Secondary Ion Mass Spectrometry

Transcription

1 Secondary Ion Mass Spectrometry A PRACTICAL HANDBOOK FOR DEPTH PROFILING AND BULK IMPURITY ANALYSIS R. G. Wilson Hughes Research Laboratories Malibu, California F. A. Stevie AT&T Bell Laboratories Allentown, Pennsylvania C. W. Magee Evans East, Inc. Plainsboro, New Jersey Fachbereieh Matsrjalwissensphaft cter Techn. Hochschule Darmstadt inv.-nr.: 3-/O New York A Wiley-Interscience Publication JOHN WILEY & SONS Chichester Brisbane Toronto Singapore

2 SUMMARY OF CONTENTS INTRODUCTION The basics of the SIMS technique are reviewed. Microelectronics processing terms used in the text are explained. 1. ANALYSIS CONDITIONS There are analysis parameters of first order and second order importance. The choice of three primary beam characteristics, species, energy, and angle of incidence, influence the selection of sputtering rate, detected area, and energy acceptance window. Knowledge of the capabilities of the instrument will help to decide which secondary ions to monitor. 1.1 Primary Beam The choice of primary ion beam depends on the elements to be analyzed and the type of information required for each element. This choice can be made with the aid of the tables of Relative Sensitivity Factor (RSF) versus element in Appendix E, and detection limit versus element in Appendix F. 1.2 Primary Beam Energy The choice of primary ion beam energy depends on the depth of the ion beam mixing that can be tolerated. Primary ion current and focusing are usually improved with higher primary ion energy. Ion beam mixing versus beam energy is illustrated with a figure showing profile distortion with increasing energy. 1.3 Angle of Incidence The primary ion angle of incidence affects the sputtering yield, the density of the primary species in the surface layer, and depth resolution. Changes in the primary energy can cause changes in the incidence angle for a magnetic sector instrument. The dependence of sputtering yield on incidence angle is shown. 1.4 Sputtering Rate Sputtering rate depends on the bombarding species and the primary current density. Sputtering rate is increased if the primary current is increased or if the rastered area is reduced. The use of a a higher sputtering rate usually improves the detection limit but may degrade depth resolution. Figures demonstrate these two effects. Aspects of primary ion current measurement are discussed. 1.5 Detected Area The detected area should be chosen to be only a small fraction of the crater area to avoid crater sidewall contributions. A very small detected area can be used to avoid defects and lateral structure and to improve depth resolution. A figure showing the effect on a profile of changing the raster size while the detected area remains constant is included.

3 1.6 Species Monitored The choice of impurity species for analysis is usually concerned with detection limit (DL), but also involves sensitivity and dynamic range. The best DL can be found with knowledge of the mass interferences and the RSFs for atomic and molecular ions. The use of rarer isotopes for analysis is shown. Reference is made to the RSF and detection limit appendices. The importance of acquiring mass spectra is demonstrated. The optimum matrix species to be monitored is also discussed. 1.7 End Point The ability to know when to stop a profile is important and depends on the analyst's knowledge of the typical background level, the sputtering rate, and the sample. 1.8 Energy Distribution The secondary ion energy distribution varies with element, with molecular versus atomic ion, and with sample charging during analysis of insulators. The choice of energy acceptance window can affect the accuracy of the profile and the detection limit. 2. PROFILE ISSUES Many characteristics affect SIMS data. Not only the properties of the materials to be analyzed but also the physics of the sputtering process play a role in the quality of the results. 2.1 Ion Beam Mixing and Depth Resolution Ion beam mixing depends on primary beam energy, species, and angle of incidence. Depth resolution is affected by ion beam mixing. Decay length and interface width are discussed. 2.2 Segregation and Charge Driven Diffusion Movement of an analyte species, such as As in S1O2, can occur during ion bombardment because of chemical gradients. Charge driven diffusion can distort the profile of some elements, such as Na. 2.3 Matrix Effects Secondary ion yields can vary with sample composition. The most frequently encountered matrix effects are caused by changes in the oxygen atom density of the sample. 2.4 Surface Effects The native oxide, the equilibration depth, and surface defects produce depth profile aberrations. The equilibration depth can be reduced with analysis at lower primary beam energy, or with the use of an oxygen leak. The equilibration depth can be zero for the analysis of oxides using an oxygen primary beam. Surface defects can be avoided by optical examination of the area to be analyzed. A plot showing the effect of surface deposits on a profile is given.

