ITTO20100396A1 - PRESSURE SENSOR - Google Patents

Description

"PRESSURE SENSOR",

TEXT OF THE DESCRIPTION

Field of invention

The present invention refers to a pressure sensor having the characteristics indicated at least in the preamble of claim 1.

More particularly, the invention relates to a pressure sensor having:

- a pressure sensitive element, comprising a die of semiconductor material, particularly silicon, defining a blind cavity having an open end at a first face of the die, the opposite end of the cavity being closed by a membrane portion defined by the die in correspondence of its face opposite to the first face;

- a substrate, crossed by a through opening and having a flat surface at which one end of the through opening opens, the sensitive element being mounted on the flat surface of the substrate with the open end the relative cavity facing the through opening of the substrate;

- an electrical circuit to which the die is electrically connected.

The invention also refers to a method for manufacturing such a sensor, according to at least the preamble of claim 10.

Prior art

Pressure sensors having the indicated structure are generally known and used in devices for detecting the pressure of fluids (liquids and gasses) in various sectors, such as the automotive sector, the domestic and household appliances sector, the environmental conditioning and hydro- thermo-sanitary in general.

In a first type of solution, the substrate is constituted by a main body of the sensor, having structural functions, for example made of metallic material, to which a closing cover is generally coupled, which protects the semiconductor die and the circuitry electrical / electrical sensor, and integrates a connector.

In another type of solution, the substrate is instead constituted by a printed circuit board or PCB (Printed Circuit Board) support in fiberglass, which therefore supports both the die and the aforementioned circuitry. In these solutions, the substrate constituted by the PCB is mounted inside a plastic or metal enclosure. The silicon die is welded to a relative glass base, hollow axially, through "anodic bonding", which is a process that involves the use of electricity for heating and fusion between silicon and glass, obtaining a chemical bond . The aforesaid base is bonded tightly to the relative substrate, by means of a silicone or epoxy resin, in correspondence with a relative through opening.

Regardless of the type of specific embodiment, and very schematically, the through opening of the substrate is placed in fluid communication with a circuit containing the medium whose pressure is to be measured, be it a liquid or an aeriform. In this way, the cavity of the die also results in fluid communication with the circuit in which the medium being measured is located. The pressure of the aforesaid medium generates a more or less marked flexion of the membrane portion of the die. The extent of the deformation, representative of the pressure, is detected by suitably making the die, for example by obtaining a miniaturized resistive bridge in correspondence with its membrane portion.

The second type of solution indicated is not very suitable for the use of semiconductor pressure sensors in combination with particularly aggressive fluids from a chemical point of view, due to the risk of deterioration of the PCB. Similar considerations apply to the case in which the sensor is used to detect the pressure of fluids having high temperatures, for example in the order of 150 ° C, or in the case in which the substrate is potentially subject to high mechanical stresses, such as in the case of high pressures.

These problems are reduced in solutions in which the glass base of the die is glued on a metal substrate, typically constituted by the main body of the sensor, having structural functions. However, even these solutions are on the whole not very suitable for use in combination with particularly aggressive fluids from a chemical point of view, since also in this case the glass base of the die is glued tightly through a silicone or epoxy resin, and these resins are poorly resistant to chemical aggression and / or to high temperatures. However, the sensor requires an electrical connection circuit, the support of which must be positioned near the die. For this reason, the face of the metal body, in correspondence with which the die and the circuit are mounted, is typically not flat, but machined in such a way as to define two distinct parallel support surfaces, at different heights. In particular, the aforesaid face of the metal body is machined so as to define a central portion in relief - crossed by a terminal portion of the pressure intake passage - on the top surface of which the glass base of the die is glued. The circuit or PCB support is formed with an opening of such dimensions as to be able to receive the aforementioned relief and rest on the lowest surface of the substrate. In this way, the PCB never comes into contact with the fluid being measured and can extend around the relief, for an easier connection to the die.

In the known configurations mentioned, with a printed circuit or PCB added on the substrate, if the substrate itself were completely flat, the chip or die glued on it would be inserted inside the aforementioned opening of the circuit support: it follows that, in presence of a relatively thick PCB, the top surface of the die - whose body is typically very thin - may be at a lower level than the top surface of the PCB, resulting in difficulty or inability to make electrical connections or a œwire bondingâ € between die and PCB, a technology that typically requires considerable precision, in particular to be able to carry out the process in an automated way. On the other hand, in case of using a thin PCB, such as a PCB in flexible material, certain positioning difficulties may arise between the PCB itself and the die, and in particular between the die and the PCB pads on which the connections must be made using the â € œwire bondingâ € technique.

This drawback is accentuated by the fact that the PCB must be glued to the substrate, with a consequent increase in thickness, due precisely to the layer of glue. In the absence of such bonding between the PCB and the substrate, the electrical connection would not be feasible, as it is made up of capillary wires - the aforementioned wire bonding - which would break at the slightest movement. This risk is also present in the presence of bonding between PCB and substrate, due to potential displacements between the two parts, for example in the bonding phase or due to subsequent expansion or deformation of the relative materials at temperature.

This type of solution, with the upper face defining two different support surfaces, complicates the realization of the device, and in particular of the metal body, which must be subjected to mechanical processing to define the two parallel surfaces, the highest one for the Support of the die base, and the lower one for the support of the circuit support. As mentioned, even with these solutions there remains the problem of deterioration of the epoxy or silicone resins used for bonding the glass base of the die to the substrate, resins that tend to degrade when in contact with aggressive fluids and / or at high temperatures. .

