MULTIPLE CD-SEM MATCHING FOR 0.18 µm LINES/SPACES AT DIFFERENT EXPOSURE CONDITIONS
André Engelen and Ingrid Minnaert-Janssen
ASML
De Run 1110
5503 LA, Veldhoven
The Netherlands
This paper was first presented at the
SPIE Microlithography Conference in March 1999
Santa Clara Convention Center, Santa Clara, California, USA.
MULTIPLE CD-SEM MATCHING FOR 0.18 µm LINES/SPACES AT DIFFERENT EXPOSURE CONDITIONS
André Engelen and Ingrid Minnaert-Janssen
ASML
De Run 1110
5503 LA, Veldhoven
The Netherlands
ABSTRACT
Critical Dimension Scanning Electron Microscopes (CD-SEMs) are used within ASML for evaluating the imaging
performance of Stepper and Step & Scan systems. This implies measuring a large number of Focus Exposure
Matrices (FEMs) and inter/intra-field CD measurements on different CD-SEMs. Therefore the CD-SEMs in ASML
are matched through focus (Best Focus ± 0.3 µm). The matching procedure is done on three steps. First, all
CD-SEMs are checked for stability. Then the magnification factor for each of the individual CD-SEMs is checked in
order to make sure that the tool is set up correctly. Finally, the CD measurements are matched through focus and
multiple exposure energies.
In this paper, we will show that the CD-SEMs of different vendors (Applied Materials and Hitachi) can be matched
using photoresist features, through focus within 5 nm for 0.18 µm features. This matching includes different
orientations (horizontal and vertical structures) and densities (isolated and nested structures) using only one
correction offset.
1. INTRODUCTION
different feature sizes, line orientations, and line
densities. In addition, we are matching CD-SEMs of
two vendors, Applied Materials and Hitachi.
Critical Dimension Scanning Electron Microscopes
(CD-SEMs) are used for evaluating the imaging
performance of Stepper and Step & Scan systems.
Typical figures of merit derived from these evaluations
are Depth of Focus (DoF), Exposure Latitude (EL) and
CD Uniformity (CDU). Typically Focus Exposure
Matrices (FEMs) involving large defocus and number
of energy steps are measured to determine these
figures of merit.
Currently eight CD-SEMs of various vendors and types
are used within ASML’s worldwide equipment set.
Therefore, tool-to-tool reproducibility, so called
matching, is very important. Additionally, the matching
contribution to the multiple tool precision will increase
when going to smaller feature sizes. As shown in table
1, the 1997 SIA roadmap predicts feature sizes of less
than 100 nm within a decade. Proportional to
decreasing feature sizes from 250 nm to 70 nm, the
precision of CD-SEMs (or alternative metrology
measurement tools) will have to improve at the same
time from 4 nm to 1 nm.
Table 1: Technology Generation and CD-SEM Precision Goals for
the next decade according to the 1997 SIA Roadmap
Year of First IC
Shipment
1997 1999 2001 2003 2006 2009 2012
Technology Generation
(nm)
250
180
150
130
100
70
50
Lithography Budget
(in nm, 3σ post etch)
20
14
12
10
7
5
4
Final CD Output
Metrology Precision
(in nm, 3σ)
4
3
2
2
1.4
1
0.8
One of the major difficulties in submicron CD
metrology is the lack of an accepted reference
standard. There are no linewidth standards in the
nanometer range that are relevant to the kind of
features measured (e.g. photoresist lines and spaces
on bare silicon wafers or anti reflective layers), and
behave similar in CD-SEM inspection (e.g. with respect
to charging). Finally, references and calibration
standards also need to be stable during use and over
time.
The matching approach for the evaluation of Stepper
and Step & Scan systems differs from the approach of
other users because their CD-SEMs are mainly based
in semiconductor production environments [2,3]. The
CD-SEM usage for system evaluation at ASML
requires multiple CD-SEM matching through focus for
The most important difference between calibrating and
matching is, that instead of using an accepted
reference, for matching a reference is determined. This
1
2. EQUIPMENT
means that one CD-SEM is chosen to be the reference
and that all other CD-SEMs are matched towards this
reference tool. As a consequence, accuracy is not an
outcome.
