PRINTHEAD PROVIDED WITH INDIVIDUAL NOZZLE ENCLOSURES

Abstract

A unit cell, a plurality of which make up a printhead, includes a substrate including an ink inlet passage; a chamber defined by chamber sidewalls and a nozzle plate defining an aperture for ejection of ink from the chamber, the chamber being in fluid communication with the inlet passage; and a nozzle enclosure comprising enclosure sidewalls and a roof defining an opening for ejection of ink, the nozzle enclosure surrounding the aperture such that ink ejected from the aperture is directed to the opening of the nozzle enclosure, thereby isolating the aperture from an adjacent aperture of an adjacent unit cell. The opening has a greater diameter than the aperture.

Description

FIELD OF THE INVENTION

The present invention relates to the field of inkjet printers and, discloses an inkjet printing system using printheads manufactured with microelectro-mechanical systems (MEMS) techniques.

BACKGROUND OF THE INVENTION

Many different types of printing have been invented, a large number of which are presently in use. The known forms of print have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.

In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.

Many different techniques on ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore, “Non-Impact Printing: Introduction and Historical Perspective”, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).

Ink Jet printers themselves come in many different types. The utilization of a continuous stream of ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.

U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electro-static field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al)

Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.

Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclosed ink jet printing techniques that rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.

As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables.

A problem with inkjet printheads, and especially inkjet printheads having a high nozzle density, is that ink can flood across the printhead surface contaminating adjacent nozzles. This is undesirable because it results in reduced print quality. Moreover, cross-contamination of ink across the printhead surface can potentially result in electrolysis and accelerated corrosion of nozzle actuators.

Previous attempts to minimize ink flooding across the printhead surface typically involve coating the printhead with a hydrophobic material. However, hydrophobic coatings have only had limited success in minimizing the extent of flooding.

A further problem with inkjet printheads, especially inkjet printheads having senstitive MEMS nozzles formed on an ink ejection surface of the printhead, is that the nozzle structures can become damaged by cleaning the printhead surface. Typically, printheads are wiped regularly to remove particles of paper dust or paper fibers, which build up on the ink ejection surface. When a wiping mechanism comes into contact with nozzle structures on the printhead surface, there is an obvious risk of damaging the nozzles.

It would be desirable to provide a printhead, which minimizes cross-contamination by ink flooding between adjacent nozzles. It would be further desirable to provide a printhead, which allows regular cleaning of the printhead surface by a wiping mechanism without risk of damaging nozzle structures on the printhead.

SUMMARY OF THE INVENTION

According to an aspect of the present disclosure, a unit cell, a plurality of which make up a printhead, includes a substrate including an ink inlet passage; a chamber defined by chamber sidewalls and a nozzle plate defining an aperture for ejection of ink from the chamber, the chamber being in fluid communication with the inlet passage; and a nozzle enclosure comprising enclosure sidewalls and a roof defining an opening for ejection of ink, the nozzle enclosure surrounding the aperture such that ink ejected from the aperture is directed to the opening of the nozzle enclosure, thereby isolating the aperture from an adjacent aperture of an adjacent unit cell. The opening has a greater diameter than the aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms that may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead according to an embodiment using a bubble forming heater element;

FIG. 2 is a schematic cross-sectional view through the ink chamber FIG. 1, at another stage of operation;

FIG. 3 is a schematic cross-sectional view through the ink chamber FIG. 1, at yet another stage of operation;

FIG. 4 is a schematic cross-sectional view through the ink chamber FIG. 1, at yet a further stage of operation; and

FIG. 5 is a diagrammatic cross-sectional view through a unit cell of a printhead in accordance with an embodiment of the invention showing the collapse of a vapor bubble.

FIG. 6 is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead.

FIGS. 7 to 20 are schematic perspective views of the unit cell shown in FIG. 6, at various successive stages in the fabrication process of the printhead.

