The present application is a nationalization under 35 U.S.C. §371 of, and claims the priority of, PCT/US2009/044570, filed May 19, 2009, entitled “Nanoflat Resistor,” which is incorporated herein by reference in its entirety.
Thermal inkjet technology is widely used for precisely and rapidly dispensing small quantities of fluid. Thermal inkjets eject droplets of fluid out of a nozzle by passing an electrical current through a heating element. The heating element generates heat which vaporizes a small portion of the fluid within a firing chamber. The vapor rapidly expands, forcing a small droplet out of the firing chamber nozzle. The electrical current is then turned off and heating element cools. The vapor bubble rapidly collapses, drawing more fluid into the firing chamber from a reservoir. During printing, this ejection process can repeat thousands of times per second. It is desirable that the heating element be mechanically robust and energy efficient in ejecting droplets.
The accompanying drawings illustrate various embodiments of the principles described herein and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the claims.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
The printhead used in thermal inkjet printing typically includes an array of droplet generators connected to one or more fluid reservoirs. Each of the droplet generators includes a heating element, a firing chamber and a nozzle. Fluid from the reservoir fills the firing chamber. To eject a droplet, an electrical current is passed through a heater element placed adjacent to the firing chamber. The heating element generates heat which vaporizes a small portion of the fluid within the firing chamber. The vapor rapidly expands, forcing a small droplet out of the firing chamber nozzle. The electrical current is then turned off and the resistor cools. The vapor bubble rapidly collapses, drawing more fluid into the firing chamber from a reservoir. During printing, this ejection process can be repeat thousands of times per second.
A minimum energy is usually required to fire ink drops of proper volume from the thermal inkjet printhead. This minimum energy is referred to as the “turn on energy”. The turn on energy must be sufficient to locally superheat the fluid to achieve reliable and repeatable vaporization. Undesirable thermal losses from the heating element lead to higher turn on energies and lower efficiency in converting the electrical pulses into mechanical forces which eject the droplet.
The mechanical robustness of the heating element is another design consideration. The heating elements are subjected to high frequency forces as a result of the vapor expansion and subsequent cavitation which occurs with each droplet ejection. These forces can result in surface erosion and failure of the heating elements. When a heating element fails, no droplets can be ejected from the firing chamber and the overall printing quality of the thermal inkjet printhead suffers.
The present specification relates to a flat heating element above nano-porous anodized alumina. This resistor design has been dubbed a “nanoflat resistor.” According to one illustrative embodiment, the nanoporous anodized alumina increases the thermal isolation of the resistive heating element. This decreases the turn on energy of the nanoflat resistor and increases the energy efficiency. The flat topography of the nanoflat resistor eliminates shoulders or other discontinuities which can be susceptible cavitation induced damage and failure. Consequently, the thermal inkjet devices which incorporate nanoflat resistors may achieve higher energy efficiency and greater reliability.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems and methods may be practiced without these specific details. Reference in the specification to “an embodiment,” “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least that one embodiment, but not necessarily in other embodiments. The various instances of the phrase “in one embodiment” or similar phrases in various places in the specification are not necessarily all referring to the same embodiment.
The energy efficiency and ejection frequency of the droplet generator (100) is at least partially determined by the efficiency of the heating element (120) in converting electrical energy into mechanical force which ejects the droplet (135). A number of energy losses can occur, including the transmission of heat (140) from the heating element upward into the body of the thermal inkjet printhead. This heat is not converted into useful energy and is lost. This lost heat can dissipate into other components within the thermal inkjet and undesirably alter their temperatures.
Lowering the amount of lost heat can make it easier to maintain the thermal inkjet printhead at a substantially isothermal state and reduce undesirable changes in the printing performance of the printhead. By increasing the proportion of the heat which passes into the fluid, less electrical current is required to fire a droplet. This increases the efficiency of the individual firing chamber (110) and reduces overall amount of heat produced by the droplet generator (100).
As shown in
A plurality of droplet generators (100) may be contained within a single inkjet die. The droplet ejection cycle described above can occur thousands of times in a second. This high frequency expansion and collapse of vapor bubble in proximity to the heating element (120) can subject it to significant mechanical stress. Particularly, the expansion and collapse of the vapor bubble can produce a shockwave which is transmitted through the liquid to the heating element. Over the design lifetime of the droplet generator (100) it can be expected eject tens of billions of droplets. Failure of the heating element (120) due to mechanical stress of repeated high frequency shock waves results in the failure of the droplet generator, with a subsequent loss of overall printing quality of the thermal inkjet printhead. Consequently, it is desirable that the heating element be mechanically robust to increase its lifetime.
