Printhead having a thin film membrane with a floating section

A printhead including a printhead substrate having at least one opening formed in a first surface to provide a fluid path through the substrate. The printhead further includes a thin film membrane formed on a second surface of the substrate. The thin film membrane includes a plurality of fluid ejection elements and has a floating section and a cantilevered section, which are detached and separated from one another by a gap.

FIELD OF THE INVENTION

Embodiments of the present invention relate to printers and, more particularly to a printhead for a printer.

BACKGROUND OF THE INVENTION

Printers typically have a printhead mounted on a carriage that scans back and forth across the width of a sheet of paper, as the paper is fed through the printer. Fluid from a fluid reservoir, either on-board the carriage or external to the carriage, is fed to fluid ejection chambers on the printhead. Each fluid ejection chamber contains a fluid ejection element, such as a heater resistor or a piezoelectric element, which is independently addressable. Energizing a fluid ejection element causes a droplet of fluid to be ejected through a nozzle to create a small dot on the paper. The pattern of dots created forms an image or text.

Hewlett-Packard is developing printheads that are formed using integrated circuit techniques. A thin film membrane, composed of various thin film layers, including a resistive layer, is formed on a top surface of a silicon substrate, and an orifice layer is formed on top of the thin film membrane. The various thin film layers of the thin film membrane are etched to provide conductive leads to fluid ejection elements, which may be heater resistor or piezoelectric elements. Fluid feed holes are also formed in the thin film layers. The fluid feed holes control the flow of fluid to the fluid ejection elements. The fluid flows from the fluid reservoir, across a bottom surface of the silicon substrate, into a trench formed in the silicon substrate, through the fluid feed holes, and into fluid ejection chambers where the fluid ejection elements are located.

The trench is etched in the bottom surface of the silicon substrate so that fluid can flow into the trench and into each fluid ejection chamber through the fluid feed holes formed in the thin film membrane. The trench completely etches away portions of the substrate near the fluid feed holes, so that the thin film membrane forms a shelf in the vicinity of the fluid feed holes.

One problem faced during development of these printheads is that the thin film membrane and the orifice layer form a composite, which when subjected to stress can crack. When the composite is placed under stress, the thin film membrane, which is the stiffer of the two components, bears the majority of the stress. Thus, when the printhead is flexed or otherwise stressed, either during assembly or operation, the thin film membrane, particularly, in the shelf portion which overlies the trench, can crack. Cracking in the thin film membrane causes reliability problems with these printheads. The problem of flexure and stresses is exacerbated in longer printheads, which typically have larger trenches.

SUMMARY

Described herein is a printhead having a printhead substrate and a thin film membrane. The printhead substrate has at least one opening formed in a first surface to provide a fluid path through the substrate. The thin film membrane is formed on a second surface of the substrate and includes a plurality of fluid ejection elements. The thin film membrane has a floating and cantilevered section, which are detached and separated from each other by a gap formed in the thin film membrane. The floating section is located over the opening of the substrate, while the cantilevered section is substantially supported by the substrate.

DETAILED DESCRIPTION

FIG. 1is a perspective view of one type of print cartridge10that may incorporate the printhead structure of the present invention. Print cartridge10is of the type that contains a substantial quantity of fluid within its body12, but another suitable print cartridge may be the type that receives fluid from an external fluid supply either mounted on the printhead or connected to the printhead via a tube.

The fluid is supplied to a printhead14. Printhead14, to be described in detail later, channels the fluid into fluid ejection chambers, each chamber containing a fluid ejection element. Electrical signals are provided to contacts16to individually energize the fluid ejection elements to eject a droplet of fluid through an associated nozzle18. The structure and operation of conventional print cartridges are very well known.

Embodiments of the present invention relate to the printhead portion of a print cartridge, or a printhead that can be permanently installed in a printer, and, thus, is independent of the fluid delivery system that provides fluid to the printhead. The invention is also independent of the particular printer, into which the printhead is incorporated.

FIG. 2is a cross-sectional view of a portion of the printhead ofFIG. 1taken generally along line2—2inFIG. 1. Although a printhead may have 300 or more nozzles and associated fluid ejection chambers, detail of only a single fluid ejection chamber need be described in order to understand the invention. It should also be understood by those skilled in the art that many printheads are formed on a single silicon wafer and then separated from one another using conventional techniques.

