U.S. patent application number 14/868056 was filed with the patent office on 2017-03-02 for cad-assisted tem prep recipe creation.
This patent application is currently assigned to FEI Company. The applicant listed for this patent is FEI Company. Invention is credited to Jason Arjavac, Matthew P. Knowles.
Application Number | 20170062178 14/868056 |
Document ID | / |
Family ID | 58017689 |
Filed Date | 2017-03-02 |
United States Patent
Application |
20170062178 |
Kind Code |
A1 |
Arjavac; Jason ; et
al. |
March 2, 2017 |
CAD-ASSISTED TEM PREP RECIPE CREATION
Abstract
An improved process workflow and apparatus for S/TEM sample
preparation and analysis is provided. Preferred embodiments provide
improved methods for an automated recipe TEM sample creation,
especially for small geometry TEM lamellae, employing CAD data to
automatically align various stages of sample preparation. The
process automatically verifies and aligns the position of
FIB-created fiducials by masking off portions of acquired images,
and then comparing them to synthesized images from CAD data. SEM
beam positions are verified by comparison to images synthesized
from CAD data. FIB beam position is also verified by comparison to
already-aligned SEM images, or by synthesizing an FIB image from
CAD using techniques for simulating FIB images. The automatic
alignment techniques herein allow creation of sample lamellas at
specified locations without operator intervention.
Inventors: |
Arjavac; Jason; (Hillsboro,
OR) ; Knowles; Matthew P.; (West Linn, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FEI Company |
Hillsboro |
OR |
US |
|
|
Assignee: |
FEI Company
Hillsboro
OR
|
Family ID: |
58017689 |
Appl. No.: |
14/868056 |
Filed: |
September 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62212352 |
Aug 31, 2015 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 2237/28 20130101;
H01J 2237/2802 20130101; H01J 2237/31745 20130101; G01N 2001/282
20130101; G01N 1/32 20130101; H01J 2237/31732 20130101; H01J
2237/2826 20130101; G01N 1/28 20130101; H01J 37/3045 20130101; H01J
37/3023 20130101; H01J 37/28 20130101 |
International
Class: |
H01J 37/28 20060101
H01J037/28 |
Claims
1. A method for automatically preparing a semiconductor sample in a
dual-beam charged particle system, the method comprising:
positioning the dual-beam charged particle system with respect to a
semiconductor die sample region of interest to be examined in a
sample chamber; with focused ion beam (FIB) deposition, forming a
first precision fiducial marker and one or more additional fiducial
markers at desired locations with respect to the region of
interest; acquiring a first scanning electronic microscope (SEM)
image of the region of interest; retrieving computer aided design
(CAD) data describing the region of interest; synthesizing a second
SEM image from CAD data describing the region of interest; masking
the one or more additional fiducial markers in the first SEM image
and comparing the masked first SEM image and the second SEM image
to determine a final correction offset for an actual position of
the precision fiducial marker; applying the final correction offset
to the location of the precision fiducial marker; based on the
corrected location of the precision fiducial marker, adjusting the
position of the FIB relative to the sample, and milling with the
FIB to create a sample lamellae for examination.
2. The method of claim 1, wherein positioning the dual-beam charged
particle system includes aligning with a desired field of view by
comparing CAD data to an acquired image.
3. The method of claim 2, wherein aligning with a desired field of
view comprises (a) acquiring a preliminary SEM image including an
alignment mark on the semiconductor die sample; (b) acquiring CAD
data describing the position of the alignment mark; (c) comparing
the preliminary SEM image with the CAD data and determining an
first alignment correction offset; and (d) based on the first
alignment correction offset, adjusting the sample position relative
to the position of the beam paths of the dual-beam system to direct
the beam paths toward the region of interest.
4. The method of claim 1, further comprising, before FIB deposition
of the fiducial markers, (a) acquiring a FIB image from the FIB
device; (b) synthesizing a FIB image from the CAD data describing
the region of interest; (c) comparing the acquired FIB image to the
synthesized FIB image to generate a FIB correction offset; and (d)
applying the FIB correction offset to the sample position.
5. The method of claim 1, further comprising, before FIB deposition
of the fiducial markers, aligning the FIB beam by comparing an
acquired SEM image to an acquired FIB image, and updating a tracked
location for the FIB beam based on the results of the
comparison.
6. The method of claim 1, further comprising, before the FIB
deposition, depositing a protective layer over at least a portion
of the region of interest.
7. The method of claim 6, wherein the protective layer is deposited
by electron beam-induced deposition (EBID).
8. The method of claim 7, wherein depositing the protective layer
with EBID further includes (a) after positioning the dual beam
charged particle system, acquiring an SEM image of the field of
view; (b) acquiring CAD data describing the position of the die
sample region of interest; (c) synthesizing a SEM image from the
acquired CAD data; (d) comparing the acquired SEM image to the
synthesized SEM image to determine an SEM alignment correction
offset; (e) applying the SEM alignment correction offset to the FIB
position; and (f) then depositing the protective layer with
EBID.
9. The method of claim 6, further comprising, when masking the one
or more additional fiducial markers, also masking the protective
layer.
