MEMS bubble generator

A MEMS vapor bubble generator with a chamber for holding liquid and a heater positioned in the chamber for heating the liquid above its bubble nucleation point to form a vapour bubble; wherein,

FIELD OF THE INVENTION

The invention relates to MEMS devices and in particular MEMS devices that vaporize liquid to generate a vapor bubble during operation.

CROSS REFERENCES TO RELATED APPLICATIONS

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 following applications have been filed by the Applicant simutaneously with the present application:

The disclosure of these co-pending applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Some micro-mechanical systems (MEMS) devices process, or use liquids to operate. In one class of these liquid-containing devices, resistive heaters are used to heat the liquid to the liquid's superheat limit, resulting in the formation of a rapidly expanding vapor bubble. The impulse provided by the bubble expansion can be used as a mechanism for moving liquid through the device. This is the case in thermal inkjet printheads where each nozzle has a heater that generates a bubble to eject a drop of ink onto the print media. In light of the widespread use of inkjet printers, the present invention will be described with particular reference to its use in this application. However, it will be appreciated that the invention is not limited to inkjet printheads and is equally suited to other devices in which vapor bubbles formed by resistive heaters are used to move liquid through the device (e.g. some ‘Lab-on-a-chip’ devices).

The resistive heaters in inkjet printheads operate in an extremely harsh environment. They must heat and cool in rapid succession to form bubbles in the ejectable liquid—usually a water soluble ink with a superheat limit of approximately 300° C. Under these conditions of cyclic stress, in the presence of hot ink, water vapor, dissolved oxygen and possibly other corrosive species, the heaters will increase in resistance and ultimately go open circuit via a combination of oxidation and fatigue, accelerated by mechanisms that corrode the heater or its protective oxide layers (chemical corrosion and cavitation corrosion).

To protect against the effects of oxidation, corrosion and cavitation on the heater material, inkjet manufacturers use stacked protective layers, typically made from Si3N4, SiC and Ta. In certain prior art devices, the protective layers are relatively thick. U.S. Pat. No. 6,786,575 to Anderson et al (assigned to Lexmark) for example, has 0.7 μm of protective layers for a ˜0.1 μm thick heater.

To form a vapor bubble in the bubble forming liquid, the surface of the protective layers in contact with the bubble forming liquid must be heated to the superheat limit of the liquid (˜300° C. for water). This requires that the entire thickness of the protective layers be heated to (or in some cases above) the liquid superheat limit. Heating this additional volume decreases the efficiency of the device and significantly increases the level of residual heat present after firing. If this additional heat cannot be removed between successive firings of the nozzle, the ink in the nozzles will boil continuously, causing the nozzles to cease ejecting droplets in the intended manner.

The primary cooling mechanism of printheads on the market is currently thermal conduction, with existing printheads implementing a large heat sink to dissipate heat absorbed from the printhead chip. The ability of this heatsink to cool the liquid in the nozzles is limited by the thermal resistance between the nozzles and the heatsink and by the heat flux generated by the firing nozzles. As the extra energy required to heat the protective layers of a coated heater contributes to an increased heat flux, more severe constraints are imposed on the density of the nozzles on the printhead and the nozzle firing rate. This in turn has an impact on the print resolution, the printhead size, the print speed and the manufacturing costs.

SUMMARY OF THE INVENTION

Accordingly the present invention provides a MEMS vapor bubble generator comprising:a chamber for holding liquid;a heater positioned in the chamber for thermal contact with the liquid; wherein,the heater is formed from a superalloy and configured to receive an actuation signal from associated drive circuitry such that, upon actuation, the heater heats some of the liquid to a temperature above its bubble nucleation point in order to generate a vapor bubble that causes a pressure pulse through the liquid.

Superalloys can offer high temperature strength, corrosion and oxidation resistance far exceeding that of conventional thin film heaters (such as tantalum aluminium, tantalum nitride or hafnium diboride) used in known thermal inkjet printheads. Their suitability in the thermal inkjet realm has, until now, gone unrecognized. The primary advantage of superalloys is that they can provide sufficient strength, oxidation and corrosion resistance to allow heater operation without protective coatings, so that the energy wasted in heating the coatings is removed from the design. As a result, the input energy required to form a bubble with a particular impulse is reduced, lowering the level of residual heat in the printhead. The majority of the remaining heat can be removed via the ejected drops, a mode of operation known as “self cooling”. The primary advantage of this mode of operation is that the design is not reliant on conductive cooling, so a heatsink is not required and the nozzle density and firing rate constraints imposed by conductive cooling are removed, allowing increased print resolution and speed and reduced printhead size and cost.

Optionally, the chamber has a nozzle opening such that the pressure pulse ejects a drop of the liquid through the nozzle opening.

Optionally the chamber has an inlet for fluid communication with a supply of the liquid such that liquid from the supply flows into the chamber to replace the drop of liquid ejected through the nozzle opening.

Optionally the heater is deposited by a sputtering process such that the superalloy has a nanocrystalline microstructure.

Optionally the heater element is deposited as a layer of the superalloy less than 2 microns thick.

Optionally the superalloy has a Cr content between 2% by weight and 35% by weight.

Optionally the superalloy has a Al content of between 0.1% by weight and 8.0% by weight.

Optionally the superalloy has a Mo content of between 1% by weight and 17.0% by weight

Optionally the superalloy has a Nb and/or Ta content totalling between 0.25% by weight and 8.0% by weight.

Optionally the superalloy has a Ti content of between 0.1% by weight and 5.0% by weight.