InFIG. 2, a silicon substrate20has an opening or trench22formed in a bottom surface thereof. Trench22provides a path for fluid to flow along the bottom surface and through substrate20.

Formed on top of silicon substrate20is a thin film membrane24. Thin film membrane24is composed of various thin film layers, to be described in detail later. The thin film layers include a resistive layer for forming fluid ejection elements or resistors26. Other thin film layers perform various functions, such as providing electrical insulation from substrate20, providing a thermally conductive path from the heater resistor elements to substrate20, and providing electrical conductors to the resistor elements. One electrical conductor28is shown leading to one end of a resistor26. A similar conductor leads to the other end of resistor26. In an actual embodiment, the resistors and conductors in a chamber would be obscured by overlying layers.

Thin film membrane24includes fluid feed holes30that are formed completely through thin film membrane24. In addition, thin film membrane24is divided into a cantilevered section32and a floating section34. Cantilevered section32is substantially supported by substrate20, while floating section34is suspended over trench22formed in substrate20. Floating section34is separated on all sides from cantilevered section32by a gap36formed in thin film membrane24. Each gap36has a width of approximately 0.1 microns. One of ordinary skill in the art will appreciate that the width of gaps36may be optimized to control the flow of fluid through printhead14. The advantages of dividing thin film membrane24into cantilevered and floating sections32and34, respectively, is described in greater detail below.

In another embodiment, floating section34is not separated on all sides from the remainder of the thin film layers but is only separated on one or both long sides to relieve stress.

An orifice layer38is deposited over the surface of thin film membrane24. Orifice layer38is adhered to the top surface of thin film membrane24, such that the two form a composite. The adhesion between thin film membrane24and orifice layer38is sufficient for orifice layer38to suspend floating section34of thin film membrane24over trench22in substrate20, however, additional structures, as described below, may be used to further secure the two together.

Orifice layer38is etched to form fluid ejection chambers40, one chamber per resistor26. A manifold42is also formed in orifice layer38for providing a common fluid channel for a row of fluid ejection chambers40. The inside edge of manifold42is shown by a dashed line44. Nozzles46may be formed by laser ablation using a mask and conventional photolithography techniques.

Trench22in silicon substrate20extends along the length of the row of fluid feed holes30so that fluid48from a fluid reservoir may enter fluid feed holes30and supply fluid to fluid ejection chambers40.

In one embodiment, each printhead is approximately one-half inch long and contains two offset rows of nozzles, each row containing 150 nozzles for a total of 300 nozzles per printhead. The printhead can thus print at a single pass resolution of 600 dots per inch (dpi) along the direction of the nozzle rows or print at a greater resolution in multiple passes. Greater resolutions may also be printed along the scan direction of the printhead. Resolutions of 1200 dpi or greater may be obtained using the present invention.

In operation, an electrical signal is provided to heater resistor26, which vaporizes a portion of the fluid to form a bubble within a fluid ejection chamber40. The bubble propels a fluid droplet through an associated nozzle46onto a medium. The fluid ejection chamber is then refilled by capillary action.

FIG. 3is a perspective view of the underside of the printhead ofFIG. 2showing trench22in substrate20, gaps36separating floating section34of thin film membrane24from cantilevered section32, and fluid feed holes30in floating section34. In the particular embodiment ofFIG. 3, a single trench22provides access to two rows of fluid feed holes30. Trench22also provides access to gaps36such that fluid may flow through gaps36and into fluid ejection chambers40. Floating section34, which is suspended over trench22, preferably has dimensions smaller than that those of trench22.

In one embodiment, the size of each fluid feed hole30is smaller than the size of a nozzle46, so that particles in the fluid will be filtered by fluid feed holes30and will not clog nozzle46. The clogging of a fluid feed hole will have little effect on the refill speed of a chamber, since there are multiple fluid feed holes supplying fluid to each chamber40. In another embodiment, there are more fluid feed holes30than fluid ejection chambers40.

FIG. 4is a cross-sectional view taken generally along line4—4inFIG. 2.FIG. 4shows the individual thin film layers which comprise thin film membrane24. In the particular embodiment ofFIG. 4, the portion of silicon substrate20shown is approximately 30 microns thick. This portion is referred to as the bridge. The bulk silicon is approximately 675 microns thick.

