Thermal ink jet printhead with suspended beam heater Number:6,755,509 from the United States Patent and Trademark Office (PTO) owispatent There is disclosed an ink jet printhead which comprises a plurality of nozzles and one or more heater elements corresponding to each nozzle. Each heater element is configured to heat a bubble forming liquid in the printhead to a temperature above its boiling point to form a gas bubble therein. The generation of the bubble causes the ejection of a drop of an ejectable liquid (such as ink) through respective corresponding nozzle, to effect printing. Each heater element is in the form of a beam suspended over at least a portion of the bubble forming liquid (which liquid can be the ink) so as to be in thermal contact therewith. This configuration of printhead provides for a relatively high efficiency of operation.<H suspended, printhead, Thermal, ink, jet, 6755509.html, Patent, pto, 6755509

Title: Thermal ink jet printhead with suspended beam heater

Abstract: There is disclosed an ink jet printhead which comprises a plurality of nozzles and one or more heater elements corresponding to each nozzle. Each heater element is configured to heat a bubble forming liquid in the printhead to a temperature above its boiling point to form a gas bubble therein. The generation of the bubble causes the ejection of a drop of an ejectable liquid (such as ink) through respective corresponding nozzle, to effect printing. Each heater element is in the form of a beam suspended over at least a portion of the bubble forming liquid (which liquid can be the ink) so as to be in thermal contact therewith. This configuration of printhead provides for a relatively high efficiency of operation.

Patent Number: 6,755,509 Issued on 06/29/2004 to Silverbrook, et al.

Inventors: Silverbrook; Kia (Balmain, AU), North; Angus John (Balmain, AU), McAvoy; Gregory John (Balmain, AU)
Assignee: Silverbrook Research Pty Ltd (Balmain, AU)

Appl. No.: 10/302,274

Filed: November 23, 2002

Current U.S. Class: 347/62 ; 347/48; 347/56; 347/65

Current International Class: B41J 2/14 (20060101); B41J 2/16 (20060101)

Field of Search: 347/47,54,56,61,62,63,65,48 219/216 252/519.12,518.1,521.5 216/27

References Cited [Referenced By]

U.S. Patent Documents


4965594	October 1990	Komuro
5534898	July 1996	Kashino et al.
5706041	January 1998	Kubby
5841452	November 1998	Silverbrook
6543879	April 2003	Feinn et al.

Other References

The Fabrication and Reliability Testing of Ti/TiN Heaters, P. DeMoor, Proceedings of SPIE, Micromachining and Microfabrication Process Technology V, vol. 3874, pp. 284-293. Please note, this document was provided in copending application 10/303, 348.* .
Copy of Examiner's amendment from 10/302,617..

Primary Examiner: Brooke; Michael S.

Claims

What is claimed is:

1. An ink jet printhead comprising: a plurality of nozzles; and at least one respective heater element corresponding to each nozzle, wherein each heater element is in the form of a suspended beam, arranged for being suspended over at least a portion of a bubble forming liquid and comprising an outer heating loop adapted to be in thermal contact with the bubble forming liquid and an inner heating loop adapted to be in thermal contact with the bubble forming liquid, and each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein, thereby to cause the ejection of a drop of the bubble forming liquid through the nozzle corresponding to that heater element.

2. The printhead of claim 1 being configured to print on a page and to be a page-width printhead.

3. The printhead of claim 1 wherein each heater element is in the form of a cantilever beam.

4. The printhead of claim 1 wherein each heater element is configured such that an actuation energy of less than 500 nanojoules (nJ) is required to be applied to that heater element to heat that heater element sufficiently to form a said bubble in the bubble forming liquid thereby to cause th ejection of a said drop.

5. The printhead of claim 1 configured to receive a supply of the bubble forming liquid at an ambient temperature, wherein each heater element is configured such that the energy required to be applied thereto to heat said part to cause the ejection of a said drop is less than the energy required to heat a volume of said bubble forming liquid equal to the volume of the said drop, from a temperature equal to said ambient temperature to said boiling point.

6. The printhead of claim 1 comprising a substrate having a substrate surface, wherein each nozzle has a nozzle aperture opening through the substrate surface, and wherein the areal density of the nozzles relative to the substrate surface exceeds 10,000 nozzles per square cm of substrate surface.

7. The printhead of claim 1 wherein each heater element has two opposite sides and is configured such that a said gas bubble formed by that heater element is formed at both of said sides of that heater element.

8. The printhead of claim 1 wherein the bubble which each element is configured to form is collapsible and has a point of collapse, and wherein each heater element is configured such that the point of collapse of a bubble formed thereby is spaced from that heater element.

9. The printhead of claim 1 comprising a structure that is formed by chemical vapor deposition (CVD), the nozzles being incorporated on the structure.

10. The printhead of claim 1 comprising a structure which is less than 10 microns thick, the nozzles being incorporated on the structure.

11. The printhead of claim 1 comprising a plurality of nozzle chambers each corresponding to a respective nozzle, and a plurality of said heater elements being disposed within each chamber, the heater elements within each chamber being formed on different respective layers to one another.

12. The printhead of claim 1 wherein each heater element is formed of solid material more than 90% of which, by atomic proportion, is constituted by at least one periodic element having an atomic number below 50.

13. The printhead of claim 1 wherein each heater element includes solid material and is configured for a mass of less than 10 nanograms of the solid material of that heater element to be heated to a temperature above said boiling point thereby to heat said part of the bubble forming liquid to a temperature above said boiling point to cause the ejection of a said drop.

