U.S. patent application number 10/485349 was filed with the patent office on 2004-12-09 for polishing method and polishing liquid.
Invention is credited to Akatsuka, Tomohiko, Hagihara, Toshiya, Sasaki, Tatsuya, Wada, Yutaka.
Application Number | 20040248415 10/485349 |
Document ID | / |
Family ID | 27750526 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040248415 |
Kind Code |
A1 |
Wada, Yutaka ; et
al. |
December 9, 2004 |
Polishing method and polishing liquid
Abstract
The present invention relates to a polishing method and a
polishing liquid for polishing a workpiece such as a semiconductor
wafer with a fixed abrasive. In the polishing method for polishing
a workpiece by pressing the workpiece to be polished against a
fixed abrasive and bringing the workpiece in sliding contact with
the fixed abrasive, the workpiece is polished while supplying a
polishing liquid which contains an anionic surface-active agent and
does not contain abrasive particles.
Inventors: |
Wada, Yutaka; (Tokyo,
JP) ; Akatsuka, Tomohiko; (Tokyo, JP) ;
Sasaki, Tatsuya; (Tokyo, JP) ; Hagihara, Toshiya;
(Wakayama, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
27750526 |
Appl. No.: |
10/485349 |
Filed: |
July 8, 2004 |
PCT Filed: |
February 20, 2003 |
PCT NO: |
PCT/JP03/01870 |
Current U.S.
Class: |
438/690 ;
257/E21.244 |
Current CPC
Class: |
B24B 49/12 20130101;
C09G 1/00 20130101; B24B 53/02 20130101; B24B 37/04 20130101; B24B
7/228 20130101; B24B 53/017 20130101; H01L 21/31053 20130101; B24B
37/044 20130101 |
Class at
Publication: |
438/690 |
International
Class: |
H01L 021/302; H01L
021/461 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 20, 2002 |
JP |
2002-43471 |
Claims
1-8. (Canceled).
9. A polishing method for polishing a workpiece by pressing the
workpiece to be polished against a fixed abrasive and bringing the
workpiece in sliding contact with the fixed abrasive, the polishing
method comprising: polishing the workpiece while supplying a
polishing liquid which contains an anionic surface-active agent and
does not contain abrasive particles, a pH of said polishing liquid
being 4 to 6.5.
10. A polishing method according to claim 9, wherein a
concentration of said anionic surface-active agent in said
polishing liquid is 0.001% by weight to 0.15% by weight.
11. A polishing method according to claim 10, wherein said anionic
surface-active agent contains an organic compound having a
hydrophilic group selected from COO.sup.- group and SO.sub.3.sup.-
group.
12. A polishing method according to any one of claims 11, wherein
said workpiece has a nitride film and a silicon oxide film
deposited on the nitride film.
13. A polishing method according to any one of claims 10, wherein
said workpiece has a nitride film and a silicon oxide film
deposited on the nitride film.
14. A polishing method according to claim 9, wherein said anionic
surface-active agent contains an organic compound having a
hydrophilic group selected from COO.sup.- group and SO.sub.3.sup.-
group.
15. A polishing method according to any one of claims 14, wherein
said workpiece has a nitride film and a silicon oxide film
deposited on the nitride film.
16. A polishing method according to any one of claims 9, wherein
said workpiece has a nitride film and a silicon oxide film
deposited on the nitride film.
17. A polishing liquid for use in a polishing method for polishing
a workpiece by pressing the workpiece to be polished against a
fixed abrasive and bringing the workpiece in sliding contact with
the fixed abrasive, wherein said polishing liquid contains no
abrasive particles and said polishing liquid has a pH of 4 to
6.5
18. A polishing liquid according to claim 17, wherein said
polishing liquid has an anionic surface-active agent at a
concentration of not less than 0.001% by weight and less than 0.15%
by weight,
19. A polishing liquid according to claim 18, wherein said anionic
surface-active agent is polyacrylic acid and/or its salt having a
weight average molecular weight of not less than 13000 and not more
than 60000.
20. A polishing liquid according to claim 17, wherein said anionic
surface-active agent is polyacrylic acid and/or its salt having a
weight average molecular weight of not less than 13000 and not more
than 60000.
Description
TECHNICAL FIELD
[0001] The present invention relates to a polishing method and a
polishing liquid, and more particularly to a polishing method and a
polishing liquid for polishing a workpiece such as a semiconductor
wafer with a fixed abrasive.
BACKGROUND ART
[0002] As semiconductor devices become more highly integrated in
recent years, circuit interconnections have become finer and
distance between those circuit interconnections becomes smaller. In
case of photolithography which can form interconnections that are
at most 0.5 .mu.m wide, it is required that surfaces on which
pattern images are to be focused by a stepper should be as flat as
possible because a depth of focus of an optical system is
relatively small. A polishing apparatus for performing chemical
mechanical polishing (CMP) has been used for planarizing a
semiconductor wafer.
