Unit cell for thermal inkjet printhead

1. A unit cell for a thermal inkjet printhead, the unit cell comprising

a substrate portion that defines an ink supply passage;

drive circuitry and interconnect layers positioned on the substrate portion;

a nozzle plate defining a nozzle;

side walls depending from the nozzle plate and positioned on the substrate portion to define an ink chamber; and

a heater element connected to the drive circuitry and positioned intermediate the nozzle plate and the substrate portion, the heater element being suspended in the ink chamber in alignment with the nozzle and being shaped to be symmetrical about two planes that intersect along an axis extending through a centre of the nozzle, wherein

the drive circuitry is partially on one side of the ink chamber with a remainder of the drive circuitry on an opposite side.

2. A unit cell as claimed in claim 1 , wherein the interconnect layers include vias for connecting the heater element to the drive circuitry, the vias being defined by open areas in an ink passivation layer formed on the interconnect layers.

3. A unit cell as claimed in claim 2 , wherein the interconnect layers include a raised metal layer on which the ink passivation layer is formed.

4. A unit cell as claimed in claim 2 , wherein the heater element is part of heater, with electrodes of the heater positioned in the open areas and connected to the interconnect layers.

5. A unit cell as claimed in claim 1 , wherein the nozzle includes a circular nozzle aperture bounded by a nozzle rim.

6. A unit cell as claimed in claim 1 , wherein the heater element is shaped so that, when an ink bubble is formed as a result of the ink being heated, a point of collapse of the bubble is spaced from the heater element itself.