Patent Publication Number: US-6988791-B2

Title: Ejection head for aggressive liquids manufactured by anodic bonding

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Application No. 10/296,629 filed Nov. 26, 2002 now U.S. Pat. No. 6,780,340, which is a U.S. National Phase Application Under 35 U.S.C. 371 and applicant herewith claims the benefit of priority of PCT/IT01/00266 filed May 25, 2001, which was published Under PCT Article 21(2) in English and Application No. TO2000A000494 filed in Italy on May 29, 2000. The said applications are hereby incorporated by reference. 

   TECHNICAL FIELD 
   This invention relates in general to the sector of ejection heads for ejecting liquids in the form of droplets, and in particular to an ejection head provided with a structure that makes this ejection head highly suited to working with liquids having a high level of chemical aggressiveness. 
   The invention also relates to a method for manufacturing an ejection head provided with a special resistance to chemically highly aggressive liquids, so as to be able to be employed advantageously in combination with this category of liquids. 
   BACKGROUND ART 
   The ejection head, also called simply ejector or injector in the following, according to the invention has characteristics that render it advantageous for use in numerous industrial sectors, even with specifics, characteristics and problems differing completely from one sector to the next. 
   In particular, among the possible sectors of application are, purely by way of example, that of ink jet printing, or that of fuel injection in an internal combustion engine. 
   As will be clear in the remainder of the description, the ejection head of the invention presents significant similarities, both structural and operational, with a thermal ink jet printhead, of the type working on the basis of the so-called bubble ink jet printing technology. Printheads of this type are widely known in the sector of ink jet printing technologies, where they are applied in a variety of solutions, and are still undergoing significant developments. 
   Therefore, for the sake of completeness and in order to facilitate the understanding of this description, and also in consideration of the fact that the ink jet printing sector constitutes, as already said, one of the possible and main fields of application of this invention, the general characteristics of these bubble type thermal ink jet printheads and some of their most recent developments will be set down in short below. As is known, in the printheads working with the bubble type ink jet technology, the ink contained in the printhead is brought to boiling point by thermal actuators consisting of electrical resistances which are powered with opportune current pulses in order to activate, inside the ink, the appearance of a bubble of vapour which, by expanding, causes ejection of the droplets through a plurality of nozzles in the printhead. 
   The printheads operating with the bubble technology may be divided into two main categories, depending on their structure, called respectively “top shooter” and “edge shooter”. In the first type, the nozzle consists of an aperture arranged immediately above the thermal actuator and separated from the latter by a small chamber filled with ink, so that the expansion of the bubble of vapour is used in a direction perpendicular to the thermal actuator so as to eject the droplet through the aperture. In the second type, the thermal actuator is disposed along the wall of a duct a short way from the duct&#39;s outlet section to the outside, so that the expansion of the bubble of steam is used in a direction transversal to the actuator to eject the drop laterally through the outlet section of the duct. 
   This bubble technology has been a standard in the printing sector for many years now, and is applied with success on numerous models of ink jet printheads, both for black and white printing and for colour printing. In particular, the ink jet printheads that work according to this technology are moving towards ever greater levels of integration and complexity, the objective being to comprise a greater number of circuits, nozzles and functions, and therefore attain ever greater printing speeds and definitions. One of the most recent examples of this technical development is represented by what are known as the monolithic printheads, i.e. by thermal ink jet heads in which the nozzle plate is made, not as a separate part, but together with the other parts of the printhead, particularly with those parts that constitute the driver circuits of the actuators and the hydraulic network for conveying the ink inside the printhead. 
   Therefore in these monolithic heads, the nozzle plate does not constitute a piece which is made separately and mounted at the end of the process of manufacturing the printheads, but rather a part which is formed progressively in the manufacturing process, so that each printhead acquires a typically monolithical structure integrating the various parts. 
   Hand in hand with the constant evolution of the bubble ink jet thermal printheads, the inks that can be used on these heads have also evolved considerably, which has led to a continuous improvement in their quality and reliability. 
   Generally speaking, evolution of the printheads has been accompanied by a corresponding evolution of the inks, the objective being to research ever better combinations between the printing media intended for receiving the droplets of ink, the structural characteristics of the head, and the chemical characteristics of the inks. 
   Typically this research into inks has been conducted with the objective of formulating inks capable both of improving the print quality on an ever broader range of print media, and of mating optimally with the new structures of printheads brought out with time. 
   In this way, both black and coloured inks have been formulated capable of minimizing the problem of clogging of the nozzles, cause by sedimentation of the pigments contained in the inks, despite the ever more intense miniaturization of the printheads and the reduction of the diameter of the nozzles in order to obtain ever smaller droplets. 
   Additionally, the research has permitted to define optimal combinations between inks and materials used in manufacturing the heads, with inks and materials compatible with one another, i.e. capable of not triggering off undesired reactions, and of maintaining their nominal characteristics in time, so as not to have negative effects on the operation and reliability of the printheads. In particular, this research into, as stated, constantly improving the combination between inks, print media, and printheads, has obviously addressed the formulation of inks having a low or practically null degree of chemical aggressiveness, namely inks free of substances capable of aggressing, corroding and reacting with, even only minimally, the various materials employed in manufacturing the heads and wetted by the inks. 
   For instance, it was attempted to avoid those inks containing substances that could interact with the organic compounds usually employed in making the junctions between the parts of the head. However, in this way, recent research in inks has in fact resulted in a certain consolidation, regarding their use on printheads, of inks with a null or practically null level of chemical aggressiveness. 
   At the same time, the possibility was ignored of employing these printheads in combination with particular types of ink and/or in general liquids which, though widely applied and capable of giving optimal results in certain fields, including different from printing true and proper, possessed however characteristics of chemical aggressiveness incompatible with the structure of the printheads that were being developed, and in particular contained aggressive substances certainly capable of corroding them and compromising their operation in time. 
   Besides, as is easy to imagine, it could be very useful and advantageous to be able to dispose of a new ink jet printhead, of the type based on the bubble technology or also on other technologies, having the ability to work with inks, perhaps already employed with success in various applications, including different from printing on paper, but unfortunately containing corrosive and/or aggressive substances likely to damage in time the structure and the materials of the currently known bubble type thermal ink jet heads. In fact, in this way the application possibilities for these printheads could be considerably extended, considering the new properties, essential characteristics and performance advantages that these corrosive substances could confer on the inks employed with them. Unfortunately however, as said, in reality the known ink jet printheads do not have a structure capable of resisting corrosive agents that may possibly be present in the inks employed with the printheads, so that in this hypothetical case they would rapidly enter decay. 
   For example, as is known, inks known to be typically aggressive, containing for instance urea, and/or having a determined acidic PH, can certainly not be used on the current thermal heads, because they would surely damage the junctions and the gluing zones between the different layers comprising the structure of the head. 
