1. Plasma Etch Sub-processes

2. Reactant Generation process Substrate Dissociation/ ionization of input gases by plasma Gas delivery RF Electrode During ionization, plasma can break down reactant gas molecules into many pieces. Each different from another. For example, CH4 can be broken down to CH3 +, CH3 2+, CH2 +, or CH 2+, etc. None of these parts of the original molecule are stable, and are thus highly reactive species. They are called reactive radicals. A plasma gas is a soup of mutant monsters. These highly reactive radicals help reactions to occur at much lower temperatures (~300C) for reactions that usually happen at 700C. To etch SiO2 we add gasses containing C and F. Such as CF4, C3F8, etc. + + 2+ CH4 CH3+ CH2+ CH3 2+CH 2+ 2+

3. Protective Polymer Film Generation Process Substrate Dissociation/ ionization of input gases by plasma Gas delivery RF Electrode + + 2+ CH4 CH3+ CH2+ CH3 2+CH 2+ 2+ Add certain gases to generate protective coating film during the etch process. Etching gas can not penetrate through the protective film. Add gases that contain C and H. They breakdown and re-link into a polymer film. This film is deposited to all the surfaces of the chamber including the wafer. Protective film Photo Resist

4. * After plasma is ignited and stabilized: * In region B, electric field is very weak * In the sheath region ions see a DC electric potential, attracting them to the lower electrode * This causes a effect called “ion bombardment” of the lower electrode +++ Ions Sheath Lower electrode RF supply B Sheath 4 Ion-bombardment Process

5. * Ion bombardment removes protective film on horizontal surfaces, exposing them to etching gas. It does not touch protective film on vertical surfaces, hence no vertical etch rate. Lower Electrode wafer +++ Protective film +++ Gas * Summary: RF plasma -- Ion Bombardment -- Anisotropic Etch Photo Resist 5 Ion-bombardment Process

6. Reactant delivery and by product removal process 1) Reacta nts enter chamber Substrate 2) Dissociation of reactants by electric fields 3) Reactant transport from bulk of plasma to surface of wafer Exhaust Gas delivery RF generator By-product removal Electrode

7. The slowest sub-process bottlenecks the overall process. It thus determines the reaction rate. It becomes the dominate sub-process– the overall process behaves like it.