Thermal inkjet head with improved jet straightness

US 2013/0293638 A1
Fluid ejection device having firing chamber with mesa
Hewlett-Packard Development Company, L.P.

There have been several thermal ink jet printhead designs in the past that have attempted to improve jet straightness.  As drops are formed, the bulk of the ink moves rapidly to the substrate.  Some of the ink is contained in the tail, which breaks off.  Often the tail will fragment into fine drops which either slowly follow the main drop to the substrate or fall back on the nozzle plate.  In addition erratic tail break-off can lead to mis-directed drops.

The proposed solution is to redesign the heater chamber floor with a central column or mesa and a circular heater.  The silicon substrate is etched to form a circular pit 230 around a central column 250.  The heater 205 is formed on the outer wall of the pit.  The heater, electrodes 208 and the exposed surface of the substrate are coated with an insulator, which also increases the robustness of the structure.

The column is centrally positioned under the nozzle.  It is thought that the tail then breaks more centrally than otherwise.   In addition, because the heater is not at the centre of the chamber, it is far less likely to be damaged by the collapsing bubble.  Therefore the overcoat layer, which has a protective function as well as being an insulator, can be reduced in thickness and therefore increase the efficiency of the device.