14. The printhead of claim 1 wherein each heater element is covered by a conformal protective coating, the coating of each heater element having been applied to all sides of the heater element simultaneously such that the coat is seamless.

15. A printer system incorporating a printhead, the printhead comprising: a plurality of nozzles; and at least one respective heater element corresponding to each nozzle, wherein each heater element is in the form of a suspended beam, arranged for being suspended over at least a portion of a bubble forming liquid and comprising an outer heating loop adapted to be in thermal contact with the bubble forming liquid and an inner heating loop adapted to be in thermal contact with the bubble forming liquid, and each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein, thereby to cause the ejection of a drop of the bubble forming liquid through the nozzle corresponding to that heater element.

16. The system of claim 15 being configured to print on a page and to be a page-width printhead.

17. The system of claim 15 wherein each heater element is in the form of a cantilever beam.

18. The system of claim 15 wherein each heater element is configured such that an actuation energy of less than 500 joules (nJ) is required to be applied to that heater element to heat that heater element sufficiently to form a said bubble in the bubble forming liquid thereby to cause the ejection of a said drop.

19. The system of claim 15, wherein the printhead is configured to receive a supply of the bubble forming liquid at an ambient temperature, and wherein each heater element is configured such that the energy required to be applied thereto to heat said part to cause the ejection of a said drop is less than the energy required to heat a volume of said bubble forming liquid equal to the volume of the said drop, from a temperature equal to said ambient temperature to said boiling point.

20. The system of claim 15 comprising a substrate having a substrate surface, wherein each nozzle has a nozzle aperture opening through the substrate surface, and wherein the areal density of the nozzles relative to the substrate surface exceeds 10,000 nozzles per square cm of substrate surface.

21. The system of claim 15 wherein each heater element has two opposite sides and is configured such that a said gas bubble formed by that heater element is formed at both of said sides of that heater element.

22. The system of claim 15 wherein the bubble which each element is configured to form is collapsible and has a point of collapse, and wherein each heater element is configured such that the point of collapse of a bubble formed thereby is spaced from that heater element.

23. The system of claim 15 comprising a structure that is formed by chemical vapor deposition (CVD), the nozzles being incorporated on the structure.

24. The system of claim 15 comprising a structure which is less than 10 microns thick, the nozzles being incorporated on the structure.

25. The system of claim 15 comprising a plurality of nozzle chambers each corresponding to a respective nozzle, and a plurality of said heater elements being disposed within each chamber, the heater elements within each chamber being formed on different respective layers to one another.

26. The system of claim 15 wherein each heater element is formed of solid material more than 90% of which, by atomic proportion, is constituted by at least one periodic element having an atomic number below 50.

27. The system of claim 15 wherein each heater element includes solid material and is configured for a mass of less than 10 nanograms of the solid material of that heater element to be heated to a temperature above said boiling point thereby to heat said part of the bubble forming liquid to a temperature above said boiling point to cause the ejection of a said drop.

28. The system of claim 15 wherein each heater element is covered by a conformal protective coating, the coating of each heater element having been applied to all sides of the heater element simultaneously such that the coating is seamless.

29. A method of ejecting a drop of bubble forming liquid from a printhead, the printhead comprising a plurality of nozzles and at least one respective heater element corresponding to each nozzle, the method comprising the steps of: providing the printhead wherein each heater element is in the form of a suspended beam comprising an outer heating loop adapted to be in thermal contact with the bubble forming liquid and an inner heating loop adapted to be in thermal contact with the bubble forming liquid; disposing a bubble forming liquid such that the heater elements are positioned above, and in thermal contact with, at least a portion of the bubble forming liquid; heating at least one heater element corresponding to a said nozzle so as to heat at least some of said portion of the bubble forming liquid which is in thermal contact with the at least one heated heater element to a temperature above the boiling point of the bubble forming liquid; generating a gas bubble in the bubble forming liquid by said step of heating; and causing the drop of bubble forming liquid to be ejected through the nozzle corresponding to the at least one heated heater element by said step of generating a gas bubble.

30. The method of claim 29 wherein the step of disposing the bubble forming liquid comprises disposing the bubble forming liquid so that it substantially surrounds the heater element.

31. The method of claim 29 wherein said step of heating at least one heater element is effected by applying an actuation energy of less than 500 nJ to each such heater element.

32. The method of claim 29, comprising, prior to the step of heating at least one heater element, the step of receiving a supply of the ejectable liquid, at an ambient temperature, to the printhead, wherein the step of heating is effected by applying heat energy to each such heater element, wherein said applied heat energy is less than the energy required to heat a volume of said bubble forming liquid equal to the volume of said drop, from a temperature equal to said ambient temperature to said boiling point.

33. The method of claim 29 wherein, in said step of providing the printhead, the printhead includes a substrate on which said nozzles are disposed, the substrate having a substrate surface and the areal density of the nozzles relative to the substrate surface exceeding 10,000 nozzles per square cm of substrate surface.

34. The method of claim 29 wherein each heater element has two opposite sides and wherein, in the step of generating a gas bubble, the bubble is generated at both of said sides of each heated heater element.

35. The method of claim 29 wherein, in the step of generating a gas bubble, the generated bubble is collapsible and has a point of collapse, and is generated such that the point of collapse is spaced from the at least one heated heater element.

36. The method of claim 29 wherein the step of providing the printhead includes forming a structure by chemical vapor deposition (CVD), the structure incorporating the nozzles thereon.

