Most members of our MatchingCube series measure the DC bias voltage that is generated by assymmetrical electrodes of the plasma chamber.
The DC bias voltage often is negative but can also be positive in some cases.
As the match is connected to an RF generator, the RF generator can monitor, and/or display, and/or control the DC bias voltage.
| Series | monitor | display | control |
| RFG-L | x | x | x |
| PowerCube | x | x | |
| Shoebox | x | | |
All barthel RF generators can monitor the DC bis via remote control.
The RFG-L series generators and the PowerCube series generators can also display the DC bias voltage.
In addition the RFG-L series generators allow to control the DC Bias.
In DCB mode the generator regulates its output power so that the DC bias voltage remains constant.
This application note describes how to do this.
Setup.
Some models have an extendet DC Bias range of +1500V.
Using a MatchingCube-A.
For the MatchingCube-A series maximum values for DC bias are
The MatchingCube translates this to the maximum values
This signal is delivered to the RFG-L generator (not PowerCube or Shoebox) by the connection cable that is delivered with the equipment.
(A minimal RS-232 cable does not work.)
The RF generator processes the signal from the match.
It can be delivered, on the display, the RS-232 interface, and the analog interface if the generator is equipped with such.
| | neg DC bias | pos DC bias |
| Display | 0 … 1000V | 0 … 1000V |
| RS-232 | 0 … 1000 | 0 … 1000 |
| Analog Interface | 0 … 10V | 0 … 10V |
The generator does not distinguish between negative and positive DC bias.
Hence, if you want to control DC bias (RFG-L in DCB mode) you also do not distinguish between positive and negative values.
The generator determines itself if the DC bias is negative or positive.
Example: For a requested DC bias setpoint of +40V you set it to 40V.
For a requested DC bias setpoint of -40V you still set it to 40V.
Altogether the DC bias scaling factor sums up to 100V/V (DC bias to analog interface).
Note: When using a MatchingCube-A do not alter the scaling factor.
Using a different barthel match.
The above description for the MatchingCube-A applies except that the maximum values for the DC bias voltage may differ.
These values are again translated to
Hence, the scaling factor may be different than with the MatchingCube-A.
Note: When using any barthel match do not alter the scaling factor.
Using a Match from a different brand.
The DC bias input impedance of the RF generator is 18kOhms.
This must be taken into account for the scaling factor.
A calibration may be neccessary.
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