ECE6450L4-Oxidation Chap 4

ECE6450L4-Oxidation Chap 4 - Lecture 4 Oxidation (applies...

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ECE 6450 - Dr. Alan Doolittle Georgia Tech Lecture 4 Oxidation (applies to Si and SiC only) Reading: Chapter 4
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Introduction 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: SiO 2 is a low vapor pressure solid while CO 2 is 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/SiO 2 interface. In theory, some Si can diffuse back out of the oxide, but in practice, this does not occur (due to Si Iinterstitial injection into the bulk). Oxidation: Si (and SiC) Only
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ECE 6450 - Dr. Alan Doolittle Georgia Tech 2 2 SiO O Si = + 2 2 2 2 0 2 H SiO H Si + = + •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/SiO 2 interface. •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/SiO 2 interface never sees the ambient, it is extremely pure (impurities must be adsorbed onto the SiO 2 and diffuse to the interface to contaminate it). •The oxidizing reaction occurs at the Si/SiO 2 interface which is continuously moving. Thus, Si material is consumed during Oxidation. From the densities and molecular weights of Si and SiO 2 , 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/SiO 2 interface, the oxidation rate depends on the thickness of the oxide and reduces as the oxidation progresses. For dry oxidations: While for wet oxidations: Oxidation: Chemistry
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ECE 6450 - Dr. Alan Doolittle Georgia Tech 3 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 O 2 or H 2 O. 3.) Reaction limited flux at the Si/SiO 2 interface. Oxidation: Chemistry C G =Concentration in Gas C S =Concentration in the stagnant layer/oxide boundary C O = Concentration in the oxide at the stagnant layer/oxide boundary C i = Concentration in the oxide at the oxide/Si boundary
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ECE 6450 - Dr. Alan Doolittle Georgia Tech + + = D t k h HkT
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ECE6450L4-Oxidation Chap 4 - Lecture 4 Oxidation (applies...

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