Patent Publication Number: US-2010110144-A1

Title: Applying a Layer to a Nozzle Outlet

Description:
This application claims the benefit of U.S. Provisional Application No. 61/110,439, filed Oct. 31, 2008, and incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure relates to fluid ejection devices. In some fluid ejection devices, fluid droplets are ejected from one or more nozzles onto a medium. The nozzles are fluidically connected to a fluid path that includes a fluid pumping chamber. The fluid pumping chamber can be actuated by an actuator, which causes ejection of a fluid droplet. The medium can be moved relative to the fluid ejection device. The ejection of a fluid droplet from a particular nozzle is timed with the movement of the medium to place a fluid droplet at a desired location on the medium. In these fluid ejection devices, it is usually desirable to eject fluid droplets of uniform size and speed and in the same direction in order to provide uniform deposition of fluid droplets on the medium. 
     SUMMARY 
     In one aspect, a nozzle layer is described that has a semiconductor body having a first surface, a second surface opposing the first surface, and a nozzle formed through the body connecting the first and second surfaces, wherein the nozzle being configured to eject fluid through a nozzle outlet on the second surface, and a metal layer around the outlet on the second surface and at least partially inside the nozzle, the metal layer inside the nozzle being completely exposed. 
     In another aspect, a method includes applying a metal layer around a nozzle outlet and at least partially inside a nozzle of a semiconductor nozzle layer, and keeping the metal layer inside the nozzle completely exposed. 
     In another aspect, a method for making nozzle layers includes measuring a plurality of nozzle outlet widths in a nozzle layer; calculating an average nozzle outlet width of the plurality of nozzles; calculating a thickness for a cover layer to be applied to the nozzle layer based on a comparison between the average nozzle width and a desired nozzle width; and applying the cover layer with the thickness around each nozzle outlet and at least partially inside each nozzle. 
     In another aspect, a kit includes a first print head including a first semiconductor body having a first surface and a first plurality of fluid flow paths through the first semiconductor body with a first plurality of apertures on the first surface, the first plurality of apertures having a first average lateral aperture dimension, and a first cover layer on the first surface and at least partially inside the first plurality of apertures to provide nozzles having a first average lateral nozzle dimension; and a second print head including a second semiconductor body having a second surface and a second plurality of fluid flow paths through the second semiconductor body with a second plurality of apertures on the second surface, the second plurality of apertures having a second lateral aperture dimension different from the first average lateral aperture dimension, and a second cover layer on the second surface and at least partially inside the second plurality of apertures to provide nozzles having a second average lateral nozzle dimension approximately equal to the first average lateral nozzle dimension. 
     Implementations may include one or more of the following features. The metal layer can include a metal selected from the group consisting of titanium, gold, platinum, rhodium, tantalum, nickel, and nickel chromium. The metal layer can be chemically resistant to alkaline fluids. The metal layer can have a thickness of about 1 micron or greater. The nozzle layer can also have a non-wetting coating on the metal layer on the second surface. The metal layer can be between about 0.1 micron and about 10 microns thick. The metal layer can be completely exposed around the outlet on the second surface and inside the nozzle. The nozzle can have tapered walls or straight walls connecting the first surface to the second surface. The metal layer can shape the outlet to have curved edges. The curved edges can have a radius of curvature of about 1 micron or greater. The outlet can be a square. The semiconductor body of the nozzle layer can comprise silicon. Applying the metal layer can comprise sputtering metal or electroplating metal on the sputtered metal. The method can further include securing the nozzle layer to a fluid flow path body. The method can also include keeping the metal layer around the nozzle outlet completely exposed. The nozzle outlet can be located on an outer surface of the nozzle layer and the metal layer around the nozzle outlet can be on the outer surface, and the method further can include applying a non-wetting coating on the metal layer on the outer surface of the nozzle layer but not inside the nozzle. The method can include shaping the nozzle outlet using the metal layer to have curved edges. Measuring a plurality of nozzle outlet widths can include using an optical measurement tool. The cover layer can comprise metal. 
     Implementations may include one or more of the following advantages. Shaping a nozzle outlet to have curved edges and/or corners can alleviate problems associated with sharp-edged outlets: nozzles can be less likely to become clogged with debris, jetting straightness can be improved, nozzles can be more durable and drop size can be more uniform. 
     Without being limited to any particular theory, the sharp edges of the nozzle outlets can act like a blade and shave off portions of a maintenance device (e.g., wiper), and the wiping action of a wiper can push this debris into the nozzles and clog them. Shaping the nozzle outlet to have curved edges can reduce the tendency of the nozzle to create and trap debris. 
