EP1637330B1

EP1637330B1 - Thermal actuator with corrugated heater element

Info

Publication number: EP1637330B1
Application number: EP05109700A
Authority: EP
Inventors: Kia Silverbrook; Gregory Mcavoy
Original assignee: Silverbrook Research Pty Ltd
Current assignee: Silverbrook Research Pty Ltd
Priority date: 1997-07-15
Filing date: 1998-07-15
Publication date: 2007-04-18
Anticipated expiration: 2018-07-15
Also published as: EP1640162B1; EP1637330A1; EP1650030A1; ATE386638T1; ATE359915T1; EP0999934A1; EP1650031B1; EP1650030B1; JP4160250B2; WO1999003681A1; EP0999934A4; EP1640162A1; JP2003521389A; EP1650031A1; EP0999934B1; ATE409119T1; ATE358019T1; ES2302134T3

Abstract

An inkjet nozzle arrangement is provided. The nozzle arrangement comprises a nozzle chamber having a slotted sidewall in a first surface and an ink ejection port defined in a second surface thereof, an ink supply channel interconnected to the nozzle chamber, a moveable vane located within the nozzle chamber and being moveable so as to cause ejection of ink from the nozzle chamber, and an actuator located outside the nozzle chamber and interconnected to the moveable vane through the slotted sidewall.

Description

Field of Invention

The present invention relates to the field of ink jet printing systems.

Background of the Art

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 of 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 utilisation of a continuous stream ink in ink jet printing appears to date back to at least 1929 wherein US Patent No. 1941001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.
US Patent 3596275 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 utilised by several manufacturers including Elmjet and Scitex (see also US Patent No. 3373437 by Sweet et al)
Piezo-electric ink jet printers are also one form of commonly utilised ink jet printing device. Piezo-electric systems are disclosed by Kyser et. al. in US Patent No. 3946398 (1970) which utilises a diaphragm mode of operation, by Zolten in US Patent 3683212 (1970) which discloses a squeeze mode of operation of a piezo electric crystal, Stemme in US Patent No. 3747120 (1972) discloses a bend mode of piezo-eleclric operation, Howkins in US Patent No. 4459601 discloses a Piezo electric push mode actuation of the ink jet stream and Fischbeck in US 4584590 which discloses a sheer mode type of piezo-electric transducer element.
Ejection can be carried out by use of electrothermal actuation as well. DE4031248 shows a thermal actuator comprising a heater element within an actuation material. The heater electrically heats the actuation material. This results in a movement parallel to the substrate of the actuator. The heater extends from the bottom to the top of the actuation material, and in a serpentine configuration along the length of the actuator.
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 US Patent 4490728. Both the aforementioned references disclosed ink jet printing techniques 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 utilising 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.
Many ink jet printing mechanisms are known. Unfortunately, in mass production techniques, the production of ink jet heads is quite difficult. For example, often, the orifice or nozzle plate is constructed separately from the ink supply and ink ejection mechanism and bonded to the mechanism at a later stage (Hewlett-Packard Journal, Vol. 36 no 5, pp33-37 (1985)). These separate material processing steps required in handling such precision devices often adds a substantially expense in manufacturing.
Additionally, side shooting ink jet technologies (U.S. Patent No. 4,899,181) are often used but again, this limit the amount of mass production throughput given any particular capital investment.
Additionally, more esoteric techniques are also often utilised. These can include electroforming of nickel stage (Hewlett-Packard Journal, Vol. 36 no 5, pp33-37 (1985)), electro-discharge machining, laser ablation (U.S. Patent No. 5,208,604), micro-punching, etc.
The utilisation of the above techniques is likely to add substantial expense to the mass production of ink jet print heads and therefore add substantially to their final cost.
It would therefore be desirable if an efficient system for the mass production of ink jet print heads could be developed.
Further, during the construction of micro electromechanical systems, it is common to utilize a sacrificial material to build up a mechanical system, within the sacrificial material being subsequently etched away so as to release the required mechanical structure. For example, a suitable common sacrificial material includes silicon dioxide which can be etched away in hydrofluoric acid. MEMS devices are often constructed on silicon wafers having integral electronics such as, for example, using a multi-level metal CMOS layer. Unfortunately, the CMOS process includes the construction of multiple layers which may include the utilization of materials which can be attacked by the sacrificial etchant. This often necessitates the construction of passivation layers using extra processing steps so as to protect other layers from possible unwanted attack by a sacrificial etchant.
In micro-electro mechanical system, it is often necessary to provide for the movement of objects. In particular, it is often necessary to pivot objects in addition to providing for fulcrum arrangements where a first movement of one end of the fulcrum is translated into a corresponding measurement of a second end of the fulcrum. Obviously, such arrangements are often fundamental to mechanical apparatuses.
Further, When constructing large integrated circuits or micro-electro mechanical systems, it is often necessary to interconnect a large number of wire to the final integrated circuit device. To this end, normally, a large number of bond pads are provided on the surface of a chip for the attachment of wires thereto. With the utilization of bond pads normally certain minimal spacings are utilized in accordance with the design technologies utilised. Where are large number of interconnects are required, an excessive amount of on chip real estate is required for providing bond pads. It is therefore desirable to minimize the amount of real estate provided for bond pads whilst ensuring the highest degree of accuracy of registration for automated attachment of interconnects such as a tape automated bonding (TAB) to the surface of a device.

