Edwin Jan Klein - Universiteit Twente

Chapter 5
5.4. Fabrication and mask design of vertically coupled
resonators defined using stepper lithography
5.4.1 Stepper lithography and mask design
As pointed out in the previous section, only 1/36 th of the total number of fabricated
devices can be properly aligned when building cross-coupled micro-resonators using
contact lithography. This is not necessarily a problem when small devices, consisting
of one or two microring-resonators, are built. Assuming a realistic average device
height of 500 µm then, when using 6 columns, 20 devices can fit in the 1 cm 2 area
allotted to a single group. Because the devices are, on average, only 1.5 mm in width
(largely due to bends, tapers etc), the number of columns can even be tripled to 18,
giving a total number of 60 functional devices per wafer, which gives more than
enough devices for redundancy and parameter variations. More complex devices such
as for instance the later discussed OADM and λ-Router, have a height between 1.5
and 2 mm and a width of about 5 mm. At most 10 to 12 of these devices can therefore
be put on 1 cm 2 which leaves almost no room for redundancy. Considering the fact
that inevitably some of these devices will be damaged during fabrication [132],
getting sufficient yield (or even functional devices) can be a problem.
In collaboration with ASML Netherlands B.V [133] a solution was therefore sought in
the use of wafer stepper lithography. Contrary to contact lithography where a single
mask exposes an entire wafer in a single step, a wafer stepper exposes only a small
fraction of the wafer by projecting a selected area (the image) of the mask (the reticle)
onto the wafer through a series of lenses as is shown in Figure 5.15. By moving the
wafer in the x- or y- direction several locations on the wafer can by exposed in this
fashion, producing many identical copies.
Figure 5.15. Wafer stepper concept: An image located on the reticle is reduced through a
series of lenses and projected on a wafer. The wafer can be moved to expose several areas
on the mask with this image.
The main benefits of using wafer stepper lithography for vertically coupled microring
resonator devices lies in the alignment accuracy of the images projected onto the
wafer and the high resolutions that can be achieved. The ASML PAS 5500/275 [134]
stepper used to fabricate the OADM and router devices has a resolution better than
120
Mirror
UV Light source
Filter
Condenser
Lens system
Reticle
Reduction
Lens system
Wafer