Abstract:
A process for manufacturing a monolithic thermal ink jet printhead ( 40 ) comprising a plurality of chambers ( 74 ) and of nozzles ( 56 ), comprises steps of ( 206 ) depositing a plurality of sacrificial layers ( 31 ), of obtaining, by means of exposure and development operations, a plurality of casts ( 156 ), of ( 215 ) applying a structural layer ( 107 ), and subsequently steps of ( 225 ) removing the casts ( 156 ) and of ( 226 ) removing the sacrificial layers ( 31 ), in order to produce a plurality of chambers ( 74 ) and nozzles ( 56 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. Ser. No. 10/297,206, filed Dec. 4, 2002, now U.S. Pat. No. 6,949,201, issued Sep. 27, 2005, which claims priority to PCT/IT01/00285, filed Jun. 4, 2004 which claims priority to Italian Patent Application T02000A000526, filed Jun. 5, 2000, all of which are incorporated herein in their entireties. 
    
    
     TECHNICAL FIELD 
     This invention relates to a manufacturing process for a printhead used in equipment for forming, through successive scanning operations, black and colour images on a print medium, usually though not exclusively a sheet of paper, by means of the thermal type ink jet technology, and in particular to the head actuating assembly and the associated manufacturing process. 
     BACKGROUND ART 
     Depicted in  FIG. 1  is an inkjet printer, on which the main parts are labelled as follows: a fixed structure  41 , a scanning carriage  42 , an encoder  44  and printheads  40  which may be either monochromatic or colour, and variable in number. 
     The printer may be a stand-alone product, or be part of a photocopier, of a “plotter”, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium  46 , normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar. 
     Also shown in  FIG. 1  are the axes of reference:
         x axis: horizontal, i.e. parallel to the scanning direction of the carriage  42 ; y axis: vertical, i.e. parallel to the direction of motion of the medium  46  during the line feed function; z axis: perpendicular to the x and y axes, i.e. substantially parallel to the direction of emission of the droplets of ink.       

       FIG. 2  is an axonometric view of the printhead  40 , showing the nozzles  56 , generally arranged in two columns parallel to the y axis, and a nozzle plate  106 . 
     The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on some only of the features of the heads and the manufacturing process, of relevance for the purposes of understanding this invention. 
     The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a definition of more than 600 dpi (dpi=“dots per inch”), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies. 
     Requirements such as these are especially important in colour printhead manufacture and make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, and narrow assembly tolerances. 
     These drawbacks are solved, for instance, by means of the monolithic printhead described in the Italian patent application TO 99A 000610, a section of which parallel to the plane z-x is illustrated in  FIG. 3 , which shows an ejector  55  comprising: a substrate  140  of silicon P, a structural layer  107 , one of the nozzles  56 ; a groove  45 ; ducts  53 ; channels  167 ; and a resistor  27  which, when current passes through it, produces the heat needed to form a vapour bubble  65  which, by expanding rapidly in a chamber  57 , results in emission of a droplet of ink  51 . Also indicated is a tank  103  containing the ink  142 . 
     Another solution is represented, for example, by a monolithic printhead described in the Italian patent application TO 2000A 000335, shown in sectional view in  FIG. 4 , which comprises the substrate  140  of silicon P, the structural layer  107 , chambers  74  arranged laterally with respect to a lamina  67 , on the bottom of which are located the resistors  27 , which are therefore external with respect to the lamina  67 . Also depicted in the figure are: the groove  45 ; two pluralities of elementary ducts  75 , for each of which only one of the elementary ducts  75  has been drawn, which convey the ink  142  from the groove  45  to the chambers  74 ; and connecting channels  68 . Also shown in the figure is a diameter D which the nozzle  56  presents to the outside of the printhead. 
     The whole comprising a chamber  74 , a nozzle  56 , a resistor  27 , a connecting channel  68  and a plurality of elementary ducts  75  is called ejector  73 . 
     Both the solutions also comprise a structural layer  107  in which the nozzles  56  are made using known techniques, such as for instance a laser drilling. These techniques have, however, a drawback described in the following: for the head to work properly, it is necessary for the nozzle  56  to have a truncated cone shape with the greater base towards the inside of the head, and the lesser base towards the outside. This is difficult to obtain using the above-mentioned techniques, whereas a nozzle with a truncated cone shape with the greater base towards the outside or, in the best case, a cylindrical shape nozzle is obtained commonly. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to produce a monolithic printhead in which the nozzles  56  are truncated cone shape with their greater base towards the inside of the head, and the lesser base towards the outside. 
