U.S. patent application number 11/532501 was filed with the patent office on 2007-01-18 for apparatus and method for in-situ endpoint detection for chemical mechanical polishing operations.
This patent application is currently assigned to Applied Materials, Inc.. Invention is credited to Manoocher Birang, Allan Gleason, Nils Johansson.
Application Number | 20070015441 11/532501 |
Document ID | / |
Family ID | 34380815 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070015441 |
Kind Code |
A1 |
Birang; Manoocher ; et
al. |
January 18, 2007 |
Apparatus and Method for In-Situ Endpoint Detection for Chemical
Mechanical Polishing Operations
Abstract
An apparatus and method of chemical mechanical polishing (CMP)
of a wafer employing a device for determining, in-situ, during the
CMP process, an endpoint where the process is to be terminated.
This device includes a laser interferometer capable of generating a
laser beam directed towards the wafer and detecting light reflected
from the wafer, and a window disposed adjacent to a hole formed
through a platen. The window provides a pathway for the laser beam
during at least part of the time the wafer overlies the window.
Inventors: |
Birang; Manoocher; (Los
Gatos, CA) ; Johansson; Nils; (Los Gatos, CA)
; Gleason; Allan; (San Jose, CA) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Applied Materials, Inc.
3050 Bowers Avenue
Santa Clara
CA
|
Family ID: |
34380815 |
Appl. No.: |
11/532501 |
Filed: |
September 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11099789 |
Apr 5, 2005 |
|
|
|
11532501 |
Sep 15, 2006 |
|
|
|
09399310 |
Sep 20, 1999 |
6876454 |
|
|
11099789 |
Apr 5, 2005 |
|
|
|
08979015 |
Nov 26, 1997 |
|
|
|
09399310 |
Sep 20, 1999 |
|
|
|
08413982 |
Mar 28, 1995 |
|
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|
08979015 |
Nov 26, 1997 |
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Current U.S.
Class: |
451/6 ;
451/41 |
Current CPC
Class: |
B24B 49/04 20130101;
G01B 11/0683 20130101; B24B 51/00 20130101; B24B 47/12 20130101;
B24B 49/12 20130101; B24B 37/205 20130101; B24B 37/013 20130101;
H01L 22/12 20130101 |
Class at
Publication: |
451/006 ;
451/041 |
International
Class: |
B24B 49/00 20060101
B24B049/00; B24B 7/30 20060101 B24B007/30 |
Claims
1. A method for chemical mechanical polishing (CMP) a wafer on a
polishing system, the method comprising the steps of: supporting a
polishing pad over a platen, the polishing pad having a polishing
surface, a bottom surface and a window from the polishing surface
to the bottom surface; holding the wafer on a polishing head
against the polishing surface; supplying a polishing liquid to the
polishing surface; polishing a surface layer on the wafer to remove
an increasing amount thereof; during at least a part of the
polishing operation, directing a light beam through a solid light
transmissive element so that the light beam impinges the wafer and
reflects back through the light-transmitting material to an optical
detector, wherein the light-transmitting element is secured to the
polishing pad so as to prevent the polishing liquid from passing
through the polishing pad, the solid light transmissive element
being more transmissive to light than the polishing surface; and
determining a point to end polishing based on a signal from the
detector.
2. The method of claim 1, further comprising rotating the platen
and the polishing pad.
3. The method of claim 1, wherein the light transmissive element
has a top surface substantially flush with the polishing
surface.
4. The method of claim 1, wherein the polishing pad includes a
polishing layer including the light transmissive element and a
backing layer having an aperture aligned with the light
transmissive element.
5. A method of endpoint detection during polishing of a wafer using
a polishing pad, said method comprising, polishing the wafer with a
polishing pad that has a polishing surface, a bottom surface and a
window from the polishing surface to the bottom surface;
transmitting detection light through a solid light transmissive
element secured to the polishing pad to a surface of the wafer
being polished, wherein the solid light transmissive element is
secured so as to prevent the polishing liquid from passing through
the polishing pad, the solid light transmissive element being more
transmissive to light than the polishing surface; and receiving a
reflection of said light reflecting off of said wafer surface and
passing through said transparent portion of said polishing pad.
6. The method of claim 5, further comprising rotating the platen
and the polishing pad.
7. The method of claim 5, wherein the light transmissive element
has a top surface substantially flush with the polishing
surface.
8. The method of claim 5, wherein the polishing pad includes a
polishing layer including the light transmissive element and a
backing layer having an aperture aligned with the light
transmissive element.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of (and claims priority
under 35 U.S.C. Section 120 to) pending U.S. patent application
Ser. No. 11/099,789, filed Apr. 5, 2005, which is a continuation
application of pending U.S. application Ser. No. 09/399,310, filed
Sep. 20, 1999, now U.S. Pat. No. 6,876,454, which is a continuation
of U.S. application Ser. No. 08/979,015, filed Nov. 26, 1997, now
abandoned, which is a file-wrapper-continuation of U.S. application
Ser. No. 08/413,982, filed Mar. 28, 1995, now abandoned. The
disclosure of each of the prior applications is considered part of
(and is incorporated by reference in) the disclosure of this
application.
BACKGROUND
[0002] 1. Technical Field
[0003] This invention relates to semiconductor manufacture, and
more particularly, to an apparatus and method for chemical
mechanical polishing (CMP) and in-situ endpoint detection during
the CMP process.
[0004] 2. Background Art
[0005] In the process of fabricating modern semiconductor
integrated circuits (ICs), it is necessary to form various material
layers and structures over previously formed layers and structures.
However, the prior formations often leave the top surface
topography of an in-process wafer highly irregular, with bumps,
areas of unequal elevation, troughs, trenches, and/or other surface
irregularities. These irregularities cause problems when forming
the next layer. For example, when printing a photolithographic
pattern having small geometries over previously formed layers, a
very shallow depth of focus is required. Accordingly, it becomes
essential to have a flat and planar surface, otherwise, some parts
of the pattern will be in focus and other parts will not. In fact,
surface variations on the order of less than 1000 .ANG. over a
25.times.25 mm die would be preferable. In addition, if the
aforementioned irregularities are not leveled at each major
processing step, the surface topography of the wafer can become
even more irregular, causing further problems as the layers stand
up during further processing. Depending on the die type and the
size of the geometries involved, the aforementioned surface
irregularities can lead to poor yield and device performance.
