Application Note
Temperature Control in Wet Cleaning
Process Systems Using High-Performance
Perfluorinated Heat Exchanger
Introduction
Many wet etch and clean processes for wafer surface cleaning require temperature conditioning
of the cleaning chemicals. The cleaning processes
are broadly grouped into two categories: Front End
of the Line (FEOL) and Back End of the Line
(BEOL). FEOL processes are primarily wafer
cleaning, etching and photoresist stripping steps
leading up to the first metal deposition. BEOL
processes start after the first metallization step.
The FEOL clean (also known as RCA clean)
primarily focuses on the removal of wafer surface
contaminants-particles, organics and metallics
and prepares the clean wafer surface for further
processing steps.
RCA FEOL processes involve the use of aboveambient temperature cleaning with aggressive,
oxidizing chemicals. Typically, these cleaning steps
start with a H2 SO4 / H2O2 treatment at high temperature, followed by dilute HF, SC1 (NH4OH / H2O2 / H2O)
and SC2 (HCl / H2O2 / H2O) steps, with water rinses
between each step. The BEOL, cleaning involves
less harsh chemicals to prevent damage to the
deposited metal layers. Solvents such as N-methyl
pyrrolidone and hydroxylamine are used, commonly
at temperatures about 70°C (158°F).
Other cleaning applications where heating chemicals/ liquids is required include hot phosphoric acid
for silicon nitride and aluminum metal etching, hot
deionized water rinses, and heated organic aminebased photoresist strippers. Electrochemical plating
baths are also maintained at sub-ambient temperatures. Heating chemical mechanical planar­ization
(CMP) liquids and abrasive slurries can also be
critical to control removal rates. In many semi­
conductor manufacturing processes liquids with
accurately controlled temperature are dispensed
onto substrates to form thin films. In these applications, the liquid temperature has an effect on
the uniformity and thickness of the final film.
Accurate and repeatable temperature conditioning
requires heating or cooling liquids such as spin on
dielectrics, photoresists, antireflective coatings
and developers prior to dispense onto a wafer.
The recent shift toward higher processing temperatures in single wafer cleaning also creates a need
for drain cooling, as hot chemical waste must be
cooled before it can be safely discharged to the
fab drainage system.
Heat exchangers are generally used for temperature
conditioning of semiconductor liquids. Fluoro­poly­
mer material based heat exchangers are preferred
because of their chemical inertness and corrosion
resistance. In this paper we discuss the design and
performance of a new, all-fluoropolymer pHasor® X
heat exchanger (HEX) that has a high-heat transfer
area (about 1.20 sq. m. per liter volume), enhanced
flow distribution and small hold up volume. Also
described are examples of temperature control
systems incorporating the HEX device for precise
temperature control.
Entegris, Inc.
1
PHASOR X HEAT EXCHANGER
High-performance pHasor X Heat Exchanger Design
The HEX is configured as a shell and tube type
device constructed using a perfluorinated thermoplastic material. Thermoplastic tubes are fusion
bonded into a thermoplastic shell / housing. The
tubes in the device are uniquely braided, twisted,
or plaited to form strands, which are thermally
annealed to set the plait of the strand in place
prior to fusion bonding. Such strands provide a
high-packing density, enhanced liquid flow distribution in the device without the need for heat
transfer enhancing baffles.
The HEX’s compact design provides a high-heat
transfer surface area in a small volume; its chemi-
cally inert materials can withstand operation at
elevated temperatures with organic as well as
corrosive and oxidizing liquids. Its high surface-tovolume design produces more efficient performance
than devices made with flat sheet materials of
similar composition.
The device can be operated in a tube side flow
or a shell side flow configuration. In the tube side
flow a process fluid flows through the lumen of the
tubes, and is separated by the fusion potted ends
from a second working fluid, which flows through
the shell (as shown in Figure 1a and 1b). In the
shell side contacting, flows are switched.
The pHasor X heat exchanger is available in two configurations and sizes:
Working liquid in
Working liquid out
U-style
Process
liquid out
Process
liquid in
Tube bundle
Figure 1a. Counter flow configuration in pHasor X U-style (PHX03U and PHX08U series)
Working liquid in
S-style
Process
liquid out
Figure 1b. Counter flow configuration in
pHasor X S-style (PHX08S series)
Process
liquid in
Tube bundle
Working liquid out
2
Entegris, Inc.
