Abstract:
Techniques are provided for forming nozzles in a microelectromechanical device. The nozzles are formed in a layer prior to the layer being bonded onto another portion of the device. Forming the nozzles in the layer prior to bonding enables forming nozzles that have a desired depth and a desired geometry. Selecting a particular geometry for the nozzles can reduce the resistance to ink flow as well as improve the uniformity of the nozzles across the microelectromechanical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. application Ser. No. 10/913,571, filed on Aug. 5, 2004. The disclosure of the prior application is considered part of and is incorporated by reference in the disclosure of this application. 
     
    
     BACKGROUND 
       [0002]    This invention relates to nozzle formation in a microelectromechanical device, such as an inkjet print head. 
         [0003]    Printing a high quality, high resolution image with an inkjet printer generally requires a printer that accurately ejects a desired quantity of ink in a specified location. Typically, a multitude of densely packed ink ejecting devices, each including a nozzle  130  and an associated ink flow path  108 , are formed in a print head structure  100 , as shown in  FIG. 1A . The ink flow path  108  connects an ink storage unit, such as an ink reservoir or cartridge, to the nozzle  130 . 
         [0004]    As shown in  FIG. 1B , a side view of a cross section of a substrate  120  shows a single ink flow path  108 . An ink inlet  118  is connected to a supply of ink. Ink flows from the ink storage unit (not shown) through the ink inlet  118  and into a pumping chamber  110 . In the pumping chamber, ink can be pressurized to flow toward a descender region  112 . The descender region  112  terminates in a nozzle that includes a nozzle opening  144 , where the ink is expelled. 
         [0005]    Various processing techniques are used to form the ink ejectors in the print head structure. These processing techniques can include layer formation, such as deposition and bonding, and layer modification, such as laser ablation, punching and cutting. The techniques that are used are selected based on a desired nozzle and flow path geometry along with the material that the ink jet printer is formed from. 
       SUMMARY 
       [0006]    In general, in one aspect, the invention features techniques, including methods and apparatus, for forming devices. An aperture is etched into a first surface of a nozzle layer of a multi-layer substrate, where the multi-layer substrate also has a handle layer. The first surface of the nozzle layer is secured to a semiconductor substrate having a chamber such that the aperture is fluidly coupled to the chamber. A portion of the multi-layer substrate is removed, including at least the handle layer of the multi-layer substrate, such that the chamber is fluidly coupled to the atmosphere through the aperture. 
         [0007]    The nozzle layer can be between about 5 and 200 microns, or less than 100 microns thick. The thickness of the nozzle layer can be reduced prior to etching, such as by grinding the nozzle layer. The nozzle layer can include silicon. The multi-layer substrate can include a silicon-on-insulator substrate. The aperture can be etched with an anisotropic etch or by deep reactive ion etch. The aperture can have tapered or straight parallel walls. The aperture can have a rectangular or round cross section. 
         [0008]    Another aspect of the invention features forming a printhead with a main portion having a pumping chamber and a nozzle portion connected to the main portion. The nozzle portion has a nozzle inlet and a nozzle outlet. The nozzle inlet has tapered walls centered around a central axis. The tapered walls lead to the nozzle outlet and the nozzle outlet has substantially straight walls that are substantially free of any surfaces that are orthogonal to the central axis. 
         [0009]    In yet another aspect, the invention features a fluid ejection nozzle layer with a body having a recess with tapered walls and an outlet. The recess has a first thickness and the outlet has a second thickness. The first and second thicknesses together are less than about 100 microns. 
         [0010]    In another aspect, the invention features a fluid ejection device with a semiconductor substrate having a chamber secured to a first surface of a semiconductor nozzle layer having an aperture. The semiconductor substrate has a chamber that is fluidly coupled to the atmosphere through the aperture. The semiconductor nozzle layer is about equal to or less than 100 microns thick. 
