Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS TO CONTINUOUSLY FORM
A UNIFORM SHEET FOR USE
AS A SEMICONDUCTOR POLISHING PAD
Cross Reference to Related Applications
The present application claims the benefit of the filing date of U.S.
Provisional
Application Ser. No. 60/527,507, filed December 5, 2003, the teachings of
which are
incorporated herein by reference.
l0 Field of the Invention
The present invention relates to the field of making polishing pads used in
polishing
and planarization, and especially in chemical-mechanical planarization (CMP)
of electronic
devices such as integrated circuits, semiconductors, hard disks, magnetic
recording heads and
silicon wafers, etc., and more specifically to a continuous process for
producing a highly
is efficient polishing pad of uniform thickness.
Bacl~round of the Invention
For many years, precision optics, quartz crystals, ceramics, metallographic
alloys,
silicon wafers, hard disks and integrated circuits, etc. have been polished by
2o chemical-mechanical means. More recently, this technique has been applied
as a means of
planarizing intermetal dielectric layers of silicon dioxide and for removing
portions of
conductive layers within integrated circuit devices as they are fabricated on
various substrates.
For example, a conformal layer of silicon dioxide may cover a metal
interconnect such that the
upper surface of the layer is characterized by a series of non-planar steps
corresponding in
25 height and width to the underlying metal interconnects. The non-planar
topography of the
upper surface is then flattened or planarized by means of chemical mechanical
planarization
(CMP) enabling subsequent layers of dielectric and interconnecting wires to be
added.
The rapid advances in semiconductor technology has seen the advent of very
large scale
integration (VLSI) and ultra large scale integration (ULSI) circuits resulting
in the packing of
3o very many more devices in smaller areas in a semiconductor substrate. The
greater device
densities combined with multiple layers of interconnect circuitry require
greater degrees of
topographic planarity to maintain the depth of focus of the photo-lithographic
processes in the
manufacture of advance integrated circuits. Moreover, copper, because of its
low resistance, is
increasingly being used as interconnects. Conventionally, etching techniques
are used to
35 planarize conductive (metals) and insulator surfaces. However, certain
metals, desirable for
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their conductive properties when used as interconnects (e.g. Au, Ag, and Cu)
are not readily
amenable to etching due to the rapid build up of complex xxide by-products,
therefore
chemical-mechanical polishing (CMP) is employed to remove these by-products
from the
polished surface.
s Typically, the various metal interconnects are formed through lithographic
or
damascene processes. For example, in a lithographic process, a first blanket
metal layer is
deposited on a first insulating layer, following which electrical lines are
formed by subtractive
etching through a first mask. A second layer is placed over the first
metalized layer, and holes
are patterned into the second insulating layer using a second mask. Metal
columns or plugs are
1o formed by filling the holes with metal. A second blanket metal layer is
formed over the second
insulating layer, the plugs electrically connecting the first and second metal
layers. The second
metal layer is masked and etched to form a second set of electrical lines.
This process is
repeated as required to generate the desired device. The damascene technique
is described in
Untied Stated Patent No. 4,789,648, to Chow, et al.
15 Presently, VLSI uses aluminum for the wiring and tungsten for the plugs
because of
their susceptibility to etching. However, the resistively of copper is
superior to either
aluminum or tungsten, making its use desirable, however copper does not have
desirable
properties with respect to etching.
Variations in the height of the upper surface of the intermetal dielectric
layer have
2o several undesirable characteristics. The depth of focus of subsequent
photolithographic
processing steps may be degraded by non-planar dielectric surfaces. Loss of
the depth of focus
lowers the resolution at which lines may be printed. Moreover, where the step
height is large,
the coverage of a second layer over the dielectric layer may be incomplete,
leading to open
circuits.
2s In view of these problems, methods have been evolved to planarize the upper
surface of
the metal and dielectric layers. One such technique is chemical-mechanical
planarization or
polishing (CMP) using an abrasive polishing agent worked by a rotating pad. A
chemical-mechanical polishing method is described in United States Patent No.
4,944,836, to
Beyer, et al. Conventional polishing pads are made of a relatively soft and
flexible material,
3o such as non-woven fibers interconnected together by a relatively small
amount of polyurethane
adhesive binder, or may comprise laminated layers with variations of physical
properties
throughout the thickness of the pad. Multilayer pads generally have a flexible
top polishing
layer backed by a layer of stiffer material.
