Technical Field of the Invention
The present invention relates to preparation of samples
for transmission electron microscopes and scanning transmission electron microscopes.
Background of the Invention
Electron microscopy provides significantly higher resolution
and greater depth of focus than optical microscopy. 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.
In a transmission electron microscope (TEM), a broad beam
impacts the sample and electrons that are transmitted through the sample are focused
to form an image of the 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. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM),
a primary electron beam is focused to a fine spot, and the spot is scanned across
the sample surface. Electrons that are transmitted through the work piece are collected
by an electron detector on the far side of the sample, and the intensity of each
point on the image corresponds to the number of electrons collected as the primary
beam impacts a corresponding point on the surface. Here as well the sample must
be sufficiently thin to allow at least a part of the electrons in the primary beam
to travel though the sample and exit on the opposite site.
There are several methods for preparing a thin sample for
viewing with a TEM or STEM. Some methods entail extracting a sample without destroying
the entire substrate from which the sample is extracted. Other methods require destroying
the substrate to extract the sample. One method, described by
Anderson et al. in "Combined Tripod Polishing and FIB Method for Preparing
Semiconductor Plan View Specimens," Materials Research Society Proceedings, Vol.
480, pp. 187-192 (1997)
, cuts a thin strip for the substrate using a diamond saw, mechanically
polishes the sample to a specified thickness, and then further thins the sample
using a focused ion beam. Another method, described in
E.C.G. Kirk et al., "Cross-Sectional Transmission Electron Microscopy of Precisely
Selected Regions from Semiconductor Devices," Inst. Phys. Conf. Ser. No. 100, Section
) entails cutting a portion from a substrate using a diamond saw and then
using a focused ion beam to produce a thin sample on a part of a substrate portion.
U.S. Pat. No. 6,841,788 to Robinson
describes using a femtosecond laser to cut through a semiconductor wafer
to free a plug or block as thick as the wafer, that is, about 750 µm thick.
A thin sample suitable for TEM or STEM viewing is formed in the top of the block.
Laser drilled guide holes are cut in the block and used to pick it up. Robinson
teaches that by removing the block, one avoids the removal of a "fragile member,"
that is, the thin sample viewable in a TEM. Although the method of Robinson does
not destroy the entire wafer, the hole in the wafer after the plug is removed renders
it unsuitable for further processing, because the hole will harbor contaminants.
One method that allows a sample to be extracted without
destroying the substrate described in
US Pat. No. 5,270,552 to Ohnishi et al.
, which describes using a focused ion beam to free a sample from a substrate
and to weld a probe to the sample using ion beam deposition to transport the sample.
Herlinger et al., "TEM Sample Preparation Using a Focused lon Beam and a Probe
Manipulator," Proceedings of the 22nd International Symposium for Testing and Failure
Analysis, p. 199-205 (1996
) describes using a focused ion beam to free a sample from a substrate,
removing the sample from the vacuum chamber, and then moving the sample to a TEM
sample holder using a probe to which the sample adheres by electrostatic attraction.
Another method include the use of tweezer-like gripper to grasp the sample. All
these methods are slow and time consuming.
In some extraction methods, such as that described by Ohnishi
et al., the extracted sample is a "chunk" that needs to be thinned extensively before
it can be viewed on a TEM or STEM. In other embodiment, such as that described in
Herlinger et al., the extracted sample is a thin lamella, that requires only minor
finishing before TEM viewing.
Summary of the Invention
An object of the invention is to provide simple and robust
methods to extract TEM or STEM samples from substrates. The samples can then be
processed on the probe or placed onto sample holders for a TEM or STEM within a
vacuum chamber or outside a vacuum chamber.
In one embodiment, a hole is drilled in the sample using
a focused ion beam. A probe is inserted into the hole, and the sample remains attached
to the probe for transport, processing, or both. Some embodiments allow knowledge
of the orientation of the sample with respect to the substrate to be preserved after
the sample is removed, so that the sample can be viewed at an appropriate angle
or further processed
In another embodiment, a sample is removed from a substrate
within a vacuum chamber by electrical attraction to a probe, and the sample is placed
into a TEM or STEM sample holder in the vacuum chamber, thereby eliminating need
to weld the sample to the probe or to the sample holder, and eliminating the requirement
to remove the sample from the vacuum chamber for placement in a sample holder.
