Patent Publication Number: US-8112889-B2

Title: Method of manufacturing an ink jet printhead

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage Application of PCT/EP2005/013999, filed Dec. 23, 2005, which designates the United States. 
     TECHNOLOGICAL FIELD OF THE INVENTION 
     This invention relates to a method of manufacturing an inkjet printhead. 
     STATE OF THE ART 
     In inkjet printers, printing is effected through a printing head comprising a plurality of nozzles capable of selectively emitting drops of black or coloured ink onto the paper while the head moves alternately (forwards and backwards) and transversely with respect to the driven movement of the paper. In the case of inkjet printers of the thermal type, the head uses heating elements, generally resistors, which heat the ink in order to boil it and therefore cause the ink to be expelled through the nozzles during the printing operation. 
     In certain applications, such as for example in points of sale (POS) printing systems for issuing receipts, tickets or bank certificates, the paper used for inkjet printing is of the ordinary type. The possibility of using ordinary paper renders inkjet technology particularly advantageous because it is relatively economical, especially when printing in black and white. 
     In other applications, high-quality colour inkjet printing is required; in order to achieve a photographic-like quality printing, especially in four ink printing systems, ink drop volume needs to be reduced significantly, for example to about 3 picoliters, wherein non-photographic quality four ink systems commonly operate with a drop volume of about 30 picoliters. U.S. Pat. No. 6,126,277 discloses a thin-film inkjet printhead being configured to eject ink droplets having a volume of about 2-4 picoliters. 
     Ink is generally ejected through an orifice or nozzle formed through an orifice plate (or nozzle plate). Build-up of material at the nozzle may affect formation of the drop, attract dust or other micro-debris, and may also cause smearing of the ink. For this reason it may be desirable that the surface of the nozzle plate should have a low wettability with respect to the fluid ejected through the nozzle. 
     U.S. Pat. No. 6,610,165 describes a method for coating a nozzle plate with a non-wetting Teflon (PFTE) material formed by thermal compression. 
     Typically, an inkjet printhead includes an array of nozzles formed through a nozzle plate that is attached to an ink barrier layer which in turn is attached to a thin film structure that includes ink firing heating resistors and the electrical interconnections suitable to control the heating of firing resistors and thus the ejection of the ink drops from the nozzles. The film structure is generally formed on or within a semiconductor substrate, typically a silicon wafer. 
     The ink barrier layer defines ink channels including ink vaporisation chambers comprising heating resistors and the nozzles which are aligned with the associated ink chambers. The ink barrier layer is typically a polymer material that is laminated as a dry film to the underlying thin film structure, and is designed to be photosensible to UV radiation and to be thermally curable. 
     Therefore, within this printhead structure, the surface of the nozzle plate opposite to the “ejection surface” (i.e. the surface through which the ink drops are ejected) needs to be bonded to the lower thin film structure of the printhead. 
     U.S. Pat. No. 6,155,674 discloses an adhesion interface between a silicon carbide (SiC) layer of a thin film substrate and a polymer ink barrier layer in the vicinity of the ink chambers formed in the polymer barrier layer and an adhesion interface between a silicon carbide layer disposed on an orifice plate and ink barrier layer. 
     Silicon carbide has been used for instance as adhesion promoter material on low-k fluorinated amorphous carbon (a-F:C) layers in the production of large scale integrated circuits. 
     WO patent application No. 01/80309 describes a method to enhance the adhesion of silicon nitride to a low-k a-F:C layer, in which silicon carbide is used to promote such adhesion; in particular the adhesion layer is obtained by depositing a relatively hydrogen-free hydrogenated silicon carbide by PECVD using silane (SiH 4 ) and methane (CH 4 ) as the deposition gases. It is believed that the low level of hydrogen results in a more compact silicon carbide structure which resists breakdown at a temperatures up to and above 400° C. 
     In semiconductor processing methods, in particular for manufacturing MRAM circuitry, silicon carbide is used as etch stop material as the lowest portion of an insulating material. US patent application 2004/0106271 discloses a chemical vapour deposition (CVD) process for depositing SiC at low temperatures over a substrate at a temperature no greater than 500° and preferably not greater than 250°. It is pointed out that silicon carbide is typically very tenaciously adhered to the substrate on which it is deposited, in part due to its exposure to high temperatures during subsequent processing. 