[0003] This type of chemical mechanical polishing (CMP) apparatus
comprises a polishing table having a polishing pad attached
thereon, and a top ring. A workpiece to be polished is disposed
between the polishing pad and the top ring and pressed against the
polishing table by the top ring, while a polishing liquid (slurry)
is being supplied onto the polishing pad, and hence the workpiece
is polished to a flat mirror finish.
[0004] The CMP apparatus is used in an STI (Shallow Trench
Isolation) forming process for forming a transistor circuit on a
lowermost layer of a semiconductor device, for example. FIGS. 3A
through 3F are schematic cross-sectional views showing an example
of the STI forming process. A nitride film 110 (generally, a film
of silicon nitride Si.sub.3N.sub.4) is formed on a silicon
substrate 100 (see FIG. 3A), and the silicon substrate 100 is
etched using the nitride film 110 as a mask, thus forming a trench
(groove) 120 (see FIG. 3B). Then, a silicon oxide film 130 made of
an insulating material is embedded in the trench 120 (see FIG.
3C).
[0005] When the silicon oxide film 130 is embedded in the trench
120, since an excessive layer of the silicon oxide film 130 is left
on the surface as shown in FIG. 3C, the excessive layer of the
silicon oxide film 130 is removed by CMP, thereby exposing the
surface of the nitride film 110 (see FIG. 3D). Then, the nitride
film 110 is etched away, thus forming a groove 130a of the silicon
oxide film (see FIG. 3E). A transistor 140 is then formed using
such groove 130a of the silicon oxide film (see FIG. 3F).
[0006] In the STI forming process described above, the purpose of
the polishing step using CMP is to completely remove the excessive
silicon oxide film formed on the nitride film. If the excessive
silicon oxide film is not removed completely, then the subsequent
etching of the nitride film will be impaired.
[0007] However, the polishing step in the above-mentioned
conventional STI forming process employs a polyurethane-based
polishing pad and a slurry with silica abrasive particles dispersed
therein. Since the nitride film 110 is less liable to be polished
than the silicon oxide film 130, the silicon oxide film 130 is
excessively polished, thus developing dishing as shown in FIG. 4.
Furthermore, the nitride film tends to be polished unevenly,
resulting in insufficient within-wafer uniformity. In the STI
forming process, it is necessary to strictly control the
thicknesses of the nitride film and the silicon oxide film in the
trench, i.e., to uniformize the film thickness within the surface
of the wafer, and it is important to suppress dishing for
within-wafer uniformity.
[0008] Recently, it has been attempted to solve the above problems
by polishing wafers using a polyurethane-based polishing pad and a
slurry with ceria abrasive particles dispersed therein, while
adding a trace amount of a high concentration surface-active agent.
If the dishing amount developed using a silica slurry is 500 .ANG.,
for example, then the dishing amount can be reduced to a range from
200 to 300 .ANG. by this process. However, this process cannot
eliminate the above dishing completely because it also uses the
soft polishing pad. Another problem of this process is that the
high concentration surface-active agent causes ceria to
agglomerate, and that the agglomerated ceria scratches the
wafer.
[0009] In an effort to solve the problems of the polishing
processes using an abrasive liquid (slurry) containing abrasive
particles and a soft polishing pad, there has been proposed a
polishing process using a fixed abrasive as disclosed in Japanese
laid-open patent publication No. 2000-173955. Specifically, in
Japanese laid-open patent publication No. 2000-173955, there has
been proposed a process for polishing a wafer with a fixed abrasive
containing ceria and a polishing liquid which contains 1% by weight
of ammonium polyacrylate having a molecular weight of 10000, and
has an alkaline pH value. The purpose of this process is to
increase the polishing rate by making the pH alkaline to
sufficiently increase the dispersion of dropped abrasive particles.
However, because ammonium polyacrylate to be added has a relatively
high concentration, aggregation is liable to occur, and hence it is
difficult to sufficiently prevent scratches from occurring.
Further, since the pH is alkaline, it is difficult to suppress
dishing and obtain good within-wafer uniformity.
DISCLOSURE OF INVENTION
[0010] The present invention has been made in view of the above
problems in the prior art. It is therefore an object of the present
invention to provide a polishing method and a polishing liquid
which can suppress dishing in a workpiece having a nitride film and
an oxide film formed on the nitride film, and obtain within-wafer
uniformity.
[0011] In order to solve the problems in the prior art, according
to a first aspect of the present invention, there is provided a
polishing method for polishing a workpiece by pressing the
workpiece to be polished against a fixed abrasive and bringing the
workpiece in sliding contact with the fixed abrasive, the polishing
method comprising: polishing the workpiece while supplying a
polishing liquid which contains an anionic surface-active agent and
does not contain abrasive particles.
[0012] The concentration of the anionic surface-active agent in the
polishing liquid is preferably 0.001% by weight to 5% by weight,
more preferably 0.001% by weight to 0.15% by weight. The pH of the
polishing liquid is preferably 5 to 10, more preferably 4 to 6.5.