Consequently, it is desirable to effect some type of planarization,
or leveling, of the IC structures. In fact, most high density IC
fabrication techniques make use of some method to form a planarized
wafer surface at critical points in the manufacturing process.
[0006] One method for achieving the aforementioned semiconductor
wafer planarization or topography removal is the chemical
mechanical polishing (CMP) process. In general, the chemical
mechanical polishing (CMP) process involves holding and/or rotating
the wafer against a rotating polishing platen under a controlled
pressure. As shown in FIG. 1, a typical CMP apparatus 10 includes a
polishing head 12 for holding the semiconductor wafer 14 against
the polishing platen 16. The aforementioned polishing platen 16 is
typically covered with a pad 18. This pad 18 typically has a
backing layer 20 which interfaces with the surface of the platen
and a covering layer 22 which is used in conjunction with a
chemical polishing slurry to polish the wafer 14. Although some
pads 18 have only the covering layer 22, and no backing layer. The
covering layer 22 is usually either an open cell foamed
polyurethane (e.g. Rodel IC1000), or a sheet of polyurethane with a
grooved surface (e.g. Rodel EX2000). The pad material is wetted
with the aforementioned chemical polishing slurry containing both
an abrasive and chemicals. One typical chemical slurry includes KOH
(Potassium Hydroxide) and fumed-silica particles. The platen is
usually rotated about its central axis 24. In addition, the
polishing head is usually rotated about its central axis 26, and
translated across the surface of the platen 16 via a translation
arm 28. Although just one polishing head is shown in FIG. 1, CMP
devices typically have more than one of these heads spaced
circumferentially around the polishing platen.
[0007] A particular problem encountered doing a CMP process is in
the determination that a part has been planarized to a desired
flatness or relative thickness. In general, there is a need to
detect when the desired surface characteristics or planar condition
has been reached. This has been accomplished in a variety of ways.
Early on, it was not possible to monitor the characteristics of the
wafer during the CMP process. Typically, the wafer was removed from
the CMP apparatus and examined elsewhere. If the wafer did not meet
the desired specifications, it had to be reloaded into the CMP
apparatus and reprocessed. This was a time consuming and
labor-intensive procedure. Alternately, the examination might have
revealed that an excess amount of material had been removed,
rendering the part unusable. There was, therefore, a need in the
art for a device which could detect when the desired surface
characteristics or thickness had been achieved, in-situ, during the
CMP process.
[0008] Several devices and methods have been developed for the
in-situ detection of endpoints during the CMP process. For
instance, devices and methods that are associated with the use of
ultrasonic sound waves, and with the detection of changes in
mechanical resistance, electrical impedance, or wafer surface
temperature, have been employed. These devices and methods rely on
determining the thickness of the wafer or a layer thereof, and
establishing a process endpoint, by monitoring the change in
thickness, In the case where the surface layer of the wafer is
being thinned, the change in thickness is used to determine when
the surface layer has the desired depth. And, in the case of
planarizing a patterned wafer with an irregular surface, the
endpoint is determined by monitoring the change in thickness and
knowing the approximate depth of the surface irregularities. When
the change in thickness equals the depth of the irregularities, the
CMP process is terminated. Although these devices and methods work
reasonably well for the applications for which they were intended,
there is still a need for systems which provide a more accurate
determination of the endpoint.
SUMMARY
[0009] The present invention is directed to a novel apparatus and
method for endpoint detection which can provide this improved
accuracy. The apparatus and method of the present invention employ
interferometric techniques for the in-situ determination of the
thickness of material removed or planarity of a wafer surface,
during the CMP process.
[0010] Specifically, the foregoing objective is attained by an
apparatus and method of chemical mechanical polishing (CMP)
employing a rotatable polishing platen with an overlying polishing
pad, a rotatable polishing head for holding the wafer against the
polishing pad, and an endpoint detector. The polishing pad has a
backing layer which interfaces with the platen and a covering layer
which is wetted with a chemical slurry and interfaces with the
wafer. The wafer is constructed of a semiconductor substrate
underlying an oxide layer. And, the endpoint detector includes a
laser interferometer capable of generating a laser beam directed
towards the wafer and detecting light reflected therefrom, and a
window disposed adjacent to a hole formed through the platen. This
window provides a pathway for the laser beam to impinge on the
wafer, at least during the time that the wafer overlies the
window.
[0011] The window can take several forms. Among these are an insert
mounted within the platen hole. This insert is made of a material
which is highly transmissive to the laser beam, such as quartz. In
this configuration of the window, an upper surface of the insert
protrudes above a surface of the platen and extends away from the
platen a distance such that a gap is formed between the upper
surface of the insert and the wafer, whenever the wafer is held
against the pad. This gap is preferably made as small as possible
but without allowing the insert to touch the wafer. Alternately
window can take the form of a portion of the polishing pad from
which the adjacent-backing layer has been removed. This is possible
because the polyurethane covering layer is at least partially
transmissive to the laser beam. Finally, the window can take the
form of a plug formed in the covering layer of the pad and having
no backing layer. This plug is preferably made of a polyurethane
material which is highly transmissive to the laser beam.
[0012] In one embodiment of the present invention, the hole through
the platen, and the window, are circular in shape. In another, the
hole and window are arc-shaped. The arc-shaped window has a radius
with an origin coincident to the center of rotation of the platen.
Some embodiments of the invention also have a laser beam whose beam
diameter that at its point of impingement on the wafer is
significantly greater than the smallest diameter possible for the
wavelength employed.
[0013] The aforementioned CMP apparatus can also include a position
sensor for sensing when the window is adjacent the wafer. This
ensures that the laser beam generated by the laser interferometer
can pass unblocked through the window and impinge on the wafer. In
a preferred embodiment of the invention, the sensor includes a flag
attached along a portion of the periphery of the platen which
extends radially outward therefrom. In addition, there is an
optical interrupter-type sensor mounted to the chassis at the
periphery of the platen. This sensor is capable of producing an
optical beam which causes a signal to be generated for as long as
the optical beam is interrupted by the flag. Thus, the flag is
attached to the periphery of the platen in a position such that the
optical beam is interrupted by the flag, whenever the laser beam
can be made to pass unblocked through the window and impinge on the
wafer.
