Yao N. Focused Ion Beam Systems: Basics and Applications
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Figure 10.19 is the classic chrome-on-glass mask. It should be obvious that
there are potential problems for repair, in that, a metal (chrome) is on top of
a very good insulator (quartz) and may lead to problems of charging. The
lower two figures are examples of phase shift mask structures. The MoSiON
(molybdenium silicide) is the phase shifting layer, which allows approxi-
mately 10% transmission through the film while imparting a 180
phase shift.
An alternative phase shift mask structure is accomplished by removing pre-
cise amounts of the substrate material to achieve the desired phase shift.
Once a mask is made it must be inspected for defects, the position and type
of defect must be cataloged and the data made available to the machine that
will effect repair. Since the price of mask-making is so high, development
costs have to be minimized, so that the number of development cycles should
be reduced and shortened in length. Efforts to this aim can be realized if the
repair system has sufficient ‘‘smarts’’ to recognize, prescribe, and effect
repairs. This requires not only a robust optical system with sufficient reso-
lution in metrology to allow milling and deposition placement accuracy and
quality, but must also possess sophisticated image recognition and pattern
generation and overlay capabilities. Thus, the repair machine must be highly
sophisticated in both hardware and software.
The system has to be calibrated and the mask must be aligned so that
navigation to the defect sites can be carried out. Once a site is within the field
of view the final alignment must be made to place the beam properly to
perform the repair. One of the serious problems encountered during this
process is charging on the sample. With so much insulating material present,
Pattern structure
Binary mask
Phase shift mask
Attenuated
Alternative
Phase shift (de
g
rees)
Phase shift (degrees) and transparency (%)
Cr
Cr
Quartz
Quartz
Quartz
MoSiON
Figure 10.19 Cross-sectional structures of different mask types. Top: Binary
chrome-on-glass. Middle and bottom: examples of two phase shift mask
designs.
Focused ion beam systems288
in lieu of charge neutralization controlled navigation, milling, and deposition
would be impossible. In general, a flood electron gun provides sufficient
negative charge to cause neutralization.
Examples of repaired defects in a mask are shown in Figures 10.20 and
10.21. In the case of clear defects, it is fairly easy to deposit carbon thick
enough to fix the defect. A variety of proprietary materials are available. The
resultant opacity is sufficient to make the mask useable As the repair systems
become more sophisticated, there must exist graphical user interfaces capable
of allowing the operator to use software that will allow a copy of a known
good area of the mask onto a defect site, so that multiple repairs at different
sites can be performed consecutively easily. This requires the system to have
the ability to generate an image of the good site with sufficient signal/noise
and then align it and overlay it onto the defect site, and if necessary modify
the image appropiately if needed, and proceed with the repair. The system
should also be able to determine what geometries are defective, and whether
(a) (b)
Figure 10.20 Images of a clear defect that was repaired by depositing a
carbon patch. The broken line in (a) was patched together as seen in (b).
(Courtesy of FEI Co.)
(a) (b)
Figure 10.21 Images of an opaque defect bridging two lines (a) that was
repaired by milling it away with the Ga ion beam (b). (Courtesy of FEI Co.)
Micro-machining and mask repair 289
the defect is clear or opaque. An example of such an operation is shown in
Figure 10.22, where a defect (which was buried) was automatically corrected
by overlaying an image of a good site onto the defect site.
On masks that employ optical proximity correction (OPC) [9–11], where
there are complex and intricate shapes, this technique is especially useful as
shown in Figures 10.23 and 10.24. OPC applies systematic changes to
photomask geometries to compensate for nonlinear distortions caused by
optical diffraction resist process effects. These distortions include line-width
(a) (b)
Figure 10.22 An example of a defect corrected by overlaying a good site
image onto the defect and automatically determining how to fix the defect.
Image (a) shows the original defect, while image (b) is the same area afte r the
repair. (Courtesy of FEI Co.)
Original Anti-serifs
Mask E-field Intensity Wafer
d = 1/2 (n–1)
Design
Reticle
Wafer
Chrome
Etched quartz (d)
Quartz
Resist
threshold
Silicon
Bright (+)
e
i
i ? e
2
Bright (–)
Dark (0)
Figure 10.23 An exampl e of the intent and nature of optical proximity
correction (OPC). The purpose is to suitably modify the design so that it will
be replicated into the wafer faithfully by correcting for both optical and
process distortions.
Focused ion beam systems290
variations dependent on pattern density, and on line-end shortening which
can break connections to contacts. Other distortions which can be reduced
are diffusion and loading effects during resist and etch processing of the
mask. OPC makes changes to the IC layout that anticipate the distortions.
To compensate for line-end shortening for example, the line is extended using
a hammerhead shape that results in a line in the resist that is much closer to
the original intended layout. To compensate for corner rounding, serif shapes
are added or subtracted from corners to produce corners in the silicon closer
to the ideal layout As masks are designed to attain the highest resolution
possible from the steppers which use them, one of the resolution enhance-
ment techniques is the embedded phase shifter mask, of which the most
common type is the MoSiON mask. A demonstration of the ability to
mill selectively the mask absorber instead of the substrate is shown in
Figure 10.25, where a contact hole in a phase shift MoSiON mask is
obscured. The gas used in this process shown in Figure 10.25 is proprietary,
(b)(a)
Figure 10.24 (a) An FIB image of a defective OPC pattern (0.2 mm defect).
