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D i a g n o s t i c s t u d i e s o f a c a p a c i t i v e l y c o u p l e d R F p l a s m a c o n t a i n i n g C H 4 - H 2 - A r P a r t I I : O n C H 4 d i s s o c i a t i o n a n d h y d r o c a r b o n p l a s m a c h e m i s t r y T . K a w e t z k i , C h . L u k a s , V . S c h u l z - v o n d e r G a t h e n , H . F . D ö b e l e , M . K ä n i n g ° , J . R ö p c k e ° I n s t i t u t f ü r L a s e r - u n d P l a s m a p h y s i k , U n i G H E s s e n , 4 5 1 1 7 E s s e n , G e r m a n y ° I n s t i t u t f ü r N i e d e r t e m p e r a t u r - P l a s m a p h y s i k , R . - B l u m - S t r . 8 - 1 0 , 1 7 4 8 9 G r e i f s w a l d , G e r m a n y I n s t i t u t f ü r L a s e r - u n d P l a s m a p h y s i k U n i G H E s s e n T O P I C : T h e c o m b i n a t i o n o f s e v e r a l i n d e p e n d e n t d i a g n o s t i c s a l l o w s a n i n s i g h t i n t o t h e p l a s m a c h e m i s t r y i n t h e r e a c t i v e p l a s m a o f a m e t h a n e c o n t a i n i n g d i s c h a r g e . KBr Window Synth.quartz window P G S 2 2m spec- trograph Optical Emission Spectroscopy Infrared- Diode Laser Absorption Spectroscopy " IRMA" Microwave Interferometry 13.56 MHz RF-Discharge with Movable Electrode System ICCD-Camera Cooler IR-ILD IR-Detector Retroreflector ch4setup.DS4 Optical Fiber Microwave Detector 0 20 40 60 80 100 0 5 10 15 power_diss The degree of dissociation of methane 50% CH 4 in Ar 50% CH 4 in H 2 CH 4 degree of dissociation [%] Power [W] 0 20 40 60 80 100 0 2 4 6 power_ch3 Concentration of the methyl radical 50% CH 4 in Ar 50% CH 4 in H 2 [CH 3 ] [10 12 molecules cm -3 ] Power [W] 0 20 40 60 80 100 0 1x10 10 2x10 10 3x10 10 4x10 10 5x10 10 power_ne 50% CH 4 in Ar 50% CH 4 in H 2 Electron density n e [cm -3 ] Power [W] 0 20 40 60 80 100 0 2 4 6 8 10 Concentration of acetylene power_c2h2 50% CH 4 in Ar 50% CH 4 in H 2 [C 2 H 2 ] [10 13 molecules cm -3 ] Power [W] 0 20 40 60 80 100 0 2 4 6 8 10 Electron density and degree of dissociation of methane ar%_ne+diss n e [10 10 cm -3 ] Ar [%] 0 10 20 30 CH 4 degree of dissociation [%] 0 20 40 60 80 100 0 1 2 3 power_c2h6 Concentration of ethane 50% CH 4 in Ar 50% CH 4 in H 2 [C 2 H 6 ] [10 14 molecules cm -3 ] Power [W] C H 4 (methane) C H C H 3 (methyle) C H 2 C 2 H 4 (ethylene) C 2 H 6 (ethane) C 2 H 2 (acetylene) + e - + e - + e - , H + C H 3 + C H 2 + C H 4 + e - ( 1 ) ( 1 ) ( 1 , 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) 0 20 40 60 80 100 0 1 2 3 4 5 [CH 3 ] [10 12 molecules cm -3 ] Ar [%] 0 2 4 6 Concentration of the methyl radical and acetylene and the relative intensity of the CH band ar%_ch3+c2h2+ch rel. CH intensity [C 2 H 2 ] [10 13 molecules cm -3 ] (1) The EEDF determines the dissociation of CH 4 (2) Chemical reaction of CH 4 with H at high H density [ W.L. Hsu, J. Appl. Phys. 72, 3102 (1992) ] (3) C 2 H 6 is formed by recombination of CH 3 [ similar dependence on power ] (4) The abstraction of H to produce C 2 H 5 and then C 2 H 4 is too slow [ different dependences on the argon mixture of CH 3 and C 2 H 2 ] (5) C 2 H 4 is produced by the rapid reaction of CH with CH 4 [ similar dependence on the argon mixture of CH and C 2 H 2 ] (6) C 2 H 2 is formed by electron impact dissociation of C 2 H 4 [ W.Y. Fan et al., J. Phys. Chem. A, submitted ] (7) C 2 H 2 does not arise from C 2 H 6 [ W.Y. Fan et al. ] - No noticeable difference in the electron density with CH 4 in Ar or H 2 , despite a big difference in pure Ar or H 2 - The electron density dependence on power exhibits two almost linear sections - The degree of dissociation of CH 4 depends nearly linearly on power - Degree of dissociation at maximum power: about 15 % - A higher degree of dissociation with CH 4 in Ar than in H 2 indicates changes in the EEDF - The CH 3 radical is the most likely precursor for forming carbon films - The CH 3 density increase with power is less pronounced than the increase of the degree of dissociation - Noticeable differences of about 25 % in the CH 3 densities in Ar or H 2 - CH 3 density in H 