U.S. patent application number 11/794915 was filed with the patent office on 2008-06-19 for substrate polishing method and apparatus.
Invention is credited to Shintaro Kamioka, Tatsuya Sasaki.
Application Number | 20080146119 11/794915 |
Document ID | / |
Family ID | 36425254 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080146119 |
Kind Code |
A1 |
Sasaki; Tatsuya ; et
al. |
June 19, 2008 |
Substrate Polishing Method and Apparatus
Abstract
A polishing apparatus is provided for optimizing a polishing
profile in consideration of even such parameters as the temperature
on the surface of an object to be polished, and the thickness of a
polishing pad, in addition to a polished amount. The polishing
apparatus for polishing the object to be polished under control of
a control unit CU has at least two pressing sections, and comprises
a top ring which can apply an arbitrary pressure to the object to
be polished from each of the pressing sections, a measuring device
IM for measuring a polished amount of the object to be polished,
and a monitoring device SM for monitoring the object to be polished
for a polishing condition. The control unit CU forces the polishing
apparatus to polish the object to be polished in accordance with a
simulation program for setting processing pressures required to
optimize a polishing profile of the object to be polished to the
top ring based on the output of the measuring device IM and the
output of the monitoring device SM.
Inventors: |
Sasaki; Tatsuya; (Tokyo,
JP) ; Kamioka; Shintaro; (Tokyo, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W., SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
36425254 |
Appl. No.: |
11/794915 |
Filed: |
January 16, 2006 |
PCT Filed: |
January 16, 2006 |
PCT NO: |
PCT/JP2006/300903 |
371 Date: |
July 9, 2007 |
Current U.S.
Class: |
451/5 ; 451/22;
451/287; 451/514 |
Current CPC
Class: |
B24B 49/16 20130101;
B24B 37/30 20130101 |
Class at
Publication: |
451/5 ; 451/22;
451/287; 451/514 |
International
Class: |
B24B 49/00 20060101
B24B049/00; B24B 49/12 20060101 B24B049/12; B24B 29/02 20060101
B24B029/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 21, 2005 |
JP |
2005-014013 |
Claims
1. A polishing apparatus for polishing an object to be polished
under control of a control unit, comprising: a top ring having at
least two pressing sections and capable of applying an arbitrary
pressure to the object to be polished from each of said pressing
sections; a measuring device for measuring a polished amount of the
object to be polished; and a monitoring device for monitoring the
object to be polished for a polishing state, said polishing
apparatus characterized in that: said control unit forces said
polishing apparatus to polish the object to be polished in
accordance with a simulation program for setting a processing
pressure required to optimize a polishing profile of the object to
be polished to said top ring based on the output of said measuring
device and the output of said monitoring device.
2. A polishing apparatus according to claim 1, characterized in
that: said at least two pressing sections include a plurality of
concentric air bags, and a retainer ring surrounding said air bags,
and the pressure of said retainer ring is kept at a value larger by
20 percent than an average value of the sum total of the pressures
applied by said air bags.
3. A polishing apparatus according to claim 1, further comprising a
polishing pad for polishing the object to be polished such that
said pad is depressed by said top ring, wherein said control unit
instructs said polishing apparatus to polish a monitor wafer
instead of the object when said monitoring device detects that said
polishing pad is cut away by a predetermined depth.
4. A polishing apparatus according to claim 2, characterized in
that when the output of said monitoring device indicates that an
abrasion loss of said retainer ring falls below a threshold, said
control unit instructs said polishing apparatus to stop
polishing.
5. A polishing apparatus according to claim 1, characterized in
that: when the output of said monitoring device indicates that the
temperature on the surface of the object to be polished exceeds a
predetermined set temperature, said control unit stops using the
simulation program or instructs said polishing apparatus to stop
polishing, and when the output of said monitoring device indicates
that the temperature on the surface falls below the set
temperature, said control unit instructs said polishing apparatus
to resume the polishing.
6. A polishing apparatus according to claim 1, further comprising a
polishing pad for polishing the object to be polished in a state in
which said polishing pad is pressed against the object to be
polished by said top ring, said polishing apparatus characterized
in that: when the output of said monitoring device indicates that
the thickness of said polishing pad falls below a threshold, said
control unit stops using the simulation program or instructs said
polishing apparatus to stop polishing.
7. A polishing apparatus according to claim 6, characterized in
that said monitoring device comprises a laser displacement gage for
measuring the thickness of said polishing pad.
8. A polishing apparatus according to claim 1, further comprising a
polishing pad for polishing the object to be polished in a state in
which said polishing pad is pressed against the object to be
polished by said top ring, and a dresser for conditioning said
polishing pad, said polishing apparatus characterized in that: when
the output of said monitoring device indicates that a cutting rate
of said dresser falls below a threshold, said control unit stops
using the simulation program, or instructs said polishing apparatus
to stop polishing.
9. A polishing apparatus according to claim 7, characterized in
that the cutting rate is monitored using a torque of a motor for
driving said dresser.
10. A polishing apparatus according to claim 1, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
11. A polishing apparatus according to claim 2, further comprising
a polishing pad for polishing the object to be polished such that
said pad is depressed by said top ring, wherein said control unit
instructs said polishing apparatus to polish a monitor wafer
instead of the object when said monitoring device detects that said
polishing pad is cut away by a predetermined depth.
12. A polishing apparatus according to claim 2, characterized in
that: when the output of said monitoring device indicates that the
temperature on the surface of the object to be polished exceeds a
predetermined set temperature, said control unit stops using the
simulation program or instructs said polishing apparatus to stop
polishing, and when the output of said monitoring device indicates
that the temperature on the surface falls below the set
temperature, said control unit instructs said polishing apparatus
to resume the polishing.
13. A polishing apparatus according to claim 2, further comprising
a polishing pad for polishing the object to be polished in a state
in which said polishing pad is pressed against the object to be
polished by said top ring, said polishing apparatus characterized
in that: when the output of said monitoring device indicates that
the thickness of said polishing pad falls below a threshold, said
control unit stops using the simulation program or instructs said
polishing apparatus to stop polishing.
14. A polishing apparatus according to claim 2, further comprising
a polishing pad for polishing the object to be polished in a state
in which said polishing pad is pressed against the object to be
polished by said top ring, and a dresser for conditioning said
polishing pad, said polishing apparatus characterized in that: when
the output of said monitoring device indicates that a cutting rate
of said dresser falls below a threshold, said control unit stops
using the simulation program, or instructs said polishing apparatus
to stop polishing.
15. A polishing apparatus according to claim 2, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
16. A polishing apparatus according to claim 3, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
17. A polishing apparatus according to claim 4, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
18. A polishing apparatus according to claim 5, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
19. A polishing apparatus according to claim 6, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
20. A polishing apparatus according to claim 7, characterized in
that said control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
Description
TECHNICAL FIELD
[0001] The present invention relates to a substrate polishing
apparatus for polishing materials to be polished, represented by a
semiconductor substrate, which can suppress a degradation in yield
rate due to non-uniformity of residual films on the surface of
substrates, mainly caused by aging changes of consumable materials,
extend the life time of such consumable materials to reduce an
operation cost, and a polishing apparatus which embodies the
method.
BACKGROUND ART
[0002] In recent years, as semiconductor devices are increasingly
more miniaturized and complicated in element structure, the
semiconductor devices tend to have increased asperities and larger
steps on the surface. As a result, thin films are formed in smaller
thicknesses on such steps, and open circuits can be developed due
to disconnections of wires, and short circuits attributable to
defective insulation between wiring layers, leading to a lower
yield rate. In a planarization technology for solving such
problems, chemical mechanical polishing (CMP) has been employed for
planarizing asperities on the surface formed in course of
deposition of insulating films and wiring metal films, for example,
during a semiconductor device manufacturing process on
semiconductor substrates.
[0003] In the CMP, a substrate, which is an object to be polished,
is pressed against a polishing pad made of unwoven fabric or the
like, and the substrate and polishing pad are slid relative to each
other with polishing slurry supplied therebetween to polish the
substrate. It has been found that concentric or lattice grooves
formed on the surface of the pad are effective for supplying a
sufficient amount of polishing slurry deep into a central region of
the substrate during the CMP-based polishing. Also, the CMP entails
so-called pad conditioning for trimming the surface of the pad with
a diamond disk or the like for purposes of removing polishing
debris which can stick to the surface of the polishing pad.
[0004] In a CMP process for polishing wiring metal and insulating
films laminated on a substrate into a flat finish, polishing
conditions applied to a manufacturing line have previously
optimized, so that the polishing is performed under the same
conditions until polishing members reach a limit consumption level
under the optimized conditions. However, as the polishing members
are consumed, the shape of the surface after polishing wiring metal
and insulating films on a substrate (called the "polishing
profile") is varying over time in step with the consumption level
of the polishing members. Generally, the polishing members are
replaced at a timing which is set before the aging changes affect
the performance of devices.
