Vacuum Technology and film growth Poly Gate pmos Polycrystaline Silicon Source Gate p-channel Metal-Oxide-Semiconductor (MOSFET) Drain polysilicon n-si ion-implanted Diffusion Resistor Poly Si Resistor Field oxide grown in steam, gate oxide made by CVD p-regions ion-implanted, Al sputter deposited or evaporated 1
Why cover vacuum science? Oxidation Sept. 14 Key advantage of Si: stable uniform oxide How control its growth, thickness, quality Ion implantation and diffusion Sept. 8 How semiconductor surfaces are doped Chemical vapor deposition (CVD) Oct 1 Most widely used method for growth of high-grade semiconductor, metals, oxide films, Physical vapor deposition (PVD) Oct. 19, 6 Growth of quality films by sputter deposition or evaporation These processes done in vacuum or controlled environment. Therefore, need to understand vacuum technology, gas kinetics.
Gas Kinetics and Vacuum Technology How far does a molecule travel between collisions? Consider a volume V of gas (e.g. N ) m 5 x 10-6 kg L velocity Snap shot number N, n = N V = N L 3 Movie Mean free path λ d molecule => impact parameter, scattering cross section = π d d λ π d Volume swept out by 1 molecule between collisions = λπd 3
Volume swept out by 1 molecule between collisions = λπd λ π d Total volume of sample L 3 = V Nλπd λ V Nπd = 1 nπd More accurately: λ = nπd Use Ideal gas: n = N/V N/V = p/k p/k B T B p λ (cm) λ = πd k B T p 1 atm 10-5 1 Torr 10-1 mt 10 4
What is flux of atoms hitting surface per unit time? v x # / vol. area Calculating gas velocities Maxwell speed distribution: m P(v) = 4π πkt 3/v exp mv kt Generally: v = vp(v)dv 1 mv 3 k BT T <=> <=> molecular molecular velocity velocity J ( # / area time) = nv x We need v x, v v x = P(v) kt πm v = v x = v / v 8kT πm, Do Do dimensional dimensional analysis analysis on on J J = nv nv Show Show JJ analogous analogous to to current current density, density, related related to to pressure pressure (elec. (elec. field) field) v vms v rms = 3kT m speed v rms 500 m/s 5
So flux of atoms hitting surface per unit time v x # / vol. area J x = nv x = n kt πm ideal gas p πmkt = J x Compare: λ = k BT πd p Dimensional analysis: (force/area = en/vol.): p = E kin Vol = n mv = Jmv Numerically, J x = 3.5 10 p(torr) MT(g /mole K) (atoms/cm sec) Pressure Pressure = (Molecular (Molecular momentum) momentum) x x flux, flux, JJ This gives a flux at 10-6 Torr of 1 monolayer (ML) arriving per sec Why Why not not per per unit unit area? area? 6
Recall diffusion in solids: Diffusivity For gas, no energy barrier, just collisions. dc n J J gas = D gas dx D n λ nv x recall λ = v x kt πd p T D gas λv x (cm /s) D gas T 3/ p or D gas T 1/ much much weaker weaker T-dep. T-dep. than than in in solid solid (which (which is is exponential) exponential) D = D 0 exp G kt Debye ν 10 13 s -1 G Figure removed for copyright reasons. Figure - in Ohring, M. The Materials Science of Thin Films. nd ed. Burlington, MA: Academic Press, 001. ISBN: 01549756. 10-6 Torr => 1 monolayer/ sec λ 7
Review Ideal gas: pv = Nk B T, λ = k B T πd p 10 cm at p = 1 mt 100 m at 10-6 Torr Generally: 1 mv 3 k BT J x = nv x = n kt πm ideal gas p πmkt = J x p = E kin Vol = n mv = Jmv J gas = D gas dc dx D n λ D gas λv x (cm /s) Weak temperature dependence relative to solid state diffusion 8
L = dimension of chamber or reactor Knudsen number p λ (cm) Recall: λ = k BT πd p 1 atm 10-5 1 Torr 10-1 mt 10 Knudsen number, N 0 = λ/l λ/l < 1 λ/l > 1 Flow is viscous; p > 1 mt Pump power must be > viscosity; Must transport large # of molecules Molecular, ballistic flow; p < 1 mt Pump efficiency is critical; Must attract and hold molecules What does this imply for pumping? 9
Gas flow and pump speed Gases are compressible, unlike liquids. express flow as number of molecules/time, not volume/t. Q = dn dt = 1 k B T V dp dt + p dv dt Conductance of Pump throughput, Q: vacuum component: dp/dt p p 1 p dp/dt = v p v p p Using Using ideal ideal gas gas law law pump Q = Cp ( p p ) Units of conductance = 1/(sec-Pa) C area/length Ohm: Ohm: I I = V/R V/R Q ps S=Q/p S=Q/punits #/(sec*pa) Pump speed units, S = V/(k B T t) sccm or L/s Std. cc/min Liters/sec 10
Gas flow and pump speed Conductance of vacuum component: Q = Cp ( p p ) (p p is pressure nearer pump) System throughput: Q ps, eff p = Q S eff p p = Q S p p chamber Q = C Q S eff Q S S eff = p CS p C + S p = 1 1 p p C + 1 S p pump Like Like parallel parallel resistors resistors Series Series conductances: conductances: 1/C 1/C = 1/C 1/C 1 + 1 1/C 1/C S = Q/p Q/pand C = Q/dp Q/dp S p S eff Effective pump speed, S eff, never exceeds conductance of worst component or pump speed, S p. C 11
