion mass spectrometry (SIMS) increases the secondary yield of molecular ions by an order of magnitude or more. This observation, coupled with the availability of an SF5 + source, has sparked renewed interest in SIMS measurements for characterizing a wide range of molecules. In this paper, we present the results of molecular dynamics simulations with Xe and SF5 projectiles that show that the molecular ion yield from bombarded organic surfaces is enhanced by the use of polyatomic projectiles. The model systems consist of a monolayer of twenty biphenyl molecules on two different substrates, Cu(001) and Si(100), and are designed as a prototype for experimentally studied systems. Our results show that the structure of the lattice is the critical factor. The breakup of the SF5 cluster within the more open lattice of the Si(100) substrate initiates collision cascades that lead to substrate atoms hitting the biphenyl molecules from below, which results in a greater yield of ejected molecules. The results are important because they predict that the nature of the substrate or matrix is a critical factor in maximizing the molecular ion yield.

An investigation of enhanced secondary ion emission under Aun+ (n=1–7) bombardment

“…Secondary ion yield enhancements are dependent on a wide range of experimental variables including the substrateÅ compositionÅ andÅ structureÅ [36].Å SiliconÅ and aluminum have similar atomic weights but very different molecular structures and bonding. Al is a facecenteredÅ cubicÅ metalÅ [45] ϩ and Au 3 ϩ primary ions in the negative ion spectrum are 1 to 2 times that observed on Al.…”

“…Molecular dynamics (MD) simulations have also been employed to examine the mechanism of nonlinear yield enhancement [31][32][33][34][35][36][37]. These simulations suggest that multiple collision cascades are required to cause the ejection of a molecule into the gas phase.…”

“…Thus there is an enhancement in the sputter yield observed due to the "nonadditive" effects of the collision cascades. Krantzman and coworkers have investgated the role of the substrate in the sputtering process by modeling Xe ϩ and SF 5 ϩ striking Cu and Si surfaces covered with a biphenyl adlayer [36,37]. They observed that the substrate significantly influenced the observed yield enhancement, with larger yield enhancements observed for less dense substrates such as Si.…”

Higher sensitivity secondary ion mass spectrometry of biological molecules for high resolution, chemically specific imaging

“…It has been demonstrated that these enhanced yields are produced with a concomitant increase in surface damage, but the yield enhancement surpasses that of damage [32]. Many studies have combined to develop the now accepted idea that the lower penetration depth and overlapping collision cascades of the polyatomic primary ion deposit more energy in the surface region, thus leading to increased sputtering [33][34][35][36][37][38][39].…”

New Cs sputter ion source with polyatomic ion beams for secondary ion mass spectrometry applications

“…Molecular dynamics simulations [11][12][13][14][15][16] indicate that the significant factors underlying the unique sputtering characteristics of polyatomic particles are ͑1͒ the reduced depth of the energy deposition in the sample during the sputtering event and ͑2͒ the time and space coincident coupling of energy transfer from the collision cascades of the individual atoms in the primary ion to the sputtered molecule. For a polyatomic projectile there is a higher probability that multiple collision cascades are generated simultaneously from the collision of the constituent projectile atoms with the substrate molecules/atoms.…”