Resistor for a fluid-jet printhead and method of its fabrication

Abstract: A fluid-jet printhead (200) has a substrate (10) having at least one layer (82) defining a fluid chamber (100) for ejecting fluid. The printhead (200) also includes a resistive layer (30) disposed between the fluid chamber (100) and the substrate (10) wherein the fluid chamber (100) has a smooth planer surface between the fluid chamber (100) and the substrate (10). The printhead (200) has a conductive layer (40) disposed between the resistive layer (30) and the substrate (10) wherein the conductive layer (40) and the resistive layer (30) are in direct parallel contact. The conductive layer (40) forms at least one void creating a planar resistor (46) in the resistive layer (30). The planar resistor (46) is aligned with the fluid chamber (100).

Description: The Field of the Invention This invention relates to printheads used in fluid jet printers, and more specifically to a fluid-jet printhead used in a fluid-jet print cartridge having improved dimensional control and improved step coverage. Background of the Invention One type of fluid-jet printing system uses a piezoelectric transducer to produce a pressure pulse that expels a droplet of fluid from a nozzle. A second type of fluid-jet printing system uses thermal energy to produce a vapor bubble in a fluid-filled chamber that expels a droplet of fluid. The second type is referred to as thermal fluid-jet or bubble jet printing systems. Conventional thermal fluid-jet printers include a print cartridge in which small droplets of fluid are formed and ejected towards a printing medium. Such print cartridges include fluid-jet printheads with orifice structures having very small nozzles through which the fluid droplets are ejected. Adjacent to the nozzles inside the fluid-jet printhead are fluid chambers, where fluid is stored prior to ejection. Fluid is delivered to fluid chambers through fluid channels that are in fluid communication with a fluid supply. The fluid supply may be, for example, contained in a reservoir part of the print cartridge. Ejection of a fluid droplet, such as ink, through a nozzle may be accomplished by quickly heating a volume of fluid within the adjacent fluid chamber. The rapid expansion of fluid vapor forces a drop of fluid through the nozzle in the orifice structure. This process is commonly known as "firing." The fluid in the chamber may be heated with a transducer, such as a resistor, that is disposed and aligned adjacent to the nozzle. In conventional thermal fluid-jet printhead devices, such as ink-jet cartridges, thin film resistors are used as heating elements. In such thin film devices, the resistive heating material is typically deposited on a thermally and electrically insulating substrate. A conductive layer is then deposited over the resistive material. The individual heater element (i.e., resistor) is dimensionally defined by conductive trace patterns that are lithographically formed through numerous steps including conventionally masking, ultraviolet exposure, and etching techniques on the conductive and resistive layers. More specifically, the critical width dimension of an individual resistor is controlled by a dry etch process. For example, an ion assisted plasma etch process is used to etch portions of the conductive and resistive layers not protected by a photoresist mask. The width of the remaining conductive thin film stack (of conductive and resistive layers) defines the final width of the resistor. The resistive width is defined as the width of the exposed resistive perpendicular to the direction of current flow. Conversely, the critical length dimension of an individual resistor is controlled by a subsequent wet etch process. A wet etch process is used to produce a resistor having sloped walls on the conductive layer defining the resistor length. The sloped walls of the conductive layer permit step coverage of later fabricated layers. As discussed above, conventional thermal fluid-jet printhead devices require both dry etch and wet etch processes. The dry etch process determines the width dimension of an individual resistor, while the wet etch process defines both the length dimension and the necessary sloped walls commencing from the individual resistor. As is well known in the art, each process requires numerous steps, thereby increasing both the time to manufacture a printhead device and the cost of manufacturing a printhead device. One or more passivation and cavitation layers are fabricated in a stepped fashion over the conductive and resistive layers and then selectively removed to create a via for electrical connection of a second conductive layer to the conductive traces. The second conductive layer is pattered to define a discrete conductive path from each trace to an exposed bonding pad remote from the resistor. The bonding pad facilitates connection with electrical contacts on the print cartridge. Activation signals are provided