3.155J/6.152J Sept. 27, Oxidation of Si. Why spend a whole lecture on oxidation of Si? Wed., Feb. 16, J/6.152J 1

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1 Sept. 27, 2004 Oxidation of Why spend a whole lecture on oxidation of? Ge has high µ e, µ h, Ge stable but no oxide GaAs has high µ e and direct band no oxide Why? is stable down to 10-9 Torr, T > 900 C can be etched with HF which leaves unaffected is a diffusion barrier for B, P, As is good insulator, ρ > Ωcm, E g = 8 ev! has high dielectric breakdown field, 500 V/µm growth on clean / interface because D through << D Oxy through dt Oxide 1 Because D through << D Oxy through growth occurs at inside surface dt Oxide + or + 2H 2 O = + 2H 2 (faster growth, more porous, lower quality) Unsaturated bond 2

2 Sept. 27, 2004 O2 O2 dtoxide Only when oxide is patterned does it cause problems with planarity. Extra free volume in dangling bonds of amorphous O2 => Implications different for field vs. patterned oxide. 3 RCA cleaning station for removing organic contaminants, Na+ and other ions as well as native oxide (by HF-dip) from wafers. Oxidation furnaces for controlled growth of oxide layer on : 1050 C and steam for field oxide. 4

3 Sept. 27, 2004 Probably safe to say that entire course of semiconductor industry would be different without good O2 growth on wafer. Device fabrication, especially MOS, would be more difficult. Depositing O2 or Al2O3 is not clean. 5 It s no accident that the world leader in chip technology, Intel, has been led by the flamboyant Hungarian, Andy Grove. As a young researcher at Fairchild Semiconductor, he wrote the book on O2 growth: the Deal-Grove model. 6

4 Sept. 27, 2004 O x growth model is similar to CVD but with extra step added: growth occurs oxygen diffusion through O x. Deal-Grove model of silicon oxidation at / interface because D ( ) >>D ( ) Growth Process limited by 1. P( ) = P g C g Concentration C g dead layer 2. Transport to surface across dead layer C s C o 3. Adhesion of C s ( ) at surface C 0 J 2 Ci 4. Diffusion through J 2 J3 5. Chemical reaction rate J 3 x 7 Deal-Grove model of silicon oxidation Ideal gas law: N V = = P g C g kt Oxide growth rate P g V = NkT Concentration C g dead layer = J 2 C J1 s C o = J 3 > D (C "C g - C s ) t dead layer Turbulence => = h g (C g - C s ) C 0 = HP s = Hk B TC s Henry s law = D J 2 ( ) C " i 0 - Diffusion (D cm 2 /s) J 2 J 3 x J 3 = k i rate constant k i (cm/s) Equate ideal gas & to J 2 & Henry Equate J 2 & Henry to J 3 ( ) " = f n P g,h g,h,d, ide,k i 8

5 Sept. 27, 2004 Deal-Grove model of silicon oxidation = J 2 = J 3 " = f n P g,h g,h,d (, ide,k i ) Concentration C g dead layer C s C o J 2 Ci HP = g /k i 1 h D k i h = h g HkT J 3 x mass transport Diffusion J 2 Reaction J 3 (k i = k s in text) Slowest process controls concentration of oxygen at interface 9 Limits: Growth limited by: Diffusion limited: D / < k i, h, HP = g /k i 1 h D k i mass transport diffusion h = h g HkT reaction very large Reaction-rate limited: k i < h, D / = HP gd k i = HP g Slower process controls concentration of oxygen at interface, which in turn controls growth rate 10

