 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The invention relates to physical vapor deposition equipment such as are
used in the fabrication of integrated circuits and semiconductor devices.
Physical vapor deposition (PVD) is a process by which a target material
(e.g. Ti) is deposited onto an object (e.g. a semiconductor wafer) by
means of a plasma. The process takes place in a vacuum chamber that
contains an inert gas (e.g. argon). The plasma, which is generated in the
chamber between the negatively biased target and the wafer, ionizes the
inert gas. The positively charged ionized gas atoms are pulled toward the
negatively biased target and impact it with sufficient energy to expel
(i.e., sputter) atoms of target material from the target. The sputtered
atoms from the target are propelled toward the wafer where they form a
layer of deposited material.
The trajectories of the material coming off from the target during
sputtering are distributed over a range of directions. Though typically
most of the sputtered material travels in a direction that is normal to
the target, a significant amount travels in other directions that diverge
from the normal direction. The sputtered material which travels along the
divergent directions tends to limit the definition that is obtainable at
discontinuities on the wafer surface. More specifically, the material
which travels along the non-normal trajectories deposits on the sidewalls
of features such as thru-holes and vias, thereby limiting how small one
can make those features. For holes that are too small, the material
deposited on the sidewalls eventually closes up the hole and prevents any
further material from being deposited at the bottom of the hole.
Collimation filters are used to filter out all sputtered material having a
trajectory that diverges from the normal direction by more than a
preselected angle. The collimation filter is placed between the target and
the wafer. In general, it is a metal plate having a particular thickness
with an array of holes passing through it. To maximize throughput, a
honeycomb structure (i.e., a pattern of hexagonal holes) is used. The
holes have a specified aspect ratio, i.e., the ratio of their length to
their diameter. The aspect ratio determines the degree of filtering which
takes place. A higher aspect ratio produces a narrower angular filter
(i.e., the preselected angle is smaller). A consequence of using a higher
aspect ratio, however, is a significantly reduced throughput. Thus,
picking the appropriate thickness and hole size for the collimation filter
is simply a question of process optimization.
Nevertheless, for a typical filter design in a conventional system, only
about 20% of the material will make it through the filter. The rest of the
material, which represents sputtered material that has a trajectory that
diverges from the normal direction by more than the preselected angle,
deposits on the filter. In this example, the collimation filter reduces
throughput by a factor of 5.
To compensate for the reduced throughput, users typically increase the
operational power on the target. For example, rather than running at a 5
kW power level, the power is increased to about 20 kW. The increased power
levels increase the rate of sputtering. They also, however, introduce
other problems, e.g. increased temperature of the wafer and unwanted
material interactions in the deposited layer.
SUMMARY OF THE INVENTION
It was discovered that much of the ionization in a plasma sputtering
chamber occurs very close to the target, leaving a considerable amount of
room for ionization enhancement throughout the rest of the plasma. It was
also discovered that RF power can be effectively coupled into the DC
generated plasma through a coil or ring antenna inside the chamber. The
coupled RF power contributes to increasing ionization efficiency rather
than causing a sputtering of the ring material.
In general, in one aspect, the invention is a plasma deposition system for
sputter depositing material from a target onto a wafer. The system
includes a chamber; a platform for holding the wafer during plasma
processing; a source onto which the target is mounted; an equipotential
conductive plane dividing the chamber into an upper cavity in which the
target is located and a lower cavity in which the wafer is located; and an
upper antenna located inside the upper cavity and surrounding the plasma.
The source generates a plasma in the chamber during operation. The
equipotential conductive plane permits material sputtered from the target
to pass into the lower cavity. And the upper antenna couples RF power into
the source-generated plasma.
In preferred embodiments, the plasma deposition system also includes a
lower antenna located inside the lower cavity for generating a second
plasma in the lower cavity. Also, the equipotential plane is a collimation
filter that is made of a refractory material (e.g. titanium).
In general, in another aspect, the invention is a plasma deposition system
for sputter depositing material from a target onto a wafer. The system
includes a chamber; a platform for holding the wafer during plasma
processing; a source onto which the target is mounted; and an antenna
located inside of the chamber and surrounding the plasma. The source
generates a plasma in the chamber during operation and the antenna is for
coupling RF power from an RF supply into the source-generated plasma.
With conventional plasma deposition techniques, the user actually has quite
limited control over the conditions under which the films are produced and
thus the properties of the generated films. For example, it is generally
not practical to lower the source power level (and thus the temperature)
beyond a certain point because this also significantly reduces throughput.
Though more desirable film properties may be achievable by operating at
low DC power levels, high throughput is essential for production purposes.
Thus, compromises must be made on what film properties are produced. The
upper antenna permits the user to have much greater control over the
plasma deposition process at throughputs that are acceptable for
production purposes and to thereby access a wider range of film
properties.