     Silicon carbide layers in thin film technology can be deposited by chemical vapour deposition (CVD) or physical vapour deposition (PVD). A deposition process that is used in the manufacture of semiconductor devices for depositing SiC on various substrates is the plasma-enhanced chemical vapour deposition (PECVD). 
     US patent application 2005/0090036 discloses a PECVD process for depositing substantially oxygen-free SiC having a dielectric constant of less than about 4 by holding the substrate at a temperature lower than 100° C., preferably at about 25° C. 
     A PECVD process is also shown in U.S. Pat. No. 6,821,571, in which an exposed surface of a carbon containing material—such as silicon carbide—is treated with an inert gas plasma such as helium and argon, or an oxygen-containing plasma such as a nitrous oxide plasma. An improvement of the adhesion and oxidation resistance of the carbon-containing layer is in this way achieved. 
     Finally, silicon carbide films are useful in the fabrication of integrated circuits and printer printheads to provide corrosion resistant and protective layers over structures formed thereon. US patent application 2003/0155074 discloses a plasma enhanced chemical vapor deposition (PECVD) of SiC, in which silane gas (SiH 4 ), methane gas (CH 4 ) and a noble gas (such as helium or argon) are used for obtaining a SiC layer having low hydrogen concentration; the temperature at which the process is carried out is comprised between 150° and 600°. 
     Both abrasion and deformation of the nozzle plate can occur during contact between the head and the other structures encountered in the printing operation, such as cleaning structures. The problem of the durability of the head is particularly present in the case of nozzle plates made of non-metal polymer material. EP patent application No. 1306215 describes a coating layer on at least one of the upper or lower surfaces of a nozzle plate to render the head more robust. Coating materials such as silicon nitride (Si 3 N 4 ), boron nitride (BN), silicon oxide (SiO 2 ), silicon carbide (SiC) and a composition known as “silicon carbon oxide” are used for this purpose. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method of manufacturing an inkjet printhead. 
     The printhead comprises a substrate, an ink barrier layer formed on the substrate and a nozzle plate arranged over the ink barrier layer. According to the preferred embodiments, the inkjet printhead comprises a metal nozzle plate, although the present invention is understood to envisage also printheads comprising a nozzle plate made of polymeric material. 
     The Applicant has considered that if the surface of the nozzle plate through which the ink drops are ejected (i.e., the ejection surface), that is the surface with which the drops come into contact, is sufficiently wetting-resistant (or anti-wetting), the drops will spread to a lesser extent, and the printing quality will significantly increase. 
     The Applicant has found that a wetting-resistant surface coating of silicon carbide on the ejection surface of the nozzle plate ensures that the nozzle plate has stable non-wettability properties in the course of the printing operation. 
     In particular, the lack of deterioration in the wetting-resistance properties of the SiC coating has the advantage of reducing the number of cleaning operations necessary in order to continue the printing operation, with consequent extension of the service life of the head. Also, if the surface of the plate has a wetting-resistant SiC coating, cleaning operations have a positive effect in removing printing residues without risking deterioration of the quality of printing subsequent to that operation. 
     When a SiC coating layer is present on the ejection surface of the nozzle plate, it has been observed that the drops remain close to the holes, and as a result of transitory hydraulics following ejection, are partly drawn back within the nozzle, with consequently less ink on the surface of the nozzle plate. 
     The property of the wettability (or non-wettability) of the surface of the nozzle plate may be evaluated by measuring the contact angle α, between a drop of ink and the surface of the nozzle plate.  FIG. 5  illustrates schematically the formation of a drop  23  on an upper surface  26  of a nozzle plate  28 . Angle α corresponds substantially to the angle which the tangent  24  to the surface of the drop  23  at a point P of the contact line between the surface of the drop  23  and the upper surface of the head  26  forms with the plane of the upper surface of the head  26 . The greater the value of α, the more the spreading of the drop is restricted, and the drop has well-defined perimeters. In other words, the higher the value of α, the more the drop is in contact with a less-wettable surface (for the same surface tension of the fluid forming the drop). 