2 at maximum power: about 5.5 x 10 12 cm -3 - CH 3 density in Ar at maximum power: about 4.0 x 10 12 cm -3 - The C 2 H 6 density and the CH 3 density behave similarly ( higher densities in H 2 than in Ar ) - The C 2 H 2 density also increases with power, but with higher densities in Ar, just opposite to C 2 H 6 and CH 3 - The electron density as a function of the admixture of Ar and H 2 exhibits a maximum at 1:1 of Ar and H 2 - The degree of dissociation is lower at higher electron densities => At high hydrogen partial pressures chemical dissociation by H is important => There have to be changes in the EEDF - In layer forming plasmas it is difficult to measure the EEDF by probes - Non-intrusive measurements of the EEDF are desirable - For a first attempt to get information about the EEDF by OES see part I - The variation of the CH 3 density with the Ar mixture is comparable to the variation of the electron density - The behaviour of the C 2 H 2 density is qualitatively different - The maximum C 2 H 2 density is at an Ar admixture of 80 % - The behaviour of the relative emission density of CH is similar to the C 2 H 2 density - Most probably CH is the dominant source of C 2 H 4 and C 2 H 2 D i a g n o s t i c s Emission Spectroscopy - The emission of various species is observed using a 2m- spectrograph and an ICCD-Camera Microwave Interferometry - A Heterodyn-Interferometer operating at a wavelength of λ = 1 mm is used to monitor the electron density For more details of the emission spectroscopy and the microwave interferometry see part I Tunable IR diode laser absorption spectroscopy (TDLAS) - powerful technique to measure absolute number densities of molecules and radicals in the electronic ground state - A new compact and transportable tunable infrared multicomponent acquisition system 'IRMA' has been used - IRMA consists of 4 independent IR diode lasers - Concentrations of several molecular species could be measured simultaneously - The IR beam of IRMA passes the plasma 2 times - For more details of IRMA: oral presentation of J.Röpcke E x p e r i m e n t a l S e t u p - capacitively-coupled parallel-plate discharge - uncooled stainless-steel electrodes diamater of electrodes: 100 mm distance of electrodes: 25 mm - electrode system displaceable in two orthogonal directions P o w e r s u p p l y - Commercial RF transceiver with matching box - Output power @ 13.56 MHz: 10 - 100 W G a s s u p p l y - Mass flow controlled gases: Argon, H 2 , CH 4 - Total pressure: 10 - 100 Pa - Total flow (here): 66 sccm Setup of the IRMA Optical Table BC: beam combiner, CS: cold station, D: detector, HC: Herriot cell, IF: intermediate focus, GM: grating monochromator, RC: reference cell. 0 50 100 150 200 250 -40 -20 0 20 40 1302.736 cm -1 1302.597 cm -1 ] ] [ ] [ [ laser off end fit begin fit CH 4 C 2 H 2 signal [m V] channel num ber Measured and fitted absorption spectrum S u m m a r y a n d C o n c l u s i o n s - For the first time three independent diagnostic methods have been applied to a CH 4 containing discharge simultaneously to get an insight into plasma physical and plasma chemical processes which could influence thin carbon film growth rate and homogeneity - Particle densities of several C-H molecules, electron densities and gas temperatures were measured by IR absorption spectroscopy, microwave interferometry and emission spectroscopy, respectively - The various plasma parameters were measured at different RF-powers and different gas mixtures - In principle all diagnostic methods are line integrated, but the special discharge chamber allows to measure radial profils by using Abel inversion (see part I) - On the basis of these measurements a model has been developed that describes the chemical processes and allows to identify the main plasma chemical reaction paths - Furthermore some informations about the EEDF were obtained (see part I) This work was funded by the DFG in the frame of the SFB191 (Essen/Bochum) and SFB198 (Greifswald) I n s t i t u t f ü r N i e d e r t e m p e r a t u r - P l a s m a p h y s i k G r e i f s w a l d