     Another object is to produce the nozzles in a precise, reliable, repetitive way and at low cost. 
     A further object is to obtain greater design freedom and a less critical photolithographic manufacturing process. 
     Another object is to obtain greater stability of the shape of the parts during the steps of the process which comprise heat proceedings. 
     These and other objects, characteristics and advantages of the invention will be apparent from the description that follows of a preferred embodiment, provided purely by way of an illustrative, non-restrictive example, and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG.  1 —is an axonometric view of an ink jet printer; 
       FIG.  2 —represents an axonometric view of an ink jet printer according to the known art; 
       FIG.  3 —represents a section view of an ejector of a first monolithic printhead, according to the known art; 
       FIG.  4 —represents a section view of an ejector of a second monolithic printhead, according to the known art; 
       FIG.  5 —represents a wafer of semiconductor material, containing dice not yet separated; 
       FIG.  6 —represents the wafer of semiconductor material, in which the dice have been separated; 
         FIGS. 7   a  and  7   b —illustrate the flow of the operations in the manufacturing process according to the invention of the ejector of  FIG. 4 ; 
       FIG.  8 —illustrates a section of the ejector of  FIG. 4  at the start of the manufacturing process; 
       FIG.  9 —illustrates a section of the ejector of  FIG. 4  in a successive phase of the manufacturing process; 
       FIG.  10 —illustrates a section of the ejector of  FIG. 4  in another phase of the manufacturing process. 
       FIG.  11 —illustrates a section of the ejector of  FIG. 4  and of a first PDMS mould in another phase of the manufacturing process. 
       FIG.  12 —illustrates a section of the ejector of  FIG. 4  in a further phase of the manufacturing process. 
       FIG.  13 —illustrates a section of the ejector of  FIG. 4  and of a mask in a further phase of the manufacturing process. 
       FIG.  14 —illustrates a section of the ejector of  FIG. 4  in a further phase of the manufacturing process. 
       FIG.  15 —illustrates a section of the ejector of  FIG. 4  and of a second PDMS mould in a further phase of the manufacturing process. 
       FIG.  16 —illustrates a section of the ejector of  FIG. 4  in a further phase of the manufacturing process. 
       FIG.  17 —illustrates a section of the ejector of  FIG. 4  at the end of the manufacturing process. 
       FIG.  18 —illustrates the flow of the operations in a second embodiment of the manufacturing process of the ejector of  FIG. 4 ; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The manufacturing process of the ejectors  73  illustrated in  FIG. 4  for the monolithic ink jet printhead  40  will now be described. This process initially comprises the production of a “wafer”  60 , as depicted in  FIG. 5 , consisting of a plurality of dice  61 , each of which comprises microelectronics  62 , an area  63 ′ suitable for accommodating microhydraulics  63  made up of a plurality of ejectors  55 , and soldering pads  77 . 
     In a first part of the process, not described as it is not essential for the understanding of this invention, when all the dice  61  are still joined in the wafer  60 , the microelectronics  62  are produced and at the same time, using the same process steps and the same masks, the microhydraulics  63  of each die  61  are produced in part. 
     In a second part of the process, on each of the dice  61  still joined in the wafer  60 , the structural layers  107  are produced and the microhydraulics  63  completed by means of operations compatible with the first part of the process. At the end of the process, the dice  61  are separated by means of a diamond wheel: the whole made up of a die  61  and a structural layer  107  thus comes to constitute an actuator  50 , as can be seen in  FIG. 6 . 
     The second part of the manufacturing process is described with the aid of the flow diagram of  FIG. 7   a  and  FIG. 7   b . The following steps, numbered from 200 to 206, have already been described in the cited Italian patent applications TO 99A 000610 and TO 2000A 000335, to which reference should be made in relation to the production details of single steps. The description that follows contains only the information needed for comprehension of the innovative aspects of this invention. 
     In a step  200 , a silicon wafer  60  is available as it is at the outcome of the first part of the process, comprising a plurality of dice  61  having their microelectronics  62  finished, protected by the protective layer  30  of Si 3 N 4  and SiC upon which the conducting layer  26  is deposited, and arranged for the successive operations in the areas of microhydraulics  63 ′ suitable for production of the plurality of ejectors  73  constituting the microhydraulics  63 . 