Optionally the superalloy has up to 5% by weight of reactive metal from the group consisting of yttrium, lanthanum and other rare earth elements

Optionally the superalloy has a Fe content of up to 60% by weight.

Optionally the superalloy has a Ni content of between 25% by weight and 70% by weight.

Optionally the superalloy has a Co content of between 35% by weight and 65% by weight.

Optionally the superalloy is MCrAlX, where M is one or more of Ni, Co, Fe with M contributing at least 50% by weight, Cr contributing between 8% and 35% by weight, Al contributing more than zero but less than 8% by weight, and X contributing less than 25% by weight, with X consisting of zero or more other elements, preferably including but not limited to Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, Hf.

In a second aspect the present invention provides a MEMS device for generating a bubble, the MEMS device comprising:a chamber for holding liquid;a heater positioned in the chamber for thermal contact with the liquid; wherein,the heater has a microstructure with a grain size less than 100 nanometers and configured to received an actuation signal from associated drive circuitry such that upon actuation the heater heats some of the liquid to a temperature above its boiling point in order to generate a vapor bubble that causes a pressure pulse through the liquid.

A grain size less than 100 nm (a “nanocrystalline” microstructure) is beneficial in that it provides good material strength yet has a high density of grain boundaries. Compared to a material with much larger crystals and a lower density of grain boundaries, the nanocrystalline structure provides higher diffusivity for the protective scale forming elements Cr and Al (more rapid formation of the scale) and a more even growth of the scale over the heater surface, so the protection is provided more rapidly and more effectively. The protective scales adhere better to the nanocrystalline structure, which results in reduced spalling. Further improvement in the mechanical stability and adherence of the scale is possible using additives of reactive metal from the group consisting of yttrium, lanthanum and other rare earth elements.

The primary advantage of an oxide scale that passivates the heater is it removes the need for additional protective coatings. This improves efficiency as there is no energy wasted in heating the coatings. As a result, the input energy required to form a bubble with a particular impulse is reduced, lowering the level of residual heat in the printhead. The majority of the remaining heat can be removed via the ejected drops, a mode of operation known as “self cooling”. The primary advantage of this mode of operation is that the design is not reliant on conductive cooling, so a heatsink is not required and the nozzle density and firing rate constraints imposed by conductive cooling are removed, allowing increased print resolution and speed and reduced printhead size and cost.

Optionally, the chamber has a nozzle opening such that the pressure pulse ejects a drop of the liquid through the nozzle opening.

Optionally the chamber has an inlet for fluid communication with a supply of the liquid such that liquid from the supply flows into the chamber to replace the drop of liquid ejected through the nozzle opening.

Optionally the heater is deposited by a super alloy deposited by a sputtering process.

Optionally the heater element is deposited as a layer of the superalloy less than 2 microns thick.

Optionally the superalloy has a Cr content between 2% by weight and 35% by weight.

Optionally the superalloy has a Al content of between 0.1% by weight and 8.0% by weight.

Optionally the superalloy has a Mo content of between 1% by weight and 17.0% by weight

Optionally the superalloy has a Nb and/or Ta content totalling between 0.25% by weight and 8.0% by weight.

Optionally the superalloy has a Ti content of between 0.1% by weight and 5.0% by weight.

Optionally the superalloy has up to 5% by weight of reactive metal from the group consisting of yttrium, lanthanum and other rare earth elements

Optionally the superalloy has a Fe content of up to 60% by weight.

Optionally the superalloy has a Ni content of between 25% by weight and 70% by weight.

Optionally the superalloy has a Co content of between 35% by weight and 65% by weight.

Optionally the superalloy is MCrAlX, where M is one or more of Ni, Co, Fe with M contributing at least 50% by weight, Cr contributing between 8% and 35% by weight, Al contributing more than zero but less than 8% by weight, and X contributing less than 25% by weight, with X consisting of zero or more other elements, preferably including but not limited to Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, Hf.

Optionally the superalloy comprises Ni, Fe, Cr and Al together with additives consisting of zero or more other elements, preferably including but not limited to Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, or Hf.

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 toFIGS. 1 to 4, the unit cell1of a printhead according to an embodiment of the invention comprises a nozzle plate2with nozzles3therein, the nozzles having nozzle rims4, and apertures5extending through the nozzle plate. The nozzle plate2is 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 nozzle3, side walls6on which the nozzle plate is supported, a chamber7defined by the walls and the nozzle plate2, a multi-layer substrate8and an inlet passage9extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element10is suspended within the chamber7, 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, ink11from a reservoir (not shown) enters the chamber7via the inlet passage9, so that the chamber fills to the level as shown inFIG. 1. Thereafter, the heater element10is heated for somewhat less than 1 microsecond (μs), so that the heating is in the form of a thermal pulse. It will be appreciated that the heater element10is in thermal contact with the ink11in the chamber7so that when the element is heated, this causes the generation of vapor bubbles12in the ink. Accordingly, the ink11constitutes a bubble forming liquid.FIG. 1shows the formation of a bubble12approximately 1 μs after generation of the thermal pulse, that is, when the bubble has just nucleated on the heater elements10. It will be appreciated that, as the heat is applied in the form of a pulse, all the energy necessary to generate the bubble12is to be supplied within that short time.

Turning briefly toFIG. 35, there is shown a mask13for forming a heater14(as shown inFIG. 34) of the printhead (which heater includes the element10referred to above), during a lithographic process, as described in more detail below. As the mask13is used to form the heater14, the shapes of several of its parts correspond to the shape of the element10. The mask13therefore provides a useful reference by which to identify various parts of the heater14. The heater14has electrodes15corresponding to the parts designated15.34of the mask13and a heater element10corresponding to the parts designated10.34of the mask. In operation, voltage is applied across the electrodes15to cause current to flow through the element10. The electrodes15are much thicker than the element10so that most of the electrical resistance is provided by the element. Thus, nearly all of the power consumed in operating the heater14is dissipated via the element10, in creating the thermal pulse referred to above.