10. A method for automatically preparing a semiconductor sample in
a dual-beam charged particle system, the method comprising:
aligning a scanning electron microscope (SEM) beam the dual-beam
charged particle system with a desired feature of interest on a
sample in a sample chamber by comparing computer aided design (CAD)
data to an acquired SEM image and applying a resulting SEM
correction offset; then, with beam-induced deposition, depositing a
protective layer over at least part of the region of interest;
then, with a focused ion beam (FIB), creating a first precision
fiducial marker and one or more additional fiducial markers at
desired locations with respect to the region of interest; acquiring
a scanning electronic microscope (SEM) image of the region of
interest; retrieving CAD data describing the region of interest;
synthesizing a second SEM image from CAD data describing the region
of interest; masking the one or more additional fiducial markers
and the protective layer in the first SEM image and comparing the
masked first SEM image and the second SEM image to determine a
final correction offset for an actual position of the precision
fiducial marker; applying the final correction offset to the FIB
position and then FIB milling with reference to the corrected
position to create a sample lamellae for examination.
11. The method of claim 10, further comprising, before creating the
fiducial markers, acquiring a FIB image from the FIB device;
synthesizing a FIB image from the CAD data describing the region of
interest; comparing the acquired FIB image to the synthesized FIB
image to generate a FIB correction offset; applying the FIB
correction offset to the FIB position relative to the sample.
12. An automated sample preparation system comprising: a dual-beam
scanning and milling system including a scanning electron
microscope (SEM), a focused ion beam (FIB) both pointing at a
sample chamber, a system controller operatively connected to the
SEM and FIB and including at least one processor and tangible
non-transitory computer media storing program instructions
executable by the at least one processor for: aligning a scanning
electron microscope (SEM) beam the dual-beam charged particle
system with a desired feature of interest on a sample in the sample
chamber by comparing computer aided design (CAD) data to an
acquired SEM image and applying a resulting SEM correction offset;
then, with beam-induced deposition, depositing a protective layer
over at least part of the region of interest; then, with a focused
ion beam (FIB), creating a first precision fiducial marker and one
or more additional fiducial markers at desired locations with
respect to the region of interest; acquiring a scanning electronic
microscope (SEM) image of the region of interest; retrieving CAD
data describing the region of interest; synthesizing a second SEM
image from CAD data describing the region of interest; masking the
one or more additional fiducial markers and the protective layer in
the first SEM image and comparing the masked first SEM image and
the second SEM image to determine a final correction offset for an
actual position of the precision fiducial marker; applying the
final correction offset to the FIB position and then FIB milling
with reference to the corrected position to create a sample
lamellae for examination.
13. The system of claim 12, wherein the program instructions are
further executable for, before FIB deposition of the fiducial
markers, (a) acquiring a FIB image from the FIB device; (b)
synthesizing a FIB image from the CAD data describing the region of
interest; (c) comparing the acquired FIB image to the synthesized
FIB image to generate a FIB correction offset; and (d) applying the
FIB correction offset to the sample position.
14. The system of claim 12, wherein the program instructions are
further executable for, before FIB deposition of the fiducial
markers, aligning the FIB beam by comparing an acquired SEM image
to an acquired FIB image, and updating a tracked location for the
FIB beam based on the results of the comparison.
15. The system of claim 12, further comprising: a plucker device
operable to remove one or more lamellas from the sample; a
transmission electron microscope (TEM), operable to receive the one
or more lamellas from the plucker device and conduct scans; a
process controller operatively connected to the dual beam scanning
and milling system, the plucker device, and the TEM and operable to
command them to execute functions in an automated workflow.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application 62/212,352, filed Aug. 31, 2015, which is hereby
incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to sample preparation
workflows with charged particle beam devices, and in particular
toward highly automated recipes for preparing transmission electron
microscope samples.
BACKGROUND OF THE INVENTION
[0003] Features on semiconductor wafers and dies are
three-dimensional structures and a complete characterization must
describe not just a surface dimension, such as the top width of a
line or trench, but a complete three-dimensional profile of the
feature. Process engineers must be able to accurately measure the
critical dimensions (CD) of such surface features to fine tune the
fabrication process and assure a desired device geometry is
obtained.
[0004] Typically, such CD measurements are made using instruments
such as a scanning electron microscope (SEM). In a scanning
electron microscope (SEM), a primary electron beam is focused to a
fine spot that scans the surface to be observed. Secondary
electrons are emitted from the surface as it is impacted by the
primary beam. The secondary electrons are detected, and an image is
formed, with the brightness at each point of the image being
determined by the number of secondary electrons detected when the
beam impacts a corresponding spot on the surface. As features
continue to get smaller and smaller, however, there comes a point
where the features to be measured are too small for the resolution
provided by an ordinary SEM.
[0005] As semiconductor geometries continue to shrink,
manufacturers increasingly rely on transmission electron
microscopes (TEMs) for monitoring the process, analyzing defects,
and investigating interface layer morphology. TEMs allow observers
to see features having sizes on the order of nanometers, and to see
the internal structure of a sample. The sample must be sufficiently
thin to allow many of the electrons in the primary beam to travel
though the sample and exit on the opposite site.
