Charge control during photomask Metrology.pdf

Charge Control During Photomask Critical Dimension (CD)
Metrology
International SEMATECH
Technology Transfer #03114452B-ENG

© 2004 International SEMATECH, Inc.
International SEMATECH and the International SEMATECH logo are registered service marks of International
SEMATECH, Inc., a wholly-owned subsidiary of SEMATECH, Inc.
Product names and company names used in this publication are for identification purposes only and may be
trademarks or service marks of their respective companies.

Charge Control During Photomask Critical Dimension (CD) Metrology
February 27, 2004
Abstract: This report from the LITJ231 project discusses experiments to control charging during critical
dimension (CD) measurements of advanced photomask structures and the development of an
advanced secondary electron detector for a low vacuum CD metrology tool. This revision updates
the sections about mask metrology in high vacuum and in the presence of gas as well as enhancing
environmental scattered electron detectors (ESEDs).
Keywords: Scanning Electron Microscopes, Critical Dimension, Masks, High Vacuum, Electron Beam
Lithography
Authors: David Joy (University of Tennessee)
Approvals: Pat Marmillion, Project Manager
Marylyn Bennett, Project Manager
Scott Hector, Program Manager
Giang Dao, Director
Laurie Modrey, Technical Information Transfer Team Leader

iii
Table of Contents
1 EXECUTIVE SUMMARY.................................................................................................1
2 INTRODUCTION ..............................................................................................................1
2.1 Background ................................................................................................................1
2.2 Mask Metrology at High Vacuum...............................................................................3
2.3 Metrology in the Presence of a Gas.............................................................................5
3 ENHANCING THE ESED ...............................................................................................10
4 SUMMARY AND CONCLUSIONS.................................................................................12
List of Figures
Figure 1 Total Electron Yield vs. Energy Profile for Quartz Showing the E1 and E2
Conditions..............................................................................................................2
Figure 2 Variation of Surface Potential as a Function of Energy for an Ungrounded
Surface Layer on an Insulating Specimen...............................................................2
Figure 3 High Vacuum Image of Mask IBM2.......................................................................4
Figure 4 Measured Variation of the Surface Potential of a Binary Mask (for 10 keV
incident energy) as a Function of the Chamber Pressure of the Air or
Helium Gas ............................................................................................................6
Figure 5 High Resolution ESED Image of the Surface Detail on the Chromium Layer
of a Binary Mask (recorded at 20 keV beam energy at a pressure of 100 Pa
of air in a Hitachi S4300 SE/N)..............................................................................7
Figure 6 BSE Image Recorded at 20 keV (and a gas pressure of 30 Pa of air) of the
Mask IBM3 in an Hitachi S4300SE/N VP-SEM.....................................................8
Figure 7 Comparison of BSE and SE Images and Signal Profiles Recorded at 20 keV
From the Mask IBM3.............................................................................................9
Figure 8 Schematic Diagram of the Operation of Ion Displacement Current GSEDs ..........11
Figure 9 ESED Image of Structures on an attPSM Recorded at 15 keV (at a gas
pressure of 100 Pa in an Hitachi S4300SE/N).......................................................12
List of Tables
Table 1 Surface Potential of Test Masks..............................................................................3
Table 2 Measured Pitch Data ..............................................................................................4
Table 3 Pitch Data Using BSE Signal .................................................................................8
Table 4 Pitch Data Using SE Signal....................................................................................9

iv
Technology Transfer #03114452B-ENG International SEMATECH
Acknowledgments
We are grateful to Dr. S. Nash (IBM), Dr. T. Liang (INTEL), and P. Marmillion (ISMT) for the
provision of mask samples and to Dr. M. Davidson for making the SPECTEL Metrologia
software available.