from the printer to the resistor via the electrical contacts. The printhead substructure is overlaid with at least one orifice layer. Preferably, the at least one orifice layer is etched to define the shape of the desired firing fluid chamber within the at least one orifice layer. The fluid chamber is situated above, and aligned with, the resistor. The at least one orifice layer is preferably formed with a polymer coating or optionally made of an fluid barrier layer and an orifice plate. Other methods of forming the orifice layer(s) are know to those skilled in the art. U.S. 5 943 076 discloses a printhead for thermal ink jet devices and a method for forming the printhead. In direct drive thermal fluid-jet printer designs, the thin film device is selectively driven by electronics preferably integrated within the integrated circuit part of the printhead substructure. The integrated circuit conducts electrical signals directly from the printer microprocessor to the resistor through conductive layers. The resistor increases in temperature and creates super-heated fluid bubbles for ejection of the fluid from the fluid chamber through the nozzle. However, conventional thermal fluid-jet printhead devices can suffer from inconsistent and unreliable fluid drop sizes and inconsistent turn on energy required to fire a fluid droplet, if the resistor dimensions are not tightly controlled. Further, the stepped regions within the fluid chamber can affect drop trajectory and device reliability. The device reliability is affected by the bubble collapsing after the drop ejection thereby wearing down the stepped regions. It is desirous to fabricate a fluid-jet printhead capable of producing fluid droplets having consistent and reliable fluid drop sizes. In addition, it is desirous to fabricate a fluid-jet printhead having a consistent turn on energy (TOE) required to fire a fluid droplet, thereby providing greater control of the size of the fluid drops. Summary of the Invention The present invention provides a method for creating a planar resistor, comprising: depositing an insulator layer on a substrate surface; depositing dielectric material on the insulator layer; patterning the dielectric material to create an area to form the planar resistor; etching the patterned dielectric material to define the length of the planar resistor; depositing a conductive layer on the insulator layer to abut the etched dielectric material defining the length of the planar resistor; planarizing the conductive layer to expose the dielectric material; depositing a resistive layer on the conductive layer and the exposed dielectric material; patterning the resistive layer to define the width of the planar resistor; and etching the resistive layer to form the width of the planar resistor. The present invention provides numerous advantages over conventional methods of forming thin film printheads. First, the present invention provides a method of forming a structure capable of firing a fluid droplet in a direction substantially perpendicular (normal or orthogonal) to a plane defined by the resistive element and ejection surface of the printhead. Second, the dimensions and planarity of the resistive material layer are more precisely controlled, which reduces the variation in the turn on energy required to fire a fluid droplet. Third, the size of a fluid droplet is better controlled due to less variation in resistor size. Fourth, the corrosion resistance, surface texture, and electro-migration resistance of the conductive layers are improved inherently by the design. Brief Description of the Drawings Figure 1 is an enlarged, cross-sectional, partial view illustrating a conventional thin film printhead substructure. Figure 2 is a flow chart of an exemplary process used to implement the conventional thin film printhead structure. Figure 3A is an enlarged, cross-sectional, partial view illustrating a thin film printhead substructure formed using the method of the present invention. Figure 3B is an overhead view of the resistor element. Figure 4 is a flowchart of a process according to the present invention used to fabricate a thin-film printhead structure. Figure 5 is a perspective view of a printhead fabricated in accordance with the invention. Figure 6 is an exemplary print cartridge that integrates and uses the printhead of Figure 5. Figure 7 is an exemplary recording device, a printer, which uses the print cartridge of Figure 6. Description of the Preferred Embodiments In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. The present invention is a method of fabricating a planar resistor. The present invention provides numerous advantages over the conventional methods of fabricating planar resistors for fluid-jet or ink-jet printheads. First, the present invention provides a method of forming a structure capable of firing a fluid droplet in a direction substantially perpendicular (normal or orthogonal) to a plane defined by the resistive