Surface Analysis Studies of Yield Enhancements in Secondary Ion Mass Spectrometry by Polyatomic Projectiles

Abstract

In this paper we examine the mechanism of secondary ion yield enhancements previously observed for polyatomic projectiles by measuring the weight loss, volume loss, and surface composition of poly(methyl methacrylate) (PMMA) films sputtered by keV SF 5 + and Ar + projectile ions. The sputter yieldsthe amount of material removed from the surface by 3.0 keV SF 5 + projectilesswas found to be 2.2 ( 0.8 higher than for Ar + projectiles, measured by weight loss in the PMMA film with a quartz crystal microbalance. This result is consistent with sputter yield measurements reported here using 5.5 keV ions and stylus profilometry. Thus, the >10× enhancement in secondary ion yield in secondary ion mass spectrometry observed for polyatomic ion projectiles is not attributable to the modest ∼2× enhancements observed in the sputter yields for this molecular solid. Surface chemical measurements by X-ray photoelectron spectroscopy also indicated fundamental differences in atomic versus polyatomic sputtering mechanisms at 3.0 keV, but not at 0.7 keV. These results provide a reasonable explanation for the depth profiling capability demonstrated here on PMMA films for 5.5 keV SF 5 + ions that is not possible with isoenergetic Ar + ions. † Part of the special issue "John T. Yates, Jr. Festschrift".

C60 molecular depth profiling of a model polymer

“…The general features of this profile are quite similar to those reported using SF 5 þ projectiles [1]. To obtain a clearer picture of how the monitored signal intensities vary as a function of depth into the sample, normalized PMMA and Au intensities are plotted in Fig.…”

“…do not yield useful depth information since too much damage is created on the surface to accurately attain molecular specificity in each layer. Fuoco et al [1] have reported that polyatomic primary ion beam sources such as SF 5 þ are much better suited for depth profiling through organic substrates. For equal beam energies, polyatomic projectiles penetrate the surface to much lower depths than their atomic counterparts and offer substantial sputter yield and secondary ion yield enhancements for molecular ions.…”

“…Here we explore the possibilities of using the C 60 þ ion source as a depth profiling tool for organic species. Poly(methyl methacrylate) (PMMA) is used as a model polymer since direct comparisons can be made to depth profiles reported by Fuoco et al [1] on PMMA using an SF 5 þ primary projectile. If depth profiling using biological cells-making it plausible to obtain 3D molecular maps of cellular constituents.…”

Microscopic Insights into the Sputtering of Ag{111} Induced by C60 and Ga Bombardment

“…[1][2][3] Cluster ion beams have been of interest in these types of experiments for many years since it has been noticed that surface molecules are desorbed with greater efficiency than with corresponding atomic ion beams. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] When C 60 strikes a surface, however, the subsequent energy dissipation processes give rise to several fascinating phenomena with significant scientific implications. Detailed studies are now possible since a stable, long-lived source of C 60 + ions has recently been developed.…”

An XPS and ToF-SIMS investigation of the outermost nanometres of a poly(vinylidene difluoride) coating

“…The use of a SF 5 + cluster ion as a depth profiling etch tool for use on polymeric samples has been demonstrated by Fuoco et al employing PMMA thin film on silicon [13], Norrman et al also used PMMA and poly(vinyl chloride) films spin coated onto Si wafer [14] and by Mahoney et al who investigated poly(L-lactic acid) (PLLA) doped with 4-acetaminodophenol [15] and PLLA blended with a triblock co-polymer [16]. Most recently, with the introduction of the Buckminsterfullerene (C 60 ) cluster ion source as an etch tool, the depth profiling of polymeric (or organic) materials has taken great strides forward.…”