The lower cavity antenna enables the user to exercise greater optimization
control over the plasma deposition process. For example, the nitrogen
sticking coefficient strongly depends upon temperature as does the ability
to react Ti with N.sub.2 to form TiN. However, these two reactions move in
opposite directions with temperature. While the sticking coefficient
improves with decreasing temperatures, the reactivity of the reactive gas
species decreases. In a conventional system, there is generally an optimum
process temperature at which one achieves the best reactivity while still
maintaining acceptable sticking coefficient. Thus, when using conventional
plasma deposition techniques, one must run at the appropriate power level
which achieves the optimum temperature. With the invention, however, a
user can increase the reactivity of the N.sub.2 at lower temperatures
where there is a better sticking coefficient without having to increase
the power level of the source-generated plasma. Thus, the invention
enables the user to run at a lower temperature (and lower power levels)
without sacrificing reactivity and thereby produce more stoichiometric
films and films with different, possibly more desirable, properties.
The lower cavity antenna also enables the user to more easily control the
energy of the arriving species that is sputtered from the target. This, in
turn, enables the user to have greater control over process optimization.
For example, in aluminum planarization work using conventional deposition
techniques, people have typically used DC or RF bias on the wafer to
further stimulate the deposited atoms to move into the contacts. But this
approach brought with it other problems. The arriving atoms, having much
higher energy, altered the properties of the resulting film and tended to
produce defects in the film. By using the ring antenna in the lower cavity
to vary the impedance above the wafer, the user can change the film
properties in an advantageous way without having to use high levels on the
wafer to do so. Rather than coupling RF or DC power to the wafer, the
lower antenna couples it to the plasma and then lets the plasma potential
enhance the bombardment energy. Thus, the user can employ very low bias on
the wafer (e.g. 10 V) so that the energy of the arriving atoms is much
lower and less likely to disrupt the film that is forming. This enables
the user to have greater control over the density of the film and the
surface states which affect the resulting barrier characteristics.
Other advantages and features will become apparent from the following
description of the preferred embodiment and from the claims.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a sputter deposition chamber including a collimation filter as
well as upper and lower cavity RF bias ring antennas that both inductively
couple to the plasma within the chamber; and
FIG. 2 shows an alternative embodiment using upper and lower chamber RF
bias ring antennas that capacitively couple to the plasma within the
chamber.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a sputter deposition system includes a deposition
chamber 10, a source assembly 12 on which a sputter target 14 is mounted,
and a movable lower platform 16 holding a wafer 18 on which material
sputtered from target 14 is deposited. The source assembly and the target
mounted thereon are electrically isolated from the rest of the chamber by
an insulator ring 20. The lower platform can be raised and lowered by a
mechanical lift mechanism 22. After the wafer is placed on the platform,
the lift mechanism raises the wafer until it contacts a clamping ring 24.
The clamping ring has a central aperture 26 that is slightly smaller than
the diameter of the wafer and it shields the platform from deposition
material during processing.
A vacuum pump 28, which is connected to the chamber through a vacuum line
30, is used to evacuate the chamber at the beginning of a process run.
Flow of inert gas (e.g. Ar) and reactive gases (e.g. N.sub.2) in and out
of the chamber is controlled by gas control circuitry 32. Power for
initiating and sustaining a plasma deposition process is supplied to the
target by a DC voltage supply 34. The negative terminal of the DC supply
is connected to the target through a line 36 and the positive terminal of
the DC supply is connected to the wall of the chamber through another line
38.
In the described embodiment, the source is a magnetron which includes a set
of magnets (not shown) positioned behind the target material. The magnets
increase the sputtering efficiency by preventing the electrons from
rapidly escaping away from the face of the target and keeping them within
a short distance of the target for a longer period of time so that they
can thereby cause multiple ionizations of the inert gas (e.g. Ar). In such
a source, the magnets rotate around the back of the target in order to
improve the uniformity of the erosion of the target during sputtering.
A collimation filter 40 separates chamber 10 into an upper cavity 42 and a
lower cavity 44. The collimation filter is connected to ground and thus
forms a ground plane separating the two cavities. Within the upper cavity
and around its inside perimeter there is a cylindrical shield 46 that
prevents material from being deposited on the walls of the chamber.
Likewise, the lower cavity also includes another cylindrical shield 48
which serves a similar purpose. Both shields are connected to ground
potential.
Inside the upper cavity, an upper ring antenna 50 couples RF power to the
plasma generated by source 12. An RF generator 52 coupled to the upper
ring antenna through an RF matching network 54 provides the RF power to
the upper ring antenna. The electrical connection is made through a
feed-through 56 in the wall of the chamber to one side of the upper ring
antenna. The other side of the upper ring antenna is electrically
connected to ground through another feed-through 58 in the chamber.
Inside the lower cavity, a second ring antenna 60 couples RF power to the
sputtered species passing through the collimation filter to the wafer. A
second RF generator 62 coupled to lower ring antenna 60 through a second
RF matching network 64 provides the RF power to the lower ring antenna. As
with the upper ring antenna, the electrical connection is made through a
feed-through 66 in the wall of the chamber to one side of the lower ring
antenna. The other side of the lower ring antenna is electrically
connected to ground through another feed-through 68 in the chamber.
