ALPHA PNEUMATICS

11-Krishna Kutir, Madanlal Dhigra Road, Panch Pakhadi

Thane 400602, INDIA

alpha_pneumatics@hotmail.com

DECOMPOSITION OF SF6-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 : 30000 RPCs of size 2m x 2m , One volume change/ day

Internal volume of each RPC : 8 ltrs., Total volume 240 m3

Minimum 12 m3 /day gas mixture discharged into Atmosphere

Consequence Of Untreated Exhaust Of Effluents

Daily CO2 Discharge Equivalent

SF6 ( 0.5%) equivalent to 1350 m3 /day

R134a (95%) equivalent to 45600 m3 /day

Contribution to Green House gases same as burning 25 tons of Carbon every day.

SF6 and R134a can not be cycled back (like CO2) , by natural processes. Will accumulate in the atmosphere .

PROBLEMS IN CONVERTING EFFLUENTS TO SAFER MATERIALS

SF6 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 CO2 emission because of cumulative effect (They can not be recycled by natural processes)

SF6 is toughest to break, R134a is relatively easier.

PLASMA DISSOCIATION OF SF6

SF6 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 SF6 molecule.

The interaction is :

SF6 + e- = (SF2)- + 4F (1)

In presence of Oxygen and Fluorine cascade of reactions take place near ground

electrode, starting from SF2 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 SF6 , R134a , O2 , and Argon .

Under plasma conditions Fluorine is liberated from break down of SF6 and R134a.

Fluorine and Fluorine based radicals react with silicon surface .

While Fluorine etches Silicon to SiF4 ( 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 SILICONWITH SIF4 ABSORPTION

RIE Process conditionsand 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 SF6.

The Concentration Measurements taken on the pump suction by IR Optical Emission Spectroscope..

REACTION WITH SILICON

Fluorine ,the product of SF6 plasma breakdown reacts with Silicon at ground electrode :

Si + 4F = SiF4 The reaction is reversible, SiF4 Is a gaseous product.

Accumulation of SiF4 , depletion of SF6 and F Pushes the net reaction to stop.

SiF4 is reacted with Aluminum Oxide at ground electrode to form AlF3…a Solid product . SiF4 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/cm2

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/cm2

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*R1 +n2*R2 *.......= m1*P1+ m2*P2 *………..

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 VFORWARD = KF *(R1)n1 * (R2)n2 * …………………

VREVERSE = KR * (P1)m1 * (P2 )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 UNITWITH 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