1
International SEMATECH Technology Transfer #03114452B-ENG
1 EXECUTIVE SUMMARY
Performing critical dimension mask metrology under high vacuum conditions inevitably leads to
significant charging of the mask, which, in turn, results in degraded precision and accuracy. As
demonstrated in the work reported here, the use of a low pressure gas environment around the
mask offers substantial benefits. The tool can be operated over a wide range of incident beam
energies, rather than being restricted to a single critical value. This enhances image resolution
and beam current from the tool without the need for any new technology developments. The
presence of the gas stabilizes and minimizes the charging, resulting in demonstrably superior
accuracy and precision in the metrology because of the elimination of temporally and spatially
dependent changes in magnification and beam position. Finally, because the microscope can be
operated at higher beam energies either secondary or backscattered electrons can be employed.
In many cases the backscattered electron (BSE) mode appears to offer valuable benefits. If
scattered electron (SE) imaging is to be retained then the current environmental scattered
electron detectors (ESEDs) are limited in bandwidth and speed and alternative detectors are
required.
2 INTRODUCTION
This project had five distinct goals:
1. Document the problems associated with performing mask metrology in a conventional
high vacuum scanning electron microscope (SEM) and determine the magnitude of
charging for a variety of masks when observed in the high vacuum SEM
2. Examine the precision of critical dimension (CD) measurements made on masks under
optimized conditions at high vacuum
3. Demonstrate how using a low vacuum (variable pressure) SEM eliminates or minimizes
the problems found in high vacuum operation
4. Determine the optimum operational conditions for the mask samples available and
determine the precision of CD measurements made on these masks under optimum low
vacuum conditions
5. Measure the efficiency of the ion SE detector systems used in the variable pressure
(VP)-SEM and investigate techniques to build a faster and more effective detector
2.1 Background
Masks have become an ever more important part of semiconductor technology, and as device
feature sizes move towards the 70 nm node the requirements on critical dimension mask
metrology, even for 4X reduction, have become much more severe. All current mask varieties
consist of metallic, or at least conductive, features positioned on an insulating substrate, such as
quartz, which causes the problems commonly encountered in electron-beam metrology. Non-
conducting materials can be examined in a stable manner under an e-beam by adjusting the beam
energy to the energies, E1 or E2, at which the total electron yield from the material equals unity
(see Figure 1). In this condition, there is a dynamic charge balance, one incident electron
producing on average one exiting electron. Consequently, the charge state of the sample does not
change. When a thin layer of conducting or insulating material is placed on top of the insulating
substrate, then the situation changes. If the beam energy is too low for the beam to penetrate the
layer, then charge balance will be obtained at E1 and E2 energies appropriate to the material
beneath the beam as before. But if the energy is increased to the point where the beam penetrates

2
the layer, then the substrate begins to affect the emission of electrons from the surface layer. As
shown in Figure 2, this leads to a condition, at energy E3, at which the layer is again in dynamic
charge balance. E3 is always higher than the E2 energy of the substrate on which the layer is
placed. Consequently choosing the beam energy to be equal to E2 to limit the charging of the
substrate will result in negative charging of the layer. Choosing the beam energy to be equal to
E3 will ensure that the layer is not charged but the substrate will charge negatively. Hence mask
charging, and the image artifacts that it produces, cannot be avoided simply by adjusting the
incident beam energy.
2.0
1.5
1.0
0.5
0
0.01 0.1 1.0 10
E1
E2
Energy (keV)
Total
Yield
Quartz
Figure 1 Total Electron Yield vs. Energy Profile for Quartz Showing the E1 and E2
Conditions
Surface
Potential
Beam Energy
Transmission
starts through
the layer
-ve
+ve
E1 E2 E3
Positive at all
energies > E3
Note: The E3 condition is indicated.
Figure 2 Variation of Surface Potential as a Function of Energy for an Ungrounded
Surface Layer on an Insulating Specimen