DETAILED DESCRIPTION

With reference to FIGS. 1 to 4, the unit cell 1 of one of the Applicant's printheads is shown. The unit cell 1 comprises a nozzle plate 2 with nozzles 3 therein, the nozzles having nozzle rims 4, and apertures 5 extending through the nozzle plate. The nozzle plate 2 is plasma etched from a silicon nitride structure which is deposited, by way of chemical vapor deposition (CVD), over a sacrificial material which is subsequently etched.

The printhead also includes, with respect to each nozzle 3, side walls 6 on which the nozzle plate is supported, a chamber 7 defined by the walls and the nozzle plate 2, a multi-layer substrate 8 and an inlet passage 9 extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element 10 is suspended within the chamber 7, so that the element is in the form of a suspended beam. The printhead as shown is a microelectromechanical system (MEMS) structure, which is formed by a lithographic process which is described in more detail below.

When the printhead is in use, ink 11 from a reservoir (not shown) enters the chamber 7 via the inlet passage 9, so that the chamber fills to the level as shown in FIG. 1. Thereafter, the heater element 10 is heated for somewhat less than 1 microsecond, so that the heating is in the form of a thermal pulse. It will be appreciated that the heater element 10 is in thermal contact with the ink 11 in the chamber 7 so that when the element is heated, this causes the generation of vapor bubbles 12 in the ink. Accordingly, the ink 11 constitutes a bubble forming liquid. FIG. 1 shows the formation of a bubble 12 approximately 1 microsecond after generation of the thermal pulse, that is, when the bubble has just nucleated on the heater elements 10. It will be appreciated that, as the heat is applied in the form of a pulse, all the energy necessary to generate the bubble 12 is to be supplied within that short time.

When the element 10 is heated as described above, the bubble 12 forms along the length of the element, this bubble appearing, in the cross-sectional view of FIG. 1, as four bubble portions, one for each of the element portions shown in cross section.

The bubble 12, once generated, causes an increase in pressure within the chamber 7, which in turn causes the ejection of a drop 16 of the ink 11 through the nozzle 3. The rim 4 assists in directing the drop 16 as it is ejected, so as to minimize the chance of drop misdirection.

The reason that there is only one nozzle 3 and chamber 7 per inlet passage 9 is so that the pressure wave generated within the chamber, on heating of the element 10 and forming of a bubble 12, does not affect adjacent chambers and their corresponding nozzles. The pressure wave generated within the chamber creates significant stresses in the chamber wall. Forming the chamber from an amorphous ceramic such as silicon nitride, silicon dioxide (glass) or silicon oxynitride, gives the chamber walls high strength while avoiding the use of material with a crystal structure. Crystalline defects can act as stress concentration points and therefore potential areas of weakness and ultimately failure.

FIGS. 2 and 3 show the unit cell 1 at two successive later stages of operation of the printhead. It can be seen that the bubble 12 generates further, and hence grows, with the resultant advancement of ink 11 through the nozzle 3. The shape of the bubble 12 as it grows, as shown in FIG. 3, is determined by a combination of the inertial dynamics and the surface tension of the ink 11. The surface tension tends to minimize the surface area of the bubble 12 so that, by the time a certain amount of liquid has evaporated, the bubble is essentially disk-shaped.

The increase in pressure within the chamber 7 not only pushes ink 11 out through the nozzle 3, but also pushes some ink back through the inlet passage 9. However, the inlet passage 9 is approximately 200 to 300 microns in length, and is only approximately 16 microns in diameter. Hence there is a substantial viscous drag. As a result, the predominant effect of the pressure rise in the chamber 7 is to force ink out through the nozzle 3 as an ejected drop 16, rather than back through the inlet passage 9.

Turning now to FIG. 4, the printhead is shown at a still further successive stage of operation, in which the ink drop 16 that is being ejected is shown during its “necking phase” before the drop breaks off. At this stage, the bubble 12 has already reached its maximum size and has then begun to collapse towards the point of collapse 17, as reflected in more detail in FIG. 21.

