A. Wucher - Academia.edu (original) (raw)
Papers by A. Wucher
Materials Science Forum, 1998
We give a short overview of recent analytical techniques for compositional surface analysis and t... more We give a short overview of recent analytical techniques for compositional surface analysis and the determination of concentration depth profiles. More specifically, the principle, the instrumentation and the performance of the routinely used electron spectroscopic and mass spectrometric methods, namely Photo-and Auger Electron Spectroscopy PES and AES, resp. Secondary Ion and Secondary Neutral Mass Spectrometry SIMS and SNMS, are described. The application of these techniques to the analysis of thin film structures is demonstrated by corresponding practical examples. The potentialities of the different techniques are specifically addressed with regard to the quantitation of the measured data, detection limits for trace analysis as well as the spatial resolution both in directions parallel and perpendicular to the sample surface.
Thin Solid Films, 1989
ABSTRACT
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015
ABSTRACT We present a model to describe the kinetic internal and external electron emission from ... more ABSTRACT We present a model to describe the kinetic internal and external electron emission from an ion bombarded metal target. The model is based upon a molecular dynamics treatment of the nuclear degree of freedom, the electronic system is assumed as a quasi-free electron gas characterized by its Fermi energy, electron temperature and a characteristic attenuation length.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1999
ABSTRACT
Physical Review B, 2003
A model is presented for the formation of excited Ag* (4d95s2) atoms during sputtering of Ag meta... more A model is presented for the formation of excited Ag* (4d95s2) atoms during sputtering of Ag metal by energetic Ar+ ions. The essential part of the formation process is the slow diffusion of 4d holes in the collision cascade from the sites of violent Ag-Ag collisions to the emitted Ag atoms. A computer simulation of Ag cascades and of the 4d-hole transport allows us to quantify the model and to describe all characteristic features of the available experimental data, in particular the fact that the sputtered Ag* atoms exhibit a narrower kinetic energy distribution than those ejected in the electronic ground state.
Physical Review B, 2002
ABSTRACT
New Journal of Physics, 2008
Page 1. On the internal energy of sputtered clusters This article has been downloaded from IOPsci... more Page 1. On the internal energy of sputtered clusters This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 New J. Phys. 10 103007 (http://iopscience.iop.org/1367-2630/10/10/103007) ...
The Journal of Physical Chemistry C, 2008
Molecular depth profiling of organic overlayers was performed using a mass selected C(60) ion bea... more 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.
The Journal of Physical Chemistry C, 2010
Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate a... more Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate are obtained using a 40 keV C 60 + cluster ion beam in conjunction with time-of-flight secondary ion mass spectrometric (ToF-SIMS) detection. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. Moreover, there is a 50% increase in fragment ion signal relative to steady state values 25 nm before reaching the guanine/Ag interface as a result of interfacial chemical damage accumulation. An altered layer thickness of 20 nm is observed as a consequence of ion beam induced chemical mixing. In general, we show that the neutral component of a molecular depth profile using the strong-field photoionization technique can be used to elucidate the effects of variations in ionization probability on the yield of molecular ions as well as to aid in obtaining accurate information about depth dependent chemical composition that cannot be extracted from ToF-SIMS data alone.
The Journal of Physical Chemistry C, 2010
ABSTRACT In this theoretical investigation, we combine the results of molecular dynamics (MD) sim... more ABSTRACT In this theoretical investigation, we combine the results of molecular dynamics (MD) simulations with a simple statistical sputtering model (SSM) in order to understand the factors limiting the optimum depth resolution achievable in sputter depth profiling experiments. The advantage of the SSM model is that it can be used to extrapolate the MD simulations toward the regime of high projectile fluence. First, a recently developed scheme [Russo, M. F., Jr.; Postawa, Z.; Garrison, B. J., J. Phys. Chem. C 2009, 113, 3270] is adapted to calculate 200 cumulative impacts of 20-keV C60 bombardment on a Si substrate, which corresponds to a projectile fluence of 3.5 × 1013 cm−2. The following results are studied as a function of fluence: the development of surface topography, the amount and depth of origin of sputtered material, and the relocation of substrate particles, which produces interlayer mixing. Data from the MD simulations are used as input parameters in the SSM, which is able to reproduce results consistent with the MD simulations. A major finding from these studies is that the statistical nature of the sputtering process has a significant effect on the achievable depth resolution. The optimum delta layer response width for Si is estimated to be about 3 nm, while corresponding values of the order of 10 nm are predicted for molecular systems.