3
The consequences of this situation have been investigated experimentally. Seven current
production mask types, including binary, attenuated phase shift masks (attPSMs), were first
measured to determine the E2 energy of the substrate (see Table 1). The data were recorded at an
incident beam current of 100 picoA in an Hitachi S4300 SE/N Schottky field emission SEM.
Table 1 Surface Potential of Test Masks
Supplier E2 (keV) Surface Potential (V)
Intel – unknown 2.4 -50
ISMT Sample 1 – binary 1.9 -320
ISMT Sample 2 – binary 2.2 -110
IBM Sample 4 – attPSM 2 -270
IBM Sample 3 – attPSM 2.3 -80
IBM Sample 2 – attPSM 2.5 +20
IBM Sample 1 – attPSM 2.2 -110
For all of the samples examined, the E2 energy for the substrate material fell between 1.9 and
2.5 keV. Images recorded at the E2 energy showed the variations in brightness anticipated from
the negative charging of the surrounding metal layers. To compare the magnitude of the charge
buildup between the different masks, the beam energy was fixed at 2.5 keV and the surface
potential was determined by noting the adjustment required to bring the image back into focus.
These potentials, tabulated in the third column of Table 1, show potentials varying from -320 V
to +20 V. While these potentials are modest in magnitude, the defocus that they produce and the
disruption in secondary electron signal collection that results from the fields that accompany
them are more than sufficient to cause images of poor quality and to produce repeated failures in
pattern recognition algorithms. Also, since the beam penetration into the substrate is less than
0.1 micrometers at these energies, a surface potential of just 100 volts results in a field of
1E+9 V/m, which is close to the dielectric breakdown value. Such charging can actually
physically damage the mask.
2.2 Mask Metrology at High Vacuum
To study the effect of charging on CD metrology, measurements were made at the E3 energy,
which was experimentally determined at each location of interest by the operator. This is
necessary since the value of E3 changes with the thickness of the layer and with its disposition
relative to surrounding areas (isolated, dense, or semi-isolated). For example, on mask sample
IBM2, parallel lines structures were examined (Figure 3) at the local E3 value of 3.0 keV.
Images recorded at this E3 condition at TV scan or slow speed scan rates were visually
acceptable, but moving the sample laterally by only a few tens of micrometers often required an
adjustment of the beam energy to match the new local E3 value before satisfactory imaging was
restored. Such effects as these cause the failure of pattern recognition routines often experienced
during automated mask metrology. In a typical experiment, the area shown in Figure 3 was
recorded as a single 20-second slow scan image. Metrology was then performed on the image
using the routines in the SPECTEL Metrologia package. Table 2 shows representative data.

4
Table 2 Measured Pitch Data
Position Pitch 1–2 (nm) Pitch 2–3 (nm)
A 1246.6 1255.4
B 1243.2 1255.7
C 1241.9 1257.0
D 1244.0 1256.0
Note: The annotation indicates the areas measured for the data in Table 2. The image was recorded at the local E3
energy value, here 3.0 keV.
Figure 3 High Vacuum Image of Mask IBM2
In the experiment, the beam scans sequentially from position A to B, C, and then to D and from
left to right across the image. The measured pitch data show that the data for the first pair of lines
1–2 falls from A to C and then recovers somewhat at D, while the data for the second pair of
lines 2–3 rises slightly between A and C before also falling back at D. This behavior occurs
because the progressive spread of charging, following the scan path of the electron beam, causes
local increases and decreases in the magnification of the image across the field of view.
Although data from the microscope used here cannot be directly compared with that from a
regular CD-SEM (because the tool lacks the same level of magnification calibration and control),
it illustrates how seriously charging degrades the quality of metrology.
Comparable results were also obtained on the other available masks. In every case, charging,
even after careful manual optimization at the E3 energy of the operating parameters, resulted in
major time-dependent distortions in the measurements. The magnitude of the distortion varied
from one situation to another but was always of the same order of magnitude as shown above. In
all cases, the visual quality of the images was degraded by the regions of positive and negative
charging outside the immediate field of view. In many cases, the magnitude of these effects was

