Patent Publication Number: US-6666947-B2

Title: Method for producing an inkjet printhead element; and an inkjet printhead element

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for producing an inkjet printhead element and an inkjet printhead element produced using said method. 
     Inkjet printheads generally comprise two members, the thinfilm die and the metal orifice plate. These members are generally manufactured separately and are subsequently brought together to form a unified inkjet printhead element. Traditionally, the attachment of the thinfilm die to the metal orifice place has been accomplished via an ink barrier layer on the thinfilm die. In the attaching process, dollops of glue are dispensed to temporarily tack the orifice plate in position before the thinfilm die and the orifice plate are permanently secured to one another by performing a so-called “stake and bake” procedure. 
     The process of attaching the thinfilm die to the orifice plate using dispensed glue has, however, been fraught with problems. It is for example very difficult to control the size and the position of the glue which is dispensed. Another grave problem which occurs is that the orifice plate, temporarily tacked in place on the liquid glue, slides laterally on the layer of liquid glue relative to the thinfilm die prior to being permanently affixed in the staking process. This shifting leads to improper alignment of the orifice plate on the thinfilm die in the staking process. 
     Generally, the inability to control the size of the dispensed glue dollops leads to a lifting of the corner of the orifice plate from the thinfilm die if too much glue is used, or complete separation of the orifice plate from the thinfilm die in the case that too little glue is used. Furthermore, the inability to control the positioning of the dispensed glue leads to the existence of glue in unintended locations on the thinfilm die and/or on the orifice plate, a situation leading to problematic smearing of the glue. This either worsens print quality or, in extreme cases, leads to rejection of the printhead, thereby increasing production costs. 
     U.S. Pat. No. 6,054,011 discloses an orifice plate with a layer of metal, for example gold, bonded thereto and an ink barrier layer. An adhesion promoter glue is provided between the ink barrier layer and a metal oxide layer which itself is applied to the metal layer which is bonded to the orifice plate. The orifice plate is therefore held to the ink barrier layer via the direct adhesion contact between the metal oxide layer and the adhesion promoter glue. According to this document, the orifice plate and the ink barrier layer are assembled by compressing the orifice plate, coated as indicated above, to the ink barrier with the polymeric adhesion promoter there between at a pressure of about 150 psi (105,555 kg/m 2 ) at a temperature of about 200° C. for about 10 minutes. Following compression and heating as above, the ink barrier layer-orifice plate assembly is heated (with the adhesion promoter glue there between) at a temperature of about 220° C. for about 30 minutes. 
     A problem with existing glue-based processes is that when the orifice plate, following temporary tacking to the thinfilm die, is subjected to the stake process at 150° C., the glue used for temporarily tacking melts and causes misalignment of the orifice plate on the thinfilm die. This can lead to smearing of glue if the amount and/or position of the glue is not correct. 
     SUMMARY OF THE INVENTION 
     It is therefore a goal of the present invention to produce an inkjet printhead element in which in which the final alignment of the orifice plate on the thinfilm die is improved. 
     This goal is met by providing a method for producing an inkjet printhead element comprising the features recited according to the independent claim. 
     A method for producing an inkjet printhead element comprises providing a thinfilm die and a barrier layer thereon, wherein the barrier layer comprises a thermoplastic component and a thermoset component. The barrier layer is heated such that the barrier layer becomes tacky. The orifice plate is aligned to the barrier layer and is brought into contact with the barrier layer. Here, the orifice plate is held in place on the barrier layer by the tackiness of the barrier layer. Finally, the assembly of the thinfilm die, the barrier layer and the orifice plate is subjected to a stake and bake process to attach the orifice plate permanently to the barrier layer and, hence, to the thinfilm die. 
     A major advantage of the inventive method as recited above is that no glue is used in the barrier layer for temporarily tacking the orifice plate to the thinfilm die. The barrier layer is heated just enough to reach the glass transition temperature of its thermoplastic component. The thermoplastic component of the barrier layer becomes highly amorphous such that the entire surface of the barrier layer becomes sticky to the touch. This stickiness will subsequently hold the orifice plate in place when the orifice plate is placed on top of the barrier layer. 