When the element10is heated as described above, the bubble12forms along the length of the element, this bubble appearing, in the cross-sectional view ofFIG. 1, as four bubble portions, one for each of the element portions shown in cross section.

The bubble12, once generated, causes an increase in pressure within the chamber7, which in turn causes the ejection of a drop16of the ink11through the nozzle3. The rim4assists in directing the drop16as it is ejected, so as to minimize the chance of drop misdirection.

The reason that there is only one nozzle3and chamber7per inlet passage9is so that the pressure wave generated within the chamber, on heating of the element10and forming of a bubble12, does not affect adjacent chambers and their corresponding nozzles. However, it is possible to feed ink to several chambers via a single inlet passage as long as pressure pulse diffusing structures are positioned between chambers. The embodiment shown inFIGS. 37 to 70incorporates these structures for the purpose of reducing cross talk to an acceptable level.

The advantages of the heater element10being suspended rather than embedded in any solid material, are discussed below. However, there are also advantages to bonding the heater element to the internal surfaces of the chamber. These are discussed below with reference toFIGS. 6 to 9.

FIGS. 2 and 3show the unit cell1at two successive later stages of operation of the printhead. It can be seen that the bubble12generates further, and hence grows, with the resultant advancement of ink11through the nozzle3. The shape of the bubble12as it grows, as shown inFIG. 3, is determined by a combination of the inertial dynamics and the surface tension of the ink11. The surface tension tends to minimize the surface area of the bubble12so that, by the time a certain amount of liquid has evaporated, the bubble is essentially disk-shaped.

The increase in pressure within the chamber7not only pushes ink11out through the nozzle3, but also pushes some ink back through the inlet passage9. However, the inlet passage9is approximately 200 to 300 microns in length, and is only about16microns in diameter. Hence there is a substantial inertia and viscous drag limiting back flow. As a result, the predominant effect of the pressure rise in the chamber7is to force ink out through the nozzle3as an ejected drop16, rather than back through the inlet passage9.

Turning now toFIG. 4, the printhead is shown at a still further successive stage of operation, in which the ink drop16that is being ejected is shown during its “necking phase” before the drop breaks off. At this stage, the bubble12has already reached its maximum size and has then begun to collapse towards the point of collapse17, as reflected in more detail inFIG. 5.

The collapsing of the bubble12towards the point of collapse17causes some ink11to be drawn from within the nozzle3(from the sides18of the drop), and some to be drawn from the inlet passage9, towards the point of collapse. Most of the ink11drawn in this manner is drawn from the nozzle3, forming an annular neck19at the base of the drop16prior to its breaking off.

The drop16requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink11is drawn from the nozzle3by the collapse of the bubble12, the diameter of the neck19reduces 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 drop16breaks off, cavitation forces are caused as reflected by the arrows20, as the bubble12collapses to the point of collapse17. It will be noted that there are no solid surfaces in the vicinity of the point of collapse17on which the cavitation can have an effect.

Manufacturing Process for Suspended Heater Element Embodiments

Relevant parts of the manufacturing process of a printhead according to embodiments of the invention are now described with reference toFIGS. 10 to 33.

Referring toFIG. 10, there is shown a cross-section through a silicon substrate portion21, 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 cell1. The description of the manufacturing process that follows will be in relation to a unit cell1, 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. 10represents 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 region22in the substrate portion21, and the completion of standard CMOS interconnect layers23and passivation layer24. Wiring indicated by the dashed lines25electrically interconnects the transistors and other drive circuitry (also not shown) and the heater element corresponding to the nozzle.

Guard rings26are formed in the metallization of the interconnect layers23to prevent ink11from diffusing from the region, designated27, where the nozzle of the unit cell1will be formed, through the substrate portion21to the region containing the wiring25, and corroding the CMOS circuitry disposed in the region designated22.

The first stage after the completion of the CMOS fabrication process consists of etching a portion of the passivation layer24to form the passivation recesses29.

FIG. 12shows the stage of production after the etching of the interconnect layers23, to form an opening30. The opening30is to constitute the ink inlet passage to the chamber that will be formed later in the process.

FIG. 14shows the stage of production after the etching of a hole31in the substrate portion21at a position where the nozzle3is to be formed. Later in the production process, a further hole (indicated by the dashed line32) will be etched from the other side (not shown) of the substrate portion21to join up with the hole31, to complete the inlet passage to the chamber. Thus, the hole32will not have to be etched all the way from the other side of the substrate portion21to the level of the interconnect layers23.

If, instead, the hole32were to be etched all the way to the interconnect layers23, then to avoid the hole32being etched so as to destroy the transistors in the region22, the hole32would have to be etched a greater distance away from that region so as to leave a suitable margin (indicated by the arrow34) for etching inaccuracies. But the etching of the hole31from the top of the substrate portion21, and the resultant shortened depth of the hole32, means that a lesser margin34need be left, and that a substantially higher packing density of nozzles can thus be achieved.

FIG. 15shows the stage of production after a four micron thick layer35of a sacrificial resist has been deposited on the layer24. This layer35fills the hole31and now forms part of the structure of the printhead. The resist layer35is then exposed with certain patterns (as represented by the mask shown inFIG. 16) to form recesses36and a slot37. This provides for the formation of contacts for the electrodes15of the heater element to be formed later in the production process. The slot37will provide, later in the process, for the formation of the nozzle walls6that will define part of the chamber7.