5
found to be scan speed-dependent. For example, on an alternating phase shift masks, the high
speed (TV rate) scan image showed both positive and negative charging with consequent image
instabilities and systematic errors in the metrology. At low scan speeds, by comparison, the
charging was stable and always positive in sign.
Metal multi-layer (MML) EUV masks are a special case because all their surface layers are
highly conductive. Nevertheless, such masks do charge under the electron beam because of the
E3 effect discussed above. For a typical MML structure with a total stack thickness of 100 nm,
the beam begins to penetrate through to the substrate at a beam energy of about 2 keV. The mask
surface acquires a positive charge as evidenced from the black “scan square” that is visible on
the image after exposure. The image darkens because the positive surface potential recollects
some of the emitted secondary electrons, reducing the signal level. In conventional SEMs, this
effect is unwelcome because it lowers the signal-to-noise ratio, but is not serious because the
positive charging is self-limiting to just a few volts. But in CD SEMs that use strong retarding
field optics, the high extraction field off the surface makes it impossible for positively charged
regions to recollect any secondary electrons. Thus their potential floats higher without any
control. Consequently, large positive potentials can be built up leading to local reductions in
apparent image magnification and the generation of contamination, or carbon carryover, in the
charged area. Contamination is a result of the fringing field from the positively charged region
attracting surface hydrocarbons towards the irradiated region where they then suffer radiolysis
(i.e., ionizing beam damage) and are deposited as carbon films of low SE yield and poor
conductivity.
2.3 Metrology in the Presence of a Gas
The VP-SEM is similar to a conventional SEM except that a pressure limiting aperture (PLA) is
placed at the bottom of the objective lens between the specimen chamber and the upper part of
the electron-optical column. A computer-controlled leak valve can then maintain a stable gas
pressure in the specimen chamber while still ensuring a high enough vacuum at the electron gun.
On the Hitachi S4300SE/N instrument used for this work, the chamber pressure can be varied
from high vacuum (below 1E-4 Pa) to 1000 Pa (8 Torr) while maintaining a gun vacuum below
1E-7 Pa (1E-9 Torr). When an e-beam is incident on a non-conducting sample in a low
atmospheric pressure, either air or some other gas, then charging can be mitigated or eliminated
by an appropriate adjustment of the operating conditions. This is because the interaction of
electrons with the gas produces both positively and negatively charged species in the gas. These
ions then drift towards surface regions of the opposite charge state and compensate them. This
process is most strongly affected by the gas pressure, as shown in Figure 4, which plots the
surface potential of a binary mask as a function of this pressure. At an incident beam energy of
10 keV, the surface potential of the mask was close to -5 kV. As gas is bled into the specimen
chamber and around the specimen the potential initially falls, at first rapidly and then more
slowly, until for pressures on the order of 25 Pa (0.2 Torr) it settles at a value close to zero. The
form of this variation depends on the choice of gas. Figure 4 shows the behavior of both
laboratory air with a relative humidity of about 55% and of dry helium. In either case, the
endpoint is the same but charge reduction is more rapidly achieved with moist air than with dry
helium. The final surface potential is independent of the choice of gas and remains constant up to
the highest pressure that can be used in the microscope (1000 Pa or 8 Torr).

6
1
0
-2
-4
-5
1 10 100
Pressure (Pa)
Surface
Potential
(keV)
-3
-1
Air
Helium
Figure 4 Measured Variation of the Surface Potential of a Binary Mask (for 10 keV
incident energy) as a Function of the Chamber Pressure of the Air or Helium
Gas
At lower beam energies, the form of the potential variation is similar except that at the highest
pressures the surface acquires a small (~50 V), stable positive potential. At a beam energy of
30 keV, the functional form of the potential variation again remains the same, but at the highest
pressures the surface now levels out at a value of -300 to -500 V. This kind of behavior was
found on all of the mask samples tested. Although the exact values of the surface potentials for a
given beam energy and gas pressure vary slightly, overall the behavior is similar in every case
and very reproducible.
These experiments demonstrate that the low pressure gas environment provides a convenient
method for controlling sample charging. Unlike the above attempts to achieve stable imaging in
high vacuum by adjusting the beam energy—which inevitably result in a distribution of surface
potentials that lead to local variations in the imaging magnification and signal level—the gas
method results in a stable surface potential over the entire field of view and some region beyond
that. This potential is not necessarily zero, but because it does not spatially vary the
magnification of any image, is constant at all points, and can be calibrated by reference to a pitch
standard. The required operating condition can be found by simply raising the gas pressure until
the image is stable. A more optimized setup can be achieved by monitoring the absorbed electron
current flowing to ground from the sample stage while adjusting the pressure. This current
averages to zero when the surface potential is at zero. At the highest beam energies where the
surface may stabilize at a negative potential, the best pressure is the lowest value that will
minimize the absorbed current.

