Thermal ink jet heater design

Details Thermal ink jet heater design Thermal ink jet heater design

1992-10-21

2001-11-13

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06315398

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to ink jet printing systems, and in particular to drop-on-demand ink jet printing systems having printheads with heater elements.
2. Description of the Related Art
Ink jet printing systems can be divided into two types. The first type is a continuous stream ink jet printing system and the second type is a drop-on-demand printing system.
In a continuous stream ink jet printing system, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed so that the stream breaks up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct the ink droplets to a gutter for recirculation or to a specific location on a recording medium.
In a drop-on-demand ink jet printing system, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless the droplet is to be placed on the recording medium. Because the drop-on-demand ink jet printing system requires no ink recovery, charging or deflection, such system is much simpler than the continuous stream ink jet printing system. Thus, ink jet printing systems are generally drop-on-demand ink jet printing systems.
Further, there are two types of drop-on-demand ink jet printing systems. The first type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. The second type uses thermal energy to produce a vapor bubble in an ink-filled channel to expel an ink droplet.
The first type of drop-on-demand ink jet printing system has a printhead with ink-filled channels, nozzles at ends of the channels and piezoelectric transducers near the other ends to produce pressure pulses. The relatively large size of the transducers prevents close spacing of the nozzles, and physical limitations of the transducers result in low ink drop velocity. Low ink drop velocity seriously diminishes the tolerances for drop velocity variation and directionality and impacts the system's ability to produce high quality copies. Further, the drop-on-demand printing system using piezoelectric transducers suffers from slow printing speeds.
Due to the above disadvantages of printheads using piezoelectric transducers, drop-on-demand ink jet printing systems having printheads which use thermal energy to produce vapor bubbles in inkfilled channels to expel ink droplets are generally used. A thermal energy generator or heater element, usually a resistor, is located at a predetermined distance from a nozzle of each one of the channels. The resistors are individually addressed with an electrical pulse to generate heat which is transferred from the resistor to the ink.
The transferred heat causes the ink to be super heated, i.e., far above the ink's normal boiling point. For example, a water based ink reaches a critical temperature of 280° C. for bubble nucleation. The nucleated bubble or water vapor thermally isolates the ink from the heater element to prevent further transfer of heat from the resistor to the ink. Further, the nucleating bubble expands until all of the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor which, of course, removes heat due to heat of vaporization. During the expansion of the vapor bubble, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus.
When the excess heat is removed from the ink, the vapor bubble collapses on the resistor, because the heat generating current is no longer applied to the resistor. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity to expel the ink droplet towards a recording medium, such as paper, in a substantially straight line direction. The entire bubble expansion and collapse cycle takes about 20 microseconds (&mgr;s). The channel can be refired after 100 to 500 &mgr;s minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to be somewhat dampened.
FIG. 1
is an enlarged, cross-sectional view of a conventional heater element design. The conventional heater element
2
comprises a substrate
4
, an underglaze layer
6
, a resistive layer
8
, a phosphosilicate glass (PSG) step region
10
, a dielectric isolation layer
12
, a tantalum (Ta) layer
14
, addressing and common return electrodes
16
,
18
, an overglaze passivation layer
20
, and a pit layer
22
. The actual heater area is determined by the length L
R
of the resistive material. However, the effective heater area is determined by the distance L
E
between the inner slanted walls of the overglaze passivation layer. In another conventional heater element design (not shown), the side walls of the overglaze passivation do not overlap the side walls of the PSG step region, and the effective heater area is determined by the distance between the inner side walls of the PSG step region. Because there is a relatively large difference L
D
between the actual heater area and effective heater area, the heat generated at the unused heater areas is lost. Further, the overglaze passivation layer
20
or PSG step region
10
alone prevents exposure of the ionic and corrosive ink to the addressing and common return electrodes and/or resistor ends.
It is generally recognized in the ink jet technology that the operating lifetime of an ink jet printhead is directly related to the number of cycles of vapor bubble expansion and collapse that the heater elements can endure before failure. Further, after extended usage, the heater robustness, i.e., the printhead's ability to produce well defined ink droplets, is degraded. Heater failures and degradation of heater robustness are due to extended exposure of the heater elements to high temperatures, frequency related thermal stresses, large electrical fields and significant cavitational pressures during vapor bubble expansion and collapse. Under such environmental conditions of the heater elements, the average heater lifetime is in the high 10
7
pulse range, i.e., number of ink droplets produced, with the first heater failure occurring as low as 3×10
7
pulse range.
Further, the bulk of all heater failures does not occur on the resistors
8
which vaporize the ink, but rather occurs near the junction between the resistor
8
and electrodes
16
,
18
. Specifically, during the collapse phase of the vapor bubble, large cavitational pressures of up to 1000 atm. impact the regions near the PSG step region
10
and overglaze passivation layer
20
of the heater. The large cavitational pressures result in attrition damage to the tantalum (Ta) layer
14
and dielectric isolation layer
12
and also attrition damage, i.e., notch damage, to the overglaze passivation layer
20
covering the PSG step region
10
. Moreover, the overglaze passivation layer
20
alone protects the electrodes
16
,
18
from the ionic ink, which is corrosive. Eventually, a hole in the Ta layer
14
, dielectric isolation layer
12
and/or passivation layer
20
allows the ionic and corrosive ink to contact the heater at the electrodes
16
,
18
to cause degradation of heater robustness and hot spot formation and eventually to heater failures.
Moreover, the heater failures are exacerbated by the problem of obtaining good conformal coverage of the Ta layer
14
over the PSG step region
10
. The problem of obtaining good conformal coverage has been corrected by using an extra processing step to taper which consequentially exten

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