4 2.5 Particulates Surface or buried particulates can distort a profile. An example is presented along with a discussion of the ability of an image depth profile to detect and correct this problem by use of selective area analysis. 2.6 Crater Shape Crater shape and bottom flatness affect the depth resolution of a profile. Diagrams are included to show profilometer measurements of different types of crater distortions. 2.7 Microtopography Topography changes can occur during ion bombardment. A figure showing a depth profile in Si and SEM micrographs of the topography formation are used. The quantification and depth resolution of the profile are affected. The importance of determining the onset depth of microtopography in a given matrix is discussed. 2.8 Memory Effect The memory effect, especially for a magnetic sector instrument, is discussed in terms of origin and dynamic range limitations. The dynamic range limitation is shown in two ways. The first is the effect of analyzing a sample matrix that is the contaminant species for a subsequent profile (heteromemory). The second is the analysis of a high concentration of a species in a depth profile where a good detection limit for that same species is desired (homomemory). In both cases, the dynamic range is limited; the limit is about 5 orders of magnitude for a magnetic sector instrument. 2.0 Count Rate Saturation Faraday cup (FC) and/or electron multiplier (EM) saturation during a. profile can cause distortion of the profile. Examples are the flattening of the peak of an implant profile or a dip at the peak. Detected area versus rastered area is discussed Sample Location and Mounting The sample position for a magnetic sector instrument can affect the ion transmission through the mass spectrometer. This is illustrated by data from profiles taken at the center and edges of the sample area. The advantage of doing all analyses at one location for highest accuracy is noted. The distance from sample to analyzer is discussed. Sample preparation and mounting techniques are described Mass Interferences Mass interferences are not always easily predicted. Mass spectra are shown that indicate problems that can occur if mass spectra are not taken. The use of high mass resolution and voltage offset techniques to resolve interferences is illustrated using profiles, alon^ with a discussion of the approximate level of sensitivity that is lost. Tlie use o\ Xppeufox G. \s &\scusse<i. 3. QUANTIFICATION 3.1 Procedure The equations for quantification are presented along with a profile of raw data and a quantified profile. Calculation of relative sensitivity factor (RSF) is shown and the use of the RSF data in Appendix E is discussed. '

5 3.2 Calibration Using Ion Implantation Ion implantation criteria for a SIMS calibration standard are discussed. More than one isotope can be implanted provided the fluences of all isotopes can be determined. The choice of the best isotope should be based on interferences and isotope abundance. The energy of the implant should carry the peak and at least some of the front side of the implant profile deeper than the equilibration zone. The optimum fluence requires not implanting a fluence that will cause detection saturation, but enough fluence to allow more than one order of magnitude of detection above background. The accuracy of the implant fluence and RBS measurement of the fluence are treated. 3.3 Systematic Trends in RSFs Patterns of RSF versus ionization potential and RSF versus electron affinity are noted. An example of RSF plots for a compound matrix is presented with reference to either matrix element. RSFs for molecular ions are discussed. A method is given to determine RSFs for an unmeasured matrix. 4. SPECIFIC APPLICATIONS 4.1 Bulk Analysis Bulk analysis can be achieved by sputtering at a high rate. Mass spectra can be used to produce quantitative results with the use of RSF values. 4.2 Metals and Rough Surfaces Analysis of metals has been difficult because of microtopography formation, inclusions, nonuniform chemistry, grains, phases, and polycrystallinity, all of which affect depth resolution. Examples are provided for Al and Au layers. Sample rotation may improve the analysis of metals. Rough surfaces can be analyzed, but the depth resolution is poor. 4.3 Insulators The parameters of interest in the analysis of bulk insulators and insulating layers are discussed. Electron beam neutralization allows the analysis of a wide range of materials. The need to flood the entire ion beam rastered area with electrons is emphasized. Example profiles are used to demonstrate the analysis of.alkali elements in SiO2- High sputtering rate analysis is discussed. Differences between quadrupole and magnetic sector methods are noted. 4.4 Interfaces The methods to measure interface width, determine interface location, and detect interface contamination are discussed. The change in relative yield across an SiO^/Si interface is also treated. 4.5 Multilayers Multilayer analysis requires consideration of the changes in secondary ion yield caused by different matrix compositions, such as GaAs and AlGaAs, along with the changes in sputtering rates. An analysis of a quantum well structure is used as an illustration.

6 4.6 Residual Gas Elements Analysis of residual gas elements (H, C, N, O) with higher sputtering, rate typically produces a better detection limit. The effect of residual gas pressure on the detection limit for these species is illustrated. 4.7 Small Areas The difficulties of analysis of small areas, particularly on patterned wafers, are described. A profile is provided to show what effect occurs if part of the crater is off the pattern. A discussion is given of ion microprobe versus ion microscope solutions to this problem and of image depth profiles. 4.8 Major Elements SIMS profiling of major constituents can supplement what Auger electron spectrometry can provide, but at a faster rate. The use of Cs + bombardment and detection of positive cluster ions is discussed. 5. APPENDICES A. Guidelines for Analysis B. SIMS and Ion Implantation Data for the Elements Table Bl. Naturally Occurring Isotopes Table B2. SIMS and Ion Implantation Data for the Elements Table B3. Ionization Potentials and Electron Affinities Table B4. First Ionization Potentials of Selected Atoms and Molecules C. Matrix Densities D. Relative Sputtering Rates E. Relative Sensitivity Factors (RSFs) Table El. Table E2. Table E3. Table E4. Table E5. Table E6. Table E7. Table E8. Table E9. Table E10. Table Ell. Table E12. Table E13. Table E14. Table E15. A 2 O 3 Diamond GaAs GaP Ge HgCdTe InP LiNbO 3 Si, O 2 +,+;Cs +,- Si,O 2 +,-;Cs\ + Si, Quadrupole Si, O 2 +, Molecular Si, Cs +, Molecijlar SiO2, Quadrupole TaSi 2

7 F. Detection Limits Table Fl: Detection Limits for Si, Ge, GaAs, GaP, InP, and InSb Table F2: Detection Limits for Diamond, SiO 2, LiNbO 3, HgCdTe/CdTe G. Mass Interferences Computer program to calculate molecular ion isotope abundances. H. Mass Spectra Magnetic Sector Quadrupole INDEX