1. Living with e-beam drift Part 1: The e-beam is not broken
Version 3
M. Rooks
Yale University

10. Other ways to minimize drift problems
3. Alignment using marks on the wafer is a very good way to correct for drift, BUT you have to write patterns that can be exposed in just a few minutes. You should be writing many small “chips” so that the system will realign often. On the EBPG, one pattern file gets one alignment. So if you need n alignments then you need n patterns.
If you have been forced to use gigantic patterns that take a long time to print, then (in theory) you would create a jobfile.ini that contains “pg set calrealign 1”, which should inspire the EBPG to realign to the chip marks each minute. Unfortunately this does not work. It’s a bug: “calrealign” will induce a one-point drift check, not a full chip alignment sequence.
You can work around this bug by splitting your pattern into n smaller patterns. This can be done (the hard way) with Beamer or with the utility gpfsplit (the easy way). Later in this presentation you’ll see an example of using gpfsplit to force realignments.

11. Living with e-beam drift
Part 2: Multi-pass writing and sorting fields

20. But the results are not so great. So let’s try a different option.
You could try…
gpfsort –v merged.gpf
But the results are not so great. So let’s try a different option.

27. Another Example: Random Walks
How can you optimize a pattern which is so dense that Yellow Brick Road sorting is ambiguous?
Here is an example of a pattern that cannot be properly reordered with the gpfsort options –ybr, –v, –h, or –ts. In this case we would like to specify the writing order manually, and we can, using the –path option.

28. Another Example: Random Walks
Simply create a text file called path.txt, and type in the vertices of the desired exposure path.
These coordinates should be the ones seen in the gpf file (using cview) or they can have the same origin as the original CAD file. The path should follow the desired field centers, but the numbers do not have to be precise. This path defines the writing order, not the field placement.

29. Another Example: Random Walks
path.txt
lower-left
upper-right #
0 -70
0 80
300 80
150 80
gpfsort –path path_example.gpf
It might be tedious to type in a long path, but you will get exactly what you want.
Look at the example path.txt file for helpful hints. If you want to use the same origin as the original CAD, then you must specify lower-left and upper-right coordinates in path.txt

30. I swear this is the last example: multi-pass writing with realignment
Suppose you have a long pattern with many writing fields, and that you have used Beamer (or whatever) to create three overlapping patterns, as we did ~11 slides ago.
Overlapping patterns (written at 1/3 dose each) wash away stitching errors, but there will still be an overall curve in the pattern from one end to the other. This “runout” error is a serious problem for multi-level devices.
Fortunately in this example you have first fabricated alignment marks in the corners. Remember each pattern file gets one alignment to the marks. If you simply do not merge together the three overlapping pattern files, then you can use cjob to align and expose three times.
600 µm

31. but
If the pattern takes (let’s say) one hour to write,* then each 1/3 pattern takes 20 minutes. We really should realign more often than that. So let’s merge and sort the pattern as before, but then split it into 12 parts so that the system will realign every five minutes. (Now click to start our lovely animation.)
align
align
File 1
align
align
File 2
File 3
File 4
The sequence for setting up this exposure is…
* Yes of course this pattern would not take an hour to write, but let’s pretend.

32. The sequence for setting this up:
Generate the pattern by whatever means – CAD or algorithmically
Beamer – generate three gpf files each with the same center but shifted lower-left corners
let’s call them wiggle1.gpf, wiggle2.gpf, wiggle3.gpf
Merge the three gpf files together with
gpfmerge wiggle*.gpf
We should sort the fields with the –ybr (default) or –h option:
gpfsort merged.gpf
I don’t like the resulting file name, so let’s rename it:
mv merged_sorted.gpf wiggle.gpf
Split the file into 12 pieces with
gpfsplit wiggle.gpf 12
Now we have ~ 12 gpf files with the names like wiggle_0.gpf and wiggle_12.gpf.

33. Finally
You can view all of these gpf files at the same time with
cview wiggle_*.gpf
Cview will display the different files as different colors:
Now you can use cjob to define the alignment marks, and to specify the exposure of all 12 patterns in the same place.

34. Use gpfsplit if you need a lot of realignments during exposure.

35. Living with e-beam drift
Part 3: Dealing with drift inside a single field

37. We could apply the same drift checks used for multi-field patterns
If there are no alignment marks, then put pg set caldrift 00:01:00 into the jobfile.ini file. This will induce the system to measure a mark position (the one on the holder, by default) each minute. This will not work as well as you hope.
If you can fabricate alignment marks on the wafer AND if the pattern takes a long time to write, then a better technique is to split the pattern in to n parts, and then have the system realign n times. You can split the pattern by cropping sections with Beamer (in the ‘extract’ module). Or you can use a smaller field size, then use gpfsplit to create n patterns. (Note that “pg set calrealign 1” does not currently work.)
Neither of these techniques will work very well. You really need to control the writing order, so read on.

42. What if your pattern is so dense that ybrsort cannot choose a sensible writing order? In that case you can use ybrsort to sort the shapes by a manually defined path (like gpfsort, but now sorting shapes inside each field.) Use the command
ybrsort -path pathfile.txt pattern.gpf
The pathfile.txt file contains points with the same origin as that of the original CAD, and you must specify the same lower-left and upper-right points you used to convert the pattern to gpf (in Beamer, most likely). The path might look like this:
# a short path for ybrsort #
lower-left
upper-right
0 0
jump
50 50
50 100

43. The path used for ybrsort can span many fields, and can be the same path used for gpfsort (field sorting). You might be thinking it would be very tedious to type in a long, complex path like this one. One trick for reducing the work is to draw the exposure path with your CAD program, using a “path” object on an unused layer. (A “path” in CAD is a segmented line of fixed width defined by a set of vertices. Look for this icon in the upper-left corner of Layout.
After drawing the path by hand inside cell “topcell” on layer 45 (for example), you can extract it to a text file with the command
extractpath cadfile.gds topcell 45
This creates the file extracted_path.txt, which you will have to edit to insert the lower-left and upper-right points.

44. Summary
To compensate for drift inside a single field the best strategies are:
Let the wafer sit on the stage for many hours before writing,
Fabricate alignment marks on the wafer then split the pattern in to n parts, allowing the system realign n times.
For single waveguides use ybrsort to re-order the fields. If the pattern has radial symmetry, use radial_sort. To force an arbitrary order on your crazy pattern, use “ybrsort –path”
Find this and other e-beam presentations at nano.yale.edu