Chapter 6 Thermal oxidation and the Si/SiO2 interface
Chapter 6 Thermal oxidation and the Si/SiO2 interface 1. SiO2 properties and applications. 2. Thermal oxidation basics. 3. Manufacturing methods and equipment. 4. Measurement methods (mechanical, optical, electrical). 5. Deal-Grove model (linear parabolic model). 6. Thin oxide growth, dependence on gas pressure and crystal orientation 7. Cl-containing gas, 2D growth, substrate doping effect . 8. Interface charges, dopant redistribution. Textbook page 313. NE 343: Microfabrication and thin film technology Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/ Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin 1 Bruce Deal and Andrew Grove Time Magazine's Man of the Year (1997) As a young researcher at Fairchild Semiconductor, he wrote the book
on SiO2 growth: the Deal-Grove model. Grove worked at Fairchild Semiconductor before becoming the fourth employee at the nascent Intel Corporation. He became Intel's president in 1979, its CEO in 1987, and its Chairman and CEO in 1997. Grove is credited with having transformed Intel from a manufacturer of memory chips into one of the world's dominant producers of microprocessors. During his tenure as CEO, Grove oversaw a 4,500% increase in Intel's market capitalization from $4 billion to $197 billion, making it, at the time, the world's most valuable company. 2 http://en.wikipedia.org/wiki/Andrew_Grove Overview Deal-Grove model: linear-parabolic model for 1D oxide growth (along z-direction) from un-patterned wafer surface. It is applicable to: Oxidation temperature 700-1200oC H2O is for wet oxidation O2 for dry oxidation Gas pressure at wafer surface 0.1-25 atm. Oxide film thickness 20-2000nm. O gets in or Si gets out for reaction?
Considering dry oxygen molecules as the oxidant species, by radio active tracer, it has been shown that oxidation proceeds by inward movement of O2 molecules through SiO2 to the Si SiO2 interface where the reaction Si(solid) + O(gas) SiO2 takes place. This forms a contrast with the case of Copper whose oxidation proceeds by the outward motion of the metallic ion and also with the case of anodic oxidation of silicon, where silicon moves outward. 3 D-G model: three flux at equilibrium F (oxidant flux): number/(cm2-s); C (oxidant concentration): number/cm3 Gas diffusion Solid state diffusion SiO2 formation Concentration of main gas flow Concentration at (inside) the oxide surface. Co Cs. Cs > C o
F1: flux of oxidizing species transported from the gas phase to the gas-oxide interface. F2: flux across the existing oxide toward the silicon substrate. F3: flux reacting at the SiO2 interface (this term is not strictly flux/flow). 4 In steady state, the three fluxes are equal. D-G model: three flux at equilibrium F1 hg (CG CS ) hg is mass transfer coefficient in cm/sec. For ideal gas, PSV=NKBT, so Cs=N/V=Ps/KT. According to Henrys law, the concentration of a gas species dissolved in a solid is proportional to the partial pressure of that species at the solid surface. So Co=HPs, where H is Henrys constant. (this is something similar to the law of segregation of dopant at an interface) PS CO F1 hg (CG ) hg (CG ) kT HkT
Define h=hg/HkT, C*=HkTCG=HPG then F1 h(C * CO ) 5 D-G model: three flux at equilibrium Using Ficks law of diffusion, D (cm2/sec) is oxidant diffusivity in the oxide, x is oxide thickness, then CO C I F2 D x The effective diffusivities of both O2 and H2O are on the same order (about 5103m2/hr at 1100oC). The rate of reaction should be proportional to the oxidant concentration at the Si/SiO 2 interface CI, Ks interface reaction rate constant (cm/sec), then F3 k s C I In steady state, the three fluxes are equal. F1 F2 F3 Now we have two equations to solve two unknown CO and CI.
