Plasma processes as advanced methods for cavity cleaning

N. Patron, R. Baracco, L. Phillips, M. Rea, C. Roncolato, D. Tonini

and V. Palmieri

… pushing the limits of RFS

Legnaro 2006

CLEANINGa post processs

• Hydrocarbons

• Water

• Oxygen, Nitrogen and other adsorbed gases

ETCHINGa main process

• Removal of ~ 100 μm

•Reduce surface roughness

• Reduce surface contamination

• Remove damaged layers

• WET ETCHING

• Chemical etching

• Electropolishing

• Electromachining

• DRY ETCHING

• PLASMA

• ION GUN

• Sputtering

• Reactive ion etching

• Ion beam cleaning

• Reactive ion beam etching

Let’s analyze one by one the different DRY ETCHING

techniques

• Reactive ion etching• DRY ETCHING

• PLASMA

• ION GUN

• Sputtering

• Ion beam cleaning

• Reactive ion beam etching

One example from our experience:

• Cu frame used in CUORE experiment for the detencion of a dobble decadiment

• We have been given the task to find a way to eliminate ppb contamination of 232 Th from the Cu surface

CUORECUORE Cryogenic Underground Observatory for Rare

Events

Dry etching methods are very clean

•Smooth surface

But Physical Methods treatment can become an enemy…..

•Thin grain boundaries

• Coarsening of grain boudaries

• Rough surface

A deeper etching

Cleaner surface, but higher demagnetization factor

Sputtering Plasma Etching

• For cleaning it might good

• It isn’t a fast routine method

• Whenever applying dry etching a fundamental comprehension of the role of Grain boundaries and grain Demagnetization factor is necessary.

(vacuum systems, flanges to be mounted…)

• Reactive ion etching• DRY ETCHING

• PLASMA

• ION GUN

• Sputtering

• Ion beam cleaning

• Reactive ion beam etching

• Mostly developed for Nb-based Josephson junctions switching devices.

• Gas mixture more frequently used are: CF4/O2

(a,b), CCl3F(c), SF6/O2(d); I2, XeF2

(e).a) M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983b) J. N. Sasserath and John Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990c) J. W. Noè, Nucl. Inst. and Meth. 212 (1083) 73d) B. J. Curtis and H. Mantle, J.Vac.Sci. Technol. A, Vol. 11, No. 5, Sep/Oct 1993e) X. L. Fu, P. G. Li, A. Z. Jin, H. Y. Zhang, H. F. Yang, W. H. Tang, J.Vac.Sci. Technol. B,

Vol. 23, No. 2, Mar/Apr 2005

• Reactive gasses are injected in the plasma

RF reactive ion etching device• Parallel plate RF powered etcher operating at 13.56 MHz

• Using CF4 and O2 as the reactive gas mixture

M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983

From Literature

Etching rates are functions of O2 percentage

M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983

J. N. Sasserath, J. Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990

From Literature

•Niobium etching rate = 30 μm/hJay N. Sasserath and John Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990

•Niobium etching rate = 2,4 μm/hM. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983

From Literature

CCl3F-vapour rf discharge processing

J. W. Noè, Nucl. Inst. and Meth. 212 (1083) 73

•Eliminate secondary electron emission problems of multipactoring from lead-plated copper quarter-wave resonators.

•Flurine ions and radicals are very agressive, Noè suggests that CF4 should work too.

LNL ACTUAL RESULTS

Etching rate: 12,7 μm/h

• Niobium DC diode sputtering with CF4

• Pressure of 410-2 mbar

• Sample voltage: - 1250 V

• Reactive ion etching• DRY ETCHING

• PLASMA

• ION GUN

• Sputtering

• Ion beam cleaning

• Reactive ion beam etching

• Two main type of sources

Kaufman sources

Broad-beam source with an extracting grid in wich a cathodic filament sustains a magnetical confined

plasma

Best confinament condition for

λ<<w

Gridless sources

Gridless sourceMAGNETRON

SOURCE

Positive ions are accelerated from the

ionization region toward the cathode’s surface by

Vdc

GRIDLESS SOURCE

It works just like a magnetron source where the anode is above ground potential and the cathode has a hole from where ions can exit and form

the ion beam

We used a gridless source

• It is more simple and it’s easier to be modified if eventually we want to reduce its dimension to use it inside of a cavity

• It needs only one power supply

Source IG1: parameters

The cathode is grounded

The anode is at +2kV

Gas process is Argon

LNL ACTUAL RESULTS

ION BEAM ETCHING

• Energy: 2 KeV

• Pressure of 410-2 mbar

• Substrate to source:170 mm

Ar

2,3 μm/h

CF4

12,7 μm/h

REACTIVE ION ETCHING

• Diode sputterind with CF4

• Pressure of 410-2 mbar

Gas flux

Plasma region

Rotational extracting

grid

A possible cavity application

• Reactive ion etching• DRY ETCHING

• PLASMA

• ION GUN

• Sputtering

• Ion beam cleaning

• Reactive ion beam etching

Atmospheric-pressure

Plasma

• AP plasma • RF • RF resonance

• AP Plasma Jet

• DC• CORONA

• MICROWAVE• MW plasma torch

Why could ATM plasma be useful…?