FIG. 21shows the stage of production after the deposition, on the layer35, of a 0.5 micron thick layer38of heater material, which, in the present embodiment, is of titanium aluminium nitride.

FIG. 18shows the stage of production after patterning and etching of the heater layer38to form the heater14, including the heater element10and electrodes15.

FIG. 20shows the stage of production after another sacrificial resist layer39, about 1 micron thick, has been added.

FIG. 22shows the stage of production after a second layer40of heater material has been deposited. In a preferred embodiment, this layer40, like the first heater layer38, is of 0.5 micron thick titanium aluminium nitride.

FIG. 23then shows this second layer40of heater material after it has been etched to form the pattern as shown, indicated by reference numeral41. In this illustration, this patterned layer does not include a heater layer element10, and in this sense has no heater functionality. However, this layer of heater material does assist in reducing the resistance of the electrodes15of the heater14so that, in operation, less energy is consumed by the electrodes which allows greater energy consumption by, and therefore greater effectiveness of, the heater elements10. In the dual heater embodiment illustrated inFIG. 42, the corresponding layer40does contain a heater14.

FIG. 25shows the stage of production after a third layer42, of sacrificial resist, has been deposited. The uppermost level of this layer will constitute the inner surface of the nozzle plate2to be formed later. This is also the inner extent of the ejection aperture5of the nozzle. The height of this layer42must be sufficient to allow for the formation of a bubble12in the region designated43during operation of the printhead. However, the height of layer42determines the mass of ink that the bubble must move in order to eject a droplet. In light of this, the printhead structure of the present invention is designed such that the heater element is much closer to the ejection aperture than in prior art printheads. The mass of ink moved by the bubble is reduced. The generation of a bubble sufficient for the ejection of the desired droplet will require less energy, thereby improving efficiency.

FIG. 27shows the stage of production after the roof layer44has been deposited, that is, the layer which will constitute the nozzle plate2. Instead of being formed from 100 micron thick polyimide film, the nozzle plate2is formed of silicon nitride, just 2 microns thick.

FIG. 28shows the stage of production after the chemical vapor deposition (CVD) of silicon nitride forming the layer44, has been partly etched at the position designated45, so as to form the outside part of the nozzle rim4, this outside part being designated4.1

FIG. 30shows the stage of production after the CVD of silicon nitride has been etched all the way through at46, to complete the formation of the nozzle rim4and to form the ejection aperture5, and after the CVD silicon nitride has been removed at the position designated47where it is not required.

FIG. 32shows the stage of production after a protective layer48of resist has been applied. After this stage, the substrate portion21is 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 hole32. The hole32is etched to a depth such that it meets the hole31.

Then, the sacrificial resist of each of the resist layers35,39,42and48, is removed using oxygen plasma, to form the structure shown inFIG. 34, with walls6and nozzle plate2which together define the chamber7(part of the walls and nozzle plate being shown cut-away). It will be noted that this also serves to remove the resist filling the hole31so that this hole, together with the hole32(not shown inFIG. 34), define a passage extending from the lower side of the substrate portion21to the nozzle3, this passage serving as the ink inlet passage, generally designated9, to the chamber7.

FIG. 36shows the printhead with the nozzle guard and chamber walls removed to clearly illustrate the vertically stacked arrangement of the heater elements10and the electrodes15.

Bonded Heater Element Embodiments

In other embodiments, the heater elements are bonded to the internal walls of the chamber. Bonding the heater to solid surfaces within the chamber allows the etching and deposition fabrication process to be simplified. However, heat conduction to the silicon substrate can reduce the efficiency of the nozzle so that it is no longer ‘self cooling’. Therefore, in embodiments where the heater is bonded to solid surfaces within the chamber, it is necessary to take steps to thermally isolate the heater from the substrate.

One way of improving the thermal isolation between the heater and the substrate is to find a material with better thermal barrier properties than silicon dioxide, which is the traditionally used thermal barrier layer, described in U.S. Pat. No. 4,513,298. The Applicant has shown that the relevant parameter to consider when selecting the barrier layer, is the thermal product; (ρCk)1/2. The energy lost into a solid underlayer in contact with the heater is proportional to the thermal product of the underlayer, a relationship which may be derived by considering the length scale for thermal diffusion and the thermal energy absorbed over that length scale. Given that proportionality, it can be seen that a thermal barrier layer with reduced density and thermal conductivity will absorb less energy from the heater. This aspect of the invention focuses on the use of materials with reduced density and thermal conductivity as thermal barrier layers inserted underneath the heater layer, replacing the traditional silicon dioxide layer. In particular, this aspect of the invention focuses on the use of low-k dielectrics as thermal barriers

Low-k dielectrics have recently been used as the inter-metal dielectric of copper damascene integrated circuit technology. When used as an inter-metal dielectric, the reduced density and in some cases porosity of the low-k dielectrics help reduce the dielectric constant of the inter-metal dielectric, the capacitance between metal lines and the RC delay of the integrated circuit. In the copper damascene application, an undesirable consequence of the reduced dielectric density is poor thermal conductivity, which limits heat flow from the chip. In the thermal barrier application, low thermal conductivity is ideal, as it limits the energy absorbed from the heater.

