Further advancing the throughput of a multi-beam SEM
Thomas Kemen1, Matt Malloy2, Brad Thiel3, Shawn Mikula4, Winfried Denk4, Gregor Dellemann1,
and Dirk Zeidler1,*
1
Carl Zeiss Microscopy GmbH, Carl-Zeiss-Strasse 22, 73447 Oberkochen, Germany
SEMATECH, 57 Fuller Rd, Suite 2200, Albany, NY 12203, USA
3
SUNY Polytechnic Institute, 257 Fuller Road, Albany, NY 12203, USA
4
Max-Planck-Institute for Medical Research, Jahnstr. 29, D-69120 Heidelberg, Germany
2
*[email protected]; phone +49 7364 20-9138; fax +49 7364 20-9456; http://www.zeiss.com/multisem
ABSTRACT
Multiple electron beam SEMs enable detecting structures of few nanometer in diameter at much higher throughputs than
possible with single beam electron microscopes at comparable electron probe parameters. Although recent multiple beam
SEM development has already demonstrated a large speed increase1, higher throughputs are still required to match the
needs of many semiconductor applications2. We demonstrate the next step in the development of multi-beam SEMs by
increasing the number of beams and the current per beam. The modularity of the multi-beam concept ensures that design
changes in the multi-beam SEM are minimized.
Keywords: Multi-beam, SEM, high speed imaging, beam splitter
1. INTRODUCTION
With the continuing decrease of structure size in semiconductor devices, there is a need for measuring and detecting
nanoscale patterns and defects3. Scanning electron microscope (SEM)-based technologies can resolve these structures,
but have not been able to achieve the throughput requirements for screening large areas yet due to physical limitations
such as the maximum achievable detector bandwidth for high-efficiency detectors and Coulomb interactions between
electrons. We have recently reported on a multi-beam SEM that uses 61 parallel electron beams in a single column to
bypass or alleviate these limitations4. Imaging of large areas such as semiconductor wafers, or large volumes of
biological tissue at high resolution, though, requires that the throughput be increased beyond the level already
demonstrated. Here, we report on the next step in the development of the multiple-beam SEM that incorporates 91
beams.
2. MULTI-BEAM ELECTRON MICROSCOPY
2.1 Throughput limitations of single-beam SEMs
The maximum achievable scan speed of any conventional SEM is ultimately limited by the electron dose per pixel
required to generate a desired minimal signal-to-noise ratio (SNR) at a pre-defined spot size5. In conventional electron
microscopic imaging, the goal is usually to obtain the optimal resolution and contrast for all images, with a beam size
smaller than the feature size. In high-throughput electron microscopy, beam size, scan pixel size, and electron dose per
pixel all have to be chosen such that a good-enough SNR is achieved at the maximum possible data acquisition rate. Two
fundamental effects limit the minimum pixel dwell time of single beam SEMs. First, reducing the dwell time per pixel
while retaining SNR requires increasing the beam current. This ultimately will lead to increasing Coulomb interactions
between the electrons, thereby blurring the electron beam and reducing the resolution at the sample. Second, efficient
Metrology, Inspection, and Process Control for Microlithography XXIX, edited by Jason P. Cain,
Martha I. Sanchez, Proc. of SPIE Vol. 9424, 94241U · © 2015 SPIE
CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2188560
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detectors for secondary electrons in an SEM cannot be operated faithfully at arbitrarily high rates because of detector
decay times. Another limit for the dwell time per pixel is therefore the bandwidth of the electron detector.
2.2 Principle of operation of the multi-beam SEM
Figure 1 demonstrates the basic principle of operation for the multi-beam SEM: An array of electron beams generated by
a multi-beam source is focused by a lens arrangement onto the specimen to form a regular pattern of N primary electron
spots. The secondary electrons (SE) that emanate from each primary electron spot are projected via another lens
arrangement onto a multi-detector that records all beams simultaneously. A magnetic beam splitter separates primary and
secondary electron beams. The bundle of electron beams is scanned over the sample, and the secondary electron yield is
recorded for each scan position (Figure 2), just as in any conventional SEM. One single scanning pass thus produces
many images in parallel, yielding a complete image of the field of view underneath the primary beam array that can
contain up to one Billion pixels.
With multiple electron beams in a single column, Coulomb interactions will be lower than within a single-beam
configuration, as the charge is distributed among many beams and therefore spread over a larger volume. Multi-beam
configurations therefore are able to maintain high resolution and high total current at the same time. Having a dedicated
detector for each beam bypasses the detector bandwidth limit. The total possible detector bandwidth of the multiple beam
SEM is the single detector bandwidth times the number of beams. With this setup, the multi-beam SEM is prepared for
future single-beam SEM detector technology improvements that might feature higher bandwidths per detector. The
electron optical design ensures that almost all secondary electrons are guided to the multi-detector to obtain the best
possible SNR at a given primary beam current. This means that primary electrons are very efficiently used to generate a
secondary electron signal, ensuring a minimum electron beam damage of the sample.
Projector Multi-Detector
Multi-Beam
Source
Beam Splitter
..
Bo-
Illumination
Detection
Objective
Sample
Figure 1. Schematic drawing of the multi-beam SEM. Primary electrons (solid lines) are focused onto the sample and
separated by a beam splitter from the secondary electrons (dotted lines) that are detected simultaneously.
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Ir
e
j.ii.
I
ir
O
I
I
Figure 2. Multi-beam SE
EM principle off operation. Thee left image shows the secondaary electron spoots at the detecto
or plane.