Polishing pads may also be made of a uniform material such as a polyurethane
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composition which is typically formed by placing reactive precursors in a
closed mold and
allowing the precursors to react and cure to form the pad material.
Subsequently, the pad
material may be die-cut to size and shape and the top surface conditioned to
form the polishing
pad. Alternatively, the reactive precursors may be placed in a cylindrical
container to form a
s log or loaf from which slices may be cut that may be subsequently used as a
polishing pad. The
formed polishing pads may also be further modified by annealing, pressing,
embossing, casting,
sintering or photolithographic processes.
The aforementioned processes are typically batch processes where one pad or
sheet of
material is produced in step function followed by another pad. This method
usually results in
t0 pad to pad or sheet to sheet variability of both physico-chemical and/or
morphological
properties and dimensions. Variability in these properties and/or dimensions
of the polishing
pads leads to non-uniformity and defects in polishing, and CMP of electronic
devices in
particular.
Some processes for producing polishing pads have been disclosed in the art.
United
is States Patent Nos. 6,126,532; 6,117,000 and 6,062,968, all assigned to
Cabot Corporation,
disclose pad manufacture using a belt line sintering method. The patents
define the
thermoplastic sintering method as one "that applies minimal or no pressure
beyond
atmospheric pressure to achieve the desired pore size, porosity and density
and thickness of the
substrate". Reference is made therein to United States Patent No. 3,835,212,
assigned to
2o Congoleum Industries, Inc. which discloses the use of an endless belt,
preheating of the belt,
depositing thermoplastic chips, compacting the chips on the belt and heating
the chips with a
heater above the belt as well as a heater positioned below the belt. This is
followed by
compressing the chips through a nip roller to form a sheet, heating the sheet,
passing it through
a calendar, and then "burnishing" the sheet via a nip roller and applying a
fluid cooling spray
25 and stripping.
United States Patent Application 09/995,025, filed November 27, 2001, is
entitled "Polishing Pad Comprising Particles With A Solid Core And Polymeric
Shell" and
relates to a pad which is described as having a solid core encapsulated by a
polymeric shell
wherein the two materials are different. The solid core is said to be an
abrasive material. The
3o application refers to a closed-mold sintering technique as well as
sintering via a continuous
process.
United States Patent No. 6,428,586, assigned to Rodel, is entitled "Method of
Manufacturing A Polymer Or Polymer Composite Polishing Pad". It is directed at
the
manufacture of a polishing pad for polishing semiconductor substrates, and
relies
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upon "transporting a continuous material forming a transported backing layer
to successive
manufacturing stations...supplying a fluid phase polymer composition onto the
transported
backing layer...shaping the polymer...and curing in a curing oven".
United States Application No. 09/766,155, also assigned to Rodel, is entitled
"Printing
Of Polishing Pad". The application states that "a pad manufacturing process is
needed that
forms a pad having a uniform surface, and in particular, a continuous process
is needed that
forms a sheet having a uniform surface". The application goes to offer a
solution to this
challenge and emphasizes the use of "flexible base sheet" that is continuously
printed upon by
passing between a cylinder (containing a pattern for printing on the pad) and
a roller.
to Each of these references disclose a rather complicated process lacking the
control of
critical process parameters to ensure optimal uniformity of product. What is
therefore needed
is a process for providing polishing pads which may be used for chemical-
mechanical
polishing which are manufactured on a continuous basis which through process
control
provides a pad having a uniform surface, uniform thickness and highly
efficient polishing
is characteristics.
Summary of the Invention
The present invention relates to a continuous process for making polishing
pads, which
are particularly useful for chemical-mechanical planarization (CMP). The
process of this
20 invention comprises a two step approach which utilizes a series of
networked process control
and feedback loops to ensure uniformity in sheet surface and thickness
resulting in efficient,
continuous production of a highly uniform polishing pad in both properties and
dimensions.