[0006] Because a sample must be very thin for viewing with
transmission electron microscopy (whether TEM or STEM), preparation
of the sample can be delicate, time-consuming work. The term "TEM"
as used herein refers to a TEM or a STEM and references to
preparing a sample for a TEM are to be understood to also include
preparing a sample for viewing on an STEM. TEM samples are
typically less than 100 nm thick, but for some applications samples
must be considerably thinner. With advanced processes at 30 nm, 22
nm, and below, the sample needs to be less than 20 nm in thickness
in order to avoid overlap among small scale structures. The
precision and accuracy involved in producing such samples is
typically very time consuming. In fact, even though the information
that can be discovered by TEM analysis can be very valuable, the
entire process of creating and measuring TEM samples has
historically been so labor intensive and time consuming that it has
not been practical to use this type of analysis for manufacturing
process control. While the use of focused ion beam (FIB) methods in
sample preparation has reduced the time required to prepare samples
for TEM analysis down to only a few hours, it is not unusual to
analyze 15 to 50 TEM samples from a given wafer. As a result, speed
of sample preparation is a very important factor in the use of TEM
analysis, especially for semiconductor process control.
[0007] FIG. 4 illustrates a prior art automated S/TEM sample
management (available commercially as the ExSolve.TM. system)
according to described in U.S. Pat. No. 8,890,064 to Arjavac et al.
The ExSolve wafer TEM prep (WTP) workflow addresses the needs of
facilities that require automated, high-throughput sampling at
advanced technology nodes. It complements the capabilities of dual
beam systems such as the FEI company's Helios NanoLab.TM.
DualBeam.TM. 1200AT, which provides more flexible,
operator-directed, sample preparation methods, along with
additional capabilities such as high-resolution scanning electron
microscopy (SEM) imaging and analysis.
[0008] In the depicted system of FIG. 4, TEM samples are processed
by a cluster of different processing tools having the capability of
sequentially processing samples (e.g., lamellae extracted from
semiconductor wafers). The S/TEM sample management tool suite 100
generally includes a Process Controller 110 and a Fab Host computer
112 operably connected to (or integrated with) a FIB system 114, a
lamella extraction tool 116 such as an Ex-Situ Plucker ("ESP"), and
a S/TEM system 118. FIB system 114 may comprise a dual beam FIB/SEM
system such as the Certus.TM./CLM available from FEI Company of
Hillsboro, Oreg., the assignee of the present invention; and S/TEM
system 118 may comprise a system such as a Tecnai.TM. G2 S/TEM also
available from FEI Company. In the system of FIG. 4, each
processing tool is operably connected to (or integrated with) a
computer station 120, which uses software 122 for implementing TEM
sample creation and processing. Any suitable software (conventional
and/or self-generated) applications, modules, and components may be
used for implementing software. For example, in the system of FIG.
4, the automated S/TEM sample management is implemented using
IC3D.TM. software for automated machine control and metrology,
which is also available from FEI Company.
[0009] However, even in such automated systems, the requirement for
manual intervention at various recipe creation steps such as
specifying and verifying fiducial locations slows down the process.
The time and number of representative samples required to
develop/create a fully automated TEM sample preparation recipe (or
"TEM prep recipe") is too long to enable leading semiconductor
manufacturers to realize "time to data" in an automated workflow
for both process monitoring and defect root cause analysis.
Foundry-type manufactures are specifically challenged due to the
large number of different wafers for different fabless customers.
By the time they develop a robust recipe, the pattern may have
changed so a new recipe would have to be developed. Recipe
development time must be reduced and optimally automated to enable
foundry customers to realize the benefits of fully automated TEM
Prep.
[0010] For TEM prep, the problem is presently solved by skilled
engineers creating the recipe in an advanced visual scripting
authoring software framework that enables automation of a wide
range of instrument control commands and imaging tasks (the
iFAST.TM. software by FEI) software on an automated,
high-throughput sample preparation system that can prepare
site-specific lamellae (the ExSolve.TM. system described briefly
above), creating test samples, and manually analyzing samples in
offline TEM. The learning is then applied to the recipe parameters
and the process is iterated. However, such a process is relatively
slow and resource intensive, and not readily scalable. Recipe
creation/development for a fully automated TEM prep processing can
be a time consuming and applications engineering intensive activity
due to lack of pre-determined knowledge of pattern information
available on wafer/sample.
SUMMARY OF THE INVENTION
[0011] The present invention provides a solution to this problem by
linking CAD, or primary circuit/layout design data to multiple
steps of the recipe creating a corrected feedback for beam
positioning to ensure accurate placement of beam for lamella
processing. The CAD helps automate and speed the TEM prep in an
automated, stepwise methodology. An improved process workflow and
apparatus for S/TEM sample preparation and analysis is provided.
Preferred embodiments provide improved methods for an automated
recipe TEM sample creation, especially for small geometry TEM
lamellae, employing CAD data to automatically align various stages
of sample preparation. The process automatically verifies and
aligns the position of FIB-created fiducials by masking off
portions of acquired images, and then comparing them to synthesized
images from CAD data. SEM beam positions are verified by comparison
to images synthesized from CAD data. FIB beam position is also
verified by comparison to already-aligned SEM images, or by
synthesizing an FIB image from CAD using techniques for simulating
FIB images. The automatic alignment techniques herein allow
creation of sample lamellas at specified locations without operator
intervention.