Further, the anionic surface-active agent preferably contains an
organic compound having a hydrophilic group selected from COO.sup.-
group and SO.sub.3.sup.- group. Further, the workpiece preferably
has a nitride film and a silicon oxide film deposited on the
nitride film.
[0013] Furthermore, the anionic surface-active agent is preferably
polyacrylic acid and/or its salt having a weight average molecular
weight of not less than 13000 and not more than 60000.
[0014] In the neutral to acidic pH region of the polishing liquid,
the surfaces of the abrasive particles of the fixed abrasive and
the surface of the nitride film are principally charged positively
(though part of the surfaces possess a negative charge, a positive
charge is predominant). Accordingly, the anionic surface-active
agent supplied onto the fixed abrasive strongly adheres to the
surfaces of the abrasive particles of the fixed abrasive and to the
surface of the nitride film electrically. Since the abrasive
particles of the fixed abrasive and the nitride film are both
covered with the anionic surface-active agent to interact
repulsively. Therefore, polishing of the nitride film is hard to
proceed and the polishing rate of the nitride film becomes
extremely low. The polishing selectivity is, for example, oxide
film:nitride film=10 or higher:1, and the nitride film acts as a
polishing stopper. Since the polishing rate of the nitride film is
extremely lowered and stock removal decreases, the resulting
nitride film can be prevented from becoming uneven in the film
thickness and within-wafer uniformity of the nitride film can be
obtained.
[0015] At this point, it may be considered that because of the
above-described polishing selectivity, the nitride film is not
polished and only the silicon oxide film is polished, and the
above-described dishing will proceed. In fact, however, dishing
does not proceed, and depth of the dishing amount can be
controlled, for example, within 100 .ANG. or less. This is
considered to be due to the following reasons:
[0016] {circle over (1)} The anionic surface-active agent adheres
not only to the surface of the nitride film, but also to the
silicon oxide film in small amount, whereby a coat of the anionic
surface-active agent is formed on the surface of the silicon oxide
film. When a surface pressure having a certain value or more is
applied to the silicon oxide, a coat on the surface of the silicon
oxide film breaks or becomes fragile, and therefore polishing of
the silicon oxide film proceeds. As the polishing proceeds to a
certain extent, however, the level of the surface of the silicon
oxide film becomes lower than that of the nitride film.
Accordingly, the surface pressure on the silicon oxide film
decreases, and hence a coat of the anionic surface-active agent is
formed thick on the silicon oxide film and the progress of
polishing is retarded.
[0017] {circle over (2)} Since the anionic surface-active agent
used here is a polymer, the anionic surface-active agent, which has
adhered to the nitride film, covers part or a considerable area of
the trench portion, and hence polishing of the silicon oxide film
by the abrasive particles in the fixed abrasive is impeded.
[0018] Further, according to the polishing method of the present
invention, the concentration of the anionic surface-active agent in
the polishing liquid is made low so that the abrasive particles,
which have dropped out of the fixed abrasive, do not agglomerate
and thus the formation of scratches can be reduced.
[0019] Further, the present invention uses a fixed abrasive as a
polishing surface. A fixed abrasive provides a hard polishing
surface which hardly deforms elastically. Such a polishing surface,
as compared to a conventional polishing pad having elasticity, is
less likely to cause dishing and can provide a flatter finished
surface.
[0020] Thus, according to the present invention, polishing of a
workpiece is carried out while supplying the polishing liquid which
contains an anionic surface-active agent at a low concentration and
does not contain abrasive particles, and has an acidic pH, onto the
fixed abrasive, whereby a nitride film, for example, is allowed to
act as a polishing stopper. Accordingly, the polishing method of
the present invention can lower the polishing rate of a nitride
film to obtain within-wafer uniformity of the nitride film and can
suppress dishing, thus enabling a high-flatness polishing with
fewer scratches.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a plan view showing an overall construction of a
polishing apparatus according to an embodiment of the present
invention;
[0022] FIG. 2 is a front view of the polishing chamber of FIG.
1;
[0023] FIGS. 3A through 3F are schematic views showing an example
of an STI forming process; and
[0024] FIG. 4 is a schematic view showing dishing caused by a
conventional polishing apparatus.
BEST MODE FOR CARRYING OUT THE INVENTION
[0025] A polishing apparatus according to embodiments of the
present invention will be described in detail below with reference
to the drawings. FIG. 1 is a plan view showing a whole arrangement
of a polishing apparatus according to an embodiment of the present
invention.
[0026] As shown in FIG. 1, the polishing apparatus comprises four
load/unload stages 2 each for receiving a wafer cassette 1 which
accommodates a number of semiconductor wafers. The load/unload
stages 2 may have a lifting and lowering mechanism. A transfer
robot 4 is provided on rails 3 so that the transfer robot 4 can
access the respective wafer cassettes 1 on the respective
load/unload stages 2.
