David Joy

Dr. David C. JoyDistinguished ProfessorMaterials Science and Engineering

University of Tennessee Knoxville, TN

D.Phil., University of Oxford (UK): A Study of Magnetic Domains in the SEMHitachi 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….

Terminology

“Low vacuum” = High pressure

“High Vacuum” = Low pressure

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

1 Atmospheric pressure is 760 mm Hg = 1 Bar = 105 Pa

1 Torr = 1 mm Hg 1 Torr = 1/760 of an

atmosphere = 132 Pa 1 milliTorr = 0.13Pa = 1 μmHg

1mbar = 1/1000 Atm = 0.76 Torr = 100Pa

1 Pa = 7.6 milliTorr = 7.6 μmHg

Pressure: Units of Measure

Pressure exerted by a column of fluid:P ≡ F/A = mg/A = ghA/A = gh h 1 Atm (mean sea level) = 760 Torr =

1013 mBar = 1.01x105 Pa = 101.3 kPa = 14.7 psi = 34 ft. water

Average atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…

“Kinds of Pressure”

Gauge Pressure: measured with respect to ambient.

Absolute pressure: measured with respect to vacuum

Car tires, basketballs, boilers, LN2 tanks, JFB/MEB compressed air supply…

Vacuum 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-cmλ = 5 x 10-3/p Torr-cmλ = 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 For each of the vacuum

ranges identified earlier there is one or more type of pump that is best

Pumps are always used in combination - one pump is used to start the next

The sequencing of the pump down is crucial and so this is done under computer control

Scroll Pumps Scroll pumps are the

foundation of clean vacuum systems

They consist of two Archimedes’ screws machined into aluminum plates mounted so that the spirals interleave

One plate is held fixed while the other oscillates. Gas is trapped between the spirals and forced out to the exit port

Pumping speed is constant from Atmospheric pressure down to about 1000Pa and the ultimate pressure is about 10Pa

Scroll pumps are oil-free and require neither inlet nor outlet valves

The world’s oldest pump technology – Archimedes’ screw

Alternatively….

Roughing can also be carried out using a diaphragm pump

Oscillation 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

RobustInexpensiveOperates to

ambient pressure

Single stage and two stage

Turbomolecular pump Archimedian screw -

runs at 20000+ rpm Needs electronic

protection / control for the bearings in case of loss of power

Produces a clean, oil-free, high vacuum down to 10-6 Pa (10-8 Torr)

Must be backed: scroll pump, diaphragm pump or rotary oil pump.

Turbo pump performance

Turbo pumps can start even at atmospheric pressure (although they labor) and they can go down to 10-8 T

It is best to pre-pump the system with a clean backing system

TMP do not pump all gases with same efficiency - large molecules are pumped faster than smaller molecules

1 milliT = 0.13Pa = 1 μmHg

Turbo pump performance

Ion Pumps Ionized molecules

spiral in magnetic field and get buried in Ti wall coating

A large number of these structures are run in parallel to improve the pumping speed

Diode pumps only handle gases that are easily ionized (no noble gases)

The triode pump If 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.

The additional electrode then makes it possible to ionize these gases

Ion pump performance

“The” UHV pump - goes to 10-9 Pa (10-11 Torr) and below in a properly designed vacuum system

Requires no backing…more than a little misleading… in fact it works best in a sealed system. Entrainment pump!

The 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

Check for electrical instability by slapping the pump with an open hand. Instability indicates need for a bakeIon Pump Performance

Cryogenic pump Cryo-pumps use liquid

helium and activated charcoal absorbers to pump to 10-12 T

Very high pumping speeds No vibration or magnetic

fields But they need periodic

bake-outs into a rough pump to clean the absorbers

They are expensive to run unless used with a closed-circuit (Stirling engine) liquid He pump

Vacuum Gauges Vacuum systems must be

monitored constantly to ensure satisfactory performance, but manufacturers seem to be reluctant to provide gauges which allow this to be done

Many different types of gauges are available because each only covers a limited range of pressures

Never trust a gauge unless you can check it independently

Range of gauge utility

Pirani gauge The Pirani is a

dedicated low vacuum gauge device

The resistance of the hot wire changes with the rate of heat loss (conduction) to the gas

