The present invention relates to printed media production and in particular ink jet printers.
Ink jet printers are a well-known and widely used form of printed media production. Ink is fed to an array of digitally controlled nozzles on a printhead. As the print head passes over the media, ink is ejected from the array of nozzles to produce an image on the media.
Printer performance depends on factors such as operating cost, print quality, operating speed and ease of use. The mass, frequency and velocity of individual ink drops ejected from the nozzles will affect these performance parameters.
Recently, the array of nozzles has been formed using microelectromechanical systems (MEMS) technology, which have mechanical structures with sub-micron thicknesses. This allows the production of printheads that can rapidly eject ink droplets sized in the picolitre (xc3x9710xe2x88x9212 liter) range.
While the microscopic structures of these printheads can provide high speeds and good print quality at relatively low costs, their size makes the nozzles extremely fragile and vulnerable to damage from the slightest contact with fingers, dust or the media substrate. This can make the printheads impractical for many applications where a certain level of robustness is necessary. Furthermore, a damaged nozzle may fail to eject the ink being fed to it. As ink builds up and beads on the exterior of the nozzle, the ejection of ink from surrounding nozzle may be affected and/or the damaged nozzle will simply leak ink onto the printed substrate. Both situations are detrimental to print quality.
To address this, an apertured guard may be fitted over the nozzles to shield them against damaging contact. Ink ejected from the nozzles passes through the apertures on to the paper or other substrate to be printed. However, to effectively protect the nozzles, the apertures need to be as small as possible to maximize the restriction against the ingress of foreign matter while still allowing the passage of the ink droplets. Preferably, each nozzle would eject ink through its own individual aperture in the guard. However, given the microscopic scale of MEMS devices, slight misalignments between the guard and the nozzles will obstruct the path of the ink droplets. Providing alignment formations on the silicon wafer substrate for engaging complementary formations on the guard can align the nozzles and respective apertures to within 0.1 xcexcm. However, while attaching the guard to the substrate, movement of the complementary formations into engagement with the alignment formations can damage the delicate nozzle structures.
Accordingly, the present invention provides a method of fabricating a printhead for an ink jet printer, the printhead including:
a substrate carrying an array of nozzles for ejecting ink onto media to be printed; and
an apertured nozzle guard to inhibit damaging contact with the nozzles, the method comprising the steps of:
forming the nozzles on the substrate using material etching and deposition techniques such that the nozzles are reinforced by sacrificial material;
positioning the apertured nozzle guard over the exterior of the nozzles such that its apertures are in close registration with the nozzles; and subsequently,
etching away the sacrificial material reinforcing the nozzles.
In this specification the term xe2x80x9cnozzlexe2x80x9d is to be understood as an element defining an opening and not the opening itself.
In a preferred embodiment, alignment formations are formed on the substrate, the alignment formations being configured for engagement with complementary formations on the apertured nozzle guard; wherein,
engagement between the alignment formations and the complementary formations holds the apertures in close registration with the nozzles such that the guard does not obstruct the normal trajectory of ink ejected from the nozzles onto the media.
Preferably, etching plasma is injected through one or more of the apertures in the nozzle guard to release the sacrificial material protecting the nozzles, the released sacrificial material and etching plasma flushing out through the apertures in the nozzle guard.
In one embodiment, the etching plasma is oxygen plasma and the sacrificial material is polyimide. In this embodiment, it is desirable to provide an inorganic seal between the alignment formation and the complementary formation.
The substrate may be a silicon wafer. The nozzle guard may have a shield containing the apertures, the shield being spaced from the silicon substrate by integrally formed struts extending from the shield for engagement with the alignment formations. In one convenient form, the alignment formations are ridges on the silicon substrate positioned to engage the struts to maintain the apertures in alignment with the nozzle array.
The alignment formations necessarily use up a proportion of the surface area of the printhead, and this adversely affects the nozzle packing density. The extra printhead chip area required for the same number of nozzles will add to the cost of manufacturing the chip. However, where assembling the printhead and the nozzle guard is not likely to be sufficiently accurate, interengaging formations on the substrate and the guard will reduce the nozzle defect rate.
The nozzle guard may further include fluid inlet openings for directing fluid through the passages to inhibit the build up of foreign particles on the nozzle array. In this embodiment, the fluid inlet openings may be arranged in the struts.
It will be appreciated that, when air is directed through the openings, over the nozzle array and out through the passages, the build up of foreign particles on the nozzle array is inhibited.
The fluid inlet openings may be arranged in the support element remote from a bond pad of the nozzle array.
The present invention, ensures that the fragile MEMS nozzles are protected during the manufacture and assembly of the printhead. By providing a nozzle guard for the printhead, the nozzle structures can be protected from being touched or bumped against most other surfaces during its operational life. To optimize the protection provided, the guard forms a flat shield covering the exterior side of the nozzles wherein the shield has an array of passages big enough to allow the ejection of ink droplets but small enough to prevent inadvertent contact or the ingress of most dust particles. By forming the shield from silicon, its coefficient of thermal expansion substantially matches that of the nozzle array. This will help to prevent the array of passages in the shield from falling out of register with the nozzle array. Using silicon also allows the shield to be accurately micro-machined using MEMS techniques. Furthermore, silicon is very strong and substantially non-deformable.