Materials Swagelok vFinal DR 18/12/08 12:22 Page 44
44
MATERIALS
Perfect gas controlled
John Baxter from Swagelok Company explains special valve
configurations and evolutions for new ALD precursors.
A
tomic layer deposition (ALD) experimental. In high productivity ALD
processing technology has applications valves are the principal component
advanced significantly in recent responsible for dispensing precursors and purge
years, enabling the use of a wider range of gases in short and frequent pulses into the
precursor materials, including liquids. The delivery chamber.
systems for these materials must be compatible Newer ultra high purity valves, such as the
with process temperatures up to 250
0
C. highly engineered Swagelok ALD valve, provide
Furthermore, for consistent precursor delivery, the high speed (total response time of <15
temperature must be controlled within +/-3
0
C. milliseconds, with up to 10 cycles per second);
Meeting the discrete needs of ALD processing is repeatable pulses with controlled flow (+/- 3%);
complicated for component manufacturers in part and an extended life cycle (>50 million). These
because applications tend to be inconsistent from valves represent a dramatic improvement over
one to the next, and in many cases are previous diaphragm valves that provided flow
control of +/- 10%, with only a few cycles per
second and a total cycle life of <2 million.
In the interest of understanding evolutions in
ALD technology and the corresponding fluid
system demands, this article will review special
valve configurations called for in applications with
temperature dependent or high temperature
precursors, as well as liquid and/or highly reactive
precursors.
Liquid precursors and
vaporisation
Liquid precursors and vaporisation technologies
have been used in the semiconductor industry for
many years, particularly in deposition, but have
only recently been applied to ALD. In ALD, timing
and temperature are critical. Employing an
unwieldy conventional assembly with many
components introduces dead legs and other sources
Figure 1 (Liquid of variability, which makes precision timing
Injection Valve) difficult. In addition, heating a non uniform mass
A specially of components is challenging. Temperatures too
configured ALD high will thermally decompose the gas;
valve vapourises temperatures too low will condense the gas. In
liquid precursor; either case, the system becomes contaminated with
mixes the precursor a bad chemistry.
with carrier gas; and The ideal solution would be a single valve or
delivers the mixture valve block performing the following functions:
to the chamber. It vapourising the liquid precursor; mixing the
also provides a vapourised precursor with a carrier gas; and
purge cycle injecting the mixture into the chamber in precisely
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