Inkjet printing has become a pervasive printing technology. Inkjet printing systems include one or more arrays of drop ejectors provided on an inkjet printing device, in which each drop ejector is actuated at times and locations where it is required to deposit a dot of ink on the recording medium to print the image. A drop ejector includes a pressurization chamber, a drop forming mechanism (such as a resistive heater) and a nozzle. In a thermal inkjet drop ejector, ink is supplied to the pressurization chamber. A resistive heater, formed for example as a patterned thin film, is at least partially enclosed within the pressurization chamber. When one or more electrical pulses of predetermined amplitude and duration are applied to the resistive heater, ink in contact with the resistive heater is vaporized to form a bubble. The bubble grows and causes a drop of ink to be ejected through a nozzle associated with the pressurization chamber. The ink vapor bubble either is vented through the nozzle or condenses within the pressurization chamber, depending upon the design of the drop ejector. Subsequently, additional ink fills the pressurization chamber and the drop ejector is ready to eject another drop of ink. Thermal inkjet printing devices, having several hundred or more drop ejectors per printing device, also typically include driver and logic electronics to facilitate electrical interconnection to the resistive heaters.
Thermal inkjet printing devices are typically fabricated as a plurality of die on a wafer. One or more die are packaged into an inkjet printhead, and the printhead is installed in an inkjet printer that includes one or more ink supplies, a pulse source, a controller, and an advance system for advancing recording medium relative to the inkjet printhead. The reliability, energy efficiency and drop volume uniformity associated with the inkjet printhead can depend upon the manufacturing variability of the resistive heaters from die to die and from wafer to wafer. In particular, as disclosed in U.S. Pat. No. 5,504,507, in determining the appropriate voltage amplitude and/or the pulse duration for the resistive heaters on a particular inkjet printhead, it is helpful to characterize the resistance of the resistive heaters on the one or more printhead die included in the printhead. As disclosed in U.S. Pat. No. 5,504,507, since the power transformed into heat by applying a voltage V to a resistive heater having a resistance R is V2/R, the higher the resistance R, the less power is available for generating heat to form the vapor bubble to eject the ink drop. As disclosed in U.S. Pat. No. 5,504,507, one or more resistive heaters on the printhead die can be tested and the test data can be encoded on the printhead die in electrically readable digital form. The data can be subsequently read in the printer and used to appropriately set the pulse amplitude and/or duration.
Typically in the printer, the voltage and/or pulse duration applied to the resistive heaters is somewhat larger than the “threshold” pulse conditions that are known to begin to eject drops of ink. For example, a pulse voltage can be set to be 10% higher than the threshold voltage. This higher voltage assures that drops are ejected even if resistances vary within the die, or if firing conditions vary (such as due to different amounts of voltage sag associated with parasitic resistances associated with firing more than one heater at a time, or firing heaters toward the center of the die as opposed to heaters nearer to the edge of the die). Although such an “overvoltage” is effective in assuring drop ejection, excessive overvoltage can result in overheating the resistive heaters, leading to premature heater burnout and lower energy efficiency. In addition, drop size uniformity from printhead to printhead can be related to the amount of energy dissipation in the resistive heaters.
Although measuring the resistance of the resistive heaters as disclosed in U.S. Pat. No. 5,504,507 provides an improved level of control of the appropriate pulsing conditions, it provides only an approximation. This is because what is more important in characterizing heating of the resistive heaters is the power density in the heater rather than the power itself. The power density in the heater is the power P dissipated in the heater divided by the area A of the heater. For a rectangular heater having a length L, a width W, a thickness t and a resistivity ρ, R=ρL/Wt, and A=LW. Therefore the power density in the resistive heater is given by:P/A=(V2/R)/LW=V2t/ρL2=V2/ρsL  (1),where ρs=ρ/t is the sheet resistivity of the resistive heater material. Due to manufacturing variability, ρs can vary due to both chemical composition and thickness of the deposited resistive heater material. The length L of the resistive heater can also vary, for example due to variation in the placement of the edges of metal electrodes contacting the resistive heater, due to variation in etching processes for example.
Therefore, what is needed for improved control of the appropriate level of pulse amplitude and/or duration for a particular printhead die, as well as for improved manufacturing control of printhead wafers, is improved test structures that are capable of determining the actual sheet resistivity ρs, the actual length L, and optionally the actual width W of the resistive heaters on a printhead die.