[0014] Further the laser interferometer includes a device for
producing a detection signal whenever light reflected from the
wafer is detected, and the position sensor includes an element for
outputting a sensing signal whenever the window is adjacent the
wafer. This allows a data acquisition device to sample the
detection signal from the laser interferometer for the duration of
the sensing signal from the position sensor. The data acquisition
device then employs an element for outputting a data signal
representing the sampled detection signal. This data acquisition
device can also include an element for integrating the sampled
detection signal from the laser interferometer over a predetermined
period of time, such that the output is a data signal representing
the integrated samples of the detection signal. In cases where the
aforementioned predetermined sample period cannot be obtained
during only one revolution of the platen, an alternate method of
piece-wise data acquisition can be employed. Specifically, the data
acquisition device can include elements for performing the method
of sampling the detection signal output from the laser
interferometer during each complete revolution of the platen for a
sample time, integrating each sample of the detection signal over
the sample time to produce an integrated value corresponding to
each sample, and storing each integrated value. The data
acquisition device then uses other elements for computing a
cumulative sample time after each complete revolution of the platen
(where the cumulative sample time is the summation of the sample
times associated with each sample of the detection signal),
comparing the cumulative sample time to a desired minimum sample
time, and transferring the stored integrated values from the
storing element to the element for calculating a summation thereof,
whenever the cumulative sample time equals or exceeds the
predetermined minimum sample time. Accordingly, the aforementioned
output is a data signal representing a series of the integrated
value summations from the summation element.
[0015] The data signal output by the data acquisition device is
cyclical due to the interference between the portion of the laser
beam reflected from the surface the oxide layer of the wafer and
the portion reflected from the surface of the underlying wafer
substrate, as the oxide layer is thinned during the CMP process.
Accordingly, the endpoint in a CMP process to thin the oxide layer
of a blank oxide wafer can be determined using additional apparatus
elements for counting a number of cycles exhibited by the data
signal, computing a thickness of material removed during one cycle
of the output signal from the wavelength of the laser beam and the
index of refraction of the oxide layer of the wafer, comparing a
desired thickness of material to be removed from the oxide layer to
a removed thickness comprising the product of the number of cycles
exhibited by the data signal and the thickness of material removed
during one cycle, and terminating the CMP whenever the removed
thickness equals or exceeds the desired thickness of material to be
removed. Alternately, instead of counting complete cycles, a
portion of a cycle could be counted. The procedure is almost
identical except that the thickness of material removed is
determined for the portion of the cycle, rather than for an entire
cycle.
[0016] An alternate way of determining the endpoint in a CMP
processing of a blank oxide wafer uses apparatus elements which
measure the time required for the data signal to complete either a
prescribed number of cycles or a prescribed portion of one cycle,
compute the thickness of material removed during the time measured,
calculate a rate of removal by dividing the thickness of material
removed by the time measured, ascertain a remaining removal
thickness by subtracting the thickness of material removed from a
desired thickness of material to be removed from the oxide layer,
establish a remaining CMP time by dividing the remaining removal
thickness by the rate of removal, and terminate the CMP process
after the expiration of the remaining CMP time. In addition this
remaining CMP time can be updated after each occurrence of the
aforementioned number of cycles, or portions thereof, to compensate
for any in the material removal rate. In this case the procedure is
almost identical except that ascertaining the thickness of the
material involves first summing all the thicknesses removed in
earlier iteration and subtracting this cumulative thickness from
the desired thickness to determine the remaining removal thickness
figure.
[0017] However, when the wafer has an initially irregular surface
topography and is to be planarized during the CMP process, the data
signal is cyclical only after the wafer surface has become smooth.
In this case an endpoint to the CMP process corresponding to a
determination that the wafer has been planarized is obtained by
employing addition apparatus elements for detecting a cyclic
variation in the data signal, and terminating the CMP whenever the
detecting element detects the cyclic variation. Preferably, the
detecting element is capable of detecting a cyclical variation in
the data signal within at most one cycle of the beginning of this
variation.
[0018] In some circumstances, it is desirable to control the film
thickness overlying a structure on a patterned wafer. This film
thickness cannot always be achieved through the aforementioned
planarization. However, this control can still be obtained by
filtering the data signal to exclude all frequencies other than
that associated with the particular structure, or group of
similarly sized structures, over which a specific film thickness is
desired. Essentially, once the signal has been filtered, any of the
previously summarized ways of determining a CMP endpoint for a
blank oxide wafer can be employed on the patterned wafer.
[0019] In addition to the just described benefits, other objectives
and advantages of the present invention will become apparent from
the detailed description which follows hereinafter when taken in
conjunction with the drawing figures which accompany it.
DESCRIPTION OF THE DRAWINGS
[0020] The specific features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0021] FIG. 1 is a side view of a chemical mechanical polishing
(CMP) apparatus typical of the prior art.
[0022] FIG. 2 is a side view of a chemical mechanical polishing
apparatus with endpoint detection constructed in accordance with
the present invention.
[0023] FIGS. 3A-C are simplified cross-sectional views of
respective embodiments of the window portion of the apparatus of
FIG. 2.
[0024] FIG. 4 is a simplified cross-sectional view of a window
portion of the apparatus of FIG. 2, showing components of a laser
interferometer generating a laser beam and detecting a reflected
interference beam.
[0025] FIG. 5 is a simplified cross-sectional view of a blank oxide
wafer being processed by the apparatus of FIG. 2, schematically
showing the laser beam impinging on the wafer and reflection beams
forming a resultant interference beam.
[0026] FIG. 6 is a simplified top view of the platen of the
apparatus of FIG. 2, showing one possible relative arrangement
between the window and sensor flag, and the sensor and laser
interferometer.
[0027] FIG. 7 is a top view of the platen of the apparatus of FIG.
2, showing relative arrangement between the window and sensor flag,
and the sensor and laser, where the window is in the shape of an
arc.
[0028] FIG. 8 is a block diagram of a method of piece-wise data
acquisition in accordance with the present invention.
[0029] FIGS. 9A-B are graphs showing the cyclic variation in the
data signal from the laser interferometer over time during the
thinning of a blank oxide wafer. The graph of FIG. 9A shows the
integrated values of the data signal integrated over a desired
sample time, and the graph of FIG. 9B shows a filtered version of
the integrated values.
[0030] FIG. 10A is a block diagram of a backward-looking method of
determining the endpoint of a CMP process to thin the oxide layer
of a blank oxide wafer in accordance with the present
invention.
[0031] FIG. 10B is a block diagram of a forward-looking method of
determining the endpoint of a CMP process to thin the oxide layer
of a blank oxide wafer in accordance with the present
invention.
