Low-Voltage Microscopy zWhen electron beams impinge on non- conducting samples a charge can build up which can make SEM imaging difficult or impossible zBy operating at low beam energies this problem can often be minimized or eliminated

Charge balance Electrons cannot be created or destroyed so currents at a point must sum to zero. The current flow to earth I sc is the difference between the in and out currents If the sample is a conductor I sc can take any value (+ve or -ve) to achieve charge balance

Non-conductors Sample can accumulate negative charges or positive charges There can be a dynamic charge balance For a non-conductor I sc is zero so charge accumulates + -

Complex materials zIn the case of complex materials (e.g. layered) then the charge balance must be considered separately for each component zIf a beam penetrates a layer then it will charge positively, a net electron emitter. substrate SE BS

Imaging non-conductors zOn a new SEM this will be the lowest available energy zOn older machines you must decide how low to go before the performance becomes too poor to be useful for the purpose intended z Set the SEM to the lowest operating energy

Negative charging z If the scan square is brighter than the background then the sample is charging negative

Positive Charging z If the scan square is dark compared to the background then the sample is charging positive

Is that all there is to it? zNo - charging is a complex phenomena and simply running the SEM at a low energy does not guarantee an image that is free from charge artifacts zTo understand why we must look in more detail at what happens when a poorly conducting specimen is hit with an electron beam

Mechanisms for Charging STATIC CHARGE Static charging depends on the net charge balance in the sample DYNAMIC CHARGING Dynamic charging comes from charge generated in the sample itself from electron-hole pairs There is no global condition where this term is zero Combining these two contributions we can synthesize a detailed model of the charging process

Charge Distribution  The net amount of negative charge injected = 1- . This is deep in the sample  The net charge that is emitted =  and gives a positive region at the surface zInduced charge occurs throughout the interaction volume and could be of either sign +ve -ve Incident beam I b Even at charge balance there is still stored charge and fields in the sample

Conductivity and Charging zThe +/- charge separation produces a field which moves the induced carriers producing conductivity (EBIC) zTraps reduce the number of electrons. If the escape time from the traps is >> than the time between electron arrivals so the charge builds- up. A charged region is therefore like a leaky capacitor zEBIC is the key to dynamic charging effects V bias L Charge e dQ Area A

Surface Potential and Electric Fields The fields produced by even small amounts of charging are very high. It is these fields which deflect the incident beam, push the secondary electrons around, move the electron-hole pairs and may even change the yield of electrons This is seen as a drifting image Monte Carlo calculation of fields in and above a resist sample

Minimizing dynamic charging zReduce the beam current as the charging varies directly with I B zChange to Ultra-High resolution operating mode and lower the emission current zReduces S/N

Dynamic Charging zReduce the magnification zDynamic charging depends on dose and on the magnification zLimits resolution by limiting magnification

Time dependent charging zDynamic charging is time dependent because of the leaky capacitor effect (EBIC) zScanning at a high speed extracts a signal before charging occurs zThe whole scanned area now floats to a uniform potential allowing stable focussing and stigmation

Coating specimens zCoating should be as thin as possible, a good conductor, and a good emitter of SE zAu/Pd, Cr are good zCarbon is bad (the filler contaminates ) and the evaporator heats the sample z Coating is effective but may hide real surface detail z May be only route if high beam energy is required e.g for EDS

How coatings work zCoatings do not make the specimen conductive zThey form a ground plane - eliminate fields due to charge zIncrease SE yield - reduce charging Charge in sample Field deflects electrons ground plane

Result of coating zBoth Au-Pd and Cr effectively eliminate charging up to about 8keV zEven at higher beam energies charge-up is minimal zThin coats do not affect EDS analysis

Other options zHeating the sample - effective for ceramics, oxides etc zUse a low pressure of a gas (VP-SEM mode or from a gas jet) zLow energy electron or ion flood beam to neutralize the charging z Use BSE detector for imaging- much less sensitive to charging z Try different SE detector, mixed or upper z Try high energy if sample is thin or on a substrate- depends on what you want to examine

Choice of detector zThe choice of the detector that is used can be very significant in determining how seriously charging will appear to be zTry biasing the sample stage zTry mixing the detector signals, or switching to the lower detector if possible

S4700 TTL detector z This detector is very efficient and gives a symmetric view z These electrons are very sensitive to chemistry and to charging effects z High energy SE, BSE, and SE3 are excluded from the signal - this improves contrast

S4700 detectors zLower (ET) and upper (TTL) detectors on S4700 have different characteristics zLower (ET) detector accepts SE1,II and III as well as some BSE zUpper (TTL) detector accepts only low energy SE1 and 2

SE spectra zThe upper detector accepts SE with energies around the peak of the SE spectrum. Peak position depends on amount of charge, chemistry, electronic structure, so these effects cause image contrast on TTL detector zLower (ET) detector accepts everything below 50eV. Much less sensitive to charging SE spectrum from Aluminum

Nonconducting samples zLatex paint at 1keV in Hitachi S4500 zUncoated, slow scan image at E2 energy z30kx original magnification zLower (ET) detector for topography, reduces visibility of charging

Lower detector zIndividual polymer macro- molecules on a silicon substrate imaged at 1.5keV zThe lower detector shows little or no contrast

Upper detector zThe upper detector easily reveals the macro- molecules zThis is because they are charging negative and the TTL detector is highly sensitive to charging effects zCharging can be a useful form of contrast

Doping contrast zChemical contrast in the SE mode zSensitive to both P- and N-type dopants zOnly visible on upper (TTL) detector Boron doping in Si 1.5keV

Upper detector zBirds-beak dopant contrast in a device zS4500 at 1keV zThis is a unique imaging capability - 2 dimensional dopant profiling at high resolution and sensitivity (1ppm)

Damage at low energies zIt is often stated that operation at low beam energies minimizes or eliminates beam induced damage zFrom casual observation this may appear to be true, but measurements show that the truth is just the opposite

Damage and beam energy zAt high energies the damage rate is low zDamage rate rises as the energy is reduced, reaching a peak at about 100eV  At still lower energies the stopping power falls again Experimental Stopping Power Data for Copper

Damage while scanning zIf the beam is scanning then the rise in damage rate is less drastic but still considerable zhowever damage is confined to the near surface and not spread through a volume