element and ejection surface of the printhead. Second, the dimensions and planarity of the resistive layer are more precisely controlled, which reduces the variation in the turn on energy required to fire a fluid droplet. Third, the size of a fluid droplet is better controlled due to less variation in resistor size. Fourth, the design inherently provides for improved corrosion resistance, improved electro-migration resistance of the conductive layers and a smoother resistor surface. Figure 1 is an enlarged, cross-sectional, partial view illustrating a conventional thin film printhead 190. The thicknesses of the individual thin film layers are not drawn to scale and are drawn for illustrative purposes only. As shown in Figure 1, thin film printhead 190 has affixed to it a fluid barrier layer 70, which is shaped along with orifice plate 80 to define fluid chamber 100 to create an orifice layer 82 (see Fig.5). Optionally, the orifice layer 82 and fluid barrier layers 70 may be made of one or more layers of polymer material. A fluid droplet within a fluid chamber 100 is rapidly heated and fired through nozzle 90 when the printhead is used. Thin film printhead substructure 190 includes substrate 10, an insulating insulator layer 20, a resistive layer 30, a conductive layer 40 (including conductors 42A and 42B), a passivation layer 50, a cavitation layer 60, and a fluid barrier structure 70 defining fluid chamber 100 with orifice plate 80. As diagrammed in Figure 2, a relatively thick insulator layer 20 (also referred to as an insulative dielectric) is applied to substrate 10 in step 110 preferably by deposition. Silicon dioxides are examples of materials that are used to fabricate insulator layer 20. Preferably, insulator layer 20 is formed from tetraethylorthosilicate (TEOS) oxide having a 14,000 Angstrom thickness. In one alternative embodiment, insulative layer 20 is fabricated from silicon dioxide. In another embodiment, it is formed of silicon nitride. There are numerous ways to fabricate insulation layer 20, such as through a plasma enhanced chemical vapor deposition (PECVD), or a thermal oxide process. Insulator layer 20 serves as both a thermal and electrical insulator for the resistive circuit that will be built on its surface. The thickness of the insulator layer can be adjusted to vary the heat transferring or isolating capabilities of the layer depending on a desired turn-on energy and firing frequency. Next in step 112, the resistive layer 30 is applied to uniformly cover the surface of insulation layer 20. Preferably, the resistive layer is tantalum silicon nitride or tungsten silicon nitride of a 1200 Angstrom thickness although tantalum aluminum can also be used. Next in step 114, conductive layer 40 is applied over the surface of resistive layer 30. In conventional structures, conductive layer 40 is formed with preferably aluminum copper or alternatively with tantalum aluminum or aluminum gold. Additionally, a metal used to form conductive layer 40 may also be doped or combined with materials such as copper, gold, or silicon or combinations thereof. A preferable thickness for the conductive layer 40 is 5000 Angstroms. Resistive layer 30 and conductive layer 40 can be fabricated though various techniques, such as through a physical vapor deposition (PVD). In step 116, the conductive layer 40 is patterned with a photoresist mask to define the resistor's width dimension. Then in step 118, conductive layer 40 is etched to define conductors 42A and 42B. Fabrication of conductors 42A and 42B define the critical length and width dimensions of the active region of resistive layer 30. More specifically, the critical width dimension of the active region of resistive layer 30 is controlled by a dry etch process. For example, an ion assisted plasma etch process is used to vertically etch portions of conductive layer 40 which are not protected by a photoresist mask, thereby defining a maximum resistor width as being equal to the width of conductors 42A and 42B. In step 120, the conductor layer is patterned with photoresist to define the resistor's length dimension defined as the distance between conductors 42A and 42B. In step 122, the critical length dimension of the active region of resistive layer 30 is controlled by a wet etch process. A wet etch process is used since it is desirable to produce conductors 42A and 42B having sloped walls, thereby defining the resistor length. Sloped walls of conductive layer 42A enables step coverage of later fabricated layers such as a passivation layer that is applied in step 124. Conductors 42A and 42B serve as the conductive traces that deliver a signal to the active region of resistive layer 30 for firing a fluid droplets Thus, the conductive trace or path for an electrical signal impulse that heats the active region of resistive layer 30 is from conductor 42A through the active region of resistive layer 30 to conductor 42B. In step 124, passivation layer 50 is then