         [0011]    Particular implementations can include none, one or more of the following advantages. Nozzles can be formed with almost any desired depth, such as around 10-100 microns, e.g., 40-60 microns. Flow path features can be formed at high etch rates and at high precision. If the nozzle layer and the flow path module are formed from silicon, the layers and module can be bonded together by direct silicon bonding or anodic bonding, thus eliminating the need for a separate adhesive layer. Forming the nozzles in a separate layer from the flow path features allows for additional processing on the back side of the layer in which the nozzles are formed, such as grinding, deposition or etching. The nozzles can be formed with a geometry that can reduce ink flow resistance. Trapping of air can be reduced or eliminated. Thickness uniformity of the nozzle layer can be controlled separately from the thickness uniformity of the substrate in which the flow path features are formed. If the nozzle layer were thinned after being connected to the flow path substrate, it could potentially be difficult to independently control the thickness of the nozzle layer. 
         [0012]    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 
         [0013]      FIG. 1A  shows a perspective view of flow paths in a substrate. 
           [0014]      FIG. 1B  is a cross-sectional view of a print head flow path. 
           [0015]      FIG. 2A  is a cross-sectional view of a print head flow path with a nozzle having walls that are substantially parallel to one another. 
           [0016]      FIG. 2B  is a cross-sectional view of a print head flow path with a nozzle having tapered walls. 
           [0017]      FIGS. 3-8  show one implementation of forming a nozzle in a nozzle layer. 
           [0018]      FIGS. 9-13  show the steps of joining a flow path module to the nozzle layer and completing the nozzle. 
           [0019]      FIGS. 14-23  show a second implementation of forming a nozzle in a nozzle layer. 
           [0020]      FIG. 24  shows a cross-sectional view of a print head flow path. 
       
    
    
       [0021]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0022]    Techniques are provided for controlling the ejection of ink from a fluid ejector or an inkjet print head by forming ejection nozzles with a desired geometry. A print head body can be manufactured by forming features in individual layers of semiconductor material and attaching the layers together to form the body. The flow path features that lead to the nozzles, such as the pumping chamber and ink inlet, can be etched into a substrate, as described in U.S. patent application Ser. No. 10/189,947, filed Jul. 3, 2002, using conventional semiconductor processing techniques. A nozzle layer and the flow path module together form the print head body through which ink flows and from which ink is ejected. The shape of the nozzle through which the ink flows can affect the resistance to ink flow. By etching the nozzle into the back side of the nozzle layer, i.e., the side that is joined to the flow path module, before the nozzle layer is secured to the flow path module, nozzles can be formed with a desired and uniform geometry. Nozzle geometries can be created that may not otherwise be achieved when the nozzle features are only etched from one side of the layer. In addition, the nozzle feature depth can be precisely selected when the back side of the nozzle layer is etched. 
         [0023]    In one implementation, the nozzle depth is selected by forming the nozzle feature in a layer of material having the thickness equal to that of the final nozzle depth, and the nozzle  224  is formed to have a cross-section with substantially consistent geometry, such as perpendicular walls  230 , as shown in  FIG. 2A . In another implementation, multiple etching techniques are employed to form a nozzle having multiple portions that each have a different geometry. The nozzle  224  is formed to have an upper portion that has a conical or pyramidal cross-section  262  and a lower portion with substantially perpendicular walls  236  that leads to the nozzle outlet  275 , as shown in  FIG. 2B . Each of the implementations will be discussed in turn below. 
         [0024]    Forming the nozzle with a substantially consistent geometry, either with perpendicular walls or a pyramidal geometry is described further below. As shown in  FIG. 3 , a multi-layer substrate, such as a silicon-on-insulator (SOI) substrate  400 , can be formed or provided. The SOI substrate  400  includes a handle layer of silicon  416 , an insulator layer  410  and a nozzle layer of silicon  420 . One method of forming an SOI substrate is to grow an oxide layer on a double side polished (DSP) silicon substrate to form the insulator layer  410 . The oxide layer can be from 0.1 to 100 microns thick, such as about 5 microns. A second double side polished silicon substrate can then be bonded to the exposed surface of the oxide layer to complete the SOI substrate  400 . When forming the oxide layer on the DSP substrate, the oxide can be grown on all exposed surfaces of the substrate. After the bonding step, any exposed oxide that is not desired can be etched away, such as by dry etching. 
