6.152J/3.155J What does Materials Science have to do with Microelectronic Processing? Need to understand DIfferences: metals, oxides and semiconductors Atomic bonding Oxidation rates, compound formation(GaAs) chemical reactions Solubility of Impurities In SI ChemIcal reactlons for CVD precursors, byproducts Need to understand Gas concentration(critical to CVD reaction rates) Gas diffusivity. Surface mobilIty (key to quallty film growth) Solid-state What controls Interfaces(e.g. SIO/Al diffusion crystal growth, and Impurities How manage grain growth, film mlcrostructure Electrical, mechanical properties depend on all of the above Wed, Sept. 10, 2003 Vacuum Technology and fllm growth (poly-Gate p-MOSFET) P++ Poly Diffusion Poly Si Resistor All layers above n-type SI made by CVD except gate oxide and Al 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 6.152J/3.155J 1 What does have to do with Microelectronic Processing? • Need to understand Di l i i i i i l ili i iti i i i l i s • Need to understand i iti l i ili li il ls i iO2 l) l i iti i il i Electrical, mechanical properties depend on all of the above chemical reactions Gas diffusivity, Solid-state diffusion Wed., Sept. 10, 2003 Materials Science fferences: meta s, ox des and sem conductors Ox dat on rates, compound format on (GaAs) So ub ty of mpur es n S Chem ca react ons for CVD precursors, byproduct Gas concentrat on (cr ca to CVD react on rates) Surface mob ty (key to qua ty f m growth) What contro nterfaces (e.g. S /A crysta growth, and mpur es How manage gra n growth, f m m crostructure Atomic bonding, Vacuum Technology and film growth ( FET) Al n Diffusion Resistor Poly Si Resistor Al Al P ly P ly p- Implant poly-Gate p-MOS -Si ++ Po ++ Po All layers above n-type Si made by CVD except gate oxide and Al 6.152J/3.155J Wed., Sept. 10, 2003 2 1
6.152J/3.155J What will we cover in next few lectures? Chemical vapor deposition(CVD) Mon. Sept 15 Most widely used method for growth of mlcro-electronlc grade semIconductor fiim also widely used for metals and oxides Oxidatio Wed. Sept 17 Key advantage of SE: stable uniform oxide How control Its growth, thickness, quallty Diffusion and ion implantation Mon. Sept 29 How semIconductor surfaces are doped Wed. Oct. 1 Physical vapor deposition(PVD) Nov 5.12 Growth of quallity films by sputter deposltlon or evaporation These processes take place in vacuum or controlled environment Therefore, need to understand vacuum technology,. gas kinetics. 6.152J3.155J Wed, Sept 10, 2003 Gas Kinetics and Vacuum Technology How far does a molecule travel between collisions? m 5x 10-26 k velocity molecule mpact parameter, scattering cross section T d2 Mean free path=x Volume swept out by 1 molecule between collisions=λxd 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 6.152J/3.155J 3 What will we cover in next few lectures? • Chemical vapor deposition (CVD) i l mi l i i il al i l l i • Oxidation Wed. Sept 17 i l i i l i i li vacuum technology,… gas kinetics. • Diffusion and ion implantation i Wed. Oct. 1 • Nov 5,12 li il iti i Wed., Sept. 10, 2003 Mon. Sept 15 Most w de y used method for growth of cro-e ectron c grade sem conductor f ms, so w de y used for meta s and ox des Key advantage of S : stab e un form ox de How contro ts growth, th ckness, qua ty These processes take place in vacuum or controlled environment. Therefore, need to understand Mon. Sept 29 How sem conductor surfaces are doped Physical vapor deposition (PVD) Growth of qua ty f ms by sputter depos on or evaporat on 6.152J/3.155J 4 i i l l l l llisi “Movie” d d l le i i i p d2 => l p d 2 Volume swept out by 1 molecule lpd2 i l V 2) L velocity “Snap shot” n = N V = N L3 m 5 x 10-26 kg Wed., Sept. 10, 2003 Gas K net cs and Vacuum Techno ogy How far does a mo ecu e trave between co ons? Mean free path ≡ l mo ecu mpact parameter, scatter ng cross sect on = between collisions = Cons der a vo ume of gas (e.g. N number N, 2
