Plasma Cleaning of SEMs and Large Vacuum Systems

By Ronald Vane XEI Scientific

Carbon – It is everywhere.

Sources of Carbon in Vacuum chambers:

Pump oils and Backstreaming.

Built into the Vacuum chamber in manufacturing

Adventitious hydrocarbons that result from various sources such as oils and solvents as well as work-pieces.

Bad Vacuum Practice - Finger prints

Carbon Lined World

Carbon Contamination artifacts in SEMS

• Free energetic electrons • Ions• Radicals – atoms, metastables, and

molecular fragments. • UV light photons

Plasma Creates Energetic Particles for Cleaning

Plasma Cleaning Mechanisms

Free Radical Chemical Etch• Ions have a limited lifetime and range are mainly useful for plasma

immersion and ion beam etching.

• Most plasma cleaning is done by free radical chemical etch.

• Chemical reaction products should be gasses to be pumped away.

• Oxygen radicals are a excellent cleaning gas for hydrocarbons and Oxygen is found in air.

• Free radical density of the plasma does not correlate with ion or electron density. O2

- ions interfere with ion and electron probe measurements of plasma.

• Free radicals of Oxygen recombine at surfaces and three body collisions. Lower pressures and larger volume s improve lifetimes and density.

Remote Air Plasma Cleaning Basic Principle

• In the air plasma, oxygen radicals are created.• They flow through the chamber from the plasma source to the pump port.• The radicals chemically etch inside the chamber any hydrocarbon contamination they encounter.• The reaction products are CO2 and H2O which are the pumped out of the system.

Plasma-driven chemistry cleans…

• Example: removal of hydrocarbon layer on EUV mirror

(Courtesy of Center for X-Ray Optics, LBNL)

Cleaning perspective

• Two cleaning needs① Contamination broadly over surfaces in low

levels/thicknesses (HC, S, residues)② Contamination spatially localized (particles)

• Plasmas excel at cleaning type 1• Plasmas poor at affecting type 2

– Hence: Ultra-/mega-sonics, laser shockwaves, electrostatics, super-critical CO2, etc. ….

• Removing type 1 should not cause type 2

Particulate Supression:Particle sources in plasma cleaning

• Feedstock gas purity• Materials, components, and assembly• Plasma discharge sputtering • Gas-phase agglomeration of reaction products

Requirements for plasma cleaning of Instrument vacuum chambers

Avoid ion sputtering etch and electron bombardment with:low voltageRFDownstream or remote plasma

Use an oxygen chemical etch to have:volatile gaseous productshighly reactive radicals produced by plasmaWell understood chemistry

Use air as an oxygen source because:widely availableNo safety concerns

Operate at low power to minimize nitrogen ionization and sputteringPlasma source should be small and light weightRemote Downstream plasma cleaning is preferable than directAvoid particulate production

Criteria for SEM Plasma Cleaner

• No damage to microscope or detectors• Mount to standard ports• Remove surface hydrocarbons in less than 5 minutes• Use air as Oxygen source• Avoid the use of argon; argon floods ion pumps.• No fluorine-containing gases.• No pure oxygen - an explosion hazard in oil filed pumps.• Low power plasma : No sputter etching or reactive ion etch

to prevent damaging the SEM.• Easy to use and install• Interlocked with vacuum system

What is new in Plasma cleaning?

Higher vacuum (<75 mTorr, < 10 Pa, <0.1 mBarr) cleaning• Better cleaning rates• Turbo pump compatible• Clean larger areas• Adds flowing afterglow UV

Requirements• Turbo Molecular pump• Low flow pressure control• Plasma ignition

Low Vacuum Plasma cleaning

• Use roughing pump – 200mTorr to 1 Torr pressure

• Plasmas ignite easily

• High Vacuum pumps and electron guns are valved off – no danger to diffusion pumps and HV parts

• Slow cleaning rates within 20 cm radius of plasma

• No direct cleaning at longer distances. Contaminates must travel to cleaning zone.

