Inspection
material being etched and will change once the
material has been removed. For example, when
etching a silicon nitride (SiN) film using CF
4
as
the etchant gas, emission from CN is observed
but is absent once the SiN has been completely
removed. Monitoring the CN emission therefore
provides a straightforward means of determining
the end point of the process.
Generally all that is necessary to implement
this approach is to provide a viewport in the
Issue VI 2009
reactor through which light can be collected and
square4
an optical system for analysis. Because of this
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simplicity, end point systems using Optical
Emission Spectroscopy (OES) have been in use
for many years and are standard components on
most plasma etch tools.
The situation with PECVD is more difficult. Experimental Fig 2: Emmission
The ideal process end point is the instant at All test work was carried out using a Plasma- spectrum of
which the desired film thickness has been Therm VERSALINE deposition tool. This system deposition plasma
oasiasemiconductor
deposited on the product wafer. Unlike etch is a parallel plate system where the substrate is
.eur
processes however, for a deposition process this placed on the lower, grounded electrode and RF
www
point in time is not associated with a change in (13.56 MHz) is applied to the upper electrode.
the plasma constituents and hence there is no The upper electrode also serves as the process
25
change in the emission spectrum. A simple OES gas inlet (“gas showerhead”) where gas is
approach is not useful for deposition processes. introduced through a large number of small
For deposition processes, techniques have (0.4mm) holes. Both electrodes are resistively
been developed which measure the light from heated (the lower up to 350
o
C) to ensure good
an external light source after reflection by the deposited film quality.
growing film. From this measurement the film The upper electrode also incorporates a
thickness can be determined based on viewport located behind the showerhead in a
interference effects. However, deposition location where plasma light reflected from the
processes by nature tend to deposit material on substrate can be viewed through one of the
all reactor surfaces, including viewports, which showerhead holes. A lens focuses the light onto
eventually become opaque and make these the end of a fiber optic which transmits it to an Fig 3: Variation of
reflectance measurements unreliable. Further, external detector for analysis. This arrangement wavelength intensity
the typical PECVD reactor configuration uses is shown schematically in Figure 1. The plasma is over time
two closely spaced electrodes, so that a simple
window will be in close proximity to the wafer
which is monitored. This inevitably disturbs the
plasma locally, giving anomalous process results
right at the point which is being measured.
Optical Emission Interferometry, or OEI is a
system which measures the reflectance of the
plasma radiation itself to calculate the film
thickness. It is implemented using a viewport
which is located within the gas introduction
system where it does not suffer from
degradation. This allows the deposited film
thickness calculation in real time and accurately
terminates a process when the required
thickness has been reached. The benefits
obtained in terms of film thickness repeatability
over the course of a 50 wafer run are shown.
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