6.152J/3.155J Gas Kinetics Volume swept out by 1 molecule ⊥3=VNrd2 More accuratelyλ deal gas: pV=NkB T, k T 6.152J3.155J Wed, Sept. 10, 2003 What is flux of atoms hitting surface per unit time? ea J(#/ area time)=s Analogous to current density related to pressure(elec field We need v. v Calculating gas velocitie Maxwell speed distribution Po)-adl2m- vexpl-2T v=vP()d 6.152J3.155 Wed, Sept. 10, 2003
6.152J/3.155J 6.152J/3.155J 5 Total volume of sample L3 = V N lpd 2 \l = V Npd2 = 1 npd2 (n = N V ) More accurately l = 1 2 npd 2 Ideal gas: pV = NkBT, n=p / kBT => \l = kB T 2pd2 p l p d 2 Volume swept out by 1 molecule between collisions = lpd 2 i i p l (cm) 1 atm 10-5 10-2 1 mT 10 Wed., Sept. 10, 2003 Gas K net cs 1 Torr 6.152J/3.155J 6 What is flux of atoms hitting surface per unit time? area # / vol. vx J / nvx 2 related to pressure (elec. field) vx , v speed P(v) vvms v v = Ú vP(v)dv Maxwell speed distribution: P(v) = 4p m 2pkT È Î Í ˘ ˚ ˙ v 2 exp - mv 2 2kT È Î Í ˘ ˚ ˙ vrms = 3kT m v = 8kT pm v , x = 2kT pm vrms ≈ 500 m/s v x = v /2 Wed., Sept. 10, 2003 ( # area time) = Analogous to current density, Calculating gas velocities We need 3/ 2 3
6.152J/3.155J So flux of atoms hitting surface per unit time k T DImensional analysts: (force/area- en/vol ) p- vol =n——=Jmv Numerically,-35×l02 Tom)(atos/ sec) This gives a flux 1 monolayer (ML) arriving per sec at 10-6 Torr 6.152J3.155J Wed, Sept. 10, 2003 Diffusivity ooO Recall for solids: Debye 101351 For gas, no energy barrier, just collisions. recall 2=- kT much weaker T-dep than In solld 6.152J3.155J Wed, Sept. 10, 2003
6.152J/3.155J 6.152J/3.155J 7 So flux of atoms hitting surface per unit time area # / vol. vx Jx = nvx 2 = n 2 2kT pm ideal gas p 2pmkT = Jx Di i l l i l p = Ekin Vol = n mv 2 2 = Jmv Numerically, Jx = 3.5 ¥1022 p(Torr) MT(g / mole ⋅K) (atoms /cm 2 sec) This gives a flux 1 monolayer (ML) arriving per sec at 10-6 Torr l = kB T 2pd 2 p Compare: Wed., Sept. 10, 2003 mens ona ana ys s: (force/area = en/vo .): Diffusivity DG D0 exp - È DG˘ Recall for solids: D = Í˙ Î kT ˚ Debye n 1013 s-1 For gas, no energy barrier, just collisions. dC n Jgas gas = D @ D dx l lvx Dgas ª nv 2 x (cm2 2 /s) kT recall l = 2pd 2 p v \D x µ T much weaker T-dep. than in so gas µT lid 3/ 2 6.152J/3.155J Wed., Sept. 10, 2003 8 4
6.152J/3.155J Knudson number L= dimension of chamber or reactor k t 1 atm 10-5 102 Flow is viscous: p>1 mT Knudsen No L Pump power >viscosity must transport lg. of molecules λ Molecular ballistic flow: p liters/min Conductance of vacuum component: Obm's law →Q=C(P-P [Units of conductance Pp pump 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 6.152J/3.155J 9 L Flow is viscous; p > 1 mT Molecular, p What does this imply for pumping? Pump power > viscosity; must transport lg. # of molecules must attract and hold molecules. Knudsen N0 ≡ l L l L 1 L L l = kB T 2pd 2 p Recall: p l (cm) 1 atm 10-5 10-2 1 mT 10 Wed., Sept. 10, 2003 = dimension of chamber or reactor ballistic flow; liters/min or sccm] Ohm’s aw: pump chamber Conductance of vacuum component: 5