     Without being limited to any particular theory, a substantially square-shaped nozzle outlet or any outlet having sharp or pointed corners can have difficulty ejecting fluid drops in a straight line because of high fluid surface tension forces in the corners. The high surface tension force in a sharp corner can pull the drop toward that corner causing the drop to be ejected at an angle. Shaping the outlet to have curved corners can reduce the tendency of the drop to be pulled toward a corner and improve jet straightness. In addition, during fluid ejection, if fluid splashes back and collects on an outer surface of the nozzle plate, then this fluid can interfere with subsequent fluid drops ejected. For example, the fluid on the surface can coalesce near the nozzle outlet and when a drop is ejected, the fluid on the nozzle surface pulls the ejected drop to one side affecting the straightness of the drop and causing drop placement errors on the printed medium. It is difficult for the coalesced fluid on the surface to enter back inside the nozzle if the edges are sharp, but with curved edges and corners, without being limited to any particular theory, the fluid can more easily re-enter the nozzle so that it does not affect the straightness of the next ejected fluid drop. 
     Without being limited to any particular theory, the sharp or pointed edges of a nozzle formed of semiconductor material can be fragile and susceptible to damage and, if damaged, the nozzle outlet can become irregularly shaped and eject drops at an angle other than straight. Further, damage to the nozzle outlet can increase the dimensions of the outlet (e.g., width or diameter) and, therefore, increase the drop volume of the ejected drops. Shaping the outlet to have curved edges and corners can improve the durability of the nozzles. 
     Twinning is the term used to describe the drop placement errors caused by jets ejecting drops at an angle rather than in a straight line. For example, when a jet ejects a drop at angle, this drop may land closer to a neighboring drop than desired. The two drops may merge together and the surface tension of the merged drops can prevent the drops from being able to completely spread leaving white space on the printed medium. Improving jet straightness, for example, by shaping the nozzles to have curved features can prevent twinning. 
     Applying a layer of an inorganic, non-metallic material, a metal layer, or both around the nozzle outlet and partially inside the nozzle can strengthen the nozzle outlet against damage and/or make the nozzle surface chemically resistant. The nozzle can be strengthened by applying one or more of these layers that are more durable than the underlying material of the nozzle layer and by increasing the radius of curvature at the edges and corners. A metal layer or oxide layer doped with a metal can reduce electric field buildup on the nozzle layer surface and/or improve galvanic compatibility in the printhead. One or more layers can be applied to the nozzle outlet with or without curved edges and/or corners. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional side view of an apparatus for fluid droplet ejection. 
         FIG. 2A  is a cross-sectional side view of an apparatus including a nozzle layer having a nozzle with tapered walls. 
         FIG. 2B  is a bottom view of a nozzle outlet formed in a nozzle layer. 
         FIG. 2C  is a cross-sectional side view of a nozzle with straight walls. 
         FIG. 3  is a scanning electron microscope (SEM) image showing a bottom view of a damaged outlet of a nozzle. 
         FIG. 4  is a flowchart of a method of making a nozzle layer. 
         FIGS. 5A-F  are diagrams of applying and removing an oxide layer to a nozzle layer, applying a protective layer, and securing the nozzle layer to a fluid path body. 
         FIG. 6A  is a cross-sectional side view of a nozzle having tapered walls. 
         FIG. 6B  is a bottom view of the nozzle in  FIG. 6A . 
         FIG. 6C  is a cross-sectional side view of a metal layer applied to the nozzle walls and around the nozzle outlet. 
         FIG. 6D  is a bottom view of a nozzle layer in  FIG. 6C . 
         FIG. 7A  is a SEM image showing a cross-sectional side view of a nozzle with tapered walls and an inorganic oxide layer grown on the surfaces of the nozzle. 
         FIG. 7B  is a SEM image showing a cross-sectional perspective view of only the right side of the nozzle after the oxide layer is removed and another oxide layer is re-grown. 
         FIG. 7C  is a cross-sectional perspective view of a nozzle with an oxide layer, the nozzle has tapered walls and curved edges and corners. 
         FIG. 7D  is a bottom view of the nozzle layer showing the nozzle outlet with curved corners. 
         FIG. 7E  is a bottom view of the nozzle layer including a protective layer showing the nozzle outlet with curved corners having a reduced radius of curvature. 