[0005] With the miniaturization of semiconductor device, an
increased number of wiring layers, and a higher processing speed in
recent years, a higher degree of flatness is required for the
surface profile, i.e., polishing profile of wiring metals and
insulating films after polishing. Specifically, allowed aging
changes in the polishing profile is increasingly narrowed in more
miniaturized devices and devices having a larger number of layers,
resulting in a higher frequency of replacement of consumed
polishing members. However, the consumable members of the CMP are
so expensive that a higher frequency of replacement due to
consumption will largely affect the cost of devices.
[0006] Generally, it is widely known that a polished amount Q can
be predicted with a certain accuracy from a relationship
Q.varies.kpv.DELTA.t (where Q represents a polished amount; k a
coefficient determined by materials of a polishing pad, a polishing
liquid, and a substrate, and the like; p a processing pressure, v a
moving speed, and .DELTA.t a polishing time) which is known as the
Preston's empirical formula in the field of polishing, and the
Preston's empirical formula is generally established in the CMP as
well. However, in the CMP, the speed of polishing based on chemical
actions is largely affected by a processing temperature, thus
making it difficult in some cases to predict the polished amount
with high accuracy only from the Preston's empirical formula. Also,
the behavior of polishing slurry within grooves in the surface of a
polishing pad is based on hydrodynamics and is therefore a factor
which is not considered in the Preston's empirical formula.
Further, the Preston's empirical formula fails to cover such
factors as insufficient dressing associated with a reduced cutting
rate of a pad conditioner, and a reduced amount of removed
polishing debris.
DISCLOSURE OF THE INVENTION
[0007] The present invention has been proposed in view of the
foregoing problems, and it is an object of the invention to
automatically optimize a processing pressure using a simulator
based on the Preston's empirical formula within a polishing
apparatus, sufficiently monitor even those parameters which cannot
be covered by the Preston 's empirical formula to improve a
correction accuracy, and accomplish a uniform polishing profile
associated with increasing miniaturization of integrated
circuits.
[0008] It is another object of the present invention to correctly
manage the state of consumable materials which have been
conventionally replaced after a certain number of substrates had
been processed to extend the life time of the consumable materials
and reduce the operational cost.
[0009] To achieve the above objects, the polishing apparatus
according to the present invention comprises a top ring for holding
an object to be polished such as a wafer while applying a pressure
to the object to be polished against a polishing member in order to
polish the object to be polished. The top ring can arbitrarily set
a pressure to the object to be polished in each of concentrically
partitioned areas, and can therefore control a pressing force on
the object to be polished. Therefore, if the object to be polished
is not polished into a flat shape, a pressing force for a required
polished amount can be additionally applied, for example, to a
portion which is not sufficiently polished, thus making it possible
to provide high polishing performance with high accuracy
flatness.
[0010] The pressure within the area of the top ring is generally
set to provide a flat surface for a wiring metal or an inter-layer
insulating film formed on a polished object to be polished.
Generally, this pressure is often set in accordance with engineer
's empirical rules, so that several objects to be polished must be
polished for adjustment before conditions are defined for polishing
the surface of the object to be polished to be flat.
[0011] Accordingly, the present invention utilizes a first
simulation program which receives a pressure setting condition for
each area in the top ring structured as described above to estimate
a polishing profile for an object to be polished. It has been found
that the result of a simulation performed by the first simulation
program has merely 1 to 5% of errors as compared with the actual
profile resulting from the polishing. The present invention can
eliminate wasteful objects to be polished which have been used at a
pressure setting stage, can instantaneously predict a polishing
profile through the simulation, and can accordingly reduce a time
required for setting the pressure as well.
[0012] Since the first simulation program can simply update a
polishing coefficient (coefficient including the influences by a
pad and a slurry) which can be found from the result of
measurements of the shape of a residual film (or polished shape) at
a relatively small number of measuring points to predict the
thickness of the residual film after polishing at a large number of
points other than the measuring points, the simulation program can
readily correct the influence caused by changes in polishing
members such as the slurry, pad and the like, and can predict a
polishing profile under a polishing condition which is set after
the correction. When the polishing coefficient is updated using the
result of polishing performed in the vicinity of a polishing
condition set value used in the first simulation program, errors
can be reduced even to 1 to 3%. When objects to be polished are
sequentially polished on an actual semiconductor production line,
there is not a large difference in the polishing condition set
value among the sequential objects to be polished, so that the
simulation can be performed with a higher accuracy. When there are
a relatively small number of points at which a polished shape is
measured, the polishing coefficient may be calculated using a curve
which is smoothly interpolated by the measuring points.
[0013] The present invention also provides a desired polishing
profile by creating a film shape on a wafer surface in a desired
thickness. For this purpose, in the present invention, a desired
polishing time, an average polished amount, and the shape of a
residual film (a polished shape may be used instead) are entered to
calculate a set pressure for each area to satisfy these conditions
by a second simulation program. The first simulation program is
incorporated into the second simulation program in the form of a
module. The first simulation program calculates a predicted value
for a polishing profile at a certain set pressure, and the second
simulation program compares this predicted value with a desired
polishing profile to calculate a modified value for the set
pressure. When the second simulation program is used to repeatedly
calculate a predicted value for the polishing profile and calculate
a modified value for the set pressure, it is possible to calculate
a set pressure which is closer to the desired polishing
profile.
[0014] Here, the set polishing time may be treated as a reference
value (target value), and the polishing may be terminated at the
time the amount of residual film actually monitored by an end point
system reaches a predetermined value.
[0015] While an average polished amount has been simply stabilized
in the past, the present invention can also control and stabilize
the flatness after polishing or a desired shape of a residual film.
Thus, in the present invention, after one test object to be
polished is preferably processed to update the polishing
coefficient, an optimized polishing condition is found by the
second simulation program to provide a desired polishing time,
average polished amount, and shape of residual film. While the
object to be polished is polished under this optimized polishing
condition, the polishing coefficient is updated as appropriate
based on the degree of consumption of the polishing members to
again optimize the polishing condition to stably provide the
desired polishing time, average polished amount, and shape of
residual film. Here, when a polishing condition under which an
object to be polished was polished may be fed back for subsequent
polishing, the quality of the polished object to be polished can be
ensured with a high accuracy in consideration of the accuracy of
the feedback control which is affected by the accuracy of the
flatness of a residual film after polishing, and the polishing
condition.
[0016] The preset invention can acquire data relating to the
polished shape not only for a generated film which can be measured
by an optical measuring device but also for a metal film using a
measuring device which can measure the metal film to conduct a
feedback control, and is rich in general-purpose properties because
it is not limited in applications of the CMP process. Also, data on
thickness can be acquired by an arbitrarily selected means such as
a measuring method using a measuring device which can make
monitoring during polishing, a method of measuring a wafer which is
transported to a measuring device after polishing, a method of
transferring data measured by a measuring apparatus external to a
CMP device to the CMP device and entering the data into the CMP
device, and the like. Also, the foregoing methods can be
arbitrarily combined to use different methods for acquiring
thickness data before polishing and after polishing, and the like,
to facilitate the operation.
[0017] Further, in the present invention, the correction accuracy
is improved by sufficiently monitoring those parameters which
cannot be covered by the Preston's empirical formula, and the
uniformization in the shape of polished wafers is realized, as
required in step with increasing miniaturization of integrated
circuits. For this purpose, the present invention controls the
polishing operation in consideration of even the temperature on a
polished surface of a wafer, the thickness of a pad, the depth of
grooves in the pad, and the cutting rate value of a dresser.
[0018] Accordingly, the invention set forth in claim 1 of the
present application provides a polishing apparatus for polishing an
object to be polished under control of a control unit, which
comprises:
[0019] a top ring having at least two pressing sections and capable
of applying an arbitrary pressure to the object to be polished from
each of the pressing sections;
[0020] a measuring device for measuring a polished amount of the
object to be polished; and
[0021] a monitoring device for monitoring the object to be polished
for a polishing state, and is characterized in that:
[0022] the control unit forces the polishing apparatus to polish
the object to be polished in accordance with a simulation program
for setting a processing pressure required to optimize a polishing
profile of the object to be polished to the top ring based on the
output of the measuring device and the output of the monitoring
device.
[0023] The invention set forth in claim 2 is characterized in that
the at least two pressing sections includes a plurality of
concentric air bags, and a retainer ring surrounding the air bags,
and the pressure of the retainer ring is kept at a value larger by
20 percent than an average value of the sum total of the pressures
applied by the air bags.
