Plasma modelling & reactor design

Transcription

1 Plasma modelling & reactor design Alan Howling, Ben Strahm, Christoh Hollenstein Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland Center for Research in Plasma Physics (CRPP) 1) The "lasma dimension" for lasma deosition of μc-si:h 2) Direct uming of a lasma reactor... and one non-intrusive lasma diagnostic for each. IWTFSSC-2 Berlin Aril 2009

2 1) Plasma-Enhanced Chemical Vaour Deosition of μc-si:h Φ H2 silane hydrogen Φ SiH4 black box PECVD

3 1) PECVD reactor setu P RF [W] f [MHz] Φ H2 silane hydrogen Φ SiH4 Plasma Box, Oerlikon rincile: Jacques Schmitt vacuum chamber RF connection gas flow showerhead RF electrode butterfly valve lasma box side view rocess ums heating grounded box glass substrate

4 c 1 1) Hydrogen dilution for lasma deosition of μc-si:h Φ H silane hydrogen Φ SiH4 low lasma box ΦSiH silane concentration c = 4 < 5% Φ is commonly used total

5 1) Reason for hydrogen dilution Need high ratio of Hto SiH x fluxes to deosit μc-si:h intuitively "Add more hydrogen than silane" Selective etching model Chemical annealing model Surface diffusion model

6 1)...but the otimal lasma control arameters are less clear c 1 Φ H2 f [MHz] P RF [W] silane hydrogen Φ SiH4 ga Φ tot? area [mbar] lasma black box: arameter inter-deendence

7 1)...because the lasma is comlex. Φ H2 Φ SiH4 P RF [W] f [MHz] { } molecules H SiH { n electron diss : n } e, Te { y sec reactions } 2 4 SiH, H, cations, anions x olysilanes, dust 2 4 H SiH olysilanes, dust { transort in the lasma and the sheath} { surface reactions } SiH x H, H2

8 1) Consider low ressures (< 2 mbar) Φ H2 Φ SiH4 P RF [W] f [MHz] { } molecules H SiH { n electron diss : n } e, Te 2 4 SiH, H, cations, anions x H 2 SiH4 { transort in the lasma and the sheath} { } surface reactions SiH x H, H 2

9 1) Consider only the majority secies Φ H2 { molecules } H 2 SiH4 H 2 SiH4 Φ SiH4 All reactive secies (SiH x, H, and ions) have very low density because of their volume and surface reactions H 2 and SiH 4 have the dominant artial ressures in the lasma reactor Only H 2 and SiH 4 leave the lasma reactor The artial ressure of SiH 4 (and H 2 ) is ~the same in the uming line as in the lasma "The lasma comosition (and deosition) is determined by the artial ressures of SiH 4 and H 2 in the lasma"

10 1) How to measure the silane artial ressure in the lasma? "inaccessible" lasma reactor vacuum chamber load lock gate valve lasma box heated closed showerhead uming line butterfly valve rocess ums substrate loading door

11 1) FTIR in um line: Non-intrusive diagnostic for SiH 4 ressure Fourier transform infrared (FTIR) absortion sectroscoy via ZnSe windows. IR source 1 m single ass extended uming line IR detector

12 1) Silane artial ressure measurement a) Silane Q-branch absorbance; b) Integrate sectrum; c) Read off a calibrated curve Silane ressure with lasma < silane ressure without lasma, SiH 0 SiH <, 4 4 due to electron dissociation of silane ( irreversible loss). Hydrogen artial ressure increases with lasma (surface recombination, & um seed adjustment)

13 1) Silane inut concentration (without lasma) Φ H2 Φ SiH4 Define 0 0 SiH H + = SiH H + = 4 2 silane inut concentration, Note 0 c 1 c = 0 SiH 4

14 1) Silane concentration with lasma Φ H2 Φ SiH4 + = SiH Define silane concentration with lasma, H 4 2 c l SiH = 4 + = SiH 4 2 H

15 1) Silane deletion due to lasma Φ H2 Φ SiH4 Define + = SiH H = SiH H 4 2 silane fractional deletion, Note 0 D 1 D = 0 SiH SiH 4 SiH

16 1) Silane concentration in lasma Φ H2 Φ SiH4 Therefore + = SiH 4 2 ( ) silane concentration in lasma, c = c 1 D H l + = SiH 4 2 H 2 arameters, c & D, define the lasma comosition

17 1) The lasma "black box" becomes a {c,d} unit box 1 ure silane the "lasma dimension" silane inut concentration, c vanishingly weak lasma concentration infinitely strong lasma infinitely diluted silane 0 0 silane deletion fraction, D 1

18 1) Contours of constant lasma comosition silane inut concentration, c 1 same lasma comosition for ( ) c = c 1 D = const. l 0 0 silane deletion fraction, D 1

19 1) Contours of constant lasma comosition silane inut concentration, c 1 ( c = 0.05 ) D = 0.1 ( ) cl = c 1 D = silane deletion fraction, D 1 5% silane 95% hydrogen strong inut dilution, weak deletion IN THE PLASMA ( c = 1 ) D = % silane IN THE 95% hydrogen PLASMA NO inut dilution, 95% deletion

20 1) Same lasma comosition = same film roerties Constant lasma comosition, c l, gives constant film roerties

21 1) Same lasma comosition = same film roerties transition material

22 1) Same lasma comosition = same film roerties c l = 1.2% c l = 0.5% Microcrystalline silicon can be deosited even with high silane concentration, rovided that the silane deletion fraction is sufficiently high, because then the lasma is dominated by hydrogen.

