Developments & Limitations in GSR Analysis

Transcription

1 Developments & Limitations in GSR Analysis ENFSI Working Group Meeting June 2006 Jenny Goulden Oxford Instruments NanoAnalysis

2 Overview Introduction Developments in GSR Software Importance of EDS Hardware Particle detection

3 What do we want to achieve with GSR Accurate particle detection Accurate particle analysis Correct identification of Unique particles Relocation of specific particles for confirmation Compliance to ASTM Standard - (E 1588) GSR Analysis by SEM/EDS

4 Requirements for GSR Accurate - MUST give the right answer Particle detection/element Identification /Quantification & Classification Flexible Fast ammunition types/ different sample preparation / different environments critical for laboratories dealing with casework Ease of reporting Data Integrity

5 Particle Detection and Measurement For ALL the particles to be detected and measured correctly : Area & Field layout must be accurate Particle detection criteria must be suitable Beam relocation must be accurate Spectrum Processing must be correct

6 GSR Software INCAGSR provides: Automated analysis Flexible detection criteria, which can be optimised for SEM and particle type Analysis conditions stored in a recipe for reuse Easy and fast data reprocessing Straightforward reporting

7 INCAGSR INCA Navigator: Data is acquired through a series of logical steps Enter sample details Define area layout Grey scale calibration Define parameters for particle detection and quantification Automatically acquire data from whole area or selected fields Data classification Data reporting

8 Automated particle analysis Sample areas are defined with the aid of a stage mimic. The positions are stored and recalled for future use: Analysis achieved by dividing the sample into rectangular fields of equal area Relocate any selected area under the beam Define up to 48 areas in any one run

9 Progress monitoring Motorised microscope stage is driven to each field position in turn Detected particles are displayed during and after acquisition By predefining the position of standards monitor microscope/system stability during a run

10 Particle Detection GSR particles are typically seen as bright particles on a dark background in the BSE image. Detection of inclusions in a typical field: (a) BSE Image (b) Grey level thresholding (c) Feature detection

11 Oxford Instruments Particle Detection Criteria Signal Source - BSE / SE Magnification / Minimum required particle size 2 pass imaging technique. Pass 1 scans entire field quickly Pass 2 scans over detected particles slowly If no particles detected pass 2 is skipped Guard Zone user defined enforced field overlap to correct for particles which occur at the field boundaries

12 Particle Relocation User selects optimum conditions, SE image may be collected in addition to BSE Morphology and chemistry are measured New data may be saved Example shows a relocated GSR particle, analysed at a high magnification

13 Data Analysis Plot all data or selected classes Identify the particle number of any point on the graph Histograms Ternary plots - select up to 4 elements/oxides at each corner

14 By class or selection of classes Select a particle from the list or the field of view Select individual samples or groups of samples from a batch run for data review Particle can be relocated under the microscope beam automatically Data Review The data for each particle can be reviewed instantly

15 Importance of EDS Hardware INCAx-sight detector INCAx-stream pulse processor Combine to give superior resolution and stability

16 Detector Performance Two main indicators of detector performance: Resolution - FWHM of an element line typically measured MnKα ISO15632:2002 recognises the importance of light element detection Stability Peak stability with count rate

17 Detector Performance

18 INCAx-sight & INCAx-stream count rate stability guaranteed Many systems claim no variation with count rate, we can prove it Our specification is: Between 1,000 and 10,000cps peak position and resolution will change by less than 1eV Measured on MnKa at Process Time 5

19 Why is this important for GSR Analysis? When peaks are well separated small changes in resolution and position can be easily compensated When peaks are close together, the position and resolution must be known for the areas of the constituents to be correctly proportioned

20 Why is this important for GSR Analysis? This is important in GSR analysis where there are some well documented overlaps: Ba&Ti Pb& S Ca&Sb

21 GSR Overlaps - Ba/Ti Sb Ba Ti particle 1 Sb Ba Pb Sb Sb Sb Sb Sb Ti Ba Ba Ba Full Scale cts Cursor: (39406 cts) kev Ti K line overlaps with Ba L

22 GSR Overlaps - Pb/S Pb particle 1 S S Sb Sb Pb Pb Sb Sb Full Scale cts Cursor: (57702 cts) kev S K lines overlap with Pb M

23 GSR Overlaps - Sb/Ca Sb particle 1 Ba Sb Ba Pb Sb Sb Sb Ba Full Scale cts Cursor: (46895 cts) kev Ca K lines overlap with Sb L

24 GSR Spectrum Peak and resolution stability are key if the peaks are to be correctly resolved and the elements correctly identified Pb Ba Sb 22 20kV PT 4 approx 7kcps 5 seconds livetime Sb Ba Pb Pb Ba Sb Ba Ba Full Scale 247 cts Cursor: (7 cts) kev

