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BACKGROUND OF THE INVENTION
1 Field of the Invention
The present invention relates to the field of semiconductor processing; and
more specifically to the field of polishing methods and apparatuses for
planarizing thin films formed over a semiconductor substrate.
2 Description of Related Art
Integrated circuits (IC's) manufactured today generally rely upon an
elaborate system of metalization interconnects to couple the various
devices which have been fabricated in the semiconductor substrate. The
technology for forming these metalized interconnects is extremely
sophisticated and well understood by practitioners in the art.
Commonly, aluminium or some other metal is deposited and then patterned to
form interconnect paths along the surface of the silicon substrate. In
most processes, a dielectric or insulated layer is then deposited over
this first metal (metal 1) layer; via openings are etched through the
dielectric layer and the second metalization layer is deposited. The
second metal layer covers the dielectric layer and fills the via openings,
thereby making electrical contact down to the metal 1 layer. The purpose
of the dielectric layer, of course, is to act as an insulator between the
metal 1 and metal 2 interconnects. Most often the intermetal dielectric
layer comprises a chemical vapor deposition (CVD) of silicon dioxide which
is normally formed to a thickness of approximately one micron.
(Conventionally the underlying metal 1 interconnects are also formed to a
thickness of approximately one micron.) This silicon dioxide layer covers
the metal 1 interconnects conformably such that the upper surface of the
silicon dioxide layer is characterized by a series of nonplanar steps
which correspond in height and width to the underlying metal 1 lines.
These step height variations in the upper surface of the interlayer
dielectric have several undesirable features. First of all, nonplaner
dielectric surfaces interfere with optical resolution of subsequent
photolithographic processing steps. This makes it extremely difficult to
print high resolution lines. A second problem involves the step coverage
of metal 2 (second metal) layer over the interlayer dielectric. If the
step height is too large there is a serious danger that open circuits will
be formed in metal 2 layer.
To combat these problems, various techniques have been developed in an
attempt to planarize the upper surface of the interlayer dielectric (ILD).
One approach employs abrasive polishing to remove the protruding steps
along the upper surface of the dielectric. According to this method, the
silicon substrate is placed face down on a table covered with a flat pad
which has been coated with an abrasive material (slurry). Both the wafer
and the table are then rotated relative to each other to remove the
protruding portions. This abrasive polishing process continues until the
upper surface of the dielectric layer is largely flattened.
One factor in achieving and maintaining a high and stable polishing rate is
pad conditioning. Pad conditioning is a technique whereby the pad surface
is put into a proper state for subsequent polishing work. In one
conditioning method, as shown in FIG. 1, the polishing pad 12 is
impregnated with a plurality of macrogrooves 14. Polishing pad 12 is shown
in FIG. 1 having a series of substantially circumferential grooves 14
formed across the portion of the pad over which polishing takes place. The
macrogrooves aid in polishing by channeling slurry between the substrate
surface and the pad. The macrogrooves 14 are formed prior to polishing by
means of a milling machine, a lathe, a press or similar method. Since
polishing does not normally occur across the entire pad surface, the
grooves are normally only formed into a portion of the pad over which
polishing takes place. This is shown in FIG. 1 by the grove path area 16.
FIG. 2 illustrates a cross section of grooved path area 16 formed on the
pad 12. As can be seen, the grooves are characteristically triangular
shaped (but may have other shapes as well), and have an initial depth
which is sufficient to allow slurry to channel beneath the substrate
surface during polishing. The depth of the macrogrooves is approximately
300 microns. The spacing of the grooves varies from about two grooves per
radial inch to 32 grooves per radial inch.
A problem with this technique of conditioning the pad is that over time,
the one time provided macrogrooves become worn down due to polishing. This
is shown by the broken line 18 in FIG. 1. As polishing occurs, pad 11 gets
worn away and the added macrogrooves become smoothed over. A smooth pad
surface results in a reduction of slurry delivery beneath the wafer. The
degradation in pad roughness over time results in low, unstable, and
unpredictable polish rates. Low polish rates decrease wafer throughput.
