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

Chapter 6
Problems and Solutions
In almost every project some unexpected problems appears, ranging from
merely inconvenient problems such as cleanroom machinery being out of
work, to more actual problems, such as fabrications steps being more complicated
than they appear on paper. In this chapter some of the problems
encountered during the process of creating the Clark sensor will be highlighted,
as well as how they were solved when possible.
6.1 Wirebonding
As mentioned in Chapter 5, the first, and at the time the only, choice for
connecting the chip to the flex print was wirebonding. Wirebonding is basically
done by having a needle, with a metal (Al) thread through it, pressing
down on the contact pad of the chip, and then through the use of ultrasound
and vibrations attach the thread. The similar process is then repeated on
the flex print, at which time the rest of the thread is snapped off. However
this would soon prove to be a lot more complicated than it sounds. For some
unexplainable reasons the thread refused to perform the second attachment,
whether it was on the gold on the chip, or on the flex print. Albeit some
were wirebondings were successful, they were few and there was far between
them.
Why this is so, remained a mystery for a while, however there was some
problems with the machine at the time. However a later examination of the
flex print revealed that the metal on it was tin covered copper. Therefor it
was impossible to bond to the flex print, unless the tin was scraped off, as
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