[0032] FIGS. 11A-C are simplified cross-sectional views of a
patterned wafer with an irregular surface being processed by the
apparatus of FIG. 2, wherein FIG. 11A shows the wafer at the
beginning of the CMP process, FIG. 11B shows the wafer about midway
through the process, and FIG. 11C shows the wafer close to the
point of planarization.
[0033] FIG. 12 is a block diagram of a method of determining the
endpoint of a CMP process to planarize a patterned wafer with an
irregular surface in accordance with the present invention.
[0034] FIG. 13 is a graph showing variation in the data signal from
the laser interferometer over time during the planarization of a
patterned wafer.
[0035] FIG. 14 is a block diagram of a method of determining the
endpoint of a CMP process to control the film thickness overlying a
particularly sized structure, or group of similarly sized
structures, in accordance with the present invention.
[0036] FIG. 15A is a simplified cross-sectional view of a wafer
with a surface imperfection being illuminated by a narrow-diameter
laser beam.
[0037] FIG. 15B is a simplified cross-sectional view of a wafer
with a surface imperfection being illuminated by a wide-diameter
laser beam.
[0038] Reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] Preferred embodiments of the present invention will now be
described with reference to the drawings.
[0040] FIG. 2 depicts a portion of a CMP apparatus modified in
accordance with one embodiment of the present invention. A hole 30
is formed in the platen 16 and the overlying platen pad 18. This
hole 30 is positioned such that it has a view the wafer 14 held by
a polishing head 12 during a portion of the platen's rotation,
regardless of the translational position of the head 12. A laser
interferometer 32 is fixed below the platen 16 in a position
enabling a laser beam 34 projected by the laser interferometer 32
to pass through the hole 30 in the platen 16 and strike the surface
of the overlying wafer 14 during a time when the hole 30 is
adjacent the wafer 14.
[0041] A detailed view of the platen hole 30 and wafer 14 (at a
time when it overlies the platen hole 30) is shown in FIGS. 3A-C.
As can be seen in FIG. 3A, the platen 30 has a stepped diameter,
thus forming a shoulder 36. The shoulder 36 is used to contain and
hold a quartz insert 38 which functions as a window for the laser
beam 34. The interface between the platen 16 and the insert 38 is
sealed, so that the portion of the chemical slurry 40 finding its
way between the wafer 14 and insert 38 cannot leak through to the
bottom of the platen 16. The quartz insert 38 protrudes above the
top surface of the platen 16 and partially into the platen pad 18.
This protrusion of the insert 38 is intended to minimize the gap
between the top surface of the insert 38 and the surface of the
wafer 14. By minimizing this gap, the amount of slurry 40 trapped
in the gap is minimized. This is advantageous because the slurry 40
tends to scatter light traveling through it, thus attenuating the
laser beam emitted from the laser interferometer 32. The thinner
the layer of slurry 40 between the insert 38 and the wafer 14, the
less the laser beam 34 and light reflected from the wafer, is
attenuated. It is believed a gap of approximately 1 mm would result
in acceptable attenuation values during the CMP process. However,
it is preferable to make this gap even smaller. The gap should be
made as small as possible while still ensuring the insert 38 does
not touch the wafer 14 at any time during the CMP process. In a
tested embodiment of the present invention, the gap between the
insert 38 and wafer 14 was set at 10 mils (250 .mu.m) with
satisfactory results.
[0042] FIG. 3B shows an alternate embodiment of the platen 16 and
pad 18. In this embodiment, the quartz insert has been eliminated
and no through-hole exists in the pad 18. Instead, the backing
layer 20 (if present) of the pad 18 has been removed in the area
overlying the hole 30 in the platen 16. This leaves only the
polyurethane covering layer 22 of the pad 18 between the wafer 14
and the bottom of the platen 16. It has been found that the
polyurethane material used in the covering layer 22 will
substantially transmit the laser beam 34 from the laser
interferometer 32. Thus the portion of the covering layer 22 which
overlies the platen hole 30 functions as a window for the laser
beam 34. This alternate arrangement has significant advantages.
First, because the pad 18 itself is used as the window, there is no
appreciable gap. Therefore, very little of the slurry 40 is present
to cause the detrimental scattering of the laser beam. Another
advantage of this alternate embodiment is that pad wear becomes
irrelevant. In the first-described embodiment of FIG. 3A, the gap
between the quartz insert 38 and the wafer 14 was made as small as
possible. However, as the pad 18 wears, this gap tend to become
even smaller. Eventually, the wear could become so great that the
top surface of the insert 38 would touch the wafer 14 and damage
it. Since the pad 18 is used as the window in the alternate
embodiment of FIG. 3B, and is designed to be in contact with the
wafer 14, there are no detrimental effects due to the
aforementioned wearing of the pad 18. It is noted that tests using
both the open-cell and grooved surface types of pads have shown
that the laser beam is less attenuated with the grooved surface
pad. Accordingly, it is preferable that this type of pad be
employed.
[0043] Although the polyurethane material used in the covering
layer of the pad is substantially transmissive to the laser beam,
it does contain certain additives which inhibit its
transmissiveness. This problem is eliminated in the embodiment of
the invention depicted in FIG. 3C. In this embodiment, the typical
pad material in the region overlying the platen hole 30 has been
replaced with a solid polyurethane plug 42. This plug 42, which
functions as the window for the laser beam, is made of a
polyurethane material which lacks the grooving (or open-cell
structure) of the surrounding pad material, and is devoid of the
additives which inhibit transmissiveness. Accordingly, the
attenuation of the laser beam 34 through the plug 42 is minimized.
Preferably, the plug 42 is integrally molded into the pad 18.
[0044] In operation, a CMP apparatus in accordance with the present
invention uses the laser beam from the laser interferometer to
determine the amount material removed from the surface of the
wafer, or to determine when the surface has become planarized. The
beginning of this process will be explained in reference to FIG. 4.
It is noted that a laser and collimator 44, beam splitter 46, and
detector 48 are depicted as elements of the laser interferometer
32. This is done to facilitate the aforementioned explanation of
the operation of the CMP apparatus. In addition, the embodiment of
FIG. 3A employing the quartz insert 38 as a window is shown for
convenience. Of course, the depicted configuration is just one
possible arrangement, others can be employed. For instance, any of
the aforementioned window arrangements could be employed, and
alternate embodiments of the laser interferometer 32 are possible.
