Cluster Secondary Ion Mass Spectrometry: Principles and Applications

Explores the impact of the latest breakthroughs in clusterSIMS technology

Cluster secondary ion mass spectrometry (SIMS) is a high spatialresolution imaging mass spectrometry technique, which can be usedto characterize the three–dimensional chemical structure in complexorganic and molecular systems. It works by using a cluster ionsource to sputter desorb material from a solid sample surface.Prior to the advent of the cluster source, SIMS was severelylimited in its ability to characterize soft samples as a result ofdamage from the atomic source. Molecular samples were essentiallydestroyed during analysis, limiting the method′s sensitivity andprecluding compositional depth profiling. The use of new andemerging cluster ion beam technologies has all but eliminated theselimitations, enabling researchers to enter into new fields onceconsidered unattainable by the SIMS method.

With contributions from leading mass spectrometry researchersaround the world, Cluster Secondary Ion Mass Spectrometry:Principles and Applications describes the latest breakthroughsin instrumentation, and addresses best practices in cluster SIMSanalysis. It serves as a compendium of knowledge on organic andpolymeric surface and in–depth characterization using cluster ionbeams. It covers topics ranging from the fundamentals and theory ofcluster SIMS, to the important chemistries behind the success ofthe technique, as well as the wide–ranging applications of thetechnology. Examples of subjects covered include:

Cluster SIMS theory and modeling Cluster ion source types and performance expectations Cluster ion beams for surface analysis experiments Molecular depth profiling and 3–D analysis with cluster ionbeams Specialty applications ranging from biological samples analysisto semiconductors/metals analysis Future challenges and prospects for cluster SIMS

This book is intended to benefit any scientist, ranging frombeginning to advanced in level, with plenty of figures to helpbetter understand complex concepts and processes. In addition, eachchapter ends with a detailed reference set to the primaryliterature, facilitating further research into individual topicswhere desired. Cluster Secondary Ion Mass Spectrometry:Principles and Applications is a must–have read for anyresearcher in the surface analysis and/or imaging mass spectrometryfields.