   There are also sectors in the art, again completely different from that of ink jet printing and the relative printheads, in which it is necessary to eject liquids in the form of droplets, preferably also very small, and in which these liquids to be ejected are particularly aggressive from the chemical viewpoint, and at any rate have a composition incompatible with the structure of the currently known printheads. 
   An important one of these sectors, briefly hinted at above, is that of the injection of a fuel, such as diesel or petrol, in the combustion chamber of an internal combustion engine. In this sector, the solutions normally adopted for fuel injection are based on mechanical type injectors, which however have the disadvantage of not reaching a sufficient degree of miniaturization of the droplets, or to put it better, that degree of miniaturization which would allow a better and more precise dosage of the fuel, and accordingly to attain better performance of the engine, such as for instance a higher thermal efficiency. 
   Therefore, potentially at least, this sector could avail of the ink jet technology which, in comparison with the traditional fuel ejectors, has been shown capable of obtaining droplets of liquid much smaller in volume, as also of obtaining in general a better and more efficient control of the quantity of liquid ejected in droplet form. 
   Yet another sector where there may be the need to dose in a precise and controlled way particularly aggressive liquids from the chemical viewpoint is the biomedical sector. 
   DISCLOSURE OF THE INVENTION 
   The general object, therefore, of this invention is to produce a new ejection head which, though bearing some similarities to the known ink jet printheads, substantially innovates with respect to the latter, and in particular possesses characteristics likely to make its use possible and advantageous in combination with particularly aggressive liquids from a chemical viewpoint, including in industrial sectors highly different from ink jet printing, and for example in the sector of injection of fuel in an internal combustion engine. 
   This object is achieved by the ejection head and corresponding manufacturing method having the characteristics defined in the main independent claims. 
   A more specific object of this invention is to produce an ink jet printhead, of the type operating with the bubble technology or other technologies, that can be used without drawbacks with aggressive inks notoriously capable of chemically reacting with and/or corroding the materials, typically organically based ones, currently used in the manufacture of printheads, so as to allow, at least potentially, an extension of the possibilities of industrial application of the technologies and concepts developed in connection with the known printheads to sectors up till now excluded from these technologies and concepts. 
   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, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG.  1 —is a schematic, sectional view of a head for the ejection of droplets of liquid according to this invention; 
     FIG.  2 —is a synthetic flow diagram of a method according to this invention for manufacturing the ejection head of  FIG. 1 ; 
       FIGS. 3A–31  are sectional views illustrating in sequence the various steps for manufacturing a plate with nozzle of the ejection head of  FIG. 1 ; 
       FIGS. 4A–4C  are sectional views illustrating the final steps for making the structure of a substrate bearing an actuator of the ejection head of  FIG. 1 ; 
     FIG.  5 —is a working diagram relating to a mounting operation, performed by means of the “anodic bonding” type technology, for soldering the nozzle plate of  FIG. 3  to the substrate of  FIG. 4 ; 
     FIG.  6 —shows a first example of application of the invention concerning a printhead provided with multiple nozzles and suitable for ejecting droplets of ink; 
     FIG.  7 —illustrates a silicon wafer used for manufacturing a plurality of nozzle plates of the printhead of  FIG. 6 ; 
     FIG.  8 —illustrates another silicon wafer used for manufacturing a plurality of substrates of the printhead of  FIG. 6 ; and 
     FIG.  9 —demonstrates a second example of application of the ejection head made with the method of the invention, in which the ejection head is arranged for ejecting droplets of fuel in an internal combustion heat engine. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   With reference to  FIGS. 1 and 2 , a head for the ejection of droplets of liquid, also called ejection head in the following, or ejection device, or more simply ejector, made according to the method of this invention, is generically depicted with the numeral  10 , and comprises a substrate  11 , also called actuation support, which bears at least one actuator  15 , also called in the following ejection actuator; a nozzle plate  12 , also called orifice plate, which is provided with at least one nozzle  13  and is permanently connected to the substrate  11  along a junction zone  25 , and a hydraulic circuit  21 , arranged inside the ejection head  10 , the function of which is to contain and convey a liquid  14  in the zone of the election head  10  between the actuator  15  and the nozzle  13 , in such a way that they are both wetted by the liquid  14 . 
   The ejection head  10  is permanently attached along the substrate  11  on a carrier  30 . The actuator  15  is positioned, along the substrate  11 , in a zone adjacent to the nozzle  13 , and is suitable for periodically activating, in the volume of liquid  14  that separates it from the nozzle  13 , a wave of pressure, or in general a pumping effect, such as to cause the emission of a plurality of droplets  16  formed by the liquid  14 , through the nozzle  13 . 
   To this end, the actuator  15  is arranged for being driven directly by means of suitable electric signals or pulses, each corresponding to an ejected drop, which are controlled by an electronic control unit  19  of the ejection head  10 . 
   The actuator  15  may also be associated with actuation circuits, arranged between the actuator and the control unit  19 , which, under the control of the control unit  19 , have the specific function of generating the pulses which directly control the actuator  15  for generating the droplets  16 . 
   In  FIG. 1 , the line  18  schematically represents the electrical connection, between the control unit  19  and the actuator  15 , the function of which is that of transmitting the signals intended for commanding the actuator  15  to cause ejection of the droplets  16 . 
   In particular, the hydraulic circuit  21  comprises a first inlet duct  24 , for conveying the liquid  14 , which extends through the substrate  11 ; a second inlet duct  22  which is formed in the nozzle plate  12  and which is in communication with one end of the first duct  24 ; and at least one chamber  20 , also formed in the nozzle plate  12 , which is adjacent to both the actuator  15  and the nozzle  13 . 
   The chamber  20  is suitable for being fed with the liquid  14  through the inlet duct  22 , and defines an internal space in which the liquid  14  is subjected to the wave of pressure generated by the actuator  15  for being ejected through the nozzle  13 . 
   In addition, the ejection head  10  is associated with a tank  17 , containing a certain quantity of liquid  14 , which constitutes a reserve for the liquid  14  to be fed to the chamber  20  of the ejection head  10 , and which for this purpose is in communication with the hydraulic circuit  21 , through a feeding duct  23 . 
   In this way, the ejection head  10  can receive the liquid  14  continuously from the tank  17 , so that it is ejected in the form of droplets  16  towards the outside of the ejection head  10  through the nozzle  13 . 
   The technologies used for generating in the liquid  14  the above-mentioned pumping effect which results in ejection of the droplets  16  of liquid may be of various types and be based on different principles. For simplicity&#39;s sake, in this description, reference will preferably be made to the bubble type ejection technology, widely known and used in the sector of printers, which is based on the generation by the actuator  15 , in the zone of the nozzle  13 , of a micro bubble of liquid vapour which, on expanding, causes the ejection of a droplet of liquid through the nozzle  13 . Clearly, however, the description that will be given must not be seen as tending to limit the scope of this invention to this particular liquid droplet ejection technology. 
