Application of High Pressure/Environmental Scanning
Electron Microscopy to Photomask Dimensional Metrology
Michael T. Postek and Andras E. Vladar [1]
National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
Abstract. The application of high pressure or environmental microscopy techniques is not new to scanning electron
microscopy. However, application of this methodology to semiconductor metrology is new because of the combined need
for implementation of high resolution, high brightness field emission technology in conjunction with large chamber and
sample transfer capabilities. This methodology employs a gaseous environment to help neutralize the charge build-up that
occurs under irradiation with the electron beam. Although potentially very desirable for the charge neutralization, this
methodology has not been seriously employed in photomask or wafer metrology until now. This is a new application of this
technology to this area, but it shows great promise in the inspection, imaging and metrology of photomasks in a charge-free
operational mode. For accurate metrology, this methodology affords a path that minimizes, if not eliminates, the need for
charge modeling.
INTRODUCTION
The application of high pressure or environmental
microscopy techniques is not new to scanning electron
microscopy (SEM). This form of microscopy was
originally proposed early in the development of SEM,
has slowly developed and has been most recently been
utilized in biological, food and chemical science
applications. The application of environmental
microscopy to production semiconductor metrology is
new because of the need for the combined
implementation of high resolution, high brightness
field emission technology in conjunction with large
chamber and sample transfer capabilities. This overall
combination of technology has not been available until
just recently.
High-pressure microscopy offers the advantage and
possible application of higher landing energies or
accelerating voltages, different contrast mechanisms
and charge neutralization [2,3]. Higher landing
energies mean higher resolution imaging is possible
rather than at the lower accelerating voltages. This
methodology employs a gaseous environment to help
neutralize the charge build-up that occurs under
irradiation with the electron beam. Although
potentially very desirable for the charge neutralization,
this methodology has not been seriously employed in
photomask or wafer metrology until now. This is a new
application of this technology to this area and much
needs to be learned. But, this technology shows great
promise in the inspection, imaging and metrology of
photomasks in a charge-free operational mode. It has
been reported that even at high accelerating voltage,
injection of air of as little as 20 Pa (0.15 Torr) into the
specimen chamber can reduce the charging potential of
an insulator at the surface by as much as an order of
magnitude [4]. In addition, this methodology affords a
path that minimizes, if not eliminates, the need for
charge modeling which is needed for higher accuracy
measurements. The modeling of charging is
exceptionally difficult since each sample, instrument
and operating mode can respond to charging in
different ways. Therefore, this methodology shows
great potential if the optimal balance can be achieved
in a reproducible manner. Further research is currently
underway to understand the ways to optimize these
operating conditions. This paper presents some new
results in high pressure SEM metrology of photomasks.
PHOTOMASK METROLOGY
Photomask dimensional metrology, especially that
associated with scanning electron microscopes, has not
evolved as rapidly as the metrology of integrated
circuit and resists features on wafers. This has largely
been due to: 1) the distinct emphasis placed on the
value of wafer production as opposed to mask
production; 2) the fact that far fewer photomask
metrology and inspection instruments are needed in
production applications, 3) photomask metrology
technology significantly leverages wafer metrology
technology improvements and 4) the distinct
technological advantage afforded by the 4x or 5x
reduction used in the optical steppers and scanners of
the lithography process and 5) there was previously a
lesser need to account for the real three-dimensionality
of the mask structures. Where photomasks are
concerned, many of the issues challenging wafer
dimensional metrology at Ix are reduced by a factor of
4 or 5 and thus have been swept aside - temporarily.
This is rapidly changing with the introduction of
advanced masks with optical proximity correction and
CP683, Characterization and Metrology for ULSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
2003 American Institute of Physics 0-7354-0152-7/03/$20.00
396
Typical SEM Chamber Pressure
Data Points
FIGURE 1. Graphical representation of the approximate location of the environmental or high-pressure SEM operating
conditions as compared to those of standard instruments. The most effective operational range is currently being investigated.
phase shifting features used in 100 nm and smaller
circuit generations. The International Technology
Roadmap for Semiconductors (ITRS) 2001 Edition
states, "mask linewidth controllability fails to meet the
requirements of the chipmakers" [5]. Fortunately,
photomask metrology generally benefits directly from
the advances made for wafer metrology so many issues
can be readily resolved.
The need for accurate measurements has been known
for many years. Accurate NIST optical photomask
standards have been available since around 1978 and
several iterations of these standards have been made
available to the industry. Accurate "wafer" standards
have not been made available for a number of technical
and monetary reasons although there is reason to
believe that a production relevant, full-wafer SEM
standard will soon become available [6]. As stated
above, electron beam-based photomask metrology has
been around for many years, but has not been as
extensively utilized in the production metrology of
photomasks. The issues regarding SEM metrology are
as complex as those for optical metrology and were
reviewed by Postek and Vladar [2], Postek and
Larrabee [7] and Postek and Joy [8]. Unfortunately,
since not as large a number of inspection instruments
for photomask metrology are required for
semiconductor manufacturing this not been as hot a
topic for industrial research and development. In fact,
finding a dedicated scanned electron beam photomask
metrology instrument today is not possible in the U. S.
at this time, since most if not all mask inspection is
done on modified wafer metrology instruments.
