Patent Publication Number: US-8109614-B2

Title: Process for protectively coating hydraulic microcircuits against aggressive liquids, particulary for an ink jet printhead

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/539,121, filed on Jun. 16, 2005, which is the U.S. national stage of International Application No. PCT/IT2003/000843, filed on Dec. 19, 2003, which claims the priority benefit of Italian Application No. TO 2002 A 001099, filed Dec. 19, 2002. The priority of each of the foregoing applications is claimed. Each of the foregoing applications is expressly incorporated herein by reference in its entirety. 
    
    
     TECHNOLOGICAL AREA OF THE INVENTION 
     This invention relates to a process for protectively coating hydraulic microcircuits against aggressive liquids, such as for example microcircuits for biomedical uses, MEMS, drinks dispensers, and microcircuits employed in various types of ink jet printheads. 
     More in particular this invention is intended for a process for producing a protective coating of the inner walls of the ink ejection chambers of an ink jet printhead, to reduce the damaging effects on the resin layers in which the ejection chambers are built, caused by the corrosive action of particularly aggressive inks. In addition, the invention relates to the process of protectively coating not only the inner walls of the ejection chambers, but also and at the same time the inner walls of the feeding ducts, hydraulically connected to the chambers and the inner walls of the nozzles ejecting the droplets of ink. 
     BRIEF DESCRIPTION OF THE CURRENT STATE OF THE ART 
     Ink jet printheads are known in the current state of the art, for which measures have been taken to limit the corrosive action of the inks on the structural layers, inside which the ejection chambers, feeding ducts and also any injection nozzles are made. 
     In the current state of the art, an ink jet printhead is known in which the structural layer encapsulating the ejection chambers, feeding ducts and injection nozzles is produced by way of the deposition of a layer of metal, for instance nickel, itself already very resistant to the aggressive agents of the inks. However this solution has the drawback of having considerable complications during its manufacturing process; for example, one difficulty is that of growing a metal uniformly starting from a substrate with existing sacrificial metallic or dielectric microstructures, which, in the case of the former, would create surface protuberances and, in the latter case, depressions in the structural layer. 
     In addition, the deposition of a metallic layer of relatively significant thickness, in the order of approximately 60-70 μm, produces strong mechanical stresses in the zones where the metallic structural layer is soldered to the layers underneath. 
     What&#39;s more, the process of making chambers and relative feeding ducts in a completely metallic structural layer, requires extremely high work times, with consequent repercussions on the final costs of a printhead obtained in this way. 
     SUMMARY DESCRIPTION OF THE INVENTION 
     The object of this invention is to present a process of coating hydraulic microcircuits to protect them from aggressive liquids, minus the drawbacks outlined above, and more in particular, to simply and effectively produce a protection for the hydraulic microcircuits against the damaging effects of the inks, for an ink jet printhead. 
     Another object of the invention is to present a manufacturing process for an ink jet printhead in which the inner walls of the chambers, feeding ducts and nozzles, made in a structural layer of dielectric material, such as non-photosensitive epoxy or polyamide resin, are treated in such a way as to offer high resistance to the aggressive agents of the inks employed. 
     Another object of the invention is to treat the inner walls of the hydraulic microcircuits of an ink jet printhead, to render them particularly insensitive to the damaging effects of the aggressive agents contained in the inks used. 
     In accordance with this invention, the process for protectively coating hydraulic microcircuits of an ink jet printhead, particularly resistant to aggressive inks and the printhead thus obtained are presented, being characterized as defined in the respective main claims. 
     This and other characteristics of the invention will appear more clearly from the following description of a preferred embodiment of an ink jet printhead and of the relative manufacturing process, provided as a non-restrictive example, with reference to the figures in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a perspective view of a silicon wafer, on which a plurality of “die” not yet separated is indicated; 
         FIG. 2  represents a plan view of a portion of a die of  FIG. 1  for an ink jet printhead, after a first manufacturing step and before building the chambers, relative feeding ducts and nozzles, using the process proposed in accordance with this invention; 
         FIG. 3  represents a section, taken according to the line III-III in  FIG. 2 ; 
         FIG. 4  shows a flow diagram of the manufacturing process of the chambers, feeding ducts and nozzles of the ink jet printhead, according to the invention; 
         FIGS. 5 to 8  illustrate the successive steps in manufacture of the chambers, feeding ducts and nozzles of the printhead of  FIG. 3 , according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Although the main object of this invention is that of producing a protective coating for hydraulic microcircuits against aggressive liquids, the following description will refer particularly to an ink jet printhead, in simplified, non-restrictive form and for reasons of simplicity and clarity of the description, it being understood in any case that this invention has a much wider relevance and is in general intended, as already said, for producing a protective coating for hydraulic microcircuits against aggressive liquids. 