Vacuum technology: Generating low pressure Two classes of vacuum pumps: 1) Molecules physically removed from chamber a) mechanical pump b) Turbo molecular pump S = 1 kt dv dt c) Oil diffusion pump p ) Molecules adsorbed on a surface, or buried in a layer a) Sputter/ion pump (with Ti sublimation) b) Cryo pump 1
1) Molecules physically removed from chamber a) Mechanical pump b) Oil diffusion pump c) Turbo molecular pump Oil contamination, Vibrations. But pumps from 1 atm to mt. S x 10 4 L/s Hot Si oil vaporized, jetted toward fore pump, momentum transfer to gas, which is pumped out. S = 1A L/s 1 atm Rotating (5 krpm) vanes impart momentum to gas, pres re incr s away from chamber, 1 Torr gas pumped by backing pump. No oil. S = 10 3 L/s (760 Torr) 1 millit Figure removed for copyright reasons. 10-6 T Figure removed for copyright reasons. Figure -9 in Ohring, 001. Figure -8 in Ohring, 001. 10-9 T Figure removed for copyright reasons. Figure -7 in Ohring, 001. 13
) Molecules adsorbed on a surface, or buried in a layer a) Sputter/ion pump (with Ti sublimation) b) Cryo pump Gas is ionized by hi-v, ions spiral in B field, embed in anode, Coated by Ti. No moving parts, no oil. S depends on pump size and S(H) >>S(O,N,H O) v Figure removed for copyright reasons. Figure -11 in Ohring, 001. Figure removed for copyright reasons. Figure -10 in Ohring, 001. Very clean, molecules condense on cold (10 K) surfaces, No moving parts. S 3A (cm )L/s 1 atm (760 Torr) 1 Torr 1 millit 10-6 T B 10-9 T 14
PUMP SUMMARY Two classes of vacuum pumps: 1) Molecules physically removed from chamber a) mechanical pump Pumps from 1 atm; moving parts, oil b) Turbo molecular pump c) Oil diffusion pump Clean, pumps lg. M well, from 1mT; low pump speed, moving parts No moving parts; oil in vac ) Molecules adsorbed on a surface, or buried in a layer a) Sputter/ion pump Clean, pumps reactants, no moving parts; (with Ti sublimation) pumps from 10-4 T down. b) Cryo pump Clean, no moving parts; pumps from 10-4 T down. Most systems use different pumps for different pressure ranges 15
Vacuum technology: Deposition chambers Standard vacuum, p 10-5 -10-6 Torr Glass or stainless steel, usually diffusion pumped, CVD, thermal evap. or sputter dep. => polycrystalline films Ultrahigh vacuum, p 10-8 -10-11 Torr; Stainless steel (bakeable); Ion and/or turbo pumped thermal evap. Sputter deposition => better quality films, epitaxial 1. Get p < 1 mt; close valve Figure removed for copyright reasons. Figure -1 in Ohring, 001.. Open backing valve, Turn on diff n pump 16
Vacuum technology: Deposition chambers Ultrahigh vacuum, p > 10-11 Torr; Stainless steel (bakeable); Ion and/or turbo pumped thermal evap. Sputter deposition => better quality films, epitaxial Baking a stainless-steel uhv system (T up to 00 C for 10 s of hrs) desorbs water vapor, organics from chamber walls; these are ion-pumped out; pressure drops as T returns to RT. 17
Thin film growth general Bonds on 3 sides R More bonds Rate of arrival Diffusion rate 3 bonds with substrate Arrival, sticking, surface diffusion, bonding arrival Film growth competes with gas arrival. diffusion Bonds on 1 side growth 1) R > 1 Non-equilibrium, fast growth, many misaligned islands form, leading to defective (high-surface-en), polycrystalline film, columnar grains, This 3-D growth is Volmer-Weber mode; Can amorphous film. ) R < 1 => Slower, more equilibrium, layer-by-layer growth, larger grains (raise surface temperature to mobility g.s. ). If film and substrate have same crystal structure, film may grow in perfect alignment with substrate ( epitaxy ). This -D growth is Frank-van der Merwe mode. 18
Thin film growth details (R < 1) R Rate of arrival Diffusion rate 1) Arrival rate, physical adsorption 3) Chemical reaction 5) Growth ) Surface diffusion 4) Nucleation 6) Bulk diffusion Better quality films; layer-by-layer growth If R > 1, processes ) - 6) have reduced probability; => poor quality, rough films 19
L = dimension of chamber or reactor Knudsen number p λ (cm) 1 atm 10-5 1 Torr 10-1 mt 10 Knudsen number, N 0 = λ/l λ/l < 1 λ/l > 1 Flow is viscous; p > 1 mt Deposited species thermalized ; Growth is from all directions, good step coverage Molecular, ballistic flow; p < 1 mt Deposited species arrives hot ; Growth ballistic, shadow effects, poor step coverage What does this imply for film growth? 0
Looking ahead Thin films made by a variety of means: thermal vapor deposition (evaporation) - for metals sputter deposition DC-magnetron- for metals -RF for oxides chemical vapor deposition - for metals, semiconductors Physical vapor deposition (PVD) Chemical vapor deposition (CVD) 1