     Preferably, the contact angle α of the wetting-resistant layer will not be less than approximately 45°. 
     The Applicant has considered that silicon carbide may exhibit adhesion properties such that a silicon carbide containing layer can be effectively used to enhance adhesion between the nozzle plate and the lower structure of the printhead, comprising a substrate and an ink barrier layer formed on the substrate. 
     The nozzle plate includes an upper surface and a lower surface, said upper surface being on the side of the ejection of ink drops and said lower surface being opposite to said upper surface. 
     According to the present invention, a wetting-resistant coating layer comprising silicon carbide is deposited on the upper surface of the nozzle plate and an adhesion promoting coating layer comprising silicon carbide is deposited on the lower surface of the nozzle plate, which is the surface that will face the underlying printhead structure. 
     Both the wetting-resistant layer and the adhesion layer are preferably obtained by chemical vapor deposition (CVD), more preferably by plasma enhanced chemical vapor deposition (PECVD). 
     The Applicant has found that adhesion of a SiC-comprising layer, in particular to the ink barrier layer, depend on the temperature a which the layer is formed. 
     The Applicant has observed that adhesion between a silicon carbide layer, which is deposited on a nozzle plate, and the ink barrier layer is not satisfactory if the silicon carbide layer has been deposited by a CVD process wherein the nozzle plate was held at about 300° C. 
     Since coating layers need to be formed on two opposite surfaces of the nozzle plate, at least two deposition process steps are to be carried out. The Applicant has found that the sequential order of the deposition process steps to be carried out to form the wetting-resistant coating layer and the adhesion coating layer is a crucial parameter in order to avoid the risk of deteriorating the adhesion properties of the SiC. 
     In particular, the Applicant has verified that if a first SiC-comprising coating layer is obtained in a first deposition step, and a second SiC-comprising coating layer is obtained in a second deposition step, following the first deposition step, the adhesion properties of the first coating layer are to a large extent lost after the second deposition step. 
     This has been found to occur also when the temperature at which the second deposition step is performed is substantially the same as the temperature at which the first deposition process step is carried out. The loss of the adhesion properties of the firstly-deposited coating layer after deposition of the second coating layer is deemed to be due to the additional thermal treatment undergone by the adhesion layer during the second deposition process step. 
     In particular, it was observed that the adhesion properties of a SiC-comprising coating layer deposited at about 150° C. were not maintained when the nozzle plate was further thermally treated during a second deposition process step, wherein the temperature of the nozzle plate was raised again and set to about 150° for more than about 20 minutes. 
     The Applicant has found that suitable adhesion properties of a SiC-containing layer can be achieved at deposition temperatures not larger than about 250° C. Preferably, deposition is carried out at a temperature comprised between 50° C. and 200° C., more preferably comprised between 100° C. and 150° C. 
     The Applicant has understood that if the wetting-resistant layer is realized first, and the adhesion layer is obtained successively, the adhesion properties of the adhesion layer are mostly maintained. 
     Advantageously, the wetting-resistant properties of the SiC-comprising layer deposited on the upper surface of the nozzle plate are not prejudiced by a successive thermal treating caused by a CVD process, e.g., carried out for deposition of a second SiC-comprising layer. 
     According to a preferred embodiment, the deposition process steps for forming the wetting-resistant coating layer and the adhesion layer are substantially identical, i.e., the deposition parameters are substantially the same for both process steps presumably resulting in two coating layers having essentially the same structural properties. In this way, the method of manufacturing an inkjet printhead can be cost-effective, since by carrying out twice the same CVD process, it is possible to obtain two coating layers on two opposite surfaces of the nozzle plate having different properties. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1-4  show a schematic detail of a printhead undergoing different steps of the method according to a preferred embodiment of the present invention. 
         FIG. 5  is a schematic representation of the contact angle of a drop on the ejection surface of a nozzle plate. 
