U.S. patent application number 16/012836 was filed with the patent office on 2020-09-24 for micro photoluminescence imaging.
The applicant listed for this patent is SEMILAB Semiconductor Physics Laboratory Co., Ltd.. Invention is credited to Laszlo Dudas, Lubomir L. Jastrzebski, Zoltan Tamas Kiss, Zsolt Kovacs, Imre Lajtos, Nicolas Laurent, Gyorgy Nadudvari.
Application Number | 20200300767 16/012836 |
Document ID | / |
Family ID | 1000005074000 |
Filed Date | 2020-09-24 |
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United States Patent
Application |
20200300767 |
Kind Code |
A9 |
Kiss; Zoltan Tamas ; et
al. |
September 24, 2020 |
MICRO PHOTOLUMINESCENCE IMAGING
Abstract
In an example implementation, a method includes illuminating a
wafer with excitation light having a wavelength and intensity
sufficient to induce photoluminescence in the wafer. The method
also includes detecting photoluminescence emitted from a portion of
the wafer in response to the illumination, and detecting excitation
light reflected from the portion of the wafer. The method also
includes comparing the photoluminescence emitted from the portion
of the wafer and the excitation light reflected from the portion of
the wafer, and identifying one or more defects in the wafer based
on the comparison.
Inventors: |
Kiss; Zoltan Tamas;
(Budapest, HU) ; Dudas; Laszlo; (Pecel, HU)
; Kovacs; Zsolt; (Budapest, HU) ; Lajtos;
Imre; (Budapest, HU) ; Nadudvari; Gyorgy;
(Pilisszentivan, HU) ; Laurent; Nicolas;
(Singapore, SG) ; Jastrzebski; Lubomir L.;
(Clearwater, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMILAB Semiconductor Physics Laboratory Co., Ltd. |
Budapest |
|
HU |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20190391079 A1 |
December 26, 2019 |
|
|
Family ID: |
1000005074000 |
Appl. No.: |
16/012836 |
Filed: |
June 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14703692 |
May 4, 2015 |
10012593 |
|
|
16012836 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 21/6489 20130101;
G01N 21/9501 20130101 |
International
Class: |
G01N 21/64 20060101
G01N021/64; G01N 21/95 20060101 G01N021/95 |
Claims
1. A method comprising: continuously illuminating a wafer
comprising a semiconductor-dielectric interface with excitation
light having a wavelength and intensity sufficient to induce
photoluminescence in the wafer and having an intensity sufficiently
low such that Auger recombination does not substantially occur of
the wafer; detecting photoluminescence emitted from the wafer in
response to the illumination, the detecting comprising integrating
a detected intensity over a period during which the wafer is
continuously illuminated; detecting excitation light reflected from
the wafer; comparing the detected photoluminescence from the wafer
and the detected excitation light reflected from the wafer; and
assessing a passivation of the semiconductor-dielectric interface
based on the comparison.
2. The method of claim 1, wherein assessing the passivation of the
semiconductor-dielectric interface comprises identifying one or
more defects at the semiconductor-dielectric interface based on the
comparison.
3. The method of claim 2, wherein the wafer comprises a plurality
of integrated circuit elements of a CMOS imaging sensor (CIS).
4. The method of claim 3, wherein the semiconductor-dielectric
interface is a wall of a trench isolating one of the integrated
circuit elements from another.
5. The method of claim 3, wherein the CIS is a back side
illuminated CIS and the semiconductor-dielectric interface is on a
back side of the back side illuminated CIS.
6. The method of claim 2, wherein identifying one or more defects
in the wafer comprises identifying one or more defects having a
dimension of 1 .mu.m or smaller.
7. The method of claim 1, further comprising varying an intensity
of the excitation light and monitoring an intensity of the
photoluminescence as a function of the intensity of the excitation
light.
8. The method of claim 7, wherein assessing the passivation of the
semiconductor-dielectric interface is based on the monitored
variation of the intensity of the photoluminescence as a function
of the intensity of the excitation light.
9. The method of claim 1, further comprising: forming a
photoluminescence intensity map of the wafer based on the
photoluminescence emitted from the portion of the wafer; and
forming a reflection intensity map of the wafer based on the
excitation light reflected from the same portion of the wafer.
10. The method of claim 9, wherein comparing the detected
photoluminescence from the wafer and the detected reflected
excitation light from the wafer comprises: determining that the
photoluminescence intensity map includes a first variation in
intensity at a first location of the wafer; upon determining that
the photoluminescence intensity map includes the first variation in
intensity at the first location of the wafer, determining whether
the reflection intensity map includes a second variation in
intensity at the first location of the wafer; upon determining that
the reflection intensity map does not include a second variation in
intensity at the first location of the wafer, determining that a
defect is present at the first location of the wafer.
11. The method of claim 10, wherein comparing the detected
photoluminescence from the wafer and the detected reflected
excitation light from the wafer further comprises: upon determining
that the reflection intensity map includes the second variation in
intensity at the first location of the wafer, determining that a
defect is not present at the first location of the wafer.
12. The method of claim 1, wherein the excitation light has a
wavelength in a range from 200 nm to 1,100 nm.
13. The method of claim 1, wherein the photoluminescence has as a
wavelength in a range from 200 nm to 1,100 nm.
14. A system comprising: one or more light sources configured to
continuously illuminate a wafer comprising a
semiconductor-dielectric interface with excitation light having a
wavelength and intensity sufficient to induce photoluminescence in
the wafer and having an intensity sufficiently low such that Auger
recombination does not substantially occur in the portion of the
wafer; a first detector configured to detect photoluminescence
emitted from the wafer in response to the illumination over a
period during which the wafer is continuously illuminated, the
detector integrating the detected intensity over the period; a
second detector configured to detect excitation light reflected
from the wafer; and one or more processors configured to: compare
the detected photoluminescence from the wafer and the detected
excitation light reflected from the wafer; and assess a passivation
of the semiconductor-dielectric interface based on the
comparison.
15. The system of claim 14, wherein the one or more processors are
configured to assess the passivation of the
semiconductor-dielectric interface by identifying one or more
defects at the semiconductor-dielectric interface based on the
comparison.
16. The system of claim 14, wherein the wafer comprises an array of
elements of a complementary metal-oxide semiconductor (CMOS)
imaging sensor.
17. The system of claim 16, wherein the semiconductor-dielectric
interface is an interface of one of the elements of the CMOS
imaging sensor.
18. The system of claim 16, further comprising an objective lens
assembly for imaging the wafer to the first detector.
19. The system of claim 18, wherein the objective lens assembly
images the wafer with spatial resolution of 1 .mu.m.times.1 .mu.m
or smaller.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of and relies on the
earlier filing date of application Ser. No. 14/703,692, filed on
May 4, 2015, the contents of which are incorporated herein by
reference in their entirety.
TECHNICAL FIELD
[0002] This disclosure relates to identifying defects in integrated
circuit devices, such as complementary metal-oxide semiconductor
imaging sensors.