[0024] The invention set forth in claim 3 is characterized in that
when the output of the monitoring device indicates that an abrasion
loss of the retainer ring falls below a threshold, the control unit
instructs the polishing apparatus to stop polishing.
[0025] The invention set forth in claim 4 is characterized in that
when the output of the monitoring device indicates that the
temperature on the surface of the object to be polished exceeds a
predetermined set temperature, the control unit stops using the
simulation program or instructs the polishing apparatus to stop
polishing, and when the output of the monitoring device indicates
that the temperature on the surface falls below the set
temperature, the control unit instructs the polishing apparatus to
resume the polishing.
[0026] In the invention set forth in claim 5, the polishing
apparatus further comprises a polishing pad for polishing the
object to be polished in a state in which the polishing pad is
pressed by the top ring, and is characterized in that when the
output of the monitoring device indicates that the thickness of the
polishing pad falls below a threshold, the control unit stops using
the simulation program or instructs the polishing apparatus to stop
polishing.
[0027] The invention set forth in claim 6 is characterized in that
the monitoring device comprises a laser displacement gage for
measuring the thickness of the polishing pad.
[0028] In the invention set forth in claim 7, the polishing
apparatus further comprises a polishing pad for polishing the
object to be polished in a state in which the polishing pad is
pressed by the top ring, and a dresser for conditioning the
polishing pad, and is characterized in that when the output of the
monitoring device indicates that a cutting rate of the dresser
falls below a threshold, the control unit stops using the
simulation program, or instructs the polishing apparatus to stop
polishing.
[0029] The invention set forth in claim 8 is characterized in that
the cutting rate is monitored using a torque of a motor for driving
the dresser.
[0030] The invention set forth in claim 9 is characterized in that
the control unit can adjust the amount of supplied slurry in
accordance with the polishing state.
[0031] Generally, the polishing apparatus is provided with a touch
panel for the operator to enter operational conditions, and when
the control unit instructs the polishing apparatus to stop using
the simulation program, this instruction is displayed on the touch
panel. In response, the operator determines whether the polishing
should be continued or stopped. Also, a setting can be previously
made through manipulations on the touch panel to select a setting
for stopping the polishing when the control unit generates an
instruction of stopping the use of the simulation program.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a top plan view generally illustrating one
embodiment of a polishing apparatus according to the present
invention;
[0033] FIG. 2 is a perspective view of the polishing apparatus in
FIG. 1;
[0034] FIG. 3 is a diagram for describing some of components in the
polishing apparatus of FIG. 1;
[0035] FIG. 4 is a diagram for describing some of components in the
polishing apparatus of FIG. 1;
[0036] FIG. 5 is a cross-sectional view illustrating the structure
of a top ring used in the polishing apparatus of FIG. 1;
[0037] FIG. 6 is a flow diagram for describing a procedure for
collecting polishing rate distribution data in the polishing
apparatus of FIG. 1;
[0038] FIG. 7(A) is a diagram generally illustrating the
configuration for detecting a change in the thickness of a
polishing pad in the polishing apparatus of FIG. 1 using a laser
displacement gage, and FIG. 7(B) is a graph showing a change in the
output of the laser displacement gage over time;
[0039] FIG. 8(A) is a table showing a comparison of measurements
when the polishing method according to the present invention is
used with those when not used, and FIG. 8(B) is a graph showing the
result of the comparison;
[0040] FIGS. 9(A) to 9(C) are diagrams showing the thickness of a
film on a wafer before CMP (A), the thickness of the film on the
wafer after CMP (B), and a polishing rate (C), respectively, when a
polishing pad is new;
[0041] FIGS. 10(A) to 10(C) are diagrams showing the thickness of a
film on a wafer before CMP (A), the thickness of the film on the
wafer after CMP (B), and a polishing rate (C), respectively, when
the polishing pad has been consumed by 0.1 mm; and
[0042] FIGS. 11(A) to 11(C) are diagrams showing the thickness of a
film on a wafer before CMP (A), the thickness of the film on the
wafer after CMP (B), and a polishing rate (C), respectively, when
the polishing pad has been consumed by 0.2 mm.
BEST MODE FOR CARRYING OUT THE INVENTION
[0043] In the following, several embodiments of a polishing method
and apparatus according to the present invention will be described
in detail with reference to the accompanying drawings. First, one
embodiment of the polishing apparatus according to the present
invention will be described with reference to FIG. 1 which is a top
plan view illustrating the layout and configuration of respective
components in the polishing apparatus, and FIG. 2 which illustrates
a perspective view of the polishing apparatus. In FIGS. 1 and 2, a
transportation mechanism common to two polishing stations installed
in areas A, B comprises separately installed linear transporters,
each of which includes two linearly reciprocating stages that are
transportation mechanisms dedicated to the two polishing stations,
respectively. Specifically, the polishing apparatus illustrated in
FIGS. 1 and 2 comprises four loading/unloading stages 2 for
carrying wafer cassettes 1 which stock multiple wafers. A carrier
robot 4 having two hands is disposed on a running mechanism 3 such
that the hands can reach each wafer cassette 1 on the
loading/unloading stages 2. The running mechanism 3 is based on
linear motors. By employing the running mechanism based on linear
motors, high-speed and stable transportation can be ensured even if
wafers are increased in diameter and weight.
[0044] In the polishing apparatus illustrated in FIG. 1, the
loading/unloading stage 2 for carrying the wafer cassette 1
comprises an SMIF (Standard Manufacturing Interface) pod or FOUP
(Front Opening Unified Pod) with a loading/unloading stage being
attached external thereto. The SMIF and FOUP are closed vessels
which receive wafer cassettes therein and cover them with
partitions to keep an environment independent of the external
space. When SMIF or FOUP is installed as the loading/unloading
stage 2 of the polishing apparatus, the polishing apparatus is
integrated with the wafer cassette by opening a shutter S arranged
on a housing H of the polishing apparatus and a shutter of the SMIF
or FOUP.
[0045] Upon termination of a polishing step, the SMIF or FOUP is
separated from the polishing apparatus by closing the shutter, and
is automatically or manually transported to another processing
step, so that it must keep the internal atmosphere clean. For this
purpose, a clean air down flow is formed through a chemical filter
above an area C through which wafers pass immediately before they
return to the cassette. Also, since a linear motor is used to move
the carrier robot 4, dust can be restrained to keep the atmosphere
of the area C more normal. Additionally, in order to keep wafers
clean within the wafer cassette 1, a clean box which contains a
chemical filter and a fan to maintain a certain degree of cleanness
by itself may be employed for a closed vessel such as SMIF and
FOUP.
[0046] Two washing machines 5, 6 are disposed on the opposite sides
to the wafer cassettes 1 in symmetric arrangement about the running
mechanism 3 of the carrier robot 4. Each of the washing machines 5,
6 is disposed at a position which can be accessed by the hands of
the carrier robot 4. A wafer station 50, which comprises four
semiconductor wafer seats 7, 8, 9, 10, is disposed at a position
between the two washing machines 5, 6, which can be accessed by the
carrier robot 4.
[0047] A barrier 14 is disposed for ranking the cleanness in an
area D in which disposed are the washing machines 5, 6 and seats 7,
8, 9, 10 and in an area C in which disposed are the wafer cassettes
1 and carrier robot 4. The barrier 14 is provided with a shutter 11
in an opening for carrying a semiconductor wafer from one area to
another. A carrier robot 20 is disposed at a position from which
the carrier robot 20 can access the washing machine 5 and three
seats 7, 9, 10, and a carrier robot 21 is disposed at a position
from which the carrier robot 21 can access the washing machine 6
and three seats 8, 9, 10.
[0048] A washing machine 22 is disposed to be adjacent to the
washing machine 5 and at a position which can be accessed by the
hand of the carrier robot 20. Also, a washing machine 23 is
disposed to be adjacent to the washing machine 6 and at a position
which can be assessed by the hand of the carrier robot 21. The
washing machines 22, 23 can wash both sides of wafers. All of these
washing machines 5, 6, 22, 23, seats 7, 8, 9, 10 of the wafer
station 50, and carrier robots 20, 21 are disposed in the area D in
which the air pressure is adjusted to be lower than the air
pressure in the area C.
