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Materials Specimen Preparation for Transmission
Electron Microscopy
PAUL E. FISCHIONE
E.A. Fischione Instruments, Inc. Export, PA USA
Abstract
To successfully prepare many of today’s electronic materials,
often non-traditional TEM specimen preparation techniques
need to be employed. Current challenges involve optimizing
analytical results from a single specimen, given that the
specimen may be comprised of materials which require
mutually exclusive preparation techniques. Advancements in
electropolishing, ultrasonic disk cutting, dimpling, and ion
milling techniques and instrumentation will be discussed.
Consideration will be given to the preparation of planar
specimens, metal-matrix composites, cross-sectional interfaces
from layered materials, blanket substrate layers for electronic
devices, and high-Tc superconductors.
SPECIMEN PREPARATION is an important aspect of the
transmission electron microscopy (TEM) analysis of electronic
materials. The recent trend in electronic devices is to utilize
more exotic substrate materials, a greater quantity of materials
for the current carrying devices, and a smaller spacing between
individual circuit components. Maximizing the quantity of
individual devices on an integrated circuit and the electron flow
through the devices results in greater processing speed. The
research detailed herein exemplifies the effort that is being
expended to enhance both specimen preparation procedures and
TEM performance.
Specimen preparation techniques are very material
dependent, therefore, it is important to initially select the
technique that is most beneficial for the individual specimen.
[l] Electropolishing is a time-proven technology for the
preparation of most metallic specimens and is also applicable
to preparing high-Tc superconductors.
For most electronic materials, a common sequence of
preparation techniques is ultrasonic disk cutting, dimpling, and
ion-milling. [2] Significant developments in disk cutting,
dimpling, and ion milling technology are presented herein. A
critical aspect of preparing a specimen from the bulk. state
. to a .
3 mm disk is the ability to rapidly capture the specific area of
interest and to preserve it in an unaltered state. When
analyzing a failed device in an electronic circuit, it is important
to position the specific area of interest in the center of the 3
mm diameter TEM specimen disk. In addition, for composite
materials, when the analysis of an interface is required, it is
beneficial to obtain a disk whereby an interface is situated near
the disk’s center.
Dimpling is a preparation technique that produces a
specimen with a thinned central area and an outer rim of
sufficient thickness to permit ease of handling. [3] This
specimen configuration is achieved by the simultaneous
rotation of both the specimen and a grinding wheel containing
an abrasive slurry (typically diamond) whose axes are
orthogonal and intersecting. Advancements in process control r
have greatly increased the capabilities and performance of the
dimpling process.
Ion milling is traditionally the final form of specimen
preparation. In this process, charged argon ions are accelerated
to the specimen surface by the application of high voltage. The
ion impingement upon the specimen surface removes material
as a result of momentum transfer.
Electropolishing
Since the mid-1960’s, electropolishing has been
successfully utilized for the preparation of metal specimens. [4]
In electropolishing, an acid electrolyte is directed at the
specimen. Electrolyte flows from nozzles containing the
cathode connections to the specimen that serves as the anode.
For peak efficiency, two jets are used for the simultaneous flow
of electrolyte at both specimen surfaces. This eliminates the
manual inversion of the specimen as is required in single-jet
polishing techniques. A typical twin-jet electropolishing
system is shown in Figure 1 and consists of the electrolytic
polishing cell and a separate power control.
Figure 1
Twin-Jet Electropolisher
Electropolishing is accomplished through the formation of
an anodic film of electrolyte on the specimen surface. Through
an electro-chemical reaction, this film removes material from
the specimen’s surface. Due to the current distribution within
the specimen, electropolishing results in an overall smoothing
of the specimen surface which is caused by higher current
densities associated with the peaks of the specimen when
compared to the valleys.
Electrolyte flow is adjustable to develop a proper anodic
film layer. Insufficient flow will result in specimen
contamination due to the redeposition of material onto the
specimen surface. Excessive flow prevents the formation of the
anodic film and will result in the specimen being etched rather
than polished, and can also distort the specimen’s thin area
once perforation occurs.