6.152J/3.155J Gas flow and pump d Conductance of vacum component: =Q-C(p-PP)(p 5 pr nearer pump chamber g%) P Effective pump speed, S, never exceeds conductance of worst component or pump speed, sp Wed, Sept. 10, 2003 Vacuum technology: Generating low pressure Two classes of vacu 1 Molecules from chamber b) Turbo molecular pump c)Ol diffusion pump 2)Molecules adsorbed on a surface, a)Sputter/on pump (with TI sublimation b) Cryo pump 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 6.152J/3.155J Wed., Sept. 10, 2003 11 Gas flow and pump speed Effective pump speed, S, never exceeds conductance of worst component or pump speed, Sp. (pp is pressure nearer pump) But throughput, Q: Q ≡ pS, p = Q S pp = Q Sp Conductance of vacuum component: fi Q = Cp - p ( ) p p pp pump chamber Q = C Q s - Q sp Ê Ë Á Á ˆ ¯ ˜ ˜ Sp S C fi s = csp c + sp = 1 1 c + 1 sp Vacuum technology: Generating low pressure Two classes of vacuum pumps: 1) Molecules physically removed from chamber a) mechanical pump b) Turbo molecular pump c) Oil diffusion pump 2) Molecules adsorbed on a surface, or buried in a layer a) Sputter/ion pump (with Ti sublimation) b) Cryo pump 6.152J/3.155J Wed., Sept. 10, 2003 12 6
6.152J/3.155J D Molecules physically removed from chamber a)MechanIcal pump b)Oll diffusion pump c) Turbo molecular pump But pumps from momentum transfer to gas, pres're Incr's away from chamber 1 atm to whIch Is pumped out. s 2X 10Us S=12AUS gas pumped by backing pump. No oll. s= 103 L/s 104T Wed, Sept. 10, 2003 2) Molecules adsorbed on a surface, or burled In a layer a)Sputter/on pump(wlth TI sublimation b)Cryo pump Gas Is ionized 760To by hI-V lons splral In b fleld embed in anode Very clean, Coated by TI molecules condense No moving parts, on cold (120 K surfaces no oll o moving parts, s 3A(cm2L/ depends on 6.152J3.155 Wed, Sept 10, 2003
(760 Torr 1 Torr 1 milli 6.152J/3.155J 6.152J/3.155J Wed., Sept. 10, 2003 13 1) Molecules physically removed from chamber a) Mechanical pump b) Oil diffusion pump c) Turbo molecular pump Hot Si oil vaporized, jetted toward fore pump, momentum transfer to gas, which is pumped out. S = 12A L/s Oil contamination, Vibrations. But pumps from 1 atm to mT. S 2 x 104 L/s 1 atm ) 1 milliT 10-6 T 10-9 T Rotating (25 krpm) vanes impart momentum to gas, pres’re incr’s away from chamber, gas pumped by backing pump. No oil. S = 103 L/s 6.152J/3.155J Wed., Sept. 10, 2003 14 2) 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,H2O) B v 1 atm (760 Torr) 1 Torr T 10-6 T 10-9 T Very clean, molecules condense on cold (120 K) surfaces, No moving parts. S 3A (cm2)L/s 7
6.152J/3.155J PUMP SUMMARY Two classes of vacuum pumps 1)Molecules physically removed from chamber umps from 1 atm: moving parts, oil b) Turbo molecular pump Clean, pumps lg M well, from 1 atm low pump speed, moving parts c)Ol dlffusion pump No moving parts; oil in vac 2)Molecules adsorbed on a surface or burled In a layer a) sputter/on pump Clean, pumps reactants, no moving parts wlth TI sublimation) pumps from 10-4T b)Cryo pump Clean, no moving parts pumps from 10-4 Wed, Sept. 10, 2003 Vacuum technology: Deposition chambers Standard vacuum, p> 10- Torr Ultrahigh vacuum, p> 10-11Torr tainless steel(bakeable) sually diffusion pumped, lon and/ CVD, thermal evap, or sputter dep hermal evap. Sputter deposition s> polycrystalline films a> better quality films, epitaxial 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 