INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 126)

MDC-370 DEPOSITION CONTROLLER
10.4 RATE CALCULATION
The deposition rate for each sensor is calculated by dividing the change in the
measured thickness by the time between measurements. The rate is then filtered
by a three pole digital filter to filter out quantizing and sampling noise introduced
by the discrete time, digital nature of the measurement process. The above filter
has an effective time constant of about 2 seconds. Following a step the displayed
rate will settle to 95% of the final value in 5 sec. The individual sensor rates are
displayed in the Source/Sensor status screen.
If only one sensor is enabled for measurement, then the displayed rate is the
measured rate for only that sensor. If multiple sensors are enabled for
measurement, then the displayed rate is the weighted average of all of the sensors
enabled for measurement.
10.5 MULTI-SENSOR AVERAGING
The MDC-370's most important feature is its ability to average multiple crystal
sensors. Most thin film monitors and controllers only measure a single crystal at
one point in the chamber which may be adequate for some processes. However,
critical processes can benefit from multi-sensor averaging because it can reduce
errors caused by changing vapor distribution which is one of the biggest error
sources in film thickness measurement.
Consider the typical case with a single crystal sensor located in the center of the
chamber up next to the planetary. Now imagine the vapor plume moving away
from the center of the chamber because of an uneven melt. Assume that the
evaporation rate at the source remains constant. As the plume moves away from
the center, a single sensor will indicate a decrease in the rate but, the actual rate
on the substrates has not changed significantly. Now consider what would
happen with addition crystal sensors located around the perimeter of the
planetary. Some of these sensors would indicate a rate increase offsetting the
decrease indicated by the sensor in the center and thereby reducing the
measurement error.
A fully configured MDC-370 can measure a maximum of six crystal sensors or
any combination of the six available sensor inputs. As with a single sensor
configuration, sensor head placement is very important. Ideally you will have one
sensor in the center of the chamber equal distance from the source as the planetary
at that point. Then, you should place at least three sensors spaced equally around
the perimeter of the planetary. It may help you to visualize the vapor distribution
if you use four equally spaced sensors around the perimeter. That way, when you
view the individual rates, you can easily see when the vapor distribution is
unequal from side to side or front to back in relation to the chamber.
Each sensor has a tooling parameter to adjust the sensor's measured rate and
thickness so that it corresponds to the rate and thickness deposited on the
substrates.
THEORY OF OPERATION 10-4