37. The method of claim 29 wherein, in the step of providing the printhead, the printhead has a structure which is less than 10 microns thick and which incorporates said nozzles thereon.

38. The method of claim 29 wherein, in the step of providing the printhead, the printhead has a plurality of nozzle chambers each chamber corresponding to a respective nozzle and wherein the step of providing the printhead further includes forming a plurality of said heater elements in each chamber, such that the heater elements in each chamber are formed on different respective layers to one another.

39. The method of claim 29 wherein, in the step of providing the printhead, each heater element is formed of solid material more than 90% of which, by atomic proportion, is constituted by at least one periodic element having an atomic number below 50.

40. The method of claim 29 wherein, in the step of providing the printhead, each heater element includes solid material and wherein the step of heating at least one heater element comprises heating a mass of less than 10 nanograms of the solid material of each such heater element to a temperature above said boiling point.

41. The method of claim 29 wherein the step of providing the printhead includes applying to each heater element, to all sides thereof simultaneously, a conformal protective coating such that the coating is seamless.

Description

FIELD OF THE INVENTION

The present invention relates to a thermal ink jet printhead, to a printer system incorporating such a printhead, and to a method of ejecting a liquid drop (such as an ink drop) using such a printhead.

BACKGROUND TO THE INVENTION

The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme).

There are various known types of thermal ink jet (bubblejet) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate, which is a separately fabricated item and which is mechanically secured to the walls of the chamber. In certain prior art devices, the top plate is made of Kapton.TM. which is a Dupont trade name for a polyimide film, which has been laser-drilled to form the nozzles. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles.

It is an object of the present invention to provide a useful alternative to the known printheads, printer systems, or methods of ejecting drops of ink and other related liquids, which have advantages as described herein.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an ink jet printhead comprising: a plurality of nozzles; and at least one respective heater element corresponding to each nozzle, wherein each heater element is in the form of a suspended beam, arranged for being suspended over at least a portion of a bubble forming liquid so as to be in thermal contact therewith, and each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein, thereby to cause the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element.

According to a second aspect of the invention there is provided a printer system incorporating a printhead, the printhead comprising: a plurality of nozzles; and at least one respective heater element corresponding to each nozzle, wherein each heater element is in the form of a suspended beam, arranged for being suspended over at least a portion of a bubble forming liquid so as to be in thermal contact therewith, and each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein, thereby to cause the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element.

According to a third aspect of the invention there is provided a method of ejecting a drop of an ejectable liquid from a printhead, the printhead comprising a plurality of nozzles and at least one respective heater element corresponding to each nozzle, the method comprising the steps of: providing the printhead wherein each heater element is in the form of a suspended beam; disposing a bubble forming liquid such that the heater elements are positioned above, and in thermal contact with, at least a portion of the bubble forming liquid; heating at least one heater element corresponding to a said nozzle so as to heat at least some of said portion of the bubble forming liquid which is in thermal contact with the at least one heated heater element to a temperature above the boiling point of the bubble forming liquid; generating a gas bubble in the bubble forming liquid by said step of heating; and

causing the drop of ejectable liquid to be ejected through the nozzle corresponding to the at least one heated heater element by said step of generating a gas bubble.

As will be understood by those skilled in the art, the ejection of a drop of the ejectable liquid as described herein, is caused by the generation of a vapor bubble in a bubble forming liquid, which, in embodiments, is the same body of liquid as the ejectable liquid. The generated bubble causes an increase in pressure in ejectable liquid, which forces the drop through the relevant nozzle. The bubble is generated by Joule heating of a heater element which is in thermal contact with the ink. The electrical pulse applied to the heater is of brief duration, typically less than 2 microseconds. Due to stored heat in the liquid, the bubble expands for a few microseconds after the heater pulse is turned off. As the vapor cools, it recondenses, resulting in bubble collapse. The bubble collapses to a point determined by the dynamic interplay of inertia and surface tension of the ink. In this specification, such a point is referred to as the "point of collapse" of the bubble.

The printhead according to the invention comprises a plurality of nozzles, as well as a chamber and one or more heater elements corresponding to each nozzle. Each portion of the printhead pertaining to a single nozzle, its chamber and its one or more elements, is referred to herein as a "unit cell".

In this specification, where reference is made to parts being in thermal contact with each other, this means that they are positioned relative to each other such that, when one of the parts is heated, it is capable of heating the other part, even though the parts, themselves, might not be in physical contact with each other.

Also, the term "ink" is used to signify any ejectable liquid, and is not limited to conventional inks containing colored dyes. Examples of non-colored inks include fixatives, infra-red absorber inks, functionalized chemicals, adhesives, biological fluids, water and other solvents, and so on. The ink or ejectable liquid also need not necessarily be a strictly a liquid, and may contain a suspension of solid particles or be solid at room temperature and liquid at the ejection temperature.

In this specification, the term "periodic element" refers to an element of a type reflected in the periodic table of elements.

DETAILED DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying representations. The drawings are described as follows.

FIG. 1 is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead according to an embodiment of the invention, at a particular stage of operation.

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.

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

FIGS. 6, 8, 10, 11, 13, 14, 16, 18, 19, 21, 23, 24, 26, 28 and 30 are schematic perspective views (FIG. 30 being partly cut away) of a unit cell of a printhead in accordance with an embodiment of the invention, at various successive stages in the production process of the printhead.

FIGS. 7, 9, 12, 15, 17, 20, 22, 25, 27, 29 and 31 are each schematic plan views of a mask suitable for use in performing the production stage for the printhead, as represented in the respective immediately preceding figures.