BACKGROUND
[0003] A complementary metal-oxide semiconductor (CMOS) imaging
sensor (CIS) is an integrated circuit (IC) device used to convert a
light intensity pattern into electric digital signals. In some
cases, a CIS is a two dimensional array of photodiodes with
accompanying CMOS logic for signal processing. Each individual
photodiode with processing CMOS logic is referred to a pixel. In
some cases, a CIS has 1,000,000 or more pixels.
[0004] A CIS is commonly fabricated on n/n++ or p/p++ wafers. As an
example, in some cases, thin lightly doped n-type or p-type
epitaxial layers (e.g., 3-5 .mu.m layers each having a dopant
concentration of 1.times.10.sup.14 to 1.times.10.sup.15 cm.sup.-3)
are grown on a highly doped n++ or p++ substrate (e.g., a substrate
having a dopant concentration of 1.times.10.sup.18 to
1.times.10.sup.20 cm.sup.-3). A CIS is formed on the epitaxial
layers, a region often referred to as the device active area.
Performance of the CIS is influenced, at least in part, by
properties of this active area.
[0005] The highly doped substrates (often referred to as handles)
provide mechanical support for the active area during the CIS
fabrication process. In some cases, the substrate also reduces the
occurrence of cross-talk in a CIS. For example, the substrate can
reduce the cross-talk that results when minority carriers generated
underneath one pixel in response to red light reach adjacent pixels
of the CIS.
[0006] A CIS can be arranged according to a variety of different
configurations. For example, a CIS can be arranged as a front side
illuminated (FSI) CIS, or as a back side illuminated (BSI) CIS.
Here, the "front" side refers to the side of the wafer on which the
IC pixel structures are fabricated. In some cases, to make a BSI
CIS, a CIS wafer first undergoes CIS processing on its front side.
The CIS wafer is then bonded along its front side to a wafer
carrier, and its backside is thinned (e.g., by a few m) until all
of its n++ or p++ substrate is removed. The surface of the CIS
wafer is then passivated and covered with an antireflection
coating, and color filters are fabricated on its back side. During
use, a light image is projected on the back side of the CIS wafer,
and the CIS converts the light image into electric digital
signals.
[0007] Light from an image projected on a CIS having photon energy
larger than the silicon band gap is primarily absorbed in the CIS
active area. This absorption generates electron and hole pairs,
resulting in photocurrent. These photo-generated minority carriers
are then collected by a p-n junction at this location. The number
of photo-generated minority carriers is proportional to the number
of photons that are absorbed in the CIS active area, and varies
according to the intensity of light. Thus, the intensity of light
incident upon the CIS active area can be deduced based on the
magnitude of the generated photocurrent. In practice, it is often
desirable for each of the pixels of a CIS to generate identical or
substantially similar photocurrent in response to uniform, low
level illumination. Otherwise, pixels having lower or higher
photocurrent (e.g., "defective" pixels) might result in bright or
dark spots in the resulting image.
[0008] In some cases, localized crystallographic defects and heavy
metal contaminations could increase or decrease photocurrent from a
given pixel, resulting in an image having bright spots or dark
spots at low illumination levels. When present in a space charge
region of the p-n junctions, these defects act as generation
centers for minority carriers. This results in an increase in the
dark current of these pixels, and if the defect is sufficiently
severe, will result in white or bright spots in the resulting
image. When present outside of the space charge region of p-n
junctions, these defects act as recombination centers for minority
carriers. This results in a decrease in the amount of photocurrent
collected by the junctions, and if the defect is sufficiently
severe, will result in as dark spots in the resulting image at low
illumination levels.
[0009] Localized crystallographic defects or heavy metal
contaminations can potentially be introduced at any step during the
fabrication process of a CIS. Thus, to improve and control the
fabrication process of a CIS, it is important to quickly identify
processing steps that are introducing these defects.
SUMMARY
[0010] Systems and techniques for identifying defects in integrated
circuit devices are described herein.
[0011] In general, in an aspect, a method includes illuminating a
wafer with excitation light having a wavelength and intensity
sufficient to induce photoluminescence in the wafer. The method
also includes detecting photoluminescence emitted from a portion of
the wafer in response to the illumination, and detecting excitation
light reflected from the portion of the wafer. The method also
includes comparing the photoluminescence emitted from the portion
of the wafer and the excitation light reflected from the portion of
the wafer, and identifying one or more defects in the wafer based
on the comparison.
[0012] In general, in another aspect, a system includes an
illumination module configured to illuminate a wafer with
excitation light having a wavelength and intensity sufficient to
induce photoluminescence in the wafer, a first detection module
configured to detect photoluminescence emitted from a portion of
the wafer in response to the illumination, a second detection
module configured to detect excitation light reflected from the
portion of the wafer, and processing module. The processing module
is configured to compare the photoluminescence emitted from the
portion of the wafer and the excitation light reflected from the
portion of the wafer, and identify one or more defects in the wafer
based on the comparison.
[0013] Implementations of these aspects may include or more of the
following features.
[0014] In some implementations, the wafer can include a
complementary metal-oxide semiconductor (CMOS) imaging sensor. The
portion of the wafer can include one or more pixels of the CMOS
imaging sensor. Identifying one or more defects in the wafer can
include identifying one or more defective pixels of the CMOS
imaging sensor. Detecting photoluminescence emitted from the
portion of the wafer can include resolving photoluminescence
emitted from the portion of the wafer at a spatial resolution of 1
.mu.m.times.1 .mu.m or smaller. Detecting excitation light
reflected from the portion of the wafer can include resolving
excitation light reflected from the portion of the wafer at a
spatial resolution of 1 .mu.m.times.1 .mu.m or smaller. Identifying
one or more defects in the wafer can include identifying one or
more defects having a dimension of 1 .mu.m or smaller. The portion
of the wafer can correspond to an active area of the CMOS imaging
sensor.
[0015] In some implementations, the method can further include
forming a photoluminescence intensity map of the portion of the
wafer based on the photoluminescence emitted from the portion of
the wafer, and forming a reflection intensity map of the portion of
the wafer based on the excitation light reflected from the portion
of the wafer. In some cases, the processing module can be further
configured to form a photoluminescence intensity map of the portion
of the wafer based on the photoluminescence emitted from the
portion of the wafer, and form a reflection intensity map of the
portion of the wafer based on the excitation light reflected from
the portion of the wafer. Comparing the detected photoluminescence
from the portion of the wafer and the detected reflected excitation
light from the region of the wafer can include determining that the
photoluminescence intensity map includes a first variation in
intensity at a first location of the wafer; upon determining that
the photoluminescence intensity map includes the first variation in
intensity at the first location of the wafer, determining whether
the reflection intensity map includes a second variation in
intensity at the first location of the wafer; and upon determining
that the reflection intensity map does not include a second
variation in intensity at the first location of the wafer,
determining that a defect is present at the first location of the
wafer. Comparing the detected photoluminescence from the portion of
the wafer and the detected reflected excitation light from the
region of the wafer can further include, upon determining that the
reflection intensity map includes the second variation in intensity
at the first location of the wafer, determining that a defect is
not present at the first location of the wafer.
