Peptide-doped trehalose thin films have been characterized by bombardment with energetic cluster ion beams of C60+ and Aux+ (x = 1, 2, 3). The aim of these studies is to acquire information about the molecular sputtering process of the peptide and trehalose by measurement of secondary ion mass spectra during erosion. This system is important since uniform thin films of approximately 300 nm thickness can be reproducibly prepared on a Si substrate, allowing detailed characterization of the resulting depth profile with different projectiles. The basic form of the molecular ion intensity as a function of ion dose is described by a simple analytical model. The model includes parameters such as the molecular sputtering yield, the damage cross section of the trehalose or the peptide, and the thickness of a surface layer altered by the projectile. The results show that favorable conditions for successful molecular depth profiling are achieved when the total sputtering yield is high and the altered layer thickness is low. Successful molecular depth profiles are achieved with all of the cluster projectiles, although the degree of chemical damage accumulation was slightly lower with C60. With C60 bombardment, the altered layer thickness of about 20 nm and the damage cross section of about 5 nm2 are physically consistent with predictions of molecular dynamics calculations available for similar chemical systems. In general, the model presented should provide guidance in optimizing experimental parameters for maximizing the information content of molecular depth profiling experiments with complex molecular thin film substrates.

A protocol for three-dimensional molecular thin-film analysis is described that utilizes imaging time-of-flight secondary ion mass spectrometry and large-area atomic force microscopy. As a test study, a 300-nm trehalose film deposited on a Si substrate was structured by bombardment with a focused 15-keV Ga+ ion beam and analyzed using a 40-keV C60+ cluster ion beam. A three-dimensional sputter depth profile was acquired as a series of high-resolution lateral SIMS images with intermittent erosion cycles. As the most important result of this study, we find that the structured film exhibits a highly nonuniform erosion rate, thus preventing a simple conversion of primary ion fluence into eroded depth. Instead, the depth scale calibration must be performed individually on each pixel of the imaged area. The resulting laterally resolved depth profiles are discussed in terms of the chemical damage induced by the Ga+ bombardment along with the physics of the C60+ induced erosion process.

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Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of LangmuirBlodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C 60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.The development of polyatomic projectiles for cluster-based secondary ion mass spectrometry (SIMS) is opening new opportunities for materials characterization. Of special interest is the emergence of molecular depth profiling whereby the projectile removes molecules in nearly a layer-by-layer fashion without the accumulation of chemical damage. [1][2][3][4][5][6][7] This problem has plagued atomic projectiles for many years 8 and has limited sensitivity. When the molecular samples are bombarded with cluster ion sources, the energy is deposited close to the surface and the chemical damage is then removed as fast as it accumulates, leaving subsurface layers relatively intact. [9][10][11][12][13][14][15] The quality of the depth profile has recently been characterized by a cleanup efficiency parameter derived from a simple erosion model for molecular solids. 16 Among all the cluster projectiles, buckminsterfullerene (C 60 + ) generally exhibits the highest cleanup efficiency. 17,18 New fundamental studies of the sputtering process are now required to optimize the experimental parameters for molecular depth profiling. The literature concerning the interactions between energetic cluster ions and molecular solids has grown rapidly, including experimental approaches 16,[19][20][21][22][23][24][25][26] and molecular dynamic (MD) simulations. [11][12][13][27][28][29] While MD simulations have provided insightful understanding, much of the experimental work lacks a quantitative understanding for comparison to the simulation results. Moreover, most of the molecular depth profiling experiments are performed on organic systems either with uniform chemical content or with unknown composition. 3,4,30,31 The analysis of buried organic layers under cluster bombardment has been shown to be feasible, but the degree of beam-induced * To whom correspondence should be addressed. nxw@psu.edu.Supporting Information Available: Representative AFM images of sample roughness ( Figure S1). This is material is available free of charge via the Internet at http://pubs.acs.org. mixing between organic layers is not fully understood. This info...

The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films ∼300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C 60 + at incident angles ranging from 5° to 73° with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73° incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at offnormal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C 60 + .The addition of cluster ion beam sources to traditional secondary ion mass spectrometry (SIMS) experiments has expanded the options for new applications. 1-5 One of the most important observations associated with these projectiles is that there is often very little chemical damage buildup during the interaction of the cluster with a molecular solid. 1-10 This effect is generally different from the behavior observed using atomic projectiles where damage buildup is usually quite severe, and the experiments must be carried out in either a low dose or very low kinetic energy (>200 eV) mode in order to retain the desired spectral information. [1][2][3][4][5]8,10,11 The high cleanup efficiency of cluster projectiles opens the possibility of molecular depth profiling through molecular solids, with many examples being reported in the past few years. 6-10,12-18 Buckminsterfullerene (C 60 ) has been shown to be particularly effective in this regard, although other cluster projectiles have also been shown to yield acceptable results. 12,14,16 If the cluster beam is focused to a submicrometer spot, it is feasible to create three-dimensional * To...

We employ a buckminsterfullerene ion source to probe the distribution of histamine molecules at the water-ice/vacuum interface. The experiments utilize secondary ion mass spectrometry to detect molecular ions that are desorbed from a frozen aqueous histamine solution. The results show that this cluster ion probe induces an extraordinarily high sputter yield of 2400 ice molecules per impact event as determined by a quartz crystal microbalance. As a consequence of this high yield, we show that it is possible to produce molecular depth profiles of the top several hundred nanometers below the ice surface without destruction of the molecular ion signal by accumulation of beam-induced chemical damage. Similar profiles are reported for desorbed neutral molecular fragments by utilizing a high-power femtosecond-pulsed laser for photoionization. While this type of information could not be achieved using atomic projectiles, it is possible to remove the damage induced by such projectiles by subsequent cluster bombardment. These experiments are particularly important for organic surface analysis since they suggest that cluster ion probes may successfully be employed to remove overlayers that may mask the desired molecular information in static secondary ion mass spectral analysis.

The bombardment of a 26 nm poly(methyl methacrylate) (PMMA) film has been studied as a model for depth profiling of polymeric samples using a newly developed C 60 þ ion source. Experiments were conducted on a ToF-SIMS instrument equipped with C 60 þ and Ga þ ion sources. A focused dc C 60 þ ion beam was used to etch through the polymer sample at specified time intervals. Subsequent spectra were recorded after each individual etching cycle using both C 60 þ 20 keV and Ga þ 15 keV ion beams at field-of-views smaller than the sputter area. PMMA fragment ion at m/z ¼ 69 and substrate Au m/z ¼ 197 were monitored with respect to primary ion doses of up to 10 14 ions/cm 2 . Depth resolution as determined by the interfacial region is found to be about 14 nm. A >10-fold increase in sputter yield for C 60 þ ion bombardment over Ga þ ions under similar conditions is observed from quartz crystal microbalance (QCM) measurements and our findings compare to enhanced SF 5 þ cluster bombardment yields of organic species. #

Molecular depth profiling of organic overlayers was performed using a mass selected C 60 ion beam in conjunction with time-of-flight (TOF-SIMS) mass spectrometry. The characteristics of sputter depth profiles acquired for a 300-nm Trehalose film on silicon were studied as a function of the kinetic impact energy of the projectile ions. The results are interpreted in terms of a simple model describing the balance between sputter erosion and ion induced chemical damage. It is shown that the efficiency of the projectile to clean up the fragmentation debris produced by its own impact represents a key parameter governing the success of molecular depth profile analysis.