PHASOR X HEAT EXCHANGER
Temperature Management with
HEX System and Process Control
Temperature Control for Recirculation
Bath System
Controlling and conditioning the liquid tempera­
ture in wet cleaning recirculating systems improves
process yield. Its fast response to temperature
changes, chemically inert heating surface, high-heat
transfer area and low volume are key advantages
of the heat exchanger in this application. Other
key components installed in the fluid circuit are
temperature and flow sensors, flow control valves,
cooling fluid reservoir and a microprocessor to
control the temperature.
Figure 2 shows the flow diagram of such a recircu­
lation test system. The objective of the system is
to control the bath liquid temperature to within a
narrow range of pre-set process temperatures by
flowing and conditioning liquids through the heat
exchanger. As seen from the test results in Figure 3,
the 45-liter bath liquid temperature is maintained
within setpoint range of 38°C (100°F) and 39.5°C
(103.1°F) using two 1000W immersion heaters.
The recirculating bath liquid flows at 7.2 L /min.
through the heat exchanger and is conditioned
with a second liquid maintained at 25°C (77°F),
flowing at 6.2 L / min. through the shell side.
Microprocessor/
controller
Heat
exchanger
Flow control valve
Recirculation
bath
6.2 L/min
7.2 L/min
Flow control valve
Pump
Entegris, Inc.
Heater/
chiller bath
Pump
3
PHASOR X HEAT EXCHANGER
7.2 L/min. Tube Flow, 6.2 L/min. Shell Flow
Omega Controller w/RTD in 45 L Bath with Two 1000W Heaters
45
No flow shell side
max hot
Setpoint 1
38°C (100.4°F)
113
Setpoint 2
39.5°C (103.1°F)
Tube inlet
104
40
Temperature (°C )
No bypass shell side–
max cool
Bath temp.
95
35
Shell
outlet
86
30
Shell
inlet
25
Temperature (°F)
Tube outlet
77
68
20
0
1000
2000
3000
4000
5000
6000
7000
Time (sec)
Figure 3. Temperature control for recirculation bath flow test results.
Controlled Volume Dispense with Precise
Temperature Control
Another application is for dispensing a controlled
liquid volume heated to a pre-set temperature.
In a typical system the heat exchanger is coupled
with a flow sensor, temperature sensor and control
valves to enable controlled volume dispense of
precisely temperature-controlled liquids.
Figure 4 shows the flow schematics of such a
system dispensing a precise volume of liquid with
controlled temperature. In this test the working
liquid contained in a 60-liter reservoir is heated to
70°C (158°F) with three 1000W heaters. Process
4
liquid at a temperature of 23°C (73.4°F) is flowed
through the heat exchanger and contacted with
the hot working liquid. A dispense consisting of
330 milliliter (mL) volume of liquid is delivered
at a flow rate of about 22 mL /sec. for 15 seconds.
The process liquid was dispensed through a flow
control valve into the heat exchanger for energy
exchange with the 70°C (158°F) working liquid
(900 mL /min.). The test results in Figure 5 show
the system can heat the volume of liquid from 23°C
(73.4°F) to about 65.7°C (150.3°F) and dispense
in a repeatable manner.
Entegris, Inc.
PHASOR X HEAT EXCHANGER
Microprocessor/
controller
Flow control valve
Recirculation
bath
Two heat
exchangers
in series
22 mL/sec
900 mL/min
60 L/min.
@ 70°C (158°F)
Flow control valve
Figure 4. Flow schematics dispense with precise temperature control.
Two Heat Exchangers in Series
Tube (COLD) Flow = 1380 mL/min
Shell (HOT) Flow = 900 mL/min
One 300 mL Dispense Every Minute: 22 mL/sec Dispense for 15 seconds
Tube Inlet Temperature = 23°C (73.4°F)
77°C
(170.6°F)
37
32
70°C working fluid
65.7°C process
outlet temperature
67°C
(152.6°F)
27
62°C
(143.6°F)
22
Process
dispense
flow rate
57°C
(134.6°F)
52°C
(125.6°F)
1500
Dispense Rate (mL/sec)
Temperature (°C )
72°C
(161.6°F)
17
12
1600
1700
1800
1900
2000
2100
2200
2300
Time (sec)
Figure 5. Controlled dispense with precise temperature control results.
Entegris, Inc.