applied uniformly over the device. There are numerous passivation layer designs incorporating various compositions. In one conventional embodiment, two passivation layers, rather than a single passivation layer are applied. In the conventional printhead example of Fig. 1, the two passivation layers comprise a layer of silicon nitride followed by a layer of silicon carbide. More specifically, the silicon nitride layer is deposited on conductive layer 40 and resistive layer 30 and then a silicon carbide is preferably deposited. With this design, electromigration of the conductive layer can intrude into the passivation layer. After passivation layer 50 is deposited, cavitation barrier 60 is applied. In the conventional example, the cavitation barrier comprises tantalum. A sputtering process, such as a physical vapor deposition (PVD) or other techniques known in the art deposits the tantalum. Fluid barrier layer 70 and orifice layer 80 are then applied to the structure, thereby defining fluid chamber 100. In one embodiment, fluid barrier layer 70 is fabricated from a photosensitive polymer and orifice layer 80 is fabricated from plated metal or organic polymers. Fluid chamber 100 is shown as a substantially rectangular or square configuration in Figure 1. However, it is understood that fluid chamber 100 may include other geometric configurations without varying from the present invention. Thin film printhead 190, shown in Figure 1, illustrates one example of a typical conventional printhead. However, printhead 190 requires both a wet and a dry etch process in order to define the functional length and width of the active region of resistive layer 30, as well as to create the sloped walls of conductive layer 40 necessary for adequate step coverage of the later fabricated layers, such as the passivation 50 and cavitation 60 layers. Figure 3 is an enlarged, cross-sectional, partial view illustrating the layers for fluid-jet printhead 200 fabricated using the method of the present invention. The thicknesses of the individual thin film layers are not drawn to scale and are drawn for illustrative purposes only. Figure 5 is an enlarged, plan view illustrating a fluid-jet printhead 200 fabricated using the method of the present invention. As shown in Figure 4 in step 110, insulative layer 20 is fabricated by being deposited through any known means, such as a plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), or a thermal oxide process onto substrate 10. Preferably, insulator layer 20 is formed from tetraethylorthosilicate (TEOS) oxide of a thickness of 9000 Angstroms. In one alternative embodiment, insulative layer 20 is fabricated from silicon dioxide. In another embodiment, it is formed of silicon nitride. In step 126, a dielectric material 44 is deposited onto the insulator layer. This dielectric material 44 is then patterned in step 128 to create a resistor area, and then dry etched in step 130 to form thin-film layers which define the resistor's length dimension L. In one preferred embodiment, dielectric material 44 is formed from silicon nitride of approximately 5000 Angstroms of thickness. In an alternative embodiment dielectric material 44 is fabricated from silicon dioxide or silicon carbide. In step 114, conductive material layer 40 is then fabricated on top of insulative layer 20 and abuts the etched dielectric material 44 to form the resistor length L. In one embodiment, conductive material layer 40 is a layer formed through a physical vapor deposition (PVD) from aluminum and copper of approximately 5000 Angstrom of thickness. More specifically, in one embodiment, conductive material layer 40 includes up to approximately two percent copper in aluminum, preferably approximately 0.5 percent copper in aluminum. Utilizing a small percent of copper in aluminum limits electro-migration. In another preferred embodiment, conductive material layer 40 is formed from titanium, copper, or tungsten. In step 132, a photoimagable masking material such as photoresist is deposited on portions of conductive layer 40, thereby exposing other portions of conductive layer 40. In step 134, the top surface of conductive layer 40 is then planarized such that the top surface of dielectric material 44 is level with the top surface of conductive layer 40. In one preferred embodiment, the top surface of conductive layer 40 is planarized through use of a resist-etch-back (REB) process. In another embodiment, the top surface of conductive layer 40 is planarized through use of a chemical/mechanical polish (CMP) process. Next in step 112, the resistive layer 30 is applied to uniformly cover the surface of the entire surface of substrate 10 and previously applied layers (wafer surface). Preferably, the resistive layer 30 is tungsten silicon nitride of a 1200 Angstrom thickness although tantalum aluminum, tantalum, or tantalum silicon nitride can also be used In step 116, a photoimagable masking material is deposited on the previously