One alternate interferometer arrangement would use a laser to
produce a beam which is incident on the surface of the wafer at an
angle. In this embodiment, a detector would be positioned at a
point where light reflecting from the wafer would impinge upon it.
No beam splitter would be required in this alternate
embodiment.
[0045] As illustrated in FIG. 4, the laser and collimator 44
generate a collimated laser beam 34 which is incident on the lower
portion of the beam splitter 46. A portion of the beam 34
propagates through the beam splitter 46 and the quartz insert 38.
Once this portion of beam 34 leaves the upper end of the insert 38,
it propagates through the slurry 40, and impinges on the surface of
the wafer 14. The wafer 14, as shown in detail in FIG. 5 has a
substrate 50 made of silicon and an overlying oxide layer 52 (i.e.
SiO.sub.2).
[0046] The portion of the beam 34 which impinges on the wafer 14
will be partially reflected at the surface of the oxide layer 52 to
form a first reflected beam 54. However, a portion of the light
will also be transmitted through the oxide layer 52 to form a
transmitted beam 56 which impinges on the underlying substrate 50.
At least some of the light from the transmitted beam 56 reaching
substrate 50 will be reflected back through the oxide layer 52 to
form a send reflected beam 58. The first and second reflected beams
54, 58 interfere with each other constructively or destructively
depending on their phase relationship, to form a resultant beam 60,
where the phase relationship is primarily a function of the
thickness of the oxide layer 52.
[0047] Although, the above-described embodiment employs a silicon
substrate with a single oxide layer, those skill in the art will
recognize the interference process would also occur with other
substrates and other oxide layers. The key is that the oxide layer
partially reflects and partially transmits, and the substrate at
least partially reflect, the impinging beam. In addition, the
interference process may also be applicable to wafers with multiple
layers overlying the substrate. Again, if each layer is partially
reflective and partially transmissive, a resultant interference
beam will be created, although it will be a combination of the
reflected beams from all the layer and the substrate.
[0048] Referring again to FIG. 4, it can be seen the resultant beam
60 representing the combination of the first and second reflected
beams 54, 58 (FIG. 5) propagates back through the slurry 40 and the
insert 38, to the upper portion of the beam splitter 46. The beam
splitter 46 diverts a portion of the resultant beam 60 towards the
detector 48.
[0049] The platen 16 will typically be rotating during the CMP
process. Therefore, the platen hole 30 will only have a view of the
wafer 14 during part of its rotation. Accordingly, the detection
signal from the laser interferometer 32 should only be sampled when
the wafer 14 is impinged by the laser beam 34. It is important that
the detection signal not be sampled when the laser beam 34 is
partially transmitted through the hole 30, as when a portion is
blocked by the bottom of the platen 16 at the hole's edge, because
this will cause considerate noise in the signal. To prevent this
from happening the position sensor apparatus has been incorporated.
Any well known proximity sensor could be used, such as Hall effect,
eddy current, optical interrupter, and acoustic sensor, although an
optical interrupter type sensor was used in the tested embodiments
of the invention and will be shown in the figures that follow. An
apparatus accordingly to the present invention for synchronizing
the laser interferometer 32 is shown in FIG. 6, with an optical
interrupter type sensor 62 (e.g. LED/photodiode pair) mounted on a
fixed point on the chassis of the CMP device such that it has a
view of the peripheral edge of the platen 16. This type of sensor
62 is activated when an optical beam it generates is interrupted. A
position sensor flag 64 is attached to the periphery of the platen
16. The point of attachment and length of the flag 64 is made such
that it interrupts the sensor's optical signal only when the laser
beam 34 from the laser interferometer 32 is completely transmitted
through the previously-described window structure 66. For example,
as shown in FIG. 6, the sensor 62 could be mounted diametrically
opposite the laser interferometer 32 in relation to the center of
the platen 16. The flag 64 would be attached to the platen 16 in a
position diametrically opposite the window structure 66. The length
of the flag 64 would be approximately defined by the dotted lines
68, although, the exact length of the flag 64 would be fine tuned
to ensure the laser beam is completely unblocked by the platen 16
during the entire time the flag 64 is sensed by the sensor 62. This
fine tuning would compensate for any position sensor noise or
inaccuracy, the responsiveness of the laser interferometer 32, etc.
Once the sensor 62 has been activated, a signal is generated which
is used to determine when the detector signal from the
interferometer 32 is to be sampled.
[0050] Data acquisition systems capable of using the position
sensor signal to sample the laser interferometer signal during
those times when the wafer is visible to the laser beam, are well
known in the art and do not form a novel part of the present
invention. Accordingly, a detailed description will not be given
herein. However some considerations should be taken into account in
choosing an appropriate system. For example, it is preferred that
the signal from the interferometer be integrated over a period of
time. This integration improves the signal-to-noise ratio by
averaging the high frequency noise over the integration period.
This noise has various causes, such as vibration from the rotation
of the platen and wafer, and variations in the surface of the wafer
due to unequal planarization. In the apparatus described above the
diameter of the quartz window, and the speed of rotation of the
platen, will determine how long a period of time is available
during any one rotation of the platen to integrate the signal.
However, under some circumstances, this available time may not be
adequate. For instance, an acceptable signal-to-noise ratio might
require a longer integration time, or the interface circuitry
employed in a chosen data acquisition system may require a minimum
integration time which exceeds that which is available in one
pass.
[0051] One solution to this problem is to extend the platen hole
along the direction of rotation of the platen. In other words, the
window structure 66' (i.e. insert, pad, or plug) would take on the
shape of an arc, as shown in FIG. 7. Of course, the flag 64' is
expanded to accommodate the longer window structure 66'.
Alternately, the window could remain the same, but the laser
interferometer would be mounted to the rotating platen directly
below the window. In this case, the CMP apparatus would have to be
modified to accommodate the interferometer below the platen, and
provisions would have to be made route the detector signal from the
interferometer. However, the net result of either method would be
to lengthen the data acquisition time for each revolution of the
platen.
[0052] Although lengthening the platen hole and window is
advantageous, it does somewhat reduce the surface area of the
platen pad. Therefore, the rate of planarization is decreased in
the areas of the disk which overlie the window during a portion of
the platen's rotation. In addition, the length of the platen hole
and window must not extend beyond the edges of the wafer, and the
data sampling must not be done when the window is beyond the edge
of the wafer, regardless of the wafer's translational position.
Therefore, the length of the expanded platen hole and window, or
the time which the platen-mounted interferometer can be sampled, is
limited by any translational movement of the polishing head.