FIG. 32 is a further schematic perspective view of the unit cell of FIG. 30 shown with the nozzle plate omitted.

FIG. 33 is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having another particular embodiment of heater element.

FIG. 34 is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of FIG. 33 for forming the heater element thereof.

FIG. 35 is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having a further particular embodiment of heater element.

FIG. 36 is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of FIG. 35 for forming the heater element thereof.

FIG. 37 is a further schematic perspective view of the unit cell of FIG. 35 shown with the nozzle plate omitted.

FIG. 38 is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having a further particular embodiment of heater element.

FIG. 39 is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of FIG. 38 for forming the heater element thereof.

FIG. 40 is a further schematic perspective view of the unit cell of FIG. 38 shown with the nozzle plate omitted.

FIG. 41 is a schematic section through a nozzle chamber of a printhead according to an embodiment of the invention showing a suspended beam heater element immersed in a bubble forming liquid.

FIG. 42 is schematic section through a nozzle chamber of a printhead according to an embodiment of the invention showing a suspended beam heater element suspended at the top of a body of a bubble forming liquid.

FIG. 43 is a diagrammatic plan view of a unit cell of a printhead according to an embodiment of the invention showing a nozzle.

FIG. 44 is a diagrammatic plan view of a plurality of unit cells of a printhead according to an embodiment of the invention showing a plurality of nozzles.

FIG. 45 is a diagrammatic section through a nozzle chamber not in accordance with the invention showing a heater element embedded in a substrate.

FIG. 46 is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a heater element in the form of a suspended beam.

FIG. 47 is a diagrammatic section through a nozzle chamber of a prior art printhead showing a heater element embedded in a substrate.

FIG. 48 is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a heater element defining a gap between parts of the element.

FIG. 49 is a diagrammatic section through a nozzle chamber not in accordance with the invention, showing a thick nozzle plate.

FIG. 50 is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a thin nozzle plate.

FIG. 51 is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing two heater elements.

FIG. 52 is a diagrammatic section through a nozzle chamber of a prior art printhead showing two heater elements.

FIG. 53 is a diagrammatic section through a pair of adjacent unit cells of a printhead according to an embodiment of the invention, showing two different nozzles after drops having different volumes have been ejected therethrough.

FIGS. 54 and 55 are diagrammatic sections through a heater element of a prior art printhead.

FIG. 56 is a diagrammatic section through a conformally coated heater element according to an embodiment of the invention.

FIG. 57 is a diagrammatic elevational view of a heater element, connected to electrodes, of a printhead according to an embodiment of the invention.

FIG. 58 is a schematic exploded perspective view of a printhead module of a printhead according to an embodiment of the invention

FIG. 59 is a schematic perspective view the printhead module of FIG. 58 shown unexploded.

FIG. 60 is a schematic side view, shown partly in section, of the printhead module of FIG. 58.

FIG. 61 is a schematic plan view of the printhead module of FIG. 58.

FIG. 62 is a schematic exploded perspective view of a printhead according to an embodiment of the invention.

FIG. 63 is a schematic further perspective view of the printhead of FIG. 62 shown unexploded.

FIG. 64 is a schematic front view of the printhead of FIG. 62.

FIG. 65 is a schematic rear view of the printhead of FIG. 62.

FIG. 66 is a schematic bottom view of the printhead of FIG. 62.

FIG. 67 is a schematic plan view of the printhead of FIG. 62.

FIG. 68 is a schematic perspective view of the printhead as shown in FIG. 62, but shown unexploded.

FIG. 69 is a schematic longitudinal section through the printhead of FIG. 62.

FIG. 70 is a block diagram of a printer system according to an embodiment of the invention.

DETAILED DESCRIPTION

In the description than follows, corresponding reference numerals, or corresponding prefixes of reference numerals (i.e. the parts of the reference numerals appearing before a point mark) which are used in different figures relate to corresponding parts. Where there are corresponding prefixes and differing suffixes to the reference numerals, these indicate different specific embodiments of corresponding parts.

Overview of the Invention and General Discussion of Operation

With reference to FIGS. 1 to 4, the unit cell 1 of a printhead according to an embodiment of the invention 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 micro second, 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.

Turning briefly to FIG. 34, there is shown a mask 13 for forming a heater 14 of the printhead (which heater includes the element 10 referred to above), during a lithographic process, as described in more detail below. As the mask 13 is used to form the heater 14, the shape of various of its parts correspond to the shape of the element 10. The mask 13 therefore provides a useful reference by which to identify various parts of the heater 14. The heater 14 has electrodes 15 corresponding to the parts designated 15.34 of the mask 13 and a heater element 10 corresponding to the parts designated 10.34 of the mask. In operation, voltage is applied across the electrodes 15 to cause current to flow through the element 10. The electrodes 15 are much thicker than the element 10 so that most of the electrical resistance is provided by the element. Thus, nearly all of the power consumed in operating the heater 14 is dissipated via the element 10, in creating the thermal pulse referred to above.

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 a 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 effect adjacent chambers and their corresponding nozzles.

The advantages of the heater element 10 being suspended rather than being embedded in any solid material, is discussed below.

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. 5.

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.

Manufacturing Process

Relevant parts of the manufacturing process of a printhead according to embodiments of the invention are now described with reference to FIGS. 6 to 29.