[0012] One embodiment provides a method for automatically preparing
a semiconductor sample in a dual-beam charged particle system. The
method includes positioning the dual-beam charged particle system
with respect to a semiconductor die sample region of interest to be
examined in a sample chamber. With focused ion beam (FIB)
deposition, the method creates a first precision fiducial marker
and one or more additional fiducial markers at desired locations
with respect to the region of interest. The fiducial may be created
by deposition or milling, but in preferred embodiments is created
with deposition. Then the position of the fiducials is verified and
aligned by acquiring a first scanning electronic microscope (SEM)
image of the region of interest, retrieving computer aided design
(CAD) data describing the region of interest, synthesizing a second
SEM image from CAD data describing the region of interest, masking
the one or more additional fiducial markers in the first SEM image,
and comparing the masked first SEM image and the second SEM image
to determine a final correction offset for an actual position of
the precision fiducial marker. The final correction offset is
applied to the tracked location of the precision fiducial marker.
Then, based on the corrected location of the precision fiducial
marker, the process adjusts the position of the FIB relative to the
FIB, and mills with the FIB to create a sample lamellae for
examination. The steps may be performed at multiple specified
locations to create multiple lamellas from a sample wafer or
device.
[0013] The invention also includes systems with automated
controllers and program products executable to conduct the
automated workflow processes herein. For example, some embodiments
provide an automated sample preparation system including a
dual-beam scanning and milling system with a scanning electron
microscope (SEM), a focused ion beam (FIB) both pointing at a
sample chamber, a system controller operatively connected to the
SEM and FIB and including at least one processor and tangible
non-transitory computer media storing program instructions
executable by the at least one processor for controlling the dual
beam scanning and milling device to conduct the processes described
above. The dual beam system may also be integrated into a larger
sample management suite with automated sample handling, including a
plucker device operable to remove one or more lamellas from the
sample, a transmission electron microscope (TEM), operable to
receive the one or more lamellas from the plucker device and
conduct scans, and a process controller operatively connected to
the dual beam scanning and milling system, the plucker device, and
the TEM and operable to command them to execute functions in an
automated workflow.
[0014] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter. It should be appreciated by those
skilled in the art that the conception and specific embodiments
disclosed may be readily utilized as a basis for modifying or
designing other structures for carrying out the same purposes of
the present invention. It should also be realized by those skilled
in the art that such equivalent constructions do not depart from
the spirit and scope of the invention as set forth in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] For a more thorough understanding of the present invention,
and advantages thereof, reference is now made to the following
descriptions and the accompanying drawings, in which:
[0016] FIGS. 1A and 1B are a connected flow chart showing a method
of automating a sample preparation recipe.
[0017] FIG. 1C is an alternative flowchart proceeding from FIG.
1A.
[0018] FIGS. 2A-J show a sequence of diagrams illustrating the
example process of FIGS. 1A-1B.
[0019] FIG. 3 is a schematic view of a dual beam system employed
according to some embodiments of the invention.
[0020] FIG. 4 illustrates a prior art automated S/TEM sample
management (available commercially the ExSolve system) into which
the dual-beam system and the system and controller processes
described herein may be integrated to improve automation.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] FIGS. 1A and 1B show a flowchart of a process for automating
a recipe for creating a sample lamella according to one embodiment.
In the preferred version, the depicted steps after loading the
sample are fully automated in a dual-beam SEM/FIB system under
control of a system controller and process controller such as those
in the example system of FIG. 3. The system is preferably included
in larger automated S/TEM sample management, such as the ExSolve
system of FIG. 4, in which capability for machine-vision based
metrology and image recognition, high-precision fiducial marks, and
automatic fiducial placement are used to significantly improve
lamella placement accuracy and precision. The techniques described
herein may be integrated into the system of FIG. 4 to improve
automation and accuracy of the lamella formation process, by
providing a process to automatically align the SEM and FIB beams at
the various automated recipe steps, so that fiducial and lamella
creation takes place at a specified location.
[0022] The process begins at box 11 where a semiconductor wafer or
portion thereof including one or more semiconductor dies is loaded
into the sample chamber of the dual beam system. The semiconductor
sample will have one or more regions of interest that are desired
to be examined, for example to determine the presence or cause of
process flaws or for quality control of critical features.
[0023] FIGS. 2A-J show a sequence of diagrams illustrating the
example process of FIGS. 1A-1B. Referring to both sets of drawings,
in FIG. 2A the depicted semiconductor wafer 201 loaded into the
dual beam system at block 11 includes one or more semiconductor die
202, which are typically not yet separated into individual chips at
this point, but may be. At block 12, the process acquires a
preliminary SEM image at a first location or area 203 depicted
enlarged as 204, the area including one or more die alignment marks
205 typically presented at the corners of die as shown in the
enlarged view of the die corners. The beam may first be positioned
using an optical imaging system or may be calibrated sufficiently
that the process can go directly to acquiring the preliminary SEM
image at the die desired to be examined.
