Depth Resolution During C₆₀⁺ Profiling of Multilayer Molecular Films

Abstract

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...

Hybrid Organic/Inorganic Materials Depth Profiling Using Low Energy Cesium Ions

“…An increasing amount of literature demonstrates that polyatomic primary ions such as SF 5 + , C 60 + , and Ar n + are suitable for this purpose [1][2][3][4][5][6]. The total energy is shared by each atom in the cluster, limiting the penetration depth and thereby the extent of the chemically damaged layer under the surface.…”

A simple erosion dynamics model of molecular sputter depth profiling

“…[26,27] In the meantime, this model has been applied several times to analyze experimentally measured molecular sputter depth profiles. [15,21,24,28,29,30] However, no rigorous derivation of the employed formulae has been published yet. The present paper is intended to close that gap by providing a detailed discussion of the underlying concepts and approximations.…”

Molecular Depth Profiling with Cluster Ion Beams

“…It can be used to compare predictions of the model to experimental data of molecular SIMS depth profiles. 3,4,22,28,33 In this form, the model contains three free parameters, namely, the total sputter yield Y tot , the damage cross section σ D and the width d of the altered layer.…”

“…SIMS is also extremely useful for the study of highly ordered and/or multilayer molecular films. 11,17,20,27,28,33,34 Figure 5.5 shows an excellent example of this, displaying molecular information as a function of depth for perfectly alternating Langmuir-Blodgett (LB) layers of arachidic acid (AA) and dimyristoyl phosphatidate (DMPA). 32 The profile shown in Figure 5.5a was acquired using a 40 keV C 60 + source, operated at glancing incidence angles of 73 • with respect to the surface normal, and the sample was maintained at liquid nitrogen temperature for the duration of the experiment.…”

“…32 The profile shown in Figure 5.5a was acquired using a 40 keV C 60 + source, operated at glancing incidence angles of 73 • with respect to the surface normal, and the sample was maintained at liquid nitrogen temperature for the duration of the experiment. A schematic view of the known 3D molecular 33 Reproduced from Zheng et al, 33 with permission from ACS publications. structure of the film is depicted in Figure 5.5b.…”