applied layers on the substrate surface. The photoimagable masking material is removed where the combined resistive layer 30 and conductive layer 60 are to be etched to define respectively the resistor width W and conductors 42A and 42B. In step 136, the exposed portions of resistive layer 30 and conductive layer 40 are removed through a dry etch process, several of which are known to those skilled in the art such as described in step 118 of Fig. 2. This etching step defines and forms the resistor width. The photoresist mask is then removed, thereby exposing an exemplary substantially rectangular-shaped conductors 42A and 42B. The passivation 50, cavitation 60, barrier 70 and orifice 80 layers are then applied as described for the conventional printhead. Conductors 42A and 42B provide an electrical connection/path between external circuitry and the formed resistive element. Therefore, conductors 42A and 42B transmit energy to the formed resistor to create heat capable of firing a fluid droplet positioned on a top surface of the formed resistive element in a direction perpendicular to the top surface of the resistive element. As shown in Figure 3B, conductors 42A and 42B define a resistor element 46 between conductors 42A and 42B. Resistive element 46 has a length L equal to the distance between conductors 42A and 42B. Resistive element 46 has a width W. However, it is understood that resistive element 46 may be fabricated having any one of a variety of configurations, shapes, or sizes, such as a thin trace or a wide trace of conductors 42A and 42B. The only requirement of the resistive element 46 is that it contacts conductors 42A and 42B to ensure a proper electrical connection. While the actual length L of resistive element 46 is equal to or greater than the distance between the outer most edges of conductors 42A and 42B, the active portion of resistive element 46 which conducts heat to a droplet of fluid positioned above resistive element 46 corresponds to the distance between the outermost edges of conductors 42A and 42B. In Figure 5, each orifice nozzle 90 is in fluid communication with respective fluid chambers 100 (shown enlarged in Figure 2) defined in printhead 200. Each fluid chamber 100 is constructed in orifice structure 82 adjacent to thin film structure 32 that preferably includes a transistor coupled to the resistive component. The resistive component is selectively driven (heated) with sufficient electrical current to instantly vaporize some of the fluid in fluid chamber 100, thereby forcing a fluid droplet through nozzle 90. Exemplary fluid-jet print cartridge 220 is illustrated in Figure 6. The fluid-jet printhead device made using the method of the present invention is a portion of fluid-jet print cartridge 220. Fluid-jet print cartridge 220 includes body 218, flexible circuit 212 having circuit pads 214, and printhead 200 having orifice nozzles 90. Fluid-jet print cartridge 220 has fluid-jet printhead 200 in fluidic connection to fluid in body 218 using a fluid delivery system 216, shown as a sponge to provide backpressure using capillary action in the sponge (preferably closed-cell foam) to prevent leakage of fluid though orifice nozzles 90 when not in use. While flexible circuit 212 is shown in Figure 6, it is understood that other electrical circuits known in the art may be utilized in place of flexible circuit 212 without deviating from the present invention. It is only necessary that electrical contacts 214 be in electrical connection with the circuitry of fluid-jet print cartridge 220. Printhead 200 having orifice nozzles 90 is attached to the body 218 and controlled for ejection of fluid droplets, typically by a printer but other recording devices such as plotters, and fax machines, too name a couple, can be used. Thermal fluid-jet print cartridge 220 includes orifice nozzles 90 through which fluid is expelled in a controlled pattern during printing. Conductive drivelines for each resistor component are carried upon flexible circuit 212 mounted to the exterior of print cartridge body 218. Circuit contact pads 214 (shown enlarged in Figure 6 for illustration) at the ends of the resistor drive lines engage similar pads carried on a matching circuit attached to a printer (not shown). A signal for firing the transistor is generated by a microprocessor and associated drivers on the printer that apply the signal to the drivelines. Fig. 7 is an exemplary recording device, a printer 240, which uses the exemplary fluid-jet print cartridge 220 of Fig. 6. The fluid-jet print cartridge 220 is placed in a carriage mechanism 254 to transport the fluid-jet print cartridge 220 across a first direction of medium 256. A medium feed mechanism 252 transports the medium 256 in a second direction across fluid-jet printhead 220. Medium feed mechanism 252 and carriage mechanism 254 form a transport mechanism to move the fluid-jet print cartridge 220 across the first and second directions of medium 256. An