DECOMPOSITION OF SF6-R134a EFFLUENT
BY RF PLASMA
AVINASH V. JOSHI
ALPHA PNEUMATICS
11-Krishna Kutir, Madanlal Dhigra Road, Panch Pakhadi
Thane 400602, INDIA
alpha_pneumatics@hotmail.com
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 SILICON
WITH SIF4 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 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.
10
DISTRIBUTION OF FLUORINE AND RADICALS ALONG THE LENGTH
‘X’ OF REACTION ZONE (1)) MEASURED BY OPTICAL EMISSION SPECTROSCOPY
EXPERIMENTAL CONDITIONS:
TOTAL GAS DENSITY : 3 x 10E16 /cm-3 (1
TORR)
ELECTRODE SPACING :10 mm,
FLOW RATE: 30 SCCM,
POWER DENSITY: 0.3W/cm2
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*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 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