Voltage adjustment is required to establish the proper
polishing condition. Figure 2 depicts the correlation between
voltage and current and its applicability to electrolytic thinning.
to between -40°C and -60°C has been found to be highly
beneficial because it slows the electropolishing process. This
minimizes the heat of reaction on the specimen and prevents
vaporization of the electrolyte, which is of particular
importance when utilizing electrolytes containing perchloric
acid that can be volatile.
A light source and photo-electric detector are utilized to
determine the precise time when a perforation occurs. Light is
directed at the specimen by a fiber optic bundle. As the
electrolytic action produces a perforation in the specimen, a
second fiber optic bundle directs the light transmitted through
the specimen to a photo detector. The detector level is
adjustable for sensing varying hole sizes.
Figure 3 is a micrograph of Superalloy 718. This highnickel content material has a complex microstructure. Coarse
and fine gamma double prime precipitates are found throughout
the matrix. Preferential precipitation of delta precipitates
occurs at the grain boundaries. A gamma-double-primeprecipitate-free zone also exists at the grain boundary. This
specimen illustrates successful electropolishing without
preferential thinning of specimen constituents.
[5] -
Figure 3
Superalloy 718
Although electropolishing has been utilized primarily for
metals, it is also applicable to high-Tc superconductors. Figure
4 depicts the orientation of “twin” crystals in YBa2Cu307 that
may play a role in the superconducting properties. Also shown
is a dislocation that rarely occurs in superconducting materials.
Figure 2
Voltage Current Distribution
In the lower portion of the curve, insufficient voltage
results in chemical etching which produces an uneven surface
due to preferential etching rates associated with varying grains,
phases, or dissimilar materials within the specimen. Excessive
voltage causes electrolysis of the aqueous solution which makes
bubbles on the specimen surface. The bubbles mask the
surface locally and result in pitting.
Electropolishing is very electrolyte dependent; therefore, it
is important to select the proper electrolyte for the particular
specimen. In addition, cooling the electrolyte and the specimen
Figure 4
YBa2Cu307
Advances in Ultrasonic Disk Cutters
Ultrasonic disk cutting is an extremely important
technique used to obtain the initial 3 mm disk from many types
of electronic materials. [2] These ceramic and semiconductor
materials are typically hard and brittle and result in rather long
cutting times when compared to rotary cutting methods.
Typical cutting times using ultrasonic cutting methods are as
follows:
Silicon
9 microns/sec
Sapphire 1 micron/sec
Glass
12 microns/sec
PZT
15 microns/sec
LiNbO 3 8 microns/sec
10 microns/sec
Cutting is produced by an ultrasonic transducer that
provides axial motion of the cutting tool. The transducer
contains lead-zirconate-titanate (PZT) piezoelectric crystals
which expand and contract at 26 kHz by the application of an
oscillating electrical voltage potential. Cutting is achieved
through the presentation of an abrasive slurry, either silicon
carbide or boron nitride, to the contact point of the cutting tool
and the specimen material. The cutting tool is typically a
hollow cylinder with a 3 mm inside diameter and a thin wall,
thus producing an appropriately sized TEM disk specimen.
Various diameter tools can be used. In addition, rectangular
tools (4 mm x 5 mm or 2 mm x 3 mm) enable the coring of
wafers that can be subsequently bonded together when
preparing cross-sectional (XTEM.) specimens.
In order to optimize disk cutter performance, some desirable
characteristics are as follows:
Minimizing Edge and Surface Damage. Due to the
aggressive nature of the ultrasonic cutting process, edge
damage and surface deformation can result. In order to
minimize damage, it is necessary to optimize the performance
of the ultrasonic transducer. This required alterations to both
the physical configuration and electrical input of previous
designs. The Si disk shown in Figure 5 demonstrates the
effects of cutting with an optimized transducer.