optional medium tray 250 is used to hold multiple sets of medium 256. After the medium is recorded by fluid-jet print cartridge 220 using fluid-jet printhead 200 to eject fluid onto medium 256, the medium 256 is optionally placed on media tray 258. In operation, a droplet of fluid is positioned within fluid chamber 100. Electrical current is supplied to resistive element 46 via conductors 42A and 42B such that resistive element 46 rapidly generates energy in the form of heat. The heat from resistive element 46 is transferred to a droplet of fluid within fluid chamber 100 until the droplet of fluid is "fired" through nozzle 90. This process is repeated several times in order to produce a desired result. During this process, a single dye may be used, producing a single color design, or multiple dyes may be used, producing a multicolor design. The present invention provides numerous advantages over the conventional printhead. First, the resistor length of the present invention is defined by the placement of dielectric material 44 that is fabricated during a combined photo process and dry etching process. The accuracy of the present process is considerably more controllable than conventional wet etch processes. More particularly, the present process is in the range of 10-25 times more controllable than a conventional process. With the current generation of low drop weight, high-resolution printheads, resistor lengths have decreased from approximately 35 micrometers to less than approximately 10 micrometers. Thus, resistors size variations can significantly affect the performance of a printhead. Resistor size variations translate into drop weight and turn on energy variations across the resistor on a printhead. Thus, the improved length control of the resistive material layer yields a more consistent resistor size and resistance, which thereby improves the consistency in the drop weight of a fluid droplet and the turn on energy necessary to fire a fluid droplet. Second, the resistor structure formed using the method of the present invention includes a completely flat top surface and does not have the step contour associated with conventional fabrication designs. A flat structure (smooth planar surface) provides consistent bubble nucleation, better scavenging of the fluid chamber, and a flatter topology, thereby improving the adhesion and lamination of the barrier structure to the thin film. Third, due to the flat topology of the present structure, the barrier structure is allowed to cover the edge of the resistor. By introducing heat into the floor of the entire fluid chamber, fluid droplet ejection efficiency is improved. Third, because there is no wet slope etch process used in the fabrication of the invention, slope roughness, and conductive layer residue on the resistive layer are no longer issues. Fourth, due to the encapsulation and cladding of conductive layer 40 by resistive layer 30, electro-migration of the conductive layer 40 is minimized into the passivation layer. Further, by attaching the printhead 200 to the fluid cartridge 220, the combination forms a convenient module that can be packaged for sale. Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the chemical, mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the appended claims.

Claims: A method for creating a planar resistor, comprising: depositing an insulator layer (110) on a substrate surface (10); depositing dielectric material (126) on the insulator layer (20); patterning the dielectric material (128) to create an area to form the planar resistor; etching the patterned dielectric material (130) to define the length of the planar resistor; depositing a conductive layer (114) on the insulator layer (20) to abut the etched dielectric material defining the length of the planar resistor; planarizing the conductive layer (134) to expose the dielectric material; depositing a resistive layer (112) on the conductive layer and the exposed dielectric material; patterning the resistive layer (116) to define the width of the planar resistor; and etching the resistive layer (136) to form the width of the planar resistor. A method for creating a printhead (200), comprising the steps of: creating a planar resistor of claim 1; applying at least one layer (82) defining a fluid chamber (100) on the planar resistor area (46). The method of claim 2, further comprising the step of depositing a planar passivation layer (124) between the planar resistor and the fluid chamber (100). The method of claim 2, further comprising the step of depositing a planar cavitation layer (60) between the planar resistor and the fluid chamber (100). A resistor (46) for a fluid-jet printhead (200) made with the method of claim 1.

Inventors: CHEN ZHIZANG, JOHNSTONE MARK ALAN, WHITLOCK JOHN P, XU GENBAO, CHEN, ZHIZANG, JOHNSTONE, MARK ALAN, WHITLOCK, JOHN P., XU, GENBAO
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 2001
Main classification: B41J2
IPC codes: B41J2