Oxidation of Si. Why spend a whole lecture on oxidation of Si? GaAs has high m and direct band no oxide

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1 Oxidation of Why spend a whole lecture on oxidation of? Ge has high m e, m h, Ge stable but no oxide GaAs has high m and direct band no oxide e 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, r > Wcm, E g = 8 ev! has high dielectric breakdown field, 500 V/mm growth on fi clean / interface because D through << D Oxy through O 2 dt Oxide Sept. 19, J/6.152J 1

2 O 2 So growth occurs at inside surface + O 2 Æ or + 2H 2 O = + 2H 2 (faster growth, more porous, lower quality) dt Oxide 2 Sept. 19, J/6.152J

3 O 2 dt Oxide Extra free volume in dangling bonds of amorphous => Implications different for field vs. patterned oxide. Sept. 19, J/6.152J 3

4 Cleaning station for removing organic contaminants and native oxide (by HF-dip) from wafers. Oxidation furnaces for controlled growth of oxide layer on : 1050 C and steam for field oxide. Sept. 19, J/6.152J 4

5 Probably safe to say that entire course of semiconductor industry would be different without. Device fabrication, especially MOS, more difficult. Depositing or Al 2 O 3 is not clean. Sept. 19, J/6.152J 5

6 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 growth: the Deal-Grove model. Sept. 19, J/6.152J 6

7 Deal-Grove model of silicon oxidation growth occurs at / interface O 2 because D O 2 ( ) >> D ( ) Growth Process limited by 1. P(O 2 ) = P g µ C g O 2 Concentration C g dead layer 2. Transport O 2 to surface across dead layer J 1 J 1 C s C o 3. Adhesion of C s (O 2 ) at surface C 0 J 2 Ci 4. Diffusion O 2 through J 2 5. Chemical reaction rate J 3 J 3 x Sept. 19, J/6.152J 7

8 N V Cg Deal-Grove model of silicon oxidation Oxide growth rate Ideal gas law: P g V = NkT O 2 Concentration = C = P g kt (C g g - C ) C 0 = HP s = Hk B TCs (C -Cs ) Henry s law J J 1 > D 3 tdead layer x CC-C 0 - C Turbulence => J i 0 2 = D O 2 ( ) s xox J 1 = h g (C g - C s ) J 2 C g Diffusion (D cm 2 /s) J 1 dead layer = J C J s 1 C o J 2 2 J 3 = J 3 C i = k i C i rate constant k i (cm/s) Sept. 19, 2003 Equate ideal gas + J 1 Equate J 2 + Henry to J 3 to J 2 + Henry fi C i = f n P g,h g,h,d O 2 (,x,k oxide i) 3.155J/6.152J 8

9 Deal-Grove model of silicon oxidation J 1 = J 2 = J 3 O 2 Concentration fi C i = f n (P g,h g,h,d O 2,ide,k i ) C g dead layer J 1 C s C o J 2 Ci C i = HP g /k i 1 h D O 2 k i h = h g HkT J 3 x mass transport J 1 Diffusion Reaction J 2 J 3 (k i = k in text) s Slowest process controls concentration of oxygen at interface Sept. 19, J/6.152J 9

10 Limits: Growth limited by: C i = Diffusion limited: D O 2/ < k i, h g, HP g /k i 1 h D O 2 k i h = mass transport diffusion reaction Reaction-rate limited: k i < h g, D O 2/ h g HkT very large C i = gd O 2 HP k i 2 C i = HPg Sept. 19, 2003 Slower process controls concentration of oxygen at interface, which in turn controls growth rate

