Sample Preparation

FIB or other ion milling techniques are essential to TEM sample preparation. The challenge faced

by engineers is how to repeatably segment a sample with extreme precision. A sharp blade

mounted to precision motion control is the instrument known as a microtome. The most

advanced microtome systems, ultramicrotome, can segment samples on the order of 10-7 m,

which is the range for TEM sampling. Microtome is a good approach when sectioning metals but

it is much more difficult to section brittle materials and soft materials. This is where an FIB tool

excels.

FIB tools perform 3 essential functions for TEM prep, imaging, cutting and depositing material.

When performing any kind of precision work there is usually a means to magnify one’s vision.

The FIB tool has a built in SEM which allows for more precise sample handling. To segment

materials, the FIB uses a highly charged beam of Ga ions. This works like a microscopic version

of a sand blaster to remove paint. Much like the sand blaster, the voltage of the Ga beam can be

controlled to increase or decrese the kintectic energy of the Ga beam. This allows for faster

coarse cuts and slower precision cuts. As can be expected from the sand blaster comparision,

higher power beams cause more damage to the sample surface. So the optimization is in

preparing the highest quality sample without excessive instrument time.

There are two common configurations for preparing materials for TEM, cross section and plan

view. Cross sections are used to visualize deposited layer thickness and lateral uniformity. Cross

sections are prepared by digging large trenches in the sample to expose the layers of interest.

Those regions are then removed from the sample in a process known as lift out. Lift out is where

the newly exposed sliver of material known as the lamella, is fastened to a transfer arm by

depositing carbon and is then cut away from the rest of the sample. After lift out, the sample can

be reinforced by depositing metal to provide rigid support around the area to be imaged. After

reinforcement, the lamella can finally be thinned then affixed to a TEM grid for imaging.

Plan view preparation is distinct from cross section views because in this case the surface is of

most interest. This sample preparation is akin to exfoliating the top most layer of material. The

challenge is to make sure that the top most layer remains structurally sound unlike most

materials which scroll after exfoliation. By tilting the sample to nearly parallel with the ion beam,

the top layer of material can be removed from the sample. As would be expected, because of

the cutting angle the process has to be done using lower beam currents so it generally takes

more time and precision to prepare a sample for plan view than for cross sectioning.

TEM Imaging Details

Once a sample has been appropriately thinned, it can be imaged by the TEM. The TEM contains

collections of magnets to focus the electron beam. Light can be bent by interacting with lenses

to change its direction. Electron beams are more easily manipulated by magnetic fields because

of the Lorentz force which curves traveling electrons in magnetic fields. Magnets consist of a

north and south end collectively referred to as a dipole. Arranging magnets symmetrically around

the electron beams leads to additional common configurations. 2 dipoles create a quadrupole, 3

dipoles create a sextupole and 4 dipoles form an octupole. Lenses in the TEM are formed by 1

or 2 sets of quadrupoles and beam collimator are formed by sextupoles. Referring to the image,

each lens will contain quadrupoles for manipulating the beam and sextupoles for keeping the

beam vertically aligned. It is clear that there are numerous components that need to work

flawlessly in unison to properly control the electron beam.

HR-TEMs

Any electron microscope that is in the transmission configuration can be called a TEM. When a

TEM contains phase contrast and can image down to the atomic length scale at 10-9 m, then it is

referred to as an HR-TEM. Most often instruments within the past 20 years will be HR-TEM but in

more recent years a new version of the TEM with even higher resolution has emerged, the

aberration corrected TEM (Cs-TEM).

Spherical Aberration:

Aberration usually refers to the optics concept that path length differences, when light passes

through lensing, can create a loss in information. To form a sharp image, all changes in the

monochromatic beam should be from sample interactions. In real life, lenses are not perfect so

they can affect the light passing through them. Consider a fun house mirror, you yourself are not

highly distorted but moving from one mirror to the next the imperfections in the glass will warp

your image. Lenses can have the same effect albeit in a much more subtle way since most

lenses’ designers aim for perfection. When electrons enter the magnetic field lenses at the edge

of the field, they take a slightly longer path than electrons entering at the center of the field. This

path difference is what makes the image blurry and the main point that the aberration correction

addresses.

