Etching

6.152J/3.155J SpringTerm 2005

Lecture 12 -- Etch and Pattern Transfer I (Wet Etch)

Etching and Pattern Transfer (1) OUTLINE
• Basic Concepts of Etching • Wet Etching • Specific Wet Etches
– Silicon – Silicon Dioxide – Aluminum

• Dry (Plasma) Etch
– Review of Plasmas

Reading Assignment:
6.152J / 3.155J -- Spring Term 2005

Plummer, Chapter 10
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Lecture 12 - Etch and Pattern Transfer I (Wet Etch)

Introduction
• Etching is selective removal of thin film(s) resulting in a desired thin film(s) pattern • The Etch Mask is usually photo-resist or oxide/nitride • Multi-layer structures can be etched sequentially using same masking layer • Etching can be done in either “wet” or “dry” environment
– Wet etching = liquid etchants – Dry etching = gas phase etchants in a plasma.

Plummer, Fig. 10-1

Wet Etch ⎯ chemical process only Dry Etch ⎯ chemical and physical (sputtering) process

6.152J / 3.155J -- Spring Term 2005

Lecture 12 - Etch and Pattern Transfer I (Wet Etch)

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6.152JST05.Lecture12-1

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6.152J/3.155J SpringTerm 2005

Lecture 12 -- Etch and Pattern Transfer I (Wet Etch)

Basic Concepts
Diffusion Reactants
Diffusion Reaction film Diffusion Reaction Products
Diffusion

Boundary Layer

substrate

• Etching process consists of three steps
– Mass transport of reactants (through a boundary layer) to the surface to be etched – Reaction between reactants and the film(s) to be etched at the surface – Mass transport of reaction products from the surface through the boundary layer

• Etching is usually done using liquid phase or gas phase reactants
– liquid phase (wet) etching —reaction products soluble in solvent or gaseous – gas phase etching — reaction products gaseous / sublimation temperature
6.152J / 3.155J -- Spring Term 2005 Lecture 12 - Etch and Pattern Transfer I (Wet Etch) 3

Definition of Terms

Plummer, Fig. 10-2

• Etch profiles do not have perfect straight walls under the edge of the mask