       FIG. 8  depicts a zone of the printhead intended to accommodate the ejectors  73 , as it is in this step, in which the following are indicated: a substrate  140  of silicon P, a protective layer  30  of Si 3 N 4  and SiC, an “interlayer”  33  of SiO 2  TEOS, a conducting layer  26 , an N-well layer  36  and regions  76  arranged for subsequent drilling, in correspondence with each of which the conducting layer  26  presents apertures  125  having the same shape as the planned elementary ducts  75  will have to have. Also indicated are an upper face  170  and a lower face  171 . 
       FIG. 9  represents the zone of the ejectors  73 , as it will appear at the end of the next steps  201 ,  202  and  203 . 
     In a step  201 , a protective photoresist  32  is applied on top of the layer  26 , in order to protect the whole wafer  60  in the successive operations. Voids are made in the protective photoresist  32  by means of known techniques, to leave the apertures  125  uncovered. 
     In a step  202 , using as the mask the conducting layer  26 , elementary holes  75 ′ are made in correspondence with the apertures  125 , for instance by means a “dry etching” technology of the ICP (“Inductively Coupled Plasma”) type, for example, known to those acquainted with the sector art. The holes  75 ′ are blind holes and partially enter into the substrate  140 . 
     In a step  203 , etching is started of the groove  45 , again using ICP technology for instance. The portion of the groove  45  made in this stage, indicated as  45 ′, presents two walls  126  substantially parallel to the plane y-z, and reaches a distance of between 100 and 150 μm, for example, from the N-well layer  36 . 
       FIG. 10  represents the area of the ejectors  73 , as it will appear at the end of the next steps  204 ,  205  and  206 . 
     In a step  204 , the protective photoresist  32  is removed. 
     In a step  205 , on the conducting layer  26  and inside the elementary holes  75 ′, a first layer is applied of positive photoresist of a thickness equal to the height that the chambers  74  will have, by means for instance of a centrifuge in a process known as “spinner coating”. With a mask not shown in any of the figures, the photoresist is exposed to ultraviolet radiation only in correspondence with windows having the shape of that section parallel to the plane x-y which the future chambers  74  and the future connecting channels  68  will have. Intensity of the ultraviolet radiation is regulated such that the positive photoresist is depolymerized only as far as the conducting layer  26 , but not inside the elementary holes  75 ′. Finally development is effected, during which the portion of depolymerized photoresist is removed, leaving in this way cavities having the shape of the future chambers  74  and of the future connecting channels  68 , whereas the elementary holes  75 ′ are still filled with the positive photoresist, indicated with the shading, which has remained polymerized as it has not been reached by the ultraviolet radiation. 
     By performing the operations in the order indicated, the advantage is obtained of effecting this step while the groove  45 ′ and the holes  75 ′ are not in communication, as they are separated by a layer of silicon of a thickness between, for instance, 100 and 150 μm, and it is therefore not necessary to fill the groove  45 ′ with a temporary layer protecting the area in which development of the positive photoresist takes place. 
     In a step  206 , electrodeposition is performed of a metal, for example copper, gold or nickel, inside the cavities produced in the step  203 , in order to form the sacrificial layers  31 , having the shape of the future chambers  74  and of the future connecting channels  68 . The positive photoresist which fills the elementary holes  75 ′ enables an outer surface of the sacrificial layer  31  of greater flatness to be obtained. 
     In a step  207 , on the upper face  170  which contains the sacrificial layers  31 , a second layer  143  is applied of positive photoresist, for instance of the type AZ 4903 by Hoechst or SPR 220 by Shipley, having a thickness s preferably between 10 and 30 μm, as shown in  FIG. 12 . The layer  143  could be applied by means of a known “spinner coating” process, but its thickness s would not be controlled with precision and its outer surface would not be flat because it would follow in part the profile of the sacrificial layers  31 . To obtain a flat surface and a controlled thickness s of the layer  143 , the positive photoresist is applied with the aid of a first mould  80  of PDMS silicon rubber, a partial section of which is shown in  FIG. 11 , in which a layer  81  of silicon rubber and a support layer  82  of glass or metal can be seen. 
     The first mould  80  is fixed in such a way as to define an interspace of thickness s with the upper face  170  of the die  61 , by means of references not shown in the figure, as these are not essential for understanding of the invention. 
     Use of the PDMS mould is known to those acquainted with the sector art having been described, for example, in the article “Fabrication of glassy carbon Microstructures by soft Lithography” published in the magazine Sensors and Actuators No A72 (1999) and in the article “Wafer-Level In-Registry Microstamping” published in the IEEE magazine Journal of Microelectromechanical Systems, vol. 8, No 1, March 1999. 
