Patent Grant
8967772
The present invention relates to the field of thermal inkjet printers. In particular, the invention reduces the resistive losses in the electrical connection between the thermal actuators and underlying drive circuitry in an inkjet printhead.
The following applications have been filed by the Applicant simultaneously with the present application:
The disclosures of these co-pending applications are incorporated herein by reference. The above applications have been identified by their filing docket number, which will be substituted with the corresponding application number, once assigned.
Various methods, systems and apparatus relating to the present invention are disclosed in the following US patents/patent applications filed by the applicant or assignee of the present invention:
The disclosures of these applications and patents are incorporated herein by reference.
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) Each pixel in the printed image is derived from ink drops ejected from one or more ink nozzles. In recent years, inkjet printing has become increasing popular primarily due to its inexpensive and versatile nature. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents.
The Applicant has developed a range of pagewidth printheads. Pagewidth printheads have an elongate array of nozzles extending the printing width of the media substrate. These printheads are faster than traditional scanning printheads as the paper continuous feeds past the printhead which remains stationary. In contrast, scanning printheads traverse the page to print successive swathes as the paper is indexed through the printer.
The large number of nozzles in a pagewidth printhead generates much more heat than a corresponding scanning printhead. This requires pagewidth printheads to be ‘self cooling’ as complex and elaborate cooling systems would not be commercially practical. Self cooling is a process whereby heat generated in the ejection process is removed from the printhead by the ejected drops of ink. Without a build up of excessive heat, the theoretical maximum firing frequency of a self cooling printhead nozzle is only restricted by the ink refill rate of the nozzle.
Low energy droplet ejection is key to the Applicants printheads self cooling operation. Reducing the energy input to each nozzle, reduces the energy that the ejected drops need to remove in order to achieve self cooling operation. Thermal inkjet uses pulses of electrical current to raise the temperature of the heaters to the superheat limit of the ink, which is typically around 300° C. for water based ink. At this temperature a high pressure vapour bubble is formed on the heater surface and expansion of the bubble forces ink out of the nozzle. Reduced energy input in thermal inkjet can be achieved through careful attention to parasitic losses in the heater contacts. Careful attention must also be given to the reliability of the heater contact design.
The heater is a film of resistive material deposited by a lithographic process of the type well known and understood in the field semiconductor fabrication. When the film is deposited on a non-planar topography, the thickness of the film varies substantially. If the film is deposited over a substantially vertical step, the film thickness on the vertical surface of the step is typically ˜⅓ of the horizontal film thickness. A conductive strip of uniform width deposited over a vertical step will therefore have ˜3 times the current density in the vertical section with ˜9 times the volumetric heating rate (the heating rate is proportional to the square of current density). The temperature of relatively thin sections of film will far exceed 300° C. during the current pulse. This causes early failure due to, inter alia, oxidation and electro-migration.
One approach to avoid this is described in the Applicant's co-pending U.S. Ser. No. 11/246,687 filed Oct. 11, 2005, the contents of which are incorporated herein by cross reference. The current density in regions with non planar topography is reduced by making the width of the conductive strip much wider in that section. The additional width compensates for areas of reduced thickness and current density remains at safe levels.
Unfortunately, the electrical current funnels from the (laterally) wide contacts of the heater to the laterally much narrower resistive element that forms the vapour bubble. If the funnelling is done over a short distance, spikes in current density and hot spots can arise at or near the ends of the resistive elements, again causing early failure. Funnelling over a longer distance avoids hot spots but the parasitic resistance of the contact (i.e. non-bubble forming) portion of the heater increases, resulting in decreased efficiency.
Another technique for addressing excess current density is described in US patent publication 2008/0259,131 assigned to Lexmark International Inc. An additional low resistivity layer is deposited on top of the resistive thin film to ‘short out’ areas of the heater contacts deposited over non-planar topography. Volumetric heating rate is proportional to resistivity and hence the contacts sections stay relatively cool. The parasitic resistance and waste heat are low, as all but the active element of the heater is shorted by the low resistivity layer.