In a first step, one or more high viscosity liquid precursor polymers are
mixed with one
or more fillers under vacuum and controlled temperature, to yield a liquid
mixture having a
25 controlled viscosity and consistency. The liquid polymers contain
functional groups which are
chemically active. Such chemically active polymers (aka precursors) encompass
the realm of
monomers, oligomers, pre-polymers and high molecular weight polymers of
organic and
inorganic origin. In addition, various modifiers such as thickening or
thinning agents,
colorants, UV and heat stabilizers, surface tension modifiers (surfactants)
etc. may be added
3o into the mix. In a second step, a hardening or curing agent is dispersed
uniformity into the
polymer mix to chemically react with the liquid polymers. The amount of the
hardening or
curing agent is precisely controlled at a specified ratio to the polymer mix
to achieve the
desired product properties. In addition, the temperature and viscosity of the
reactant mixture as
well as the ambient pressure and atmosphere are precisely controlled to a pre-
determined
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specification. Typically, a high vacuum no less than 28 in. of water may be
applied during the
entire mixing process, and nitrogen gas may be used to 'blanket' the mix if it
has to be stored
for an extended period of time.
The reactive mix may then be dispensed into a preset gap space formed between
a
vertically stacked pair of endless steel belts with heaters on the back of one
or both belts,
accompanied by feedback control of such parameters as conveyor speed, belt
pressure,
temperature and belt gap to form a uniform sheet. Subsequently, the sheet may
be cut to length,
post cured in an oven and die cut to form the polishing pads of the present
invention.
It is therefore an object of the present invention to provide a continuous
process
to comprising a series of in-line automatic and/or manual networked process
controls and testing
and inspection metrology equipment, in order to form uniform polishing pads
particularly for
Chemical-Mechanical Planarization.
It is a further object of the present invention to provide an apparatus for
use in a
continuous process comprising a series of networked process controls to form a
uniform
polishing pad, particularly for chemical-mechanical planarization.
It is still a further object of the present invention to provide polishing
pads with a high
degree of uniformity within each pad and from pad to pad, particularly for
chemical-mechanical planarization, which are produced by a continuous process
comprising a
series of process controls to ensure product uniformity.
2o It is still a further object of the present invention to provide an
automated two-step
process for providing polishing pads of high uniformity.
It is also an object ofthe present invention to produce a polishing pad where
all raw and
in-process materials are maintained in a well controlled enclosed environment
to avoid the
inadvertent contamination by foreign substances which may be detrimental to
the final
polishing application.
Still other objects and advantages of the present invention will become
readily apparent
to those skilled in the art from the following detailed description, wherein
it is shown and
described preferred embodiments of the invention, simply by way of
illustration of the best
mode contemplated of carrying out the invention. As will be realized the
invention is capable
of other and different embodiments, and its several details are capable of
modification in
various obvious respects, without departing from the invention. Accordingly,
the description is
to be regarded as illustrative in nature and not as restrictive.
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Brief Description of the Drawings
The features, operation and advantages of the invention may be better
understood from
the following detailed description of the preferred embodiments taken in
conjunction with the
attached drawings, in which
FIG. 1 is a block diagram of the general features of the continuous process of
the
present invention.
FIG. 2 is a simplified schematic view of the apparatus for continuous
manufacture of a
polishing pad of the present invention.
Description of Preferred Embodiments
The present invention is directed at a high volume essentially continuous
manufacturing process for forming polishing pads to be used for chemical-
mechanical
polishing. One key to the process are numerous networked process controls
which are used to
monitor and feed back information in real time as the product is being
produced. This provides
Is extremely consistent sheet material in terms of composition, properties,
thickness and surface
uniformity, resulting in polishing pads which provide a high degree of
polishing consistency
and efficiency.
As diagrammed in FIG. 1, the manufacturing process of the present invention
comprises two basic steps: Step A, the compounding of the preferable high
viscosity liquid
2o polymer precursor and Step B, the continuous dispensing and sheet formation
of the liquid
polymer precursor mixed with a hardening agent. Subsequently, the sheet
material may be cut
to length, postcured to establish optimal properties and cut to shape to form
polishing pads.
In step A (see FIG. 1), the compounding step, a highly viscous mixture of
liquid
polymer, preferably polyurethane is first equilibrated under vacuum and
elevated temperature
25 for several hours. Examples of such sources of polymers are the Adiprene
Polyurethane
Pre-polymer offered by the Crompton Corporation, and the Airthane Polyurethane
Pre-polymer from Air Products, respectively. Any required additives such as
antifoaming
agent, UV and heat stabilizers, surface tension modifiers, etc. are also
blended uniformity into
the polymer mix. Any required organic or inorganic filler materials, soluble
or insoluble, and
3o in various particulate configurations and sizes are then added preferably
under vacuum or a
blanket of nitrogen gas and dispersed uniformity within the mix. Again the
temperature,
viscosity of the entire mixture, the vacuum or ambient nitrogen (in the
headspace of the mixing
vessel) are kept precisely controlled throughout the mixing, storing and
subsequent operations.