[0053] Accordingly, a more preferred method of obtaining adequate
data acquisition integration time is to collect the data over more
than one revolution of the platen. In reference to FIG. 8, during
step 102, the laser interferometer signal is sampled during the
available data acquisition time in each rotation of the platen.
Next, in steps 104 and 106, each sampled signal is integrated over
the aforementioned data acquisition time, and the integrated values
are stored. Then, in steps 108 and 110, a cumulative sample time is
computed after each complete revolution of the platen and compared
to a desired minimum sample time. Of course, this would constitute
only one sample time if only one sample has been taken. If the
cumulative sample time equals or exceeds the desired minimum sample
time, then the stored integrated values are transferred and summed,
as shown in step 112. If not, the process of sampling, integrating,
storing, computing the cumulative sample time, and comparing it to
the desired minimum sample time continues. In a final step 114, the
summed integrated values created each time the stored integrated
values are transferred and summed, are output as a data signal. The
just-described data collection method can be implemented in a
number of well known ways, employing either logic circuits or
software algorithms. As these methods are well known, any detailed
description would be redundant and so has been omitted. It is noted
that the method of piece-wise data collection provides a solution
to the problem of meeting a desired minimum sample time no matter
what the diameter of the window or the speed of platen rotation. In
fact, if the process is tied to the position sensor apparatus, the
platen rotation speed could e varied and reliable data would still
be obtained. Only the number of platen revolutions required to
obtain the necessary data would change.
[0054] The aforementioned first and second reflected beams which
formed the resultant beam 60, as shown in FIGS. 4 and 5, cause
interference to be seen at the detector 48. If the first and second
beams are in phase with each other, they cause a maxima on detector
48. Whereas, if the beams are 180 degrees out of phase, they cause
a minima on the detector 48. Any other phase relationship between
the reflected beams will result in an interference signal between
the maxima and minima being seen by the detector 48. The result is
a signal output from the detector 48 that cyclically varies with
the thickness of the oxide layer 52, as it is reduced during the
CMP process. In fact, it has been observed that the signal output
from the detector 48 will vary in a sinusoidal-like manner, as
shown in the graphs of FIGS. 9A-B. The graph of FIG. 9A shows the
integrated amplitude of the detector signal (y-axis) over each
sample period versus time (x-axis). This data was obtained by
monitoring the laser interferometer output of the apparatus of FIG.
4, while performing the CMP procedure on a wafer having a smooth
oxide layer overlying a silicon substrate (i.e. a blank oxide
wafer). The graph of FIG. 9B represents a filtered version of the
data from the graph of FIG. 9A. This filtered version shows the
cyclical variation in the interferometer output signal quite
clearly. It should be noted that the period of the interference
signal is controlled by the rate at which material is removed from
the oxide layer during the CMP process. Thus, factors such as the
downward force placed on the wafer against the platen pad, and the
relative velocity between the platen and the wafer determine the
period. During each period of the output signal plotted in FIGS.
9A-B, a certain thickness of the oxide layer is removed. The
thickness removed is proportional to the wavelength of the laser
beam and the index of refraction of the oxide layer. Specifically
the amount of thickness removed per period is approximately
.lamda./2n, where .lamda. is the freespace wavelength of the laser
beam and n is the index of refraction of the oxide layer. Thus, it
is possible to determine how much of the oxide layer is removed,
in-situ, during the CMP process using the method illustrated in
FIG. 10A. First, in step 202, the number of cycles exhibited by the
data signal are counted. Next, in step 204, the thickness of the
material removed during one cycle of the output signal is computed
from the wavelength of the laser beam and the index of refraction
of the oxide layer of the wafer. Then, the desired thickness of
material to be removed from the oxide layer is compared to the
actual thickness removed, in step 206. The actual thickness removed
equals the product of the number of cycles exhibited by the data
signal and the thickness of material removed during one cycle. In
the final step 208, the CMP process is terminated whenever the
removed thickness equals or exceeds the desired thickness of
material to be removed.
[0055] Alternately, less than an entire cycle might be used to
determine the amount of material removed. In this way any excess
material removed over the desired amount can be minimized. As shown
in the bracketed portions of the step 202 in FIG. 10A, the number
of occurrences of a prescribed portion of a cycle are counted in
each iteration. For example, each occurrence of a maxima (i.e.
peak) and minima (i.e. valley), or vice versa, would constitute the
prescribed portion of the cycle. This particular portion of the
cycle is convenient as maxima and minima are readily detectable via
well know signal processing methods. Next, in step 204, after
determining how much material is removed during a cycle, this
thickness is multiplied by the fraction of a cycle that the
aforementioned prescribed portion represents. For example in the
case of counting the occurrence of a maxima and minima, which
represents one-half of a cycle, the computed one-cycle thickness
would be multiplied by one-half to obtain the thickness of the
oxide layer removed during the prescribed portion of the cycle. The
remaining steps in the method remain unchanged. The net result of
this alternate approach is that the CMP process can be terminated
after the occurrence of a portion of the cycle. Accordingly, any
excess material removed will, in most cases, be less than it would
have been if a full cycle where is as the basis for determining the
amount of material removed.
[0056] The just-described methods look back from the end of a
cycle, or portion thereof, to determine if the desired amount of
material has been removed. However, as inferred above, the amount
of material removed might exceed the desired amount. In some
applications, this excess removal of material might be
unacceptable. In these cases, an alternate method can be employed
which looks forward and anticipates how much material will be
removed over an upcoming period of time and terminates the
procedure when the desired thickness is anticipated to have been
removed. A preferred embodiment of this alternate method is
illustrated in FIG. 10B. As can be seen, the first step 302
involves measuring the time between the first occurrence of a
maxima and minima, or vice versa, in the detector signal (although
an entire cycle or any portion thereof could have been employed).
Next, in step 304, the amount of material removed during that
portion of the cycle is determined via the previously described
methods. A removal rate is then calculated by dividing the amount
of material removed by the measured time, as shown in step 306.
This constitutes the rate at which material was removed in the
preceding portion of the cycle. In the next step 308, the thickness
of the material removed as calculated in step 304 is subtracted
from the desired thickness to be removed to determine a remaining
removal thickness. Then, in step 310, this remaining removal
thickness is divided by the aforementioned removal rate to
determine how much longer the CMP process is to be continued before
it termination.
