Thermal inkjet printers have experienced a great deal of commercial success since their inception in the early 1980's. The fundamental principles of how thermal inkjet printers work is analogous to what happens when a pot of coffee is made. Using the electric drip coffee maker analogy, water is poured into a container (reservoir) and is channeled towards a heating element that is located at the base of the container. Once the coffee has been placed in the filter, the coffee maker is turned on and power is supplied to the heating element that is surrounded by water. As the heating element reaches a certain temperature, some of the water surrounding it changes from a liquid to a gas, thus, creating bubbles within the water. As these "super heated" bubbles are formed, heated water surrounding these bubbles is pushed from the reservoir into a tube and finally into the carafe. Referring now to the thermal printhead, ink is located in a reservoir that has a heating element (heater resistor) at its base. When the heater resistor is turned on for a certain amount of time (pulsed by electronic circuitry) corresponding to a certain temperature, the ink surrounding the heater resistor changes from a liquid to gas phase, thus, creating a bubble that pushes surrounding ink through an orifice and finally onto a printing medium (carafe). The aforementioned example radically simplifies inkjet technology. For a more detailed treatment of the history and fundamental principles of thermal inkjet technology, refer to the Hewlett-Packard Journal, Vol. 36, No. 5, May 1985.
In the coffee maker analogy, the water was poured into a container (reservoir) and channeled to a heating element located at its base. This channeling, for an inkjet cartridge, may be accomplished in a variety of different ways with the objective being to simultaneously provide the ink ejecting heater resistors with a continuous supply of ink.
The ink channel has traditionally been a challenging feature to fabricate both in terms of manufacturing repeatability and manufacturing cost. When manufacturing a multiplicity of printheads, variation in critical dimensions can be cataclysmic. For example, if a channel's width is too narrow, it may restrict the flow of ink to the heater resistor(s) consequently causing variations in the volume of ink ejected onto the printing medium. Likewise, if the channel width is too large, ink may be more readily supplied to some heater resistors than others thus creating variations in the rate at which ink may be ejected from the printhead nozzles (hence, the distance through which ink travels before reaching the heater resistor impacts the speed/frequency at which the printhead operates).
In terms of cost, traditional techniques of fabricating ink feed channels involved "sand blasting" holes into a substrate as disclosed in U.S. Pat. No. 5,681,764. This technique, although effective, required very specialized equipment that varied significantly from conventional IC processing thus requiring special facilities, personnel, and equipment. Consequently, there has been many efforts in the inkjet printing community to develop techniques for fabricating ink feed channels wherein the channel dimensions could be accurately controlled using standard IC manufacturing equipment and methodology. The following US patents describe such methods and techniques in an attempt to remedy the aforementioned problem.
U.S. Pat. No. 5,308,442 illustrates a method for isotropically etching ink feed channels employing wet chemical etching. This technique incorporates standard integrated circuit (IC) photolithography and wet etch processing methodology and provides an alternative to the traditional sand blasting approach. Additionally, it provides an improvement over the sand blasting technique wherein the path through which ink flows prior to reaching the heater resistor is shortened. This technique, however, is based purely on conventional anisotropic wet chemical etching (hereafter referred to as wet etching) from the backside of the wafer/wafer substrate subsequently limiting the dimensional control of the ink feed channel. The backside of the wafer refers to the side opposite of where nozzles will be formed.
U.S. Pat. No. 5,387,314 discloses a technique for channeling ink from a reservoir to a heater resistor by utilizing photolithography techniques with a combination of wet etching and plasma etching (a conventional gaseous etching technique hereafter referred to as dry etching). A semiconductor wafer, such as a silicon wafer, is used with a known crystallographic orientation to accommodate channels through which ink flows to the heater resistor. Such a wafer can be etched in two prominent process steps: Firstly, trenches are anisotropically etched part way into the semiconductor from the backside of the substrate. Secondly, an isotropic dry etch is used to etch from the front side (the side upon which nozzles are formed) of the substrate thus creating a channel through the substrate. The advantages of this technique as compared to that previously described in U.S. Pat. No. 5,308,442, is that the front side dry etch offers a greater degree of dimensional control. As this is well know in the semiconductor industry, isotropic wet etch processes are, in general, more variable than dry etch processes. Combining both dry and wet etch processing was a major step whereupon dimensional control of the ink feed channel was improved. However, the aforementioned process introduces an isotropic dry etch step from the front side of the wafer thus requiring the substrate above the ink feed channel to be void of active devices or signal lines.
Many of the aforementioned challenges associated with the fabrication of ink feed channels still persist. Consequently, there remains an opportunity to develop a manufacturing process and apparatus wherein: (1) ink feed channels dimensions can be precisely controlled, (2) the distance through which ink flows before reaching the heater resistor can be minimized, (3) and the time required to form the ink feed channel is reduced.