U.S. patent application number 12/341604 was filed with the patent office on 2010-06-24 for method of observing pattern evolution using variance and fourier transform spectra of friction forces in cmp.
This patent application is currently assigned to ARACA, Inc.. Invention is credited to Takenao Nemoto, Ara Philipossian, Yasa Sampurno, Akinobu Teramoto.
Application Number | 20100159804 12/341604 |
Document ID | / |
Family ID | 42266807 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100159804 |
Kind Code |
A1 |
Sampurno; Yasa ; et
al. |
June 24, 2010 |
METHOD OF OBSERVING PATTERN EVOLUTION USING VARIANCE AND FOURIER
TRANSFORM SPECTRA OF FRICTION FORCES IN CMP
Abstract
A method of determining pattern evolution of a semiconductor
wafer during chemical mechanical polishing prior to polishing end
point by determining the periodic change in the variance and FT or
FFT frequency spectra of shear force and change in variance and FT
or FFT frequency spectra of COF, shear force and/or down force
between the semiconductor wafer and the polishing pad. By comparing
features of the data and spectra thus obtained, analysis leading to
a deeper understanding of the changes that occur as CMP processes
occur as well as diagnostic analysis of specific CMP processes and
specific wafers can be accomplished
Inventors: |
Sampurno; Yasa; (Tucson,
AZ) ; Philipossian; Ara; (Tucson, AZ) ;
Teramoto; Akinobu; (Sendai-shi, JP) ; Nemoto;
Takenao; (Tokyo, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
ARACA, Inc.
Tucson
AZ
|
Family ID: |
42266807 |
Appl. No.: |
12/341604 |
Filed: |
December 22, 2008 |
Current U.S.
Class: |
451/5 ;
451/8 |
Current CPC
Class: |
B24B 37/042 20130101;
B24B 49/16 20130101 |
Class at
Publication: |
451/5 ;
451/8 |
International
Class: |
B24B 49/00 20060101
B24B049/00; B24B 49/16 20060101 B24B049/16 |
Claims
1. A method of determining pattern evolution of a semiconductor
wafer during chemical mechanical polishing prior to polishing end
point by measuring the change in the shear force and the down force
between the semiconductor wafer and the polishing pad over
time.
2. The method according to claim 1, wherein the said pattern
evolution is determined according to the change in the coefficient
of friction (COF) calculated between the semiconductor wafer and
the polishing pad calculated from the SF and the DF over time.
3. The method according to claims 1 and 2 wherein measurements are
made periodically.
4. The method according to claims 1 and 2, wherein shear force (SF)
is determined by measurement of two components of the SF
perpendicular to each other and calculation of the resultant shear
force.
5. The method according to claims 1 and 2, wherein down force (DF)
is determined by measuring the load applied to the polishing pad
and the semiconductor wafer.
6. The method according to claim 1, wherein pattern evolution of
the wafer is determined by extracting frequency components using
fourier transform (FT) of DF, SF and COF during different time
periods prior to polishing end point and determining the change in
intensity change of extracted frequency components.
7. The method according to claim 6 wherein the fourier transform
used is fast fourier transform (FFT).
8. The method according to claim 1, wherein the surface of the film
to be polished is irregular when polishing is initiated.
9. The method according to claim 1, wherein a CMP slurry containing
at least one member selected from the group consisting of silicon
dioxide, cerium oxide particles and ammonium polyacrylate or an
ammonium acrylate copolymer is used.
10. The method according to claim 1 wherein the variance of the
shear force is determined.
11. The method according to claim 1 wherein the variance of the
down force is determined.
12. The method according to claim 2 wherein the variance of COF is
determined.
13. The method according to claim 1, wherein the film to be
polished contains tantalum, tantalum nitride, silicon oxide,
silicon nitride, silicon oxynitride or other types of low k
dielectrics.