(b) The same area after the repair. (Courtesy of FEI Co.)
(a) (b)
Figure 10.25 Repair of an opaque defect in a MoSiON phase shift mask. In
(a) there is an obscured hole that must be opened up. This flaw is shown
corrected in (b). (Cou rtesy of FEI Co.)
Micro-machining and mask repair 291
and demonstrates the kind of high selectivity necessary in order to minimize
the effect of the incident ion beam on the clear parts of the mask. One of the
concerns in using Ga ion beams to remove opaque mask material is that
implantation of Ga into the quartz substrate can cause staining in the clear
areas which modifies the optical properties of the quartz. There are known
post processing techniques to remove the staining, but they add time and
complexity to the repair.
10.13 Future FIB mask repair machines
As feature sizes become smaller and masks become more complex in order to
compensate for distortions in processing, repairs to the masks become more
difficult to perform. In addition, what have in the past been two-dimensional
repairs are quickly becoming three-dimensional in nature. As lithography
heads below the printing of 0.13 mm lines to the 65 nm node, strong phase
shifting techniques will be imperative. One such technique uses alternating
phase shift masks (see Figure 10.19), which are very costly and difficult to
manufacture, and have complex three-dimensional structures. Since the
quartz substrate has varying thickness, repairs of the bumps in the quartz
become problematic. Clearly a process not involving Ga would be welcome.
However, a most efficaceous method for the removal of chrome at high
resolution is found in the use of the Ga beam. To resolve this, a suitable
combination of three techniques (FIB, SEM, and AFM) appears to be a good
solution. The dual-beam machines discussed in Chapter 5 offer the possibility
of suitable mask repair solutions into the future. The SEM is capable of high
resolution imaging without implanting baryons into the sample, and under
suitable conditions can compensate for sample surface charging and also can
initiate deposition of materials if gases are introduced. Figure 10.26
demonstrates the capability of an ‘‘environmental’’ SEM to form pristine
images in cases where normal SEMs exhibit serious charging. Thus the SEM
can be an adjunct partner to the repair process, both as a navigation tool and
a means of performing gas assisted operations.
In addition to the association of SEM technology with FIB technology,
there also is another nanoscale metrology tool available for integration: the
AFM. The AFM is capable of profiling a surface at the nanometer scale, so
that the three-dimensional profiles of defects can be accurately mapped and
measured, and the data used to arrive at an optimal repair strategy. The
AFM can also be used to scrape away and remove material, either substrate
or absorber. Figure 10.27 shows the possibility of such technology.
Focused ion beam systems292
10.14 Conclusion
FIB technology, by utilizing the interactions of ions with matter, is capable of
performing micro-machining and deposition of material at the micro- and
nano-scales. FIB has been invaluable in the development and production of
microelectronics through failure analysis and circuit modification, but also in
repairing the masks that generate the devices in Si. The use of FIB will
continue, and mask repair is a potential future area of contribution for FIB,
but fundamental physical limitations may require that it be integrated with
SEM and AFM technology to be able to be useful in the future.
Acknowledgements
It is always a pleasure to thank those who share their knowledge and skills
freely. Too many people to list here have taken their time to show and
(a) (b)
Figure 10.26 Two images of the same area on a photomask. (a) A normal
high vacuum SEM image exhibiting serious charging of the sample,
rendering the image useless. (b) An ESEM (environmental SEM) image of
the same sample taken at elevated pressures, showing that it is possible to
completely remove the sample charging problem. (Courtesy of FEI Co. )
(a) (b)
Figure 10.27 The removal of material with an AFM. The defect in (a) has
been removed by the AFM, as shown in (b). (Courtesy of FEI Co.)
Micro-machining and mask repair 293
explain things to me. Diane Stewart of FEI Co. is an expert in the repair of
masks, and she gave me invaluable assistance here.
References
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Focused ion beam systems294
11
Three-dimensional visualization of nanostructured
materials using focused ion beam tomography
derren dunn
IBM Microelectronics
alan j. kubis and robert hull
University of Virginia
11.1 Introduction
One of the fundamental goals of materials science is to establish structure
property relationships. Historically, structure property relationships in
materials have been established with great success using imaging and
spectroscopic methods that measure signals that vary in one and two
dimensions. For example, X-ray and neutron diffraction techniques are
typically used to measure average physical and crystallographic properties
of relatively large volumes of material. While different modes exist for these
techniques such as spot modes and surface reflectance modes, they
effectively average over volumes of materials properties and produce
information that varies in at most two-dimensions. Auger electron spec-
troscopy (AES) and secondary ion mass spectroscopy (SIMS), while cap-
able of high spatial resolution in surface mapping modes and depth
profiling modes, are traditionally used to produce maps and profiles in one
and two dimensions.