Referring to FIG. 6, there is shown a cross-section through a silicon substrate portion 21, being a portion of a Memjet printhead, at an intermediate stage in the production process thereof. This figure relates to that portion of the printhead corresponding to a unit cell 1. The description of the manufacturing process that follows will be in relation to a unit cell 1, although it will be appreciated that the process will be applied to a multitude of adjacent unit cells of which the whole printhead is composed.

FIG. 6 represents the next successive step, during the manufacturing process, after the completion of a standard CMOS fabrication process, including the fabrication of CMOS drive transistors (not shown) in the region 22 in the substrate portion 21, and the completion of standard CMOS interconnect layers 23 and passivation layer 24. Wiring indicated by the dashed lines 25 electrically interconnects the transistors and other drive circuitry (also not shown) and the heater element corresponding to the nozzle.

Guard rings 26 are formed in the metallization of the interconnect layers 23 to prevent ink 11 from diffusing from the region, designated 27, where the nozzle of the unit cell 1 will be formed, through the substrate portion 21 to the region containing the wiring 25, and corroding the CMOS circuitry disposed in the region designated 22.

The first stage after the completion of the CMOS fabrication process consists of etching a portion of the passivation layer 24 to form the passivation recesses 29.

FIG. 8 shows the stage of production after the etching of the interconnect layers 23, to form an opening 30. The opening 30 is to constitute the ink inlet passage to the chamber that will be formed later in the process.

FIG. 10 shows the stage of production after the etching of a hole 31 in the substrate portion 21 at a position where the nozzle 3 is to be formed. Later in the production process, a further hole (indicated by the dashed line 32) will be etched from the other side (not shown) of the substrate portion 21 to join up with the hole 31, to complete the inlet passage to the chamber. Thus, the hole 32 will not have to be etched all the way from the other side of the substrate portion 21 to the level of the interconnect layers 23.

If, instead, the hole 32 were to be etched all the way to the interconnect layers 23, then to avoid the hole 32 being etched so as to destroy the transistors in the region 22, the hole 32 would have to be etched a greater distance away from that region so as to leave a suitable margin (indicated by the arrow 34) for etching inaccuracies. But the etching of the hole 31 from the top of the substrate portion 21, and the resultant shortened depth of the hole 32, means that a lesser margin 34 need be left, and that a substantially higher packing density of nozzles can thus be achieved.

FIG. 11 shows the stage of production after a four micron thick layer 35 of a sacrificial resist has been deposited on the layer 24. This layer 35 fills the hole 31 and now forms part of the structure of the printhead. The resist layer 35 is then exposed with certain patterns (as represented by the mask shown in FIG. 12) to form recesses 36 and a slot 37. This provides for the formation of contacts for the electrodes 15 of the heater element to be formed later in the production process. The slot 37 will provide, later in the process, for the formation of the nozzle walls 6, that will define part of the chamber 7.

FIG. 13 shows the stage of production after the deposition, on the layer 35, of a 0.25 micron thick layer 38 of heater material, which, in the present embodiment, is of titanium nitride.

FIG. 14 shows the stage of production after patterning and etching of the heater layer 38 to form the heater 14, including the heater element 10 and electrodes 15.

FIG. 16 shows the stage of production after another sacrificial resist layer 39, about 1 micron thick, has been added.

FIG. 18 shows the stage of production after a second layer 40 of heater material has been deposited. In a preferred embodiment, this layer 40, like the first heater layer 38, is of 0.25 micron thick titanium nitride.

FIG. 19 then shows this second layer 40 of heater material after it has been etched to form the pattern as shown, indicated by reference numeral 41. In this illustration, this patterned layer does not include a heater layer element 10, and in this sense has no heater functionality. However, this layer of heater material does assist in reducing the resistance of the electrodes 15 of the heater 14 so that, in operation, less energy is consumed by the electrodes which allows greater energy consumption by, and therefore greater effectiveness of, the heater elements 10. In the dual heater embodiment illustrated in FIG. 38, the corresponding layer 40 does contain a heater 14.

FIG. 21 shows the stage of production after a third layer 42, of sacrificial resist, has been deposited. As the uppermost level of this layer will constitute the inner surface of the nozzle plate 2 to be formed later, and hence the inner extent of the nozzle aperture 5, the height of this layer 42 must be sufficient to allow for the formation of a bubble 12 in the region designated 43 during operation of the printhead.

FIG. 23 shows the stage of production after the roof layer 44 has been deposited, that is, the layer which will constitute the nozzle plate 2. Instead of being formed from 100 micron thick polyimide film, the nozzle plate 2 is formed of silicon nitride, just 2 microns thick.

FIG. 24 shows the stage of production after the chemical vapor deposition (CVD) of silicon nitride forming the layer 44, has been partly etched at the position designated 45, so as to form the outside part of the nozzle rim 4, this outside part being designated 4.1

FIG. 26 shows the stage of production after the CVD of silicon nitride has been etched all the way through at 46, to complete the formation of the nozzle rim 4 and to form the nozzle aperture 5, and after the CVD silicon nitride has been removed at the position designated 47 where it is not required.

FIG. 28 shows the stage of production after a protective layer 48 of resist has been applied. After this stage, the substrate portion 21 is then ground from its other side (not shown) to reduce the substrate portion from its nominal thickness of about 800 microns to about 200 microns, and then, as foreshadowed above, to etch the hole 32. The hole 32 is etched to a depth such that it meets the hole 31.