[0024] Next at block 13 the process acquires CAD data describing a
portion or snippet of the layout of the examined die, including the
position of the alignment mark. This data may also include the
position of the region of interest, but the location of that may
already be provided by the process inputs. At block 14, the process
next compares the preliminary SEM image 204 with the CAD data
depicted by the overlaid image 206 (FIG. 2B) to determine whether
the dual beam system, particularly the SEM beam, is correctly
aligned with the die. Block 14 may first include processing the CAD
data 206 to synthesize a SEM image suitable for direct comparison
by known image feature alignment techniques. The images 204 and 206
are compared to determine whether there is an offset, shown by
arrow 207, between where the system controller beam location
tracking process has stored or "believes" the location of the SEM
beam path to be relative to the die, and what it is actually
measuring on the die. If an offset 207 is detected by the
comparison, the system controller at block 15 applies the offset
207 by updating its stored position and orientation to reflect the
more accurate known present position and orientation of the SEM
device with respect to the die. The offset may include rotation as
well. Preferably, the system at block 15 merely stores the correct
position, and then proceeds to drive or move the beam path to place
the beam at the desired location to examine the present region of
interest. This may involve moving the sample stage or adjusting the
beam position mechanically or with its beam steering control
voltages.
[0025] Next at block 16, the process acquires a SEM image 208 at
the location of the region of interest, depicted as image 208 in
FIG. 2C. While this embodiment uses an SEM, other versions may use
an optical image or other particle beam image such as an FIB image.
To determine that the beam is properly aligned at the location, the
process next at block 17 acquires or recalls the CAD data
describing the region of interest for comparison, and synthesizes
from this CAD data an SEM image 210 suitable for comparison with
the acquired image. The synthesized image 210 is preferably of the
field of view expected from the acquired image, but may be larger
to facilitate locating the actual area of the acquired image on the
synthesized image. Generally the synthesis employs the dimensions
of features defined in the CAD data, along with their defined
materials, to model or approximate the SEM beam imaging process to
create an image, typically luminance data in grayscale form. If an
FIB image or optical image was used at block 16, the synthesis
process at block 17 will synthesize an appropriate image for
comparison. Next at block 18, the process compares the acquired SEM
image to the synthesized image to determine whether there is any
offset in alignment between the actual position of the SEM beam and
the stored position where the system assumes the SEM image to have
been acquired. This produces an offset distance and direction 212
to correct any offset of the beam from its desired location. The
offset is applied in block 19, typically by updating the location
of the beam to reflect the present actual location relative to the
sample.
[0026] Next, the process at block 21 starts at the adjusted
location, depicted as 214 in FIG. 2E, and performs an SEM
deposition (electron beam induced deposition, EBID) of a protective
layer 216 over the region of interest, to protect from damage or
contamination from FIB deposition and milling that will be needed
to complete the sample preparation. This deposition occurs
according to known methods which typically employ one or more
precursor gasses, such as Tungsten hexacarbonyl and Napthalene,
acted upon by the scanned electron beam to deposit material, such
as Tungsten or Carbon, respectively, on the surface of the sample.
The protective layer 214 is preferably large enough to completely
cover the surface of the sample volume that will be milled and
removed as a lamella or other extracted sample such as a wedge or
chunk.
[0027] The flowchart continues at connector A in FIG. 1B. After
depositing the protective layer with electron beam deposition, the
process may align the focused ion beam to improve accuracy for the
ion based deposition and milling that is to follow. Importantly,
the FIB alignment may not always be in synch or known relative to
the alignment of the SEM. FIG. 1B shows one automated alignment
process for the FIB, while FIG. 1C shows another. The preferred
process at block 22 acquires an FIB image of the region of
interest. This image is formed according to known FIB imaging
techniques, typically scanning the FIB at a lower beam current than
the milling steps and detecting secondary electrons or secondary
ions to form the image. Next at block 23, the process acquires CAD
data for the region of interest in the FIB image, and synthesizes
and FIB image for comparison to the acquired image. The synthesis
of the FIB image includes selecting all the features in the CAD
layout that are within the penetration range of the FIB, based upon
the settings of the FIB as used to acquire the image. These
features are layed-out or modeled and the layout created therefrom
is modeled at each location (pixel) according to a material model
indicating an expected emission amount (secondary electron
emission) that the material has when exposed to a FIB at the
current employed. These modeled emissions may be further filtered,
transformed, or scaled to achieve the desired synthetic image. Next
at block 24, the process compared the acquired FIB image with the
synthesized FIB image to determine a location correction offset for
the FIB. The correction offset is applied at block 25 to align the
system controller's tracked location for the FIB with the actual
location determined by the comparison. As with the SEM, this is
preferably done by adjusting the tracked location in memory at the
system controller, but may also be done with a movement of the beam
or sample. With the FIB beam aligned, it can be driven to precise
locations to create fiducials for aligning the lamella milling
process. The fiducial locations are preferably specified
automatically using CAD data to specify the location of the
fiducial with respect to a particular structure on the wafer
surface. This may be done in a pre-processing step associated with
each particular feature to be examined. In other embodiments,
automated FIB or SEM metrology can also be used to identify or help
identify the lamella site, or to confirm that the site is correct.
Such metrology may consist of image-based pattern recognition, edge
finding, ADR, center-of-mass calculations, blobs, etc.