Two examples of low-k dielectrics suitable for application as thermal barriers are Applied Material's Black Diamond™ and Novellus' Coral™, both of which are CVD deposited SiOCH films. These films have lower density than SiO2(˜1340 kgm−3vs ˜2200 kgm−3) and lower thermal conductivity (˜0.4 Wm−1K−1vs ˜1.46 Wm−1K−1). The thermal products for these materials are thus around 600 Jm−2K−1s−1/2, compared to 1495 Jm−2K−1s−1/2for SiO2i.e. a 60% reduction in thermal product. To calculate the benefit that may be derived by replacing SiO2underlayers with these materials, models using equation 3 in the Detailed Description can be used to show that ˜35% of the energy required to nucleate a bubble is lost by thermal diffusion into the underlayer when SiO2underlayers are used. The benefit of the replacement is therefore 60% of 35% i.e. a 21% reduction in nucleation energy. This benefit has been confirmed by the Applicant by comparing the energy required to nucleate a bubble on1. heaters deposited directly onto SiO2and2. heaters deposited directly onto Black Diamond™.

The latter required 20% less energy for the onset of bubble nucleation, as determined by viewing the bubble formation stroboscopically in an open pool boiling configuration, using water as a test fluid. The open pool boiling was run for over 1 billion actuations, without any shift in nucleation energy or degradation of the bubble, indicating the underlayer is thermally stable up to the superheat limit of the water i.e. ˜300° C. Indeed, such layers can be thermally stable up to 550° C., as described in work related to the use of these films as Cu diffusion barriers (see “Physical and Barrier Properties of Amorphous Silicon-Oxycarbide Deposited by PECVD from Octamethylcycltetrasiloxane”, Journal of The Electrochemical Society, 151 (2004) by Chiu-Chih Chiang et. al.).

Further reduction in thermal conductivity, thermal product and the energy required to nucleate a bubble may be provided by introducing porosity into the dielectric, as has been done by Trikon Technologies, Inc. with their ORION™ 2.2 porous SiOCH film, which has a density of ˜1040 kgm−3and thermal conductivity of ˜0.16 Wm−1K−1(see IST 2000 30043, “Final report on thermal modeling”, from the IST project “Ultra Low K Dielectrics For Damascene Copper Interconnect Schemes”). With a thermal product of ˜334 Jm−2K−1s−1/2, this material would absorb 78% less energy than a SiO2underlayer, resulting in a 78*35% =27% reduction in the energy required to nucleate a bubble. It is possible however that the introduction of porosity may compromise the moisture resistance of the material, which would compromise the thermal properties, since water has a thermal product of 1579 Jm−2K−1s−1/2, close to that of SiO2. A moisture barrier could be introduced between the heater and the thermal barrier, but the heat absorption in this layer would likely degrade overall efficiency: in the preferred embodiment the thermal barrier is directly in contact with the underside of the heater. If it is not in direct contact, the thermal barrier layer is preferably no more than 1 μm away from the heater layer, as it will have little effect otherwise (the length scale for heat diffusion in the ˜1 μs time scale of the heating pulse in e.g. SiO2is ˜1 μm).

An alternative for further lowering thermal conductivity without using porosity is to use the spin-on dielectrics, such as Dow Corning's SiLK™, which has a thermal conductivity of 0.18 Wm−1K−1. The spin-on films can also be made porous, but as with the CVD films, that may compromise moisture resistance. SiLK has thermal stability up to 450° C. One point of concern regarding the spin-on dielectrics is that they generally have large coefficients of thermal expansion (CTEs). Indeed, it seems that reducing k generally increases the CTE. This is implied in “A Study of Current Multilevel Interconnect Technologies for 90 nm Nodes and Beyond”, by Takayuki Ohba, Fujitsu magazine, Volume 38-1, paper 3. SiLK, for example, has a CTE of ˜70 ppm.K−1. This is likely to be much larger than the CTE of the overlying heater material, so large stresses and delamination are likely to result from heating to the ˜300° C. superheat limit of water based ink. SiOCH films, on the other hand, have a reasonably low CTE of ˜10 ppm.K−1, which in the Applicant's devices, matches the CTE of the TiAlN heater material: no delamination of the heater was observed in the Applicant's open pool testing after 1 billion bubble nucleations. Since the heater materials used in the inkjet application are likely to have CTEs around ˜10 ppm.K−1, the CVD deposited films are preferred over the spin-on films.

One final point of interest relating to this application relates to the lateral definition of the thermal barrier. In U.S. Pat. No. 5,861,902 the thermal barrier layer is modified after deposition so that a region of low thermal diffusivity exists immediately underneath the heater, while further out a region of high thermal diffusivity exists. The arrangement is designed to resolve two conflicting requirements:1. that the heater be thermally isolated from the substrate to reduce the energy of ejection and2. that the printhead chip be cooled by thermal conduction out the rear face of the chip.

Such an arrangement is unnecessary in the Applicant's nozzles, which are designed to be self cooling, in the sense that the only heat removal required by the chip is the heat removed by ejected droplets. Formally, ‘self cooled’ or ‘self cooling’ nozzles can be defined to be nozzles in which the energy required to eject a drop of the ejectable liquid is less than the maximum amount of thermal energy that can be removed by the drop, being the energy required to heat a volume of the ejectable fluid equivalent to the drop volume from the temperature at which the fluid enters the printhead to the heterogeneous boiling point of the ejectable fluid. In this case, the steady state temperature of the printhead chip will be less than the heterogenous boiling point of the ejectable fluid, regardless of nozzle density, firing rates or the presence or otherwise of a conductive heatsink. If a nozzle is self cooling, the heat is removed from the front face of the printhead via the ejected droplets, and does not need to be transported to the rear face of the chip. Thus the thermal barrier layer does not need to be patterned to confine it to the region underneath the heaters. This simplifies the processing of the device. In fact, a CVD SiOCH may simply be inserted between the CMOS top layer passivation and the heater layer. This is now discussed below with reference toFIGS. 6 to 9.

