 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The invention relates to apparatuses for processing of substrates using
radiofrequency induced plasma in a plasma chamber. In particular, the
invention provides apparatuses and methods for generating a plasma of a
uniform plasma density.
Gaseous plasma technology is a well known technique used for the
fabrication of integrated circuits. Parallel plate reactors have been used
extensively for exciting the gases in the reaction chamber to generate the
chemical reactions required for thin film etching and deposition of
wafers. In general, when coupling power through an insulator, previous
hardware setups have used 13.56 MHz as the exciting frequency for the
gases due to a higher excitation efficiency. For instance, see U.S. Pat.
No. 4,948,458 ("Ogle"), the disclosure of which is hereby incorporated by
reference.
In apparatuses such as that shown by Ogle, a radiofrequency magnetic field
is induced in a low pressure reaction chamber by sending a radiofrequency
resonant current through an external planar coil and passing the generated
radiofrequency energy through a dielectric window in the chamber. The
magnetic field generates a plasma by causing a circulating flux of
electrons in a process gas introduced into the chamber to produce a region
of ionic and radical species. The plasma so generated is used to etch or
deposit materials on a wafer in the chamber.
It has been found that the plasma density across the surface area of the
wafer is highly variable in such apparatuses, with densities measured
across 150 mm and 200 mm wafer areas being as much as two times as great
in some areas than in others. This non-uniform plasma density causes
significantly non-uniform oxide and resist etch rates over measured wafer
areas and makes it extremely difficult to control critical dimensions of
fine line geometry on the wafer.
Normally, a flat dielectric window is used with the apparatuses. It has
been observed that the magnetic flux of the planar coil is highest near
the window center and, with a flat window, the induced electric field is
consequently higher near the window center. The apparatuses and methods of
the present invention utilize a dielectric window having a characteristic
cross section, wherein the window is thicker at the center and thinner at
the edges, to decrease the higher induced electric field near the window
center.
SUMMARY OF THE INVENTION
An apparatus according to one aspect of the present invention includes a
housing having a chamber in which a semiconductor wafer can be treated
with plasma, the housing including at least one inlet port connected to an
interior of the chamber through which process gas can be supplied to the
chamber. The apparatus further includes a radiofrequency energy source
that is arranged so as to pass radiofrequency energy into the chamber and
induce plasma in the interior of the chamber by activating, with an
electric field induced by the radiofrequency energy source, process gas
supplied to the chamber through the inlet port. A dielectric window having
an inner surface thereof forming part of an inner wall of the chamber is
arranged such that radiofrequency energy from the radiofrequency energy
source can be passed to the interior of the chamber through the dielectric
window. The dielectric window has a thickness which varies at different
points along the inner surface thereof such that the thickness is largest
at a central portion of the dielectric window, the dielectric window being
effective to decrease the induced electric field in the interior of the
chamber near the central portion of the dielectric window.
The radiofrequency energy source can comprise a substantially planar plasma
generating electrode having one planar face thereof facing an outer planar
surface of the dielectric window. The dielectric window can be circular in
shape. The dielectric window can comprise a plurality of layers of the
same or different dielectric materials. The dielectric window can also
include at least one step therein such that the dielectric window has a
region of reduced thickness surrounding the central portion. The
dielectric window can include at least one tapered surface surrounding the
central portion or the dielectric window can be convex in shape.
The invention also provides a method for treating an article with plasma
comprising steps of placing an article within a chamber and introducing
process gas into the chamber and generating a uniform electric field in
the chamber by passing radiofrequency energy through a dielectric window
in the chamber. The dielectric window has a thickness which varies at
different points along an inner surface thereof such that the thickness is
largest at a central portion of the dielectric window. As a result, the
uniform electric field generates a uniform electron flow in the process
gas and thereby generates a plasma of uniform plasma density. The process
further includes the step of plasma treating an article by exposing a
surface of the article to the plasma generated in the chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will be well
understood by reading the following detailed description in conjunction
with the drawings in which like numerals indicate similar elements and in
which:
FIG. 1 is an isometric view of an apparatus for producing a planar plasma
in accordance with the present invention;
FIG. 2 is a cross-sectional view of the apparatus of FIG. 1;
FIG. 3 is a schematic view of the circuitry of the apparatus in FIG. 1;
FIG. 4 is a schematic view illustrating the magnetic field profile produced
by the apparatus of FIG. 1;
FIG. 5 is a graphic representation of ion current density versus distance
from a center of a wafer in an apparatus having a dielectric window with a
flat cross-section;
FIG. 6 is a graphic representation of ion current density versus distance
from a center of a wafer in an apparatus according to the present
invention;
FIG. 7 is a side view of an embodiment of a dielectric window according to
the present invention;
FIG. 8 is a side view of another embodiment of a dielectric window
according to the present invention; and
FIG. 9 is a side view of a further embodiment of a dielectric window
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, apparatus and methods are provided for
