Capacitance sensing systems and methods
Systems, methods, and apparatuses for measuring capacitance of a load. An apparatus includes a ground connector, an output connector configured to couple to the load, and a time-varying signal source configured to inject a time-varying voltage signal onto a conduction path between the ground connector and the output connector. A DC power source or amplifier is configured to apply a DC offset to the time-varying voltage signal, and a current monitor is configured to measure time-varying current in the conduction path. A capacitance module is configured to determine the capacitance based upon at least one of the time-varying current, and accuracy of this measurement is improved by arranging an AC shunt across the DC power source or amplifier.
1 . An apparatus for measuring a capacitance of a first load, the apparatus comprising:
a ground connector configured to couple to ground;
a first output connector configured to couple to the first load;
a first conduction path between the ground connector and the first output connector;
a first DC power source or amplifier configured to DC bias the first conduction path;
a first time-varying signal source configured to inject a first time-varying signal onto the first conduction path between the first DC power source or amplifier and the first output connector;
a first AC shunt arranged across the first DC power source or amplifier;
a current monitor configured to measure a first current in the first conduction path or in the first AC shunt; and
a capacitance module coupled to the current monitor, the capacitance module configured to determine the capacitance of the first load based upon the first current.
2 . The apparatus of claim 1 , wherein most time-varying signal drops across the capacitance of the first load and no more than 1% of the time-varying signal drops across the first DC power source or amplifier.
3 . The apparatus of claim 1 , wherein the first AC shunt further comprises a resonant circuit.
4 . The apparatus of claim 3 , wherein the resonant circuit has a resonant frequency substantially equal to a frequency of the first time-varying signal.
5 . The apparatus of claim 3 , wherein the first AC shunt further comprises a first resistance.
6 . The apparatus of claim 5 , wherein the current monitor is coupled across the first resistance.
7 . The apparatus of claim 1 , wherein the first time-varying signal source couples the first time-varying signal to the first conduction path via a first transformer.
8 . The apparatus of claim 7 , wherein the current monitor couples to the first conduction path via a second transformer.
9 . The apparatus of claim 8 , wherein the first time-varying signal source couples the first time-varying signal to a second conduction path via a second secondary of the first transformer that is inverted from a first secondary of the first transformer, the second conduction path coupled to a second output connector configured for coupling to a second load capacitively coupled to the first load.
10 . The apparatus of claim 9 , wherein the current monitor is configured to differentially sample currents on the first conduction path and the second conduction path.
11 . The apparatus of claim 9 , wherein the current monitor is configured to differentially sample currents in the first AC shunt and a second AC shunt coupled between the second conductive path and the ground connector.
12 . The apparatus of claim 7 , further comprising:
a second output connector configured to couple to a second load;
a second conduction path between the ground connector and the second output connector;
second DC power source or amplifier in parallel with the first DC power source or amplifier and in series with the first time-varying source, wherein the second DC power source or amplifier is configured to apply a second DC voltage onto the second conduction path, and wherein the first time-varying signal source is configured to inject the first time-varying signal onto the second conduction path as well as the first conduction path;
a second AC shunt for shunting AC across the second DC power source or amplifier;
the current monitor configured to measure a second current in the second conduction path; and
the capacitance module coupled to the current monitor, the capacitance module further configured to determine a second capacitance of the second load, based upon the second current in the second conduction path.
13 . The apparatus of claim 12 , wherein the first and second currents are out of phase, and the current monitor is configured to differentially measure the first current and the second current.
14 . The apparatus of claim 13 , wherein the current monitor measures the first and second currents via a single transformer having two secondaries, one for each of the two conduction paths.
15 . A method of determining capacitance of an unknown load, the method comprising:
combining a DC signal from a power source and a time-varying signal from a time-varying source to an output connection configured for coupling to the unknown load;
monitoring the time-varying signal provided to the output connector; and
providing an AC shunt across the power source or amplifier configured to minimize a voltage drop of the time-varying signal across the power source or amplifier.
16 . The method of claim 15 , wherein the power source is a DC power source or an amplifier.
17 . The method of claim 15 , wherein the monitoring the time-varying signal is performed across a resistor of the AC shunt.
18 . The method of claim 15 , wherein the monitoring the time-varying signal comprises inductively measuring current between the time-varying source and the output connector.
19 . The method of claim 15 , wherein the output connector is configured for coupling to an electrostatic chuck of a plasma processing chamber.
20 . A non-transitory, tangible computer readable storage medium, encoded with processor readable instructions to perform a method for capacitance sensing, the method comprising:
generating a DC signal;
injecting a time-varying signal into the DC signal;
providing the DC signal and the time-varying signal to an output connector configured for coupling to one or more segments of a multi-segmented electrostatic chuck;
shunting most of the time-varying signal around a source of the DC signal;
monitoring the time-varying signal;
converting the time-varying signal to a capacitance; and
adjusting a parameter of the multi-segmented electrostatic chuck based on the capacitance.