   For instance, by way of alternative to the bubble technology, the pumping effect for ejection of the droplets could be obtained from the deformation of a piezoelectric type actuator. 
   This much said, in the bubble technology mentioned, the actuator  15  consists of a resistor which, in practice, is driven by the control unit  19  with a brief current pulse sufficient to determine, by the joule effect, a rapid heating of the same resistor  15 . 
   Accordingly the liquid  14  arranged in the immediate vicinity of the resistor  15  is brought to evaporation, and therefore causes the appearance of a vapour bubble, derived from the liquid  14 , which by expanding exerts a pumping effect in the direction of the nozzle  13  to determine, through the latter the ejection of a droplet  16 . 
   Then, at the end of the pulse, on account of the simultaneous cooling of the resistor  15 , the vapour bubble collapses, so that the liquid  14  adjacent to the resistor  15  returns to its starting conditions, and the resistor  15  can once again be activated with a new pulse to cause the ejection of a new droplet  16 . In short, this cycle is repeated periodically, driving the resistor  15  with a predetermined succession of pulses which result in the generation of a like number of vapour bubbles adjacently to the resistor  15 , and the ejection of corresponding droplets  16  through the nozzle  13 . 
   As illustrated in  FIG. 1 , the nozzle  13  is arranged to the front with respect to the resistor  15 , so that the expansion of the vapour bubble is used in the normal direction to the resistor  15  to eject the droplet  16 . This disposition, as already said, is often called “top shooter” type, and is typical of an important category of ejection heads which are based on the bubble technology. However the relative disposition between the ejection actuator and the nozzle may also be different from that shown in  FIG. 1 , without departing from the scope of this invention. 
   As described in detail later, the liquid  14  used on the ejection head  10  for being ejected in the droplet form may also be of different types, and have completely different compositions from one type of liquid to the next, depending on the specific sector in which the ejection head  10  is applied, and therefore of the specific characteristics that the liquid must possess in relation to that given sector. The nozzle plate  12  and the substrate  11  constitute the essential parts of this ejection head  11 , and are produced in two distinct processes, indicated in  FIG. 2  with the numerals  31  and  32  respectively, before subsequently being assembled and connected permanently together, during a step  33 , in order to form the ejection head  10 . 
   For clarity&#39;s sake, the two manufacturing processes  31  and  32 , respectively of the nozzle plate  12  and of the substrate  11 , will be described separately, starting with that of the nozzle plate  12 . 
   With reference to  FIG. 3   FIGS. 3A–3I , this process comprises an initial step, represented in  FIG. 3A , wherein a wafer of silicon  51 , having two opposite faces indicated respectively  51   a  and  51   b , is stuck using an adhesive substance on a carrier  52 , for example on the side  51   b.    
   The wafer  51  may readily be found in commerce and has a standard shape, for example round shape having diameter 3″ and approximate thickness 75 μm. 
   The carrier  52  too may consist of a known type wafer, even if considerably thicker than the wafer  51  used to make the nozzle plate  12 . 
   For example the carrier  52  may be made of a round wafer of diameter 4″, thickness 0.5 mm, either of standard silicon type, or of glass or ceramic. 
   The wafer  51  is oxidised on the outside, so as to present on its two opposite faces,  51   a  and  51   b , a thin layer  55  silicon dioxide SiO 2 , of thickness 0.3÷0.4 μm for example. 
   After being mounted on the carrier  52 , the wafer  51  is covered in a known way, on its free face  51   a  opposite that  51   b  stuck on the carrier  52 , with a thin layer  53  of a light-sensitive substance, called “photoresist”, 1–3 μm thick. 
   In particular the photoresist constituting the layer  53  is positive type, i.e. it is such as to be, under normal conditions, resistant and not subject to attack from certain substances, and as to become, on the other hand, easy to dissolve and remove by these substances, if exposed to light radiation. 
   According to known techniques and as illustrated in  FIG. 3B , after application on the wafer  51  this layer  53  of positive photoresist is subsequently illuminated with light  49  coming through a suitable mask  50  having a given configuration which corresponds to the positive image of those parts of the hydraulic circuit  21 , namely the inlet duct  22  and the chamber  20 , that will be formed in the nozzle plate  12 . 
   In this way, the layer  53  is impressioned in such a way as to become removable in the subsequent operation only in the areas illuminated by the light  49 . 
   Conveniently, for the purpose of reaching economies of scale and improving the efficiency of the production process, the wafer  51  can be used for manufacturing a plurality of nozzle plates  12 , each corresponding to an elementary area of the wafer  51 . 
   To this end, the mask  50  is arranged with a configuration which is made up of a plurality of equal profiles, each reproducing a hydraulic circuit  21  to be made on a corresponding elementary area of the wafer  51 . Accordingly the positive photoresist  53  is illuminated through the mask  50 , and therefore becomes removable, along a plurality of equal zones, one for each elementary area of wafer  51 , which correspond to the profiles of the mask  50 . 
   For simplicity&#39;s sake,  FIG. 3B , as also the following ones, refer to and represent the structural changes which occur only in one elementary area of the wafer  51 , though it will be clear that what is depicted in each of these figures is to be considered as repeated exactly in each of the other elementary areas of the wafer  51 . 
   Therefore, using known techniques, the layer  53  of photoresist is developed, removing therefrom the zones impressioned by the light and accordingly non-resistant, in order to uncover, in correspondence with these zones, the underlying layer  55  of SiO 2 , as illustrated in  FIG. 3C . 
   Later, the wafer  51  is subjected to an etching operation, the object of which is to remove, in correspondence with the areas not protected by the upper layer  53  of photoresist, the surface thickness  55  of SiO 2 , in order to uncover the underlying silicon part. 
   Typically this etching operation to remove the SiO 2  is effected in a liquid bath, or at any rate in a humid environment, and accordingly is also often called “wet etching” or “wet”. Then the external layer  53  of photoresist is removed. In this way the layer  55  of SiO 2  forms the protective mask for the successive operation of etching the silicon constituting the wafer  51 . 
   According to a variant of the process described up to now, the starting wafer may be exempt, on its faces, of the surface layer of SiO 2 , and therefore consist solely of pure silicon. In the latter case, the layer of photoresist is deposited directly on the silicon of the wafer and subjected to the same operations of illumination, development, and removal already described in relation to the previous case of the wafer with oxidised surface, in order to form a protective mask for the subsequent step of etching the silicon of the wafer, which is exactly equivalent to that performed through the layer of SiO 2 , relative to the earlier case. For simplicity&#39; sake, only the case of the wafer  51  provided with the two surface layers of SiO 2  is depicted in  FIG. 3 . 