         FIG. 8  is a SEM image showing a cross-sectional side view of a nozzle layer secured to a descender layer. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Fluid droplet ejection can be implemented with a substrate, for example a microelectromechanical system (MEMS), including a fluid flow path body, a membrane, and a nozzle layer. The flow path body has a fluid flow path formed therein, which can include a fluid fill passage, a fluid pumping chamber, a descender, and a nozzle having an outlet. An actuator can be located on a surface of the membrane opposite the flow path body and proximate to the fluid pumping chamber. When the actuator is actuated, the actuator imparts a pressure pulse to the fluid pumping chamber to cause ejection of a droplet of fluid through the outlet. Frequently, the flow path body includes multiple fluid flow paths and nozzles. 
     A fluid droplet ejection system can include the substrate described. The system can also include a source of fluid for the substrate. A fluid reservoir can be fluidically connected to the substrate for supplying fluid for ejection. The fluid can be, for example, a chemical compound, a biological substance, or ink. 
     Referring to  FIG. 1 , a cross-sectional schematic diagram of a portion of a microelectromechanical device, such as a printhead in one implementation is shown. The printhead includes a substrate  100 . The substrate  100  includes a fluid path body  102 , a nozzle layer  104 , and a membrane  106 . A fluid reservoir supplies a fluid fill passage  108  with fluid. The fluid fill passage  108  is fluidically connected to an ascender  110 . The ascender  110  is fluidically connected to a fluid pumping chamber  112 . The fluid pumping chamber  112  is in close proximity to an actuator  114 . The actuator  114  can include piezoelectric material, such as lead zirconium titanate (PZT), sandwiched between a drive electrode, and a ground electrode. An electrical voltage can be applied between the drive electrode and the ground electrode of the actuator  114  to apply a voltage to the actuator and thereby actuate the actuator. A membrane  106  is between the actuator  114  and the fluid pumping chamber  112 . An adhesive layer (not shown) can secure the actuator  114  to the membrane  106 . 
     A nozzle layer  104  is secured to a bottom surface of the fluid path body  102  and can have a thickness between about 1 and 100 microns (e.g., between about 5 and 50 microns or between about 15 and 35 microns). A nozzle  117  having an outlet  118  is formed in an outer surface  120  of the nozzle layer  104 . The fluid pumping chamber  112  is fluidically connected to a descender  116 , which is fluidically connected to the nozzle  117 . While  FIG. 1  shows various passages, such as a fluid fill passage, pumping chamber, and descender, these components may not all be in a common plane. In some implementations, two or more of the fluid path body, the nozzle layer, and the membrane may be formed as a unitary body. 
       FIG. 2A  shows a module  200  including a nozzle layer  201  attached to a fluid path body  210 . The nozzle layer  201  includes a nozzle  202  having tapered walls  204  connecting an inlet  206  on a first surface  207  to an outlet  208  on a second surface  209 . The outlet  208  can be narrower than the inlet  206 . The first surface  207  of the nozzle layer  201  can be secured to the fluid path body  210  (e.g., bonding such as anodic bonding, silicon-to-silicon direct wafer bonding, or bonding with an adhesive like BCB). Anodic bonding and examples of materials used in anodic bonding are described in U.S. Pat. No. 7,052,117, the entire contents of which are incorporated by reference. The nozzle layer and fluid flow path body can be made of a semiconductor material, such as silicon, e.g., single crystal silicon. Fluid drops can be ejected through the outlet  208  formed in the second surface  209 .  FIG. 2B  shows a square-shaped outlet  208  having a side with a width, W,  212 , such as between about 1 microns and about 100 microns, such as between about 1 and 10 microns, about 10 and 30 microns, or about 5 and 50 microns. 
     Alternatively,  FIG. 2C  shows a nozzle  202  having straight walls  214  connecting the nozzle inlet  216  to the nozzle outlet  218 . In general, the edge of the outlet can have an angle of about 90 degrees or less (e.g., 45 degrees or less) measured from the plane of the outer surface of the nozzle layer.  FIG. 2A  shows a nozzle having an outlet edge  220  with an angle  222  of about 54 degrees, whereas  FIG. 2C  shows an outlet edge  224  having an angle  226  of about 90 degrees. 
