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This description of EBES format is taken from Appendix E of Computer Aids
for VLSI Design by Steven M. Rubin [207].
One of the most popular formats for producing integrated-circuit masks is the
Electron Beam Exposure System (EBES), which can specify the very small
features needed in high-density chips. Although the actual format varies with
each different company's mask-making machine, an original standard, designed
at Bell Laboratories, forms the basis on which extensions are made. This basic
format is described here.
The electron beam is controlled in a digital raster fashion, such that a line
of 256 points (called address units) can be written at a time. By convention,
this sweep is run vertically and the mask is moved horizontally to make the
sweep cover the top 256 rows of the chip (see Figure B.4). It thus takes
multiple passes across the width of the chip to write a complete pattern. When
multiple copies of an IC die are being produced on a wafer, a single sweep of
256 rows is made on every die position before advancing to the next 256
rows. This means that the pattern file which is organized in 256 row stripes,
needs to be read only once.
Figure B.4:
EBES raster motion, actual mask making sweeps out 256 rows as it advances horizontally
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The EBES file is organized about these 256 row stripes. The beginning of the
file contains a START DRAWING command followed by the stripes and an END
DRAWING command. Each stripe consists of a START STRIPE command, a series of
figure commands, and an END STRIPE command. The figures can be either
RECTANGLEs, PARALLELOGRAMs, or TRAPEZOIDs.
EBES files are binary, with 16 bits per word. Commands can take any number of
words but must be aggregated into 1024-word blocks. If a command is near the
end of a block and would span into the next block, then an END OF BLOCK
command must appear followed by pad data to the block end. All blocks must end
with the END OF BLOCK command except for the last block, which ends with the
END DRAWING command.
The START DRAWING command is the first in an EBES file and contains 16
words. The first word has a 2 in the high byte and a code in the low byte that
gives the size of an address unit. The address-unit size will be 1 micron if
the code is 0, one-half micron if the code is 3, and one-quarter micron if the
code is 6. The second and third words of the START DRAWING command are the x
and y size of the entire pattern. Words 4, 5, and 6 contain the EBES file
creation date: Word 4 is the month, word 5 is the day, and word 6 is the
year. For these fields, the high byte is the first digit and the low byte is
the second digit in ASCII. Words 7 through 13 contain the name of this file by
having a name length in word 7 (always the value 6) and by having the next six
words (8 to 13) contain an ASCII string with the format
"XXXXXXXXX.XX". Finally, words 14 through 16 describe the current mask layer
by having word 14 contain the value 2 and words 15 and 16 contain four ASCII
characters.
After the START DRAWING command comes the first START STRIPE command. This
command is one word with the value 7 in the low byte and the stripe number
(from 1 to 255) in the high byte. Note that the word and byte sizes form
restrictive limits on the overall pattern size. For larger dies to be made,
multiple pattern files must be abutted.
After the START STRIPE come the figures. When all figures in a stripe have
been listed, an END STRIPE command appears. The END STRIPE is a single word
with the value 8. At the end of a block is an END OF BLOCK command which is a
single word with the value 9. At the end of the EBES file is an END DRAWING
command which is a single word with the value 4.
The RECTANGLE is one of the figures allowed in a stripe. It contains four
words that describe the width, height, and corner position within the
stripe. The first word contains the value 16 in the low 6 bits and contains
the rectangle height (minus one) in the high 10 bits. Since the rectangle must
fit in a 256-tall stripe, this height can have only the values 0 to 255. The
second word of the rectangle command is the width, the third word is the
starting x position, and the fourth word is the starting y position. Again,
the y position can range only from 0 to 255 since it must be within the
stripe.
The PARALLELOGRAM command is similar to the RECTANGLE command except that it
has two more words to give a skew distance and extra precision bits (see
Figure B.5). Word 1 has the value 17 in the low 6
bits and a value that is one less than the height in the top 10 bits. Words 2, 3 and 4 define the width, x
position, and y position as in the RECTANGLE command, except words 2 and 3 are
multiplied by 16. Word 5 contains the x offset between the bottom and the top
(also multiplied by 16). The sixth word contains extra precision bits for the
width (the low 5 bits), the x location (the next 5 bits), and the x offset
(the high 6 bits). These extra precision bits are high bits for the three
horizontal position values, and define 21- or 22-bit fields for each. The
reason for this extra precision is not to extend the addressing range but to
allow fractional coordinates: These three values are in 1/16 address
units. Thus there is an implied decimal point 4 bits from the right on all
these numbers.
Figure B.5:
EBES parallelogram example. (a)
Figure (b) Record.
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There are three types of TRAPEZOIDs, as illustrated by
Figure B.6. Type 1 has a vertical right edge, Type 2 has a
vertical left edge, and Type 3 has no vertical edges. The Type 1 and Type 2
TRAPEZOIDs can be described in exactly the same format as is used for the
PARALLELOGRAM because they have the same fields: x, y, width, height, and x
offset. Type 1 TRAPEZOIDs have the value 18 in the low 6 bits of the first word, and
Type 2 TRAPEZOIDs have the value 19 in this field.
Figure B.6:
EBES trapezoid types: (a) Type 1 has vertical edge on right
(b) Type 2 has vertical edge on left (c) Type 3 has no vertical edges.
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Type 3 TRAPEZOIDs require a seven-word description because of the extra x
offset field. The first Word has a 20 in the low 6 bits and the height minus one in
the top 10 bits. The second Word is the width, the third word is the x
location, the fourth word is the y location, the fifth word is the first x
offset, the sixth word is the second x offset, and the last word contains
additional precision bits for the width (low 5 bits), the x location (next 5
bits), and the first x offset (top 6 bits). The extra precision bits for the
second x offset are in the fourth word (the top 6 bits), which is otherwise
used only for its low 8 bits to contain the y location.
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