ASML uses worldwide different types of CD-SEMs:
- Automated top down CD-SEMs:
- Applied Materials 7830Si (4x)
- Hitachi S-8840 (2x)
- Hitachi S-8C40 (1x).
The terminology used in this work conforms to ISO
[4,5].
measurement
terminology
The
overall
measurement error is the square root of the sum of all
variances, which originate from all sources of
variability influencing the measurement. These
variances are gathered in two terms: repeatability and
reproducibility. Repeatability is the variance on
measurements taken under the same conditions and
reproducibility is the variance on measurements under
different conditions. Long term repeatability is referred
to as single-tool stability with a typical time period
being one month.
- Tiltable process diagnostic CD-SEM:
- Hitachi S-7800H (1x).
The CD-SEMs that are used in this paper are named
according to Table 2.
Table 2: Overview of ASML’s CD-SEM equipment used in this
paper
single tool reproducibility or
stability
tool-to-tool
reproducibility or
matching
repeatability or
single tool precision
I-12531.ILL
In this study, the main contributors to the measurement
error were determined as: dynamic repeatability, and
tool-to-tool reproducibility or matching (Figure 1).
TYPE
LOCATION
AMAT #1
Applied Materials 7830Si
Veldhoven
AMAT #2
Applied Materials 7830Si
Veldhoven
AMAT #3
Applied Materials 7830Si
Veldhoven
AMAT #4
Applied Materials 7830Si
Tempe
S-7800H
Hitachi S-7800H
Veldhoven
S-8C40
Hitachi S-8C40
Veldhoven
To match all CD-SEMs, a reference CD-SEM has to be
determined. For this matching procedure, AMAT #1
was selected to be the reference CD-SEM for the
following reasons:
multiple tool
precision
Figure 1:
NAME
- This CD-SEM is used to optimize all algorithms
using a cross-section SEM for top, sidewall and
foot verification.
Measurement terminology. In a stable environment,
multiple tool precision has been determined from
repeatability and tool-to-tool reproducibility
- Half of the equipment set, are identical Applied
Materials systems which are used throughout
ASML.
[1],
According to the 1997 SIA Roadmap
the target for
total multiple tool precision is 3 nm for 0.18 µm
technology. The Advanced Metrology Advisory Group
(AMAG) divided this budget for the total multiple tool
precision equally between single-tool precision
(dynamic repeatability) and tool-to-tool reproducibility
(matching) [4]. This indicates a budget of 2.1 nm for
each variable. These budgets are very close to the
resolution and performance limits of the state of the art
CD-SEMs as used within ASML.
Evaluating imaging performance implies measuring
FEMs using large defocus values (typically 1.0 µm)
and large energy ranges (typically 3 mJ for APEX-E).
The recipes, which are used on the CD-SEMs, can be
divided into three groups:
1)
2)
2
The FEM recipes, with which a complete FEM is
measured (up to 1176 data points). This
measurement results in a value for Best Energy
(BE) and Best Focus (BF) of the tested Stepper
or Step & Scan system.
The Usable Depth of Focus (UDoF)
measurements and 5-bar measurements. To
measure UDoF, nine FEMs divided over the
Stepper/Step&Scan
exposure
field
are
3)
measured through focus and at best energy. The
5-bar measurements are very similar to the
UDoF measurements but put extra attention to
the differences in CD-value between the first,
third and fifth line of a 5 bar structure in order to
determine lens aberrations and proximity effects.
The CD Uniformity test, which measures the CD
range for both vertical and horizontal lines over a
focus range of BF ± 0.3 µm in 6 fields across the
wafer. This implies that an offset between vertical
and horizontal measurements (HV-differences)
of the CD-SEM influences the result of the
CD-Uniformity test.
of the CD measurements. In contrast with the first two
steps, this step requires unique wafers which have to
be circulated among the several CD-SEMs. This is
critical since exactly the same features have to be
measured.