     The outlets  208  and  218  shown in  FIGS. 2A and 2C  can be square-shaped (as shown in  FIG. 2B ), circular, elliptical, polygonal, or any other shape suitable for droplet ejection. If the outlet is other than square, the longest dimension can be, for example, between about 1 micron and about 100 microns, such as between about 1 and 10 microns, about 10 and 30 microns, or about 5 and 50 microns. This outlet size can produce a useful fluid droplet size for some implementations. The nozzle layer can be formed in a semiconductor body, such as silicon, and the nozzle can be formed in the semiconductor body by plasma etching (e.g., deep reactive ion etching), wet etching (e.g., KOH etching), or another process. A plurality of nozzle layers can be formed in a single silicon wafer and processed together. The silicon wafer including the plurality of nozzle layers can also be bonded to other wafers, such as a wafer including a plurality of fluid flow path bodies. The wafer including the plurality of flow path bodies can also be bonded to another wafer including a plurality of membranes. 
     The nozzles in  FIGS. 2A-2C  include outlets having sharp edges, which can be broken or chipped, such as during maintenance operations or handling of the printhead. Sharp edges can include an edge having a radius of curvature less than 0.1 micron. During maintenance operations, a wiper can be used to wipe off excess fluid from the outer surface of the nozzle layer. Since the outlet has sharp edges, the edges can act like a blade and shave off portions of the wiper, subsequently, leaving debris in the nozzle and/or damaging the edges of the nozzle outlet. In other cases, the fluid being ejected may attack the material of the nozzle layer and etch away the edges of the outlet. 
       FIG. 3  is a SEM image showing a nozzle layer  300  with a square-shaped nozzle outlet  302  that has been damaged. For example, the right side of the nozzle outlet has been chipped and broken and is now irregularly shaped. Such irregular shapes no longer eject fluid drops in a straight line. Rather the drops will be ejected at an angle, causing drop placement errors on the printed medium. In the case of a nozzle with tapered walls, the width of the nozzle outlet can significantly increase as the edges of the outlet are chipped away, causing not only drop placement errors due to trajectory errors and decreases in velocity but also undesirable increases in fluid drop volumes. 
       FIG. 4  is a flowchart  400  of a method of making a nozzle layer, such as the nozzle layers in  FIGS. 2A-2C .  FIGS. 5A-5E  are diagrams illustrating the fabrication of a nozzle layer, for example, for a printhead.  FIGS. 5A-5E  show a nozzle layer  500  separate from a fluid flow path body, e.g., the fluid flow path body  210  in  FIG. 2A . Initially, as shown in the cross-sectional view of  FIG. 5A , a nozzle layer  500  having a depth, D,  501  and a nozzle  502  having an outlet  504  is fabricated (step  401 ). The nozzle layer  500  and nozzle  502  can be fabricated with conventional techniques and can have features discussed above with respect to  FIGS. 2A-2C . In particular, the outlet  504  can have sharp edges  506 . As shown in  FIG. 5B , a layer of an inorganic oxide  508  is thermally grown on the exposed surfaces of the nozzle layer  500  (step  402 ). In some implementations, the inorganic oxide  508  can be grown on only a portion of the nozzle layer, such as around the outlet  504  on the outer surface  510  and at least partially inside the nozzle  502 . Next, the inorganic oxide  508  is removed (step  404 ), for example, by using hydrofluoric acid, as shown in  FIG. 5C . 
     The inorganic oxide (e.g., silicon dioxide) can have a thickness of about 0.5 microns or greater, such as about 1 micron or greater, for example, between about 1 and 10 microns or between about 2 and 5 microns. 
     Without being limited to any particular theory, when thermal oxide is grown on a semiconductor (e.g., silicon, e.g., single crystal silicon) surface, the oxide both grows on the silicon surface and into the silicon surface, such that about 46% of the oxide thickness is below the original silicon surface and 54% is above it. When growing thermal oxide, an oxidant (e.g., water vapor or oxygen) combines with silicon atoms at the silicon surface to form a layer of silicon oxide on the silicon surface. As the silicon oxide layer increases in thickness, the oxidant has a longer distance to travel to reach the silicon surface. Again without being limited to any particular theory, the distance the oxidant has to travel at the corners and edges of the nozzle outlet is even greater than the distance the oxidant has to travel at the straight or flat surfaces. Since the oxidant has a longer distance to travel at the corners and edges, the silicon surface at the corners is eroded slower causing the corners and edges to be rounded or curved. Along with the corners, the silicon edges of the outlet are also eroded at a different rate than the flat surfaces causing the edges to be curved, but not as much as the corners.  FIG. 5C  shows the curved edges  512  and  FIG. 5  D shows the curved corners  514 . In an implementation, a layer of silicon oxide (e.g., 5 microns thick) is thermally grown on a silicon nozzle layer (e.g., 30 microns thick) at a temperature between about 800° C. and 1200° C. and, subsequently, placed in a bath of hydrofluoric acid (e.g., for about 7 minutes) to remove the silicon oxide. In some implementations, after removing the oxide layer, a subsequent oxide layer can be re-grown and removed. With each oxide layer that is grown and removed, the radius of curvature of the edges and corners can be further increased. 