The collapsing of the bubble 12 towards the point of collapse 17 causes some ink 11 to be drawn from within the nozzle 3 (from the sides 18 of the drop), and some to be drawn from the inlet passage 9, towards the point of collapse. Most of the ink 11 drawn in this manner is drawn from the nozzle 3, forming an annular neck 19 at the base of the drop 16 prior to its breaking off.

The drop 16 requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink 11 is drawn from the nozzle 3 by the collapse of the bubble 12, the diameter of the neck 19 reduces thereby reducing the amount of total surface tension holding the drop, so that the momentum of the drop as it is ejected out of the nozzle is sufficient to allow the drop to break off.

When the drop 16 breaks off, cavitation forces are caused as reflected by the arrows 20, as the bubble 12 collapses to the point of collapse 17. It will be noted that there are no solid surfaces in the vicinity of the point of collapse 17 on which the cavitation can have an effect.

Referring to FIG. 6, an embodiment of the unit cell 1 according to the invention is shown. The aperture 5 is surrounded by a nozzle enclosure 60, which isolates adjacent apertures on the printhead. The nozzle enclosure 60 has a roof 61 and sidewalls 62, which extend from the roof to the nozzle plate 2 and form a seal therewith. An opening 63 is defined in the roof 61, which allows ink droplets (not shown) to pass through the nozzle enclosure and onto a print medium (not shown).

The nozzle enclosure 60 minimize cross-contamination between adjacent apertures 5 by containing any flooded ink in the immediate vicinity of each nozzle. Flooding of ink from each nozzle may be caused by a variety of reasons, such as nozzle misfires or pressure fluctuations in ink supply channels. The nozzle enclosure may be formed from or coated with a hydrophobic material during the fabrication process, which further minimizes the risk of cross-contamination.

A further advantage of the printhead according to the invention is that it allows the nozzle plate 2 of the printhead to be wiped without risk of damaging the sensitive nozzle structures. Typically, inkjet printheads are cleaned by a wiping mechanism as part of a warm-up cycle. The nozzle enclosures 60 provide a protective barrier between the nozzles and the wiping mechanism (not shown).

In the interests of brevity, the fabrication stages have been shown for the unit cell of FIG. 6 only (see FIGS. 7 to 20). It will be appreciated that the other unit cells will use the same fabrication stages with different masking.

Referring to FIG. 7, there is shown the starting point for fabrication of the thermal inkjet nozzle shown in FIG. 13. CMOS processing of a silicon wafer provides a silicon substrate 21 having drive circuitry 22, and an interlayer dielectric (“interconnect”) 23. The interconnect 23 comprises four metal layers, which together form a seal ring for the inlet passage 9 to be etched through the interconnect. The top metal layer 26, which forms an upper portion of the seal ring, can be seen in FIG. 7. The metal seal ring prevents ink moisture from seeping into the interconnect 23 when the inlet passage 9 is filled with ink.

A passivation layer 24 is deposited onto the top metal layer 26 by plasma-enhanced chemical vapour deposition (PECVD). After deposition of the passivation layer 24, it is etched to define a circular recess, which forms parts of the inlet passage 9. At the same as etching the recess, a plurality of vias 50 are also etched, which allow electrical connection through the passivation layer 24 to the top metal layer 26. The etch pattern is defined by a layer of patterned photoresist (not shown), which is removed by O₂ ashing after the etch.

Referring to FIG. 8, in the next fabrication sequence, a layer of photoresist is spun onto the passivation later 24. The photoresist is exposed and developed to define a circular opening. With the patterned photoresist 51 in place, the dielectric interconnect 23 is etched as far as the silicon substrate 21 using a suitable oxide-etching gas chemistry (e.g. O₂/C₄F₈). Etching through the silicon substrate is continued down to about 20 microns to define a front ink hole 52, using a suitable silicon-etching gas chemistry (e.g. ‘Bosch etch’). The same photoresist mask 51 can be used for both etching steps. FIG. 9 shows the unit cell after etching the front ink hole 52 and removal of the photoresist 51.

