About Bruker Nano
Nano provides Atomic Force Microscope/Scanning Probe Microscope (AFM/SPM)
products that stand out from other commercially available systems for their
robust design and ease-of-use, whilst maintaining the highest resolution. The NANOS
measuring head, which is part of all our instruments, employs a unique
fiber-optic interferometer for measuring the cantilever deflection, which makes
the setup so compact that it is no larger than a standard research microscope
The firm basis for the quality of our microscopes is a team of experienced
scientist and engineers with a background of more than 15 years in the AFM
The development of modern technology affects the science of small objects in
two ways. On one hand better means for handling, imaging and analysis of miniature
objects are provided, which means we can try and understand our world on a much
smaller scale. On the other hand further miniaturization in manufacturing necessitates
the control of technological processes at a minimum of one order of magnitude
below the aspired device size.
The need for rapid and efficient nanoanalysis is growing very quickly. The
next generation 22 nm node in microelectronics architecture is approaching.
New solutions for electronic interconnects, capacitors, denser data storage
and solar cells are currently under development. This requires atomic scale
analysis of a wide range of materials such as functionalized carbon nanotubes
(CNTs), various perovskites and three-dimensional nanostructures.
Another important field of miniaturization is modern medicine. It strives to
identify toxic nanoparticles and transfer medication and operation tools precisely
to the place where they are needed in the body. For all of this nanoanalysis
is irreplaceable. To understand and control the function of miniature sized
natural and artificial objects we need to know their element distribution.
The task for scientists and manufacturers is to find the most efficient way
to characterize small objects with high quality data at high spatial resolution.
One of many steps in this direction in analytical electron microscopy (AEM)
is aberration correction. Instead of turning to higher and higher accelerating
voltages in larger and larger microscopes the correction of the spherical and
chromatic aberration has become reality now and atomic resolution below 80 keV
This is necessary for the investigation of radiation sensitive materials like
CNTs. AEM employs different types of spectroscopy. Energy dispersive X-ray analysis
(EDS) is used to distinguish between different elements in the sample utilizing
the generated X-rays. In both scanning and transmission electron microscopy
(SEM/TEM) the aim is to acquire EDS spectra with high signal to background ratio
in a short time from small sample areas. This article discusses an efficient
way to meet these requirements.
For EDS with high spatial resolution, the excited sample volume generating
the radiation needs to be small. Unfortunately, the smaller the excited volume
the smaller the total EDS signal is, providing the electron dose stays the same.
This is the case in SEM, where lower accelerating voltages are used to decrease
the excited sample volume and to an even larger extent in TEM, where only the
neck of the tear drop shaped scattering volume is available for X-ray generation
in the electron transparent sample slices (fig. 1).
Among the obvious solutions are higher current density in the probe -
which is limited by the beam sensitivity of the samples -, longer measurement
times - which are limited by the stability of the microscope and detector
system as well as the beam sensitivity - or a larger solid angle for X-ray
collection. A comprehensive overview about how the detection limit is influenced
by these factors is provided by M. Watanabe and D.B. Williams. O. Krivanek et
al. explain how aberration correction contributes to the improvement of the
peak to background ratio, which is so important for advancing the detection
Figure 1. Excitation volume that contributes
to initial X-ray generation.
Conventionally, lithium drifted liquid nitrogen
cooled silicon devices (Si(Li)) were
used to convert X-ray quanta into electric
charge. Recently Peltier cooled silicon
drift detectors (SDD) were developed
and Bruker AXS Microanalysis (formerly
RÖNTEC) was the first to use and optimize
this new liquid nitrogen free technology
for commercial EDS analysis in
SEM. This has several advantages
compared to conventional Si(Li) technology,
particularly for nanoanalysis. SDDs
provide a drift field, generated by drift
rings on the back side of the active crystal,
to guide and collect the charge cloud
generated by each photon (fig. 2).
Figure 2. 30 mm2 chip used in TEM-detectors.
Data can be collected much faster and much more efficiently than with conventional
Si(Li) detectors. Bruker's hybrid signal processing unit developed especially
for SDD readout makes sure the superb collection capabilities of the detectors
are properly exploited. XFlash Silicon Drift Detectors show very little dead
time, extreme count rate capabilities, and are - unlike most Si(Li) -
immune to overload conditions. Furthermore they don't require liquid nitrogen
for cooling, making vibrations associated with heavy dewars on the microscope
column, microphony and icing problems obsolete. Additionally, the smaller temperature
gradient between the ambient area and the detector chip, which only needs to
be cooled down to -25 to -30 °C for 30 mm2 XFlash
SDDs, ensures high stability and very little drift. These are the reasons why
Bruker decided to adapt its XFlash Detectors for TEM as well (fig. 3).
