H. Oechsner

Sputtering—a review of some recent experimental and theoretical aspects

After a brief outline of the present sputtering theory for a random solid, recent results of the sputtering yieldS for polycrystalline targets are discussed, in particular in view of the influence of the projectile mass and the bombarding angle. The angle dependence ofS at low bombarding energies, and results on the angular distribution of sputtered particles for oblique ion incidence point out necessary modifications of present sputtering theories with respect to the anisotropy of the collision cascades in the solid and the influence of the target surface. The energy distribution of the neutral particles ejected along the target normals is related to the theoretically predictedE−2-distribution of low energy recoils in the Recent mass spectrometric studies of postionized sputtered neutrals are discussed in view of the formation of sputtered molecules and the application of sputtered neutral mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface effects on non-elementary sputtering targets, and the protracted removal of foreign atoms from a matrix.

Thin film and depth profile analysis

1. Introduction..- 1.1 Requirements for Thin Film and In-Depth Analysis.- 1.2 Object and Outline of the Book.- References.- 2. The Application of Beam and Diffraction Techniques to Thin Film and Surface Micro-Analysis..- 2.1 Methods to Determine Chemical Structures in Material Research.- 2.2 Selected Analytical Features Used to Determine Chemical Structures.- 2.2.1 Depth Profiling.- a) Destructive Depth Profiling.- b) Nondestructive Methods for Depth and Thin Film Analysis.- 2.2.2 Microspot Analysis and Element Imaging.- 2.3 Determining Physical Structures in Material Research.- 2.3.1 X-Ray Diffraction.- 2.3.2 X-Ray Double Crystal Diffraction.- 2.3.3 Ultrasonic (Acoustic) Microscopy.- 2.4 Application of Different Microanalytical Techniques to Specific Analytical Problems.- 2.4.1 AES and TEM-EDX in Interface Analysis of MnZn Ferrites.- 2.4.2 Interfaces of SrTiO3 Boundary Layer Capacitor Material Studies by TEM and Selected-Area EDX.- 2.4.3 Analysis of GaAlAs with SIMS, X-Ray Diffraction and AES.- 2.5 Future Prospects.- References.- 3. Depth Profile and Interface Analysis of Thin Films by AES and XPS.- 3.1 Quantification from First Principles.- 3.2 Initial Transient Layer.- 3.3 Steady-State Region.- 3.4 Film-Substrate Interface.- References.- 4. Secondary Neutral Mass Spectrometry (SNMS) and Its Application to Depth Profile and Interface Analysis..- 4.1 Background.- 4.2 Experimental Method.- 4.2.1 Related Techniques.- 4.2.2 Performance of SNMS.- a) The Postionizing Method.- b) Operation Modes of SNMS.- 4.3 Quantification of SNMS.- 4.3.1 Quantification for Atomic Sputtering.- 4.3.2 Quantification Using Molecular SNMS Signals.- 4.3.3 Sensitivity of SNMS.- 4.4 Applications of SNMS to Depth Profile Analysis.- 4.4.1 General Considerations.- 4.4.2 Examples of Depth Profiling by SNMS.- 4.5 Concluding Remarks.- References.- 5. In-Situ Laser Measurements of Sputter Rates During SIMS/AES In-Depth Profiling..- 5.1 Background.- 5.2 Principles of Laser Technique.- 5.2.1 Laser Optical Arrangement.- 5.2.2 Phase and Reflectance Measurement.- 5.2.3 Fundamentals of Sputter-Rate Determination.- 5.3 Experiments.- 5.4 Results and Discussion.- 5.4.1 Signal Artefacts.- 5.4.2 Phase and Reflectance Measurements During Sputtering.- a) Sputtering of Silicon Surfaces.- b) Doped Silicon.- c) Metal Film Material.- d) Transparent Material.- e) Opaque and Transparent Multilayers.- 5.5 Conclusion.- 5.A. Appendix.- A.1. Opaque Material.- A.2. Transparent Material.- References.- 6. Physical Limitations to Sputter Profiling at Interfaces - Model Experiments with Ge/Si Using KARMA..- 6.1 Background.- 6.1.1 General Problems Encountered in Sputter Profiling.- 6.1.2 Requirements for a Model Experiment.- 6.2 Experimental Approach.- 6.2.1 KARMA.- 6.2.2 Sample Preparation.- 6.3 Conversion of Raw Sputter Profiles into Depth Profiles.- 6.3.1 Establishing the Depth Scale.- 6.3.2 Escape-Depth Correction.- 6.3.3 Self-Consistent Determination of Effective Mean Free Paths.- 6.4 Depth Profiles of the Ge/Si Interface.- 6.4.1 Asymmetry of Depth Profiles.- 6.4.2 Broadening as a Function of Ion Mass and Energy.- 6.5 Dose Effects and Preferential Sputtering.- 6.5.1 Dose Effects.- 6.5.2 Preferential Sputtering.- 6.6 Depth Resolution in Sputter Profiling.- 6.6.1 Depth Resolution Limits.- 6.7 Summary and Outlook.- References.- 7. Depth Resolution and Quantitative Evaluation of AES Sputtering Profiles.- 7.1 Background.- 7.2 Calibration of the Depth Scale.- 7.3 Calibration of the Concentration Scale.- 7.4 Depth Resolution in Sputter Profiling.- 7.5 Determination of the Resolution function.- 7.5.1 Definition of Depth Resolution.- 7.5.2 Experimental Determination of Depth Resolution.- 7.5.3 Model Descriptions of Depth Resolution.- 7.6 Deconvolution Procedures.- 7.7 Conclusion.- References.- 8. The Theory of Recoil Mixing in Solids.- 8.1 Background.- 8.1.1 Nomenclature.- 8.2 Review of Recoil Mixing Models.- 8.2.1 Primary Recoil Implantation and Mixing.- 8.2.2 Cascade Mixing.- a) Random-Walk Models.- b) Transport Theory Approach.- c) Miscellaneous Approaches.- 8.3 General Formulation of Atomic Relocation Phenomena.- 8.3.1 Target Description.- a) Unbounded Total Density N(?,x).- b) Total Density Bounded to N(x) = No.- 8.3.2 Description of Atomic Relocation.- 8.3.3 Balance Equation for Atomic Relocation.- a) The Diffusion Approximation.- 8.4 Solutions to the Specific Mixing Models.- 8.4.1 Thermal Mixing and Thermal Diffusion.- 8.4.2 Recoil Mixing.- a) Cascade Mixing, Diffusion Approaches.- b) Cascade Mixing, Forthright Solutions.- 8.5 Summary and Outlook.- 8.6 List of Symbols.- References.- Additional References with Titles.