[0016] In some implementations, the method can further include
detecting photoluminescence emitted from a second portion of the
wafer, where the second portion of the wafer is larger than the
first portion of the wafer. The method can also include forming a
photoluminescence intensity map of the second portion of the wafer
based on the photoluminescence emitted from the second portion of
the wafer. The method can also include determining the presence of
one or more curved regions of contrast in the photoluminescence
intensity map, and upon determining the presence of one or more
curved regions of contrast in the photoluminescence intensity map,
adjusting a property of the excitation light. In some cases, the
system can further include a third detection module configured to
detect photoluminescence emitted from a second portion of the
wafer, where the second portion of the wafer is larger than the
first portion of the wafer. The processing module can be further
configured to form a photoluminescence intensity map of the second
portion of the wafer based on the photoluminescence emitted from
the second portion of the wafer, determine the presence of one or
more curved regions of contrast in the photoluminescence intensity
map, and upon determining the presence of one or more curved
regions of contrast in the photoluminescence intensity map, adjust
a property of the excitation light. Adjusting the property of the
excitation light can include adjusting a wavelength of the
excitation light to increase the photoluminescence emitted from the
second portion of the wafer. Adjusting the property of the
excitation light can include adjusting an intensity of the
excitation light.
[0017] In some implementations, illuminating the wafer with
excitation light can include illuminating the wafer with excitation
light such that Auger recombination does not substantially occur in
the portion of the wafer.
[0018] In some implementations, the excitation light can have a
wavelength in a range from 200 nm to 1100 nm.
[0019] In some implementations, the photoluminescence can have as a
wavelength in a range from 200 nm to 1100 nm.
[0020] Among other advantages, embodiments may be used to identify
localized defects in a CIS device during the manufacturing process
(e.g., during or between intermediate steps of the manufacturing
process of the CIS device) and/or after the completion of the
manufacturing process (e.g., as a part of a post-manufacturing
inspection). In some cases, embodiments can be used to identify
defects associated with a single pixel of the CIS device, such that
one or more individual defective pixels can be identified in a CIS
device. In some cases, embodiments can be used to reduce the number
of positives that might otherwise result during the defect
detection process due to the presence of particulate matter on a
CIS device.
[0021] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other features and
advantages will be apparent from the description and drawings, and
from the claims.
DESCRIPTION OF DRAWINGS
[0022] FIG. 1 shows an example system for detecting defects in a
CIS sample.
[0023] FIG. 2 shows a photoluminescence intensity map having
variations in photoluminescence intensity characteristic of defects
in a silicon substrate.
[0024] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0025] Defects in a CIS device can be identified by inducing
photoluminescence in the active region of the CIS device, and
examining the photoluminescence for localized variations in
intensity.
[0026] In general, photoluminescence can be induced in a silicon
wafer with light having photon energy larger than the energy gap of
silicon (e.g., more than 1.1 eV). As this light is absorbed in the
silicon, electron-hole pairs are generated in the silicon. Some of
these photo-generated carriers will recombine through radiative
recombination and release photons of light, a phenomenon known as
photoluminescence.
[0027] As the intensity of photoluminescence varies depending on
the composition of the wafer, localized variation in the
composition of the wafer (e.g., resulting from material defects or
contaminations) will result in localized variation in the induced
photoluminescence. Thus, defects in a CIS device can be identified,
at least in part, by illuminating a CIS device with excitation
light sufficient to induce photoluminescence, and examining the
photoluminescence for localized variations in intensity.
[0028] An example system 100 for identifying defects in a CIS is
shown in FIG. 1. The system 100 includes a stage assembly 110, an
illumination assembly 130, an optical assembly 150, and an imaging
assembly 170. In an example usage of system 100, a CIS sample 190
is placed on the stage assembly 110 and is positioned for
examination. The illumination assembly 130 generates excitation
light suitable for inducing photoluminescence in the CIS sample
190. The optical assembly 150 directs the excitation light
generated by the illumination assembly 130 onto the CIS sample 190,
thereby inducing photoluminescence in the CIS sample 190 and/or
causing excitation light to be reflected by the CIS sample 190. The
optical assembly directs the photoluminescence generated by the CIS
sample 190 and/or light reflected by the CIS sample 190 towards the
imaging assembly 170. The imaging assembly 170 detects the
photoluminescence and reflected excitation light, and identifies
defects in the CIS sample based 190 on the detected light.
[0029] The stage assembly 110 supports the CIS sample 190 during
examination by the system 100. In some cases, the stage assembly
110 can move along one or more axes, such that the CIS sample 190
can be moved related to the illumination assembly 130, optical
assembly 150, and/or the imaging assembly 170. For example, in some
cases, the CIS can move along the x, y, and z axes of a Cartesian
coordinate system in order to move the CIS sample 190 along any of
three dimensions related to the other components of the system
100.
[0030] The illumination assembly 130 generates excitation light
that, when incident upon the CIS sample 190, induces
photoluminescence in the CIS sample 190. The illumination assembly
130 includes light sources 132a-b, collimating lenses 134a-b,
filters 136a-b, dichroic beam splitter 138, and focusing lens
140.
[0031] Light sources 132a-b generate light having particular
properties suitable for inducing photoluminescence in the CIS
sample 190. In some cases, the light sources 102a-b are laser light
sources that generate light having a particular wavelength and
intensity. In some cases, the light sources 132a-b each generate
light having a different wavelengths, such that the illumination
assembly 130 can provide different types of light. For example, the
light source 132a can generate light having a first wavelength
(e.g., 532 nm), and the light source 132b can generate light having
a second wavelength (e.g., 880 nm). The light sources 132a-b can be
operated independently from one another, such that light at each of
the different wavelengths can be individually or simultaneously
generated. Light sources 132a-b include any component capable to
generating light at a specified wavelength. For example, in some
cases, the light sources 132a-b can include one or more lasers or
light emitting diodes (LEDs).
[0032] In some cases, the light sources 132a-b each generate light
having a different wavelengths, such that the illumination assembly
130 can provide different types of light. For example, the light
source 132a can generate light having a first wavelength (e.g., 532
nm), and the light source 132b can generate light having a second,
different wavelength (e.g., 880 nm). As another example, either or
both of the light sources 132a-b can generate light having a
wavelength less than 532 nm (e.g., 300 nm, 350 nm, 400 nm, 450 nm,
500 nm, or any intermediate wavelength thereof). As yet another
example, either or both of the light sources 132a-b can generate
light having a wavelength between 200 nm and 1100 nm (e.g., 200 nm,
300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm,
1100 nm, or any intermediate wavelength thereof). Although example
wavelengths are described above, these are merely illustrative
examples. In practice, the light sources 132a-b can each generate
light having any other wavelengths, depending on the
implementation.
[0033] The light generated by the light sources 132a-b can also
vary in intensity, depending on the implementation. As an example,
in some cases, the light sources 132a-b can each generate light
having a power between 0.02 W and 20 W. In some cases, the
intensity of light generated by the light sources 132a-b can also
be adjusted during use of the system 100. For example, in some
cases, the light generated by the light sources 132a-b can be
adjusted between 0.02 W and 20 W during operation of the system
100. As another example, in some cases, the light generated by the
light sources 132a-b can be adjusted during operation of the system
100 such that they generate light having a power less than 0.02 W
(e.g., 0.015 W, 0.010 W, or 0.005 W). Although example intensities
are described above, these are merely illustrative examples. In
practice, the light sources 132a-b can each generate light having
other intensities, depending on the implementation.
