TECHNICAL NOTE
Statistical Process Control
with FabGuard®
FabGuard Sensor Integration and Analysis System is a fully
automated, real-time early fault detection and analysis system
for improving semiconductor equipment and process productivity
and various INFICON in situ diagnostic sensors. The powerful
analysis techniques of FabGuard are capable of "smart
diagnostics" by combining sensor and tool data for fault
detection and classification. FabGuard puts in situ sensors
to work to:

Figure 2 shows the FabGuard database query filters used tocreate an SPC control rule. The control variable is Wafer
Temperature. The data used to create the control limits isfrom one chamber on one tool, for one recipe, for one day.

• Baseline normal process and tool behavior• Analyze process data in real-time to detect problems andpinpoint problem sources• Issue warnings and alarms
One of the benefits of FabGuard is the application of
Statistical Process Control (SPC) to tool data for a PVD
process. The FabGuard database is also mentioned, because
the database is used to select which data is used for SPC.
Figure 1 shows raw tool data from one of the wafers used tocreate the SPC control charts shown later in this example.
The lower plot shows the time series data for 3 variables: DCPower (red), Pressure Manometer (green), and Wafer
Temperature (blue).

Figure 1. Raw tool data from one of the wafers used to create SPC controlcharts.

Figure 2. FabGuard database query used to create an SPC rule for Wafer
Temperature.

Figure 3 shows the X-bar S control chart created from thedatabase query of Figure2. In an X-bar S control chart, “X-bar”
means that the controlled quantity is the average value of thecontrol variable and “S” means that the control limits arecalculated from the standard deviation of the control variable.
In this case, the control variable is Wafer Temperature andthe average and standard deviation are calculated in the timeinterval of 30 to 90 seconds after the start of each wafer. In
the plot, X-bar for each wafer is plotted as a solid circle andthe upper and lower control limits (UCL and LCL) are plotted
as horizontal lines. The numerical values of UCL and LCL areestimates of ±3 sigma from the average. UCL and LCL varyfor each wafer, because a slightly different number of samples
was used for each wafer. The color of the symbols changes
every time a new lot starts.

Figure 3. X-bar S control chart for Wafer Temperature.

Figure 4. X-bar S control chart for DC Power.

A summary of the SPC statistics is given below the plot.
X-bar-bar is the average of the X-bar values for each wafer. In
the plot, X-bar-bar is the narrow horizontal line at the midpointof the data. S-bar is the average standard deviation withineach wafer. For Wafer Temperature, X-bar-bar is 481.66
degrees C and S-bar is 1.49. The ratio of S-bar to X-bar-bar isonly 0.3%, which means that the Wafer Temperature varied by
an extremely small relative amount.

limits. However, starting in the fifth lot (around wafer 100) there
is a significant shift in the mean. The mean shift is large
enough that all of the wafers in the next 6 lots are above theupper specification limit, so the process is performing quite
poorly with respect to DC Power.

Several wafers are outside of the SPC control chart limits,
UCL and LCL. This implies that the process is out of control,
in an SPC sense. However, all of the wafers are inside thespecification limits, USL and LSL, so the process is performing
acceptably well with respect to Wafer Temperature.

Figures 3 and 4 were selected specifically to demonstrate thatcare must be taken when applying SPC. Although some of thedata in Figure 3 is outside its SPC control chart limits, theprocess is still performing in spec. In contrast, although all of
the data in Figure 4 is inside its SPC control chart limits, theprocess is performing out of spec.

The control chart for Pressure Manometer is similar to the onefor Wafer Temperature, so it does not need to be discussed
here. The control chart for DC Power, shown in Figure 4, ismore interesting. All of the data is inside the SPC control chart

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