These three steps for matching will now be discussed
individually in more detail:
Step 1: CD-SEM Stability
Prior to matching, it has to be assured that the stability
of the CD-SEMs which are included in the matching
procedure is within the control limits. Applying
correction offsets to tools that are not stable in time,
can result in incorrect adjustments of the CD
measurements. The stability of the individual
CD-SEMs is checked daily by measuring an etched
polysilicon reference wafer. On this wafer, both vertical
and horizontal lines are measured at five sites across
the wafer. In this way, both the separate CD values for
vertical and horizontal lines can be monitored as well
as the horizontal-vertical differences (HV-differences).
It should also be noted that with the advent of Step &
Scan systems, the metrology requirements on
accurate H-V control is essential in assessing the
‘scanning’ performance of advanced lithography
exposure tools [6]. The currently allowed range for the
0.30 µm CD values is 6 nm, while the range for the
HV-difference is set to 3 nm. A part of the CD range
over time can be attributed to charging and
contamination effects of the measurement sample.
These three groups of recipes, FEM, UDoF and
CD-Uniformity measurements, show the importance of
CD-SEM matching through focus.
ASML’s system evaluations are performed on bare
silicon photoresist wafers using a BARC. This means
that next to matching over a large focus range,
matching for different photoresist types is also very
important. Various photoresist processes have
different photoresist profiles and different charging
characteristics in the CD-SEM. Therefore CD-SEM
algorithms need to be optimized in order to measure
correctly. This optimization needs to be done for each
different type of CD-SEM separately, since each type
can respond different to the same photoresist profiles
due to different hard- and software.
3. METHOD
Step 2: Magnification matching
or pitch calibration
The CD-SEM matching procedure at ASML consists of
three steps:
1) CD-SEM Stability
2) Magnification matching or pitch calibration
3) Matching of Critical Dimension through
focus.
The magnification factors of all CD-SEMs have to be
checked in order to make sure that the tool setup is
correct. Magnification matching is a relative simple
way to make sure that the basic machine performance
is identical on the various CD-SEMs.
Since pitch measurements are relatively insensitive to
charging, focus, energy and the measurement
algorithm, magnification matching is done by
measuring pitch linearity over a large pitch range of
0.5 µm to 2.4 µm on an etched polysilicon wafer for
both horizontal and vertical features. This pitch range
was chosen in order to check linearity over a large CD
range.
This step 2 matching must be done before step 3 of the
matching procedure is started. Adjustment of the
magnification factor afterwards, results in a different
value for the measured CD.
The first two steps involve measurements using an
etched polysilicon wafer. Since pitches are determined
by the reticle, these measurements can be performed
on different wafers as long as they are exposed with
the same reticle. These two steps make sure that the
operating condition of all individual CD-SEMs is
optimal.
Matching within tight specification limits can only be
done when the stability over time is better than the
repeatability of the individual CD-SEM. Additionally, it
is obvious that unstable tools and tools with an
incorrect magnification factor cannot be included in the
matching procedure until these errors are corrected.
The last step of matching involves the actual matching
3
Step 3: Matching of Critical Dimension
through focus
Before the offsets between the several tools can be
determined and corrected, the repeatability budget has
to be quantified. Since a limited number of data points
is used, matching cannot correct beyond the
repeatability
limits
of
the
individual
tools.
Quantification of the dynamic repeatability through
focus is done by measuring a particular feature ten
times where the energy used remains constant at a
nominal or best energy while focus is varied in a FEM
layout. To correct for charging and contamination
effects, for each focus step a linear equation is fitted
through the ten data points. The dynamic repeatability
is defined as three times the standard deviation of the
residual errors from the fit averaged over a focus range
of BF ± 0.4 µm. The slope of the linear regression line
characterizes the charging and contamination.
Because photoresist wafers are measured eight times
on the several tools, during the matching procedure,
analysis of the matching results will include the
correction for charging and contamination to judge on
the absolute offset between the CD-SEMs.
