5) Chemical analysis in SEM

What are the two sources of xrays in a spectrum

characteristic; bremsstrahlung

What are the labels of the shells in an atom (in the context of xrays)

K, L, M, N

At what energy does the most efficient xray production occur

primary e is 3x xray energy

Which xrays are used for analysis and why

Kalpha for light elements; higher Z element won’t produce these so use L or M instead

Describe the sampling volume for xrays

almost as large as interaction volume as can escape from deep

The volume analysed and the fraction of xrays measured depend on

E of e beam (penetration); E of particular xray (large or small transition E); local Z number (higher Z absorb xrays more)

What are the two types of xray detectors

energy dispersive; wavelength dispersive

Energy dispersive xray spectrometers (EDX/EDS)

detects number of xrays as a function of E for whole spectrum at once

Wavelength dispersive xray spectrometers (WDX/WDS)

measure number of xrays of a single energy at higher energy resolution and counting accuracy

Why is quantitative analysis hard for lighter elements and where does it start

boron onwards; BCNO are difficult due to absorption of the low E e produced

Describe and explain the position of the xray detector

xrays can’t be focused so must be line of sight; ~20mm from specimen often close to the SE detector

Explain how an EDX detector works

xrays produced e/h pairs in the semiconductor detector; E field moves e/h generating current proportional to xray E;

Explain the set-up of an EDX detector

detector cooled to reduce noise and spurious current; Be window prevents contamination condensing on cold detector but absorbs BCNO xrays; use windowless detector in high vacuum for light elements;

How are EDX pulses recorded and what is the counting time

one at a time; counting time = live time (time counting) + dead time (pulses analysed and other incoming xrays ignored); count rate of 1-10,000 /s

What is a spot spectrum

peaks in xray spectrum attributed to specific spot position on the specimen

Problems with EDX

spurious xrays from microscope column; BCNO need special detector; must be kept at 77K (even when off); poor E resolution; close xrays cannot be separated and electronic noise causes poor signal to background ratio

What are two types of spurious xray peaks in EDX

sum peaks: two xrays arrive at same time recorded as sum of their energies; escape peaks: incoming xray ionises a Si atom losing 1.74keV causing a lower peak

Describe how WDX works

xrays reflected off crystal with know interplanar spacing so only specified wavelengths are diffracted; these xrays detected by simple gas proportional counter with very fast detection rates

Advantages of WDX

high energy resolution (up to 5eV with high peak to background ratio); high count rate; good for light elements; detect low concentrations ~0.01% (EDX 0.1%)

Advantages of EDX

cheap and simple; quick (whole spectrum in parallel); better for thin foil analysis due to higher % of xrays collected

Measuring chemistry at one location

spot spectra (EDX and WDX); e beam focused on one spot, count xrays as function of E; average composition in interaction volume can be determined (~1um³)

Measuring chemistry at different locations

BSE compositional mapping to find areas of different comp; many spot spectra at different locations and compare intensities/energies

Measuring 2D xray maps

choose xray E or element; count xrays as a function of beam position; generate 2D map of element distribution; EDX allows maps of several elements at once; limited resolution due to large sampling volume

What are some key points for xray analysis

conductive coatings must be thin and low Z so no peak; specimens flat as possible as need line of sight; rough specimens can generate secondary xrays from outside analysis zone; sub surface grains can produce xrays