[0049] The polishing apparatus illustrated in FIGS. 1 and 2 has a
housing H which surrounds the respective devices, and the housing H
is partitioned into a plurality of chambers (including the areas C,
D) by partitions 14, 24A, 24B. The partitions 24A, 24B define two
areas A, B, separated from the area D, which form two polishing
chambers. Each of the two areas A, B comprises two polishing
tables, and a top ring for holding a semiconductor wafer and
polishing the semiconductor wafer while pressing the same against
the polishing table. Specifically, polishing tables 34, 36 are
disposed in the area A, while polishing tables 35, 37 are disposed
in the area B. Also, a top ring 32 is disposed in the area A, while
a top ring 33 is disposed in the area B. Further disposed in the
area A are an abrasive liquid nozzle 40 for supplying a polishing
abrasive liquid to the polishing table 34, and a mechanical dresser
38 for dressing the polishing table 34, while disposed further in
the area B are an abrasive liquid nozzle 41 for supplying a
polishing abrasive liquid to the polishing table 35, and a
mechanical dresser 39 for dressing the polishing table 35. In
addition, a dresser 48 is disposed for dressing the polishing table
36, in the area A, while a dresser 49 is disposed for dressing the
polishing table 37 in the area B.
[0050] The polishing tables 34, 35 comprise atomizers 44, 45, which
are dressers based on liquid pressure, in addition to mechanical
dressers 38, 39. The atomizer mixes a liquid (for example, pure
water) with a gas (for example, nitrogen) into a sprayed fluid
mixture which is blown from a plurality of nozzles onto a polishing
surface to wash out polishing grounds and slurry grains deposited
or clogged on the polishing surface. With the cleaning of the
polishing surface by the fluid pressure of the atomizer, and the
dressing of the polishing surface by the dressers 38, 39 which
involve mechanical contacts, more desirable dressing, i.e.,
restoration of the polishing surface can be achieved.
[0051] FIG. 3 is a diagram illustrating the relationship between
the top ring 32 and the polishing tables 34, 36. As appreciated, a
similar relationship is established between the top ring 33 and the
polishing tables 35, 37. As illustrated in FIG. 3, the top ring 32
is suspended from a top ring head 31 by a rotatable top ring
driving shaft 91. The top ring head 31 is supported by a
positionable rocking shaft 92, and the top ring 32 is made
accessible to the polishing tables 34, 36. The dresser 38 is
suspended from a dresser head 94 by a rotatable dresser driving
shaft 93. The dresser head 94 is supported by a positionable
rocking shaft 95, and the dresser 38 can move between a standby
position and a dresser position on the polishing table 34. The
dresser head (rocking arm) 97 is supported by a positionable
rocking shaft 98, and the dresser 48 can move between a standby
position and a dresser position on the polishing table 36.
[0052] The dresser 38 has an elongated shape longer than the
diameter of the table 36, and the dresser head 97 rocks about the
rocking shaft 98. The dresser 48 is suspended from the dresser head
97 by a dresser fixing mechanism 96 such that the dresser fixing
mechanism 96 and the dresser 48, opposite to the dresser head 97
from the rocking shaft 98, make pivotal movements, there by
permitting the dresser 48 to dress on the polishing table 36
through motions just like those of wipers of a car, without
revolutions. Scroll type polishing tables can be used for the
polishing tables 36, 37.
[0053] Turning back to FIG. 1, an inverter 28 is installed at a
position to which the hands of the carrier robot 20 are accessible
for inverting a semiconductor wafer within the area A separated
from the area D by the partition 24A. Similarly, an inverter 28' is
installed at a position to which the hands of the carrier robot 21
are accessible for inverting a semiconductor wafer within the area
B separated from the area D by the partition 24B. The partitions
24A, 24B which separate the areas A, B from the area D, are formed
with opening for transporting a semiconductor wafer therethrough,
and shutters 25, 26 dedicated to the inverters 28, 28' are disposed
on the respective openings.
[0054] Each of the inverters 28, 28' comprises a chucking mechanism
for chucking a semiconductor wafer, an inverting mechanism for
inverting a semiconductor wafer upside down, and a wafer presence
detecting sensor for confirming whether a semiconductor wafer is
chucked by the chucking mechanism. A semiconductor wafer is carried
to the inverter 28 by the carrier robot 20, while a semiconductor
wafer is carried to the inverter 28' by the carrier robot 21.
[0055] In the area A which defines one polishing chamber, a linear
transporter 27A is installed for providing a transportation
mechanism for transporting a semiconductor wafer between the
inverter 28 and the top ring 32. In the area B which defines the
other polishing chamber, a linear transporter 27B is installed for
providing a transportation mechanism for transporting a
semiconductor wafer between the inverter 28' and the top ring 33.
The linear transporters 27A, 27B comprise two stages which linearly
reciprocate, and a semiconductor wafer is passed between the linear
transporter and the top ring or inverter through a wafer tray.
[0056] The right-hand area of FIG. 3 illustrates the positional
relationship among the linear transporter 27A, lifter 29, and
pusher 30. A similar positional relationship to that illustrated in
FIG. 3 is established among the linear transporter 27B, lifter 29',
and pusher 30'. Accordingly, the following description will be made
only on the linear transporter 27A, lifter 29, and pusher 30. As
illustrated in FIG. 3, the lifter 29 and pusher 30 are disposed
below the linear transporter 27A. The inverter 28 is disposed above
the linear transporter 27A. The top ring 32, when rocking, can be
positioned above the pusher 30 and linear transporter 27A.
[0057] FIG. 4 is a diagram for describing how a semiconductor wafer
is passed between the linear transporter and the inverter and
between the linear transporter and the top ring. As illustrated in
FIG. 4, a semiconductor wafer 101 before polishing, carried to the
inverter 28 by the carrier robot 20, is inverted by the inverter
28. As the lifter 29 moves up, a wafer tray 925 on a loading stage
901 is transferred onto the lifter 29. As the lifter 29 further
moves up, the semiconductor wafer 101 is transferred from the
inverter 28 to the wafer tray 925 on the lifter 29. Subsequently,
the lifter 29 moves down, and the semiconductor wafer 101 is placed
on the loading stage 901 together with the wafer tray 925. The
wafer tray 925 and semiconductor wafer 101 are transported above
the pusher 30 by a linear movement of the loading stage 901. In
this event, an unloading stage 902 receives a polished
semiconductor wafer 101 from the top ring 32 through the wafer tray
925, and moves toward the lifter 29. The loading stage 901 and
unloading stage 902 pass each other halfway in their movements.
When the loading stage 901 reaches above the pusher 30, the top
ring 32 has previously rocked to the position indicated in FIG. 4.
Next, the pusher 30 moves up, and further moves up, after it has
received the wafer tray 925 and semiconductor wafer 101 from the
loading stage 901, to reach the top ring 32 to which the
semiconductor wafer 101 alone is transferred.
[0058] The wafer 101, which has been transferred to the top ring
32, is sucked by a vacuum sucking mechanism of the top ring 32,
transported to the polishing table 34 while it is still sucked.
Next, the wafer 101 is polished by a polishing surface which has a
polishing pad, a grindstone, or the like mounted on the polishing
table 34. The second polishing table 36 is disposed at a location
to which the top ring 32 can access. After the wafer has been
polished on the first polishing table 34 in this way, the wafer is
again polished on the second polishing table 36. However, depending
on the type of a film formed on the semiconductor wafer, the
semiconductor wafer may be first polished on the second polishing
table 36 and then polished on the first polishing table 34.
[0059] The polished wafer 101 is returned to the inverter 28
through the opposite path to the aforementioned. The wafer, which
has returned to the inverter 28, is rinsed with pure water or
washing chemical liquid from a rinse nozzle. Also, the wafer
sucking surface of the top ring 32, from which the wafer has been
removed, is washed with pure water or chemical liquid from a top
ring washing nozzle.
[0060] Now, a general description will be given of processing steps
performed in the polishing apparatus illustrated in FIGS. 1 to 4.
In two-stage washing, two-cassette parallel processing, one wafer
follows a path which passes wafer cassette (CS1).fwdarw.carrier
robot 4.fwdarw.seat 7 of the wafer station 50.fwdarw.carrier robot
20.fwdarw.inverter 28.fwdarw.loading stage 901 of the linear
transporter 27A.fwdarw.top ring 32.fwdarw.polishing table
34.fwdarw.polishing table 36 (as required) .fwdarw.unloading stage
902 of linear transporter 27A.fwdarw.inverter 28.fwdarw.carrier
robot 20.fwdarw.washing machine 22.fwdarw.carrier robot
20.fwdarw.washing machine 5.fwdarw.carrier robot 4t.fwdarw.wafer
cassette (CS1). The other wafer in turn follows a path which passes
wafer cassette (CS2).fwdarw.carrier robot 4.fwdarw.seat 8 of the
wafer station 50.fwdarw.carrier robot 21.fwdarw.inverter
28'.fwdarw.loading stage 901 of linear transporter 27B .fwdarw.top
ring 33.fwdarw.polishing table 35.fwdarw.top ring
33.fwdarw.unloading stage 902 of linear transporter
27B.fwdarw.inverter 28'.fwdarw.carrier robot 21.fwdarw.washing
machine 23.fwdarw.carrier robot 21.fwdarw.washing machine 6
.fwdarw.carrier robot 4.fwdarw.and wafer cassette (CS2).
