U.S. patent application number 10/924833 was filed with the patent office on 2006-05-11 for method of chemical mechanical polishing, and a pad provided therefore.
Invention is credited to Peter Renteln.
Application Number | 20060099891 10/924833 |
Document ID | / |
Family ID | 36316936 |
Filed Date | 2006-05-11 |
United States Patent
Application |
20060099891 |
Kind Code |
A1 |
Renteln; Peter |
May 11, 2006 |
Method of chemical mechanical polishing, and a pad provided
therefore
Abstract
In a chemical mechanical polishing for removing a barrier and
subsequent buffing a polyurethane polishing pad is used which is
composed of for removal a barrier and buffing after a bulk copper
removal to the barrier in a chemical mechanical polishing of a
copper film deposited on a surface of a semiconductor wafer, at
least one layer of the polishing pad is made from a mix composed of
the prepolymer with an isocyanate concentration of between
substantially 6.5% and 11.0% or from a monomer and an addition of
isocyanate required to achieve a same molal concentration, with a
shore D hardness less than substantially 35%.
Inventors: |
Renteln; Peter; (San Ramon,
CA) |
Correspondence
Address: |
SNELL & WILMER;ONE ARIZONA CENTER
400 EAST VAN BUREN
PHOENIX
AZ
850040001
US
|
Family ID: |
36316936 |
Appl. No.: |
10/924833 |
Filed: |
November 9, 2004 |
Current U.S.
Class: |
451/57 |
Current CPC
Class: |
B24B 37/24 20130101 |
Class at
Publication: |
451/057 |
International
Class: |
B24B 1/00 20060101
B24B001/00 |
Claims
1. A method of chemical mechanical polishing of a copper film
deposited on a surface of a semiconductor wafer, comprising the
steps of removing a bulk copper to a barrier; removing the barrier;
and subsequently buffing, wherein said barrier removal and buffing
including using a polyurethane polishing pad at least one layer of
which is made of a mix composed of a prepolymer or a monomer and an
addition of an isocyanate at a concentration of between
substantially 6.5% and 11.0% or from a monomer and an addition of
isocyanate required to achieve a same molal concentration, in which
a shore D hardness is less than substantially 35%.
2. A method as defined in claim 1, wherein the pad has an additive
including a methyl alcohol.
3. A method as defined in claim 1, wherein the pad has an additive
which includes water.
4. A method as defined in claim 1, wherein the pad has an additive
which includes starch.
5. A method as defined in claim 1, wherein the pad has a thickness
between 10 mils and 200 mils.
6. A method as defined in claim 5, wherein the pad has a thickness
of between 40 and 100 mils.
7. A method as defined in claim 1, wherein the pad is a single
open-layered pad.
8. A method as defined in claim 1, wherein the pad is stacked on
and attached to a sub pad.
9. A method as defined in claim 1, wherein said is a pad selected
from the group consisting of a groove pad and an ungrooved pad.
10. A method as defined in claim 1, wherein said pad is a pad
selected from the group consisting of a perforated pad and an
unperforated pad.
11. A method as defined in claim 1, wherein said layer is composed
of said polyurethane selected from the group consisting of a
polyether polyurethane and polyester polyurethane.
12. A method as defined in claim 1; and further comprising
introducing in said layer abrasive particles selected from the
group consisting of silica, alumina, ceria, titania, diamond and
silicon carbide.
13. A method as defined in claim 1; and further comprising
introducing a filler into said mix.
14. A polishing pad for removal of a barrier and buffing after a
bulk copper removal to the barrier in a chemical mechanical
polishing of a copper film deposited on a surface of a
semiconductor wafer, the polishing pad having at least one layer
made of a mix is composed of a prepolymer with an isocyanate
concentration of between substantially 6.5% and 11.0% or from a
monomer and an addition of isocyanate required to achieve a same
molal concentration, with a shore D hardness less than
substantially 35%.
15. A polishing pad as defined in claim 14, wherein the pad has an
additive including a methyl alcohol.
16. A polishing pad as defined in claim 14, wherein the pad has an
additive which includes water.