[0034] Excitation light generated by the light sources 132a-b are
directed towards collimating lenses 134a-b, respectively. The
collimating lenses 134a-b narrows the beam of passing light, such
that the light exiting the collimating lenses 134a-b are aligned
along the optical axes of collimating lenses 134a-b,
respectively.
[0035] The collimated excitation light from collimating lenses
134a-b are directed into filters 136a-b, respectively. The filters
136a-b filter the passing light, such that only light having
particular wavelengths or range of wavelengths are substantially
transmitted through the filters 136a-b, respectively. The filters
136a-b can be used to "clean" the light generated by the light
sources 132a-b. For example, if the light source 132a generates
light having a first wavelength (e.g., 532 nm), the filter 136a can
be a band-pass filter that transmits light having a range of
wavelengths that includes the first wavelength (e.g., 522 nm to 542
nm), while light having wavelengths outside of this range are not
substantially transmitted. As another example, if the light source
132b generates light having a first wavelength (e.g., 880 nm), the
filter 136b can be a band-pass filter that transmits light having a
range of wavelengths that includes the second wavelength (e.g., 870
nm to 890 nm), while light having wavelengths outside of this range
are not substantially transmitted. In some cases, for example in
implementations in which the light sources 132a-b include one or
more lasers, the filters 136a-b can also include speckle-reducing
elements (e.g., a moving diffuser element) in order to reduce the
effects of interference effects in the laser beam.
[0036] The filtered excitation light from the filters 136a-b are
directed into a dichroic beam splitter 138. The dichroic beam
splitter 138 reflects light and/or transmits light, depending on
the wavelength of light incident upon it. For example, if the light
source 132a generates light having a first wavelength (e.g., 532
nm) and the light source 132b generates light having a second
wavelength (e.g., 880 nm), the dichroic beam splitter 138 can
transmit light having the first wavelength and reflects light
having the second wavelength. As a result, although light generated
by each of the light sources 132a-b are initially directed in
substantially different directions, dichroic beam splitter 138
redirects the light in a substantially similar direction.
[0037] The excitation light from the dichroic beam splitter 138 is
directed to a focusing lens 140. The focusing lens 140 focuses the
light towards the optical assembly 150.
[0038] The optical assembly 150 directs the excitation light
generated by the illumination assembly 130 towards the CIS sample
190, and directs photoluminescence generated by the CIS sample 190
and/or light reflected by the CIS sample 190 towards the imaging
assembly 170. The optical assembly 150 includes dichroic beam
splitters 152 and 156, an objective lens 154, a filter 158, and
field lenses 160a-b.
[0039] The excitation light from the focusing lens 140 is directed
to the dichroic beam splitter 152. The dichroic beam splitter 152
reflects light and/or transmits light, depending on the wavelength
of light incident upon it. For example, if the light source 130a
generates excitation light having a first wavelength (e.g., 532
nm), the light source 130b generates excitation light having a
second wavelength (e.g., 880 nm), and photoluminescence induced in
the CIS sample 190 has a third wavelength (e.g., 1100 nm), dichroic
beam splitter 152 can partially reflect and partially transmit the
light at each of these wavelengths. Thus, at least some of the
excitation light received from the illumination assembly 130 is
redirected by the dichroic beam splitter 152 towards the objective
lens 154, at least some of the photoluminescence induced in the CIS
sample 190 is transmitted by the dichroic beam splitter 152 towards
the imaging assembly 170, and at least some of the excitation light
reflected by the CIS sample 190 is also transmitted towards the
imaging assembly 170.
[0040] The excitation light from the dichroic beam splitter 152 is
directed to an objective lens assembly 154. The objective lens
assembly 154 directs the excitation light onto the CIS sample 190.
In some cases, the objective lens assembly 154 can direct the
excitation light onto a particular region of the CIS sample 190
(e.g., a region of the CIS sample 190 that is being examined), such
that the intensity of excitation light incident upon that region of
the CIS sample 190 is uniform or substantially uniform. This region
can be, for example, the entirety of the CIS sample 190 or a
portion of the CIS sample 190.
[0041] The excitation light incident on the CIS sample 190 can
induce photoluminescence in the CIS sample 190. In some cases, the
photoluminescence in the CIS sample 190 can have a wavelength
between 200 nm and 1100 nm (e.g., 200 nm, 300 nm, 400 nm, 500 nm,
600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, or any
intermediate wavelength thereof).
[0042] The objective lens assembly 154 can focus on particular
regions of the CIS sample 190 in order to obtain photoluminescence
from these regions. In some cases, the objective lens assembly 154
can focus on a region of the CIS sample 190 that includes one or
more pixels of the CIS sample 190, and the objective lens assembly
154 can include a lens element having a wide angle and shallow
depth of field, such that it resolves photoluminescence from each
of the pixels within that region. In some cases, the objective lens
assembly have a lens element having a focal length between 0.5 mm
and 550 mm, and a depth of field between 1 .mu.m and 400 .mu.m. In
some cases, the objective lens assembly 154 can resolve light with
sufficient resolution to distinguish photoluminescence from each of
the pixels. For example, if the CIS sample 190 includes pixels
having dimensions of 1 .mu.m.times.1 .mu.m along the surface, the
objective lens assembly 154 can resolve photoluminescence at a
spatial resolution of 1 .mu.m.times.1 .mu.m or finer.
[0043] The excitation light incident on the CIS sample 190 can also
result in the reflection of excitation light from the CIS sample
190. The objective lens assembly 154 can also focus on particular
regions of the CIS sample 190 in order to obtain excitation light
reflected from these regions. In an similar manner as above, in
some cases, the objective lens assembly 154 can focus on a region
of the CIS sample 190 that includes one or more pixels of the CIS
sample 190, and the objective lens assembly 154 can include a lens
element having a wide angle and shallow depth of field, such that
it resolves excitation light reflected from each of the pixels
within that region. In a similar manner as above, some cases, the
objective lens assembly 154 can resolve light with sufficient
resolution to distinguish excited light reflected from each of the
pixels. For example, if the CIS sample 190 includes pixels having
dimensions of 1 .mu.m.times.1 .mu.m along the surface, the
objective lens assembly 154 can resolve reflected light at a
spatial resolution of 1 .mu.m.times.1 .mu.m or finer.
[0044] In some cases, the objective lens assembly 154 can be
refocused in order to resolve light from different regions of the
CIS sample 190. For example, in some implementations, the focal
depth of objective lens assembly 154 can be varied in order to
examine photoluminescence from varying depths from the surface of
the CIS sample 190 (e.g., from the back surface of the CIS sample
190 to the front surface of the CIS sample 190).