14. The method according to claim 1 wherein the determination of
the pattern evolution from changes in the SF and DF and numbers or
quantities derived from them is accomplished by data processing
means.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of determining the
pattern evolution of the down force (DF) and shear force (SF) and
from them the coefficient of friction (COF) during chemical
mechanical polishing between a semiconductor wafer and the
polishing pad using Fast Fourier Transform (FFT) or Fourier
Transform (FT) spectra of periodic down force, shear force and/or
coefficient of friction measurements.
[0003] 2. Description of the Prior Art
[0004] Currently under research and development are processing
methods for improvement in density and miniaturization in
production of ULSI semiconductor devices. One of the methods, CMP
(chemical mechanical polishing) technology, is now a technology
essential in production of semiconductor devices; for example, for
polishing an interlayer dielectric film, isolating a shallow trench
device, forming a plug or preparing embedded metal wiring.
[0005] Generally, in chemical mechanical polishing, a polishing pad
is first fixed on the rotary polishing pad of a polisher, while an
irregular-surfaced semiconductor wafer is fixed on a polishing
head. Chemical mechanical polishing is performed by pressing the
polishing head onto the revolving polishing pad, while CMP slurry
is supplied to the polishing pad. Irregularity on the wafer present
before polishing is eliminated by chemical mechanical polishing,
and the wafer surface is planarized. The polishing should be
terminated immediately after the surface is planarized to ensure
maximum uniformity or until unwanted top layers are removed. Until
today, the only way to do this job in microelectronic manufacturing
is CMP.
[0006] A time management method for maintaining a constant
polishing period and a method of determining the polishing endpoint
detection has been used for making the thickness of the
surface-planarized film constant after polishing of a semiconductor
wafer, but a method for monitoring pattern evolution in SF, DF or
COF is advantageous because it allows the operator greater
flexibility in when to end or alter the operation conditions of a
process and provides additional useful and precise information as
to the progress of the polishing. Such a method can also be used as
a diagnostic tool for the polishing process. When polishing using
the same set of consumables, one should expect a similar pattern
evolution progress. If the pattern evolution is shown to be altered
during polishing, it indicates that there is something wrong during
polishing making it possible for the operator to take action
without further harming the overall process. In polishing a
semiconductor wafer carrying an integrated circuit pattern the
shear force will vary, according to the material composition,
structure and surface conditions of the film being polished.
Heretofore, methods of using shear force in the endpoint detection
method have been disclosed; for example, in U.S. Pat. No. 5,046,015
and Japanese Patent Application Laid Open No. 8-197417 both
incorporated herein by reference and endpoint detection has lead to
improvement in the reproducibility of the extent of and thickness
of material removed by chemical mechanical polishing.
[0007] In the polishing method above, the shear force provides a
torque to the polishing table. Thus, it is possible to determine
the shear force by measuring the electric current of the motor
driving the polishing table. The shear force F, the torque Tq
generated on the polishing table, and the distance r between the
position of the shear force applied to the polishing table and the
rotational center of the polishing table have the relationship:
Tq=F.times.r. However, the position r of the semiconductor wafer on
the polishing table is variable as it moves during polishing, and
thus, the shear force F cannot be determined only by motor current.
As described above no method of directly measuring the shear force
generated between a revolving semiconductor wafer and a polishing
pad that can be performed easily industrially has yet been
disclosed.
[0008] For example, when the conditioning, that is to say, the
surface roughening of the polishing pad is performed simultaneously
with polishing, a torque is applied to the motor driving the
polishing table, and the motor current changes. In addition, the
load created by the polishing table itself is applied to the motor,
and contribution of the shear force between the semiconductor wafer
and the polishing pad to the motor torque becomes relatively
smaller. Thus, determination of the shear force between
semiconductor wafer and polishing pad from the motor current leads
to expansion of error.
[0009] In US Patent Application No. 2008/0200032, incorporated
herein by reference, a polishing method for measuring the COF
during polishing of a semiconductor wafer and using the change
thereof in determining the polishing end point is disclosed. This
patent detects the endpoint by means of significant alterations in
the amplitude of frequency peaks in fast fourier transform spectra
of shear force versus time. However, this patent application
provides no method or disclosure concerning the analysis, based on
variance of shear force, variance of down force, FFT of shear
force, FFT of down force and FFT of COF of polishing pattern
evolution prior to polishing endpoint.