[0061] In three-stage washing, two-cassette parallel processing,
one wafer follows a path which passes wafer cassette
(CS1).fwdarw.carrier robot 4.fwdarw.seat 7 of wafer station
50.fwdarw.carrier robot 21.fwdarw.washing machine 6.fwdarw.carrier
robot 21.fwdarw.seat 9 of wafer station 50.fwdarw.carrier robot
20.fwdarw.inverter 28.fwdarw.loading stage 901 of linear
transporter 27A.fwdarw.top rig 32.fwdarw.polishing table
34.fwdarw.polishing table 36 (as required).fwdarw.unloading stage
902 of the linear transporter 27A.fwdarw.inverter 28.fwdarw.carrier
robot 20.fwdarw.washing machine 22.fwdarw.carrier robot
20.fwdarw.seat 10 of wafer station 50.fwdarw.carrier robot
20.fwdarw.washing machine 5.fwdarw.carrier robot 4.fwdarw.wafer
cassette (CS1). The other wafer in turn follows a path which passes
wafer cassette (CS2).fwdarw.carrier robot 4.fwdarw.seat 8 of wafer
station 50.fwdarw.carrier robot 21 .fwdarw.inverter
28'.fwdarw.loading stage 901 of linear transporter 27B.fwdarw.top
ring 33.fwdarw.polishing table 35.fwdarw.polishing table 37 (as
required) .fwdarw.unloading stage 902 of linear transporter
27B.fwdarw.inverter 28'.fwdarw..fwdarw.carrier robot
21.fwdarw.washing machine 23.fwdarw.carrier robot 21.fwdarw.washing
machine 6.fwdarw.carrier robot 21.fwdarw.seat 9 of the wafer
station 50.fwdarw.carrier robot 20.fwdarw.washing machine
5.fwdarw.carrier robot 4.fwdarw.wafer cassette (CS2).
[0062] Further, in the three-stage washing series processing, a
wafer follows a path which passes wafer cassette
(CS1).fwdarw.carrier robot 4.fwdarw.seat 7 of wafer station
50.fwdarw.carrier robot 20.fwdarw.inverter 28.fwdarw.loading stage
901 of linear transporter 27A.fwdarw.top ring 32 .fwdarw.polishing
table 34.fwdarw.polishing table 36 (as required).fwdarw.unloading
stage 902 of linear transporter 27A.fwdarw.inverter
28.fwdarw.carrier robot 20, washing machine 22.fwdarw.carrier robot
20.fwdarw.seat 10 of wafer station 50.fwdarw.inverter
28'.fwdarw.loading stage 901 of linear transporter 27B
.fwdarw.polishing table 35.fwdarw.polishing table 37 (as
required).fwdarw.unloading stage 902 of linear transporter
27B.fwdarw.top ring 33.fwdarw.inverter 28' .fwdarw.carrier robot
21.fwdarw.washing machine 23.fwdarw.carrier robot 21.fwdarw.washing
machine 6.fwdarw.carrier robot 21.fwdarw.seat 9 of wafer station
50.fwdarw.carrier robot 20.fwdarw.washing machine 5.fwdarw.carrier
robot 4.fwdarw.wafer cassette (SC1).
[0063] According to the polishing apparatus illustrated in FIGS. 1
to 4, since the polishing apparatus comprises the linear
transporter having at least two stages (seats), which linearly
reciprocate, as a transportation mechanism dedicated to each
polishing station, the polishing apparatus can reduce a time
required to transfer an object to be polished between the inverter
and the top ring, and can increase the number of objects to be
polished which can be processed per unit time. Also, when an object
to be polished is transferred between a stage of the linear
transporter and the inverter, the object to be polished is
transferred between the wafer tray and the inverter, and when the
object to be polished is transferred between a stage of the linear
transporter and the top ring, the object to be polished is
transferred between the wafer tray and the top ring, so that the
wafer tray can absorb impacts during the transfer, thus making it
possible not only to increase the speed at which the object to be
polished can be transferred, but also to improve the throughput, of
the object to be polished. Also, since the transfer and placement
of the wafer from the inverter to the top ring are performed
through the tray which is removably held on each stage of the
linear transporter, it is possible to eliminate a transfer of the
wafer, for example, between the lifter and the linear transporter,
and between the linear transporter and the pusher, thus preventing
damages possibly caused by produced dust and failure in holding the
wafer.
[0064] Further, since the polishing apparatus has a plurality of
trays classified into two groups, i.e., a group dedicated to
loading for holding objects to be polished before polishing, and a
group dedicated to unloading for holding polished objects, a wafer
before polishing is passed from the tray dedicated to loading,
rather than from the pusher, to the top ring, while a polished
wafer is passed from the top ring to the tray dedicated to
unloading, rather than to the pusher. Thus, the loading of a wafer
to the top ring is performed using a jig or member different from
that which is used for the unloading of a wafer from the top ring,
making it possible to solve a problem that a polishing liquid or
the like sticking to a polished wafer sticks to and solidifies on a
wafer supporting member common to the loading and unloading, and
scratches or sticks to a wafer before polishing.
[0065] An inline monitor IM is installed at an appropriate location
in the area C of the polishing apparatus described above, such that
a polished and washed wafer is carried to the inline monitor IM by
a carrier robot for measuring the thickness and profile of the
wafer. The inline monitor IM can also measure a wafer before
polishing, and the difference in thickness before polishing and
after polishing can be regarded equal to a polished amount. Thus,
the inline monitor IM acts as a thickness measuring device.
Actually, the inline monitor IM is disposed above the robot 3.
Further, the polishing apparatus comprises a state monitor SM for
monitoring parameters representative of the operating state of the
polishing apparatus such as the temperature on the polishing
surface, the thickness of the polishing pad, the cutting rate of
the dresser, and an abrasion loss of the retainer ring. The
operation of the overall polishing apparatus is controlled by a
control unit CU. The control unit CU stores a simulation program,
later described, and a control flow program for measuring an
arbitrary value among the temperature on the polishing surface, the
thickness of the pad, the depth of the grooves in the pad, the
cutting rate value of the dresser, and the abrasion loss of the
retainer ring in the top ring to optimize the polishing. The
control unit CU may be contained within the polishing apparatus as
illustrated in FIG. 1 or may be separated from the polishing
apparatus. The state monitor SM, inline monitor IM, and control
unit CU are omitted in FIG. 2.
[0066] It is known from the Preston's formula that a pressing force
for pressing the surface of a wafer against a polishing pad is
generally proportional to a polished amount. However, an
appropriate pressing force must be found by modeling a top ring
which has a complicated structure, and taking into consideration
the non-linearity of the polishing pad which is made of an elastic
material, a large deformation of the wafer which is a thin plate,
particularly, stress concentration which conspicuously appears on
the end face of the wafer. Thus, difficulties would be encountered
in analytically finding a mathematical solution. On the other hand,
the use of a finite element method or a boundary element method to
find the pressing force involves dividing an object into a large
number of elements, requiring an extremely large amount of
calculations, a long calculation time, and high calculation
capabilities. In addition, for achieving appropriate results, the
operator is required to have expertism on numerical analysis, so
that it is virtually infeasible, from viewpoints of the cost and
practice to reference the mathematically derived pressing force in
making a simple adjustment in the field, and incorporate the same
in a CMP apparatus for utilization.
[0067] Bearing the foregoing discussion in mind, the top ring in
the polishing apparatus in the configuration described above is
implemented by a profile control type one. The profile control type
top ring, herein referred to, is a generic name for a top ring
which has a plurality of pressing sections. Specifically, the
profile control type top ring may be one having a plurality of
pressing sections comprised of air bags or water bags
concentrically partitioned by a plurality of membranes, one having
a plurality of sections which directly press the back surface of a
wafer with an air pressure by applying a pressure to partitioned
air chambers, one having a section which generates a pressure with
the aid of a spring, one having a local pressing section by
disposing one or a plurality of piezo-electric elements, or a
combination of those.
