ALPHA PNEUMATICS 11-Krishna Kutir, Madanlal Dhigra Road, Panch Pakhadi Thane , INDIA DECOMPOSITION OF SF 6 -R134a EFFLUENT BY RF PLASMA AVINASH V. JOSHI

RPC GAS EFFLUENTS : FACTS  Efficiency of gas recovery /recycling process, for open loop or close loop method : In the range of 90 % to 95%.,  At least 5% gas to be let out in to Atmosphere  The proposed INO set up : RPCs of size 2m x 2m, One volume change/ day  Internal volume of each RPC : 8 ltrs., Total volume 240 m 3  Minimum 12 m 3 /day gas mixture discharged into Atmosphere

Consequence Of Untreated Exhaust Of Effluents  Daily CO 2 Discharge Equivalent SF 6 ( 0.5%) equivalent to 1350 m 3 /day R134a (95%) equivalent to m 3 /day  Contribution to Green House gases same as burning 25 tons of Carbon every day.  SF 6 and R134a can not be cycled back (like CO 2 ), by natural processes. Will accumulate in the atmosphere.

PROBLEMS IN CONVERTING EFFLUENTS TO SAFER MATERIALS  SF 6 and R134a are highly stable compounds.Can not be decomposed or converted by simple chemistry.  Being saturated compounds, they can not be adsorbed or polymerized to form solid products.  Gases are more harmful than equivalent CO 2 emission because of cumulative effect (They can not be recycled by natural processes)  SF 6 is toughest to break, R134a is relatively easier.

PLASMA DISSOCIATION OF SF 6  SF 6 at 1 torr pressure fills the space between Electrodes across which RF ( 13.56MHz) power is applied.  Electron moves from surface of the plate and is accelerated towards ground electrode.  The Mean Free path being sufficiently long at 1 torr, the Electron acquires necessary energy for interaction with SF 6 molecule.  The interaction is : SF 6 + e - = (SF 2 ) - + 4F (1)  In presence of Oxygen and Fluorine cascade of reactions take place near ground electrode, starting from SF 2 and resulting in release of large number of Fluorine atoms and Radicals. (1)

REACTIVE ION ETCHING (RIE)  Similarity of situation was noticed while working on Reactive Ion Etching (RIE) process used in solar cell manufacturing.  Purpose of RIE : to obtain highly textured anti reflection surface  Uses a gas mixture of SF 6, R134a, O 2, and Argon.  Under plasma conditions Fluorine is liberated from break down of SF 6 and R134a.  Fluorine and Fluorine based radicals react with silicon surface.  While Fluorine etches Silicon to SiF 4 ( gas) and Radicals react at silicon surface to form solid polymers.  Polished silicon surface changes to anti-reflecting texture.

SCHEMATIC OF REACTIVE ION ETCHING OF SILICON WITH SIF 4 ABSORPTION

RIE Process conditions and Etch rate variations  It was noticed that at certain process conditions, the Silicon etch rates were found to be highly Non-uniform due to Fluorine Depletion along length of process zone X.  This is not a good result from RIE process point of view.  Whereas this is an ideal condition for extraction of fluorine and fluorine radicals from SF 6.  The Concentration Measurements taken on the pump suction by IR Optical Emission Spectroscope..

REACTION WITH SILICON  Fluorine,the product of SF 6 plasma breakdown reacts with Silicon at ground electrode : Si + 4F = SiF 4  The reaction is reversible, SiF 4 Is a gaseous product.  Accumulation of SiF 4, depletion of SF 6 and F Pushes the net reaction to stop.  SiF 4 is reacted with Aluminum Oxide at ground electrode to form AlF 3 …a Solid product. SiF 4 is depleted and the gas phase reaction rate is restored.  End products are absorbed in liquid chemical scrubber at pump exhaust.

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DISTRIBUTION OF FLUORINE AND RADICALS ALONG THE LENGTH ‘X’ OF REACTION ZONE (1)) MEASURED BY OPTICAL EMISSION SPECTROSCOPY TOTAL GAS DENSITY : 3 x 10E16 /cm -3 (1 TORR) ELECTRODE SPACING :10 mm, FLOW RATE: 30 SCCM, POWER DENSITY: 0.3W/cm 2 EXPERIMENTAL CONDITIONS:

DEPENDENCE OF RADICAL CONCENTRATION ON OXYGEN MOLAR CONCENTRATION IN THE INPUT GAS (1) (2) TOTAL DENSITY :3 x 10E16 /cm -3 (1 TORR), ELECTRODE SPACING :10 mm, FLOW RATE: 30 SCCM, POWER DENSITY: 0.3W/cm 2 NOTE: TO OBTAIN MAXIMUM FLOURINE FROM THE SF6,NEED TO OPERATE WITH SF6 to O2 in the 1:1 ratio

FACTORS CONTROLLING THE EXTENT OF REMOVAL OF EFFLUENTS  ELECTRODE DESIGN : Electric field is responsible for generation of electrons, d/L Ratio of injector holes and the distance between electrodes has a strong influence field strength.  GAS PRESSURE : Efficiency of Ionization is maximum at 700 mtorr to 1200 mtorr  CONCENTRATION ( ACTIVITY) In a reversible chemical reaction of type n1*R 1 +n2*R 2 * = m1*P 1 + m2*P 2 *……….. Where n1,n2, m1, m2 are stoichiometric coefficients, R1, R2,P1,P2 are Activities ( Concentrations) of Reactants and Products respectively. Reaction rate V is expressed as V FORWARD = K F *(R 1 ) n1 * (R 2 ) n2 * ………………… V REVERSE = K R * (P 1 ) m1 * (P 2 ) m2 *……………….. ….. Net forward reaction rate is difference of Forward and Reverse rates It is necessary to avoid Product pile-up and maintain net Forward reaction rate.

REACTIVE ION ETCHING SETUP GAS MIXTURE INLET VACUUM CHAMBER RF ELECTRODE OPTICAL EMISSION SPECTROSCOPE/ LANGMUIR PROBE PORT PUMPING PORT

PROPOSED SETUP FOR PLASMA DECOMPOSITION UNIT WITH WET SCRUBBER

REFERENCES (1) K.R.Ryan And I.C.Plumb, Plasma Chemistry Of SF6/O2 Mixtures, Plasma Chem.Plasma Process, VOL.6 (1986) pp247 (2) Y.Tzeng And T.H.Lin,Dry Etching Of Silicon Material In SF6 Based Plasma, J. Electrochemical Society (Solid State Science & Technology), volume 134 No. 9 pp 2304, September 1987

ACKNOWLEDGEMENTS  PROF. NABA K. MONDAL (TIFR – MUMBAI)  Dr. ANIL KUMAR SAXENA, DGM,AMORPHOUS SILICON SOLAR CELL PLANT B.H.E.L., GURGAON INDIA