U.S. patent application number 14/542580 was filed with the patent office on 2015-03-12 for multi-spectral defect inspection for 3d wafers.
The applicant listed for this patent is KLA-Tencor Corporation. Invention is credited to Steven R. Lange.
Application Number | 20150069241 14/542580 |
Document ID | / |
Family ID | 50727633 |
Filed Date | 2015-03-12 |
United States Patent
Application |
20150069241 |
Kind Code |
A1 |
Lange; Steven R. |
March 12, 2015 |
Multi-Spectral Defect Inspection for 3D Wafers
Abstract
Multi-spectral defect inspection for 3D wafers is provided. One
system configured to detect defects in one or more structures
formed on a wafer includes an illumination subsystem configured to
direct light in discrete spectral bands to the one or more
structures formed on the wafer. At least some of the discrete
spectral bands are in the near infrared (NIR) wavelength range.
Each of the discrete spectral bands has a bandpass that is less
than 100 nm. The system also includes a detection subsystem
configured to generate output responsive to light in the discrete
spectral bands reflected from the one or more structures. In
addition, the system includes a computer subsystem configured to
detect defects in the one or more structures on the wafer using the
output.
Inventors: |
Lange; Steven R.; (Alamo,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KLA-Tencor Corporation |
Milpitas |
CA |
US |
|
|
Family ID: |
50727633 |
Appl. No.: |
14/542580 |
Filed: |
November 15, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13742315 |
Jan 15, 2013 |
8912495 |
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14542580 |
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61729330 |
Nov 21, 2012 |
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Current U.S.
Class: |
250/338.1 |
Current CPC
Class: |
G01N 21/3563 20130101;
G01N 2201/0612 20130101; G01N 21/359 20130101; G01N 21/55 20130101;
G01N 21/9503 20130101; G01N 2201/105 20130101; G01N 21/9501
20130101 |
Class at
Publication: |
250/338.1 |
International
Class: |
G01N 21/95 20060101
G01N021/95; G01N 21/55 20060101 G01N021/55 |
Claims
1. A system configured to detect defects in one or more structures
formed on a wafer, comprising: an illumination subsystem configured
to direct light in discrete spectral bands to the one or more
structures formed on the wafer, wherein at least some of the
discrete spectral bands are in the near infrared wavelength range,
and wherein each of the discrete spectral bands has a bandpass that
is less than 100 nm; a detection subsystem configured to generate
output responsive to light in the discrete spectral bands reflected
from the one or more structures; and a computer subsystem
configured to detect defects in the one or more structures on the
water using the output.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to multi-spectral
defect inspection for 31) wafers.
[0003] 2, Description of the Related Art
[0004] The following description and examples are not admitted to
be prior art by virtue of their inclusion in this section.
[0005] Semiconductor memory manufacturers, particularly Flash
memory now and DRAM in the near future, have had trouble extending
their shrinking design rule roadmap to smaller dimensions due to
rapidly increasing costs for lithography and multiple process steps
associated with pitch splitting techniques. The concept of going
vertical has taken off with 3D or VNAND memory leading the way by
building transistors (bits) vertically rather than orienting them
in a planar manner which historically has been the rule. Early
VNAND devices have 116 to 24 vertical bits with roadmaps quickly
extending to 48 and 64 bits vertically and beyond. The changes are
achieved with fewer process steps, relaxed lithography sizes, and
lower manufacturing costs compared with the planar approach; hence,
they're being quickly adopted.
[0006] The deposition of these thick stacks typically occurs in a
single process step with subsequent processing on the whole stack
to create the vertical transistors and connections. Wafer
inspection for defects needs then to inspect the whole stack. For
water inspection, these changes result in much thicker stacks of
materials and structures, with the early VNAND devices having 2 um
to 3 um thick stacks and eventual stacks in the 6 um to 8 um range
(typical planar thicknesses are about 0.1 um to 1 um depending upon
the process step). Defects in the processing steps can occur
throughout these stacks and need to be detected and their source
identified and corrected to ensure high manufacturing yields.
[0007] Accordingly, it would be advantageous to develop methods and
systems for detecting defects in one or more structures on a wafer
that have characteristics described above,
SUMMARY OF THE INVENTION
[0008] The following description of various embodiments is not to
be construed in any way as limiting the subject matter of the
appended claims.
[0009] One embodiment relates to a system configured to detect
defects in one or more structures formed on a wafer. The system
includes an illumination subsystem configured to direct light in
discrete spectral bands to the one or more structures formed on the
wafer. At least some of the discrete spectral bands are in the near
infrared (NIR) wavelength range. Each of the discrete spectral
bands has a bandpass that is less than 100 nm. The system also
includes a detection subsystem configured to generate output
responsive to light in the discrete spectral bands reflected from
the one or more structures. In addition, the system includes a
computer subsystem configured to detect defects in the one or more
structures on the wafer using the output. The system may be further
configured as described herein.
[0010] Another embodiment relates to a method for detecting defects
in one or more structures formed on a wafer. The method includes
directing light in discrete spectral bands to the one or more
structures formed on the wafer. At least some of the discrete
spectral bands are in the NIR wavelength range. Each of the
discrete spectral bands has a bandpass that is less than 100 nm.
The method also includes generating output responsive to light in
the discrete spectral bands reflected from the one or more
structures. In addition, the method includes detecting defects in
the one or more structures on the wafer using the output.