   In both the cases described above, after formation of the protective mask for the silicon of the wafer  51 , as said, either through the layer of SiO 2 , or through a layer of photoresist, the wafer  51  is subjected to one or more further etching operations, which have the purpose of selectively removing the silicon of the wafer  51  down to a given depth, in order to form the chamber  20  and the inlet duct  22 , of the hydraulic circuit  21 , which are present on the nozzle plate  12 . 
   This etching step, showin in  FIG. 3D , is performed by means of appropriate equipment in a vacuum environment, where the wafer  51  is subject to the action of agents in the gaseous or plasma state which combine with the non-protected silicon of the wafer  51 , corroding it and removing it down to the desired depth. 
   Therefore, by contrast with the etching step previously referred and performed in a humid environment, or “wet etching”, this etching step is often referred to as “dry etching”. 
   For example, in this step the wafer  51  is hollowed for a depth of approx. 10÷25 μm, in order to form a recess  54  made of two portions  54   a  and  54   b , corresponding respectively to the chamber  20  and to the inlet duct  22 , in which the portion  54   a  has a roughly square plan shape. 
   Subsequently, a thick layer  56  of negative photoresist, consisting for instance of SU8 type negative photoresist, from the name of its producer, is deposited, in a known process, along the entire extension of the unstuck side  51   a  of the wafer  51 , in order to completely cover the recess  54  as well. Indicatively this layer  56  is approximately 15÷30 μm thick, permitting it to cover the step defined by the recess  54 . 
   It is emphasised that this negative photoresist constituting the layer  56  has the opposite behaviour to that of the positive photoresist constituting the previous layer  53 , and therefore under normal conditions it may melt in contact with certain substances, whereas, if illuminated, it acquires a certain resistance to these substances. 
   Then, as illustrated in  FIG. 3E , this thick layer  56  is illuminated, through a given mask  59 , so as not to receive the light  49  in correspondence with that portion of the same layer  56  indicated with the numeral  58  and having a square shape in plan view, which fills the portion  54   a  of the recess  54 , corresponding roughly to the chamber  20 . 
   Later, as illustrated in  FIG. 3F , the layer  56  of negative photoresist is developed and hollowed, using known techniques, in order to remove the non-illuminated portion  58  and thereby delimit, along the bottom of the recess  54 , adjacent to the chamber  20 , a confined area  61 , of square shape and not protected by the layer  56 , corresponding to the zone of the nozzle  13  that will be formed. 
   At this point, as illustrated in  FIG. 3G , the wafer  51  is subjected to another etching process, the object of which is to hollow the silicon of the wafer  51  only in correspondence with the confined, square area  61 , defined on the bottom of the recess  54 . 
   This is a wet etching, being performed in a damp environment for example using a compound such as KOH, and is also called anisotropic, as it is developed on the crystallographic axes of the silicon constituting the wafer  51 . 
   In particular, this etching causes the formation of a blind hole  62 , of pyramid shape, as illustrated in the plan view of  FIG. 3G . 
   In greater detail, taking into account the side of the uncovered square area  61 , of the thickness, of approximately 50 μm, of the silicon wall to be etched, and of the incline, of roughly 54° of the crystallographic axes of the silicon, the etching is conducted in such a way as to form in the wall a pyramid-shaped blind hole  62 , leaving a thin residual layer of silicon, indicated with the numeral  60 , at the bottom of the blind hole  62 . 
   At this point, after the thick layer  56  of photoresist has been removed, the wafer  51  is unstuck, along the side  51   b , from the carrier  52 , cleaned and then stuck again, this time on the opposite side  51   a  of the same carrier  52  or on another similar carrier. 
   Subsequently, as illustrated in  FIG. 3H , the wafer  51  is covered on the side  51   b , now free, with a layer  57  of positive photoresist, represented with the dot and dash line, which is later illuminated with a suitable mask, impressioned and developed with the same techniques as already seen earlier, in such a way as to protect the entire extension of the layer  55  of silicon dioxide SiO 2  arranged along the side  51   b , with the exception of a limited circular area adjacent to the wall  60  and corresponding to the nozzle  13 . 
   The wafer  51  is then subjected to another “wet” etching process, i.e. in a chemical bath, to remove the circular, unprotected area of the layer  55  of silicon dioxide SiO 2 , and uncover an underlying and corresponding circular zone of the silicon of the wafer  51 . 
   In this way, the layer  55  forms a protective mask for the silicon of the wafer  51  during the subsequent dry etching operation. 
   Naturally if originally the wafer  51  was not provided with the layer of SiO 2 , this protective mask is made with a layer of photoresist, in the same way as already seen earlier. 
   In particular, in this case, the layer of photoresist is selected with a suitable thickness, in relation to the thickness of silicon to be etched in the following step, to permit a correct conduction of this etching step. 
   Then, in a dry type etching process, the circular uncovered area of the silicon of the wafer  51 , i.e. not protected by the layer  55 , is etched, in such a way as to hollow the wall  60  and form in it a pass-through hole  63  corresponding to the nozzle  13 . 
   Finally the wafer  51  which, it will be recalled, has undergone the operations described earlier for each of its elementary areas, is cut into single units corresponding to these areas, and each constituting a nozzle plate  12 . 
   Following this, the single nozzle plates  12  are washed and inspected to check that they do not contain defects, and that they have been formed correctly. In this way, from the wafer  51 , the structure is obtained that constitutes the nozzle plate  12 , which is shown in  FIG. 3I , both in lateral section and in plan view. 
   The process  32  for manufacturing the substrate  11  in large part follows a known sequence and employs technologies that are also known, and will not therefore be described in detail. 
   It is recalled simply that this process  32  starts with the availability of a carrier or wafer of silicon  70 , similar to the one used for manufacturing the nozzle plate  12 , but of significantly greater thickness, for example 0.5 mm, and has the object of making on the carrier  70 , as well as the actuator  15 , certain protective layers having the function of protecting the actuator  15  itself so as to prolong its working life. 
   In the process  32 , a suitable track, or tracks, are also made, for the electric connection of the actuator  15  with the circuits arranged for driving it. 
   In particular, as anticipated above, the process  32  may also include the production, on the silicon wafer  70 , of specific auxiliary circuits, often called “drivers”, suitable for being conditioned by the control unit  19  for generating the pulses to be sent directly to the actuator  15  for activating ejection of the droplets  16 . 
   In the same way as the nozzle plate  12 , and with the purpose of creating economies of scale and improving the efficiency of the productive cycle of the substrate  11 , a single wafer of silicon  70  may be used to simultaneously produce a plurality of substrates  11 , each identical and corresponding to an elementary area or portion of the original silicon wafer  70 . 
   For clarity&#39;s sake, the structure of the substrate  11  which is produced via the known operations mentioned above and which corresponds to an elementary portion of the wafer  70  is represented in  FIG. 4A . 