Figure 3.SDD technology in TEM: Bruker
XFlash 5030 installed on a Jeol2200 FS,
Humboldt University, Berlin.
Today, the challenge in EDS is to maximize the solid angle for radiation detection
in such a way that fast and complete charge collection as well as energy resolution
don't suffer. The solid angle (Ω) can be described as Ω =
A (cos δ)/d2. A is the active detector area, d its distance
to the sample and δ is the angle between the normal of the detector surface
and the line of shortest distance between sample surface and detector centre.
If the detector is tilted towards the sample so that δ
= 0, the solid angle equals A/d2. One approach
for improving the solid angle for
radiation collection is to increase the active
detector area. Since larger chip areas
are difficult to readout, this has shortcomings
like pile up, incomplete charge collection,
the necessity of stronger cooling,
worse energy resolution and geometric
constraints. Hence Bruker
smaller detector areas closer to
the sample or several small detectors at
once, as both are much more efficient.
In this way a high count rate efficiency with almost no pile up, low dead time
and no peak broadening is achieved, resulting in a clean high throughput and
a count rate capability of up to several million counts per second (cps). This
makes the approach ideal for high brightness electron sources, radiation sensitive
samples and in-situ analysis. Excellent elemental mapping for all count rates
and therefore all magnifications is also guaranteed. Superior energy resolution
supports the analysis of light elements. Heavy elements with N-lines in the
same low energy range can be identified using the company's comprehensive
atomic data library. Furthermore the low temperature gradient between the environment
and a small chip area provides stable experimental conditions.
The first example shows nickel catalyst particles (with a diameter of 20 nm)
in carbon nanotubes. The sample was analyzed in a Zeiss Supra55 SEM at 20 keV
in transmission using a 10 mm2 XFlash SDD at a solid angle of 0,01
sr and a beam current of 500 pA . The 1024 x 220 pixel HyperMap was acquired
in 15 minutes using a pixel dwell time of 4096 µs. Mirroring the well
resolved EDS HyperMap the nickel particles are evident as strong emitters in
the secondary electron image (fig. 4).
Figure 4. Ni catalyst particles in carbon nanotubes.
Ni-Kα lines were used for identification in the HyperMap (4a). 4b shows
a secondary electron image. Sample courtesy of: S. Hermann, T. Geßner,
Center for Microtechnologies at the Chemnitz University of Technology.
An experimental result of an AlGaAs (P, In) quantum well research project is
shown in figure 5. The data was acquired using a 30 mm2 XFlash 5030
SDD for TEM with a 0.12 sr solid angle in a Jeol2200FS TEM. A 210 pA probe current
in a 0.7 nm spot was used. The 244 by 342 pixel map was acquired in 6 minutes
using 4096 µs dwell time per pixel. The distribution of the heavy element
indium (shown in yellow) correlates well with the high angle annular dark field
The latter increases with the atomic number of the scattering elements, provided
the whole mapped area is equally thick. The quantification of the element map
was performed using 8 by 8 pixel binning and theoretical Cliff- Lorimer factors.
The elemental profile was generated by adding up all the 8 by 8 pixel data perpendicularly
to the layers in the marked region. Just like the acquired raw data the profile
provides nm resolution. In order to deliver even more precise data Bruker also
offers drift correction options for longer acquisition times and AutoPhase (its
principle component analysis solution).
Figure 5. Quantum well research sample. AlGaAs
(P, In) as deposited: AlGaAs, 5 nm GaAsP, 7 nm InGaAs, 5 nm GaAsP, AlGaAs. 5a:
Elemental distribution of In, P and Al. 5b: The corresponding HAADF image. 5c:
Elemental profile. Sample courtesy of: G. Tränkle, Ferdinand Braun Institute,
Berlin and A. Mogilatenko, W. Neumann, Humboldt University, Berlin.
High spatial resolution EDS results using SDD technology in electron microscopy
were described above. It is evident that a combination of electron dose and
detection efficiency influences the data quality. A large solid angle using
small active detector areas guarantees good detector performance, which includes
fast readout, low dead time, no pile up, high energy resolution, a stable instrumental
environment and excellent performance at high and low count rates.
A good detector performance at high count rates is very useful for elemental
imaging and for finding the right sample area in low magnification mode. It
is also ideal for employing high brightness electron sources and for elemental
mapping in in-situ experiments. In addition fast high quality readout is important
when low electron doses have to be used for beam sensitive samples and when
large data sets for 3-D-characterization need to be acquired. On the nano-scale
in high magnification mode, where the number of generated X-rays drops dramatically,
such detectors deliver excellent results as well.
Source Bruker AXS - AFM and SPM
For more information on this source please visit Bruker AXS - AFM and