Concept, Construction and Commissioning

1
Concept, Construction and Commissioning of an
Alignment System for Deep X-Ray Lithography O.
Wilhelmi, S. Peredkov and A. Bogdanov MAX-lab,
Lunds Universitet, S-221 00 Lund, Sweden
O. W. is employed within the TMR-network program
microsync of the European Commission.
1 http//www.maxlab.lu.se/beamlines/bld811/resul
ts.html
PRINCIPLE OF OPERATION Alignment marks are
produced on mask and substrate. The design of the
alignment marks and their position on a
4-mask are shown below (Fig. 1).
The substrate is moved by a kinematic stage
relative to the fixed mask in order to maximize
the photocurrent, i.e. obtain best alignment.
The sequence in the middle of the poster (Fig. 5)
illustrates the alignment process. A
(pico-)motorized kinematic stage was chosen as
the most appropriate device for this application.
The selected stage is shown in Fig. 6 below, the
decisive criteria are summarized in Tab. 1.
Fig. 5
STEP 4
Illustration of different stages in the
alignment process.
STEP 1
Fig. 1
Fig. 6
The motorized kinematic stage 8095M from New
Focus, Inc. which moves the substrate. The red
pico-motors can easily be recognized on the
left-hand side.
alignment marks
Alignment marks on a 4-mask (left) and alignment
mark design (right). The area inside the reticles
is x-ray transparent, the outside area is
covered by absorber)
X-ray sensitive diodes behind the transparent
reticles on mask and substrate (resist on wafer)
register the transmitted intensity (Fig. 2
3). The photocurrent is the measure for the
overlay. The wafer is thinned from the backside
underneath the alignment marks.
STEP 2
mask
mask
resist
substrate
wafer
Tab. 1 Specifications of the stage 8095M by New
Focus, Inc. .
diode
diode
GOAL Overlay accuracy 200 nm
Mounting of mask, substrate and a diode opposed
to the x-ray beam (Fig. 2) and as cross section
(Fig. 3).
Since the absorber around the reticles does not
completely absorb the x-rays, tiny changes in the
photocurrent have to be detected. The
photocurrent is to be read out by a multimeter
(PREMA DMM 5017) which provides 6 1/2 digit
resolution at 5 Hz and 5 1/2 digits at 50 Hz in
the 200-?A-measurement range .

• SOME FURTHER FEATURES
• Vacuum-in-vacuum technique for substrate-fixing
  in an ambient pressure
• between 100 mbar and 200 mbar. The substrate
  is fixed on a carrier before
• mounting both to the stage. A bayonet-type
  mount allows fast and easy
• assembly of substrate carrier and stage.
• Lean construction 7.8 kg total weight of
  alignment system and chamber.
• Both have to be moved on the scanner.
• Last but not least Money ! Total Costs of
  commercial components 13550
• (the costs include the production of the
  precision parts for the alignment-
• system in an external workshop, and exclude
  the costs for production of the
• chamber in the MAX-lab workshop). The
  screen-unit is also excluded.

The alignment procedure is carried out with mask
and substrate in exposure position ?
in-line. During the alignment procedure, the
resist in the pattern area has to be protected
from exposure. A screen with a vertical slit can
therefore be moved into the x-ray beam, covering
the pattern area while alignment marks are
irradiated. The screen unit is shown in Fig.
4. The screen will be swayed out of the x-ray
beam when the alignment is accomplished.
Surveillance of the alignment throughout pattern
irradiation remains feasible.
STEP 3
Fig. 7
Fig. 8
Screen-unit which protects the resist in the
pattern area as long as the alignment takes
place.
Fig. 4
View into the chamber with the mounted alignment
system. Fig. 7 Substrate (blue disc) fixed on
the substrate carrier. Fig. 8 Mask (brown disk)
mounted in front of the resist.