11 Oxide growth rate C i k i Rate of growth = d = J 3 =, dt N N HP g /k i Ci = 1 x 1 + ox + h D O 2 k i ( N = # O 2 molecules incorporated / cm 3 ) N = 2.2 x / cm 3, dry 4.4 x / cm 3, H 2 O d dt = HP g /N 1 h + D + 1 k i rate depends on ide Ê t 1 ox g Ú Á + x + 1 ˆ d = Ú HP dt Ë h D k x 0 i 0 N t 0 x ox ( ) 2 + Ax = B t + t ox x Ê 1 1 ˆ A = 2DÁ + (length) Ë h k i B = 2DH P N ( length2 ) t > 0 g time 2 t = (x 0 + Ax 0 ) B (time) Sept. 19, J/6.152J 11 o

12 Ê 1 h + D + 1 ˆ Ú Á d = Ë k i x 0 t Ú 0 HP g N dt t 0 2 x 2 ox + A = Bt+ ( t) Ê A = 2D 1 h + 1 ˆ Á Ë k i B = 2DH P g N t = x Ax 0 Parabolic and linear growth rates ( ) B t >0 = -A + A2 + 4B(t + t) 2 Rate constants A and B known experimentally; both µ D = D 0 e -Ea/kT Thick oxide => parabolic rate constant, B ( ) Quad.Eq >> A ææææ = Bt+ t t t Thin oxide => linear rate constant, B/A x 2 ox << A ææææ 2 B ( A t + t ) Sept. 19, J/6.152J t 12

13 ( wet oxidation is much faster than dry ) D O 2 ( ) < D H 2 O ( ) ºC, 1 atm, 0.1 mm / hr fi dry oxide, denser, use for gate oxide C, 25 atm, 1 mm / hr => wet oxide, more porous, poorer diffusion barriers; use for etch oxide, field oxide. Dry O % Cl; Cl is a metal getter fi cleaner oxide. Sept. 19, J/6.152J 13

14 Exercise: calculate x OX grown for 1 hr. in dry oxidation at 1100 C. From table, A = 0.09 mm, B = mm 2 / hr, t = hr. x = ox -A + A 2 + 4B( t + t) 2 = 0.14 mm ( mm / 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. Sept. 19, J/6.152J 14

15 / interface, local charges O 2 Oxide near the interface is a sub oxide, O x, x < 2. C O2 O, which is often + charged. WHY? Electronegativity O 2-4+ e O 2- O 2- O 2- O O O O O O + +e - e - Sept. 19, J/6.152J 15

16 / interface and dry vs. wet oxidation O 2 C O + O2 e -. Gases unstable at 100 o C, dissociate at surface. Outside in oxide O -> - + h + 2 O + O 2H - 2 O -> O + O + h + + n(h +, H, H - ) Which species diffuse quickly? Large small O, - O, H -, H, H +, h + Slow fast O + C O2 - e. H +, h + O + C O2 - e. O - O - +O + -> Sept. 19, J/6.152J 16

17 Initial oxidation regime. 2 x + Axox = B ( t + t) ox Deal - Grove: at small, = B A t + t ( ) d dt = B A = const. O 2 To explain this many models proposed. It appears that / interface is not sharp. small x Oxide grows not just at but also at x ox - ox 2 dx Sept. 19, J/6.152J 17

18 Structure of Quartzite O O O bridging oxygen O disorder Amorphous tetrahedral network fewer bridging O s, some non-bridging. Network modifyers B, P replace Sept. 19, J/6.152J 18

19 Effects of Dopants on Oxidation of Æ We will see segregation coefficient for crystal growth: k = C S generally < 1 CL Related parameter C X (in ) for segregation of impurity X on oxidation: m = CX (in ) Impurity concentration profiles depend on m, D x in, D x in and growth rate (not shown below): m > 1 ( oxide rejects impurity, X) D x ( ) < D x () J ( ) = J () C X (0) x int x x D ( ) > D x () C X (0) D x ( ) < D x () C X (0) m < 1( oxide consumes X ) D x ( ) > D x () C X (0) Sept. 19, J/6.152J 19

20 Common dopants in enhance oxidation at higher concentration Oxide thickness vs. wet oxidation time For three different boron concentrations Linear, B/A, and parabolic, B, rate constants vs. phosphorus concentration. Sept. 19, J/6.152J 20