However, the beveled ends of the electrodes (220, 230) create shoulders which protrude into the firing chamber (110,
A number of additional overcoat layers can be formed over the WSiN resistor layer (250) to provide additional structural stability and electrically insulate fluid in the firing chamber from the resistor layer (250). In this embodiment, a silicon nitride/silicon carbide overcoat (260) and a tantalum overcoat (270) are deposited over the resistor layer (250). As discussed above, the shoulders (225) can be more susceptible to cavitation damage (227) or other surface erosion. The additional layers (260, 270) are specifically designed to protect the underlying resistor layer (250) from mechanical and other damage. However, due to the beveled topography the additional layers (260, 270) may be weaker in the shoulder regions. For example, tantalum overcoat is susceptible to failure under the impact of bubble collapse in the shoulder region (225). This is related to structural properties of sputter deposited tantalum, and the line-of-sight nature of the sputtering process. The sloped edges of aluminum terminations are almost 45 degree from the normal to the substrate, creating a considerable degree of shadowing among the columnar grains of tantalum as they grow away from the substrate. This promotes inter-granular porosity and weak bonds among the tantalum grains which are susceptible to stresses exerted during bubble collapse. Also, the tantalum layer is almost 30% thinner in these areas. This is because of the almost 45 degree topography in these areas. Since resistor life is proportional to the thickness of Ta, this adversely impacts the reliability of the TIJ device.
Thicker overcoat layers could increase the reliability of the device. However, the additional layers (260, 270) separate the resistor layer (250) from the fluid in the firing chamber and reduce the efficiency and firing frequency in proportion to their thickness.
In the embodiment illustrated in
Throughout the specification and appended claims, the term “nanoflat resistor” refers to a resistive material which is substantially planar, a portion of which overlies a thermally and electrically insulating substrate. According to one illustrative embodiment, a nanoflat resistor includes a nanoporous anodized alumina layer and an overlying planar resistor layer.
A voltage is applied across the aluminum electrodes (360, 370). In
The reduction of thermal energy stored under the resistive layer (350) allows for faster thermal recovery and cool down between firings. More rapid cool down can significantly increase the frequency at which the droplet generator can operate and increase the printing speed of the thermal ink jet device.
Additionally, the nanoflat resistor (390) has a substantially planar surface which can be more robust than resistor configurations with discontinuities such as shoulders or beveled geometries. The planar surface of the nanoflat resistor (390) can be more robustly constructed and more uniformly distributes stresses from vapor bubble expansion and collapsing. This can increase the lifetime of the resistor and the thermal inkjet print head. In some embodiments, the number or thickness of protective overcoats can be reduced, which can increase the thermal efficiency and firing frequency of the droplet generator.
The figures are not drawn to scale and are not representative of the thickness of layers or relative thickness of layers. Further, the figures are not meant to be an accurate representation of all the layers used to form a thermal ink jet printhead. For example, one or more layers which protect against cavitation damage may be present.
As mentioned above, the relative dimensions in the figure are not necessarily to scale. The thickness of each layer will have various effects on the efficiency of the nanoflat resistor. For example, the thickness of the resistor layer (440) will determine the exact resistivity of the resistor. The thickness of the aluminum layer (425) will determine how well the aluminum will conduct electrical current. The thickness of overlying layers may be determined by balancing any increase in the life of the nanoflat resistor against the thermal resistance the overlying layers introduce between the resistor layer (440) and the fluid in the firing chamber.
According to one illustrative embodiment, the anodization of a thermal inkjet die may be performed using a 2% oxalic acid solution at room temperature and applying 30 volts across the electrolytic solution, with the aluminum serving as the cathode.
The exact diameter of the nano-pores (604) may depend on the type of electrolytic solution, applied voltage, current density, temperature, and other factors. The more porous the anodized aluminum (600) is, the lower its thermal conductivity will be, thus increasing the thermal isolation of the resistor layer and lowering the amount of energy which is required to propel a droplet of ink onto a substrate. Further, by making the anodized aluminum more porous, its heat capacity is decreased, which leads to more rapid droplet ejection cycles.
According to one illustrative embodiment, the heat capacity and the thermal conductivity of the nanoporous alumina (608) can be further lowered by enlarging the pore diameters.
According to one illustrative embodiment, a wet etchant such as phosphoric acid can be used to increase the pore diameters.