     
    
    
     DETAILED DESCRIPTION 
     In the preferred embodiment of the invention the method of printing uses an inkjet printing head of the “top shooter” thermal type, that is one which emits ink drops in a direction substantially perpendicular to the ejection members, i.e., the nozzles. 
     With reference to the figures, an inkjet printhead manufactured by the method according to the present invention is generally indicated at  1 . 
       FIGS. 1-4  show a schematic section of two portions of the nozzle plate  30 , between which a nozzle  82  is defined. 
     With reference to  FIG. 1 , the method according to the present invention comprises a step of arranging a nozzle plate  30  comprising a plurality of nozzles  82  (only one nozzle is shown in  FIG. 1 ) from which ink droplets directed against a printing medium, which is generally paper (not shown), are ejected. The nozzle plate comprises an upper surface  31  and a lower surface  32 , said upper surface  31  being the surface facing the side where ink droplets are emitted. 
     The lower surface  32  is the surface of the nozzle plate  30  which is opposite to the upper surface  31  and which will be placed into contact with the remaining portion of the printhead  1  in a successive process step. 
     Nozzle plate  30  is preferably of metal, more preferably of Au-coated nickel. In  FIG. 1 , a galvanic nickel plate (grown for example by electroforming)  80  is coated with a layer of galvanic gold  81  again obtained, for example, by electroforming. Preferably, a layer of gold  52  and  72 , respectively, having a thickness of some nm (for example 2-5 nm), is deposited by sputtering onto both the upper and lower surfaces of the Au-coated plate, i.e., on layer  81 . Preferably the surface of the galvanic Au layer  81  is treated by sputter etching using argon gas plasma in order to clean the surface before deposition of Au layers  52  and  72  by sputtering. 
     According to a preferred embodiment, the method comprises a first deposition step on the upper surface  31  of a first coating  40 , including a first layer  41  comprising silicon carbide. Said first coating layer is a wetting-resistant layer. Preferably, the first coating layer  41  is formed by PECVD. During deposition of the first coating layer  41 , the nozzle plate  30  is held at a first temperature not larger than 250° C. 
     In the example illustrated in  FIG. 2 , the nozzle plate  30  is held during the first deposition step by means of a holder  42  in order to secure a mask  43  (e.g., a metallic masking layer) on the upper surface  31  of the nozzle plate. Mask  43  protects some areas of the nozzle plate  30  from the deposition of the first SiC-comprising layer  41 , for instance the surface areas where alignment marks (not shown) are present. Alignment marks can be optionally used to align the nozzle plate with the underlying substrate before bonding of the nozzle plate to the ink barrier layer, as described more in detail in the following. Alignment between the nozzle plate and the underlying structure can be carried out by means of a standard optical alignment technique. It has been observed that alignment marks tend to be difficult to detect under optical beam through SiC layers. 
     The holder can also function as supporting substrate during deposition for more than one nozzle plate. 
     The first coating layer comprising silicon carbide  41  deposited on the upper surface  31  of the nozzle plate functions as wetting-resistant layer. It is deposited at least on the surface areas in the vicinity of and corresponding to the nozzles  82 . In this way, the ink-contact surface on which the ink droplets is in contact with has wetting resistant properties. 
     Preferably, the first SiC-containing coating layer  41  is deposited on substantially the whole upper surface  31  of the nozzle plate  80 , optionally with the exception of very small surface areas (e.g., not larger than 1-2% of the upper surface of the nozzle plate) containing alignment marks, which are not in the vicinity of the ink-ejection areas. 
     Preferably, the first SiC layer  41  can be approximately 30-40 nm thick. 
     Preferably, the temperature at which the nozzle plate is maintained during the deposition of the first SiC-comprising coating layer  41 , i.e., the first deposition temperature, is comprised between 50° C. and 200° C., and more preferably is comprised between 100° C. and 150° C. 
     Precursor gases for forming SiC-comprising coating layer  41  comprise methane gas (CH 4 ), and silane gas (SiH 4 ) 5% diluted with Argon (SiH 4 /Ar 5%). 
