1. Field of the Invention
The present invention relates to an ink jet print head that ejects ink utilizing thermal energy generated by a heating resistor as an electrothermal converter, and a method for manufacturing the ink jet print head. More specifically, the present invention relates to an ink jet print head comprising an electrothermal converter (heater) composed of a heating resistant film, the ink jet print head being able to print characters, symbols, images, and the like on various print media by ejecting ink to the print media, as well as a method for manufacturing the ink jet print head. The print media include paper, plastic sheets, clothes, and other various articles.
2. Description of the Related Art
An ink jet print head using heating resistors as means for generating ejection energy for ejecting ink bubbles the ink by thermal energy generated by the heating resistors so that the bubbling of the ink can be utilized to eject the ink from ejection ports.
For such ink jet printing apparatuses, there has been a growing demand for the improved quality of printed images and improved functions such as faster printing. To improve the quality of printed images, the size of dots can be reduced by decreasing the size of the electrothermal converters and thus the amount of ink per dot. For the dot size, the size of droplets tends to be decreased in order to reduce the granularity of a half tone portion of a gray scale and a half tone portion and a high light portion of a color photo image. In particular, the amount of ink droplets provided by a print head that ejects color ink tend to decrease year by year from about 5 pl to 2 or 1 pl. Furthermore, evolution and prevalence of digital input apparatuses has resulted in a strong demand for high-definition images such as photographic images. As a result, the size of ink droplets is desired to be further reduced.
The increasingly reduced size of ink droplets serves to improve the quality of images for which a high contrast such as that of photographic images is required. However, if characters are printed, the reduced size of ink droplets may reduce printing speed. To avoid this problem, for example, the same print head comprises nozzles and electrothermal converters which allow larger ink droplets are ejected, and nozzles and electrothermal converters which allow smaller ink droplets to be ejected. That is, the former nozzles and electrothermal converters are used to eject the larger ink droplets in order to print characters and the like, whereas the latter nozzles and electrothermal converters are used to eject the smaller ink droplets in order to print high-quality images. Thus, both high image quality and appropriate printing speed can be achieved by using the different nozzles and electrothermal converters to selectively eject the larger or smaller ink droplets.
However, if the size of electric resistors constituting the electrothermal converters is reduced so as to correspond to the reduced size of ink droplets, the electric resistors (heating resistors) need to have an increased electric resistance value so as to be driven under the same driving conditions as those in the conventional art.
With reference to FIGS. 16A, 16B, and 16C, description will be given of the relationship between the size of heating resistors constituting electrothermal converters and the driving conditions for the heating resistors. A large-sized heating resistor A and a small-sized heating resistor B were prepared as shown in FIG. 16A. FIG. 16B shows variations in the resistance values (solid lines A and B) and current values (dotted lines A and B) of the heating resistors A and B with respect to driving pulse width which variations are observed when a fixed driving voltage is used for the heating resistors A and B. FIG. 16C shows variations in the resistance values (solid lines A and B) and current values (dotted lines A and B) of the heating resistors A and B with respect to the driving voltage which variations are observed when a fixed driving pulse width is used for the heating resistors A and B. As is apparent from FIGS. 16B and 16C, with the reduced size of the heating resistor, the resistance value needs to be increased in order to allow the hating resistor to be driven under the same conditions as those in the conventional art.
Japanese Patent Laid-Open No. 10-114071 describes an ink jet print head in which each of the heating resistors is composed of a thin film to provide a high heating resistance characteristic corresponding to the reduced size of ink droplets. The heating resistor is made up of a thin film of TaxSiyNz in which x=20 to 80 at %, y=3 to 25 at %, and z=10 to 60 at %.
As described above, the resistance of the heating resistor can be effectively increased in order to reduce the size of ink droplets. On the other hand, the increased resistance of the heating resistor makes the adverse effect of the film formation tolerance of the heating resistor more serious. As a result, in the same nozzle row with a plurality of nozzles arranged therein, the film formation tolerance of the electrothermal resistors corresponding to the respective nozzles may vary bubbling energy generated by the electrothermal resistors. In this case, the amount of ink ejected may vary among the nozzles in the same nozzle row, degrading the quality of high-definition images.
Furthermore, ink ejection speed may vary among the nozzles in the same nozzle row, causing ink droplets to impact a print medium at incorrect positions. This may degrade the quality of high-definition images. Further, if the bubbling energy varies among the electrothermal resistors corresponding to the respective nozzles in the same nozzle row, energy of a magnitude equal to or larger than a set value is applied to the heating resistors. Thus, the heating resistors may be disconnected before meeting the predetermined number of required ink ejections.
Furthermore, in order to allow the larger and smaller ink droplets to be ejected from the print head, large- and small-sized heating resistors (heaters) need to be provided so as to provide different amounts of ink droplets ejected. In order to drive small-sized heating resistors using the same driving voltage as that for conventional large-sized heating resistors, it is necessary to increase the resistance value of the small-sized heating resistors or to reduce the driving pulse width. Thus, in order to drive the heating resistors of different sizes mixed in the same print head, it is necessary to provide the heating resistors with different resistance values or to drive the heating resistors using different driving pulses of respective pulse widths.
For example, if a large-sized heating resistor allowing 5 pl of ink droplets to be ejected is driven using a driving pulse of pulse width 0.8 μs, a small-sized heating resistor allowing 2 pl of ink droplets to be ejected must be driven using a driving pulse of pulse width 0.4 μs. It is conventionally known that a pulse width of at most 0.6 μs results in unstable ink ejection, affecting the use of the print head in a normal environment. That is, it is difficult to drive heating resistors of different sizes using driving pulses of different pulse widths.
On the other hand, to provide heating resistors of different sizes with respective resistance values, it is possible to vary the shape or the sheet resistance value (resistance value per unit area) among the heating resistors.
With a print head that ejects larger and smaller ink droplets, if the same driving voltage is used for both the heating resistor allowing the larger ink droplets to be ejected and the heating resistor allowing the smaller ink droplets to be ejected, the same sheet resistance value (resistance value of the heating resistor per unit area) is conventionally used on the same substrate in the print head. Thus, if one of the large- and small-sized heating resistors is substantially square, the other heating resistor must be rectangular. For the print head, the heating resistor located immediately below (downstream of) the ejection port is ideally square. If the heating resistors are not symmetric with respect to the ejection port, an adverse effect may be exerted on the ejection angle of main ink droplets ejected from the ejection port and on the bending of satellite droplets ejected after the main droplets. This may reduce the accuracy with which ink droplets impact the print medium as well as the print quality.
Furthermore, if the heating resistor allowing the larger ink droplets to be ejected and the heating resistor allowing the smaller ink droplets to be ejected each have an ideal square shape, different driving voltages must be set for the respective heating resistors. This results in the need for one more power supply unit, increasing the costs of the ink jet printing apparatus main body.
Different wiring resistors may be connected to the heating resistor allowing the larger ink droplets to be ejected and the heating resistor allowing the smaller ink droplets to be ejected, in order to provide each of the heating resistors with an ideal square shape and to allow the same driving voltage to be used for both heating resistors. However, this may pose problems such as too small a wiring width and a significant variation in wiring resistance.