U.S. patent application number 09/737738 was filed with the patent office on 2001-04-26 for method for wafer polishing and method for polishing-pad dressing.
This patent application is currently assigned to Matsushita Electronics Corporation. Invention is credited to Hashimoto, Shin, Hidaka, Yoshiharu.
Application Number | 20010000490 09/737738 |
Document ID | / |
Family ID | 16024316 |
Filed Date | 2001-04-26 |
United States Patent
Application |
20010000490 |
Kind Code |
A1 |
Hashimoto, Shin ; et
al. |
April 26, 2001 |
Method for wafer polishing and method for polishing-pad
dressing
Abstract
It is arranged such that the least common multiple of two
numbers m and n of which one is prime to the other, is made as
large as possible where the number m is the rotational speed (rpm)
of a platen with a polishing pad affixed thereto and the number n
is the rotational speed (rpm) of a carrier with a wafer mounted
thereon. As a result of such arrangement, it is not until the
platen completes m revolutions that a point on the polishing pad
that comes into contact with a fixed point on the wafer returns to
the original contact point with the fixed point at the start of
polishing, and the resulting trajectory is therefore spread
uniformly over the polishing pad. Each point on the wafer is
brought into contact with most surface regions of the polishing
pad, therefore preventing the wafer from undergoing deterioration
in planarity uniformity due to a particular point on the wafer, on
one hand, frequently coming into contact with low polishing-rate
regions in the polishing pad and due to the other points on the
wafer, on the other hand, less frequently coming into contact with
the regions.
Inventors: |
Hashimoto, Shin; (Osaka,
JP) ; Hidaka, Yoshiharu; (Toyama, JP) |
Correspondence
Address: |
McDermott, Will & Emery
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Matsushita Electronics
Corporation
Osaka
JP
|
Family ID: |
16024316 |
Appl. No.: |
09/737738 |
Filed: |
December 18, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09737738 |
Dec 18, 2000 |
|
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09108323 |
Jul 1, 1998 |
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6180423 |
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Current U.S.
Class: |
438/14 ; 216/52;
257/E21.237; 451/28; 451/288 |
Current CPC
Class: |
H01L 21/30625 20130101;
H01L 21/02024 20130101; B24B 37/042 20130101; B24B 53/017
20130101 |
Class at
Publication: |
438/14 ; 438;
216/52; 451/28; 451/288 |
International
Class: |
H01L 021/66; B44C
001/22; B24B 005/00; B24B 029/00; G01R 031/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 2, 1997 |
JP |
9-177053 |
Claims
What is claimed is:
1. A wafer polishing method comprising the steps of: (a) rotating a
polishing pad affixed to a platen at a first rotational speed; (b)
supplying an abrasive material over a surface of said polishing
pad; and (c) pressing a wafer to be polished against said polishing
pad surface while at the same time rotating said wafer at a second
rotational speed; wherein the ratio of said first rotational speed
to said second rotational speed is controlled such that a
trajectory, formed by points on said polishing pad that come into
contact with a fixed point on said wafer, is distributed uniformly
on said polishing pad.
2. The wafer polishing method of claim 1 wherein said rotational
speed ratio is controlled such that said points on said polishing
pad do not form a substantially fixed trajectory during
polishing.
3. The wafer polishing method of claim 1 wherein said rotational
speed ratio is controlled such that, when said ratio is expressed
using two natural numbers m and n of which one is prime to the
other, the least common multiple of said numbers m and n is ten or
beyond.
4. The wafer polishing method of claim 1 wherein said rotational
speed ratio is controlled to be an approximate irrational
number.
5. The wafer polishing method of claim 1 wherein said polishing pad
is formed of a closed-cell-foam type polyurethane resin.
6. The wafer polishing method of claim 1 wherein said polishing pad
is provided with periodically-formed grooves or pores.
7. A polishing-pad dressing method comprising the steps of: (a)
rotating a polishing pad affixed to a rotary platen, at a first
rotational speed; and (b) pressing a dresser against a surface of
said polishing pad while at the same time rotating said dresser at
a second rotational speed for activation of said polishing pad
surface; wherein the ratio of said first rotational speed to said
second rotational speed is controlled such that a trajectory,
formed by points on said polishing pad that come into contact with
a fixed point on said dresser, is distributed uniformly on said
polishing pad.
8. The polishing-pad dressing method of claim 7 wherein said
rotational speed ratio is controlled such that said points on said
polishing pad do not form a substantially fixed trajectory during
polishing.
9. The polishing-pad dressing method of claim 7 wherein said
rotational speed ratio is controlled such that, when said ratio is
expressed using two natural numbers m and n of which one is prime
to the other, the least common multiple of said numbers m and n is
ten or beyond.
10. The polishing-pad dressing method of claim 7 wherein said
rotational speed ratio is controlled to be an approximate
irrational number.
Description
BACKGROUND OF THE INVENTION
1. This invention relates to a method for polishing a surface of a
wafer by CMP (chemical-mechanical polishing) and to a method for
dressing a polishing pad which is used in such CMP.
2. Chemical-mechanical polishing (CMP), a combination of chemical
and mechanical polishing, is an attractive polishing process for
planarizing wafers incorporating therein semiconductor integrated
circuits, to such an extent that the wafers are provided with
almost perfect surface flatness. In a typical CMP technique, a
wafer to be polished is mounted onto a polishing pad attached to a
platen. The wafer is then rotated, during which a slurry-like
abrasive liquid (dispersion of a colloidal silica in a liquid) is
supplied between the wafer and the polishing pad, to polish a
surface of the wafer.
