What is Thin Film

"Thin film" refers to both a process of manufacturing and also a description of heater elements on a thermal printhead.

The Thin Film Process

Kyocera's thin film process starts with an alumina ceramic substrate. A glaze pattern is then placed on the substrate to form a mound of glass beneath the location of the heater line.

An extremely thin layer of resistor material is then deposited on the entire surface of the substrate, followed by a thin layer of metal. Both of these layers are considered to be "thin film." These thin films are deposited by sputtering or by chemical vapor deposition. Diagram 1 below shows these layers covered by yet another layer, the photoresist. Next, the photoresist light-sensitive material is exposed in the first masking process and developed. Then portions of the thin films are etched away, resulting in a pattern of metal conductors, as shown in diagram 2.

(Figure 1) ceramic substrate before patterning
ceramic substrate after electrode patterning
ceramic substrate heater processinga

 
A second photolithographic process is used with the heater mask to etch away only the metal layer in a straight line across the conductor patterns over the glaze mound, exposing a line of thin-film high-resistance heater elements. The heater elements and the adjacent portions of the metal conductors are then covered by another thin film of abrasion-resistant ceramic coating, which is thin enough to efficiently conduct heat into the thermal paper or thermal transfer ribbon that is pressed against it.

Thin Film Heater Elements

Each heater element has high electrical resistance, due to its extremely thin cross section and the nature of the proprietary ceramic heater material. The surface of each heater element is a precise rectangular shape, so it is useful to mention energy density - mJ/mm2. The entire surface of each element is in contact with the thermal media, so the cross sectional area of thermal conductivity is maximized. The layer of ceramic resistor material is so thin that there is little heat build-up in the middle of the layers.

Comparison to Thick Film Technology

An alternative technology, which is not used by Kyocera, screen prints a thick film mound of resistor material across the width of the printhead. This mound is laid on top of the conductor pattern without a distinct gap between heaters. As a result, sometimes a printed dot has a slight butterfly shape in contrast to the precise rectangular shape from Kyocera thin film printheads. The more complicated conductor pattern of thick film printheads, as shown below, requires that the conductors be fabricated at twice the resolution of the printed dots. This limits the print resolution of thick film printheads.

(Figure 2) concept of printhead heater

(Diagram made by Kyocera)

In both examples above, the common bus is located along the top, providing heater voltage at all points. Each electrode along the bottom connects to a transistor in the driver IC, so when switched on, permits current to flow through the resistor / heater. Both examples in the above diagram show the heaters for three printed dots.

Another effect of the precise heater shape of Kyocera thin film printheads is that adjacent heaters have the same resistance. The dimensional variations between thin film heaters from photolithography and etching are much smaller than the dimensional variations in width and height between thick film heaters. Less variation in the amount of resistor material results in less variation in the resistance of the heaters. Heater-to-heater resistance variation directly leads to dot-to-dot print energy variation. This makes it very difficult to print acceptable continuous tone images such as color photographs or monochrome gray-scale video output with thick film printheads.

A third difference is heat build-up within the resistor material. A thick film heater will be hotter in the middle of the mound, compared to a thin film heater, when both are at the same surface temperature. High temperature works against long pulse life. Of course there are counter measures, such as interrupting the pulse width, but these add further conplications. As the heat in the center takes longer to flow out, thermal response is not as high as in thin film. Another way of looking at it is that it takes longer to print a white dot. The maximum speed without history control is less than for thin film, and the history control needs to be more complicated at the same speeds.


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