Etching

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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 • Etching occurs both vertically and laterally – Undercutting of photoresist mask and non vertical sidewall

• Erosion of resist could also occur
– More lateral etching of resist

• Etchant could attack substrate leading to changes in profile 6.152J / 3.155J -- Spring Term 2005 Lecture 12 - Etch and Pattern Transfer I (Wet Etch) 4

6.152JST05.Lecture12-1

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6.152J/3.155J SpringTerm 2005

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

Definition of Terms
Etch Directionality
Measure of relative etch rates in different directions usually vertical vs. lateral Etch rates are same in all directions. It is usually related to chemical processes different etch rates in different directions. It is usually related to physical processes such as ion bombardment and sputtering

Isotropic Etching

Anisotropic Etching Highly directional etching with

• Anisotropic etching is the preferred process • Step coverage problems ?? Plummer, Fig. 10-3
6.152J / 3.155J -- Spring Term 2005 Lecture 12 - Etch and Pattern Transfer I (Wet Etch) 5

Etch Figures of Merit
Etch Rate (R) Etch Uniformity (U) Selectivity(Sfm, Sfs)
Rate of film removal, typically 1000 A/min. % change in etch rate across a wafer, lot, etc Ratio of the etch rate of various materials e.g. Film to PR Sfm : Film to mask selectivity; Sfs : Film to substrate selectivity Measure of directionality of the etch. A=1 corresponds to perfect anisotropic etch A=0 corresponds to isotropic etch Measure of the lateral extent of the etch per side Physical and/or chemical damage of the substrate.

Anisotropy, A Undercut Substrate Damage

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)

Etch Figures of Merit
Uniformity
U= R high − R low R high + R low

Rhigh = maximum etch rate Rlow = minimum etch rate

Selectivity

Sfm =

Rf...
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