Different types of CD-SEMs use different hardware
(e.g. columns, detectors) and software (e.g.
measurement algorithms). As a consequence, they
respond differently to photoresist images, sidewall
profiles and substrates. Therefore a stable CD-SEM
and magnification matching is insufficient. The
matching of CD measurements is done by measuring
Focus Exposure Matrices (FEMs).
These photoresist FEMs are measured for:
- feature sizes:
- 0.35 µm (365nm I-line)
- 0.25 µm, 0.22 µm and 0.18 µm (248nm
DUV).
- line orientations:
- horizontal
- vertical.
- line densities:
- isolated
- nested.
4. RESULTS
Step 1: CD-SEM Stability
Additionally, pitch measurements (vertical and
horizontal), and measurements for the first, third and
fifth line of a vertical 5-bar structure are included. This
so-called 5-bar measurement is only performed on the
AMAT systems since these systems can measure the
three lines in one single pass with minimal throughput
penalty.
The stability of the CD-SEMs is monitored by
measuring, on a daily basis, horizontal and vertical
features on an etched polysilicon wafer. Figure 2
shows a typical example of these measurements over
a month for both the CD values and the HV-difference,
including the horizontal (HCL) and vertical control
limits (VCL). In this figure all measurements are
shown, including the out of specification situations. It
should be noted that the results are not corrected for
charging and contamination, which result in an
increase of CD values after some measurements. The
charging and contamination can be removed by putting
the wafer in an acid bath, after which the measured CD
value returns to its lower, pre-charged value.
All CD-SEMs offer the possibility to include offsets.
During normal operation, the offsets obtained in the
matching procedure are used. During initial matching
for each CD-SEM, the applied offsets were set to zero
in the measurement parameter files used in the
CD-SEM matching recipes. This makes it possible to
compare the absolute offset between the various
CD-SEMs which are caused by hardware and/or
software differences. Additionally, the current offsets
can be input into the analysis software to determine
the current performance.
After measuring the same FEMs on the different
CD-SEMs, the data is subtracted from each other. This
results in a FEM containing only differences between
systems. From this, the average differences or offsets
are calculated over a focus range of Best Focus ± 0.3
µm (i.e. a focus range of 0.6 µm). For the normal FEMs
that are typical for Stepper and Step & Scan system
evaluation, this focus range contains 63 data points.
When calculating the stability of the CD-SEM, the CD
measurements are corrected for charging and
contamination effects. Stability, long term repeatability,
has been determined in a similar way as the dynamic
repeatability determination. A typical value for the
stability is 1.7 nm.
Figure 2b shows the HV-differences, which are
controlled with the stigmation settings and the beam
alignment. The relative change in HV-difference is
important and it is apparent that the sample has an
inherent HV-difference. Because of this HV-difference
the horizontal lines are approximately 6 nm wider
compared to the vertical ones.
4
292
288
after acid bath
284
HCL
VCL
horizontal
vertical
280
2
6
10
14
18
26
30
2
215
7
8
9
10
time [hours]
Figure 3:
14
18
22
26
30
The magnification matching results of all CD-SEMs are
very similar, they are actually plotted on top of each
other. The fitted line in Figure 4 shows the linear
regression results of the average of all CD-SEMs. As
expected the slope equals one and the intercept
equals zero. Additionally, in both cases the fitted line
has a coefficient of correlation, R2, of 0.99999.
.
6
10
Before the actual CD matching, the magnification
matching on all CD-SEMs was done. The pitches,
measured on an etched polysilicon wafer, ranged from
0.5 to 2.4 µm. We assumed that the real physical
feature size of the measured pitches equals the
targeted feature size on the reticle divided by four.
Therefore we expect the slope of the fitted line to be
one and the intercept zero.
The results for vertical and horizontal magnification
matching are shown in Figure 4. In these graphs the
measured pitches are plotted as a function of the
assumed physical pitches
218
5
6
Step 2: Magnification matching or
pitch calibration.