Roof Bonded and Floor Bonded Heater Elements

FIGS. 6 to 9schematically show two bonded heater embodiments; inFIGS. 6 and 7the heater10is bonded to the floor of the chamber7, andFIGS. 8 and 9bonded the heater to the roof of the chamber. These figures generally correspond withFIGS. 1 and 2in that they show bubble12nucleation and the early stages of growth. In the interests of brevity, figures corresponding toFIGS. 3 to 5showing continued growth and drop ejection have been omitted.

Referring firstly toFIGS. 6 and 7, the heater element10is bonded to the floor of the ink chamber7. In this case the heater layer38is deposited on the passivation layer24after the etching the passivation recesses29(best shown inFIG. 10), before etching of the ink inlet holes30and31and deposition of the sacrificial layer35(shown inFIGS. 14 and 15). This re-arrangement of the manufacturing sequence prevents the heater material38from being deposited in the holes30and31. In this case the heater layer38lies underneath the sacrificial layer35. This allows the roof layer50to be deposited on the sacrificial layer35, instead of the heater layer38as is the case in the suspended heater embodiments. No other sacrificial layers are required if the heater element10is bonded to the chamber floor, whereas suspended heater embodiments need the deposition and subsequent etching of the second sacrificial layer42above described with reference toFIGS. 25 to 35. To maintain the efficiency of the printhead, a low thermal product layer25can be deposited on the passivation layer24so that it lies between the heater element10and the rest of the substrate8. The thermal product of a material and its ability to thermally isolate the heater element10is discussed above and in greater detail below with reference to equation3. However, in essence it reduces thermal loss into the passivation layer24during the heating pulse.

FIGS. 8 and 9show the heater element10is bonded to the roof of the ink chamber7. In terms of the suspended heater fabrication process described with reference toFIGS. 10 to 36, the heater layer38is deposited on top of the sacrificial layer35, so the manufacturing sequence is unchanged until after the heater layer38is patterned and etched. At that point the roof layer44is then deposited on top of the etched heater layer38, without an intervening sacrificial layer. A low thermal product layer25can be included in the roof layer44so that the heater layer38is in contact with the low thermal product layer, thereby reducing thermal loss into the roof50during the heating pulse.

Bonded Heater Element Manufacturing Process

The unit cells shown inFIGS. 6 to 9are largely schematic and purposely correspond to the unit cells shown inFIGS. 1 to 4where possible so as to highlight the differences between bonded and suspended heater elements.FIGS. 37 to 70show the fabrication steps of a more detailed and complex bonded heater embodiment. In this embodiment, the unit cell21has four nozzles, four heater elements and one ink inlet. This design increases the nozzle packing density by supplying a plurality of nozzle chambers from a single ink inlet, using elliptical nozzle openings, thinner heater elements and staggering the rows of nozzles. The greater nozzle density affords greater print resolution.

FIGS. 37 and 38show the partially complete unit cell1. In the interests of brevity, this description begins at the completion of the standard CMOS fabrication on the wafer8. The CMOS interconnect layers23are four metal layers with interlayer dielectric in between. The topmost metal layer, M4layer50(shown in dotted line) has been patterned to form heater electrode contacts covered by the passivation layer24. M4layer is in fact made up of three layers; a layer if TiN, a layer of Al/Cu (>98% Al) and another layer of TiN which acts as an anti-reflective coating (ARC). The ARC stops light from scattering during subsequent exposure steps. A TiN ARC has a resistivity suitable for the heater materials (discussed below).

The passivation layer may be a single silicon dioxide layer is deposited over the interconnect layers23. Optionally, the passivation layer24can be a silicon nitride layer between two silicon dioxide layers (referred to as an “ONO” stack). The passivation layer24is planarised such that its thickness on the M4layers50is preferably 0.5 microns. The passivation layer separates the CMOS layers from the MEMS structures and is also used as a hard mask for the ink inlet etch described below.

FIGS. 39 and 41show the windows54etched into the passivation layer24using the mask52shown inFIG. 40. As usual, a photoresist layer (not shown) is spun onto passivation layer24. The clear tone mask52—the dark areas indicate where UV light passes through the mask—is exposed and the resist developed in a positive developing solution to remove the exposed photoresist. The passivation layer24is then etched through using an oxide etcher (for example, a Centura DPS (Decoupled Plasma Source) Etcher by Applied Materials). The etch needs to stop on the top, or partly into the TiN ARC layer but not the underlying Al/Cu layer. Then the photoresist layer (not shown) is stripped with O2plasma in a standard CMOS asher.

FIGS. 42 and 43show the deposition of a 0.2 micron layer of heater material56. Suitable heater materials, such as TiAl, TiAlN and Inconel™ 718, are discussed elsewhere in the specification. As shown inFIGS. 44 and 46, the heater material layer56is patterned using the mask58shown inFIG. 45. As with the previous step, a photoresist layer (not shown) is exposed through the mask58and developed. It will be appreciated that mask58is a clear tone mask, in that the clear areas indicate where the underlying material is exposed to UV light and removed with developing solution. Then the unnecessary heater material layer56is etched away leaving only the heaters. Again, the remaining photoresist is ashed with O2plasma.

After this, a layer photoresist42is again spun onto the wafer8as shown inFIG. 47. The dark tone mask60(dark areas block the UV light) shown inFIG. 48, exposes the resist which is then developed and removed to define the position of the ink inlet31on the passivation layer24. As shown inFIG. 49, the removal of the resist42at the site of the ink inlet31exposes the passivation layer24in preparation for the dielectric etch.

