The need for Cochlear Implants (CI's) with improved functionality and quality requires new techniques to be used to manufacture the electrode array. Titanium nitride (TiN) is one of the contenders for use as metal in the future CI's, but its characteristics need to be verified, measured and tested. A metal should comply to certain requirements like the ability to deliver enough charge to the nerve ending in the saline environment of the cochlea. TiN is able to withstand a high current density (2.8 mA µm2 ), while aluminium failed due to electromigration; even when coated with TiN. The resistance of a 10mm long and 5µm wide track decreases from 1.08 . 10^4? to 6.9. 10^2? when a combination of Al and TiN is made. This solves the reasonably high resistance of TiN. The self-heating and the change in resistance due to temperature changes (TCR) are measured, because the amount of dissipated heat should stay as low as possible. TiN has a low TCR, 5.9 . 10^-4. Furthermore, the metal must not dissolve in the environment, which can be checked by a (cyclic) voltammetry and by endurance tests. TiN was able to withstand the harsh tests. The improved functionality and quality asks for the addition of transistors to the electrode array. Furthermore, this thesis shows that it is possible to create working npn, NMOS and PMOS transistors with TiN using only 5 masks.

Characterization of metals for Cochlear Implants

Lower thresholds and smaller stimulus currents may allow for the use of smaller cochlear implant stimulus electrodes and potentially more channels of stimulation within a given cochlea region. With high (2 kpps) compared to low (125 pps) pulse rate stimuli, inferior colliculus (IC) units have a significantly lower stimulus threshold. The provision of a background "spontaneous" stimuli may provide, a further decrease in threshold current, a unit population with a larger range of thresholds and a more physiologically natural stimulus response. A low threshold, low current level, high pulse rate processing strategy capable of evoking background "spontaneous" firing in units is described. This background stimulus strategy has the potential to deliver more speech information to cochlear implant patients.

Potential advantages of high pulse rate stimulation of cochlear implants

Inferior colliculus (IC) units have a significantly lower stimulus threshold when driven with a cochlear implant using bipolar stimuli at high (2 k pulses/s) compared to low (125 pulse/s) pulse rates. At high pulse rates temporal summation is used to explain this threshold shift.

B. Lithgow

Papers

Cochlear implant threshold changes at high electric stimulus pulse rates