     As anticipated, this description refers to a process relating to an inkjet printhead for treating the inner walls of the chambers, feeding ducts and nozzles of said head, in such a way as to offer high resistance against the aggressive agents of the inks employed; it is clear that the process mainly, though not exclusively, concerns the final part of manufacture of the head. 
     In the description that follows, therefore, the initial steps of manufacture of the printhead will not be described in detail, as these belong to the state of the art, well-known to those acquainted with the sector art, but the process of manufacturing the chambers, relative feeding ducts and injection nozzles, according to the invention, may be considered as applying to a conventional ink jet printhead, made in a first step in a way known in the state of the art. 
     Depicted in  FIG. 1  by way of an example is a wafer  10  of crystalline silicon, on which die  12  are indicated, constituting a like number of conventional type ink jet printheads, not yet separated; the figure represents one of the die, in enlarged view, in which two zones  13  are indicated in which the driving microcircuits are arranged and the zone  14  enclosing the nozzles  15 . 
     In  FIG. 2 , represented by way of non-restrictive example is the section of a conventional ink jet printhead, in the state it is in after a first manufacturing phase, known in itself, in which the manufacturing process has come to the deposition of a sacrificial layer of copper in the zone where the chambers, relative feeding ducts and nozzles will be made; in particular,  FIG. 2  shows this printhead, in which a die  20  can be seen which is made up of a substrate of silicon  21  covered by a plurality of metallic and dielectric layers, in which an array of microcircuits has been made for driving thermal elements  22 , or resistors, for expulsion of said ink. This plurality of layers, known in themselves in the sector art, is represented for simplicity of the description, by a single layer  23 , on top of the silicon layer  21 . 
     The thermal elements  22  are covered by a protective layer  24 , consisting of a deposit of silicon nitride and carbide (Si 3 N 4 , SiC), which is in turn covered by a layer  25  made of tantalum and gold, forming the so-called “seed layer”. Deposited on the layer  25  is a sacrificial metallic layer  26 , provided with a protuberance  27 , constituting the cast of at least one ejection nozzle, not depicted. 
     Also visible in  FIGS. 2 and 3  are two feeding holes  28 , suitable for bringing the ink into the ejection chambers, not shown in the figures, as they are the object of this invention and are described later; the holes  28  will subsequently be put in hydraulic communication with a slot  29 , not shown in the figures, as it is made later in a step of this process and also described later. 
     The object of this invention, as stated in the early part of the description, consists in coating the inner walls of the chambers, of the relative feeding ducts connected to them and of the nozzles, with one or more protective layers of noble metals, for the purpose of eliminating the damaging effects produced by particularly aggressive inks. 
     All this is obtained by depositing on the outer surface of the sacrificial layer, already present, one or more layers of noble metals, such as for example nickel-gold, palladium-gold, ruthenium, etc. Said layers, after the removal of the sacrificial layer, will remain adhering to the inner walls of the chambers and of the other adjacent compartments, created in a structural layer of resin deposited previously. 
     At the end of this operation, chambers, feeding channels and nozzles are obtained with inner walls completely coated by the layer of noble metals, and therefore effectively protected from the aggressive action of the inks employed. 
     Naturally the inner shape of the chambers, feeding ducts and nozzles represents the true impression of the sacrificial layer, because the upper surface of the chambers and the ducts connected to them faithfully reproduce the outer surface of the sacrificial layer. 
     In particular, where the ink jet printhead used is that described in the Italian patent application entitled “Inkjet printhead and relative manufacturing process,” corresponding to the International Patent Application WO 2004/056574 A1, filed by the same applicant, and the manufacturing process that this invention refers to is applied, concave-shaped upper inner walls of the chambers and of the feeding ducts connected to them would be obtained, a faithful copy of the corresponding shape of the sacrificial layer made using the process described in the already cited International patent application. 
     In the latter case, the twin advantage would be obtained of great resistance of the chambers and feeding ducts to the aggressive agents in the inks and a more effective prevention of air bubbles becoming attached to particular points of the walls, with optimization of the phase in which the expulsion bubble is developed. 