Clearly, defect analysis and repair of reticles are
exceptions to this trend and are not a topic of this
current work.
397
HIGH PRESSURE MICROSCOPY
High pressure or environmental microscopy is an
approach to photomask inspection that has until now
not been fully explored [9,10]. The methodology
employs a gaseous environment surrounding the
sample to help neutralize the charge. Typically the gas
used for photomask inspection is water vapor (although
other gasses can be used). As shown in Figure 1, a
typical SEM operates with a sample chamber pressure
of about 6.7xlO"3 Pa (5xlO~ 5 Torr). For high-pressure
microscopy work, the chamber pressure is allowed to
rise to the realm of about 20 to 160 Pa by the injection
of the water vapor (as compared to atmospheric
pressure of 101,325 Pa). As shown in the figure, these
operating conditions are magnitudes different than
current standard SEM operating parameters.
The primary electron beam passes through the water
vapor, interacts with it and creates positive ions. The
primary electron beam continues on and strikes the
surface of the mask and undergoes the typical
sample/beam interactions. Electrons from the primary
beam can create a negative charge on the insulating
mask surface. The negative charge developed on the
mask cause the positive ions from the gas interaction to
drift toward the mask, neutralizing the negative charge
on the sample. Concurrently, the signal electrons are
accelerated toward the detector by an electric field. As
they proceed, they also interact with and ionize
additional water vapor, creating additional ions and
electrons. This multiplication enhances both the charge
neutralization process and the signal collected.
Depending upon the design of the instrument either the
secondary electrons, backscattered electrons or light is
collected as the signal forming mechanism. The
resulting process eliminates charging effects (beam
FIGURE 2. Scanning electron micrographs of binary photomask structures using the environmental SEM. (a) Image taken at 160
Pa (1.2 Torr) at 100,000x magnification showing the lack of sample charging at 10 keV accelerating voltage, (b) Micrograph of a
single photomask line using high-pressure microscopy conditions showing the extent of the line edge roughness present.
distortion and shift) and can provide high quality
images of dielectric samples as shown in Figure 2.
Figure 2 shows two images of photomasks taken at
high accelerating voltages with the pressure levels
between 160 Pa (1.2 Torr) and 107 Pa (0.7 Torr).
For various technical reasons, high-pressure
microscopy has mostly been employed on specimens
of biological nature and not on many semiconductor
samples. Although potentially desirable for charge
neutralization, this methodology has not been seriously
employed in photomask or wafer metrology until now.
High-pressure microscopy offers advantages of the
possible application of higher accelerating voltages and
different contrast mechanisms [2]. This is a new
application of this technology to this area, but it shows
great promise in the inspection, imaging and metrology
of the photomasks.
One significant benefit afforded by this technology is
that for accurate metrology, this methodology affords a
potential path that minimizes, // not eliminates, the
need for charge modeling. The modeling of charging
is exceptionally difficult since each sample, instrument
and operating mode can respond to charging in
different ways. This methodology shows great
potential if the optimal balance can be achieved in a
reproducible manner.
metrology has generally benefited from the advances
made for wafer metrology. In this instance, the
converse might eventually prove to be true if these
methods can be successfully transferred to wafer
applications. International SEMATECH and NIST are
currently investigating the potential this affords to
production photomask metrology.
ACKNOWLEDGEMENTS
The authors would like to thank and acknowledge the
excellent collaboration and technical support provided
by Trisha Rice, Ralph Knowles, Ed Griffith and others
at FEI Company in obtaining the high
pressure/environmental micrographs from the MDA
600 tool [11]. The authors would like to thank Marylyn
Bennett (Texas Instruments/I SMT), Pat Marmillion
(IBM/ISMT), Bill Banke (IBM) and Bhanwar Singh
(AMD) for supplying the photomasks, the Office of
Microelectronics Programs for their support during this
research and Dr. Robert Larrabee, and Mr. Samuel
Jones for their technical comments and assistance.
REFERENCES
1.
CONCLUSION
Environmental or high pressure scanning electron
microscopy affords a new approach in to the accurate
inspection and metrology of photomask samples. The
minimization if not elimination of the charging
currently limiting the inspection of photomasks in the
SEM is a significant step forward for this work. Mask
398
Contribution of the National Institute of Standards
and Technology, not subject to copyright.
2. Postek, M. T. and A. E. Vladar. In Handbook of
Silicon Semiconductor Metrology (Alain Diebold,
ed.) Marcel Dekker, New York,.2000, pp. 295333.
3. Postek, M. T., Vladar A. E. and Bennett, M. H.
SPIE Photomask Symposium 2003, in press.
4. Joy, D. C. Proc. SPIE 4689, 2002, pp.1-10.
5.
Semiconductor Industry Association. International
Technology Roadmap for Semiconductors 2001
Edition. 2001. http://public.itrs.net.
6. Postek, M. T., A. E. Vladar and J. Villarrubia.
Proc. SPIE 3988, 2001, pp.42-56.
7. Postek, M. T. and Larrabee, R. D. In. Fine-Line
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11. Certain commercial equipment is identified in this
report to adequately describe the experimental
procedure. Such identification does not imply
recommendation or endorsement by the National
Institute of Standards and Technology, nor does it
imply that the equipment identified is necessarily
the best available for the purpose.