Then, the sacrificial resist of each of the resist layers 35, 39, 42 and 48, is removed using oxygen plasma, to form the structure shown in FIG. 30, with walls 6 and nozzle plate 2 which together define the chamber 7 (part of the walls and nozzle plate being shown cutaway). It will be noted that this also serves to remove the resist filling the hole 31 so that this hole, together with the hole 32 (not shown in FIG. 30), define a passage extending from the lower side of the substrate portion 21 to the nozzle 3, this passage serving as the ink inlet passage, generally designated 9, to the chamber 7.

While the above production process is used to produce the embodiment of the printhead shown in FIG. 30, further printhead embodiments, having different heater structures, are shown in FIG. 33, FIGS. 35 and 37, and FIGS. 38 and 40.

Control of Ink Drop Ejection

Referring once again to FIG. 30, the unit cell 1 shown, as mentioned above, is shown with part of the walls 6 and nozzle plate 2 cut-away, which reveals the interior of the chamber 7. The heater 14 is not shown cut away, so that both halves of the heater element 10 can be seen.

In operation, ink 11 passes through the ink inlet passage 9 (see FIG. 28) to fill the chamber 7. Then a voltage is applied across the electrodes 15 to establish a flow of electric current through the heater element 10. This heats the element 10, as described above in relation to FIG. 1, to form a vapor bubble in the ink within the chamber 7.

The various possible structures for the heater 14, some of which are shown in FIGS. 33, 35 and 37, and 38, can result in there being many variations in the ratio of length to width of the heater elements 10. Such variations (even though the surface area of the elements 10 may be the same) may have significant effects on the electrical resistance of the elements, and therefore on the balance between the voltage and current to achieve a certain power of the element

Modem drive electronic components tend to require lower drive voltages than earlier versions, with lower resistances of drive transistors in their "on" state. Thus, in such drive transistors, for a given transistor area, there is a tendency to higher current capability and lower voltage tolerance in each process generation.

FIG. 36, referred to above, shows the shape, in plan view, of a mask for forming the heater structure of the embodiment of the printhead shown in FIG. 35. Accordingly, as FIG. 36 represents the shape of the heater element 10 of that embodiment, it is now referred to in discussing that heater element. During operation, current flows vertically into the electrodes 15 (represented by the parts designated 15.36), so that the current flow area of the electrodes is relatively large, which, in turn, results in there being a low electrical resistance. By contrast, the element 10, represented in FIG. 36 by the part designated 10.36, is long and thin, with the width of the element in this embodiment being 1 micron and the thickness being 0.25 microns.

It will be noted that the heater 14 shown in FIG. 33 has a significantly smaller element 10 than the element 10 shown in FIG. 35, and has just a single loop 36. Accordingly, the element 10 of FIG. 33 will have a much lower electrical resistance, and will permit a higher current flow, than the element 10 of FIG. 35. It therefore requires a lower drive voltage to deliver a given energy to the heater 14 in a given time.

In FIG. 38, on the other hand, the embodiment shown includes a heater 14 having two heater elements 10.1 and 10.2 corresponding to the same unit cell 1. One of these elements 10.2 is twice the width as the other element 10.1, with a correspondingly larger surface area. The various paths of the lower element 10.2 are 2 microns in width, while those of the upper element 10.1 are 1 micron in width. Thus the energy applied to ink in the chamber 7 by the lower element 10.2 is twice that applied by the upper element 10.1 at a given drive voltage and pulse duration. This permits a regulating of the size of vapor bubbles and hence of the size of ink drop ejected due to the bubbles.

Assuming that the energy applied to the ink by the upper element 10.1 is X, it will be appreciated that the energy applied by the lower element 10.2 is about 2X, and the energy applied by the two elements together is about 3X. Of course, the energy applied when neither element is operational, is zero. Thus, in effect, two bits of information can be printed with the one nozzle 3.

As the above factors of energy output may not be achieved exactly in practice, some "fine tuning" of the exact sizing of the elements 10.1 and 10.2, or of the drive voltages that are applied to them, may be required.

It will also be noted that the upper element 10.1 is rotated through 180.degree. about a vertical axis relative to the lower element 10.2. This is so that their electrodes 15 are not coincident, allowing independent connection to separate drive circuits.

Features and Advantages of Particular Embodiments

Discussed below, under appropriate headings, are certain specific features of embodiments of the invention, and the advantages of these features. The features are to be considered in relation to all of the drawings pertaining to the present invention unless the context specifically excludes certain drawings, and relates to those drawings specifically referred to.

Suspended Beam Heater

With reference to FIG. 1, and as mentioned above, the heater element 10 is in the form of a suspended beam, and this is suspended over at least a portion (designated 11.1) of the ink 11 (bubble forming liquid). The element 10 is configured in this way rather than forming part of, or being embedded in, a substrate as is the case in existing printhead systems made by various manufacturers such as Hewlett Packard, Canon and Lexmark. This constitutes a significant difference between embodiments of the present invention and the prior ink jet technologies.

The main advantage of this feature is that a higher efficiency can be achieved by avoiding the unnecessary heating of the solid material that surrounds the heater elements 10 (for example the solid material forming the chamber walls 6, and surrounding the inlet passage 9) which takes place in the prior art devices. The heating of such solid material does not contribute to the formation of vapor bubbles 12, so that the heating of such material involves the wastage of energy. The only energy which contributes in any significant sense to the generation of the bubbles 12 is that which is applied directly into the liquid which is to be heated, which liquid is typically the ink 11.