[0028] Next, at block 26, the process use focused ion beam (FIB)
induced deposition to deposit a first precision fiducial marker and
one or more additional fiducial markers at desired locations with
respect to the region of interest. Preferably, a combination of
high precision (fine) fiducials and low precision (bulk) fiducials
are used to optimize lamella placement precision and accuracy, as
described, for example, in U.S. Pat. No. 8,134,124, for "Method for
Creating S/TEM Sample and Sample Structure" to Blackwood et al,
which is assigned to the assignee of the present application and
which is hereby incorporated by reference. In the preferred
fiducial arrangement shown in FIG. 2G, the high-precision fiducial
219 is created at one end of the region of interest, and two
additional fiducials 217 and 218, which may be low-precision
fiducial marks or a mixture of the different types, are located at
either end of the region of interest. This is not limiting and any
suitable number and shape of high-precision and low-precision
fiducials may be employed. The drawing is not exactly to scale and
typically low-precision fiducials will be larger to facilitate
location on lower resolution scanned images for rough milling when
the lamella milling process is conducted. These low-precision
fiducials are used for gross-structure pattern recognition, such as
quickly re-finding the approximate lamella location and determining
the location for bulk milling of the lamella. Because a larger beam
size will be used for the bulk milling, a suitable low precision
fiducial should be easily identified by pattern recognition
software even in lower resolution images.
[0029] With the fiducials created, the process now prepares to mill
out the sample lamella from the region of interest. To best do so,
it must determine the precise location at which the high-precision
fiducial has actually been deposited, versus the desired target
location, which may not be the same because of sample drift or
other alignment problems that may occur during FIB deposition. To
determine this actual location, the process first acquires a SEM
image including the region of interest and the fiducials at block
27. Next, the process retrieves CAD data describing the same area
for comparison at block 28. Then the process synthesizes another
SEM image 222 (FIG. 2i) from the CAD data, using SEM synthesis
techniques as discussed above.
[0030] To compare the synthesized image with the acquired image,
the process at block 31 first masks the one or more additional
fiducial markers 217 and 218 in the acquired SEM image, so that
they will not interfere with the image comparison, while leaving
the high-precision fiducial 219 unmasked as depicted in FIG. 2H.
For versions that use an SEM deposition to provide a protective
layer (like that done in this embodiment at block 21), the masking
also includes masking off the protective layer deposit 216. The
masking preferably eliminates the image portions including shapes
of the deposited structures from the image in some manner so they
do not introduce errors into the image comparison, with example
`masking` shown by the blanked-out areas 220 and 221 depicted in
FIG. 2H. For example, the image data may be removed from the image,
or a label or other instruction may be created for input to the
image comparison process at block 32 causing the process to ignore
data from the masked regions during the comparison. The masked data
may also be nulled out or replaced with the average luminance value
of the entire image. The masked areas 220 and 221 may be located
for masking by searching the images for the fiducial marks, with
the FIB protection layer 216 located from its location relative to
the surrounding fiducials. Or the masked features may be located
simply by masking off their best known locations, which may include
masking off a larger area than their known sizes to account for
possible location errors. With the masking created at block 31, the
process at block 32 compares the masked SEM image and the
synthesized SEM image to determine a final correction offset such
as the depicted offset 222 for determining an actual position of
the precision fiducial marker. Next at block 33, the process
applies the final offset correction such as to a stored actual
location of the precision fiducial marker, again preferably
applying the offset to the location of the precision fiducial
relative to the sample, but optionally using other methods of
applying the offset. Next at block 34, the process, with reference
to the corrected location the precision fiducial marker, mills with
the FIB to create a sample lamellae for examination. The lamella
milling may be conducted with suitable known techniques that
automatically mill a lamella of desired size and shape with
reference to a local precision fiducial. These techniques typically
include rough milling steps (relatively large beam) that reference
the position of the low precision fiducials, and fine milling steps
that reference the position of the high-precision fiducial.
[0031] The above-described embodiment provides a manner to
automatically and accurately create lamellas or other sample
portions from a wafer, preferably without human intervention as
each step can be automatically controlled. It is noted that while
the process steps in the depicted process go in order for the
creation of a single lamella, in actuality the process is typically
applied to create multiple lamellas on a sample wafer, and
therefore the steps in the process may be applied to multiple
locations, and the process for any particular lamella formation may
be interrupted to conduct similar steps at other locations. In some
versions, the SEM protective layers are deposited at all locations,
then the fiducials are formed at all locations, then the lamella
milling is conducted at all locations. In some scenarios, the
automated beam alignment steps discussed above may be sufficient to
keep the beam aligned for multiple locations. For example, aligning
the FIB beam position may be done once, and then fiducials created
at all desired locations. Or, the beam alignment procedure may be
done periodically for processing larger numbers of locations,
without requiring automatic beam alignment at every location.
[0032] FIG. 1C is a flowchart showing an alternative method of
calibrating the FIB beam from that found in FIG. 2B. The depicted
method begins at connector A from FIG. 2A. Before the fiducials can
be formed, the location shift from the SEM beam location to the FIB
location must be corrected so that the system controller tracks the
actual location of the FIB for deposition and milling. The process
of FIG. 1B performs such calibration by acquiring FIB image and
aligning it with a synthesized FIB image. In this version, the FIB
location is compared to the already-calibrated SEM beam location by
comparing an image acquired with each beam. An SEM image is
acquired at block 41, which may be done at the region of interest
or another suitable area nearby. Next at block 42, the process
acquires an FIB image of the same area. At block 43, the acquired
FIB image may optionally be processed to make its luminance
properties more similar to an SEM image for comparison. Next at
block 44, the process compares the two images to determine the
offset distance and direction between them. Next, at block 45, the
process applies this location correction offset to the FIB tracked
location. With the FIB now aligned, the process continues with the
fiducial creation and lamella milling as described with respect to
FIG. 1B steps 26-34.