SUMMARY OF THE INVENTION
[0010] The present invention is method of determining pattern
evolution of a semiconductor wafer during chemical mechanical
polishing prior to polishing end point by measuring the periodic
change in the variance and FT or FFT frequency spectra of shear
force and change in variance or FT or FFT frequency spectra of COF,
shear force and/or down force between the semiconductor wafer and
the polishing pad.
[0011] Using the method of the present invention, it is possible to
determine these pattern evolutions within a single film or
sequentially within multiple films during the chemical mechanical
polishing of a wafer easily and determine characteristics of
individual wafers or series of wafers to aid manufacturers in wafer
development or to help determine optimal polishing conditions for
particular wafers or types of wafers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic view illustrating an example of the
method of measuring shear force according to the present invention.
The same apparatus was used as in US Patent Application No.
2008/0200032 and that the description and figures depicting that
apparatus are incorporated by reference.
[0013] FIG. 2 is a sectional view illustrating a semiconductor
wafer of a shallow trench isolation film having a test pattern
formed on the surface used in Examples of the present
invention.
[0014] FIG. 3 is a graph showing the transient variance over time
in the shear force and down force obtained in the practice examples
of the present invention.
[0015] FIG. 4 shows examples of the graph of variance of shear
force and variance of COF as well as spectra of fast Fourier
transformation (FFT) of the shear force and COF respectively
obtained in an Example of the present invention.
[0016] FIG. 5 is a graph showing the change over time in the down
force obtained in the practice examples of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0017] In the method of polishing a semiconductor wafer according
to the present invention, a semiconductor wafer is polished while
pressed on a polishing pad fixed on a revolving polishing table.
Simultaneously, a slurry is supplied to the space between the
polishing pad and the semiconductor wafer. The semiconductor wafer
may be held by a polishing head, and the polishing head may be
rotated by a driving unit, independently from the polishing
table.
[0018] In the polishing method according to the present invention,
the pattern evolution is determined from the change in variance and
the frequency peaks of the FFT spectra of the shear force and COF
between the wafer and polishing pad during polishing. The
coefficient of friction COF between the wafer and polishing pad
during polishing is represented by the ratio of the shear force
F.sub.shear applied to the wafer and the polishing pad to the load
applied to the wafer F.sub.normal (F.sub.shear/F.sub.normal).
F.sub.normal is a value in proportion to the load applied to the
polishing head, and thus, the coefficient of friction COF is in
proportion to the shear force F.sub.shear when F.sub.normal is
constant.
[0019] In directly determining the shear force F.sub.shear applied
to the wafer and the polishing pad (hereinafter referred to also as
SF), a force in the "horizontal" direction parallel to the surface
of the polishing head (x-y direction) generated on the polishing
table or the polishing head may be measured. The method of
determining the coefficient of friction COF by the force in the x-y
direction generated on the polishing head and the load applied via
the polishing head onto the polishing table will be described with
reference to the drawings. FIG. 1 is a schematic view illustrating
the measuring method according to the present invention. A
polishing pad 13 is fixed on a polishing table 12, and the
polishing table 12 (diameter 500 mm) is rotated, driven by a drive
motor 11. A CMP slurry is supplied through a slurry-supply tube 14.
The polishing table 12 and the drive motor 11 are fixed on a stand
3, and stored in a CMP polisher 1 via load cells 19a. A
semiconductor wafer 15 is fixed on the polishing head 16 and its
slide plate 17 movable only in one direction are mounted on a stand
18 mechanically separated from the polishing table 12. A load
applied from the polishing head 16 in the "vertical" direction
perpendicular to the plane of the semiconductor wafer surface
("z-direction") is transmitted to the polishing table 12, the stand
3, and the load cells 19a. The load cells 19a detect the pressure
in the "z-direction", and the electrical signals generated in the
load cells 19a are transmitted to a recorder 20 and Fourier
transform (FT) or fast Fourier transform device (FFT 21).
