Presentation on theme: "David Joy zDr. David C. Joy zDistinguished Professor zMaterials Science and Engineering University of Tennessee Knoxville, TN zD.Phil., University of Oxford."— Presentation transcript:
David Joy zDr. David C. Joy zDistinguished Professor zMaterials Science and Engineering University of Tennessee Knoxville, TN zD.Phil., University of Oxford (UK): z A Study of Magnetic Domains in the SEM zHitachi payroll
A Clean Machine The FEGSEM cannot operate except in a clean, ultra- high vacuum. (Numerous caveats for low vacuum and ESEM operation.) This requires items such as: Scroll Pumps Turbo Molecular Pumps Ion Pumps as well as associated gauges and even with the cleanest of vacuums there is still The Dark Side of SEM that must be faced….
Common Vacuum Units There are many varied units that are used to specify pressures The Torr, the Bar and the Pascal are in common use..... but the Pascal is the SI recommended unit for pressure and so is the best choice for documentation l 1 Atmospheric pressure is 760 mm Hg = 1 Bar = 10 5 Pa l 1 Torr = 1 mm Hg l 1 Torr = 1/760 of an atmosphere = 132 Pa l 1 milliTorr = 0.13Pa = 1 μmHg l 1mbar = 1/1000 Atm = 0.76 Torr = 100Pa l 1 Pa = 7.6 milliTorr = 7.6 μmHg
Pressure: Units of Measure zPressure exerted by a column of fluid: zP F/A = mg/A = ghA/A = gh h z1 Atm (mean sea level) = 760 Torr = 1013 mBar = 1.01x10 5 Pa = 101.3 kPa = 14.7 psi = 34 ft. water zAverage atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…
Kinds of Pressure zGauge Pressure: measured with respect to ambient. zAbsolute pressure: measured with respect to vacuum zCar tires, basketballs, boilers, LN2 tanks, JFB/MEB compressed air supply… zVacuum systems, cathode ray tubes, light bulbs, barometers
Mean Free Path in Gases With sufficient accuracy for approximate calculations we may take: λ = 7 x 10 -3 /p mbar-cm λ = 5 x 10 -3 /p Torr-cm λ = 5/p μmHg-cm
Qualitative Vacuum Ranges Low vacuum (SEC) 760 to 1 Torr Medium vacuum (SEC) 1 to 10 -3 Torr High vacuum (Chamber) 10 -3 to 10 -6 Torr Very high (Column) 10 -6 to 10 -9 Torr Ultra-high (Gun) 10 -9 and lower FEGSEMs contain examples of each vacuum level laminar molecular
Vacuum pumps zFor each of the vacuum ranges identified earlier there is one or more type of pump that is best zPumps are always used in combination - one pump is used to start the next zThe sequencing of the pump down is crucial and so this is done under computer control
Scroll Pumps zScroll pumps are the foundation of clean vacuum systems zThey consist of two Archimedes screws machined into aluminum plates mounted so that the spirals interleave zOne plate is held fixed while the other oscillates. Gas is trapped between the spirals and forced out to the exit port zPumping speed is constant from Atmospheric pressure down to about 1000Pa and the ultimate pressure is about 10Pa zScroll pumps are oil-free and require neither inlet nor outlet valves The worlds oldest pump technology – Archimedes screw
Alternatively…. zRoughing can also be carried out using a diaphragm pump zOscillation of the diaphragm alternately pulls gas in one port and then expels it through the other. Oil free pumps are clean but typically a factor of 3x slower, and 3x more expensive, than pumps containing oil – but worth the wait and expense
Rotary Vane Mechanical Pump zRobust zInexpensive zOperates to ambient pressure zSingle stage and two stage
Turbomolecular pump zArchimedian screw - runs at 20000+ rpm zNeeds electronic protection / control for the bearings in case of loss of power zProduces a clean, oil- free, high vacuum down to 10 -6 Pa (10 -8 Torr) zMust be backed: scroll pump, diaphragm pump or rotary oil pump.
