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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing
techniques and, more particularly, to a technique for dispensing slurry
for polishing wafers.
2. Prior Art
The art abounds with references pertaining to techniques for polishing a
surface. Various semiconductor polishing techniques today can be traced
back to the polishing methods employed to polish optical lenses. Similar
techniques have been utilized in the semiconductor field to polish wafers,
which are used for manufacturing integrated circuit devices. Thus, a
number of methods are known in the prior art for polishing wafers.
The manufacture of an integrated circuit device requires the formation of
various layers (both conductive and non-conductive) above the base
substrate to form the necessary components and interconnects. During the
manufacturing process, removal of a certain layer or portions of a layer
must be achieved in order to pattern and form various components and
interconnects. Generally, this removal process is termed "etching."
One of the techniques available for removing material is the
chemical-mechanical polishing (CMP) process in which a chemical slurry is
used along with a polishing pad. The mechanical movement of the pad
relative to the wafer provides the abrasive force for removing the exposed
surface of the wafer. A variety of CMP tools are available to perform this
type of polishing. Improvements are currently being sought in the areas of
pad design, chemical compositions of the slurry and forces exerted on the
wafer by the pad, all to obtain better control over existing CMP
practices. However, one area lacking consideration is the area pertaining
to the storage, mixing and transport of slurry to the pad for use on the
tool.
The current practice entails premixing of a slurry in relatively large
quantities (1 to 15 +gallons) prior to use. Generally, bulk slurry is
taken from storage in large quantities and premixed to a desired
concentration. The mixing will depend on the type of use for the slurry.
For example, in performing CMP on oxides, generally, slurries are diluted
to desired concentration, typically by the use of de-ionized water. In
performing CMP on metals, oxidizers and/or etchants are generally used to
control the chemical component of the polishing process. Dilution by the
use of de-ionized water can also be practiced.
Furthermore, additives can be introduced as well, for both oxide and metal
polishing, if required. These additives are utilized to enhance removal
rate, uniformity, selectivity, etc. Then the slurry is heated, if
necessary, and retained in a container for transport to the pad. This
results in variable heat loss during distribution to systems located in
different environments or at varying distances. The technology is further
complicated by a potentially short lifetime of the slurry after addition
of chemical oxidizer, etchant and/or additive. Additional risks are
encountered if either the slurry or oxidizer/etchant/additive (or the
combination of slurry and oxidizer/etchant/additive) experiences
degradation at elevated temperatures or over prolonged period of time.
Also, to change concentration of the dilution or chemicals, usually the
entire system must be purged and the mixture disposed of and remixed at
the desired concentrations. In some instances, concentration levels will
change due to evaporation of the diluting agent, such as water. This
constant remixing of the chemicals can be a significant cost burden in
manufacturing.
Therefore, it is appreciated that a novel technique for providing slurry
distribution in order to reduce waste and improve control over the slurry
is desirable. The present invention addresses these needs.
SUMMARY OF THE INVENTION
A point of use slurry dispensing system with controls for dilution,
temperature and oxidizer/etchant/additive infusion is described. A slurry
in unmixed form is pumped to a pad on a CMP tool by a liquid pump. A
liquid heater is used to heat the slurry to a desirable temperature. A
liquid dispensing line from the heater to the pad where the slurry is
dispensed is encased in a line heater to maintain the temperature of the
slurry at the desired level. The liquid heater is located proximal to the
tool so that the dispensing line is made short as possible or practical.
The slurry is dispensed onto the polishing pad in its original form.
A second dispensing line similar to the first is utilized to distribute a
diluting agent, such as de-ionized water, to the pad as well. The diluting
agent is dispensed onto the pad at the dispensing point of the slurry. The
mixing of the slurry and the diluting agent occurs at the point of use on
the pad. Flow control is maintained by the liquid pump of the second line
so that the amount of diluting agent flow determines the concentration
level of the slurry when mixed. In an alternative embodiment, the two
liquids are mixed in the dispensing line just prior to the point of use.