Unfortunately, both the resistive heater film and the low resistivity layer must be coated with an insulating layer to prevent contact with ink, or a corrosive galvanic cell will form (two dissimilar metals in contact in the presence of an electrolyte). Also, the traditional material for the low resistivity layer (aluminium) chemically corrodes if exposed to ink.
Coating with insulating layers increases the thermal mass that must be heated to the superheat limit to form a bubble, so this coating will increase the energy required to jet ink. As such, insulating coatings are contrary to energy efficient droplet ejection and therefore counter to self cooling operation.
A second drawback relates to patterning the low resistivity layer without damaging the underlying heater material film. Dry etches are preferred in most semiconductor fabrication facilities, but dry etches with suitable selectivity between the two materials, both likely to contain aluminium, are unlikely to exist. Finding a wet etch that can etch the low resistivity layer without etching the resistive thin film is likely to be easier, but that would impose significant constraints on the selection of the heater film material. These selection constraints may be contrary to the goal of self cooling, which requires thin film materials with particular properties, such as very high oxidation resistance.
According to a first aspect, the present invention provides an inkjet printhead comprising:
a supporting substrate;
a conductive layer deposited in a pattern on one side of the supporting substrate;
an insulating layer deposited such that the conductive layer is between the insulating layer and the supporting substrate;
an ink chamber supported on the supporting substrate such that the conductive layer is between the ink chambers and the supporting substrate;
a nozzle in fluid communication with the ink chamber;
a heater on the insulating layer configured to vaporize some ink in the ink chamber such that a droplet of ink is ejected through the nozzle, the heater having a resistive element extending between a pair of contacts; and,
at least one metallic via in each of the contacts respectively, the metallic vias extending through the insulating layer to establish an electrical connection between the conductive layer and the contacts; wherein,
the insulating layer has a planar surface on which the heater is supported.
The invention is predicated on the realisation that the areas of high current density can be avoided by supporting the heater on a planar surface and electrically connecting the contacts to the underlying CMOS with metallic vias.
Preferably, the resistive element is an elongate strip extending between the contacts and the at least one metallic via in each of the contacts has a width substantially equal to the width of the strip.
Preferably, the metallic vias contain tungsten, copper or aluminium.
Preferably, one end of each of the vias is planar and co-planar with the planar surface on which the heater is supported.
Preferably, the heater is less than 2 microns thick and in a further preferred form, the heater is less than 1 micron thick.
Preferably, the heater is an alloy containing titanium and aluminium.
Preferably, the thickness of the insulating layer between the conductive layer and the contacts is between 1.2 microns and 1.8 microns.
Preferably, the insulating layer is a laminate of different materials. In a further preferred form, the laminate is a layer of silicon nitride between two outer layers of silicon dioxide.
Preferably, the conductive layer is a top-most metal layer in a stack of CMOS layers on the supporting substrate. Preferably, the CMOS layers provide the heater with an electrical pulse of energy to generate the vapour bubble, the electrical pulse generating less than 250 nano-joules. Preferably, the CMOS has a drive transistor through which the electrical pulse flows, the drive transistor having a drive voltage less than 5V.
According to a second aspect, the present invention provides a method of fabricating an inkjet printhead comprising the steps of:
providing a supporting substrate;
depositing and patterning a conductive layer on one side of the supporting substrate;
depositing an insulating layer on the conductive layer;
etching holes through the insulating layer to the conductive layer;
depositing metal in the holes to form metallic vias;
planarizing an outer surface of the insulating layer and one end of each of the metallic vias respectively; and,
depositing and patterning a layer of heater material on the outer surface to form a heater with a resistive element extending between a pair of contacts; wherein,
the metallic vias electrically connect the contacts to the conductive layer.
Preferably, the step of planarizing the outer surface is a chemical, mechanical planarization process.
Preferably, the resistive element is an elongate strip extending between the contacts and the at least one metallic via in each of the contacts has a width substantially equal to the width of the strip.
Preferably, the metallic vias contain tungsten, copper or aluminium.
Preferably, one end of each of the vias is planar and co-planar with the planar surface on which the heater is supported.
Preferably, the heater is less than 2 microns thick and in a further preferred form, the heater is less than 1 micron thick.
Preferably, the heater is an alloy containing titanium and aluminium.
