Direct comparison of Au₃⁺ and C₆₀⁺ cluster projectiles in SIMS molecular depth profiling

Abstract

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C 60 ϩ and Au 3 ϩ , have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au 3 ϩ bombardment is found to induce more chemical damage as well as Au implantation when compared with C 60 ϩ . Moreover, C 60 ϩ is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS. (J Am Soc Mass Spectrom 2007, 18, 406 -412) © 2007 American Society for Mass Spectrometry E nergetic cluster ion bombardment and secondary ion mass spectrometry (SIMS) experiments have opened new applications in molecular depth profiling and 3D chemical imaging [1][2][3]. With this approach, it has been reported for many systems, that cluster projectiles remove large amounts of material from molecular solids without the damage accumulation associated with atomic projectiles [4 -12]. Moreover, erosion of the material occurs without significant interlayer mixing and/or the formation of topographical features, allowing depth resolution of 10 to 30 nm to be achieved [8 -10]. These attributes have compelled a majority of SIMS workers to quickly adopt this new technology.Although many different cluster projectiles have been examined, practical considerations have led to wide adaptation of liquid metal ion sources consisting of Au or Bi projectiles (Au 3 ϩ and Bi 3 ϩ , respectively) [13][14][15][16], and gas ion sources consisting of SF 5 ϩ or C 60 ϩ projectiles [6,17,18]. The physics of the ion/solid interaction is likely to be quite different between the carbon and metal-type sources since at 20 keV incident energy, each carbon atom carries 333 eV of kinetic energy while each metal atom in a trimer carries 6667 eV of kinetic energy. Moreover, there is a large mass variation between the atoms comprising these cluster species.The imaging capability of the two types of sources is quite different. The liquid metal ion gun (LMIG) technology yields very bright ion beams with a probe size of less than 100 nm in diameter [19]. The gas ion source requires apertures to define the beam size, sacrificing beam current. Recently, a C 60 source has been introduced [20] that achieves a submicron probe size with enough current to acquire images in a reasonable amount of time. In general, however, higher lateral resolution is achieved with the LMIG design if there is enough sensitivity in the mass...

Cluster secondary ion mass spectrometry of polymers and related materials

“…It was found that damage accumulated under Au cluster bombardment (signal decreased with fluence), but not under C þ 60 bombardment, which maintained relatively constant signals with increasing fluence. Similar results were found in other kinds of molecular samples, such as trehalose (Cheng et al, 2007). Finally, the sputter rates were found to be enhanced significantly for PS 2000 when employing C þ 60 ion beams as opposed to Au þ 3 beams (Hill & Blenkinsopp, 2004).…”

“…Cheng and Winograd (2005, Cheng, Wucher, and Winograd (2006), and Cheng et al (2007) performed a series of studies on depth profiling in polypeptide films. The first article in this series (Cheng & Winograd, 2005) shows the feasibility of using C þ 60 to depth profile through trehalose films (300-1,000 nm) (naturally occurring sugar, which has shown to enhance the stability of biomaterials) containing different polypeptides.…”

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

“…Apparently, using cluster primary ions is the key to in situ liquid SIMS analysis. It should be noted that unlike C 60 or Ar cluster ions, molecular depth profiling of organic thin films using metal cluster primary ions is not feasible because of accumulation of beam damage [34]. Therefore, the self-renewable feature of liquid interfaces (attributable to liquid diffusion and evaporation in many cases) plays an important role to compensate beam damage in in situ liquid SIMS analysis.…”

A Computational Investigation of C60 Depth Profiling of Ag: Molecular Dynamics of Multiple Impact Events

“…1,2 Their abilities of nonlinear enhancement of yield, reduced chemical damage, and reduced damage depth have opened the door to a wide array of depth profiling capabilities. [3][4][5][6][7][8][9][10][11][12] For example, it is possible to analyze multilayered structures used in the electronics/semiconductor industry, 6,13 as well as perform molecular specific depth profiles of cells and other biologically important systems. [13][14][15][16][17][18][19][20] The critical issue for quantitative interpretation of depth profiles is the interface width, a quantity that reflects how precisely one can measure a change in composition.…”