A field oxide layer50, having a thickness of 1.2 microns, is formed over silicon substrate20using conventional techniques A tetraethyl orthosilicate (TEOS) layer52, having a thickness of 1.0 microns, is then applied over the layer of oxide50. A boron TEOS (BTEOS) layer may be used instead.

A resistive layer of, for example, tantalum aluminum (TaAl), having a thickness of 0.1 microns, is then formed over TEOS layer52. Other known resistive layers can also be used.

A patterned metal layer, such as an aluminum-copper alloy, having a thickness of 0.5 microns, overlies the resistive layer for providing an electrical connection to the resistors. The conductive AlCu traces are etched to reveal portions of the TaAl layer to define a first resistor dimension (e.g., a width). A second resistor dimension (e.g., a length) is defined by etching the AlCu layer to cause a resistive portion to be contacted by AlCu traces at two ends. This technique of forming resistors26and electrical conductors is well known in the art.

TEOS layer52and field oxide layer50provide electrical insulation between resistors26and substrate20, as well as an etch stop when etching substrate20. In addition, TEOS layer52and field oxide layer50provide a mechanical support for an overhang portion54of cantilevered section32and for floating section34. The TEOS and field oxide layers also insulate polysilicon gates of transistors (not shown) used to couple energization signals to the resistors26.

Referring back toFIG. 4, over the resistors26and AlCu metal layer is formed a silicon nitride (Si3N4) layer56, having a thickness of 0.25 microns. This layer provides insulation and passivation. Prior to nitride layer56being deposited, the resistive and patterned metal layers are etched to pull back both layers from fluid feed holes30so as not to be in contact with any fluid. This is because the resistive and patterned metal layers are vulnerable to certain fluids and the etchant used to form trench22. Etching back a layer to protect the layer from fluid may also apply to the polysilicon layer in the printhead.

Over the nitride layer56is formed a layer58of silicon carbide (SiC), having a thickness of 0.125 microns, to provide additional insulation and passivation. Other dielectric layers may be used instead of nitride and carbide.

Carbide layer58and nitride layer56are also etched to expose portions of the AlCu traces for contact to subsequently formed ground lines (out of the field ofFIG. 4).

On top of carbide layer58is formed an adhesive layer60of tantalum (Ta), having a thickness of 0.3 microns. The tantalum also functions as a bubble cavitation barrier over the resistor elements. This layer60contacts the AlCu conductive traces through the openings in the nitride/carbide layers.

Gold (not shown) is deposited over tantalum layer60and etched to form ground lines electrically connected to certain ones of the AlCu traces. Such conductors may be conventional.

The AlCu and gold conductors may be coupled to transistors formed on the substrate surface. Such transistors are described in U.S. Pat. No. 5,648,806, assigned to the present assignee and incorporated herein by reference. The conductors may terminate at electrodes along edges of substrate20.

A flexible circuit (not shown) has conductors, which are bonded to the electrodes on substrate20and which terminate in contact pads16(FIG. 1) for electrical connection to the printer.

Fluid feed holes30and gaps36are formed by etching through the layers that form thin film membrane24. In one embodiment, a single feed hole and gap mask is used. In another embodiment, several masking and etching steps are used as the various thin film layers are formed.

Orifice layer38is then deposited and formed, followed by the etching of the trench22. In another embodiment, the trench etch is conducted before the orifice layer fabrication. Orifice layer38may be formed of a spun-on epoxy called SU-8. Orifice layer38in one embodiment is approximately 30 microns.

A backside metal may be deposited, if necessary, to better conduct heat from substrate20to the fluid.

FIG. 5is a top-down view of the structure ofFIG. 2. The dimensions of the elements may be as follows: fluid feed holes30are 10 microns×20 microns; fluid ejection chambers40are 25 microns×25 microns; nozzles46have a diameter of 16 microns; heater resistors26are 20 microns×20 microns; and manifold42has a width of approximately 20 microns. The dimensions will vary depending on the fluid used, operating temperature, printing speed, desired resolution, and other factors.