[0068] In the following, a pressing section will be described with
reference to a top ring which has a plurality of concentrically
partitioned air bags. As illustrated in FIG. 5, the top ring
comprises a plurality of concentric air bags, and adjusts a
pressure applied from each air bag to an associated area of a
wafer. In the following, the side of the wafer facing the air bags
is called the "wafer back surface," and the side facing the
polishing pad the "wafer surface." FIG. 5 illustrates a
cross-sectional view taken along a plane including the axis of
rotation of the top ring used in the polishing apparatus of the
present invention, where the top ring T has a central discoidal air
bag E1, a troidal air bag E2 surrounding the air bag E1, a troidal
air bag E3 surrounding the air bag E2, a troidal air bag E4
surrounding the air bag E3, and a troidal retainer ring E5
surrounding the air bag E4. As illustrated, the retainer ring E5 is
designed such that it can come into contact with the pad, and a
wafer W carried on a polishing table is fitted in a space defined
by the retainer ring E5, and applied with pressures from the
respective air bags E1-E4.
[0069] As will be appreciated, the number of air bags which make up
the top ring T is not limited to four, but may be increased or
decreased in accordance with the size of a wafer. Also, though not
shown in FIG. 5, an air pressure feeder is disposed at an
appropriate location of the top ring T for each air bag in order to
adjust the pressure applied to the back surface of the wafer W by
the associated air bag E1-E4. Also, a pressure applied to the
retainer ring E5 may be controlled by an air bag placed on the
retainer ring E5 in a manner similar to the other air bags, or may
be controlled by directly transmitting a pressure from a shaft
which supports the top ring T. In the present invention, a
combination of pressures applied by the respective air bags E1-E4
and retainer ring E5 to the back surface of the wafer W and the
polishing pad around the wafer W, and a resultant distribution of
pressing forces on the surface of the wafer W have been previously
stored in a memory of the control unit CU of the polishing
apparatus. Preferably, the pressing force of the retainer ring E5
is set to 20% or more of an average value of the sum total of
pressing forces applied by the air bags E1-E4 in order to prevent
the wafer from slipping out.
[0070] By using the structure as described above, assuming that a
practical pressure setting range for the pressures applied from the
air bags to the back surface of the wafer and the pressure applied
from the retainer ring to the polishing pad (hereinafter called the
"back surface pressure") is from 100 to 500 hPa, the air pressure
is in a range of .+-.200 hPa, and the pressing force distribution
on the surface of the wafer W can be regarded to be substantially
linear (i.e., the principle of superposition is substantially
established), the pressing force distribution on the surface of the
wafer resulting from a desired pressure applied by each air bag to
an associated area on the back surface of the wafer can be found in
a back surface pressure setting range of .+-.200 hPa by
synthesizing pressing force distributions on the surface of the
wafer by a combination of three different pressures of 100 hPa, 300
hPa, and 500 hPa applied to the back surface.
[0071] In other words, by dividing set pressures on the back
surface in a range in which a change in the surface pressing force
can be regarded as substantially linear (the principle of
superposition is established) , preparing previously calculated
data on a pressing force distribution on the surface of the wafer
for a plurality of cases, and synthesizing data appropriately
selected from the prepared data, the pressing force distribution on
the surface of the wafer corresponding to an arbitrary set pressure
on the back surface of the wafer is found without complicated
calculations based on the finite element method or the like. By
storing a procedure for finding the pressing force distribution on
the surface of the wafer in a computer, a simulation tool can be
created for finding a pressing force distribution on the surface of
a wafer for a set pressure on the back surface of the wafer.
[0072] Once the pressing force distribution on the surface of the
wafer is found in this way, a predicted polishing profile can be
found for the wafer by multiplying this pressing force distribution
by data on a distribution of a polishing coefficient on the wafer
surface, previously found for the wafer to be polished. It is known
from the aforementioned Preston's empirical formula that the amount
Q of polished wafer is generally proportional to the product of the
pressure applied to the wafer by each air bag, i.e., the pressing
force P, a moving speed v on the contact plane, and a polishing
time .DELTA.t. While the moving speed (i.e., a relative speed of
the wafer surface to the polishing pad) v of the contact plane on
the wafer surface differs from one location to another on the wafer
surface, and the polishing time .DELTA.t also differs depending on
polishing conditions, the polishing coefficient corresponds to kv
if the polishing rate per unit pressure is defined to be the
polishing coefficient. When a distribution of a value corresponding
to kv in the Preston's formula has been found for the wafer
surface, the polished amount Q on the wafer surface, and a
distribution of the polished amount Q per unit time, i.e., the
polishing rate Q/.DELTA.t can be found from the pressing force P.
Since the amount of polished wafer (polishing rate) can be found by
such a simple calculation, the result of the calculation by the
simulation tool can be referenced for a simple adjustment in the
field, and incorporated in a CMP device for utilization.
[0073] FIG. 6 illustrates an exemplary procedure for finding data
on a distribution of the polishing coefficient on the surface of a
wafer. First, at step S1, the shape of a film deposited on a
certain wafer is previously measured. Next, at step S2, the
measured wafer is actually polished under a particular set pressure
condition for a particular polishing time. In this event, at step
S3, a distribution of a pressing force on the surface of the wafer
under this pressure condition is calculated using the simulation
tool. The shape of the film on the surface of the wafer thus
polished is again measured, and a distribution of the amount of
polishing on the surface of the wafer is calculated from the
difference in the shape before polishing and after polishing (step
S4). Next, at step S5, the calculated distribution of the polished
amount is divided by the polishing time and the calculated
distribution of the pressing force to find a distribution of the
polishing rate per unit pressure at each point on the surface of
the wafer, i.e., a distribution of the polishing coefficient on the
surface of the wafer. Here, instead of dividing by the polishing
time, a distribution of the polished amount may be found per unit
pressure. Alternatively, an initial condition of the polishing pad,
a situation after it has been used for a certain time, and a
distribution of the polishing coefficient near a use limit may be
previously calculated and stored within the control unit CU as data
on aging changes of the polishing coefficient.
[0074] As has been so far described, the present invention is not
limited to the profile control type top ring using air bags, but it
is apparent that only if a force acting from the back surface of
the wafer is found, the profile can be predicted by calculating a
distribution of a pressing force on the surface of the wafer based
on the acting force. Therefore, a top ring to which the present
invention can be applied may be made up of respective pressing
sections which comprise liquid bags that can accept a pressurized
liquid therein, partitioned air chambers that directly press a
wafer with a pressurized gas, resilient bodies that generate
pressures using, for example, springs, piezo-electric elements that
press a wafer, or a combination of those options.
[0075] In the present invention, the simulation tool as described
above is used to configure the top ring such that a polishing
pressure can be set for each area, estimate a pressure which must
be set for each area to accomplish a target polishing profile, and
feed a calculated pressure value back to a wafer which is to be
subsequently polished. In this way, even if the polishing profile
varies over time as the polishing member consumes more and more,
the variations can be corrected as appropriate to stably ensure a
desired polishing profile.
[0076] To realize the foregoing, the present invention executes the
following control flow:
[0077] 1. A wafer is polished under an arbitrary polishing
condition.
[0078] 2. A distribution of the thickness of a wiring metal or an
insulating film is measured on the polished wafer. This measurement
can be made with a thickness measuring device contained in the
polishing apparatus or a measuring apparatus external to the
polishing apparatus, and measured data may be fetched online, or
measured data recorded on another storage medium may be retrieved.
The measurement should be made at at least one location within each
area.
[0079] 3. Based on the result of the measurements, a polishing
pressure condition is calculated in order to create a target
polishing profile. This step is performed in the following
procedure:
[0080] 3-1) A target polishing profile is set. For example, a
plurality of arbitrary points at which a polished amount should be
managed are specified on the surface of the wafer, and the polished
amount QT is set at each of the specified points, or the polishing
rate Q.sub.T.DELTA.t=Q.sub.T/.DELTA.t is set at each point. The
processing can be carried out by any method. Here, a description
will be given of a method of setting the polished amount.
[0081] 3-2) The polished amount Q.sub.poli is calculated for each
of the areas of the actually polished wafer. The calculation of the
polished amount requires data on an initial thickness of the wafer
before polishing, and the initial thickness is measured using one
of the measuring device contained in the polishing apparatus and
the measuring device installed external to the polishing apparatus.
The initial thickness data may be fetched by any of the methods
described in Step 2.
[0082] 3-3) The polished amount calculated at each point is divided
by a pressure P applied to an area which includes the point to
calculate the polished amount per unit contact pressure
Q.sub.poli.DELTA.p=Q.sub.poli/P.
[0083] 3-4) The target polished amount Q.sub.T at the point closest
to the point at which the distribution was measured at step 2 is
extracted. Alternatively, the target polished amount Q.sub.T may be
approximated from two locations near the measuring location in a
linear fashion.
