first lab reports
DESCRIPTION
First lab reports. Grading Explanation of “soft windows” in upper right corner and how mouse is used to change entities therein: 5 points Adjustment of gun tilt and gun shift: 3 points Need for diagram of sample locations: 2 points Other details: 15 @ 1 point 25 point total - PowerPoint PPT PresentationTRANSCRIPT
First lab reportsGrading
Explanation of “soft windows” in upper right corner and how mouse is used to change entities therein: 5 points
Adjustment of gun tilt and gun shift: 3 points Need for diagram of sample locations: 2 points Other details: 15 @ 1 point 25 point total -1 for each incorrect statement Average was 20
Only two people turned in prelabs for lab 2
Meeting place update
Monday classes: WEB 103Friday classes: WEB 112
Currents in an SEM (W-filament)
Filament current: Current that heats a tungsten filament, typically 2.6-2.8 A. Strongly affects filament lifetime. Similar for Schottky FEG, but only heated to 1700 K
Emission current: total current leaving the filament, typically about 400 μA for W-filament, 40 μA for FEG.
Beam current: Portion of emission current that transits the anode aperture; decreases going down the column.
Probe current: a calculated number related to the current on the sample, typically 10 pA – 1 nA.
Specimen current: the current leaving the sample through the stage, typically about 10% of the probe current. Remember that one electron incident on the sample can generate many in the sample…a 20 keV electron can generate hundreds at 5 eV.
FEI also defines a parameter called “spot size” which is proportional to the log2(probe current); proportionality constant depends on aperture size.
Surface Emissions
Specimen current
X-raysCathodoluminescence
Pole Piece, etc
SE3
≈ 1 nm for metals upto 10 nmfor insulators
Interaction volume The interaction volume
falls with beam energy E as about 1/E5
(dE/ds ~ [ln(E)]/E) The interaction volume
no longer samples the bulk of the specimen but is restricted to near-surface regions only
The signal is therefore much more surface oriented at low energies than at high
Monte Carlo simulations of interactions in silicon
What happens at low energy?
At low energies the electron range falls from the micrometer values found above 10keV to just a few nanometers at energies of 0.5keV
This variation has profound implications for every aspect of scanning microscopy
Range from modified Bethe equation
Spatial resolution….. At high energy the SE1
signal typically comes from a volume 3-5nm in diameter, but the SE2 signal from a volume of 1-3µm in diameter
High resolution contrast information is therefore diluted by the low spatial resolution SE2 background
SE2 come from the full width of interaction volume
But at low energies…... ..the SE1 and SE2
electrons emerge from the same volume because of the reduction in the size of the interaction volume
So SE1, SE2 and BSE images can all exhibit high resolution….
the interaction volume shrinks
Seeing is believing The sample is a 30nm
film of carbon on a copper grid
At 20keV the carbon film is transparent because it is penetrated by the beam.The SE signal comes from the carbon film but is produced by electrons backscattered from the copper
SE image of TEM grid 20keV
Electron range at low energy
Carbon film completely covers grid!!
At 1keV -by comparison - the carbon appears solid and opaque because the beam does not penetrate through the film
This variation of beam range with energy is dramatic and greatly affects what we see in the low voltage SEM
Same area as before but 1keV beam
Some consequences of low energy operation
The interaction volume decreases in size and shrinks towards the top surface as the energy falls
High Energy Images At high energies the
beam travels for many micrometers giving the sample a translucent appearance
The SE image information is mostly SE2 and so copies the BSE signal.
The information depth is ~Range/3 and so is often a micron or more
MgO cubes 30keV S900
Low Energy Images At low energy the beam
only penetrates a few tens of nanometers.
The image now only contains information about the surface and the near surface regions of the specimen
The signal information depth (SE1,SE2 and even BSE) is only nanometers
Silver nanocrystals 1keV
0.1µm
as a result. . . .as a result. . . .
Indium Tin Oxide (ITO) Indium Tin Oxide (ITO)
the SE signal (in the LVSEM can produce)
high contrast nm resolution easy to interpret
surface images from crystals & nano-particles…
Silver Nanocrystals 1keV
and ..