3. SOG (spin-on-glass) and etch back are known in the art as a
process for planarizing an upper surface of a film such as an
interlayer dielectric film of a wafer. In the former process a
wafer is spin-coated with a glass solution prepared by dissolution
of glass in an organic solvent. In the latter process a film of
photo resist is deposited on an interlayer dielectric film and
these films are thereafter subjected to simultaneous etch back
processing. The CMP process has the advantage over these two
processes in that wafers can be planarized more perfectly because
the CMP process combines both chemical polishing and mechanical
polishing. However, the current technology of the CMP process is
not satisfactory. Achieving ideal planarity everywhere in a wafer
is still difficult and there is yet room for improvement in the CMP
process. Various approaches have been made with a view to improving
the uniformity of in-wafer planarity.
4. One of the approaches is set forth in Japanese Patent
Publication (KOKAI) No. 8-339979. This application describes a
technique for supporting a wafer lower surface with the aid of
fluid, to improve the in-wafer planarity uniformity.
5. Another approach is described in Japanese Patent Publication
(KOKAI) No. 9-225812. This application provides means for
maintaining the degree of planarity at an adequate level while
performing a CMP process, to improve the in-wafer planarity
uniformity.
6. The following Preston equation is known and is generally used to
calculate the CMP polishing rate (Rpo).
Rpo=k*P*V,
7. where k is the Preston coefficient, P is the pressure, and V is
the polishing pad/wafer relative speed.
8. In order to improve in-plane uniformity of the polishing rate,
based on the Preston equation, equalization in time quadrature of V
(the polishing pad/wafer relative speed) at any points on the wafer
is desired. In other words, it has been determined from the Preston
equation that such equalization is achieved at an arbitrary point
on the wafer to provide best in-plane polishing rate uniformity if
both a polishing pad and a wafer rotate at the same speed.
9. However, CMP is a combination of chemical polishing and
mechanical polishing, which makes, in actual process, variations in
polishing state complicated. It is difficult to constantly place a
polishing pad in an ideal state during a period of polishing. As
described in a paper reported in VLSI Multilevel Interconnection
Conference (1997), pp. 175-179, not every condition derived from
the Preston equation yields best in-plane polishing rate
uniformity. Although some reasons why the best conditions sometimes
happen to differ from the Preston equation-based conditions may be
pointed out, no novel guidelines for improving in-plane polishing
rate uniformity are proposed in the foregoing paper.
10. For example, when mounting a polishing pad of closed-cell-foam
type polyurethane onto a platen, the inventors of the present
invention believe that the polishing rate varies for the following
mechanical reason.
11. The closed-cell-foam type polyurethane polishing pad, as
illustrated in FIG. 10, has at its surface a great number of recess
portions with a diameter in a range of 50-100 .mu.m. The recess
portions result from the breaking of closed cells at the surface,
and a slurry-like abrasive is held in the recess portions. During
polishing, the abrasive is supplied between a polishing pad and a
wafer little by little. If polishing debris, formed as a result of
polishing of the wafer and the pad, is collected in a recess
portion, or if recess-portion blocking occurs locally owing to the
load of the wafer, polishing is not performed on a portion of the
wafer corresponding to such a recess portion filled with polishing
debris. Because of the foregoing, the polishing pad will undergo a
local variation in polishing rate, resulting in a drop in overall
polishing rate. To cope with this problem, the surface of the
polishing pad is grounded with a dressing disk having abrasive
particles such as diamond after the polishing pad has been used for
a certain length of time. This allows the entire polishing pad to
become re-activated, and there are formed new recess portions at
the surface. However, to date, it is difficult to completely
prevent clogged recess portions and deterioration in planarity
between one dressing and the next dressing.
12. The inventors of the present invention noted that the following
points suggest that the foregoing in-pad local polishing rate
variation adversely affects uniformity of the wafer planarity.
13. Specifically, after a polishing pad is subjected to a dressing
process, it sometimes occurs that grains of diamond, dettached from
a dressing disk and then remaining on a polishing pad, produce in
the wafer a deep, large scratch visible to even the naked eye. This
scratch was observed and the observation result shows that the size
of the scratch is large and deep as compared with those of the
diamond grain. The reason why such a large scratch is created may
be explained as follows. A grain of diamond, cut into a wafer,
passes through a fixed trajectory many times with pad/wafer
relative rotational motion, as a result of which the original micro
scratch gradually develops until visible to the naked eye.
14. To summarize, in the case there exists the foregoing
non-uniformity of polishing rate in a polishing pad, if there is
locally created a low polishing-rate portion in the polishing pad
which frequently passes through a corresponding wafer region (in
other words if a fixed point on the wafer frequently passes through
a specific region on the polishing pad) the variation in polishing
rate of the polishing pad gradually promotes deterioration in wafer
planarity uniformity. However, the relationship between polishing
pad rotation and wafer rotation has been little considered in
conventional CMP processing.
SUMMARY OF THE INVENTION
15. Based on the appreciation of the foregoing problems, the
present invention was made. Apart from the problem of polishing-pad
planarity and the problem of accuracy (e.g., parallelism between
wafer and polishing pad), a major object of the invention is
therefore to provide a method capable of improving uniformity in
the wafer planarity by achieving uniform distribution of regions of
a polishing pad that come into contact with each point of the wafer
without ill effect (i.e., non-uniformity in the polishing rate in
the polishing pad).
16. The present invention provides a wafer polishing method
comprising the steps of:
17. (a) rotating a polishing pad affixed to a platen at a first
rotational speed;
18. (b) supplying an abrasive material over a surface of said
polishing pad; and
19. (c) pressing a wafer to be polished against said polishing pad
surface while at the same time rotating said wafer at a second
rotational speed;
20. wherein the ratio of said first rotational speed to said second
rotational speed is controlled such that a trajectory, formed by
points on said polishing pad that come, in turn, into contact with
a fixed point on said wafer, is distributed uniformly on said
polishing pad.
21. One important aspect of the method of the present invention is
that, during polishing, an arbitrary point on the wafer is brought
into contact with as many points on the polishing pad as possible.