Referring to FIG. 10, in the next stage of fabrication, the front ink hole 52 is plugged with photoresist to provide a front plug 53. At the same time, a layer of photoresist is deposited over the passivation layer 24. This layer of photoresist is exposed and developed to define a first sacrificial scaffold 54 over the front plug 53, and scaffolding tracks 35 around the perimeter of the unit cell. The first sacrificial scaffold 54 is used for subsequent deposition of heater material 38 thereon and is therefore formed with a planar upper surface to avoid any buckling in the heater element (see heater element 10 in FIG. 10). The first sacrificial scaffold 54 is UV cured and hardbaked to prevent reflow of the photoresist during subsequent high-temperature deposition onto its upper surface.

Importantly, the first sacrificial scaffold 54 has sloped or angled side faces 55. These angled side faces 55 are formed by adjusting the focusing in the exposure tool (e.g. stepper) when exposing the photoresist. The sloped side faces 55 advantageously allow heater material 38 to be deposited substantially evenly over the first sacrificial scaffold 54.

Referring to FIG. 11, the next stage of fabrication deposits the heater material 38 over the first sacrificial scaffold 54, the passivation layer 24 and the perimeter scaffolding tracks 35. The heater material 38 is typically a monolayer of TiAlN. However, the heater material 38 may alternatively comprise TiAlN sandwiched between upper and lower passivating materials, such as tantalum or tantalum nitride. Passivating layers on the heater element 10 minimize corrosion of the and improve heater longevity.

Referring to FIG. 12, the heater material 38 is subsequently etched down to the first sacrificial scaffold 54 to define the heater element 10. At the same time, contact electrodes 15 are defined on either side of the heater element 10. The electrodes 15 are in contact with the top metal layer 26 and so provide electrical connection between the CMOS and the heater element 10. The sloped side faces of the first sacrificial scaffold 54 ensure good electrical connection between the heater element 10 and the electrodes 15, since the heater material is deposited with sufficient thickness around the scaffold 54. Any thin areas of heater material (due to insufficient side face deposition) would increase resistivity and affect heater performance.

Adjacent unit cells are electrically insulated from each other by virtue of grooves etched around the perimeter of each unit cell. The grooves are etched at the same time as defining the heater element 10.

Referring to FIG. 13, in the subsequent step a second sacrificial scaffold 39 of photoresist is deposited over the heater material. The second sacrificial scaffold 39 is exposed and developed to define sidewalls for the cylindrical nozzle chamber and perimeter sidewalls for each unit cell. The second sacrificial scaffold 39 is also UV cured and hardbaked to prevent any reflow of the photoresist during subsequent high-temperature deposition of the silicon nitride roof material.

Referring to FIG. 14, silicon nitride is deposited onto the second sacrificial scaffold 39 by plasma enhanced chemical vapour deposition. The silicon nitride forms a roof 44 over each unit cell, which is the nozzle plate 2 for a row of nozzles. Chamber sidewalls 6 and unit cell sidewalls 56 are also formed by deposition of silicon nitride.

Referring to FIG. 15, the nozzle rim 4 is etched partially through the roof 44, by placing a suitably patterned photoresist mask over the roof, etching for a controlled period of time and removing the photoresist by ashing.

Referring to FIG. 16, the nozzle aperture 5 is etched through the roof 24 down to the second sacrificial scaffold 39. Again, the etch is performed by placing a suitably patterned photoresist mask over the roof, etching down to the scaffold 39 and removing the photoresist mask.

