Abstract:
Fluid ejection nozzles having a tapered section leading to a straight walled bore are described. Both the tapered section of the nozzle and the straight walled bore are formed from a single side of semiconductor layer so that the tapered section and the bore are aligned with one another, even when an array of nozzles are formed across a die and multiple dies are formed on a semiconductor substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/346,698, filed Dec. 30, 2008, which is incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to forming etched features in a semiconductor material. 
       BACKGROUND 
       [0003]    Dies having multiple structures for jetting fluid can be formed in semiconductor material. Each structure can include a fluid path that leads to a nozzle. An actuator forces fluid in the flow path out the nozzles when actuated. Some types of nozzles have a tapered portion that leads to an oval or circular outlet. To form the die, an array of nozzles can be partially formed in a nozzle plate that is attached to a device body in which the fluid path is located. In forming a device, the nozzle plate can be formed separate from the device body. In some devices, the tapered portion is formed in the nozzle plate prior to attaching the nozzle plate to the device body. The circular holes are then formed in the nozzle plate after attaching the nozzle plate to the device body. Circular nozzles in a nozzle plate have more uniform ejection qualities, such as drop volume and ejection direction, than nozzles with a rectangular or square shape. 
       SUMMARY 
       [0004]    In one aspect, a method of forming a nozzle plate is described. The method includes etching a first side of a semiconductor layer to form a tapered feature, wherein the semiconductor layer is part of a multilayer substrate and an etch stop layer is adjacent to the semiconductor layer and etching the first side of the semiconductor substrate to form a straight walled feature wherein walls of the straight walled feature intersect walls of the tapered feature and the walls of the straight walled feature are perpendicular to a surface of the first side of the semiconductor layer. 
         [0005]    In another aspect, a method of forming a nozzle plate of a fluid ejection device is described that includes etching a first surface of a semiconductor layer to form a straight walled feature that extends vertically and only part way to an etch stop layer adjacent to the semiconductor layer. A protecting layer is applied on sidewall surfaces of the straight walled feature. A bottom of the straight walled feature is etched to extend the feature to the etch stop layer and to form a bore. Side surfaces of the bore are extended to cause a portion of the bore to widen. 
         [0006]    Fluid ejection devices are described. In one aspect a device includes a silicon body having a descender formed therein. A nozzle plate has a nozzle formed therein, wherein the nozzle has an inlet adjacent to the descender, the inlet having walls that are substantially parallel to one another, the inlet leading to a tapered section, the tapered section leading to a passage and the passage leading to an outlet, wherein the inlet is in fluid communication with the descender but has a cross sectional dimension different from the descender. 
         [0007]    Embodiments of the techniques described herein may include one or more of the following. A cross section of the straight walled features may be either circular or oval and the straight walled feature may extend to the etch stop layer. A cross section of the straight walled feature parallel to the first side may be less than a cross section of the tapered feature parallel to the first side. The straight walled feature may be etched prior to etching to form the tapered feature and a location of the straight walled feature in the semiconductor layer may be used to position the tapered feature. Etching the first side of the semiconductor substrate to form a straight walled feature may include etching a topside layer of oxide or nitride on the first side of the semiconductor substrate to form a mask with a first aperture having a round or oval shape. An overcoat layer of oxide or nitride may be to walls of a hole formed in the semiconductor layer. An overcoat layer of oxide or nitride may be applied to the topside layer and to walls of a hole formed in the semiconductor layer, wherein the hole has a cross section substantially equal to a cross section of the first aperture and subsequently part of the overcoat layer may be removed to leave a sidewall layer of oxide or nitride and the topside layer and to expose the semiconductor layer around the hole. Etching a first side of a semiconductor layer may include etching the semiconductor layer in a region that is exposed around the hole to form the tapered feature. Etching the semiconductor layer may leave a cylindrical tube of oxide between the hole and the tapered feature. The tube of oxide may be removed. Etching to form the tapered feature may form a square pyramid. Etching the semiconductor layer may include etching a silicon layer. Etching the semiconductor layer may include etching a silicon layer that is about 50 microns thick or less. The straight walled feature may be round and the bore may be substantially cylindrical. Extending side surfaces of the bore may include forming walls that are not parallel to walls of the bore. Extending side surfaces of the bore may include performing an anisotropic etch. Extending side surfaces of the bore may also include etching from a second surface of the semiconductor layer that is opposite to the first surface to form walls that are parallel to walls of the bore. Etching a first surface of a semiconductor layer can include etching silicon. The inlet can have a square cross section. The descender can be wider than the inlet of the nozzle. The passage can be cylindrical. 