• To clean surfaces from carbon contamination or adsorbed gases.

• To etch surfaces using plasma activated chemicals, without any need of a vacuum system.

• To add an efficient cleaning step to the cavities surface treatments

• To substitute some dungerous steps of Nd cavity chemistry

An example of a surface treatment

• AP plasma • RF • RF resonance

• AP Plasma Jet

• DC• CORONA

• MICROWAVE• MW plasma torch

DC corona plasma• Corona discharges accur only if the electric field is sharply NONUNIFORM, typically where the size “r” of one electrode is much lower than the distance. It’ may be seen as luminous glow around the more curved electrode. The electric field’s minimun value for the ignition is around 30 kV/cm.

Electrodes

High field gradientLow field gradientDischargeCorona

DC Corona discharge

• A non-self-sustaining current of 10-14 A can be detected.

• It is due to ions produced by cosmic rays.

• The corona is ignited.• A luminous layer around

the electrode where the E field is the highest can be seen.

• A self sustaining discharge makes the current jump to ~10-6 A.

• Massive production of O3

Vapplied << Vcorona Vapplied > Vcorona

Coronas are operated at currents/voltages below the onset of arcing

The Corona Mechanism• The extablisment of a corona begins with an external ionization event generating a primary electron and it is followed by an electron avalanche.

• The second avalanches are due to energetic photons :NEGATIVE CORONA POSITIVE CORONA

Positive Corona• It appears more uniform than the corresponding negative corona thanks to the homogeneous source of secondary avalanche electrons (photoionization).

• The electrons are concentrated close to the surface of the curved conductor, in a region of high-potential gradient and therefore the electrons have a higher energy than in negative corona.

• Produce O3

Negative Corona• It appears a little larger as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it.

• The electron density is much greater than in the corresponding positive corona but they are of a predominantly lower energy, being in a region of lower potential-gradien.

• The lower energy of the electrons will mean that eventual reactions which require a higher electron energy may take place at a lower rate.

• Produce a larger amount of O3

Why could corona plasma be useful?

• UV/O3 treatments has been proved to be capable of producing clean surfaces in less than 1 minute(f).

• Ozone production could be easily used to clean the cavities surfaces from carbon contaminants.

f) J. R. Vig, J.Vac.Sci. Technol. A, Vol. 3, No. 3, May/Jun 1985

The early stage of our studies•Negative Corona inside a 1,5 GHz cavity

•Discharge voltage 30kV

•Strong production of O3

1,5 GHz seamless Cu Cavity

•Positive Corona inside a 1,5 GHz cavity

•Discharge voltage 25kV

•Production of O3

1,5 GHz seamless Cu Cavity

• To have a more uniform corona plasma it is necessary to have the same electrode distance along all the lenght of the cavity.

• It is important to verify if the 2-6 eV electron and ion energy could be used for surface chemical etching or cleaning using reactive gases.

Attempts for understanding and studies

Cavity

Cavity shaped catode

Catode’s edges facing the cavity

Corona ignited at the edges

Cathode cavity shaped

Negative corona inside the cavity

• AP plasma • RF • RF resonance

• AP Plasma Jet

• DC• CORONA

• MICROWAVE• MW plasma torch

RF Resonance plasma•Our purpose was to ignite an atmosferic resonance plasma inside a cavity.

• Relate the mode exctitation to the shape of the plasma inside the cavity in order to control and eventually direct the plasma more or less close to the internal surface of the cavity.

•Study the surface modification due to the plasma physical or chemical action.

Excitation mode TM010

Electric field Module of Magnetic field

Module of Electric field

Magnetic field

Lateral view

Base view

6 GHz cavityCavity

TM010 plasma at a power of 50 W

1,5 GHz cavity

upper iris

lower iris

Plasma at a

power of 150 W

antenna

Pill-box cavity for the excitation mode TE111

RF power supply frequency range

Excitation mode TE111

Module of Electric field

Magnetic field

Module of Magnetic field

Electric fieldBase View

Lateral View

What do we expect

•A plasma ball in the center of the cavity when we excite the TM010 mode, as we have seen in the 6 GHz cavity.

•A rod of plasma along a diameter at the center of the cavity pointing to the surface, when we excite the TE111.

view port

Loop antenna

Al Pill-Box

•We found the resonance frequencies of the modes TM010 and TE111.

•Using a loop antenna we tried to ignite the plasma by exciting at the TE111 mode’s resonance frequency.

•We found out by observing that the plasma shape wasn’t changing while moving away from the resonance frequency that we weren’t observing a plasma due to a resonance mode excitation.