[0084] 3-5) At each point, a difference Q.sub.T-Q.sub.poli is
calculated between the target polished amount Q.sub.T set at 3-1
and the polished amount Q.sub.poli calculated at 3-2, and the
polished amount corresponding to the difference is divided by the
polished amount per unit contact pressure calculated at 3-3 to
calculate a correction polishing pressure
(Q.sub.T-Q.sub.poli)/Q.sub.poli.DELTA.p.
[0085] 3-6) The correction polishing pressure calculated at 3-5 is
added to the pressure which was set upon polishing to find a
pressure value P.sub.input. When a plurality of measuring points
are included in an area, pressure values calculated at the
plurality of points are averaged, and the average is set to the
pressure value P.sub.input of the area.
[0086] 3-7) The pressure value P.sub.input calculated at 3-6 is
entered into the simulation tool according to the present invention
to estimate the polished amount at each of the points specified at
3-1 to find a estimate of the polished amount Q.sub.est.
[0087] 3-8) The difference Q.sub.T-Q.sub.est is calculated between
the estimate of the polished amount Q.sub.est and the target
polished amount Q.sub.T.
[0088] 3-9) The polished amount Q.sub.est calculated at 3-7 is
divided by the pressure value P.sub.input to calculate a polished
amount Q.sub.est.DELTA.p (=Q.sub.est/P.sub.input) per unit contact
pressure.
[0089] 3-10) The difference Q.sub.T-Q.sub.est calculated at 3-8 is
divided by the polished amount Q.sub.est.DELTA.p per unit contact
pressure to find a correction pressure value
(Q.sub.T-Q.sub.est)/Q.sub.est.DELTA.p which is then added to the
pressure value P.sub.input. The calculated pressure values at
points within the area are averaged, and the resultant average is
defined to be a recommended pressure value P.sub.output in each
area.
[0090] 3-11) The recommended pressure value P.sub.output calculated
at 3-10 is again entered into the simulation tool. If the
difference between the estimate of the polished amount at each
point and the target polished amount falls within a previously set
arbitrary allowable range, this recommended pressure value
P.sub.outout is applied (fed back) to wafers which are to be
actually polished from then on. If the difference falls out of the
allowable range, steps 3-7 to 3-10 are repeated until the
difference falls within the allowable range to find the recommended
pressure value.
[0091] The period of the feedback may be freely set, and an
exemplary method of setting the period may involve measuring all
wafers and feeding the recommended pressure value back to wafers
which are to be subsequently polished, or not conducting the
feedback when the polishing member is not so consumed because of
small variations in the polishing profile, and conducting the
feedback when the polishing member has been much consumed. Further,
the period set by the latter method may also be measured every
arbitrary number of wafers, and a polishing condition fed back
immediately before the measurement is continuously applied from the
time the measurement is once made to the time the wafer is next
measured. As the polishing member is more consumed, the period can
be set shorter. Alternatively, for setting the polishing rate, each
polished amount may be divided by the polishing time at the
aforementioned step 3.
[0092] Further, instead of correcting the polishing coefficient due
to the influence of the edge shape, which has been made for
predicting the polishing profile, the pressure on the back surface
resulting from the measurement of the edge shape can be corrected
after the calculation of the recommended pressure value to correct
the edge polishing profile, restraining variations in polishing of
an outer peripheral region of a wafer due to the edge shape. For
example, for an oxide film on a wafer, a recommended pressure value
for the retainer ring (E5) may be multiplied by a pressure
correction coefficient in accordance with the magnitude of roll-off
(Corrective Pressure Value for Retainer Ring=Pressure Correction
Coefficient.times.Recommended Pressure Value for Retainer Ring).
Here, the pressure correction coefficient is created by actually
polishing a wafer having, for example, a previously known roll-off
while varying the retainer ring pressure. Alternatively, the finite
element method may be relied on to calculate the relationship
between the pressing pressure and the magnitude of roll-off to
create the pressure correction coefficient.
[0093] Since the magnitude of roll-off varies from minute to
another as the polishing is advanced, the magnitude of roll-off can
be measured during polishing by a measuring device associated with
the polishing apparatus to correct the pressure during polishing.
Alternatively, the pressure can also be corrected by creating the
pressure correction coefficient in consideration of the polishing
time without measuring the magnitude of roll-off during
polishing.
[0094] For the shape at an end of a metal film on a wafer, the
correction can be made in a similar method to the oxide film
roll-off correcting method. The method of correcting the edge shape
using the pressure correction coefficient can also be applied when
the recommended pressure value is not calculated.
[0095] The polishing apparatus illustrated in FIG. 1 can be applied
to a variety of objects to be polished by exchanging the top ring.
When the top ring is exchanged in order to change an object to be
polished, it is necessary to change a set of pressing force
distributions on the surface of an object to be polished which have
been previously calculated in conformity to the shape of the top
ring. In this regard, the result of calculations of separately and
previously calculated pressing force distributions may be set, or
parameters such as the number of air bags of the top ring, an
available pressure range and the like may be entered when the
polishing apparatus is initially activated, and a plurality of
pressing force distributions on the surface of an object to be
polished, corresponding to the entered parameters, maybe calculated
within the polishing apparatus and stored in the control unit.
[0096] In this way, in the polishing apparatus of FIG. 1, a recipe
can be created not only to polish a wafer to be flat but also to
polish a wafer into a particular shape. Even when a film surface
shape of a wafer before polishing is not flat, a recipe can be
created to make the shape of the residual film flat after polishing
in consideration of the original shape. Also, the polishing
condition can be optimized without relying on empirical rules of
engineers as before, but an optimal condition can be calculated to
polish into a desired polishing profile. As compared with the prior
art which sets a polishing condition after a plurality of test
wafers have been polished, efforts, time, and cost can be
reduced.
[0097] In the foregoing description, the simulation program has
used two variables which are the thicknesses of an initial wafer
and the polished wafer and the pressing force of the top ring.
Further, in the present invention, the accuracy for the correction
is improved by sufficiently monitoring those parameters which
cannot be covered by the Preston's empirical formula, and the
temperature on the polishing surface, the thickness of the pad, the
depth of the grooves in the pad, the cutting rate value of the
dresser, and the amount of worn retainer ring in the top ring are
also reflected to the polishing in order to accomplish uniform
shapes, resulting from the polishing, in association with further
miniaturization of integrated circuits.
[0098] To implement the foregoing, the state monitor SM (FIG. 1) in
the polishing apparatus according to the present invention performs
the following operations, and supplies resultant outputs to the
control unit CU to further optimize the polishing using parameters
which are not considered by the simulation program.
[0099] (1) In regard to the temperature on the polishing surface, a
temperature range in which the polishing may be continued is set,
and the temperature on the polishing surface is monitored by the
state monitor SM. This can be implemented by providing the state
monitor SM, for example, with a radiation temperature. As a result
of the monitoring, when the state monitor SM detects that the
temperature on the polishing surface exceeds an upper limit or a
lower limit of the set temperature range, the control unit CU stops
the polishing and cools down the polishing surface. The polishing
surface is cooled down in the following manner. A flow path is
provided within the polishing table for communicating a cooling
medium such as water therethrough. As a polishing stop signal is
output from the control unit, the flow rate of the cooling medium
is increased or the temperature of the cooling medium itself is
reduced. Here, while the flow rate or temperature of the cooling
medium is manipulated on the basis of the stop signal from the
control unit, the flow rate and temperature of the cooling medium
may be manipulated in accordance with the output of the state
monitor SM, i.e., a change in the temperature on the polishing
surface. Subsequently, when the state monitor SM detects that the
temperature on the polishing surface falls within the temperature
range, the control unit CU resumes the polishing. In this event,
the simulation program may be paused in a period in which the
polishing is stopped.
[0100] (2) The state monitor SM also monitors the thickness of the
polishing pad or the depth of the grooves in the polishing pad on
the polishing table (described in greater detail in connection with
FIG. 7). Each time the state monitor SM detects that the thickness
of the polishing pad or the depth of the grooves in the polishing
pad is reduced by 0.1 mm, a monitoring wafer is polished instead of
the wafer which has been so far polished, and the state monitor SM
modifies a default value of the simulation application from the
result of the polishing to optimize a pressure balance of the
retainer ring and air bags in the top ring for a wafer which is to
be next polished. When the state monitor SM detects that the
thickness of the polishing pad or the depth of the grooves falls
below a predetermined threshold while the wafer is thus being
polished, the control unit CU stops the polishing. In response, the
operator replaces the polishing pad.
[0101] While the state monitor SM comprises a laser displacement
gage so that the thickness of the polishing pad can be monitored by
directly monitoring the surface of the polishing pad by the laser
displacement gage or by measuring the distance to a member which
comes into contact with the polishing pad by the laser displacement
gage, the present invention is not so limited.
