Transcript Plasma processes as advanced methods for cavity cleaning

N. Patron, R. Baracco, L. Phillips, M. Rea, C. Roncolato, D. Tonini
and V. Palmieri
Plasma processes as advanced
methods for cavity cleaning
… pushing the limits of RFS
Legnaro 2006
ETCHING
CLEANING
a main process
a post processs
• Removal of ~ 100 μm
• Hydrocarbons
•Reduce surface roughness
• Water
• Reduce surface contamination • Oxygen, Nitrogen
and other adsorbed
gases
• Remove damaged layers
• Sputtering
• PLASMA
• Reactive ion etching
• DRY ETCHING
• ION GUN
• Ion beam cleaning
• Reactive ion beam etching
• Chemical etching
• WET ETCHING
• Electropolishing
• Electromachining
Let’s analyze one by one the different
DRY ETCHING
techniques
• PLASMA
• Sputtering
• Reactive ion etching
• DRY ETCHING
• ION GUN
• Ion beam cleaning
• Reactive ion beam etching
One example from our experience:
CUORE
Cryogenic 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
Dry etching methods are
very clean
•Smooth surface
•Thin grain boundaries
But Physical Methods treatment
can become an enemy…..
A deeper etching
• Coarsening of grain
boudaries
• Rough surface
Cleaner surface, but higher
demagnetization factor
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.
• Sputtering
• PLASMA
• Reactive ion etching
• DRY ETCHING
• ION GUN
• Ion beam cleaning
• Reactive ion beam etching
• 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
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
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/h
M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983
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
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
• Sputtering
• PLASMA
• Reactive ion etching
• DRY ETCHING
• ION GUN
• 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
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
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
REACTIVE ION ETCHING
• Energy: 2 KeV
• Diode sputterind with CF4
• Pressure of 410-2 mbar
• Pressure of 410-2 mbar
• Substrate to source:170 mm
Ar
CF4
2,3 μm/h
12,7 μm/h
A possible cavity application
Gas flux
Plasma
region
Rotational
extracting
grid
• Sputtering
• PLASMA
• Reactive ion etching
• DRY ETCHING
• ION GUN
• Ion beam cleaning
• Reactive ion beam etching
Atmospheric-pressure
Plasma
• DC
• AP plasma
• RF
• CORONA
• RF resonance
• AP Plasma Jet
• 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
• DC
• AP plasma
• RF
• CORONA
• RF resonance
• AP Plasma Jet
• 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.
High
Low field
Corona
Discharge
gradient
gradient
Electrodes
DC Corona discharge
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.
• A self sustaining
discharge makes the
current jump to ~10-6 A.
• Massive production of O3
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
1,5 GHz seamless Cu Cavity
•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
• 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
• DC
• AP plasma
• RF
• CORONA
• RF resonance
• AP Plasma Jet
• 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
Lateral view
Electric field
Module of Magnetic
field
Base view
Magnetic field
Module of Electric
field
6 GHz cavity
Cavity
TM010
plasma at
a power of
50 W
1,5 GHz cavity
upper iris
antenna
Plasma
at a
power of
150 W
lower iris
Pill-box cavity for the excitation
mode TE111
RF power supply
frequency range
Excitation mode TE111
Lateral View
Base View
Magnetic field
Module of Electric
field
Electric field
Module of Magnetic
field
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.
• DC
• AP plasma
• RF
• CORONA
• RF resonance
• AP Plasma Jet
• MICROWAVE
• MW plasma torch
Atmospheric Pressure Plasma Jet
Gas
mixture
O2+He2
O2+He2
+CF4
O2+He2+ O2+He2+ O2+He2+
CF4
CF4
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
Ionization
space
Inner
electrode
Water in
Gas in
RF connection
Outer
electrode
Current density (μA/mm2)
13,56 MHz
Current density VS distance from the exit
0,50
100 W
0,40
30 W
0,30
0,20
0,10
0,00
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Distance (mm)
Future APPJ source developement
Plasma and chemicals exit radially from the nozzle
• DC
• AP plasma
• RF
• CORONA
• RF resonance
•APPJ
• MICROWAVE
• MW plasma torch
MW Atmospheric Plasma Torch
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
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 Guides
Resonant Cavity.
Complex
Frequency
2-60 MHz RF
1-100 KHz AC
DC/Pulsed Pwr
2.45 GHz
Plasma Density
Electrons/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 Insitu
In-situ
In-situ
Downstream
Flexible Shapes
Yes
Yes
No
No
Hazards
Low
High Ozone
Substrate 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
-Electron emission from the
cathode
-Photoionization
POSITIVE CORONA
-Photoionization
Future developements and studies
Cavity
Catode
Catode’s edges
facing the
cavity where
the corona will
be ignited
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?
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 It is more simple has a Struttura
lifetime limited by the sputtering robusta e semplice da revisionare
process
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
Magnetic
extractor
Coil
Teflon
chamber
Cooled
anode
Ionization
area
Inlet gas
6 GHz cavity
Cavity
TM010
plasma at
a power of
50 W
Excitation mode TM010
Electric field
Module of Magnetic
field