U.S. patent application number 09/750593 was filed with the patent office on 2001-05-31 for chemical-mechanical polishing slurry that reduces wafer defects.
Invention is credited to Beckage, Peter J., Burke, Peter A..
Application Number | 20010002357 09/750593 |
Document ID | / |
Family ID | 25430786 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010002357 |
Kind Code |
A1 |
Burke, Peter A. ; et
al. |
May 31, 2001 |
Chemical-mechanical polishing slurry that reduces wafer defects
Abstract
A method of making a chemical-mechanical polishing slurry
includes mixing a ferric salt oxidizer with a solution to produce a
mixture with a dissolved ferric salt oxidizer, filtering the
mixture to remove most preexisting particles therein that exceed a
selected particle size, adding a suspension agent to the mixture,
and adding abrasive particles to the mixture after filtering the
mixture. Advantageously, when polishing occurs, scratching by the
preexisting particles is dramatically reduced.
Inventors: |
Burke, Peter A.; (Austin,
TX) ; Beckage, Peter J.; (Austin, TX) |
Correspondence
Address: |
David G. Dolezal
Skjerven Morrill MacPherson LLP
Suite 700
25 Metro Drive
San Jose
CA
95110
US
|
Family ID: |
25430786 |
Appl. No.: |
09/750593 |
Filed: |
December 28, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09750593 |
Dec 28, 2000 |
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09280391 |
Mar 29, 1999 |
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09280391 |
Mar 29, 1999 |
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08911744 |
Aug 15, 1997 |
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5934978 |
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Current U.S.
Class: |
451/41 ;
451/285 |
Current CPC
Class: |
C09G 1/02 20130101; C09K
3/1463 20130101 |
Class at
Publication: |
451/41 ;
451/285 |
International
Class: |
B24B 001/00; B24B
029/00 |
Claims
What is claimed is:
1. A method of making a chemical-mechanical polishing slurry,
comprising the following steps: mixing an oxidizer with a solution
to produce a mixture with a dissolved oxidizer; filtering the
mixture to remove a substantial amount of preexisting particles
therein; adding a suspension agent to the mixture; and adding
abrasive particles to the mixture after filtering the mixture.
2. The method of claim 1, wherein the oxidizer is a ferric salt
oxidizer.
3. The method of claim 1, wherein the abrasive particles are
selected from the group consisting of Al.sub.2O.sub.3, SiC,
SiO.sub.2, CeO.sub.2 and Si.sub.3N.sub.4.
4. The method of claim 1, wherein the suspension agent is an
aqueous surfactant that inhibits growth of the preexisting
particles.
5. The method of claim 1, wherein the preexisting particles include
reaction product particles formed in the mixture.
6. The method of claim 1, including adding the suspension agent to
the mixture before filtering the mixture.
7. The method of claim 1, including adding the suspension agent to
the mixture after filtering the mixture.
8. The method of claim 7, including adding the suspension agent to
the mixture immediately after filtering the mixture.
9. The method of claim 7, including pre-mixing the suspension agent
and the abrasive particles, and then adding the suspension agent
and the abrasive particles to the mixture.
10. The method of claim 7, including adding the abrasive particles
to the mixture after adding the suspension agent to the
mixture.
11. A method of making a chemical-mechanical polishing slurry,
comprising the following steps: mixing a ferric salt oxidizer with
a solution to produce a mixture with a dissolved ferric salt
oxidizer; filtering the mixture to remove most preexisting
particles therein that exceed a selected particle size; adding a
suspension agent to the mixture; and adding abrasive particles to
the mixture after filtering the mixture.
12. The method of claim 11, wherein the selected particle size is
at most about 0.1 microns.
13. The method of claim 11, wherein the selected particle size is
about 0.1 microns.
14. The method of claim 11, wherein most of the abrasive particles
have a larger particle size than the selected particle size.
15. The method of claim 11, wherein the abrasive particles are
selected from the group consisting of Al.sub.2O.sub.3, SiC,
SiO.sub.2, CeO.sub.2 and Si.sub.3N.sub.4.
16. The method of claim 11, wherein the ferric salt oxidizer is
selected from the group consisting of
Fe(NO.sub.3).sub.3.multidot.9H.sub.2O,
FeCl.sub.3.multidot.6H.sub.2O,
Fe.sub.2(SO.sub.4).sub.3.multidot.5H.sub.2- O and
FeNH.sub.4(SO.sub.4).sub.2.multidot.12H.sub.2O.
