Inspection
Real time film thickness
monitoring for PECVD
Although optical emission techniques have been used for many years to
monitor plasma etch processes, the use for PECVD processes is less well
accepted. This is primarily because there is no inherent change in plasma
emission at endpoint. Dr. Dave Johnson, PhD, Dr. Ken Mackenzie, PhD
and Chris Johnson of Plasma-Therm describe here a technique which
uses plasma emission reflections from the substrate to generate an
interference signal from which thickness of the film can be computed.
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lasma processing is extensively used Chemical Vapour Deposition (PECVD) is used to
in the manufacturing of silicon-based deposit films of silicon dioxide and silicon
oasiasemiconductor
semiconductors and other semiconductor nitride, which are used for their insulating and
products such as LED’s and solar devices. Plasma dielectric properties or as encapsulation layers to
etching is used to define patterns in a wide provide environmental protection.
range of thin film materials at all stages in device Like all manufacturing, plasma processes
fabrication. These thin film materials include have some variability which is caused by both
silicon and poly-silicon, silicon dioxide and variations in operating parameters (e.g. gas flow
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nitride, aluminum, chromium and other metals, or pressure) and by variations in the quality of
Fig 1: PECVD gallium arsenide, gallium nitride and other incoming material (e.g. material composition or
schematic compound semiconductors. Plasma Enhanced film thickness). As device performance becomes
square4
Issue VI 2009
more demanding, the tolerable process variation
is also decreasing. The most frequently used
strategy to compensate for these variations is to
adjust the process termination time to maintain a
consistent output quality. A means to determine
when the process is at the optimum time (end
point) is therefore essential.
Plasmas are used in semiconductor
processing due to their unique chemical and
physical properties. Such plasmas also emit light
in the UV and visible regions at wavelengths
which are characteristic of the plasma
constituents. These include process precursors
and process by-products. Also ion and neutral
species caused by dissociation/recombination
reactions within the plasma.
In an etch process, this mix of components
is strongly influenced by the presence of the
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