     Alternatively, to shape the sharp edges and corners to be curved, an etchant (e.g., KOH) can be used to etch the sharp features of the semiconductor nozzle layer to create curved edges and corners, for example, by placing the nozzle layer in a KOH bath for a predetermined time. 
       FIG. 5C  shows a cross-sectional view of the nozzle layer  500  after the oxide layer  508  has been removed leaving a nozzle  502  that now has an outlet  504  with curved edges  512 . The curved edges can have a radius of curvature greater than 0.1 micron, such as 0.4 microns or greater. The edges  513  of the nozzle inlet are also curved when the oxide is removed. The amount of curvature of the edges and corners can depend on the thickness of the oxide grown on the semiconductor nozzle layer. As the thickness of the oxide increases the curvature of the edges and corners can also increase. 
       FIG. 5D  is an optical microscope photograph showing a bottom view of the nozzle outlet  504  having curved corners  514 . Without being limited to any particular theory, the curved corners can improve the straightness of the drop trajectory by reducing the high fluid surface tension forces in the corners and/or by allowing fluid on an outer surface of the nozzle layer to more easily re-enter the nozzle outlet. The outlet  504  in  FIG. 5D  has straight sides  516  connected by curved corners  514  that can have a radius of curvature  518  of about 0.5 microns or greater, such as 1 micron or greater, for example, between about 1 and 10 microns or between about 2 and 5 microns. 
     After the oxide is removed,  FIG. 5E  shows a protective layer  522  (e.g., an inorganic, non-metallic layer, such as oxide, a metal layer, or a conductive layer) applied to the nozzle layer  500  (step  406 ). The protective layer can be a material more durable than the semiconductor material and can strengthen the semiconductor material, especially the sharp features that are susceptible to damage, such as during maintenance and handling. Inorganic, non-metallic materials can include oxide, diamond-like carbon, or a nitride like silicon nitride or aluminum nitride. Applying a protective layer, for example, re-growing another oxide layer or sputtering a metal layer can increase the curvature of the edges  523  more so than the curvature of the silicon edges  512  in  FIG. 5C . The radius of curvature of edges  523  can be of about 0.5 microns or greater, such as 1 micron or greater, for example, between about 1 and 10 microns or between about 2 and 5 microns. However, if the nozzle outlet is, for example, square-shaped, then the re-grown oxide can reduce the curvature of the corners, and if too much oxide is re-grown, then the oxide can re-square the corners. Therefore, in some implementations, to avoid re-squaring the corners  514  of  FIG. 5D , the thickness of the re-grown oxide can be less than the thickness of the removed oxide  508  in  FIG. 5B . For example, the re-grown oxide can be about 50% or less than the thickness of the removed oxide layer. The curved edges  523  can be less susceptible to chipping and breaking and can prevent the nozzle  502  from being clogged because the curved edges  523  are less likely to shave off debris from a maintenance device. 
     While  FIG. 5E  shows a protective layer  522  covering the surfaces of the nozzle layer  500 , the protective layer can cover only a portion of the nozzle layer, such as the areas around the nozzle outlet and partially inside the nozzle  504 . Alternatively, the protective layer can be only on the outer surface of the nozzle layer around the nozzle outlet and not inside the nozzle. In the case of a nozzle layer having a low surface energy (e.g., a contact angle of about 20° or less), such as silicon, the outer surface of the nozzle layer can be contaminated by process contaminants, like low tack tape, silicones, and outgassing polymers. These contaminants can create non-wetting areas near the nozzle outlets having contact angles of about 70° or greater. A protective layer having a high surface energy (e.g., a contact angle of about 70° or greater), such as gold, can be applied on the outer surface of the silicon nozzle layer, such that the contaminants and the protective layer have about the same surface energy. By including a protective layer having a high surface energy on the outer surface of the nozzle layer, the nozzle layer can be contaminant resistant. 
