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Description  |
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CROSS-REFERENCE TO RELATED APPLICATION
The invention disclosed in this application is related to that of U.S.
patent application Ser. No. 07/301,933 filed Jan. 25, 1989, by D. K.
Coultas and J. H. Keller and assigned to the assignee of this application.
The subject matter of that application is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to plasma processing of
semiconductor wafers in the manufacture of integrated circuits (ICs) and,
more particularly, to a plasma processing device using an optimized
helical resonator to achieve efficient processing.
2. Description of the Prior Art
Plasma processing is an indispensable technology in the manufacture of very
large scale integrated (VLSI) circuits. Plasma-assisted etching techniques
have replaced chemical etching, and sputtering has replaced evaporation
for the deposition of metals. Plasma enhanced chemical vapor deposition
(PECVD) is an alternative to conventional and low pressure CVD techniques.
In a reactive ion etcher (RIE), for example, silicon wafers are loaded on
a radio frequency (RF) powered electrode and the chamber is grounded. In
typical sputtering and PECVD devices, the wafers are loaded onto a
revolving turntable in the chamber.
A number of schemes have been used to generate the plasma used for
processing the wafers, including magnetrons and capacitive devices. More
recently, helical resonators (i.e., inductors) have been used to generate
the plasma. An example is disclosed, for example, in U.S. Pat. No.
4,368,092 to Steinberg et al. This device uses a downstream, high Q
helical resonator to produce the plasma that is introduced to the reaction
chamber. A disadvantage of the Steinberg et al. device is that it is
radially asymmetric resulting in nonuniformities of the etching of the
workpiece. U.S. Pat. No. 4,918,031 to Flamm et al. discloses another
example of a high Q, helical resonator wherein, in one embodiment, the
substrates are held within the resonator, thus avoiding the
nonuniformities produced by the Steinberg et al. device. However, because
of the high Q, the source efficiency is low, and wall sputtering of the
source is increased, leading to damage to the chamber and particulate
generation.
The ratio of the reactance (inductive or capacitive) of a circuit to the
series resistance of the circuit is called the Q (for quality factor) of
the circuit. Helical resonators for producing plasmas are typically high Q
devices with an unloaded Q (i.e., plasma attenuated) of about 1000. These
devices are capable of producing large voltages and can produce plasmas at
low pressures. However, these same properties can cause them to operate in
a capacitively coupled mode instead of an inductive mode. Capacitive
coupling is much less efficient than inductive coupling for generating
high plasma densities at low pressures (e.g., 1-20 mTorr).
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optimized
helical resonator to increase the source efficiency and plasma density for
more efficient processing of semiconductor wafers.
According to a first embodiment of the invention, an optimized
configuration is used for generating a down stream plasma. This embodiment
of the invention uses a lumped capacitance to reduce the value of .omega.L
to less than about 200 Ohms. The value of the lumped capacitance is
determined by taking into account the stray capacitance between the coil
and the grounded enclosure. A second embodiment of the invention is an
optimized configuration for single wafer high plasma density reactive ion
etching (RIE). This embodiment uses a flat spiral coil having a low value
of .omega.L and resonant as a 1/4 or 1/2 wave resonator. On the 1/4 wave
resonator, one side of the coil is grounded and the other is at a high
voltage. On the 1/2 wave resonator, both sides of the coil are grounded
and the center of the coil is at a high voltage. The third embodiment of
the invention combines features of the first two embodiments using both
spiral and solenoid coils as the helical resonator.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects and advantages will be better
understood from the following detailed description of a preferred
embodiment of the invention with reference to the drawings, in which:
FIG. 1 is a cross-sectional view of a conventional helical resonator
characterized by a high Q;
FIG. 2 is a cross-sectional view of a first embodiment of the optimized
helical resonator according to the invention;
FIG. 3 is a cross-sectional view of a second embodiment of the optimized
helical resonator according to the invention; and
FIG. 4 is a cross-sectional view of a third embodiment of the optimized
helical resonator according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 1, there is
shown a conventional high Q helical resonator 10 comprising an inductive
coil 11 wound about a gas discharge tube 12. At the upper end of the gas
discharge tube 12, there is a gas inlet 13 for admitting a plasma forming
gas. At the lower or discharge end of the tube 12 is an electrode 14 for
supporting a semiconductor wafer to be processed. The ends of the coil 11
are connected to a ground shield 15, shown schematically in the FIG. 1. A
tap 16 on coil 11 is connected to a first RF source 17. The tap location
is adjusted to provide optimum impedance matching between the RF source 17
and the helical resonator. The wafer supporting electrode 14 can be biased
by a second RF source 18, as is a common practice in high density plasma
processing. The RF source 17 dominantly controls the plasma density, while
the RF source 18 mainly controls the ion energy.
