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Description  |
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BACKGROUND OF THE INVENTION
Plasma etching systems have included reactor chambers including a pair of
electrodes to which R.F. power is applied. A wafer including a film
thereon to be etched is generally placed on one of the electrodes.
Suitable gases are injected into the chambers and plasma is formed to
provide the etching of the film. High or low radio frequencies are used in
the etching process dependent upon the type of film being etched.
One of the problems found in R.F. plasma etching involves excessive stray
electrical discharges from the electrodes to the reactor chamber and other
parts in the system. When the wafer is on a grounded chuck electrode and
the voltage is applied to an upper counter electrode, the voltage between
the upper electrode and the wafer is always less than the voltage between
the upper electrode and the walls of the system which are generally
grounded. This is because the wafer itself is never exactly at chuck
potential, but is electrically isolated from the chuck electrode by an
insulating coating on the back of the wafer. The result is that some of
the current goes from the upper electrode to various grounded surfaces in
the system instead of to the wafer where it will do some good in the
etching process.
The higher the power the worse the problem of stray discharges becomes, and
the more R.F. current is diverted to grounded surfaces. The stray
discharges become a problem because a certain amount of power still has to
reach the wafer. Stray discharges tend to be erratic, and are unstable.
This means that it is difficult to always predict how much power will go
to the wafer and how much will go into stray discharges.
OBJECTS OF THE INVENTION
It is an object of this invention to provide an improved plasma etching
system in which stray electrical discharges between an R.F. powered
electrode and other parts of the system is minimized.
It is a further object of this invention to provide an improved plasma
etching system in which the R.F. power requirements are minimized.
It is still a further object of this invention to provide an improved R.F.
plasma etching system with improved process consistency and reliability,
and which provides improved spatial etch uniformity.
It is still a further object of this invention to provide an improved R.F.
plasma etching system in which stray electrical discharges are minimized
to minimize likelihood of damage to the reactor and which permits use of
higher R.F. power densities than would normally otherwise be possible.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a grounded plasma etching chamber
includes insulated counter and chuck electrodes, with a wafer having a
film to be etched being disposed on the chuck electrode. A radio frequency
voltage is applied across the electrodes. Means are provided to split the
voltage so that substantially equal voltages are applied to the two
electrodes.
Other objects and advantages of the present invention will be apparent and
suggest themselves to those skilled in the art, from a reading of the
following specification and claims, taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are schematic diagrams, partly in block diagram form,
illustrating two different embodiments of the present invention; and
FIGS. 3 and 4 are waveforms shown for purposes of explanation of the
embodiments illustrated in FIGS. 1 and 2, respectively.
DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a low frequency plasma etching system comprises a
plasma etching chamber 10. The walls of the chamber are grounded, or
returned to a point of some reference potential. Gases are injected into
the chamber and suitable cooling is provided in conventional manners.
Because many of the details relating to such things as vacuum pumping
lines and other elements in the plasma etching system involved are
conventional and only indirectly related to the invention, they are not
illustrated or described in detail for purposes of clarity.
A counter electrode 12 and a chuck electrode 14 within the chamber 10 are
isolated from the walls of the chamber. A wafer 16 is disposed on the
chuck electrode 14 and capacitively coupled thereto through an insulating
film which may normally be on the wafer or on the chuck electrode. The
capacitive coupling maintains the R.F. potential of the wafer 16 close to
that of the chuck electrode 14.
A source of low frequency R.F. signals 18 is connected to a matching
transformer 20 including a primary winding 22 and a secondary winding 24,
which is connected across the electrodes 12 and 14. A tuning inductor 26
is connected across the secondary winding 24 to improve the matching of
the transformer 20 and to tune out the capacitance of the chamber 10 and
also the discharge capacitance.
It is noted that both electrodes 12 and 14 are ungrounded and driven with
oppositely phased R.F. signals from the source 18. Because of this, the
R.F. potential at each electrode is about half that of a driven electrode
in a conventional reactor used heretofore.
The entire secondary circuit of the transformer 20 is permitted to float
both as to the R.F. potential and any D.C. potential in such a way as to
equalize capacitive and stray discharge currents to ground from the two
electrodes 12 and 14. As mentioned, the total R.F. voltage is split evenly
between two electrodes. It was found that etch performance, for SiO.sub.2
etching at 400 kHz was basically unaffected compared to a single ended
system provided, of course, that the same R.F. voltage was applied across
the electrodes. However, it was found that the R.F. power requirements
were less and that other advantages were attained through use of the
present invention illustrated in FIG. 1.