Turbo pump performance zTurbo pumps can start even at atmospheric pressure (although they labor) and they can go down to 10 -8 T zIt is best to pre-pump the system with a clean backing system zTMP do not pump all gases with same efficiency - large molecules are pumped faster than smaller molecules l 1 milliT = 0.13Pa = 1 μmHg Turbo pump performance
Ion Pumps zIonized molecules spiral in magnetic field and get buried in Ti wall coating zA large number of these structures are run in parallel to improve the pumping speed zDiode pumps only handle gases that are easily ionized (no noble gases)
The triode pump zIf noble or unusual gases are expected to be found in the SEM (nitrogen, helium, counter gases from a WDS system etc.) then a triode pump must be used. zThe additional electrode then makes it possible to ionize these gases
Ion pump performance zThe UHV pump - goes to 10 -9 Pa (10 -11 Torr) and below in a properly designed vacuum system zRequires no backing…more than a little misleading… in fact it works best in a sealed system. Entrainment pump! zThe IP requires a periodic bake-out into rough pumped system to clean the buried gas from the pump. This is done during the gun bake procedure zCheck for electrical instability by slapping the pump with an open hand. Instability indicates need for a bake Ion Pump Performance
Cryogenic pump zCryo-pumps use liquid helium and activated charcoal absorbers to pump to 10 -12 T zVery high pumping speeds zNo vibration or magnetic fields zBut they need periodic bake- outs into a rough pump to clean the absorbers zThey are expensive to run unless used with a closed- circuit (Stirling engine) liquid He pump
Vacuum Gauges zVacuum systems must be monitored constantly to ensure satisfactory performance, but manufacturers seem to be reluctant to provide gauges which allow this to be done zMany different types of gauges are available because each only covers a limited range of pressures zNever trust a gauge unless you can check it independently Range of gauge utility
Pirani gauge zThe Pirani is a dedicated low vacuum gauge device zThe resistance of the hot wire changes with the rate of heat loss (conduction) to the gas zThe Wheatstone bridge then measures the change in resistance of the hot wire zPiranis are rugged and generally reliable and rarely need attention Schematic Circuit for a Pirani (hot wire) gauge
Pirani calibration zThe calibration of a Pirani depends on thermal conductivity and so on the actual gas in the system zBeware when using a crystal spectrometer as gases leaking from the counter tubes will degrade the accuracy of the Pirani gauge Correction Curve for Pirani Gauges
Penning (Cold cathode) Gauge zA Penning gauge measures the ion current flowing from the cathode to the anode zThe magnetic field increases sensitivity by making the ions spiral as they travel to cause secondary ionization zBeware - a Penning gauge reads zero current when the pressure is both very low and very high. The gauge must strike to be operational zCheck with a Pirani gauge if in doubt Penning gauges require routine cleaning and testing
Capacitance Manometer Gauge head on chamberController and digital read-out
Capacitance Manometer zA = Annular electrode zD = Disk electrode zS = Substrate zG = Getter (in vacuum space) zDifferential capacitance between annulus and disk depends on pressure difference between Test Chamber and Getter. (Earlier reference to unbacked getter pump)
Ion gauges zPressures lower than 10 -5 Torr can be measured with ion gauges (which are miniature ion pumps) or (more usually) directly from the actual ion pump zMass spectrometer gauges (residual gas analyzers) are a desirable extra. These can measure partial pressures of e.g helium (for leak testing) or of water vapor.
O-ring seals zO-rings (from the 1950s) made it possible to build demountable vacuum systems zThe rings are now made of high tech polymers such as VITON zTwo kinds are in common use... 4Black/shiny - has filler. Low vacuum only. Lubricate with finger grease to prevent cracking 4Brown/dull - high vacuum, and bakeable. Do not grease zDo not crush or cut the ring - ensure that it is in the groove designed for it
UHV metal to metal seals zFirst used in the 1960s zKnife edges on the flanges cut into OFHC (oxygen free high conductivity) copper rings about 5mm thick to make an impermeable metal to metal seal zGood down to pressures as low as 10 -10 Pa zBakeable, clean, long lasting and (with care) reusable zExpensive - an 8 inch gasket costs ~ $100 zDont touch !
