INTEGRATED CIRCUITS Dr. Esam Yosry Lec. #7
INTEGRATED CIRCUITSDr. Esam Yosry
Lec. #7
Etc
hin
g Introduction
Etching
Wet Etching
Dry Etching
Plasma Etching
Wet vs. Dry Etching
Physical vs. Chemical Processes
Intr
od
uct
ion
(Chip
Fabri
cati
on C
ycl
e)
Intr
od
uct
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 Etching
Resist Mask
Insulator Film
Isotropic
Anisotropic
Semiconductor
hf
Mask
Film
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
Etc
hin
gPla
sma E
tch
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.