In one preferred embodiment, as illustrated in FIG. 1, the heater element 10 is suspended within the ink 11 (bubble forming liquid), so that this liquid surrounds the element. This is further illustrated in FIG. 41. In another possible embodiment, as illustrated in FIG. 42, the heater element 10 beam is suspended at the surface of the ink (bubble forming liquid) 11, so that this liquid is only below the element rather than surrounding it, and there is air on the upper side of the element. The embodiment described in relation to FIG. 41 is preferred as the bubble 12 will form all around the element 10 unlike in the embodiment described in relation to FIG. 42 where the bubble will only form below the element. Thus the embodiment of FIG. 41 is likely to provide a more efficient operation.

As can be seen in, for example, with reference to FIGS. 30 and 31, the heater element 10 beam is supported only on one side and is free at its opposite side, so that it constitutes a cantilever.

Efficiency of the Printhead

The feature presently under consideration is that the heater element 10 is configured such that an energy of less than 500 nanojoules (nJ) is required to be applied to the element to heat it sufficiently to form a bubble 12 in the ink 11, so as to eject a drop 16 of ink through a nozzle 3. In one preferred embodiment, the required energy is less that 300 nJ, while in a further embodiment, the energy is less than 120 nJ.

It will be appreciated by those skilled in the art that prior art devices generally require over 5 microjoules to heat the element sufficiently to generate a vapor bubble 12 to eject an ink drop 16. Thus, the energy requirements of the present invention are an order of magnitude lower than that of known thermal ink jet systems. This lower energy consumption allows lower operating costs, smaller power supplies, and so on, but also dramatically simplifies printhead cooling, allows higher densities of nozzles 3, and permits printing at higher resolutions.

These advantages of the present invention are especially significant in embodiments where the individual ejected ink drops 16, themselves, constitute the major cooling mechanism of the printhead, as described further below.

Self-Cooling of the Printhead

This feature of the invention provides that the energy applied to a heater element 10 to form a vapor bubble 12 so as to eject a drop 16 of ink 11 is removed from the printhead by a combination of the heat removed by the ejected drop itself, and the ink that is taken into the printhead from the ink reservoir (not shown). The result of this is that the net "movement" of heat will be outwards from the printhead, to provide for automatic cooling. Under these circumstances, the printhead does not require any other cooling systems.

As the ink drop 16 ejected and the amount of ink 11 drawn into the printhead to replace the ejected drop are constituted by the same type of liquid, and will essentially be of the same mass, it is convenient to express the net movement of energy as, on the one hand, the energy added by the heating of the element 10, and on the other hand, the net removal of heat energy that results from ejecting the ink drop 16 and the intake of the replacement quantity of ink 11. Assuming that the replacement quantity of ink 11 is at ambient temperature, the change in energy due to net movement of the ejected and replacement quantities of ink can conveniently be expressed as the heat that would be required to raise the temperature of the ejected drop 16, if it were at ambient temperature, to the actual temperature of the drop as it is ejected.

It will be appreciated that a determination of whether the above criteria are met depends on what constitutes the ambient temperature. In the present case, the temperature that is taken to be the ambient temperature is the temperature at which ink 11 enters the printhead from the ink storage reservoir (not shown) which is connected, in fluid flow communication, to the inlet passages 9 of the printhead. Typically the ambient temperature will be the room ambient temperature, which is usually roughly 20 degrees C. (Celsius).

However, the ambient temperature may be less, if for example, the room temperature is lower, or if the ink 11 entering the printhead is refrigerated.

In one preferred embodiment, the printhead is designed to achieve complete self-cooling (i.e. where the outgoing heat energy due to the net effect of the ejected and replacement quantities of ink 11 is equal to the heat energy added by the heater element 10).

By way of example, assuming that the ink 11 is the bubble forming liquid and is water based, thus having a boiling point of approximately 100 degrees C., and if the ambient temperature is 40 degrees C., then there is a maximum of 60 degrees C. from the ambient temperature to the ink boiling temperature and that is the maximum temperature rise that the printhead could undergo.

It is desirable to avoid having ink temperatures within the printhead (other than at time of ink drop 16 ejection) which are very close to the boiling point of the ink 11. If the ink 11 were at such a temperature, then temperature variations between parts of the printhead could result in some regions being above boiling point, with the unintended, and therefore undesirable, formation of vapor bubbles 12. Accordingly, a preferred embodiment of the invention is configured such that complete self-cooling, as described above, can be achieved when the maximum temperature of the ink 11 (bubble forming liquid) in a particular nozzle chamber 7 is 10 degrees C. below its boiling point when the heating element 10 is not active.

The main advantage of the feature presently under discussion, and its various embodiments, is that it allows for a high nozzle density and for a high speed of printhead operation without requiring elaborate cooling methods for preventing undesired boiling in nozzles 3 adjacent to nozzles from which ink drops 16 are being ejected. This can allow as much as a hundred-fold increase in nozzle packing density than would be the case if such a feature, and the temperature criteria mentioned, were not present.

Areal Density of Nozzles

This feature of the invention relates to the density, by area, of the nozzles 3 on the printhead. With reference to FIG. 1, the nozzle plate 2 has an upper surface 50, and the present aspect of the invention relates to the packing density of nozzles 3 on that surface. More specifically, the areal density of the nozzles 3 on that surface 50 is over 10,000 nozzles per square cm of surface area.

In one preferred embodiment, the areal density exceeds 20,000 nozzles 3 per square cm of surface 50 area, while in another preferred embodiment, the areal density exceeds 40,000 nozzles 3 per square cm. In a preferred embodiment, the areal density is 48 828 nozzles 3 per square cm.

