On-site Generated Fluorine as an Alternative to
Greenhouse Gases: Part II
BY PAUL STOCKMAN
In Part II, we discuss the fundamental chemical properties of fluorine that
allow it to deliver significant process and cost of ownership benefits while at the
same time eliminating the need for the majority of greenhouse gas usage
in high-tech manufacturing.
Introduction
In Part I of this article,1 we described the safe and reli- able on-demand production of fluorine as demon- strated by Linde’s on-site fluorine generators, with an
installed base of more than 30 systems over the last 10
years. With production capacities of 1 to 100s of tons per
year, these systems initially displaced cylinder sources of
F2 and ClF3 used for chamber cleaning of semiconductor
processes, and subsequently have become the preferred
bulk supply of chamber cleaning agents for large LCD
and thin-film photovoltaic manufacturing. In Part II, we
discuss the fundamental chemical properties of fluorine
that allow it to deliver significant process and cost of
ownership benefits while at the same time eliminating
the need for the majority of greenhouse gas usage in
high-tech manufacturing.
CVD Process Cleaning
Chemical vapor deposition (CVD) processes used in
high-tech thin film manufacturing require periodic cleaning to remove particles and films from the surfaces of the
vacuum chamber and process equipment. The frequency
of the cleaning can be as short as once per deposition
cycle, or may range up to once per several days, depending upon the thickness of the film deposited and the
sensitivity of the devices being made. Without cleaning,
these films and particles lead to defects that render semiconductor chips inoperable, displays with dark pixels, and
solar modules with degraded efficiencies.
Most high throughput CVD processes employ auto-
mated, on-line cleaning with gas-phase chemicals as a
time-saving alternative to off-line manual and wet pro-
cesses. These gases must not only react with compounds
in the deleterious films and particles but also must be
non-reactive towards the chamber materials of construc-
tion. In order to maximize the availability of the costly
CVD equipment, chamber cleaning processes must be
quick. And increasingly, device manufacturers are look-
ing for processes that have low environmental impact
and long-term sustainability. But most importantly, the
cleaning processes must be very low cost per process
cycle: it’s cleaning, after all.
Chemical Nature of Fluorine
Fluorine-containing gases, or F-gases, meet all of the
technical and cost requirements for chamber cleaning
agents. The gases are activated to release the fluorine
atoms as neutral radicals, which subsequently react with
residual thin-films to form a gas-phase waste stream that
is removed through the vacuum exhaust system of the
chamber. While very reactive towards the thin-film compounds, fluorine radicals are inert towards most of the
metals and ceramics used in CVD equipment, provided
moisture and oxygen are excluded.
Historically, the electronics industry has used fluorocarbons (CxFy), sulfur hexafluoride (SF6), and more recently
nitrogen trifluoride (NF3) as feedstocks to supply the
fluorine radicals. These gases are easy to compress and
transport in cylinders, and the cost to supply has been
reduced through higher-volume manufacturing and
packaging. However, these gases have certain process
inefficiencies inherent to the high bond strengths and
side reactions associated with the carbon / sulfur / nitrogen carrier atoms. These gases have also been the target
