A Wavelength Converter Integrated with a Discretely Tunable Laser ...

74 5. Technology
only a thin layer of resist (typically 300 nm) remains on the top of the SOA. During
the lithography, all negative resist outside the active regions is exposed. Therefore, only
the resist in the active regions dissolves during development. However, the whole resist
column in the active regions is dissolvable and the development process must last just
long enough to open the top of the ridge. The key parameters that must be controlled
in this process are the uniformity of the thickness of the resist over the wafer and the
uniformity and reproducibility of the development speed over the wafer. An advantage
of this process is that if the resist is not developed deep enough, it can be developed
further.
d) Plasma. A third self aligned method uses an O ¦ plasma in stead of developer to remove
resist from the SOA waveguide tops. This method does not use a mask nor UV-exposure.
A relatively thin layer of resist is spun such that only about 100 nm of resist or even no
resist on top of the SOA mesa is left. Next, the resist is etched from the SOA tops in
an O ¦ plasma, followed by a buffered HF etch that removes the SiN from the tops. In
this method there is no distinction between active and passive regions apart from their
geometry, i.e. the passive waveguides are about 600 nm lower than the SOAs. The main
problem is that the resist should not be removed from the passive waveguides, but this is
very hard to verify using a microscope.
This process does not work on wide ( waveguides 4 ), because during spinning the
resist sees them as planar surfaces, and the resist on top the ridges will be as thick as
next to the ridges. Only the SiN on top of the ridges is removed and it remains on the
rest of the wafer. This may cause the wafer to bow the SiN is strained.
If contact openings are defined on narrow SOAs using a self-aligned contact opening process,
then it must be taken into account that the metalization step that follows must be compatible
with narrow ridges. This excludes a planarization step with low resolution polyimide on ridges
of £¥ , and only a metalization that results in step coverage can be used (sputtering or
evaporation on a tilted an rotating wafer).
In summary, for the fabrication of contact openings in SiN on SOA ridges 2.0 , the
standard method ”a” works fine. Methods ”c” and ”d” were found to be intolerant and hard
to reproduce. For SOA ridges 2.0 method ”b” is suggested as the most tolerant and
reproducible method.