Spherical Aberration Corrected TEM (Cs-TEM)

At the end of this journey in lenses and collimators, a final image is produced. The result of the

additional aberration correction components is a higher resolution image that more accurately

depicts the atomic locations. This is demonstrated in the figure where the corrected image has

additional visible diffraction planes and sharper resolution in the atomic locations. The Ga and N

atoms are now also more clearly resolved in their orientation relative to each other.

Scanning Tunneling Electron Microscopy (STEM)

Now for many engineers the location of atoms is good but insufficient. For elemental analysis

there are 3 commonly used techniques with TEM. The first option is STEM, which is a Z contrast

mode that uses inelastically scattered electrons along with detectors that are located far from the

electron beam to be able to resolve the subtle differences in proton density between elements.

STEM mode provides excellent contrast between light and heavy elements because the

difference in nucleus size is the source of the contrast. This makes STEM the ideal mode for

imaging thin interfaces that are sub nanometer in thickness. STEM works particularly well for thin

samples, 40 to 60 * 10-9 m.

Even though STEM is a powerful technique there are still additional ways to gain more

information about a sample using the characteristic x-rays that the elements emit.

Energy Dispersive x-ray Spectroscopy (EDS)

EDS is an attachment to both an SEM and TEM that allows for the capture of x-rays emitted

from the electron beam’s interaction with the sample. These x-rays energies can be used to

identify the particular atoms in the sample. There are some overlapping energies so the

identification of elements is not 100% guaranteed. Overall, the ease of acquisition of EDS has

led to its widespread use for elemental analysis. An example EDS shows fine changes in material

in the different layers that comprise 3D flash memory. An important point is that EDS works

better with thicker samples so 200 * 10-9 m gives much more counts than thin samples. Luckily

there are dual EDS detectors which can increase the overall signal intensity by a factor of 10.

The basis for contrast in STEM can be used for elemental analysis as a technique called EELS.

Electron Energy Loss Spectroscopy (EELS)

This brings us to the final elemental analysis technique EELS. EELS is the quantitative version of

STEM, where specific energy peaks identify each element. Unlike in EDS, multiple peak locations

are used to identify each element so it becomes much more improbably that an element will be

misidentified. The drawback of EELS is that only a few elements can be scanned at the same

time, which makes EELS the best choice for identifying known elements in a sample while EDS

can be used to identify unexpected elemental impurities. There are also dual EELS detectors to

further increase the signal intensity and improve the speed of the technique. EELS is the

appropriate choice for very thin samples 40 * 10-9 m.

Overall, we hope that you enjoyed our review of TEM and have a better appreciation for the

usefulness of STEM, EDS and EELS. Outermost Technology is highly skilled in FIB and ion mill

preparation which is why we consistently deliver high quality TEM images across a variety of

materials. Outermost Technology has proven expertise in TEM imaging for semiconductor

devices as shown by the 3D NAND images for both Si and Ga based devices. Please consider

Outermost Technology for your next TEM job.

First

Microscope

Microscopes had been

around since 1595 but

Antonie van Leeuwenhoek

(Netherlands-1632) highly

successful as a merchant is

credited for developing the

first practical microscope in

1673 to fulfill his passion for

nature observation.

First TEM Prototype

For 300 years, the observation with

microscopes was limited to objects

bigger than the wavelength of the visible

light. In 1904, Pr. August Köhler

(Germany-1866) broke this barrier by

introducing the first UV microscope

while working for Carl Zeiss AG.

Moving Electron

Wavelength

In his 1924 thesis, Louis de

Broglie (France-1892)

formalized the wave nature of

the electron. His theory, proven

in 1927, was rewarded by the

1929 Nobel prize. Knowing the

wavelength of the electron

(~1nm) would be the first step

towards the development of

electron microscopes.

First UV Microscope

In 1931, Ernst August Friedrich Ruska (right),

(Germany-1906) collaborated with Max Knoll

(left) at the Technical University of Berlin to

create the first electron microscope then helped

Siemens AG release the first commercial TEM

in 1939. He received the Nobel prize in 1986 for

his work on electron optics.

1904

1924

1931

1673 1997

HOW DID WE GET HERE?

HOW DOES IT WORK?