       FIG. 5F  shows the nozzle layer  500  secured to a fluid path body  524  (e.g., carbon body or silicon body) (step  408 ). The nozzle layer can be secured to the fluid path body by anodic bonding, silicon-to-silicon direct wafer bonding, using an adhesive, such as an epoxy like benzocyclobutene (BCB), or other securing means. 
     Protective layer  522  can be silicon nitride, which can be tougher and more wear resistant than silicon or silicon oxide, especially if processed at higher temperatures (e.g., 1000° C. or greater). Processing at higher temperatures creates a nitride layer that is denser and has fewer pinholes. Since the nitride is tougher than oxide, a thinner layer can be applied to a nozzle, for example, the nitride layer can have a thickness less than 0.5 micron, such as between about 0.05 and 0.2 micron. If necessary, silicon nitride can also be deposited at a lower temperature (e.g., 350° C.), which can be important if the nozzle layer is connected to other heat-sensitive components, such as a piezoelectric actuator that can depole if exposed to temperatures above its Curie temperature. 
     The protective layer (e.g., non-metallic layer or metal layer) can be selected based on its chemical resistance to the fluid being ejected. A protective layer is chemically resistant, for example, if the layer does not react with the fluid. For instance, the fluid does not significantly attack, etch, or degrade the protective layer. The protective layer can also be selected for its durability against maintenance operations, such as wipers, and/or its robustness compared to the underlying material of the nozzle layer (e.g., silicon). 
     Protective layers with fewer pinholes can better protect the semiconductor material from being attacked by aggressive fluids like alkaline inks The protective layer  522  can be about 10 nanometers or greater, such as between about 10 nanometers and 20 microns thick. 
     In some implementations, the protective layer can include a conductive material (e.g., non-metallic or metallic) so as to reduce electric field buildup due to electrostatic charges developed on the nozzle surface, for example, by connecting the conductive material to ground. Conductive materials can also be used to improve the galvanic compatibility in a printhead. The conductive material can be an oxide, such as indium tin oxide (ITO), potentially doped with metal such as cesium or lead. 
     In some implementations, the protective layer can include be a metal layer. The metal can be tougher than the semiconductor material (e.g., silicon) of the nozzle layer. Metal layers can, for example, include titanium, tantalum, platinum, rhodium, gold, nickel, nickel chromium, and combinations thereof. In some implementations, the protective layer can be applied to a nozzle outlet with or without curved edges and/or corners. For example, a protective layer can be applied to the nozzle outlet without first growing and removing an oxide layer. 
       FIGS. 6A-6D  show diagrams of a metal layer (e.g., titanium) being applied to a nozzle layer, in which the nozzle outlet does not have curved edges or corners.  FIG. 6A  shows a nozzle layer  600  having a nozzle  602  with tapered walls  604 , and  FIG. 6B  shows a bottom view of the nozzle outlet  606 , which is square-shaped having a side with a length, L,  607 . Other nozzle outlet shapes are possible, such as circular, elliptical, or polygonal.  FIG. 6C  shows a metal layer  608  applied to a few surfaces of the nozzle layer  600  including inside the nozzle on the tapered walls  604 , around the nozzle outlet  606 , and on the outer surface  612  of the nozzle layer  600 . The metal layer on the inside of the nozzle may be thinner than the metal layer on the outer surface  612  due to the deposition process (e.g., sputtering). For a metal layer with a more uniform thickness, a thin metal layer can be sputtered on the nozzle layer (e.g., about 200 Angstroms or greater) and a second metal layer can be electroplated on the sputtered metal layer (e.g., 980 nm or greater).  FIG. 6D  shows the nozzle outlet  606  having a metal layer  608  applied to the outer surface  612  of the nozzle layer. 
     In some implementations, the metal layer of  FIGS. 6C and 6D  is exposed meaning that subsequent layers are not applied on top of the metal layer. The metal layer can be completely exposed both on the outer surface and inside the nozzle. While a native oxide layer may grow on the surface of the metal, this layer is on the Angstrom level and for purposes of this application would still be considered exposed metal. For some metals, such as titanium, the native oxide layer provides the chemical inertness that makes the metal layer resistant to aggressive fluids. 