Typically, the RF power supplied to the upper and lower ring antennas will
be less than about 5 kW, though in some applications it may be desirable
to use higher power levels (e.g. 20 kW). The precise power levels will
depend upon the results that are desired and thus will vary from one
application to the next.
Each RF matching network creates a resonant circuit with its associated
ring antenna so as to achieve high current in the antenna (i.e., efficient
coupling of energy into the antenna). They include variable components
that enable the user to tune them for optimum coupling to the plasma
during a processing run. Since such tunable matching networks are well
known to persons of ordinary skill in the art, they will not be described
in greater detail here.
In the described embodiment, which processes 8 inch wafers, the target size
is about 13 inches. The target material may be any of a wide variety of
materials including, for example, Ti, TiN, or W-materials which are
typically used for producing ohmic contacts or device barriers. The
spacing between the target and the wafer when the wafer is in a fully
raised position is typically about 94 mm. The collimation filter is
positioned approximately midway between the target and the wafer. It has a
honeycomb structure, it is made of a refractory material such as titanium,
it has a thickness of about 0.950 inch and the holes are about 0,625 inch
in diameter. The upper and lower antennas are positioned at approximately
the midpoints of in their respective cavities and are located around the
inside periphery of the chamber.
The upper and lower antennas provide a means by which the user can pump
energy into the plasma and thereby dramatically affect the total
production of ions. The upper and lower ring antennas are made of a
conductive material (e.g. aluminum or copper) that is also resistant to
the conditions existing in the plasma sputtering chamber. They may be
single or multiple turn coils (e.g. 2-3 turns), the number of turns, of
course, depending upon the frequency of the RF generators. In the
described embodiment, RF frequencies within the range of 400 kHz to 40 MHz
may be used.
In the upper cavity, the plasma is generated by the DC voltage supply and
the upper antenna provides a way to couple further energy into that
plasma. By pumping further energy into the plasma, the user can increase
the number of inert gas ions that are available for sputtering the target
at the power level determined by the DC voltage supply. That is, the upper
antenna serves to lower the source impedance of the target. Thus, with the
aid of the upper antenna, the user can increase throughput at a given
power level and thereby compensate for the reduced throughput resulting
from the collimation filter. Also, since sputter distribution is dependent
upon the source impedance, the upper antenna enables the user to control
the shape of the sputter distribution. In addition, the upper antenna
tends to ionize the material that is sputtered from the target, which
tends to straighten the trajectories of the sputtered material and to
further concentrate more of the plasma along the direction normal to the
target. Thus, it becomes possible to reduce the amount of material that is
deposited on the filter and further increase throughput.
It should be noted that the improved sputtering efficiency achieved by the
upper antenna may make it more practical to use sources other than a
magnetron. The loss in efficiency attributable to some less efficient
sources will be more than compensated by the beneficial effects of the
upper antenna. Eliminating the magnetron has the added benefit of
improving the erosion uniformity of the target.
Since the lower cavity is separated from the upper cavity by a ground plane
(i.e., the collimation filter), the plasma from the upper cavity will
typically not penetrate into the lower cavity. Thus, the lower antenna and
the RF generator driving it are used to strike and maintain a separate
plasma in the lower cavity. Through the second plasma generated in the
lower cavity, the user can enhance the ionization of the reactive species
(e.g. N.sub.2), which in turn will alter its reaction rate with the
sputtered species. This capability may be particularly useful in, for
example, coherent TiN deposition processes, in which it may be important
to control the reactivity of the reactive gas species and to thereby
produce films with different properties.
Note that turning on the RF power to the lower antenna does not affect the
sputter rate of the target but it does increase the bombardment energy and
ionization of the sputtered species onto the wafer. Thus, the lower
antenna can be used to optimize the bombardment energy to control the
characteristics of the deposited material and the barrier characteristics.
Referring to FIG. 2, in a capacitively coupled system, the upper and lower
ring antennas are formed by continuous cylindrical plates or rings 80 and
82, respectively, rather than by coils. Through the cylindrical plates, RF
power is capacitively coupled into the plasma. In all other respects,
however, the internal design of the chamber is similar to what was
previously described. Of course, different RF matching circuits 84 and 86
are provided which are designed for efficiently coupling power into a
capacitive load rather than an inductive load. Also, in the capacitively
coupled system, it may be desirable to produce a magnetic field in the
chamber pointing from the target to the wafer. Such a magnetic field would
assist in confining the plasma and enhancing its density. It can be
generated by a set of Helmholtz coils 90 that are located outside of and
surround the chamber, as shown diagrammatically in the figure.
Other embodiments are within the following claims. For example, the upper
antenna can also be used in a deposition system by itself with no
collimation filter or lower antenna. In that application, it would serve
to control the target impedance and ionization efficiency as previously
described. Also, though the described embodiment used a DC voltage supply
as the power source, it could instead use an RF source or a hybrid DC
voltage supply and RF power source. In addition, a DC bias could be
applied to the wafer.
* * * * *
|
|
 |
|
 |
Description |
|