Figure 5
Si Disk cut with Optimized Tool Excursion
Specimen
e is also a function of the lateral
movement of the specimen in relation to the cutting tool. This
has been alleviated by rigidly attaching the specimen container
to the force-applying stage using four (4) Samarium/Cobalt
rare earth magnets. Sample material is glued onto an aluminum
slide with a low melting point polymer. The aluminum slide is
securely held into the specimen container by two locking
screws.
The Ability to Cut Through Thick Material.
Typical ultrasonic cutting instruments used in TEM specimen
preparation advance the specimen material into the cutting tool
by means of a cantilever stage. With the specimen attached to
the cantilever stage at a fixed distance from the stage pivot
point, the path of the specimen in relation to the cutting tool is
circular. Two negative aspects exist in this configuration. [6]
1. The specimen has a non-circular cross-section.
2. Sufficient lateral force is exerted on the cutting tool to
stall the transducer.
For this research, the sample stage shown in Figure 6 was
designed to advance the material parallel to the cutting tool.
This factor enables cylindrical rods to be cored, even from thick
(10 mm) bulk materials. The cylindrical rods can subsequently
be sliced into 3 mm disks.
TANIUM CUTTING TOOL
GROUND POTENTIAL)
SPECINEN
CONTAINER
TRANSLATION (0.8)
vdc)
Figure 6
Parallel Motion Sample Stage
Precise Specimen Feature Positioning. A critical
aspect of obtaining the initial disk specimen from electronic
materials, and in particular failed devices within an integrated
circuit, is having the ability to obtain the disk from a precisely
selected, specific area of interest. To accomplish this, the
ultrasonic cutting tool head is mounted to a vertical post with a
mechanism that permits its rotation. Mounted perpendicular to
the cutting tool is an optical microscope that is used for both
bulk material positioning and specimen observation. For
repeatable specimen material cutting, the microscope is rotated
over the bulk material and locked in place as shown in Figure
7. To ensure precise angular positioning and repeatability, a
locking mechanism was developed to provide repeatability of
better than 0.01 mm. A specific area of the specimen is
selected by moving the specimen material in relation to the
microscope until the desired feature is centered in the
microscope’s reticle. Rotating the ultrasonic cutting tool head
into the cutting position as shown in Figure 8, and initiating the
cutting process, produces the specimen disk from the predetermined location on the bulk material.
and ultrasonic transducer assembly are maintained at ground
potential.
Specimen material is glued to a thin aluminum plate using
a low melting point polymer as shown in Figure 9. This
provides a conductive surface onto which the specimen is
attached. The plate is in electrical contact with the specimen
container which is both magnetically and electrically coupled to
the force-applying stage that carries the 0.8 vdc signal.
When preparing conductive materials, an aluminum
specimen plate containing a thin layer of epoxy paint was
utilized. The non-conductive epoxy coating prevents continuity
between the specimen and the specimen plate.
Once cutting is initiated, the process will continue until the
tool penetrates through the specimen material and contacts the
aluminum plate. At that time, electrical continuity is detected
as a result of current flow in the sensing circuitry and the
process is automatically terminated. To ensure that the
specimen has been completely cut, an override switch defeats
the termination circuitry and allows cutting to be continued.
Figure 7
Alignment Microscope
Position
Figure 9
Mounted Specimen Material
A Preparation Process for XTEM
Specimens
Figure 8
Ultrasonic Cutting
Head Position
Automatic Process Termination. A crucial aspect of
ultrasonic cutting is to determine process completion. To sense
termination, an electrical continuity detector was developed.
In the instrument configuration shown in Figure 6, an +0.8 vdc
signal is placed on the specimen stage, while the cutting tool
To successfully prepare specimens from thin film or
electronic materials, it is necessary to prepare cross-sectional
interface (XTEM) specimens. This process affords specific
feature or interface analysis.