Summary of the invention

Accordingly the invention provides a thermal actuator according to claim 1. Advantageous embodiments are provided in the dependent claims thereto. The invention also provides an ink jet nozzle according to claim 9.

Brief Description of the Drawings

Notwithstanding any other forms which 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. 170 is a cross-sectional view of a single ink jet nozzle constructed in accordance with an embodiment, in its quiescent state;
Fig. 171 is a cross-sectional view of a single ink jet nozzle constructed in accordance with an embodiment, in its activated state;
Fig. 172 is an exploded perspective view illustrating the construction of a single ink jet nozzle in accordance with an embodiment;
Fig. 173 is a cross-sectional schematic diagram illustrating the construction of a corrugated conductive layer in accordance with an embodiment of the present invention;
Fig. 174 is a schematic cross-sectional diagram illustrating the development of a resist material through a halftoned mask utilised in the fabrication of a single ink jet nozzle in accordance with an embodiment;
Fig. 175 is a top view of the conductive layer only of the thermal actuator of a single ink jet nozzle constructed in accordance with an embodiment;
Fig. 176 provides a legend of the materials indicated in Fig. 177 to Fig. 188; and
Fig. 177 to Fig. 188 illustrate sectional views of the manufacturing steps in one form of construction of an ink jet printhead nozzle.

Description of the Preferred and Other Embodiments

The preferred embodiments and other embodiments will be discussed under separate headings with the heading including an IJ number for ease of reference. The headings also include a type designator with T indicating thermal, S indicating shutter type and F indicating a field type.

A Description of IJ23 T

In an embodiment, ink is ejected from a nozzle through the utilisation of the bending of a thermal actuator so as to eject the ink.
Turning now to Fig. 170, there is illustrated a single nozzle arrangement 2301 of an embodiment. The nozzle arrangement 2301 includes a thermal actuator 2302 located above a nozzle chamber 2303 and nozzle 2304. The thermal actuator 2302 includes an electrical circuit comprising leads 2306, 2307 connected to a serpentine resistive element 2308. The resistive element 2308 can comprise the copper layer in this respect, a copper stiffener 2309 is provided to provide support for one end of the thermal actuator 2302.
The copper resistive element 2308 is constructed in a serpentine manner to provide very little tensile strength along the length of the thermal actuator panel 2302.
The copper resistive element is embedded in a polytetrafluoroethylene (PTFE) layer 2312. The PTFE layer 2312 has a very high coefficient of thermal expansion (approximately 770 x 10^-6). This layer undergoes rapid expansion when heated by the copper heater 2308. The copper heater 2308 is positioned closer to the top surface of the PTFE layer 2312, thereby heating the upper level of the PTFE layer 2312 faster than the bottom level, resulting in a bending down of the thermal actuator 2302 towards the nozzle 2304 in the nozzle chamber 2303.
The operation of the nozzle arrangement 2301 is as follows:

1) When data signals distributed on the print head indicate that a particular nozzle is to eject a drop of ink, a drive transistor for that nozzle is turned on. This energises the leads 2306, 2307, and the heater 2308 in the paddle for that nozzle. The heater 2308 is energised for approximately 3 µs, with the actual duration depending upon the design chosen for the actuator nozzle.
2) The heater heats the PTFE layer 2312, with the top level of the PTFE layer 2312 being heated more rapidly than the bottom level. This causes the paddle to bend generally in the direction towards the nozzle 2304 in the nozzle chamber 2303, as illustrated in Fig. 171. The bending of the paddle pushes ink from the ink chamber 2303 out of the nozzle 2304.
3) When the heater current is turned off, the paddle 2302 begins to return to its quiescent position. The paddle return 'sucks' some of the ink back into the nozzle 2304 into the nozzle chamber, causing the ink ligament connecting the ink drop to the ink in the nozzle 2304 to thin. The forward velocity of the drop and backward velocity of the ink in the chamber are resolved by the ink drop breaking off from the ink in the nozzle. The ink drop then continues towards the recording medium.
4) The paddle 2302 is at the quiescent position until the next drop ejection cycle. Construction

In order to construct a series of nozzle arrangements 2301 having an actuator associated with each of the nozzles, the following major parts need to be constructed:

A liquid ink print head has one actuator associated with each of a multitude of nozzles. The actuator has the following major parts:
1. 1) Drive circuitry to drive the arrangement 2301.
2. 2) The nozzle tip 2304. The radius of the nozzle tip 2304 is an important determinant of drop velocity and drop size.
3. 3) The paddle 2302 is made of a heater layer 2308 embedded into PTFE layer 2312. The paddle 2302 is fixed to one end of the ink chamber, and the other end is suspended 'over' the nozzle. Approximately half of the paddle contains the copper heater 2308. The heater section is at the fixed end of the paddle.
4. 4) The nozzle chamber 2303. The nozzle chamber 2303 is slightly wider than the paddle. The gap between the paddle and the nozzle chamber is determined by the fluid dynamics of the ink ejection and refill process. If the gap is too large, much of the paddle force will be wasted on pushing ink around the edges of the paddle. If the gap is too small, the ink refill time will be too long. Also, if the gap is too small, the crystallographic etch of the nozzle chamber will take too long to complete. A 2µm gap will usually be sufficient. The nozzle chamber is also deep enough so that air ingested through the nozzle tip when the plunger returns to its quiescent state does not extend to the piston. If it does, the ingested bubble may form a cylindrical surface instead of a hemispherical surface. If this happens, the nozzle will not refill properly. A depth of approximately 20µm is suitable.
5. 5) Nozzle chamber ledges 2313. As the paddle 2302 moves approximately 10µm, and the crystallographic etch angle of chamber surface 2315 is 54.74 degrees, a gap of around 7µm is required between the edge of the paddle 2 and the outermost edge of nozzle chamber. The walls of nozzle chamber must also clear the nozzle hole. This requires that the nozzle chamber 3 be approximately 52vm wide, whereas the paddle 2302 is only 30 µm wide. Were there to be an 11µm gap around the paddle, too much ink would flow around to the sides of the paddle when the actuator is energised. To prevent this, the nozzle chamber 2303 is undercut 9µm into the silicon surrounding the paddle, leaving a 9µm wide ledge 2313 to prevent ink flow around the paddle.

Example Basic Fabrication Sequence

Two wafers are required: a wafer upon which the active circuitry and nozzles are fabricated (the print head wafer) and a further wafer in which the ink channels are fabricated. This is the ink channel wafer. One form of construction of print head wafer will now be discussed with reference to Fig. 171 which illustrates an exploded perspective view of a single inkjet nozzle constructed in accordance with an embodiment.

