Data Input: Seven datasets, representing
CD-SEM metrology of six BARC and one bare wafer layer, are
combined into a single data analysis workbook for VR
the optimal BARC film for an 80 nm process from six vendor
Data file: Weir_Process_BARC.XLS
Raw Data Configuration
Seven wafers were exposed. One wafer was
processed without an anti-reflection coating and each of the
remaining six wafers was coated with a different commercial
Bottom Anti-Reflection Coating (BARC). The objective of the
analysis was to competitively compare Process Window and
performance of each commercial BARC.
Each wafer contained a focus-exposure matrix of
nested features as shown in Figure 1. The wafers were process
identically with the exception of the BARC layer. One nested
site of 80 nm features was measured on each exposure field. The
features were measured using a commercial CD-SEM. The objective
was to determine the most robust BARC coating by measuring the
extent of a +/-10% Process Window of Bottom Critical Dimensions
(CDs). Since the sites were able to be viewed by the CD-SEM, the
operator also recorded the number of features that were resolved
in the line-space pairs by counting the number of profiles
standing from 1 to a maximum of 7 peaks.
Figure 1: Exposure Dose and Focus Matrix values shown
from the Weir Main interface layout.
Data was assembled into one lot of seven wafers
for this analysis. Assembly was simple since the imported CD-SEM
data of Vector-Raptor could be easily copied and pasted into a
master Weir Workbook. Each BARC coating here is designated using
the Wafer ID for the dataset.
A total of 836 measurements for the seven wafers
is displayed in Figure 2. The process target size is 80 nm with
a +/-10% Process Window. Process Window visualization here is
enhanced by right clicking the graphic and setting the bar-code
range key to the 72 to 88 nm settings shown in the figure. Any
exposure-site that is not dark blue or deep red will be within
the Process Window.
Figure 2: Bottom Critical Dimension (BCD) SEM
Measurements for all seven wafers Color bar-code has
been set to 80 nm +/- 10% range of process window
Figure 3: “Collapsed Line” settings for seven BARC
Features included nested pairs of seven
line-space profiles. This data was also gathered by the SEM
operator and manually entered into the Weir data using the FEM
Format editor of Weir. Figure 3 displays the number of standing
lines in the exposure data with values that range from 1 to 7
representing the number of well defined and standing line/space
Vector Raptor was able to use the “Collapse”
variable as a data-culling tool by setting the features minimum
threshold to 3 lines and therefore excluding poor exposures
outside of the process definition.
The Process Window response of any two BARC
constituents can be easily compared using the VR Matching as
shown in Figure 4. This method is much more direct than the
complexity of adding in a classic Process Window analysis and
visualization of the process response as well as the observation
of poor metrology points is superior to classic methods since
the analysis is not dependant upon a complex polynomial model
interpretation. Finally, although this data has only one point
per exposure field, the same plot could have been employed on a
mult-field-site analysis of the entire exposure field’s Process
Window response thereby incorporating the influence of lens
aberrations and edge-field effects.
Figure 4: Wafer
without BARC (left) compared to wafer with Expo4073D BARC.
Visualization in Figure 4 used the Color-Bar Key
to help in determining the Process Window extent. The user could
also have used the Variable Culling tool to set the Process
Windows maximum and minimum values thereby eliminating
“out-of-specification” metrology points.
The areas of Figure 4 that are not deep red or
blue define the process window. Placing the process exposure at
the center of the window is as simple as either selecting the
exposure site on the graph or using the mouse to box-in the
desired process space and viewing the resulting box-enclosed
data set. The advantage of using a BARC coating in the process
is clearly seen in this data since the Process Window of the
right vector plot of Figure 4 is much greater than that of the
Figure 5: Difference Comparison Setup
EXPO4008 BARC (Left) as “Reference” compared with remaining
six BARC responses (Right).
Difference Comparison of BARC Process Windows
Expo4008 BARC was selected, by visual comparison using the
comparative display method of the previous section, as a film
exhibiting one of the largest Process Window responses. Figure 5
displays the Reference data of Expo4008 in comparison with the
remaining six film responses that will comprise the “Matching”
lot. Once again the bar-key coding in this figure is set to the
limits of the desired Process Window of 80 nm +/- 10% allowing
both the Process Window as well as the remaining well-defined,
but out-of-specification, line-space pairs to be viewed.
Selections were performed by loading the same data lot into both
Reference and Matching lot objects. Individual films for each
were then selected using the “Wafer ID” checkbox. VR Matching
can now be used in the Difference Calculation mode to observe
the process variation of each wafer.