When referring to the areal density, each nozzle 3 is taken to include the drive-circuitry corresponding to the nozzle, which consists, typically, of a drive transistor, a shift register, an enable gate and clock regeneration circuitry (this circuitry not being specifically identified).

With reference to FIG. 43 in which a single unit cell 1 is shown, the dimensions of the unit cell are shown as being 32 microns in width by 64 microns in length. The nozzle 3 of the next successive row of nozzles (not shown) immediately juxtaposes this nozzle, so that, as a result of the dimension of the outer periphery of the printhead chip, there are 48,828 nozzles 3 per square cm. This is about 85 times the nozzle areal density of a typical thermal ink jet printhead, and roughly 400 times the nozzle areal density of a piezoelectric printhead.

The main advantage of a high areal density is low manufacturing cost, as the devices are batch fabricated on silicon wafers of a particular size.

The more nozzles 3 that can be accommodated in a square cm of substrate, the more nozzles can be fabricated in a single batch, which typically consists of one wafer. The cost of manufacturing a CMOS plus MEMS wafer of the type used in the printhead of the present invention is, to a some extent, independent of the nature of patterns that are formed on it. Therefore if the patterns are relatively small, a relatively large number of nozzles 3 can be included. This allows more nozzles 3 and more printheads to be manufactured for the same cost than in a cases where the nozzles had a lower areal density. The cost is directly proportional to the area taken by the nozzles 3.

Bubble Formation on Opposite Sides of Heater Element

According to the present feature, the heater 14 is configured so that when a bubble 12 forms in the ink 11 (bubble forming liquid), it forms on both sides of the heater element 10. Preferably, it forms so as to surround the heater element 10 where the element is in the form of a suspended beam.

The formation of a bubble 12 on both sides of the heater element 10 as opposed to on one side only, can be understood with reference to FIGS. 45 and 46. In the first of these figures, the heater element 10 is adapted for the bubble 12 to be formed only on one side as, while in the second of these figures, the element is adapted for the bubble 12 to be formed on both sides, as shown.

In a configuration such as that of FIG. 45, the reason that the bubble 12 forms on only one side of the heater element 10 is because the element is embedded in a substrate 51, so that the bubble cannot be formed on the particular side corresponding to the substrate. By contrast, the bubble 12 can form on both sides in the configuration of FIG. 46 as the heater element 10 here is suspended.

Of course where the heater element 10 is in the form of a suspended beam as described above in relation to FIG. 1, the bubble 12 is allowed to form so as to surround the suspended beam element.

The advantage of the bubble 12 forming on both sides is the higher efficiency that is achievable. This is due to a reduction in heat that is wasted in heating solid materials in the vicinity of the heater element 10, which do not contribute to formation of a bubble 12. This is illustrated in FIG. 45, where the arrows 52 indicate the movements of heat into the, solid substrate 51. The amount of heat lost to the substrate 51 depends on the thermal conductivity of the solid materials of the substrate relative to that of the ink 11, which may be water based. As the thermal conductivity of water is relatively low, more than half of the heat can be expected to be absorbed by the substrate 51 rather than by the ink 11.

Prevention of Cavitation

As described above, after a bubble 12 has been formed in a printhead according to an embodiment of the present invention, the bubble collapses towards a point of collapse 17. According to the feature presently being addressed, the heater elements 10 are configured to form the bubbles 12 so that the points of collapse 17 towards which the bubbles collapse, are at positions spaced from the heater elements. Preferably, the printhead is configured so that there is no solid material at such points of collapse 17. In this way cavitation, being a major problem in prior art thermal ink jet devices, is largely eliminated.

Referring to FIG. 48, in a preferred embodiment, the heater elements 10 are configured to have parts 53 which define gaps (represented by the arrow 54), and to form the bubbles 12 so that the points of collapse 17 to which the bubbles collapse are located at such gaps. The advantage of this feature is that it substantially avoids cavitation damage to the heater elements 10 and other solid material.

In a standard prior art system as shown schematically in FIG. 47, the heater element 10 is embedded in a substrate 55, with an insulating layer 56 over the element, and a protective layer 57 over the insulating layer. When a bubble 12 is formed by the element 10, it is formed on top of the element. When the bubble 12 collapses, as shown by the arrows 58, all of the energy of the bubble collapse is focussed onto a very small point of collapse 17. If the protective layer 57 were absent, then the mechanical forces due to the cavitation that would result from the focussing of this energy to the point of collapse 17, could chip away or erode the heater element 10. However, this is prevented by the protective layer 57.

Typically, such a protective layer 57 is of tantalum, which oxidizes to form a very hard layer of tantalum pentoxide (Ta.sub.2 O.sub.5). Although no known materials can fully resist the effects of cavitation, if the tantalum pentoxide should be chipped away due to the cavitation, then oxidation will again occur at the underlying tantalum metal, so as to effectively repair the tantalum pentoxide layer.

Although the tantalum pentoxide functions relatively well in this regard in known thermal ink jet systems, it has certain disadvantages. One significant disadvantage is that, in effect, virtually the whole protective layer 57 (having a thickness indicated by the reference numeral 59) must be heated in order to transfer the required energy into the ink 11, to heat it so as to form a bubble 12. This layer 57 has a high thermal mass due to the very high atomic weight of the tantalum, and this reduces the efficiency of the heat transfer. Not only does this increase the amount of heat which is required at the level designated 59 to raise the temperature at the level designated 60 sufficiently to heat the ink 11, b