     Adhesion in the inventive method recited above takes place directly between the orifice plate and the barrier layer, i.e. no glue is needed, the temporary adhesion of the orifice plate on the barrier layer prior to curing instead depending on the tackiness of the barrier layer upon reaching its glass transition temperature T g . Since the orifice plate is directly contacted lightly to the already tacky barrier layer instead of using glue to temporarily tack the orifice plate, the danger of glue being squeezed out from in between the orifice plate and the barrier layer into undesired regions is essentially eliminated. This leads to a lower reject ratio, thereby streamlining and economizing the entire production process. 
     The fact that the orifice plate is directly tacked to the barrier layer offers greater resistance to a shifting of the orifice plate on the barrier layer prior to staking and/or curing the orifice plate on the barrier layer. This means that proper alignment of the orifice plate on the barrier layer can be controlled to be more precise and more reproducible in the absence of the shifting of the orifice plate and the barrier layer relative to one another. 
     The method according to the invention as recited above comprises two heating phases. 
     The first of these two heating phases is intended to prepare the barrier layer for the subsequent contacting of the orifice plate, and therefore takes place prior to such contacting. During this first of two heating phases, the barrier layer is heated to within approximately 2% of the glass transition temperature of its thermoplastic component. This first heating phase is therefore not intended to liquefy the entire barrier layer but rather only to render one of the barrier layer&#39;s components, the thermoplastic component, highly amorphous so that the entire surface of the barrier layer becomes tacky to the touch. Since it is not intended that the barrier layer be melted prior to contacting the orifice plate on the barrier layer, the heating conditions to which the barrier is subjected in this first heating phase can and, advantageously, should be kept gentle. Especially characteristic of the gentleness of this first heating phase is the fact that the heating of the barrier layer to only its approximate glass transition temperature can be accomplished very quickly. Once the approximate glass temperature of the thermoplastic component of the barrier layer is reached, an orifice plate can be contacted with the (now tacky) barrier layer. 
     Between the two heating phases, the orifice plate is first aligned with the barrier layer with which it is to be brought in contact and is then brought in contact with the (tacky) barrier layer. The danger of glue being displaced from in between the barrier layer and the orifice plate during the process of contacting the orifice plate to the barrier layer is essentially ruled out by two characteristic features of the invention: 
     a) The barrier layer is merely tacky and is not in liquid form, so that it cannot flow into unintended regions, and 
     b) The contacting of the orifice plate to the barrier layer is itself accomplished very gently, preferably by merely touching the orifice plate to the barrier layer; the tackiness of the barrier layer engendered during the first heating phase as described above is sufficient to hold the incoming orifice plate securely, thereby obviating any active compression of the orifice plate onto the barrier layer. It is such active compression which is in part responsible for the undesired displacement of adhesion material in other related methods in the prior art. 
     The second heating phase of the method according to the invention is carried out following the contacting of the orifice plate to the barrier layer by a stake process at a pressure of 150 psi and a temperature of 200° C. and then a bake process of 200° C. for 20-30 min to set the thermoset component of the barrier layer. During this second heating phase, the duration of which will depend on the time necessary for the particular thermoset component used in the barrier layer to cure, the orifice plate therefore becomes permanently attached to the thinfilm die via the interposed barrier layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a preferred embodiment of an apparatus for producing an inkjet printhead element according to the method of the invention. 
     FIG. 2 shows an intermediate process step during the production of a plurality of inkjet printhead elements according to a preferred embodiment of the method of the invention. 
     FIG. 3 shows two graphs in which the pre-stake misalignment and the post-stake misalignment are compared for methods for producing inkjet printhead elements in the prior art (FIG. 3 a ) and for the glueless method for producing an inkjet printhead element according to the invention (FIG. 3 b ). 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     The invention will now be described in detail with reference to the figures. 