17. The method of claim 11, wherein the suspension agent is an
aqueous surfactant.
18. The method of claim 11, wherein the solution is water.
19. The method of claim 11, wherein the preexisting particles
include reaction product particles formed by reacting the ferric
salt oxidizer in the mixture with at least carbon in the
mixture.
20. The method of claim 11, wherein the reaction product particles
are an organic nitro compound.
21. The method of claim 11, wherein the preexisting particles are
mostly reaction product particles formed by reacting the ferric
salt oxidizer in the mixture.
22. The method of claim 11, wherein the preexisting particles
include undissolved ferric salt oxidizer particles in the
mixture.
23. The method of claim 11, including adding the suspension agent
to the mixture before filtering the mixture.
24. The method of claim 11, including adding the suspension agent
to the mixture after filtering the mixture.
25. The method of claim 24, including adding the suspension agent
to the mixture immediately after filtering the mixture.
26. The method of claim 24, including pre-mixing the suspension
agent and the abrasive particles, and then adding the suspension
agent and the abrasive particles to the mixture.
27. The method of claim 24, including adding the abrasive particles
to the mixture after adding the suspension agent to the
mixture.
28. The method of claim 11, wherein most of the abrasive particles
have a larger particle size than that of the preexisting particles
remaining in the mixture after filtering the mixture.
29. The method of claim 11, wherein most of the preexisting
particles are reaction product particles formed in the mixture.
30. The method of claim 11, wherein the slurry is adapted for
polishing a metal selected from the group consisting of tungsten,
aluminum and copper.
31. A method of polishing a metal layer during the fabrication of
an integrated circuit device, comprising the following steps:
providing a chemical-mechanical polishing slurry, including: mixing
an oxidizer with a solution to produce a mixture with a dissolved
oxidizer; filtering the mixture to remove most preexisting
particles therein that exceed a selected particle size; adding a
suspension agent to the mixture; and adding abrasive particles to
the mixture after filtering the mixture; and then polishing the
metal layer with the slurry.
32. The method of claim 31, wherein the selected particle size is
at most about 0.1 microns.
33. The method of claim 31, wherein the abrasive particles are
selected from the group consisting of Al.sub.2O.sub.3, SiC,
SiO.sub.2, CeO.sub.2 and Si.sub.3N.sub.4; the oxidizer is a ferric
salt oxidizer selected from the group consisting of
Fe(NO.sub.3).sub.3.multidot.9H.sub.2O,
FeCl.sub.3.multidot.6H.sub.2O,
Fe.sub.2(SO.sub.4).sub.3.multidot.5H.sub.2- O and
FeNH.sub.4(SO.sub.4).sub.2.multidot.12H.sub.2O; the suspension
agent is an aqueous surfactant; the solution is water; and the
metal layer is predominantly tungsten.
34. The method of claim 33, wherein the abrasive particles are
Al.sub.2O.sub.3, and the ferric salt oxidizer is
Fe(NO.sub.3).sub.3.multi- dot.9H.sub.2O.
35. A method of polishing a semiconductor wafer, comprising:
providing a polishing pad with a polishing surface; mounting a
semiconductor wafer on a wafer holder; rotating the polishing
surface; introducing a slurry onto the polishing surface, wherein
the slurry includes: abrasive particles, most of which are larger
than a selected particle size; an oxidizer; a suspension agent; and
preexisting particles, having been filtered, so that most are
smaller than the selected particle size and; planarizing the wafer
using the rotating polishing surface and the slurry.
36. A chemical-mechanical polishing slurry, comprising: abrasive
particles, most of which are larger than a selected particle size;
an oxidizer; a suspension agent; and preexisting particles, having
been filtered, so that most are smaller than the selected particle
size.
37. The slurry of claim 36, wherein the selected particle size is
at most about 0.1 microns.
38. The slurry of claim 36, wherein the abrasive particles are
selected from the group consisting of Al.sub.2O.sub.3, SiC,
SiO.sub.2, CeO.sub.2 and Si.sub.3N.sub.4.
39. The slurry of claim 36, wherein the oxidizer is a ferric salt
oxidizer selected from the group consisting of
Fe(NO.sub.3).sub.3.multidot.9H.sub.- 2O,
FeCl.sub.3.multidot.6H.sub.2O,
Fe.sub.2(SO.sub.4).sub.3.multidot.5H.su- b.2O and
FeNH.sub.4(SO.sub.4).sub.2.multidot.12H.sub.2O.