17. A polishing pad as defined in claim 14, wherein the pad has an
additive which includes starch.
18. A polishing pad as defined in claim 14, wherein the pad has a
thickness between 10 mils and 200 mils.
19. A polishing pad as defined in claim 14, wherein the pad has a
thickness of between 40 and 100 mils.
20. A polishing pad as defined in claim 17, wherein the pad is a
single open-layered pad.
21. A polishing pad as defined in claim 14, wherein the pad is
stacked on and attached to a sub pad.
22. A polishing pad as defined in claim 14, wherein said is a pad
selected from the group consisting of a groove pad and an ungrooved
pad.
23. A polishing pad defined in claim 14, wherein said pad is a pad
selected from the group consisting of a perforated pad and an
unperforated pad.
24. A polishing pad as defined in claim 14, wherein said layer is
composed of said polyurethane selected from the group consisting of
a polyether polyurethane and polyester polyurethane.
25. A polishing pad as defined in claim 14; and further comprising
in said layer, abrasive particles selected from the group
consisting of fssilica, alumina, ceria, titania, diamond and
silicon carbide.
26. A polishing pad as defined in claim 14; and further comprising
a filler introduced into said mix.
27. A method of chemical mechanical polishing of a copper film
deposited on a surface of a semiconductor wafer, comprising the
steps of removing bulk copper to a barrier; removing the barrier;
and subsequently buffing, wherein said barrier removal and buffing
include using a polyurethane polishing pad at least one layer of
which is made from a mix composed of a prepolymer with an
isocyanate concentration of between substantially 6.5% and 8.5% or
from a monomer and an addition of isocyanate required to achieve a
same molal concentration, in which a shore D hardness is less than
substantially 35%.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method of chemical
mechanical polishing of semiconductor wafers, and to a polishing
pad for performing the same.
[0002] Polishing of a semiconductor substrate for the purpose of
planarization of the wafer surface is known in the art of
semiconductor processing, and is referred to as CMP, or
Chemical-Mechanical Planarization. A specific application of CMP is
known as just "copper CMP", which entails the polishing of a copper
film deposited on the work surface of a semiconductor wafer. The
copper is typically deposited through the use of electrodeposition,
and is typically deposited on a surface in which submicron channels
in the underlying oxide layer have been created. The copper fills
the channels as well as on top of the areas between channels (the
"field"). The copper above the areas between channels is often
called the "overburden". Since copper would otherwise diffuse into
the oxide layer and potentially reach the sensitive silicon
substrate below, destroying the transistors, a "barrier layer" must
be made available on the oxide surface before depositing the
copper. Typical materials for the barrier layer are tantalum (Ta)
and tantalum nitride (TaN). During copper CMP, both the copper
overburden and the barrier layer must be polished away, leaving the
copper in the channels to act as conducting interconnecting lines.
The barrier then clads the copper channels, preventing diffusion of
the copper into the oxide.
[0003] The objective of the CMP process is to remove all the copper
from the field area and leave the copper in the trenches at the
level of the oxide. The surface of the copper must be left in
pristine condition; it must be absolutely flat and without
oxidation, contamination or corrosion on its surface. This must be
the case for larger areas of copper as well, including areas known
as "bond pads", which are used to allow probes to make contact with
the conductor, and are typically 100 um by 50-75 um in dimension.
The process is usually performed in three steps, involving the
three rotating platens normally found on commercially available CMP
tools. The steps include: 1) Removal of the bulk copper; 2) Removal
of the barrier layer; and 3) A buff of the final surface to remove
all residue and passivate the copper surface. After these three
steps the wafer is cleaned and dried and is ready for further
processing. Often, step 1, the removal of the copper, is divided
into two steps. In the first step, half or more of the copper film
is removed leaving the surface as planar as possible. In the second
step, the remainder of the copper is removed, leaving only barrier
material.