First Generation Evactron®plasma cleaners (Decontaminators)

Second Generation

• Microprocessor Control – May be connected to external computer and integrated into system

• Simple operation for repetitive cleaning.• Interlock: Enable with vacuum level detection• Operates at roughing pump pressures above high

vacuum crossover.• Preset vacuum, ignition power, cleaning power,

Cleaning cycle time.• N2 post plasma purge possible• Better impedance matching and RF power supply

for faster cleaning.• Rack mount or desktop controllers

Evactron Second Generation Features

Understanding plasma cleaning

Key parameters that affect plasma cleaning• Quantifying plasma cleaning• Distance from plasma• Gas pressure in plasma volume• Energy delivery to plasma• Plasma gas media• Gas throughput (residence time )• Chamber specifics/geometry

Quantifying plasma cleaning: QCMQuantifying Hydrocarbon Cleaning

• Use established quartz crystal microbalance (QCM) tools• Deposit hydrocarbon-based (HC) solid film on QCMs • Remove HC layer using Evactron plasma cleaner• Record Δ(thickness) versus Δ(time)) to calculate cleaning rate

Plasma Source

QCM crystal with hydrocarbon coating

Adjustable support arm

Flowing plasma afterglow

QCM holder

Using QCMs:

Quantifying Hydrocarbon Cleaning

Results from preferred method:• Apply Hydrocarbon by spray, heat to cure.• High linearity over entire thickness• Increase in film thickness (several microns)• Results repeatable and consistent• Parametric studies w/o breaking vacuum

Slope of line is proportional to cleaning rate

Quantifying Hydrocarbon Cleaning Rates

Finding a better plasma cleaning system

• Lower pressure operation –longer MFP• Lower pressure leads to…

– Longer “reaching distance” (this is the mfp if elastic collisions in gas are ignored)

– Fewer gas-phase 3 body collisions, less quenching• Lower pressure also leads to…

– Fewer gas-phase 2 body collisions (scattering), more line-of-sight cleaning

– Less Oxygen available for excitation– Less TMP heating and more ability for high RPM

Simple 2D Monte Carlo Model: shows transition from scattering to line-of-sight

• 30 cm x 80 cm chamber

• Zero-width (point) source

• Only elastic collisions, 20 total

• Reflective walls

• Group drift omitted Reaching distance λ (red) and 10λ (black)

Initial radical position (launch site) Reducing source

aperture causes narrowing of coverage area

Besides scattering we need to understand Radical Production vs. Pressure. Consider a concentration of

radicals produced at plasma output (Nrad)

Nrad (x) = Nrad (x=0) * exp(-x/R)

But R ≈ k/P and Nrad (x=0) ≈ ϒ*P

So… Nrad (x,P)/γ = P *exp(-x*P/k) The right side allows us to estimate relative cleaning versus pressure and distance

R = “reaching distance”

ϒ = radical generation efficiency

Model of Radical Transport vs. Pressure:Each graph shows same data presented two different ways

As expected flux falls off exponentially with distance, falling

faster with increasing pressure

Less expectedly, because of trade-off in pressure (radicals versus reaching)

optimum pressure to use is set by distance

0 5 10 15 20 25 30 35 40 45 500.001

0.01

0.1

1

10

Flux vs. position as f (pressure in mT)1510204060

Position (cm)

Rela

tive

flux

0 20 40 60 80 100 1200.0001

0.001

0.01

0.1

1

10

Flux vs. pressure as f(distance)

1510203050

Pressure (mT)Re

lativ

e Fl

ux

Radical transport: experimental

As expected flux falls off exponentially with distance, falling faster with

increasing pressure

Distances of 20 and 25 cm peak respectively around 50 and 35 mTorr;

40 and 50 cm away may also have maxima but data getting noisy

10 15 20 25 30 35 40 45 50 550

102030405060708090

Removal Rate vs Distance 10 mTorr20 mTorr50 mTorr100 mTorr

Distance (cm)

Rela

tive

Rem

oval

Rat

e

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

Removal Rate vs. Pressure50 cm from PRS25 cm from PRS40 cm from PRS20 cm from PRS

Pressure (mTorr)

Rela

tive

rem

oval

Rat

e

XEI Test Chambers for Plasma cleaning

Which pressure to compare?