   In particular, this structure comprises a base layer  71  of silicon corresponding substantially to the thickness of the initial starting wafer  70 ; a zone  72 , made in MOS technology, which comprises a series of circuits or drivers for controlling operation of the ejection head  10 ; a thin layer  73  of silicon dioxide SiO 2  selectively grown on the layer of silicon  71 , and in particular lacking along the zone  72  with the MOS circuits; a thin resistive film of limited extent or resistor  74 , constituting the actuator  15 ; one or more tracks, not shown on the drawings and extending in the normal direction to the plane of  FIG. 4 , for electrically connecting the resistor  74  to the circuits of the zone  72 ; a protective layer  76  made of silicon nitride and silicon carbide and deposited on the resistor  74 ; and a layer  77 , made of tantalum Ta, arranged over the nitride/carbide layer  76  in the area of the resistor  15 . 
   The layer  77  of Ta has essentially the function of protecting the resistor  74  against wear caused by the mechanical stresses to which the resistor  74  is subjected, during operation of the ejection head  10 . 
   Typically these stresses are caused by the phenomenon of cavitation that occurs due to the pumping effect of the liquid  14 , caused by the resistor  74 , for ejecting the droplets  16 . 
   As will be seen more clearly below, this layer  77  of tantalum is arranged for also being used advantageously during the successive operation of joining the substrate  11  with the nozzle plate  12 , to form the ejection head  10 , and to this end the layer  77  of tantalum is deposited on the silicon wafer  70  in order to cover not only the area of the resistor  74 , but to extend laterally along the zone where the junction will be made. 
   Also, to this same end, the layer  77  is formed in such a way as to have, along its edge, a portion  77   a , which is disposed externally with respect to the junction zone. 
   Differently from the known art and with the purpose of arranging the substrate  11  for the next operation, described below, of joining with the nozzle plate  12 , the structure of the substrate  11  also comprises, along given junction zones, an outer surface layer  78  of borosilicate glass, deposited on the layer  77  of tantalum. 
   As illustrated in  FIG. 4B , this layer  78  of borosilicate glass is initially deposited continuously on all the areas of the original wafer  70 , in order to completely cover the layer  77  of tantalum provided on these areas. 
   More particularly, the layer  78  is of a thickness of between 1÷5 μm, and is made of Pyrex 7740, or Schott 8329 borosilicate glass, containing ions of sodium and lithium, with thermal expansion coefficient of 2.3*10 6 K −1  and therefore very close to that of the silicon which is of 2.3*10 6 K −1 . 
   Accordingly the layer  78  of borosilicate glass and the silicon of the wafer  70  mate together optimally without causing the occurrence of mechanical stresses in the junction area. 
   Deposition of the outer layer  78  of borosilicate glass on the substrate  11  is performed in a known way, for instance by way of the process known as “RF sputtering”, in which the borosilicate glass is atomized and sprayed on the substrate  11 . 
   The layer  78  may also be deposited by way of the process known as “electron-beam evaporation”, in which an electronic ray is radiated upon an electrode consisting of borosilicate glass, so that the borosilicate glass evaporates and is deposited on the substrate  11 . 
   With respect to sputtering, the electron-beam evaporation process has the advantage of being faster, i.e. of being able to deposit a greater quantity of material per unit of time, and in addition of being able to ensure a greater stechiometric control of the deposited layer  78  of borosilicate glass. 
   This continuous layer  78  of borosilicate glass is then etched with known techniques in order to uncover the area of the resistor  74 , and to restrict the layer  78  to the area of the substrate  11  intended for coupling with the nozzle plate  12 . 
   In this way, the layer of borosilicate glass  78  forms a kind of frame around the resistor  74 . To this end, the continuous layer  78  is first covered with a layer of positive photoresist, which is then selectively illuminated, and finally removed in correspondence with the illuminated zones, in order to define a protective mask for the underlying layer  78 . 
   Later, again with known techniques and for instance by way of a dry etching step, the layer  78  of borosilicate glass is removed along the areas not protected at the top by the photoresist. 
   Accordingly the structure depicted in  FIG. 4C  and which constitutes the substrate  11  is obtained. 
   Naturally, where a single original wafer  70  is used to produce numerous substrates  11 , this structure is duplicated into the various elementary areas of the silicon wafer  70 . 
   In short, this structure comprises by way of example a residual layer  78   a  of borosilicate glass, which is obtained from selective etching of the original continuous layer  78  and is disposed laterally with respect to the resistor  74 , in order to uncover the portion of the layer  77  of tantalum which protects the resistor  74 , and to also define a junction or soldering surface  79  for the coupling of the substrate  11  with the nozzle plate  12 . 
   In order to ensure the best results during the subsequent step of joining the substrate  11  with the nozzle plate  12 , step which is carried out by means of the anodic bonding technology as will be described in detail below preferably the layer  78  of borosilicate glass is subjected to a planarzation operation along the free surface intended for coupling with the nozzle plate  12 . 
   The object of this operation is to reduce to a minimum roughness of the surface of the layer  78  and it is carried out, for instance, using a planarization process called CMP, or “Chemical-Mechanical Polishing”. 
   In fact, as is known, the anodic bonding process requires an exceptional degree of planarity of the surfaces that have to be coupled by means of this process. 
   Unfortunately the wafer  70 , during the operations for forming the substrate  11 , which precede the depositing of the layer of borosilicate glass  78 , inevitably acquires a certain degree of roughness, which the same layer  78  of borosilicate glass necessarily reproduces and amplifies. 
   Therefore the CMP planarization process has the object of remedying this progressive increase in roughness of the wafer  70 , ensuring a very high degree of planarity of the surface of the layer  78  of borosilicate glass intended for contact coupling with the nozzle plate  12 . 
   In particular, this CMP process may be carried out following application of the continuous layer  78  of borosilicate glass, and before its etching to define the residual layer  78   a  and the corresponding junction surface  79 . 
   As anticipated above, and according to a characteristic of this invention, the plate  12  with the nozzle  13  and the substrate  11 , after being manufactured separately from one another as described earlier, are joined permanently in a joining process based on the anodic soldering technology, frequently also called “anodic bonding”. 
   For information, it is pointed out that anodic bonding constitutes a joining technology which has been developed and perfected in recent years, and which at present is being applied to an ever greater extent in numerous sectors of the art, in particular in the field of microstructures, also abbreviated MEMS standing for “Micro ElectroMechanical Systems”, for the purpose of achieving a stable and efficacious junction between two parts making up a microstructure. 
   For instance this joining technology based on anodic bonding is used to advantage to structurally join together two silicon wafers, in which case it is also known as “silicon-to-silicon anodic bonding”. 
   As is known, the anodic bonding technology is employed to join two surfaces having a high degree of planarity, and is based essentially on the principle of putting the two surfaces to be joined into reciprocal contact at a suitable pressure and temperature, and of then applying a certain potential to them. 
