Presentation on theme: "ALPHA PNEUMATICS 11-Krishna Kutir, Madanlal Dhigra Road, Panch Pakhadi Thane 400602, INDIA DECOMPOSITION OF SF 6 -R134a EFFLUENT."— Presentation transcript:
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.
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