[0011] Each of the steps of the method described above may be
further performed as described herein. In addition, the method
described above may include any other step(s) of any other
method(s) described herein. Furthermore, the method described above
may be performed by any of the systems described herein,
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Further advantages of the present invention will become
apparent to those skilled in the art with the benefit of the
following detailed description of the preferred embodiments and
upon reference to the accompanying drawings in which:
[0013] FIG. 1 is a schematic diagram illustrating a cross-sectional
view of one embodiment of a structure in which defects may be
detected by the embodiments described herein;
[0014] FIG. 2 is a plot illustrating the penetration depth of
different wavelengths of light into amorphous silicon;
[0015] FIG. 3 is a plot illustrating the penetration depth of
different wavelengths of light into undoped polysilicon;
[0016] FIG. 4 is a plot illustrating simulated defect signals for
an oxide-polysilicon-oxide-polysilicon (OPOP) deposition with a
particle at various depths;
[0017] FIG. 5 includes plots illustrating simulated defect signals
for different VNAND structures as a function of wavelength and
aperture;
[0018] FIG. 6 is a plot illustrating simulated defect signals for
an OPOP deposition with a particle at various depths;
[0019] FIG. 7 is a schematic diagram illustrating a side view of
one embodiment of a s system configured to detect defects in one or
more structures formed on a wafer;
[0020] FIG. 8 is a schematic diagram illustrating a cross-sectional
view of one embodiment of a spectral filter wheel that includes two
or more spectral filters and that may be included in the system
embodiments described herein;
[0021] FIG. 9 is a plot illustrating the discrete spectral bands of
light provided by two light emitting diodes (LEDs) that may be
included in the system embodiments described herein and the
transmission characteristics of a beam splitter that may be used to
direct the light provided by both of the LEDs into a common
illumination path;
[0022] FIG. 10 is a schematic diagram illustrating a side view of
one embodiment of two light sources that may be included in the
system embodiments described herein and a beam splitter that may be
used to direct the light provided by both of the light sources into
a common illumination path;
[0023] FIG. 11 is a plot illustrating the output spectrum of the
LEDs shown in FIG. 9 after the light from the LEDs has been
directed into a common illumination path as shown in FIG. 10;
[0024] FIG. 12 is a schematic diagram illustrating a side view of
one embodiment of 8 LEDs that may be included in the system
embodiments described herein and beam splitters that may be used to
direct the light provided by each of the LEDs into a common
illumination path;
[0025] FIG. 13 includes plots illustrating different combined
spectrums using LEDs having bandwidths spaced 30 nm apart with
varying power on selected LEDs; and
[0026] FIG. 14 is a Hock diagram illustrating one embodiment of a
non-transitory computer-readable medium.
[0027] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof are shown by
way of example in the drawings and are herein described in detail
The drawings may not be to scale. It should be understood, however,
that the drawings and detailed description thereto are not intended
to limit the invention to the particular form disclosed, but on the
contrary, the intention is to cover all modifications, equivalents
and alternatives falling within the spirit and scope of the present
invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Turning now to the drawings, it is noted that the figures
are not drawn to scale. In particular, the scale of some of the
elements of the figures is greatly exaggerated to emphasize
characteristics of the elements. It is also noted that the figures
are not drawn to the same scale. Elements shown in more than one
figure that may be similarly configured have been indicated using
the same reference numerals.
[0029] One embodiment relates to a system configured to detect
defects in one or more structures formed on a wafer. In one
embodiment, the one or more structures are formed through a stack
of two or more layers, and the stack has a thickness greater than
at least 1 um. For example the one or more structures that can be
inspected by the embodiments described herein include structures
that are oriented and built vertically such as 3D or VNAND memory
rather than being oriented and built in a planar manner. In one
such example, as shown in FIG. 1, the one or more structures may
include trench 100 formed through stack 102 of layers 106, 108,
110, 112, 114, and 116 and even into bottommost layer 104. As
described further herein, layers 104, 106, 108, 110, 112, 114, and
116 may include alternating layers of materials that have different
properties including different optical properties.
[0030] In one embodiment, the one or more structures are formed
through a stack of two or more layers of material, and at least
some of the two or more layers include polysilicon. For example, as
shown in FIG. 1, the stack may include layers 106, 108, 110, 112,
114, and 116, and layers 106, 110, and 114 may be polysilicon white
layers 104, 108, 112, and 116 may be formed of oxide. Such a
structure may be further configured as described herein,
[0031] VNAND stacks can be divided into two architectures, gate
first and gate last. Gate first architecture has a stack of
alternating layers of oxide and polysilicon (OPOP) to make up each
bit, and the gate last has alternating layers of oxide and silicon
nitride (ONON). The important part is that polysilicon is
substantially opaque for wavelengths below 450 nm and gradually
becomes more transparent at longer wavelengths and completely
transparent at about 1.06 um. Wafer inspection systems will need to
get light into the stack of polysilicon to the defect location and
then back out to obtain a signal. This implies that longer
wavelengths are needed for these wafer types.
[0032] In one embodiment, the one or more structures include one or
more opaque metallic structures such as tungsten structures formed
through a stack of two or more layers of material. In another
embodiment, the one or more structures are formed through a stack
of two or more layers of material, and at least some of the two or
more layers comprise an opaque conductor such as tungsten. For
example, for the ONON stack, all of the materials are transparent
above about 240 nm; so, commonly available wafer inspection systems
can see well into these stacks. However, later on in the
fabrication process, the silicon nitride is replaced by tungsten,
which is opaque to all wavelengths. The process where this
replacement occurs involves etching a deep trench through all of
the layers, using a wet etch chemistry to remove the silicon
nitride, and then depositing the tungsten using the trench. In one
such example, as shown in FIG. 1, layers 104, 108, 112, and 116 may
be silicon oxide while layers 106, 110, and 114 may be silicon
nitride. Therefore, layers 106, 110, and 114 may be replaced by
tungsten using a process such as that described above, and tungsten
may also be formed in trench 100, which has been formed through the
stack, to form a tungsten plug.
