Definitions
Usually the definitions of two important
quality factors - homogeneity and uniformity - are easily mixed up in a
deposition process no matter which coating technologies are involved. Note that many articles, papers or technical
notes made the same confusion. So, the
definitions must be clear.
Homogeneity generally
considers a very local spot area and measure/check the layer properties along
the direction of the layer grown.
Uniformity considers the
differences or tolerances among several measured data along one axis or on a
big XY-plane area.
PEM system
normally takes the responsibility of layer’s homogeneity control.
Uniformity can
be achieved if the process chamber has a good layout or configuration for
reactive plasma process.
Homogeneity
To achieve a good homogeneity layer, PEM system was introduced to reactive plasma processês to ensure the composition fraction (or alloy fraction) in each compound or molecules formed by the reactive plasma process is nearly constant.
In general, a
PEM system is able to handle magnetron reactive sputtering in a distance range
500mm-600mm along the long axis of magnetron for a good PID closed loop control
under the operating pressure range: approx. 0.8mTorr~10mTorr. In this example,
a SiOx layer is grown
by a dual magnetron sputtering sources with a process control by a PEM
system. PEM system varies the oxygen
flow quickly to fit the setpoint by a fast response PID calculation to give a feedback
voltage to control the oxygen flow rate.
A good PID closed loop control can bring the alloy fraction factor x in
each grown SiOx layer has an
almost constant x value in different growth time, for example: T1, T2, T3 and T4.
This is what PEM
system contributes to get a high homogeneity layer.
Uniformity
Uniformity is a very important factor to be
concerned and it’s very complicated because there are many factors that can
influence it. Generally, process
chamber’s configuration is most critical to get a good uniformity of the grown
layer.
This example has
3 measuring points over the width 1300mm of the PET film to compare the
thickness, refractive index and other data corresponding to the uniformity in 3
locations in the same XY-plane on the surface of PET film.
The factors
often influencing the uniformity include:
-Pumping: if the
pumping ports can not provide good pumping speed and the gas distribution
inside the process chamber is not good, the uniformity would be influenced.
-Gas piping: if
the gas piping delivering the reactive gas can not deliver the gas into the
reactive plasma zone in the shortest time evenly, the uniformity would be
influenced.
-Magnetrons: if
magnetic constraint for electrons is not uniform, the uniformity would be
influenced.