[0020] The center of the semiconductor wafer 15 is fixed by the
polishing head 16 and is placed at a distance from the center of
the polishing pad 13 on the polishing table 12, and thus, a shear
force in the x-y direction is applied by friction with the
polishing pad 13. The shear force generated on the semiconductor
wafer 15 is transmitted, through the polishing head 16, motor 2,
and slide plate 17, to the load cells 19b and 19c. The load cell
19b detects the depth-direction component ("y component") of shear
force, while the load cell 19c the lateral-direction component
("x-component") of shear force perpendicular thereto; and these
components are transmitted to the recorder 20 and FFT 21.
[0021] The ratio F.sub.shear/F.sub.normal and the coefficient of
friction COF are calculated from the shear force in combination of
these two components and the load in the "z-direction".
[0022] The method of measuring the COF in the x-y plane generated
on the polishing table is the same in principle as method of
determining the SF described above. The voltage signals obtained
for shear force and down force are transmitted to the recorder 20
and the FFT signal processing unit 21 and processed to output COF
and variance of COF.
[0023] Although the polishing head presses the polishing table
downward in FIG. 1, the present invention may be applied similarly
to a CMP polisher wherein the polishing head and the polishing
table are placed upside down or at any angle.
[0024] The shear force is measured in real time, and all components
including direct-current to high frequency components are
determined according to the frequency characteristics of the load
cell or strain gauge. The friction coefficient obtained from the
shear force also includes a high frequency component, and it is
possible to analyze the down force, shear force and coefficient of
friction at each frequency by FT or FFT.
[0025] The COF depends on the physical properties of the film to be
polished, the CMP slurry, and the polishing pad. Conditioning using
a dresser such as a diamond conditioner disc may be needed to
maintain the roughness of the polishing pad surface constant, and
such conditioning is performed during or after polishing.
[0026] When there is irregularity on the surface of the film to be
polished, the load concentrates on the prominent or elevated
regions. The area upon which load is concentrated widens as surface
irregularity is reduced by the progress of polishing, and the load
is at last applied uniformly on the entire semiconductor wafer
surface after the surface has been planarized completely. The
measured shear force varies by the change of the area exposed to
concentrated load by planarizing resulting from polishing, and of
course the various features of the pattern evolution prior to
endpoint can be determined and using this change.
[0027] For example, as shown in part 1 of FIG. 2, in the silicon
wafer 31 with trenches 34, there are a silicon nitride stopper
layer 33 and a pad oxide or silicon dioxide layer 32 and these are
covered overall by an HDP SiO.sub.2 film 35. Endpoint of the CMP
process in this example would occur when a planarized surface
broadly exposing the silicon nitride layer is obtained as shown in
parts 3a and 3b. The difference between 3a and 3b is the existence
of "dishing" or similar surface anomaly 36 that occurs because the
physical properties of the silicon nitride stopper layer 33 and the
HDP SiO.sub.2 film 35 or other covering layer as the case may be
are different and have responded non-uniformly to the CMP
process.
[0028] The pattern evolution of the present invention which
comprises the change in pattern of the wafer surface over time and
the change over time of the FT or FFT spectra of the frequency of
shear force measurements but which may also include the change in
variance over time of both the SF, DF and COF is due to a
combination of physical changes that occur at the wafer surface
prior to endpoint as CMP polishing proceeds that include but are
not limited to elimination of the initial surface roughness, the
representation of which may be observed between Parts 1 and Parts
2a and 2b but additionally to uneven features or dishing 36 that
develop as a result of the depth and physical properties of
structures 38 that have not yet been uncovered by CMP. Ideally the
progress of CMP would flow from Part 1 to Part 2a and then to 3a
with the result of an entirely flat surface both during the process
and at the end. Analysis of the pattern evolution can indicate how
well this is progressing and also provide information about the
wafer being polished which can lead to alterations or improvements
in the settings or conditions of polishing for that wafer. What
often tends to happen is that some surface features persist as in
3b. Determination of the endpoint is useful in its own right but
does not provide the diagnostic and potentially corrective enabling
information of the present invention.