Energieverteilungen bei der Festkörperzerstäubung durch Ionenbeschuß

The energy distributions of the neutral particles sputtered from polycrystalline targets of Al, Ti, Ni, Cu and Ag by normally incident Ar+-ions in the 1 keV region have been determined for ejection energies below 20 eV in a direction close to the normal to the target surface. The experimental method employed is strongly connected to the characteristic properties of a low pressure electrodeless hf plasma used as an effective ion source as well as an ionizing medium for the ejected target atoms. The resulting curves always show a maximum at most probable ejection energies between 1 and 5 eV, being approximately half of the surface binding energy. These curves are converted to the corresponding energy distributions for the recoil atoms within the target by an energy dependent factor. It is found that the energy distribution within the bombarded solid decreases monotonically with the inverse square of the energy of the recoils.

Sputtered neutral silver clusters up to Ag18

Abstract Neutral silver clusters Agn with n ≤ 18 sputtered from a polycrystalline silver surface under bombardment with 5 keV Ar+ ions were detected by means of energy resolved time-of-flight mass spectrometry. Postionization of the ejected neutral particles was performed by an excimer laser operated with ArF (hv = 6.4 eV). The kinetic energy of the detected particles was determined from their flight time interval between the sputter pulse and the ionizing laser pulse. By saturating the ionization of the sputtered neutrals, relative cluster sputtering yields Y(Agn) were determined as a function of the cluster size. For n ≤ 6, the value of Y(Agn) is found to drop by approximately one order of magnitude if n is increased by one atom, whereas for cluster sizes larger than six atoms the decrease becomes significantly less pronounced and an odd-even intensity alternation similar to that observed for sputtered ion clusters becomes visible. The kinetic energy distributions of silver atoms, dimers, trimers, tetramers and pentamers determined from the time-of-flight method all look very similar and, in particular, show essentially the same high-energy dependence for sputtered atoms and molecules. In comparison, corresponding data taken by electron impact ionization with subsequent electrostatic energy analysis exhibit a significantly different asymptotic behaviour, the high energy slope of the energy spectra becoming steeper as the cluster size increases. From a detailed analysis of the measured signals, it is concluded that due to the concurrence of single- and multiphoton absorption processes the laser postionization results may be corrupted by photo-frag- mentation.

