《Microelectronics Process》lecture4

Chemical Vapor Deposltlon(CVD) Processes: gift of Slo2-Expose Si to steam = uniform insulating layer or metal film growth: high vacuum, single element Contrast with CVD: toxic, corrosive gas flowing through valves T up to 1000 C, multiple simultaneous reactions, gas dynamics, dead layers.. whose ldea was It? DIELECTRIC SIN Sio POLY- SI GATE OXIDE BARRIER METAL SOURCE DRAIN WAFER All layers above poly-Si made by cvD, except gate oxide and aluminum Mon.. Sept. 15. 2003

Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single element… CVD: toxic, corrosive gas flowing through valves, T … Contrast with up to 1000°C, multiple simultaneous reactions, gas dynamics, dead layers… whose idea was it? All layers above poly-Si made by CVD, except gate oxide and aluminum Mon., Sept. 15, 2003 1

CVD reactor reacton chamber (similar to those Control for si oxidation module Control T as motul low rate Mon., Sept. 15, 2003

CVD reactors Control module Four reaction chambers (similar to those for Si oxidation) Control T, gas mixture, pressure, flow rate Mon., Sept. 15, 2003 2

CVD is film growth from vapor/gas phase via chemical reactions in gas and on substrate e.g. SiH4 (g)- Si(s)+2H2(g) Do not want Si to nucleate above substrate(homogeneous nucleation) but on substrate surface(heterogeneous nucleation wall Reactor Transport of precursor Removal of across by-products dead layer to Susceptor substrate olysls: thermal/aun say decomposed species ond to subs decomposition More details at substrate Mon.. Sept. 15. 2003

CVD is film growth from vapor/gas phase via chemical reactions in gas and on substrate: homogeneous nucleation), e.g. SiH4 (g) Æ Si (s) + 2H2 (g) Do not want Si to nucleate above substrate ( but on substrate surface (heterogeneous nucleation). Twall Reactor Transport of precursors across dead layer to substrate Pyrolysis: thermal Susceptor film T sub> Twall Chemical reaction: Decomposed species bond to substrate decomposition at substrate More details… by-products Removal of Mon., Sept. 15, 2003 3

CVD Processes Bulk transport tansport of byproduct Reactan molecule ○ Duluson of 2) Transport Carrier gas (g)byproduct across andry (Maintain hi p, cO slow reaction d layer Decomposition 1∝D△C sorption O Reaction with film Surface diffusion Mon., Sept. 15, 2003

CVD Processes 8 1 Bulk Bulk transport transport of byproduct Reactant molecule 7 Diffusion of Transport Carrier gas 2 across bndry 4 (g) byproduct (Maintain hi p, layer Decomposition slow reaction) 6 Desorption 3 Adsorption 5 J1 µDgDC Reaction with film J2 ~ kiCi Surface diffusion Mon., Sept. 15, 2003 4

Hi vel low P Gas transport Low vel, hi P Laminar flow across plate Transport Hi vel. low acrosS boundary ∝D△C Plate Knudsen Nk Viscous flow 2 Tube Wall Laminar flow pipe Conductance aA/L Mon., Sept. 15, 2003

Gas transport J1 µDgDC Transport across boundary layer 2 Knudsen NK ≡ lL <1 L Viscous flow Dgas ª lvx 2 Mon., Sept. 15, 2003 5

Revisit gas d c D J=D C-C dynamics. dx d(x Boundary layer Layer thickness, 8(x) 入 And we saw gas dIffuSivity D (unlike solid 2 gas vel: uo boundary layer wafer water X=L Fuld dynamics:6(*=nr p=mass density, n= viscosity puo 6(x)a=2r/ 2 L Reynolds #:Re Puo L 3√ Re ease of gas flc So √Re 2 Mon.. Sept. 15. 2003

Revisit gas J1 = hg(Cg - Cs) dC D J1 = D = (Cg - Cs) dynamics: dx d(x) Boundary layer Layer thickness, d(x) lvx And we saw gas diffusivity D = (unlike solid) 2 z u gas vel: u0 boundary layer Cg d (x) d (x) us = 0 Cs wafer wafer x x = L hx Fluid dynamics: d(x) = r = mass density, h = viscosity ru0 L 1 h 2 L Reynolds #: Re = ru0L d = Ú d(x)dx = 23 L ru0L ≡ 3 Re ease of gas flow h L 0 D 3 D Æ Re So: hg = d 2 L Mon., Sept. 15, 2003 6

Several processes In serles SImplify cvD to 2 steps: △C B J,=kc 2 Reaction rate constant, ks Stlcking coeficient yr as in oxidation but no 0≤%AB≤ sold-state diffusion here reaction occurs at surface AB bounces Good off surface ad heston Let's analyze, solve for J2 Mon.. Sept. 15. 2003

Several processes in series Simplify CVD to 2 steps: Boundary AB layer Dg J1 = d DC J2 B A J2 = k Cs s Reaction rate constant, k Sticking coefficient gAB, s …as in oxidation, but no 0 ≤ gAB ≤ 1 sold-state diffusion here, reaction occurs at surface. AB bounces Good off surface adhesion Let’s analyze, solve for J2… Mon., Sept. 15, 2003 7

Two man CVD layer =2△C1=bC8=C B =kC In steady state: J-J2n C, J,=kC h,(Co-C)=kcs tk h tk ectrical analog小=h R-R++R2 G=1/RG2G1+2) Two processes In serles; slowest one Imts film growth Mon.. Sept. 15. 2003

J1 = J2, hg ( Cg - Cs ksCs J2 = ksCs = hgks hg + ks Cs = Cg hg hg + ks Cg , Boundary layer J2 = ksCs B A AB J2 J1 = hg( ) Cg - Cs process: J1 = Dg d DC In steady state: ) = Two main CVD J1 = J2, 1+R2 1G2 /(G1+G2) Electrical analogy: R = R G = 1/R= G Two processes in series; slowest one limits film growth Mon., Sept. 15, 2003 8

Two men cVD Boundary A: layer J1=△CJ=1(C2-C) 6 A J2=kc hk 2 h tk hk cc FIm growth rat≡y=J # h +k N h k Slower process controls growth Mon.. Sept. 15. 2003

Boundary layer J2 = ksCs B A AB J2 J1 = hg( ) Cg - Cs Two main CVD process: J1 = Dg d DC J2 = ksCs = hgks hg + ks Cg ≡ v = J # area - t Ê ËÁ ˆ¯˜ 1 N # vol Ê ËÁ ˆ¯˜, v = hg ks hg + ks Cg Nf = Cg Nf 1 hg + 1 ks Film growth rate Slower process controls growth Mon., Sept. 15, 2003 9

Two men cVD pIocesS: AB leyer J ,=h c-C, A kC ExamIne these 2 mlts of growth, k, h, or k Transport LImited growth Reactlon lImlted growth, h《k k《h kC c AG C 3DO 3万5CJRe √Re KT 2LN 4LN ease of gas flow Mon.. Sept. 15. 2003

Bou nd a ry laye r J 2 = k s C s B A AB J2 J1 = h g( ) Cg - C s Two m ain CVD p rocess: Exa mine these 2 limits of g rowth, hg or ks limited … Tra nspo rt limited g rowth, Reactio n limited g rowth, k < < h : v = Cg N f 1 h g + 1 ks s g h < < k : g s h C 3DC g kT g xg 3 lv C Re v = k s Cg = C k 0 e - D G v = g g Æ Re = N f 2LN f 4LN f N f N f ease of gas flow Mon., Sept. 15, 2003 10