Each spott corresponds too one emitted seecondary beam acquired by on
ne detector. All beams are scannned concurrenttly. As an
example, the beams markked in dark andd light blue simuultaneously acq
quire the imagess marked in darrk and light bluee,
respectiveely. The right im
mage shows a montage
m
of the 91
9 single-beam
m images recordeed in one shot w
with a total field
d of view
of about 200
2 µm (samplee: etched siliconn test chip, desccribed in more detail
d
in figure 4).
2.3 Scaling of
o the multiple beam apprroach
To advance thhe throughputt of the multi--beam SEM, we
w further inccreased the nuumber of beam
ms and detecto
ors from 61 too
91. At the saame time, the current per beam
b
could bee increased frrom about 600 pA to moree than 3 nA per
p beam. Thee
higher beam current ensuures that a goood SNR is achieved
a
even
n for fast scaan rates. The total usable beam currennt
increases by about
a
a factorr of seven, thee total field off view (FoV) by factor of tw
wo, as the disstance between
n the beams at
a
the sample haas been increaased to 18 µm
m. Due to the higher Coulo
omb effect andd the larger F
FoV, the beam
m probe size iss
expected to increase slighhtly. This hass been experiimentally verrified, as show
wn by the appplication datta in the nexxt
paragraph. Other
O
than incrreasing the nuumber of deteectors and insserting a 91-bbeam source into the electrron optics thaat
features higher current perr beam, no chaanges to the multi-beam
m
SE
EM design haave been requiired. This dem
monstrates thaat
the single-collumn, multiplle beam conceept ensures siimple scalabillity. We expecct this conceppt to be adaptable for muchh
higher beam numbers
n
still.
3. APPL
LICATION RESULTS
R
The further increase of imaging
i
speeed of the 91--beam SEM provides thee next step fo
for electron microscopy
m
too
applications where
w
both high
h
resolutionn and large scan
s
area are required. Wee demonstratee this at two examples, thee
imaging of brrain tissue andd the imaging of a semicondductor test retiicle.
3.1 Imagingg of brain tissue
For imaging of biologicall samples theere is an incrreasing need to image largge volumes oof biological tissue at highh
gans or evenn whole orgaans. Examples include thee
resolution too gain insightt into the fuunctioning off parts of org
understandingg of neural ciircuits 6,7 and the analysis of extended cellular
c
structuures 8. The reesolution that is required too
obtain inform
mation at suffiiciently detaileed level can only
o
be obtain
ned by electroon microscopyy. With singlee beam SEMss,
the currently achievable daata acquisitionn rates are tooo low to accom
mmodate even modestly siized volumes. For examplee,
imaging a bloock of tissue of
o 2 mm side length with ann isotropic vo
oxel size of 100 nm would reesult in about 8 Petabytes of
data. At a datta acquisitionn rate of 20 MHz,
M
this resullts in a total acquisition
a
tim
me of about 12 years, even
n before takingg
into account any overheadd times. As a consequence,, EM studies are restrictedd to smaller arreas, giving only
o
a keyholee
view of a hugge cellular andd macroscopicc context, whiich in the brain
n spans over several
s
mm. F
Figure 3 show
ws a sub-regionn
from a coronnal block-face of an osmium
m-stained moouse brain imaaged with thee 91-beam SE
EM at a pixel dwell time of
50ns. The strructures of intterest are resoolved well, andd, due to the increased currrent, a good S
SNR is achiev
ved at this fasst
scan rate.
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a
,_,
,,.4
..,;
Figure 3. Cereebral cortex of mouse
m
brain (bblock-face), sam
mple by Winfried Denk and Shawn
S
Mikula, Max Planck So
ociety, showingg
unmyelinated neuronal
n
and gllial processes and neuronal nuuclei, acquired by the 91-beam
m multi-beam S
SEM with 273 nA
n total currentt,
dwell time 50nns. A 1-2 nm cooat of palladium
m has been evapporated onto thee block-face to dissipate
d
chargiing 9. Inset lower right: 6 µm x
5 µm single-beeam sub-image,, detail of the fuull multi-beam image.
i
3.2 Imagingg of semicond
ductors samplles
To demonstrrate the capabbilities of thee multi-beam SEM for sem
miconductor samples,
s
we imaged etcheed silicon tesst
calibration strructures. Figuure 4 shows thhat, despite thhe much higheer current in thhe electron opttical column, the test reticlee
is resolved well
w and that the resolutionn variation beetween the beeams is small.. Due to the combination of high probee
current and high detection efficiency, aggain a good SN
NR is achieved
d even for sm
mall pixel dwelll times.
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n
Figure 4: Test chip show
wing a hexagonaal arrangement of calibration structures for tool adjustments.. The structuress are
printed inn an e-beam direect write lithogrraphy process with
w a high placcement precisionn, etched in SiO
O2 on a Si-substtrate, and
finally coaated with a com
mpletely conducctive layer, width of image: 20
00 µm. Inset low
wer right: 18 µm
m x 16 µm sing
gle-beam
sub-imagee, detail of the full
f multi-beam
m image. Imaginng has been perfformed with thee 91-beam SEM
M at a total currrent of
273nA annd a dwell time of 50ns.
4. SUMM
MARY AND OUTLOOK
K
We have dem
monstrated thaat the multi-beeam SEM conccept that uses a single electtron optical coolumn and mu
ultiple electronn
beams for paarallel image acquisition
a
is scalable to higher
h
beam numbers
n
and beam
b
currentss while mainttaining a goodd
image qualityy. This scalability ensures that
t
the multi--beam SEM approach
a
will be able to meeet future thro
oughput needss
in neurosciennce research ass well as otherr fields, such as
a semiconducctor applicatioons.
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