Furthermore, the amounts and relative ratios of all components in the entire
mixture are
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precisely measured and controlled.
As noted, a filler component may also optionally be included, preferably the
hollow
Expancel~ microspheres from Expancel, Inc. having diameters from 20 to 90
microns, at a
level of about 1-5 wt. percent, and preferably at 3 wt. percent of the
formulation. The dry filler
component preferably has a specific weight of from about 0.03 to about 0.12
grams per cubic
centimeter. One purpose of the dry filler component is to provide a porous
surface on the
polishing pad after it is conditioned for use. The conditioning of the surface
of the pad by
abrading its top surface removes a thin layer of reacted polymer and breaks
the microspheres at
the surface providing a controlled level of porosity. Alternatively, other
materials may be used
to as the filler component to provide specific abrasive or porous properties
of the polishing pad,
and include, but are not limited to, water soluble fibers and soluble products
such as salts that
may be washed out after surface buffing to create a porous surface,
particulate or powder
polymers, etc.
The homogenous liquid precursor compounded in Step A, preferably has a
specific
weight from about 0.50 to about 1.2 grams per cubic centimeter and is stored
for use in the
continuous process phase (Step B) at 25 - 40 degrees C. under vacuum or
nitrogen and constant
but gentle blending action.
Turning now to the step of dispensing and the sheet forming continuous
process, Step B
is shown in simplified schematic representation in FIG. 2. The liquid polymer
precursor
comprising, preferably, a blend of polyurethane pre-polymers with an
additives) and filler
components dispersed therein, can be pumped or gravity fed to a continuously
replenished feed
tank 10 which is preferably maintained under vacuum conditions or inert gas
(e.g. nitrogen)
and preferably at about 20 - 40 degrees C. A hardening or curing agent, for
example (MOCA
or Ethacure), can be stored in an ambient or nitrogen blanketed tank 20. For
reacting, the
curing agent and the liquid polymer precursor are precisely metered by
Coriolis mass flow
regulations 12, 22 and pumped into, preferably, a continuous static or a
dynamic mixer 30. The
stoichiometric ratio of the curing agent to the polyurethane precursor is
preferably between
0.85 to 1.05. This ratio and set of processing conditions preferably provides
a viscosity of the
mixture in the range of 20,000 - 400,000 Pascal seconds.
3o Dynamic or mechanical mixers may be used providing that minimal air is
introduced.
Upon exiting the mixer, the reacting composition 32 is uniformly fed into the
feed end 42 of a
double steel belt press 40. The feed end of the double belt press as well as
the double belt press
itself is preferably kept under an atmospherically controlled chamber 60 at
uniform
temperature and pressure prior to the compressed heated zones and to ensure a
more efficient
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polymerization during heating.
The double belt press 40 will generally comprise two endless steel belts
driven over
rollers revolving in opposing directions, with mutually facing outer sides
thereof. pressed
against material passed there between. Heating can be accomplished by means of
one or more
temperature controlled press plates, pressure chambers or IR heaters at the
back sides of the
compressed areas. The steel belts which form the top and bottom surface of the
continuously
molded urethane sheet can be heated and maintained preferably at 80-110
degrees C to provide
polymerization of the urethane into a solid compound under isobaric
conditions. The pressing
zone 44 is formed between mutually opposing outer sides of the pressing belts.
1o The endless belts are preferably of stainless steel and can be maintained
at a precisely
controlled pressure level preferably about 4 MPa (S50 - 600 psi) although in
the broad context
of the present invention, the pressure may range from 1-10 MPa (145 psi - 1450
psi). The
pressure may be generated by the weight of the upper unit plates of the
endless belt and/or by
mechanical or hydraulic means. As the molded sheet moves between the opposing
steel belts,
the sheet is compressed, over a distance of preferably about 4.5 meters at 44,
to its final
thickness and solidified by the heat provided from the heated belts. In the
preferred
embodiment of the present invention, the curing process can be initiated by
the exothermic heat
of reaction of the urethane and amine components and by the heat from the
endless belts.