High-resolution imaging techniques, such as scanning and transmission
electron microscopy, generally produce two-dimensional real space
intensity maps of surfaces or an image that has been averaged through a
sample thickness, as is the case in conventional transmission electron
microscopy.
Scanning probe microscopy techniques, such as atomic force microscopy
and scanning tunneling microscopy, may be regarded as providing a measure
of surface variation in three dimensions. In particular, both techniques are
capable of measuring in-plane variations in position with near atomic scale
Focused Ion Beam Systems: Basics and Applications, ed. N. Yao.
Published by Cambridge University Press. ª Cambridge University Press 2007.
295
lateral resolution as well as sub-atomic scale measurements parallel to a local
surface normal. These scanning probe techniques are limited to surface
measurements, however, and in general cannot directly measure sub-surface
information.
In an effort to investigate structural relationships in three dimensions,
several two-dimensional characterization techniques have been adapted to
incorporate tomographic reconstruction principles to reconstruct object
volumes and chemical maps in three dimensions. There are primarily two
methods employed to generate three-dimensional reconstructions of materi-
als, methods that reconstruct three-dimensional objects from projections and
direct sectioning methods. Projection methods make use of images and
chemical maps collected from an object at many different angles. These
images and maps are then transformed using Radon transforms and back-
plane projections to reconstruct volumes and form a three-dimensional
representation of an object. One of the most common probes for tomographic
reconstructions that use projection methods are X-rays. Copley et al.[1] give
an overview of this technique and a comparison of various instrumental
variables. X-ray tomography records X-ray attenuation as a function of
angle to produce a two-dimensional image. A series of these images can be
obtained at various positions and then a three-dimensional volume is
reconstructed using projection methods.
A new X-ray tomography technique, cone-beam X-ray tomography, is
currently being investigated for medical applications [2]. The advantage of
this technique is that an array detector is implemented which has a higher
effective photon collection efficiency so that there is no need for interpola-
tion. There is an added complexity to this technique in that complex
deconvolution algorithms are needed and scattered photons contribute to the
noise. Whether this technique will find applications in materials analysis is
yet to be seen. In some cases elemental distributions in a material can be
inferred [3], but since attenuation is dependent on the density of the material
and the linear attenuation coefficient of elemental constituents, care should
be taken in interpreting data.
Many different types of X-ray sources can be used to perform X-ray
tomography but to achieve high spatial resolutions, a synchrotron source is
used to obtain a monochromic primary beam. With synchrotron radiation
spatial resolution of up to 1 mm can be obtained. The major advantage of this
technique over most other micro-tomographic techniques is that it is non-
destructive. This allows a sample to be used for other testing or it allows the
evolution of a material to be followed as a function of time and experimental
conditions [4–9].
Focused ion beam systems296
Transmission electron microscopy (TEM) imaging techniques have also
been adapted to produce tomographic reconstructions in several modes.
Bright-field imaging has been used in tilt series modes to collect through
thickness projections of objects as a function of angle. Back projection and
Radon transform methods are then applied to these tilt series images to
reconstruction volume representations of an object. For example, this
method has been used successfully to reconstruct vias used in wiring struc-
tures in integrated circuits [10]. In particular, reconstructions using BF
images were used to investigate the roughness and step coverage of liner
materials in via structures, which is a critical area of investigation in product
failure analysis for the semiconductor industry.
Recently, another application of TEM to reconstruct three-dimensional
objects has been realized using electron holography. In electron holography,
a bi-prism is used in conjunction with field-emission electron sources to
measure both the amplitude and phase in transmitted images. As part of
holographic image processing, phase information is used to produce three-
dimensional reconstructions of an object. Electron holography has been used
successfully to elucidate the three-dimensional structure of ultra-fine particles
used in catalysis [11].
Direct section methods are typically done in two modes, atom-by-atom
methods and serial sectioning. Atom-by-atom methods typically employ field
ion microscopy (FIM) [12–14]. In FIM, atomic scale reconstructions of three-
dimensional elemental positions are performed with the aid of time of flight
mass spectrometry (TOFMS) and a position sensitive detector. A sample is
formed into a probe with a tip radius of less than 50 nm and a taper angle of
less than 10
. Monolayers of atoms are field desorbed in a pulse mode and
spatially and mass analyzed in the TOFMS. This information is then
reconstructed into a three-dimensional volume with close to atomic resolu-
tion. The field of view of this technique is limited to less than a cubic
micrometer, and requires specialized sample fabrication into very specialized
geometries. Recent advances in sources, detectors, and sample preparation
methods have eased constraints for this technique and have shown con-
siderable promise for ultra-high-resolution characterization of materials with
0.2 nm resolution [15–17].
Serial sectioning methods have found wide application and involve the
slicing of an object at regular increments and the collection of images and
chemical maps from each slice. Volume reconstructions from these images
and chemical maps are then created by interpolating image and chemical
signals between slices to form a three-dimensional representation.
Three-dimensional visualization of nanostructured materials 297