Vacuum Hygiene zAlways keep vacuum systems running 24/7/365 zUse LN 2 cooled maze traps, and fore-line traps, to reduce backstreaming in older machines zDo not overpump the specimen exchange chamber (SEC) as this can result in backstreaming (unique to Hitachi FEG systems) zKeep your fingers away from samples and from the specimen chamber area - wear gloves zIf column contamination occurs try nitrogen purging (laminar flow) over a weekend Maze trap fitted to a rotary pump (mfp discussion)
Cleaning samples zDo not use organic solvents as these are always contaminated, even the fresh electronic grade material in brown glass bottles zNever, never, use squeeze or spray bottles as the TEFLON filler goes into solution 4Use detergents instead e.g. Alconox Detergent 8 which are bio-degradeable and leaves no residue 4Carbon Dioxide snow cleaning - no residue and good solvent action but expensive to set up. www.co2clean.com CO 2 snow gun for sample cleaning
Clean is not for ever... zAs soon as a specimen is prepared for observation it begins to get dirty again (CCW rule: one monolayer/sec at 10 -6 Torr) zEven storing the sample in a vacuum dessicator will not prevent the growth of bacterial or microbial surface contaminant films because the source of the problem is carried in by the specimen itself Remedial action is required As prepared After one week
Plasma cleaning zPlasma cleaning is a rapid and easy way of removing the build-up of surface contaminants zFast and non-destructive Same sample after plasma cleaning
The Dark Side of SEM zThe interaction of electrons with solids results in a variety of interactions which give us uniquely valuable information about the sample zBut these same interactions can also result in either temporary or permanent damage to the sample zKnow your enemy!
Unwanted Beam Interactions Intrinsic to electron beam irradiation Radiation Damage Ionization Displacement Heating Contamination Etching Results from vacuum problems Both are usually important
Unwanted beam interactions zElectron beams have bad effects on organic, polymeric, and ionic materials zThis is radiolysis Effect of 0.01 C/cm on protein protoxin 500nm Shrinkage of ArF resist 1mC/cm 2
Radiolysis is…. zRadiolysis is the breaking of bonds as the result of ionization by the electron. zElectrons are the most intense source of ionizing radiation available - the typical dose in an SEM is equivalent to standing 6 foot from a 10 megaton H-bomb Compare SEM to Sun and SPEAR
Radiolysis damage in Polymers zIn polymers radiolysis produces swelling or shrinking in the material and the actual loss of the sample zDespite appearances this damage is not due to heating in the sample zThe effect may be reduced by coating with metal or a thin carbon layer Courtesy Dale Newbury NIST
Dose does matter zA typical SEM dose for a photo-record is about 0.1 C/cm 2 or 100 el/Å 2 zTypically at 1 -10el/Å 2 we see a loss of crystallinity at 10-100 el/Å 2 mass loss and above 100 el/Å 2 limiting mass loss Dose from a single photo scan
Is a high beam energy bad? zIt is often said that low beam energies minimize or eliminates beam induced damage zFrom casual observation this statement may appear to be true, but physics and measurements show that the truth is just the opposite zAnd note - even a very low energy electron (1eV) has an equivalent temperature of 10,000 o K, which is hotter than the surface of the sun
Mythbuster fact zAll electrons damage zAt low energies the damage is high but limited by the range zDamage is a maximum when range & feature sizes are similar zAt higher energies damage falls - energy deposition occurs outside the feature Adapted from Egerton (2004) Range<>size damage limited
Damage in semiconductors ze- beam damage of devices shifts the threshold voltage zdamage is localized in gate oxides and is usually reversible zDamage depends on the beam energy…. and generally appears to get worse as the energy is increased
Thermal damage? zNot usually a serious problem as the energy deposited is quite small zFor a typical material of medium density and thermal diffusivity the temperature rise with energy and beam dose is minimal Magnification 5keV15keV30keV 400x 0.1C/nA 0.24C/nA 0.56C/nA 4000x 0.15C/nA 0.34C/nA 0.79C/nA
Other beam induced damage zIn addition to Radiolysis the beam can produce knock-on damage zIn this the incident electron strikes an atom head on and knocks it out of position generating vacancies, e.g. Frenkel defects zThis requires a minimum beam energy before it can occur, the value varying with the atomic number of the sample zFor Carbon (Z=6) the knock-on threshold energy is about 80keV, for Silicon (Z=14) the knock-on threshold is 220keV zNot currently a problem with SEMs
Contamination and Etching zContamination is beam induced polymerization of the hydrocarbons present on the sample surface zEtching is the removal of surface layer by impact of ions (C + H 2 O 2- --> CO + H 2 ) zBoth phenomena are affected by surface charging and often occur together zBoth are temperature dependent zYour microscope is not to blame!