         [0008]    The methods described herein may provide one or more of the following advantages. A bore and a tapered portion of a nozzle can both be formed by etching a layer of semiconductor material from a single side, rather than from opposite sides of the layer. The cylindrical bore may therefore be easier to align with the tapered portion. If the tapered portion and circular portion are etched from a single side prior to bonding the nozzle plate to a device body, alignment problems associated with shifting of a distance from one feature to a next feature along a plane due to deformation when the nozzle plate is attached to a device body can be avoided. If nozzles in an array of nozzles have circular outlets, there may be more consistency, e.g., in nozzle size and fluid ejection performance, across a series of outlets than when the nozzles are formed with square outlets. This is because etching to form circular outlets can be done by a method that is indifferent to the crystal planes of the layer. If the nozzles in an array are uniformly sized, the size of droplets that are jetted from each nozzle can be more uniform. Uniformly sized droplets can improve jetting precision and image quality when the nozzle is part of a printing device. In addition, square nozzles can have a surface tension across the meniscus that is not uniform, especially near the corners of the square shape nozzle opening. Lack of uniformity can cause inaccurate jetting direction. A circular outlet may allow for a more accurate and more uniform jetting direction of fluid drops. The tapered portion of the feature may be precisely aligned with the cylindrical portion. Alignment can allow the droplet to be ejected perpendicular to the nozzle plate surface. If the cylindrical bore and tapered portion are not aligned with one another, droplet ejection may be at an angle. Further, if the alignment is not consistent across an array of nozzles, the ejection direction across the array may not be consistent. Better alignment and more consistent quality of alignment therefore can result in more consistent direction of droplet ejection and improved image quality. 
         [0009]    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 
         [0010]      FIGS. 1-15  illustrate the steps of one embodiment of a method of forming a nozzle plate and attaching it to a device layer. 
           [0011]      FIGS. 16-27  illustrate the steps of another embodiment of a method of forming a nozzle plate and attaching it to a device layer. 
           [0012]      FIGS. 28-29  illustrate a nozzle plate adhered to a device body. 
       
    
    
       [0013]    Many of the layers and features are exaggerated to better show the process steps and results. Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0014]    Forming a nozzle plate for a fluid ejection device includes forming a number of nozzles each having a cylindrical bore section and a tapered section. One difficulty in forming such a feature with this shape is aligning the cylindrical portion of the nozzle with the tapered portion. Methods for performing the etch of both the cylindrical portion and the tapered portion from a single side of a layer are described that can eliminate the alignment problems encountered when the cylindrical bore is etched from a side of the layer opposite to the tapered recess. 
         [0015]    Referring to  FIG. 1 , a silicon-on-oxide (“SOI”) wafer  10  has a layer of a single crystal material, such as a silicon layer  40 , a handle layer  20  and an oxide layer  30  between the silicon layer  40  and handle layer  20 . The silicon layer  40  can have a &lt;100&gt; crystal orientation. The handle layer  20  can be formed of silicon. A second oxide layer  50  is on a side of the silicon layer  40  opposite to the handle layer  20 . Optionally, a third oxide layer (not depicted) is on a side of the handle layer  20  opposite to the silicon layer  40 . While the layers can have just about any thickness, the oxide layers  30 ,  50  are thinner than the silicon layer  40  and handle layer  20 . In an exemplary SOI wafer  10 , the oxide layers are less than a few microns thick, such as about 1 micron thick. The handle layer can have a thickness of greater than 200 microns, such as about 600 microns. The silicon layer  40  has the thickness of the final desired thickness of the nozzle plate. Here, the silicon layer  40  is about 30 microns thick. In other embodiments, the silicon layer  40  is about 50 microns. Only a portion of a wafer  10  is shown in the figures for the sake of simplicity. That is, the creation of a single nozzle is shown, but in most cases a plurality of nozzles will be formed in a plurality of nozzle plates formed in the wafer  10 . 
         [0016]    Referring to  FIG. 2 , a layer of photoresist  65  is applied to the oxide layer  50  that is over the silicon layer  40 . The layer of photoresist  65  is patterned to create a square hole  73 . The photoresist layer  65  with the square hole  73  is used as a mask for etching a hole  75  into oxide layer  50 , as shown in  FIG. 3 . The layer of photoresist  65  is then stripped from the wafer  10 , as shown in  FIG. 4 . A second layer of photoresist  85  is applied to the top of the etched oxide layer  50 . Referring to  FIGS. 5-6 , the second layer of photoresist  85  is patterned with a circular hole  77 . A stepper can be used to perform precise alignment of a mask to a target on the wafer, such a stepper can have an alignment accuracy of about 15 nm. As shown in  FIG. 7 , the second layer of photoresist  85  is used as a mask to etch into the silicon layer  40 , thereby forming a cylindrical hole  83 . The cylindrical hole  83  can be formed, such as by using a dry reactive ion etching method, such as a low frequency etch process that does not undercut, for example, the Bosch process. The silicon oxide layer  30  acts as an etch stop during the etching of silicon layer  40 . The second layer of photoresist  85  is then stripped, as shown in  FIG. 8 . Silicon nitride (Si 3 N 4 ) could alternatively be used in layer  30  or layer  50 . 