ERBIUM LUMINESCENCE IN POROUS SILICON DOPED FROM SPIN-ON FILMS

Erbium photoluminescence at room temperature and at 77 K has been observed from porous silicon doped with erbium from a spin‐on silica gel film. Erbium incorporation into silicon dioxide at the surface of porous silicon and rapid thermal processing at temperatures higher than 1223 K were found to be a necessary prerequisite for erbium‐related luminescence in porous silicon. No erbium diffusion into monocrystalline silicon from the spin‐on films was observed. The depth‐dependent erbium concentration in the bulk of porous silicon was determined by secondary‐neutral‐ and secondary‐ion‐mass spectrometry depth profiling. The laterally resolved erbium distribution in the porous silicon was derived from energy‐dispersive x‐ray analysis. Possible mechanisms of erbium‐related luminescence in porous silicon are discussed.

Sputtering of Ta2O5 by Ar+ ions at energies below 1 keV

The sputtering of anodically formed Ta2O5 layers of about 3500 A thickness has been studied by Sputtered Neutral Mass Spectroscopy (SNMS). For perpendicular bombardment with Ar+ ions up to 900 eV the flux of ejected neutral particles is found to consist almost exclusively of metal atoms Ta and Oxide specific molecules TaO and TaO2 with intensity ratios in the order 1 : 1 : 10−1. From depth profiling measurements with SNMS, and from the intensity ratios in the SNMS spectra the total sputtering yield of Ta2O5 and the partial yields of Ta, TaO and of oxygen have been determined for normally incident Ar+ ions of 100 to 600 eV. After an initial increase the TaO intensity in the SNMS spectra remains constant during the sputter removal of the whole layer. A simple model is derived by which the preferred emission of TaO molecules, and the initial increase of the TaO intensity is referred to ion induced variations of the surface stoichiometry of Ta2O5. For optimum TaO production the model predicts equal atomic surface concentrations of Ta and O.

Phase separation in magnetron sputtered superhard BCN thin films

Abstract The similar crystalline structure of diamond and cubic boron nitride suggests the synthesis of superhard thin films containing boron, carbon and nitrogen. BCN thin films have been prepared by reactive r.f.-magnetron sputtering of a hexagonal boron nitride target in an argon/acetylene atmosphere of a constant pressure of 2 × 10−2 mbar. A deposition temperature of 600 °C leads to nearly hydrogen-free BCN films. At a flow ratio of 0.05% C2H2/Ar, the composition of the deposited BCN films is close to B5CN3 as a result of energy dispersive X-ray (EDX) and elastic recoil detection (ERD) measurements. By applying a r.f.-substrate bias, the ion energy has been varied at a constant ratio of ions to film-forming particles of 4. The BCN films show a maximum in stress at an ion energy of 110 eV due to knock-on subplantation of argon ions and a phase separation into carbon, boron and cubic boron nitride regions as a result of X-ray diffraction investigations. Auger electron- and infra-red spectroscopy.

Sputtered neutral mass spectrometry (SNMS) as a tool for chemical surface analysis and depth profiling

An experimental system for mass spectrometry of supttered neutral particles involving a hf plasma operated in Ar at several 10−4 Torr is described. The potentialities of the method for quantitative surface analysis are reasoned. Depth profiling by sputtered neutral mass spectrometry is demonstrated for anodic oxide layers on Nb and Ta.

Mass spectrometry of neutral molecules sputtered from polycrystalline metals by Ar+-ions of 100–1000 eV

The flux of neutral particles sputtered from clean polycrystalline targets of 11 metals has been studied by mass spectrometry after being postionized in a low pressure hf plasma. Besides atoms Me1, postionized metal molecules Me2 and Me3 have been detected. The energyE0 of the bombarding Ar+ ions has been varied between 100 and 1000 eV. Considering the different plasma influences, an attempt is made to obtain the initial composition of the beam of sputtered neutrals from the ratiosR21 andR31 of postionized molecules Me2 and Me3 to postionized atoms, Me1, respectively. The influence of the bombarding energyE0 onR21 has been measured for all target elements, onR31 for Pd and Ag. The yields of Me2 atE0=1 keV approximately appear to be proportional to the square of the total sputtering yieldS for the different target materials. This behaviour and the energy dependence ofR21 andR31 are discussed in view of a possible molecule formation process in sputtering.

SIMS depth profile analysis using MCs+ molecular ions

SummaryThe use of Cs+ primary ions in conjunction with the detection of MCs+ molecular ions (where M is the element to be monitored) in SIMS depth profiling is shown to be an efficient method of minimizing the variations of ion yields with sample composition, e.g., at the interface of multilayer structures. Depth profiles of several such samples demonstrate that MCs+ intensities follow closely the concentrations of the respective elements, providing the possibility of a (semi)quantitative analysis of major components by means of secondary ion mass spectrometry. As indicated by the similarity of their energy distribution data, the formation and emission process of MCs+ molecules seems coupled to that of Cs+ ions.