To prepare an XTEM specimen, the desired area of interest
is initially obtained from the bulk material. With a 4 mm x 5
mm rectangular cutting tool installed onto the Ultrasonic Disk
Cutter, a series of rectangular wafers are produced. These
wafers are then inserted into a Teflon chuck jaw contained in a
spring-loaded vise assembly as shown in Figure 10. The
wafers are subsequently bonded together using a specialized
epoxy. This epoxy is vacuum compatible and cures at relatively
low temperature (60°C), and does not erode as rapidly as other
types of epoxy when subjected to energetic ion beams. When
preparing XTEM specimens, it is highly recommended to
produce blank specimens from Si. In this manner, the silicon
serves as a thickness indicator, changing color in the presence
of transmitted light as the specimen thins.
TEFLON CWCK
/
XTEH
STACK
Figure 10
XTEM Clamping Vise
Following the curing of the epoxy, the wafer stack is
removed from the Teflon chuck and transferred to the
Ultrasonic Disk Cutter. A cylindrical core with a diameter of
either 3.0 mm or 2.3 mm and having a length of 4 mm is then
obtained.
For very fragile specimens, it is necessary to provide
support for the XTEM materials rather than relying solely on
the epoxy for mechanical integrity. In this case, a 2.3 mm
cylinder is produced, then is glued into a thin-wall brass tube
having an outside diameter of 3.0 mm using F-l epoxy as
shown in Figure 11.
Following curing, the cylinder is then cut into 500 micron
thick disks using a low speed diamond cut-off saw. For
specimens having a high degree of structural integrity, a 3 mm
diameter cylinder can be produced, then directly cut into disk
form using low speed diamond sawing techniques.
After acquiring the 3 mm diameter disk, subsequent planar
polishing, dimpling, and ion beam thinning techniques as
described in the following sections are readily employed.
Figure 11
XTEM Disk Preparation
Advances in Precision Dimpling
Grinders
For the successful analysis of electronic materials,
especially those comprised of materials of varying hardness, it
is important to mechanically pre-thin the specimen prior to ion
beam milling. Pre-thinning greatly increases the uniformity of
the electron transparent area following ion milling, which is
particularly important when analyzing specimens comprised of
materials with radically dissimilar milling rates. [7]
Prior to this research two significant disadvantages were
associated with the dimpling process. [2]
1. The reliance on the amount of applied force to
determine the rate of specimen material removal. This
results in uneven wheel wear and excessive wheel
vibration.
2. A minimal amount of electron transparent area that
exists at the bottom of the dimple, which is a function of
the diameter of the grinding wheel.
These issues were addressed and additional features such as
a microscope attachment and specimen holder with a sliding
magnetic base to facilitate positioning, back-lighting of the
specimen for color change observation in Si specimens, and
automatic process termination were incorporated into the
instrument shown in Figure 12.
Figure 12
Ultra-precision Dimple Grinder
Rate Control Stage. To improve specimen thickness
measurement accuracy when compared to previously existing
methods, an electro-mechanical stage as shown in Figure 13
was developed. [8] This stage utilizes a microprocessor
controlled stepper motor with a system resolution of 37 nm per
step of the motor An
. algorithm was developed to equate the
number of steps to specimen thickness by providing a precise
indication of wheel height and, therefore, specimen thickness.
This technique yields grinding rates as low as 0. lµm/min and a
total system accuracy of one micron in thickness. In addition,
by maintaining precise process control, incremental grinding
(removing one micron at a time) was made possible.
GRINDING WHEEL
GRINDING WHEEL
significant increase in transparent area due to simultaneous
oscillation and rotation.
ELCCli?IC& PIN
LIMM IUJVWENT
MCHANISM
STEPPER MITChI/
GEAR k?mJcTlaJ
Figure 13
Rate Control Stage
During dimpling, the abrasive media gradually decreases
the grinding wheel diameter. When a wheel wears unevenly,
the level of vibration increases and subsequent specimen
deformation or fracture occurs. In the dimple grinders that use
weight to control the grinding rate, uneven wheel wear is a
common problem. Because the electro-mechanical stage shown
in Figure 13 physically supports the grinding wheel stage and
advances it into the material at the programmed rate, wheel
vibration and, therefore, specimen deformation are
eliminated. In fact, the grinding wheel tends to become more
concentric as the grinding progresses in this type of process.
