1. Field of the Invention
This invention relates to bubble ink jet printing systems and, more particularly, to an improved integrated circuit chip for use in a thermal ink jet printhead which contains active driver, logic and resistive heater elements and a method for making the chip.
2. Description of Related References
Drop-on-demand thermal ink jet printers are generally well known, and in such systems a thermal printhead comprises one or more ink filled channels communicating with a relatively small ink supply chamber and a linear array of orifices, generally referred to as nozzles. A plurality of thermal transducers, usually resistors, are located in the channels at a predetermined location relative to the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink in contact therewith and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink 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 separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity required for the droplet to proceed in a substantially straight line direction towards a recording medium, such as paper.
In order to generate the resistor current pulses, some type of active driver device must be employed. Preferably, the driver circuitry should be formed on the same chip as the resistive elements. It is generally known to utilize bipolar or less expensive MOS type circuitry as the active driver devices. A typical device which utilizes bipolar type circuitry is disclosed in U.S. Pat. No. 4,429,321 to Matsumoto. In the Matsumoto patent, a liquid jet recording device is disclosed, wherein a method of fabricating the device is shown which incorporates a control unit and a transducer on a single substrate. The control unit in this recorder is a bipolar type of transistor. A method of doping using various implants to create a resistor is shown (see Table 1). A base region of the bipolar transistor is fabricated using boron doping. Unfortunately, bipolar transistors exhibit destructive thermal run away when switching high currents. Therefore, it is desirable and generally more cost effective to have a resistor structure which is immediately and simply integrated on the same wafer with an accompanying MOS driver.
For example, U.S. Pat. No. 4,947,192 to Hawkins et al. discloses a monolithic silicon integrated circuit chip for a thermal ink jet printer wherein a MOS transistor and a resistor are formed on the same substrate. A lightly doped source and drain layer is shown. The relevant portions of the disclosure of U.S. Pat. No. 4,947,192 issued to Hawkins et al., are hereby incorporated by reference into this specification.
The Hawkins et al. reference describes the importance of combining driver and transducer elements on a single chip. Moreover, the reference indicates the potential for adding logic circuitry capable of addressing an arbitrarily large number of ink jets with minimal electrical connections. Such a monolithic device, having logic elements, drivers, and transducers incorporated therein, would generally require added photoresist masking and implant steps to produce enhancement and depletion mode logic devices. While such a structure is desirable, the added processing steps result in potentially higher yield losses and manufacturing costs. A device of this type may be achieved using a single polysilicon layer, as indicated by Hawkins et al. Specifically, The source-drain n+ contacts are doped with arsenic, while polysilicon is doped with phosphorous to create low resistivity (25 .OMEGA./.quadrature.) material at the ends of the transducers. Unfortunately, such a structure would require at least two additional processing steps to implant the arsenic and phosphorous in their respective locations. This would result in an eleven mask step process to create the structure previously disclosed in the Hawkins et al. reference.