Such a method eliminates a harmful influence due to a variation in
local polishing rate occurring in the polishing pad, therefore
improving the post-polishing uniformity of planarity of a wafer
surface to be polished.
22. A variation to the foregoing method can be made in which said
rotational speed ratio is controlled such that said points on said
polishing pad do not form a substantially fixed trajectory during
polishing.
23. One important aspect of the foregoing variation is that every
point on the wafer is brought into contact with many regions on the
polishing pad. Such arrangement eliminates a harmful influence due
to a variation in local polishing rate occurring in the polishing
pad, therefore improving the post-polishing uniformity of planarity
of a wafer surface to be polished.
24. Another variation to the foregoing method can be made in which
said rotational speed ratio is controlled such that, when said
ratio is expressed using two natural numbers m and n of which one
is prime to the other, the least common multiple of said numbers m
and n is ten or beyond.
25. One important aspect of the foregoing variation is as follows.
When the platen makes m revolutions, the wafer makes mn/m (=n)
revolutions, and a point on the polishing pad that comes into
contact with the fixed point on the wafer returns to its home
position and then moves over a fixed trajectory. When the least
common multiple of the numbers m and n is large, however, the
overall length of the fixed trajectory extends. Corresponding to
such an extension, the fixed trajectory passes through a greater
number of regions on the polishing pad. This avoids a situation of
an arbitrary point on the wafer coming into contact with a low
polishing-rate region, that is locally created in the polishing
pad, with considerable frequency as compared with other points on
the wafer. This improves the post-polishing uniformity of planarity
of a wafer surface to be polished.
26. Yet another variation to the foregoing method can be made in
which said rotational speed ratio is controlled to be an
approximate irrational number.
27. On important aspect of the foregoing variation is that, even
when the ratio of the platen rotational speed to the wafer
rotational speed is approximately expressed by a ratio represented
by integers, the least common multiple of these integers is
considerably large. Accordingly, until the time when points on the
polishing pad that come into contact with a certain point on the
wafer enter a fixed trajectory, the fixed trajectory have passed
through every region on the polishing pad. Additionally within a
given polishing time that is practically limited, points on the
polishing pad that come into contact with one point on the wafer
will not get into a fixed trajectory. Accordingly, the foregoing
operation and effects of the present invention can be significantly
obtained.
28. Another variation to the foregoing method can be made in which
said polishing pad is formed of a closed-cell-foam type
polyurethane resin.
29. One important aspect of the foregoing variation is as follows.
Even when a low polishing-rate portion is locally created because a
recess portion, which is formed by breaking of a closed cell,
becomes blocked with polishing debris or is broken, a case, in
which influence by the presence of such a low polishing-rate
portion is strongly exerted on only a specific point on the wafer,
does not occur by virtue of the foregoing operation. This ensures
that the foregoing operation and effects of the present invention
can be obtained.
30. In another variation to the foregoing method, said polishing
pad is provided with periodically-formed grooves or pores.
31. One important aspect of the foregoing variation is that smooth
supply and discharge of a slurry-like abrasive material are carried
out through grooves or pores. Although a clogged groove or pore can
be taken as an inactive region that does not substantially
contribute to the action of polishing, a case, in which influence
by the presence of such a clogged groove is strongly exerted on
only a specific region, does not occur by virtue of the foregoing
operation. This ensures that the foregoing operation and effects of
the present invention can be obtained.
32. The present invention also discloses a polishing-pad dressing
method comprising the steps of:
33. (a) rotating a polishing pad affixed to a rotary platen, at a
first rotational speed; and
34. (b) pressing a dresser against a surface of said polishing pad
while at the same time rotating said dresser at a second rotational
speed for activation of said polishing pad surface;
35. wherein the ratio of said first rotational speed to said second
rotational speed is controlled such that a trajectory, formed by
points on said polishing pad that come into contact with a fixed
point on said dresser, is distributed uniformly on said polishing
pad.
36. A variation to the foregoing method can be made in which said
rotational speed ratio is controlled such that said points on said
polishing pad do not form a substantially fixed trajectory during
polishing.
37. Another variation to the foregoing method can be made in which
said rotational speed ratio is controlled such that, when said
ratio is expressed using two natural numbers m and n of which one
is prime to the other, the least common multiple of said numbers m
and n is ten or beyond.
38. Yet another variation to the foregoing method can be made in
which said rotational speed ratio is controlled to be an
approximate irrational number.
39. Important aspects of the foregoing variations to the aforesaid
method are as follows. A case is considered in which a polishing
pad is subjected to dressing with the aid of a dresser with very
fine diamond grains embedded therein. If diamond grains that differ
from one another in shape and in dimensions draw the same
trajectories respectively on the polishing pad, this results in
deterioration in dressing uniformity. However, such deterioration
can be suppressed by the present invention, resulting in providing
almost uniform polishing pad activation.
BRIEF DESCRIPTION OF THE DRAWINGS
40. FIG. 1 is a perspective view outlining a way of polishing a
wafer by a CMP polishing apparatus of a first embodiment of the
invention.
41. FIG. 2 is a complex plane representation useful for
understanding relative motion between a polishing pad and a wafer
in the first embodiment.
42. FIG. 3 is a top view showing respective contact positions at
which a fixed point on the wafer contacts with the polishing pad
when the polishing pad makes revolutions an integral number of
times, namely one revolution, two revolutions, and m revolutions in
the first embodiment.
43. FIG. 4 is a top view of a trajectory drawn on the polishing pad
in Experimental Example 1.
44. FIG. 5 is a top view of a trajectory drawn on the polishing pad
in Experimental Example 2.
45. FIG. 6 is a top view of a trajectory drawn on the polishing pad
in Comparative Example 1.