The Wheatstone bridge then measures the change in resistance of the hot wire

Pirani’s are rugged and generally reliable and rarely need attention

Schematic Circuit for a Pirani (hot wire) gauge

Pirani calibration The calibration of a

Pirani depends on thermal conductivity and so on the actual gas in the system

Beware 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

A Penning gauge measures the ion current flowing from the cathode to the anode

The magnetic field increases sensitivity by making the ions spiral as they travel to cause secondary ionization

Beware - a Penning gauge reads zero current when the pressure is both very low and very high. The gauge must ‘strike’ to be operational

Check with a Pirani gauge if in doubt

Penning gauges require routine cleaning and testing

Capacitance Manometer

Gauge head on chamber Controller and digital read-out

Capacitance Manometer

A = Annular electrode D = Disk electrode S = Substrate G = Getter (in vacuum

space) Differential capacitance

between annulus and disk depends on pressure difference between Test Chamber and “Getter”. (Earlier reference to “unbacked getter pump”)

Ion gaugesPressures 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

Mass 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 O-rings (from the 1950s) made

it possible to build demountable vacuum systems

The rings are now made of high tech polymers such as VITON

Two kinds are in common use... Black/shiny - has filler. Low

vacuum only. Lubricate with finger grease to prevent cracking

Brown/dull - high vacuum, and bakeable. Do not grease

Do not crush or cut the ring - ensure that it is in the groove designed for it

o-ring compressed to fill groove

remove dust,hairs andlubricate withfinger

UHV metal to metal seals

Knife edges

copper gasket

bolt

Tighten boltsin sequence1,4,2,5,3,6

1

2

3

4

5

6

First used in the 1960s Knife edges on the flanges

cut into OFHC (oxygen free high conductivity) copper rings about 5mm thick to make an impermeable metal to metal seal

Good down to pressures as low as 10-10 Pa

Bakeable, clean, long lasting and (with care) reusable

Expensive - an 8 inch gasket costs ~ $100

Don’t touch !

Vacuum Hygiene Always keep vacuum systems

running 24/7/365 Use LN2 cooled maze traps, and

fore-line traps, to reduce backstreaming in older machines

Do not overpump the specimen exchange chamber (SEC) as this can result in backstreaming (unique to Hitachi FEG systems)

Keep your fingers away from samples and from the specimen chamber area - wear gloves

If column contamination occurs try nitrogen purging (laminar flow) over a weekend

Maze trap fitted to a rotary pump

(mfp discussion)

Cleaning samples Do not use organic solvents as

these are always contaminated, even the fresh ‘electronic grade’ material in brown glass bottles

Never, never, use squeeze or spray bottles as the TEFLON filler goes into solution

Use detergents instead e.g. Alconox ‘Detergent 8’ which are bio-degradeable and leaves no residue

Carbon Dioxide ‘snow’ cleaning -no residue and good solvent action but expensive to set up. www.co2clean.com

CO2 “snow gun” for sample cleaning

Clean is not for ever ...

As soon as a specimen is prepared for observation it begins to get dirty again (CCW rule: “one monolayer/sec at 10-6 Torr)

Even 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

Plasma cleaning is a rapid and easy way of removing the build-up of surface contaminants

Fast and non-destructiveSame sample after plasma cleaning

The Dark Side of SEM

The interaction of electrons with solids results in a variety of interactions which give us uniquely valuable information about the sample

But these same interactions can also result in either temporary or permanent damage to the sample

Know your enemy!

Unwanted Beam Interactions

Intrinsic to electron beam irradiation

Radiation Damage

IonizationDisplacementHeating

ContaminationEtching

Results fromvacuum problems

Both are usually important

Unwanted beam interactions

Electron beams have bad effects on organic, polymeric, and ionic materials

This is ‘radiolysis’

Effect of 0.01C/cm on

protein protoxin

500nmShrinkage of ArF resist 1mC/cm2

Radiolysis is….Radiolysis is the breaking

of bonds as the result of ionization by the electron.

Electrons 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

In polymers radiolysis produces swelling or shrinking in the material and the actual loss of the sample

Despite appearances this damage is not due to heating in the sample

The effect may be reduced by coating with metal or a thin carbon layer

Courtesy Dale Newbury NIST

Dose does matter A typical SEM dose for

a photo-record is about 0.1 C/cm2 or 100 el/Å2

Typically at 1 -10el/Å2 we see a

loss of crystallinity at 10-100 el/Å2 mass loss and above 100 el/Å2 limiting mass loss

Dose from a single photo scan

Is a high beam energy bad?

