Electric heating – Metal heating – By arc
Reexamination Certificate
1999-03-18
2001-01-16
Pelham, Joseph (Department: 3742)
Electric heating
Metal heating
By arc
C315S039000
Reexamination Certificate
active
06175095
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and more particularly to plasma sources that can be operated at high powers for increased processing rates.
2. Description of the Prior Art
Microwave energy is conventionally coupled to gas flows to generate plasmas useful in semiconductor wafer processing. Prior art equipment couples as much as 1200 watts of microwave energy centered at 2.45 GHz from a ringed “applicator” through and into a quartz plasma tube that passes through the rings.
Carl F. Weissfloch, et al., describes one such applicator in U.S. Pat. No. 3,814,983, issued Jun. 4, 1974, and states that the use of microwave plasmas has been severely limited by the small size of plasma volumes achievable with conventional microwave applicators, namely antennas, waveguides, and cavity resonators. So a slow wave structure illustrated in FIGS.
6
-
8
of such patent is offered. A thirty-six inch long semi-radiant slow wave applicator is described that operates in the degenerate half wavelength (&lgr;/2) mode. Slow wave structures such as microwave applicators are supposedly dividable into two types, resonant slow wave structures and traveling slow wave structures. A rectangular waveguide
2
transitions to the slow waveguide structure with a doubly tapered inner conductor
24
that connects to a tapered parallel plane transmission line
25
. A set of twelve parallel bars
26
,
31
are arranged in a fence-line in FIG.
6
and in a circle in
FIGS. 7 and 8
. Each bar
26
,
31
terminates in a top shorting plane
29
,
34
and at the opposite ends in a bottom shorting plane
29
,
34
. Some prior art equipment uses fourteen such bars in a circle. A central hole in each of the ring shorting planes
34
allows for the insertion of a plasma tube in which the microwave energy is delivered. In between the planes
29
,
34
, the odd numbered bars
26
,
31
connect together with a strap
27
,
32
and the even numbered bars
26
,
31
connect together with a strap
28
,
33
. The transmission line
25
is connected to each of the straps
27
,
32
and
28
,
33
and enters perpendicular to the bars
26
,
31
.
In practice, such prior art structures have not worked very well. The power delivery is usually very uneven and concentrates near the first three sets of bars
31
in circular structures. The power delivered by the microwave system is usually limited to 1200 watts because spot heating of the quartz, sapphire or ceramic plasma tubes is too severe to be handled adequately by a cooling system. The terminating impedance presented to the waveguide and microwave source has not been good, and much of the power input is reflected back. The relatively close physical spacing of adjacent bars
26
and
31
on opposite phases carried by the straps
27
,
28
,
31
and
32
tend to arc together and pitting results.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a microwave energy plasma source that can increase semiconductor process production rates.
It is another object of the present invention to provide a higher power microwave energy plasma source than is conventionally possible.
It is a further object of the present invention to provide a microwave energy plasma source that produces a more uniform plasma.
It is another object of the present invention to provide a microwave energy plasma source that provides an efficient terminating impedance.
Briefly, a microwave applicator embodiment of the present invention comprises six, and only six, equal length parallel rods equally distributed in 60° angular intervals in a circle, and at circumferential intervals that are one half the wavelength of operation of a microwave power source. The circumference is therefore preferred to be three wavelengths. The top ends of every rod terminate in a top planar shorting ring. The bottom ends of every rod terminate in a bottom planar shorting ring which is perpendicular to each of the rods and which is parallel to the top planar shorting ring. Intermediate to the two planar shorting rings are upper and lower planar feed rings that are each parallel to the outside planar shorting rings. The upper planar feed ring connects to odd numbered rods and the lower planar feed ring connects to even numbered rods. A ridged waveguide is connected to feed the feed rings at a point centered where any one of the rods is located. A quartz, ceramic, or sapphire plasma tube is passed through the central axis of the planar shorting rings and feed rings. The applicator is tuned by either internal or external means to match its resonant frequency to that of the microwave source such that impedance mismatch reflections are minimized and the energy transferred to the plasma generated within the plasma tube is maximized.
An advantage of the present invention is a microwave energy plasma source is provided that can increase semiconductor wafer processing production rates.
Another advantage of the present invention is that a microwave energy plasma source is provided that can be operated at two to three times the plasma power levels of conventional equipment.
A further advantage of the present invention is that a microwave energy plasma source is provided that produces a more uniform plasma density within the plasma tube.
Another advantage of the present invention is that a microwave energy plasma source is provided that provides a better terminating impedance to connecting waveguides and microwave sources.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the drawing figures.
REFERENCES:
patent: 3646388 (1972-02-01), Dudley et al.
patent: 3814983 (1974-06-01), Weissfloch
patent: 5284544 (1994-02-01), Mizutani et al.
patent: 5536921 (1996-07-01), Hedrick et al.
Caughran James W.
MacDonald Niles K.
White Terry L.
Gasonics International
Pelham Joseph
Townsend and Townsend / and Crew LLP
Van Quang
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