Modern SEMs are very clean RGA of S4300 chamber just before the specimen is inserted H2OH2ON hydrocarbon
but samples are not.. S4300 chamber vacuum just after sample insertion hydrocarbons from the SEC pump
Contamination and Etching Electrons break down the hydrocarbon film by radiolysis. The residue charges +ve and the field pulls in fresh material for radiolysis. If water vapor is present then negative drift to + ve charged regions and can etch that area away
Low magnification zAt low magnifications the hydrocarbon film is polymerized into a thin sheet. zThis will charge positive (and so look black in the SE image) but is not a serious problem zMinimize by pre-exposing the sample at the lowest possible magnification prior to examination Schematic of contamination build-up at low magnification scans
Black squares... zThe black squares are visible evidence of the charging that occurs zPost facto in situ removal of contamination is possible using plasma sources in the chamber although the process is slow zUse plasma cleaning before observation for best results zExample shown is by courtesy of Dr. Bryan Tracy, Spansion Inc.
High magnification zAt high magnification the contamination grows a cone which scatters the beam zAvoid spot mode - always keep the beam scanning the sample zPre-pump samples before use zKeep your hands off the sample zAvoid the use of dirty solvents zPlasma clean before use if possible ~ 0.03 Ant-hill contamination
Virtue of necessity.. zContamination cones can grow to a height of tens of nanometers and are so tough they are used for high resolution AFM tips zMeasured diameters of carbon nanotubes can be high by half an order of magnitude! zCan prevent this growth by pre-irradiating the area at low magnification before going up to a high magnification 30nm high cones grown on a silicon wafer in spot mode - 1min
Temperature effects zAltering both the temperature of the sample and its surroundings will switch contamination to etching as the temperature falls zThis is because water vapor condenses out on the sample surface and etches the contaminant zBut the situation is unstable and leads to sample erosion
Temperature Effects II zHolding the sample at room temperature, but placing a cold surface close to it, can dramatically reduce the contamination rate zAt a low enough temperature the situation becomes stable and non-contaminating zSuch a device is called a Cold Finger zIt is actually a disc placed just above the sample surface operate here
The Cold Finger zStandard fitting on S4700, and beyond and available as an option for the S4500 zThe finger is held at LN2 temperatures, a few mm from the specimen surface zAfter allow the sample enough time to reach thermal equilibrium before starting to image
Without a cold finger zThis high resolution image of gold on carbon disappears in just a few seconds of observation because of the contamination build-up that occurs
With a cold finger in use... zThe equivalent area stays clean and high in contrast for an extended period of time. zRemember to give the sample time enough to reach thermal equilibrium before trying to achieve high resolution
Controlling contamination zCold fingers are a good start zBeam blanking during flyback and settling periods reduces LHS edge contamination in the S5500 zAnything that reduces charging also reduces contamination and/or etching - so coating samples, pre-cleaning them, heating them prior to observation etc. all help zKeep beam currents and magnifications low, use minimum dose procedures, work fast.
50 nm The combination of a cold finger and maintaining the sample at a low temperature (-90C) eliminates contamination