7
The pressure (P0) required to achieve stable imaging is found to depend on the gas path length
(GPL), which is the distance that the electron beam travels through the gas atmosphere. The
pressure for charge control follows the relationship
P0 × GPL ~ constant Eq. [1]
This can be explained by noting that a longer gas path allows for more secondary ionizations to
occur, enhancing the efficiency of the charge reduction. This relation is important because it
indicates how to achieve the best imaging. The incident e-beam is scattered as it passes through
the gas. Although the majority of the beam remains in a small focused probe, the scattered
electrons form a skirt or halo around the central probe, reducing both the incident beam current
and the contrast in images. The radius (r) of this skirt can be estimated by the equation
2
/
3
2
/
1
364
GPL
T
P
E
Z
rs ∗






∗
= Eq. [2]
where rs is in micrometers, Z is the atomic number of the gas allowing for mono- or divalent
groupings of atoms, E is the beam energy in keV, P is the pressure in Pa, and T is the
temperature in °K. For typical values of E, P, T, and GPL, this equation yields a beam skirt
radius of 10 to 100 micrometers. Comparing Eq. [2] to Eq. [1] shows that the best balance
between charge control and skirt radius is achieved by minimizing the GPL and then adjusting
the pressure to reach the charge balance condition. It is important to note that the presence of the
scattered skirt does not degrade the image resolution. As shown in Figure 5, high resolution
imaging is not affected by the skirt since this merely reduces the contrast and signal-to-noise
ratio.
Figure 5 High Resolution ESED Image of the Surface Detail on the Chromium Layer
of a Binary Mask (recorded at 20 keV beam energy at a pressure of 100 Pa of
air in a Hitachi S4300 SE/N)
Different gases result in larger or smaller amounts of beam scattering (i.e., depending on the
atomic number Z ) and, as noted earlier, in some variation in the amount of charge control
achieved. The atmosphere that provides the best combination of imaging resolution and charge
control is laboratory air because of the large amounts of water vapor that it typically contains.
However, the water vapor is itself ionized by the electron beam to form aggressive radicals that
can produce significant radiation damage on polymer resists and other organic materials. In
situations where this could be a problem, the optimum choice is helium since its low atomic
number and single atomicity result in only low levels of beam scattering and no attack on the

8
polymers or biological tissue. However, helium is not readily ionized when it is pure. Its efficacy
for charge control can be enhanced by adding a few percent of sulfur dioxide. Many other gases
have been investigated, but none shows any definite advantages over air or helium.
The quality of metrology in the microscope with gas present was evaluated by a procedure
similar to the one employed in the high vacuum case. On areas of the type shown in Figure 6, the
apparent pitch and linewidth of arrays were measured under slow scan conditions. In each case,
the pressure was set, as described above, to achieve zero or at least the minimum absorbed
current corresponding to the minimum surface potential. A beam voltage of 20 keV was chosen
for this exercise. Conventional CD-SEM metrology is always performed at low beam energies,
typically below 1 keV, mostly to minimize the charging and to restrict the electron-solid beam
interaction volume. Under gas environmental conditions, such a choice would not be appropriate
because of the high amount of scattering that would be experienced by the beam (see Eq. [2]).
Although VP-SEM imaging is possible at energies below 3 keV, moving to 20 keV or a similar
energy minimizes the gas scattering and substantially enhances the electron-optical performance
of the SEM, providing both a smaller spot size and more beam current. This choice of beam
energy also makes it possible to collect and analyze both SE and BSE signals.
Figure 6 BSE Image Recorded at 20 keV (and a gas pressure of 30 Pa of air) of the
Mask IBM3 in an Hitachi S4300SE/N VP-SEM
For example, metrology was performed on IBM Mask 3 at 20 keV, using the BSE signal in
laboratory air at a pressure of 30Pa. As before, pitch data were compared for adjacent pairs of
lines and at different positions in the scan cycle. Linewidth was also measured (see Table 3).
Table 3 Pitch Data Using BSE Signal
Position Pitch 1–2 (nm) Pitch 2–3 (nm) Linewidth (nm)
1 748.2 754.4 366.2
2 751.6 753.6 363.1
3 748.3 752.4 364.8
4 751.3 752.3 367.8