[0045] In some cases, the magnification of the objective lens
assembly 154 also can be changed in order to examine particular
regions of the CIS sample 190 in greater or lesser detail. In some
cases, the magnitude of the objective lens assembly 154 can be
changed by moving lens elements of the objective lens assembly 154
relative to one another (e.g., a "zoom" lens), or by otherwise
modifying the light path of light through the objective lens
assembly 154.
[0046] The photoluminescence and reflected excitation light is
directed by the objective lens assembly 154 to the dichroic beam
splitter 152. As described above, the dichroic beam splitter 152
reflects light and/or transmits light, depending on the wavelength
of light incident upon it. For example, if the light source 130a
generates excitation light having a first wavelength (e.g., 532
nm), the light source 130b generates excitation light having a
second wavelength (e.g., 880 nm), and photoluminescence induced in
the CIS sample 190 has a third wavelength (e.g., 1100 nm), dichroic
beam splitter 152 can partially reflect and partially transmit the
light at each of these wavelengths. Thus, at least some of the
photoluminescence and reflected excitation light is transmitted by
the dichroic beam splitter 152 towards the imaging assembly
170.
[0047] At least a portion of the photoluminescence and reflected
excitation light is directed by the dichroic beam splitter 152 to
the dichroic beam splitter 156. The dichroic beam splitter 156 also
reflects light and/or transmits light, depending on the wavelength
of light incident upon it. For example, if the light source 130a
generates excitation light having a first wavelength (e.g., 532
nm), the light source 130b generates excitation light having a
second wavelength (e.g., 880 nm), and photoluminescence induced in
the CIS sample 190 has a third wavelength (e.g., 1100 nm), dichroic
beam splitter 156 can reflect excitation light having the first and
second wavelengths, and transmit photoluminescence having the third
wavelength. As a result, photoluminescence and reflected excitation
light are redirected along different optical paths.
[0048] Photoluminescence transmitted by the dichroic beam splitter
152 is directed through a filter 158. The filter 158 filters the
passing light, such that only light having particular wavelengths
or range of wavelengths are substantially transmitted through the
filter 158. The filter 158 can be used to "clean" the output of the
dichroic beam splitter 152. For example, if photoluminescence from
the CIS sample 190 is expected to have a particular wavelength
(e.g., 1100 nm), the filter 158 can be a band-pass filter that
transmits light having a range of wavelengths that includes the
photoluminescence wavelength (e.g., 1000 nm to 1200 nm), while
light having wavelengths outside of this range are not
substantially transmitted. As another example, in some cases, the
filter 158 can be a long-pass filter that attenuates light having
relatively shorter wavelengths, while transmitting light having
relatively longer wavelengths. This can be helpful, for example, in
filtering out reflected excitation light, which in many cases has a
shorter wavelength than the photoluminescence from the CIS sample
190.
[0049] Photoluminescence and the reflected excitation light are
then directed to field lenses 160a-b respectively. The field lenses
160a-b focus the photoluminescence and the reflected excitation
light towards the detectors 172a-b, respectively, of the optical
assembly 170.
[0050] The imaging assembly 170 detects the photoluminescence and
reflected excitation light, and identifies defects in the CIS
sample based 190 on the detected light. The imaging assembly 170
includes detectors 172a-b, and a processing module 174.
[0051] The detectors 172a-b measure the photoluminescence and the
reflected excitation light, respectively, from the dichroic beam
splitter 152. In some cases, the detectors 172a-b are configured to
measure the intensity of light at a sufficiently high spatial
resolution to resolve photoluminescence and the reflected
excitation light for a single pixel of the CIS sample 190. For
example, if the CIS sample 190 includes pixels having dimensions of
1 .mu.m.times.1 .mu.m along the surface, the detectors 172a-b can
each resolve photoluminescence at a spatial resolution of 1
.mu.m.times.1 .mu.m or finer. In some cases, the detectors 172a-b
can include a single detection element that measures the intensity
of light incident upon it, or several such detection elements. For
example, in some cases, the detectors 172a-b can include a line of
detection elements (e.g., a "line" detector), or a two dimensional
array of detection elements. In some cases, the detectors 172a-b
can be include one or more InGasAs line cameras or arrays, or Si
line cameras or arrays.
[0052] In some cases, the detectors 172a-b measure the
photoluminescence and the reflected excitation light, respectively,
by integrating the intensity of light received over a period of
time. This integration time can depend, at least in the part, on
the intensity of light that is applied to the CIS sample 190. For
example, in some cases, reducing the intensity of light incident on
the CIS sample 190 by a factor of two can result in an increase in
the integration time by a factor of two. As measurement noise from
the detector increases with integration time, in some cases, the
intensity of light that is applied to the CIS sample 190 can be
adjusted in order to limit the resulting measurement noise of the
detectors 172a-b to appropriate levels. In some cases, the
detectors 172a-b can be cooled in order to further reduce
measurement noise. For example, in some cases, either or both of
the detectors 172a-b can be cooled (e.g., by a Peltier cooler) to a
particular temperature (e.g., 100K) in order to reduce the amount
of noise in the resulting measurements.
[0053] Measurements from the detectors 172a-b are transmitted to
the processing module 174 for interpretation. In some cases,
processing module 174 can generate one or more multiply dimensional
maps that represent the intensity of photoluminescence and
reflected excitation light for a particular portion of the CIS
sample 190. For example, in some cases, the detectors 172a-b can
include a two dimensional array of detection elements that each
measure the intensity of light incident upon that detection
element. Using this information, the processing module 174 can
generate spatial maps that represent the intensity of
photoluminescence and reflected excitation light for at specific
locations on the CIS sample 190.
[0054] The processing module 174 can also identify defects in the
CIS sample 190 based on the measurements from the detectors 172a-b.
For example, the processing module 174 can identify regions of the
CIS sample 190 with a localized variation in photoluminescence
(e.g., a spot, blotch, line, curve, or other region having
photoluminescence that more intense or less intense than the
surrounding regions). The processing module 174 can identify these
regions of the CIS sample 190 as having defects. In some cases, the
processing module 174 can identify one or more specific pixels of
the CIS sample 190 as being defective (e.g., pixels associated with
the localized variation in photoluminescence).
[0055] In some cases, localized variation in photoluminescence
might not be the result of defects in the CIS sample, but rather
might be the result of particulate matter on the surface of the CIS
sample. As particulate matter can block or otherwise attenuate
light, the presence of these particulates can locally affect the
intensity of excitation light incident on the CIS sample, and can
result in localized variation in photoluminescence. To distinguish
between localized variations in photoluminescence as a result of
defects in the CIS sample from those as a result of particulate
matter, the processing module 174 can determine if regions of the
CIS sample 190 identified as having localized variations in
photoluminescence also have corresponding localized variations in
reflected excitation light.
[0056] As an example, if a region has both a localized variation in
photoluminescence and a corresponding localized variation in
reflected light, the processing module 174 determines that the
variation in photoluminescence is the result of particulate matter
on the surface of the CIS sample, and not a defect or contamination
in the CIS sample. Thus, the processing module 174 can determine
that no defect exists in this region.
[0057] As another example, if the region has a localized variation
in photoluminescence, but does not have a corresponding localized
variation in reflected light, the processing module 174 determines
that the variation in photoluminescence is not the result of
particulate matter on the surface of the CIS sample. Thus, the
processing module 174 can determine that a defect exists in this
region.