The typical Q for the conventional helical resonator shown in FIG. 1 is on
the order of 1000. The unloaded coil resistances are typically about 0.1
Ohm. When the plasma is on, the resistance increases to about one Ohm,
dropping the Q by a factor of ten. The problem with the high Q helical
resonator of the prior art shown in FIG. 1 is that the large inductance
associated with standard helical 1/4 or 1/2 wave resonators produces a
large voltage across the coil 11. This, in turn, produces a capacitive
coupling of the power to the plasma inside the reactor chamber 12. As a
result, the source will operate at reduced efficiency due to the drop in
current in the coil 11 limiting power inductively coupled to the plasma.
The inventors have shown that for dominantly inductive coupling, the power
in the discharge is proportional to the square of the coil current minus a
constant A, i.e., P.sub.ind .varies.I.sup.2 -A, where A is defined by the
following expression:
##EQU1##
where L.sub.c is the coil inductance, L.sub.plasma is the plasma
inductance, k is the coupling coefficient between the plasma and the coil,
and .omega. is 2 .pi.f, the applied frequency. For dominantly capacitive
coupling, the power into the plasma is proportional to the square of the
voltage V on the electrode or coil, i.e., P.sub.cap .varies.V.sup.2, where
V.sup.2 =.omega..sup.2 L.sub.c.sup.2 I.sup.2. If I.sup.2 .perspectiveto.A,
then P.sub.cap becomes
##EQU2##
With capacitive coupling, most of the power into the discharge goes into
ion bombardment of the electrodes. Thus, to get high density plasmas, the
capacitive coupling and L.sub.c could be reduced. This implies lower Q
operation of a helical resonator for achieving high density plasmas. High
density plasmas are needed in single wafer etching in order to achieve
reasonable process times. In addition, the reduced inductance L.sub.c can
be used in conjunction with multi-pole magnets to confine the plasma and
further increase its density. In addition, magnets may be inside or
outside the coil to confine the plasma. The basic structure of a single
wafer plasma processing tool is disclosed in the above-referenced
application Ser. No. 07/301,933.
Referring now to FIG. 2, there is shown a first embodiment of the present
invention. In FIG. 2, there is shown a low Q helical resonator 20
comprising an inductive coil 21 wound about a gas discharge tube 22. At
the upper end of the gas discharge tube 22, there is a gas inlet 23 for
admitting a plasma forming gas, and at the lower or discharge end of the
tube 22 in an electrode 24 for supporting a semiconductor wafer to be
processed. One end of the coil 21 is connected to ground shield 25, shown
schematically in FIG. 2. The ground shield 25, and hence one end of the
coil 21, is connected to ground. A tap 26 on coil 21 is connected to a
first RF source 27, while the electrode 24 on which a semiconductor wafer
is loaded is connected to a second RF source 28. If desired, RF sources 27
and 28 may include matching networks, as is common for RF sources. In
addition, a variable capacitor 29 is connected between the ungrounded end
(i.e., the "hot" end) of coil 21 and the ground shield 25.
In this optimized helical resonator, the number of turns of the coil 21 is
significantly less than the number of turns typically used in the high Q
coil 11 shown in FIG. 1. As a result, the inductance is less resulting in
a Q preferably in the range of 150 to 200. As in the conventional helical
resonator, when the plasma is on, the resistive load increases to about
one Ohm. However, in the case of the optimized helical resonator, the Q
drops to about 15 to 20 in contrast to 100 for the conventional helical
resonator. The capacitor 29 is adjustable to permit tuning the system to
be resonant at the operating frequency.
A second embodiment of the invention is shown in FIG. 3. In this
embodiment, the coil 31 is in the form of a flat spiral having its
interior end connected to the ground shield 35. A tap 36 of the coil 31 is
connected to a first RF source 37, and a second RF source 38 is connected
to the electrode 34. A capacitor 39 is connected between the ungrounded
end of coil 31 and ground. In this embodiment, magnetic confinement of the
plasma is accomplished by means of multi-dipole magnets 32 arranged about
the periphery of the reaction chamber, in the classic magnetic confinement
cylindrical arrangement. That is, magnetic fields of the magnets 32 are
directed inwardly and alternate in polarity.
The third embodiment of the invention, shown in FIG. 4, combines features
of the first two embodiments. More particularly, the coil 41 is formed of
a flat spiral part 41A and a solenoid part 41B. As in the second
embodiment shown in FIG. 3, the flat spiral part 41A has its interior end
connected to a ground shield 45. A tap 46 of the coil 41 is connected to a
first RF source 47, and a second RF source 48 is connected to the
electrode 44. The tap 46 is shown on the flat spiral part 41A of the coil,
but it will be understood that the tap could be on the solenoid part 41B
depending on the specific design, the principle criteria being to provide
a good impedance match with the RF source 47. A capacitor 49 is connected
between the ungrounded end of coil 41 and ground. Magnets 42 are used for
magnetic field confinement of the plasma, as in the embodiment of FIG. 3.
While the invention has been described in terms of a single preferred
embodiment, those skilled in the art will recognize that the invention can
be practiced with modification within the spirit and scope of the appended
claims.
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