….organics such as polymer resists
The best approach - try a wide range of energies and modes
CNT with intercalated iron
Some consequences of low energy operation
Spatial resolution is improved in all image modes
Low Voltage BSE imaging
At a WD of 1.5 or 2mm high resolution BSE imaging is readily possible and is very efficient
‘Z’ contrast may be less evident at low energies than at high Ta barrier under copper
seed
Some consequences of low energy operation
Changes in SE and BSE generation lead to differences in image detail and interpretation
SE yield variation The rapid change in the
incident electron beam range causes a large, characteristic variation in the SE yield
Typically the yield rises from ~0.1 at 30keV to in excess of 1 at around 1keV, and as high as 100 for some materials
Experimental SE yield data for Ag
Why the SE yield changes
SE escape depth is ~ 3-5nm At high energies most SE
are produced too deep to escape so the SE yield is low
But at lower energies the incident range is so small that most of the SE generated can escape so the SE yield rises rapidly
At very low energies fewer SE are produced because less energy is available so the SE yield falls again interaction volumes
low voltagehigh
voltage
BS yields at Low Voltage
The BSE yield varies with energy as well as with atomic number
Above ~2keV the yield rises steadily with Z
But at low energies the BSE yield for low Z elements rises, and for high Z elements it falls
Below 100eV the situation is more complex
Experimental BSE yield data
Do high and low kV SE images look the same?
•No..compared to the high energy
norm…
•The image looks less 3-D
•Highlighting is absent
•Surface junk is more visible
•Interpretation is essential
Device images at 20keV and 1keV
Origin of topographic contrast
Topographic contrast weakens and ultimately disappears as the beam energy is reduced.
At high energy tilting the sample puts more of the interaction volume in the SE escape zone
SE escape
But at low energy all the SE always escape
Beam penetration effects
At high energy the interaction volume fills features on the surface - SE2 emission leads to enhanced SE emission making objects look almost 3-dimensional
But at low energies the reduced interaction volume means that only the edges of features are enhanced
SE emission
High energy
Low energy
Some consequences of low energy operation Less charge is deposited in the sample
This is the real advantage of a FEG over a W-filament: FEG has almost as much resolution at 1 kV as at 15 kV
FEI now has landing energies as low as 50 eV!!!
The LVSEM and charging When electron beams
impinge on non-conducting samples a charge can build up inside the specimen which can make SEM imaging unstable, difficult, or even in extreme cases, impossible
By operating at low beam energies this problem can often be minimized or eliminated
Low voltage SEM has now become the norm for many users because of this effect Pathological charging artifacts
Charge Balance
I bI b
I b
sc
Electrons cannot be created or destroyed so currents at a pointsum to zero (Kirchoff’s Law)
Where are the BSE and SE yields respectively, and Q is the charge on the specimen at some time t. For a conductor this equation is always balanced by Isc
Working with Conductors
If the sample is a conductor then it cannot charge and Q=0 at all times
In this case at high energies where electron yields are small excess current flows to ground as specimen current ISC
At low energies where yields are high current flows from ground to make up the deficit
But the charge is always balanced and stable imaging is possible
..but in an insulator
ISC is zero If the sample is not to charge thenThis is achieved when
1..)( ifeiII BB
0dt
dQ
This condition represents a dynamic charge balance
If ()<1 then negative charging will occur and
If ()>1 then positive charging will occur
The charge balance condition
The variation of the () yield curve is about the same for all materials
In most cases there are energies for which () = 1
These are called the E1 and E2 or ‘crossover’ energies
Total yield data for quartz (SiO2)
Positive charge
Negative charge
NEUTRAL
E1 and E2 values for pure elements
E1 and E2 both increase with atomic number Z
E2 may also depend on the density (e.g diamond, graphite, and dry biological tissue have very different E2 values)
A few elements never reach charge balance (e.g Li, Ca)
Low Z elements need low keV. Since these elements so important the goal has been to make SEMs work at 0.5 - 2keV
Computed E1 and E2 energies
E2 values
Material E2(keV) Material E2 (keV)
Resist 0.55 Kapton 0.4 Resist on Si 1.10 Polysulfone 1.1 PMMA 1.6 Nylon 1.2 Pyrex glass 1.9 Polystyrene 1.3 Cr on glass 2.0 Polyethylene 1.5 GaAs 2.6 PVC 1.65 Sapphire 2.9 PTFE 1.8 Quartz 3.0 Teflon 1.8
Determining E2 in the SEM
Negative E>E2
Positive E<E2
Charging in Complex materials
In the case of complex materials (e.g. layered) then the charge balance must be considered separately for each component
If a beam penetrates a layer then it will charge positively (net electron emitter). The E3 energy at which this first occurs is typically <1keV for 3nm of hydrocarbon, and a few keV for a 250nm thick passivation layer.
substrate
SE BS
Thin film charging (E3)
SE Image of Chip covered by a 1m passivation layer imaged at 15keV -
above the E3 energyHow a thin metal film on top of an
insulator charges with energy
Imaging non-conductors
On a new SEM this will be the lowest available energy
On older machines you must decide how low to go before the performance becomes too poor to be useful for the purpose intended
The goal is to avoid implanting charge deep beneath the surface. If this is allowed to occur then stable imaging may never be achieved.