[0058] In some implementations, the processing module 174 can be
implemented using digital electronic circuitry, or in computer
software, firmware, or hardware, or in combinations of one or more
of them. For example, in some cases, the processing module 174 can
be implemented, at least in part, as one or more computer programs
(e.g., one or more modules of computer program instructions,
encoded on computer storage medium for execution by, or to control
the operation of, a data processing apparatus). A computer storage
medium can be, or can be included in, a computer-readable storage
device, a computer-readable storage substrate, a random or serial
access memory array or device, or a combination of one or more of
them. The term "processing apparatus" encompasses all kinds of
apparatus, devices, and machines for processing data, including by
way of example a programmable processor, a computer, a system on a
chip, or multiple ones, or combinations, of the foregoing. The
apparatus can include special purpose logic circuitry, e.g., an
FPGA (field programmable gate array) or an ASIC (application
specific integrated circuit). The apparatus can also include, in
addition to hardware, code that creates an execution environment
for the computer program in question, e.g., code that constitutes
processor firmware, a protocol stack, a database management system,
an operating system, a cross-platform runtime environment, a
virtual machine, or a combination of one or more of them. The
apparatus and execution environment can realize various different
computing model infrastructures, such as web services, distributed
computing and grid computing infrastructures.
[0059] Although an example system 100 is shown and described, this
is merely an illustrative example. In practice, the system 100 can
have other arrangements, depending on the implementation.
[0060] Implementations of the system 100 can be used to identify
localized defects in a CIS device during the manufacturing process
of the CIS device (e.g., before, during, or after any step during
the manufacturing process), and/or after the completion of the
manufacturing process. For example, in some cases, implementations
of the system 100 can be used to monitor one or more intermediate
steps in the manufacturing process of one or more CIS devices,
and/or to inspect one or more completed CIS devices.
[0061] In some cases, when defects are detected in a CIS device,
information regarding the location and nature of the defects can be
used to modify the manufacturing process such that fewer and/or
less severe defects are introduced into CIS devices in the future.
For example, information regarding the detection of defects can be
used to identify particular manufacturing equipment or processes
that are partially or wholly responsible for the defects. This
information can then be used to repair and/or replace that
equipment, or to modify the processes in order to improve the
manufacturing process. In some cases, information regarding the
location and nature of the defects can also be used to identify
wafers or portions of wafers that are defective, such that these
wafers or portions of wafers can be discarded or otherwise not used
in future processes.
[0062] Implementations of the system 100 can be used to identify
localized defects in a CIS device with a spatial resolution of at
least a single pixel of the CIS device. For example, in some cases,
a CIS device build with 32 nm technology has pixels of
approximately 0.9.times.0.9 .mu.m; implementations of the system
100 can be used to identify localized defects in this CIS device
with a spatial resolution of 0.9.times.0.9 .mu.m or finer.
[0063] In general, the system 100 can generate various different
types of excitation light, depending on the application. For
instance, in some cases, the system 100 can vary the wavelength of
excitation light generated by the illumination module 130 in order
to probe different depths under the surface of the CIS sample 190.
As an example, in some cases, the illumination module 130 can
generate green light (e.g., having a wavelength of approximately
532 or 540 nm) in order to generate minority carriers and
photoluminescence close to the surface of the CIS sample 190 (e.g.,
where 1/(absorption coefficient)=1.5 .mu.m). On the other hand, in
some cases, the illumination module 130 can generate near infrared
illumination (e.g., having a wavelength of approximately 880 nm) in
order to generate minority carriers and photoluminescence further
away from the silicon surface. As described above, the illumination
module 130 can include multiple light sources, and each light
source can be selectively activated in order to product light
having a different wavelength. For instance, in the example system
100 shown in FIG. 1, the illumination module 130 can include two
light sources, 132a-b, each configured to product light having a
different wavelength. The light sources 132a-b can thus be
selectively switching on or off in order to probe different depths
below the surface of the CIS sample 190. Although two light sources
132a-b are shown, in practice, a system 100 can include any number
of light sources, depending on the implementation.
[0064] In some cases, the system 100 can vary the intensity of
excitation light generated by the illumination module 130. In some
cases, the excitation light generated by the illumination module
130 can have an intensity that is sufficiently high to induce
photoluminescence in the CIS sample 190, and also sufficiently low
to such that Auger recombination does not substantially occur in
the illuminated portion of the CIS sample 190.
[0065] In general, for a low injection level (e.g., when the
minority carrier concentration is less than the majority carrier
concentration), the intensity of photoluminescence is proportion to
a product of the minority carrier concentration and the majority
carrier concentration at a particular location. For example, this
can be expressed as:
PL=A*C.sub.minority carrier*C.sub.majority carrier,
where PL is the intensity of induced photoluminescence at a
particular location (expressed as a number of photons),
C.sub.minority carrier is the minority carrier concentration at
that location, C.sub.majority carrier is the majority carrier
concentration at that location, and A is a constant.
[0066] The minority carrier concentration C.sub.minority carrier
(referred to as an injection level) is proportional to an effective
life time of minority carriers and a generation rate (i.e., the
number of photons adsorbed in the silicon normalized to the volume
of silicon where minority carriers are present). For example, this
can be expressed as:
C.sub.minority carrier=R.sub.generation*t.sub.life,effective,
where R.sub.generation is the generation rate at the location and
t.sub.life,effective is the effective life time of minority
carriers at the location.
[0067] The intensity of induced photoluminescence at a particular
location PL is proportional to the intensity of
photoluminescence-inducing light absorbed in the silicon at that
location (expressed as a number of photons), the effective life
time of minority carriers at that location, and the dopant
concentration in the silicon at that location. For example, this
can be expressed as:
PL=A*I.sub.absorbed*t.sub.life,effective*C.sub.majority
carrier,
where I.sub.absorbed is the intensity of photoluminescence-inducing
light absorbed in the silicon at the location (expressed as a
number of photons).
[0068] The effective life time t.sub.life,effective has
contributions from various recombination channels, in particular
the bulk recombination, the recombination at the interfaces, and
the Auger recombination. For example, this can be expressed as:
1 / t life , effective = 1 / t recombination , bulk + 1 / t
recombination , interfaces + 1 / t recombination , Auger ,
##EQU00001##
where t.sub.recombination,bulk is the bulk recombination life time,
t.sub.recombination,interfaces is the interfaces recombination life
time, and t.sub.recombination,Auger is the Auger recombination life
time.
[0069] Defects at interfaces of a pixel (e.g., at the front
surface, the back surface, or the walls of the deep trench
insulation (DTI) reduces the effective life time
t.sub.life,effective in a given pixel, and will cause a reduction
of photoluminescence intensity from this pixel. For example, this
can be expressed as:
1 / t life , interfaces = 1 / t recombination , front + 1 t
recombination , back + 1 t recombination , DTI , ##EQU00002##
where t.sub.recombination,front is the front surface recombination
life time, t.sub.recombination,back the back surface recombination
life time, and t.sub.recombination,DTI is the Deep Trench Isolation
(DTI) wall recombination life time.