   In this way, in fact, the junction zone becomes the seat of opportune electrostatic charges tending to reciprocally attract and co-penetrate the molecules of the two surfaces, so as to produce a structural cohesion between the two. 
   Often this technology requires that the surfaces intended to be contact coupled be adequately prepared, for instance by means of depositing on at least one of them a suitable layer of material. 
   Further, as already said, this technology also requires the two surfaces to be coupled to be extremely flat and without roughness, i.e. mating perfectly along the zone of contact, so that the phenomenon of co-penetration and structural cohesion between the respective molecules can take place. 
   Further details and information about the anodic bonding technology may be obtained in the following publications, quoted below by way of reference: 
   “Field Assisted Glass-Metal Sealing”, published on page 3946, of volume 40, No. 10, September 1969, of the magazine “Journal of applied physics”; 
   “Fabrication of a silicon-Pyrex-silicon stack by a.c. anodic bonding” published on page 219 et seq, of No. A 55, 1996, of the magazine “Sensors and Actuators”; 
   “Anodic bonding technique under low temperature and low voltage using evaporated glass”, published in Vol. 15, No. 2, March/April 1997, of the magazine “Journal of Vacuum Science Technology”; 
   “Silicon-to-silicon wafer bonding using evaporated glass”, published on page 179 et seq, of No. A 70, 1998, of the magazine “Sensors and Actuators”. 
   For completeness,  FIG. 5  schematically represents the step of joining the nozzle plate  12  with the substrate  11  using the anodic bonding technique, and the anodic bonding equipment or machine, generically indicated with the numeral  85 , used to make the junction. 
   In particular, the anodic bonding equipment  85  comprises two counter-electrodes, generically indicated with the numerals  81  and  82 , adapted for working respectively as the anode and the cathode in the anodic bonding step. In detail, initially the nozzle plate  12  and the substrate  11  are arranged in reciprocal contact on the smooth surface  79  defined by the layer of borosilicate glass  78   a , and in addition aligned with precision with respect to one another. Thus, during a punching operation, the nozzle plate  12  and the substrate  11  are temporarily connected one to the other, for instance with a laser ray, or by means of a suitable adhesive, so that they are held together, at least until the definitive junction is made. Then the assembly formed by the nozzle plate  12  and the substrate  11  is loaded on the anodic bonding machine  85 , setting the substrate  11  on a heating element  83  the object of which is to heat and maintain the substrate  11  at a temperature between 200 and 400° C., during the anodic bonding. 
   Moreover, the assembly formed by the nozzle plate  12  and the substrate  11  is disposed on the bonding machine  85  setting the anode  81  of the latter on top of the nozzle plate  12 , with a certain pressure, and also electrically connecting the cathode  82  of the anodic bonding machine  85  with the portion  77   a , of the tantalum layer  77 , which extends to the outside of the zone of contact between the substrate  11  and the nozzle plate  12 . In particular, the anode  81  is plate-shaped so as to practically cover the nozzle plate  12  over its entire extent. 
   The cathode  82  of the bonding machine  85  is also connected to the main layer of silicon of the substrate  11 , and to the heating element  83 , to keep them at the same potential during the bonding operation. At this point, the anodic bonding machine  85  applies, for instance during a period of 15 minutes, a potential defined by a voltage V, of indicatively between 50 and 500 volt, between the anode  81  and the cathode  82 , thus activating that phenomenon called, as already stated, anodic bonding which gives that structural cohesion between the borosilicate glass of the layer  78   a  and the silicon dioxide SiO 2  on the surface of the nozzle plate  12 . 
   As tantalum is conductive, the layer  77  operates in this anodic bonding step as a cathode plate true and proper which distributes the potential difference generated by the anodic bonding machine  85  through the junction zone, so that the bonding assumes uniform characteristics over its full extent. 
   Accordingly the substrate  11  and the nozzle plate  12  are joined permanently and structurally through a junction, indicated with the numeral  25 , which extends along a corresponding junction zone defined by the layer  78   a  of borosilicate glass deposited on the substrate  11 . 
   In this way, the ejection head  10  is formed, with the relative internal hydraulic circuit  21  intended for conveying the liquid  14  inside the ejection head  10 . 
   The ejection head  10  manufactured in the above way with the junction  25  presents numerous and important innovative aspects with respect to the known way. 
   First and foremost, unlike what happens in the known art, the substrate  11  and the nozzle plate  12  of the ejection head  10  are bound closely together in a joining process that does not involve the use of additional substances, such as binders or other compounds, generally of the organic type, liable to cause a certain structural discontinuity in the junction zone. 
   In fact, the anodic bonding technology, via which the junction  25  is produced, is characterized precisely by its ability to produce a complete continuity and structural co penetration between the materials of the parts that are being joined, in the specific case between the silicon of the nozzle plate  12  and the borosilicate glass deposited on the substrate  11 . 
   In particular, the structure of the ejection head  10  obtained through this method does not present, either in the parts that comprise it, or on the junction  25 , organic type substances, or other similar materials, so that the ejection head  10  can advantageously be employed, without suffering damage, such as for instance corrosion, and/or unsticking, which would compromise its operation, even with liquids that are especially aggressive vis-a-vis organic compounds. 
   As a general concept, it may be said that the ejection head  10  of the invention is characterized by the fact of comprising, between the nozzle plate  12  and the substrate  11  bearing the ejection actuator  15 , a junction  25  which has the property of being substantially inert from the chemical point of view. 
   In other words, this junction  25 , in relation with the liquid  14  contained in the hydraulic circuit  21  of the ejection head  10  and thereby wetting the zone of the same junction  25  in being ejected in droplet form by the ejection head  10 , possesses special properties of resistance to chemical corrosion by the liquid  14 , and also of non combining chemically with the latter, which are at least equal and equivalent, and at any rate not inferior, to those of the materials, in particular silicon, and/or of the parts that comprise the structure of the nozzle plate  12  and of the substrate  11 , and which are also wetted by the liquid  14 . 
   Description of a First Example of Application of the Invention for Producing an Ink Jet Printhead 
     FIG. 6  shows in section view an ink jet printhead, indicated generically with the numeral  110  and suitable for being fed with ink  140 , which is produced in accordance with the method of the invention. Where possible, the parts of the printhead  110  corresponding to those of the ejection head  10  are indicated with reference numerals incremented by  100  with respect to the ejection head  10 . 
   In particular, the printhead  110  comprises a nozzle plate  112  and a substrate  111 , also called “die”, which are made separately from one another and then joined permanently together via a junction  125 , in a similar way to the manufacturing process described in connection with the ejection head  10 . More particularly, the junction  125  is manufactured with the anodic bonding technology, after appropriately preparing the substrate  111  by depositing on it a layer  178  of borosilicate glass. 
