Patent Publication Number: US-7713456-B2

Title: Drop generator die processing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a division of U.S. patent application Ser. No. 10/286,125, filed Oct. 31, 2002, now U.S. Pat. No. 6,885,083 which is hereby incorporated by reference. 

   TECHNICAL FIELD 
   This invention relates to the production of wafer dies that are eventually used as drop generators in devices such as thermal inkjet printheads, and to a way of processing the dies to reduce the likelihood of delamination of thin film layers on the dies. 
   BACKGROUND OF THE INVENTION 
   Drop generators, such as used with inkjet printers for ejecting droplets of ink, are generally formed over an insulated, rigid substrate to define a printhead. The substrate is often part of a conventional silicon wafer that is delineated into an array of individual dies. Each die on the wafer is processed to produce a single printhead. The wafer printhead dies are thereafter separated and incorporated into print cartridges or carriers that connect the printhead with an ink supply. 
   The printheads are manufactured from selected combinations of thin film layers of material that are deposited or grown on the substrate using processes often adapted from conventional semiconductor component fabrication. In particular, drop generators and associated control circuitry of the printhead are incorporated into and carried on the front surface of the rigid substrate mentioned above. In certain designs, the material comprising at least one of the thin film layers may be permeable to moisture. If portions of such layers are exposed to moisture (such as may occur when the printhead is mounted to the print cartridge), it is possible for the printhead layers to delaminate as the absorbed moisture penetrates and degrades the moisture permeable layer. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a diagram representing a cross section of part of a die that is processed in a manner consistent with an embodiment of the present invention. 
       FIG. 2  is another diagram representing a top view of some dies of a wafer, which dies are processed in accord with an embodiment of the present invention. 
       FIGS. 3A-3F  are detailed diagrams illustrating one preferred method of processing a die in accordance with an embodiment of the present invention. 
       FIG. 4  is a diagram illustrating an alternative application of the present invention in a part of a die that carries a fusible link. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Reference is made first to  FIG. 1 , which diagrammatically illustrates the primary components of concern in describing a preferred embodiment of the present invention. Specifically, the diagram shows a cross section taken at the junction of two adjacent dies  22 ,  24  that are part of an array of dies on a wafer  20 . The dies are depicted in their orientation prior to being separated by conventional techniques, such as by sawing of the wafer. 
   In a preferred embodiment, each die  22 ,  24  will eventually be used as an inkjet printhead. Thus, each die carries layers of material, in addition to those shown, that are adapted for moving and chambering ink in the printhead, and for controlled ejections of drops from the ink chamber. These layers, which are generally applied using thin film techniques, include mechanisms for controlling the firing of the resistor that expels the ink drops. Such mechanisms include transistors and associated conductors between the printhead and a controller that is normally carried in the printer. Where pertinent to the present invention, certain of these additional layers are discussed below, primarily in connection with  FIGS. 3A-3F . The reader may, however, refer to additional U.S. Patents for more information about such printhead construction. Two of these patents are U.S. Pat. Nos. 6,336,714 and 5,635,966. 
   Before continuing with this description, it is pointed out that  FIG. 1  shows only a small portion of the wafer and two adjacent dies  22 ,  24 , including two, substantially parallel adjacent edges  30 ,  32  of the respective dies  22 ,  24 . In this embodiment, the space between the two edges is removed by conventional wafer sawing techniques to physically define the respective die edges  30 ,  32  after the die fabrication steps are complete. (Other conventional techniques may be employed for separating the dies from the wafer.) This space aligns with what is designated as a saw street on the wafer prior to separation of the dies. As noted above, for the purposes of explaining this embodiment of the invention, only the die layers adjacent to the edges  30 ,  32  are discussed in this portion of the description. 
   In the embodiment shown the wafer  20  generally comprises a silicon substrate  26  upon which is grown a thin silicon oxide  28 . A layer of phosphosilicate glass (PSG)  40  covers the oxide on the substrate in the vicinity of the die edges  30 ,  32  such that, prior to separation of the dies, the layer of PSG  40  extends from one die to the next across the saw street. 