         [0025]    Different types of SOI substrates can also be used. For example, the SOI substrate  400  can include an insulator layer  410  of silicon nitride instead of an oxide. As an alternative to bonding together two substrates to form the SOI substrate  400 , a silicon layer can be formed on the insulator layer  410 , such as by a deposition process. 
         [0026]    As shown in  FIG. 4 , the nozzle layer  420  of the SOI substrate  400  is thinned to a desired thickness  402 . One or more grinding and/or etching steps, such as a bulk grinding step, can be used to achieve the desired nozzle layer thickness  402 . The nozzle layer  420  is ground as much as possible to achieve the desired thickness, because grinding can control thickness precisely. The nozzle layer thickness  402  can be about 10 to 100 microns, e.g., between about 40 and 60 microns. Optionally, a final polish of the back side  426  of the nozzle layer  420  can decrease surface roughness. Surface roughness is a factor in achieving a silicon to silicon bond, as described below. The polishing step can introduce uncertainty in thickness and is not used for achieving the desired thickness. 
         [0027]    Referring to  FIG. 5 , once the desired thickness of the nozzle layer  420  has been achieved, the back side  426  of the nozzle layer  420  is prepared for processing. The processing can include etching. One exemplary etching process is described, however, other methods may be suitable for etching the nozzle layer  420 . If the nozzle layer  420  does not already have an outer oxide layer, the SOI substrate  400  can be oxidized to form a back side oxide layer  432  and a front side oxide layer  438 . A resist layer  436  is then coated on the back side oxide layer  432 . 
         [0028]    The resist  436  is patterned to define the location  441  of the nozzle. Patterning the resist  436  can include conventional photolithographic techniques followed by developing or washing the resist  436 . The nozzle can have a cross section that is substantially free of corners, such as a circular, elliptical or racetrack shape. The back side oxide layer  432  is then etched, as shown in  FIG. 6 . The resist layer  436  can optionally be removed after the oxide etch. 
         [0029]    The silicon nozzle layer  420  is then etched to form the nozzle  460 , as shown in  FIG. 7A . During the etch process, the insulator layer  410  serves as an etch stop. The silicon nozzle layer  420  can be etched, for example, by deep reactive ion etching (DRIE). DRIE utilizes plasma to selectively etch silicon to form features with substantially vertical sidewalls. DRIE is substantially insensitive to silicon geometry and etches a straight walled hole to within ±1°. A reactive ion etching technique known as the Bosch process is discussed in Laermor et al. U.S. Pat. No. 5,501,893, the entire contents of which is incorporated hereby by reference. The Bosch technique combines an etching step with a polymer deposition to etch relatively deep features. Because of the alternative etching and deposition, the walls can have a slight scallop contour, which can keep the walls from being perfectly flat. Other suitable DRIE etch techniques can alternatively be used to etch the nozzle layer  420 . Deep silicon reactive ion etching equipment is available from Surface Technology Systems, Ltd., located in Redwood City, Calif., Alcatel, located in Plano, Tex., or Unaxis, located in Switzerland and reactive ion etching can be conducted by etching vendors including Innovative Micro Technology, located in Santa Barbara, Calif. DRIE is used due to its ability to cut deep features of substantially constant diameter. Etching is performed in a vacuum chamber with plasma and gas, such as, SF 6  and C 4 F 8 . 
