The second-generation thermal inkjet structure - effects of materials and processes changes

Hewlett-Packard Journal, August, 1988 by Ronald A. Askeland, Winthrop D. Childers, William R. Sperry

The Second-Generation Thermal InkJet Structure

THE PRINCIPLES OF OPERATION of the HP PaintJet print cartridge are identical to those of the HP ThinkJet print cartridge.sup.1 Ink is channeled to specified chambers containing a thin-film resistor on the floor and a small orifice on the ceiling. The thin-film resistors are rapidly heated to temperatures exceeding 400[deg.]C. The ink directly over an excited resistor is vaporized and a bubble is formed. As this vapor bubble grows, momentum is transferred to the ink above the bubble, which causes this ink to be propelled through the orifice onto the paper. Ink is refilled automatically to the resistor area by capillary action.

The performance specifications of the PaintJet printer required a second-generation material set rather than further tuning of what had been developed for the ThinkJet program. Print resolution is increased from96 dots per inch to 180 dots per inch. This requirement increased the resistor count from 12 to 30 resistors per printhead. Usable ink volume is increased from 3.5 ml to 12 ml. The ink volume for each drop is reduced from 220 picoliters to 100 pl. These changes require a 3.5-fold increase in resistor life.

The most striking performance improvement offered by the PaintJet printer is its ability to generate over 330 different colors. This is achieved by combining patterns of magenta, yellow, and cyan droplets, all generated from a single cartridge. This requires significantly different ink management schemes.

Material Selection

A cross-sectional view of the PaintJet printhead is shown in Fig. 1. Silicon has replaced glass as the substrade material. Although substantially more expensive, it offers performance improvements in the areas of defect density and thermal capacity. Thin-film parameters and photolithography parameters are also more easily controlled on silicon.

A thermal capacitor is required so that the heat generated by the thin-film resistor is transferred to the ink. this heat pulse lasts less than 5 [mu]s and creates a vapor bubble. The capacitor thickness is selected so that excess heat is removed after bubble formation. Excess heat can cause unwanted secondary nucleation and drop performance degradation. The main sources for heat removal are the silicon substrate and the ejected ink. Several thermal barrier materials were tested, including A1.sub.2.O.sub.3., Si.sub.3.N.sub.4., and SiC. The optimum barrier was found to be a thin layer of SiO.sub.2.. This material was found superior for thickness uniformity, defect density, and etch resistance to the chemicals used to define the various thin-film materials.

The resistor film is TaAl and the conductor film is Al doped with a small percentage of copper. These films have been used in several printhead applications at HP over the years. The resistor material is less than 0.1 [mu]m thick and therefore does not substantially contribute to the thermal capacitance.

NExt come SiNsub.3.N.sub.4., SiC, and tantalum passivation layers. These layers protect the resistor and conductor materials from chemical attack by the ink. In addition, protection from severe hydraulic forces induced by the collapsing vapor bubble is provided. It is desirable to keep these layers as thin and uniform as possible because they can negatively affect thermal response. The design thicknesses are optimized for step coverage and pinhole density factors.

Contact between the print cartridge and the PaintJet electronics was a major challenge. The interconnect design was driven by the requirements that contact be made directly to the silicon substrate and that the cartridge be inserted easily by the customer several times throughout its life. Each time, a low-resistance contact to each of 30 resistors must be achieved. In addition, this contact scheme had to have an extremely low profile so that the minimum print-head-to-paper spacing of less than 1 mm could be maintained. Durable, low-resistance contact pads are designed along the periphery of the silicon substrate. Each pad has a contact area of 1 x 1.4 mm and consists of a sandwich of Ta and Au. The pad is electrically connected to a conductor trace by a series of small vias etched through the Si.sub.3.N.sub.4 and SiC layers (see Fig. 2). The Ta film offers a very tough, scratch-resistant base, and the Au film ensures a chemically inert, low-contact-resistance surface. The multiple-via concept offers some basic design advantages. Multiple parallel, low-contact-resistance paths are formed. High reliability is achieved since a failure at any of these independent sites will not affect the overall performance of a trace. Interconnect insertion tests have shown that cartridges can survive more than 15 insertions and environments such as 65[deg.]C at 90% relative humidity without performance degradation.

Ink management was a major design consideration in the PaintJet print cartridge. In the color cartridge each group of ten resistors requires an isolated ink supply system (see Fig. 3). The material selection constraints for the printhead were driven by performance issues. To achieve 180 dpi and conserve silicon real estate, the spacing between resistors is 200 [mu]m. The plated metal wall design used in the ThinkJet printhead cannot meet this spacing requirement. To deliver a 100-pl drop, a channel thickness of approximately 50 [mu]m is required. Fig. 4 is a micrograph showing the resistor area and ink channel geometry. Channel features of less than 100 [mu]m are also required to achieve both drop volume control and fluidic impedance balance. The adhesion of the channel material to the silicon substrate is very important. Finally, this material must be relatively chemically inert since it will be in contact with the various inks for periods as long as 2-1/2 years .