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Description  |
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FIELD OF THE INVENTION
This invention relates to ion-assisted plasma etching of semiconductor
wafers in remote source plasma reactors with powered wafer chucks. More
particularly, it relates to equipment improvements designed to improve
etch uniformity over the surface of a wafer.
BACKGROUND OF THE INVENTION
Integrated circuits are typically fabricated on a wafer of semiconductor
material such as silicon or gallium arsenide. During the fabrication
process, the wafer is subjected to an ordered series of steps, which may
include photomasking, material deposition, oxidation, nitridization, ion
implantation, diffusion and etching, in order to achieve a final product.
There are two basic types of etches: ion-assisted etches (also called
reactive-ion, plasma or dry etches) and solution etches (also called wet
etches). Solution etches are invariably isotropic (omnidirectional) in
nature, with the etch rate for a single material being relatively constant
in all directions. Reactive-ion etches, on the other hand, are largely
anisotropic (unidirectional) in nature. Reactive ion etches are commonly
used to create spacers on substantially vertical sidewalls of other
layers, to transfer a mask pattern to an underlying layer with little or
no undercutting beneath mask segment edges, and to create contact vias in
insulative layers.
A plasma etch system (often referred to as a reactor) is primarily a vacuum
chamber in which a glow discharge is utilized to produce a plasma
consisting of chemically reactive species (atoms, radicals, and ions) from
a relatively inert molecular gas. The gas is selected so as to generate
species which react either kinetically or chemically with the material to
be etched. Because dielectric layers cannot be etched using a
direct-current-induced glow discharge due to charge accumulation on the
surface of the dielectric which quickly neutralizes the dc-voltage
potential, most reactors are designed as radio-frequency diode systems and
typically operate at a frequency of 13.56 MHz, a frequency reserved for
non-communication use by international agreement. However, plasma etch
processes operating between 100 KHz-80 MHz have been used successfully.
The first ionization potential of most gas atoms and molecules is 8 eV and
greater. Since plasma electrons have a distribution whose average energy
is between 1 to 10 eV, some of these electrons will have sufficient energy
to cause ionization of the gas molecules. Collisions of these energized
electrons with neutral gas molecules are primarily responsible for the
production of the reactive species in a plasma. The reactive species,
however, can also react among themselves in the plasma and alter the
overall plasma chemistry.
Since plasmas consisting of fluorine-containing gases are extensively used
for etching silicon, silicon dioxide, and other materials used in VLSI
fabrication, it is instructive to examine the glow-discharge chemistry of
CF.sub.4. Initially, the only species present are CF.sub.4 molecules.
However, once a glow discharge is established, a portion of the CF.sub.4
molecules dissociated into other species. A plasma is defined to be a
partially ionized gas composed of ions, electrons, and a variety of
neutral species. The most abundant ionic specie found in CF.sub.4 plasmas
is CF.sub.3.sup.+, such ions being formed by the electron-impact reaction:
e+C.sub.F .fwdarw.C.sub.F.sup.+ +F+2e. In addition to CF.sub.4 molecules,
ionic species, and electrons, a large number of radicals are formed. A
radical is an atom, or collection of atoms, which is electrically neutral,
but which also exists in a state of incomplete chemical bonding, making it
very reactive. In CF.sub.4 plasmas, the most abundant radicals are
CF.sub.3 and F, formed by the reaction: e+CF.sub.4 .fwdarw.CF.sub.3 +F+e.
Radicals are generally thought to exist in plasmas in much higher
concentrations than ions, because they are generated at a faster rate, and
they survive longer than ions in the plasma.
Plasma etches proceed by two basic mechanisms. The first, chemical etching,
entails the steps of: 1) reactive species are generated in the plasma; 2)
these species diffuse to the surface of the material being etched; 3) the
species are adsorbed on the surface; 4) a chemical reaction occurs, with
the formation of a volatile by-product; 5) the by-product is desorbed from
the surface; and 6) the desorbed species diffuse into the bulk of the gas.
The second, reactive-ion etching, involves ionic bombardment of the
material to be etched. Since both mechanisms occur simultaneously, the
complete plasma etch process would be better aptly identified as an
ion-assisted etch process. The greater the chemical mechanism component of
the etch, the greater the isotropicity of the etch.
