Inkjet printhead assembly for symmetrical vapor bubble formation

1. An inkjet printhead assembly comprising:

a substrate defining a plurality of ink inlet passages;

drive circuitry arranged on the substrate;

a plurality of nozzle chamber structures arranged on the substrate and defining (i) nozzle chambers in fluid communication with respective ink inlet passages, and (ii) ink ejection ports in fluid communication with respective nozzle chambers; and

a heater element suspended within and spanning each nozzle chamber, the heater element connected to the drive circuitry to heat ink in the nozzle chamber to form a vapor bubble around the heater element, the heater element being positioned within the nozzle chamber such that a distance between a point of collapse of the vapor bubble and respective ink ejection ports is less than 50 microns, wherein

the heater element is provided with a double omega configuration having symmetrical gaps at opposed sides thereof, whereby symmetrical formation of the vapor bubble centered on an axis of the nozzle is effected.

2. An ink jet printhead assembly as claimed in claim 1 , wherein the heater element is positioned such that a distance between a point of collapse of the vapor bubble and respective ink ejection ports is less than 25 microns.

3. An ink jet printhead assembly as claimed in claim 1 , wherein the heater element is positioned such that a distance between a point of collapse of the vapor bubble and respective ink ejection ports is less than 5 microns.

4. An ink jet printhead assembly as claimed in claim 3 , wherein the heater element require less than 200 nanojoules (nJ) to form the vapor bubble.

5. An ink jet printhead assembly as claimed in claim 3 , wherein the heater element require less than 80 nanojoules (nJ) to form a vapor bubble.

6. An ink jet printhead assembly as claimed in claim 1 , wherein the heater element is configured so that when a bubble forms in the ink, it forms on both sides of each heater element.