[0033] It turns out that light at longer wavelengths (greater than
600 nm) can penetrate into the trench if the light is polarized in
a direction perpendicular to the trench. So, again, longer
wavelengths are advantageous for inspecting the ONON layers after
the tungsten replacement. Thus, for many inspection layers on VNAND
devices, longer wavelength light is needed for inspection, but
currently available wafer inspection systems typically have
wavelengths between 260 nm and 450 nm and thus are not well suited
for inspecting these devices. In particular, currently available
inspection systems that have wavelengths between 260 nm and 450 nm
can only inspect a few of the layers for the VNAND devices. For
example, simulations of the e-field penetration into a tungsten
slot, that is 1 um thick, as a function of wavelength for
polarization perpendicular to the trench showed that longer
wavelengths (e.g., longer than 450 nm) penetrate to the bottom of
the trench while shorter wavelengths cannot penetrate to the bottom
of the trench and, in some cases, cannot penetrate at all. In
addition, thee-field a 1 um high tungsten trench was simulated with
HEF polarization (horizontal electric field perpendicular to the
trench), which showed good penetration at 633 nut, and with VEF
(vertical electric field parallel to the trench), which showed zero
penetration at the same wavelength.
[0034] Taking all of the above into consideration, the system
embodiments described herein include an illumination subsystem
configured to direct light in discrete spectral bands to the one or
more structures formed on the wafer, at least some of the discrete
spectral bands are in the near infrared (NIR) wavelength range, and
each of the discrete spectral bands has a bandpass that is less
than 100 nm. Such an illumination subsystem may be configured as
shown in FIG. 7 described further herein, for example. In one
embodiment, the discrete spectral bands are in a wavelength range
from 500 nm to 950 nm. For example, the system may be implemented
by altering a basic brightfield (BF) wafer inspection system to
have longer wavelengths (e.g., about 500 nm or 600 nm to about 950
nm), optional polarization control for parallel and perpendicular
e-field as described further herein, and a set of spectral
subbands, which may in some embodiments be provided by a set of
subband wavelength filters, across the entire operating wavelength
range. As will be described further herein, the set of spectral
subbands is used to provide information about the depths of
defects, the types of defects, and to increase, and even maximize,
the defect signal due to the defect signal variation with
wavelength.
[0035] In one embodiment, each of the discrete spectral bands is
separated by a wavelength of at least 1 nm. In other words, the
discrete spectral bands of the light that are directed to the wafer
are not continuous over an entire larger wavelength range that
encompasses all or some of the discrete spectral bands. For
example, the discrete spectral bands of the light are not directed
to the wafer as broadband light. In other words, in the embodiments
described herein, broadband light is not incident on the wafer
(although as described further herein a broadband source can be
used to provide at least some of the light in the discrete spectral
bands). In still other words, although light such as broadband
light that has some range of wavelengths may be considered to have
a number of sub wavelength bands within that entire range, those
subbands are not necessarily or inherently discrete unless the
light from the source is acted upon by some other optical
element(s) such as those described further herein.
[0036] In one embodiment, the light directed by the illumination
subsystem to the one or more structures penetrates to different
depths in the one or more structures depending on the discrete
spectral bands of the light. For example, the penetration depth in
polysilicon varies with wavelength with more penetration at longer
wavelengths. FIG. 2 shows amorphous silicon (aSi) penetration depth
as a function of wavelength. As shown in FIG. 2, if only a
wavelength of 600 nm was chosen for a wafer inspection system, the
light will penetrate only about 160 nm into the amorphous silicon.
In addition, different wavelengths may be used to penetrate more or
less into the material. Thus, if one performs multiple inspections
at longer and longer wavelengths, the inspections will be sensitive
to defects at deeper and deeper depths. So, looking at the signal
as a function of wavelength is equivalent to looking at different
depths in the wafer stack as a function of wavelength. FIG. 3 shows
the penetration depth for undoped polysilicon, which shows similar
depth versus wavelength characteristics as described above.
Therefore, the embodiments described herein can use multiple
wavebands to view different depths into OPOP stack wafers.
[0037] To understand how these material properties and wafer stacks
with structures affect the inspection light, one can use various
simulation codes (such as Finite Difference Time Domain or Rigorous
Coupled Wave Algorithm) which solve Maxwell's equations for the
propagation of electric fields and apply them to the inspection
wavelengths and stacks of wafer materials. The output from these
simulators can show the electric field penetration into the layers
and the expected signal a wafer inspection tool will have.
:Inspection tool recipe options like wavelength, polarization, and
illumination and collection aperture can be simulated as well to
predict the tool's performance.
[0038] Another feature of the relatively thick stacks of the
structures described herein is the interference causes standing
e-field waves that show up as oscillating bands vertically in the
trench. The spatial frequency of the bands is a function of
wavelength with shorter wavelengths having more bands. What this
means for wafer inspection is that the defect signal wilt oscillate
as the defect varies its depth in the trench. So, if a single
wavelength was used, the defect signal can change its sign relative
to the background and can disappear at a null. To avoid this lack
in sensitivity, one can scan the wafer with different wavelengths
so that a null is not encountered between the wavelengths.
[0039] The signal as a function of depth was simulated with results
shown in FIG. 4 for an OPOP deposition with a particle at various
depths (upper, top middle, middle, lower middle, and lower) in the
stack. These simulations were performed for a wafer inspection
system aperture that causes illumination through only the outer
part of the illumination pupil at longer wavelengths from 500 nm to
900 nm. (Light passing through such an aperture will appear, in
cross-section, to have an annular shape, and thus such an aperture
wilt be referred to herein as an "annular" aperture.) For the
particle at the highest position in the OPOP stack (the "upper"
particle), the signals at these wavelengths are generally strong
with even the 500 nm wavelength having relatively good signals the
depth of the particle increases (e.g., from upper to top middle and
so forth), the signals at shorter wavelengths decrease but remain
stronger at the longer wavelengths. The signal oscillation with
wavelength that can be seen in this figure illustrates the changing
of the standing-wave effect where wavelength changes the location
of standing nodes causing the signal to vary. Similarly, wavelength
can be used to discriminate depths in tungsten trenches with longer
wavelengths seeing further into the trenches as described further
herein.