FIG. 1 is a diagrammatic representation of a typical parallel-plate plasma
etch reactor. To perform a plasma etch, a wafer 11 is loaded in the
reactor chamber 12 and precisely centered on a disk-shaped lower electrode
13L, thereby becoming electrically integrated therewith. A disk-shaped
upper electrode 13U is positioned above the wafer (the number 13* applies
to either 13L or 13U). The flow of molecular gas into the chamber 12 is
automatically regulated by highly-accurate mass-flow controllers 14. A
radio-frequency voltage 15 is applied between electrodes 13L and 13U.
Chamber pressure is monitored and maintained continuously through a
feedback loop between a chamber manometer 16 and a downstream throttle
valve 17, which allows reactions products and surplus gas to escape in
controlled manner. Spacing of the electrodes is controlled by a
closed-loop positioning system (not shown). At a particular voltage known
as the breakdown voltage, a glow discharge will be established between the
electrodes, resulting in a partial ionization of the molecular gas. In
such a discharge, free electrons gain energy from the imposed electric
field and lose this energy during collisions with molecules. Such
collisions lead to the formation of new species, including metastables,
atoms, electrons, free radicals, and ions. The electrical discharge
between the electrodes consists of a glowing plasma region 18 centered
between lower electrode 13L and upper electrode 13U, a lower dark space
19L between the lower electrode 13L and plasma region 18, and an upper
dark space region 19U between the upper electrode 13U and plasma region
18. Dark space regions 19, are also known as "sheath" regions. Electrons
emitted from the electrodes 13* are accelerated into the discharge region.
By the time the electrons have reached plasma region 18, they have
acquired sufficient kinetic energy to ionize some of the molecular gas
molecules and raise the electrons of other molecular gas molecules to
less-stable atomic orbitals of increased energy through a mechanism known
as electron impact excitation. As each of these excited electrons
"relaxes" and falls back to a more stable orbital, a quantum of energy is
released in the form of light. This light gives the plasma region its
characteristic glow. Free electrons may also collide with species already
formed by collisions between free electrons and gas molecules, leading to
additional subspecies. Because free electrons have little mass, they are
accelerated much more rapidly toward the electrodes than are ionized gas
molecules, leaving the plasma with a net positive charge. The voltage drop
through the plasma is small in comparison to the voltage drops between the
plasma and either of the plates at any given instant of an AC voltage
cycle. Therefore, plasma ions which are accelerated from the plasma to one
of the plates are primarily those that happen to be on the edge of one of
the dark spaces. Acceleration of ions within the plasma region is minimal.
Although ions are accelerated toward both electrodes, it is axiomatic that
the smaller of the two electrodes will receive the greatest ionic
bombardment. Since the ions are accelerated substantially perpendicularly
between the two electrodes (parallel to the electric field), the ions will
collide with the wafer perpendicularly to the wafer's surface. As an ion
collides with an atom or molecule of reactive material on the wafer, the
two may react to form a reaction product. Because ion bombardment of the
electrodes with ions and electrons causes an elevation of electrode
temperature, both electrodes are normally cooled by the circulation of
deionized water through the electrodes and an external temperature control
unit (not shown). Water cooling prevents elevation of wafer temperature to
levels which would destabilize photoresist. Typical plasma reactors
consist of a single process chamber flanked by two loadlock chambers (one
for wafer isolation during loading, the other for isolation during
unloading).
Parallel-plate etch reactors have fallen into disfavor for certain
applications. For example, the voltage required to sustain the plasma is
far higher that is required to etch polycrystalline silicon or
single-crystal silicon. In fact, the voltage is so high that ions are
accelerated to energies sufficient to etch silicon dioxide. For this
reason, it is very difficult to perform an etch of silicon that stops on a
silicon dioxide layer using a parallel-plate reactor. For such
applications, a new type of plasma reactor has been developed. In this
type of reactor, the plasma is generated in a source chamber remote from
the wafer (typically at the very top of the chamber, and the wafer chuck
is powered separately from the plasma source generator. Such a reactor is
generally called a high-density source plasma reactor. Examples of sources
used to create the high-density plasma are: a Mori source, a helicon
source, and an electron cyclotron resonance (ECR) source. A description of
the operation of such sources is beyond the scope of this disclosure, and
not particularly relevant thereto.