     FIG. 1  illustrates the adjacent dies  22 ,  24  with the layers that pass through the saw street shown in horizontal dotted lines that represent the position of those layers before the dies are sawn apart. The PSG layer  40  is characteristically moisture permeable. As a result, the edge  40 E of that layer that is exposed after the dies  22 ,  24  are sawn apart is susceptible to the penetration of ambient moisture, which penetration is illustrated in  FIG. 1  by the wavy arrow  45 . The moisture may originate in the ambient air or, in instances where the die is used as a printhead, in the liquid ink or vapor that is in the vicinity of the edge  40 E. 
   In one embodiment, the movement of moisture into a die can have the deleterious effect of disintegrating the PSG layer  40 , leading to a delamination of other thin film layers on the die. For example, delamination of the die can cause failure of electrical-signal carrying layers, such as shown as the conductive layer  42  in  FIG. 1 . As will become clear, embodiments of the present invention tend to prevent such delamination. 
   It is useful to describe next an exemplary way of fabricating the just-mentioned conductive layer  42 , which in  FIG. 1  extends through a hole or via  43  in the PSG layer and in the oxide  28  to contact the substrate  26 . This via  43  is made by patterning a layer of photoresist material that is laid over the PSG layer  40 , and thereafter etching the PSG and oxide to form the via  43 . The conductive layer  42  is thereafter deposited over the PSG and into the via  43 , and then patterned and etched to the configuration shown in  FIG. 1 . 
   In one embodiment, the uppermost layer of the dies  22 ,  24 , such as appears near their respective edges  30 ,  32 , can be referred to as a protective layer  44  comprising, for example, a deposit of passivation material such as SiN covered with SiC. 
   In accordance with this embodiment, the present invention, the dies  22 ,  24  are processed with the goal of interrupting the continuity of the moisture permeable PSG layer  40  near the location where that layer may be exposed to moisture, such as the near the edges  30 ,  32  of the dies. The interruption has the effect of blocking movement of the moisture through the PSG layer  40  (or any other moisture permeable layer interrupted in accord with an embodiment of the present invention). 
   In one embodiment, a barrier  50  is provided for interrupting or separating the PSG layer  40 . In this embodiment, the barrier  50  is very near the exposed edge  30 ,  32  of each die and, therefore, the path of movement of the moisture  45  is very short, and any attendant delamination of the die near the edge is inconsequential to the operation of the die components. 
   One way of positioning the barrier  50  to interrupt the PSG layer  40  is to first remove a portion of the PSG layer at a boundary near the edge of the die. In one approach, this is done by further patterning of the photoresist material that is laid over the PSG layer for making the via  43  mentioned above. The PSG layer  40  is then etched to form a gap  52  in that layer (as well as the via), which gap is illustrated in  FIG. 1  as the space removed from the PSG layer  40 . Depending on the properties of the selected etchant, the underlying oxide layer  28  may also be removed as is also illustrated in  FIG. 1 . Alternatively, therefore, the oxide layer  28  may remain after the gap  52  is etched. 
   The gap  52  in the PSG layer is located near the edges  30 ,  32  of the respective dies and, therefore, underlies the protective layer  44 . Accordingly, the deposition of the protective layer (which occurs after the formation of the gap  52 ) substantially fills the gap with the protective material, thus forming the barrier  50 . 
   It is contemplated that the gap  52  may be located (or the die layers selected) such that material other than that of the protective layer  44  fills the gap  52  to form the barrier  50 . For instance, the gap  52  may completely or partially underlie a subsequently deposited metal layer. Accordingly, all or some of the barrier  50  may be metal. It will be appreciated that such barrier material will serve to block moisture movement. For that matter, any material that forms a solid barrier and is not moisture permeable (that is, material that has no affinity for absorbing liquid) will suffice for this embodiment. 
