U.S. patent application number 10/186797 was filed with the patent office on 2004-01-01 for undercut measurement using sem.
Invention is credited to Grella, Luca, Lorusso, Gian Francesco.
Application Number | 20040000638 10/186797 |
Document ID | / |
Family ID | 29735250 |
Filed Date | 2004-01-01 |
United States Patent
Application |
20040000638 |
Kind Code |
A1 |
Lorusso, Gian Francesco ; et
al. |
January 1, 2004 |
UNDERCUT MEASUREMENT USING SEM
Abstract
The disclosure relates to measuring an undercut of a feature on
a specimen using a scanning electron microscope (SEM). In
accordance with one embodiment, a method for measuring the undercut
includes illuminating the feature with a primary electron beam at
an incident angle, changing the incident angle of the primary
electron beam over a set of angles, measuring an intensity of
scattered electrons from the feature for each incident angle in the
set of angles, and determining a discontinuity in the intensities
as a function of the incident angle.
Inventors: |
Lorusso, Gian Francesco;
(Fremont, CA) ; Grella, Luca; (Gilroy,
CA) |
Correspondence
Address: |
OKAMOTO & BENEDICTO, LLP
P.O. BOX 641330
SAN JOSE
CA
95164
US
|
Family ID: |
29735250 |
Appl. No.: |
10/186797 |
Filed: |
July 1, 2002 |
Current U.S.
Class: |
250/310 ; 850/10;
850/18; 850/9 |
Current CPC
Class: |
H01J 2237/2611 20130101;
G01N 23/2251 20130101; H01J 37/28 20130101; H01J 2237/2815
20130101 |
Class at
Publication: |
250/310 |
International
Class: |
G21K 007/00; G01N
023/00 |
Claims
What is claimed is:
1. A method for measuring an undercut of a feature on a specimen
using a scanning electron microscope (SEM), the method comprising:
illuminating the feature with a primary electron beam at an
incident angle; changing the incident angle of the primary electron
beam over a set of angles; measuring an intensity of scattered
electrons from the feature for each incident angle in the set of
angles; and determining a discontinuity in the intensities as a
function of the incident angle.
2. The method of claim 1, wherein the set of angles is oriented
such that at higher incident angles the primary electron beam
impinges directly upon a sidewall of the feature.
3. The method of claim 2, wherein the discontinuity occurs when the
primary electron beam begins to impinge directly upon the
sidewall.
4. The method of claim 3, wherein the measured undercut angle
corresponds to the incident angle at which the discontinuity is
determined to occur.
5. The method of claim 1, wherein the determining is performed
using a computer-implemented process.
6. The method of claim 1, wherein the intensities are measured
based on peak heights.
7. The method of claim 1, wherein the intensities are measured
based on integrated peak areas.
8. The method of claim 1, wherein the incident angle is changed by
tilting a primary electron beam.
9. The method of claim 1, wherein the incident angle is changed by
tilting a specimen stage.
10. The method of claim 1, wherein the SEM comprises an electron
beam (e-beam) inspection apparatus for inspection of semiconductor
wafers.
11. The method of claim 10, wherein the e-beam inspection apparatus
comprises an SEM configured for measuring critical dimensions
(CD-SEM).
12. A method for measuring an undercut of a feature on a specimen
using a scanning electron microscope (SEM), the method comprising:
illuminating the feature with primary electrons at an incident
angle; measuring an intensity of scattered electrons from the
feature by a plurality of detectors at a set of scattering angles;
and determining a discontinuity in the intensities as a function of
the scattering angle.
13. The method of claim 12, further comprising: applying a
non-uniform extracting field to differentiate between scattered
electrons at different scattering angles.
14. The method of claim 13, wherein the plurality of detectors are
oriented such that a sidewall of the feature is within direct
line-of-sight of detectors at higher detecting angles.
15. The method of claim 14, wherein the discontinuity occurs
between a first detector without direct line-of-sight and a second
detector with direct line-of-sight.
16. The method of claim 15, wherein the measured undercut angle
corresponds to the detecting angle at which the discontinuity is
determined to occur.
17. The method of claim 12, wherein the determining is performed
using a computer-implemented process.