         [0030]    In one implementation, rather than etching with DRIE the silicon nozzle layer  420 , an etch is performed to create tapered walls, as shown in  FIG. 7B . Tapered walls can be formed by anisotropically etching the silicon substrate. An anisotropic etch, such as a wet etch technique, can include, but is not limited to, a technique that uses ethylenediamine or KOH as the etchant. Anisotropic etching removes molecules from the 100 plane much more quickly than from the 111 plane, thus forming the tapered walls. An anisotropic etch on a substrate with the 111 plane at the exposed surface exhibits a different etch geometry than a substrate with a 100 plane at the surface. 
         [0031]    When the nozzle is complete, the back side oxide layer  432  is stripped from the substrate, such as, by etching, as shown in  FIG. 8 . 
         [0032]    The etched silicon nozzle layer  420  is then aligned to a flow path module  440  that has the descender  512  and other flow path features in preparation for bonding, as shown in  FIG. 9 . The surfaces of the flow path module  440  and the nozzle layer  420  are first cleaned, such as by reverse RCA cleaning, i.e., performing an RCA2 clean consisting of a mixture of DI water, hydrochloric acid and hydrogen peroxide followed by an RCA1 clean in a bath of DI water, ammonium hydroxide and hydrogen peroxide. The cleaning prepares the two elements for direct silicon bonding, or the creation of Van der Waal&#39;s bonds between the two silicon surfaces. Direct silicon bonding can occur when two flat, highly polished, clean silicon surfaces are brought together with no intermediate layer between the two silicon layers. The flow path module  440  and the nozzle layer  420  are positioned so that the descender  512  is aligned with the nozzle  460 . The flow path module  440  and nozzle layer  420  are then brought together. Pressure is placed at a central point of the two layers and allowed to work its way toward the edges. This method reduces the likelihood of voids forming at the interface of the two layers. The layers are annealed at an annealing temperature, for example, around 1050° C.-1100° C. An advantage of direct silicon bonding is that no additional layer is formed between the flow path module  440  and the nozzle layer  420 . After direct silicon bonding, the two silicon layers become one unitary layer such that no or virtually no delineation between the two layers exists when the bonding is complete, as shown in  FIG. 10  (the dotted line shows the former surfaces of the flow path module  440  and nozzle layer  420 ). 
         [0033]    As an alternative to directly bonding two silicon substrates together, a silicon layer and an oxide layer can be anodically bonded together. The anodic bonding includes bringing together the silicon and oxide layers and applying a voltage across the substrates to induce a chemical bond. 
         [0034]    Once the flow path module  440  and nozzle layer  420  are bonded together, the handle layer  416  is removed. Specifically, the handle layer  416  can be subjected to a bulk polishing process (and optionally a finer grinding or etching process) to remove a portion of the thickness, as shown in  FIG. 11 . 
         [0035]    As shown in  FIG. 12 , the oxide layer can be completely removed by etching, thus exposing the nozzle opening. Although this implementation has parallel side walls, the nozzle could have tapered walls if the etching process shown in  FIG. 7B  were to be used. 
         [0036]    As shown in  FIG. 13 , alternatively, the insulator layer  410  can be left on the nozzle layer  420  and etched through from the outer surface to form a part of the nozzle opening. 
         [0037]    In one implementation, the back side etch process is performed to create a nozzle with multiple portions having different geometries. 
         [0038]    The nozzle can be formed in either a 100 plane DSP wafer or a SOI substrate with a nozzle layer  500  that is a 100 plane silicon, as shown in  FIG. 14 . The nozzle layer  500  can be thinned to the desired thickness, as described above. The thickness can be between around 1 and 100 microns, such as between about 20 and 80 microns, e.g., around 30 to 70 microns. 
         [0039]    Referring to  FIG. 15 , an oxide layer is grown on the silicon nozzle layer  500  to form a back side oxide  526 . An insulator layer  538  and a handle layer  540  are on the opposite side of the nozzle layer  500  from the back side oxide  526 . A resist can be formed on the back side oxide  526 , such as by spinning-on the resist. The resist can be patterned to define the location of the nozzle. The location of the nozzle is formed by creating an opening  565  in the back side oxide  526 . 
