Materials Swagelok vFinal DR 18/12/08 12:23 Page 46
46
MATERIALS
Figure 3 ( however, seat swelling may occur, which can
Thermal profiles reduce flow in the valve. At high temperatures,
from an infrared PEEK; polyimide, such as Vespel; or
camera demonstrate polychlorotrifluoroethylene (PCTFE) are materials
the effectiveness of with good mechanical properties, superior to PFA.
a thermal isolator They can be applied in ALD valves, provided they
between the valve are chemically compatible with precursors. In
body and the selecting seat materials, relative performance is
actuator. The valve evaluated. There are always trade offs. PEEK,
on the left exhibits polyimide, and PCTFE do not offer the chemical
a narrower inertness of PFA.
temperature
gradient in the valve Rapid cycles at high
body temperature
Rapid cycling of ALD valves at high temperature
(up to 10 cycles per second) is made possible
through a combination of pneumatic actuation and
a solenoid pilot valve. Direct electric solenoid
energy consumption. A thermal isolator between actuators cannot operate at high temperatures, but
the valve body and actuator contains heat in the solenoid pilot valves are effective and can be
valve body, reducing the amount of heat positioned strategically relative to temperature
transferred to the actuator, which in some cases, is gradient. Pneumatic actuation can yield response
not high temperature tolerant. times of less than 15 milliseconds. Pneumatic
In Figure 3, thermal profiles compare the heat actuation remains preferable because of the
distribution in a conventional diaphragm valve and actuator’s small size, lower cost, safety, and high
a valve with a thermally isolated actuator. Both temperature capabilities.
valves are heated with heater cartridges inserted The key to fast pneumatic actuation is a
into the valve body and controlled to a target combination of a fast, relatively high flow solenoid
temperature of 200
0
C at the valve seat. pilot valve controlling the air supply to the
The valve with the thermally isolated actuator pneumatic actuator; minimised air volume
(left) exhibits a smaller thermal gradient in the required to achieve actuation; and properly sized
valve body. tubing to transport air between the solenoid pilot
Figure 4 (Pneumatic and actuator. Figure 4 shows the command signal,
Actuation Profiles) Seat materials for high solenoid pilot valve response, and pneumatically
Twenty profiles taken temperature actuated valve response of a Swagelok ALD3
at random The upper temperature limit of an ultra high series valve, which achieves a full stroke
demonstrate the purity diaphragm valve is typically dictated by the approximately 12 milliseconds after the command
consistent, quick fluoropolymer valve seat. Fluoropolymers, such as is triggered. In Figure 4, twenty profiles taken at
response time of perfluoroalkoxy (PFA), provide a broad chemical random are superimposed to illustrate
pneumatically and temperature compatibility and resist thermal repeatability.
actuated ALD valves decomposition to temperatures beyond 200
0
C.
for high temperature They are thus desirable for valve seals. Conclusion
applications As temperatures climb to 200
0
C and beyond, ALD technologies are developing at an impressive
pace and with multiple variations. At this time of
rapid development and experimentation, there are
no blanket solutions. Each solution is different,
and customised valve configurations are critical to
success. Tool manufacturers are pursuing exotic
chemical mixtures, high temperature applications
in excess of 200
0
C, liquid precursors, and
vapourisation techniques; it is up to the industryís
component manufacturers to accommodate these
disparate approaches to advancing ALD
technology. The critical role of ALD valves
occasions close working relationships between tool
manufacturers and fluid system component
providers. Valve manufacturers must exhibit a
readiness to reconfigure their products to the
unique needs of the tool manufacturer. Conversely,
tool manufacturers must trust valve manufacturers
with their challenges and expect real innovation.
www.euroasiasemiconductor.com December 2008 / January 2009
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