INTEGRATED CIRCUITSDr. Esam Yosry

Lec. #7

Etch

ing

Introduction

Etching

Wet Etching

Dry Etching

Plasma Etching

Wet vs. Dry Etching

Physical vs. Chemical Processes

Intro

duct

ion

(Chi

p Fa

brica

tion

Cycle

)

Intro

duct

ion

(Pro

cess

es)

Oxidation

Diffusion

Ion Implantation

Deposition

Etching

Lithography

Deposition Removal Patterning Modification of

electrical properties

Etching

• We want to transfer the photoresist pattern to the underlying material.

• Removal of material by chemical or physical means is called etching.

• Process must be selective to the material you are removing.

Etching

Etching

• Want faster etches so process is done quicker.

• Too fast process will be difficult to control.

• Ratio of etch rates of other materials referenced to the material you are etching.

• Lateral extent of the etch under the resist mask.

Etching

Etching

• Usually selectivity and directionality are the first order issues.

• Selectivity comes from chemistry process.

• Directionality comes from physical process.

• Modern etching try to optimize both.

Etching

• Two main types of etching: Wet and Dry (plasma).

• Plasma etching dominates today.

• Etching of thin films and sometimes the silicon substrate are very common process steps.

Wet Etching

• Wafers submerged in specific chemical baths.

• Purpose is to selectively remove materials (Si, SiO2, Al) that are not covered by a Photoresist.

• Etch Rate is the rate at which the material is removed (measured in Å/min).

• Isotropic Etch proceeds in all directions at the same rate.

• Anisotropic Etch is preferential in one direction.

Wet EtchingResist Mask

Insulator Film

Isotropic

Anisotropic

Semiconductor

hf

MaskFilm

Substrate

Anisotropic

Isotropic

Wet Etching

• Oxide, Poly, and Metal layers are to be etched through a developed PR window.

• Wet etchant acids : HNO3, H3PO4 and HF acids.

• Wet etching is isotropic (only in large windows).

Wet Etching

Wet Etching Substrate Selectivity

Etch StopWe want the etchant to remove SiO2 and stop at Si.

StringerWhen material is cleared from a planar region -

residual material remains at steps.

MaskSiO2

Si

FILM II

FILM I

SUBSTRATE

Prior to Etch

FILM I II

SUBSTRATE

Etched to “Endpoint”

“Residue”

Wet Etching Wet Etching Silicon Dioxide

• Solution of Hydrofluoric Acid is normally used.Reaction is: SiO2 + 6HF H2 + SiF6 + 2H2O

• Etch Rate ~ 1000Å/min at 25°C.

Wet Etching Aluminum• Phosphoric Acid is used to etch Aluminum.• The reaction forms Al2O3 which dissolves in

water.• Etch rate = 2000Å/min at 25°C

Dry Etching • Removing by exposing the mask to bombardment of ions.• The dry etching process typically etches directionally or anisotropically.• Dry etching is useful for materials which are chemically resistant and could not be wet etched.• Dry etchant : Plasma and Reactive Ion Etching.• Dry etching is anisotropic (for small windows).

Dry

Etch

ing

Plas

ma

Etch

ing

• It is a faster and simpler chemical process.

• More directional (anisotropic).

• Both chemical and ionic species play a role.

• Reactant ions in the RF induced plasma diffuse to the wafer.

• Reaction products are volatile:CF4 for Si SiF4 (volatile)

CHF3 for SiO2 SiF4+CO2+ HF

Dry EtchingPlasma Etching

Plasma Etching Mechanics

• Three mechanisms:Chemical etching (isotropic, selective).Physical etching (anisotropic, less selective).

Ion-enhanced etching (anisotropic, selective).

Plasma Etching

Plasma Etching

Wet vs. Dry Etching

Characteristic Dry Etching Wet Etching

cost high lowcontrol easy not

easyetch rate slow fastselectivity moderate highresolution good fairanisotropy good

difficultwaste disposal problem low high

Physical vs. Chemical Processes

Thanks

Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.