         [0040]    Referring to  FIGS. 16A ,  16 B and  16 C, the nozzle layer  500  is etched using an anisotropic etch, such as a wet etch technique. The etch defines a recess  566  in the silicon nozzle layer  500  that has an inverted pyramid shape, or is the shape of a pyramidal frustum with a base, a recessed surface  557  parallel to the base and sloped walls  562 . The tapered wall  562  meets the recessed surface  557  at an edge having a length  560 . The recess  566  can be etched through to the insulator layer  538 , as shown in  FIG. 16A . Alternatively, the recess  566  can extend only partially through the nozzle layer  500 , as shown in  FIG. 16B . If the recess  566  is not etched through to the insulator layer  538 , substantially constant recess depths can be achieved by controlling the etch time and rate. A wet etch using KOH has an etch rate that is dependent on temperature. The recess  566  can be about 1 to about 100 microns deep, such as about 3 to 50 microns. 
         [0041]    As shown in  FIG. 17 , the etched nozzle layer  500  is joined with a flow path module  440 . The nozzle layer  500  is joined with the flow path module  440  so that the descender  512  is aligned with the recess  566 . The nozzle layer  500  and the flow path module  440  can be bonded together with an adhesive, an anodic bond or a direct silicon bond (fusion bond). If a direct silicon bond is selected, the back side oxide  526  is removed prior to bonding. 
         [0042]    As shown in  FIG. 18 , the handle layer  540  is removed. The handle layer  540  can be removed, such as by grinding, etching or a combination of grinding and etching. 
         [0043]    To achieve the desired nozzle geometry, the front side of the nozzle layer  500  is also etched. As shown in  FIG. 19 , the front side is prepared for etching by coating a resist  546  on the insulator layer  538  and patterning the resist  546 , as described above. The resist is patterned such that the underlying insulator layer  538  is exposed in areas that correspond to the recesses  566  formed in the back side of the nozzle layer  500 . 
         [0044]    As shown in  FIGS. 20A and 20B , respectively, a view of the front side of the nozzle layer  500  shows that the resist  546  can be patterned with a circular opening  571  or a rectangular opening  572 . Other opening geometries may be suitable, such as a polygon with five or more sides. The exposed oxide is etched in a location  559  corresponding to the recess  566  to expose the underlying nozzle layer  500 , as shown in  FIG. 21 . 
         [0045]    Referring to  FIG. 22 , the nozzle layer  500  is etched to form a nozzle outlet  575 . The etch process used can be DRIE, so that the nozzle outlet  575  has substantially straight walls, as described above. This can form a nozzle outlet  575  that converges at a point beyond the exterior of the nozzle outlet  575 . The nozzle outlet can be about 5 to 40 microns in diameter, such as about 25 microns in diameter. The diameter  577  of the nozzle outlet  575  is sufficient to intersect the tapered walls  562  of the recess  566 . The nozzle recess  566  forms the nozzle entry. 
         [0046]    Referring to  FIGS. 23A and 23B , a side cross sectional view of the nozzle layer shows the intersection of the tapered walls  562  and the nozzle outlet  575 . The diameter of the nozzle outlet  575  is large enough so that the intersection between the recess  566  and the nozzle outlet  575  can remove any portion of the recessed surface  557 , even if the recess  566  did not extend to the insulator layer when the recess was formed. Therefore, the nozzle outlet  575  is formed to have a dimension  577  that is equal to or greater than the length  560  of the wall  562  where the wall  562  meets the recessed surface  557 . In one implementation, the diameter of the nozzle outlet  575  is less than the recessed surface of the pyramidal frustum and a portion of the recessed surface remains after the outlet  575  is formed. 
         [0047]    As shown in  FIG. 24 , the nozzle layer processing is completed. The back side oxide layer  526  is removed. The pyramidal nozzle inlet can have a depth of between about 10 to 100 microns, such as about 30 microns. The nozzle outlet  575  can have a depth of between about 2 and about 20 microns, such as about 5 microns. 