[0027] The transfer robot 4 has upper and lower hands. The lower
hand of the transfer robot 4 is a vacuum attraction-type hand for
holding a semiconductor wafer under vacuum, and is used only for
removing a semiconductor wafer from a wafer cassette 1. The vacuum
attraction-type hand can hold and transport the semiconductor wafer
even if the semiconductor wafer is not located at a normal position
in the wafer cassette due to a slight displacement. The upper hand
of the transfer robot 4 is a recess support-type hand for
supporting a peripheral edge of a wafer, and is used only for
returning the semiconductor wafer to the wafer cassette 1. The
recess support-type hand can transport the semiconductor wafer
while keeping the semiconductor wafer clean because dust is not
collected, unlike the vacuum attraction-type hand. In this manner,
since a clean wafer which has been cleaned is held by the upper
hand, the clean semiconductor wafer is not further
contaminated.
[0028] Two cleaning units 5, 6 for cleaning a semiconductor wafer
are disposed at an opposite side of the wafer cassettes 1 with
respect to the rails 3 of the transfer robot 4. The cleaning units
5, 6 are disposed at positions accessible by the hands of the
transfer robot 4. Each of the cleaning units 5, 6 has a spin-dry
mechanism for drying a wafer by spinning the wafer at a high speed,
and hence two-stage cleaning and three-stage cleaning of a wafer
can be performed without replacing any cleaning module.
[0029] Between the two cleaning units 5 and 6, a wafer station 12
having four semiconductor wafer supports 7, 8, 9 and 10 is disposed
at a position accessible by the transfer robot 4. A transfer robot
14 having two hands is disposed at a position where hands of the
transfer robot 14 can access the cleaning unit 5 and the three
supports 7, 9 and 10. A transfer robot 15 having two hands is
disposed at a position where hands of the transfer robot 15 can
access the cleaning unit 6 and the three supports 8, 9 and 10.
[0030] The support 7 is used to transfer a semiconductor wafer
between the transfer robot 4 and the transfer robot 14, and the
support 8 is used to transfer a semiconductor wafer between the
transfer robot 4 and the transfer robot 15. These supports 7, 8
have sensors 16, 17 for detecting existence of a semiconductor
wafer, respectively.
[0031] The support 9 is used to transfer a semiconductor wafer from
the transfer robot 15 to the transfer robot 14, and the support 10
is used to transfer a semiconductor wafer from the transfer robot
14 to the transfer robot 15. These supports 9, 10 have sensors 18,
19 for detecting existence of a semiconductor wafer, and rinsing
nozzles 20, 21 for preventing the semiconductor wafer from being
dried or rinsing the wafer.
[0032] The supports 9 and 10 are disposed in a common
water-scatter-prevention cover which has an opening defined therein
for transferring wafers therethrough. At the opening, there is
provided a shutter 22. The support 9 is disposed above the support
10. The upper support 9 serves to support a wafer which has been
cleaned, and the lower support 10 serves to support a wafer to be
cleaned. With this arrangement, the wafer is prevented from being
contaminated by rinsing liquid which would otherwise fall thereon.
The sensors 16, 17, 18 and 19, the rinsing nozzles 20, 21, and the
shutter 22 are schematically shown in FIG. 1, and their positions
and shapes are not exactly illustrated.
[0033] A cleaning unit 24 is disposed at a position adjacent to the
cleaning unit 5 and is accessible by the hands of the transfer
robot 14. Further, a cleaning unit 25 is disposed at a position
adjacent to the cleaning unit 6 and is accessible by hands of the
transfer robot 15. These cleaning units 24 and 25 are capable of
cleaning both surfaces of a wafer.
[0034] The respective upper hands of the transfer robot 14 and the
transfer robot 15 are used for transporting a semiconductor wafer
that has been cleaned to the cleaning units or the supports of the
wafer station 12. On the other hand, the respective lower hands of
the transfer robot 14 and the transfer robot 15 are used for
transporting a semiconductor wafer that has not cleaned or a
semiconductor wafer to be polished. Since the lower hands are used
to transfer a wafer to or from a reversing device (described later
on), the upper hands are not contaminated by drops of rinsing
liquid which fall from an upper wall of the reversing device.
[0035] As shown in FIG. 1, the cleaning units 5, 6, 24 and 25 have
shutters 5a, 6a, 24a and 25a at respective wafer entrances for
transferring wafers therethrough, respectively. The shutters 5a,
6a, 24a and 25a are opened only when wafers are transferred through
the shutters 5a, 6a, 24a and 25a.
[0036] The polishing apparatus has a housing 26 for enclosing
various components therein. An interior of the housing 26 is
partitioned into a plurality of areas (including areas A and B) by
partition walls 28, 30, 32, 34 and 36.
