This invention relates generally to an integrated thin-film drive head for thermal ink-jet printing and more particularly, to a drive head wherein the driver transistor, the resistor and the interconnect are comprised of the same thin-film layer.
An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include printheads with orifice plates having very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles inside the printhead are ink chambers, where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The rapid expansion of ink vapor forces a portion of the ink in the chamber through the nozzle in the form of a droplet. This process is called xe2x80x9cfiring.xe2x80x9d The ink in the chamber is heated with a heat transducer that is aligned adjacent to the nozzle. Typically, the heat transducer is a resistor, or piezoelectric transducer, but may comprise any substance or device capable of quickly heating the ink. Such printers are known as thermal ink-jet printers.
Thin-film resistors are typically used in drive heads of thermal ink-jet printers. In such a device, a resistive heating material is typically disposed on an electrically and thermally insulated substrate. Conventional fabrication techniques allow placement of a substantial number of resistors on a single drive head substrate.
In the past, the number of resistors applied to the substrate was limited by the conductive components used to electrically connect the drive head to external pulse driver circuitry for selectively heating the resistors. Thus, thermal ink-jet drive heads have been developed which incorporate pulse driver circuitry directly on the drive head substrate with the resistors. The incorporation of the pulse driver circuitry on the drive head substrate reduces the number of interconnect components needed to electrically connect the pen to the printer, thereby allowing fabrication of smaller ink-jet pens.
The pulse driver circuitry located on the substrate, typically comprises MOSFET drive transistors (metal oxide semiconductor field effect transistors). The integrated circuitry (i.e., resistors, drive transistors and interconnects) are dimensionally defined on the substrate by conductive trace patterns, lithographically formed using conventional masking, ultraviolet exposure and etching techniques known in the art.
The pulse driver circuitry (hereafter referred to as a driver head) is affixed adjacent to an ink barrier and an outer orifice plate. The internal geometry of the ink barrier defines the shape of the ink chamber. The ink chamber is situated above, and aligned with, a corresponding resistor which, when fired (heated), ejects a droplet through the chamber nozzle.
The integration of driver components and heater resistors on the same drive head substrate requires multi-layer connective circuitry so that the driver transistors can communicate with the resistors and other portions of the printing system. Typically, this circuitry involves a plurality of separate material layers, each formed using conventional circuit fabrication techniques.
Fabrication of conventional ink-jet drive heads generally requires at least four metal depositions and nine lithographic masks (excluding an ink barrier mask) to define the above-described driver circuit components. This procedure results in high production costs and relatively low manufacturing efficiency.
The amount of energy necessary for ejection of ink droplets from the chamber is known in the art as xe2x80x9cturn on energyxe2x80x9d or TOE. A higher TOE may result in excessive printhead heating. Excessive printhead heating generates bubbles from air dissolved in the ink and causes the ink vapor bubble for form prematurely. Air bubbles within the ink and premature formation of the vapor droplet result in a poor ink droplet formation and, thus, poor print quality. Print speed must be slowed to a rate that prevents excessive printhead heating.
Ink-jet drive head systems that can operate at higher temperatures than the operating temperatures of conventional (aluminum-based) drive heads, without electrical or thermal degradation, improve drive head reliability. Allowing operation at higher temperatures increases print speed without sacrificing print quality.
Current thermal ink-jet drive head devices use aluminum as one of the basic components for the formation of the resistors in the drive head integrated circuitry. Although aluminum resistors are acceptable for most applications, they suffer from two major drawbacks: (1) electromigration, or physical movement, of the aluminum in the driver head, which may cause failure at relatively high current densities and (2) relatively complex fabrication processes, as mentioned above.
The present invention provides integrated, thin-film driver heads for a thermal ink-jet printhead, the fabrication of which eliminates the above described disadvantages of conventional drive head devices. The present invention provides smaller resistors that require a low TOE. Smaller drive head circuitry also allows fabrication of smaller printhead, which in turn allows printers to run at faster print speeds with excellent print quality, while minimizing printhead heating.
The present invention also provides an improved drive head manufacturing process, resulting in faster, more cost effective fabrication, producing thin-film ink-jet drive heads superior in both operation and reliability.
According to the present invention, a preferred thin-film ink-jet drive head provides a MOSTFT (metal oxide semiconductor thin-film transistor) transistor, a resistor and the interconnect between the two electrical components, all comprised of the same, multi-functional thin-film layer. Thus, differing portions of the multi-functional thin-film layer of the present invention functions as (1) heating resistors to propel ink droplets from the printhead (2) ink MOSTFT transistors to selectively drive (fire) the resistors, and (3) direct conductive pathways between the drive transistors and the resistors. The present invention substantially eliminates the need to use numerous thin-film layers for carrying out these functions.
The preferred manufacturing process of the present invention requires only five lithographic masks and two metal depositions. The time and materials efficiency of the present drive head fabrication process makes it significantly more economical than previous processes. Moreover, the process of the present invention results in smaller printheads with the advantages described above.
Additionally, when the multi-functional thin-film layer comprises polysilicon or a similar material, the transistor gate and resistor dimensions can be made even smaller, resulting in printheads with very low TOEs. As discussed above, a lower TOE allows faster print speed with improved print quality. A smaller resistor will also allow ejection of smaller ink bubbles. A smaller ink bubble increases resolution.