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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. - PowerPoint PPT Presentation
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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
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 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
• 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/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/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 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
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 Nb 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 cavity
Cavity
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
(μ
A/m
m2 )
Future APPJ source developement
Plasma 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 cavity
Cavity
TM010 plasma at a power of 50 W
Excitation mode TM010
Electric field Module of Magnetic field