[0037] Area A in which the wafer cassettes 1 and the transfer robot
4 are disposed, and area B in which the cleaning units 5 and 6 and
the supports 7, 8, 9 and 10 are disposed, are partitioned by the
partition wall 28 so that cleanliness of area A and area B can be
separated from each other. The partition wall 28 has an opening for
transferring semiconductor wafers between area A and area B, and a
shutter 38 is provided at the opening of the partition wall 28. All
of the cleaning units 5, 6, 24 and 25, the supports 7, 8, 9 and 10
of the wafer station 12, and the transfer robots 14 and 15 are
placed in area B. Pressure in area B is adjusted so as to be lower
than pressure in area A.
[0038] As shown in FIG. 1, in area C separated from area B by the
partition wall 34, a reversing device 40 for reversing a
semiconductor wafer is provided at a position accessible by the
hands of the transfer robot 14. The semiconductor wafer is
transferred to the reversing device 40 by the transfer robot 14.
Further, in area C, a reversing device 41 for reversing a
semiconductor wafer is provided at a position accessible by the
hands of the transfer robot 15. The semiconductor wafer is
transferred to the reversing device 41 by the transfer robot 15.
Each of the reversing devices 40 and 41 has a chuck mechanism for
chucking a semiconductor wafer, a reversing mechanism for reversing
the semiconductor wafer, and a detecting sensor for detecting
whether or not the chuck mechanism chucks the semiconductor
wafer.
[0039] The partition wall 34 forms a polishing chamber which is
separated from area B. The polishing chamber is further divided
into two areas C and D by the partition wall 36. The partition wall
34 between area B and areas C, D has openings for transferring
semiconductor wafers therethrough. Shutters 42, 43 for the
reversing devices 40, 41 are respectively provided at the openings
of the partition wall 34.
[0040] As shown in FIG. 1, each of areas C and D has two polishing
tables and one top ring for holding and pressing one semiconductor
wafer against the polishing tables to polish the wafer.
Specifically, area C has a top ring 44, polishing tables 46, 48, a
polishing liquid supply nozzle 50 for supplying a polishing liquid
onto the polishing table 46, an atomizer 52 having a plurality of
ejection nozzles (not shown) connected to a nitrogen gas supply
source and a liquid supply source, a dresser 54 for dressing the
polishing table 46, and a dresser 56 for dressing the polishing
table 48. Similarly, area D has a top ring 45, polishing tables 47,
49, a polishing liquid supply nozzle 51 for supplying a polishing
liquid onto the polishing table 47, an atomizer 53 having a
plurality of ejection nozzles (not shown) connected to a nitrogen
gas supply source and a liquid supply source, a dresser 55 for
dressing the polishing table 47, and a dresser 57 for dressing the
polishing table 49.
[0041] The polishing liquid supply nozzles 50, 51 supply polishing
liquids, used for a polishing process, and dressing liquids (e.g.,
water) used for a dressing process, onto the polishing tables 46,
47, respectively. The atomizers 52, 53 eject liquids composed of a
mixture of nitrogen gas with pure water or a chemical liquid onto
the polishing tables 46, 47, respectively. Nitrogen gas from the
nitrogen gas supply source and pure water or a chemical liquid from
the liquid supply source are passed through a regulator or air
operated valve (not shown) to regulate pressure thereof to a
predetermined value, and supplied to the ejection nozzles in the
atomizers 52, 53 in a mixed state. In this case, the liquid should
preferably be ejected from the ejection nozzles of the atomizers
52, 53 toward outer peripheral edges of the polishing tables 46,
47. Other inert gases may be used instead of nitrogen gas. Further,
the atomizers 52, 53 may eject only a liquid of pure water or a
chemical liquid. The polishing tables 48, 49 may have atomizers as
with the polishing tables 46, 47, respectively. With atomizers for
the polishing tables 48, 49, surfaces of the polishing tables 48,
49 can be kept clean.
[0042] The mixture of nitrogen gas with pure water or the chemical
liquid is supplied in a state of {circle over (1)} liquid fine
particles, {circle over (2)} solid fine particles as a result of
solidification of the liquid, or {circle over (3)} gas as a result
of vaporization of the liquid. These states {circle over (1)},
{circle over (2)} and {circle over (3)} are referred to as
atomization. In these states, the mixture is ejected from the
ejection nozzles of the atomizers 52, 53 toward the polishing
tables 46, 47. For example, pressure or temperature of the nitrogen
gas and/or the pure water or the chemical liquid, or the shape of
the nozzles determines which state of the mixed liquid is to be
ejected, i.e., the liquid fine particles, the solid fine particles,
or gas. Therefore, the state of the liquid to be ejected can be
varied, for example, by properly adjusting pressure or temperature
of the nitrogen gas and/or the pure water or the chemical liquid
with use of a regulator or the like, or by properly adjusting the
shape of the nozzles.
[0043] The polishing tables 48, 49 may be replaced with wet-type
thickness measuring devices for measuring a thickness of a film
formed on a wafer. With such wet-type thickness measuring devices,
the thickness of a film formed on a wafer can be measured
immediately after the wafer is polished, and hence it is possible
to further polish the polished wafer or to control a polishing
process for polishing a subsequent wafer based on measured
results.