1) Starting with a single crystal silicon wafer, which has a buried epitaxial layer 2316 of silicon which is heavily doped with boron. The boron should be doped to preferably 10²⁰ atoms per cm³ of boron or more, and be approximately 3µm thick. The lightly doped silicon epitaxial layer 2315 on top of the boron doped layer should be approximately 8µm thick, and be doped in a manner suitable for the active semiconductor device technology chosen. This is the print head wafer. The wafer diameter should preferably be the same as the ink channel wafer.
2) The drive transistors and data distribution circuitry layer 2317 is fabricated according to the process chosen, up until the oxide layer over second level metal.
3) Next, a silicon nitride passivation layer 2318 is deposited.
4) Next, the actuator 2302 (Fig. 170) is constructed. The actuator comprises one copper layer 2319 embedded into a PTFE layer 2320. The copper layer 2319 comprises both the heater portion 2308 and planar portion 2309 (of Fig. 170). Turning now to Fig. 173, the corrugated resistive element can be formed by depositing a resist layer 2350 on top of the first PTFE layer 2351. The resist layer 2350 is exposed utilising a mask 2352 having a half-tone pattern delineating the corrugations. After development the resist 2350 contains the corrugation pattern. The resist layer 2350 and the PTFE layer 2351 are then etched utilising an etchant that erodes the resist layer 2350 at substantially the same rate as the PTFE layer 2351. This transfers the corrugated pattern into the PTFE layer 2351. Turning to Fig. 174, on top of the corrugated PTFE layer 2351 is deposited the copper heater layer 2319 which takes on a corrugated form in accordance with its under layer. The copper heater layer 2319 is then etched in a serpentine or concertina form. In Fig. 175 there is illustrated a top view of the copper layer 2319 only, comprising the serpentine heater element 2308 and stiffener 2309. Subsequently, a further PTFE layer 2353 is deposited on top of layer 2319 so as to form the top layer of the thermal actuator 2302. Finally, the second PTFE layer 2352 is planarised to form the top surface of the thermal actuator 2302 (Fig. 170).
5) Etch through the PTFE, and all the way down to silicon in the region around the three sides of the paddle. The etched region should be etched on all previous lithographic steps, so that the etch to silicon does not require strong selectivity against PTFE.
6) Etch the wafers in an anisotropic wet etch, which stops on <111> crystallographic planes or on heavily boron doped silicon. The etch can be a batch wet etch in ethylenediamine pyrocatechol (EDP). The etch proceeds until the paddles are entirely undercut whereby forming nozzle chamber 2303. The backside of the wafer need not be protected against this etch, as the wafer is to be subsequently thinned. Approximately 60µm of silicon will be etched from the wafer backside during this process.
7) Permanently bond the print head wafer onto a pre-fabricated ink channel wafer. The active side of the print head wafer faces the ink channel wafer. The ink channel wafer is attached to a backing plate, as it has already been etched into separate ink channel chips.
8) Etch the print head wafer to entirely remove the backside silicon to the level of the boron doped epitaxial layer 2316. This etch can be a batch wet etch in ethylenediamine pyrocatechol (EDP).
9) Mask the nozzle rim 2311 (Fig. 170) from the underside of the print head wafer. This mask is a series of circles approximately 0.5µm to 1µm larger in radius than the nozzles. The purpose of this step is to leave a raised rim 2311 around the nozzle tip, to help prevent ink spreading on the front surface of the wafer. This step can be eliminated if the front surface is made sufficiently hydrophobic to reliably prevent front surface wetting.
10) Etch the boron doped silicon layer 2316 to a depth of 1µm.
11) Mask the nozzle holes from the underside of the print head wafer. This mask can also include the chip edges.
12) Etch through the boron doped silicon layer to form nozzles 2304.
13) Separate the chips from their backing place. Each chip is now a full print head including ink channels. The two wafers have already been etched through, so the print heads do not need to be diced.
14) Test the print heads and TAB bond the good print heads.
15) Hydrophobise the front surface of the print heads.
17) Perform final testing on the TAB bonded print heads.

It would be evident to persons skilled in the relevant arts that the arrangement described by way of example in an embodiments will result in a nozzle arrangement able to eject ink on demand and be suitable for incorporation in a drop on demand ink jet printer device having an array of nozzles for the ejection of ink on demand.
Of course, alternative embodiments will also be self-evident to the person skilled in the art. For example, the thermal actuator could be operated in a reverse mode wherein passing current through the actuator results in movement of the paddle to an ink loading position when the subsequent cooling of the paddle results in the ink being ejected. However, this has a number of disadvantages in that cooling is likely to take a substantially longer time than heating and this arrangement would require a constant current to be passed through nozzles when not in use.
One form of detailed manufacturing process which can be used to fabricate monolithic ink jet print heads operating in accordance with the principles taught by the present embodiment can proceed utilizing the following steps:

