The
avoidance
of
contamination is extremely important to semiconductor processing and
the
wafers will undergo many cleaning operations in the course of the
process. Most of
the processing will be
done in the clean-room
environment. The
wafers must be kept
covered as much
as possible and constant attention is required to prevent them from
coming
into contact with contaminated surfaces.
The
success
of the process will depend largely on your ability to keep wafers free
from
contamination. Remember
that impurity
levels, which
are typically measured in terms of atoms per cubic centimeter, can have
very
significant effects.
1.1 Caro’s Etch (or Piranha)
The Caro’s etch is a general cleaning procedure. It is especially useful for removing hardened photo resist.
The
wafers
are
immersed in a hot mixture of hydrogen peroxide (H2O2)
and sulfuric acid (H2SO4). This
can range from a 1:1 ratio by volume to a 1:3 ratio by volume. The standard process is
1:1, but 1:3 is more
aggressive. This is
followed by a thorough
rinse in deionized (DI)
water. Then the wafers are spun dry and are ready for the next
processing
step. The following
step by step procedure
can be used
for 5” wafers in the
7.5”x7.5”x8.5” quartz tank.
1.2 Standard RCA Clean for 5” (8") RCA bench. Note: For the 8" RCA Bench, use the quantities in parentheses ()
An
RCA clean
is required
immediately before any high temperature process step.
This
is necessary to avoid diffusion of surface contaminants into the wafers. After an RCA clean, wafers
must be loaded into
a furnace
within 24 hours. If
more than 24 hours
pass after
an RCA clean, the wafers must be considered contaminated and may not be
loaded
into a furnace. The
RCA clean is one of
many effective
processes that are currently in use at semiconductor processing
facilities.
The
wafers
are
first immersed in a hot mixture of ammonium hydroxide (NH4OH),
hydrogen peroxide (H202)
and deionized (DI) water
in
a ratio of 1:1:5. This
removes traces of
organic residues
and when done on exposed silicon surfaces, traps ionic contaminants
such
as sodium and potassium in a thin oxide.
Wafers
are
then rinsed in a quick dump rinser (QDR) which provides a fast method
of
rinsing, allowing wafers to be moved quickly from one step to the next.
Next,
the
thin
layer of silicon dioxide (SiO2) is removed by
dipping in a
dilute
hydrofluoric acid (HF). Another
QDR cycle
follows.
Wafers
are
then
immersed in a mixture of hot hydrochloric acid (HCL), H2O2,
and DI water. The ratio is again 1:1:5.
This
mixture
is effective for removal of ionic contaminants, especially metals. Wafers are again rinsed in
the QDR. A
final HF dip and QDR step is optional.
Then
the wafers
are spun dry (or dried with N2 gun) and are
ready for the
next
process step.
Note:
If
performing this procedure on the 8" RCA Bench, use the numbers in bold
print and in parentheses ()
1.
Start
logbook
entry.
2.
Wear
gloves
when
handling cleaning equipment.
3.
Rinse
all
tanks
and beakers once with DI water.
4.
Add
10 (14)
liters of
DI water to each RCA1 and RCA2 tank
5.
Add
2 (3)
liters of
NH4OH to RCA1 tank
6.
Turn
on
temperature
controller for RCA1 and set to heat bath to 75 degrees C.
- The feedback loop controls the temperature of the liquid in the tank, not the temperature of the heater. If the level of liquid in the tank is insufficient to cover the sensors and the heaters, the control loop will be unstable and will likely damage the heaters and/or tanks. The liquid level sensors should not allow the heaters to turn on if the level is not at a sufficient starting point, and should turn the heaters off if level falls too low during operation.
7. Add 2 (3) liters of HCl to RCA2 tank
8. Turn on temperature controller for RCA2 and set to heat bath to 75 degrees C.
9. Load RCA clean Teflon carrier with wafers.
10. Add 2 (3) liters of H2O2 to RCA1 bath once it has reached stable temperature
11. Place carrier and wafers in NH4OH mixture for 10-15 minutes.
12.
Rinse
wafers
in
QDR tank.
13.
Turn
off
heater
for RCA1
14.
Mix
20:1 HF
dip
if necessary (commonly, the HF in the tank will suffice)
a. Add 2.5 (10) liters of DI water to HF tank marked for RCA use.
b. Add 250 (1000) ml of 49% HF.
c. Add 2.5 (10) liters of DI water to HF tank. (Provides mixing action).
d. If precise control of oxide removal is required, use more dilute HF. Etch rate is approximately linear with concentration. At room temperature, the following rates may be expected:
- 20:1 125 angstroms/min
- 50:1 50 angstroms/min
- 100:1 30 angstroms/min
15.
Dip
wafers in
HF
solution for 60 seconds.
16.
Rinse
wafers
in
QDR.
17. Add 2 (3) liters of H2O2 to RCA2 tank once temperature has stabilized.
18. Place carrier and wafers in HCl mixture for 10-15 minutes.
a. Agitate initially by withdrawing and reimmersing carrier 2 or 3 times.
b. Leave handle attached – use caution, fumes from bath will condense on handle.
c. Rinse gloves with DI water.
19.
Rinse
wafers
in
QDR.
20.
Turn
off RCA2
heat
controller.
21. Optional step: dip wafers in HF solution for 60 seconds.
22.
Dry
wafers in
SRD
or with N2 gun.
23.
Place
dry
wafers
in clean, dry RCA Clean Box for transport to next process.
24.
Clean
bench.
a. Aspirate solutions out of RCA1 and RCA2 tanks.
b. Rinse RCA1 and RCA2 tanks with DI water three times.
c. Rinse entire bench well with DI water spray.
d. Verify that everything is off and that all chemicals and wafer carriers are stored properly.
25. Complete logbook entry.