- Electron interactions
- Elastically scattered electrons
- No energy lost
- Electrons emerge at high angle
- Inelastically scattered electrons
- Energy lost (0 < E < EO)
- No direct replacement
- Electrons emerge at low angle
- X-rays generated (Bremsstrahlung)
- Hole creation
- K. L. M shell holes
- Hole filling
- energy released as x-ray
- x-ray energy quantum dependent
- filled from different shell
- can create additional holes
- Auger electron production
- X-ray provides escape energy
- electron gains characteristic energy level relating to Z#
- element characteristic image
- Characteristic x-rays
- labeled by:
- element producing x-ray
- shell producing the vacancy
- shells traversed in filling vacancy
- number of electrons producing vacancy
- greater energy in inner shells, distance
- less energy in first electron ejected
- foreign electron replacement is continuum
- always less energy than in incident beam
- intensity of produced x-rays depend on:
- atomic number
- ionization cross section
- fluorescent yield
- x-ray absorption & fluorescence
- dependent on beam current
- X-ray detection
- Geometry of detection
- solid angle of detection
- area of detector, A
- detector angle to specimen, alpha
- distance to source, D
- omega = [A * cos(alpha)] / D2
- take-off angle
- angle of x-ray relative to surface
- absorption and secondary fluorescence
- increases sharply below 30 degrees
- incidence angle
- angle of incident beam to surface
- affects depth of beam penetration
- Contamination:
- Occurs on cryogenic surfaces and sample
- Invalidates absorption corrections
- Energy-dispersive x-ray detectors
- Detector construction
- Pure silicon wafer (10-30 mm2 area)
- Always impurities are present
- Extra holes occupied by drifted Li
- Electrons are shared in valence bands
- Energy required to dislodge electron: -3.8 eV
- Gold coating on both sides
- Leakage current decreased by cryogenics
- Dead layer at crystal surface
- Physics:
- Wafer is reverse-bias diode at 100-1000 volts
- Incoming x-ray interactions:
- Hit gold, creating photoelectrons
- Displaces electrons. creating holes
- Hole creation continues to the energy of the x-ray
- Current passage in wafer:
- Proportionate to e-hole pairs
- Current measured at peak
- Holes are refilled by e-current bias
- Energy-dispersive x-ray electronics
- Preamplifier
- FET: field effect transistor
- Operational at cryogenic temperatures
- Measures current required to reset Si
- At high range, this is reset by LED
- Transfers sawtooth waveform
- Relates directly to energy of x-ray
- Amplifier/pulse processor
- Integrates sawtooth into bell-shape
- Counts rate of FET resets
- Filters the pulses (time constant)
- Time constant is reading time
- Short times create:
- Lower resolution
- High processing speed
- Worse x-ray statistics
- Pulse pile-up rejection
- Fast channel amplifier
- Finds start of pulse
- Rejects coinciding pulses
- Most efficient at high energy
- Inefficient below 1 KeV
- Measures height of pulse
- Result is transferred as volts
- EDC and Multichannel Analyzer
- EDC: Engery-to-digital converter
- Timed capacitive discharge converter
- Voltage pulse charges capacitor
- Time to discharge is measured
- Time converted to x-ray energy
- MCA: Multichannel analyzer
- Counts in channel increased by 1
- Usually in computer firmwire
- Deadtime is result of FET & pulse processor
- Maximum rate of count accumulation: 60%
- Nominal deadtime-. 30% --best: 5%
- Livetime = Realtime minus deadtime
- Can preset # of seconds of livetime for acquiring spectrum
- Analysis of spectra -- Qualitative
- Result of analysis: x-ray energy histogram
- Channel count distributions form peaks
- Sum peaks
- Result of two coinciding pulses
- Adds energies of two pulses together
- Increases with high count rates
- Escape peaks
- At peak minus 1.74 KeV (Si-Ka)
- Remove before interpretation
- Peak overlap depends on element distribution
- Can be determined on relative peaks
- Check higher energy peaks (K, L, M)
- Scaling of peaks should coincide
- May require changes in resolution
- Effects of voltage
- Voltage must be more than the energy needed
- Overvoltage improves signal
- Ideally 3 times peak of interest
- Higher voltage adds fluorescence
- Detector efficiency:
- Limited by window & PP under 700 eV
- Most efficient between 7 to 20 KeV
- X-rays exit back of wafer > 20 KeV
- Output methods:
- Histogram (on EOA monitor)
- Line profiles (on SEM CRT)
- Dot maps (on SEM CRT)
- Computer dot maps (on EDA monitor)
- Analysis of spectra -- Quantitative
- Be careful, because garbage-in is garbage-out
- Requirements:
- Topographic Differences < 1 µm
- Reproducible:
- Microscope and detector geometry
- Angles of electron incidence (tilt)
- Angle of detector relative to specimen
- Take-off angle
- Livetime counts
- Acquisition rates
- Beam current density
- Appropriate:
- Specimen preparation
- Accelerating voltage
- Background removal
- Estimated or "modeled"
- Automated or manual
- Deconvolution
- Removal of peak overlaps
- Gaussian modeling
- Use of elemental standards
- Reference scaling
- Least squares fit
- Quantitative calculations
- All assume homogeneity at point analyzed!
- ZAF corrections
- Applied to K-ratios
- K-ratio (standard) compared
- Calculates expected values for:
- Z: x-ray production estimates - Absorption of x-rays
- Fluorescence of substance
- Quality of standard spectra crucial
- Standardless analysis
- All corrections are based on theory
- Elements below Na cannot be done
- Programs: MAGIC V and FRAME
- Hall method
- Applicable to biological thin sections
- Standards needed
- Scale k-ratio to background
- Thin enough that no ZAF needed
- Minimum detectable concentration
- Z number dependent
- Statistics dependent
- 1000 second livetime
- integral size