[0004] There are, therefore, two primary process sequences used on
a three-platen tool during copper CMP. They are either (Sequence
#1): Platen 1) bulk copper removal to barrier, Platen 2) barrier
removal and Platen #3) buff; or (Sequence #2), Platen 1) incomplete
bulk copper removal, Platen 2) final copper removal, and Platen 3)
barrier removal and buff. There also exist small variations to
these two sequences that could include variations in the
chemistries and slurries used, the processing conditions and times,
and the exact division of all the required steps. The polishing
pads and slurries/chemistries used in these three steps are
generally different. At this point, a typical way to carry out
process sequence #1 could be to use the following: Platen 1) An
IC1000 pad (such as supplied by Rohm and Haas) and a copper slurry
such as Cabot 5001; Platen 2) An IC1000 pad and a barrier slurry
such as Hitachi T-605; and Platen 3) A Politex pad (such as
supplied by Rohm and Haas) and water or water and a passifying
chemical such as BTA. Conversely, a typical way to carry out
process sequence #2 could be to use the following: Platen 1) An
IC1000 pad and a copper slurry such as Cabot 5001; Platen 2) An
IC1000 pad and a copper slurry such as Cabot 5001; Platen 3) A
Politex pad and a barrier slurry such as Hitachi T-605, followed by
a rinse with water or water and BTA for passivation.
[0005] Step 3 in the last case could also be done with an IC1000
pad, but is much more typically done with a Politex pad. The
disadvantage of using the Politex for this purpose is that the pad
is basically too soft for the application. A typical Politex pad is
composed of several layers. The bottom layer is a polyurethane
impregnated polyester felt. The next layer is a very porous
urethane, and the top layer consists of vertical urethane
structures having tapered pores which are wider on the bottom and
narrower on top.
[0006] Although the softness of the pad is advantageous in
delivering a very nice final surface (buff), both clean and
scratch-free, the net result of using this pad for the barrier
removal step is that the resulting planarity of the wafer is poor.
As a barrier remover, the pad is exposed initially to both copper
and tantalum simultaneously and later when the barrier is cleared,
to oxide, copper and tantalum simultaneously. A pad with poor
planarizing properties will do a poor job of removing these
materials uniformly. This nonplanarity can result in bridging (a
conductive material remaining where it is supposed to be removed
resulting in undesired electrical contact), dishing/erosion
(material being removed by the process that should remain in place
causing an increase in resistivity if it is copper or shorts if it
is oxide), or undesirable surface features which encourage
bridging.
[0007] Another issue with the use of the Politex pad is the
relatively low lifetime of the pad. In the presence of certain
chemicals, the pad will tend to lose integrity, resulting in
shortened lifetime. In general, the blown polyurethane material is
chemically inert, and is not known to disintegrate in the presence
of any of the chemicals it is normally exposed to in CMP.
[0008] The alternative case, using the highly planarizing, highly
inert, IC1000 on the third platen will result in a very planar
surface without the problems listed above, but with a much higher
rate of scratching. Scratching, of course, is unacceptable as it
can result in shorts (bridging) and opens (a conductive line
interrupted).
[0009] In light of these issues sequence #1 would apparently
satisfies all requirements. However, it is very slow. It has a much
slower throughput (tpt), and thus the polishing equipment is poorly
utilized. Just like any assembly line, the tpt of a tool is a
function of its slowest step. Generally, the robots moving the
wafers from station to station on such tools (see, for example, the
Mira Mesa, supplied by Applied Materials) are fast. The slowest
step is typically the slowest of the platen steps. Since Sequence
#1 requires that all of the copper be removed at the first platen,
then that platen becomes the tpt limiter. It is not uncommon for
that first step to take 3 or 4 minutes, resulting in a maximum tpt
of the tool of 15-20 wafers per hour (wph). A higher tpt is
generally desired.
[0010] The required planarization is somewhere between that which
is offered by the Politex and that which is offered by the IC1000.
It would seem as though the ideal pad for this application is one
made of blown polyurethane, but such that it is significantly
softer than the IC1000.
SUMMARY OF THE INVENTION
[0011] Accordingly, it is an object of the present invention to
provide a method of chemical mechanical polishing and a polishing
pad, which are further improvements of the existing methods and
polishing pads.