5
PHASOR X HEAT EXCHANGER
Temperature Stability in Response to
Fluctua­tions in Inlet Temperature
To extend the utility of 193 nm illumination
beyond 65 nm technology, the industry is gearing
up for a radical move to switch from “193 nm dry
lithography” to the “193 nm immersion lithography”
using ultrapure water (UPW). In immersion lithography, a higher refractive index liquid (e.g., DI
water, index = 1.44) is placed between the final
lens and the wafer (replacing the lower index air,
index = 1). The liquid’s refractive properties are
used to create higher resolution images than a
“dry” lens system will allow. The higher refractive
index of the DI water delivers two benefits:
improved resolution and increased depth-of-focus
of up to 50% for printing the finer circuit lines
onto wafers.
To maintain stable optical (refractive index) and
physical properties (surface tension, density), it
is critical to precisely and accurately control the
process water temperature in the immersion lithography process. Figure 6 shows a flow schematic of
a temperature control system that is designed to
maintain the outlet UPW temperature to an immersion tool constant despite fluctuations in the
incoming UPW flow rate and temperature. The
system uses two heat exchangers with associated
sensors and controllers. The incoming water to
the heat exchangers was intentionally subjected to
temperature fluctuations for the varying amounts
of time at several flow rates. The inlet process
water temperature variation is at least 8 times the
variation of the inlet cooling water temperature.
The results (see Figures 7 and 8) show the dual
HEX pHasor X heat exchanger configuration is
able to maintain the UPW temperatures within
±0.1°C (0.18°F) in a single pass process, handling
repeated fluctuations in the inlet temperature
of up to ±0.4°C (0.72°F) and flow rates up to
3 L / min. for prolonged periods of time. Temper­
ature controlla­bility improves signi­ficantly as the
process flow rates decrease due to smaller heat
loads. The device responds efficiently with minimal
delays to changes in the inlet water temperature.
Two pHasor X Heat Exchangers (HEX) in Series
Counter-Current Flow, Single Pass
Temperature
probe
Heater/chiller
Set temp. = 22°C (71.6°F)
Cooling water flow
fixed @ 4 L/min
Incoming DI water
@ 22°C (71.6°F) +
sinusoidal variations
to immersion
lithography tool
Temperature
probe
Figure 6. Temperature control testing setup.
6
Temperature
probe
Target process
water temperature =
22°C (71.6°F)
Entegris, Inc.
PHASOR X HEAT EXCHANGER
2-Minute Period
Shell Inlet: 22°C (71°F) ±0.003°C (0.00°F)
HEX Outlet: 22°C (71°F) ±0.002°C (0.00°F)
HEX Inlet: 22°C (71°F) ±0.158°C (0.28°F)
22.4
72.3
Shell inlet
Tube outlet
Tube inlet
Temperature (°C )
22.2
72.1
71.9
22.1
71.7
22.0
71.6
21.9
71.4
21.8
71.2
21.7
71.0
21.6
Temperature (°F)
22.3
70.8
0
100
200
Time (sec)
300
400
500
Figure 7. Process DI water flow rate = 0.5 L/min.; Tube side = process fluid; Shell side = working fluid.
2-Minute Period
Shell Inlet: 22°C (71°F) ±0.013°C (0.02°F)
HEX Outlet: 22°C (71°F) ±0.050°C (0.09°F)
HEX Inlet: 22°C (71°F) ±0.216°C (0.39°F)
22.4
72.3
Shell inlet
Tube outlet
Tube inlet
Temperature (°C )
22.2
72.1
71.9
22.1
71.7
22.0
71.6
21.9
71.4
21.8
71.2
21.7
71.0
21.6
Temperature (°F)
22.3
70.8
0
100
200
Time (sec)
300
400
500
Figure 8. Process DI water flow rate = 3 L/min.; Tube side = process fluid; Shell side = working fluid.
Entegris, Inc.
7
Conclusions
Design of temperature control subsystems and
com­ponents for liquid chemicals used in wet
cleaning applications are demonstrated. A key
component of the subsystem is the perfluorinated
material-based, compact pHasor X heat exchanger.
It provides a high-heat transfer surface area in a
small volume; its chemically inert materials can
withstand operations at elevated temperatures
with corrosive and oxidizing liquids as well as
organics. And its high surface-to-volume design
produces more efficient performance than devices
made with flat sheet materials of similar compo­
sition. The pHasor X-based temperature control
systems have demonstrated ability to precisely
control liquid temperatures in applications for
recirculation bath systems, dispense of controlled
volume and immersion lithography.
pHasor X heat exchanger preserves absolute fluid purity with
maximum heat transfer.
For Additional Information
For more information on controlling liquid tem­
perature in wet cleaning process systems using
the high-performance perfluorinated pHasor X
heat exchanger, contact Entegris, Inc.
Entegris® and pHasor® are registered trademarks of Entegris, Inc.
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