[0040] As noted above, each of the discrete spectral bands of light
directed to the wafer by the illumination subsystem has a bandpass
that is less than 100 nm. For example, scanning with a relatively
small wavelength range has the advantage of collecting a relatively
good signal, either positive or negative. If a broadband
illumination source is used for the structures described herein,
positive signals can cancel negative ones and leave the inspection
system with substantially little overall signal. FIG. 5 shows
simulations of defect signals in two different VNAND structures as
a function of wavelength and different apertures (full
(illumination across the entire available illumination pupil), tow
sigma (illumination through the center, but not the edge, of the
pupil), and annular (described further above)). As can be seen in
FIG. 5, the defect signal changes phase from positive to negative
with a wavelength period of about 100 nm. So, an inspection system
with a waveband of 100 nm would average the positive and negative
signals almost to zero, and an inspection system with a waveband
greater than 100 nm will average more than 1 period resulting in a
substantially low overall signal. The optimum waveband is possibly
in the 50 nm range where one is likely to find either a maximum or
minimum in the signal, albeit there will be cases where the signal
can cancel out. It can be further minimized if repeat scans are
performed with different apertures, like tow-sigma and annular or
full, which tend to be out of phase with each other,
[0041] One desirable output of inspection is the location of the
defect in the stack and the type of defect since that information
can aid in the correction or minimization of the defects to ensure
higher yields. Therefore, one advantage of the embodiments
described herein is that they can give information on the location
of the defect in the stack and the type of defect. For example, the
system includes a computer subsystem described further herein
configured to detect defects in the one or more structures on the
wafer using the output generated by a detection subsystem of the
system, also described further herein. In one embodiment, the one
or more structures are formed through a stack of two or more layers
of material, and the computer subsystem is configured to determine
a location of the defects within the stack.
[0042] To identify the depth of a defect fairly accurately, a test
of the signal vs. wavelength can be performed for a given aperture
and then this result can be compared with a set of simulations of
the signal vs. various depths to match the observed signal to the
simulated signal. While such a test may take too much time during
an inspection, the test may be performed on selected defects (e.g.,
by running an Optics Selector session, which can be performed on
tools commercially available from KLA-Tencor, Milpitas, Calif., and
as described in U.S. Pat. No. 8,073,240 issued on Dec. 6, 2011 to
Fischer et al. and U.S. Patent Application Publication No.
2011/0320149 published on Dec. 29, 2011 to Lee et al, which are
incorporated by reference as if fully set forth herein) by varying
the wavelength (e.g., using spectral filters such as those
described further herein) and recording the signals of a defect
paying attention to the sign of the signals. The BF simulations
shown in FIG. 6 show that the signal changes fairly dramatically
with wavelength when the defect is moved deeper into the structure
(e.g., from the upper to the top middle to the middle to the lower
middle and to the lower portions of the structure). To provide even
more data to compare, apertures could also be incorporated in
addition to the wavelength using the plot for the annular aperture
described above. This technique is somewhat similar to the optical
critical dimension (OCD) process of simulating thousands of
structure variations and then matching observed BF reflection
spectra to the simulated spectra and determining the CD from the
closest matching spectra, which may be performed as described in
U.S. Pat. No. 6,483,580 to Xu et al. issued Nov. 19, 2002 and U.S.
Pat. No. 6,900,892 to Shchegrov et al. issued May 31, 2005, both of
which are incorporated by reference as if fully set forth herein,
in the embodiments described herein, however, a defect depth is
matched to a simulated condition rather than the structure shape
(e.g., CD). In a similar manner, a defect type may be determined by
comparing the signal vs. wavelength and/or aperture to simulated
signals. For example, in one embodiment, the one or more structures
are formed through a stack of two or more layers of material, and
the computer subsystem described herein is configured to determine
a type of the defects.
[0043] FIG. 7 illustrates one embodiment of a system configured to
detect defects in one or more structures formed on a wafer. As
described further herein, the system includes an illumination subs
stem configured to direct light in the discrete spectral bands
described herein to the one or more structures formed on the wafer.
In this embodiment, the illumination subsystem includes light
source 700 configured to generate the light, mirror 702 configured
to collect the light from the light source, and condenser lens 704
configured to focus the light from the light source and the mirror
to the opening of light pipe 706, which scrambles the light to
create a uniform distribution at the exit of the pipe. The
illumination subsystem may also include collimating lens 708
configured to collect the light exiting the light pipe and to
direct the light through pupil image plane 710, spectral filter
712, and optional polarizer 714 of the illumination subsystem. In
addition, the illumination subsystem includes reflective optical
element 716 configured to direct the light from the polarizer to
pupil relay 718 of the illumination subsystem. The illumination
subsystem further includes beam splitter 720 (e.g., a 50/50 beam
splitter) configured to direct the light through system pupil 722
and objective lens 724 of the illumination subsystem to wafer
726.
[0044] Mirror 702, condenser lens 704, light pipe 706, collimating
lens 708, reflective optical element 716, pupil relay 718, beam
splitter 720, and objective lens 724 may include any suitable such
elements known in the art. In addition, although some of the
elements of the illumination subsystem are shown as single optical
elements or formed of multiple optical elements, the number of
optical elements included in any one element may vary depending on
the configuration of the system. For example, the number of
refractive optical elements included in objective lens 724 shown in
FIG. 7 may vary from that shown. :In addition, objective lens 724
and other optical elements of the illumination subsystem may be
designed for NIR wavelengths with appropriate anti-reflective (AR)
and high-reflective (RR) coatings for the optics in the path of the
light,
[0045] In one embodiment, the illumination subsystem includes a
broadband light source configured to generate at least two of the
discrete spectral bands of light. For example, in one embodiment,
light source 700 may be a Xenon broadband arc lamp or another
similar lamp that can be used to provide the longer (e.g., NIR)
wavelengths described herein. The broadband light source may
generate tight having wavelengths that span all of the discrete
spectral bands of light, or multiple tight sources may be used,
each providing light having wavelengths that span only some of the
discrete spectral bands of tight,
[0046] In one such embodiment, the illumination subsystem includes
two or more spectral filters configured to be positioned in a path
of the light generated by the broadband light source, different
spectral filters correspond to different discrete spectral bands,
and the illumination subsystem is configured to sequentially
position the spectral filters in the path such that the light in
the discrete spectral bands is sequentially directed to the wafer.