46. FIG. 7 is a top view of a trajectory drawn on the polishing pad
in Comparative Example 2.
47. FIG. 8 is a top view of a trajectory drawn on the polishing pad
in Comparative Example 3.
48. FIG. 9 is a diagram showing both variations in average
polishing rate and variations in average polishing rate of
different wafers prepared by performing CMP processing under
conditions of Experimental Example 2 and under conditions of
Comparative Example 3 respectively.
49. FIG. 10 shows in cross section a surface state of a polishing
pad under polishing by CMP.
50. FIG. 11 is a perspective view outlining a polishing pad
dressing method of a second embodiment of the invention.
51. FIG. 12 is a complex plane representaion useful in
understanding relative motion between a polishing pad and a dresser
in the second embodiment.
52. FIG. 13 is a top view showing respective contact positions at
which a fixed point on the dresser contacts with the polishing pad
when the polishing pad makes revolutions an integral number of
times, namely one revolution, two revolutions, and m revolutions in
the second embodiment.
DETAILED DESCRIPTION OF THE INVENTION
53. Preferred embodiments of the present invention are now
described below.
54. Embodiment 1
55. FIG. 1 is a perspective view of an exemplary structure of a
polishing apparatus used in a CMP process in accordance with a
first embodiment of this invention. A CMP polishing apparatus of
the present embodiment, manufactured by Speedfam Company Limited,
includes a disk-like platen 1 rotatable about its central axis, a
platen shaft 2 which centrally supports the platen 1, a polishing
pad 3 affixed onto the platen 1 and formed of a closed-cell-foam
type polyurethane resin and an unwoven cloth, a disk-like carrier 4
on which is mounted a silicon wafer 6, a carrier shaft 5 which
centrally supports the carrier 4, and a polishing liquid supply
apparatus 7 for supplying a slurry-like polishing liquid 8 the main
component of which is colloidal silica. Both the platen shaft 2 and
the carrier shaft 5 are forcefully rotated by servomotor or the
like. The rotational speed of the platen shaft 2 and the rotational
speed of the carrier shaft 5 are variably controlled independently
of each other.
56. FIG. 2 is a complex plane representation showing a positional
relationship between the polishing pad 3 and the wafer 6 in the CMP
polishing apparatus of the present invention. Making reference to
FIG. 2, the manner of relative motion resulting from the rotation
of the polishing pad 3 (i.e., the rotation of the platen 1) and the
rotation of the wafer 6 (i.e., the rotation of the carrier 4), is
illustrated.
57. Parameters of FIG. 2 concerning both the rotational motion of
the carrier 2 and the rotational motion of the platen 1 are as
follows.
58. r: distance between wafer center P (i.e., the wafer rotational
center) and point Z on the wafer
59. R: distance between P and polishing pad center O (i.e., the
polishing pad rotational center)
60. w: wafer rotational angular speed
61. .theta.: wafer initial phase angle
62. wp: platen rotational angular speed
63. The carrier 4 can hold thereon a plurality of wafers in some
cases. However, for convenience the present embodiment is
illustrated in terms of a case in which only one wafer 6 is mounted
on the carrier 4. Additionally, the center of the wafer 6 coincides
with that of the carrier 4 and the centre of the polishing pad 3
coincides with that of the platen 1.
64. Defining a state, in which the wafer center P and the polishing
pad center O are offset in y (imaginary) axis direction in a
complex plane, as an initial state, and if the initial phase angle
e of the point Z on the wafer and the polishing time t in a rest
frame are variables, the position of the point Z in rotational
motion in clockwise (CCW) direction is given by the following
equation (1). In the equation (1), i represents the imaginary
unit.
Z(t, .theta.)= i R+r Exp {i (-wt+.theta.)} (1)
65. If such wafer motion is observed on the polishing pad which
rotates in CCW direction, this determines a trajectory formed or
drawn by the point Z on the polishing pad. What is required to find
the trajectory is to find a CCW rotational mapping of Equation (1),
which is given by the following equation (2).
Z.fwdarw.Zp (t, .theta.)
Exp (i wp t) [i R+r Exp {i (-wt+.theta.)}]
i R Exp (i wp t)+r Exp [i {(wp-w) t+.theta.}] (2)
66. How the trajectory varies is determined by the ratio of w to wp
on the basis of Equation (2). For example, when wp is twice w, the
following equation (4) is derived from substitution of Equation (3)
in Equation (2).
wp=2w (3)
Zp=i R Exp (i 2wt)+r Exp {i (wt+.theta.)} (4)
67. If the location of a point X on the wafer which is advanced in
initial phase angle by pi (180.degree.) with respect to the initial
phase angle .theta. of the point Z is obtained from Equation (4),
the result is given by the following equation (5).
Zp(t, .theta.+pi)
=i R Exp (i 2wt)+r Exp [{i (wt+.theta.+pi)})] (5)
68. Further, if the location of the point X after a lapse of a time
pi/w is obtained from Equation (4), this results in the following
equation (6). Equations (4) and (6) completely agree with each
other.
Zp (t+pi/w, .theta.+pi)=i R Exp {i (2wt+2pi)
+r Exp [i {w (t+pi/w)+.theta.+pi)}]
=i R Exp (i 2wt)+r Exp [{i (wt+.theta.+pi)}]
=Zp (t, .theta.) (6)
69. In other words, if w/wp=1/2, a trajectory drawn on the pad by
contact between the point Z and the pad completely agrees with
another drawn on the pad by a contact point between the point X and
the pad (these two points Z and X are in symmetry with respect to
P, in other words they differ from each other in phase by
180.degree.), with only a time lag of w/pi (1/2 of the period of
w).
70. A commonly-used condition of w/wp=1/2 (for example, w= 30 rpm
and w=60 rpm) means that both an arbitrary point on the wafer and
another point opposite thereto with respect to the wafer center
keep rotating along the same trajectory thirty times per
minute.