     FIG. 1 shows a heating wafer chuck  100 , a heating element  101  of the heating wafer chuck  100 , a wafer frame  102 , a thinfilm wafer  103  comprising a plurality of thinfilm die regions  105  and a plurality of barrier layers  104 , each of which rests on a respective thinfilm die region  105 . A thinfilm die  105  is defined by the region of the thinfilm wafer  103  covered by a barrier layer  104 . It is important for effectively executing the method according to the invention that the wafer  102  be in conductive thermal contact with the heating wafer chuck  100 , that the thinfilm wafer  103  be in conductive thermal contact with the wafer frame  102 , and that the respective barrier layers  104  composed of a thermoplastic component and a thermoset component are each in conductive thermal contact with the thinfilm wafer  103  via a respective region  105  designating the region of the thinfilm die. A preferred material for the thermoplastic component of the barrier layer is poly(methylmethacrylate) (PMMA) and a preferred material for the thermoset component of the barrier layer is an epoxy compound. The heating element  101  of the heating wafer chuck  100  is preferably controlled electrically (electrical contacts not shown here), and is preferably made of a material which allows rapid heating to a desired temperature. Furthermore, the wafer frame  102  is preferably made of a material which can effectively, i.e. rapidly, conduct heat from the heating wafer chuck  100  into the thinfilm wafer  103  so that this heat may ultimately be transmitted, through each of the thinfilm die regions  105 , to each of the corresponding barrier layers  104 . In this way, a respective barrier layer  104  can rapidly be rendered tacky by heating to the glass transition temperature of its thermoplastic component. 
     FIG. 2 shows a heating wafer chuck  200 , a heating element  201  of the heating wafer chuck  200 , a wafer frame  202 , a thinfilm wafer  203  with a plurality of thinfilm dies  205  upon each of which a barrier layer  204  is located. A thinfilm die  205  is defined by the region of the thinfilm wafer  203  covered by a barrier layer  204 . FIG. 2 shows an intermediate step in the production of a plurality of inkjet printhead elements on a single thinfilm wafer  203 . In this figure, one orifice plate  206  has already been brought into contact with the tacky barrier layer  204  and another orifice plate  207  has yet to be brought into contact with a barrier layer  204 . The orifice plate  207  will be brought into contact with a barrier layer  204  using a place chuck  208  which carries the orifice plate  207  to its intended location on a barrier layer  204 . The place chuck  208  is positioned by using a positioning apparatus  209 , shown here in cutaway. The positioning apparatus  209  is capable of moving relative to a given barrier layer  204  in the lateral directions indicated by the double-headed arrow  210  as well as in the vertical directions indicated by the double-headed arrow  211 . 
     According to a preferred embodiment of the invention, the orifice plate  207  is aligned to the barrier layer  204  based on a visual correlation of the position of the place chuck  208  which holds the orifice plate  207  and/or of the orifice plate itself  207  relative to the position of the (already tacky) barrier layer  204  on the thinfilm die  203 . In this way, a precise and, hence, reproducible placement of the orifice plate  207  onto the barrier layer  204  is made possible. 
     According to a preferred embodiment of the method according to the invention, the visual correlation is accomplished by optical imaging or by electronic imaging of the place chuck  208  and/or of the orifice plate  207  itself relative to the barrier layer  204 , wherein an especially preferred mode of electronic imaging is that performed using a CCD (charged coupled device) camera. 
     It should be noted here that the movement of the positioning apparatus  209  is of such a nature as to allow the place chuck  208  to lightly bring the orifice plate  207  into contact with an intended barrier layer  204 . That is to say that, due to the tacky nature of the intended barrier layer  204 , very little pressure needs to be exerted via the place chuck  208 , itself driven by the positioning apparatus  209 . 
     As explained above for FIG. 2, the arrows  210  and  211  can be understood as indicating the direction of movement of the positioning apparatus  209  and, hence, that of the place chuck  208  and the orifice plate  207 , relative to a barrier layer  204  which, due to its being mounted on a stationary heating wafer chuck  200 , is itself also stationary. 