18. A scanning electron microscope (SEM) for measuring an undercut
of a feature on a specimen, the SEM comprising: an electron
illumination system for illuminating the feature with a primary
electron beam at an incident angle; a mechanism for changing the
incident angle of the primary electron beam over a set of angles; a
detector for measuring an intensity of scattered electrons from the
feature for each incident angle in the set of angles; and a
processor for determining a discontinuity in the intensities as a
function of the incident angle.
19. The SEM of claim 18, wherein the mechanism for changing the
incident angle comprises an adjustable electromagnet lens system
for tilting the primary electron beam with respect to the
specimen.
20. The SEM of claim 18, wherein the mechanism for changing the
incident angle comprises a tiltable stage for tilting the specimen
with respect to the primary electron beam.
21. An apparatus for measuring an undercut of a feature on a
specimen, the apparatus comprising: means for illuminating the
feature with a primary electron beam at an incident angle; means
for changing the incident angle of the primary electron beam over a
set of angles; means for measuring an intensity of scattered
electrons from the feature for each incident angle in the set of
angles; and means for determining a discontinuity in the
intensities as a function of the incident angle.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to specimen
inspection. More particularly, the present invention relates to
e-beam inspection systems.
[0003] 2. Description of the Background Art
[0004] Semiconductor manufacturing processes include deposition and
etching of various material layers on a semiconductor wafer. During
the processing, various microscopic features (trenches, islands,
and so on) are created on the wafer. Often times, the
cross-sectional profile of a microscopic feature may be of interest
to the manufacturer. In particular, the angle of undercut of a
sidewall of the feature may be of interest.
[0005] Scanning electron microscopy (SEM) may be used to inspect a
wafer, and the rock angle of the incident beam may be varied in an
attempt to view an undercut. Unfortunately, SEM images tend to have
significant resolution degradation when taken at large rock angles.
This blurring of the images makes impractical the determination of
large undercut angles by viewing SEM images at large rock
angles.
[0006] Another conventional technique for determining undercut
angles is by way of focus ion beam (FIB) sections. FIB systems
impinge a focused beam on ions (for example, gallium ions) onto a
specimen. The focused ion beam may act to precision mill the
specimen at high beam currents or to image the specimen at low beam
currents (in which case less material is sputtered). Hence, an FIB
system may be used in preparing a cross-section specimen for
transmission electron microscope (IEM) imaging. Recent FIB systems
may be utilized for in-situ cross-section preparation and
high-resolution imaging. However, FIB techniques are
disadvantageously destructive due to the sputtering or milling of
material from the sample.
[0007] FIG. 1A is a conventional image of a cross section of a
feature 170 that is slightly undercut on both left and right sides.
The image of the cross section was obtained by the conventional
focused ion beam (FIB) technique. As mentioned above, the FIB
technique is disadvantageous in that it requires destruction of the
specimen. This is because the FIB technique thins the sample by ion
milling.
[0008] FIG. 1B shows a conventional analysis of the cross-sectional
FIB image of the feature 170 to determine the undercut angles. The
analysis gives an outline of the feature 170. Using the outline of
the feature 170, the undercut angle may be determined by comparing
the actual left 172-L and right 172-R sidewalls to vertical
reference lines 174-L and 174-R, respectively. (The slight
asymmetry seen in the reference lines is thought to be due to the
milling of the sample.) Analysis of this FIB image indicates a left
undercut of about five (5) degrees and a right undercut of about
two (2) degrees.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1A is a conventional image of a cross section of a
feature that is slightly undercut on both left and right sides.
[0010] FIG. 1B shows a conventional analysis of the cross-sectional
FIB image of the feature to determine the undercut angles.
[0011] FIG. 2 is a diagram providing an overview of the technique
for measuring undercut angles in accordance with an embodiment of
the invention.
[0012] FIGS. 3A and 3B shown experimental electron scans of the
undercut feature depicted in FIGS. 1A and 1B.
[0013] FIG. 4A is a graph showing the analysis of data from a
series of electron scans at different incidence angles to measure
the left undercut in accordance with an embodiment of the
invention.
[0014] FIG. 4B is a graph showing the analysis of data from a
series of electron scans at different incidence angles to measure
the right undercut in accordance with an embodiment of the
invention.