For example, spectral filter 712 shown in FIG. 7 may be one of
several spectral titters that may be positioned in the path of the
light generated by light source 700, which may be a broadband light
source as described above. In one such example, the light source
may be configured to provide light at wavelengths between 700 nm
and 850 nm, and the center wavelength of each spectral filter may
be spaced apart by 50 nm at 700 nm, 750 nm, 800 nm, and 850 nm with
a 40 nm full width half maximum (FWHM) bandpass.
[0047] in some such embodiments, the spectral fitters may be
arranged on a spectral filter wheel such as that shown in FIG. 8.
For example, as shown in Fig, 8, spectral filter wheel 800 includes
5 different spectral filters (spectral filters 802, 804, 806, 808,
and 810), and the spectral filter wheel may be coupled to axle 812
in any suitable manner such that the spectral filter wheel can be
rotated by one or more elements (not shown) of the illumination
subsystem (such as a mechanical, motorized, or robotic assembly).
In this manner, the illumination subsystem may be configured to
alter the spectral filter that is positioned in the path of the
light from the light source. In addition, by rotating the spectral
filter wheel, the illumination subs can sequentially move each of
the spectral filters into the path of the light from the light
source. Each of the spectral filters may be a bandpass filter whose
wavelength range is matched to one of the discrete spectral bands.
In this manner, light in the discrete spectral bands may be
separately and sequentially directed to the wafer.
[0048] In one embodiment in which the one or more structures are
formed through a stack of two or more layers of material, the
illumination subsystem includes a polarizing component configured
to alter a polarization of the light directed by the illumination
subsystem to the one or more structures such that the polarization
of the light is substantially perpendicular to the one or more
structures. In another embodiment, the one or more structures
include one or more trenches formed through a stack of two or more
layers of material, and the illumination subsystem includes a
polarizing component configured to alter a polarization of the
light directed by the illumination subsystem to the one or more
trenches such that the polarization of the light is substantially
perpendicular to the one or more trenches. Such structures that
include trenches may be further configured as described herein. For
example, in one embodiment, at least one of the two or more layers
of material in the stack includes tungsten. In both such
embodiments, polarizer 714 shown in FIG. 7 may be used as such a
polarizing component. Polarizer 714 may be optional, and the
illumination subsystem may be configured to rotate the polarizer in
any suitable manner for parallel and perpendicular orientations
with respect to the wafer. In addition, as shown in FIG. 7,
polarizer 714 may be located relatively near the spectral
filterr(s).
[0049] The system also includes a detection subsystem configured to
generate output responsive to light in the discrete spectral bands
reflected from the one or more structures. For example, as shown in
FIG. 7, the detection subsystem includes objective lens 724 that is
configured to collect the light reflected by the one or more
structures. Therefore, the illumination and detection subsystems
may share a common objective. The detection subsystem also includes
beam splitter 720 that is configured to transmit the light
collected by the objective tens. Beam splitter 720 may be further
configured as described above. In addition, the detection subsystem
includes a pupil relay magnification tube that is formed by
refractive optical elements 728 and 730, which may include any
suitable refractive or reflective optical elements known in the
art. The detection subsystem may also include pupil image plane 732
and polarizer 734, which may include any suitable polarizing
component. The detection subsystem further includes a zooming tens
group made up of refractive optical elements 736, 738, and 740,
which may include any suitable refractive or reflective optical
elements known in the art. In addition, the detection subsystem may
include trombone mirrors 742 and 744, which may include any
suitable reflective optical elements. The detection subsystem
further includes detector 746, which may include any suitable
detector such as a charge coupled device (CCD). Other embodiments
may not include the pupil relay or zoom and have a fixed
magnification.
[0050] this manner, light reflected by the one or more structures
may be collected by objective lens 724, transmitted through beam
splitter 720, collected from the beam splitter by the pupil relay
magnification tube that directs the light through pupil image plane
732 and polarizer 734 to the zooming lens group. The zooming lens
group directs the light to the trombone mirrors, which reflect and
focus the light to the detector, which generates the output
responsive to the light in the discrete spectral bands reflected
from the one or more structures. The output may include any
suitable output known in the art such as signals, image signals,
data, image data, and the like. The detector shown in FIG. 7 may be
capable of detecting light in all of the discrete spectral bands
and producing output in response thereto. However, the detector may
be replaced with multiple detectors (not shown), each used to
detect light in only some of the discrete spectral bands.
[0051] The system may be designed with various magnifications to
sample the BF or dark field (DF) image that is created by various
apertures in the incident and reflected light paths. For example,
as shown in FIG. 7, the pupil relay magnification tube, the zooming
lens group, and the trombone mirrors may be used to alter the
magnification of the detection subsystem depending on the aperture
used for inspection and/or to optimize the performance of the
system for any other factors. In addition, the system may be
configured to move the trombonk mirrors back and forth as shown by
arrow 748 to compensate for changes in the magnification of the
pupil relay magnification tube and/or the zooming lens group. Like
the objective tens, the magnification tube should be designed for
NIR wavelengths with appropriate AR and HR coatings for the optics
in the path.
[0052] The illumination and detection subsystems may include
additional optical elements such as apertures, which may be
positioned, for example, at the position(s) pupil image plane 710
and/or pupil image plane 732. In addition, the system may be
configured such that different apertures can be used for the same
structures or different apertures can be used for different
structures. For example, the system may be configured to move
apertures into and out of the path of light as described above with
respect to the spectral filters and the aperture used is
independent of the spectral filter used.