Unstable and unpredictable polish rates make the planarization process
unmanufacturable since one can only estimate the amount of ILD removed
from wafer to wafer. Additionally, when the pad roughness becomes "glazed"
or "smoothed" over time, rough wafers polish at a different, higher rate
than do smooth wafers. That is, wafers which have rough surfaces from, for
example, laser scribe lines, polish at faster rates because their surfaces
"rough" the pad surface while they polish. This increases slurry delivery
beneath these wafers which accounts for the rise in polish rate. Thus, the
polish rate of wafers polished with the earlier method is dependant upon
wafer type. Different polish rates for different types of wafers make the
polishing process unmanufacturable.
Thus, what is desired is an apparatus and method for mechanically polishing
a thin film wherein the polish rate is high, stable, and independent of
wafer type.
SUMMARY OF THE INVENTION
An apparatus for polishing a thin film formed on a semiconductor substrate
is described. The apparatus has a rotatable table and a means for rotating
the table. A polishing pad with a plurality of preformed, circumferential,
triangular grooves of about 300 microns deep covers the table. The
preformed grooves facilitate the polishing process by creating a
corresponding plurality of point contacts at the pad/substrate surface.
Means is provided for depositing an abrasive slurry on the upper surface
of the pad. Means is also provided for forcibly pressing the substrate
against the pad such that the rotational movement of the table relative to
the substrate together with the slurry results in planarization of the
thin film. Additionally, while wafers are polished a pad conditioning
apparatus generates a plurality of radial microchannel grooves with a
triangular shape and with a depth of about 40 microns. The microchannel
grooves aid in facilitating polishing by channeling slurry between the
substrate and the polishing pad. The pad conditioning apparatus comprises
a diamond block holder having a plurality of threaded diamond tipped
shanks embedded into a substantially planar surface of the block. A
conditioner arm is coupled at one end to the diamond block holder and at
the other end to a variable speed oscillating motor. The motor pivots the
arm about a fixed point which sweeps the holder block in a radial
direction across a predetermined portion of the polishing pad. The
embedded diamond tipped threaded shanks generate the microchannel grooves
as the holder block is swept across the pad surface.
A goal of the present invention is to provide an apparatus for planarizing
a thin film by polishing, wherein the polish rate is high, stable, and
wafer independent.
Another goal of the present invention is to continually and consistently
channel slurry between the polishing pad and substrate by continually
conditioning the pad surface during polishing.
Still another goal of the present invention is to provide means to
adequately and continually condition the polishing pad without providing
undo wear on the pad surface.
Still yet another goal of the present invention is to be able to condition
predetermined portions of the polishing pad more than other portions of
the pad.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overhead view of a polishing pad which has been preconditioned
with macrogrooves.
FIG. 2 is a cross-sectional view of a polishing pad which has been
preconditioned with macrogrooves. FIG. 2 also shows the "smoothing" of the
preformed macrogrooves due to polishing.
FIG. 3 is a side view of the wafer polishing apparatus of the present
invention.
FIG. 4 is an overhead view of the wafer polishing apparatus of the present
invention.
FIG. 5(a) is a cross-sectional view of the diamond block holder of the pad
conditioning assembly of the present invention.
FIG. 5(b) is a bottom view of the diamond block holder of the pad
conditioning assembly of the present invention.
FIG. 5(c) is an illustration of the threaded diamond tipped stainless steel
shank used in the pad conditioning assembly of the present invention.
FIG. 6 is a cross-sectional view of a polishing pad showing preformed
macrogrooves and the pad conditioning assembly generated microgrooves.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
An improved polishing apparatus utilized in the polishing of a thin film
formed on a semiconductor substrate is described. In the following
description numerous specific details are set forth, such as specific
equipment and material, etc. in order to provide a thorough understanding
of the invention. It will be obvious, however, to one skilled in the art,
that the present invention may be practiced without these specific
details. In other instances, other well known machines and processing
steps have not been described in particular detail in order to avoid
unnecessarily obscuring the present invention.