The Journal of Physical Chemistry C, 2012
The development of cluster beam sources for secondary ion mass spectrometry (SIMS) and the realiz... more The development of cluster beam sources for secondary ion mass spectrometry (SIMS) and the realization that they can be used for molecular depth profiling 1 has stimulated the interest in finding a way of converting the information obtained from simulations of individual impacts of the clusters into the actual depth profiles. Modeling of single impacts by molecular dynamics (MD) simulations requires a modest sized sample of several hundred thousand particles. Modeling of depth profiling, that is, dynamic SIMS, increases the size of the sample by at least an order of magnitude. Consequently, modeling even a single impact on the large sample requires a very long time to complete. In addition, as hundreds to thousands of sequential impacts are needed to calculate depth profiles, the simulations could last for years. Recently, however, a "divide and conquer" approach was developed to make dynamic SIMS MD simulations possible. 2 It is now feasible to model up to fluences approaching 10 14 impacts/cm 2 in several months of computer time. 3À6 The amount of material removed with these heroic calculations, however, is still only less than a handful of monolayer equivalents, which is far from the amount of material removed in a typical depth profiling experiment. Thus, it is not possible to extract a depth profile or an interface width, key experimental metrics of depth profiling quality. The issue is how to take the wealth of simulation data that should contain all the physics relevant for the depth profile and create the appropriate depth profile.
Surface Science, 1992
The emission of neutral and positively charged silver clusters during sputtering of a polycrystal... more The emission of neutral and positively charged silver clusters during sputtering of a polycrystalline silver target by 5 keV Ar + ion bombardment has been studied and the Sputter ejected silver flux has been characterized.
Surface Science, 1999
Sputtering from an oxygen-dosed calcium surface under 4 keV Ar+ ion bombardment has been studied.... more Sputtering from an oxygen-dosed calcium surface under 4 keV Ar+ ion bombardment has been studied. Both the secondary neutral and positive secondary ion fluxes have been analyzed using time-of-flight mass spectrometry to characterize the abundance yields of atoms and molecules. The neutrals were probed using laser post-ionization by an ArF excimer laser. A large number of molecules of the form Ca m O n (m=1-5 and n=0-5) were observed in both the neutral and positive ion channels. Measurements to characterize the sputter yield and abundance distributions of these species were performed at a series of oxygen doses ranging from 0.03 to 300 L. Energy distributions for the secondary neutral species have also been measured as a function of oxygen coverage. The results for neutral atomic calcium agree well with previous measurements. A simple model to describe mono-atomic oxide formation was used to relate oxygen dose to oxygen surface concentration. It was shown that sputter-cleaning the calcium surface could reduce the oxygen surface atom fraction concentration to about 0.01 and that at this level the sputter yield and abundance distribution for Ca clusters was essentially the same as for an oxygen-free calcium surface. The model was extended to larger molecules and found to describe their formation in the Ca-O system quite well. The model offers some insights into the unexpected result that substantial numbers of Ca 2 dimers form during the sputtering process.
Surface and Interface Analysis, 2004
We have investigated the merits of fullerene cluster ions as projectiles in time-of-flight second... more We have investigated the merits of fullerene cluster ions as projectiles in time-of-flight secondary neutral mass spectrometry (ToF-SNMS) sputter depth profiling of an Ni : Cr multilayer sample similar to the corresponding NIST depth profiling standard. It is shown that sputter erosion under bombardment with C 60 + ions of kinetic energies between 10 and 20 keV provides good depth resolution corresponding to interface widths of several nanometres. This depth resolution is maintained during the complete removal of the multilayer stack with a total thickness of 500 nm. This finding is in contrast to the case where atomic Ga + projectile ions of comparable kinetic energy are used, demonstrating the unique features of cluster projectiles in sputter depth profiling.
Surface and Interface Analysis, 2013
The delta layer depth response predicted by a simple statistical sputtering model is compared wit... more The delta layer depth response predicted by a simple statistical sputtering model is compared with molecular sputter depth profile data obtained on Langmuir-Blodgett delta layer systems. All input parameters of the statistical sputtering model are determined from low-fluence molecular dynamics simulations performed for 20-keV C 60 cluster bombardment of silicon, making the model de facto parameter free. It is found that both calculated and measured depth response functions can be parametrised by the semiempirical Dowsett expression. The resulting parameters (leading and trailing edge slope, full-width at half-maximum) agree surprisingly well with those determined from the measured depth profiles.