[0015] FIGS. 5A through 5D depict simulated electron scans based on
a hypothetical feature with ten-degree undercuts on each side.
[0016] FIG. 6 is a graph showing the analysis of the simulated data
from FIGS. 5A through 5D in accordance with an embodiment of the
invention.
[0017] FIG. 7 depicts one type of SEM system with which the
invention may be utilized.
[0018] FIG. 8 depicts another type of SEM system with which the
invention may be utilized.
SUMMARY
[0019] Embodiments of the invention relates to methods for
measuring an undercut of a feature on a specimen using a scanning
electron microscope (SEM). One method includes illuminating the
feature with a primary electron beam at an incident angle, changing
the incident angle of the primary electron beam over a set of
angles, measuring an intensity of scattered electrons from the
feature for each incident angle in the set of angles, and
determining a discontinuity in the intensities as a function of the
incident angle. Another method includes illuminating the feature
with primary electrons at an incident angle, measuring an intensity
of scattered electrons from the feature by a plurality of detectors
at a set of scattering angles, and determining a discontinuity in
the intensities as a function of the scattering angle.
[0020] Another embodiment of the invention relates to a scanning
electron microscope (SEM) for measuring an undercut of a feature on
a specimen. The SEM includes an electron illumination system for
illuminating the feature with a primary electron beam at an
incident angle, a mechanism for changing the incident angle of the
primary electron beam over a set of angles, a detector for
measuring an intensity of scattered electrons from the feature for
each incident angle in the set of angles, and a processor for
determining a discontinuity in the intensities as a function of the
incident angle.
[0021] Another embodiment of the invention relates to an apparatus
for measuring an undercut of a feature on a specimen. The apparatus
includes means for illuminating the feature with a primary electron
beam at an incident angle, means for changing the incident angle of
the primary electron beam over a set of angles, means for measuring
an intensity of scattered electrons from the feature for each
incident angle in the set of angles, and means for determining a
discontinuity in the intensities as a function of the incident
angle.
DETAILED DESCRIPTION
[0022] The present invention relates to a technique for measuring
undercut angles in an advantageously non-destructive manner. The
technique may be performed using a scanning electron microscope and
may be applied to measure undercut angles of features on a
semiconductor wafer or other types of specimens. In accordance with
one embodiment, even if the images have substantial resolution
degradation (due to large rock angles), the technique may still be
applied to measure undercut angles.
[0023] FIG. 2 is a diagram providing an overview of the technique
for measuring undercut angles for an undercut feature 202 in
accordance with an embodiment of the invention. An electron beam
204 with an incident angle of less than the undercut angle (for
example, zero degrees) is illustrated on the left side (situation
labeled "a"), and an electron beam 210 with an incident angle that
is greater than the undercut angle is illustrated on the right side
(situation labeled "b"). Corresponding electron intensity profiles
206 (for situation "a") and 214 (for situation "b") are depicted
below.
[0024] For situation "a", the electron intensity profile 206 for
the feature 202 is seen to be relatively symmetrical. There is a
peak 208-L on the left side of the profile 206 and a nearly equal
sized peak 208-R on the right side. These peaks in electron
intensities are believed by the applicants to be due to emission
and collection of scattered electrons from the sidewalls of the
feature 202.
[0025] For situation "b", the electron intensity profile 214 for
the feature 202 is seen to be substantially asymmetrical. The
right-side peak 216-R is now substantially higher than the
left-side peak 216-L. This asymmetry is believed by the applicants
to be due to the electron beam 210 being incident from the right
side at an angle exceeding the undercut angle of the right
sidewall. As a result, it is believed that the incident beam 210
directly illuminates the right sidewall and so causes the emission
of a greater number of scattered electrons 212.
[0026] FIGS. 3A and 3B show experimental electron scans of the
undercut feature 170 depicted in FIGS. 1A and 1B. The beam tilt was
at zero degrees for FIG. 3A and was at 6.6 degrees for FIG. 3B.
