Near-infrared diode laser in situ monitoring and control of chemical vapour deposition processes

The objective of the experiments described here
was to demonstrate the application of a relatively
new technique, near-IR diode laser absorption
spectroscopy (NIRDLAS), to the monitoring and
control of chemical vapour deposition processes.
The technique is entirely non-invasive and
detects gas phase species in an in situ manner
with a time resolution of the order of
milliseconds.
Approach. In this illustration of the technique,
the atmospheric pressure MOCVD reaction of tin
oxide deposition on a glass substrate was
studied. The precursors were dimethyl tin
dichloride (DMT), water and oxygen with a
substrate temperature of 650 °C. Methane was
selected as the product species to monitor. The
high spectral resolution of the laser allows
unambiguous, selective detection of methane in
the 2n3 band around 1665 nm. The high spatial
resolution of the beam and the ability of the
beam to pass directly through the walls of a
quartz reactor leads to the possibility of spatial
monitoring of reactors. By measuring the ratio of
absorption lines arising from different vibration
or rotation states it is also possible to extract
temperature information across the reactor.
Although not as sensitive as mid-infrared diode
laser spectroscopy, the technique has many
practical advantages, for example, fibre optics
can also be used to launch and receive the beam.
Results It was shown that the evolved methane
concentration was directly correlated with the
deposition rate of the tin oxide film. This
important result enables the monitoring of thin
film production and properties in situ during
production. Preliminary results will also be
presented of near-infrared laser reflectance
measurements taken during the deposition of InP
films by APCVD.
Conclusions The technique is generic and can be
applied to in situ monitoring of gas phase CVD,
PVD and PECVD reactions either for diagnostic
purposes or for process control.