[0070] The interface recombination life time t.sub.life,interfaces
is inversely proportional to surface (i.e., interface)
recombination velocity at this interface and the distance between
these interfaces. For example, this can be expressed as:
1 / t life , interfaces = 1 2 d interfaces v recombination ,
interface , ##EQU00003##
where d.sub.interfaces is the distance between interfaces, and
v.sub.recombination,interface is the recombination velocity at the
interface.
[0071] In some cases, for the DTI interfaces, the distant between
interfaces d.sub.interfaces can be approximately 1 .mu.m (i.e., for
a pixel having a dimension of 1 .mu.m). The interfacial
recombination rate for a well-passivated interface is in the range
of 1 to 10 cm/sec. Assuming an interfacial recombination rate of 10
cm/sec, one can expect that the DTI interfacial recombination life
time t.sub.recombinaton,DTI to be about 5.times.10.sup.6 sec. This
will control the effective life time t.sub.life,effective in a
pixel if there are no bulk defects and the surfaces are well
passivated.
[0072] The effective life time t.sub.life,effective depends on the
injection level (e.g., the intensity of light incident on the CIS
active area). Thus, the effective life time t.sub.life,effective
can be controlled, at least in part, by adjusting the injection
level (e.g., by adjusting the intensity of the excitation light
illuminating the CIS sample). The injection level can be adjusted
according to one or more criteria.
[0073] For example, in some cases, the injection level can be
adjusted in order to reduce Auger recombination contribution to the
effective life time in the CIS active area. At high injection
levels, the effective life time can be controlled by Auger
recombination. For example, in some cases, for a 1.times.10.sup.17
cm.sup.-3 injection level, the Auger recombination limits the
effective life time in p-type silicon to 1.times.10.sup.-4 seconds.
As another example, in some cases, for a 1.times.10.sup.18
cm.sup.-3 injection level, the Auger recombination limits the
effective life time in p-type silicon to 1.times.10.sup.-6 seconds.
Since Auger recombination is not sensitive to defects, the
injection level can be adjusted such that Auger recombination in
the CIS active area does not dominate the recombination
processes.
[0074] Further, Auger recombination also controls the effective
life time in the highly doped substrate. For instance, for p++ and
n++ substrates, Auger recombination can limit the effective life
time for all injection levels. As an example, in some cases, in a
p++ substrate having a 1.times.10.sup.20 cm.sup.-3 carrier
concentration, the effective life time is limited to
1.times.10.sup.-9 sec. The Auger recombination life time changes
abruptly with dopant concentration. For example, a ten times
reduction of the dopant concentration (e.g., from a dopant
concentration of 1.times.10.sup.20 to 1.times.10.sup.19) will
increase the effective life time one hundred times (e.g., to
1.times.10.sup.-7 sec).
[0075] The effective life time in the CIS active area also depends
on the injection level at a low injection level regime (e.g., when
the contribution of Auger recombination is negligible). Thus, it is
important to monitor photoluminescence intensity as a function of
the illumination level (i.e., injection level), since the life time
response to the injection level could be different for various
defects. For example, given a p-type silicon, a defect such as
interstitial Fe will result in an increase in the effective life
time as the injection level is increased. However, a defect such as
a Fe--B pair will result in a decrease in the effective life time
with an increase of injection level. For a defect which increases
recombination at the interface (e.g., SiO.sub.2 interfaces or walls
of the DTI), the effect of the changing injection level on the
interfacial recombination life time will depend on a state of a
space charge region at this interface. For the interface in an
inversion at low injection levels, the interface recombination
lifetime does not change with an increasing injection level. When
the injection level becomes high (e.g., larger than the majority
carrier concentration), then the life time decreases with the
increasing injection level. For the interface in a depletion for
the low injection levels, the interfacial recombination life time
increases with the increasing injection level. But, for high
injection levels, the interfacial recombination life time does not
change with an increase in injection level. Therefore, for a given
defective pixel, the dependence of the photoluminescence intensity
on the injection level could provide an important clue to a nature
of this defect.
[0076] As discussed above, in some cases, it is important to
contain photoluminescence to the CIS active area and use
appropriately low injection levels. Further, it is important to
minimize or otherwise appropriate reduce the amount of light that
is absorbed in the highly doped substrates. Photoluminescence
intensity is proportional to the majority carrier concentration.
Thus, for the same amount of photons absorbed in the substrate and
the epitaxial layer (e.g., the CIS active area), photoluminescence
from the substrate could be stronger than from the CIS active
region. For instance, in an example CIS device, the CIS active
region has an average dopant concentration of 1.times.10.sup.16
cm.sup.-3 at the beginning of processing, and has an effective life
time of 5.times.10.sup.-6 sec. The substrate in this example has an
average dopant concentration of 1.times.10.sup.19 cm.sup.-3, and
has an effective life time of 1.times.10.sup.-7 (controlled by the
Auger recombination) which, in some cases, can result in a
diffusion length (e.g., the distance that minority carriers will
diffuse) of approximately 20 .mu.m. Given similar amounts of
absorbed photons in the CIS active region and the highly doped
substrate, the photoluminescence from the substrate will be five
times more intense than for the CIS active area. To minimize the
background photoluminescence from the substrate to less than 5% of
the photoluminescence from the CIS active area, the amount of
photons absorbed in the substrate can be limited to less than 1% of
the photons absorbed in the CIS active area. Therefore, the
wavelength of the photoluminescence generating light (e.g.,
absorption coefficient) can be chosen appropriately.
[0077] As described above, in some cases, it is also important to
induce photoluminescence using an injection level at which the
Auger recombination does not control the life time in the active
area of the CIS sample (e.g., when the contribution of Auger
recombination in the CIS active area is negligible). As discussed
above, for an injection level of 1.times.10.sup.17 cm.sup.-3, the
Auger life time is 1.times.10.sup.-4 sec. But, for an injection
level of 1.times.10.sup.18 cm.sup.-3, the Auger life time is
1.times.10.sup.-6. As an example, in some cases, for an CIS active
area with an effective (i.e., bulk and interfacial recombination)
life time in the range of 5.times.10.sup.-6, the injection level of
1.times.10.sup.18 cm.sup.-3 can be avoided, as the sensitivity to
the bulk and the interfacial recombination would be lost at this
injection level, and will contribute less than 20% to the measured
effective lifetime.
[0078] In some cases, it is also important to induce
photoluminescence using an injection level that will prevent out
diffusion of minority carriers from the CIS active area to the
highly doped substrate. An important concern regarding the
injection level (e.g., the minority carrier concentration generated
in the CIS active region) is related to out diffusion of minority
carriers generated in the CIS active area into the highly doped
substrate. Due to the dopant concentration difference between the
highly doped p++ substrate and the lighter doped p-type materials
in the CIS active area, an electric field exists at the p/p++
interface which, for low injection levels, will impede diffusion of
the minority carriers from the CIS active area into the substrate.