Precise process control enabled the GaInP/InGaAsP/InGaAs
specimen shown in Figure 14 to be dimpled to near electron
transparency and necessitated an ion-beam milling duration of
less than 10 minutes.
Figure 15
Dimple w/o Specimen Oscillation
Figure 16
Dimple with 300 um Oscillation
Figure 14
GaInP/InGaAsP/InGaAs specimen
Specimen Stage. To increase the amount of thin area, a
stage which holds the specimen and produces a simultaneous
oscillation and rotation was developed. The length of
oscillation is user programmable and corresponds to the size of
the flat bottom on the dimple. Examples of the thin area are
demonstrated in Figures 15 and 16 and clearly indicate a
Specimen Preparation. Prior to dimpling, it is
desirable to reduce the specimen thickness to approximately 75
to 100 microns .This is achieved through the use of classic
metallographic polishing techniques utilizing a precision,
planar specimen grinder as shown in Figure 17. This device
allows the rapid removal of large amounts of material in a
controlled manner while inducing minimal specimen damage.
Figure 17
Planar Polisher
Polishing is conducted on a wet-type of rotary
grinding/polishing wheel. The abrasive size is progressively
decreased from 600 grit to 0.5 micron. Final polishing is
achieved with Syton on a cloth pad. It is important to
remember that this surface serves as one side of the final TEM
specimen; therefore, preparation induced artifacts must be
minimized. Following planar polishing, the specimen is demounted, turned over, and reattached to the platen with a low
melting point polymer. The platen fits into a magnetic base,
coupled to the specimen stage with a rare-earth magnet. To
select a specific area on the specimen surface to be dimpled,
the magnetic base is easily slid into the appropriate position.
This process is conducted while the specimen is being
observed through an optical microscope as depicted in Figure
18.
The platen contains a glass center portion that permits light
to be transmitted from a source located beneath the specimen
stage. This feature is particularly important when preparing Si
specimens that experience a change in the color of transmitted
light as the specimen thickness is reduced to less than 5
microns.
the grinding rate is reduced to 1 .O micron/minute; the grinding
force is reduced to 20 grams, and the diamond abrasive is
changed to a one micron grit size. At a specimen thickness of
10 microns, the grinding rate is reduced to 0.5 microns/minute;
the grinding force is reduced to 10 grams and the diamond
abrasive is changed to a 0.25 micron grit size. Final polishing is
accomplished with a grinding force of 5 grams and a grinding
rate of 0.2 um/min. Syton is used as the final abrasive
medium.
Through the incremental reduction of abrasive grit size,
grinding force, and grinding rate, specimens are prepared
without inducing artifacts as demonstrated by the integrated
circuit shown in Figures 19 and 20. This material was dimpled
to approximately 3 microns in thickness.
Figure 19
Integrated Circuit XTEM Specimen
Figure 20 is the XTEM specimen showing the aluminum
layer between two layers of tungsten. It is important to note
that the Al contained in this structure is free of any
mechanically induced deformation.
Figure 18
Specimen Mount
For optimum results, a progressive sequence of operational
parameters is employed. Initially, an aggressive grinding rate
of 4.0 microns/minute with a force of 30 grams is used. Three
micron diamond abrasive compound serves as the grinding
medium. As the specimen thickness approaches 25 microns,
Figure 20
XTEM of W/Al in an Integrated Circuit
Ion Milling
For many types of electronic materials, ion beam milling is
an excellent final preparation technique for TEM specimens.
[5] To meet the changing needs of TEM specimen preparation,
a versatile, compact, table-top, precision ion milling/polishing
system was designed to consistently produce high-quality TEM
specimens with large electron transparent areas from a wide
range of materials. The system as shown in Figure 21 was
designed to be fully programmable for ease of use. Features
include two independently adjustable Hollow Anode Discharge
(HAD) ion sources for either rapid milling or more gradual
specimen polishing, automatic gas control, a turbo-pumped
vacuum system, a milling angle range of 0 o to 45o, specimen
rotation or rocking, a liquid nitrogen cooled specimen stage,
automatic termination, and chemically-assisted etching.