The reduction of stray discharges at 400 kHz, almost amounting to
elimination of such discharges, results from the non-linearity of stray
discharge power with respect to the electrodes to ground R.F. potential.
It was found that the stray power increases at least as the square of the
voltage with a relatively sharp cut-off at an electrode voltage V.sub.c.
Therefore, cutting the peak electrode to ground voltage in half helps
considerably.
Referring to FIG. 4, curve 28 illustrates the dependence of stray discharge
R.F. current upon electrode to ground R.F. potential. The R.F. voltage
indicated at 29 corresponds to a typical single ended etching condition,
with considerable stray discharge current to grounded surfaces. The
voltage indicated at 30 is the voltage present on each electrode when the
electrodes are split as in the present invention. The etch rate is the
same, but the stray discharge currents are nearly zero. Below a certain
power density on the wafer, splitting the R.F. drive, virtually eliminates
stray discharges.
Referring to FIG. 2, a second embodiment of the present invention includes
a high R.F. frequency plasma etching system where the technique of
achieving quasisymmetric excitation of the electrodes applies only when
the discharge load impedance is highly capacitive.
A grounded reactor chamber 32 includes a counter electrode 34 and a chuck
electrode 36. A wafer 38 having a film thereon to be etched is disposed on
the chuck electrode 36.
A source of high R.F. power 40, which may generate signals in the frequency
range of about 13 MHz, is connected through a single ended matching
network 42 across the electrodes 34, 36 and an inductor 44. The inductor
44 is connected between the electrode 36 and ground. In this embodiment,
the source of power 40 is also returned to ground, or other point of
reference potential. In the embodiment illustrated, with a frequency of 13
MHz with discharges at O.3 to 5 torr, the load impedance is highly
capacitive.
The electrode 34 is driven in a conventional way using an adjustable
matching network of single ended design. The electrode 36 is connected to
ground through the adjustable inductor 44.
The value of the inductor 44 is chosen so as to tune with the plasma series
capacitance at the R.F. drive frequency. This capacitance comprises
primarily the capacitance between the electrodes 34 and 36 and that of the
plasma sheath. The inductor 44 is adjusted to substantially equalize the
R.F. potentials on the electrodes 34 and 36 at voltages somewhat higher
than half the usual single-sided excitation voltage. The operation of FIG.
2 is illustrated in the vector diagram in FIG. 3.
The R.F. series current i through the reactor 44 is used as the phase
reference. The voltage across the plasma is V.sub.uc =V.sub.u -V.sub.c,
where V.sub.u =voltage on counter electrode 34 and V.sub.c =voltage on the
chuck electrode 36. V.sub.c lags almost 90.degree. behind i, due to the
highly capacitive nature of the plasma discharge. The chuck voltage
V.sub.c, however, leads by 90.degree. in fact,
V.sub.c =j.omega.Li
Referring to FIG. 4, it is clear that .vertline.V.sub.u
.vertline.<.vertline.V.sub.uc .vertline., and that L (the inductor 44) can
be adjusted to give .vertline.V.sub.c
.vertline..congruent..vertline.V.sub.u
.vertline..congruent.0.5.vertline.V.sub.uc .vertline..
When L, i.e., the inductor 44, is so adjusted, the effective load impedance
seen by the match network 42 is V.sub.u /i, which is both smaller in
magnitude and less capacitive than is the case with the chuck grounded
(L=0). Thus, the impedance transformation demands on the match network are
reduced and in some cases the network may be eliminated. Also, circulating
currents and peak voltages are reduced. Stray discharges, though not
eliminated, are significantly suppressed, particularly when operating at
high plasma power densities (>2 W/cm.sup.2).
Basically, the present invention has provided circuitry for minimizing
stray electrical discharges in plasma etching systems for both low and
high frequency R.F. systems. This was accomplished by lowering the voltage
requirements at the electrodes to approximately one-half their normal
levels with the total voltage between the two electrodes being
substantially the same as single ended etching systems so that the overall
etching operation with respect to time and quality is not affected.
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