First lab reports zGrading yExplanation of “soft windows” in upper right corner and how mouse is used to change entities therein: 5 points yAdjustment of gun tilt and gun shift: 3 points yNeed for diagram of sample locations: 2 points yOther details: 1 point y25 point total y-1 for each incorrect statement yAverage was 20 zOnly two people turned in prelabs for lab 2

Meeting place update zMonday classes: WEB 103 zFriday classes: WEB 112

Currents in an SEM (W-filament) zFilament current: Current that heats a tungsten filament, typically A. Strongly affects filament lifetime. Similar for Schottky FEG, but only heated to 1700 K zEmission current: total current leaving the filament, typically about 400 μA for W-filament, 40 μA for FEG. zBeam current: Portion of emission current that transits the anode aperture; decreases going down the column. zProbe current: a calculated number related to the current on the sample, typically 10 pA – 1 nA. zSpecimen 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. zFEI also defines a parameter called “spot size” which is proportional to the log 2 (probe current); proportionality constant depends on aperture size.

Surface Emissions Specimen current X-rays Cathodoluminescence Pole Piece, etc SE3  ≈ 1 nm for metals up to 10 nm for insulators

Interaction volume  The interaction volume falls with beam energy E as about 1/E 5  (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.... Indium Tin Oxide (ITO) the SE signal (in the LVSEM can produce) zhigh contrast znm resolution zeasy 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 voltage high 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 z 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 Electrons cannot be created or destroyed so currents at a point sum 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 I sc

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 I SC  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 zI SC is zero zIf the sample is not to charge then zThis is achieved when 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 zThese are called the E1 and E2 or ‘crossover’ energies Total yield data for quartz (SiO 2 ) 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 keV 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 1  m passivation layer imaged at 15keV - above the E3 energy How 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. z 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 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. z 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)/cos 2  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 4The sample acts like a leaky capacitor which charges more quickly than it discharges 4At slow scan speeds each pixel charges and then discharges before the beam reaches it again 4this fluctuating potential affects SE emission, signal collection, scan raster etc 4At 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

I B =100pA Vacc. : 1.5kV Mag. : x200k 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 I B  the charging varies directly with I B 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 E 0 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 SiO 2 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 Conductivity Thickness Minimum useful thickness is about 10nm Conductivity Temperature -150CRT 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 energy Experimental Charging Data from Alumina (Sapphire)

Radio Shack Special zIf 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 zResistivity <0.1ohm.cm and dries quickly zNo 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 effects 25keV2.5keV1.0keV0.5keV 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 E 0, then the landing energy is E f =E 0 -V B and it can be shown that C c ’ = -C s = L.E f /E 0  So if E o =5000V, the landing energy is 50eV, and L ~ 1mm then C s and C c 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 V acc VRVR e e Accelerating Voltage V acc VRVR Retarding Voltage V acc VRVR Landing Voltage (ex) 2000V – 1500V = 500V Retarding system V acc Normal Accelerating Voltage Accelerating V acc 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 E L = 100eV => 1600 eV beam in volts retarding potential Sample : Membrane Filter 0.1kV

Disadvantages of Retarding System Sample Electron beam without retarding Electron beam with Retarding Not usable for general Depth of Focus become shallow (SE/BSE) Signal Control cannot be used 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 frontier 500eV25eV14eV 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 zBecause C s and C c decrease with the landing energy the imaging resolution is only limited by diffraction 500eV 30eV 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