Step #1 - Set the SEM to the lowest operating energy
Failure to follow this advice...
If a poorly conducting sample is irradiated with a high energy beam then the implanted charge may prevent a low energy beam from reaching the surface at all
In that case it acts as a mirror giving a birds’ eye view of the inside of the SEM
Mirror image of sample chamber in an SEM
Next……...
If the sample is charging positively (i.e. a dark scan square) then E1< E<E2 or E>E3. Increase the beam energy and proceed to image
If sample is charging negatively (i.e. bright scan square) then E>E2.
Since we cannot reduce the beam energy any further we go on to step 3.
Step #2 - Determine the charging state of the sample using the scan square test
Step 3
Tilt the sample to 45 degrees and repeat the usual scan square test
Can E2 be reached now? E2() = E2(0)/cos2
so tilting by 45 degrees raises E2 by a factor of 2x
But ..because E2 varies with the angle of incidence the ‘no charge’ condition can never be satisfied everywhere on the surface at the same time and charging will always occur
Tilting the sample reduces charging at all energies
So does charge balancing help ?
In some cases - yes But because the E2
‘charge balance’ condition can never be simultaneously satisfied everywhere on a surface with topography - hence charging will always be present
Phase Shift Lithography mask slow scan imaged at E2
A better strategy Go to E2 and then scan at
high rates The sample acts like a
leaky capacitor which charges more quickly than it discharges
At slow scan speeds each pixel charges and then discharges before the beam reaches it again
this fluctuating potential affects SE emission, signal collection, scan raster etc
At high scan speeds (TV) there is less time to discharge so the potential stabilizes TIME
Beam dwell time on pixel
Potential
Slow scan
Fast scan
Forget eliminating charge – stabilize it then live with it
IIBB=100pA=100pA
Vacc.Vacc. : : 1.5kV 1.5kV
Mag.Mag. : : x200kx200k
Scan stabilized imaging
Uncoated photoresist
Imaged at E2 and scanned
at TV rate
the choice of detector
Pure SE signal – Thru-lens upper detector
ET lower detector SE + BSE + scattered electrons
Single polymer macro-molecules
makes a difference
Uncoated Teflon tape adhesive BSE image at 2keV
..so does reducing IB
the charging varies directly with IB so reducing the current cuts the charge
Use a smaller aperture, or reduce the gun emission current
Reduces the S/N ratio so longer scan times may be required
..and lowering the magnification
This minimizes Dynamic Charging (internal charge production from electron-hole pairs). The magnitude of this depends on the dose and hence on the magnification
Dynamic charging is worst when E0 is close to E2
Limits resolution by limiting magnification
Choosing a detector
The choice of detector can have a significant effect on the apparent severity of charging
The conventional ET (Everhart - Thornley) detector sees more topography but is much less sensitive to charging than... Individual polymer macro-
molecules on Si at 1.5keV -Lower (ET) detector
Upper detector
…a through the lens detector. This is because TTL systems act as simple SE spectrometers and preferentially select low energy electrons
Note however that charging can be a useful form of contrast mechanism when properly employed Same area as before, TTL detector
Comparing upper and lower detectors
Poly2 with CoSi on Top
rougher
SiO2
Si substrate with CoSi2
smoother
What is this residue??
Missing CoSi!!
Side Detector - Topography In-Lens Detector – Chemistry Image
BSE imaging to avoid charging
Backscattered electrons are less affected by charging and offer the same resolution as SE at LV
Newer technologies such as conversion plates, and ExB filters, for BSE actually improve in efficiency as the beam energy is reduced, so using this mode to avoid charging problems becomes a good choice
Uncoated Teflon S4700 ExB BSE image
Controlling Charging by Coating
The oldest method for controlling charging is to put a coat of carbon or metal on the surface of the sample
Coatings do not make the sample a conductor except in the limiting case when the surface is buried by a thick layer of metal
Instead the coating forms a ground plane - a localized equipotential region. In this area the free electrons in the metal re-arrange so as to eliminate the external field. The sample remains charged but incident and emitted electrons are unaffected
ground plane
Field lines do not leak away from the surface
Charge in sample
Field deflects electrons
If you must use carbon..
Carbon is not an ideal coat because it must be quite thick before it becomes a good conductor and has a low SE yield. Do not use evaporated carbon as this contains a lot of filler, instead use ion deposition
Thickness - probably 10 to 20nm minimum
How to check - shadow on the filter paper is light to dark grey
Con
du
ctiv
ity
Thickness
Minimum useful thickness is about 10nm
Con
du
ctiv
ity
Temperature-150C RT
300M-ohm
1 M-ohm
How effective is coating?