producing highly uniform, planar plasmas over relatively large areas. The
ionic and radical species produced in the plasma experience minimum
acceleration in non-planar directions, and the resulting plasma thus has
very low desired kinetic energy. In addition, uniform planar plasma can be
produced over very wide pressure ranges, typically from 10.sup.-5 Torr to
5 Torr, and greater.
The apparatus of the present invention comprises a housing having an
interior chamber bounded at least in part by a dielectric window. A planar
coil is disposed proximate the window, and a radiofrequency source is
coupled to the coil. Usually, the radiofrequency source is coupled through
an impedance matching circuit to maximize power transfer and a tuning
circuit to provide for resonance at the operating frequency, typically
13.56 MHz. Inlet ports are provided for supplying a process gas to the
chamber. By resonating a radiofrequency current through the coil, an
electromagnetic field is induced which extends into the chamber through
the dielectric window. In this way, a flow of electrons may be induced.
Moreover, as the electrons are closely confined to a planar parallel to
the planar coil, transfer of kinetic energy in non-planar directions is
minimized.
The chamber includes a support surface for a planar article, typically a
semiconductor wafer. The surface supports the wafer in a plane which is
parallel to the plane of the coil, and hence, also parallel to the plane
of the plasma. Thus, the semiconductor wafer is exposed to a uniform
plasma flux, ensuring uniform plasma treatment. As the plasma species have
minimum kinetic velocities in non-planar directions, their kinetic impact
on the wafer is minimized. Thus, the treatment can be generally limited to
the chemical interaction of the plasma species with the wafer.
A velocity component in the direction normal to the surface of the
semiconductor wafer may be provided by applying a radiofrequency potential
in a direction normal to the plane of the plasma. Conveniently, such a
potential may be applied by the support surface upon which the wafer is
maintained. For instance, the support surface can include a conventional
bottom electrode for supplying such a potential.
The method and apparatus of the present invention are useful in a variety
of semiconductor processing operations, including plasma etching such as
etching of an aluminum layer on a semiconductor substrate, deposition
processes, resist stripping, plasma enhanced chemical vapor depositions,
and the like.
The housing defines a generally air-tight interior chamber wherein the
planar plasma is to be generated. The housing includes at least one inlet
port for introducing a process gas and at least one outlet port for
connection to a vacuum system for maintaining a desired operation pressure
within the chamber. Systems for supplying a preselected process gas and
for maintaining a preselected pressure within the chamber are well known
in the art and need not be described further. One or more surfaces within
the chamber support the articles to be treated. Typically, the surfaces
will be disposed in a preselected orientation relative to the planar
plasma which is to be generated within the chamber, usually being
generally parallel to the plane of the plasma.
In order to induce the desired planar plasma, an electrically-conductive
coil is disposed adjacent to the exterior of the dielectric window. The
coil is substantially planar and generally comprises a single conductive
element formed into a planar spiral or a series of concentric rings. By
inducing a radiofrequency current within the coil, an electromagnetic
field is produced which will induce a flow of electrons within a planar
region parallel to the plane of the coil.
The planar coil is generally circular, although ellipsoidal patterns and
other deviations from true circularity can be tolerated. Moreover, the
coil may be truly planar across its diameter, or may deviate somewhat from
true planarity. Deviations from planarity can be less than 0.2 of the
diameter of the coil, usually being less than 0.1 of the diameter.
Adjustments to the profile of the coil may be made to modify the shape of
the field which is generated. The diameter of the coil will generally
correspond to the size of the plasma which is to be generated. Coil
diameters may range from about 8 cm to 30 cm, usually from about 13 cm to
18 cm. For the treatment of individual semiconductor wafers, the coil
diameter will generally be from about 13 to 18 cm.
