Efficient and accurate method for real-time prediction of the self-bias voltage of a wafer and feedback control of ESC voltage in plasma processing chamber
In a plasma reactor having an electrostatic chuck, wafer voltage may be determined from RF measurements at the bias input using previously determined constants based upon transmission line properties of the bias input, and this wafer voltage may be used to accurately control the DC wafer clamping voltage.
1. A method of processing a wafer while controlling an electrostatic chuck clamping voltage, comprising:
applying a first RF current to the electrostatic chuck through an RF match network at a first frequency from a first RF power source;
applying a second RF current to the electrostatic chuck through the RF match network at a second frequency from a second RF power source;
applying DC voltage to the electrostatic chuck;
measuring a third RF current at the match network attributable to the first frequency;
measuring a fourth RF current at the match network attributable to the second frequency;
calculating a wafer voltage at the first and second frequencies based upon the measured third and fourth currents;
calculating a DC component of the wafer voltage attributed to each frequency;
calculating a total DC voltage at the wafer based upon the DC component attributable to each frequency and a DC component attributable to an intermodulation of the first and second frequency; and
adjusting the applied DC voltage to maintain a predetermined difference between the applied DC voltage and the total DC voltage at the wafer.
2. The method of claim 1 , further comprising flowing a cooling fluid through the electrostatic chuck.
3. The method of claim 1 , further comprising etching the wafer.
4. The method of claim 1 , wherein the wafer is disposed in a plasma reactor having a grounded gas distribution showerhead.
5. The method of claim 4 , further comprising flowing a cooling fluid through the electrostatic chuck.
6. The method of claim 4 , wherein the total DC voltage at the wafer is calculated using the equation:
V
D
C
=
-
3
4
*
(
A
g
A
p
)
2
-
1
(
A
g
A
p
)
2
+
1
[
V
RF
(
f
1
)
+
V
RF
(
f
2
)
-
2
3
V
RF
(
f
1
)
*
V
RF
(
f
2
)
V
RF
(
f
1
)
+
V
RF
(
f
2
)
]
,
where A g represents an area of the gas distribution showerhead, A p represents an area of the electrostatic chuck, V RF (f1) represents the DC component of the wafer voltage attributed to the first frequency, and V RF (f 2 ) represents the DC component of the wafer voltage attributed to the second frequency.
7. The method of claim 6 , further comprising flowing a cooling fluid through the electrostatic chuck.
8. The method of claim 6 , further comprising etching the wafer.