With reference to FIG. 3, the polishing apparatus of the present invention
is illustrated. The polishing apparatus is used to planarize a thin film
layer formed over a semiconductor substrate. The thin film is typically an
interlayer dielectric (ILD) formed between two metal layers of a
semiconductor device. The thin film, however, need not necessarily be an
ILD, but can be any one of a number of thin films used in semiconductor
circuit manufacturing such as, but not limited to: metal layers, organic
layers, and even the semiconductor material itself. In fact, the pad
conditioning technique of the present invention can be generally applied
to any polishing process which uses similar equipment and where polishing
pad "smoothing" causes the polish rate to decline. For example, the
present invention may be useful in the manufacture of metal blocks,
plastics, and glass plates.
During planarization, a silicon substrate 25 is placed face down on pad 21
which is fixedly attached to the upper surface of table 20. In this
manner, the thin film to be polished is placed in direct contact with the
upper surface of the pad 21. According to the present invention, pad 21
comprises a relatively hard polyurethane, or similar material, capable of
transporting abrasive particulate matter such as silica particles. In the
currently preferred embodiment of the present invention, an initially
nonperforated pad manufactured by Rodel, Inc. known by the name "IC60" is
employed. It is appreciated that similar pads having similar
characteristics may also be used in accordance with the invented method.
Carrier 23, also know as a "quill", is used to apply a downward pressure F1
against the backside of the substrate 25. The backside of substrate 25 is
held in contact with the bottom of carrier 23 by a vacuum or simply by wet
surface tension. Preferably, an insert pad 27 cushions wafer 25 from
carrier 23. An ordinary retaining ring is employed to prevent wafer 25
from slipping laterally from beneath carrier 23 during processing. The
applied pressure F1 is typically on the order of 5 lbs per square inch and
is applied by means of a shaft 22 attached to the back side of carrier 23.
This pressure is used to facilitate the abrasive polishing of the upper
surface of the thin film. Shaft 22 may also rotate to impart rotational
movement to substrate 25. This greatly enhances the polishing process.
Additionally, a pad conditioning assembly 30 is provided for generating
microchannels 50 in pad 21. The microchannels 50 are generated while
wafers are being planarized. The pad conditioner assembly 30 comprises a
conditioner arm 32 wherein one end of arm 32 is coupled by means of a ball
and socket joint 34 to a diamond holder block 36. The ball and socket
joint 34 helps to ensure that the bottom surface 37 of holder block 36 is
uniformly in contact with pad 21 when undulations in pad 21 are present.
In the preferred embodiment the diamond holder block 36 has five threaded
stainless steel diamond tipped shanks 38 embedded into the bottom surface
37 of holder block 36. The diamond tips 44 of shanks 38 protrude a
distance of 40 microns from the bottom plane 37 of the holder. The weight
of the conditioning assembly 30 provides a downward force F2 of
approximately 16 ounces. Such a force is adequate to embed the diamond
tips 44 of the stainless steel shanks 38 into pad 21. The bottom surface
37 of the diamond holder block 36 acts as a mechanical stop to ensure that
the diamond tips 44 are embedded into 21 pad at the preferred depth of 40
microns.
FIG. 4 is an overhead view of the polishing apparatus of the present
invention. In the preferred embodiment of the present invention the
polishing pad 21 is initially conditioned prior to polishing by
impregnating the surface with a plurality of circumferential macrogrooves
47. It is to be appreciated that macrogrooves other than circumferential
macrogrooves can be utilized. The one-time provided macrogrooves are
formed be means of a milling machine, lathe, or press, or similar method.
There are between 2-32 macrogrooves per radial inch. The macrogrooves are
dimensioned so as to facilitate the polishing processing by creating point
contact at the pad/substrate interface. The grooves also increase the
available pad area and allow more slurry to be applied to the substrate
per unit area. Although the preferred embodiment of the present invention
preconditions pad 21 with macrogrooves prior to polishing, one need not
necessarily precondition pad 21. That is, a smooth pad 21 can be utilized
in the present invention because the pad conditioning apparatus 30 of the
present invention adequately conditions the pad surface during the
planarization process.