[0029] It is clear that the present invention monitors at least two
processes that occur during polishing: the elimination of initial
roughness of the wafer surface and the development of dishing or
other surface features while still removing the initial layer that
are indicative of material conditions of the layer or the presence
and dimensions of deeper structures.
[0030] SF and DF are measured over a series of short intervals.
These intervals are preferably between 2 and 15 seconds in length
and have the same length. Variance is calculated for each of them.
The longer the interval the better the variance figure but the
shorter the interval, the sharper the picture of the SF and COF
pattern evolution that emerges. There is a point beyond which
lengthening of the interval ceases to generate improvements in the
variance figure commensurate with the loss of pattern definition.
That point is typically about 5 seconds and that is the preferred
value for interval length.
[0031] When the shear force at each frequency is determined by fast
FT or FFT of the shear force, the maximum (peak) intensities appear
at a particular frequencies. The peak intensity varies in relation
to the degree of irregularity of the wafer surface, and thus, it is
possible to determine the evolution in pattern by monitoring the
change in peak intensity. The peak frequency, which is influenced
by the location, shape and dimension of irregularities of the
surface of the wafer and CMP conditions is determined separately
for each semiconductor wafer produced.
[0032] Before the CMP end-point, the fluctuation in certain
frequency peaks is related to the pattern evolution or, as we can
also say, the step height evolution, or even more basically, step
height reduction and it is also related to processes or features at
the polishing surface that can be used as a diagnostic of the
polishing process in real time.
[0033] In regard to the type of slurry used in the present
invention, typically, the selection of the slurry is not
particularly limited and is based on the slurry performance, that
is to say the removal rate, within reasonable limits of
wafer-non-uniformity. It is possible to detect the pattern
evolution with the present invention when slurries are used that
are effective for various CMP processes. Any slurry used in CMP
processing may be used as the slurry of the present invention.
[0034] The COF and variance of shear force figures results obtained
using the present invention may either increase, decrease or
otherwise vary with time depending upon the wafer and the
conditions under which CMP is carried out.
[0035] FIG. 3 shows the COF and the variance of a system described
by the following conditions: All polishing was performed with a
200-mm polisher and tribometer. Shear force and down force were
acquired in real-time at a frequency of 1,000 Hz. A 100-grit
diamond disc was used to condition the pad at 5.8 lb during wafer
polishing. The conditioner disc rotated at 30 RPM and swept 10
times per minute. During polishing, the diamond disc, the pad and
the wafer carrier rotated in counterclockwise fashion. A CMP slurry
was used to polish STI patterned wafers. The slurry flow rate was
set at 200 ml/min. Polishing was done on an IC1000 A2 k-groove pad
at a pressure and sliding velocity of 3 PSI and 1.37 m/s,
respectively.
[0036] The polishing conditions may be altered from those specified
in the above embodiment; however, once conditions are established,
it is preferred to maintain them constant during the CMP operation
and for comparative runs. However, for example, the load applied to
the wafer or the rotation rate of the polishing pad, conditioner or
wafer may be altered. In the event that the rate of any of these is
altered during the run it is preferred that they be altered in a
constant or at least consistent manner.
[0037] Results (spectra) obtained by Fast Fourier transformation of
the shear forces acquired during the polishing intervals in FIG. 3
are shown in FIG. 4. There are many peaks observed in FIG. 4.
Several of the largest peaks in this example, in which the variance
of shear force and coefficient of friction increase with time tend
to decrease and in some cases become sharper as the surface is
planarized and the thickness of the SiO.sub.2 layer decreases.
[0038] The polishing method according to the present invention is
not limited to the materials described in the foregoing example and
is suitable for determining pattern evolution for any overburden
removed by CMP processes including for example copper and Ta/TaN.