71. If a polishing pad, on which two different points on a wafer
move along the same trajectory, has a local factor that contributes
to a variation in polishing rate, such a micro factor (i.e., the
local factor) will gradually develop on the surface of the wafer
thereby finally producing an unwanted macro phenomenon that
deteriorates planarity uniformity of the wafer. For instance, if a
notch present on one point causes a polishing pad surface to
undergo a non-uniform variation thereby resulting in an abnormal
polishing rate, this has an effect on the opposite side to the one
point; for example, the polishing pad is compressed
excessively.
72. Generally, in a case in which the polishing pad angular speed
wp is 2n times as large as the wafer angular speed w where the
number n is any integer, points of the wafer (the phase difference
in wafer rotation therebetween being pi/n) move along the same
trajectory over the polishing pad.
73. For example, if wp : w=4:1, four points of the wafer, i.e., an
arbitrary point, a second point (rotated 90.degree. from the
arbitrary point around the wafer center), a third point (rotated
180.degree.), and a fourth point (rotated 270.degree.), move along
the same trajectory. Such a situation must be avoided in CMP recipe
preparation. However, the relationship between platen rotational
speed and carrier rotational speed has been little considered until
the present invention.
74. Inconveniences occur, even when the platen-carrier rotational
speed ratio is simply expressed by integers, let alone when the
ratio is expressed in a simple ratio form such as (integer) : 1. An
exemplary case is now considered in which the ratio of the
polishing pad angular speed and the wafer angular speed is m:n
where the number m is prime to the number n and the numbers m and n
are positive integers other than 1. FIG. 3 is a diagram showing
rotational states of the polishing pad and rotational states of the
wafer when the polishing pad makes one revolution, when the
polishing pad makes two revolutions, and when the polishing pad
makes m revolutions respectively. Suppose here that Point Z on the
wafer is in contact with Point S on the polishing pad in the
initial state (see FIG. 3). Point S is a start point at which a
fixed trajectory starts. When the platen makes one revolution from
the initial state, the carrier makes n/m revolutions. Since the
number m is prime to the number n, division of n by m, i.e., n/m,
never produces an integral result, and Point S' on the polishing
pad that comes into contact with Point Z on the wafer will never
conform to Point S. When the platen makes m revolutions, the
carrier makes mn/m (=n) revolutions, and Point S' on the polishing
pad that comes into contact with Point Z on the wafer conforms to
Point S (i.e., the original contact point) for the first time, in
other words a point on the polishing pad that comes into contact
with Point Z on the wafer arrives at Point S where the fixed
trajectory starts. The polishing pad and the wafer thereafter
repeat the same relative motion, as a result of which Point Z on
the wafer moves along the fixed trajectory on the polishing
pad.
75. When the least common multiple (B.C.M.) of the integers m and n
(the number m is prime to the number n), i.e., mn, is large, the
length of the fixed trajectory extends. In addition to making
contact with a specific point on the polishing pad, Point Z on the
wafer is evenly brought into contact also with many other points on
the polishing pad. On the other hand, when the L.C.M. is small,
Point Z soon returns to Start Point and thereafter moves along the
same trajectory, in other words Point Z comes into contact with
limited regions. This produces the possibility that a specific
point on the wafer is frequently brought into contact with a low
polishing-rate region that is locally created in the polishing pad
while other points on the wafer infrequently come into contact with
the region.
76. The following conclusions are drawn from the above
consideration.
77. 1. When the platen-carrier rotational speed ratio is expressed
by two natural numbers, i.e., n and m (the number n is prime to the
number m), it is preferable to make the L.C.M. of these numbers m
and n, i.e., mn, as large as possible. The experiments, which are
described later, show that the L.C.M. of the numbers m and n (mn)
is preferably 10 or beyond.
78. 2. It is particularly preferred that the platen-carrier
rotational speed ratio, i.e., the platen-carrier angular speed
ratio, nearly corresponds to, or is approximated to an irrational
number that cannot be expressed in the form of a natural (rational)
number m/n. Practically it is difficult to allow the ratio to
exactly correspond to an irrational number; however, if the ratio
is an approximate irrational number this makes it possible for
Point Z on the wafer to come into contact with almost every point
on the polishing pad without travelling on the fixed trajectory on
the polishing pad in a limited polishing period of time. Setting of
such a rotational speed ratio which is an approximate irrational
number can be achieved easily by setting rotational speeds for
motors that drive the carrier shaft 5 and the platen shaft 2.
79. 3. Taking into account a practically limited period of time
taken for polishing, it is sufficient that the platen-carrier
rotational speed ratio is set in such a way as to prevent entrance
of a point on the polishing pad to a fixed trajectory during
polishing. In other words the rotational speed ratio is set in
order for a point on the polishing pad that comes into contact with
Point Z on the wafer not to conform to Start Point S, with the
polishing pad and the wafer rotated an integral number of
times.
80. 4. Taking into account the state of the foregoing trajectory,
it is preferred that, although the surface region of the polishing
pad is divided into sub-regions by a trajectory drawn by a point on
the polishing pad that comes into contact with Point Z on the wafer
(see FIG. 5), these sub-regions are uniform in size and fine within
concentric ring-like regions, i.e., within ring-like regions having
identical radii from the polishing pad center O. In other words, it
is preferred that the foregoing trajectory is evenly distributed on
the polishing pad, to densely form lattice-like patterns
thereon.
81. Substantially, the number of revolutions of a CMP polishing
apparatus is m (rpm) where m is an integer. Points on the polishing
pad that come into contact with Point Z on the wafer repeatedly
form the same trajectory at polishing time intervals of about one
minute. The process of polishing is carried out for about four
minutes at most, and it is therefore preferred that the same fixed
trajectory is not drawn four times or more during polishing.