9
These data and the other comparable sets that have been collected are free from the distortion
that was evident in the high vacuum data. Instead of a systematic variation in the pitch caused by
the buildup of charge, the low vacuum data show only random variations in pitch and linewidth
values of a magnitude consistent with the errors to be anticipated from this tool.
The BSE data can also be compared with the corresponding values from the SE detector as
shown in Table 4.
Table 4 Pitch Data Using SE Signal
Position Pitch 1-2 (nm) Pitch 2-3 (nm) Linewidth (nm)
1 752.2 756.7 392.5
2 750.4 755.8 383.8
3 751.3 756.1 390.6
4 752.8 755.7 392.9
The pitch values once again display only a random variation in value as a function of their
position in the raster. The linewidth is similarly well controlled, confirming that either imaging
mode can be applied equally successfully. However, a comparison of the BSE and the SE
linewidth data shows that the SE values are always about 8% higher than the corresponding BSE
values. The BSE linewidth values are also found to be much less dependent on the measurement
algorithm (e.g., peak, threshold, regression to baseline) than the SE profiles. This can be
understood by comparing the line profiles in the two cases (see Figure 7). The SE image
delineates the edges very clearly, but these display a high value of apparent beam width. The
profiles on either side of a line appear to be affected by residual charging, resulting in a baseline
that is far from flat. By comparison, the BSE profile shows close to vertical edges and a flat
baseline. It can therefore be counted as an additional advantage of the variable pressure mode of
operation that the higher beam energy permits the effective use of the BSE detector for
metrology.
Figure 7 Comparison of BSE and SE Images and Signal Profiles Recorded at 20 keV
From the Mask IBM3

10
Further Monte Carlo simulations will need to be performed to identify the characteristics of
metrology performed at high energy, in the presence of a gas atmosphere, and with a variety of
detectors other than standard SE collectors. As is evident from the figures, the form of the signal
profile under these new operational parameters is different from that of conventional low energy,
secondary electron metrology. Consequently, an important task is to apply Monte Carlo
simulations to derive edge positions from the profiles and hence to develop more appropriate
algorithms for these conditions. In addition, it is necessary to determine what effect on the
metrology of the scattering of the electron beam in the gas atmosphere. Inelastic scattering in the
gas results in some amount of degradation of the beam profile and diameter, while elastic
scattering produces a “skirt” surrounding the beam and a fall in incident beam current. This
model must now be coupled to the profile simulation to provide a detailed overview of the
complete imaging process.
The above experiments have demonstrated that the VPSEM can perform mask metrology in the
presence of a gas atmosphere with higher precision and reproducibility than under comparable
high vacuum conditions. In addition, the VPSEM approach eliminates contamination (carbon
carryover) during measurements. This benefit, which accrues because contamination is driven by
charging so the control of one leads to the elimination of the other, can be expected to improve
precision in metrology. Although at this time no VPSEM is fitted with industrial strength pattern
recognition and focusing capability, it is evident that the ability of the gas interaction to stabilize
and control charging would also improve these facilities compared to the high vacuum condition.
A variable pressure SEM designed for mask metrology could, therefore, offer better accuracy
and precision and higher throughput, with less need for operator intervention, than tools
currently used for this purpose. An additional advantage is that a VP CD-SEM can also be used
for in situ mask editing and repair. These capabilities are obtained by injecting precursor gases,
such as WF6 or XeF2, that upon interaction with the electron beam can provide controlled and
selective etching and deposition of metal layers. A single tool could then handle all aspects of
mask metrology, editing, and repair. The modifications required to convert an existing VPSEM
platform into a prototypical VP CD-SEM would mostly consist of engineering a stage stable
enough for metrology, adding laser interferometry for stage positioning, and providing a more
flexible and responsive gas handling and control system.
3 ENHANCING THE ESED
An advantage of the low pressure, high beam energy approach to metrology is that backscattered
imaging can be used. Although the BSE mode offers significant benefits for metrology, many
users might still prefer to use the secondary electron signal. In gas, however, the conventional
Everhart Thornley SE detector cannot be used because the voltages that must be applied to the
scintillator to ensure efficient operation would result in a destructive flashover. Some other form
of detector must therefore be used. The most common alternatives, ESEDs or gaseous SE
detectors (GSEDs), rely instead on converting the secondary electrons into ions and then
detecting the ion signal. Figure 8 shows the general form of such a detector. Ions produced by
secondary electron interactions with the gas drift towards the electrode, generating a secondary
ion cascade as they accelerate in the field. The motion of this cloud of charge induces a
displacement current that can be detected by high gain amplifiers at either of the points shown.