40. The slurry of claim 36, wherein the suspension agent is an
aqueous surfactant.
41. A polishing system for polishing a semiconductor wafer,
comprising: a polishing pad with a polishing surface; a rotatable
platen for removably securing the polishing pad; a rotatable wafer
holder for removably securing a wafer such that the wafer can be
pressed against the polishing surface; a chemical-mechanical
polishing slurry, including: abrasive particles, most of which are
larger than a selected particle size; an oxidizer; a suspension
agent; and preexisting particles, having been filtered, so that
most are smaller than the selected particle size; and a dispenser
for dispensing the slurry onto the polishing surface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to polishing slurry, and more
particularly to a chemical-mechanical polishing slurry that reduces
wafer defects and its method of making.
[0003] 2. Description of Related Art
[0004] In the manufacture of integrated circuits, the planarization
of semiconductor wafers is becoming increasingly important as the
number of layers used to form integrated circuits increases. For
instance, metallization layers formed to provide interconnects
between various devices may result in nonuniform surfaces. The
surface nonuniformities may interfere with the optical resolution
of subsequent lithographic steps, leading to difficulty with
printing high resolution patterns. The surface nonuniformities may
also interfere with step coverage of subsequently deposited metal
layers and possibly cause open or shorted circuits.
[0005] Various techniques have been developed to planarize the top
surface of a semiconductor wafer. One such approach involves
polishing the wafer using a polishing slurry that includes abrasive
particles mixed in a suspension agent. With this approach, the
wafer is mounted in a wafer holder, a polishing pad has its
polishing surface coated with the slurry, the pad and the wafer are
rotated such that the wafer provides a planetary motion with
respect to the pad, the polishing surface is pressed against an
exposed surface of the wafer, and the slurry is used as a
hydrodynamic layer between the polishing surface and the wafer. The
polishing erodes the wafer surface, and the process continues until
the wafer topography is largely flattened.
[0006] In chemical-mechanical polishing (CMP), the abrasive
particles provide friction while oxidants and/or etchants cause a
chemical reaction at the wafer surface. Additives can also be added
to enhance the removal rate, uniformity, selectivity, etc., and
dilution by deionized water is also practiced.
[0007] CMP is becoming a preferred method of planarizing tungsten
interconnects, vias and contacts.
[0008] Tungsten CMP slurries typically include abrasive particles
such as alumina, a ferric salt oxidizer such as ferric nitrate, a
suspension agent such as propylene glycol, and deionized water.
With proper process parameters, CMP tungsten processing has shown
significantly improved process windows and defect levels over
standard tungsten dry etching. One significant advantage of CMP
tungsten processing is that it has a highly selective polish rate
for tungsten as compared to the dielectric. This selectivity allows
for over-polishing while still achieving a flat tungsten stud. When
overetching occurs using dry etching, the contact or via becomes
further recessed, which creates a serious disadvantage since
overetching is frequently required to remove defects.
[0009] The advantages of CMP, however, can be offset by the
creation of significant defects during polishing, such as
scratches. The prior art teaches that scratching can be controlled
by the proper manufacturing, size control and filtering of the
abrasive particles. The prior art also teaches that the proper
mining sequence of the abrasive particles with the suspension agent
leads to lower defects.
[0010] Unfortunately, for various reasons, prior CMP slurries have
not been as effective as needed. In particular, deep or wide
scratch defects in the polished surface continue to cause problems.
This may arise since conventional slurry filtering tends to remove
only those particles that are significantly larger than most of the
abrasive particles. Therefore, a need exists for an improved CMP
slurry that reduces scratching defects.
SUMMARY OF TEE INVENTION
[0011] An object of the invention is to provide a CMP slurry which
enables planarization of a polished layer and reduces scratching
detects. These objects are achieved by filtering a solution with a
dissolved oxidizer before adding the abrasive particles to the
mixture, thereby removing a substantial amount of preexisting
particles in the solution.
[0012] In accordance with one aspect of the invention, a method of
making a CMP slurry includes mixing a ferric salt oxidizer with a
solution to produce a mixture with a dissolved ferric salt
oxidizer, filtering the mixture to remove most preexisting
particles therein that exceed a selected particle size, adding a
suspension agent to the mixture, and adding abrasive particles to
the mixture after filtering the mixture. Advantageously, when
polishing occurs, scratching by the preexisting particles is
dramatically reduced due to the filtering operation.