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 embodiment 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
For a more thorough understanding of the present invention,
and advantages thereof, reference is now made to the following descriptions taken
in conjunction with the accompanying drawings, in which:
Detailed Description of Preferred Embodiments
- FIG. 1 shows a preferred focused ion beam system for implementing the present
- FIG. 2 is a flow chart showing a first embodiment of the present invention,
- FIG. 3 shows a top view of a substrate with a sample area indicated by a dashed
- FIG. 4 shows a top view of a substrate with a sample freed from the substrate,
- FIG. 5 shows a cross-sectional view of the substrate and sample of FIG. 4,
- FIG. 6 shows the sample of FIG. 5 with a probe inserted,
- FIG. 7 shows a top of view of a substrate with two rectangular cuts,
- FIG. 8 shows a cross-sectional view of the substrate of FIG. 7 with the sample
- FIG. 9 shows a top view of the sample of FIG. 8 with the sample freed,
- FIG. 10 is a flow chart showing another embodiment of the present invention,
- FIG. 11 shows a probe approaching a freed sample,
- FIG. 12 shows the probe in contact with the sample of FIG. 11,
- FIG. 13 shows the sample attached to the probe moving from the substrate surface,
- FIG. 14 shows the sample approaching a TEM sample holder,
- FIG. 15 shows the sample contacting the TEM sample holder, and
- FIG. 16 shows the sample separated from the probe and remaining in the TEM sample
This disclosure relates to novel methods to remove small
chunks of material from a substrate. This method could be used for an ex-situ or
in-situ lift out of chunks or lamella.
FIG. 1 shows a typical ion beam system, focused ion beam
(FIB) system 10, suitable for practicing the present invention. FIB system 10 includes
an evacuated envelope 11 having an upper neck portion 12 within which are located
a liquid metal ion source 14 and a focusing column 16 including extractor electrodes
and an electrostatic optical system. Other types of ion sources, such as multicusp
or other plasma sources, and other optical columns, such as shaped beam columns,
could also be used, as well as electron beam and laser system.
An ion beam 18 passes from liquid metal ion source 14 through
ion beam focusing column 16 and between electrostatic deflection means schematically
indicated at deflection plates 20 toward sample 22, which comprises, for example,
a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26.
A system controller 19 controls the operations of the various parts of FIB system
10. Through system controller 19, a user can control ion beam 18 to be scanned in
a desired manner through commands entered into a conventional user interface (not
shown). Alternatively, system controller 19 may control FIB system 10 in accordance
with programmed instructions.
For example, a user can delineate a region of interest
on a display screen using a pointing device, and then the system could automatically
perform the steps described below to extract a sample. In some embodiments, FIB
system 10 incorporates image recognition software, such as software commercially
available from Cognex Corporation, Natick, Massachusetts, to automatically identify
regions of interest, and then the system can manually or automatically extract samples
in accordance with the invention. For example, the system could automatically locate
similar features on semiconductor wafers including multiple devices, and take samples
of those features on different (or the same) devices.
An ion pump 28 is employed for evacuating upper neck portion
12. The lower chamber 26 is evacuated with turbomolecular and mechanical pumping
system 30 under the control of vacuum controller 32. The vacuum system provides
within lower chamber 26 a vacuum of between approximately 1 x 10-7 Torr
(1.3 x 10-7 mbar) and 5 x 10-4 Torr (6.7 x 10-4
mbar). If an etch-assisting gas, an etch-retarding gas, or a deposition precursor
gas is used, the chamber background pressure may rise, typically to about 1 x 10-5
Torr (1.3 x 10-5 mbar).
High voltage power supply 34 is connected to liquid metal
ion source 14 as well as to appropriate electrodes in ion beam focusing column 16
for forming an approximately 1 keV to 60 keV ion beam 18 and directing the same
toward a sample. Deflection controller and amplifier 36, operated in accordance
with a prescribed pattern provided by pattern generator 38, is coupled to deflection
plates 20 whereby ion beam 18 may be controlled manually or automatically to trace
out a corresponding pattern on the upper surface of sample 22. In some systems the
deflection plates are placed before the final lens, as is well known in the art.
Beam blanking electrodes (no shown) within ion beam focusing column 16 cause ion
beam 18 to impact onto blanking aperture (not shown) instead of target 22 when a
blanking controller (not shown) applies a blanking voltage to the blanking electrode.
The liquid metal ion source 14 typically provides a metal
ion beam of gallium. The source typically is capable of being focused into a sub
one-tenth micrometer wide beam at sample 22 for either modifying the sample 22 by
ion milling, enhanced etch, material deposition, or for the purpose of imaging the
sample 22. A charged particle detector 40, such as an Everhart Thornley or multi-channel
plate, used for detecting secondary ion or electron emission is connected to a video
circuit 42 that supplies drive signals to video monitor 44 and receiving deflection
signals from controller 19.