In another embodiment, a third independent distribution line is utilized to
dispense oxidizer/etchant/additive chemical(s) at the point of use or in
the dispensing line just prior to the point of use. By mixing the slurry
with the diluting agent and/or the oxidizer/etchant/additive at the point
of use, mixing of the liquids occurs just prior to use. Furthermore,
improved dilution, temperature and chemical infusion controls are
achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a CMP tool utilizing a slurry dispensing
system of the present invention in which two separate liquid delivery
lines mixes two liquids at the point of use.
FIG. 2 is an illustration of a CMP tool utilizing a slurry dispensing
system of the present invention in which two separate liquid delivery
lines mixes two liquids in a dispensing line just prior to the point of
use.
FIG. 3 is an illustration of a CMP tool utilizing a slurry dispensing
system of FIG. 2, but with an additional liquid delivery line for
delivering an additional chemical for mixing at the point of use.
FIG. 4 is an illustration of a CMP tool utilizing a slurry dispensing
system of FIG. 2, but with an additional liquid delivery line for
delivering an additional chemical for mixing with the other two liquids in
a dispensing line just prior to the point of use.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention describes a point of use slurry dispensing system
with controls for dilution, temperature and chemical infusion. In the
following description, numerous specific details are set forth, such as
specific devices, temperatures, chemicals, etc., in order to provide a
thorough understanding of the present invention. However, it will be
obvious to one skilled in the art that the present invention may be
practiced without these specific details. In other instances, well known
processes and structures have not been described in detail in order not to
unnecessarily obscure the present invention. Additionally, it is to be
noted that the description refers to the dispensing of a slurry onto a
pad, simply because the drawings illustrate a wafer residing above a pad.
However, if a tool incorporates a design in which the pad resides above
the wafer, then it is understood that the slurry is dispensed onto the
wafer.
The technique described herein is referred to as a point of use slurry
dispensing system. Although a novel apparatus can be designed to
incorporate the method of the present invention, it is appreciated that a
variety of prior art polishing equipment can be readily adapted to
implement the method of the present invention as well. Once the technique
described herein is disclosed, those ordinarily skilled in the art can
readily implement the method in a variety of ways. Furthermore, the
preferred embodiment is described in reference to polishing semiconductor
wafers, especially silicon wafers, utilizing a chemical-mechanical
polishing (CMP) process, but the invention need not be limited only to
wafers or to CMP.
Referring to FIG. 1, a setup of a tool for performing polishing is shown.
An item to be polished, such as a silicon wafer 10, is supported by a
carrier 11 and placed face-down on polish platen 12 so that a surface
which is to be polished rests against the polish platen 12. Typically, a
polishing pad 13 forms or resides on the surface of the platen 12 upon
which the wafer surface resides. It is to be appreciated that the
polishing device of FIG. 1 can be utilized for polishing a variety of
objects other than semiconductors. However, the preferred embodiment is
for the purpose of polishing semiconductor wafers. More specifically, a
silicon wafer is polished by the use of a CMP process.
In most CMP applications, the carrier 11 is rotated in order to rotate the
wafer 10. Usually, the polishing pad 13 is also rotated as well.
Furthermore, the carrier 11 can be made to traverse across the surface of
pad 13 in order to prevent the wafer 10 from being stationary over one
area of the pad 13 during use. It is to be noted that the carrier 10, pad
13, or both, can be made to move horizontally. This practice allows for
relative horizontal movement of the carrier 11 in relation to pad 13, so
that the wafer 10 moves across the surface of pad 13 during use.
Alternatively, the carrier 11 and wafer 10 can be made stationary over an
area of pad 13. What is important is that there be mechanical force
(usually, a rotating force) applied to the surface of the wafer being
polished.
In general usage, CMP requires the use of a chemical slurry 14 in order to
achieve the desired polishing of the wafer. The slurry 14 is generally
made to flow across the surface of the pad 13 in order to have the slurry
form a hydrodynamic layer between the wafer 10 and pad 13. There are a
variety of slurries for use in CMP applications to polish semiconductor
wafers and these slurries (and their use) are well known in the prior art.