Both FIGS. 3A and 3B show electron intensity versus position in the
region of the undercut feature 170. The electron scans of FIGS. 3A
and 3B do not require destruction of the specimen and were
performed prior to the destructive FIB cross sectioning of FIGS. 1A
and 1B. By using the present invention, undercut angles may be
determined without the use of destructive FIB sectioning.
[0027] As can be seen from FIG. 3A, the electron intensity profile
302 for the feature 170 is seen to be relatively symmetrical. There
is a peak 304-L on the left side of the profile 302 and a nearly
equal sized peak 304-R on the right side. The symmetry shown is
expected given the zero degree incidence angle of the primary
electron beam.
[0028] On the other hand, in FIG. 3B, the electron intensity
profile 312 for the feature 170 is seen to be asymmetrical. The
right-side peak 314-R is now substantially higher than the
left-side peak 314-L. This asymmetry is believed by the applicants
to be due to the electron beam being incident from the right side
at an angle (6.6 degrees) that exceeds the undercut angle of the
right sidewall (approximately 4 degrees). As a result, it is
believed that the incident beam directly illuminates the right
sidewall and so causes the emission of a greater number of
scattered electrons. In accordance with an embodiment of the
invention, it may be determined from FIGS. 3A and 3B that the
undercut angle is somewhere between zero degrees and 6.6
degrees.
[0029] Note that the peaks are wider in FIG. 3B than the peaks in
FIG. 3A. This broadening of the peaks corresponds to the blurring
of the detected image that occurs when the incident beam is tilted
in the SEM. It is believed that this blurring that makes the
asymmetry between right and left intensities difficult to detect
visually from the scanned image. Hence, a preferred embodiment of
the invention quantitatively analyzes the scanned data.
[0030] FIG. 4A is a graph showing the analysis of data from a
series of electron scans at different incidence angles to measure
the left undercut in accordance with an embodiment of the
invention. The y-axis shows the ratio between the left peak
intensity l(left) and the right peak intensity l(right). The x-axis
shows the incidence angle of the primary beam (the rock angle) in
degrees. In a preferred embodiment, the peak intensities are
measured by the peak heights. However, in other embodiments, it may
be possible also to measure the peak intensities using integrated
peak areas. In the graph of FIG. 4A, we see that there is a
discontinuity in the slope of the data at an incident angle of
about 4 degrees. This indicates that the left undercut is
approximately 4 degrees. This is in relatively close agreement with
the FIB cross section measured left undercut of about 5
degrees.
[0031] FIG. 4B is a graph showing the analysis of data from a
series of electron scans at different incidence angles to measure
the right undercut in accordance with an embodiment of the
invention. The y-axis shows the ratio between the right peak
intensity l(right) and the left peak intensity l(left). The x-axis
shows the incidence angle of the primary beam (the rock angle) in
degrees. In the graph of FIG. 4B, we see that there is a
discontinuity in the slope of the data at an incident angle of
about +3 degrees. This indicates that the right undercut is
approximately 3 degrees. This is in relatively close agreement with
the FIB cross section measured left undercut of about 2
degrees.
[0032] FIGS. 5A through 5D depict simulated electron scans based on
a hypothetical feature with ten-degree undercuts on each side. Each
of these figures depicts electron intensity (in arbitrary units) on
the y-axis as a function of position on the x-axis. FIG. 5A
corresponds to low incident angles of six degrees or less and shows
a relatively symmetrical profile. FIG. 5B corresponds to an
incident angle of eleven degrees and shows asymmetry in that the
right peak is higher than the left peak. FIG. 5C corresponds to an
incident angle of seventeen degrees and shows the asymmetry
increasing as the right peak becomes even higher. Finally, FIG. 5D
corresponds to an incident angle of twenty-five degrees and shows
that a reduction in asymmetry as the right peak is lower than in
FIG. 5C. Thus, FIG. 5D shows that the asymmetry does not continue
to increase at very high angles.
[0033] FIG. 6 is a graph showing the analysis of the simulated data
from FIGS. 5A through 5D in accordance with an embodiment of the
invention. Data points in FIG. 6 are at zero degrees (from FIG.