It is often said that low beam energies minimize or eliminates beam induced damage

From casual observation this statement may appear to be true, but physics and measurements show that the truth is just the opposite

And note - even a very low energy electron (1eV) has an equivalent temperature of 10,000oK, which is hotter than the surface of the sun

‘Mythbuster’ fact All electrons damage At low energies the

damage is high but limited by the range

Damage is a maximum when range & feature sizes are similar

At higher energies damage falls - energy deposition occurs outside the feature

Adapted from Egerton (2004)

Range<<size damage limited

Range~size damage is maximum

Range>>size damage limited

Damage in semiconductors

e- beam damage of devices shifts the threshold voltage

damage is localized in gate oxides and is usually reversible

Damage depends on the beam energy…. and generally appears to get worse as the energy is increased

Thermal damage?Not usually a serious problem as the

energy deposited is quite smallFor a typical material of medium density

and thermal diffusivity the temperature rise with energy and beam dose is minimal

Magnification 5keV 15keV 30keV

400x 0.1C/nA 0.24C/nA 0.56C/nA

4000x 0.15C/nA 0.34C/nA 0.79C/nA

Other beam induced damage

In addition to Radiolysis the beam can produce ‘knock-on’ damage

In this the incident electron strikes an atom head on and knocks it out of position generating vacancies, e.g. Frenkel defects

This requires a minimum beam energy before it can occur, the value varying with the atomic number of the sample

For Carbon (Z=6) the knock-on threshold energy is about 80keV, for Silicon (Z=14) the knock-on threshold is 220keV

Not currently a problem with SEMs

Contamination and Etching

Contamination is beam induced polymerization of the hydrocarbons present on the sample surface

Etching is the removal of surface layer by impact of ions (C + H2O 2- --> CO + H2 )

Both phenomena are affected by surface charging and often occur together

Both are temperature dependentYour microscope is not to blame!

Modern SEMs are very clean

RGA of S4300 chamber just before the

specimen is inserted

H2O Nhydrocarbon

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 At low magnifications the

hydrocarbon film is polymerized into a thin sheet.

This will charge positive (and so look black in the SE image) but is not a serious problem

Minimize by pre-exposing the sample at the lowest possible magnification prior to examination

Schematic of contamination build-up at low magnification

scans

Black squares... The black squares are visible

evidence of the charging that occurs

Post facto in situ removal of contamination is possible using plasma sources in the chamber although the process is slow

Use plasma cleaning before observation for best results

Example shown is by courtesy of Dr. Bryan Tracy, Spansion Inc.

High magnification

At high magnification the contamination grows a cone which scatters the beam

Avoid spot mode - always keep the beam scanning the sample

Pre-pump samples before use Keep your hands off the sample Avoid the use of dirty solvents Plasma clean before use if

possible

~ 0.03

Ant-hill contamination

Virtue of necessity..

Contamination cones can grow to a height of tens of nanometers and are so tough they are used for high resolution AFM tips

Measured diameters of carbon nanotubes can be high by half an order of magnitude!

Can 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 Altering both the

temperature of the sample and its surroundings will switch contamination to etching as the temperature falls

This is because water vapor condenses out on the sample surface and etches the contaminant

But the situation is unstable and leads to sample erosion

Temperature Effects II

Holding the sample at room temperature, but placing a cold surface close to it, can dramatically reduce the contamination rate

At a low enough temperature the situation becomes stable and non-contaminating

Such a device is called a “Cold Finger”

It is actually a disc placed just above the sample surface

operate here

The Cold Finger Standard fitting on

S4700, and beyond and available as an option for the S4500

The finger is held at LN2 temperatures, a few mm from the specimen surface

After allow the sample enough time to reach thermal equilibrium before starting to image

Without a cold finger

This 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...

The equivalent area stays clean and high in contrast for an extended period of time.

Remember to give the sample time enough to reach thermal equilibrium before trying to achieve high resolution

Controlling contamination “Cold fingers” are a good start Beam blanking during flyback and

settling periods reduces LHS edge contamination in the S5500

Anything that reduces charging also reduces contamination and/or etching - so coating samples, pre-cleaning them, heating them prior to observation etc. all help

Keep 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