     For example, methane is introduced in the deposition chamber with a flow rate of about 50 sccm, whereas the flow rate of the SiH 4 /Ar mixture is of about 150 sccm. Pressure in the deposition chamber can be of about 750 milli Torr, while the power supplied (at low frequency) is of about 44 W. Deposition temperature is of about 150° C. 
     Due to the relatively low deposition temperature and the deposition parameters, it is believed that the film deposited using the parameters of the above described example is substantially an hydrogenated silicon carbide (SiC x H y ) layer. 
     According to a preferred embodiment, before the deposition of the first coating SiC-comprising layer  41 , the step of depositing said first coating  40  comprises a step of covering the upper surface  31  of the nozzle plate  30  with a first intermediate layer  50  for improving the adhesion between sputtered Au-film  52  and the first SiC-comprising coating layer  41 . 
     The first intermediate layer  50  comprises a film of tantalum  51 . The tantalum film can be deposited by sputtering with a thickness comprised between 30 nm and 50 nm. 
     By means of the above described first deposition of the first SiC-coating layer  41 , a wetting-resistant coating is realized on the upper surface  31  of the nozzle plate  30 , thereby forming a wetting resistant ejection surface. The contact angle α of the first coating layer  41  was measured to be comprised between 40° and 500°. Measurements of the contact angle mentioned in the present description can be obtained at ambient temperature (22-25° C.) using a commercial OCA 20 static angle measuring system distributed by FKV, depositing a drop of liquid on the surface of the nozzle plate using a micropipette. 
     Following the deposition of a first SiC-comprising coating layer, the method according to the present invention further comprises a step of depositing on the lower surface  32  of the nozzle plate  30  a second coating  60  including a second SiC-comprising layer  61  ( FIG. 3 ). 
     During deposition of the second coating layer  60  the nozzle plate  30  is maintained at a temperature (i.e., the second deposition temperature) not larger than about 250° C. 
     Before the second deposition step, the nozzle plate is positioned preferably in the deposition chamber so as to have the lower surface  32  facing the gases used during deposition. After the first deposition step, the nozzle plate  30  is removed from holder  42  and it is placed on a heater block (not shown) inside the deposition chamber with the surface coated by layer  41  facing the heater block. 
     Preferably the second deposition temperature is comprised between 50° C. and 200° C., more preferably is comprised between 100° C. and 150° C. 
     According to a preferred embodiment, the first and second deposition temperatures are substantially equal to each other. 
     By means of the above described second deposition, an adhesive coating is realized on the lower surface  32  of the nozzle plate  30 , so that the latter can be reliably engaged with the underlying portion of the printhead  1 , as described more in detail in the following. 
     In the preferred embodiment, the deposition of the second layer  61  is obtained by means of a Chemical Vapor Deposition (CVD) process and, in particular, by means of a Plasma Enhanced Chemical Vapor Deposition (PECVD) process. 
     Precursor gases for forming SiC-comprising coating layer  41  comprise methane gas (CH 4 ), and silane gas (SiH 4 ) 5% diluted with Argon (SiH 4 /Ar 5%). 
     For example, methane is introduced in the deposition chamber with a flow rate of about 50 sccm, whereas the flow rate of the SiH 4 /Ar mixture is of about 150 sccm. Pressure in the deposition chamber can be of about 750 milli Torr, while the power supplied (at low frequency) is of about 44 W. Deposition temperature is of about 150° C. 
     At the end of the second deposition step, the second SiC layer  61  can be approximately 30-40 nm thick. 
     Preferably, the deposition parameters defining the second deposition step are substantially the same as the deposition parameters defining the first deposition step for forming the first SiC-coating layer. 
     According to a preferred embodiment, before the deposition of the second SiC-comprising layer  61 , the step of depositing said second coating  60  comprises a step of covering the lower surface  32  of the nozzle plate  30  with a second intermediate layer  70 , for improving the adhesion between sputtered Au-film  72  and the second SiC-comprising coating layer  61 . 
     The second intermediate layer  70  comprises a film of tantalum  71 . The tantalum film can be deposited by sputtering with a thickness comprised between 30 nm and 50 nm. 