5A), six degrees (from FIG. 5A), eleven degrees (from FIG. 5B),
seventeen degrees (from FIG. 5C), and twenty-five degrees (from
FIG. 5D). As indicated from FIG. 6, the undercut angle is between
six degrees and eleven degrees. This is a good result because the
simulations are based on an undercut angle of ten degrees. Of
course, further simulations at closer angles may be performed to
more precisely determine the undercut angle.
[0034] Scanning electron microscope (SEM) systems are shown in
FIGS. 7 and 8 as examples of SEM systems with which the invention
may be utilized. The present invention may be utilized in other
types of SEM systems as well.
[0035] The SEM system 10 depicted in FIG. 7 is particularly
suitable for measurement of critical dimensions and is described in
detail in U.S. Pat. No. 5,869,833, entitled "Electron Beam Dose
Control for Scanning Electron Microscopy and Critical Dimension
Measurement Instruments," issued to Richardson et al. and assigned
to KLA-Tencor Corporation of San Jose, Calif. The disclosure of
U.S. Pat. No. 5,869,833 (the Richardson patent) is hereby
incorporated by reference.
[0036] The SEM 10 of FIG. 7 includes an electron beam source 12, a
focusing column and lens assembly 14, and a scan controller 16. The
scan controller 16 scans an electron beam across selected regions
of the specimen 20. Also included is a detector subsystem 24 to
detect secondary and backscattered electrons from the specimen
20.
[0037] The electron beam source 12 at the top of the SEM system 10
produces an electron beam 34. One implementation that could be used
includes an electron source 36 that consists of a thermal field
emitter with electrons accelerated by a surface field generated by
power supply 32. Alternative electron source embodiments may
instead be employed. The electrons emitted by the electron source
36 are then, within the beam source 12, directed through the
electrodes 38 and the source lens 39 (each also controlled by the
power supply 32) to form the electron beam 34 that enters the
focusing column and lens assembly 14 to be directed to the specimen
20.
[0038] In the focusing column and lens assembly 14, the electron
beam 34 passes through an aperture 41, reducing the beam current.
For example, the beam current may be reduced from approximately 300
pA (pico Amperes) to a range of 5 to 100 pA forming the electron
beam that is labeled 34'. The electron beam 34' then passes through
an objective lens 42 that includes magnetic coils 43 and pole
pieces 44 to generate a strong magnetic field. That magnetic field
is used to focus beam 34' into an electron beam 18 with a spot size
that may be, for example, about 5 nm (nanometers) when directed at
the specimen 20. A bias may be applied by a power supply 52 to the
specimen 20 to create a decelerating field to slow down the
electrons in the beam 18 as the electrons approach the specimen
20.
[0039] In operation, the electron beam 18 may be raster scanned
over the specimen 20 and the secondary and backscattered electron
signal 28 may be detected by the detector subsystem 24. The
secondary and backscattered electrons 28 are released as a result
of the interaction of the electron beam 18 with specimen 20 and are
directed back toward the objective lens 42. As electrons 28 are
released, they may spiral through the objective lens 42 (as a
result of the magnetic field), and then travel toward the detector
subsystem 24 as they leave the field within the lens 42. Typically,
the specimen 20 may be comprised of a variety of materials that may
be conductive, insulating, or semiconducting. A sub-area within the
specimen 20 may be of particular interest for scanning to determine
features of that sub-area. An image processor and display subsystem
26 may develop the image of the sub-area. For example, the specimen
20 may be a semiconductor wafer and a sub-area of the wafer may be
a portion of a circuit die on the wafer.
[0040] The detector subsystem 24 may be selected to have a
bandwidth that is at least adequate to detect the secondary and
backscattered electrons that form the electron signal 28. For
example, the detector subsystem 24 may include a micro-channel
plate, micro-sphere plate, semiconductor diode, or a scintillator
and photo-multiplier assembly. The detector subsystem 24
illustrated in FIG. 7 includes a detector 55 and collector plat 56.
The secondary and backscattered electron signal 28 is received by
the detector 55 and then collected by the collector plate 56. The
collector plate 56 generates a signal that is received by the image
processor and dispay subsystem 26. The image processor and display
subsystem 26 may amplify the signal by an amplifier 58 before the
signal is input into an image generator 59.