The present invention provides a printhead with improved reliability. Since the composite formed by thin film membrane24and orifice layer38is not continuous throughout, due to gaps36in thin film membrane24, it is less sensitive to the loads imposed by flexure of printhead14. When flexure occurs, gaps36stop the propagation of stress through thin film membrane24and allow the lower modulus SU-8 material of orifice layer to bear the imposed load. Thus, by isolating floating section34of thin film membrane24from loads created by flexure of the die, the thin film membrane can remain over trench22in substrate, thereby taking advantage of the smaller features and tighter tolerances offered by integrated circuit techniques. Adjusting the width of gaps36also provides a way to control fluid refill other than through barrier architecture or through shelf length. In addition, the present invention requires no additional process steps, as gaps36may be formed simultaneously with fluid feed holes30. Finally, the present invention enables the use of the thin film membrane in larger printheads that have a greater potential for flexure.

As discussed above, adhesion between the top layer of thin film membrane24and orifice layer38enables orifice layer38to suspend floating section34of thin film membrane24over trench22in substrate20. Orifice layer38may also be further secured to thin film membrane24.FIGS. 6A–6Cillustrate a method of forming rivet-like structures to secure orifice layer38to thin film membrane24. These structures may be formed, as needed, in floating section34of thin film membrane24. InFIG. 6A, thin film membrane24is etched to form one or more openings62at desired locations for the rivets. Thin film membrane24is then used as a mask, and silicon substrate20is exposed to an anisotrophic etchant, such as TMAH. The etchant attacks the exposed silicon and undercuts the thin film membrane, as illustrated inFIG. 6B. Next, SU-8, the epoxy which forms orifice layer38, is spun on. The epoxy material flows into the cavity created by the etchant, as illustrated inFIG. 6C. The SU-8 is then exposed and baked to cure, and the rivet is complete.

FIG. 7is a cross-sectional view of an embodiment of the invention without fluid feed holes. The layers of thin film membrane24are similar to those inFIG. 4. UnlikeFIG. 4, there is no fluid feed hole30. Rather, fluid flows through gaps36.

FIG. 8illustrates one embodiment of a printer70that can incorporate various embodiments of printheads. Numerous other designs of printers may also be used. More detail of a printer is found in U.S. Pat. No. 5,582,459, to Norman Pawlowski et al., incorporated herein by reference.

Printer70includes an input tray72containing sheets of paper74, which are forwarded through a print zone76using rollers78for being printed upon. Paper74is then forwarded to an output tray80. A moveable carriage82holds print cartridges82,84,86and99, which respectively print cyan (C), black (K), magenta (M), and yellow (Y) fluid.

In one embodiment, fluids in replaceable fluid cartridges92are supplied to their associated print cartridges via flexible fluid tubes94. The print cartridges may also be the type that hold a substantial supply of fluid and may be refillable or non-refillable. In another embodiment, the fluid supplies are separate from the printhead portions and are removably mounted on the printheads in carriage82.

Carriage82is moved along a scan axis by a conventional belt and pulley system and slides along a slide rod96. In another embodiment, the carriage is stationary, and an array of stationary print cartridges print on a moving sheet of paper.

Printing signals from a conventional external computer (e.g., a PC) are processed by printer70to generate a bitmap of the dots to be printed. The bitmap is then converted into firing signals for the printheads. The position of the carriage82as it traverses back and forth along the scan axis while printing is determined from an optical encoder strip98, detected by a photoelectric element on carriage82, to cause the various fluid ejection elements on each print cartridge to be selectively fired at the appropriate time during a carriage scan.

The printhead may use resistive, piezoelectric, or other types of fluid ejection elements.

As the print cartridges in carriage82scan across a sheet of paper, the swaths printed by the print cartridges overlap. After one or more scans, the sheet of paper74is shifted in a direction towards output tray80, and carriage82resumes scanning.

The present invention is equally applicable to alternative printing systems (not shown) that utilize alternative media and/or printhead moving mechanisms, such as those incorporating grit wheel, roll feed, or drum or vacuum belt technology to support and move the print media relative to the printhead assemblies. With a grit wheel design, a grit wheel and pinch roller move the media back and forth along one axis while a carriage carrying one or more printhead assemblies scan past the media along an orthogonal axis. With a drum printer design, the media is mounted to a rotating drum that is rotated along one axis while a carriage carrying one or more printhead assemblies scans past the medial along an orthogonal axis. In either the drum or grit wheel designs, the scanning is typically not done in a back and forth manner as is the case for the system depicted inFIG. 8.

Multiple printheads may be formed on a single substrate. Further, an array of printheads may extend across the entire width of a page so that no scanning of the printheads is needed; only the paper is shifted perpendicular to the array.

Additional print cartridges in the carriage may include other colors or fixers.