Transcript Undergraduate Admissions & College of Engineering

Etching
ECE/ChE 4752: Microelectronics
Processing Laboratory
Gary S. May
February 19, 2004
Outline
 Introduction
Wet Chemical Etching
 Plasma Etching

Definition



Recall: Photolithography = process of
transferring patterns (on masks) onto a thin
layer of photoresist
Photoresist patterns must be transferred once
more onto the underlying layers to produce
circuit features
Pattern transfer accomplished by selectively
etching unmasked portions of a layer
Etching Hierarchy
Etching
Wet
isotropic
anisotropic
Dry
anisotropic
(plasma etching)
Outline
Introduction
 Wet Chemical Etching
 Plasma Etching

Uses
Prior to thermal oxidation or epitaxial
growth, wafers are chemically cleaned to
remove contamination
 Especially suitable for blanket etches (i.e.,
over the whole wafer surface) of
polysilicon, oxide, nitride, metals, and III-V
compounds.

Mechanism
1. Reactants
transported
by diffusion
to surface
2. Reactions
occur at
surface
3. Products
from surface
removed by
diffusion
Uniformity

“Equality” of vertical etch rates at different sites
on the wafer surface
Etch rate uniformity (%) 


(maximum etch rate - minimum etch rate)
 100%
maximum etch rate  minimum etch rate
This is actually non-uniformity
Alternative definitions:
s/m
 | Rcenter  Redge | 
 100%
U  
Rcenter


Silicon Etching

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Most etchants are mixtures of HNO3 and HF in
water or acetic acid (CH3COOH).
HNO3 oxidizes silicon to form an SiO2 layer:
Si + 4HNO3 → SiO2 + 2H2O + 4NO2
HF is used to dissolve the SiO2 layer:
SiO2+ 6HF → H2SiF6 +2H2O
Water can be used as a diluent for this etchant, but
acetic acid is preferred.
Orientation-Dependent Etching
Some etchants dissolve a certain crystal
plane of Si faster than another plane
 For Si, the (111) plane has more available
bonds per unit area than the (110) and (100)
planes
 Therefore, etch rate is slower for the (111)
plane.

KOH Etching

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KOH is an orientation-dependent etchant for Si.
Solution with 19 wt % KOH in deionized water at
80 oC removes the (100) plane at a much higher
rate than the (110) and (111) planes [ratio of etch
rates for (100):(110):(111) planes = 100:16:1].
SiO2 Etching
 Commonly etched in a dilute solution of HF with
or without NH4F
 Adding NH4F is called a buffered HF solution
(BHF), also called buffered-oxide-etch (BOE)
 Reaction for SiO2 etching:
SiO2+ 6HF → H2SiF6 +2H2O
 SiO2 can also be etched in vapor-phase HF.
Outline
Introduction
 Wet Chemical Etching
 Plasma Etching

Anisotropy

Vertical features are desirable to increase circuit density.
photoresist
undercut
photoresist
etched material
etched material
substrate
Anisotropic

Quantitatively:
substrate
Isotropic
 RL 
 100%
Af  1 
 RV 
where RL = lateral etch rate, RV = vertical etch rate
Plasma Fundamentals
 Plasma: ionized gas composed of equal
numbers of positive and negative charges and a
different number of unionized molecules
 Produced when electric field is applied to a gas,
causing gas to break down and become ionized
 Initiated by free electrons that gain kinetic
energy from electric field, collide with gas
molecules, and lose energy.
 Energy transferred causes the gas molecules to
be ionized (i.e., to free electrons).
 Free electrons gain kinetic energy from the
field, and the process continues.
Plasma Etching

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Plasma etching - chemical reaction combined with
physical ion bombardment
Other names:
 ion milling
 sputter etching
 reactive ion etching
 reactive ion beam etching
First explored as a cheaper alternative to wet
solvent resist stripping in 1960’s and 70’s
Etch
Equipment
Etch Mechanism
F  D
C
x
1.
2.
3.
4.
5.
Etchant species generated in plasma.
Reactant transported by diffusion to surface.
Reactant adsorbed on the surface.
Chemical reaction (along with ion bombardment) forms volatile
compounds.
Compounds desorbed from surface, diffused into the bulk gas, and
pumped out by vacuum system.
End-Point Control
 Dry etching has less etch selectivity than wet.
x j  1.1 Ds t
 Plasma reactor must be equipped with a
monitor that indicates when the etching
process is to be terminated (“end point
detection” system).
 Laser interferometry is used to determine the
end point.
Laser Interferometry

Dd  l / 2n

Intensity of laser light reflected off thin film
surface oscillates.
Period of the oscillation related to change in
film thickness
Dd  l / 2n
where Dd = change in film thickness, l is the
wavelength, and n is the refractive index
Interferometry Example


Typical signal from a silicide/polycrystalline Si
gate etch
Dd for polysilicon = 80 nm (measured by using
a He-Ne laser with l = 632.8 nm)