         [0048]    Modifications can be made to the above mentioned processes to achieved the desired nozzle geometry. In one implementation, all of the etching is performed from the back side of the nozzle layer  500 . In another implementation, the insulator layer  538  is not removed from the nozzle. To complete the nozzle, the insulator layer  538  can be etched so that the walls of the opening are substantially the same as the walls of the nozzle outlet  575 , as shown in  FIG. 22 . Alternatively, the walls of the opening in the insulator layer  538  can be different from the walls of the nozzle outlet  575 . For example, the nozzle opening  575  can have tapered walls that lead into a straight walled portion formed in the insulator layer  538 . Forming the opening in the insulator layer  538  can either occur before or after attaching the nozzle layer  500  with a flow path module  440 . 
         [0049]    One potential disadvantage of forming the nozzles in a separate substrate is that the depth of the nozzles may be limited to a particular range of thicknesses, such as more than about 200 microns. Processing substrates thinner than about 200 microns can lead to a drop in yield, because of the increased likelihood of damaging or breaking the substrate. A substrate generally should be thick enough to facilitate substrate handling during processing. If the nozzles are formed in a layer of an SOI substrate, the layer can be ground to the desired thickness prior to formation while still providing a different thickness for handling. The handle layer also provides a portion that can be grasped during processing without interfering with the processing of the nozzle layer. 
         [0050]    Forming the nozzle in a layer of a desired thickness can obviate the step of reducing the nozzle layer after the nozzle layer has been joined with the flow path module. Grinding away the handle layer after the nozzle layer is joined with the flow path module does not leave the flow path features open to grinding solution or waste grinding material. When the insulator layer is removed after the nozzle layer is joined to the flow path module, the insulator layer can be selectively removed so that the underlying silicon layer is not etched. 
         [0051]    A nozzle formation process that uses two types of processing can form nozzles with intricate geometries. An anisotropic back side etch can form a recess in the shape of a pyramidal frustum having a base at the surface of the substrate, sloped or tapered walls and a recessed surface in the substrate. A front side etch that is configured so that the diameter is greater than the diameter of the recessed surface of the pyramidal frustum removes the recessed surface of the pyramidal frustum shape from the recess and the nozzle. This technique removes any substantially flat surface that is orthogonal to the direction of ink flow from the nozzle. This can reduce the incident of trapped air in the nozzle. That is, tapered walls that are formed by the anisotropic etch can keep the ink flow resistance low, while accommodating a large amount of meniscus pull-back during fill without air ingestion. The tapered walls of the nozzle smoothly transitions into the straight parallel walls of the nozzle opening, minimizing the tendency of the flow to separate from the walls. The straight parallel walls of the nozzle opening can direct the stream or droplet of ink out of the nozzle. 
         [0052]    The depth of the anisotropic etch directly affects the length of both the nozzle entry and the nozzle outlet if the nozzle opening is not formed with a diameter greater than the diameter of the recessed surface of the pyramidal frustum. The anisotropic etch depth is determined by the length of time of the etch along with the temperature at which the etch is performed and can be difficult to control. The geometry of a DRIE etch may be easier to control than the depth of an anisotropic etch. By intersecting the walls of the nozzle outlet with the tapered walls of the nozzle entry, variations in depth of the anisotropic etch do not affect the final nozzle geometry. Therefore, intersecting the walls of the nozzle outlet with the tapered walls of the nozzle entry can lead to higher uniformity within a single print head and across multiple print heads. 
         [0053]    A number of implementations of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Exemplary methods of forming the aforementioned structures have been described. However, other processes can be substituted for those that are described to achieve the same or similar results. For example, tapered nozzles can be formed by electroforming, laser drilling or Electrical Discharge Machining. The apparatus described can be used for ejecting fluids other than inks. Accordingly, other embodiments are within the scope of the following claims.