[0013] These and other objects, features and advantages of the
invention will be further described and more readily apparent from
a review of the detailed description of the preferred embodiments
which follow.
BRIEF DESCRIPTION OF TIE DRAWINGS
[0014] The following detailed description of the preferred
embodiments can best be understood when read in conjunction with
the following drawings, in which:
[0015] FIG. 1 is a chart of particle size versus particle count for
filtered and unfiltered ferric nitrate solutions;
[0016] FIG. 2 is a chart of total particles for samples of filtered
and unfiltered ferric nitrate solutions;
[0017] FIG. 3 is an EDX graph of amorphous white particles filtered
from a ferric nitrate solution;
[0018] FIG. 4 is an FTIR graph of amorphous white particles
filtered from a ferric nitrate solution;
[0019] FIG. 5 is an EDX graph of amber flakes filtered from a
ferric nitrate solution;
[0020] FIG. 6 is an FIR graph of amber flakes filtered from a
ferric nitrate solution;
[0021] FIG. 7 is an EDX graph of a survey scan of a filter used for
a ferric nitrate solution; and
[0022] FIG. 8 is an FTIR graph of a survey scan of a filter used
for a ferric nitrate solution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Commercially available CMP equipment and slurries are
currently available for planarization of integrated circuits with
tungsten vias through silicon dioxide layers. The commercially
available slurries, however, exhibit problems such as high scratch
counts. Our slurry substantially addresses and reduces these
problems.
[0024] We have discovered that preexisting particles in slurries
can significantly contribute to scratching. As used herein,
"preexisting particles" generally refer to unwanted particles that
exist in a mixture of an oxidizing agent and a solution before the
desired abrasive particles (such as alumina) are added. We believe
the preexisting particles include undissolved oxidizing agent,
contaminants and/or reaction products formed in the mixture. We
specifically believe, for instance, that a contaminant is dust, and
a reaction product of a ferric nitrate oxidizer is an organic nitro
compound. Moreover, as the slurry ages, the preexisting particles
tend to grow and/or coalesce. As a result, when polishing occurs,
the preexisting particles can cause substantial wafer damage.
[0025] In accordance with one aspect of the invention, a slurry is
prepared by mixing a ferric salt oxidizer with a solution to
produce a mixture with a dissolved ferric salt oxidizer, filtering
the mixture to remove a substantial amount of the preexisting
particles therein, adding a suspension agent to the mixture, and
adding abrasive particles to the mixture after filtering the
mixture.
[0026] Preferably, the filtering removes most of the preexisting
particles that exceed a particle size of about 0.1 microns, most of
the abrasive particles have a particle size in the range of about
0.2 to 0.7 microns, and the slurry is used for polishing within one
day of the filtering operation. In this manner, the preexisting
particles that remain in the slurry exhibit relatively little
growth or coalescence before polishing occurs. Furthermore, as
polishing occurs, the preexisting particles cause very little
scratching since most of the abrasive particles have a far larger
particle size than that of the preexisting particles.
[0027] Preferably, the ferric salt oxidizer and the solution are
thoroughly mixed before filtering occurs so that essentially all of
the ferric salt oxidizer is dissolved. However, if undissolved
ferric salt oxidizer remains, the filtering removes most of these
particles which exceed a selected particle size.
[0028] Preferably, the suspension agent and the abrasive particles
are thoroughly pre-mixed so that the suspension agent wets the
abrasive particles. It is also preferred that the pre-mixed
suspension agent and abrasive particles are added to the mixture
immediately after filtering the mixture, especially if polishing
occurs several days after the filtering operation, and that the
suspension agent inhibits the growth and/or coalescence of the
preexisting particles.
[0029] Finally, it is preferred that most of the preexisting
particles are Fe-containing particles (such as undissolved ferric
salt oxidizer), that the preexisting particles have a different
composition than the abrasive particles, and that none of the
abrasive particles (or particles with the same composition) are in
the mixture before filtering the mixture.
[0030] Other sequences can be used. For instance, the suspension
agent can added to the mixture before filtering the mixture.
Likewise, the suspension agent can be added to the mixture after
filtering the mixture, and then the abrasive particles can be added
to the mixture. It is essential, however, that the abrasive
particles be added to mixture after filtering the mixture.