The location of charged particle detector 40 within lower
chamber 26 can vary in different embodiments. For example, a charged particle detector
40 can be coaxial with the ion beam and include a hole for allowing the ion beam
to pass. In other embodiments, secondary particles can be collected through a final
lens and then diverted off axis for collection. A scanning electron microscope (SEM)
41, along with its power supply and controls 45, are optionally provided with the
FIB system 10.
A gas delivery system 46 extends into lower chamber 26
for introducing and directing a gaseous vapor toward sample 22.
U.S. Pat. No. 5,851,413 to Casella et al.
for "Gas Delivery Systems For Particle Beam Processing," assigned to the
assignee of the present invention, describes a suitable gas delivery system 46.
Another gas delivery system is described in
U.S. Pat. No. 5,435,850 to Rasmussen
for a "Gas Injection System," also assigned to the assignee of the present
invention. For example, iodine can be delivered to enhance etching, or a metal organic
compound can be delivered to deposit a metal.
A micromanipulator 47, such as the AutoProbe 200™
from Omniprobe, Inc., Dallas Texas, or the Model MM3A from Kleindiek Nanotechnik,
Reutlingen, Germany, can precisely move objects within the vacuum chamber. Micromanipulator
47 may comprise precision electric motors 48 positioned outside the vacuum chamber
to provide X, Y, Z, and theta control of a portion 49 positioned within the vacuum
chamber. The micromanipulator 47 can be fitted with different end effectors for
manipulating small objects. In the embodiments described below, the end effector
is a thin probe 50 having a tapered end. The thin probe 50 may be electrically connected
to system controller 19 to apply an electric charge to the probe 50 to control the
attraction between a sample and the probe.
A door 60 is opened for inserting sample 22 onto X-Y stage
24, which may be heated or cooled, and also for servicing an internal gas supply
reservoir, if one is used. The door is interlocked so that it cannot be opened if
the system is under vacuum. The high voltage power supply provides an appropriate
acceleration voltage to electrodes in ion beam focusing column 16 for energizing
and focusing ion beam 18. When it strikes sample 22, material is sputtered, that
is physically ejected, from the sample. Alternatively, ion beam 18 can decompose
a precursor gas to deposit a material. Focused ion beam systems are commercially
available, for example, from FEI Company, Hillsboro, Oregon, the assignee of the
present application. While an example of suitable hardware is provided above, the
invention is not limited to being implemented in any particular type of hardware.
FIG. 2 describes the steps of a first embodiment of the
invention. In step 202, the outline of a sample to be extracted from a substrate
is determined. For example, the substrate may be a semiconductor wafer or portion
thereof and the portion to be extracted may include a portion of an integrated circuit
that is to be observed using a TEM. FIG. 3 shows a portion of a substrate 302 with
a sample area 304 indicated by a dashed line. In step 204, a hole is milled in the
substrate within the portion that will be extracted as a sample. FIG. 3 shows a
hole 306 in sample area 304. The hole should be positioned so that it does not interfere
with the portion of the sample to be observed, for example, if a particular cross
section of an integrated circuit is to be the target of observation, the hole should
not damage that area of the circuit.
The hole 306 is preferably sufficiently deep so that when
a probe is inserted into the hole, the sample adheres to a probe when the probe
is withdrawn from the substrate. In most embodiments, the hole preferably does not
extend through the sample. The depth, diameter, and orientation of the hole will
vary, therefore, depending on the type of sample to be extracted. For a sample of
an integrated circuit being extracted for viewing on a TEM, the hole is typically
about 1 or 2 microns in diameter, about 3 µm deep, and may be oriented at a
non-normal angle to the sample surface. The walls of the hole will typically have
a slight taper as a natural result of the focused ion beam milling process. A desired
taper can also be produced by controlling the FIB beam path, for example, by controlling
the ion dose so that the portion of the hole near the outer circumference receives
less ions than the portion of the hole near its center, so the hole is deeper near
In step 206, sample 304 is partially freed from a substrate
using a focused ion beam. The sample can be, for example, a "chunk," that requires
shaping and thinning before viewing with an electron microscope, or the sample could
be, for example, a thin lamella, which requires little or no processing before viewing
with an electron microscope. FIGS. 4 and 5 shows a "chunk" sample 304 freed from
substrate 302. Sample 304 can be freed, for example, as described by
U.S. Pat. No. 6,570,170 to Moore
by directing a focused ion beam from two directions, or as shown in
U.S. Pat. No. 5,270,552 to Ohnishi et al.