[0033] FIG. 3 is a schematic diagram of a one embodiment of an
exemplary dual beam SEM/FIB system 302 that is equipped to carry
out a method according to the present invention. As discussed
above, embodiments of the present invention can be used in a wide
variety of applications. Suitable dual beam systems are
commercially available, for example, from FEI Company, Hillsboro,
Oreg., the assignee of the present application. While an example of
suitable hardware is provided below, the invention is not limited
to being implemented in any particular type of dual beam device.
The system controller 338 controls the operations of the various
parts of dual beam system 302. Through system controller 338, a
user can cause ion beam 352 or electron beam 316 to be scanned in a
desired manner through commands entered into a conventional user
interface (not shown). In the preferred embodiments herein, system
controller 338 controls dual beam system 302 to perform the
techniques discussed herein automatically in accordance with
programmed instructions, some of which may be issued by the process
controller 110, connected to network 130. CAD database 110 is also
operatively connected to dual beam system 302 and process
controller 110 over network 130. The CAD database 110 may be
provided on a system computer such as a Fab Host Controller 112
(FIG. 4), a dedicated CAD database computer, or the process
controller 110. What is important is the CAD database is available
to supply requested layouts to the system controller to conduct the
automated processes described above.
[0034] Dual beam system 302 has a vertically mounted electron beam
column 304 and a focused ion beam (FIB) column 306 mounted at an
angle of approximately 52 degrees from the vertical on an evacuable
specimen chamber 308. The specimen chamber may be evacuated by pump
system 309, which typically includes one or more, or a combination
of, a turbo-molecular pump, oil diffusion pumps, ion getter pumps,
scroll pumps, or other known pumping means.
[0035] The electron beam column 304 includes an electron source
310, such as a Schottky emitter or a cold field emitter, for
producing electrons, and electron-optical lenses 312 and 314
forming a finely focused beam of electrons 316. Electron source 310
is typically maintained at an electrical potential of between 500 V
and 30 kV above the electrical potential of a work piece 318, which
is typically maintained at ground potential.
[0036] Thus, electrons impact the work piece 318 at landing
energies of approximately 500 eV to 30 keV. A negative electrical
potential can be applied to the work piece to reduce the landing
energy of the electrons, which reduces the interaction volume of
the electrons with the work piece surface, thereby reducing the
size of the nucleation site. Work piece 318 may comprise, for
example, a semiconductor device, micro-electromechanical system
(MEMS), or a lithography mask. The impact point of the beam of
electrons 316 can be positioned on and scanned over the surface of
a work piece 318 by means of deflection coils 320. Operation of
lenses 312 and 314 and deflection coils 320 is controlled by
scanning electron microscope power supply and control unit 322.
Lenses and deflection unit may use electric fields, magnetic
fields, or a combination thereof.
[0037] Work piece 318 is on movable stage 324 within specimen
chamber 308. Stage 324 can preferably move in a horizontal plane (X
and Y axes) and vertically (Z axis) and can tilt approximately
sixty (60) degrees and rotate about the Z axis. A door 327 can be
opened for inserting work piece 318 onto X-Y-Z stage 324 and also
for servicing an internal gas supply reservoir (not shown), if one
is used. The door is interlocked so that it cannot be opened if
specimen chamber 308 is evacuated.
[0038] Mounted on the vacuum chamber are multiple gas injection
systems (GIS) 330 (two shown). Each GIS comprises a reservoir (not
shown) for holding the precursor or activation materials and a
needle 332 for directing the gas to the surface of the work piece.
Each GIS further comprises means 334 for regulating the supply of
precursor material to the work piece. In this example the
regulating means are depicted as an adjustable valve, but the
regulating means could also comprise, for example, a regulated
heater for heating the precursor material to control its vapor
pressure.
[0039] When the electrons in the electron beam 316 strike work
piece 318, secondary electrons, backscattered electrons, and Auger
electrons are emitted and can be detected to form an image or to
determine information about the work piece. Secondary electrons,
for example, are detected by secondary electron detector 336, such
as an Everhart-Thornley detector, or a semiconductor detector
device capable of detecting low energy electrons. STEM detector
362, located beneath the TEM sample holder 318 and the stage 324,
can collect electrons that are transmitted through a sample 318
mounted on the TEM sample holder 318. Signals from the detectors
336, 362 are provided to a system controller 338. Said controller
338 also controls the deflector signals, lenses, electron source,
GIS, stage and pump, and other items of the instrument. Monitor 340
is used to display user controls and an image of the work piece
using the signal
[0040] The chamber 308 is evacuated by pump system 309 under the
control of vacuum controller 341. The vacuum system provides within
chamber 308 a vacuum of approximately 3.times.10-6 mbar. When a
suitable precursor or activator gas is introduced onto the sample
surface, the chamber background pressure may rise, typically to
about 5.times.10-5 mbar.