[0053] In one such embodiment, the illumination subsystem includes
first and second apertures that are different from each other. The
illumination subsystem is configured to scan the light in the
discrete spectral bands across the wafer using the first aperture
and then the second aperture, and the computer subsystem described
further herein is configured to detect the defects in the one or
more structures on the wafer using the output generated by scanning
performed with the first and second apertures. In this manner,
different scans of the wafer can be performed with different
apertures. For example, one scan may be performed with a low sigma
aperture, and a second scan may be performed with an annular or BF
aperture. Such scans may be performed because they tend to be out
of phase with each other and can therefore minimize the chance that
the signals from any one defect within any one of the discrete
spectral bands will cancel themselves out. In this manner, the
output acquired by each scan may be a separate data set. The
separate data sets may be used to try to determine what the defect
was, based upon the results of the scans. In addition, repeating
scans with different apertures (and/or different wavelengths as
described further herein) may be performed to get a higher capture
rate of the defects in the stack.
[0054] The system and each of the subsystems may include any other
suitable optical elements. For example, the detection subsystem may
include an IR filter (not shown). In addition, the detection
subsystem may include a Y-mirror (not shown) that tilts the image
of the wafer relative to the detector and is used to align a swath
across dies during scanning so that the image of each part of the
die lands on exactly the same pixel in the detector (which may be
part of run time alignment (RTA)). The system may also include an
auto-focus subsystem. For example, as shown in FIG. 7, the system
may include beam splitter 750 positioned in the path of the light
from the wafer. The beam splitter may be coupled to auto-focus
subsystem 752, which may include any suitable combination of
optical elements. Beam splitter 750 may direct light from the
auto-focus subsystem to the wafer and direct that light reflected
from the wafer back to the auto-focus subsystem such that it can be
used to determine and correct the focus position of the system.
[0055] The system shown in FIG. 7 also includes a computer
subsystem (such as computer system 1404 shown in FIG. 14)
configured to detect defects in the one or more structures on the
wafer using the output. For example, the computer subsystem may be
coupled to the detector(s) of the detection subsystem by one or
more transmission media, which may include "wired" and/or
"wireless" transmission media, such that the computer subsystem can
receive the output generated by the detector(s). The computer
subsystem may be configured to use the output and any suitable
algorithm and/or method to detect the defects.
[0056] Using spectral filter(s) with a broadband light source as
described above may be advantageous since it has relatively low
coherence and would typically have less wafer noise. However, other
light sources can be used in the embodiments described herein. For
example, in one embodiment, the illumination subsystem includes
light emitting diodes (LEDs), each of the LEDs is configured to
provide the light in only one of the discrete spectral bands, and
each of the LEDs has a bandwidth that is spaced from bandwidths of
other LEDs in the illumination subsystem by at least 20 nm. In this
manner, the embodiments may use a set of LEDs across the NIR range
spaced 20 nm to 70 nm apart. In other words, the center wavelength
of one LED may be spaced by about 20 nm to 70 nm from the center
wavelength of another LED. LEDs are relatively incoherent sources
with bandwidths on the order of 10 nm and would not contribute to
excessive wafer noise. LEDs are unpolarized light sources and like
the arc lamp described above may be used with a separately
polarizing filter (e.g., polarizer 714 shown in FIG. 7) that can be
rotated.
[0057] in another embodiment, the illumination subsystem includes
laser diodes, each of the laser diodes is configured to provide the
light in only one of the discrete spectral bands, and each of the
laser diodes has a bandwidth that is spaced from bandwidths of
other laser diodes in the illumination subsystem by at least 20 nm.
For example, the embodiments described herein may use a set of NIR
laser diodes spaced out 20 nm to 70 nm apart. Light from the laser
diodes may be speckle busted in any suitable manner. Laser diodes
are polarized no a method of rotating the polarization by wave
plates may be used with these light sources. For example, polarizer
714 shown in FIG. 7 may be a wave plate suitable for rotating the
polarization of the tight produced by the laser diodes.
[0058] In one embodiment, the illumination subsystem includes two
or more light sources, and each of the two or more light sources is
configured to provide light in only one of the discrete spectral
bands. The two or more light sources may include any of the light
sources described herein such as two or more broadband light
sources, LEDs, or laser diodes. In addition, each of the two or
more light sources may be configured to provide tight in only one
of the discrete spectral bands as described above. In other words,
a first light source may provide light in only a first discrete
spectral band, a second light source may provide light in only a
second discrete spectral band different from the first, and so
on.
[0059] In one such embodiment, the illumination subsystem includes
one or more beam splitters configured to direct the light provided
by each of the two or more light sources into a common illumination
path. For example, the two or more light sources may include LEDs
and, as shown in FIG. 9, one of the LEDs may provide light at
shorter wavelengths (i.e., LED Short shown in FIG. 9), and another
of the LEDs may provide light at longer wavelengths (i.e., LED Long
.lamda. shown in FIG. 9). As further shown in FIG. 9, the shorter
wavelength LED may have a center wavelength at about 780 nm while
the longer wavelength LED may have a center wavelength at about 820
nm. In addition, as shown in FIG. 9, the spectral bands of the two
LEDs are discrete and separated in wavelength from one another.
[0060] For such light sources, the illumination subsystem may
include a dichroic beam splitter that has the spectral
characteristics shown in FIG. 9. For example, the dichroic beam
splitter may have substantially zero transmission for wavelengths
below about 800 nm and approximately 100% transmission for
wavelengths above about 800 nm. In this manner, the dichroic beam
splitter will reflect wavelengths below about 800 nm and transmit
wavelengths above about 800 nm. As such, the beam splitter can
direct the light provided by the two light sources shown in FIG. 9
into a common illumination path by reflecting the light provided by
the shorter wavelength LED and transmitting the light provided by
the longer wavelength LED.
[0061] As further shown in FIG. 10, the illumination subsystem may
include LED 1000 configured to provide light at shorter wavelengths
and LED 1002 configured to provide light at longer wavelengths.
These LEDs may be further configured as described herein. In
addition, as shown in FIG. 10, each of the LEDs may be coupled to a
refractive optical element to collimate the light from the LED such
as lens 1004 coupled to LED 1000 and lens 1006 coupled to LED 1002.