However, the difference in the minority carrier concentrations in
the CIS active region and the substrate will generate a diffusion
field which will be the driving force for the minority carrier
diffusion from the CIS active region into the substrate. As the
injection level increases, this diffusion driving force will also
increase. As long as the diffusion force is smaller than the
electric repulsion, the minority carriers will be contained in the
epitaxial layer. For high injection levels, this diffusion gradient
can overcome the electric repulsion and some minority carriers
could enter into the substrate. As a result, a strong background
photoluminescence will be generated from the substrate. This will
reduce the sensitivity of detecting photoluminescence changes from
the CIS active area.
[0079] Therefore, in order to enhance the detection sensitivity of
the system 100 and to minimize photoluminescence generation in the
highly doped substrate of a CIS sample, it is important to select
an appropriate wavelength and intensity for the excitation light
that is applied to the CIS sample.
[0080] In some cases, the wavelength and intensity of the
excitation light can evaluated empirically for each application.
For example, the empirical determination can be made regarding
whether the wavelength and intensity of the excitation light
results in photoluminescence that is predominantly from the active
area of the CIS sample, or whether the resulting photoluminescence
has a large contribution from the substrate of the CIS sample.
[0081] In an example evaluation process, the CIS sample is
illuminated with excitation light having a particular wavelength
and intensity, and the resulting photoluminescence is detected over
a relatively large portion of the CIS sample (e.g., a "macro"
region). In some cases, the portion of the CIS sample that is
examined in this manner can be larger than the portions of the CIS
sample measured by the detectors 172a-b, as described above. In
some cases, this "macro" region can have an area of approximately 1
cm.sup.2 or larger (e.g., 1 cm.sup.2, 2 cm.sup.2, 3 cm.sup.2, 4
cm.sup.2 or larger). In some cases, this "macro" region can include
the entire CIS sample.
[0082] In some cases, the photoluminescence of this "macro" region
can be determined using a detector that is separate from the
detectors 172a-b. For example, a separate detector can be directed
towards the CIS sample 190 in order to acquire photoluminescence
measurements of the "macro" region alongside the detectors 172a-b.
In some cases, the photoluminescence of this "macro" region can be
determined by one of the detectors 172a-b. For example, in some
implementation, the optical path between the CIS sample 190 and the
detectors 172a-b can be varied while imaging the "macro" region,
such as by using a different objective lens 154 or adjusting the
optical properties of the objective lens 154 while imaging the
"macro" region.
[0083] The resulting photoluminescence map is examined for
variations in intensity indicative of defects typically associated
with the silicon substrate of a CIS device. For instance, silicon
wafers manufactured by the Czochralski process (i.e., "CZ wafers")
often include curved variations in photoluminescence when
illuminated by excitation light. As an example, FIG. 2 shows an
example photoluminescence map 200 for a CIS device. In this
example, the photoluminescence map 200 includes several variations
in photoluminescence intensity 210, appearing as circular or curved
bands of intensity variation. In some cases, variations similar to
those shown in FIG. 2 are characteristic of silicon wafers
manufactured by the Czochralski process, and this characteristic
pattern is not present in the epitaxial layers of the device (e.g.,
CIS active area).
[0084] The contribution of photoluminescence from the substrate to
the total detected photoluminescence can be quantitatively
calculated by illuminating the CIS sample with long wavelength
illumination (e.g., near infrared illumination with an energy large
than approximately 1.1 eV, the energy gap of Si), such that a large
portion of the carriers (e.g., substantially most or substantially
all of the carriers) are generated in the substrate. The changes in
this photoluminescence intensity due to the characteristic
substrate defects are used as a reference for calculations in the
substrate photoluminescence contribution to the total
photoluminescence detected for the short wavelength.
[0085] As an example, when the CIS sample is illuminated with
excitation light having a relatively long wavelength (e.g.,
excitation light having a wavelength that causes substantially most
or substantially all of the carriers to be generated in the
substrate of the CIS sample), the defect contrast is 50% (e.g., the
photoluminescence intensity map includes localized variations in
intensity of that differ the surrounding intensity by 50%).
However, when the CIS sample is illuminated with excitation light
having a relatively shorter wavelength, the defect contrast is 5%
(e.g., the photoluminescence intensity map includes localized
variations in intensity of that differ the surrounding intensity by
5%). Thus, in this example, one can estimate that the substrate
contribution to the total photoluminescence for the relatively
shorter wavelength is about 10% (e.g., 5% divided by 50%).
[0086] If the substrate contribution to photoluminescence is too
large, then the measurement conditions can be adjusted (e.g., by
reducing the wavelength or light intensity to the excitation light
applied to the CIS sample) until the defect pattern is reduced or
eliminated.
[0087] In some cases, the upper injection limit that can be used
for the detection of defects in a CIS sample can be determined, at
least in part, based on the Auger recombination life time, which
decreases with increasing injection level. As an example, for an
injection level of 1.times.10.sup.17 cm.sup.-3, the Auger life time
in an example device is 100.times.10.sup.-6 sec. For a CIS active
region with a 5.times.10.sup.-6 sec effective recombination life
time from bulk and interfacial recombination, this Auger
recombination will contribute about 5% to the effective life
time.
[0088] The sensitivity of the system 100 to defects in the CIS
sample depends, at least in part, on the bulk and interfacial
recombination lifetime being shorter than the Auger recombination
life time. In some cases, the wavelength and intensity of the
excitation light can be varied in order to obtain a particular
percentage of contribution Auger recombination to the effective
life time. For example, in some implementations, the wavelength and
intensity of the excitation light can be varied such that the Auger
life time of the CIS active region is less than or equal to 5% of
the effective recombination life time from bulk and interfacial
recombination of the CIS active region. In some implementation,
this threshold corresponds to an injection level of approximately
1.times.10.sup.17 cm.sup.-3 or less. For the CIS active area of
approximately 5 .mu.m thickness, in some cases, this will
correspond to the absorption of about 1.times.10.sup.19
photons/cm.sup.2 sec (e.g., corresponding to a power of 100
mW/cm.sup.2) in the CIS active region. Although an example
threshold is described above, this is merely an illustrative
example. In some cases, the wavelength and intensity of the
excitation light can be varied such that the Auger life time of the
CIS active region is less than or equal to some other percentage of
the effective recombination life time from bulk and interfacial
recombination of the CIS active region (e.g., 1%, 5%, 10%, 15%, or
any other percentage).
[0089] Although implementations for detecting defects in CIS
devices are described herein, this is merely one illustrative
application. In practice, implementations can be used to detect
defects in other devices or circuit in which generated minority
carriers and photoluminescence are substantially confined to device
active region. For example, in some cases, implementations can be
used to detect defects in CMOS circuits built using fully depleted
silicon on insulator (SOI) technologies.
[0090] While this specification contains many details, these should
not be construed as limitations on the scope of what may be
claimed, but rather as descriptions of features specific to
particular examples. Certain features that are described in this
specification in the context of separate implementations can also
be combined. Conversely, various features that are described in the
context of a single implementation can also be implemented in
multiple embodiments separately or in any suitable
sub-combination.
[0091] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made without departing from the spirit and scope of the
invention. Accordingly, other implementations are within the scope
of the following claims.
* * * * *