MKS 937B - Set Mfcs for Ratio Flow and Pressure Control; Figure 6-14 System Setup Display

MKS 937B Operation Manual
57
There are two relays available for each MFC. These relays are normally used to
provide an alarm signal. For example, a relay can be used to report a low flow
possibly caused by an empty gas cylinder.
6.8.2 Set MFCs for ratio flow and pressure control
In the Ratio mode of operation, MFCs that are set to ratio control are tied together during the
pressure control process. The flow rates for all ratio controlled MFCs vary (either increase or
decrease) simultaneously at the same rate. This operating mode can maintain the composition of
different gases during the process while the total flow rate varies. This technique is widely used for
processes such as reactive sputter process.
The maximum gas flow rate for a MFC is limited by its full scale and this, in turn, limits the settings for
gas flow ratios., Therefore, the maximum ratio initial setpoint for a MFC in the 937B must be less than
½ of the full scale. For example, if full scale for a particular MFC is 1000 sccm, its maximum ratio
initial setpoint is 500 sccm. This enables a scaling of 0 to 200% in flow rate for all MFCs during the for
ratio control of system prressure
.
During the ratio control of system pressure, a master channel is assigned, and the flow rates for rest
of channels will match the master channel to maintain the gas ratio unchanged. In order to achieve
best control stability, the channel with maximum absolute flow rate is set internally as the master
channel for the ratio control.
The 937B controller is capable of simultaneously manual ratio control of up to 6 MFCs. However, if it
is set for auto pressure control (PID or RatioA), only up to 4 MFCs can be simultaneously controlled
since at least one slot will be taken by the pressure control board to measure the system pressure.
To set the ratio control, press the
System
Setup
button. The following information will be displayed:
System Setup
P Unit
Address
CommType
Set MFC Ratio Ctrl
System FV Information
253
OFF
Baud Rate
Com Mode
Set Combination Ch
RS232
ON
OFF
937B
9600
Torr
Set DAC Parameter OFF
Set Param Enable User Cal Enable
Parity None
Figure 6-14 System setup display.
The screen shown in Figure 6-15 will be displayed after changing the Set MFC Ratio Ctrl value to ON
and pressing the
Enter
key. The available MFC channels and their ranges (always in sccm, not the
Nominal Range) are detected and displayed automatically.