Development of a Oxygen Sensor for Marine ... - DTU Nanotech

44 CHAPTER 5. FABRICATION
Thermometer), Ag (Reference Electrode), and Au (Contacts) are deposited.
Why these 3?
As discussed previously Pt and Ag have been used almost exclusively for
Clark Sensors, and referenced as the best suited metals from a dissolved oxygen
sensor[37].
Where as Au gold provides an excellent surface for bonding to wirebond, as
well as being able resist some exposure to saltwater.
Figure 5.7: Illustration of the wafer with the three metals added.
Figure 5.8: Alignment marks after the metals are added. From left to right,
ignoring the first one, it is Pt(brown), Ag(light brown), Au(red).
Each metal have double alignment marks, two boxes and a crisscross
pattern. This is mainly to have a little extra safety when
aligning the masks, so that there is always another alignment
mark to check with.
Pt will be put on first for two reasons; It is the metal with the smallest
structure, and hence the metal where the lithography step is most sensitive,
and therefore where it is most like to go wrong. Also since the Au-Contacts
and the Pt-Thermometer crosses, having the PT buried beneath the Au will
ensure an unbroken line (Especially as the Au, 3000 ˚A, layer is over 10 times
as thick as the Pt, 200 ˚A, layer).
A minor problem however is that a lot of metals have poor adhesion to
many forms of Silicon (including Silicide), hence a titanium layer is added