Vacuum Technology and film growth. Diffusion Resistor

Transcription

1 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

2 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.

3 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

4 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 Torr 10-1 mt 10 4

5 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

6 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 = 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

7 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 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: Torr => 1 monolayer/ sec λ 7

8 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

9 L = dimension of chamber or reactor Knudsen number p λ (cm) Recall: λ = k BT πd p 1 atm 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

10 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

11 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 /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

12 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

13 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 T Figure removed for copyright reasons. Figure -9 in Ohring, 001. Figure -8 in Ohring, T Figure removed for copyright reasons. Figure -7 in Ohring,

14 ) 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

15 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

16 Vacuum technology: Deposition chambers Standard vacuum, p Torr Glass or stainless steel, usually diffusion pumped, CVD, thermal evap. or sputter dep. => polycrystalline films Ultrahigh vacuum, p 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, Open backing valve, Turn on diff n pump 16

17 Vacuum technology: Deposition chambers Ultrahigh vacuum, p > 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

18 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

19 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

20 L = dimension of chamber or reactor Knudsen number p λ (cm) 1 atm 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

21 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