[0044] A rotary transporter 60 is disposed below the reversing
devices 40 and 41 and the top rings 44 and 45 to transfer wafers
between the cleaning chamber (area B) and the polishing chamber
(areas C, D). The rotary transporter 60 has four stages for placing
wafers W at equal angular intervals, and can hold a plurality of
wafers thereon at the same time.
[0045] A wafer which has been transferred to the reversing device
40 or 41 is transferred to the rotary transporter 60 by elevating
and lowering a lifter 62 or 63 disposed below the rotary
transporter 60 when a center of a stage of the rotary transporter
60 is aligned with a center of the wafer held by the reversing
device 40 or 41. A wafer placed on the stage of the rotary
transporter 60 is transported to a position below the top ring 44
or the top ring 45 by rotating the rotary transporter 60 by an
angle of 90.degree.. At this time, the top ring 44 or the top ring
45 is positioned above the rotary transporter 60 beforehand by a
swinging motion of the top ring. A wafer held on the stage of the
rotary transporter 60 is transferred to the top ring 44 or 45 by
elevating and lowering a pusher 64 or 65 disposed below the rotary
transporter 60 when a center of the top ring 44 or 45 is aligned
with a center of the wafer.
[0046] Next, the polishing chamber will be described below.
Although only area C will be described below, the following
description can be applied to area D. FIG. 2 shows a relationship
between the top ring 44 and the polishing tables 46, 48 in area
C.
[0047] As shown in FIG. 2, the top ring 44 is supported from a top
ring head 72 by a top ring drive shaft 70 which is rotatable. The
top ring head 72 is supported by a support shaft 74 which can
angularly be positioned, and the top ring 44 can access the
polishing tables 46 and 48.
[0048] The dresser 54 is supported from a dresser head 78 by a
dresser drive shaft 76 which is rotatable. The dresser head 78 is
supported by a support shaft 80 which can angularly be positioned,
and the dresser 54 can be moved between a standby position and a
dressing position above the polishing table 46. The dresser 56 is
similarly supported from a dresser head 84 by a dresser drive shaft
82 which is rotatable. The dresser head 84 is supported by a
support shaft 86 which can angularly be positioned, and the dresser
56 can be moved between a standby position and a dressing position
above the polishing table 48.
[0049] The polishing table 46 has an upper surface composed of a
fixed abrasive 46a having abrasive particles and pores or a pore
agent, which are fixed by a binder (resin). The fixed abrasive 46a
serves as a polishing surface for polishing a semiconductor wafer
held by the top ring 44. Such a fixed abrasive 46a can be obtained,
for example, by spray-drying a mixed liquid which is a mixture of a
slurry of abrasive (dispersion of abrasive particles in a liquid)
and a resin emulsion, filling the resulting mixed powder into a
molding jig, and subjecting the mixed powder to a pressure/heat
treatment. Ceria (CeO.sub.2) or silica (SiO.sub.2) having an
average particle diameter of not more than 0.5 .mu.m is preferably
used as the abrasive particles. A thermoplastic resin or a
thermosetting resin may be used as the binder. The thermoplastic
resin is preferably used.
[0050] The polishing table 48 has an upper surface composed of a
soft nonwoven fabric. The nonwoven fabric serves as a cleaning
surface for cleaning a semiconductor wafer after a polishing
process to remove abrasive particles attached to a surface of the
semiconductor wafer.
[0051] Next, a polishing process for polishing a semiconductor
wafer with the use of a polishing apparatus according to the
present invention will be described below. Although a polishing
process only in area C will be described below, the following
description can be applied to a polishing process in area D.
Further, though the following description illustrates application
of the present invention to a polishing step in an STI formation,
the present invention is also applicable, for example, to polishing
of a semiconductor wafer having a pattern of interlayer dielectrics
(ILD).
[0052] 1) Polishing Process
[0053] While the polishing table 46 and the top ring 44 are rotated
about their own axes, respectively, a semiconductor wafer is
pressed against the polishing table 46 to thereby polish a silicon
oxide film on the semiconductor wafer until the fixed abrasive
reaches a nitride film. During the polishing, a polishing liquid
which contains an anionic surface-active agent and does not contain
abrasive particles is supplied from the polishing liquid supply
nozzle 50 onto the fixed abrasive 46a. The concentration of the
anionic surface-active agent is preferably 0.001% by weight to 5%
by weight, more preferably 0.001% by weight to 0.15% by weight.
