Apparatus and methods for improving the stability of RF power delivery to a plasma load
Methods for improving the stability of RF power delivery to a plasma load are disclosed. The method includes adding an RF resistor and/or a power attenuator at one of many specific locations in the RF power system to lower the impedance derivatives while keeping the matching circuit substantially in tune with the RF transmission line.
1. A method of manufacturing a given radio frequency (RF) generator configured for use to at least generate a plasma in a plasma processing system, the method comprising:
receiving a plurality of data points, said plurality of data points representing a first set of values of
P
G
∂
Z
T
∂
P
G
for a set of stable plasmas of interest and a given RF cable length associated with said plasma processing system, a term P G in said first set of values of
P
G
∂
Z
T
∂
P
G
representing output power of a first RF generator, a term
∂
Z
T
∂
P
G
in said first set of values of
P
G
∂
Z
T
∂
P
G
representing derivative of a load impedance with respect to said output power of said first RF generator;
obtaining a second set of values of
P
G
∂
Z
T
∂
P
G
,
said second set of values of
P
G
∂
Z
T
∂
P
G
representing values of
P
G
∂
Z
T
∂
P
G
in a stable operating region of said given RF generator, a term P G in said second set of values of
P
G
∂
Z
T
∂
P
G
representing output power of said given RF generator, a term
∂
Z
T
∂
P
G
in said second set of values of
P
G
∂
Z
T
∂
P
G
representing derivative of a load impedance with respect to said output power of said given RF generator;
changing design parameters for said given RF generator to cause said second set of values of
P
G
∂
Z
T
∂
P
G
to substantially encompass said first set of values of
P
G
∂
Z
T
∂
P
G
;
and
designing and manufacturing said given RF generator using said design parameters.
2. The method of claim 1 further comprising:
plotting said first set of values of
P
G
∂
Z
T
∂
P
G
on a plot wherein real parts of said first set of values of
P
G
∂
Z
T
∂
P
G
are plotted on a x-axis of said plot and imaginary parts of said first set of values of
P
G
∂
Z
T
∂
P
G
are plotted on a y-axis of said plot.
3. The method of claim 1 wherein said first set of values of
P
G
∂
Z
T
∂
P
G
represents a first set of measured values.
4. The method of claim 1 wherein said set of measured values is obtained by varying said output power of said first RF generator while tuning elements values associated with a matching network of said plasma processing system is held fixed.
5. The method of claim 1 wherein said plasma processing system represents an inductively coupled plasma processing system.
6. The method of claim 1 wherein said plasma processing system represents an capacitively coupled plasma processing system.
7. The method of claim 2 further comprising:
plotting said second set of values of
P
G
∂
Z
T
∂
P
G
on said plot wherein real parts of said second set of values of
P
G
∂
Z
T
∂
P
G
are plotted on said x-axis of said plot and imaginary parts of said second set of values of
P
G
∂
Z
T
∂
P
G
are plotted on said y-axis of said plot.
8. The method of claim 4 wherein said first set of values of
P
G
∂
Z
T
∂
P
G
is obtained by measuring input impedance values Z M while said output power of said first RF generator is varied, thereby obtaining values for
∂
Z
M
∂
P
G
and transforming said values for
∂
Z
M
∂
P
G
to values for
∂
Z
T
∂
P
G
.
9. The method of claim 4 wherein said transforming said values for
∂
Z
M
∂
P
G
to values for
∂
Z
T
∂
P
G
is performed using an equation
ⅆ
Z
T
ⅆ
Z
M
=
(
1
+
tan
2
θ
)
(
1
+
j
tan
θ
)
2
,
where said θ≡2πL/λ, said L represents the length of the transmission line, and said λ is the wavelength of the RF signal in said transmission line.
10. The method of claim 7 wherein said second set of values of
P
G
∂
Z
T
∂
P
G
is considered to substantially encompass said first set of values of
P
G
∂
Z
T
∂
P
G
when a first region on said plot that contain plotted values of said second set of values of
P
G
∂
Z
T
∂
P
G
substantially encompasses a second region on said plot that contain plotted values of said first set of values of
P
G
∂
Z
T
∂
P
G
.