Figure 2 1
Variable Energy/Angle Ion Mill
Ion Source. For this research, a HAD ion source was
developed. [9] A schematic representation is shown in Figure
22. This type of ion source was chosen for its minimal gas
consumption, large range of ion beam current density, long
component life, and overall ionization efficiency. HAD ion
sources provide an ion accelerating voltage range of 0.5 kv to
6.0 kv and an internal plasma current range of 3 mA to 10 mA.
Ion beam current densities of up to 400 microamps/cm2 are
readily produced. Due to the focusing effect of the extraction
aperture, 85% of the ion beam is concentrated into a 1.5 mm
diameter spot size; therefore, specimens with large electron
transparent areas are prepared with minimal risk of
contamination from either the holding plates or the specimen
stage.
Argon gas is introduced through an orifice located in the
center of the cathode that is maintained at ground potential.
Gas flow is automatically regulated by two independently
controlled mass flow controllers. The controllers interface with
the microprocessor in a feedback control loop. Typical gas flow
ranges from 0.4 sccm to 1.0 sccm
Figure 22
Hollow Anode Discharge Ion Source
Ionization of the argon gas is initiated by the application of a
voltage between the anode and cathode. The cathode surface is
concave with a precise radius to focus both ions and electrons
at the hollow anode discharge portion of the anode whereby a
secondary plasma is formed. It is from this secondary plasma
that the positively charged ions are extracted by the application
of a negative voltage potential applied to the extraction
aperture. This aperture focuses and accelerates the ions to the
specimen. In order to compensate for minute differences in
component tolerances, a means was developed to allow the
adjustment of the ion beam position in relation to the specimen
from outside of the vacuum
For effective ionization, the chamber is evacuated by a 70
lps turbomolecular vacuum pump that maintains a system
vacuum of between 1 x 10-4 and 5 x 10-5 torr under normal
milling conditions.
Specimen Mounting. Specimen mounting is
accomplished by either of two methods. For traditional
milling angles of 8o to 45o, the specimen is conveniently
contained between two plates that are fit into the specimen gear
as shown in Figure 23. The plates are fabricated from either
stainless steel or tantalum and are secured into the specimen
gear by means of a single spring clip.
PLATE
Figure 23
Plate Type Milling
For composite materials or cross-sectional interface
specimens comprised of materials with radically dissimilar
milling rates, it is advantageous to ion mill at low angles of
incidence. In these cases the slower milling materials
contained in the specimen act as a beam block, protecting the
softer materials from the thinning effects of the energetic ion
beam. This factor results in more uniform milling when
compared to milling at higher angles of incidence. Milling
angles of between 2o and 8o have been shown to be highly
effective in preparing electronic specimens comprised of
materials with varying milling rates. [10]
For low angle applications, the specimen is affixed to the
top surface of the specimen post attachment by a low melting
point polymer as shown in Figure 24. This low angle
attachment is necessary to prevent the shadowing effects of
the specimen holder on the specimen and eliminates the
possibility of material from the specimen holding mechanism
from being sputtered onto the specimen surface.
In the case of either the specimen plate or the low angle
milling attachment, the specimen is loaded into the milling
chamber through an access port containing a hinged door. A
single o-ring seal located in the door ensures vacuum integrity.
The chamber is fitted with observation windows and
halogen lamp assemblies for specimen viewing in either
reflected or transmitted light. A stereo microscope is utilized
for in-situ specimen monitoring.
LOW ANGLE
MILLING
ATTACHMENT
Figure 24
Low Angle Milling Attachment
Specimen Rocking. For the preparation of XTEM
specimens, a rocking mechanism was developed. When ion
milling planar specimens, the specimen is fully (360o) rotated.
However, when preparing either composite materials or XTEM
specimens, the rapidly milling materials contained in the
specimen need to be protected from the ion beam to preserve
both structural integrity and the interface between the materials
of interest. The rocking angle control is adjustable from 0o to
360o in 1o increments and prevents either the glue line or an
interface from being parallel to the ion beam as shown in
Figure 25.
