Howling, Alan

Infoscience - École polytechnique fédérale de Lausanne

1 aah IV ICANS23IV. Uniformity in time: rapid equilibration to  steady-state process parameters. Direct pumping.V. So where is the problem?  - Causes of non-uniformity. Some recommendations.A. Howling  ICANS-23 Utrecht  August 23 (2009)2 aah IV ICANS23Plasma chemistry equilibration time to steady-state depletiontime (s)0 20 40 60SiHnormalized intensityM. N. van den Donker, B. Rech, F. Finger, W. M. M. Kessels, and M. C. M. van de Sanden, Appl. Phys. Lett. 87, 263503 (2005)L. Feitknecht, J. Meier, P. Torres, J. Zuercher, and A. Shah,Sol. Energy Mater. Sol. Cells 74, 539 (2002)... thick amorphous incubation layer ... deteriorates solar cell performance.μc-Si:HAlmost a minute to reach the plasma composition suitable to grow μc-Si:H...3 aah IV ICANS23OES: Non-intrusive, rapid diagnostic for equilibration timeOpticalEmissionSpectrometerpumpgridplasma emission(combined with IR depletion measurement)4 aah IV ICANS23Time-resolved optical emission spectrum from plasma ignitionEmission from silane radicals, SiH*, falls as the silane becomes depletedEmission from excited molecular and atomic hydrogen rises as its partial pressure risesThe plasma chemistry equilibration time is much less than one second!A. A. Howling, B. Strahm, P. Colsters, L. Sansonnens, and Ch. Hollenstein, Plasma Sources, Sci. Technol. 16, 679 (2007).5 aah IV ICANS23The gas residence time...res 0-3 -1-10totThe gas residence time,  1 ,  is the time to replace all the gas in the reactor by flow in and pumping out:molecules m sThe inverse gas residence time (no plasma),  smoleculaanτΦ=⎡ ⎤⋅ ⋅⎣ ⎦⎡ ⎤ =⎣ ⎦[ ] [ ][ ]gas tot6-3 3box6 -10res 0K sccm6.1 10 .es m Pa m500 500For this example,  6.1 10 5.7 s .40 0.0067The gas residence time (no plasma), 1 0.18 seconds.T Fp Vaaτ−−= ×⎡ ⎤ ⎡ ⎤⋅⎣ ⎦ ⎣ ⎦×= × =×= =6 aah IV ICANS23- the zero-dimensional model was used to predict the required time resolution (~ 20 ms) of the optical emission spectrometer for this experiment.( )( )( )44 4440SiH4 SiH e 0 SiHSiHSiH eqAssuming constant pumping speed,  ,the reaction rate balance for silane isSiH :    1 1 exp ,  0where the plasma chemistry equilibration 1/a adnkn a ndtn t tDnΦτ=− + =⎡ ⎤⎛ ⎞∴ = − − −⎢ ⎥⎜ ⎟⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦( )( )eq res0 0eq1 1e time,      .1 0.57For this example,  0.076 s.5.7Da aτ ττ−= < =−= =... and the plasma chemistry equilibration 1/e timeUse the zero-dimensional, time-dependent analytical model to calculate the 1/e equilibration time :00.20.40.60.810 0.2 0.4 0.6 0.8ratio SiH*/Fulchermodels for constantpumping speed & pressuretime [s]experimental datagood agreement with experiment(for constant pressure calculations, see A. A. Howling, B. Strahm, P. Colsters, L. Sansonnens, and Ch. Hollenstein, Plasma Sources, Sci. Technol. 16, 679 (2007))7 aah IV ICANS23Compare open and closed reactors (descriptive)Black arrows: convective flowWhite arrows: diffusive flow (disappear in steady state)• showerhead reactor• open electrodes• indirect pumping at the wall• showerhead reactor• closed electrodes• directly pumped plasma reactor• differential pumping of vacuum chamberDon't design the reactor around your exotic diagnostic technique; design the reactor for the process!8 aah IV ICANS23Open laboratory reactor (axial symmetry) Closed, directly-pumped plasma box(lateral view)Plasma zone (20 cm in diameter)“Dead” volume57 cmPlasma zoneindirect pumpingdirectpumpingt=0-1 stime to steady-state > 100 s ! time to steady-state < 1 s !Silane back-diffusion from any dead volume between the reactor and the pumps increases the equilibration time.t=0-100 sCompare open and closed reactors (numerical)9 aah IV ICANS230.30.40.50.60.70.80.91-0.4 -0.2 0 0.2 0.4(a)silane density (normalized)distance x [m]Vxplasma reactor pumping linepumpgrid1 ms50 ms0.1s0.15s0.2sxmin=DAB /Vx• An advantage of direct pumping is that the concentration gradient is convected downstream away from the plasma. • Retro-diffusion is strongly reduced.• Equilibration time is the minimum theoretically possible.Dispersive axial flow model (analytic)0.30.40.50.60.70.80.91-0.5 0 0.5 1 1.5 2 2.5Longer time axial dispersionx [m]1 ms0.2 s 0.5 s1 splasmaregion10 aah IV ICANS23But the door is normally closed, the amorphous incubation layer is then thinner than 10 nm. substratePractical consequence of the pumping configuration409 nm96 nmAmorphousincubation layers with open door<10 nmplasma box side view11 aah IV ICANS23Intermediate  ConclusionsFor plasma conditions uniform in time:• Rapid equilibration of plasma chemistry requires a closed, directly-pumped showerhead reactor with a uniform plasma.• Avoid gas circulation between the plasma and any dead volumes.- monitored non-intrusively by optical emission spectroscopy via the pumping grid.Ref. A. A. Howling, B. Strahm, P. Colsters, L. Sansonnens and Ch. Hollenstein, Plasma Sources, Sci. Technol. 16, 679 (2007).

Reactor design for large area thin film silicon deposition (Invited tutorial)

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Reactor design for large area thin film silicon deposition (Invited tutorial)

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