1. Advanced e-beam lithography overview
positioning and alignment
complications
from design to EBPG gpf workfile

2. Simplest case of direct write (without marker searches and alignment)
just exposure at ‘some’ position
holder centre
alignment microscope
general layout:
array (n,m) of pattern centre positions
pitch SRD = srd_x, srd_y
(srd= step and repeat distance)
job definition file type:
*.JOB (or *.LAYOUT)

3. Marker search positioning & alignment
manually
with SEM (not in batch jobs!)
any recognisable structure with known position
automatically
more accurate
physical principle
contrast in backscattered electron yield
RECT or TOPO type
POS or NEG tone
best markers are rectangles, max size 100 µm
crosses can be used, but aligning is harder
default marker RECT POS 20,20 (holder)

4. marker search with image grabbing
1. the reference image (template image): an image of (another) marker of an artificial image obtained from e.g. pattern data
2. image of the marker (marker image)
3. offset between template image and marker image is calculated

5. Marker search positioning & alignment
marker search parameters
expected contrast CONTRA
max radius ISRAD (default 50µm, max half scanfield)
step ISXSTP, ISYSTP (default 30% marker size)
keep free area (> 100µm) around marker
confusing of search failure by other features
searching = high exposure
backscattered electrons

6. Marker search positioning & alignment
prerequisites (use alignment microscope)
loading of wafer, rotation < 0.2º
position of first (reference) marker known
next marker within search radius
array of markers (≥2x2) preferable along x and y
typical alignment accuracy
3 sigma = 30 nm due to stage movement
3 sigma = 10 nm obtainable with in-field alignment
(which needs special .com-file)
NB!
wafer cq. mounting plate connected to holder with springs
holder with springs to stage

7. Alignment positioning & alignment
alignment is done in steps
each step improves accuracy
in most cases 2 steps are sufficient (global, layout and pattern markers): coarse and matrix
at least 3 markers needed for correcting:
rotation, shift and scaling
at least 4 markers allow also keystone to be corrected.

8. Coarse-alignment positioning & alignment
check on prerequisites
improves effectiveness actual marker search
with premarker

9. Matrix-alignment positioning & alignment
3 or 4 markers in orthogonal arrangement
markers from one cell or from different cells
more closely spaced markers = better overlay accuracy
*.LAYOUT files: pattern should be within the marker field
die-to-die
in-die

10. extra flexibility with CJOB positioning & alignment
pattern can be outside marker field
markers not necessarily in orthogonal system
several patterns and pattern layers can be exposed after one alignment
But: in CJOB no array of markers possible

11. Problems positioning & alignment
features close to the marker
design larger free area around marker, at least 100 µm
layers not well-aligned to each other
ensure the centres of the patterns coincide.
actions:
1) pattern sizes (in x and y) are equal, or
2) put layers in one design and process them together
small alignment errors
ASCII (accuracy) definition of patterns?
drift problems?
substrate well mounted?
charging problems?
resist heating problems?
marker positions and pitch OK?
use marker CENTRE, not a corner!

12. Proximity effect (1) complications
the developed pattern is wider than the scanned pattern, due to the interactions of primary beam electrons with the resist and substrate → resist outside scanned pattern receives a non-zero dose

13. Proximity effect (2) complications
Forward scattering:
due to electron-electron interactions
 deflect primary electrons by a small angle, leads to broadening of the beam [~1 … 10nm]
depends on:
- resist material
resist thickness
acceleration voltage

14. Proximity effect (3) complications
Backscattering:
electrons do not stop in the resist but penetrate in the substrate and still contribute to exposure by scattering back into the resist causing inelastic / exposing processes
lateral range [~10 µm] depends on:
substrate material
acceleration voltage

15. Proximity effect (4) complications
Consequence:
dose variations
pattern distortions
Correction methods:
Local
Dose test critical dimension
Ghost exposure
Global
Shape correction
Dose correction

16. Proximity effect (5) complications
Forward and backscattering can be approximated by a two- gaussian model:

17. Proximity correction (6) complications
Dose correction
EBPG offers infinite number of different relative doses in pattern; trial and error process
Layout Beamer can handle backscatter dose in more analytical way with input from Monte Carlo calculation by program TRACER

18. Charging complications
drain of electrons needed
requires conductive layer otherwise unwanted beam deflections
proven solutions:
intermediate layer in multilayer resist system (e.g. Ge or Si)
thin conductive Ge layer on top of resist and dry (plasma) removal before development
metal underlayer like chromium of mask plates
SnO2 or InSnO2 (transparent substrates)
Elektra92 (conductive polymer)
Some recipes available on request

19. Miscellaneous complications
frequency out of range
too high  smaller aperture, defocus
too low  multiple pass writing (more sensitive to instabilities!)
vibrations
if writing frequency close to 800Hz from turbo instabilities may occur  multiple pass writing
stitching
take critical nano-details in centre of a scan field

20. Layout Beamer from design to EBPG gpf workfile
Software
Design Autocad/DesignCAD/LDM-file/L-Edit/Other
Output formats DXF/ GDSII/ CIF/ TXL/ other formats
Conversion LayoutBeamer  gpf pattern data
Gpf pattern data  exposure EBPG
 Cview inspection
Computer
Layout Beamer PG5000+
USER pegasus EPIC-ALFA
(design) (job)
pegasus.kavli.tudelft.nl epic-alfa.kavli.tudelft.nl
cad/&KN-lab
or: PG5200+
EPIC-BETA
epic-beta.kavli.tudelft.nl

21. Layout Beamer from design to EBPG gpf workfile
important commands
demonstration