The lenses may be, for example, relay lenses configured to direct
the light from the light sources to beam splitter 1008. Beam
splitter 1008 may be further configured as described above. For
example, beam splitter 1008 may be a dichroic beam splitter that is
configured to reflect the light from the longer wavelength LED and
to transmit the light from the shorter wavelength :LED such that
the light from both LEDs is directed along common illumination path
1010. In this manner, the illumination subsystem may be configured
to combine the output from two LEDs with a dichroic beam splitter
that transmits the shorter wavelength LED source and reflects the
longer wavelength LED source. Therefore, as shown in FIG. 11, the
output spectrum from the LED pair after the dichroic beam splitter
will be light in two discrete spectral bands 1100 and 1102 that are
spaced apart in wavelength and may be centered, for example, at
wavelengths of about 780 nm and about 820 nm. Any and all of the
light sources described herein may, however, be selected for any
suitable discrete spectral bands.
[0062] The optical configuration described above may, of course, be
easily expanded to include more than two light sources that are
combined by more than one beam splitter. For example, as shown in
FIG. 12, the illumination subsystem my have an 8 LED combined
spectral source layout. Multiple pairs of LED sources may be
combined with multiple pairs of dichroic beam splitters. For
example, the illumination subsystem may include LEDs 1200, 1202,
1204, 1206, 1208, 1210, 1212, and 1214, which may be configured as
described above. Each of the LEDs may be coupled to at least one
refractive optical element such as lenses 1216, 1218, 1220, 1222,
1224, 1226, 1228, and 1230, which may be configured as described
above,
[0063] Beam splitter 1232 is configured to direct light from LEDs
1200 and 1202 along a common illumination path, and beam splitter
1234 is configured to direct light from LEDs 1204 and 1206 along a
common illumination path. In this manner, the light exiting each of
these beam splitters may include two discrete spectral bands. In
addition, the light exiting both of these beam splitters may be
directed along a common illumination path by beam splitter 1236.
The light exiting this beam splitter will then have four discrete
spectral bands.
[0064] In a similar manner, the light from LEDs 1208 and 1210 may
be combined by beam splitter 1238, and the light from LEDs 1212 and
1214 may be combined by beam splitter 1240. The light exiting beam
splitters 1238 and 1240 may then be combined by beam splitter 1242
into a common illumination path, and the light in that path will
have four discrete spectral bands. The light from beam splitters
1236 and 1242 may then be further combined into another common
illumination path by beam splitter 1244, and the light exiting this
beam splitter will then have 8 discrete spectral bands. The beam
splitters shown in FIG. 12 may include any suitable commercially
available or specially designed dichroic beam splitters. In
addition, each of the beam splitters shown in FIG. 12 may have
different transmission/reflection characteristics from each other
since each of the beam splitters will be reflecting or transmitting
different wavelengths of light depending on its position within the
optical configuration.
[0065] This light may then be directed to other components of the
illumination subsystem described herein such as condenser 704 or
polarizer 714 shown in FIG. 7. In other words, the light source
shown in FIG. 7 may be replaced with the light source/beam splitter
combinations shown in FIG. 10 or 12. In addition, the optical
configuration shown in FIG. 12 can be made more compact than that
shown in FIG. 12 by rotating some of the elements by 90 degrees.
Furthermore, the configuration shown in FIG. 12 can be expanded to
include more light sources or can be made smaller by including
fewer light sources. Light from laser diodes or other sources can
be combined in a similar manner to that shown in FIG. 12.
[0066] In another such embodiment, at least some of the two or more
light sources are configured to emit different powers. For example,
the two or more light sources may be selected for different powers
or the powers of the light sources may be variable or controllable
(e.g., by varying one or more parameters of the light sources
themselves). In this manner, the discrete spectral bands of light
directed to the wafer may have different powers.
[0067] FIG. 13 shows examples of different combined spectrums using
LEDs spaced 30 nm apart with varying power on selected LEDs to
create various spectral outputs that are suited to the layer being
inspected. For example, as shown in spectral output 1300, the power
of the light in the middle discrete spectral bands may be the
lowest of all of the spectral bands and may increase as the
discrete spectral bands move away from the center overall
wavelength. In contrast, as shown in spectral output 1302, the
power of the light in the middle discrete spectral bands may be the
highest of all the spectral bands and may decrease as the discrete
spectral bands move away from the center overall wavelength. In
another possibility shown in spectral output 1304, light in
adjacent discrete spectral bands may have powers that alternate
between the highest provided power and the lowest provided power.
More specifically, the power of the spectral bandwidth centered at
700 nm may be low, while the power of the spectral bandwidth
centered at 730 nm may be high, the power of the spectral bandwidth
centered at 760 nm may be tow, and so on.
[0068] As further shown in FIG. 13, some of the spectral bands have
the same power while other spectral bands have different powers.
However, each of the spectral bands may have a power that is
different than the power of each other spectral band. Furthermore,
the power of one or more of the spectral bands may be altered
depending on the wafer (or layer on the wafer) being inspected. In
this manner, the overall spectral shape provided by a combination
of light sources in the system may be variable across wafers. In
addition, as shown in FIG. 13, the illumination subsystem may be
configured such that at least some light from all of the light
sources included in the system is directed to the wafer. However,
the power used for one or more of the spectral bands for any one
wafer being inspected may be reduced to zero in some cases. In
other words, inspection of a wafer may include illuminating the
wafer with light from fewer than all of the light sources included
in the system. In a similar manner, if the illumination subsystem
includes a broadband light source as described further above, not
all of the wavelengths of light produced by the broadband source
may be directed to any given wafer being inspected. Instead, the
wavelengths of the broadband source that are used for inspection
may be selected by altering the spectral filter(s) positioned in
the path of the light produced by the source.