Preferably, the thickness of the insulating layer between the conductive layer and the contacts is between 1.2 microns and 1.8 microns.
Preferably, the insulating layer is a laminate of different materials. In a further preferred form, the laminate is a layer of silicon nitride between two outer layers of silicon dioxide.
Preferably, the conductive layer is a top-most metal layer in a stack of CMOS layers on the supporting substrate. Preferably, the CMOS layers provide the heater with an electrical pulse of energy to generate the vapour bubble, the electrical pulse generating less than 250 nano-joules. Preferably, the CMOS has a drive transistor through which the electrical pulse flows, the drive transistor having a drive voltage less than 5V.
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. The smallest repeating units of the printhead will have an ink supply inlet feeding ink to one or more chambers. The entire nozzle array is formed by repeating these individual units. Such an individual unit is referred to herein as a “unit cell”.
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, medicaments, 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.
Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
In the description than follows, corresponding reference numerals relate to corresponding parts. For convenience, the features indicated by each reference numeral are listed below.
The MEMS manufacturing process builds up nozzle structures on a silicon wafer supporting substrate, after the completion of CMOS processing.
During CMOS processing of the wafer, four metal layers are deposited onto a silicon wafer 2, with the metal layers being interspersed between interlayer dielectric (ILD) layers. The four metal layers are referred to as M1, M2, M3 and M4 layers and are built up sequentially on the wafer during CMOS processing. These CMOS layers provide all the drive circuitry and logic for operating the printhead.
In the completed printhead, each heater element actuator is connected to the CMOS via a pair of electrodes defined in the outermost M4 layer. Hence, the M4 CMOS layer is the foundation for subsequent MEMS processing of the wafer. The M4 layer also defines bonding pads along a longitudinal edge of each printhead integrated circuit. These bonding pads (not shown) allow the CMOS to be connected to a microprocessor via wire bonds extending from the bonding pads.
Before MEMS processing of the unit cell 1 begins, bonding pads along a longitudinal edge of each printhead integrated circuit are defined by etching through the passivation layer 4. This etch reveals the M4 layer 3 at the bonding pad positions. The nozzle unit cell 1 is completely masked with photoresist for this step and, hence, is unaffected by the etch.
Turning to
In the next step (
Typically, when filling trenches with photoresist, it is necessary to expose the photoresist outside the perimeter of the trench in order to ensure that photoresist fills against the walls of the trench and, therefore, avoid ‘stringers’ in subsequent deposition steps. However, this technique results in a raised (or spiked) rim of photoresist around the perimeter of the trench. This is undesirable because in a subsequent deposition step, material is deposited unevenly onto the raised rim—vertical or angled surfaces on the rim will receive less deposited material than the horizontal planar surface of the photoresist filling the trench.
As shown in
After exposure of the SAC1 photoresist 10, the photoresist is reflowed by heating. Reflowing the photoresist allows it to flow to the walls of the pit 8, filling it exactly.
Referring to
This etch is defined by a layer of photoresist (not shown) exposed using the dark tone mask shown in
In the next sequence of steps, an ink inlet for the nozzle is etched through the passivation layer 4, the oxide layer 5 and the silicon wafer 2. During CMOS processing, each of the metal layers had an ink inlet opening (see, for example, opening 6 in the M4 layer 3 in
Referring to
In the first etch step (
In the second etch step (
In the next step, the ink inlet 15 is plugged with photoresist and a second sacrificial layer (“SAC2”) of photoresist 16 is built up on top of the SAC1 photoresist 10 and passivation layer 4. The SAC2 photoresist 16 will serve as a scaffold for subsequent deposition of roof material, which forms a roof and sidewalls for each nozzle chamber. Referring to
As shown in
Referring to
Referring to
Referring to
With all the MEMS nozzle features now fully formed, the next stage removes the SAC1 and SAC2 photoresist layers 10 and 16 by O2 plasma ashing (
Referring to
Bonded Heater Connected by Vias to Top Metal of CMOS
In the above described embodiment, electrical contact between the heater and top metal layer of the CMOS is provided by selectively etching the passivation layer and depositing the heater material directly on the exposed areas of the top metal layer. Whilst this provides reliable electrical connection it is possible to provide greater control of the characteristics of the connection by forming contact elements between the heater and the CMOS.
