The present invention relates to thermal ink jet printers, and more particularly, to printheads incorporating a plurality of heater resistors which are selectively addressed to heat and expel ink from adjacent ink channels.
Thermal ink jet printing utilizes printheads which use thermal energy to produce vapor bubbles in ink-filled channels to expel ink droplets. 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.degree. 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 (.mu.s). The channel can be refired after 100 to 500 .mu.s minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to be somewhat dampened.
To eject an ink droplet, each heater element must become hot enough to cause the ink to reach a bubble nucleation temperature of at least 280.degree. C. for water based ink. In order for the heater element to generate the thermal energy to cause bubble nucleation, an operating voltage is applied to a resistor of the heater element. Typically, the operating voltage is proportional to the resistance of the resistor, i.e., the higher the resistance, the higher the operating voltage.
Conventionally, polysilicon is used to form the resistors of the heater elements. The resistance value of the resistors is chosen based on the actual required power (Power=V.times.I=I.sup.2 .times.R=V.sup.2 /R) for ejection of the ink droplet through bubble nucleation. Once the required power and voltage has been chosen, the resistance value is determined. The fabrication of the determined resistance is controlled by the sheet resistance (ohms/square; .OMEGA./.quadrature.) of the polysilicon and the size of the resistor. The size of the resistor can be tightly controlled by photolithography and polysilicon etch techniques. The sheet resistance of the polysilicon is primarily controlled by impurity doping, preferably by ion implantation, and annealing of the doped polysilicon.
As drop ejection becomes more efficient, the heater element size required decreases and the resistance required increases. The more resistive the heaters, the lower the doping level, and the more difficult it is to accurately control the resistance. When resistance is too high, threshold voltage is too high and drops are not ejected. When resistance is too low, threshold voltage is too low, and the extra voltage applied to the printhead results in excess heat, which causes ink to bake on the heater. This results in an extra insulting layer on the heater, decreasing energy transferred to the ink, creating smaller drops and eventually completely failing to eject drops. In addition, "over voltage" operation results in decreased heater lifetime. Conventional techniques use doping to introduce impurities in the polysilicon heater structure. One method disclosed in U.S. Pat. No. 5,639,386, whose contents are incorporated by reference, teaches methods of fabricating polysilicon resistors with improved threshold uniformity which includes steps for forming resistors with two doped regions, a lightly doped center region and more heavily doped end region. The heater element is formed on a polysilicon layer 8 (see FIG. 3 of the patent). N-type dopants, typically phosphorus, are ion implanted into the polysilicon layer to form a lightly doped heater center region. A photoresist layer 8 is patterned to protect the heater center region followed by heavier doping at the ends. This results in lightly doped center regions 8A and heavily doped end regions 8B. Regions 8B are heavily doped to make good electrical contacts to address leads and for low resistance to form transistor gates. The photoresist layer is removed, and the polysilicon is plasma etched to form the final resistor structure.