11. A method of obtaining stable plasma when using a given RF generator to process a substrate in a plasma processing chamber of a plasma processing system, the method comprising:
generating a plurality of data points using a first RF generator different from said given RF generator, said plurality of data points representing a first set of values of
P
G
∂
Z
T
∂
P
G
for a set of stable plasmas of interest and a first RF cable length associated with said first RF generator in said plasma processing system, a term P G in said first set of values of
P
G
∂
Z
T
∂
P
G
representing output power of said first RF generator, a term
∂
Z
T
∂
P
G
in said first set of values of
P
G
∂
Z
T
∂
P
G
representing derivative of a load impedance with respect to said output power of said first RF generator;
obtaining a second set of values of
P
G
∂
Z
T
∂
P
G
,
said second set of values of
P
G
∂
Z
T
∂
P
G
representing values of
P
G
∂
Z
T
∂
P
G
in a stable operating region of said given RF generator, a term P G in said second set of values of
P
G
∂
Z
T
∂
P
G
representing output power of said given RF generator, a term
∂
Z
T
∂
P
G
in said second set of values of
P
G
∂
Z
T
∂
P
G
representing derivative of a load impedance with respect to said output power of said given RF generator; and
conducting, employing a RF transmission line of a second RF cable length, RF signals from said given RF generator when said given RF generator is employed with said plasma processing system, said second RF cable length being selected such that said second set of values of
P
G
∂
Z
T
∂
P
G
substantially encompasses said first set of values of
P
G
∂
Z
T
∂
P
G
.
12. The method of claim 11 further comprising:
plotting said first set of values of
P
G
∂
Z
T
∂
P
G
on a plot wherein real parts of said first set of values of
P
G
∂
Z
T
∂
P
G
are plotted on a x-axis of said plot and imaginary parts of said first set of values of
P
G
∂
Z
T
∂
P
G
are plotted on a y-axis of said plot.
13. The method of claim 11 wherein said first set of values of
P
G
∂
Z
T
∂
P
G
represents a first set of measured values.
14. The method of claim 11 wherein said set of measured values is obtained by varying said output power of said first RF generator while tuning elements values associated with a matching network of said plasma processing system is held fixed.
15. The method of claim 11 wherein said plasma processing system represents an inductively coupled plasma processing system.
16. The method of claim 11 wherein said plasma processing system represents an capacitively coupled plasma processing system.
17. The method of claim 12 further comprising:
plotting said second set of values of
P
G
∂
Z
T
∂
P
G
on said plot wherein real parts of said second set of values of
P
G
∂
Z
T
∂
P
G
are plotted on said x-axis of said plot and imaginary parts of said second set of values of
P
G
∂
Z
T
∂
P
G
are plotted on said y-axis of said plot.
18. The method of claim 14 wherein said first set of values of
P
G
∂
Z
T
∂
P
G
is obtained by measuring input impedance values Z M while said output power of said first RF generator is varied, thereby obtaining values for
∂
Z
M
∂
P
G
and transforming said values for
∂
Z
M
∂
P
G
to values for
∂
Z
T
∂
P
G
19. The method of claim 14 wherein said transforming said values for
∂
Z
M
∂
P
G
to values for
∂
Z
T
∂
P
G
is performed using an equation
ⅆ
Z
T
ⅆ
Z
M
=
(
1
+
tan
2
θ
)
(
1
+
j
tan
θ
)
2
,
where said θ≡2πL/λ, said L represents the length of the transmission line, and said λ is the wavelength of the RF signal in said transmission line.
20. The method of claim 17 wherein said second set of values of
P
G
∂
Z
T
∂
P
G
is considered to substantially encompass said first set of values of
P
G
∂
Z
T
∂
P
G
when a first region on said plot that contain plotted values of said second set of values of
P
G
∂
Z
T
∂
P
G
substantially encompasses a second region on said plot that contain plotted values of said first set of values of
P
G
∂
Z
T
∂
P
G
.