   The location and size of the barrier  50  may be selected to conform with manufacturing constraints such as mask layout limitations. For example, in a typical inkjet printhead embodiment, the barrier may be 2 μm wide (as measured, for example, left to right in  FIG. 1 ), but can also be much narrower or wider. 
   Moreover, rather than forming two barriers  50  (that is, one on each die  22 ,  24 ) so that the saw street is bounded by a discrete pair of parallel barriers, it is contemplated in one embodiment that all of the PSG layer  40  between the two dies (and across the street) could be removed (as by the patterning and etching steps illustrated in  FIGS. 3B and 3C  discussed below), thereby providing a single gap within which a strip of barrier material extends continuously from die  22  to die  24  and across the street. 
   Alternatively, the gap  52  in each die may be formed so that one, inner side of the gap (“inner” being the right side of the gap  52  in die  24 ; the left side of the gap of die  22 ) is on one side of the respective die edge, and the other side of the gap resides in the saw street so that in one embodiment after the dies  22 ,  24  are separated there is no PSG layer remaining at the edges of the dies  22 ,  24 . This approach completely eliminates any path through moisture permeable material at the edge of the die. 
   In one embodiment, the above-mentioned boundary along which the gap  52  is formed should have an innermost part (that is, the part most distant from the die edge) that is sufficiently spaced from the saw street to ensure that, due to manufacturing tolerances, the actual sawn edge of the die does not reach the layer of PSG  40  that is just inside the barrier  50 . Put another way, the barrier should be adequately spaced from the saw street to ensure that the barrier is not inadvertently cut away when the dies are separated. In one embodiment of a printhead die, this space (shown as dimension  48  in  FIG. 1 ) is about 20 μm. 
     FIG. 2  diagrams an embodiment of how the barriers on each die are arranged to extend around the periphery of each die  22 ,  24 . That figure shows (at a much smaller scale as compared to that of  FIG. 1 ) the above described exemplary dies  22 ,  24 , as well as cutaway portions of two other dies D 3  and D 4 , in a view that illustrates how the barrier  50  is placed (that is, how the moisture permeable layer  40  is interrupted) along the periphery of three of the illustrated dies  22 , D 3 , D 4  of an array of dies carried on the wafer  20 . The barrier illustrated on the periphery of die  24  in  FIG. 2  is configured in a manner somewhat differently from that of the other dies on the wafer for illustrating another way of providing the barrier. The barrier on that die  24  is formed of two discrete segments  51 ,  55 . This embodiment is used in some applications where it is difficult to form a single continuous barrier around the entire periphery of the die. 
   In this two-segment arrangement, one barrier segment  55  is formed to define a U-shape substantially around all but one side (the top side in  FIG. 2 ) of the die  24 . The other barrier segment  51  is separately formed also to define a U-shape (inverted in  FIG. 2 ) around substantially all but one side of the die (the bottom side in  FIG. 2 ). In this embodiment, the barriers  51 ,  55 , therefore, overlap along the entire length of each opposing side edge of the die. It will be appreciated that in this configuration moisture can move from a side edge into the die by following a very lengthy path along and between the entire overlapped parts of the barriers  51 ,  55 . In one embodiment, this path is sufficiently long for preventing moisture from reaching the interior of the die during the useful life of the die. 
   This description now turns to the particulars of how die components of interest here are fabricated in a way to carry out the present invention, and reference is made to  FIGS. 3A-3F . 
     FIG. 3A  represents a partial assembly of a die  124  corresponding to an intermediate step in one embodiment of the fabrication process. The die  124  is adapted to include the present invention. Any of a number of fabrication methods can be followed to arrive at what is shown and next described with reference to  FIGS. 3A-3F . One such process is described in the previously mentioned reference, U.S. Pat. No. 5,635,966. 
     FIG. 3A  shows the front surface  134  of the upper portion of a silicon substrate  130  that is like the substrate  26  described above in connection with  FIG. 1 . Only a portion of the thickness of the substrate  130  (that is, the upper portion) is depicted in  FIGS. 3A-3F . 