It is a characteristic of the method of the present invention that
it is primarily concerned with pattern evolution during the period
of removal of overburden before the underlying layer in wafers
subjected to CMP polishing have been exposed.
EXAMPLES
Example 1
[0039] Hereinafter, the present invention will be described with
reference to Examples. FIG. 1 is a schematic view illustrating the
shear force measurement method used in the present example.
Polishing pad 13 is fixed on polishing table 12, and the polishing
table 12 (diameter 500 mm) is rotated, as driven by drive motor 11.
The polishing table 12 and the drive motor 11 are fixed on stand 3,
and placed in CMP polisher 1 resting on load cells 19a.
Semiconductor wafer 15 is fixed on polishing head 16 and pressed
downward by polishing head 16. Motor 2 rotating polishing head 16
and its slide place 17 movable only in one direction are mounted on
stand 18, which is mechanically separated from polishing table 12.
A pressure applied from polishing head 16 in the "z-direction" is
transmitted to polishing table 12, stand 3 and load cells 19a. Load
cells 19a detect the pressure in the "z-direction", and the
electrical signals generated in load cells 19a are transmitted to
recorder 20 and FFT device 21.
[0040] The position of semiconductor wafer 15 is fixed by polishing
head 16 and is placed away from the center of polishing table 12,
and thus, a shear force in the x-y direction is applied by friction
with polishing pad 13. The shear force generated on semiconductor
wafer 15 is transmitted, through polishing head 16, motor 2, and
slide plate 17, to load cells 19b and 19c. The load cell 19b
detects the depth direction component of shear force, while the
load cell component 19c the width-direction component of shear
force, and these components are transmitted to the recorder 20 and
FFT device 21.
[0041] FIG. 2 is a cross-sectional view of a semiconductor wafer
carrying a test pattern for shallow trench isolation film on the
surface during different phases of polishing.
[0042] A test pattern having the cross sectional structure shown in
FIG. 2 was used for evaluation of the CMP for shallow trench
isolation. A pad oxide layer 32 and a SiN stopper film 33 were
formed one by one on a silicon wafer 31, and trenches 34 were
formed thereon. HDP SiO.sub.2 film 35 was formed thereon, and the
product was used as the test pattern wafer for evaluation of CMP.
The depth of the trench 34 h1 was 400 nm; the stopper layer 33
thickness t2 was 110 nm; the thickness of the pad oxide layer 32
t3, 12.5 nm, and the thickness of the HDP SiO.sub.2 layer 35
thickness t1, 670 nanometers. The width of the shallow trench 34
isolation w1 was 50 microns, and the width of the active element w2
was 50 microns. The difference in surface level before CMP was 542
nm. IC-1000/Suba 400 laminate pad manufactured by Rohm and Haas
having concentric grooves processed on the surface was used as
polishing pad 13. A diamond conditioner disk (not shown in the
Figure) was used for making the polishing pad surface uniform. The
diamond conditioner disk of diameter of 100 mm carried #100 grit
diamond particles. A dispersion of 1 wt % of cerium oxide particles
(volumetric median diameter (d50): 0.25 microns, d99: 0.67 microns)
and 0.3 wt % ammonium polyacrylate (weight average molecular weight
Mw as determined by gel permeation measurement: 8000) in purified
water at pH 5.0 was used as the CMP slurry supplied from the slurry
supplying tube 14. An analyzer LA-920 manufactured by Horiba, Ltd.
Was used for measurement of the particle size distribution of the
CMP slurry, under the condition of a refractive index of 2.138 and
an absorption coefficient of 0. The value d99 represents a particle
diameter at an accumulated total volume of 99% when the volumes of
the particles are measured from the particles smallest in
volume.
[0043] The operational condition of the CMP polisher is as follows:
polishing table rotational frequency 93 per minute, polishing head
rotational frequency 97 per minute, polishing head pressure 22 kPa,
diamond conditioning disc load: 26 N, and diamond conditioner disc
rotation rate: 30 per minute. Conditioning was performed
simultaneously during polishing. The rate of application of CMP
slurry was 200 ml/min.