         [0017]    As shown in  FIG. 9 , a thin silicon nitride or silicon oxide layer  91  is applied over the oxide layer  50  and on the sidewalls of hole  83  and the bottom of hole  83 . In some embodiments, the thin oxide layer  91  is a thermal oxide layer that is grown using a furnace or silicon nitride deposited using low pressure chemical vapor deposition. The thin oxide layer  91  can have a thickness of about 0.5 micron and can be thinner than oxide layer  50 . Because the etched oxide layer  50  leaves a portion of silicon layer  40  surrounding hole  83  exposed, the effect of forming oxide layer  91  is to create a thin region of oxide that has a square perimeter surrounded by a thick layer of oxide. 
         [0018]    Some of the oxide is then removed from horizontal surfaces of the wafer by an anisotropic process such as dry etching, as shown in  FIG. 10 . The oxide removal is performed until the square shaped region of thin oxide surrounding the cylindrical hole is removed and allows the silicon layer  40  to be exposed in the square region. Oxide is also removed from the bottom of the cylindrical hole. In some embodiments, the thickness of the original oxide layer  50  remains at the bottom of the cylindrical hole  83 . The oxide on the sidewalls of the cylindrical hole remains intact. 
         [0019]    As shown in  FIG. 11 , a wet etch is then performed, using the remaining oxide from oxide layer  50  on top of silicon layer  40  as a mask. The anisotropic etch, for example, a tetramethylammonium hydroxide (TMAH) or potassium hydroxide (KOH) etch, stops on the  111  crystal plane to form angled sidewalls  101  in the square region that is devoid of oxide around the cylindrical hole. This forms the tapered region of the nozzle. In some embodiments a dry etch could be used to form the taper and not necessarily stop on the &lt;111&gt; crystal plane. In some embodiments, this etch step leaves a cylindrical tube of oxide  97  in the center of a square pyramidal shaped recess in the silicon layer  40 . A picture of the etched feature in  FIG. 12  shows a tube of oxide material and angled sidewalls  101 . 
         [0020]    Referring to  FIG. 13 , the cylindrical tube of oxide  97  is removed with an oxide wet etch, such as a hydrofluoric acid etch or a buffered oxide etch. If silicon nitride is used as the material  97  in  FIG. 10 , the silicon nitride can be removed with hot phosphoric acid. The cylindrical portion  103  of the feature has walls that are substantially perpendicular to a bottom surface  105  of the silicon layer  40  and meet the sidewalls  101 . Because these features can form nozzles, the silicon layer  40  can form a nozzle plate. Referring to  FIG. 14 , the nozzle plate can then be attached to a device body  130 . The device body  130  can include descenders  132 . When the device body  130  is aligned with the etched silicon layer  40 , the descenders  132  are aligned with nozzle features  134 . The device body  130  and silicon layer  40  are bonded together, such as with an adhesive or a silicon to silicon bond. Suitable device bodies and adhesive methods are described in U.S. Publication No. 2005-0099467, published on May 12, 2005, which is incorporated by reference herein for its disclosure and figures. The handle layer  20  and any oxide or nitride layers can be removed, such as by grinding, polishing, or dry etching. In some embodiments, a final oxide layer  33  can be applied to exposed surfaces, such as a layer 0.4 or 0.5 micron thick, as shown in  FIG. 15 . 
         [0021]    In another embodiment of forming a nozzle plate with nozzles, an SOI wafer  210  is provided that has a handle layer  220 , an oxide layer  230  on the handle layer, a silicon layer  240  on the oxide layer  230  and an oxide layer  250  on the silicon layer  240 , as shown in  FIG. 16 . This process can use materials and techniques that are similar to those described above. For example, this SOI wafer  210  can have the same layers and thicknesses as mentioned above for SOI wafer  10 . A circular hole  255  is etched into oxide layer  250 , as shown in  FIGS. 17 and 18 . 
         [0022]    Referring to  FIG. 19 , the hole  255  is used as a mask to etch into silicon layer  240 . The etch that is performed only extends part way through the total thickness of the silicon layer  240 . The depth of the etch is controlled by timing the etching process. The etch depth is determined by determining the length of the straight walled, i.e., cylindrical, portion of the bore that is desired. An oxide layer  270  is then applied to the bottom and sidewalls of cylindrical hole  265  and on top of the remaining oxide layer  250 . 