[0012] In keeping with these objects and with others which will
become apparent hereinafter, one feature of the present invention
resides, briefly stated, in a method of chemical mechanical
polishing of a copper film deposited on a surface of a
semiconductor wafer, comprising the steps of removing a bulk copper
to a barrier; removing the barrier; and subsequently buffing,
wherein said barrier removing and buffing includes using a
polyurethane polishing pad at least one layer of which is
fabricated from a mix composed of a prepolymer with an isocyanate
concentration of between substantially 6.5% and 11.0% or from a
monomer and an addition of isocyanate required to achieve a same
molal concentration, in which a shore D hardness of the layer is
less than substantially 35%.
[0013] Another feature of the present invention resides, briefly
stated, in a polishing pad for removal of a barrier and subsequent
buffing in a chemical mechanical polishing of a copper film
deposited on a surface of a semiconductor wafer, the polishing pad
is fabricated from a mix composed of the polymer with an isocyanate
concentration of between substantially 6.5% and 11.0% or from a
monomer and an addition of isocyanate required to achieve a same
molal concentration, and has a shore D hardness less than
substantially 35%.
[0014] The inventive method is performed so that the polishing and
the inventive polishing pad has such material properties.
[0015] The novel features which are considered as characteristic
for the present invention are set forth in particular in the
appended claims. The invention itself, however, both as to its
construction and its method of operation, together with additional
objects and advantages thereof, will be best understood from the
following description of specific embodiments when read in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 of the drawings is a view showing a relationship
between a hardness of a pad and a number of scratches.
DESCRIPTION WITH THE PREFERRED EMBODIMENTS
[0017] The present invention resides in a method of chemical
mechanical polishing of a copper film deposited on a surface of a
semiconductor wafer, comprising the steps of removing the bulk
copper to a barrier; removing the barrier; and subsequently
buffing, wherein said barrier removal and buffing include using a
polyurethane polishing pad fabricated from a mix composed of a
prepolymer or a monomer and an addition isocyanate required to
achieve a same molal concentration, in which a shore D hardness is
less than substantially 35%. In a preferred embodiment, the
isocyanate concentration is between 6.5% and 8.5%, the smaller
value generally leading to a softer polishing pad.
[0018] The present invention also relates in a polyurethane
polishing pad for removal of a barrier and subsequent buffing in a
chemical mechanical polishing process of a copper film deposited on
a surface of a semiconductor wafer, the polishing pad fabricated
from a mix composed of prepolymer with an isocyanate concentration
of between substantially 6.5% and 11.0% or from a monomer and an
addition of isocyanate at a concentration required to achieve a
same molal concentration, with a shore D hardness less than
substantially 35%. In a preferred embodiment, the pad is made from
a polyurethane mix in which the isocyanate concentration is between
6.5% and 8.5%, resulting in pads on the softer side of the hardness
spectru.
[0019] In developing the inventive method and the inventive pad it
was determined that hardness is the best parameter to predict
scratching. This is illustrated in FIG. 1.
[0020] On the y-axis is number of scratches and on the x-axis is
Shore D hardness. The number of scratches comes from a test we
developed which was designed to elicit scratches in all cases, even
from the softest of pads. The test consisted of using a known
aggressive barrier slurry, which is virtually incapable of removing
copper, and using it to "polish"--at fairly high downforce (4
psi)--a pristine blanket copper film in which the number of
pre-polish scratches has already been determined. The copper wafer
is polished for 1 minute and then remeasured for scratches (using
dark-field microscopy at low magnification). The increase in the
number of scratches is then plotted:
[0021] The graph indicates the desirable hardness of the pad is
approximately under 35 on the Shore D scale. The Politex is
measured to be 25 on this scale by our measurements (indicated on
graph), and for reference, the IC1000 is measured to be 56 by our
measurements (indicated on graph) and 55 from information published
by Rohm and Haas Corporation.
[0022] The next objective to create the material is to determine
exactly what material or materials govern planarization. To this
end, we develop a planarization figure of merit, called GPE, or
General Planarization Efficiency. It was determined that GPE is a
function of the NCO (isocyanate) concentration of the base resin.