[0031] The ferric salt oxidizer can be formulated from suitable Fe
compounds, such as ferric nitrate (Fe(NO.sub.3).sub.319 9H.sub.2O),
ferric chloride hexahydrate (FeCl.sub.3.multidot.6H.sub.2O), ferric
sulfate pentahydrate (Fe.sub.2(SO.sub.4).sub.3.multidot.5H.sub.2O)
and ferric ammonium sulfate dodecahydrate
(FeNH.sub.4(SO.sub.4).sub.2.multido- t.12H.sub.2O).
[0032] The preferred solution in which the ferric salt oxidizer is
initially mixed is ultra-pure, deionized water.
[0033] The suspension agent (also referred to as a dispersion
agent) is preferably an aqueous surfactant that improves the
colloidal behavior of the abrasive particles in deionized water,
and inhibits the growth and/or coalescence of the preexisting
particles. For instance, the suspension agent can be a commercially
available aqueous mixture of propylene glycol and methyl
paraben.
[0034] The suspension agent can be formulated from the following
classes:
[0035] (1) glycols such as ethylene glycol, propylene glycol and
glycerol;
[0036] (2) polyethers such as polyethylene glycol;
[0037] (3) aliphatic polyethers; and
[0038] (4) akoxylated alkyphenols.
[0039] The abrasive particles can be any of the commonly used
abrasives such as alumina (Al.sub.2O.sub.3), silicon carbide (SiC),
silicon dioxide (SiO.sub.2), ceria (CeO.sub.2) and silicon nitride
(Si.sub.3N.sub.4).
[0040] The resultant slurry is well-suited for CMP polishing a
predominantly tungsten layer during the fabrication of an
integrated circuit device. We believe that CMP of tungsten films
takes place by a chemical oxidation of the tungsten surface with a
suitable ferric salt oxidizing agent in an aqueous solution,
followed by mechanical abrasion of the more brittle metal oxide
which has formed on the surface by the solid abrasive particles
present in the aqueous suspension Both the oxidation and the
abrasion continue simultaneously and continuously. The reaction for
tungsten by the ferric ion is
W+6Fe.sup.+++3H.sub.2O.fwdarw.WO.sub.3+6Fe.sup.+++6H.sup.+
[0041] and occurs in an acid solution.
[0042] Several CMP experiments (Experiments 1, 2 and 3) as set
forth below were performed to determine the affects of filtering a
ferric nitrate solution on particle counts and scratches of a
subsequently polished wafer. A chemical analysis (Experiment 4) of
filtered material from aged ferric nitrate solution was also
performed as set forth below.
[0043]
[0044] Experiment 1
[0045] In this experiment, the affects of filtering a ferric
nitrate solution were investigated. An unfiltered ferric nitrate
solution with 0.33M concentration was prepared by adding 500 grams
of ferric nitrate crystals to 1 gallon of ultra-pure water (UPW).
The solution was agitated slightly by either shaking the bottle or
with the aid of a small mixing blade to ensure all the ferric
nitrate was dissolved. A filtered ferric nitrate solution was
initially prepared the same way as the unfiltered solution, and
then filtered through a 0.1 micron filter. A surfactant, which was
not mixed with the other solutions, was a commercially available
aqueous mixture of propylene glycol and methyl paraben from
Universal Photonics, Inc., sold under the trade name "Everflo
White".
[0046] Particle tests were performed by pouring about 2500 ml of
each solution onto a polishing pad while running a particle monitor
wafer loaded on a polisher. The order of particle tests was as
follows:
[0047] 1. UPW (to verify the polisher was clean)
[0048] 2. Surfactant
[0049] 3. Filtered ferric nitrate solution (tested immediately
after filtering)
[0050] 4. Unfiltered ferric nitrate solution
[0051] The metrology tool measured defects, which included
particles and scratches. The number of defects added in the 0.2 to
0.3 micron range, 0.3 to 0.4 micron range, and above 0.4 micron
were measured, and then summed to provide total defects added, as
listed below in Table 1. A negative number indicates the removal of
particles from the initial count on the particle monitor wafer.
1TABLE 1 Defects Defects Defects Added Added Added Total 0.2 to 0.3
0.3 to 0.4 Above 0.4 Defects Solution Micron Micron Micron Added
UPW -2 -7 -3 -12 Surfactant 12 0 1 13 Filtered Ferric Nitrate 11 0
1 12 Unfiltered Ferric Nitrate 1767 91 171 2029
[0052] Table 1 indicates that filtering the ferric nitrate solution
drastically reduced defects on a wafer polished immediately after
the filtering operation.