[0044] FIG. 3 shows the change of the shear force obtained over
time. FIG. 3 showed that the time T2 when the shear force
F.sub.shear is maximal was 70 sec. The thickness of respective
films and the level differences before polishing and 5, 25, 50 and
70 seconds after polishing were as follows: [0045] T=0 (before
polishing) [0046] Thickness of the stopper layer (SiN) t2: 101 nm.
[0047] Thickness of dent layer (SiO2) t1: 678 nm. [0048] Level
difference h2: 542 nm. [0049] No stopper film exposed. [0050] T=5
(before polishing) [0051] Thickness of the stopper layer (SiN) t2:
101 nm. [0052] Thickness of dent layer (SiO2) t1: 669 nm. [0053]
Level difference h2: 515 nm. [0054] No stopper film exposed. [0055]
T=25 [0056] Thickness of the stopper layer (SiN) t2: 101 nm. [0057]
Thickness of dent layer (SiO2) t1: 631 nm. [0058] Level difference
h2: 398 nm. [0059] No stopper film exposed. [0060] T=50 [0061]
Thickness of the stopper layer (SiN) t2: 101 nm. [0062] Thickness
of dent layer (SiO2) t1: 581 nm. [0063] Level difference h2: 229
nm. [0064] No stopper film exposed. [0065] T=70 (SiN) t2: 101 nm.
[0066] Thickness of the stopper layer (SiN) t2: 101 nm. [0067]
Thickness of dent layer (SiO2) t1: 540 nm. [0068] Level difference
h2: 4 nm [0069] Part of stopper film exposed.
[0070] Spectra obtained by FFT of the shear force are shown in FIG.
4. The frequency (Hz) is plotted on the abscissa and shear force
intensity ratio (logarithm) on the ordinate in FIG. 4. Ten or more
peaks are observed in the frequency range of 5 to 100 Hz, 50
seconds after initiation of polishing Start (T1). Observation of
the alterations among the width and height of peaks A, B, C, D, E,
F, G, H, I and J for FFT of the shear force and K, L, M, N, O, P
and Q for FFT of COF at T2, T3 and T4 comprise the pattern
evolution associated with the disc under these CMP conditions.
[0071] Observation of peak A at about 2 Hz shows that it remained
consistent over time but increased slightly in intensity. Peak B at
about 5 Hz increased in intensity from T2 to T3 and then declined
at T4, Peak C at about 8 Hz differentiated from Peak B from T2 to
T3 and then increased in intensity at T4. Peak D demonstrated
increased shouldering at T3 and then increased in intensity by T4.
Peaks E and F appear to have fused by T3 and then differentiated
even more significantly by T4. This may in fact be complete shifts
of minor peaks from overlapping one peak to overlapping an entirely
different peak or peaks by another time period. Peak G is not
strongly represented at T2 but appears to represent a modest
intensity by T4 and may represent a process indicating changes in
surface features of the wafer as the outer layer is removed. Peak H
is not clearly present at T2 but becomes very sharp though of large
intensity by T4. Peak I remains sharp throughout the process until
T4 and then diminishes indicating that some feature of the wafer
surface has perhaps disappeared by this point. Finally, Peak J is
all but nonexistent at T2, prominent at T3 and returns to being a
shoulder to Peak I by T4. It is interesting that there are peaks
that appear to exist during certain time periods in the polishing
process and not in others and also that at T3 and perhaps over the
entire period there is considerable shifting of the maximum Hz
values for the peaks.