   The substrate  111  and the nozzle plate  112  define a plurality of ejection units, indicated with numerals  110   a ,  110   b ,  110   c , etc., which are arranged along an ejection side  150  of the printhead  110  and have, each one, a structure corresponding to that of the ejection head  10 . 
   Each ejection unit  110   a ,  110   b ,  110   c , etc., comprises a respective nozzle, indicated in order with numerals  113   a ,  113   b ,  113   c , etc., a respective actuator  115   a ,  115   b ,  115   c , etc. and a respective ejection chamber  120   a ,  120   b ,  120   c , etc. 
   The printhead  110  is also provided internally with a hydraulic circuit  121  the function of which is to feed the ink  140  from a single tank  117  to the different ejection units  110   a ,  110   b ,  110   c , etc., and which comprises, in addition to the chambers  120   a ,  120   b ,  120   c , etc., a plurality of inlet ducts  122 , each communicating with a respective ejection chamber  120   a ,  120   b ,  120   c , etc., and a central slot  123  made through the substrate  111 . 
   In particular, the central slot  123  communicates at one end with the tank  117 , and at the opposite end with the plurality of inlet ducts  122 , which in turn are arranged both on one side and the other of the slot  123  in order to put the slot  123  in communication with the ejection chambers  120   a ,  120   b ,  120   c , etc. of the different ejection units  110   a ,  110   b ,  110   c , etc. 
   In this way, the ink  140  can flow from the tank  117  to each single ejection unit  110   a ,  110   b ,  110   c , etc. through the hydraulic circuit  121 . As already intimated, the method for manufacturing the printhead  110  is substantially similar to that for manufacturing the ejector  10 . 
   Again in this case, with a view to improving efficiency of the industrial mass production of these printheads  110 , a single silicon wafer may be used in order to produce multiple substrates  111  and also to produce multiple nozzle plates  112 , with obvious advantages in terms of industrial production at lower costs. 
   In detail, as shown schematically in  FIG. 7 , multiple nozzle plates  112 , corresponding to elementary portions  112   a ,  112   b ,  112   c , etc., of an original silicon wafer  151 , are produced together on the original silicon wafer, in the steps described with reference to the nozzle plate  12 , so as to form for each nozzle plate  112  the respective ejection chambers  120   a ,  120   b ,  120   c , etc. and the respective nozzles  113   a ,  113   b ,  113   c , etc. 
   Finally, in accordance with what is indicated by the arrow  160 , this wafer  151  is cut or singularized into units each of which constituting a nozzle plate  112 . 
   Similarly and as illustrated in  FIG. 8 , multiple substrates  111 , each corresponding to an elementary portion  111   a ,  111   b ,  111   c , etc., of a single original silicon wafer  170 , are simultaneously formed on the latter in the steps already described with reference to the substrate  11 . 
   In particular, these elementary portions or areas  111   a ,  111   b ,  111   c , etc. of the silicon wafer  170  are subjected to a series of operations in order to produce, in correspondence with each of these, a structure of the type depicted in FIG.  4 —section (c), with a layer of borosilicate glass  178  defining a junction zone for the next anodic bonding operation. 
   Conveniently, for the purpose of preparing the silicon wafer  170  for the subsequent joining operation with anodic bonding, the conductive layers of tantalum in the areas  111   a ,  111   b ,  111   c , etc. are interconnected to one another and to a conductive ring  177   a  made along the edge of the wafer  170 , so as to form, on the surface of the wafer  170 , a mesh  177 , also called equipotential mesh or network on account of its ability to keep the elementary areas  111   a ,  111   b ,  111   c , etc. at a same potential during joining with the nozzle plates  112 . 
   An equipotential network of the type of the mesh  177  is described in the Italian patent application No. T099A000987, filed on 15 Nov. 1999 on behalf of the Applicant, the said application being cited here for reference for all details, not found in this description, of the configuration and characteristics of the mesh  77 . 
   In this way, the silicon wafer  170  acquires a structure which encompasses a plurality of elementary areas  111   a ,  111   b ,  111   c , etc., each corresponding to a substrate  111 , which are already prepared for joining with the respective nozzle plates  112 . 
   Then the single nozzle plates  112  which, as already said, have been made separately, are mounted, aligned, and temporarily affixed, one by one, on the different elementary areas  111   a ,  111   b ,  111   c , etc., defined on the silicon wafer  170  and therefore still permanently interconnected to one another. At this point, it is possible to proceed with the anodic bonding step true and proper, in which each nozzle plate  112  is joined with the corresponding elementary area  111   a ,  111   b ,  111   c , etc. of the silicon wafer  170 , by applying a given potential between the same using an appropriate anodic bonding machine. 
   In order to permit a correct locating of the anode on the different nozzle plates  112  and therefore optimal bonding thereof with the respective areas  111   a ,  111   b ,  111   c , etc. of the silicon wafer  170 , this anodic bonding machine has a specially modified anode, divided in particular into a plurality of elements, each corresponding to a nozzle plate  112 , which are mounted on a sprung structure that permits limited movements between one anode element and another. 
   In fact, in this way, each of these anode elements is capable of adapting, independently from the others, to the corresponding nozzle plate  112 , so as to set perfectly on the latter with the right pressure, when the anode of the anodic bonding machine is brought globally into contact against the various nozzle plates  112 . 
   In turn, the cathode of the bonding machine is brought into contact, possibly at numerous points, with the outer conducting ring  177   a , to which the various layers of tantalum, forming the mesh  177  and arranged on the elementary areas of the silicon wafer  170  are connected. 
   In this way, all these layers of tantalum are brought to and maintained at the same potential, in the anodic bonding step. 
   In particular, this anodic bonding step consists, as stated earlier, in putting into reciprocal contact at a given pressure and temperature each nozzle plate  112  with the respective area  11   a ,  111   b ,  111   c , etc. and in applying a suitable potential between them, through the anode which presses with its elements on each nozzle plate  112 , and the cathode which is connected via the mesh  177  to the tantalum layers arranged on each area  111   a ,  111   b ,  111   c , etc. 
   Accordingly, that close structural cohesion, typical of the anodic bonding technology, is achieved between each nozzle plate  112  and the corresponding elementary area  111   a ,  111   b ,  111   c , etc. of the silicon wafer  170 . 
   Finally, after the junction has been made, the silicon wafer  170  is cut or singularized into single blocks, each of which formed by a nozzle plate  112  and a substrate  111  permanently and structurally interconnected, and constitutes an ejection assembly suitable for being subsequently assembled with a tank for forming a printhead  110  such as the one shown in  FIG. 6 . 
