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

CLEANING a post processs Hydrocarbons Water Oxygen, Nitrogen and other adsorbed gases ETCHING a 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 CUORE CUORE 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: CF 4 /O 2 (a,b), CCl 3 F (c), SF 6 /O 2 (d) ; I 2, XeF 2 (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 MHz Using CF 4 and O 2 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 O 2 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 From Literature

CCl 3 F-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 CF 4 should work too.

LNL ACTUAL RESULTS Etching rate: 12,7 μm/h Niobium DC diode sputtering with CF 4 Pressure of 4  mbar Sample voltage: 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 V dc 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 4  mbar Substrate to source:170 mm Ar 2,3 μm/h CF 4 12,7 μm/h REACTIVE ION ETCHING Diode sputterind with CF 4 Pressure of 4  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 gradient Low field gradient Discharge Corona

DC Corona discharge A non-self-sustaining current of 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 O 3 V applied << V corona V applied > V corona 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 O 3

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 O 3

Why could corona plasma be useful? UV/O 3 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 O 3 1,5 GHz seamless Cu Cavity

Positive Corona inside a 1,5 GHz cavity Discharge voltage 25kV Production of O 3 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 fieldModule 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 O 2 +He 2 O 2 +He 2 +CF 4 MaterialKaptonSiO 2 TaW 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)

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 Current density VS distance from the exit Distance (mm) Current density (μA/mm 2 )

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 ApjetDiffuse Dielectric Barrier CoronaMicrowave MethodHelium Process Gas with added reactive gas Dielectric Cover on Electrode with He process gas Sharply Pointed Electrode at HV Wave Guides Resonant Cavity. Complex Frequency2-60 MHz RF1-100 KHz ACDC/Pulsed Pwr2.45 GHz Plasma Density Electrons/cm 3 (volume average) Reactive Species: O/cm ? (Limited due to ozone generation) Undesirable byproducts: Ozone/cm High TemperatureLow High at edgeRF Substrate Heating Uniform GlowYesYes?NoPoint Source Process MethodsDownstream or In- situ In-situ Downstream Flexible ShapesYes No HazardsLowHigh Ozone Substrate Damage High Voltage High OzoneSignficant Health & Safety (microwave) + High Ozone Scalable to large area? Yes No

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

V applied << V corona a non-self-sustaining current of A can be detected. It is due to ions produced by cosmic rays V applied > V corona 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 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 CORONAPOSITIVE CORONA -Electron emission from the cathode -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 I 3 = 72 μm 3 /min Niobium etching rate using XeF 2 = 60 μm 3 /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 mesuredCorrente 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 fieldModule of Magnetic field