[0041] Focused ion beam column 306 comprises an upper neck portion
344 within which are located an ion source 346 and a focusing
column 348 including extractor electrode 350 and an electrostatic
optical system including an objective lens 351. Ion source 346 may
comprise a liquid metal gallium ion source, a plasma ion source, a
liquid metal alloy source, or any other type of ion source. The
axis of focusing column 348 is tilted 52 degrees from the axis of
the electron column. An ion beam 352 passes from ion source 346
through focusing column 348 and between electrostatic deflectors
354 toward work piece 318.
[0042] FIB power supply and control unit 356 provides an electrical
potential at ion source 346. Ion source 346 is typically maintained
at an electrical potential of between 1 kV and 60 kV above the
electrical potential of the work piece, which is typically
maintained at ground potential. Thus, ions impact the work piece at
landing energies of approximately 1 keV to 60 keV. FIB power supply
and control unit 356 is coupled to deflection plates 354 which can
cause the ion beam to trace out a corresponding pattern on the
upper surface of work piece 318. In some systems, the deflection
plates are placed before the final lens, as is well known in the
art. Beam blanking electrodes (not shown) within ion beam focusing
column 348 cause ion beam 352 to impact onto blanking aperture (not
shown) instead of work piece 318 when a FIB power supply and
control unit 356 applies a blanking voltage to the blanking
electrode.
[0043] The ion source 346 typically provides a beam of singly
charged positive gallium ions that can be focused into a sub
one-tenth micrometer wide beam at work piece 318 for modifying the
work piece 318 by ion milling, enhanced etch, material deposition,
or for imaging the work piece 318.
[0044] A micromanipulator 357, such as the AutoProbe200.TM. from
Omniprobe, Inc., Dallas, Tex., or the Model MM3A from Kleindiek
Nanotechnik, Reutlingen, Germany, can precisely move objects within
the vacuum chamber. Micromanipulator 357 may comprise precision
electric motors 358 positioned outside the vacuum chamber to
provide X, Y, Z, and theta control of a portion 359 positioned
within the vacuum chamber. The micromanipulator 357 can be fitted
with different end effectors for manipulating small objects. In the
embodiments described herein, the end effector is a thin probe 360.
As is known in the prior art, a micromanipulator (or microprobe)
can be used to transfer a TEM sample (which has been freed from a
substrate, typically by an ion beam) to a TEM sample holder 318 for
analysis.
[0045] It should be noted that FIG. 3 is a schematic
representation, which does not include all the elements of a
typical dual beam system for the sake of simplicity, and which does
not reflect the actual appearance and size of, or the relationship
between, all the elements.
[0046] One suitable S/TEM sample management tool suite in which the
present invention may be employed is described in U.S. Pat. No.
8,890,064 to Arjavac et al., which is commonly owned by the
assignee of the present invention, and is hereby incorporated by
reference. The sample management suite generally includes a Process
Controller and a Fab Host computer operably connected to (or
integrated with) a dual-beam or FIB system for creating lamella, a
lamella extraction tool, and a S/TEM system for examining the
lamella. Integrating the present system with the prior art tool
suite involves installing the improved dual beam system as
described herein, with programming to provide the automated
workflow. The process controller may also be programmed to specify
defect locations and desired lamella sizes and orientations for
each defect or region of interest to be studied. The process host
or other machine may need programming adjustments so that it can
provide detailed CAD data responsive to requests from the dual beam
system controller at the various steps described herein. However,
some existing systems already include a CAD database operatively
connected to the network and able to respond to requests to provide
specified CAD data. The system controller or process controller may
further be programmed to synthesize FIB images as described
herein.
[0047] While the automated workflow processes described above focus
on the beam alignment and automated lamella creation, the recipe
for the defect review process may further include some or all of
the following parameters for a scanning electron microscope (SEM)
defect review tool as described in the incorporated U.S. Pat. No.
8,890,064: Wafer Rotation (if applicable); Wafer alignment points
(optical and electron beam); add/remove test dies; Wafer Tilt (if
applicable); SEM Column--Landing Energy; SEM Column--current; SEM
Column--extraction current; Automatic Defect Locator (ADL)
parameters such as video levels, focusing parameters, initial field
of view (FOV); FOV; Acquisition times or frames; Automatic Defect
Classification (ADC); Auto-focus; Charge Control; Contrast and
brightness settings (if applicable); and Defect sampling. Each of
these different parameters may be determined from the inspection
results as described above. Obviously, the parameters that are
included in the recipe may vary depending on, for example, the
configuration of the defect review tool. For instance, the above
listed parameters may be suitable for an electron beam-based defect
review tool, and a recipe for a different type of defect review
tool (e.g., a high resolution optical imaging system) may include a
different set of parameters.
[0048] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims. The combinations of features described herein
should not be interpreted to be limiting, and the features herein
may be used in any working combination or sub-combination according
to the invention. Further, the various novel workflow processes
herein may be employed to improve prior art workflows, such as the
processes described in the incorporated patent, and the description
should be interpreted as supporting such an incorporation where
fiducial location needs to be verified in a workflow, or where the
SEM or FIB beam alignment needs to be verified. This description
should therefore be interpreted as providing written support, under
U.S. patent law and any relevant foreign patent laws, for any
working combination or some sub-combination of the features
herein.
[0049] Moreover, the scope of the present application is not
intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *