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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 410-2 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 410-2 mbar • Pressure of 410-2 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