[0041] The location of the electron beam 18 on the specimen 20 is
controlled by the scan controller 16. The scan controller 16
illustrated in FIG. 7 includes scan plates 45 that are located
within the magnetic field created by coils 43 and pole pieces 44.
The scan plates 45 are powered by a raster generator 48 (via
signals on lines 46 and 47) to direct the electron beam 18 in both
the x and y directions across the specimen 20.
[0042] The SEM system depicted in FIG. 8 is described in U.S. Pat.
No. 5,578,821, entitled "Electron Beam Inspection System and
Method," issued to Meisberger et al. and assigned to KLA-Tencor
Corporation of San Jose, Calif. The disclosure of U.S. Pat. No.
5,578,821 (the Meisberger patent) is hereby incorporated by
reference.
[0043] FIG. 8 is a simplified schematic representation of the paths
of the primary, secondary, backscattered and transmitted electrons
through the electron optical column and collection system for
electron beam inspection. In brief, FIG. 8 shows a schematic
diagram of the various electron beam paths within the column and
below substrate 57. Electrons are emitted radially from field
emission cathode 81 and appear to originate from a very small
bright point source. Under the combined action of the accelerating
field and condenser lens magnetic field, the beam is collimated
into a parallel beam. Gun anode aperture 87 masks off electrons
emitted at unusable angles, while the remaining beam continues on
to beam limiting aperture 99. An upper deflector (not depicted) is
used for stigmation and alignment, ensuring that the final beam is
round and that it passes through the center of the objective lens
104 comprising elements 105, 106 and 107. A condenser lens (not
depicted) is mechanically centered to the axis defined by cathode
81 and beam limiting aperture 99. The deflection follows the path
shown, so that the scanned, focused probe (beam at point of impact
with the substrate) emerges from the objective lens 104.
[0044] In High Voltage mode operation, Wien filter deflectors 112
and 113 deflect the secondary electron beam 167 into detector 117.
When partially transparent masks are imaged, the transmitted beam
108 passes through electrode system 123 and 124 that spreads the
beam 108 before it hits the detector 129. In Low Voltage mode
operation, the secondary electron beam is directed by stronger Wien
filter deflections toward the low voltage secondary electron
detector 160 that may be the same detector used for backscatter
imaging at high voltage. Further detail on the system and its
operation is described in the Meisberger patent.
[0045] In accordance with one embodiment of the invention, the
incidence angle of the primary electron beam may be varied by
appropriate adjustment of the currents in the objective lenses that
focus the beam onto the specimen. In accordance with another
embodiment of the invention, the incidence angle of the primary
electron beam may be varied by tilting the stage holding the
specimen Advantageously, this embodiment may avoid the blurring of
the scanned image that occurs when the incident beam is tilted.
[0046] In accordance with an alternate embodiment, electron
detection at a range of angles may be used to measure the undercut
angle. Such an embodiment may include multiple electron detectors
at different scattering angles and application of a non-uniform
extracting field to differentiate between scattered electrons at
the different scattering angles. The multiple detectors would be
oriented such that the sidewall of interest is within direct
line-of-sight of the detectors at higher detecting angles so that a
discontinuity occurs between a first detector without direct
line-of-sight and a second detector with direct line-of-sight. In
that case, the measured undercut angle may correspond to the
detecting angle at which the discontinuity is determined to
occur.
[0047] In the above description, numerous specific details are
given to provide a thorough understanding of embodiments of the
invention. However, the above description of illustrated
embodiments of the invention is not intended to be exhaustive or to
limit the invention to the precise forms disclosed. One skilled in
the relevant art will recognize that the invention can be practiced
without one or more of the specific details, or with other methods,
components, etc. In other instances, well-known structures or
operations are not shown or described in detail to avoid obscuring
aspects of the invention. While specific embodiments of, and
examples for, the invention are described herein for illustrative
purposes, various equivalent modifications are possible within the
scope of the invention, as those skilled in the relevant art will
recognize.
[0048] These modifications can be made to the invention in light of
the above detailed description. The terms used in the following
claims should not be construed to limit the invention to the
specific embodiments disclosed in the specification and the claims.
Rather, the scope of the invention is to be determined by the
following claims, which are to be construed in accordance with
established doctrines of claim interpretation.
* * * * *