25 Pulse Processor Performance Peak position must be reproducible for: peak shape and area to be correctly resolved, element to be correctly identified and quantified Accurate analysis requires: when count rate changes, peaks must not shift or change in resolution Key for GSR applications: possibility of peak overlaps, relatively high count rates are often used coupled with a short analysis time

26 Benefits of Peak Stability Accurate & Reliable AutoID at productive count rates even for spectra containing difficult overlaps results do not change with count rate reliable AutoID at high count rates and with the short livetimes typical for GSR analysis No need to know the elements in your sample Unexpected / Unusual ammunitions will not be overlooked

27 Benefits of Peak Stability Accurate & Reliable Quantification Correct elements are identified then the quant will be accurate Accurate & Reliable Classification whatever your ammunition

28 Particle Detection Requirements for particle detection are increasing to the sub-micron range for many applications The use of SEM with field emission sources has made the imaging of samples on the nano-scale at all kvs a reality Plano standard now includes particles in sub-micron range In GSR is the routine analysis of sub-micron particles is becoming a more common requirement?

29 Particle Detection & Analysis Factors that control the particle detection: Beam conditions - kv, spot size, beam current (W-SEM or FEG-SEM) Spot or raster beam for analysis Stage reproducibility and calibration Detection System & Sample BSE detector solid state 2 Segments or 4 Segments Background of GSR sample e.g. carbon tape or cloth Optimum time for analysis

30 Spatial Resolution of Interaction Volume at Different kvs 800 nm 300 nm 100 nm 200 nm 80 nm 550 nm 12 kev 7 KeV 3 kev Material: Fe

31 Spatial Resolution of Interaction Volume at Different kvs Higher kv larger interaction volume X-ray signal from background as well as particle The smaller the particle - the greater the X- ray signal from the background

32 Spatial Resolution of Interaction Volume at Different kvs For some particle analysis applications a lower kv is used For GSR applications kv of 20 or 25kV is typical To excite the Pb L line Achieve adequate backscatter contrast for particle detection

33 100nm Conventional W- SEM 100nm full & 4 partial hits of the beam on the particle nA; ~ 80nm Spot Size; 100nm Pixel Size,

34 Hot Field Emission SEM 100nm 100nm hits of beam on the particle nA; < 8nm Spot Size ; 100nm Pixel Size

35 Effect of Spot Size on Particle Detection Larger Spot size higher count rate poorer image quality smallest particles may be missed

36 Beam Conditions Effect of kv & spot size on image, 20kV 4nA -> good statistics (spectrum) poor quality image with W - SEM better image with FEG SEM 25kV 1nA -> acceptable statistics image acceptable on both W and FEG SEM 20kV 0.5nA -> analytical statistics poor - possibly use a large detecting crystal (30mm²), good quality image

37 Spot or Area Analysis INCAGSR option: Spot analysis - centre of the longest chord Scan over entire particle

38 Stage and Beam Calibration When detecting small particles accuracy of stage calibration and stage movement are critical The scanned raster and the stage movements must be orthogonal i.e. The sides of the image area must be parallel with the stage X and Y axes correctly tiled fields

39 Stage and Beam Calibration If the image and stage are not orthogonal the fields will not be properly tiled Gaps where particles are missed Overlapping fields were particles are counted twice particles missed particles double counted

40 System Calibration & Validation GSR a measure of confidence in your system is required INCAGSR dedicated stage and beam calibration This is then validated using a supplied particle standard

41 Regular arrays System Validation Regular grid of Au particles with a known size and position Au particles 5,10,15,20 µm Used to validate field tiling particle detection particle measurement

42 Random arrays - System Validation e.g.plano series of standards which are specifically designed for GSR validation Particle positions and chemistry known For example Plano SPS 521C with 43 Sb/Pb particles precipitated onto the surface of a silicon chip 6, 2.5 and 1.2µm size Additional Fe, Cu and Pb particles

43 System validation - Standards Conditions of analysis 20kV, 1nA probe current 2048x2048 image resolution 5 seconds live time minimum size 0.5µm 43 unique particles detected and measured correctly

44 Detection System Range of BSE-detectors are available, for example: Solid state detectors ( 2 quadrants, 4 quadrants) Robinson type detectors Scintillator based systems Micro channel detectors

45 Detection System BSE detector must be: fast response high signal/noise ratio e.g. 4 quadrant solid state detectors

46 Particle detection Find global dynamic range for BSE detector using a suitable standard e.g Mn/Rh or Cu/C Test this on known particles, of a suitable size Verify your system on a real particle

47 Analysis Time Any system can detect and measure all particles correctly slow scan speed high magnifications high magnification All result in an overall increase in analysis time Optimum conditions, for time and quality of data (e.g. 98% of particles detected and measured correctly)

48 Conclusion Software developments have created powerful and sophisticated tools for GSR applications easy of use powerful data processing Hardware is at least as important as software

49 Conclusion Particle Detection critical Influenced by a number of parameters Will differ from system to system Difficult to standardise on set of conditions that will work on every system