In thermal inkjet printheads the heater is activated with electrical pulses to raise its temperature to the superheat limit of the ink (typically around 300° C. for water based ink). At this temperature a high pressure vapour bubble is formed on the surface of the resistive element of the heater. Expansion of the bubble forces ink out of the associated nozzle.
As discussed above, when the heater material is deposited on a non-planar topography, the thickness of the film varies substantially. If the film is deposited over a substantially vertical step, the film thickness on the vertical surface of the step is typically ˜⅓ of the horizontal film thickness. A conductive strip of uniform width deposited over a vertical step will therefore have ˜3 times the current density in the vertical section with ˜9 times the volumetric heating rate (the heating rate is proportional to the square of current density). The temperature of relatively thin sections of film will far exceed 300° C. during the current pulse. This causes early failure due to, inter alia, oxidation and electro-migration.
To avoid this is, the contacts for each heater can be much wider than the resistive element. The additional width compensates for areas of reduced thickness and current density remains at safe levels.
Unfortunately, the electrical current funnels from the (laterally) wide contacts of the heater to the (laterally) much narrower resistive element that forms the vapour bubble. If the funnelling is done over a short distance, spikes in current density and hot spots can arise at or near the ends of the resistive elements, again causing early failure. Funnelling over a longer distance avoids hot spots but the parasitic resistance of the contact (i.e. non-bubble forming) portion of the heater increases, resulting in decreased efficiency.
Many of the currently available thermal inkjet printheads, use an additional low resistivity layer is deposited on top of the resistive thin film to ‘short out’ areas of the heater contacts deposited over non-planar topography. Volumetric heating rate is proportional to resistivity and hence the contacts sections stay relatively cool. The parasitic resistance and waste heat are low, as all but the active element of the heater is shorted by the low resistivity layer. However, the heater and the additional layer need to be coated with an insulating layer to prevent galvanic corrosion. Also, the traditional material for the additional low resistivity layer is aluminium which is prone to corrode if exposed to ink.
Coating with insulating layers increases the thermal mass that must be heated to the superheat limit to form a bubble, so this coating will increase the energy required to jet ink. As such, insulating coatings are contrary to energy efficient droplet ejection and therefore counter to self cooling operation.
Furthermore, patterning the low resistivity layer without damaging the underlying heater material film is difficult. Dry etches are preferred in most semiconductor fabrication facilities, but dry etches with suitable selectivity between the two materials, both likely to contain aluminium, are unlikely to exist. Finding a wet etch that can etch the low resistivity layer without etching the resistive thin film is likely to be easier, but would impose significant constraints on the selection of the heater film material. These selection constraints may be contrary to the goal of self cooling, which requires thin film materials with particular properties, such as very high oxidation resistance.
The technique illustrated in
Referring to
The use of CMP to substantially flatten the passivation layer and the contact elements on the device scale leaves about 0.6 microns thickness variation on the wafer scale. Conventionally, about 0.5 microns of a oxide is deposited which is then capped with 0.5 microns of a nitride, such that conventional devices have about a one micron variation in topography caused by the patterning of the underlying top metal layer of the CMOS leading to the above-discussed non-planar topography which cannot be tolerated by the subsequently formed heaters.
In the illustrated embodiment, current density in the contact elements is minimized by forming the contact openings as a series of lines instead of single, large vias, as is conventional. Current crowding into each heater is also minimized by forming the contact opening lines to be shorter than the heater width, positioned symmetrically about the heater and away from the ends of the heater perpendicular to the longitudinal axis of the heater.
Referring to
Forming each contact element as a series of conductive via lines can lead to the conductive via closest to the heater carrying most of the current and heat. Thus, providing the multiple conductive vias may extend the life of the contact element if the closest conductive via fails due to the extra current and heat load. Referring to
Although the invention is described above with reference to specific embodiments, it will be understood by those skilled in the art that the invention may be embodied in many other forms.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20070153064 | Miyamoto et al. | Jul 2007 | A1 |
| 20080311307 | Bulovic et al. | Dec 2008 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20110096124 A1 | Apr 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61254138 | Oct 2009 | US |