Contributors xi

About the Editor xiii

1 AN INTRODUCTION TO CLUSTER SECONDARY ION MASS SPECTROMETRY(CLUSTER SIMS) 1
Christine M. Mahoney and Greg Gillen

1.1 Secondary Ion Mass Spectrometry in a Nutshell 2

1.1.1 SIMS Imaging 4

1.1.2 SIMS Depth Profiling 4

1.2 Basic Cluster SIMS Theory 5

1.3 Cluster SIMS: An Early History 6

1.3.1 Nonlinear Sputter Yield Enhancements 6

1.3.2 Molecular Depth Profiling 7

1.4 Recent Developments 8

1.5 About this Book 9

Acknowledgment 11

References 11

2 CLUSTER SIMS OF ORGANIC MATERIALS: THEORETICAL INSIGHTS13
Arnaud Delcorte, Oscar A. Restrepo, and BartlomiejCzerwinski

2.1 Introduction 13

2.2 Molecular Dynamics Simulations of Sputtering with Clusters15

2.2.1 The Cluster Effect 15

2.2.2 Computer Simulations and the Molecular Dynamics Experiment 18

2.2.3 Light and Heavy Element Clusters, and the Importance ofMass Matching 20

2.2.4 Structural Effects in Organic Materials 21

2.2.4.1 Amorphous Molecular Solids and Polymers 21

2.2.4.2 Organic Crystals 26

2.2.4.3 Thin Organic Layers on Metal Substrates 28

2.2.4.4 Hybrid Metal Organic Samples 32

2.2.5 Induced Chemistry 34

2.2.6 Multiple Hits and Depth Profiling 36

2.2.7 From Small Polyatomic Projectiles to Massive Clusters38

2.2.7.1 Light–Element Clusters 38

2.2.7.2 Large Argon Clusters 41

2.2.7.3 Massive Gold Clusters 45

2.3 Other Models 46

2.3.1 Analytical Models: From Linear Collision Cascades to FluidDynamics 46

2.3.2 Recent Developments and Hybrid Approaches 47

2.4 Conclusions 50

Acknowledgments 51

References 51

3 ION SOURCES USED FOR SECONDARY ION MASS SPECTROMETRY57
Albert J. Fahey

3.1 Introduction 57

3.2 Research Needs that have Influenced the Development ofPrimary Ion Sources for Sputtering 58

3.3 Functional Aspects of Various Ion Sources 59

3.3.1 Energy Spread in the Beam 59

3.3.2 Point–Source Ionization 60

3.3.3 Stable Emission 60

3.3.4 Ion Reactivity 60

3.3.5 Source Lifetime 60

3.3.6 Penetration Depth and Surface Energy Spread of theProjectile 61

3.4 Atomic Ion Sources 61

3.4.1 Field Emission 61

3.4.2 Radio Frequency (RF) Ionization 62

3.4.3 Electron Impact 63

3.4.4 Thermal Ionization 64

3.4.5 DC–Glow Discharge 65

3.4.6 Sputtering 66

3.5 Molecular Ion Sources 66

3.5.1 Field Emission 66

3.5.2 Radio Frequency Discharge 67

3.5.3 Electron Impact 68

3.5.4 DC–Glow Discharge 69

3.5.5 Sputtering 69

3.6 Cluster Ion Sources 70

3.6.1 Jets and Electron Impact (Massive Gas Clusters) 71

3.6.2 Field Emission 72

3.7 Summary 73

References 74

4 SURFACE ANALYSIS OF ORGANIC MATERIALS WITH POLYATOMICPRIMARY ION SOURCES 77
Christine M. Mahoney

4.1 Introduction 77

4.2 Cluster Sources in Static SIMS 78

4.2.1 A Brief Introduction to Static SIMS 78

4.2.2 Analysis beyond the Static Limit 79

4.2.3 Increased Ion Yields 80

4.2.4 Decreased Charging 81

4.2.5 Surface Cleaning 82

4.3 Experimental Considerations 83

4.3.1 When to Employ Cluster Sources as Opposed to AtomicSources 83

4.3.2 Type of Cluster Source Used 84

4.3.2.1 Liquid Metal Ion Gun (LMIG) 84

4.3.2.2 C + 60 for Mass Spectral Analysis and ImagingApplications 85

4.3.2.3 The Gas Cluster Ion Beam (GCIB) 86

4.3.2.4 Au 4+ 400 86

4.3.2.5 Other Sources 88

4.3.3 Cluster Size Considerations 88

4.3.4 Beam Energy 90

4.3.5 Sample Temperature 92

4.3.6 Matrix–Enhanced and Metal–Assisted Cluster SIMS 92

4.3.7 Matrix Effects 95

4.3.8 Other Important Factors 96

4.4 Data Analysis Methods 96

4.4.1 Principal Components Analysis 96

4.4.1.1 Basic Principles of PCA 97

4.4.1.2 Examples of PCA in the Literature 98

4.4.2 Gentle SIMS (G–SIMS) 101

4.5 Other Relevant Surface Mass–Spectrometry–Based Methods101

4.5.1 Desorption Electrospray Ionization (DESI) 103

4.5.2 Plasma Desorption Ionization Methods 105

4.5.3 Electrospray Droplet Impact Source for SIMS 107

4.6 Advanced Mass Spectrometers for SIMS 108

4.7 Conclusions 109

Appendix A: Useful Lateral Resolution 110

References 110

5 MOLECULAR DEPTH PROFILING WITH CLUSTER ION BEAMS117
Christine M. Mahoney and Andreas Wucher

5.1 Introduction 117

5.2 Historical Perspectives 120

5.3 Depth Profiling in Heterogeneous Systems 123

5.3.1 Introduction 123

5.3.2 Quantitative Depth Profiling 125

5.3.3 Reconstruction of 3D Images 127

5.3.4 Matrix Effects in Heterogeneous Systems 128

5.4 Erosion Dynamics Model of Molecular Sputter Depth Profiling130

5.4.1 Parent Molecule Dynamics 131

5.4.2 Constant Erosion Rate 134

5.4.3 Fluence–Dependent Erosion Rate 136

5.4.4 Using Mass Spectrometric Signal Decay to Measure DamageParameters 138

5.4.5 Surface Transients 141

5.4.6 Fragment Dynamics 141

5.4.7 Conclusions 145

5.5 The Chemistry of Atomic Ion Beam Irradiation in OrganicMaterials 146

5.5.1 Introduction 146

5.5.2 Understanding the Basics of Ion Irradiation Effects inMolecular Solids 146

5.5.3 Ion Beam Irradiation and the Gel Point 147

5.5.4 The Chemistry of Cluster Ion Beams 150

5.5.5 Chemical Structure Changes and Corresponding Changes inDepth Profile Shapes 152

5.6 Optimization of Experimental Parameters for Organic DepthProfiling 156