     It is, however, also within the scope of the invention that the arrows  210  and  211  in FIG. 2 can represent the directions of movement of the heating wafer chuck  200  mounted on the positioning apparatus of its own relative to a stationary place chuck  208  and, hence, stationary orifice plate  207 . In such a scenario, one can dispense with the positioning apparatus  209  to drive the place chuck  208 . 
     Arrows  210  and  211  can also be understood to represent the movement of both the heating wafer chuck  200  and the positioning apparatus  209  relative to one another until proper alignment of the orifice plate  207  with the intended barrier layer  204  is achieved. Such a movement of both the heating wafer chuck  200  and the positioning apparatus  209  is of such a nature that, during this movement, each of their positions changes with respect to a given point in the stationary frame of the room in which the production is performed. 
     However, since in most practical applications the thinfilm wafer  203  will comprise a plurality of thinfilm die regions  205 , each with a corresponding barrier layer  204 , it will prove most expedient in the majority of cases to keep the heating wafer chuck  200  stationary and to render the place chuck  208  mobile so that the latter is free to retrieve new orifice plates  207  from a location remote to the thinfilm wafer  203 , returning after each retrieval to bring the retrieved orifice plate  207  into contact with a free (i.e. as yet bearing no orifice plate  206 ) barrier layer  204 . 
     As in FIG. 1, the heating element  201  of the heating wafer chuck  200  is advantageously controlled so as to allow rapid and precise adjustment to a desired temperature. This will normally be achieved by some means of electrical control (not shown). 
     According to a further embodiment of the method of the invention, bringing the orifice plate  207  in contact with the barrier layer  204  comprises staking the orifice plate  207  to the barrier layer  204 . Such a staking process benefits from the fact that the entire barrier layer  204  in its tacky state is used to immobilize the orifice plate  207  subsequent to the orifice plate&#39;s  207  being brought in contact with the barrier layer  204 . In this way, between the time of bringing the orifice plate  207  into contact with the barrier layer  204  and the time of staking the orifice plate  207  to the barrier layer  204 , minimal shifting of the orifice plate  207  on the barrier layer  204  takes place. 
     According to a preferred embodiment of the method as recited above, the barrier layer  204  is composed of the material IJ5000 (DuPont), for which the thermoplastic temperature is about 90° C. and the thermoset temperature is about 200® C. Using this material, a thermoplastic temperature of about 90° C. can be reached within a matter of seconds using the heating wafer chuck  200 , at which point the material IJ5000 becomes tacky to the touch. Following contacting the orifice plate  207  to the barrier layer  204 , further treatment at the thermoset temperature, i.e. the temperature at which the thermoset component of the barrier layer  204  cures, permanently attaches the orifice plate  207  to the barrier layer  204 . Here as well, a staking procedure can precede the treatment at the thermoset temperature. For the material IJ5000, the thermoset temperature is about 200° C., and heating the assembly of the thinfilm die  203 , the barrier layer  204  and the orifice plate  207  at about this temperature takes place for about 20-30 minutes. 
     It is important that the thermoplastic component and the thermoset component of the barrier layer  204  are mutually compatible, i.e. that the thermoplastic component and the thermoset component can be mixed with one another to form a homogeneous material and, therefore, a uniform barrier layer composition. Regions of nonuniformity throughout the barrier layer  204  might be expected to result from an inhomogeneous mixing of the thermoplastic component with the thermoset component of the barrier layer  204  and would be expected to decrease the overall adhesion and/or curing aptitude of the barrier layer  204 . 
     As mentioned above, a thinfilm die according to such an embodiment is defined by the region of the thinfilm wafer  203  covered by a barrier layer  204 . In this way, production of multiple inkjet printhead elements can take place in a single production run during which the place chuck  208  places one orifice plate  206 ,  207  after the next on each of the plurality of barrier layers  204  of the thinfilm wafer  203 . Following production of the plurality of inkjet printhead elements on a single thinfilm wafer  203 , the individual inkjet printhead elements can be separated from the surrounding region of the thinfilm wafer  203  and can be further used in the production of an inkjet printhead. 
     FIG. 3 shows two graphs, FIG. 3 a  and FIG. 3 b , depicting the pre- and post-stake misalignment for existing and for the inventive processes of attaching an orifice plate, respectively. 