• AP plasma • RF • RF resonance

• AP Plasma Jet

• DC• CORONA

• MICROWAVE• MW plasma torch

Atmospheric Pressure Plasma Jet

Gas mixture

O2+He2 O2+He2 +CF4

O2+He2+CF4

O2+He2+CF4

O2+He2+CF4

Material Kapton SiO2 Ta W Ta

Etching Rate

8 μm/min(g)

1,5 μm/min(g)

2 μm/min (g)

1 μm/min (g)

6 μm/min (h)

g) V. J. Tu, J. Y. Jeong, A. Schutze, S. E. Babayan, G. Ding, G. S. Selwyn, R. F. Hicks, J.Vac.Sci. Technol. A, Vol. 18, No. 6, Nov/Dec 2000

h) J. Y. Jeong, S. E. Babayan, V. J. Tu, J. Park, I. Henins, R. F. Hicks, G. S. Selwyn, Plasma Sources Sci. Technol. 7 (1998) 282-285

13,56 MHz / 2,45 GHz APPJ Device

Water out

Water in

Gas in

RF connection

Inner electrode

Ionization space

Outer electrode

13,56 MHz

0,00

0,10

0,20

0,30

0,40

0,50

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

100 W

30 W

Current density VS distance from the exit

Distance (mm)

Cur

rent

den

sity

(μA

/mm

2 )

Future APPJ source developementPlasma and chemicals exit radially from the nozzle

• AP plasma • RF • RF resonance

• A P P J

• DC• CORONA

• MICROWAVE• MW plasma torch

MW Atmospheric Plasma Torch

Gas Inlet

MW 2,45 GHz waveguide

Quarz tube placed at

MW 2,45 GHz

•Plasma ignited inside a quartz tube at 500W

SO…• Different etching methodes and devices has

been explored.

• There are some ideas of exploring the use of reactive gases like CF4 or NF3 in both the vacuum and plasma processes.

• Still a lot of studies needs to be done…

Advice and suggestions

THANK YOU

The End? or the beginning

Paschen curve

Factors/Systems

Apjet Diffuse Dielectric Barrier

Corona Microwave

Method Helium Process Gas with added reactive

gas

Dielectric Cover on Electrode with He

process gas

Sharply Pointed Electrode at HV

Wave GuidesResonant Cavity.

Complex

Frequency 2-60 MHz RF 1-100 KHz AC DC/Pulsed Pwr 2.45 GHz

Plasma DensityElectrons/cm3

(volume average)

1011-1012 109 108 1011

Reactive Species: O/cm3

1016 1013 1013 ? (Limited due to ozone generation)

Undesirable byproducts:Ozone/cm3

1016 1018 1013 High

Temperature Low Low High at edge RF Substrate Heating

Uniform Glow Yes Yes? No Point Source

Process Methods Downstream or In-situ

In-situ In-situ Downstream

Flexible Shapes Yes Yes No No

Hazards Low High OzoneSubstrate Damage

High Voltage

High Ozone Signficant Health & Safety (microwave) +

High Ozone

Scalable to large area?

Yes Yes No No

•If the applied voltage V is less than the ignition voltage for a Corona discherge Vc than a non-self-sustaining current of 10-14

A can be detected. It is due to ions produced by cosmic rays.

•If the applied voltage V is less than the ignition voltage for a Corona discherge Vc than a non-self-sustaining current of 10-14

A can be detected. It is due to ions produced by cosmic rays.

•Vapplied << Vcorona

a non-self-sustaining current of 10-14 A can be detected. It is due to ions produced by cosmic rays

•Vapplied > Vcorona

The corona is ignited, a luminous layer around the electrode where the E field is the highest can be seen. The discherge current jump to 10-6 A. It is a self sustaining discharge.

The Corona Mechanism• The extablisment of a corona begins with an external ionization event generating a primary electron and followed by an electron avalanche.

•The second avalanches process is due to :

NEGATIVE CORONA POSITIVE CORONA

-Electron emission from the cathode

-Photoionization

-Photoionization

Future developements and studies

Cavity

Catode

Catode’s edges facing the cavity where the corona will be ignited

Future developements and studies

Catode

Catode’s edges facing the cavity where the corona will be ignited

Cavity

What’s next on LNL superconductivity group?

Focused Ion Beam

•Niobium etching rate using I3 = 72 μm3/min

•Niobium etching rate using XeF2 = 60 μm3/min

Which source to be used?Kaufman Gridless

Fragile and expensive grids with a lifetime limited by the sputtering

process

It is more simple has a Struttura robusta e semplice da revisionare

Multiple power supplies are necessary to obtain a good

control of the energy and current of the ion beam

Necessario un unico generatore di potenza, a discapito del controllo

dell’energia e della corrente ionica

The system of energy power supplies give a sharp energy

distribution

Profilo di energia degli ioni debolmente definito

Ion current can easily be mesured Corrente ionica proveniente dalla sorgente deve essere dedotta

Difficoult to decrease the source’s dimention

Sorgente riscalabile a dimensioni molto maggiori

Gridless source IG1: technical design

Coil

Cooled anode Inlet gas

Ionization area

Magnetic extractor

Teflon chamber

6 GHz cavityCavity

TM010 plasma at a power of 50 W

Excitation mode TM010

Electric field Module of Magnetic field