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

5.4 Contact openings and metalization 73
The definition of the contacts openings in the SiN isolation layer is normally done using contact
lithography. A narrow opening in resist on top of the SOA is defined and in the next step the
SiN in the opening is etched away. This is an increasingly difficult process if the SOA ridges
become narrower (
SOAs benefits from a self-aligned process. In the list below, four ways of fabricating contact
openings on SOA ridges are compared. The first paragraph describes the standard method.
The other three describe self-aligned methods.
2.0 ), depending on the technology that is used. Therefore, narrow
a) Standard. The standard process for fabricating contact openings uses a mask in combination
with lithography on positive resist (HPR-504) (Fig. 5.7a). The contact openings are
1.7 wide on a 2.0 wide ridge waveguide. The total width of the SOA ridge is
2.6 , because of § the 300 nm thick SiN passivation layer on sides both Therefore,
the alignment tolerance is 0.5 on both sides of the contact opening, which the lower
limit. After exposure with UV-light and the development of the resist, the SiN is etched
in buffered HF and the resist is removed using acetone and an ¦ O plasma. Finally, after
the SiN etch, the wafer is cleaned from fluorides in a RBS solution. This contact opening
process can be used for waveguide widths 2.0 , but for smaller widths the mask
alignment in the lithography step becomes cumbersome.
b) Positive resist. In this self-aligned process, positive photo resist is used in combination
with lithography and a mask. The mask allows for exposure in the area near the SOAs
only (Fig 5.7b). Upon UV light exposure, the photo resist becomes dissolvable in a
developer up to a depth that scales linearly with the exposure time. If the positive resist
is exposed for a duration that makes it dissolvable beyond the top of the ridge, then
development of the resist results in step 2 in Fig 5.7b. Subsequently, the SiN is etched
away in a buffered HF solution and the wafer is cleaned from fluorides in a RBS solution.
The SiN isolation should not be removed more than § nm below the top of the ridge,
because this may cause optical loss in the Au metalization that will be applied later.
Experimentally it was found that for each second of exposure of the HPR-504 resist,
the dissolvability increased with 200 nm . This process has not been tested on the actual
SOA ridges, because the necessary mask (an inverted version of the active-passive
mask) was not available. It is important that the thickness of the resist on the wafer in
combination with the exposure depth is controlled better then § nm. This should
be feasible because the thickness of the resist in the spinning process is typically better
controlled than 100 nm. Note that the SiN is removed from the top of the SOAs only
and that it remains on the rest of the wafer. This may cause problems if the SiN layer is
strained.
c) Negative resist. A second self aligned method to fabricate contact openings uses negative
photo resist in combination with the active-passive mask that was used in the wafer
epitaxy. The negative resist (MaN-440) is spun on the wafer at a high speed, such that
350 nm of SiN is deposited on the chip, but side walls are covered with a 10-20% thinner layer.
The intensity UV light was 7.5 mJ/cm s at a wavelength of 405 nm.