82. In the above-described analyses the carrier (wafer) is
subjected to rotational motion only. The carrier may be subjected,
in addition to rotational movement, to reciprocating motion
(translational motion) either in a direction perpendicular to the
direction in which the platen rotates, in a direction corresponding
to the direction in which the platen rotates, or in a direction
diagonally intersecting with the direction in which the platen
rotates. Additionally, by sufficiently expanding the range of such
translational motion to replace the number of times per unit time
reciprocate motion is carried out with the number of revolutions
per unit time of the carrier, the same effects as the above can be
achieved without having to rotate the carrier depending on the
case.
83. In the present embodiment, the number of wafers mounted on the
carrier is one. The present invention is also applicable to cases
in which a plurality of wafers are mounted on a single carrier. In
such a case each wafer point rotates on the carrier center and the
analyses of the foregoing equations (1) to (6) can be utilized, by
taking (i) r=the distance from the carrier center to Point Z of the
wafer and (ii) R (offset distance)=the distance from the polishing
pad center O to the carrier center.
84. Experimental examples and comparative examples (conventional
conditions) for analyzing trajectories on polishing pads (platens)
are now described below. A polishing apparatus by Speedfam Company
Limited was used. In each example, the initial phase angle .theta.
is 45 degrees, the wafer diameter r is 100 mm, the offset distance
R is 162. 5 mm, and the polishing time is 60 seconds. A target for
polishing is a p-type TEOS film or a p-type BPSG film formed on a
wafer.
85. Experimental Example 1
86. FIG. 4 illustrates a trajectory drawn on a polishing pad by a
single point (a fixed point) on a wafer in Experimental Example 1
in accordance with the present invention. Polishing conditions of
the present experimental example are CMP conditions for
planarization of BPSG films and the platen rotational speed (wp) is
23 rpm. The carrier rotational speed (w) is 17 rpm. In other words,
the platen-carrier rotational speed ratio is expressed by two
numbers, 23 and 17, of which one number is prime to the other
number, and the L.C.M of these two numbers, i.e., 23.times.17
(=391), is large. As can be seen from the figure, within a time
less than the polishing time (60 seconds) there is drawn no fixed
trajectory on the polishing pad and a certain point on the wafer
does not move along a fixed trajectory on the polishing pad. When
polishing is carried out for 60 seconds, i.e., when the platen
makes 23 revolutions, the fixed point on the wafer returns to the
original contact point on the polishing pad, in other words a point
on the polishing pad that comes into contact with the fixed point
on the wafer does not enter the fixed trajectory. Even in such a
case the fixed point on the wafer equally comes into contact with
many regions on the polishing pad, therefore avoiding ill effects
due to a local variation in polishing rate.
87. Experimental Example 2
88. FIG. 5 is a diagram showing a trajectory drawn on a polishing
pad by a fixed point on a wafer in Experimental Example 2 in
accordance with the present invention. Polishing conditions of the
present experimental example are CMP conditions for planarization
of p-type TEOS films. The platen rotational speed (wp) is 61 rpm
and the carrier rotational speed (w) is 43 rpm. In other words, the
platen-carrier rotational speed ratio is expressed by two numbers,
61 and 43, of which one number is prime to the other number, and
the L.C.M of these two numbers, i.e., 61.times.43 (=2643), is
large. As can be seen from the figure, within a time less than the
polishing time (60 seconds), a point on the polishing pad that
comes into contact with the fixed point on the wafer does not enter
a fixed trajectory. When polishing is carried out for 60 seconds,
i.e., when the platen makes 61 revolutions, a certain point on the
wafer returns to the original contact point on the polishing pad.
Even in such a case, the point on the wafer equally comes into
contact with many regions on the polishing pad, therefore avoiding
ill effects due to a local variation in polishing rate.
89. Comparative Example 1
90. FIG. 6 is a diagram of a trajectory drawn on a polishing pad by
a point on a wafer in Comparative Example 1. Polishing conditions
of Comparative Example 1 are conventional CMP conditions used for
planarization of BPSG films. The platen rotational speed wp is 20
rpm and the carrier rotational speed w is 20 rpm. As can bee seen
from FIG. 6, resonance occurs so that a fixed trajectory, which is
in an almost perfect circular form, is drawn on the polishing pad.
This shows that a certain point on the wafer frequently comes into
contact with limited regions on the polishing pad.
91. Comparative Example 2
92. FIG. 7 is a diagram of a trajectory drawn on a polishing pad by
a point on a wafer in Comparative Example 2. Polishing conditions
of Comparative Example 2 are conventional CMP conditions used for
planarization of p-type TEOS films. The platen rotational speed wp
is 60 rpm and the carrier rotational speed w is 30 rpm. In other
words, the platen-carrier rotational speed ratio is expressed by
two numbers, 2 and 1, of which one number is prime to the other
number, and the L.C.M of these two numbers, i.e., 2.times.1 (=2),
is small. As shown in the figure, a point on the polishing pad that
comes into contact with a fixed point on the wafer returns to the
start point of a fixed trajectory every time the platen makes two
revolutions and thereafter resonance occurs resulting in motion
along the fixed trajectory. This shows that a certain point on the
wafer frequently (but less frequently than in Comparative Example
1) comes into contact with only limited regions on the polishing
pad.
93. Comparative Example 3
94. FIG. 8 is a diagram of a trajectory drawn on a polishing pad by
a fixed point on a wafer in Comparative Example 3. Polishing
conditions of Comparative Example 3 are conventional CMP
conditions. The platen rotational speed wp is 60 rpm and the
carrier rotational speed w is 40 rpm. In other words, the
platen-carrier rotational speed ratio is expressed by two numbers,
3 and 2, of which one number is prime to the other number, and the
L.C.M of these two numbers, i.e., 3.times.2 (=6), is small. As
shown in the figure, a point on the polishing pad that comes into
contact with the fixed point on the wafer returns to the start
point of a fixed trajectory every time the platen makes three
revolutions and thereafter resonance occurs resulting in motion
along the fixed trajectory. This shows that a certain point on the
wafer frequently (but less frequently than in Comparative Examples
1 and 2) comes into contact with only limited regions on the
polishing pad.