FIG. 2 is a diagrammatic representation of a typical high-density-source
plasma reactor. The reactor comprises an etch chamber 21 formed by a
cylindrical sidewall 22, which is grounded, a floor wall 23, and a ceiling
wall 24. A source chamber 25 adjoins the etch chamber 21. A disc-shaped
wafer chuck 26 is concentrically mounted within the lower portion of the
etch chamber 21. A wafer 27 is precisely centered on the wafer chuck 26,
thereby becoming electrically integrated therewith. The wafer may be held
in place against the wafer chuck 26 by any one of a variety of known
techniques, such as the use of a clamping ring 28, or an electrostatic
chuck (not shown). The flow of molecular gas, which is depicted as being
introduced into the source chamber 25 through a primary manifold 29, is
automatically regulated by highly-accurate mass-flow controllers 30.
However, molecular gases and/or atomic gases may be introduced at other
locations in either the source chamber 25 or the etch chamber 21. A
high-density plasma 31 is generated within the source chamber 25 by either
a Mori source, a helicon source, or an ECR source (not shown). A
radio-frequency voltage generator 32 is coupled between the wafer chuck 26
and ground. Chamber pressure is monitored and maintained continuously
through a feedback loop between a chamber manometer 33 and a downstream
throttle valve 34, which allows reactions products and surplus gas to
escape in controlled manner. As the high-density plasma escapes from the
source chamber 25 and migrates toward the etch chamber 21, its density
usually decreases and it usually becomes more spacially uniform. The less
dense plasma 35 within the etch chamber 21 receives additional power from
the powered wafer chuck during the etch process. Power coupling between
the wafer chuck 26 and the cylindrical sidewall 22 causes reactive ions to
be accelerated through a dark space that is established just above the
surface of the wafer 27, permitting ion-assisted etching of etchable
material on the surface of the wafer to occur. The amount of power
supplied to the wafer chuck 26 greatly influences etch rates, etch
uniformity, and profile control. The discussion of ion-assisted etching
relative to the parallel-plate etch reactor is also largely applicable in
the case of a high-density source plasma reactor.
Still referring to FIG. 2, it should be noted that the cylindrical sidewall
22 is normally fitted with a large number of vertical magnetic strips of
alternating polarity, thus creating a magnetic field wall on the interior
surface of cylindrical sidewall 22. Such an arrangement is known as a
"McKenzie bucket" or simply a "confinement bucket", and was devised as a
means to more evenly distribute reactive ions which were generated within
the source chamber 25 and which have migrated downward to the etch chamber
21. This feature is not depicted, as it is not relevant to this invention.
One of the problems associated with high-density source plasma etch
reactors is that etching is not uniform across the surface of the wafer.
Nonuniformpower coupling between the wafer and the walls of the etch
chamber can be the dominant cause of nonuniform etching rates across the
surface of the wafer. Nonuniformpower coupling occurs because regions near
wafer edge are physically closer to the grounded walls of the chamber than
are regions closer to the wafer center. Thus, higher power is coupled to
the walls through a unit area near the edge of the wafer than is coupled
by a unit area located nearer the center of the wafer. This radially
nonuniform coupling of the rf power to the chamber walls results in lower
etch rates near the center of the wafer than near the edge. It can also
adversely affect other process results such as feature profile and/or
selectivity.