Materials Science Forum, 1998
We give a short overview of recent analytical techniques for compositional surface analysis and t... more We give a short overview of recent analytical techniques for compositional surface analysis and the determination of concentration depth profiles. More specifically, the principle, the instrumentation and the performance of the routinely used electron spectroscopic and mass spectrometric methods, namely Photo-and Auger Electron Spectroscopy PES and AES, resp. Secondary Ion and Secondary Neutral Mass Spectrometry SIMS and SNMS, are described. The application of these techniques to the analysis of thin film structures is demonstrated by corresponding practical examples. The potentialities of the different techniques are specifically addressed with regard to the quantitation of the measured data, detection limits for trace analysis as well as the spatial resolution both in directions parallel and perpendicular to the sample surface.
Thin Solid Films, 1989
ABSTRACT
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015
ABSTRACT We present a model to describe the kinetic internal and external electron emission from ... more ABSTRACT We present a model to describe the kinetic internal and external electron emission from an ion bombarded metal target. The model is based upon a molecular dynamics treatment of the nuclear degree of freedom, the electronic system is assumed as a quasi-free electron gas characterized by its Fermi energy, electron temperature and a characteristic attenuation length.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1999
ABSTRACT
Physical Review B, 2003
A model is presented for the formation of excited Ag* (4d95s2) atoms during sputtering of Ag meta... more A model is presented for the formation of excited Ag* (4d95s2) atoms during sputtering of Ag metal by energetic Ar+ ions. The essential part of the formation process is the slow diffusion of 4d holes in the collision cascade from the sites of violent Ag-Ag collisions to the emitted Ag atoms. A computer simulation of Ag cascades and of the 4d-hole transport allows us to quantify the model and to describe all characteristic features of the available experimental data, in particular the fact that the sputtered Ag* atoms exhibit a narrower kinetic energy distribution than those ejected in the electronic ground state.
Physical Review B, 2002
ABSTRACT
New Journal of Physics, 2008
Page 1. On the internal energy of sputtered clusters This article has been downloaded from IOPsci... more Page 1. On the internal energy of sputtered clusters This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 New J. Phys. 10 103007 (http://iopscience.iop.org/1367-2630/10/10/103007) ...
The Journal of Physical Chemistry C, 2008
Molecular depth profiling of organic overlayers was performed using a mass selected C(60) ion bea... more 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.
The Journal of Physical Chemistry C, 2010
Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate a... more Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate are obtained using a 40 keV C 60 + cluster ion beam in conjunction with time-of-flight secondary ion mass spectrometric (ToF-SIMS) detection. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. Moreover, there is a 50% increase in fragment ion signal relative to steady state values 25 nm before reaching the guanine/Ag interface as a result of interfacial chemical damage accumulation. An altered layer thickness of 20 nm is observed as a consequence of ion beam induced chemical mixing. In general, we show that the neutral component of a molecular depth profile using the strong-field photoionization technique can be used to elucidate the effects of variations in ionization probability on the yield of molecular ions as well as to aid in obtaining accurate information about depth dependent chemical composition that cannot be extracted from ToF-SIMS data alone.
The Journal of Physical Chemistry C, 2010
ABSTRACT In this theoretical investigation, we combine the results of molecular dynamics (MD) sim... more ABSTRACT In this theoretical investigation, we combine the results of molecular dynamics (MD) simulations with a simple statistical sputtering model (SSM) in order to understand the factors limiting the optimum depth resolution achievable in sputter depth profiling experiments. The advantage of the SSM model is that it can be used to extrapolate the MD simulations toward the regime of high projectile fluence. First, a recently developed scheme [Russo, M. F., Jr.; Postawa, Z.; Garrison, B. J., J. Phys. Chem. C 2009, 113, 3270] is adapted to calculate 200 cumulative impacts of 20-keV C60 bombardment on a Si substrate, which corresponds to a projectile fluence of 3.5 × 1013 cm−2. The following results are studied as a function of fluence: the development of surface topography, the amount and depth of origin of sputtered material, and the relocation of substrate particles, which produces interlayer mixing. Data from the MD simulations are used as input parameters in the SSM, which is able to reproduce results consistent with the MD simulations. A major finding from these studies is that the statistical nature of the sputtering process has a significant effect on the achievable depth resolution. The optimum delta layer response width for Si is estimated to be about 3 nm, while corresponding values of the order of 10 nm are predicted for molecular systems.