11
Note: The signal can be collected either at the electrode or in the return line of the specimen to ground.
Figure 8 Schematic Diagram of the Operation of Ion Displacement Current GSEDs
Measurements of the performance of such displacement detectors show that they have an
excellent detective quantum efficiency (DQE). At a GPL of 5 mm, the detector used here had a
DQE of 0.45, which rivals that of a conventional SE detector. Such detectors are not, however,
very sensitive because they need high gain, DC-coupled amplifiers to operate. A more critical
problem is that these detectors are slow because they depend on the drift of the ions rather than
the electrons. Whereas transit times for secondary electrons to the detector are measured in
nanoseconds, the corresponding time for ions is on the order of hundreds of microseconds. This
manifests itself under the imaging conditions used for metrology as shown in Figure 9. First,
only slow scan speeds (frame raster times of a few seconds) are useful. Higher scan speeds result
in a rapid loss of image detail as ion transit times become comparable with pixel dwell times.
Even then, each feature edge parallel to the line scan direction produces a long decay tail. This is
symptomatic of poor slew rate response in the DC amplifiers and of a time-dependent change in
the gain of the ESED/GSED. The gain of these amplifiers depends critically on the electric field
near the sample surface. An abrupt change in SE signal, as occurs at an edge, rapidly increases
the ion production. This ion space charge partially screens the field experienced at the specimen,
leading to a drop in gain until ion drift and recombination restore the original steady-state
condition.
The behavior of these detectors has been modeled to identify how they can be improved.
Simulations show that while the gain can be improved by optimized electrode design, there is
little opportunity to increase the speed of signal response because the ion drift velocity cannot be
raised by any significant amount as ion-ion scattering and recombination becomes dominant at
higher fields. However, scan speed is less important in the VPSEM than it is in a high vacuum
tool since there is no need to “outrun” the charging effects. However, if SE rather than BSE
imaging is to be used for a VP CD-SEM, then other strategies will probably be preferable. One
suggestion is to use a standard SE detector but to place it inside a differentially pumped chamber
opened to receive the signal by a high transmission mesh grid. Because the local pressure around
the biased scintillator need be reduced to only about 0.01 Torr to avert flashover, not much
pumping is required. The result is a true SE detector that has above average sensitivity and a very
high speed compared to the gaseous “ion drift” SE detectors. Alternatively, a cathodo-

12
Note: The image was recorded for a scan time of 20 seconds. Note the artifacts extending in the line direction from
each horizontal line feature.
Figure 9 ESED Image of Structures on an attPSM Recorded at 15 keV (at a gas
pressure of 100 Pa in an Hitachi S4300SE/N)
luminescent (CL) detector that responds to the luminescence generated in the gas by the
secondary electrons is also feasible. Such an approach has been demonstrated on at least one
commercial tool with reasonable success. Such a detector is fast and requires no differential
pumping because it is collecting photons rather than charge particles. However in the usual
atmospheres of air or helium, the CL detector is inefficient: the images are noisy unless the raster
scan time is increased. This limitation could be removed by spiking the gas with a suitable
fluorescent agent, which can enhance the signal by many orders of magnitude, although it will be
necessary to show that no adverse effects on the beam profile or charge control are experienced.
4 SUMMARY AND CONCLUSIONS
Performing CD mask metrology under high vacuum conditions inevitably leads to significant
charging of the mask, which, in turn, results in degraded precision and accuracy. As
demonstrated in the work reported here, a low pressure gas environment around the mask offers
substantial benefits. The tool can be operated over a wide range of incident beam energies, rather
than being restricted to a single critical value. This enhances image resolution and beam current
from the tool without the need for any new technology developments. The presence of the gas
stabilizes and minimizes the charging, resulting in demonstrably superior accuracy and precision
in the metrology because temporally and spatially dependent changes in magnification and beam
position are eliminated. Finally, because the microscope can be operated at higher beam
energies, either secondary or backscattered electrons can be employed. In many cases, the BSE
mode appears to offer valuable benefits. If SE imaging is to be retained, then the current
environmental SE detectors will be limited in bandwidth and speed and alternative detectors will
be required.

International SEMATECH Technology Transfer
2706 Montopolis Drive
Austin, TX 78741
http://www.sematech.org
e-mail: info@sematech.org

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