SUMMARY OF THE INVENTION
This invention is a hardware modification which permits greater uniformity
of etching to be achieved in a high-density-source plasma reactor (i.e.,
one which uses a remote source to generate a plasma, and which also uses
high-frequency bias power on the wafer chuck). The invention addresses the
uniformity problem which arises as the result of nonuniformpower coupling
between the wafer and the walls of the etch chamber. The solution to
greatly mitigate the nonuniformity problem is to increase the impedance
between the wafer and the chamber walls. This may be accomplished by
placing a cylindrical dielectric wall around the wafer. Quartz is a
dielectric material that is ideal for the cylindrical wall if silicon is
to be etched selectively with respect to silicon dioxide, as quartz it is
virtually inert under such conditions. Any dielectric material can be
used, including those which are etchable, provided that they do not have a
negative impact on the etch process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of a typical parallel-plate plasma
etch reactor;
FIG. 2 is a diagrammatic representation of a typical remote plasma source
etch reactor having a powered wafer chuck;
FIG. 3 is a diagrammatic representation of the remote plasma source etch
reactor of FIG. 2, which has been fitted with a cylindrical dielectric
wall decoupler; and
FIG. 4A is a plot of the etch rate for polycrystalline silicon, in
.ANG./min., as a function of location on the wafer for a power setting of
3,000 watts for the source plasma generator and about 95 watts for the
wafer chuck power setting for both a standard high-density source plasma
reactor and a high-density source plasma reactor which incorporates the
invention;
FIG. 4B is a plot of the etch rate for polycrystalline silicon, in
.ANG./min., as a function of location on the wafer for a power setting of
3,000 watts for the source plasma generator and about 60 watts for the
wafer chuck power setting for both a standard high-density source plasma
reactor and a high-density source plasma reactor which incorporates the
invention; and
FIG. 4C is a plot of the etch rate for polycrystalline silicon, in
.ANG./min., as a function of location on the wafer for a power setting of
3,000 watts for the source plasma generator and about 31 watts for the
wafer chuck power setting for both a standard high-density source plasma
reactor and a highdensity source plasma reactor which incorporates the
invention.
PREFERRED EMBODIMENT OF THE INVENTION
Referring now to FIG. 3, a conventional high-density-source plasma reactor
has been fitted with a device which uniformly increases the impedance
between the wafer and the chamber walls. The impedance-increasing device
is a cylindrical dielectric wall 37 that, like the wafer 27, is precisely
centered (i.e., concentrically mounted) on the wafer chuck 26. Quartz is a
dielectric material that is ideal for the cylindrical wall 35 if silicon
is to be etched selectively with respect to silicon dioxide, as quartz it
is virtually inert under wafer chuck power settings of 60 watts. It is
hypothesized that power coupling between the wafer and the chamber wall 22
is uneven because the electrical paths from the wafer surface, through the
dark space above the wafer surface, through the plasma 35, and, finally,
to the chamber wall 22, are of different lengths, depending on the radial
location on the surface of the wafer. The center of the wafer is the
farthest from the wall, so one would expect power coupling for the wafer's
center region to be less than for the wafer's edge. Actual etch rates do
support this hypothesis. It is assumed that the dielectric wall 37 is
successful in improving the uniformity of etch rate because it increase
the power coupling path for all portions of the wafer 27. However, the
increase in path length is greater for portions of the wafer nearest the
edge.
FIGS. 4A, 4B, and 4C demonstrate the effectiveness of the invention at a
plasma source power setting of 3,000 watts, but at different wafer chuck
power settings. In these figures, the circular data points represent
measured data for the standard reactor without the dielectric wall 37, and
the square data points represent measured data for the reactor with the
dielectric wall 37.
FIG. 4A is a plot of the etch rate for polycrystalline silicon, in
.ANG./min., as a function of location on a six-inch wafer. Position 3
represents the center of the wafer, and positions 0 and 6 represent the
edges of the wafer. For the data represented by FIGS. 4A, 4B, and 4C the
wafer chuck power settings were approximately 95 watts, 60 watts, and 31
watts, respectively. For a six inch diameter wafer, as portrayed in FIGS.
4A-4C, the 95 watts, 60 watts, and 31 watts power settings provide
associated power densities across the wafer of 520 (mW/cm.sup.2) 330
(mW/cm.sup.2) and 170 (m/W/cm.sup.2) respectively. It will be noted that
at a wafer chuck power setting of approximately 95 watts, etch uniformity
is measurably improved with the dielectric wall 35 installed in the
reactor. However, substantial nonuniformity is still present. Although the
higher the wafer chuck power setting, the more rapid the etch, this
improvement in etch uniformity may not be sufficient for the process
concerned. At a wafer chuck power setting of approximately 60 watts, etch
uniformity with the dielectric wall 35 installed is greatly improved over
that obtained with the standard reactor. At a wafer chuck power setting of
approximately 31 watts, nonuniformity of the etch with the dielectric wall
35 installed may not even be accurately measurable.
Although only a single embodiment of the invention has been disclosed
herein, it will be obvious to those having ordinary skill in the art of
ion-assisted etching that changes and modifications may be made thereto
without departing from the scope and the spirit of the invention as
hereinafter claimed.
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