[0069] In a further such embodiment, the illumination subsystem
includes one or more elements configured to after the output power
of the light provided by at least one of the two or more light
sources such that the tight in at least two of the discrete
spectral bands directed to the wafer has different tight output
powers. For example, as described above, the illumination subsystem
may alter one or more parameters of the light sources themselves to
alter the output power of the light sources. In one such example,
the one or more elements may include individual controllers (not
shown), one coupled to each light source, that can separately and
independently alter the output power of the light sources. In
another example, the one or more elements may include optical
elements that are or can be positioned in the path of the light
from each of the light sources independently. For example, the
refractive optical elements shown in FIG. 12 (e.g., lenses 1216,
1218, etc.) may be replaced with spectral filters such as multiple
neutral density filters that can be independently positioned by the
system in the path of each of the light sources. The refractive
optical elements may also be replaced with optical elements such as
liquid crystal devices that can change the power of the light
transmitted to other optical elements of the illumination subsystem
without being moved into and out of the path of the light.
[0070] In summary, therefore, the system embodiments described
herein provide wafer inspection systems configured to operate at MR
wavelengths using optionally polarized light with multiple spectral
bands, each having a bandpass less than 100 nm, for the inspection
of 3D wafer structures. Such embodiments have a number of
advantages over currently available systems. For example, as
described further herein, the embodiments can be used to
distinguish defect depth by wavelength penetration, where longer
wavelengths "see" defects lower in the structure (either a trench
etch in an opaque material or penetrating silicon, amorphous,
and/or polysilicon materials). Since the wavelengths used by the
embodiments described herein can penetrate into such materials, the
defects detected by the embodiments may include no called "buried"
defects, or defects that are located completely within a
material.
[0071] Any of the defect information generated by the embodiments
described herein may be used to generate a imp of the defects
across the wafer (a "wafer" or "defect" map). Different wafer maps
can be generated for defects detected at different depths in the
wafer. In this manner, the different wafer maps can be used to
identify different signatures in the defects that are present at
different depths in the wafer. The signature can change quite
dramatically with wavelength over the NIR range as the embodiments
view further into the wafer finding different populations of
defects. For example, unique signatures can be found in different
portions of the wafer (near the center versus near the edge) for
different discrete spectral bands. Such information about the
defects versus in depth in the wafer can provide highly useful
information about the process(es) used to form structures on the
wafer.
[0072] In addition, the relatively small bandpass allows higher
signals to pick a maximum signal wavelength, since if the signals
oscillate rapidly with wavelength, a larger waveband will average
the oscillations with a lower signal. Furthermore, the embodiments
may be used to identify the depth of a defect and/or its type by
comparing an observed signal vs. wavelength and/or aperture to a
set of simulated signal vs. wavelength and/or aperture and using
the best match to determine the defect depth and/or type.
[0073] Each of the system embodiments described above may be
configured to perform any step(s) of any method(s) described
herein. In addition, each of the system embodiments described
herein may be configured according to any other embodiments or
systems described herein.
[0074] Another embodiment relates to a method for detecting defects
in one or more structures formed on a wafer. The method includes
directing light in discrete spectral bands to the one or more
structures formed on the wafer, which may be performed according to
any embodiments described herein. At least some of the discrete
spectral bands are in the NIR wavelength range in addition, each of
the discrete spectral bands has a bandpass that is less than 100
nm. The discrete spectral bands may be further configured according
to any of the embodiments described herein. The one or more
structures may include any of the one or more structures described
herein. The method also includes generating output responsive to
light in the discrete spectral bands reflected from the one or more
structures, which may be performed according to any of the
embodiments described herein. In addition, the method includes
detecting defects in the one or more structures on the wafer using
the output, which may be performed according to any of the
embodiments described herein.
[0075] Each of the embodiments of the method described above may
include any other step(s) of any other method(s) described herein.
In addition, each of the embodiments of the method described above
may be performed by any of the systems described herein,
[0076] Another embodiment relates to a non-transitory
computer-readable medium containing program instructions stored
therein for causing a computer system to perform a
computer-implemented method for detecting defects in one or more
structures formed on a wafer. One embodiment of such a
computer-readable medium is shown in FIG. 14. In particular,
computer-readable medium 1400 contains program instructions 1402
stored therein for causing computer system 1404 to perform a
computer-implemented method for detecting defects in one or more
structures formed on a wafer.
[0077] The computer-implemented method includes any step(s)
described above with respect to the computer subsystem of the
system. For example, the computer-implemented method may include
detecting defects in the one or more structures using output that
is responsive to light in the discrete spectral bands reflected
from the one or more structures. The computer-implemented method
may also include any other step(s) of any other method(s) described
herein. In addition, the computer-readable medium may be further
configured as described herein.
[0078] Program instructions 1402 implementing methods such as those
described herein may be stored on computer-readable medium 1400.
The computer-readable medium may be a non-transitory
computer-readable storage medium such as a magnetic or optical
disk, a magnetic tape, or any other suitable non-transitory
computer-readable medium known in the art.
[0079] The program instructions may be implemented in any of
various ways, including procedure-based techniques, component-based
techniques, and/or object-oriented techniques, among others. For
example, the program instructions may be implemented using ActiveX
controls, C++ objects, JavaBeans, Microsoft Foundation Classes
("MFC"), or other technologies or methodologies, as desired.
[0080] Computer system 1404 may take various forms, including a
personal computer system, mainframe computer system, workstation,
image computer, parallel processor, or any other device known in
the art. In general, the term "computer system" may be broadly
defined to encompass any device having one or more processors,
which executes instructions from a memory medium.
[0081] Further modifications and alternative embodiments of various
aspects of the invention will be apparent to those skilled in the
art in view of this description. For example, multi-spectral defect
inspection for 3D wafers is provided. Accordingly, this description
is to be construed as illustrative only and is for the purpose of
teaching those skilled in the art the general manner of carrying
out the invention. It is to be understood that the forms of the
invention shown and described herein are to be taken as the
presently preferred embodiments. Elements and materials may be
substituted for those illustrated and described herein, parts and
processes may be reversed, and certain features of the invention
may be utilized independently, all as would be apparent to one
skilled the art after having the benefit of this description of the
invention. Changes may be made in the elements described herein
without departing from the spirit and scope of the invention as
described in the following claims,
* * * * *