Homogeneity & Uniformity

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.