Plasma processes as advanced methods for cavity cleaning
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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. ETCHING a main process. CLEANING a post processs. Hydrocarbons. Removal of ~ 100 μm. Water.

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Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

ETCHING

a main process

CLEANING

a post processs

  • Hydrocarbons

  • Removal of ~ 100 μm

  • Water

  • Reduce surface roughness

  • Reduce surface contamination

  • Oxygen, Nitrogen and other adsorbed gases

  • Remove damaged layers


Plasma processes as advanced methods for cavity cleaning

  • Sputtering

  • PLASMA

  • Reactive ion etching

  • DRY ETCHING

  • Ion beam cleaning

  • ION GUN

  • Reactive ion beam etching

  • Chemical etching

  • WET ETCHING

  • Electropolishing

  • Electromachining


Let s analyze one by one the different dry etching techniques

Let’s analyze one by one the different DRY ETCHINGtechniques


Plasma processes as advanced methods for cavity cleaning

  • Sputtering

  • PLASMA

  • Reactive ion etching

  • DRY ETCHING

  • Ion beam cleaning

  • ION GUN

  • Reactive ion beam etching

One example from our experience:


Plasma processes as advanced methods for cavity cleaning

CUORECryogenic Underground Observatory for Rare Events

  • 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


Plasma processes as advanced methods for cavity cleaning

Dry etching methods are very clean


Plasma processes as advanced methods for cavity cleaning

  • Smooth surface

  • Thin grain boundaries

But Physical Methods treatment can become an enemy…..


Plasma processes as advanced methods for cavity cleaning

A deeper etching

  • Coarsening of grain boudaries

  • Rough surface

Cleaner surface, but higher demagnetization factor


Sputtering plasma etching

Sputtering Plasma Etching

  • For cleaning it might good

  • It isn’t a fast routine method

(vacuum systems, flanges to be mounted…)

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


Plasma processes as advanced methods for cavity cleaning

  • Sputtering

  • PLASMA

  • Reactive ion etching

  • DRY ETCHING

  • Ion beam cleaning

  • ION GUN

  • Reactive ion beam etching


Plasma processes as advanced methods for cavity cleaning

  • Reactive gasses are injected in the plasma

  • 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 1983

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

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

d) B. J. Curtis and H. Mantle, J.Vac.Sci. Technol. A, Vol. 11, No. 5, Sep/Oct 1993

e) 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


Rf reactive ion etching device

From Literature

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


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

From Literature

  • Niobium etching rate = 30 μm/h

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

  • Niobium etching rate = 2,4 μm/h

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


Plasma processes as advanced methods for cavity cleaning

CCl3F-vapour rf discharge processing

  • 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.

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


Plasma processes as advanced methods for cavity cleaning

LNL ACTUAL RESULTS

  • Niobium DC diode sputtering with CF4

  • Pressure of 410-2 mbar

  • Sample voltage: - 1250 V

Etching rate: 12,7 μm/h


Plasma processes as advanced methods for cavity cleaning

  • Sputtering

  • PLASMA

  • Reactive ion etching

  • DRY ETCHING

  • Ion beam cleaning

  • ION GUN

  • Reactive ion beam etching


Plasma processes as advanced methods for cavity cleaning

  • Two main type of sources


Kaufman sources

Kaufman sources

Broad-beam sourcewith an extracting grid in wich a cathodic filament sustains a magnetical confined plasma


Plasma processes as advanced methods for cavity cleaning

Gridless sources

Best confinament condition for

λ<<w


Gridless source

MAGNETRON 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

Gridless source


We used a gridless source

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

Source IG1: parameters

The cathode is grounded

The anode is at +2kV

Gas process is Argon


Plasma processes as advanced methods for cavity cleaning

LNL ACTUAL RESULTS

  • ION BEAM ETCHING

  • Energy: 2 KeV

  • Pressure of 410-2 mbar

  • Substrate to source:170 mm

  • REACTIVE ION ETCHING

  • Diode sputterind with CF4

  • Pressure of 410-2 mbar

Ar

CF4

2,3 μm/h

12,7 μm/h


Plasma processes as advanced methods for cavity cleaning

A possible cavity application

Gas flux

Plasma region

Rotational extracting grid


Plasma processes as advanced methods for cavity cleaning

  • Sputtering

  • PLASMA

  • Reactive ion etching

  • DRY ETCHING

  • Ion beam cleaning

  • ION GUN

  • Reactive ion beam etching


Plasma processes as advanced methods for cavity cleaning

Atmospheric-pressure

Plasma


Plasma processes as advanced methods for cavity cleaning

  • CORONA

  • DC

  • RF resonance

  • AP plasma

  • RF

  • AP Plasma Jet

  • MICROWAVE

  • MW plasma torch


Why could atm plasma be useful

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 Nb cavity chemistry


Plasma processes as advanced methods for cavity cleaning

An example of a surface treatment


Plasma processes as advanced methods for cavity cleaning

  • CORONA

  • DC

  • RF resonance

  • AP plasma

  • RF

  • AP Plasma Jet

  • MICROWAVE

  • MW plasma torch


Dc corona plasma

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.