During polishing operations, carrier 23 typically rotates at approximately
40 rpms in a circular motion relative to table 20. This rotational motion
is easily provided by coupling an ordinary motor to shaft 22. In the
currently preferred embodiment, table 20 also rotates at approximately 15
rpms in the same direction relative to the movement of the substrate.
Again, the rotation of table 20 is achieved by well-known mechanical
means. As table 20 and carrier 23 are rotated, a silica based solution
(frequently referred to as "slurry") is dispensed or pumped through pipe
28 onto the upper surface of pad 21. Currently, a slurry known as SC3010,
which is manufactured by Cabot Inc. is utilized. In the polishing process
the slurry particles become embedded in the upper surface of pad 21. The
relative rotational movements of carrier 23 and table 20 then facilitates
the polishing of the thin film. Abrasive polishing continues in this
manner until a highly planar upper surface is produced and the desired
thickness reached.
FIG. 5a is a cross sectional view of diamond holder block 36 of the pad
conditioner apparatus 30. The diamond block holder 36 is made of stainless
steel. The block holder 36 has a substantially planar bottom surface 37.
The bottom surface 37 has two silicon carbide wear plates 39 recessed
within holder 36 and flush with bottom surface 37. The silicon carbide
wear plates 39 prevent diamond holder block 36 from becoming worn out
during continuous polishing. Embedded within holder 36 are a plurality of
stainless steel threaded shanks 38. The tops of the threaded shanks 38 are
accessible at top surface 42 of the holder 36. In this way the length at
which diamond tips 44 of the threaded shanks 38 protrude from surface 37
can be easily controlled. In the preferred embodiment of the present
invention the diamond tips 44 protrude about 40 microns from surface 37.
FIG. 5b is a view of the bottom surface 37 of the holder 36. Five diamond
tipped threaded shanks are shown arranged in the preferred pattern. Four
of the five shanks 38a, 38b, 38c, and 38d are arranged in a parallelogram
configuration around a center axis 40 of bottom surface 37. The shanks
38a, 38b, 38c, and 38d are separated from one another by a distance of
approximately 0.15 inches. The fifth shank 38e is placed on the center
axis 40 about an inch from shank 38d. Although the exact number and
placing of the shanks need not be as shown, and in fact can be quite
arbitrary, the present number and placing works well in providing adequate
spacing and arrangement of microchannels 50 in pad 21. The microchannels
50 provided by such arrangement and number provide adequate roughing of
pad 21 in order to continually channel slurry beneath the wafer without
providing undue wear on pad 21.
FIG. 5c is a detail of the diamond tipped stainless steel threaded shank 38
used in the present invention. The shank 38 in the preferred embodiment is
approximately 0.4 inches long and has a diameter of about 1/8 inch. The
shank is made of stainless steel. The shank 40 has a cone shaped base 42
of about 0.05 inches. A grade A or AA diamond tip 44 without cracks or
major flaws is welded onto base 42 of shank 38. The point of diamond tip
44 is ground to a 90.degree. angle. The shank 38 is threaded so that the
length at which shank 38 protrudes from holder 36 may be variably
controlled and so that shank 38 can be securely fastened within holder 36.
The diamond tipped threaded shank 38 of the present invention is
manufactured by makers of diamond tools with well know techniques.
Referring back to FIG. 4, in order to polish wafers and thereby smooth the
thin film layer, table 20 and pad 21 rotate in a clockwise direction as
does quill 23. As wafers are polished the conditioning assembly 30
oscillates so that diamond holder block 36 sweeps back and forth across
the previously provided macrogrooves 47 with a fixed downward pressure.