The Journal of Physical Chemistry C, 2012
The development of cluster beam sources for secondary ion mass spectrometry (SIMS) and the realiz... more The development of cluster beam sources for secondary ion mass spectrometry (SIMS) and the realization that they can be used for molecular depth profiling 1 has stimulated the interest in finding a way of converting the information obtained from simulations of individual impacts of the clusters into the actual depth profiles. Modeling of single impacts by molecular dynamics (MD) simulations requires a modest sized sample of several hundred thousand particles. Modeling of depth profiling, that is, dynamic SIMS, increases the size of the sample by at least an order of magnitude. Consequently, modeling even a single impact on the large sample requires a very long time to complete. In addition, as hundreds to thousands of sequential impacts are needed to calculate depth profiles, the simulations could last for years. Recently, however, a "divide and conquer" approach was developed to make dynamic SIMS MD simulations possible. 2 It is now feasible to model up to fluences approaching 10 14 impacts/cm 2 in several months of computer time. 3À6 The amount of material removed with these heroic calculations, however, is still only less than a handful of monolayer equivalents, which is far from the amount of material removed in a typical depth profiling experiment. Thus, it is not possible to extract a depth profile or an interface width, key experimental metrics of depth profiling quality. The issue is how to take the wealth of simulation data that should contain all the physics relevant for the depth profile and create the appropriate depth profile.
Surface Science, 1992
The emission of neutral and positively charged silver clusters during sputtering of a polycrystal... more The emission of neutral and positively charged silver clusters during sputtering of a polycrystalline silver target by 5 keV Ar + ion bombardment has been studied and the Sputter ejected silver flux has been characterized.
Surface Science, 1999
Sputtering from an oxygen-dosed calcium surface under 4 keV Ar+ ion bombardment has been studied.... more Sputtering from an oxygen-dosed calcium surface under 4 keV Ar+ ion bombardment has been studied. Both the secondary neutral and positive secondary ion fluxes have been analyzed using time-of-flight mass spectrometry to characterize the abundance yields of atoms and molecules. The neutrals were probed using laser post-ionization by an ArF excimer laser. A large number of molecules of the form Ca m O n (m=1-5 and n=0-5) were observed in both the neutral and positive ion channels. Measurements to characterize the sputter yield and abundance distributions of these species were performed at a series of oxygen doses ranging from 0.03 to 300 L. Energy distributions for the secondary neutral species have also been measured as a function of oxygen coverage. The results for neutral atomic calcium agree well with previous measurements. A simple model to describe mono-atomic oxide formation was used to relate oxygen dose to oxygen surface concentration. It was shown that sputter-cleaning the calcium surface could reduce the oxygen surface atom fraction concentration to about 0.01 and that at this level the sputter yield and abundance distribution for Ca clusters was essentially the same as for an oxygen-free calcium surface. The model was extended to larger molecules and found to describe their formation in the Ca-O system quite well. The model offers some insights into the unexpected result that substantial numbers of Ca 2 dimers form during the sputtering process.
Surface and Interface Analysis, 2004
We have investigated the merits of fullerene cluster ions as projectiles in time-of-flight second... more We have investigated the merits of fullerene cluster ions as projectiles in time-of-flight secondary neutral mass spectrometry (ToF-SNMS) sputter depth profiling of an Ni : Cr multilayer sample similar to the corresponding NIST depth profiling standard. It is shown that sputter erosion under bombardment with C 60 + ions of kinetic energies between 10 and 20 keV provides good depth resolution corresponding to interface widths of several nanometres. This depth resolution is maintained during the complete removal of the multilayer stack with a total thickness of 500 nm. This finding is in contrast to the case where atomic Ga + projectile ions of comparable kinetic energy are used, demonstrating the unique features of cluster projectiles in sputter depth profiling.
Surface and Interface Analysis, 2013
The delta layer depth response predicted by a simple statistical sputtering model is compared wit... more The delta layer depth response predicted by a simple statistical sputtering model is compared with molecular sputter depth profile data obtained on Langmuir-Blodgett delta layer systems. All input parameters of the statistical sputtering model are determined from low-fluence molecular dynamics simulations performed for 20-keV C 60 cluster bombardment of silicon, making the model de facto parameter free. It is found that both calculated and measured depth response functions can be parametrised by the semiempirical Dowsett expression. The resulting parameters (leading and trailing edge slope, full-width at half-maximum) agree surprisingly well with those determined from the measured depth profiles.