High field gradient

Low field gradient

Corona

Discharge

Electrodes


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 jumpto ~10-6 A.

Massive production of O3

DC Corona discharge

Vapplied << Vcorona

Vapplied > Vcorona

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


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

The early stage of our studies

1,5 GHz seamless Cu Cavity

  • Negative Corona inside a 1,5 GHz cavity

  • Discharge voltage 30kV

  • Strong production of O3


Plasma processes as advanced methods for cavity cleaning

  • Positive Corona inside a 1,5 GHz cavity

1,5 GHz seamless Cu Cavity

  • Discharge voltage 25kV

  • Production of O3


Plasma processes as advanced methods for cavity cleaning

  • 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.


Plasma processes as advanced methods for cavity cleaning

Corona ignited at the edges

Cavity

Cavity shaped catode

Catode’s edges facing the cavity

Attempts for understanding and studies


Plasma processes as advanced methods for cavity cleaning

Cathode cavity shaped

Negative corona inside the cavity


Plasma processes as advanced methods for cavity cleaning

  • CORONA

  • DC

  • RF resonance

  • AP plasma

  • RF

  • AP Plasma Jet

  • MICROWAVE

  • MW plasma torch


Plasma processes as advanced methods for cavity cleaning

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.


Plasma processes as advanced methods for cavity cleaning

Excitation mode TM010

Electric field

Module of Magnetic field

Lateral view

Module of Electric field

Base view

Magnetic field


Plasma processes as advanced methods for cavity cleaning

6 GHz cavity

Cavity

TM010 plasma at a power of 50 W


Plasma processes as advanced methods for cavity cleaning

1,5 GHz cavity

antenna

upper iris

Plasma at a power of 150 W

lower iris


Plasma processes as advanced methods for cavity cleaning

Pill-box cavity for the excitation mode TE111

RF power supply frequency range


Plasma processes as advanced methods for cavity cleaning

Excitation mode TE111

Module of Electric field

Lateral View

Magnetic field

Module of Magnetic field

Base View

Electric field


What do we expect

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.


Plasma processes as advanced methods for cavity cleaning

Al Pill-Box

view port

Loop antenna


Plasma processes as advanced methods for cavity cleaning

  • 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.


Plasma processes as advanced methods for cavity cleaning

  • CORONA

  • DC

  • RF resonance

  • AP plasma

  • RF

  • AP Plasma Jet

  • MICROWAVE

  • MW plasma torch


Plasma processes as advanced methods for cavity cleaning

Atmospheric Pressure Plasma Jet

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


Plasma processes as advanced methods for cavity cleaning

13,56 MHz / 2,45 GHz APPJ Device

Ionization space

Water out

Inner electrode

Outer electrode

Water in

Gas in

RF connection


Plasma processes as advanced methods for cavity cleaning

Current density VS distance from the exit

Current density (μA/mm2)

Distance (mm)

13,56 MHz


Plasma processes as advanced methods for cavity cleaning

Future APPJ source developement

Plasma and chemicals exit radially from the nozzle


Plasma processes as advanced methods for cavity cleaning

  • CORONA

  • DC

  • RF resonance

  • AP plasma

  • RF

  • A P P J

  • MICROWAVE

  • MW plasma torch


Plasma processes as advanced methods for cavity cleaning

MW Atmospheric PlasmaTorch

Gas Inlet

  • Plasma ignited inside a quartz tube at 500W

Quarz tube placed at l / 4

MW 2,45 GHz waveguide

MW 2,45 GHz


Plasma processes as advanced methods for cavity cleaning

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…


Plasma processes as advanced methods for cavity cleaning

Advice and suggestions

THANK YOU


Plasma processes as advanced methods for cavity cleaning

The End? or the beginning


Plasma processes as advanced methods for cavity cleaning

Paschen curve


Plasma processes as advanced methods for cavity cleaning

  • If the applied voltage V is less than the ignition voltage for a Corona discherge Vcthan 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 Vcthan a non-self-sustaining current of 10-14 A can be detected. It is due to ions produced by cosmic rays.


Plasma processes as advanced methods for cavity cleaning

  • 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.


Plasma processes as advanced methods for cavity cleaning

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


Plasma processes as advanced methods for cavity cleaning

Future developements and studies

Cavity

Catode

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


Plasma processes as advanced methods for cavity cleaning

Future developements and studies

Cavity

Catode

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


What s next on lnl superconductivity group

What’s next on LNL superconductivity group?


Plasma processes as advanced methods for cavity cleaning

Focused Ion Beam

  • Niobium etching rate using I3 = 72 μm3/min

  • Niobium etching rate using XeF2 = 60 μm3/min


Which source to be used

Which source to be used?


Gridless source ig1 technical design

Gridless sourceIG1: technical design

Magnetic extractor

Coil

Ionization area

Teflon chamber

Cooled anode

Inlet gas


Plasma processes as advanced methods for cavity cleaning

6 GHz cavity

Cavity

TM010 plasma at a power of 50 W


Plasma processes as advanced methods for cavity cleaning

Excitation mode TM010

Electric field

Module of Magnetic field


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