     In FIG. 3 a , one sees a unimodal peak corresponding to a misalignment, taking place prior to the stake process, of about 1 micron. In contrast thereto, one sees in curve  300  for the post-stake misalignment (curve  301 ) a multimodal distribution of extents of misalignment, in which the maximum (i.e. for the greatest number of thinfilm dies tested) misalignment is 2.5 microns. This 2.5 micron misalignment indicates the distance by which an orifice plate is offset from its ideal position of perfect alignment on a barrier layer/thinfilm die. Both the magnitude of this offset, i.e. about 2.5 microns as well as the statistical multimodality of the distribution of the offsets obtained with respect to the number of dies sampled imply a high degree of uncertainty in existing glue-based methods for producing inkjet printhead elements. This uncertainty ultimately manifests itself as an undesirable lack of reproducibility in the production of such inkjet printhead elements. 
     FIG. 3 b  shows the corresponding pre-stake misalignment (curve  302 ) and post-stake misalignment (curve  303 ) of the orifice plate relative to its proper position on the barrier layer for the method of the invention. The pre-stake misalignment in FIG. 3 b  (curve  302 ) exhibits, as in FIG. 3 a , an essentially unimodal distribution. As with the results for the pre-stake misalignment depicted in FIG. 3 a , the essentially unimodal distribution for the pre-stake misalignment in FIG. 3 b  also centers on approximately 1 micron. The post-stake misalignment shown in FIG. 3 b  (curve  303 ), however, is radically different from that of FIG. 3 a . The statistical distribution of the post-stake misalignment results obtained with the method of the present invention define an essentially unimodal distribution centering on about 1.5 microns of misalignment distance of the orifice plate on the barrier layer. This represents an approximately 40% better post-stake alignment as compared to the maximum post-stake alignment value of FIG. 3 a  (i.e. about 2.5 microns). 
     In addition, one notes that the post-stake misalignment curve  303  of FIG. 3 b  not only mirrors the statistical center of mass of the pre-stake misalignment curve of FIG. 3 b  to a greater extent than is seen in FIG. 3 a , but is also much more closely superimposed with the general shape of the pre-stake misalignment curve  302  of FIG. 3 b  than is the case for the two curves of FIG. 3 a . This means that the method according to the invention as recited above is capable of greatly stabilizing the position of the orifice plate on the barrier layer following contacting but prior to staking the orifice plate on the barrier layer. This stabilization is due to the large resistive force restricting the lateral movement of the orifice plate relative to the barrier layer after the two have been brought in contact. This large restrictive force relative to existing glue-based production methods is attributable to the fact that the entire barrier layer is used in a thermally activated, i.e. tacky or sticky, form to hold the contacted orifice plate firmly in place. 
     As demonstrated by a comparison of the results in FIG. 3 a  with those of FIG. 3 b  it becomes clear that the present method for producing an inkjet printhead element is capable of generating highly reproducible orifice plate-barrier layer assemblies in a well-aligned fashion. The minimal misalignment of the orifice plate relative to the barrier layer in these assemblies make them extremely amenable to demanding printing applications such as increased resolutions and print speeds. 
     List of Reference Numbers 
       100  heating wafer chuck 
       101  heating element of heating wafer chuck 
       102  wafer frame 
       103  thinfilm wafer 
       104  barrier layer 
       105  region designating thinfilm die 
       200  heating wafer chuck 
       201  heating element of heating wafer chuck 
       202  wafer frame 
       203  thinfilm wafer 
       204  barrier layer 
       205  region designating thinfilm die 
       206  orifice plate tacked to barrier layer  204   
       207  orifice plate not yet tacked to barrier layer  204   
       208  place chuck carrying orifice plate  207   
       209  positioning apparatus (in cutaway) 
       210  horizontal direction of movement 
       211  vertical direction of movement 
       300  pre-stake misalignment curve in prior art method 
       301  post-stake misalignment curve in prior art method 
       302  pre-stake misalignment curve in method of invention 
       303  post-stake misalignment curve in method of invention