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 Clean, pumps lg. M well, from 1 atm; low pump speed, moving parts c) Oil diffusion pump No moving parts; oil in vac 2) 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. b) Cryo pump Clean, no moving parts; pumps from 10-4 T. 6.152J/3.155J Wed., Sept. 10, 2003 15 Vacuum technology: Deposition chambers Standard vacuum, p > 10-6 Torr Ultrahigh vacuum, p > 10-11 Torr; Glass or stainless steel, Stainless steel (bakeable); usually diffusion pumped, Ion and/or turbo pumped CVD, thermal evap. or sputter dep. thermal evap. Sputter deposition => polycrystalline films => better quality films, epitaxial 6.152J/3.155J Wed., Sept. 10, 2003 16 8
6.152J/3.155J Vacuum technology: Deposition chambers Ultrahigh vacuum, p>10-1lTorr: Stainless steel(bakeable); thermal evap. Sputter deposition a> better quality films, epitaxial BakIng a stalnless-steel uhv system (T up to 200 C for 10s of hrs) desorbs water vapor, organics om chamber walls these are ion-pumped out pressure drops as T returns to RT. 6.152J3.155J Wed, Sept. 10, 2003 Thin film growth general Bonds on 3 sic 3 bonds with substrate urface diffusion Rate of arrival Bonds on 1 side a Diffusion rate Film growth competes with gas arrival ading to defective(high-surface-en), polycrystalline film, columnar grains. This 3-D growth is the Volmer-Weber mode: Can. amorphous film 2)R Slower, more equilibrium, layer-by-layer growth, larger grains rature to↑ mobility→↑g.s.). if film and substrate have same crystal structure, film may grow in perfect alignment with substrate(epitaxy"). This 2-d growth is the Frank-van der Merwe mode 6.152J3.155 Wed, Sept 10, 2003
6.152J/3.155J 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 200 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. 6.152J/3.155J Wed., Sept. 10, 2003 17 Thin film growth general Bonds on 3 sides More bonds 3 bonds with substrate Arrival, sticking, surface diffusion, bonding Bonds on 1 side Rate of arrival R ≡ Diffusion rate Film growth competes with gas arrival. 1) R > 1 fi Non-equilibrium, fast growth, many misaligned islands form, leading to defective (high-surface-en), polycrystalline film, columnar grains, This 3-D growth is the Volmer-Weber mode; Can fi amorphous film. 2) R Slower, more equilibrium, layer-by-layer growth, larger grains (raise surface temperature to ↑ mobility fi ↑ g.s. ). If film and substrate have same crystal structure, film may grow in perfect alignment with substrate (“epitaxy”). This 2-D growth is the Frank-van der Merwe mode. 6.152J/3.155J Wed., Sept. 10, 2003 18 9
6.152J/3.155J Thin fllm growth detalls (R 1, these processes have reduced probablity Wed, Sept. 10, 2003 Looking ahead Thin films made by a variety of means: thermal vapor deposltlon(evaporation) for metals Physlcal vapor deposition (PVD) sputter deposition DC-magnetron- for metals .RF for oxides chemIcal vapor deposltlon ChemIcal vapor deposition for metals, semiconducto (CVD) 6.152J3.155 Wed, Sept 10, 2003 10
6.152J/3.155J Thin film growth details (R 1, these processes have reduced probability 6.152J/3.155J Wed., Sept. 10, 2003 19 Looking ahead… Thin films made by a variety of means: thermal vapor deposition (evaporation) Physical vapor deposition - for metals (PVD) sputter deposition DC-magnetron- for metals -RF for oxides chemical vapor deposition Chemical vapor deposition - for metals, semiconductors (CVD) 6.152J/3.155J Wed., Sept. 10, 2003 20 10