There is only one point per field so interpolation during the
“Difference” calculation is not needed; the “Point-for-point”
option was selected. During the Difference calculation, VR
Matching will find the nearest field value on the Reference
wafer to any fields on the Matching lot wafers. So if a
match-wafer, or in this case BARC coating, is on an
exposure-field that is not contained in the reference, the
program will search for the nearest reference exposure and use
Tip: Including all valid
data points, both in the process window and external to it,
as in Figure 5, is an excellent method for discovering
process extension capabilities. Any process capable of
moving to smaller feature sizes can easily be evaluated and
centered using this method by simply changing the color-bar
However, when the process limits are well defined we can obtain
a precise measurement of the extent of the Process Window for
each film by restricting the feature values to these limits. In
this dataset we used the variable-culling tool to restrict BCD
values to the process-window range of 72 < BCD < 88 nm and
re-plotted the graphs of Figure 5 into those of Figure 6. Now
the exact shape and extent of the process-exposure space can be
comparatively seen. In addition to shape and range the stability
of critical feature size within the space can also be evaluated
as a measure of the robustness of the process.
Figure 6: Difference Comparison Setup of the exact Process
EXPO4008 BARC (Left) as reference compared with remaining
six wafers (Right).
Bottom CD values here are restricted to the process window
range of 72 < BCD < 88 nm
Since the exposure layout centers the Process Window near the
physical center of the wafer, we’ll observe BCD variation of
BARC as a function of the exposure radius from the wafer center
by selecting the “Radial Plot” drop-down graph. This method
implies that the greater the radial distance, the greater the
extant of the film’s Process Window. We generate the Difference
Calculation by simply pressing the “Difference” Command button
to create the plot interface of Figure 7.
the graphic of Figure 7 the response of each BARC film can be
easily viewed by clicking the appropriate tab on the interface.
There is one tab set up for each wafer, in this case BARC film,
of the data lot. The final three (3) tabs of the interface show
the Trend Plots associated with each BARC’s response.
Figure 7: Process Window, Difference Comparison for All
six remaining BARC Process Window plots, which are accessed
using each of the interface tabs, are shown in Figure 8. The
comparison is now restricted to only points whose feature size
lies within the desired Process Window so we can see the
performance quality and extent of each BARC clearly defined as
the number of data points actually on the chart. That is, the
greater the number of plotted points and their range from
Radius=0, the greater the extent of the Process Window.
Based purely on Process Window size the Expo4073D BARC exhibits
the largest window. Looking at the approximate average CD
difference value of this film, we can see that it also exhibits
a small feature size shift from the bare, or “No BARC”, process
wafer. This suggests that exposure times should not have to
significantly increase using this BARC. Feature size ranges can
be estimated by an examination of the population spread, or
envelope, of points along the ordinate of the graph. The
Expo4073D range of CD values appears to be approximately the
same as those of the No-BARC or bare wafer. This implies that
the process window has been expanded but process performance is
not improved beyond bare wafer values.
Examination of the “envelope” of the plotted data points yields
the comparative performance of each BARC film. A film that
exhibits a point-spread envelope centered nearest the zero
ordinate value exhibits a closer match performance in exposure
response to the reference BARC. An example of a good match to
the reference is found in the Expo4061H_vk90 plot.
different mean value implies an offset in the average feature
size from the selected reference BARC. This is commonly observed
between any BARC film and a process not using BARC as shown in
the graph of the upper left of the figure. The greater the shift
in this mean value from the bare wafer exposure, the greater the
exposure change needed for the film.
plot whose population contains the minimum envelope-spread along
the BCD ordinate values along with the greatest range along the
abscissa, or exposure values will sustain the greatest process
and the most robust response of critical feature profile size.
This film will not only exhibit a large process window but will
also support the tightest histogram-spread of points within that
window. For example, although the Expo4037D BARC exhibits the
greatest process window size, the EXPO40618_vk90 film supports a
window that is only slightly smaller while at the same time
exhibiting a much tighter variation in BCD values across the
window and a closer average value to that of the bare wafer
last three tabs of the Difference Interface exhibit the
statistical trend response of each film relative to the
reference BARC using summary statistics displayed on Trend
Figure 8: Process Window Difference Comparison for each BARC
Film Radial range along the abscissa indicates the extent of
exposure size of the process window.
Spread of the population along the ordinate indicates the
robustness of the film to process variance.
Mean Difference +/- Standard Deviation for each film, shown in
Figure 9, can be selected from the “Mean” tab. In this instance,
the shift in exposure or BCD offset values shows the match
similarity of the film to that of the reference. The error-bars
show the robustness of the film and process to exposure
Figure 9: Mean-Difference Tab Plot showing BCD Mean
Difference from the Reference and Standard Deviation for
each film within the defined Process Window.
we can see that the closest match to the bare wafer CD values is
the Expo4073D film. The closest match to our reference BARC is
found on the Expo4061B_vk90 film. An examination of the error
bars for each process window provides instant evaluation of the
standard deviation of features sizes within the process window.
Here the Expo4061B_vk90 BARC film also exhibits the tightest
population spread of feature sizes and will result in the most
robust process performance.
Figure 10 further clarifies the process robustness of each film
by plotting the range of BCD variation from the reference values
for each film.
Range of BCD Differences within the Process Window from the
reference film for each BARC tested.