     Preferably, the second coating layer (i.e., the adhesive layer)  61  covers substantially the whole lower surface  32  of the nozzle plate  30 . It is to be noted that the first coating layer  41  is deposited before the second coating layer  61 . In other words, the deposition of the second layer  61  is carried out after the deposition of the first layer  41  is completed. 
     The nozzle plate  30  comprising a wetting-resistant coating layer on its upper surface and an adhesion-promoting layer on its lower surface is brought into contact to the underlying portion of printhead  1 . In particular, the lower surface of the nozzle plate coated with SiC-comprising layer  61  is brought into contact to the underlying portion of the printhead. 
       FIG. 4  illustrates a partial transverse cross-section of a printhead  1  obtained by a process according to a preferred embodiment of the invention illustrated in  FIGS. 1-3 . The same reference numerals are given to elements of the nozzle plate corresponding to those shown in  FIGS. 1-3  and their description is omitted. 
     The printhead  1  comprises substrate  10  and an ink barrier layer  20  formed on such a substrate  10 . 
     Preferably, substrate  10  comprises a silicon substrate  12  (typically formed from a crystalline silicon wafer) on which there is formed a thin-film structure  11 . The thin-film structure  11  comprises a layer of silicon oxide  6  formed within the upper surface of the silicon substrate  12  and a plurality of heating elements  2  (only one element is illustrated in  FIG. 3 ), for example resistors of Ta/Al, which are deposited on the silicon oxide surface  6 . The film-film structure  11  further comprises a layer or a plurality of protective layers  3 , for example a Ta/SiC/Si 3 N 4  multilayer, which covers the resistors  2  in order to protect them. 
     Each nozzle  82  is positioned in relation to a chamber  5  where a bubble of vapour forms following heating of resistor  2 . 
     The ink barrier layer  20  in which are provided chambers  5  and conduits (not shown) through which the ink flows to the chambers from an ink reservoir fed by a cartridge (not shown). 
     Preferably, the ink barrier layer  20  is a polymeric layer laminated as a dry film on the thin-film structure  11 . More preferably, the polymeric layer is photosensitive and a pattern can be defined in the layer by exposure to UV radiation and subsequent thermal curing. 
     After the deposition of the second coating layer  61  is completed, the nozzle plate  30  is arranged onto the ink barrier layer  20 , by bringing into contact the second coating layer  61  with the ink barrier layer  20 . Thanks to the adhesive properties of the silicon carbide included in the second coating layer  61 , the nozzle plate  30  and the ink barrier layer  20  can be reliably bonded to one another. 
     Preferably, bonding between the second coating layer  61  and the ink barrier layer  20  is obtained by a thermo-compression process. During this process, the SiC-coating layer  61  is urged against the upper surface of the ink barrier layer  20  by means of known spring devices, such as one or more spring clips. After the mechanical contact between the layers is achieved, the printhead  1  preferably undergoes an additional thermal treatment, during which the nozzle plate  30  (and the underlying layers) is heated at a annealing temperature. 
     The annealing temperature is advantageously higher than said first and second temperatures; in the preferred embodiment, the third temperature is comprised between 140° C. and 180° C., more preferably between 155° C. and 165° C. 
     According to a preferred embodiment, first and second deposition temperatures are of about 150° C., whereas the annealing temperature is of about 160° C. Annealing time can be of about 1 h. 
     At the end, the nozzle plate  30  is properly bonded to the underlying portion of the printhead  1  (namely, the ink barrier layer  20  and the substrate  10 ). 
     It has been noted that a post-deposition annealing of the nozzle plate can improve the wetting-resistant properties of the first SiC-comprising coating layer. 
     An increase of the contact angle α of the first layer  41  by approximately 10° was observed after annealing at 160° C. for about 1 h, so that contact angles between the ejection surface and the ink droplets of about 50°-60° could be obtained. 
     If post-deposition annealing is carried out while the second SiC-comprising coating layer (i.e., the adhesion layer) is maintained in mechanical contact to the ink barrier layer, it has been noted that the adhesion properties of the adhesion layer do not significantly deteriorate. In fact, a reliable bonding (supposedly through a chemical bonding reaction) between the two layers has been observed to take place.