FIGS. 50 and 51shows the dielectric etch through the passivation layer24, the CMOS interconnect layers23and into the underlying wafer8. This is a deep reactive ion etch (DRIE) using any standard CMOS etcher (e.g. Applied Materials Centura DPS (Decoupled Plasma Source) Etcher), and extends about 20 microns to 30 microns into the wafer8. In the embodiment shown, the front side ink inlet etch is about 25 microns deep. The accuracy of the front side etch is important as the backside etch (described below) must be deep enough to reach it in order to establish an ink flow path to the nozzle chamber. After the front side etch of the ink inlet31, the photoresist42is ashed away with O2plasma (not shown).

Once the photoresist layer42is removed, another layer of photoresist35is spun onto the wafer as shown inFIGS. 52 and 53. The thickness of this layer is carefully controlled as it forms a scaffold for the subsequent deposition of the chamber roof material (described below). In the present embodiment, the photoresist layer 35 is 8 microns thick (except where it plugs the ink inlet31as best shown inFIG. 53). Next the photoresist layer35is patterned according to the mask62shown inFIG. 55. The mask is a clear tone mask in that the dark areas indicate the areas of exposure to UV light. The exposed photoresist is developed and removed so that the layer35is patterned in accordance withFIG. 54.FIG. 56is a section view of the patterned photoresist layer35.

With the photoresist35defining the chamber roof and support walls, a layer of roof material, such as silicon nitride, is deposited onto the sacrificial scaffolding. In the embodiment shown inFIGS. 57 and 58, the layer of roof material44is 3 microns thick (except at the walls or column features).

FIGS. 59,60and61show the etching of the nozzle rims4. A layer of photoresist (not shown) spun onto the roof layer44and expose under the clear tone mask64(the dark areas are exposed to UV). The roof layer44is then etched to a depth of 2 microns leaving the raised nozzle rims4and the bubble vent feature66. The remaining photoresist is then ashed away.

FIGS. 62,63and64show the nozzle aperture etch through the roof layer44. Again, a layer of photoresist (not shown) is spun onto the roof layer44. It the then patterned with the dark tone mask68(clear areas exposed) and then developed to remove the exposed resist. The underlying SiN layer is then etched with a standard CMOS etcher down to the underlying layer of photoresist35. This forms the nozzle apertures3. The bubble vent hole66is also etched during this step. Again the remaining photoresist is removed with O2plasma.

FIGS. 65 and 66show the application of a protective photoresist overcoat74. This prevents the delicate MEMS structures from being damaged during further handling. Likewise, the scaffold photoresist35is still in place to provide the roof layer44with support.

The wafer8is then turned over so that the ‘backside’70(seeFIG. 67) can be etched. Then the front side of the wafer8(or more specifically, the photoresist overcoat74) is stuck to a glass handle wafer with thermal tape or similar. It will be appreciated that wafers are initially about 750 microns thick. To reduce the thickness, and therefore the depth of etch needed to establish fluid communication between the front and the back of the wafer, the reverse side70of the wafer is ground down until the wafer is about 160 microns thick and then DRIE etched to remove any pitting in the ground surface. The backside is then coated with a photoresist layer (not shown) in preparation for the channel32etching. The clear tone mask72(shown inFIG. 68) is positioned on the back side70for exposure and development. The resist then defines the width of the channel32(about 80 microns in the embodiment shown). The channels32are then etched with a DRIE (Deep Reactive Ion Etch) down to and marginally beyond the plugged front side ink inlets31. The photoresist on the backside72is then ashed away with O2plasma, and the wafer8is again turned over for the front side ashing of the protective overcoat74and the scaffold photoresist35.FIGS. 69 and 70show the completed unit cell1. WhileFIG. 70is a plan view, the features obscured by the roof have been shown in full line for the purposes of illustration.

In use, ink is fed from the backside70into the channel32and into the front side inlet31. Gas bubbles are prone to form in the ink supply lines to the printhead. This is due to outgassing where dissolved gasses come out of solution and collect as bubbles. If the bubbles are fed into the chambers7with the ink, they can prevent ink ejection from the nozzles. The compressible bubbles absorb the pressure generated by the nucleating bubbles on the heater elements10and so the pressure pulse is insufficient to eject ink from the aperture3. As the ink primes the chambers7, any entrained bubbles will tend to follow the columnar features on either side of the ink inlet31and be pushed toward the bubble vent66. Bubble vent66is sized such that the surface tension of the ink will prevent ink leakage, but trapped gas bubbles can vent. Each heater element10is enclosed on three sides by chamber walls and by additional columnar features on the fourth side. These columnar features diffuse the radiating pressure pulse to lower cross-talk between chambers7.

Superalloys are a class of materials developed for use at elevated temperatures. They are usually based on elements from Group VIIA of the Periodic Table and predominantly used in applications requiring high temperature material stability such as jet engines, power station turbines and the like. Their suitability in the thermal inkjet realm has until now gone unrecognized. Superalloys can offer high temperature strength, corrosion and oxidation resistance far exceeding that of conventional thin film heaters (such as tantalum aluminium, tantalum nitride or hafnium diboride) used in known thermal inkjet printheads. The primary advantage of superalloys is that they can have sufficient strength, oxidation and corrosion resistance to allow heater operation without protective coatings, so that the energy wasted in heating the coatings is removed from the design - as discussed in the parent specification U.S. Ser. No. 11/097,308.