   The substrate in this embodiment is doped to form a source region  138  and drain region  139  of a transistor for controlling an adjacent firing resistor (not shown) of an inkjet printhead. A gate oxide (GOX) layer  147  is provided for defining the transistor gate dielectric layer. Atop the GOX layer  147  there is deposited and patterned a layer of polysilicon  145  to define the gate region of the transistor. 
   Away from the transistor region, the oxide layer is grown thicker to provide a field oxide (FOX) layer  128  that provides in a printhead the electrical and thermal insulation for isolating individual transistors on the die. In some embodiments, this FOX layer is not required. 
   The assembly of  FIG. 3A  also shows a layer of phosphosilicate glass (PSG)  140  that is deposited using, for example, plasma-enhanced chemical vapor deposition (PECVD). The PSG layer  140  can be about 8000 Å thick (the layers not being shown to scale in the figures). Respective to the printhead components of the dies the PSG layer  140  serves as a dielectric layer for isolating the transistor gate  145 , source  138 , and drain  139  on the substrate  130 . 
   The PSG layer  140  extends over the FOX layer  128 , beyond the future edge  132  of the die (that is, the edge that is formed after the die is sawn from the wafer), and across the saw street between adjacent dies and across the future edge of the adjacent die (not shown), as is described above in connection with  FIG. 1 . 
   In accordance with this embodiment, and with reference to  FIGS. 3B and 3C , the moisture permeable PSG layer  140  is patterned ( FIG. 3B ) and etched ( FIG. 3C ) to form the gap  152  in the PSG layer. This patterning and etching preferably is done at the same time (and using the same photomask to create the photoresist layer  141 ,  FIG. 3B ) that the PSG layer is patterned and etched to form other components of the die, such as the vias  143  depicted in  FIG. 3C . As noted, these vias  143  provide openings where a subsequently deposited metals layer can contact the transistor source, drain, and gate, as well as the substrate. The etching of the PSG layer  140  may be carried out using, for example, a combination of CF 4 , CHF 3 , and Ar. 
     FIG. 3D  illustrates a layer  142  comprising two metals. The layer  142  is deposited over the PSG layer  140 , patterned using a photomask, and later etched (as at  151 ,  FIG. 3E ) for the purpose of providing the conductive lines to carry power to the above mentioned firing resistor, and establish the width of that resistor. Preferably, the metals  142  are deposited in sequence using the same metal deposition tool, with one metal comprising TaAl (about 900 Å thick) and the other comprising AlCu (about 5000 Å thick). 
   In one preferred embodiment, the metals layer  142  is etched away from the edge  132  of the die ( FIG. 3E ) and, therefore, does not form part of the material that forms the barrier  250 . It is contemplated, however, that the metal layers  142  can be retained in the gap  252  and, along with the protective layer  144  described below, form an effective barrier  250 . 
     FIG. 3F  illustrates the deposition of a protective layer  144 . This layer, among other things, covers and protects the printhead resistors from corrosion and other effects that might occur if the resistor were exposed to ink. The protective material may be made up of a deposit of SiN (about 2,500 Å) covered with a deposit of SiC (about 1,250 Å). A conventional PECVD reactor may be employed for this deposition. 
   In this embodiment of the invention, the protective layer  144  provides the barrier  250  ( FIG. 3F ) that, as described above, is located and sized for interrupting the moisture permeable layer of PSG  140  and, thus, limiting the length of the possible path for moisture to move in that PSG layer. 
   In the embodiment shown in  FIG. 3F , the barrier  250  seals the PSG layer  140  at the gap  252 , extending from the substrate  130  through the gap  252 , and over the top surface of the PSG layer  140  in the vicinity of the gap.  FIG. 3F  also shows the edge of the die  124  after its edge  132  is sawn from the wafer. 