ROCKING
CONTROL
Figure 25
Variable Rocking Angle
Liquid Nitrogen Cooling In cases when materials
experience a phase transformation at relatively low
temperatures, it is essential to employ liquid nitrogen specimen
cooling. Liquid nitrogen prevents the energetic ion beam from
thermally altering the material’s properties. For this research,
a liquid nitrogen specimen cooling system was designed. A
liquid nitrogen dewar located within the instrument enclosure
is thermally coupled to the specimen stage, which lowers the
stage and specimen temperature during milling.
The specimen stage was designed to include a resistive
temperature detector (RTD) and a resistive heater assembly,
both of which are linked through the microprocessor. The
RTD provides a continuous indication of the specimen stage
temperature which is displayed on the system monitor. When
utilizing liquid nitrogen specimen cooling, it is imperative that
venting does not occur until the specimen stage is at room
temperature. Failure to meet this condition will result in
specimen contamination from condensation forming on the
specimen and stage surface. To alleviate this situation, an
interlock was developed so that when the RTD senses a
temperature of less than 20°C, the resistive heater is
automatically energized to elevate the specimen stage
temperature prior to venting.
Chemical Etching. To minimize ion-beam-induced
artifacts when preparing II-VI or III-V materials, a chemicallyassisted etching attachment was designed. This device
provides a stream of molecular iodine vapor at the impingement
point of the ion beams. [11]
Iodine gas flow is generated by sublimation of iodine vapor
from iodine crystals contained in a vessel located outside of the
milling chamber.
Ion Milling Results. The effectiveness of low angle,
variable-energy ion milling is exemplified in the preparation of
specimens from electronic materials. The specimen shown in
Figure 26 is an Al/Cu metal layer deposited on a SiO2
substrate. Although these materials exhibit dissimilar milling
rates, a large electron transparent area with a uniform thickness
was produced.
These factors yield specimens with large amounts of thin area
without preparation-induced artifacts and, therefore, TEM
performance and the corresponding analysis are greatly
enhanced.
The methods of electropolishing, ultrasonic cutting, ultraprecision dimpling, and ion beam milling have been shown to
be effective for producing high quality TEM specimens from
electronic materials. Improvements implemented into
Ultrasonic Disk Cutters yield specimens with minimal damage
from precisely selected positions on the bulk material.
Ultra-precision dimpling grinders have demonstrated the
importance of high precision process control over dimpling
and enable mechanical pre-thinning to either near or at electron
transparency resulting in greatly reduced ion milling times and
superior samples.
Advances in ion milling instrumentation enable effective
milling for a wide range of materials including composites and
XTEM specimens. Improvements in instrument process control
afford ion milling with minimal user intervention. Significant
developments in ion source technology have resulted in both
increased milling rates and reduced levels of ion induced
damage.
Figure 26
Al/Cu on SiO2
The specimen shown in Figure 27 is comprised of a
polycrystalline Cu film on an epitaxial Ti 0.5W0.5N/TiN
superlattice on MgO. This specimen is a clear demonstration
of the ability to uniformly thin materials of varying hardness.
In this specimen the Cu has the tendency to mill very rapidly
while the Ti0.5W0.5N/TiN super-lattice exhibits a very low
milling rate. Low angle milling, combined with the ability to
adjust the ion milling energies enabled the successful
preparation of this specimen.
Acknowledgment
The authors would like to thank Dr. M.G. Burke and J.J.
Haugh for the preparation of the Superalloy 718 specimen, Dr.
Ivan Petrov for the ion milled specimen preparation, and Teng
Chen-Ming for the preparation of the GaInP/InGaAsP/InGaAs
sample.
This research was supported in part by the Pennsylvania
Ben Franklin Partnership Program.
References
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2.
Figure 27
Multi-layered
Specimen
Conclusion
Many advancements in instrument performance and
preparation techniques have occurred in the past few years.
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