220
4
CL
Figure 2b: HV-differences as a function of time (measured
daily).
I-12744.ILL
Critical Dimensions [nm]
Hitachi S-7800H
223
3
-8
number of measurement [days]
225
2
-6
H-V
22
Figure 3 shows stability results for 0.22 µm features as
a function of time, corrected for charging and
contamination effects, for both the Hitachi S-7800H
and Hitachi S-8C40.
The same feature has been regularly measured over a
time frame of approximately 8 hours. Comparison of
the results indicate that the Hitachi S-7800H is not
suitable for automated SEMing within ASML
(measuring time is typically 3-4 hours). This is caused
by the fact that this microscope is equipped with a cold
field-emission electron gun that has to be flashed once
a day [7]. Therefore, the Hitachi S-7800H is mainly
used for tilted profile images and excluded from the
matching procedure.
1
-4
stigmator not optimal
Figure 2a: CD of horizontal and vertical features as a function
of time (measured daily).
0
-2
-10
number of measurement [days]
Hitachi S-8C40
I-12482.ILL
CD (H-V) [nm]
296
I-12481.ILL
Critical Dimension [nm]
0
300
Measured CD as a function of time for the Hitachi
S-7800H and the Hitachi S-8C40.
5
2.5
2.0
1.5
AMAT #1
AMAT #2
AMAT #3
AMAT #4
S-8C40
0.5
0
0
0.5
1.0
1.5
2.0
2.5
2.0
1.5
AMAT #1
AMAT #2
AMAT #3
AMAT #4
S-8C40
1.0
0.5
0
0
3.0
Y = 1.005X - 0.002
R2 = 0.99999
0.5
1.0
1.5
Figure 4a: Vertical magnification matching. Measured pitches as
a function of the assumed physical pitches.
AMAT #1
AMAT #2
AMAT #1
AMAT #2
AMAT #3
frequency
frequency
AMAT #4
S-8C40 #1
10
0.35
0.36
3.0
30
AMAT #3
0
0.34
2.5
Figure 4b: Horizontal magnification matching. Measured pitches
as a function of the assumed physical pitches.
I-12540.ILL
30
20
2.0
pitch on reticle /4 [µm]
pitch on reticle /4 [µm]
0.37
I-12539.ILL
1.0
I-12477.ILL
Y = 1.002X - 0.001
R2 = 0.99999
measured pitch [µm]
measured pitch [µm]
2.5
3.0
I-12478.ILL
3.0
20
S-8C40 #1
10
0
0.34
0.38
AMAT #4
0.35
0.36
0.37
0.38
critical pitch dimension [ m]
critical pitch dimension [ m]
Figure 5a: Vertical pitch measurements for 0.18 µm on 0.5 µm thick
APEX-E represented in histograms.
Figure 5b: Horizontal pitch measurements for 0.18 µm on 0.5
µm thick APEX-E represented in histograms.
Special attention is given to HV-difference because of
its importance when evaluating Step & Scan systems.
Figure 6 shows that the maximum HV-difference is
1 nm when the magnification factor is set up correctly.
Magnification matching can be verified by measuring
these pitches. Therefore pitch measurements for both
vertical and horizontal features are measured on a
FEM. Pitches are independent of focus and energy,
therefore results can be presented in a histogram.
Figure 5 shows the histograms of the vertical and
horizontal measurements for 0.18 µm features
measured on 0.5 µm thick APEX-E.
It can be concluded that after magnification matching
the setup for the individual CD-SEMs independent of
vendor is good with respect to pitch measurements.
This includes HV-differences, which are smaller than
1% of the target CD-value.
After magnification matching, the pitch differences for
both orientations are less than 2 nm for all CD-SEMs,
independent of the vendor. Thus, when the
magnification factors are setup correctly, the pitches
match within the dynamic repeatability of our
CD-SEMs. The minor differences can be caused by
small hardware differences (e.g. detector efficiency, tip
condition and electron optics).
