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Sources of metal ions in front-end
processes include process hardware (SiC paddles, boats, and tube), diffusion
through quartz tubes, and unintended contamination of the wafers from
etch, resist strip, ion implantation, or other process steps. In conventional
processing these contaminants transport through the process ambient by
diffusion, resulting in contamination of the wafers in process.
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Promecon
technology imposes an electric field between the wafers and the
ambient, by applying a voltage between the wafer boat and the process
tube.
The electric field is
concentrated in the region between the wafer edges and the tube
or liner, with negligible electric fields at the sensitive surface
of the wafers. Positive ions formed by electron transfer at surfaces
are driven away from the wafer by the electric field, and must be
replaced by slow diffusion of neutrals, causing the concentration
of metals near the wafers to fall.
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Promecon protection is
particularly effective with alkali metals, which are volatile and
easily ionized; the electric field is also useful in reducing iron
and copper contamination. Promecon can be used to reduce or eliminate
the need for chlorine-based cleaning, reducing materials costs,
increasing uptime, and decreasing equipment corrosion. Operation
of Promecon™ during oxidation can be employed with no degradation
of oxide quality.
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