The diamond tips 44 of the shanks 38 located in holder 36 generate
microchannel grooves 50 into pad 21 and thereby condition pad 21 for
maximum slurry transport. In the preferred embodiment the microgrooves 50
are radial in direction and extend the entire distance across the
macrochannelled grooved path area 42. The diamond holder block makes
approximately 3.5 cycles (sweeps back and forth) per revolution of pad 21.
The rate is chosen to adequately condition pad 21 for optimal slurry
transport but yet not to overly degrade pad 21. Additionally, a fractional
number of cycles is chosen so that diamond holder block 36 does not
continually condition the same area of pad 21 time after time. In this
way, over time the entire grooved path area 42 is uniformly conditioned
with microchannels.
The holder 36 is swept across pad 21 by means of an oscillating motor
coupled to conditioner arm 32 at pivot point 52. The motor in the
preferred embodiment is a variable-speed oscillating motor. A
variable-speed motor allows holder 36 to move across different radii of
pad 21 at different rates. This allows holder 36 to spend more time at
certain radii of pad 21 than at other radii, thereby conditioning specific
radii of pad 21 more than other radii. This is useful when pad 21 wears at
specific radii more than at other radii. In this way pad conditioner
assembly 30 can spend more time conditioning those areas of pad 21 which
become worn down or smoothed quicker that other areas of pad 21. The
variable speed motor also allows pad conditioner assembly 30 to operate
synchronously with different table 20 rotation rates.
FIG. 6 is a cross-sectional view of pad 21. The one time provided preformed
macrogrooves 47 are shown having a triangular shape and a depth of
approximately 300 microns. It is to be appreciated that although the
macrogrooves 47 characteristically have a triangular cross-sectional
shape, other shapes such as U's and sawtoothed can be used as well. The
microgrooves 50 generated by the diamond tips 44 of shanks 38 during wafer
planarization are shown having a triangular shape with a depth of about 40
microns and a spacing of approximately 0.15 inches. Although the
microgrooves 50 are generated radially in the preferred embodiment, it is
to be appreciated that other directions may also be used. The radial
direction of microgrooves 50 is preferred because it aids in the delivery
of slurry into the preformed macrogrooves 47. What is most important,
however, is to continually form microgrooves 50 which adequately and
continually condition pad 21 during wafer planarization so that slurry can
be readily and continually supplied between the wafer being planarized and
pad 21.
The pad conditioner assembly 30 continually conditions pad 21 with
microgrooves 50 as wafers are being planarized. The continual generation
of microgrooves 50 increases and stabilizes the wafer polishing rate. In
the present invention a dielectric layer of a wafer is removed at a rate
of approximately 2,500 .ANG. per minute. It is to be appreciated that this
is a fast rate allowing for good wafer throughput. More importantly, with
the apparatus of the present invention the polish rate remains stable from
wafer to wafer, making the present invention much more manufacturable than
earlier techniques. Because pad 21 is continually conditioned with
microchannel grooves 50, a continual and consistent flow of slurry is
delivered between the wafer being planarized and pad 21. In the earlier
method, the one time generated macrogrooves 47 become "smooth" or "glazed"
over time, resulting in a decrease in slurry delivery over time which
causes a slow and unstable polishing rate. Additionally, in the present
invention the polish rate is not dependant upon the type of wafers being
polished. That is, wafers with rough surfaces (i.e. with bumpy surfaces or
with laser scribe marks) have substantially the same polish rates as do
smooth wafers. This is because in the present invention all wafers receive
substantially the same amount of slurry delivery due to the continual
conditioning of pad 21 by the pad conditioning assembly 30. The polishing
rate of the polishing apparatus of the present invention is essentially
wafer independent, making the polishing apparatus of the present invention
much more reliable and manufacturable than previous designs.
Thus, an apparatus and method for planarizing a thin film of a
semiconductor device has been described. The apparatus continually
generates microgrooves into a polishing pad surface while wafers are
polished. The generated microgrooves provide a consistent supply of slurry
between wafers and the polishing pad, resulting in a high, stable, and
wafer independent polish rate.
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