6 Sept. 27, 2004 Oxide growth rate τ is the time corresponding to growth of pre-existing oxide Rate of growth= d dt = J 3 n Ox = k i n Ox, HP = g /k i 1 h D k i ( n O = # molecules in oxide / cm 3 ) n O = 2.2 x / cm 3, dry 4.4 x / cm 3, H 2 O d dt = HP g /n Ox 1 h + D + 1 k i rate depends on ide " 1 h + D + 1 % t HP ( $ ' d = ( g dt # k i & n Ox x 0 x 2 ox + A = B( t + ") 0 " A = 2D 1 h + 1 % $ ' (length) # k i & B = 2DH P g n Ox ( length2 time ) " = x Ax 0 ( ) B (time) 11 x o t 0 t > 0 " 1 h + D + 1 % t HP ( $ ' d = ( g dt # k i & n Ox x 0 0 t 0 2 x 2 ox + A = B( t + ") " A = 2D 1 h + 1 % $ # k ' i & B = 2DH P g " = x Ax 0 = "A + A2 + 4B(t + #) 2 Rate constants A and B known experimentally; both D = D 0 e -Ea/kT Parabolic and linear growth rates Thick oxide => parabolic rate constant, B n Ox ( ) B Quad.Eq >> A """# = B( t + $ ) τ t > 0 t Thin oxide => linear rate constant, B/A ( ) 2 2 Quad.Eq << A """# $ B A t + % t 12

7 Sept. 27, 2004 How thick is thick, how thin is thin?? x 2 ox + A = B( t + ") 2 x ox > A or > A " thick ( wet oxidation is much faster than dry) > µm,dry; > µm,wet " thick 13 ( wet oxidation is much faster than dry) D ( ) < ºC, 1 atm, 0.1 µm / hr => dry oxide, denser, use for gate oxide. D H 2 O ( ) C, 25 atm, 1 µm / hr => wet oxide, more porous, poorer diffusion barriers; use for etch oxide, field oxide. Dry + 1-3% Cl; Cl is a metal getter cleaner oxide. 14

8 Sept. 27, 2004 Exercise: calculate x OX grown for 1 hr. in dry oxidation at 1100 C. From table, A = 0.09 µm, B = µm 2 / hr, τ = hr. = "A + A2 + 4B( t + #) = 0.14 µm 2 ( µm / hr is typical) This is the oxide thickness grown over any thin native oxide present. Now you calculate for steam oxidation at same time and temp. 15 Why is wet oxidation faster? / interface, local charges Oxygen gradient exists in O x. Oxide near the interface is a sub oxide, O x, x < 2. c O2 O, which is often + charged. WHY? Electronegativity - 4+ O e - - O 2+ O - O e O O + + e - 16

9 Sept. 27, 2004 / interface and dry vs. wet oxidation Which species diffuse quickly? c O2 O + e -. Large small O -, O, H -, H, H +, h + Slow fast c O2 O + e -. Gases unstable at 100 o C, dissociate at surface. Outside in oxide -> O + O - + h + 2H 2 O -> O + O - + h + + n(h +, H, H - ) What does dipole layer do to ions? H +, h + c O2 O + e O - O - + O + -> Initial oxidation regime. x 2 ox + A = B( t + ") Deal - Grove: at small, = B A ( t + ") d dt = B A = const. c Ox δx To explain this many models have been proposed. It appears that / interface is not sharp. small Oxide grows not just at but also at " #x 2 18

10 Sept. 27, 2004 Structure of Quartzite O O O bridging oxygen O disorder Amorphous tetrahedral network fewer bridging O s, some non-bridging. Network modifiers B, P replace 19 Effects of Dopants on Oxidation of We will see segregation coefficient for crystal growth: Related parameter for segregation of impurity X on oxidation: k = c max solid generally < 1 max c liquid m = c X c X (in ) (in ) Impurity concentration profiles depend on m, D x in, D x in and growth rate (not shown below): D x ( ) < D x ( ) J x ( ) int = J x ( ) C X (0) m > 1 ( oxide rejects impurity, X) D x ( ) > D x ( ) C X (0) D x ( ) < D x ( ) m < 1( oxide consumes X ) C X (0) D x ( ) > D x ( ) C X (0) 20

11 Sept. 27, 2004 Common dopants in enhance oxidation at higher concentration boron phosphorus CP Oxide thickness vs. wet oxidation time For three different boron concentrations Linear, B/A, and parabolic, B, rate constants vs. phosphorus concentration. 21 Now, go grow some glass! 22