F1 h(C * CO ) CO C I F2 D x F3 k s C I 6 F1 F2 F3 Deal-Grove model C I C O C *
C * ( h k s ) ks ksx ksx 1 1 h D D ksx * 1 C D C * , since h k s
ks ksx 1 h D This means the gas absorption rate at the oxide surface is much faster than chemistry occurring at the Si/SiO 2 interface. Cg Cs Cs C* CI=C* C* x
Cg x CI=0 SiO2 Si Figure 6-16 ksx/D << 1, reaction rate limited SiO2 ksx/D1 corresponds to oxide thickness 50-200 nm. Si ksx/D >> 1, diffusion limited 7
Deal-Grove model Assume N1 as the number of oxidant molecules incorporated per unit volume of oxide grown. For O2 (dry) oxidation, N1=2.21022 cm-3; for H2O (wet) oxidation, N1=4.41022 cm-3 (2 that of dry, since H2O has only one oxygen atom). Then the oxidation rate R (F is flux, F=F1=F2=F3=KSCI, unit is number/(cm2s); unit for R is cm/s). F dx R N1 dt kS C * kS kS x N1 1 h D x0 t kS kS x * N1 1
dx k C dt S h D xi 0 Let B=2DC /N1 A=2D(1/ks+1/h), B/AC*ks/N1,(h>>ks) * xo2 xi2 xo xi t B B/ A Xi account for any oxide present at the start of the oxidation. 8
Deal-Grove model Rewrite the above equation, xo2 xo t B B/ A xi2 Axi B xo A 1 t2 1 2 A / 4B B =2DC*/N1parabolic rate constant, contribution of flux F2. B/A C*ks/N1linear rate constant, contribution of flux F3. Two limiting forms, when one of the two terms dominates. For short time/thin oxide, linear. For long time/thick oxide, parabolic.
x0 t xo B (t ) A 2 o x B (t ) 9 Determine B and B/A from experiment It is difficult to calculate B and B/A, mainly because we dont know the K S value. The oxidation rate depends on the processes of Si-Si bond breaking, Si-O bond formation, and possibly O2 or H2O dissociation. All of these effects and others are lumped into KS, the interaction rate constant (cm/sec). So B and B/A is determined experimentally. xo2 xo
t B B/ A xo B t A xo Assume =0? Figure 6-17 Extraction of rate constants from oxide thickness versus time experimental data. 10 Arrhenius expression for B and B/A D-G model is applicable for: oxidation on flat un-patterned surface, lightly doped substrates, using simple O2 or H2O ambient, and when the oxide thickness is larger than about 20nm. B=2DC*/N1D, D depends on T exponentially. Experimentally it is found: B/AC*ks/N1ks. From table below, E2 is independent of whether O2 or H2O oxidation, as well as crystalline direction E2 represents a fundamental process only related to the substrate (e.g. Si-Si bond breaking). Table 6.2 Rate constants describing (111) silicon oxidation kinetics at 1 atm total pressure. For the corresponding values for (100) silicon, all C2 values should be divided by 1.68. O2 bubble through
95C H2O H2+O2 reacts to form H2O Mostly H2O, but some O2, no longer popular H2O only, more popular now 11 Arrhenius expression for B and B/A 12 Calculated dry oxidation rate from D-G model xi 0 Figure 6-19 (100) silicon. The initial fast oxidation for the first 20nm is not included (i.e. xi, =0). 13 Calculated wet (H2O) oxidation rate from D-G model xi 0
Figure 6-20 14 Dry vs. wet oxidation Wet oxidation: a mixture of O2 and H2O (O2 bubble through hot water) used as oxidant. Advantage: higher growth rate than dry oxidation. Reason for higher rate: much larger solubility in oxide (Henrys constant H) for H 2O compared with O2. C*(bulk concentration, =HPG) for H2O: 3x1019/cm3; and for O2: 5x1016/ cm3. Disadvantage: oxides grown wet are less dense, with a more open structure, because out-diffusion of H2 creates voids along its path. Thus wet oxidation is typically used when a thick oxide is required that will not be subjected to any significant electrical stress that may lead to electrical breakdown. Dry oxidation: slow, higher quality than wet oxidation, used for gate oxide. Note that dry oxidation appears to always have some initial oxide present (i.e. x i0). 15 Thermal oxidation example A <100> silicon wafer has a 2000- oxide on its surface (a) How long did it take to grow this oxide at 1100o C in dry oxygen? (b) The wafer is put back in the furnace in wet oxygen at 1000o C. How long will it take to grow an additional 3000 of oxide?
Graphic solution a) According to the figure, it would take 2.8hr to grow 0.2m oxide in dry oxygen at 1100o C. b) The total oxide thickness at the end of the oxidation would be 0.5 m which would require 1.5hr to grow if there was no oxide on the surface to begin with. However, the wafer thinks it has already been in the furnace 0.4hr (to grow the first 200nm oxide). Thus the additional time needed to grow the 0.3 mm oxide is 1.5-0.4 = 1.1 hr. 16 Thus we need to grow a total of 0.91m of SiO2. At 1000oC in H2O we have 17
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