The sample is freed, for example, by undercutting the sample from opposite
directions to form two planes that intersect each other and the surface, and then
milling the sides of the sample to free it.
FIG. 5 is a cross sectional view of sample 304 in substrate
302 after sample 304 has been freed. FIG 5 shows the angle 502 of the hole 306 with
respect to the surface of sample 304. Angle 502 allows the probe to be inserted
into the hole in a crowded FIB vacuum chamber, where the probe may not have sufficient
clearance for vertical insertion. Inserting the probe at an angle may also provide
some additional friction between the probe and the sample, to help keep the sample
on the probe as the probe is moved. In optional step 208, the sample is removed
from the vacuum chamber of the focused ion beam. In step 210, a probe tip is inserted
into the hole 306. FIG. 6 shows a sample 304 with a probe 602 whose tip 604 inserted
into the hole 306. Probe tip 604 has a taper that is about the same as or slightly
steeper than the taper of hole 306 to ensure sufficiently close contact between
probe tip 604 and the interior of hole 306. To maximize contact between the sides
of hole 306 and probe tip 604, probe tip 604 preferably does not contact the bottom
of hole 306. The taper of probe tip 604 can be created for example, by milling with
the focused ion beam. While a round hole 306 and probe tip 604 are shown, other
mating shapes can be used for the hole and probe. For example, the hole could have
two parallel walls and two tapering walls to form a wedge-shaped point. Mechanical
friction and/or electrostatic forces to keep the chunk adhered to the probe tip.
Additionally a direct current, alternating current, or radio frequency signal can
be applied to the probe to better adhere the tip to substrate.
FIGS. 7, 8, and 9 show an embodiment of the invention in
which a lamella is cut from the substrate 700, for example, using the method described
by Herlinger et al. The ion beam cuts a small rectangular trench 702 on one side
of the lamella 704 and a wider rectangular trench 706 on the opposite side of the
lamella. The lamella is formed of the material left between the rectangles. A hole
708, similar to hole 306 described above, is milled in the top of the lamella 704.
The substrate or the beam is then tilted and the lamella is either entirely or partially
freed. The lamella can be partially freed by cutting with the ion beam along most
of its perimeter, but leaving tabs attaching the lamella to the substrate on either
side at the top. FIG. 8 shows a cross sectional view of a partially freed lamella
704 that remains attached to the substrate by tabs 810. The probe can be inserted
into hole 708, and then the tabs 810 can be cut using the focused ion beam. In some
embodiments, if the tabs are sufficiently thin and the contact between the probe
and the hole sufficiently strong. The tabs can be snapped off mechanically by motion
of the probe with the sample attached. Alternatively, the lamella can be entirely
freed by cutting along its entire perimeter, without leaving tabs. FIG. 9 shows
an embodiment in which the lamella is completely freed from the substrate and remains
in trench 702 or trench 706. The probe can be inserted either before or after the
lamella is freed.
The ion beam can be used with or without an etch-enhancing
gas. A focused beam or a shaped beam can be used. The order of the steps can be
varied without departing from the scope of the invention. For example, the probe
can be attached to the sample before or after the sample is freed from the substrate.
The probe can be attached to the sample either in the vacuum chamber or outside
of the vacuum chamber. While attached to the probe, the sample can be further processed
by the focused ion beam, for example, for thinning. While attached to the probe,
the sample could also be viewed in an electron microscope, such as an SEM, TEM,
or STEM. Because the hole is milled into the sample at a known angle with respect
to the substrate surface (which may be 90°), the orientation of the separated
sample with respect to the substrate surface before separate can be readily determined.
The sample can therefore be readily maintained in a preferred orientation for further
processing or viewing.
Alternatively, the sample can be removed from the probe
and placed on a TEM sample holder. The sample can be freed from the probe, for example,
by neutralizing static charge on the probe to eliminate attraction between the probe
and the sample, so that the sample slips from the probe. In some embodiments, a
static charge can be provided on the TEM sample holder to attract the sample to
the sample holder to provide an additional force to urge the sample from the probe.
In other embodiments, the sample can be adhered to the TEM sample holder by ion
beam deposition or by an adhesive before or after the probe is removed from the
sample. Alternatively, a portion of the probe can remain attached to the sample
as described in
U.S. Pat. App. Pub. No. 20040251412 of Tappel
, which application is assigned to the assignee of the present application.
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. 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.