[0072] Interestingly the FFT graph of the COF shows much the same
features of pattern evolution as that of the shear force
suggesting, perhaps unsurprisingly, that in this example variation
of frequency maxima over time is related to the shear force. The
development of peaks K, J and L roughly parallel those of A, B and
C and M, N and O those of D, E and F. Finally, Peaks P and Q
parallel peaks H and I in development and peaks for G and J are not
clearly paralleled in the COF FFT graphs suggesting possibly that
they are not due to the shear force component of COF. FIG. 5, the
FFT graph of the down force again shows much the same features of
pattern evolution as that of the shear force and COF though with
fewer peaks indicating that some frequencies seem less dependent on
direction and allowing perhaps isolation of those in the horizontal
direction that are more direction specific. The development of
peaks S, T and U over time roughly parallel those of A, B and C,
whereas the remaining peaks at the high end of the frequency
spectra again exhibit greater mutability, and mobility though
substantially less sharpness than lower frequency peaks and here it
is harder to observe with confidence any correlation with the
change in shear force peaks over time.
[0073] Finally, the transient variance of the shear and down forces
suggest a decrease in shear forces from T0 to about T2 and from
there a continuous increase through T3 with a brief drop and
recovery in shear force prior to T4. It is important that the FFT
or FT spectra of COF and SF be considered in light of and in
conjunction with the variance of shear force because this allows an
added dimension to efforts to tie physical effects at the wafer
surface to frequencies and to identify and explain these physical
effects.
DETAILED DESCRIPTION OF THE DRAWINGS
[0074] FIG. 1 is a schematic view illustrating an example of the
method of measuring shear force according to the present
invention
[0075] 1 is a CMP polisher
[0076] 2 is a Motor
[0077] 3 is a stand
[0078] 11 is a drive motor
[0079] 12 is a polishing table
[0080] 13 is a polishing pad
[0081] 15 is a semiconductor wafer
[0082] 16 is a polishing head
[0083] 17 is a slide plate
[0084] 19a is a load cell
[0085] 19b is a load cell
[0086] 19c is a load cell
[0087] 20 is a recorder
[0088] 21 is an FFT or data processing device
[0089] FIG. 2 is a sectional view illustrating a semiconductor
wafer of a shallow trench isolation film having a test pattern
formed on the surface used in Examples of the present
invention.
[0090] 31 is a silicon wafer
[0091] 32 is a pad oxide layer
[0092] 33 is a silicon nitride stopper film
[0093] 34 are trenches
[0094] 35 is an HDP SiO.sub.2 film
[0095] 36 is dishing or other surface anomaly
[0096] 38 is an underlying structure
[0097] FIG. 3 is a graph showing the transient variance over time
in the shear force and down force obtained in the practice examples
of the present invention.
[0098] FIG. 4 shows examples of the graph of variance of shear
force and variance of COF as well as spectra of fast Fourier
transformation (FFT) of the shear force and COF respectively
obtained in an Example of the present invention.
[0099] FIG. 5 is a graph showing the change over time in the down
force obtained in the practice examples of the present
invention.
EFFECTS OF THE INVENTION
[0100] The method of the present invention allows considerable
detailed investigation to be carried out in real time into the
factors controlling the removal of material and the structure and
characteristics of the wafer surface during CMP. Once the spectra
are analyzed and features of the spectra coupled to some degree
with surface phenomena on the wafer during CMP it is possible to
understand how the wafer surface changes under given CMP conditions
and ultimately in addition to providing a deeper understanding into
the mechanics of the CMP process this will allow a certain amount
of process optimization and, since the method may be used in real
time, feedback for the process in situ may be possible as well once
the characteristics of a particular wafer surface under particular
conditions are well understood using the method of the present
invention. So in essence the method of the present invention
provides both extremely in depth and cost effective monitoring and
ultimately a means of controlling critical aspects of the CMP
process to improve it and obtained better and more consistent
results.
[0101] The method of the present invention also provides a
diagnostic measure of the `health` of the polishing process. When
polishing using the same set of consumables, one should expect the
similar pattern evolution progress. If the pattern evolution is
shown to be significantly altered during polishing, it indicates
that there is a potential problem or anomaly occurring during
polishing and this allows the operator to take the prompt action
minimizing further harm to the overall process.
* * * * *