[0057] It must be noted, however, that the period of the detector
signal, and so the removal rate, will typically vary as the CMP
process progresses. Therefore, the above-described method is
repeated to compensate. In other words, once a remaining time has
been calculated, the process is repeated for each occurrence of a
maxima and minima, or vice versa. Accordingly, the time between the
next occurring maxima and minima is measured, the thickness of
material removed during the portion of the cycle represented by
this occurrence of the maxima and minima (i.e. one-half) is divided
by the measured time, and the removal rate is calculated, just as
in the first iteration of the method. However, in the next step
308, as shown in brackets, the total amount of material removed
during all the previous iterations is determined before being
subtracted from the desired thickness. The rest of the method
remains the same in that the remaining thickness to be removed is
divided by the newly calculated removal rate to determine the
remaining CMP process time. In this way the remaining process time
is recalculated after each occurrence of the prescribed portion of
a cycle of the detector signal. This process continues until the
remaining CMP process time will expire before the next iteration
can begin. At that point the CMP process is terminated, as seen in
step 312. Typically, the thickness to be removed will not be
accomplished in the first one-half cycle of the detector signal,
and any variation in the removal rate after being calculated for
the preceding one-half cycle will be small. Accordingly, it is
believe this forward-looking method will provide a very accurate
way of removing just the desired thickness from the wafer.
[0058] While the just-described monitoring procedure works well for
the smooth-surfaced blank oxide wafers being thinned, it has been
found that the procedure cannot be successfully used to planarize
most patterned wafers where the surface topography is highly
irregular. The reason for this is that a typical patterned wafer
contains dies which exhibit a wide variety of differently sized
surface features. These differently sized surface features tend to
polish at different rates. For example, a smaller surface feature
located relatively far from other features tends to be reduced
faster than other larger features. FIGS. 11A-C exemplify a set of
surface features 72, 74, 76 of the oxide layer 52 associated with
underlying structures 78, 80, 82, that might be found on a typical
patterned wafer 14, and the changes they undergo during the CMP
process. Feature 72 is a relatively small feature, feature 74 is a
medium sized feature, and feature 76 is a relatively large feature.
FIG. 11A shows the features 72, 74, 76 before polishing, FIG. 11B
shows the features 72, 74, 76 about midway through the polishing
process, and FIG. 11C shows the features 72, 74, 76 towards the end
of the polishing process. In FIG. 11A, the smaller feature 72 will
be reduced at a faster rate than either the medium or large
features 74, 76. In addition, the medium feature 74 will be reduced
at a faster rate than the large feature 76. The rate at which the
features 72, 74, 76 are reduced also decreases as the polishing
process progresses. For example, the smaller feature 72 will
initially have a high rate of reduction, but this rate will drop
off during the polishing process. Accordingly, FIG. 11B shows the
height of the features 72, 74, 76 starting to even out, and FIG.
11C shows the height of the features 72, 74, 76 essentially even.
Since the differently sized features are reduced at different rates
and these rates are changing, the interference signal produced from
each feature will have a different phase and frequency.
Accordingly, the resultant interference signal, which is partially
made up of all the individual reflections from each of the features
72, 74, 76, will fluctuate in a seemingly random fashion, rather
than the previously described periodic sinusoidal signal.
[0059] However, as alluded to above, the polishing rates of the
features 72, 74, 76 tend to converge closer to the point of
planarization. Therefore, the difference in phase and frequency
between the interference beams produced by the features 72, 74, 76
tend to approach zero. This results in the resultant interference
signal becoming recognizable as a periodic sinusoidal wave form.
Therefore it is possible to determine when the surface of a
patterned wafer has become planarized by detecting when a
sinusoidal interference signal begins. This method is illustrated
in FIG. 12. First, in step 402, a search is made for the
aforementioned sinusoidal variation in the interferometer signal.
When the sinusoidal variation is discovered, the CMP procedure is
terminated, as shown in step 404.
[0060] FIG. 13 is a graph plotting the amplitude of the detector
signal over time for a patterned wafer undergoing a CMP procedure.
The sampled data used to construct this graph was held at its
previous integrated value until the next value was reported, thus
explaining the squared-off peak values shown. A close inspection
shows that a discernible sinusoidal cycle begins to emerge at
approximately 250 seconds. This coincides with the point where the
patterned wafer first became planarized. Of course, in real-time
monitoring of the interferometer's output signal, it would be
impossible to know exactly when the cycling begins. Rather, at
least some portion of the cycle must have occurred before it can be
certain that the cycling has begun. Preferably, no more than one
cycle is allowed to pass before the CMP procedure is terminated. A
one-cycle limit is a practical choice because it provides a high
confidence that the cycling has actually begun, rather than the
signal simply representing variations in the noise caused by the
polishing of the differently sized features on the surface of the
wafer. In addition, the one-cycle limit ensures only a small amount
of material is removed from the surface of the wafer after it
becomes planarized. It has been found that the degree of
planarization is essentially the same after two cycles, as it was
after one. Thus, allowing the CMP procedure to continue would only
serve to remove more material from the surface of the wafer. Even
though one cycle is preferred in the case where the CMP process is
to be terminated once the patterned wafer becomes planarized, it is
not intended that the present invention be limited to that
timeframe. If the signal is particularly strong, it might be
possible to obtain the same level of confidence after only a
portion of a cycle. Alternately, if the signal is particularly
weak, it may take more than one cycle to obtain the necessary
confidence. The choice will depend on the characteristics of the
system used. For instance, the size of the gap between the quartz
window and the surface of the wafer will have an effect on signal
strength, and so the decision on how many cycles to wait before
terminating the CMP process.
[0061] The actual determination as to when the output signal from
the laser interferometer is actually cycling, and so indicating
that the surface of the wafer has been planarized can be done in a
variety of ways. For example, the signal could be digitally
processed and an algorithm employed to make the aforementioned
determination. Such a method is disclosed in U.S. Pat. No.
5,097,430, where the slope of the signal is used to make the
determination. In addition, various well known curve fitting
algorithms are available. These methods would essentially be used
to compare the interferometer signal to a sinusoidal curve. When a
match occurs within some predetermined tolerance, it is determined
that the cycling has begun.
[0062] Some semiconductor applications require that the thickness
of the material overlying a structure formed on a die of a
patterned wafer (i.e. the film thickness) be at a certain depth,
and that this film thickness be repeatable from die to die, and
from wafer to wafer. The previously described methods for
planarizing a typical patterned wafer will not necessarily produce
this desired repeatable film thickness. The purpose of the
planarization methods is to create a smooth and flat surface, not
to produce a particular film thickness. Accordingly, if it is
desirable to control the film thickness over a specific structure,
or group of similarly sized structures, an alternate method must be
employed. This alternate method is described below.