     In some implementations, only the metal layer inside the nozzle is completely exposed while a non-wetting coating is applied to the metal layer on the outer surface. The non-wetting coating provides a hydrophobic surface that causes fluid on the outer surface to bead up rather than form a puddle near the nozzle outlet. The non-wetting coating is not inside the nozzle because a non-wetting coating inside the nozzle can affect the position of the meniscus and the ability of the fluid to properly wet the area around the nozzle outlet. Non-wetting coatings are described in U.S. Patent Publication Nos. 2007/0030306 (entitled “Non-Wetting Coating on a Fluid Ejector” filed by Okamura et al. on Jun. 30, 2006 and published on Feb. 8, 2007), 2008/0150998 (entitled “Pattern of Non-Wetting Coating on a Fluid Ejector” filed by Okamura on Dec. 18, 2007 and published on Jun. 26, 2008), and 2008/0136866 (entitled “Non-Wetting Coating on a Fluid Ejector” filed by Okamura et al. on Nov. 30, 2007 and published on Jun. 12, 2008), the entire contents of which are incorporated by reference. Although  FIG. 6C  shows the metal layer  608  covering entire surfaces, the metal layer can be applied such that it covers only a portion of the nozzle layer, for example, the area around the nozzle outlet and at least partially inside the nozzle near the outlet. The metal layer can be selected to be chemically resistant to a particular fluid (e.g., alkaline fluid with a high pH or acidic fluid with a low pH). Examples of chemically resistant metals can include titanium, gold, platinum, rhodium, and tantalum. In an implementation, a titanium or tantalum metal layer, which is chemically resistant to alkaline fluids, can be applied to a silicon nozzle layer of a printhead to protect the nozzle outlets from being etched when ejecting drops of an alkaline fluid. 
     The metal layer can be about 0.1 micron or greater, such as about 0.2 to 5 microns thick (e.g., 2 to 2.5 microns). For durability, the metal layer can be about 1 micron or greater, such as about 1 to 10 microns thick. The metal layer can be electrically conductive. Along with making the nozzle layer more durable, the metal layer can be applied, for example, by vacuum deposition (e.g., sputtering) or by a combination of vacuum deposition and electroplating, such that the metal layer shapes the edges of the nozzle outlet to be curved. Electroplated metal can provide a more conformal, uniform layer than sputtered metal and can increase the radius of curvature of the nozzle outlet edges. For example, the metal layer on the outlet edges can have a radius of curvature of 1 micron or greater, such as 2 to 5 microns. 
     When applying a protective layer (e.g., metal layer), additional material can be added to change the width of the nozzles to make the nozzles more uniform from printhead to printhead. For example, if the desired nozzle outlet width is 10 microns, and a first nozzle layer of a first print head has an average outlet width of 11 microns and the a second nozzle layer of a second print head has an average outlet width of 12 microns, then an additional 1 micron of material (e.g., metal) can be applied around the nozzles of the first nozzle layer and 2 microns on the second nozzle layer, such that the first and second nozzle plates both have an average outlet width of 10 microns. The width of the individual nozzles can be measured using an optical measurement tool available from JMAR Technologies or Tamar Technology. 
     Other combinations are possible, such as a first layer of an inorganic, non-metallic material (e.g., oxide, silicon nitride, or aluminum nitride) and a second layer of a metal. With a nozzle layer made of silicon, precise nozzle features can be etched into the silicon, for example, by photolithography and dry or wet etching that may not be possible with a metal nozzle layer, especially thicker nozzle layers (e.g., 3-100 microns). By depositing a thin metal layer on the silicon, the nozzle plate can not only have fine features, but also be durable and chemically inert. 
     The non-metallic and metal layer(s) can be applied, for example, by PVD, CVD like PECVD, or thermally grown in the case of thermal oxide, and can have the same thickness as the removed oxide layer, or it can be thicker or thinner, for example, the thickness can be between about 0.1 micron or greater, about 0.5 to 20 microns, such as about 1 to 10 microns. When applying the layer(s) to sharp edges, the layer(s) can provide a radius of curvature of about 0.5 micron or greater, such as 1 micron or greater, such as about 1 to 5 microns. In the case of nozzles with corners, the additional layer(s) may slightly reduce the curvature in the corners. Thus, the layer(s) should be thin enough to avoid re-squaring the corners of the nozzle outlet. 
       FIG. 7A  is a SEM image of a nozzle layer  700  showing a cross-sectional side view of a nozzle  702  formed in a semiconductor nozzle layer (e.g., silicon). The outlet  704  of the nozzle  702  is located near the top of the picture and the inlet  706  is closer to the bottom. The nozzle  702  has tapered walls  708  and edges  710  that have been eroded slightly from the growth of the thermal oxide layer  712  such that the edges  710  are slightly curved. As explained above, growing the oxide layer  712  on the surfaces of the nozzle layer  702  shapes the edges and the corners to be curved. 