The use of oxidizers, etchants and/or chemical additives to enhance the
polishing process or to obtain desired planarization of the surface is
also known in the prior art. Thus, the practice of CMP utilizing items
10-14 in FIG. 1 is known in the prior art and is described here as one
example of performing CMP on an object, which in this instance is a
semiconductor wafer 10. Additionally, although FIG. 1 shows the wafer 10
above the pad 13, it is generally understood that the two can be reversed.
In that instance, the pad 13 will be located above the wafer 10.
As shown in FIG. 1, a novel aspect of the present invention is in the
dispensing of the slurry 14 onto platen 12 (or pad 13) by a dispensing
system 20. A concentrated liquid slurry obtained from a bulk slurry supply
is introduced into a holding container 21. A pump 22 is coupled to
container 21 by a liquid flow line for pumping the liquid slurry from
container 21. Pump 22 is utilized for the purpose of controlling the flow
rate of the concentrated slurry being drawn from container 21. The output
from pump 22 is coupled to a liquid heater 23. Heater 23 is utilized to
heat the flowing slurry to a desirable temperature required for the
dispensed slurry. Although a variety of liquid heating devices can be
used, the preferred approach is a tank type heater capable of maintaining
temperature of a small volume of liquid (100-1000 ml) from 70.degree.
F.-212.degree. F.
A dispensing line (or tube) 24 is then coupled from heater 23 to transport
the slurry to a location proximal to platen 12. The open end of dispensing
line 24 is noted as a dispensing point 19 for dispensing the slurry 14
onto platen 12. The dispensing point is made proximal to the surface of
the pad 13 that does not interfere with the movement of pad 13 or carrier
11. The intent is to place the dispensing point of the slurry as close as
possible (or practical) to platen 12. In some instances, a line heater 25
is used to encase dispensing line 24. The purpose of the line heater is to
prevent any heat loss from the slurry after the slurry leaves liquid
heater 23.
It is to be noted that either or both of the heaters 23 and 25 can be used.
Furthermore, if the desirable temperature of the slurry is at or near room
temperature, then heaters 23 and 25 would not be needed (assuming that the
slurry in the container is also at room temperature). It should also be
noted that (although not shown) heaters 23 and 25, along with pump 22, are
typically coupled to a monitor and control system for monitoring the flow
rate and slurry temperature. If an automated monitor and control system is
utilized, the flow rate and temperature of the slurry can be controlled by
a processor. Monitoring of these parameters during operation of the CMP
equipment will allow immediate feedback and automated response to correct
for any deviations in the set parameters for the slurry.
Also shown in FIG. 1 is another liquid container 26, pump 27, liquid heater
28, dispensing line 29 and line heater 30. These units 26-30 function
equivalently to the above-described units 21-25 to provide a duplicate
dispensing system for dispensing another liquid onto platen 12. Typically,
the liquid introduced into container 26 is a diluting agent, such as
de-ionized (d.i.) water, which is used to control the concentration level
of slurry 14 on platen 12. The dispensing point of line 29 is placed
essentially at the dispensing point of line 24 so that the concentrated
slurry from container 21 mixes with the diluting agent of container 26 at
the dispensing point 19. Thus, the concentration level of the slurry 14 on
platen 12 can be controlled by adjusting the flow rate of the diluting
agent from line 29.
It is to be noted that the containers 21 and 26, pumps 22 and 27, and
heaters 23 and 28 can be located at any distance from the platen 12. In
some instances, these units may not need to reside in the actual CMP tool
itself. However, it is preferred to have the heaters 23 and 28 proximal to
platen 12 in order to reduce the line length of dispensing lines 24 and 29
to reduce heat loss from the liquids, especially if line heaters 25 and 30
are not used. It is preferred to locate heaters 23 and 28 so that the
dispensing line from each heater to the dispensing point 19 is no more
than 70 cm.
The dispensing system 20 of FIG. 1 is flexible in that each of the liquid
dispensing line is independent of the other. Thus, liquid flow rates and
temperature of the each liquid can be set at different levels and
controlled separately. Additionally, if desired, other agents (such as
oxidizers, etchants and/or chemical additives) can be introduced and
combined with the liquids in container 21 and/or container 26.