Testing has indicated that superalloys can in some cases have far superior lifetimes compared to conventional thin film materials when tested without protective layers.FIG. 71is a Weibull Plot of heater reliability for two different heater materials tested in open pool boiling (the heaters are simply actuated in an open pool of water i.e. not within a nozzle). Skilled artisans will appreciate that Weibull charts are a well recognized measure of heater reliability. The chart plots the probability of failure, or unreliability, against a log scale of the number of actuations. It should be noted that the Key shown inFIG. 71also indicates the number of failed and suspended data points for each alloy. For example, F=8 below Inconel 718 in the key indicates that eight of the heaters used in the test were tested to the point of open circuit failure, while S=1 indicates that one of the test heaters was suspended or in other words, still operating when the test was suspended. The known heater material, TiAlN is compared with the superalloy Inconel 718. The registered trademark Inconel is owned by Huntington Alloys Canada Ltd 2060 Flavelle Boulevard, Mississauga, Ontario L5K 1Z9 Canada.

The applicant's prior work indicates that oxidation resistance is strongly correlated with heater lifetime. Adding Al to TiN to produce TiAlN greatly increased the heater's oxidation resistance (measured by Auger depth profiling of oxygen content after furnace treatment) and also greatly increased heater lifetime. The Al diffused to the surface of the heater and formed a thin oxide scale with a very low diffusivity for further penetration of oxygen. It is this oxide scale which passivates the heater, protecting it from further attack by an oxidative or corrosive environment, permitting operation without protective layers. Sputtered Inconel 718 also exhibits this form of protection and also contains Al, but has two other advantageous properties that further enhance oxidation resistance; the presence of Cr, and a nanocrystalline structure.

Chromium behaves in a similar fashion to aluminium as an additive, in that it provides self passivating properties by forming a protective scale of chromium oxide. The combination of Cr and Al in a material is thought to be better than either in isolation because the alumina scale grows more slowly than the chromia scale, but ultimately provides better protection The Cr addition is beneficial because the chromia scale provides short term protection while the alumina scale is growing, allowing the concentration of Al in the material required for short term protection to be reduced. Reducing the Al concentration is beneficial because high Al concentrations intended for enhanced oxidation protection can jeopardize the phase stability of the material.

X-ray diffraction and electron microscope studies of the sputtered Inconel 718 showed a crystalline microstructure, with a grain size less than 100 nm (a “nanocrystalline” microstructure). The nanocrystalline microstructure of Inconel 718 is beneficial in that it provides good material strength yet has a high density of grain boundaries. Compared to a material with much larger crystals and a lower density of grain boundaries, the nanocrystalline structure provides higher diffusivity for the protective scale forming elements Cr and Al (more rapid formation of the scale) and a more even growth of the scale over the heater surface, so the protection is provided more rapidly and more effectively. The protective scales adhere better to the nanocrystalline structure, which results in reduced spalling. Further improvement in the mechanical stability and adherence of the scale is possible using additives of reactive metal from the group consisting of yttrium, lanthanum and other rare earth elements.

It should be noted that superalloys are typically cast or wrought and this does not yield a nanocrystalline microstructure: the benefits provided by the nanocrystalline structure are specific to the sputtering technique used in the MEMS heater fabrication of this application. It should also be noted that the benefits of superalloys as heater materials are not solely related to oxidation resistance: their microstructure is carefully engineered with additives to encourage the formation of phases that impart high temperature strength and fatigue resistance. Potential additions comprise the addition of aluminium, titanium, niobium, tantalum, hafnium or vanadium to form the gamma prime phase of Ni based superalloys; the addition of iron, cobalt, chrome, tungsten, molybdenum, rhenium or ruthenium to form the gamma phase or the addition of C, Cr, Mo, W, Nb, Ta, Ti to form carbides at the grain boundaries. Zr and B may also be added to strengthen grain boundaries. Controlling these additives, and the material fabrication process, can also act to suppress undesirable age-induced Topologically Close Packed (TCP) phases, such as sigma, eta, mu phases which can cause embrittlement, reducing the mechanical stability and ductility of the material. Such phases are avoided as they may also act to consume elements that would otherwise be available for the favoured gamma and gamma prime phase formation. Thus, while the presence of Cr and Al to provide oxidation protection is preferred for the heater materials, superalloys in general can be considered a superior class of materials from which selection of heater material candidates may be made, since considerably more effort has been put into designing them for high temperature strength, oxidation and corrosion resistance than has been put into improving the conventional thin film heater materials used in MEMS.

The Applicant's results indicate that superalloysa Cr content between 2% by weight and 35% by weight;a Al content of between 0.1% by weight and 8% by weight;a Mo content of between 1% by weight and 17% by weight;a Nb+Ta content of between 0.25% by weight and 8.0% by weight;a Ti content of between 0.1% by weight and 5.0% by weight;a Fe content of up to 60% by weight;a Ni content of between 26% by weight and 70% by weight; and or,a Co content of between 35% by weight and 65% by weight;are likely to be suitable for use as a thin film heater element within a MEMS bubble generator and warrant further testing for efficacy within the specific device design (e.g. suspended heater element, bonded heater element and so on).

Superalloy's having the generic formula MCrAlX where:M is one or more of Ni, Co, Fe with M contributing at least 50% by weight;Cr contributing between 8% and 35% by weight;Al contributing more than zero but less than 8% by weight; and,X contributing less than 25% by weight, with X consisting of zero or more of Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, Hf;provide good results in open pool testing (described above).

Brightray, Ferry and Nimonic are the registered trademarks of Special Metals Wiggin Ltd Holmer Road HEREFORD HR4 9FL UNITED KINGDOM.

Thermo-Span is a registered trademark of CRS holdings Inc., a subsidiary of Carpenter Technology Corporation

The present invention has been described h1erein by way of example only. Ordinary workers in this field will readily recognize many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.