Furthermore, the magnification factors of all CD-SEMs
included in the matching procedure are correct. In
combination with the correct long term repeatability,
stability, the operating condition of the CD-SEMs is
good and the last step of the matching procedure,
matching of critical dimension through focus, can be
done on these tools.
6
20
10
0.35
0.36
0.37
critical pitch dimension [ m]
Figure 6:
0.38
vertical
horizontal
20
10
0
0.34
0.35
0.36
0.37
S-8C40
vertical
horizontal
20
10
0
0.34
0.38
critical pitch dimension [ m]
0.35
0.36
0.37
0.38
critical pitch dimension [ m]
Vertical and horizontal pitch measurements for three CD-SEMs included in the matching procedure.
Critical Dimension [nm]
Step 3: Matching of Critical Dimension
through focus.
The last and most important part of the matching
procedure is the actual matching of CD
measurements. In this step the CD differences or
offsets will be quantified in order to determine the
offsets which have to be applied on the several
CD-SEMs. This paragraph will be divided into two
parts; first the quantification of repeatability and
charging budgets will be discussed followed by the
results of CD matching.
200
Y = 0.7X + 184
190
180
0.4 µm
0.3 µm
0.2 µm
170
1
Figure 7:
Repeatability and Charging
The CD at best energy has been measured ten times
through focus. An example of an experiment done on
the AMAT #4 for 0.18 µm features on 0.5 µm thick
APEX-E is shown in Figure 7, the CD is plotted as a
function of the run number for BF ± 0.4 µm.
According to the method described in the previous
section the repeatability and charging budget is
determined. The charging budget, slope of the fitted
line, per measurement for AMAT #4 equals 0.7 nm. For
the other CD-SEMs similar contributions have been
found. After correction for charging and contamination
effects the dynamic repeatability has been determined
over a focus range of BF ± 0.4 µm on two randomly
chosen Applied Materials systems (AMAT #1, AMAT
#4) and on the Hitachi S-8C40. The values determined
for the dynamic repeatability are 2.3 nm, 2.5 nm and
2.6 nm respectively.
The good repeatability of the CD-SEMs, in
combination with the good stability results, allow us to
match our CD-SEMs.
7
I-12541.ILL
0
0.34
30
AMAT #3
frequency
vertical
horizontal
frequency
frequency
AMAT #1
I-12536.ILL
30
30
2
3
0.1 µm
BF
-0.1 µm
4
5
6
run number
-0.2 µm
-0.3 µm
-0.4 µm
7
8
9
10
Averaged CD over a focus range of Best Focus ± 0.4
µm for 0.18 µm features on 0.5 µm thick APEX-E as
a function of the run number. The slope of the line
fitted equals the charging contribution.
Critical dimension matching
for charging and contamination effects (0.7 nm per
measurement). For all feature sizes, CD-SEMs of the
same vendor (Fig. 9a) show similar results. All offsets
are smaller than 5 nm and independent of orientation
(horizontal or vertical structures) and density (isolated
or nested structures). The differences between CD
measurements can be explained by minor differences
between CD-SEMs of the same type due to e.g.
different detector efficiency, different tip condition,
beam current or operator dependent setup (stigmation,
first aperture).
CD-SEMs of different vendors show larger offsets
(Figure 9b). The DUV features, using APEX-E
photoresist, all show similar offsets independent of the
orientation and density, namely averaged 20 nm.
These differences arise from the usage of different
software (e.g. measurement algorithms) and hardware
(e.g. columns, electron optics) by the compared
vendors. These differences are corrected by applying
a constant correction offset to the CD-SEM recipes of
the different CD-SEMs. The i-line features, however,
show different offsets. This illustrates the importance
of matching all feature sizes, especially when different
photoresists are used.
I-12542.ILL
The final step is the matching of the critical dimension
measurements. The FEM results of all CD-SEMs are
compared and eventually corrected towards the
reference CD-SEM, AMAT #1. The CD matching is
done according to the method as described in
section 2.