5.6.1 Introduction 156

5.6.2 Organic Delta Layers for Optimization of ExperimentalParameters 157

5.6.3 Sample Temperature 159

5.6.4 Understanding the Role of Beam Energy During Organic DepthProfiling 167

5.6.5 Optimization of Incidence Angle 171

5.6.6 Effect of Sample Rotation 174

5.6.7 Ion Source Selection 178

5.6.7.1 SF + 5 and Other Small Cluster Ions 178

5.6.7.2 C n+ 60 and Similar Carbon Cluster Sources 179

5.6.7.3 The Gas Cluster Ion Beam (GCIB) 180

5.6.7.4 Low Energy Reactive Ion Beams 188

5.6.7.5 Electrospray Droplet Impact (EDI) Source for SIMS189

5.6.7.6 Liquid Metal Ion Gun Clusters (Bi + 3 and Au + 3 )193

5.6.8 C + 60 /Ar+ Co–sputtering 195

5.6.9 Chamber Backfilling with a Free Radical Inhibitor Gas197

5.6.10 Other Considerations for Organic Depth ProfilingExperiments 197

5.6.11 Molecular Depth Profiling: Novel Approaches and Methods198

5.7 Conclusions 198

References 200

6 THREE–DIMENSIONAL IMAGING WITH CLUSTER ION BEAMS207
Andreas Wucher, Gregory L. Fisher, and Christine M.Mahoney

6.1 Introduction 207

6.2 General Strategies 210

6.2.1 Three–Dimensional Sputter Depth Profiling 210

6.2.2 Wedge Beveling 216

6.2.3 Physical Cross Sectioning 217

6.2.4 FIB–ToF Tomography 219

6.3 Important Considerations for Accurate 3D Representation ofData 225

6.3.1 Beam Rastering Techniques 225

6.3.2 Geometry Effects 226

6.3.3 Depth Scale Calibration 228

6.4 Three–Dimensional Image Reconstruction 233

6.5 Damage and Altered Layer Depth 238

6.6 Biological Samples 242

6.7 Conclusions 243

References 244

7 CLUSTER SECONDARY ION MASS SPECTROMETRY (SIMS) FORSEMICONDUCTOR AND METALS DEPTH PROFILING 247
Greg Gillen and Joe Bennett

7.1 Introduction 247

7.2 Primary Particle Substrate Interactions 248

7.2.1 Collisional Mixing and Depth Resolution 248

7.2.2 Transient Effects 249

7.2.3 Sputter–Induced Roughening 251

7.3 Possible Improvements in SIMS Depth Profiling The Useof Cluster Primary Ion Beams 253

7.4 Development of Cluster SIMS for Depth Profiling Analysis255

7.4.1 CF + 3 Primary Ion Beams 255

7.4.2 NO + 2 and O + 3 Primary Ion Beams 256

7.4.3 SF + 5 Polyatomic Primary Ion Beams 257

7.4.4 CSC 6 and C 8 Depth Profiling 258

7.4.5 Os3(CO)12 and Ir4(CO)12 Primary Ion Beams 262

7.4.6 C + 60 Primary Ion Beams 263

7.4.7 Massive Gaseous Cluster Ion Beams 265

7.5 Conclusions and Future Prospects 266

References 266

8 CLUSTER TOF–SIMS IMAGING AND THE CHARACTERIZATION OFBIOLOGICAL MATERIALS 269
John Vickerman and Nick Winograd

8.1 Introduction 269

8.2 The Capabilities of TOF–SIMS for Biological Analysis 270

8.3 New Hybrid TOF–SIMS Instruments 270

8.3.1 Introduction 270

8.3.2 Benefits of New DC Beam Technologies 271

8.4 Challenges in the Use of TOF–SIMS for Biological Analysis273

8.4.1 Sample Handling of Biological Samples for Analysis inVacuum 273

8.4.2 Analysis is Limited to Small to Medium Size Molecules274

8.4.3 Ion Yields Limit Useful Spatial Resolution for MolecularAnalysis to not Much Better than 1 m 275

8.4.4 Matrix Effects Inhibit Application in Discovery Mode andGreatly Complicates Quantification 275

8.4.5 The Complexity of Biological Systems can Result in DataSets that Need Multivariate Analysis (MVA) to Unravel 276

8.5 Examples of Biological Studies Using Cluster–TOF–SIMS276

8.5.1 Analysis of Tissue 277

8.5.2 Drug Location in Tissue 285

8.5.3 Microbial Mat Surface and Subsurface Analysis inStreptomyces 289

8.5.4 Cells 291

8.5.5 Depth Scale Measurement 302

8.5.6 High Throughput Biomaterials Characterization 306

8.6 Final Thoughts and Future Directions 310

Acknowledgments 310

References 310

9 FUTURE CHALLENGES AND PROSPECTS OF CLUSTER SIMS313
Peter Williams and Christine M. Mahoney

9.1 Introduction 313

9.2 The Cluster Niche 314

9.3 Cluster Types 314

9.4 The Challenge of Massive Molecular Ion Ejection 315

9.4.1 Comparing with MALDI: The Gold Standard 316

9.4.2 Particle Impact Techniques 317

9.5 Ionization 318

9.5.1 Preformed Ions 319

9.5.2 Radical Ions and Ion Fragments 319

9.5.3 Ionization Processes for Massive Clusters 320

9.6 Matrix Effects and Challenges in Quantitative Analysis321

9.7 SIMS Instrumentation 322

9.7.1 Massive Cluster Ion Source Technology 323

9.8 Prospects for Biological Imaging 324

9.9 Conclusions 325

References 326

Index 329

Christine M. Mahoney, PhD, is a recognized expert and leader in the field of Secondary Ion Mass Spectrometry (SIMS). Throughout her career, she has focused primarily on the application of SIMS to molecular targets, and has played a significant role in the development of cluster SIMS for polymer depth profiling applications. She received her PhD in analytical chemistry from SUNY Buffalo in 1993. She spent the following eight years at the National Institute of Standards and Technology (NIST), where much of her molecular depth profiling work was performed. Christine is currently employed as a senior research scientist at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL), where she continues to lead research in the field of SIMS.