The coil includes a sufficient number of turns in order to produce a
relatively uniform magnetic field across its entire diameter. The number
of turns will also depend on the diameter of the coil. A coil sized for
treating individual semiconductor wafers usually has from abut 5 to 8
turns. The resulting inductance of the coil will usually be from 1.2 to
3.5 .mu.H, with an impedance in the range from about 20 to 300 Ohms.
Conveniently, the planar coil may be formed from any electrically
conductive metals, usually being formed from copper. The coil can have a
load carrying capacity in the range from abut 5 to 100 amps.
The planar coil is disposed next to a dielectric window forming part of the
treatment chamber. The dielectric window maintains the isolation of the
interior of the chamber, while allowing penetration of the magnetic field
produced by the planar coil. The remainder of the housing can be metal.
The dielectric window can be composed of quartz, although other dielectric
materials, particularly ceramics which do not absorb energy at the
frequency of operation, may be used. Conveniently, a dielectric window may
be placed adjacent to a port formed in a wall of the housing. The geometry
of the port usually corresponds to that of the planar coil, typically
being circular. The planar coil can be disposed very close to or touching
the dielectric window in order to maximize the intensity of the magnetic
field produced within the chamber. The thickness of the dielectric window
is thin enough to transmit the energy to the plasma, usually being
selected to be sufficient to withstand the differential pressure created
by the vacuum within the chamber. For example, the window can be at least
one-half inch thick or thicker.
The planar coil is driven by a radiofrequency (RF) generator of a type
which is generally utilized in the operation of semiconductor processing
equipment. The RF generator will usually operate at a frequency in the
range from about 13.56 MHz to 100 MHz, typically being operated at 13.56
MHz. The RF generator usually has a low impedance, typically about 50
Ohms, and will be capable of producing from about 1 to 6 amps, usually
from about 2 to 3.5 amps, with an RMS voltage of at least about 50 volts,
usually being at least about 70 volts, or more. Conveniently, the RF
generator can have an output connector in the form of a coaxial cable
which may be connected directly to the circuitry operating the planar
coil.
Referring to FIGS. 1 and 2, a plasma treatment system 10 suitable for
etching individual semiconductor wafers W includes a chamber 12 having an
access port 14 formed in an upper wall 16. A dielectric shield/window 18
is disposed below the upper wall 16 and extends across the access port 14.
The dielectric window 18 is sealed to the wall 16 to define a vacuum-tight
interior chamber 19 of the chamber 12.
A planar coil 20 is disposed within the access port 14 adjacent to
dielectric window 18. Coil 20 is formed as a spiral having a center tap 22
and an outer tap 24. The plane of the coil 20 is oriented parallel to both
the dielectric window 18 and a support surface 13 upon which the wafer W
is mounted. In this way, the coil 20 is able to produce a planar plasma
within the chamber 19 of the chamber 12 which is parallel to the wafer W.
The distance between the coil 20 and the support surface 13 can be in the
range from about 3 to 15 cm, more usually from about 5 to 20 cm with the
exact distance depending on the particular application.
Referring now to FIGS. 1-3, the planar coil 20 is driven by an RF generator
30 of the type described above. The output of the generator 30 is fed by a
coaxial cable 32 to a matching circuit 34. The matching circuit 34
includes a primary coil 36 and a secondary loop 38 which may be mutually
positioned to adjust the effective coupling of the circuit and allow for
loading of the circuit at the frequency of operation. Conveniently, the
primary coil 36 is mounted on a disk 40 which may be rotated about a
vertical axis 42 in order to adjust the coupling.
A variable capacitor 44 is also provided in series with the secondary loop
38 in order to adjust the circuit resonant frequency with the frequency
output of the RF generator 30. Impedance matching maximizes the efficiency
of power transfer to the planar coil 20. An additional variable capacitor
46 can be provided in the primary circuit in order to cancel part of the
inductive reactance of coil 36 in the circuit. However, other circuit
designs may also be provided for resonantly tuning the operation of planar
coil 20 and for matching the impedance of the coil circuit with the RF
generator.