     So that the positive photoresist fills the PDMS mould  80  uniformly and completely by capillarity, reaching the most hidden recesses and avoiding air inclusions, it must necessarily have a low viscosity and must, where possible, be applied in a vacuum (pressure of a few mm of Hg). 
     In a step  210 , a prepolymerization of the layer  143 , called “soft bake” by those acquainted with the sector art, is performed with a very slow rise in temperature, in order to permit a gradual elimination of the solvent. 
     In a step  211 , the PDMS mould  80  is removed. 
     In a step  212 , exposure of the layer  143  of positive photoresist is performed by means of ultraviolet radiation (UV) and a mask  144 , as can be seen in  FIG. 13 . Covers  145  in the mask, opaque to the ultraviolet radiation, are aligned with the resistors  27 , have a generally though not exclusively round shape, and have diameter d substantially equal to the diameter D of the future nozzles  56 . 
     During this operation, portions  156 ′ of the layer  143 , which do not receive the ultraviolet radiation, remain polymerized, bound off by a transition surface  147 . The portions  156 ′ must take on a truncated cone shape equal to that of the future nozzles  56 , having their greater base towards the inside of the head and their lesser base towards the outside. If the covers  145  have distinct edges, the ultraviolet radiation undergoes diffraction at the edges, rendering gradual the depolymerization of the positive photoresist local to the transition surfaces  147 , which accordingly assume a truncated cone shape, though this is however rarely identical to the shape designed. To obtain a truncated cone shape identical to the design shape, it is usually necessary to add grey areas  146  in the mask  144  around the covers  145 , which partially and in a predefined way intercept the ultraviolet radiation, in order to graduate in a controlled manner the depth of the action of the ultraviolet radiation and obtain the truncated cone shape desired. 
     In a step  213 , a complete polymerization, called “post-bake” by those acquainted with the sector art, is performed of the layer  143  in order to render the transition surfaces  147  better defined. 
     In a step  214 , development of the layer  143  is performed, as can be seen in  FIG. 14 . The depolymerized part of the positive photoresist is removed from the layer  143 . Casts  156  adhering to the sacrificial layers  31 , having an outer face  157  and a shape equal to that of the future nozzles  56 , are left after this operation. 
     In a step  215 , the structural layer  107  shown in  FIG. 16  is applied on the upper face  170  which contains the sacrificial layers  31  and the casts  156 . It has an outer surface  101  and is made of a compound polymer, for example, an epoxy resin or a mix of epoxy resin and methacrylates. To obtain a flat outer surface  101  and a controlled thickness of the structural layer  107 , the polymer is applied using a second PDMS silicon rubber mould  85 , known to those acquainted with the sector art, a partial section of which is shown in  FIG. 15  in which a layer  86  of silicon rubber and a support layer  87  of glass or metal can be seen. 
     The second mould  85  is put in contact with the outer face  157  of the casts  156 , and defines an interspace of thickness s with the upper face  170  of the die  61 : in this way, the outer surface  101  is co-planar with the outer face  157  of the casts  156 . 
     In a variant of this step  215 , the second mould  85  coincides with the first mould  80  used in the step  207 , as in both steps the same interspace of thickness s is defined with the upper face  170  of the die  61 . 
     So that the polymer fills the PDMS mould uniformly and completely by capillarity, reaching the most hidden recesses and avoiding air inclusions, it must necessarily have a low viscosity and must, where possible, be applied in a vacuum (pressure of a few mm of Hg). 
     In a step  216 , prepolymerization of the layer  107  is performed by means, for instance, of heating between 60° C. and 80° C., with a very slow rise in temperature, the purpose of which is to liberate the gaseous products of the polymerization. 
     The steps that follow are described with reference to  FIG. 17 , which represents a section parallel to the plane z-x of the head according to the invention, as it will appear at the end of the manufacturing process. 
     In a step  217 , etching of the groove  45  is completed by means of a “wet” type technology using, for example, a KOH (Potassium Hydroxide) or TMAH (Tetrametil Ammonium Hydroxide) bath, as is known to those acquainted with the sector art. Etching of the groove  45  is conducted according to geometric planes defined by the crystallographic axes of the silicon and accordingly forms an angle α=54.7°. The etching is stopped automatically when the N-well layer  36  is reached by means of a method, called electrochemical etch stop, known to those acquainted with the sector art. At the end of this operation, the groove  45  is delimited by the lamina  67 , and the holes  75 ′ are through holes, their blind bottom having been removed. 