Resins that fell within the 6.5% to 11.0% range delivered high GPE
and resins outside that range resulted in low GPE. Therefore, a
resin from the range is proposed.
[0023] Finally, since in a blown polyurethane, the chemical
inertness property comes inherently with the use of this material,
it is selected from the inventive pad.
[0024] Therefore, in accordance with the present invention the
above described polyurethane polishing pad is proposed as well as
the above described method of removing barrier and buffing.
[0025] The polyurethane polishing pad can have an additive which
includes methyl alcohol, or water, or starch of any type including
corn starch. The pad can have a thickness of between 10 mils and
200 mils, preferably 80 mils.
[0026] The pad in accordance with the present invention can be
stacked on a subpad but a preferred embodiment is that the pad is a
single-layered pad.
[0027] The pad in accordance with the present invention can be
grooved or ungrooved, perforated or unperforated.
[0028] Herein below an example of how to manufacture the pad in
accordance with the present invention is presented.
[0029] First, as with all urethanes, start with either a prepolymer
resin with an NCO concentration of between 6.5% and 11.0%, or a
polyol and an amount of isocyanate required to achieve the same
molal concentration. Preheat the prepolymer to about 100 deg F., or
mix until said temperature is achieved. Start to add additives as
desired. These may include initiators such as water, blowing agents
such as Vazo, catalysts such as a tertiary amine, and surfactants
such as L-6100, a silicone surfactant typically used to regulate
cell size supplied by Crompton. An exothermic reaction begins once
an accelerant, an example of which accelerant is MOCA
(4,4'-Methylenebis-[o-chloroaniline]), also known as a chain
extender, is added. The pores are created either by the
incorporation of air during the mixing or by the intentional
addition of pore creators, such as microballoons as taught in
Reinhardt, U.S. Pat. No. 5,578,362. The mix is then pored into a
mold, which may be preheated. While the exothermic reaction is
sufficient to bring the temperature of the center of the cake to
some 270 degF and thus to a complete reaction, surface cooling
would prevent the outside of the cake from reacting in a reasonable
period of time. Therefore curing in an oven is required. This is
typically done at a temperature of about 240 deg F. for about 6
hours. The cake is then cooled and sliced to fabricate the
polishing pads.
[0030] After curing, the cake is allowed to cool. Once cool, it can
be sliced using equipment consisting of a stationary skiving blade
and movable table. Usable slices are then made into pads by
applying adhesive to one face, mounting a subpad if desired,
grooving the usable surface, finishing with a smoothing process,
inspecting and packaging.
[0031] If a pad can both planarize and pass the scratch-inducing
test, then it can also be used in applications where a buff alone
is needed. As such, it might have additional applications such as
post-oxide buff. Further, one could conceive of other non-CMP
simple cleaning applications commonly carried out in a
semiconductor fab environment.
[0032] Also, again since the pad planarizes, one could conceive of
using it for 2.sup.nd step copper, namely the step where the
remaining copper is removed down to the barrier. In this case,
since it is softer than the primary pad it won't have the same long
range planarization. However, if the first step copper is done with
the hard pad, the copper surface should be well planarized, making
the use of a slightly softer pad feasible. Another potential
application of this pad is therefore also primary copper CMP.
[0033] It will be understood that each of the elements described
above, or two or more together, may also find a useful application
in other types of constructions differing from the types described
above.
[0034] While the invention has been illustrated and described as
embodied in a method of chemical mechanical polishing, and a pad
provided therefore, it is not intended to be limited to the details
shown, since various modifications and structural changes may be
made without departing in any way from the spirit of the present
invention.
[0035] Without further analysis, the foregoing will so fully reveal
the gist of the present invention that others can, by applying
current knowledge, readily adapt it for various applications
without omitting features that, from the standpoint of prior art,
fairly constitute essential characteristics of the generic or
specific aspects of this invention.
[0036] What is claimed as new and desired to be protected by
Letters Patent is set forth in the appended claims.
* * * * *