95. Next, the difference in uniformity of the wafer planarity
between the Experimental Examples polishing conditions utilizing
the present invention and the Comparative Examples polishing
conditions is described below. FIG. 9 graphically compares
Experimental Example 2 (wp=61 rpm; w=43) and Comparative Example 3
(wp=60 rpm; w=40 rpm). In other words, SSR (nm/min) indicated by
.box-solid. and WIWNU (Within-Wafer-Non-Uniform- ity) (%) indicated
by .quadrature. of Experimental Example 2 are compared with
polishing rates indicated by .circle-solid. and polishing rate
variations indicated by .smallcircle. of Comparative Example 3,
where SSR indicates the average wafer polishing rate and WIWNU
indicates the wafer in-plane polishing rate variation. A load of
140 KgG was applied to the carrier, and wafer polishing rate
measurement is carried out at several locations, exclusive of
regions laying within 5 mm from the periphery of the wafer. A
variation in polishing rate is expressed by a value obtained as a
result of division of a difference between the maximum and minimum
of measured values at locations in the same wafer by an average
measured value. FIG. 9 shows that there is no difference in average
polishing rate between Comparative Example 3 and Experimental
Example 2; however, if CMP processing is performed using
Experimental Example's 2 conditions in accordance with the present
invention, this achieves further reductions in in-wafer polishing
rate variation as compared with Comparative Example 3. Uniformity
of the wafer planarity is clearly improved by utilizing the present
invention.
96. It follows from the foregoing experimental and comparative
examples that, when the ratio of wp (the platen rotational speed)
to w (the wafer rotational speed) is expressed by natural numbers m
and n of which one is prime to the other, the L.C.M. of these two
numbers is preferably 10 or beyond in order to obtain the present
invention's benefits.
97. In such a case it is much preferred that none of the numbers m
and n are 1 (for example, m=5 and n=2), which is however not
absolutely necessary. For instance, in case m is assigned a value
of 10 and n a value of 1, such a value setting allows the carrier
to make one revolution when the platen makes ten revolutions, and,
at this point in time, a contact point with a fixed point on the
wafer returns to the start point of a fixed trajectory on the
polishing pad. In other words, a fixed trajectory formed on the
polishing pad comprises a first spiral that gets to an
internal-diameter portion from an external-diameter portion when it
makes five revolutions and a second spiral which rotates in a
direction opposite to that of the first spiral and which gets to an
outer peripheral portion from an inter peripheral portion when it
makes five revolutions. On the other hand, if m and n are assigned
a value of 1 and a value of 10 respectively (m=1 and n=10), this
value setting allows the platen to make one revolution when the
carrier makes ten revolutions, and, at this point in time, a
contact point returns to the start point of a fixed trajectory. The
resulting trajectory is in the form of a small coil of ten rounds.
In both of the foregoing settings (i.e., the L.C.M. of the numbers
m and n is ten or greater), a fixed trajectory is formed on the
polishing pad in order that it can pass through many regions,
therefore avoiding circumstances in which there is an increase in
the probability that a specified point on the wafer frequently
comes into contact with regions that are low in polishing rate as
compared with other points.
98. Embodiment 2
99. The present invention can be applied to a process, i.e., a
dressing process step, for dressing of a polishing pad with a
diamond dresser to make polishing-pad activation (polishing rate
recovery). Grains of diamond, which are very fine, are embedded
into a surface of a diamond dresser. These diamond grains have
different shapes and dimensions. If each diamond grain draws the
same trajectory with considerable frequency at the time of
performing a dressing on a polishing pad that is being rotated
while rotating a diamond dresser, this becomes a bar to obtaining
the uniformity of dressing, as in the case of polishing. As a
result, polishing-pad activation is not carried out in uniform
fashion, in addition to which local variations in polishing rate
occur in the polishing pad. The foregoing first embodiment can be
applied intact to dressing uniformity processing.
100. A second embodiment of the invention is now described. The
second embodiment provides a method for dressing of a polishing pad
which uses the first embodiment of the present invention.
101. FIG. 11 is a perspective view of a polishing apparatus when a
dressing according to the second embodiment is performed on a
polishing pad. A CMP polishing apparatus, used in the present
embodiment as well as in the first embodiment, is illustrated by
reference to FIG. 11. The CMP polishing apparatus, on one hand,
includes a disk-like platen 1 rotatable about its central axis, a
platen shaft 2 which centrally supports the platen 1, a polishing
pad 3 affixed onto the platen 1 and formed of a closed-cell-foam
type polyurethane resin and an unwoven cloth. A dresser apparatus,
on the other hand, includes a disk-like dresser 11 and a dresser
shaft 12 which centrally supports the dresser 11. Both the platen
shaft 2 and the dresser shaft 12 are forcefully rotated by
servomotor or the like. The rotational speed of the platen shaft 2
and the rotational speed of the dresser shaft 12 are variably
controlled independently of each other.
102. FIG. 12 is a complex plane representation useful in
understanding the positional relationship between the polishing pad
3 in the polishing apparatus and the dresser 11. As can be seen
from FIG. 12, the manner of relative motion between the rotation of
the platen 1 and the rotation of the dresser 11 is basically
identical with the manner of relative motion between the rotation
of the platen 1 and the rotation of the carrier 4 in the first
embodiment of the present invention.
103. Parameters of FIG. 2 concerning both the polishing-pad
rotational motion and the dresser rotational motion are as follows.
The same signs as the first embodiment are used.