Technical Background

Following several years of

research in Heidelberg,

Maximilian Haider (Austria-1950)

presented in 1997, with his

colleagues Knut Urban and Harald

Rose, the first pictures taken with

a spherical aberration

Cs-corrected TEM allowing the

development of sub-ångström

resolution TEM.

Aberration-

Corrected TEM

There are only a handful of technologies that image

the location of individual atoms which include atom

probe tomography (APT), scanning tunneling

microscopy (STM) and transmission electron

microscopy (TEM). TEM is the most versatile tool that

can deliver images and high quality elemental analysis

with atomic precision. The TEM ecosystem is vast

with customized sample holders and additional

detectors all aligned to push the limit of resolution and

increase the level of high technology. Join us as we

highlight the key components in a modern TEM.

Microscopy uses small particles/waves, such as

photons or electrons, to image larger objects. Light

microscopes have benefited from brighter sources,

polarizers and filters to make the beam as uniform as

possible when it interacts with a specimen. In an

analogous way, intense electron beams are aligned,

filtered and collimated to generate high resolution

images. The product of decades of development has

pushed the resolution from light microscopes range of

microns (10-6 m) to TEM range of Angstroms (10-10 m).

SEM VS TEM

Analogous to light microscopes, there are two

orientations for the electron beam, reflection and

transmission. Scanning electron microscopes (SEM)

use reflected electron beams to gather information

about the samples. The resolution of the SEM is on

the order of 10-9 m which is less than that of TEM, but

combined with the facile sample prep it has proven to

be a cost-effective tool for measurements of relatively

large features. For SEM, the sample needs only to be

mildly compatible with vacuum and slightly

conducting to be imaged. For TEM, the sample must

be sectioned and thinned to under 10-7 m for imaging.

This sample preparation is accomplished using a

Focused Ion Beam (FIB). FIB preparation has kept

pace with TEM’s as the requirements continue to grow

stricter for higher resolution.

Transmission Electron Microscopy

2975 Scott Blvd., Ste 115

Santa Clara, CA, 95054, USA

T: +1-408-889-1019

E: contact@outermost-tech.com

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www.outermost-tech.com

Figure 2. Yearly Limit on Microscope Resolution

in Å (Angstrom 10-10 m). Image from

Appalachian State University 2016

Figure 1. Modern TEM tools are easily larger

than an average adult in height.

Image from Thermo Scientific

Figure 3. Focused Ion Beam (FIB) equipment

used for TEM preparation. Image

from Thermo Scientific.

Figure 4. Preparation of TEM lamella. A) Depositing Pt, B) Trench milling, C) U-cut, D) Lift-out,

E) Welding to TEM grid, F) Lamella thinning. Image from Regina Seidl, 2016

Figure 5. Method to prepare plan-view lamella for TEM. MRS Bulletin, Volume 32, May 2007

Figure 6. Method to prepare plan-view lamella for TEM.

MRS Bulletin, Volume 32, May 2007

Figure 7. Ray diagram depicting high resolution HR-TEM and spherical aberration

correction Cs-TEM. Image from Appalachian State University 2016

Figure 8. Difference between non corrected HR-TEM (Left image) and the

aberration corrected Cs-TEM (Right image). Image from Thermo Scientific

Figure 9. Example TEM image followed by two STEM images in dark and bright field modes.

Elemental data is shown in figure 10. Images from Outermost Technology

Figure 11. High resolution EDS line scan that shows the elemental concentration

of a transistor gate structure. Image from Outermost Technology

Technologies for Composition Analysis

Figure 10. Showing changes in atomic concentration across atomic length

scales in the inner section of 3D-Nand memory using Cs-TEM.

Figure 12. 10 nm scale bar demonstrating the strength of EELS for

analysis of semiconductor gate structures with HR-TEM.

Access to a variety of TEM’s. HR-TEM’s for routine samples that

need more resolution than SEM

Access to top of the line Cs-TEM’s for very narrow features imaging

Access to twin EDS and twin EELS detectors with higher resolution

and signal intensity than single detectors

Benefit from excellent FIB preparation skills applied to many

semiconductor, metals and soft materials

Benefit from competitive pricing and in-depth analysis report

WHY CHOOSE OUTERMOST TECHNOLOGY FOR TEM?

*Valid until May 15 with a minimum quantity of 3 samples

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