As used in the specification and appended claims, the term “turn on energy” refers to the minimum amount of electrical energy applied to a nanoflat resistor or other heating element that produces an ink droplet of a predetermined size. The vertical axis of graph shows turn on energy in micro-Joules. The horizontal axis of the graph shows the porosity of the nanoporous alumina, with a porosity of 0% indicating an alumina layer without pores and a porosity of 100% indication an air space under the nanoflat resistor.
Two horizontal dashed lines show the Turn On Energy (TOE) for various alternative heating element configurations. The upper dashed line, labeled “STD, TOE=0.494 μJ” indicates that the turn on energy for a standard configuration, such as that illustrated in
As can be seen from the graph in
A layer of aluminum is then deposited and appropriately patterned (step 905). The layer of aluminum can be pure aluminum or aluminum alloys. For example, a small amount of copper may be included in the aluminum to make the metal better suited to conduct an electrical current. According to one illustrative embodiment, a continuous planar layer of aluminum extends under the area where the nanoflat resistor will be formed. The mask is then applied and patterned (step 910) to expose one or more portions of the aluminum layer. The exposed portions of the aluminum layer are then anodized (step 915) as described above. According to one illustrative embodiment, the aluminum is anodized to create a nanoporous structure which extends through the thickness of the aluminum layer. This is to prevent current from leaking through the aluminum as opposed to flowing through the resistor material. The anodizing process may slightly increase the thickness of the anodized aluminum relative to the non anodized aluminum. This change in thickness is typically small and gradual.
The nanoporous structure may then be wet etched as described above to enlarge the pore diameters of the nanoporous structure (step 920). Various parameters can be controlled during the wet etching process to obtain the nanoporous structure. For example, the composition of the etchant solution, the time, temperature, and other factors may be controlled. In some circumstances, the wet etching process may be omitted and the anodized nanoporous structure may be used without pore enlargement.
The mask is removed (step 925) to expose two aluminum electrodes which are separated by the anodized nanoporous section. A layer of resistive material may then be deposited over the aluminum to form a nanoflat resistor (step 930). According to one illustrative embodiment, the resistive material is sputtered onto the underlying layers. As mentioned above, the anodizing process may slightly increase the thickness of the anodized alumina relative to the non anodized aluminum. This increase in height can be naturally compensated during the deposition of the resistor layer. During deposition, the resistor material extends a short distance into the nanopores. This naturally reduces the thickness of the resistor layer to compensate for the increased height of the anodized alumina and produces a smooth monolithic surface resistor surface. According to one illustrative embodiment, the pore sizes may be selected to produce this natural compensation for the increased height of the anodized alumina.
In optional steps, the surface may be planarized or a capping layer can be formed over the nanoporous section prior to the deposition of the resistive layer. The capping layer may serve as a sealant which closes the nanopores before the resistive material layer is in place. According to one illustrative embodiment, the capping layer may be used with larger pore sizes. This can help protect the nanopores from any unwanted material getting inside and reducing the effectiveness of the pores. As mentioned above, the sealant step may be skipped and the resistive material can serve as a sealant.
By way of example and not limitation, the resistive material may be tungsten silicon nitride. Additional insulating and/or protecting layers may then be deposited over the nanoflat resistor (step 935). For example, these insulating/protective layers may include silicon nitride, silicon carbide, tantalum, other materials, or combinations thereof.
An additional advantage to the fabrication of a heating resistor embodying principles described in this specification is that many of the steps are similar to the fabrication of traditional dry etch heating resistors. According to one illustrative embodiment, the anodization process can be substituted for the dry etching process, with the remainder of the steps remaining the same. Thus the cost to implement manufacturing of nanoflat resistors is minimized.
In sum, to increase the performance of a thermal inkjet device heating resistor, two main factors are considered. First, the efficiency at which the resistor transfers electrical energy into thermal energy, and second, the reliability of the resistor. The efficiency at which energy is transferred can be accomplished by reducing the heat capacity of the material underneath the resistor. The heat capacity can be reduced by making the material more porous. The aluminum underneath the resistor can be made porous through anodizing. This decreases the turn on energy of the droplet generator and increases the frequency at which the droplet generator can operate. The life of the nanoflat resistor is extended by the flat monolithic topography of the resistor layer.
The preceding description has been presented only to illustrate and describe embodiments and examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2009/044570 | 5/19/2009 | WO | 00 | 11/18/2011 |
Publishing Document | Publishing Date | Country | Kind |
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WO2010/134910 | 11/25/2010 | WO | A |
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Number | Date | Country | |
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20120062355 A1 | Mar 2012 | US |