ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 4Oxidation (applies to Si and SiC only)Reading: Chapter 4ECE 6450 - Dr. Alan DoolittleGeorgia TechIntroduction discussion:The ability to grow a high quality thermal oxide has propelled Si into the forefront of all semiconductor technology. Ge allows faster transistors (due to it’s much higher mobility) , dissipates much less heat and was used first, before Silicon. However, Ge-oxides are much more unstable, much poorer quality and very difficult to form. Some present day efforts are being made to produce SiGe channel transistors to marry the benefits of Si (good oxides) with the speed of Ge.High power devices are being developed in SiC. One key advantage of SiC over other material alternatives is the ability to grow high quality oxides on the Si face of SiC. (Note: SiO2is a low vapor pressure solid while CO2is a high vapor pressure gas).During the oxidation of Si, the Oxidizing Species defuses through the oxide to react with the Si at the Si/SiO2interface. In theory, some Si can diffuse back out of the oxide, but in practice, this does not occur (due to SiIinterstitialinjection into the bulk).Oxidation: Si (and SiC) OnlyECE 6450 - Dr. Alan DoolittleGeorgia Tech22SiOOSi=+222202 HSiOHSi+=+•Typically, some hydrogen is introduced (even in a dry oxidation) to allow the monovalent hydrogen to passivate (chemically satisfy) broken interface bonds at the Si/SiO2interface. •The stability of this passivation is an issue of increasing concern as E-fields increase due to decreasing device dimensions. Electrons tend to be accelerated into the Hydrogen, breaking the H-Si Bond. These same broken bonds can then trap electrons, preventing or slowing their conduction.•Since the Si/SiO2interface never sees the ambient, it is extremely pure (impurities must be adsorbed onto the SiO2and diffuse to the interface to contaminate it). •The oxidizing reaction occurs at the Si/SiO2interface which is continuously moving. Thus, Si material is consumed during Oxidation. From the densities and molecular weights of Si and SiO2, we find that the thickness of the Si consumed is 0.44d, where d is the oxide thickness.•Likewise, since the oxygen must diffuse through the oxide to react at the Si/SiO2interface, the oxidation rate depends on the thickness of the oxide and reduces as the oxidation progresses.For dry oxidations:While for wet oxidations:Oxidation:ChemistryECE 6450 - Dr. Alan DoolittleGeorgia Tech3 flow regimes occurring during oxidation:1.) Stagnant Gas Flow: occurs due to finite gas flow in the bulk gas, and zero flow at the wafer surface.2.) Diffusion through the oxide: Molecular diffusion of O2or H2O.3.) Reaction limited flux at the Si/SiO2interface.Oxidation:ChemistryCG=Concentration in GasCS=Concentration in the stagnant layer/oxide boundaryCO= Concentration in the oxide at the stagnant layer/oxide boundaryCi= Concentration in the oxide at the oxide/Si boundaryECE 6450 - Dr. Alan DoolittleGeorgia Tech⎟⎟⎠⎞⎜⎜⎝⎛++=DtkhHkTkHPCoxidesGsGi1⎟⎟⎠⎞⎜⎜⎝⎛++⎟⎠⎞⎜⎝⎛+=DtkhHkTkDtkHPCoxidesGsoxidesG110⎥⎦⎤⎢⎣⎡++==DtkhHkTkNPHkdtdtoxidesGsGsoxide1 RateOxidation 1where H is Henry’s gas constant, kSis the chemical rate constant for the reaction at the Si/SiO2interface, k is Boltzman’s constant, PGis the partial pressure of the oxidizing species, T is absolute temperature, D is the diffusion coefficient for the oxidant, hGis the mass transport coefficient in the stagnant layer, toxideis the oxide thickness, and N1is the number of molecules of oxidizing species per unit volume of SiO2.Given that the concentration of the oxygen at the Si/SiO2interface is,If D--->0 (diffusion controlled) Ci--->0, CO--->HPG.If D--->infinity (reaction controlled) Ci=CO=HPG/ (1 + kSHkT/hG)The rate of oxidation can be expressed as,and the concentration of the oxygen in the oxide at the stagnant gas/oxide boundary is,Oxidation:Chemistry(*)ECE 6450 - Dr. Alan DoolittleGeorgia Tech22SiOOSi=+22222 HSiOOHSi+=+What is N1: Since SiO2has a molecular density of 2.2x1022molecules/cm3and N1= 2.2x1022molecules/cm3for dry oxidations.While for wet oxidations,N1= 4.4x1022molecules/cm3for wet oxidationsN1is the number of molecules of oxidizing species per unit volume of SiO2.Oxidation:ChemistryECE 6450 - Dr. Alan DoolittleGeorgia Tech()()τ+=+ tBAttoxideoxide2⎟⎟⎠⎞⎜⎜⎝⎛+=GshkDA11212NDHPBG=,2BAttoo+=τThe general solution of this equation (*), under the assumptions of at t=0, toxide= to,Oxidation:Thickness -Time RelationshipTime shift due to initial oxide thickness, toA and B or B/A are usually quoted, not the fundamental constants (D, ksetc...).A more useful form of this equation is,()⎥⎥⎦⎤⎢⎢⎣⎡⎟⎠⎞⎜⎝⎛+++−=241121)(AtBAttoxideτECE 6450 - Dr. Alan DoolittleGeorgia Tech()τ+≈ tABtoxide()τ+≈ tBtoxide2Consider two limiting cases, Case I.) Thin oxides, ===> Neglect quadratic term,in which case B/A is termed the linear rate coefficient.Case II: Thick oxides ===> Neglect the linear term,in which case B is called the parabolic rate coefficient.Oxidation:Thickness -Time RelationshipECE 6450 - Dr. Alan DoolittleGeorgia TechAdvantages DisadvantagesDry (O2) Better Electrical Breakdown, Dense, Used for Gates Slow Growth RateWet (O2+H2O) Fast Growth Rate, good for device isolation, contact isolation, etc... Somewhat Porous (sponge analogy) Not used for gate oxidesPractical considerations:Metallic impurity gettering:Halogen species (Cl, F, etc...) are often introduced to getter metallic impurities from the tube during an oxidation. This also tends to increase the oxidation rate for thin oxides (linear term). HCL is the safest to use (bubbled into the furnace as described in the diffusion discussion) but is highly corrosive to the gas tubing etc.... However, Trichloroethylene (TCE) and Trichloroethylane (TCA) are less corrosive but can be toxic (TCE is, while TCA can form phosgene at high temperatures).Thick oxides:Due to the growth rate dependence on thickness, thick oxides must be performed at high pressures. These furnaces look like submarine torpedo tubes and raise the partial pressure of oxygen that in turn raises the parabolic rate coefficient, B.Thin Oxides:Most thin (toxide<~ few hundred angstroms) dry oxides grow at a rate faster than Deal & Grove predicts. Corrections can be made to work down to ~ 300 angstroms, but modern MOS gate oxides are <100 angstroms.Orientation