         [0023]    Referring to  FIG. 20 , an oxide etch is performed to remove oxide from the bottom of cylindrical hole  265 . This also removes some of the oxide thickness from over the silicon layer  240 . However, oxide remains on the top of the silicon layer  240  at this stage due to original oxide layer  250 . The resulting layer is oxide layer  280 . As shown in  FIG. 21 , the cylindrical hole  265  or tube in the silicon layer  240  is then extended down to the oxide layer  230 , such as by performing a dry etch to form holes  267 . The geometry of the hole  267  is now a cylindrical tube. While the walls of the cylindrical substantially form a right cylinder, the top of the cylinder is lined with oxide and the bottom is not. 
         [0024]    Referring to  FIG. 22 , an anisotropic or wet etch is performed in hole  267  to create hole  269 , which has a portion with angled walls  270 . The portion of the hole  279  that is protected by the oxide layer  280  is not affected by the wet etch. Referring to  FIG. 23 , the oxide layer is stripped, such as by performing a buffered oxide etch. The silicon layer  240  can be cleaned and reoxidized to form oxide layer  285 , as shown in  FIG. 24 . The oxide protects the structure from attack during a subsequent etch. This oxide layer can be removed, if desired, at a later step. 
         [0025]    A top of the silicon layer  240  is then bonded, such as by direct bonding, to a sacrificial wafer  290 , as shown in  FIG. 25 . The sacrificial wafer  290  can be formed of silicon. The silicon layer  220  can be removed from the back side of silicon layer  220 . A layer of photoresist  292  is applied to the backside of the silicon layer  240 . The photoresist is patterned with a hole  283 . The hole can be a square hole. The edges of the hole align approximately with the greatest dimension or width of the hole  259 , i.e., the hole formed by the anisotropic etch. The alignment does not need to be accurate. The silicon layer can be seen through the oxide layer such as with an infrared (IR) camera. This allows for alignment of hole  283  with hole  259 . The hole  283  is then used as a mask and the silicon layer  240  is etched from a backside. The oxide layer  294  from inside the hole  259  is removed by a wet or dry etch. A portion  299  of the oxide layer  294  is not removed if the oxide is removed by a dry etch, as shown in  FIG. 26 . 
         [0026]    Referring to  FIG. 27 , the photoresist  292  is removed and a device layer  130  is bonded to the backside of silicon layer  240 . A descender  132  in the device layer  130  is aligned with hole  261 . However, it is not crucial the that sidewalls of the descender are co-planar with the sidewalls of the hole  261 . That is, the descender can be somewhat wider or narrower than hole  261 . 
         [0027]    Referring to  FIGS. 28 and 29 , a device body  130  attached to a nozzle plate  350 ,  360  that is formed according to the methods described herein is completed, such as by adding additional silicon layers and actuators  370 . An actuator  370 , such as a piezoelectric type actuator, is adjacent to a pumping chamber. Actuation of the actuator  370  when fluid is in the pumping chamber causes fluid to be expelled through nozzle  380 . As shown, the descender is wider than the inlet of the nozzle. 
         [0028]    When nozzles having a tapered portion that leads to a cylindrical outlet are formed where the taper is etched from one side of the substrate and the outlet is etched from the opposite side, it can be difficult to etch the outlet so that it is aligned with the tapered recess. The problem can be exacerbated by bowing in the SOI wafer or stress, stretching or compression that can be caused in the nozzle plate layer by attaching the SOI wafer to the device body. It can be very difficult to apply a mask and locally align each circular hole with a tapered inlet. That is, if the SOI wafer is bowed at all, it may be possible to align a mask with some of the circular holes on a substrate, but other holes can be out of alignment. Ideally, all of the holes across the substrate could be aligned with their respective tapered portions. Etching both the tapered portion and the circular portion using the same mask can eliminate this problem, particularly if one etched feature can create a location in which the other etched feature is etched. When both the tapered portion and the round bore portion are etched from the same side of the nozzle plate layer, an operator can visually inspect whether a mask to form one of the features is aligned with the features that are already formed. There is no need to guess whether all of the features are aligned. Also, because this method completes the nozzle etching prior to bonding the nozzle plate to the device body, if there are any defects caused by etching the nozzle plate, only the nozzle plate needs to be discarded, rather than the nozzle plate and the device body. 
         [0029]    A number of embodiments 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. For example, although circular nozzle outlets and cylindrical bores are described, the outlets and bores can have an oval shape or cross section or a bore with a rectangular or square cross section. Accordingly, other embodiments are within the scope of the following claims.