     Accordingly the process for producing chambers, relative feeding ducts and protected nozzles, according to this invention, continues starting from the state of progress of manufacture of a printhead, by way of non-restrictive example, of the type described in the cited International Patent Application WO 2004/056574 A1, shown in  FIG. 3 , and proceeds in the steps described in the flow diagram of  FIG. 4 , integrated with the explanatory drawings in  FIGS. 5 to 8 . 
     In step  40 , a wafer  10  ( FIG. 1 ) is made available, comprising a plurality of partially constructed die  12 , up to the stage depicted in  FIG. 2 , in which, as already recalled, a still uncovered sacrificial layer  26 ,  27  of copper is present. 
     In step  41 , illustrated in  FIG. 5 , a coating layer  30  of noble metals, such as for example nickel-gold is deposited on the sacrificial layer  26  and on the cast  27  of the nozzle. Alternatively, the coating layer  30  may be of palladium-gold, or of ruthenium, etc.; the deposition is performed through an electrochemical process, of a type known to those acquainted with the sector art. 
     In step  42 , an adhesion layer  31  is applied on the layer  30  of noble metals to promote perfect adhesion, through molecular bonds, of the layer of resin, which will be applied in the next step. 
     In step  43 , a structural layer  32  ( FIG. 6 ), made of a film of non-photosensitive epoxy or polyamide resin, is deposited through lamination on the coating layer  30 , covered by the adhesion layer  31 ; this type of material is used to advantage to offer greater resistance to the aggressive environment created by particularly aggressive inks. 
     In step  44 , polymerization is performed of the structural layer  32  to increase its resistance to the mechanical and thermal stresses, that develop during operation of the head. 
     In step  45 , illustrated in  FIG. 7 , lapping is performed of the outer surface  33  of the structural layer  32  so as to completely uncover the upper cap  34  of the cast of copper  27  of the nozzles and to produce a perfectly flat surface of the structural layer  32 . This is done by means of a mechanical lapping and simultaneous CMP type chemical treatment (Chemical-Mechanical-Polishing), or other similar process, known to those acquainted with the sector art. 
     In step  46 , anisotropic etching of the slot  29  is performed in the bottom part of the layer of silicon  30  ( FIG. 7 ), by means of a “wet” type technology that uses for instance KOH, or TMHA. Etching of the silicon continues right up to the aperture of the holes  28 , so that the thickness of the remaining layer  38  of silicon, in correspondence with the slot  48 , is of approximately 10 μm. 
     In step  47 , the sacrificial layer  26 ,  27  is removed with a chemical etching, conducted by means of a highly acid bath, for example made of a mix of HCl and HNO3 in a solution. Composition of the bath is prepared in such a way as not to attack the metallic layer  30 , which adheres tightly to the resin of the structural layer,  32 . At the end of this operation, illustrated in  FIG. 8 , chambers  35 , ducts  36  and nozzles  37  are obtained with their inner walls completely coated by the layer  30  of noble metals, and thus effectively protected against the aggressive action of the inks employed. 
     In step  48 , illustrated in  FIG. 8 , a metallic layer  39  to protect the resin, consisting of a noble metal, preferably chromium, and having a thickness of approximately 1000° A, is deposited on the outer surface of the structural layer  32  by means of vacuum evaporation. Its function is to create a water-repellent outer surface (anti-wetting), offering the resin scratch-proofing and corrosion-proofing properties. 
     In step  49 , the final operations, known to those acquainted with the sector art, are conducted, these are: 
     dicing of the “wafer”  10  into the single die  12 ; 
     soldering of a flat cable, not shown, to the pads on each die  12 , through the known TAB process; 
     mounting of the die with relative flat cable on the container-tank of the head; 
     filling of the tank with ink and final testing. 
     It is important to observe that the presence of a layer of noble metal, such as for instance nickel-gold on the copper surface of the sacrificial layer, facilitates its etching by electrochemical means as well, since it forms a continuous electrode inside the chambers and the feeding ducts, preventing the creation of “dead zones” that are isolated from the electrical connection with the “seed layer”. 
     It remains understood that changes, additions, or part substitutions may be made to the ink jet printhead and to the relative manufacturing process, or variants of the manufacturing process, according to this invention, without departing from the scope of the invention. 
     For instance the protective layer  39 , deposited on the structural layer  32  in step  49 , may consist of, instead of chromium, magnesium fluoride and oxygen (MgF 2 +O 2 ), or of silicon dioxide and chromium (SiO 2 +Cr). 
     Also, according to another embodiment, the protective layer  39  may be formed of two overlapping deposits, made of the components indicated above.