104. r: distance between dresser center P and point Z on the
dresser
105. R: distance between P and polishing pad center O
106. w: dresser rotational angular speed
107. .theta.: dresser initial phase angle
108. wp: platen rotational angular speed
109. Equations (1)-(6) of the first embodiment are applicable to
the second embodiment without having to make any changes
thereto.
110. Thus, if w/wp=1/2, a trajectory drawn on the pad by contact
between the point Z and the pad completely coincides with another
drawn on the pad by contact between the point X and the pad (these
two points Z and X are in symmetry with respect to P, in other
words they differ from each other in phase by 180.degree.), with
only a time lag of w/pi (1/2 of the period of w).
111. Inconveniences occur when the platen-dresser rotational speed
ratio is simply expressed by integers and when the ratio is
expressed in a simple ratio form such as (integer): 1. An exemplary
case is now considered in which the ratio of the polishing-pad
angular speed and the dresser angular speed is m:n where the number
m is prime to the number n and the numbers m and n are positive
integers, exclusive of 1. FIG. 13 is a diagram showing rotational
states of the polishing pad and rotational states of the dresser
when the polishing pad makes one revolution, when the polishing pad
makes two revolutions, and when the polishing pad makes m
revolutions respectively. Let us assume here that Point Z on the
dresser is in contact with Point S on the polishing pad in the
initial state (see FIG. 13). Point S is a start point at which a
fixed trajectory starts. When the platen makes one revolution from
the initial state, the dresser makes n/m revolutions. Since the
number m is prime to the number n, division of n by m, i.e., n/m,
never yields an integral quotient, and Point S' on the polishing
pad that comes into contact with Point Z on the dresser will never
conform to Point S. When the platen makes m revolutions, the
dresser makes mn/m (=n) revolutions, and Point S' on the polishing
pad that comes into contact with Point Z on the dresser now
conforms to Point S (i.e., the original contact point) for the
first time, in other words a point on the polishing pad that comes
into contact with Point Z on the dresser arrives at Start Point S
(the fixed trajectory start point). The polishing pad and the wafer
thereafter repeat the same relative motion, as a result of which
Point Z on the on the dresser moves along the fixed trajectory on
the polishing pad.
112. When the least common multiple (L.C.M.) of the integers m and
n (the number m is prime to the number n), i.e., mn, is large, the
length of the fixed trajectory extends. In addition to contact with
a specific point on the polishing pad, Point Z on the dresser is
evenly brought into contact also with many other points on the
polishing pad. On the other hand, when the L.C.M. is small, Point Z
soon returns to Start Point S and thereafter moves along the same
trajectory, in other words Point Z comes into contact with limited
regions. This produces the possibility that a specific point on the
dresser is brought, with considerable frequency, into contact with
a low polishing-rate region that is locally created in the
polishing pad while other points on the dresser infrequently come
into contact with the region in question.
113. By making a change in platen-dresser angular speed ratio in
the present embodiment, trajectories similar to FIGS. 4-8 are drawn
or formed on polishing pads respectively, as in the first
embodiment.
114. The following conclusions are drawn from the above
consideration.
115. 1. When the platen-dresser rotational speed ratio is expressed
by two natural numbers, i.e., n and m (the number n is prime to the
number m), it is preferred to make the L.C.M. of these numbers m
and n, i.e., mn, as large as possible, preferably 10 or beyond.
116. 2. It is particularly preferred that the platen-dresser
rotational speed ratio, i.e., the platen-dresser angular speed
ratio, nearly corresponds to, or is approximated to an irrational
number that cannot be expressed in the form of a natural (rational)
number m/n.
117. 3. It is sufficient that the platen-dresser rotational speed
ratio is set in such a way as to prevent entrance of a point on the
polishing pad to a fixed trajectory during polishing. In other
words, the ratio is set in order for a point on the polishing pad
that comes into contact with Point Z on the dresser not to conform
to Start Point S with the polishing pad and the dresser rotated an
integral number of times.
118. 4. It is preferred that, although the surface region of the
polishing pad is divided into sub-regions by a trajectory drawn by
a point on the polishing pad that comes into contact with Point Z
on the dresser (see FIG. 5), these sub-regions are uniform in size
and fine within a concentric ring-like region, i.e., within a
ring-like region having identical radii from the polishing pad
center O. In other words, it is preferred that the foregoing
trajectory is evenly distributed on the polishing pad, to densely
form lattice-like patterns thereon.
119. Dressing is not a process step belonging in the manufacture of
semiconductor devices (a step for dresser surface planarization),
and it is therefore required that polishing rate activation be
completed as quickly as possible. The present invention is
available to realizing polishing pad surface activation in a short
time.
120. Other Embodiments
121. In the foregoing embodiments of the invention, it is arranged
such that the direction of rotation of the platen and the direction
of rotation of the carrier (dresser) are the same. It is to be
noted that the invention is not limited to such embodiments. For
instance, even when they rotate in opposite directions, the same
effects that the first and second embodiments achieve can be
obtained by, for example, arrangement that the platen-carrier
(dresser) rotational speed ratio is expressed by two natural
numbers of which one is prime to the other and the L.C.M. of these
numbers is 10 or greater.
122. In a variation to the foregoing, a polishing pad (formed of
closed-cell-foam type polyurethane or the like material) is
employed, having a series of about 1-mm grooves or pores formed, at
a pitch in the range of from about 5 mm to about 10 mm, on a
surface thereof. These grooves (pores) are provided for smooth
supply and discharge of a slurry-like abrasive liquid. Although,
when local groove (pore) blocking occurs to cause a certain groove
to become blocked, such a clogged groove can be taken as an
inactive region that does not contribute to the action of
polishing, even in such a case it is possible to improve uniformity
of the in-wafer planarity by utilizing the present invention.
* * * * *