[0053] The experimental procedure and equipment for Experiment 1
were as follows:
2 Equipment: IPEC 472 Avanti Polisher. Wafer Carrier: Standard
design. Polish Pad: Industry standard. Particle Monitor: 6000 .ANG.
of plasma TEOS (CVD deposited) on prime silicon wafer. Dummy Wafer:
20,000 .ANG. of plasma TEOS (CVD deposited) on prime silicon wafer.
Metrology Tool: Tencor 6400 Surfscan. Pad Condition: New pad prior
to particle tests. Process Cycle: Set time of 3O sec per run, 5
psi, 25 rpm carrier, 100 rpm platen. Loading Sequence: Ran 1 dummy
wafer under followed by the particle monitor wafer for each
solution tested.
[0054] Experiment 2
[0055] In this experiment, the affects of aging unfiltered ferric
nitrate solution, filtered ferric nitrate solution, and filtered
ferric nitrate solution mixed with a surfactant were investigated.
A ferric nitrate solution with a 0.98M concentration was prepared
by adding 5 kg of ferric nitrate crystals to 3.33 gallons of UPW.
The solution was agitated with the aid of a small mixing blade to
ensure all the ferric nitrate was dissolved. The solution was then
divided three ways. A first gallon of solution was filtered through
a 0.1 micron filter and then mixed immediately with 1 gallon of
surfactant (Everflo White), bottled and stored. A second gallon of
solution was filtered through a 0.1 micron filter, bottled, and
stored. A third gallon of the solution was left unfiltered and
stored in a bottle.
[0056] The solutions were particle tested after aging for 14 days.
The particle tests were performed in a similar manner as the
previous experiment. The order the particle tests was as
follows:
[0057] 1. UPW (to verify the polisher was clean)
[0058] 2. Surfactant and filtered ferric nitrate solution
[0059] 3. Filtered ferric nitrate solution
[0060] 4. Unfiltered ferric nitrate solution
[0061] The results of the particle tests are listed below in Table
2.
3TABLE 2 Defects Defects Defects Added Added Added Total 0.2 to 0.3
0.3 to 0.4 Above 0.4 Defects Solution Micron Micron Micron Added
UPW 17 2 -8 11 Surfactant and Filtered 120 3 14 137 Ferric Nitrate
Filtered Ferric Nitrate 281 5 -4 282 Unfiltered Ferric Nitrate 4810
509 922 6241
[0062] Table 2 indicates that aging the ferric nitrate solutions
led to far more defects. In addition, the shelf life was improved
by filtering the ferric nitrate, and further improved by mixing the
filtered ferric nitrate solution with the surfactant immediately
after the filtering operation.
[0063] The experimental procedure and equipment for Experiment 2
were as follows:
4 Equipment: IPEC 472 Avanti Polisher Wafer Carrier: Standard
design. Polish Pad: Industry standard. Particle Monitor: 6000 .ANG.
of plasma TEOS (CVD deposited) on prime silicon wafer. Dummy Wafer:
20,000 .ANG. of plasma TEOS (CVD deposited) on prime silicon wafer.
Metrology Tool: Tencor 6400 Surfscan. Pad Condition: New pad prior
to particle tests. Process Cycle: Set time of 3O sec per run, 5
psi, 25 rpm carrier, 100 rpm platen. Loading Sequence: Ran 1 dummy
wafer under followed by the particle monitor wafer for each
solution tested.
[0064] Experiment 3
[0065] In this experiment, the affects of aging filtered and
unfiltered ferric nitrate solutions were investigated by liquid
particle counting. The filtered and unfiltered ferric nitrate
solutions used in Experiment 2 (without the surfactant) were
diluted to a concentration of about 0.002M by adding about 1 part
solution to about 500 parts UPW. Liquid particle counting was
performed on the diluted filtered and unfiltered ferric nitrate
solutions after 4 days of aging, and on the diluted filtered ferric
nitrate solution after 10 days of aging.
[0066] FIGS. 1 and 2 show the results of the liquid particle
counting. FIG. 1 is a chart of particle count versus particle size,
and FIG. 2 is a chart of total particles for the samples used.
FIGS. 1 and 2 demonstrate that filtering the ferric nitrate leads
to dramatic improvements in shelf life as compared to unfiltered
ferric nitrate.