From the viewpoints of suppressing dishing to obtain within-wafer
uniformity and dispersing abrasive particles, which have dropped
out of the fixed abrasive, to prevent scratches, the concentration
is more preferably not less than 0.002% by weight, further more
preferably not less than 0.003% by weight, and especially
preferably not less than 0.004% by weight. Further, from the
viewpoints of achieving a sufficiently high polishing rate and
dispersing the dropped abrasive particles to prevent scratches, the
concentration is preferably less than 0.15% by weight, more
preferably less than 0.10% by weight, further more preferably less
than 0.08% by weight, especially preferably less than 0.05% by
weight. Accordingly, the concentration of the anionic
surface-active agent is preferably not less than 0.001% by weight
and less than 0.15% by weight, more preferably not less than 0.002%
by weight and less than 0.10% by weight, further more preferably
not less than. 0.003% by weight and less than 0.08% by weight,
especially preferably not less than 0.004% by weight and less than
0.05% by weight. The purpose of the addition of the surface-active
agent is, besides allowing the nitride film to function as a
stopper, to control the polishing rate of the silicon oxide film to
thereby suppress dishing (in the trench portion). A rise in the
concentration of the surface-active agent can lower the polishing
rate of the silicon oxide film and can therefore suppress dishing.
If the concentration is excessively increased, the polishing rate
is extremely lowered, thus requiring a long polishing time. This
leads to a lowering of the productivity. For example, when ammonium
polyacrylate is added at a concentration of 0.05% by weight, the
polishing rate of the silicon oxide film decreases by about 30%, as
compared to the case of pure water. There is a linear relationship
between the surface-active agent concentration and the decrease in
the polishing rate. Thus, the polishing rate decreases by about 60%
at a concentration of 0.10%, and by about 90% at a concentration of
0.15%. In consideration of the productivity, it is desirable to set
the upper limit of the concentration of the surface-active agent at
0.15,%.
[0054] The pH of the polishing liquid is preferably in the range of
from 4 to 10, more preferably from 4 to 6.5. In particular, from
the viewpoint of dispersing dropped abrasive particles to prevent
scratches, the pH is more preferably over 4.5, further more
preferably over 4.8, especially preferably over 5.0. Further, from
the viewpoint of suppressing dishing to obtain within-wafer
uniformity, the pH is more preferably less than 5.5, further
preferably less than 5.4, especially preferably less than 5.3.
Accordingly, the pH value is preferably higher than 4.5 and less
than 5.5, more preferably higher than 4.8 and less than 5.4,
especially preferably higher than 5.0 and less than 5.3.
[0055] The anionic surface-active agent preferably contains an
organic compound having a hydrophilic group which is either a
COO.sup.- group or a SO.sub.3.sup.- group. From the viewpoints of
dispersing dropped abrasive particles to prevent scratches and
suppressing dishing to obtain within-wafer uniformity, an organic
compound having a COO.sup.- group is particularly preferred.
Specific examples of such organic compounds include: sulfuric acid
compounds, such as alkylsulfuric acid, alkyl ether sulfuric acid,
and their salts; sulfonic acid compounds, such as alkylsulfonic
acid, alkylbenzene sulfonic acid, alkylnaphthalene sulfonic acid,
sulfosuccinic acid, a naphthalenesulfonic acid-formalin condensate,
lignin sulfonic acid, polystyrene sulfonic acid, and their salts;
carboxylic acid compounds, such as a soap, alkyl ether carboxylic
acid, polyacrylic acid, polymethacrylic acid, a copolymer
containing acrylic acid or metacrylic acid, and their salts; and
phosphoric acid compounds, such as alkylphosphoric acid, alkyl
ether phosphoric acid, and their salts. Of these compounds,
polyacrylic acid and its salts are most preferable. Among
polyacrylate, an ammonium salt is preferable.
[0056] From the viewpoint of dispersing dropped abrasive particles
to prevent scratches, the weight average molecular weight (Mw) of
polyacrylic acid and its salt is preferably not less than 13000,
more preferably not less than 14000, and most preferably not less
than 15000. On the other hand, from the viewpoint of suppressing
dishing to obtain within-wafer uniformity, the molecular weight is
preferably not more than 60000, more preferably not more than
50000, and most preferably not more than 45000. Accordingly, the
molecular weight is preferably not less than 13000 and not more
than 60000, more preferably not less than 14000 and not more than
50000, and most preferably not less than 15000 and not more than
45000. The molecular weight values of polyacrylic acid and its salt
were measured by gel permeation chromatography (GPC) carried out
under the following condition:
[0057] Pump: CCPD (Tosoh Corporation)
[0058] Detector: RI-71 (Shodex)
[0059] Data processing software: Gpc-8020 (Tosoh Corporation)
[0060] Column: G4000PW.sub.XL (Tosoh Corporation) was used as a
first-stage column, and G2500PW.sub.XL (Tosoh Corporation) was used
as a second-stage column
[0061] Eluent: A mixture of 0.2 M phosphoric acid buffer solution
and CH.sub.3CN in the ratio of 9:1 (volume ratio) was used. The 0.2
M phosphoric acid buffer solution is a mixture of 0.2 mol/L aqueous
KH.sub.2PO.sub.4 solution and 0.2 mol/L aqueous Na.sub.2HPO.sub.4
solution in the ratio of 1:1 (volume ratio).