     In a step  220 , the photoresist is removed from the holes  75 ′, in such a way as to obtain the elementary ducts  75 . 
     In a step  221 , a complete polymerization is performed of the structural layer  107  by means, for instance, of heating to a temperature of between 80 and 100° C. lasting for a few hours. 
     In a step  222 , the surface  101  of the structural layer  107  is cleaned with, for instance, an oxygen plasma process, for the purpose of removing any residues of the layer  107  which could partially or totally cover the casts  156 , so that the outer faces  157  are clean. Alternatively a lapping operation may be performed. 
     In a step  223 , etching is performed of the protective layer  30  of Si 3 N 4  and SiC in correspondence with the soldering pads, not shown in any of the figures. 
     In a step  224 , the wafer  60  is cut into the single die  61  by means of a diamond wheel, not shown in any of the figures. 
     In a step  225 , the casts  156  of positive photoresist are removed by means of a bath in a solvent suitable for the photoresist itself and which does not eat into the structural layer  107 . Turnover of the solvent may be stimulated by using ultrasound agitation or a spray jet. When this operation is completed, the nozzles  56  are obtained, shaped exactly like the casts  156 . 
     In a step  226 , the sacrificial layer is removed by means of a chemical process. The cavities left empty by the sacrificial layer thus come to form the chambers  74  and the connecting channels  68 . 
     The technology described from step  205  to step  226  is known to those acquainted with the sector art, as it is employed in the production of MEMS/3D (MEMS: Micro Electro Mechanical System). 
     Finally, in a step  227 , the finishing operations, known to those acquainted with the sector art, are performed:
         soldering of a flat cable on the dice  61  in a TAB (Tape Automatic Bonding) process, for the purpose of forming a subassembly;   mounting of the subassembly on the container of the head  40 ;   filling with ink  142 ;   testing of the finished head  40 .       

     The step  206 , electrodeposition of the sacrificial layer  31 , and the step  217 , wet etching of the oblique walls of the groove  45  with an electrochemical etch stop, require operations performed by means of electrochemical processes, during which specific layers belonging to all the dice  61  of the wafer  60  and, where applicable, all the segments into which the dice  61  are subdivided must be put at the same electrical potential. 
     This may be done advantageously as described in the Italian patent application TO 99A 000987, which is incorporated herein. 
     Second Embodiment 
     In a second embodiment, the steps from 207 to 216 inclusive are carried out in the same order as already described for the preferred embodiment, whereas the steps from 217 to 227 are carried out in an order indicated below, with the aid of the flow diagram in  FIG. 18 . The different steps correspond to those already described in relation to the preferred embodiment, and accordingly are designated with the same numerals followed by a single inverted comma. 
     After the step  216 , the step  222 ′ is carried out, in which cleaning is performed of the surface  101  of the structural layer  107 , for example with an oxygen plasma process, or a lapping operation. 
     In a step  225 ′ the casts  156  of positive photoresist are removed by means of a solvent bath. On completion of this operation, the nozzles  56  are obtained. 
     In a step  217 ′, etching of the groove  45  by means of the wet technology is completed. On completion of this operation, the groove  45  is bound off by the lamina  67 , and the holes  75 ′ are through holes, their blind bottom having been removed. 
     In a step  220 ′, the photoresist is removed from the holes  75 ′, so that the elementary ducts  75  are obtained. 
     In a step  221 ′, a complete polymerization, called “post-bake” by those acquainted with the sector art, is performed of the structural layer  107 . 
     In a step  226 ′, the sacrificial layer  31  is removed. In this second embodiment, an electrolytic process as described in the already quoted patent applications TO 99A 000610 and TO 99A 000987 may be used for the purpose, as the dice are still joined in the wafer  60 , and the equipotential surface constituted by the conducting layer  26  is accordingly available. The cavities left empty by the sacrificial layer come to form the chambers  74  and the connecting channels  68 . 
     In a step  223 ′ etching of the protective layer  30  of Si 3 N 4  and SiC in correspondence with the soldering pads is performed. 
     In a step  224 ′, the wafer  60  is cut into the single dice  61  by means of the diamond wheel. 
     Finally, in a step  227 ′ the finishing operations, known to those acquainted with the sector art, are performed:
         soldering of a flat cable on the die  61  in a TAB (Tape Automatic Bonding) process, for the purpose of forming a subassembly;   mounting of the subassembly on the container of the head  40 ;   filling with ink  142 ;   testing of the finished head  40 .