The process to form a continuous uniform sheet may be fully PLC controlled
with
2o interactive information exchange between mechanical components, each having
built-in tools
for process control, data trending and process enhancements. All liquid as
well as solid raw
material components are monitored as to precise temperature and viscosity, and
dosed for
compounding and mixing. The system preferably uses a fast communication
protocol
-PROFIBUS- to monitor and control all system components. The system is set up
for easy
operator control and interface, via touch screens, to monitor and adjust
process parameters. An
in-line, real-time testing and inspection metrology equipment for monitoring
material
properties such as thickness, density, hardness, compressibility and surface
roughness, as well
as inspection with an imaging system for contaminants, can be fully
integrated. The system
also incorporates online continuous data storage, statistical analysis,
trending and CRT display
3o as well as operator alert on process excursions.
The belt press 40 can produce a continuous urethane sheet 32A having a uniform
density of preferably less than 1.0 g/cc due to the process controls in use.
Density variation
measured on a square yard is less than 2.0 percent. Thickness of the sheet is
controlled by
micrometer adjustment at 50 to +/- O.OSmm. The nominal thickness of the sheet
32A formed as
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described above, may preferably be between lmm to 3mm or higher.
The resulting urethane sheet 32A is discharged from the belt press 40 at the
takeoff end
and is cut to length using preferably a roller blade cutter at 62. Other types
of cutters may also
be used. The cut to length sheets 32B are next stacked at 70 and transferred
to an oven 80 for
post curing for preferably 16-24 hours at a temperature specified for a given
type of urethane
and curing agent, said temperature typically varies from 150 to 450 degree F.
The cured urethane sheets can be subsequently cooled to ambient temperature
under
controlled cooling rates, tested and inspected again, and buffed or napped to
remove any
surface polymeric 'skin' to expose the bulk structure of the sheet. The
buffing or napping is
1o also designed and controlled to impart a pre-determined micro texture to
the urethane sheet
surface which reduces the 'break-in' time in end-use polishing applications.
While the preferred material composition to continuously produce polishing
pads of the
present invention is a blend of liquids with filler(s), other material
precursor compositions may
also be processed through the apparatus of the present invention, including,
but not limited to,
dry polymers, powders and other solid compounds and fillers, both soluble and
insoluble. The
incorporation of these materials may require corresponding change in
processing temperature
and other parameter.
Table I lists some properties of the polishing pads contemplated and targeted
for
manufacture by the continuous process of the present invention.
TABLE I
Thickness: 10 - 130 mils; preferably 50 - 100 mils
Density: 0.3 - 1.2 g/cc; preferably 0.5 - 0.9 g/cc
Pore Size distribution: 20 - 100 micrometers; preferably 20 - 60 micrometers
Pore or Void Volume: 15 - 60 % of total volume; preferably 20 - 40 % of total
volume
Hardness: 30 - 80 Durometer Shore D; preferably 45 - 75 Durometer Shore D
Compressibility: 0 - 10 %; preferably 0 - 4
Glass Transition (Softening) Temperature (Tg): 50 - 200 degree C
3o Surface Roughness, Ra: 0 - 10 micrometers
Abrasion Resistance: Ranking > 2 (for Tabor Abrasion Tester)
Compression Modulus: 1 - 8 MPa; preferably 3 - 5 MPa
Deflection from lay flat: edge and bulk waviness < 0.5"; preferably < 0.1 "*
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*It should be noted that the values reported for such deflection from lay flat
are measured over
a 1 meter by 1 meter area of the pad. Accordingly, when the edge and bulk
waviness are < 0.5"
it corresponds to a 1 meter by 1 meter area of the pad that does not indicate,
when laying flat, an
upward deflection or wave in the pad that exceeds 0.5", and is preferably <
0.1 ".
Due to the scale and nature of this continuous process, variations in the
composition
and properties of the finished product will be minimized. The raw materials
can be blended in
large quantities to provide a recirculating feed system that continuously
supplies the mixer.
These materials can therefore be blended under precise process control of
temperature, vacuum,
ratio and viscosity. The continuous nature of the sheet forming process
enables precise control
to for producing a uniform sheet as process controls monitor and feed back
data regarding feed
temperature, belt temperature, pressure, feed rate, conveyor speed, sheet
thickness and density.
From the sheets, large quantities of polishing pads having little variability
can be cut.
The foregoing embodiments have been described merely as examples of the
invention
and are not intended to limit its scope. Since modification of the described
embodiments may
is occur to persons skilled in the art, the scope of the invention is intended
to cover all such
modifications which come within the true spirit and full scope of the
invention.