[0067] The experimental procedure and equipment for Experiment 3
were as follows:
5 Equipment: CLS-700 liquid particle counter manufactured by
Particle Monitoring System and an NEC laptop computer. Purge Gas:
N.sub.2. Specimen Container: Squeeze bottle. Measured Particle
Size: 0.2 to 2.0 microns. Sampling Rate: Standard.
[0068] Experiment 4
[0069] In this experiment, the composition of the preexisting
particles in aged ferric nitrate solution was investigated. An
unfiltered ferric nitrate solution of 0.6M concentration was
prepared by adding 460 grams of ferric nitrate crystals to 0.5
gallons of UPWin a polypropylene bottle. The solution was shaken
for about I minute, stored for 45 days, and then filtered through a
0.8 micron gold-coated membrane filter. After air drying, the
filter was examined by optical and SEM microscopy to identify
characteristic particles. Many of the particles were amorphous
white particles resembling polymers, while several others were
amber flakes. Few smaller particles (i.e., 1-5 micron diameters)
were observed.
[0070] The amorphous white particles, the amber flakes, and a
survey scan of the filter (in an area free from larger particles)
were analyzed by EDX spectroscopy to determine elemental
composition and by FTIR spectroscopy to identify chemical
structure. The results are listed below in Table 3.
6TABLE 3 Elemental Chemical Particle Composition Structure
Amorphous Major: C polyethylene and polypropylene and White Minor:
O, Si, Fe inorganic silicates Particles Trace: Al, Cl, Ti Amber
Major: O, Fe, Si polydimethylsiloxane (silicone) and Flakes Minor:
C, Cl polymer and ferric nitrate and water Trace: Al, S of
hydration and organic nitro compound Survey Scan Major: Au (filter)
polydimethylsiloxane (silicone) and Minor: C, O polymer and
inorganic silicates and Trace: Al, Si trace polyethylene
[0071] FIG. 3 shows the EDX graph for the amorphous white
particles, FIG. 4 shows the FTIR graph for the amorphous white
particles, FIG. 5 shows the EDX graph for the amber flakes, FIG. 6
shows the FTIR graph for the amber flakes, FIG. 7 shows the EDX
graph for the survey scan, and FIG. 8 shows the FTIR graph for the
survey scan.
[0072] The EDX and FTIR analysis indicate that most of the filtered
particles were polymeric. In the amorphous white particles, the
polyolefins (polyethylene and polypropylene) probably came from the
polypropylene bottle used to mix and store the solution, and the
inorganic silicates probably came from airborne dust but might have
been added as fillers to polymer products. Thus, the amorphous
white particles appear to be contaminants that can be substantially
reduced or eliminated by preparing the solution in a cleaner
environment. In the amber flakes, the ferric nitrate appears to be
undissolved crystals, the organic nitro compound appears to be a
reaction product formed in the solution, and the silicone polymers
probably came from sealants, lubricants, defoaming agents, o-rings
or gaskets.
[0073] FIGS. 1 and 2 demonstrate that the preexisting particles get
larger as the solution ages. (That is, the median preexisting
particle size increases, although it is entirely possible that many
of the preexisting particles do not get larger.) The exact
mechanism by which the preexisting particles grow and/or coalesce
is not presently understood.
[0074] Accordingly, our slurry includes abrasive particles, most of
which are larger than a selected particle size, an oxidizer, a
suspension agent, and preexisting particles, having been filtered,
so that most are smaller than the selected particle size. Although
the preferred oxidizer is a ferric salt, other oxidizers such as
ammonium persulfate are suitable. The slurry can be used to polish
various metals such as tungsten, aluminum and copper.
[0075] The slurry is well-suited for use in a polishing system for
polishing a semiconductor wafer that includes a polishing pad with
a polishing surface, a rotatable platen for removably securing the
polishing pad, a rotatable wafer holder for removably securing a
wafer such that the wafer can be pressed against the polishing
surface, and a dispenser for dispensing the slurry onto the
polishing surface. A method of polishing a semiconductor wafer with
the slurry includes providing a polishing pad with a polishing
surface, mounting a semiconductor wafer on a wafer holder, rotating
the polishing surface, introducing the slurry onto the polishing
surface, and planarizing the wafer using the rotating polishing
surface and the slurry.
[0076] Other variations and modifications of the embodiments
disclosed herein may be made based on the description set forth
herein, without departing from the scope and spirit of the
invention as set forth in the following claims.
* * * * *