It should also be noted that if the slurry and the diluting agent are to
have the same dispensing temperature, then the dispensing lines 24 and 29
could be enclosed within a single line heater. That is only one line
heater is needed for heaters 25 and 30. Likewise, as is shown in a
dispensing system 33 of FIG. 2, the two liquids can be combined together
prior to the dispensing this alternative embodiment shown in FIG. 2, the
actual mixing of the two liquids is achieved after the liquid heaters 23
and 28, but before the dispensing point 19. A variety of techniques known
in the prior art can be used for combining the two liquids in the
dispensing line. One of the simplest approaches is to place a "T"
connection at the mixing point.
Referring to FIG. 3, another alternative embodiment is shown. In FIG. 3, an
additional liquid container 40, pump 41, liquid heater 42 and dispensing
line 43 are shown. These units 40-43 are again a duplication of units
21-24 shown in FIGS. 1 and 2 and function in a similar manner. Instead of
introducing an oxidizer, etchant and/or additive (hereinafter referred to
as oxidizer/etchant/additive) in the slurry in container 21, the
oxidizer/etchant/additive can be kept separate from the slurry until mixed
at the point of use. Therefore, an oxidizer/etchant/additive is introduced
into container 40 and transported to platen 12 by the dispensing tube 43.
A dispensing point 45 for this liquid is disposed proximal to opening 19
for slurry. Thus, a fully independent flow and temperature control is
available for the oxidizer/etchant/additive required by slurry 14.
It is to be noted that this third liquid feed is shown added to the system
33 of FIG. 2. However, it can be readily added to the system of FIG. 1 as
well. Furthermore, line heaters are not shown in FIG. 3, but as described
above, such line heaters can be utilized as well. It is also appreciated
that another embodiment is to combine all three liquids prior to the
dispensing point. For example, as is shown in FIG. 4, all three liquids
can be combined in a dispensing tube 48, similar to the approach
illustrated in FIG. 2. In this instance, the three liquids are combined
prior to being ejected at a single dispense opening 49.
It is appreciated that many different design variations can be implemented
to practice the present invention without departing from the spirit and
scope of the present invention. For example, at the dispensing point (of
either the single dispensing line embodiment or the multiple dispensing
line embodiment), instead of dispensing directly onto the pad, a container
can be used to collect the liquid(s) for further dispensing onto the pad.
Furthermore, instead of using liquid heaters and line heaters prior to the
dispensing point as shown in the Figures, this additional container can be
heated to control the temperature of the slurry being dispensed.
Additionally, it is understood that the makeup of the various liquids
described can be readily modified, including pre-diluting slurries,
oxidizers, etchants and additives utilizing the technique of the present
invention. For example, the oxidizer/etchant/additive can be diluted
similar to the dilution scheme for slurries. In that instance, a diluting
liquid container would be coupled to the oxidizer/etchant/additive line
similar to the lines for containers 21 and 26 used for slurries.
An advantage of this invention over the prior art is in the ability to
control any one or all of temperature, dilution and/or
oxidizer/etchant/additive concentration at the point of dispense on the
polishing pad. The actual mixing of the liquids can occur on the pad or in
the dispensing line. Temperatures of the liquids can be controlled
independently and maintained at temperature during use or during delays in
application of the slurry. Time at temperature and mixing of the liquids
can be delayed until the liquid is within centimeters of actual dispense
onto the polishing pad. Thus, the mixing is achieved at or near the last
possible moment before the slurry is used.
The present invention address concerns with the short shelf life of the
chemicals after they are heated and mixed, or changes in concentration due
to evaporation in a mixture. Variations in slurry concentration or
chemicals can be generated at the point of use, which also lessens the
amount of chemicals wasted. Thus, the practice of the present invention
provides for substantial cost and time reduction in the use of slurries,
as well as allowing for improved controls over the use of the slurry.
Thus, a point of use slurry dispensing system is described.
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