Referring to FIG. 17, in the next stage a third sacrificial scaffold 64 is deposited over the roof 44. The third sacrificial scaffold 64 is exposed and developed to define sidewalls for the cylindrical nozzle enclosure over each aperture 5. The third sacrificial scaffold 64 is also UV cured and hardbaked to prevent any reflow of the photoresist during subsequent high-temperature deposition of the nozzle enclosure material.

Referring to FIG. 18, silicon nitride is deposited onto the third sacrificial scaffold 64 by plasma enhanced chemical vapour deposition. The silicon nitride forms an enclosure roof 61 over each aperture 5. Enclosure sidewalls 62 are also formed by deposition of silicon nitride. Whilst silicon nitride is deposited in the embodiment shown, the enclosure roof 61 may equally be formed from silicon oxide, silicon oxynitride etc. Optionally, a layer of hydrophobic material (e.g. fluoropolymer) is deposited onto the enclosure roof 61 after deposition. This extra deposition step may be performed at any stage after deposition (e.g. after etching or after ashing).

Referring to FIG. 19, the nozzle enclosure 60 is formed by etching through the enclosure roof layer 61. The enclosure opening 63 is defined by this etch. In addition, the enclosure roof material which is located outside the enclosure sidewalls 62 is removed. The etch pattern is defined by standard photoresist masking.

With the nozzle structure, including nozzle enclosure 60, now fully formed on a frontside of the silicon substrate 21, an ink supply channel 32 is etched from the backside of the substrate 21, which meets with the front plug 53.

Referring to FIG. 20, after formation of the ink supply channel 32, the first, second and sacrificial scaffolds of photoresist, together with the front plug 53 are ashed off using an O₂ plasma. Accordingly, fluid connection is made from the ink supply channel 32 through to the nozzle aperture 5 and the nozzle enclosure opening 63.

It should be noted that a portion of photoresist, on either side of the nozzle chamber sidewalls 6, remains encapsulated by the roof 44, the unit cell sidewalls 56 and the chamber sidewalls 6. This portion of photoresist is sealed from the O₂ ashing plasma and, therefore, remains intact after fabrication of the printhead. This encapsulated photoresist advantageously provides additional robustness for the printhead by supporting the nozzle plate 2. Hence, the printhead has a robust nozzle plate spanning continuously over rows of nozzles, and being supported by solid blocks of hardened photoresist, in addition to support walls.

Claims

1. A printhead comprising a plurality of unit cells, at least one of the plurality of unit cells comprising: a substrate including an ink inlet passage;a chamber defined by chamber sidewalls and a nozzle plate defining an aperture for ejection of ink from the chamber, the chamber being in fluid communication with the inlet passage; anda nozzle enclosure comprising enclosure sidewalls and a roof defining an opening for ejection of ink, the nozzle enclosure surrounding the aperture such that ink ejected from the aperture is directed to the opening of the nozzle enclosure, thereby isolating the aperture from an adjacent aperture of an adjacent unit cell, whereinthe opening has a greater diameter than the aperture.

2. The printhead of claim 1, wherein the enclosure sidewalls abut or are integrally formed with the at least part of the nozzle plate.

3. The printhead of claim 1, including a plurality of formations about the aperture, the formations assisting to isolate the aperture from the adjacent aperture.

4. The printhead of claim 3, wherein the nozzle enclosure also surrounds the formations.

5. The printhead of claim 3, wherein the formations each have a hydrophobic surface.

6. The printhead of claim 3, wherein the formations include at least a rim about the aperture.

7. The printhead of claim 1, wherein the roof is spaced apart from the at least part of the nozzle plate.

8. The printhead of claim 1, wherein the roof opening is spaced apart from and aligned with the nozzle aperture, thereby allowing ejected ink droplets to pass therethrough onto the print medium.

9. The printhead of claim 1, wherein the enclosure sidewalls extend from a perimeter region of the roof.

10. The printhead of claim 1, wherein the chamber includes a heater element.

11. The printhead of claim 1, wherein the printhead has a nozzle density sufficient to print at up to 1600 dpi.