FIG. 4 shows a desired magnetic field profile 60 in a conventional
apparatus using a flat dielectric window 18 and a planar coil 20. At the
edges of the coil 20, the magnetic field strength is less than at the
center. The induced magnetic field causes a circulating flux of electrons
in the plasma created by collision of the electrons with the individual
molecules of the process gas, and in turn produces a region of ionic and
radical species. FIG. 5, however, shows that, with such an apparatus
having a window of uniform thickness, ion current density drops off
sharply as ion current density is measured farther and farther from the
center position 0 of the window 18. The dotted line in FIG. 5 corresponds
to the outer edge of a 6 inch wafer.
If the wafer W is small enough, then the reduced ion current density at
extreme edges should not adversely affect oxide and resist etch rates on
the wafer W. However, it is common to use wafers W having sufficiently
large diameters that the reduced ion current density at distances from the
center of the coil 20 and window 18 does adversely affect oxide and resist
etch rates on the wafer W.
Substantially uniform ion current density, usually within .+-.5% over the
entire diameter of 150 mm and 200 mm diameter wafers, as shown by the
graph of test results set out in FIG. 6, is made possible by the present
invention by providing a window 18 having a thickened center portion. In
particular, the ion current density is substantially uniform from center
position 0 to distances of at least 750 mm from the center position 0. The
dotted line in FIG. 6 corresponds to an edge of an 8 inch wafer.
As is shown in FIGS. 7-9, the window 18 according to the invention can have
various cross sections. Several different types of window material may be
used for the dielectric window 18, including ceramic, quartz or glass
materials. The most advantageous window cross section under the particular
intended use conditions will be a function of the dielectric constant of
the particular window material that is chosen and power supplied to the
coil. For instance, in the case where 500 Watts is supplied to the coil,
the ratio (t.sub.c /t.sub.e) of center thickness t.sub.c to edge thickness
t.sub.e is about 3:1. If the power is increased to 1000 Watts, the ratio
t.sub.c /t.sub.,e is preferably about 1.5:1. On the other hand, if the
power is lowered to 200 Watts, the ratio t.sub.c /t.sub.e is preferably
about 6:1.
The window 18 having a thickened center may be formed by machining or
molding a particular dielectric material such as Al.sub.2 O.sub.3,
ZrO.sub.2, SiO.sub.2, etc. to form a particular lens cross section. For
instance, window 18 can be formed by laminating (such as by sintering)
together a series of progressively smaller window portions 181, 182, 183
which form a series of steps, as shown in FIG. 7. In FIG. 7, portions 181
and 183 are one-half inch in thickness and portion 182 is one-quarter inch
in thickness. The progressively smaller window portions 181, 182, 183 may,
of course, also be machined or molded from a single piece of dielectric
material. Alternatively, window 18b can have the convex cross section
shown in FIG. 8 or the truncated cone cross section of the window 18c
shown in FIG. 9. In FIG. 9, window 18c includes a tapered surface
surrounding a thicker central portion of the window. The window 18 can
also be made by laminating together materials having different dielectric
properties, such as ceramic materials laminated together preferably
without adhesive.
According to a preferred embodiment of the invention, dielectric window 18
comprises a flat disc of Al.sub.2 O.sub.3 having a diameter of 9 to 10
inches. Such a window can be held by suitable seal means in a 12 inch
diameter opening in a plasma chamber. The thickened central portion of the
window is preferably formed by a flat disc of Al.sub.2 O.sub.3 having a
diameter of about 5 to 6 inches. The two pieces of Al.sub.2 O.sub.3 can be
laminated together by sintering and the ratio of diameters of the two
discs can be about 2:1. If the coil is supplied with 500 Watts, the
thickness t.sub.e at the outer edge of the window is preferably 1.0 inch
and the thickness t.sub.c of the center of the window is preferably 1.5
inch.
The thickened central portion of the window 18 is ordinarily disposed on
the inside of the chamber 12, with a flat outer surface of the window 18
facing outwardly from the chamber. Nonetheless, different characteristic
cross sections, configurations, materials, and window thicknesses may be
found to be more efficacious for particular applications.
It is, of course, possible to embody the invention in specific forms other
than those described above without departing from the spirit of the
present invention. The embodiments described above are merely illustrative
and should not be considered to be restrictive in any way. The scope of
the invention is given in the appended claims, rather than the preceding
description, and all variations and equivalents which fall within the
range of the claims are intended to be embraced therein.
* * * * *
|
|
 |
|
 |
Description |
|