[0063] As alluded to previously, each differently sized surface
feature resulting from a layer of oxide being formed over a
patterned structure on a die tends to produce a reflected
interference signal with a unique frequency and phase. It is only
close to the point of planarization that the frequency and phase of
each differently sized feature converges. Prior to this convergence
the unique frequency and phase of the interference signals caused
by the various differently sized features combine to produce a
detector signal that seems to vary randomly. However, it is
possible to process this signal to eliminate the interference
signal contributions of all the features being polished at
different rates, except a particularly sized feature, or group of
similarly sized features. Once the interference signal associated
with the particularly sized feature, or group of features, has been
isolated, the methods discussed in association with the removal of
material from a blank oxide disk are employed to remove just the
amount of material necessary to obtain the desired film
thickness.
[0064] Of course, the frequency of the interference signal
component caused by the feature of interest must be determined
prior to the signal processing. It is believed this frequency can
be easily determined by performing a CMP process on a test specimen
which includes dies exclusively patterned with structures
corresponding to the structure which is to have a particular
overlying film thickness. The detector signal produced during this
CMP process is analyzed via well known methods to determine the
unique frequency of the interference signal caused by the surface
features associated with the aforementioned structures.
[0065] The specific steps necessary to perform the above-described
method of controlling the film thickness over a specific structure,
or group of similarly sized structures on a die, in situ, during
the CMP processing of a wafer, will now be described in reference
to FIG. 14. In step 502, the detector signal is filtered to pass
only the component of the signal having the predetermined frequency
associated with the structure of interest. This step is
accomplished using well known band-pass filtering techniques. Next
in step 504 a measurement is made of the time between the first
occurrence of a maxima and minima, or vice versa, in the detector
signal (although an entire cycle or any portion thereof could have
been employed). The amount of material removed during that portion
of the cycle (i.e. one-half cycle) is determined in step 506 via
previously described methods. Then, a removal rate is then
calculated by dividing the amount of material removed by the
measured time, as shown in step 508. This constitutes the rate at
which material was removed in the preceding portion of the cycle.
In the next step 510, the thickness of the material removed as
calculated in step 506 is subtracted from the desired thickness to
be removed (i.e. the thickness which when removed will result in
the desired film thickness overlying the structure of interest), to
determine a remaining removal thickness. Then, this remaining
removal thickness is divided by the aforementioned removal rate to
determine how much longer the CMP process is to be continued before
it termination, in step 512. Once a remaining time has been
calculated, the process is repeated for each occurrence of a maxima
and minima, or vice versa. Accordingly, the time between the next
occurring maxima and minima is measured, the thickness of material
removed during the portion of the cycle represented by this
occurrence of the maxima and minima (i.e. one-half) is divided by
the measured time, and the removal rate is calculated, just as in
the first iteration of the method. However, in the next step 510,
as shown in brackets, the total amount of material removed during
all the previous iterations is determined before being subtracted
from the desired thickness. The rest of the method remains the same
in that the remaining thickness to be removed is divided by the
newly calculated removal rate to determine the remaining CMP
process time. This process is repeated until the remaining time
expires before the next iteration can begin. At that point, the CMP
process is terminated, as seen in step 514.
[0066] It is noted that although the method for controlling film
thickness described above utilizes the method for determining the
CMP process endpoint illustrated in FIG. 10B, any of the other
endpoint determination methods described herein could also be
employed, if desired.
[0067] It is further noted that the beam diameter (i.e. spot) and
wavelength of the laser beam generated by the laser interferometer
can be advantageously manipulated. As shown in FIGS. 15A and 15B, a
narrow beam 84, such as one focused to the smallest spot possible
for the wavelength employed, covers a smaller area of the surface
of the wafer 14 than a wider, less focused beam 86. This narrow
beam 84 is more susceptible to scattering (i.e. beam 88) due to
surface irregularities 90, than the wider beam 86, since the wider
beam 86 spreads out over more of the surface area of the wafer 14,
and encompasses more of the surface irregularities 90. Therefore, a
wider beam 86 would have an integrating effect and would be less
susceptible to extreme variations in the reflected interference
signal, as it travels across the surface of the wafer 14.
Accordingly, a wider beam 86 is preferred for this reason. The
laser beam with can be widened using well known optical
devices.
[0068] It must also be pointed out that the wider beam will reduce
the available data acquisition time per platen revolution since the
time in which the beam is completely contained within the
boundaries of the window is less than it would be with a narrower
beam. However, with the previously described methods of data
acquisition, this should not present a significant problem. In
addition, since the wider beam also spreads the light energy out
over a larger area than a narrower beam, the intensity of the
reflections will be lessen somewhat. This drawback can be remedied
by increasing the power of the laser beam from the laser
interferometer so that the loss in intensity of the reflected beams
is not a factor in detection.
[0069] As for the wavelength of the laser beam, it is feasible to
employ a wavelength anywhere from the far infrared to ultraviolet.
However, it is preferred that a beam in the red light range be
used. The reason for this preference is two-fold. First, shorter
wavelengths result in an increase in the amount of scattering
caused by the chemical slurry because this scattering is
proportional to the 4th power of the frequency of the laser beam.
Therefore, the longer the wavelength, the less the scattering.
However, longer wavelengths also result in more of the oxide layer
being removed per period of the interference signal, because the
amount of material removed per period equals approximately
.lamda./2n. Therefore, the shorter the wavelength, the less
material removed in one period. It is desirable to remove as little
of the material as possible during each period so that the
possibility of any excess material being removed is minimized. For
example, in a system employing the previously described method by
which the number of cycles, or a portion thereof, are counted to
determine the thickness of the oxide layer removed, any excess
material removed over the desired amount would be minimized if the
amount of material removed during each cycle, or portion thereof,
is as small as possible.
[0070] It is believed these two competing factors in the choice of
wavelength are optimally balance if a red light laser beam is
chosen. Red light offers an acceptable degree of scattering while
not resulting in an unmanageable amount of material being removed
per cycle.
[0071] While the invention has been described in detail by
reference to the preferred embodiment described above, it is
understood that variations and modifications thereof may be made
without departing from the true spirit and scope of the invention.
Wherefor, what is claimed is:
* * * * *