   It is contemplated that the edge of the die may be one other than that formed when the die is sawn. For instance, such an edge in a substrate may be formed by etching the substrate to make a slot or hole in the substrate for directing ink therethrough. Such an ink-directing slot is illustrated in dashed lines at  60  of the die  24  of  FIG. 2 . The slot  60  is surrounded with an adjacent barrier  53  that apart from its location otherwise matches the construction of a peripheral barrier  50  as discussed above. Also, openings (such as through substrate interconnects) may be formed from the back to the front of the substrate (through the oxide layer) to pass conductive traces. Such openings also have the potential for exposing part of the moisture permeable material to ambient moisture and also may be isolated with a barrier in accord with the present invention. In any event, the method of the present invention is applicable in any situation where moisture permeable material is exposed, such as may result from any mechanical or chemical action in the vicinity of that material. 
   It is noteworthy that for dies having a central ink slot (such as appears at  60  in  FIG. 2 ), with the barrier  53 , in one embodiment, barriers at the side edges of the dies are also. The side-edge barriers are employed for preventing penetration of ambient moisture into the moisture permeable layer. Also, in a printhead application, the side edges of these dies may be repetitively brushed with the wiper mechanisms of printhead service stations, which can have the effect of delivering small amounts of residual ink into direct contact with the edge. Accordingly, using only a single barrier to surround an ink slot in one embodiment does not address the edge delamination problem identified here. 
   It is contemplated that there are many possible ways of implementing embodiments of the present invention to limit or prevent the movement of moisture in a moisture permeable layer of material, such as PSG, in instances where that material may become exposed to moisture. One alternative embodiment of the present invention is illustrated in  FIG. 4 , which shows a cross sectional diagram of a portion of a printhead die  224  that carries a fusible link  300 . Such links are sometimes used in printhead encoding systems as explained in detail in U.S. Pat. No. 6,325,483. 
   Respective to embodiments of the present invention, a fusible link  300  is deposited and patterned to reside atop a layer of PSG  240  in a die  224  that may be otherwise constructed in accord with the above discussion of the dies  24 ,  124 . The link  300  is covered with a protective layer  244  similar to the protective layer  144  described above in this embodiment. One part of the link is in electrical communication with a sense line or current source (not shown), such as through contact pad  302 . Another part of the link  300  is connected, as by conductor  242 , to the encoding circuitry (not shown) on the die  224 . 
   In one embodiment, the identification aspect of some selected links (such as the link  300  under consideration here) is carried out by applying sufficient current through the link to destroy the link in a manner akin to blowing a fuse. The physical effect of blowing the link  300  is to disintegrate part of the link as well as a portion of the protective layer  244  that is adjacent to the link. The absence of this material creates a void (shown as dashed lines  304 ) that exposes a portion  306  of the PSG layer  240  to ambient moisture, which moisture may include a small amount of residual ink in the vicinity of the fuse. The moisture, if left unchecked, would be absorbed by the moisture permeable PSG layer and penetrate along paths  245  in that layer, thus causing delamination problems in layers elsewhere in the die, as mentioned above. 
   In accordance with an embodiment of the present invention, the portions of the PSG layer  240  that underlie fusible links  300  are provided with gaps  252  that are filled with material to form a barrier  250 . The barrier is formed in substantially the same manner as described above in connection with barrier  152  of  FIG. 3 , including the etching of the PSG layer  240  to form the gap  252  that is filled and overlaid with another layer to form the barrier  250 . In this instance, it will be appreciated that some conductive material  242  may be part of the material that makes up the barrier  250  as is seen in the rightmost part of barrier  250  in  FIG. 4 . In any event, the boundary of the barrier  250  is established to surround the fusible link  300  so that any moisture penetrating the PSG layer  240  as a result of a blown fusible link will be blocked by the barrier from moving outside of the barrier to other functional parts of the die. 
   Although the foregoing description has focused on the processing of dies for use in printheads in inkjet printing, it will be appreciated that the present invention may also be applied to the production of dies used in drop generators for any of a variety of applications or fluids. Moreover, although the embodiment of a printhead die was described as incorporating a silicon substrate, it is possible that other rigid substrates, such as glass, will suffice for supporting the remaining layers. 
   Thus, having here described embodiments of the present invention, the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents of the invention defined in the appended claims.