Thin films of either Au-Pd and Cr can effectively eliminate charging up to 8keV
Even at higher beam energies charge-up is minimal
Thin metal coats do not degrade EDS analysis - they improve it because they stabilize the beam landing energyExperimental Charging Data from
Alumina (Sapphire)
Radio Shack Special
If you prefer too make a ground line, or provide a ground plane the Circuit Writer, or Artic Silver, pens which deposit a silver loaded polymer work very well
Resistivity <0.1ohm.cm and dries quickly
No vapor in vacuum
Building a real low voltage SEM
There are several problems in achieving competitive electron-optical performance at low energies
Gun brightness falls linearly with energy. A FEG at 500eV is only as bright as a tungsten hairpin at 20keV
It is increasingly difficult to shield the column against outside electro-magnetic interference
The electron wavelength gets larger so diffraction is significant
Depth of field decreases Chromatic aberration is the killer
Chromatic aberration effects25keV 2.5keV 1.0keV 0.5keV
Kenway-Cliff numerical ray-tracing simulations of electron arrivals with a lens Cs=3mm,Cc=3mm, =7 m.rads
5nm
The energy spread of the beam causes a chromatic error in the focus. Even with a cold FEG source (~ 0.3eV wide) this greatly degrades the probe at 0.5 keV and below. Both the source and
the objective lens are important factors
Building a ULV CD-SEM
Decelerating the electrons just before they strike the sample reduces the landing energy and improves the optics
If the beam voltage is E0, then the landing energy is
Ef =E0-VB and it can be shown that
Cc’ = -Cs = L.Ef/E0
So if Eo=5000V, the landing energy is 50eV, and L ~ 1mm then Cs and Cc are reduced from mm to micrometers
Retarding on the S4800
Retarding Field Operation can be used in two ways (a) to enhance the imaging performance at an energy that is already available or (b) reach beam energies below the lowest value available on the microscope
Vacc
VR
ee
Accelerating VoltageVacc
VR Retarding Voltage
Vacc VR Landing Voltage
(ex)
2000V – 1500V = 500V
Retarding system
Vacc
Normal Accelerating Voltage
AcceleratingVacc
Voltage
Keeping 2kV spot size and beam current condition, accelerating voltage of 500V condition is obtained.
Mode (1) uses the retarding field effect to enhance resolution.
A retarding field image at 500eV has better resolution than a standard image at 500eV because the aberrations are smaller.
Here EL = 100eV => 1600 eV beam in - 1500 volts retarding potential
Sample : Membrane Filter
0.1kV 0.1kV
Disadvantages of Retarding System
Sample
Electron beam without retarding
Electron beam with Retarding
1 2
3 4
5
Not usable for general Depth of Focus become shallow
(SE/BSE) Signal Control cannot be used
1
2
3
4
5
Sample edge area
Pre-Tilted sample
Rough surface sample
Tilting stage
Cross-section
Secondary electrons are accelerated by retarding voltage and have the same energy level as backscattered electrons. So, it becomes impossible to detect each signal separately. As a result, always mixed signal of SE and BSE is detected and its mixing ration cannot be controlled.
sample observation
Mode 2 - ultra-low voltage use Retarding field can also be
used to reach ultra-low energies
Below about 200eV SE and BSE cannot be separated and so we must consider them together
The total yield (SE+BSE) varies rapidly with beam energy as shown but significant signal is still present at energies <10eV
Note that the total signal level at 100eV is about the same as that at 2keV so the signal to noise ratio should be acceptable
Total yield data for Copper
The new frontier500eV 25eV 14eV
Topographic contrast disappears at ultra-low energies but strong shadow (detector) contrast remains visible. Contamination is minimal. Many
of these effects remain to be explained
Resolution at Ultra-Low Energies
The resolution can be maintained at a very good level using the retarding field approach
Down to energies of 30-40eV the resolution is approximately independent of the choice of landing beam energy
In this example images at up to 300kx are shown at 100eV from a Hitachi S4800
Courtesy Bill Roth HHTA
Resolution at ultra-low energies
Because Cs and Cc decrease with the landing energy the imaging resolution is only limited by diffraction
500eV30eV
100nm
Contrast changes with energy
As the landing energy is reduced from 300eV the contrast in this example changes in many different ways. For example, note the change in contrast of the ‘black dots’ below 60eV - first they disappear then they reappear in opposite contrast.
Retarding Field ULV operation is a powerful new mode on the S4800 microscopes