SIMS Flashcards
3 steps in SIMS
- ion generation - ionisation
- Ion analysis - mass separation
- Ion detection - micro channel plates
Method for ion generation
desorption ionisation, best because sample can remain in solid phase
methods for mass analysis
time of flight
Quadrupole
Magnetic
MCP?
Microchannel plates
million independent channel and each channel acts as an electron multiplier
2D periodic array of channels fused together and sliced into a thin plate
Types of analysis in SIMS
surface spectroscopy (mass spectra, static SIMS, mass resolution surface imaging - ion mapping depth profiling - dynamic SIMS/ ion beam broadening
UHV, what and whyq
Ultra high vacuum,
monolayers form easily on samples that could inhibit analysis - we need to know how quickly it’ll form
gives us high sensitivity
smaller pressure - lower density of molecules present, higher mean free path of electrons, lower force - reduced collisions with surface to be analysed
Strength of SIMS
Can analyse all elements
molecular species can also be detected
info form solid surface - we know where it came from
UHV - gives sensitivity so high detection levels
analysis can be performed both on surface and in bulk
what do O2+ and Cs2+ favour (ion species)
O - positive ions
Cs - negative ions
chemical effect due to the presence of oxygen
- oxide surface layer
- oxygen primary ion beam
- oxygen flood
2+3 can be done if a layer of oxygen doesn’t form naturally
depth of altered region depends on…
type of ion beam
its energy and
angle of incidence
ion beam interactions
- recoil mixing
- cascade mixing
both result in feature such as interfaces, or buried features broadening
SRIM
Stopping and range of ions in matter
model impact on matter
Reducing ion beam interaction
reduce ion impact per atom
for inorganic materials low energy beam are good such as O2+ or Cs2+
For organic materials Ar clusters are needed
depth profiling vs mass spectromerty modes
depth - dynamic mode
mass - static mode
surface spectrometry - interferences
become greater with larger mass
C-H interactions become more visible as well
surface spectrometry
mass spectra - static SIMS - fragmentation - mass resolution
fragmentation - if charge spread around molecule - less likely to fragment
lowest Energy C-H bonds broken
oxygen bonds, and benzene rings stable
mass resolution
typically 28 amu
R=M/deltam
depth profiling intricacies
chemical effects - correct ion beam has to be selected
physical effects - suitable ion beam energy and ion beam current
speed of analysis - appropriate area chosen
Ion beam effects play a role
charging - what if a sample is insulating?
+ve charge builds up on surface
Cover it with a conductive layer gold
Use a flood gun - low beam of ions directed over surface to dissipate charge
RSF
relative sensitivity factor