[0102] (3) In order to prevent insufficient dressing of the dresser
and a reduction in the amount of removed polishing debris, the
state monitor SM monitors the dresser for the cutting rate when the
pad is conditioned. When the state monitor SM detects that the
cutting rate falls below a predetermined threshold, the control
unit CU stops the polishing, or extends a conditioning time for the
dresser, i.e., a time for which the polishing pad is cut. In this
way, since the polishing pad is uniformly cut away at all times,
the polishing can be performed with a high accuracy. Variations in
the cutting rate can be detected by monitoring the torque of a
motor used by the dresser for the conditioning.
[0103] (4) Further, the state monitor SM can monitor the retainer
ring in the top ring for an abrasion loss. Then, the control unit
CU instructs the polishing apparatus to stop the polishing when the
state monitor SM detects the abrasion loss of the retainer ring
falls below a certain threshold.
[0104] When a desired result cannot be achieved even if the
polishing is performed in consideration of those parameters which
cannot be covered by the Preston's basic formula, the amount of
supplied slurry is preferably adjusted. The control flow from the
foregoing (1) to (4) is stored in the control unit CU as a
program.
[0105] FIG. 7(A) generally illustrates an exemplary configuration
for measuring a relative change in the positions of the mechanical
dressers 38, 39 (FIG. 1) by the laser displacement gage associated
with the state monitor SM for detecting the thickness of the
polishing pad. As illustrated, a bar member 1001 is attached to an
appropriate location of the driving shaft 93 of each dresser. The
bar member 1001 is formed of a material which can reflect laser
light, or has a film formed on the surface thereof and made of a
material which can reflect laser light. A laser displacement gage
1002 is attached by an appropriate attaching means at a position at
which the laser displacement gage 1002 can receive laser light
which is irradiated to the bar member 1001 and reflected from the
bar member 1001. In this way, as the thickness of the polishing pad
is reduced with the advancement of the conditioning, the laser
displacement gage 1002 outputs a signal corresponding to a change
in the distance between the bar member 1001 and the laser
displacement gage 1002, i.e., a reduction in the thickness of the
polishing pad.
[0106] FIG. 7(B) shows the relationship between a conditioning time
and a reduction in the thickness of the polishing pad, derived by
making use of the output from the laser displacement gage 1003. It
can be understood from this graph that the thickness of the
polishing pad substantially linearly reduces as the conditioning
advances. By utilizing this relationship, a temporal changing rate
of the thickness of the polishing pad, i.e., the cutting rate of
the dresser can be found.
[0107] When the polishing apparatus as described above was used to
actually polish wafers, the following results were obtained. For
reference, the polishing was performed using IC1000/Suba400(K-gr)
for the polishing pad, and SS-25 for slurry with the rotational
speed of the polishing table set at 70/71 rpm, the rotational speed
of the top ring set at 71 rmp, and a default value for the pressure
of the air bags set at 250 hPa, and a pressing force of the dresser
set at 200 N.
[0108] Under the foregoing conditions, the polishing was performed
in the following procedure. First, after a polishing pad is
replaced, a monitoring wafer is polished. The pressure balance of
the air bags within the top ring is optimized from the result of
the polishing to polish a wafer. Next, after the polishing pad is
cut away by 0.1 mm, the monitoring wafer is polished. The pressure
balance of the air bags within the top ring is optimized from the
result to polish a wafer. After the polishing pad is further cut
away by 0.1 mm, the monitoring wafer is polished. The pressure
balance of the air bags is optimized from the result to polish a
wafer. In the following, this procedure is repeated a required
number of times.
[0109] When the mechanical dressers 38, 39 are fed by such a
mechanism as ball screws, a number of pulses required for driving a
motor for feeding can be measured to calculate the amount by which
the mechanical dresser is fed.
[0110] FIG. 8(A) is a diagram for describing the state of a
residual film before and after CMP when the present invention is
applied and not applied. The surface of the wafer is not flat but
partially has asperities and steps. The difference between a
maximum value and a minimum value of the thickness of a film to be
polished in the wafer is called the "thickness difference." When a
polished surface of the wafer is flat, the thickness difference is
zero. Also, the difference between the "thickness difference" after
the polishing and the "thickness difference" before the polishing
is called the "residual film difference."
[0111] FIG. 8(A) shows the residual film difference .DELTA. when
the present invention is applied and when not applied, when the
grooves in the polishing pad have the depth of 0.4 mm, 0.3 mm, and
0.2 mm, respectively, under conditions in which the pressures of
the air bags E1-E5 in the top ring are set as illustrated.
Specifically, the residual film difference .DELTA. was:
[0112] 3.3 nm when the grooves had a depth of 0.4 mm and the
present invention was not applied;
[0113] -43.5 nm when the grooves had a depth of 0.4 mm and the
present invention was applied;
[0114] 7.2 nm when the grooves had a depth of 0.3 mm and the
present invention was not applied;
[0115] -29.4 nm when the grooves had a depth of 0.3 mm and the
present invention was applied;
[0116] 68.6 nm when the grooves had a depth of 0.2 mm and the
present invention was not applied; and
[0117] -65.3 nm when the grooves had a depth of 0.2 mm and he
present invention was applied.
[0118] FIG. 8(B) is a graphic representation of the foregoing
result. A minus residual film difference means that the "thickness
difference" after polishing is smaller than the "thickness
difference" before polishing, so that the difference in thickness
was improved as compared with before polishing, i.e., the flatness
was improved. It can therefore be understood that the difference in
thickness after CMP was largely reduced by applying the present
invention.
[0119] Next, FIG. 9 shows the thickness and polishing rate when the
polishing pad has not at all consumed, where indicates a value when
the present invention was applied, and .diamond-solid. indicates a
value when the present invention was not applied. FIG. 9(A) is a
graph showing the relationship between a radial distance from the
center of a 300-mm wafer and the thickness before CMP; and FIG.
9(B) is a graph showing the relationship between a radial distance
from the center of the wafer and the thickness after CMP in FIG.
9(A). Then, when the polishing rate was derived from the thickness
before and after CMP when the present invention was applied and
when not applied, a graph shown in FIG. 9(C) was obtained. When the
result of a simulation for the polishing rate (indicated by
.largecircle.) was plotted on this graph, it was understood that
the polishing rate when the present invention was applied was
fairly consistent with the result of the simulation.
[0120] FIG. 10 shows the thickness and polishing rate when the
polishing pad has been consumed by 0.1 mm, where indicates a value
when the present invention was applied, and .diamond-solid.
indicates a value when the present invention was not applied. FIG.
10(A) is a graph showing the relationship between a radial distance
from the center of a 300-mm wafer and the thickness before CMP; and
FIG. 10(B) is a graph showing the relationship between a radial
distance from the center of the wafer and the thickness after CMP
in FIG. 10(A). Then, when the polishing rate was derived from the
thickness before and after CMP when the present invention was
applied and when not applied, a graph shown in FIG. 10(C) was
obtained. When the result of a simulation for the polishing rate
(indicated by .largecircle.) was plotted on this graph, it was
recognized that the polishing rate was reduced, though slightly, at
the center as the polishing pad was consumed more, i.e., the
polishing pad has shallower grooves, but the polishing rate was
fairly consistent with the result of the simulation at the center,
while in an outer peripheral region, actual values were slightly
different from the result of the simulation.
[0121] FIG. 11 shows the thickness and polishing rate when the
polishing pad has been consumed by 0.2 mm, where indicates a value
when the present invention was applied, and .diamond-solid.
indicates a value when the present invention was not applied. Like
FIGS. 9 and 10, FIG. 11(A) is a graph showing the relationship
between a radial distance from the center of a 300-mm wafer and the
thickness before CMP; and FIG. 11(B) is a graph showing the
relationship between a radial distance from the center of the wafer
and the thickness after CMP in FIG. 11(A). Then, when the polishing
rate was derived from the thickness before and after CMP when the
present invention was applied and when not applied, a graph shown
in FIG. 11(C) was obtained. When the result of a simulation for the
polishing rate indicated by .largecircle. was plotted on this
graph, it was recognized that the polishing rate was largely
reduced at the center, and largely differed from the result of the
simulation in an outer peripheral region. A default value of the
simulation application should be modified.
INDUSTRIAL AVAILABILITY
[0122] As will be understood from the foregoing description, since
the present invention optimizes a processing pressure using a
simulation program based on the Preston's basic formula, and
performs the polishing in consideration of even those parameters
which cannot be covered by the Preston's basic formula, it is
possible to realize the uniformization in the shape of polished
wafers, as required in step with increasing miniaturization of
integrated circuits. It is further possible to extend the life time
of a consumable material by correctly managing the state of the
consumable material to reduce the operation cost.
* * * * *