23 1) Two regimes of oeration silane inut concentration, c 1 REGIME 1 "conventional" : Film crystallinity governed by hydrogen dilution of silane, 'indeendent' of lasma. 0 0 silane deletion fraction, D 1 REGIME 2 "recent" : Film crystallinity governed by strong lasma deletion, even for high silane concentration.

24 1) Simle lasma chemistry model Based on a reduced set of gas hase reactions k1 e + SiH 4 SiH 2 + 2H + e e e e + kh H2 2H + e e + + e and simlified surface reactions. SiH + 2 S Sisurf H2 H + H surf H H + vacancy H R 1 H H vacancy surf +

25 1) Simle, analytical lasma chemistry model IN Plasma reactions & deosition OUT Φ Φ = kn n + a n SiH e SiH SiH ( 2 ) + Sn + R n = k n n + an H SiH H H e H H { radical fluxes } Sn SiH 2 Rn H { surface reaction} deosition H+H recombination A zero-dimensional model is aroriate to showerhead reactors

26 1) Simle, analytical lasma chemistry model Just one silane dissociation channel here: e + SiH 4 SiH 2 + 2H + e More detailed lasma chemistry only changes the numerical constants:... the general conclusions remain the same. radical flux ratio Γ Γ Rn k nh 2 k 1 = = = Sn k n k c H H H H SiH SiH SiH l Hydrogen and silane are the dominant artial ressures; so we can exect radical densities to deend on them. "This shows why the lasma deosition is determined by c l, the silane concentration in the lasma."

27 lasma black box f [MHz] P RF [W] ga Φ tot 1) Deletion accounts for all of the lasma arameters? area [mbar] analytical model D = 1+ akn e ( 1+ c) 1 Φ -1 6 Tgas total a s = effective uming seed = ; ga area ( [ ]) -1 kn e s = silane dissociation rate = function of PRF, f MHz ; ΦSiH 4 c = silane inut concentration,. Φ total { } { Φ } D if any of, ga, area, c, P, f &/or, T RF total gas

28 1) The "lasma dimension" for lasma deosition of μc-si:h 2) Direct uming of a lasma reactor IWTFSSC-2 Berlin Aril 2009

29 2) Plasma chemistry equilibration time to steady-state deletion van den Donker et al (2005) Feitknecht et al (2002) SiH normalized intensity Almost a minute to reach the lasma comosition suitable to grow μc-si:h time (s) time (s) μc-si:h... thick amorhous incubation layer... deteriorates solar cell erformance.

30 2) OES: Non-intrusive, raid diagnostic for equilibration time lasma emission um grid Otical Emission Sectrometer

31 2) Time-resolved otical emission sectrum from lasma ignition Emission from silane radicals, SiH*, falls as the silane becomes deleted Emission from excited molecular and atomic hydrogen rises as its artial ressure rises The lasma chemistry equilibration time is less than one second!

32 2) Comare oen and closed reactors (numerical) Oen laboratory reactor (axial symmetry) Closed, directly-umed lasma box (lateral view) Plasma zone (20 cm in diameter) Dead volume 57 cm Plasma zone indirect uming t=0-100 s time to steady-state > 100 s! direct uming t=0-1 s time to steady-state < 1 s! Silane back-diffusion from any intermediate dead volume increases the equilibration time.

33 2) Practical consequence of the uming configuration Amorhous incubation layers with oen door 409 nm 96 nm lasma box side view <10 nm su bs tra te But the door is normally closed, the amorhous incubation layer is then thinner than 10 nm.

34 Two Conclusions 1. Plasma comosition and deosition deend on the silane concentration in the lasma, c l =c(1-d), and not only on the silane concentration in the inut flow, c. Strong hydrogen dilution in the lasma, and μc-si:h deosition, can be obtained with high inut concentration of silane. - monitored non-intrusively by FTIR in the uming line. 2. Raid equilibration of lasma chemistry requires a closed, directly-umed showerhead reactor with a uniform lasma - avoid gas circulation between the lasma and any dead volumes. - monitored non-intrusively by OES in the uming line. Refs. B. Strahm et al, Plasma Sources, Sci. Technol (2007). A. A. Howling et al, Plasma Sources, Sci. Technol (2007). Acknowledgements: Swiss Federal Research Grant CTI ; and Dr. U. Kroll and colleagues, Oerlikon-Solar Lab SA, Neuchâtel, Switzerland.