Two FOCAL data sets measured by the scanner tool. Best Focus
for both chucks plus the stage-precision data has been
Examine across field focus uniformity for the tool prior to
FOCAL® optimization. Chuck performance is determined and the
source of overall performance degradation is calculated.
Next the data taken by FOCAL after correction is analyzed.
The performance after correction is shown to have improved
center-field performance but also resulted in a greater
range in overall full-field focus error. The Before & After
data sets are then subtracted to calculate the corrections
actually applied to the tool by FOCAL®.
FOCAL® Data Layout: Before Correction
FOCAL Data imported by "Family" using VR Matching
An ASML Focal Analysis was performed on an
immersion scanner. The metrology is converted from registration
data to three distinct sets of focus data. The focus data
consists of full field, or “Best Focus”, for each scanner chuck,
and a second focus set of focus stage-precision data.
The two sets of “Scanner Chuck” data comprise
that of the measured focus uniformity on the pre-alignment chuck
(“Chuck 1”) and the field exposure chuck (“Chuck 2’).
VR Matching imports this data as recorded by
metrology but the Stage Precision data is condensed to a single
field on the wafer for convenience as shown in Figure 1. The
data is identified by the software as three families as shown in
the “Family” checkbox from the interface in the lower right side
of the figure.
We will examine only the Best Focus data in the
remainder of this analysis. Statistics shown in Figure 1 include
not only the chuck-response but also the stage precision
Variation between Wafer Stages
Figure 2 illustrates both the method and data
gathered for the focus data before correction. The same data
set, ”FODSC_FocalImmersion_1stSet.XLS” , was loaded into
the Reference and the Matching interfaces. The “Before
Correction” “Family” check box was used to restrict the
reference data to only that measured on chuck #1 as shown in the
top half of Figure 2. Similarly the matching data shown in the
bottom half of the figure presented the focus uniformity of the
Chuck #1 (top) compared to Chuck #2 (bottom).
Note that the focus ranges and standard deviation
on both stages is rather large but very repeatable for both
chuck #1 and chuck #2. The average focus value between the two
chucks is about -5 nm lower on chuck #2. The source of the large
range in focus values becomes apparent if the vector-plotted
field graphs are observed. Chuck-to-chuck differences, other
than the 5 nm offset, are easily seen. The major variance in
this data is caused by the left-most site-column of focus data
with some lesser but still strong contribution from that on the
right side of the field. The field uniformity without these
edge-columns is much more uniform across the majority of the
field exposure’s center area. This response suggests that the
source of the focus error may in fact be aperture vignetting of
the scan-slit but the more likely cause is the limitation of the
FOCAL sensor array, which is results in poor metrology at the
extremities of the scan slit.
FOCAL however does not have the capability of
noticing the field-location based source of variance and simply
chose to correct the overall field focus in an attempt to
minimize the variance. This resulted in a minimization of the
errors for the field edge while moving the corresponding error
values into field center as is shown in Figure 3. The actual
mechanism for this will be shown in the Difference measurement
calculation presented later in this section.
Figure 3: Focus
uniformity Before (top) and After (Bottom)
correction. Data shown overlays chuck 1 & 2.
Histograms represent data exclusive of the
left-and-right most focus columns illustrating the
superior performance of the tool prior to FOCAL
correction across field center.
Figure 3 plots the focus uniformity across each
field for both before and after focus correction on the scanner.
Each field displays both chuck #1 & #2 data. Therefore, each
site on the field displays two vector focus values. Scaling on
each plot has been set to 10 nm for each comparison. Scaling was
fixed by right-clicking the image to use the plot editor.
The mean focus value on the corrected field has
shifted down to values about 4 nm more negative in focus. X and
Y focus means are closer than before the correction but still
exhibit the significant shift. Unfortunately the range of focus
values across the field has significantly risen after correction
as has the variance. So while the focus correction improved the
difference between X and Y average values the full-field range
and variance has degraded. This effect on focus becomes apparent
if the two columns of sites located at X=+/- 12 mm from field
center are excluded and plotted on histograms as shown in the
figure. Quite clearly the before-correction spread of focus has
been degraded and will result in greater critical feature
variation across the exposure field in the field-center critical
Examination of the after-correction field in the
lower half of Figure 3 reveals both a general tilt of the field
resulting in over 10 nm of variance from upper left to lower
right. Also notice the chuck-to-chuck variation between focus
vectors in the upper left quadrant of the exposure.
We can investigate this across-field variation
with greater detail by using the left mouse-button to box in the
center row of data in each field plot to obtain response
corresponding to focus uniformity across the field-center slit
location. Generate the graphs of Figure 4 by selecting the XY
Plot from the pop-up menu that appears when the button is
Chuck #1 X & Y focus values are plotted as
circles in these plots while the Chuck #2 data are displayed as
filled squares. X-focus corresponds to the focus variation seen
by the edges of a vertically-oriented feature and is therefore
called “dz-V” by the metrology. Similarly “dz-H” will sometimes
be called the Y focus value. Variable names are dictated by the
user metrology rather than Vector Raptor.
The offset in average focus and field-center-slit
improvement in focus range resulting from the focus improvement
is easily seen in Figure 4. Also notice that the chuck-#1 to #2
splitting has been removed.
Examining field response on the graphs on the
right side of Figure 4 for the row located at +10 mm above field
center we now increase the plot scales to almost double those of
the field center. This graph clearly shows the degradation in
performance resulting from the correction.
Across-slit focus uniformity before (top) & after
Difference Measurement of the FOCAL Correction Applied
We next select the “Difference” tab in the vector
raptor interface. The “before correction” data is used as the
reference and subtracted from the ‘after correction” data. No
actual data culling is needed except for the removal of the
precision focal data family. To calculate the data we selected a
the “Point-for-point” subtraction option and the pressed the
“Calculate” command button. The results, representing the focus
change actually made to the tool, are shown in figures 22 and
The matching report shown in Figure 22
illustrates the mean -5 nm shift imparted by the correction. The
overall correction range of values added removed a 12 nm spread
of values with only 0.6 nm of difference between X and Y
correction values. However the vector plot of figure 23 is more
interesting since the source of the focus-error signature seen
in the bottom plot of Figure 3 now becomes apparent.
Figure 23 clearly shows the induced field tilt.
The barrel distortion layout of the plot was used to correct for
the error contributed by the high focus errors seen in the sites
located at the field edges, +/- 12 mm from field center. If the
-12 mm left-hand site-column data is closely examined, we can
see that the correction tracked the general signature of the
columns focus error going from high values at the top of the
column at the top of the exposure to small values with opposite
sign at the bottom. Examine the top plot of Figure 3 in
comparison with that of figure 23 to see this signature. However
the large values of this column’s distortions were not damped
quickly enough as the focus moved to those located nearer the
center of the field.
The overall effect of the FOCAL correction was to
reduce the focus errors at the extremities of the field while
increasing focus non-uniformity, and therefore increasing
critical feature dimensional nonuniformity, in the field center.