   The method of the invention can be used for producing a printhead capable of working with inks decidedly more aggressive than those neutral ones, generally water or alcohol based, used on traditional ink jet heads. In fact, the so-called aggressive inks, while fully innocuous in relation to the head of the invention, are capable, if used with traditional printheads, of irreparably damaging the structure in a very short time, particularly in the junction zone or zones between the parts that comprise the traditional printheads, these junctions, as is known, being made with substances easily attacked by and/or combinable with these aggressive inks. Furthermore, this method which adopts the anodic bonding technology has the additional advantage over the traditional methods of involving the occurrence of lesser heat expansions and in general lesser deformation during the joining step between the nozzle plate and the substrate, both of silicon, in forming the ink jet printhead. 
   On the contrary, with the traditional method, the nozzle plate and the substrate, as also the hydraulic circuit are normally made of different materials, such as for example: metal, silicon, and plastic, so that these parts, when connected together to form the printhead, may give rise to reciprocal deformations likely to have a negative influence on manufacturing precision of the printhead. 
   Therefore, in short, the method of the invention enables compliance to be guaranteed with extremely low manufacturing and assembly tolerances, and accordingly decidedly much higher production precision levels to be reached than with the traditional method. 
   Description of a Second Example of Application of the Invention Concerning an Injector for Internal Combustion Engines 
     FIG. 8  illustrates schematically an application in which the ejection head of the invention constitutes a fuel injector for an internal combustion engine, indicated generically with the numeral  200 , and comprising at least one cylinder  201  with a piston  202  and a combustion chamber  203 ; an inlet duct  204  bringing fresh air to the combustion chamber  203 , and an exhaust duct  206  for the fumes from the combustion chamber  203 . 
   For simplicity&#39;s sake, a single cylinder  201  is depicted in  FIG. 9 , even if it is clear that the engine  200  may comprise multiple cylinders, according to types widely known in the art. 
   A valve  207  is disposed in correspondence with the outlet zone of each of the ducts  204  and  206  in the combustion chamber  203 , for the purpose of excluding or otherwise the flow of air to and the flow of fumes from the latter-named. The inlet duct  204  is suitable for receiving the air from a filter zone  208 , where the fresh air is suitably filtered, and accommodates on its inside a butterfly valve  209  with the function of controlling the flow of filtered air in the direction of the arrow  205  towards the combustion chamber  203 . 
   The injector, indicated with the numeral  210 , has the function of ejecting droplets of fuel, such as petrol or diesel, in the inlet duct  204 , in quantities controlled exactly by a control unit  211 , associated with the ejector  210 , so as to form with the filtered air coming from the filter zone  208  an air-fuel mix which feeds the combustion chamber  203 . 
   In particular, the optimal quantities of fuel to be injected in droplet form are determined by the control unit  211  on the basis of data sent to the latter, on lines  212 , by suitable sensors in the engine. 
   The injector may be mounted in the position indicated with the letter A after the butterfly valve  209 , in the case of Multipoint injection (or MPI, “Multi Point Injection”), i.e. with one injector for each cylinder; or also alternatively in the position indicated with B, before the butterfly valve  209 , in the case of Single Point injection (SPI), i.e. with a single injector generating the air-fuel mix which is then shared between the cylinders. In the latter case, the air inlet duct divides into numerous ducts corresponding to the cylinders of the engine, immediately after the butterfly valve  209 . 
   In this way, the injector  210  of the invention permits to dose with great precision the quantity of fuel delivered to the cylinder, or cylinders, of the engine, so as to obtain better performances from the engine, such as for example a higher thermal efficiency, than the traditional engines. 
   Furthermore the injector has a particularly robust structure, suitable for resisting efficaciously the system of thermal and mechanical stresses and the corrosive actions of a chemical nature depending on the fuels used, typically present in internal combustion engines. 
   Other Possible Applications of the Injection Head According to the Invention 
   The forms of application of the ejection head manufactured in accordance with this method are not limited to those described above. 
   In fact, this ejection head, by virtue of its chemically inert structure in the junction zone between the actuation support and the nozzle plate, is suitable for being used in multiple sectors which require precise injection of special liquids, sometimes specifically developed for these sectors, and decidedly more aggressive from the chemical viewpoint than the inks, both water-based and even alcohol-based, which are usually employed for printing on paper media with the conventional ink jet printheads. 
   One particular example that springs to mind is the industrial marking field in general in which this ejection head could be used to advantage for ejecting liquids, such as special paints or inks, capable of adhering stably also to non-paper media, such as plastic or metallic laminates, in order to produce particular markings on these media. 
   For example, the ejection head could be used for making custom images on plastic media, such as those generically designated with the word “badge”, or on numerous consumer products, such as skis, helmets, tiles, gift objects, and still others. In fact, the liquids currently used for these marking applications, and probably also those that will be developed in the future, are incompatible with use on the traditional printheads, since they are prepared with substances or solvents which would irreparably damage the structure of the traditional heads, whereas on the contrary these could be employed without any drawback on this ejection head. 
   Purely by way of example, quoted below are some types of solvents which already today are of wide scale application in products such as fuels, paints and printing inks, and which could be used for preparing liquids to be used, without drawbacks, in combination with the ejection head of the invention, thanks to the latter&#39;s chemically inert structure: 
   aliphatic and aromatic hydrocarbons such as: liquid paraffins, toluene, xylene; 
   aliphatic and aromatic alcohols such as: methyl alcohol, isopropyl alcohol, n-propyl alcohol, sec-butyl alcohol, isobutyl alcohol, n-butyl alcohol, benzyl alcohol, cydohexanol; 
   esters such as: methyl acetate, ethyl acetate, isopropyl acetate, n-propyl acetate, sec-butyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, 2-ethoxy ethyl acetate; 
   glycol esters such as: 2-methoxyethanol, 2-ethoxyethanol, 2-butoxyethanol; 
   ketones such as: acetone, methy ethyl ketone, methyl isobutyl ketone, methyl isoamyl ketone, cyclohexanone; 
   lactones such as: 6-caprolactone monomer. 
   Another possible application of this ejection head is that of microdosing, in particular though not exclusively in the biomedical sector. In fact this ejection head, thanks to its chemically inert structure without organic substances, may be used without drawbacks for ejecting and dosing a vast range of liquids used in the medical field, for instance organic liquids in general and more particularly liquids containing urea, or liquids such as insulin, or still other medical liquids which need to be dosed with special precision in certain medical functions. Even use of this ejection head for ejecting in a controlled manner edible liquids, i.e. foodstuffs, may be numbered among the possible forms of application of the invention. In general, it may be said that this ejection head has a chemically inert structure which, as well as the advantage of not being subject to corrosion by a vast range of liquids used in the medical field, has the further advantage of not combining with these liquids, and therefore of not altering and offending even minimally the characteristics while they are maintained in this ejection head. 
   It remains understood that changes and/or improvements may be made to the method for manufacturing a head for ejecting a liquid in droplet form, as also to the ejection head manufactured in accordance with the method, described up to this point, without exiting from the scope of the invention.