TMP Chamber PRS

Pump Speed Flow (sccm) TMP (mTorr) Chamber (mTorr) PRS (mTorr)Low 20 3.0-4.0 4.0-5.0 18

High 20 2.5-2.9 3.0-4.0 18

Low 14 2.0-2.7 2.0-3.5 12

High 14 1.0-1.9 2.0-3.0 12

High Vacuum Speeds Cleaning

0 100 200 300 400 500 600 7000

5

10

15

20

25

30

Etch Rate vs Pressure in PRS

Zephyr, 20 cm from plasmaZephyr, 40 cm from plasma Zephyr, 60 cm from plasma

Pressure (mTorr)

Etch

Rat

e (A

ng./

min

)

←Evactron Turbo Plasma cleaning

↓Evactron Classic Plasma Ceaning↓

Plasma Power is not EqualCapacitive power vs ICP power

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70Cleaning rates for different sources at 20W

Zephyr

Pressure (mTorr, corrected)

Clea

ning

rate

(Ang

/min

)

Short mean free paths drop ICP radical production and raise heat.

ICP Cleaning is slower at low vacuum

Energy distribution of Plasmas

Capacitive coupled ICP Radical productionCross Section Edge

Electron energy distribution in the plasma sheath• Capacitive coupled plasmas have fewer electron collisions in the sheath.• ICP electrons are tumbled within the plasma, producing many collisions and heat.• Electrons with energy below the cross-section energy edge are non productive

Energy distribution of PlasmasCapacitive coupled ICP Radical production

Cross Section Edge

Electron energy distribution in the plasma sheath• Capacitive coupled plasmas have fewer electron collisions in the sheath.• ICP electrons are tumbled within the plasma, producing many collisions and heat.• Electrons with energy below the cross-section energy edge are non productive

At higher vacuum more energy is above absorption edge because of fewer collisions and less thermalization

Energy distribution of PlasmasCapacitive coupled ICP Radical production

Cross Section Edge

Electron energy distribution in the plasma sheath• Capacitive coupled plasmas have fewer electron collisions in the sheath.• ICP electrons are tumbled within the plasma, producing many collisions and heat.• Electrons with energy below the crossection energy edge are non productive

Low vacuum:Energy is thermalized below Absorption edgeMore heat –less radicals

0 10 20 30 40 50 60 70 80 90 100 1100

10

20

30

40

50

60

70

80

90

100Clean rates vs Pressure for different

sourcesZephyrExponential (Zephyr)ICP-1 20WPolynomial (ICP-1 20W)ICP-1 30W

Pressure (mTorr, corrected)

Clea

ning

rate

(Ang

/min

)

Effect of power on cleaning volume

Conclusion: Higher power does not increase cleaning volume at high vacuum.

10 15 20 25 30 350

20

40

60

80

100

120

140

160

180

200

Cleaning rates vs Power at 20 sccm flow rate in different chamber volume and pumping speeds

20cm from plasma40cm from plasma60cm form plasma

Power (Watts)

Clea

ning

Rat

e (A

ng/m

in)

Plasma Afterglow

UV Lines

UV spectrum from N2 afterglow

Afterglow UV Speeds Cleaning

• Nitrogen afterglow fills chamber• UV from all angles on surfaces• Desorbs water vapor and HC• Activates HC for O radical oxidation

RGA of contamination

RGA after

3rd Generation Evactron® EPHigh Speed Turbo Vacuum Cleaning

• Fixed input flow rate of air• No Vacuum gauge• Push Button operation• Computer or Bluetooth interface• Lower price

Conclusions Plasma Cleaning rates increase at lower pressures if input gas flow is

constant.

Radical flux scatters less at low pressure

Cleaning increases at locations far from plasma radical source at low pressure.

Cleaning rate and radical density is dependent on:• Pressure• Source type and efficiency• Chamber geometry and distance• Radical production and recombination rates.• Flowing afterglow and UV intensity