1. Using a double sided polished wafer deposit 3 microns of epitaxial silicon heavily doped with boron.
2. Deposit 10 microns of epitaxial silicon, either p-type or n-type, depending upon the CMOS process used.
3. Complete drive transistors, data distribution, and timing circuits using a 0.5 micron, one poly, 2 metal CMOS process. This step is shown in Fig. 177. For clarity, these diagrams may not be to scale, and may not represent a cross section though any single plane of the nozzle. Fig. 176 is a key to representations of various materials in these manufacturing diagrams, and those of other cross referenced ink jet configurations.
4. Etch the CMOS oxide layers down to silicon or aluminum using Mask 1. This mask defines the nozzle chamber, and the edges of the print heads chips. This step is shown in Fig. 178.
5. Crystallographically etch the exposed silicon using, for example, KOH or EDP (ethylenediamine pyrocatechol). This etch stops on <111> crystallographic planes, and on the boron doped silicon buried layer. This step is shown in Fig. 179.
6. Deposit 0.5 microns of low stress silicon nitride.
7. Deposit 12 microns of sacrificial material (polyimide). Planarize down to nitride using CMP. The sacrificial material temporarily fills the nozzle cavity. This step is shown in Fig. 180.
8. Deposit 1 micron of PTFE.
9. Deposit, expose and develop 1 micron of resist using Mask 2. This mask is a gray-scale mask which defines the heater vias as well as the corrugated PTFE surface that the heater is subsequently deposited on.
10. Etch the PTFE and resist at substantially the same rate. The corrugated resist thickness is transferred to the PTFE, and the PTFE is completely etched in the heater via positions. In the corrugated regions, the resultant PTFE thickness nominally varies between 0.25 micron and 0.75 micron, though exact values are not critical. This step is shown in Fig. 181.
11. Etch the nitride and CMOS passivation down to second level metal using the resist and PTFE as a mask.
12. Deposit and pattern resist using Mask 3. This mask defines the heater.
13. Deposit 0.5 microns of gold (or other heater material with a low Young's modulus) and strip the resist. Steps 2311 and 2312 form a lift-off process. This step is shown in Fig. 182.
14. Deposit 1.5 microns of PTFE.
15. Etch the PTFE down to the nitride or sacrificial layer using Mask 4. This mask defines the actuator paddle and the bond pads. This step is shown in Fig. 183.
16. Wafer probe. All electrical connections are complete at this point, and the chips are not yet separated.
17. Plasma process the PTFE to make the top and side surfaces of the paddle hydrophilic. This allows the nozzle chamber to fill by capillarity.
18. Mount the wafer on a glass blank and back-etch the wafer using KOH with no mask. This etch thins the wafer and stops at the buried boron doped silicon layer. This step is shown in Fig. 184.
19. Plasma back-etch the boron doped silicon layer to a depth of 1 micron using Mask 5. This mask defines the nozzle rim. This step is shown in Fig. 185.
20. Plasma back-etch through the boron doped layer and sacrificial layer using Mask 6. This mask defines the nozzle, and the edge of the chips. At this stage, the chips are still mounted on the glass blank. This step is shown in Fig. 186.
21. Etch the remaining sacrificial material while the wafer is still attached to the glass blank.
22. Plasma process the PTFE through the nozzle holes to render the PTFE surface hydrophilic.
23. Strip the adhesive layer to detach the chips from the glass blank. This process completely separates the chips. This step is shown in Fig. 187.
24. Mount the print heads in their packaging, which may be a molded plastic former incorporating ink channels which supply different colors of ink to the appropriate regions of the front surface of the wafer.
25. Connect the print heads to their interconnect systems.
26. Hydrophobize the front surface of the print heads.
27. Fill with ink and test the completed print heads. A filled nozzle is shown in Fig. 188.

Claims

A thermal actuator comprising a heater element encased within an actuation material, said actuator operating by means of said heater element electrically heating said actuation material, wherein said heater element has a corrugated structure so as to improve thermal distribution of heat from said heater element to said actuation material.
A thermal actuator as claimed in claim 1, wherein said actuation material has a higher coefficient of thermal expansion than said heater element.
A thermal actuator as claimed in any one of the preceding claims, wherein said heater element has a serpentine or concertinaed form so as to allow substantially unhindered expansion of said actuation material during heating.
A thermal actuator as claimed in any one of the preceding claims, wherein one surface of said actuator is hydrophilic.
A thermal actuator as claimed in claim 4 wherein said hydrophilic surface is formed by means of processing of a hydrophobic surface.
A thermal actuator as claimed in any one of the preceding claims, wherein one surface of said actuator is hydrophobic and the other surface is hydrophilic.
A thermal actuator as claimed in any one of the preceding claims, wherein said heater material comprises substantially copper.
A thermal actuator as claimed in any one of the preceding claims, wherein the heater element is positioned nearer to one surface of the actuator than the other, thereby causing bending of the actuator when the heater element is electrically heated.
An inkjet nozzle comprising a thermal actuator according to any one of the preceding claims.