Figure 8 shows the Bossung curves for nine different
energies (for vertical dense lines) as measured on
AMAT#1 and AMAT #2, left and right corner
respectively. These two FEMs are subtracted from
each other in the center graph. This figure clearly
shows that the difference between both systems is
constant through focus. Due to the optimized
measurement algorithms, this is typical for all CD-SEM
comparisons independent of vendor. This justifies that
a constant offset through focus can be applied.The
critical dimension matching procedure includes
different feature sizes, 0.35 µm (365 nm I-line) and
0.25 µm, 0.22 µm and 0.18 µm (248 nm DUV). Figure
9 shows the absolute offsets of AMAT #1 and AMAT #3
(Figure 9a) and AMAT #1 and Hitachi S-8C40
(Figure 9b) for all these feature sizes after correction
Figure 8:
Comparison of a FEM measured on AMAT#1 (reference) and AMAT#2. The small corner graphs show
Bossung curves for nine energies, while these FEMs are subtracted from each other in the center graph.
8
I-12534.ILL
0.18 m
0.22 m
0.25 m
0.35 m
vertical dense L3
vertical dense L2
vertical dense L1
vertical isolated
horizontal isolated
I-12760.ILL
15
10
5
0
AMAT #1 - S-8C40
-10
AMAT #1 - AMAT #4
-5
vertical dense
AMAT #1 - S-8C40
AMAT #1 - AMAT #3
AMAT #1 - AMAT #2
vertical dense L3
vertical dense L1
vertical dense L2
vertical isolated
horizontal isolated
horizontal dense
AMAT #1 - AMAT #4
-5
20
AMAT #1 - AMAT #3
0
25
AMAT #1 - AMAT #2
5
Absolute differences between CD-SEMs of different
vendors, AMAT #1 and Hitachi S-8C40 for different
features (0.35 µm (365nm I-line) and 0.25 µm, 0.22 µm
and 0.18 µm (248nm DUV)), orientations (vertical and
horizontal structures), densities (isolated and nested
structures).
vertical dense L3
10
vertical dense
-10
vertical dense L2
15
Figure 10a:
-5
vertical dense L1
absolute CD offset [nm]
20
-10
0
Figure 9b:
I-12535.ILL
Absolute differences between CD-SEMs of the same
vendor, AMAT #1 and AMAT #3 for different features
(0.35 µm (365nm I-line) and 0.25 µm, 0.22 µm and
0.18 µm (248nm DUV)), orientations (vertical and
horizontal structures), densities (isolated and nested
structures) and 5-bar measurements.
25
5
vertical dense
0.18 m
0.25 m
0.35 m
vertical dense L3
vertical dense L1
vertical dense L2
vertical isolated
horizontal isolated
horizontal dense
-10
0.22 m
-5
10
vertical isolated
0
15
horizontal isolated
5
20
horizontal dense
10
25
horizontal dense
15
Figure 9a:
absolute CD offset [nm]
absolute CD offset [nm]
I-12533.ILL
20
vertical dense
absolute CD offset [nm]
25
Figure 10b: Differences after applying one constant correction
offset between all CD-SEMs with the reference
CD-SEM, AMAT #1, for 0.18 µm features for different
orientations (vertical and horizontal structures),
densities (isolated and nested structures) and 5-bar
measurements.
Absolute differences between all CD-SEMs with the
reference CD-SEM, AMAT #1, for 0.18 µm features for
different orientations (vertical and horizontal
structures), densities (isolated and nested structures)
and 5-bar measurements.
9
Critical dimension matching becomes more important
when going to smaller feature sizes.
Figure 10 shows the 0.18 µm matching results for all
CD-SEMs compared to the reference system. All
Applied Materials CD-SEMs show offsets less then 5
nm towards the reference system, since all these
systems are equal with respect to hard- and software.
As discussed previously, the Hitachi S-8C40 shows an
averaged offset of 20 nm. It should be noted that this is
not a problem because they match through focus and,
thus, can be corrected using only one constant offset.