       FIG. 7B  is a SEM image showing a cross-sectional perspective view of only the right side of the nozzle  702  after the oxide layer  712  is removed and an oxide layer  715  is re-grown on the silicon surface. The edge  713  has a radius of curvature greater than the curvature of the silicon edge  710  in  FIG. 7A . 
       FIG. 7C  is a schematic of a cross-sectional perspective top view of a nozzle  702  formed in a nozzle layer  700  having tapered walls  708  starting with an inlet  706  on a first surface  714  and ending in an outlet  704  on a second surface  716 . The tapered walls  708  form a truncated-pyramid shape, which can be formed by KOH etching. The nozzle inlet  706  and outlet  704  have straight sides  718  connected by curved corners  720  and the inlet  706  is connected to the outlet  704  by tapered walls  708 . A protective layer  722 , such as an inorganic, non-metallic and/or metal layer, is applied to the nozzle layer  700  having curved features. In some implementations, the tapered walls can be conical or polygonal rather than pyramidal. Alternatively, the nozzle can have a combination of tapered walls and straight walls, for example, a first portion of the nozzle starting at the nozzle inlet can have tapered walls that connect to a second portion of the nozzle having straight walls that end at the nozzle outlet, such as the nozzles described in U.S. Pat. No. 7,347,532, the entire contents of which are incorporated by reference. 
     Referring back to  FIGS. 7A and 7B , in an implementation, the oxide layer  712  (shown in  FIG. 7A ) can be thermally grown to a thickness of about 5 microns and subsequently removed, which shapes the silicon edge  710  to have a radius of curvature of about 0.4 micron. An oxide layer  715  (shown in  FIG. 7B ) having a thickness of about 2 microns is re-grown on the silicon surface such that the radius of curvature at the oxide edge  713  is about 2.5 microns. As mentioned before, while re-growing an oxide layer increases the radius of curvature of the edges  713 , it can decrease the radius of curvature of the corners. For example,  FIG. 7D  shows the nozzle outlet  702 , after growing and removing the 5 micron thick oxide layer  712  (from  FIG. 7A ), with corners  724  having a radius of curvature  726  of about 5 microns at the silicon surface  727 . In some implementations, the radius of curvature of the corner  724  can be about equal to the thickness of the removed oxide layer  712 .  FIG. 7E  shows the nozzle outlet  702  after the 2 micron thick oxide layer  715  is re-grown, the radius of curvature  728  at the corner  730  is reduced to about 3 microns. To limit the reduction in curvature of the corners, the re-grown oxide can be thinner than the removed oxide layer. 
     The nozzle layer can be processed separately as shown in  FIGS. 5A-5E  or secured to another part for processing. For example, if the nozzle layer is not thick enough to be processed separately, then the nozzle layer can be bonded to another part (e.g., bonded to a fluid path body without the membrane and actuator or bonded to a descender layer) by, for example, anodic bonding, silicon-to-silicon direct wafer bonding, or using an adhesive (e.g., BCB).  FIG. 8  is a SEM image showing a cross-sectional side view of a combination part  800  including a nozzle layer  801  (e.g., silicon) secured to a descender layer  802  (e.g., silicon). The nozzle layer  801  includes a plurality of nozzles  804  that are aligned with a plurality of descenders  806  formed in the descender layer  802 . Similar to the process described above, an oxide layer can be applied to the combination part  800  and subsequently removed, and a second layer (e.g., a protective layer like oxide or metal) can be applied to the combination part  800 , and finally it can be secured to a fluid flow path body (not shown). 
     In some implementations, the nozzle layer can be partially processed by itself, and completely processed after bonding the nozzle layer to another part. For example, the thermal oxide layer can be grown on and removed from the nozzle layer, and then the nozzle layer can be bonded to a fluid flow path body, after which, a protective layer can be applied to the nozzle layer. In other implementations, a nozzle layer is not oxidized rather a protective layer excluding thermal oxide can be applied to the surfaces of the nozzle layer that is already bonded to a fluid path body. 
     The use of terminology such as “inner” and “outer” and “top” and “bottom” in the specification and claims is to illustrate relative positioning between various components of the substrate, nozzle layer, and other elements described herein. The use of “inner” and “outer” and “top” and “bottom” does not imply a particular orientation of the substrate or nozzle layer. Although specific embodiments have been described herein, other features, objects, and advantages will be apparent from the description and the drawings. All such variations are included within the intended scope of the invention as defined by the following claims.