Edwin Jan Klein - Universiteit Twente

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Fabrication
280 nm and an alignment accuracy better than 40 nm. The problems related to the
accurate positioning of the resonator with respect to the waveguides are therefore
essentially removed. The downside of using a stepper however lies in the maximum
mask area that can be exposed. To achieve the high optical resolution the wafer
stepper reduces the image(s) present on the reticle linearly by a factor of five. Due to
this reduction and the image space available on the reticle the maximum area that can
be exposed on the wafer is 22 x 27.4 mm for this specific stepper tool. This, however,
is the maximum area that can be exposed in a single exposure step and uses up all the
image space on the reticle. This poses a problem since the fabrication of a typical
microring resonator based device with heaters requires at least three or four mask
exposures. The solution is then to either use 4 expensive reticles or use a single
reticle with 4 smaller images instead of a single large image. The latter solution then
still allows for a roughly 10 by 10 mm area to be exposed by these images.
Router image, guide layer Router image, ring layer
1
ROADM image, guide layer ROADM image, ring layer
2
Marker image
Figure 5.16. Layout of a mask (reticle) used in the PAS 5500/275 wafer stepper. In total 5
images are present in this reticle. The stepper can select one of these images to expose a
certain area on a wafer.
Instead of using the stepper lithography for the definition of all layers in the device a
more efficient method can also be found, based on alignment accuracy requirements.
Based on the results of the OADM devices fabricated in the “Vertical I” process the
decision was made to improve the OADM and router devices built using the stepper
lithography by adding an additional gold electrode layer on top of the heaters. In
addition the newer designs added vertical tapers to reduce the fiber–chip coupling
losses. A total of 5 mask exposure steps are therefore required:
5
3
4