Lecture 19
Etching
EE 441 Spring 2009: Tadigadapa
Etching is the removal of a material in
the exposed areas using:
–
–
–
–
Wet Chemicals & Solutions
Gases
Plasma
Milling
For Example:
– Wet etching has high selectivity, low
physical damage, and low anisotropy
– Ion milling has low selectivity, high
damage and high anisotropy
Etching Metrics:
1. Etch Rate (μ
(μm/s))
2. Etch Rate Uniformity (%
of etch rate)
3. Selectivity (Ratio of etch
rate
t off ttwo materials)
t i l)
4. Undercut (lateral
distance per side)
Anisotropy is given by:
RL
A = 1−
Rv
RL is the lateral etch rate
Rv is the vertical etch rate
For Perfect Anisotropic etch A=1
For Perfect Isotropic etch A=0
Lecture 19
Wet Etching
EE 441 Spring 2009: Tadigadapa
• Wet
W t etching
t hi consists
i t off three
th processes:
– Movement of etchant of species to the surface of the wafer
– A chemical reaction with the exposed surface
– Movement of the reaction products away from the surface
• The slowest of these processes is the “Rate
Rate Limiting Step
Step”.
It is also assumed that the chemical reaction produces
soluble products which can be moved away!
• In general, agitation is necessary to maintain uniformity in
etch rate which aids the movement of the material.
• If the etch rate produces gaseous products, these bubbles
can stick to the surface of the wafer and cause nonuniformity in etch. This pattern is most pronounced at
pattern edges.
Lecture 19
EE 441 Spring 2009: Tadigadapa
Wet Etching Methods
• Si
Simple
l dip
di in
i an etchant
t h t (with
( ith magnetic
ti stirrer)
ti )
• Can result in extreme non-uniformities due to non-uniform
temperatures
• Etch time control
• Changing concentration of etch chemical
• A simple variation is to combine this process with
ultrasonic agitation but can cause cavitation!
• A more elaborate set-up is spray etching. The wafer is
sprayed with the correct concentration and temperature
etchant
t h t while
hil wafer
f is
i slowly
l l rotated.
t t d This
Thi ensures very
high process control but creates enormous waste
chemicals!
• May also be useful in single-sided etching.
Lecture 19
EE 441 Spring 2009: Tadigadapa
Wet Etching of Some Materials
• R
Remember:
b in
i order
d for
f the
th etch
t h step
t to
t be
b successful
f l we
need good selectivity between photoresist and the material
etch rate in a specific
p
etchant!
• In some cases: we first transfer the pattern into a material
of high selectivity – then use this material as the mask.
• Most commonly etched materials are: SiO2, Si3N4, Al,
and Polysilicon and Silicides (Si compounds with
transition metals)
metals).
• SiO2: Wet etching of silicon dioxide is accomplished in a
dilute solution of Hydrofluoric Acid (HF).
• Dilutions in water of H2O:HF::6:1, 10:1 & 20:1 are
commonly used.
• Etch rate of thermal oxide in 6:1 HF solution is about
about 1200Å/min.
Lecture 19
Wet Etching
EE 441 Spring 2009: Tadigadapa
• In general the etch rate of SiO2 follows the following trend:
Increasing Etch Rate
Thermal Oxide
Deposited Oxide
(LPCVD)
Doped
p Oxide
PSG/BSG
PECVD Oxide
• Selectivity of better than 100:1 can be obtained between Si and
SiO2. HF is an isotropic
p oxide etch.
• The reaction pathway depends upon the ionic strength, the
solution pH, and the composition of etchant solution.
SiO2 + 6HF → H2 + SiF6 + H2O
• Since the reaction consumes HF, the reaction rate will decrease
with time. To avoid this HF is buffered with NH4F which
maintains a constant concentration of HF through
NH4F ↔ NH3 + HF
• Buffering controls the pH of the etchant and minimizes
photoresist attack!
Lecture 19
EE 441 Spring 2009: Tadigadapa
Wet Etching of Silicon Nitride
• A 20:1 BHF (Buffered HF) or BOE (Buffered Oxide Etch)
etches thermal oxide at ~ 300Å/min but the etch rate for
silicon nitride is only ~10Å/min!
• Practical
P ti l etch
t h rates
t for
f Si3N4 are achieved
hi d using
i phosphoric
h h i
acid (H3PO4) @ 140-200°C.
• Typical selectivities in phosphoric etch are 10:1 for nitride
over oxide and 30:1 for nitride over Si!
• If nitride is exposed to high-temperature oxidation step, then a
BHF dip must precede the nitride etch step.
• Wet Etch of Aluminum:
• Common aluminum etchant is a mixture of 20% Acetic acid
(CH3COOH),
COOH) 77% Ph
Phosphoric
h i acid
id (H3PO4) and
d 3% Nit
Nitric
i
Acid (HNO3) by volume.
• Or NaOH (Sodium Hydroxide) @ 60-90°C can also etch
aluminum.
Lecture 19
Sili
Silicon
Iso-etch
I
t h Curves
C
EE 441 Spring 2009: Tadigadapa
Wet Etching
Wet etching is a purely chemical
process:
• Lack of anisotropy
• Poor pprocess control
• Particle contamination
• Commonly used in the
fabrication of
Microelectromechanical
Systems (MEMS).
• Isotropic Wet Etching of Silicon:
• Most techniques use strong oxidants to chemically oxidize the
Si and HF is used to etch the oxide. A common etchant
solution is HF and Nitric acid in water.
water
• Si + HNO3 + 6HF → H2SiF6 + HNO2 +H2 +H2O
• Acetic acid is used as a diluent than water. The maximum
etch rate for this solution ~ 470μm/min i.e. a hole in wafer can
be etched in 90 seconds!!
Lecture 19
EE 441 Spring 2009: Tadigadapa
Anisotropic Etching of Silicon
• For example: to isolate devices it may be necessary to make
0.25μm wide several microns deep etch into the wafer.
• Some etchants etch certain crystal
y
directions much faster. These
etches produce sharp facets with well controlled angles in single
crystal substrates.
• Anisotropic etchants of Si:
Etchant
Etch Rate
Etch Rate Ratio
(against 100)
KOH/H2O/IPA @ 80-85°C
1.4μm/min
400 for (110)
600 for (111)
Ethylene Diamine Pyrocatechol
(water) @ 115°C
1.2μm/min
35 for (111)
Tetramethyl Ammonium Hydroxide
@ 90 °C
1μm/min
12.5-50 for (111)
Lecture 19
EE 441 Spring 2009: Tadigadapa
Anisotropic Etching of Silicon
Lecture 19
EE 441 Spring 2009: Tadigadapa
Anisotropic Etching of Silicon
For a (100) oriented Si wafer,
afer the Primary
Primar flat is oriented along
the (110) direction.
The {111} planes will be along the 110 direction at an angle of
54.74 to the {100} plane.
<100>
<111>
[001]
(100)
<111>
Wm
<110>
54.74
54
74
•
z
[100]
[010]
Then
W0
W0 = Wm − 2 z
Lecture 19
Anisotropic Etching of Silicon
EE 441 Spring 2009: Tadigadapa