CD-SEMs of one vendor show only small differences
due to minor hardware differences (e.g. different
detector efficiency, different tip condition, beam current
or operator dependent setup). By comparing CD
measurements of CD-SEMs of different vendors
(Applied Materials and Hitachi) larger difference are
observed, namely 20 nm. These offsets, caused by
major hardware and software differences, can easily
be corrected by applying one constant offset through
focus.
6. FUTURE WORK
Additionally, figure 9 and 10 show that the same
correction offsets can be applied for all measured
features (e.g. horizontal and vertical, isolated and
nested). The differences in CD offsets for all these
features is smaller than 4 nm. Therefore, after applying
an averaged correction offset, the CD-SEMs are
matched within specification and smaller than the
dynamic repeatability of the used CD-SEMs, namely
generally significant smaller than 2 nm.
The main concern is maintaining the tight matching
specifications over time. It is unfeasible to perform the
matching procedure on regular basis, since a lot of
expensive CD-SEM time will be consumed. Therefore,
once CD-SEMs are matched, we’re looking into the
possibility to study the resolution targets using Fourier
transformation [8]. By this, the performance of all
CD-SEMs can be monitored and adjusted if necessary.
Next to that, all CD-SEMs will be linked to a central
recipe server in order to ensure usage of common
releases of CD-SEM recipes. Finally, accuracy will
continue to become more important with decreasing
feature sizes. Therefore, a lot of effort has to be put into
absolute calibration and correlation with using
cross-section SEM and AFM.
Finally, in agreement with the pitch measurements, no
significant HV-differences have been observed.
HV-difference is very near to the dynamic repeatability
of the CD-SEMs since the magnification matching for
both horizontal and vertical features is performed prior
to CD matching.
Thus, it can be concluded that CD-SEMs of different
vendors can be matched through focus within 5 nm for
0.18 µm lines/spaces, using only one correction offset
and identical recipe database on CD-SEMs of one
vendor.
When going to smaller feature sizes the demands on
the CD-SEMs become more demanding. According to
the 1997 SIA Roadmap [1], for 0.13 µm technology
CD-SEM precision must decrease to 2 nm. To reach
this goal, a lot of effort has to be put in increasing the
stability of the CD-SEMs and decreasing both the
dynamic repeatability and the matching budget. By
matching smaller features additional problems will
arise such as severe charging and contamination,
photoresist profile deviation due to e-beam sensitivity.
An example of charging and photoresist deviation
effects on 0.13 µm features using 193nm photoresist is
shown in Figure 11.
5. CONCLUSIONS
Critical Dimension Scanning Electron Microscopes
can be matched through focus within 5 nm for 0.18 µm
lines/spaces, using only one correction offset which is
constant through focus. Thus, the offset is independent
of orientation (vertical and horizontal structures) and
density (isolated and nested structures).
The three step matching procedure as followed within
ASML shows good matching results. Firstly, the
stability of the CD-SEMs included in the matching
procedure is continuously monitored. Since the CD
change over a month is less than 2 nm, it allows us to
match our systems within tight specifications.
Secondly, the magnification factors, vertical as well as
horizontal, are checked and corrected on every
CD-SEM. As a result, the pitch measurements are
matched and the HV-differences are minimized.
Thirdly, the actual CD measurements are matched.
a
b
Figure 11: A 0.13 µm photoresist line measured for the first
time (CD = 130 nm) (a) and after 4 measurements
(CD = 147 nm) (b).
10
7. ACKNOWLEDGEMENTS
8. REFERENCES
The authors wish to thank Erik van Brederode, Yin
Fong Choi , Frank Duray, Mariëtte Hoogendijk, Ted der
Kinderen, Bart Rijpers and Jenny Swinkels in the
Veldhoven office and Shawn Cassel of the Demo Lab
in Tempe for their support. Furthermore, they wish to
thank Guy Davies, Jo Finders and Paul Luehrmann for
their useful discussions with the authors and inputs
during the course of this work.
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‘National Technology Roadmap for
Semiconductors’ (1997).
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[5] Semiconductor Equipment and Materials
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