[0062] Flow rate: 1.0 mL/min
[0063] Column temperature: 40.degree. C.
[0064] Sample amount: Aqueous solution of 0.5 wt % polyacrylic acid
and its salt 100 .mu.L
[0065] Molecular weight conversion: using seven species of
polyethylene glycol with known molecular weights (12000, 26000,
95000, 145000, 250000, 510000, 920000), a calibration curve was
prepared for conversion of molecular weight.
[0066] It is preferred that the polishing liquid of the present
invention contains at least two types of anionic surface-active
agents: polyacrylic acid and its salt which, from the viewpoint of
dispersing dropped abrasive particles to prevent scratches, has a
weight average molecular weight of not less than 25000 and not more
than 60000; and polyacrylic acid and its salt which, from the
viewpoint of suppressing dishing to obtain within-wafer uniformity,
has a weight average molecular weight of not less than 13000 and
less than 25000.
[0067] The first polyacrylic acid and its salt, has a weight
average molecular weight of not less than 25000 and not more than
60000, more preferably has a weight average molecular weight of not
less than 27000 and not more than 45000, and most preferably has
not less than 28000 and not more than 35000. From the viewpoints of
dispersing dropped abrasive particles to prevent scratches and
suppressing dishing to obtain within-wafer uniformity, the
surface-active agent is added preferably in an amount of not less
than 0.0005% by weight and less than 0.03% by weight, more
preferably not less than 0.001% by weight and less than 0.02% by
weight, most preferably not less than 0.005% by weight and less
than 0.01% by weight.
[0068] The second polyacrylic acid and its salt, has a weight
average molecular weight of not less than 13000 and less than
25000, more preferably has a weight average molecular weight of not
less than 14000 and not more than 24000, most preferably has not
less than 15000 and not more than 23000. From the viewpoints of
suppressing dishing to obtain within-wafer uniformity and achieving
a sufficiently high polishing rate, the surface-active agent is
added preferably in an amount of not less than 0.003% by weight and
less than 0.12% by weight, more preferably not less than 0.01% by
weight and less than 0.08% by weight, most preferably not less than
0.02% by weight and less than 0.04% by weight.
[0069] The experimental results of polishing of STI substrates
(SKW3-1, using MIT964 mask) by using a polishing apparatus
according to the present invention which includes a fixed abrasive
pad comprising ceria abrasive particles having an average particle
diameter of 0.2 .mu.m and a thermoplastic resin as a binder will be
described. The following polishing liquids 1, 2 and 3 were used to
polish the STI substrates: polishing liquid 1 containing 0.008% by
weight of polyacrylic acid having a molecular weight of 31200 and
0.032% by weight of polyacrylic acid having a molecular weight of
17700, the pH of the polishing liquid being adjusted to 5.1 with
ammonia; polishing liquid 2 containing 1.6% by weight of
polyacrylic acid having a molecular weight of 31200, the pH of the
polishing liquid being adjusted to 6.0 with ammonia; and polishing
liquid 3 containing 1.0% by weight of polyacrylic acid having a
molecular weight of 31200, the pH of the polishing liquid being
adjusted to 9.0 with ammonia. When the STI substrate was polished
using the polishing liquid 1, dishing was observed but the depth of
dishing was not more than 100 .ANG., while no scratch was observed,
thus good polishing was achieved. When the STI substrate was
polished using the polishing liquid 2, though dishing was at the
same level as the polishing liquid 1, some scratches were observed.
When the STI substrate was polished using the polishing liquid 3,
some scratches were observed and no improvement was found in terms
of dishing.
[0070] In this manner, by polishing a substrate while supplying a
polishing liquid which contains not less than 0.001% by weight and
less than 0.15% by weight of anionic surface-active agent and does
not contain abrasive particles and has a pH of 4 to 6.5, onto the
fixed abrasive 46a, it becomes possible to allow a nitride film to
act as a stopper, as described above. Accordingly, the polishing
rate of the nitride film can be lowered to thereby obtain
within-wafer uniformity of the nitride film and dishing of an oxide
film can be suppressed. A high-flatness polishing with fewer
scratches can thus be effected.
[0071] As described above, according to the present invention, a
workpiece is polished while supplying the polishing liquid which
contains an anionic surface-active agent at a low concentration of
not less than 0.001% by weight and less than 0.15% by weight and
does not contain abrasive particles and has a pH of 4 to 6.5, onto
a fixed abrasive. This makes it possible to allow a nitride film,
for example, to act as a polishing stopper. Accordingly, the
present invention makes it possible to lower the polishing rate of
a nitride film, thereby obtaining within-wafer uniformity of the
nitride film, and to suppress dishing, thus enabling a
high-flatness polishing with fewer scratches.
INDUSTRIAL APPLICABILITY
[0072] The present invention can be advantageously applied to a
polishing method and a polishing liquid for polishing a workpiece
such as a semiconductor wafer by using a fixed abrasive.
* * * * *