Summary and purpose of the invention

The present invention essentially proposes to provide a pressure sensor, which can also be used with aggressive fluids and / or with high temperatures, with an improved structure and reliability with respect to the prior art cited.

This and still other objects, which will result in the following, are achieved according to the invention by a pressure sensor and by a process for manufacturing a pressure sensor having at least the characteristics indicated in the attached claims, which form an integral part of the Technical teaching provided in relation to the invention.

Brief description of the drawings

Further objects, characteristics and advantages of the present invention will become clear from the following detailed description and from the attached drawings, provided purely by way of non-limiting example, in which:

- figure 1 is a perspective view, partial and schematic, of a pressure sensor according to a first embodiment of the invention;

- figure 2 is a schematic section of the sensor of figure 1;

- figure 3 is an enlarged detail of figure 2;

- figure 4 is a perspective view, partial and schematic, of a pressure sensor according to a second embodiment of the invention;

- figure 5 is a schematic section of the sensor of figure 4, on a larger scale; - figure 6 is a perspective view, partial and schematic, of a pressure sensor according to a third embodiment of the invention;

- figure 7 is a schematic section of the sensor of figure 6, on a larger scale; - figure 8 represents, through partial and schematic sections, the main phases of a gluing and sealing process used in the production of the pressure sensors according to figures 1-7;

- Figures 9 and 10 are views similar to those of Figures 1 and 2, but relating to a pressure sensor according to a fourth embodiment of the invention;

- Figures 11 and 12 are views similar to those of Figures 4 and 5, but relating to a pressure sensor according to a fifth embodiment of the invention;

- figures 13 and 14 are views similar to those of figures 6 and 7, but relating to a pressure sensor according to a sixth embodiment of the invention;

- figure 15 represents, through partial and schematic sections, an example of a succession of phases of a gluing and sealing process used in the production of the pressure sensors according to figures 9-14.

Description of preferred embodiments of the invention

The reference to "an embodiment" within this description indicates that a particular configuration, structure, or feature described in relation to the embodiment is included in at least one embodiment. Thus, the terms â € œin one embodimentâ € and the like, present in different parts within this description, are not necessarily all referring to the same embodiment. Furthermore, the particular configurations, structures or features can be combined in any suitable way in one or more embodiments. The references used below are for convenience only and do not define the scope of protection or the scope of the forms of implementation.

In the remainder of this description, terms such as â € œsuperiorâ € and â € œinferiorâ € are intended as a simple non-limiting spatial reference, to facilitate the description of the details illustrated in the figures.

With particular reference to Figures 1, 2 and 3, 1 indicates as a whole an example of a pressure sensor according to the present invention. Note that in the various figures the sensor 1 is shown limitedly to the parts of immediate interest for the purposes of understanding the invention, and therefore without a relative body or cover which - depending on the case - partially covers or completely encloses the parts here represented in the figures.

In the exemplified embodiment, the sensor 1 has a main body 2, having structural functions, with a threaded cylindrical lower part 2a, above which a seat for a sealing ring 3 is defined. The part 2a can be used , for example, to directly fix the main body 2 in a threaded seat which is in fluid communication with the circuit in which the medium whose pressure is to be measured is located.

The body 2 is crossed by an axial passage, indicated with 4 in figures 2 and 3, which creates a pressure point, this passage being preferably, but not necessarily, substantially coaxial to the axis of the body 2. The passage 4 crosses the body 2 completely, opening at the extremity or upper face of the body itself, indicated by 5, which is flat. An electric circuit is associated with this upper flat face 5.

In one embodiment, the body 2 - or at least its part defining the flat face 5 - therefore also fulfills the function of circuit support and is preferably formed with an electrically insulating material, preferably a ceramic material, for example obtained by means of sintering of alumina powders, or a polymer; the part of the body 2 which fulfills the function of supporting the circuit could also be formed in suitable glass.

As will be seen below, in a different embodiment, the body 2 or its part defining the face 5 can also be formed with an electrically conductive material, such as a metallic material, for example steel or aluminum, or a polymer conductive: in these cases, a layer of insulating material is preferably deposited on face 5, such as an electrically insulating metal oxide layer, over which electrical connections and / or tracks are then further deposited, as described below.

As mentioned, the surface or face 5 is substantially flat, and in it opens the upper end of the passage 4. A pressure sensitive element is mounted on face 5, including a die, that is a small block or plate at the base of semiconductor material, typically silicon, which is die-bonded to face 5. In the aforementioned die, indicated as a whole with 6 in the figures, the integrated circuit that supervises the general operation of the pressure sensor can be obtained directly in miniaturized form 1. The die 6 can be configured as a single silicon plate or block having a quadrangular section, but this realization must not be construed as limiting, since the die 6 may have different shapes from the one illustrated and be formed by a plurality of parts or layers in mutually joined silicon. Die 6 can be obtained with a technique known per se in the semiconductor chip manufacturing sector.

Unlike a common integrated circuit, die 6 is preferably devoid of its own casing or package, and therefore also devoid of related connection terminals (pins or leads), typically made of relatively rigid metal elements. For this purpose, contacts or pads, not shown, are directly affixed to the upper semiconductor face in the form of thin films of electrically conductive material deposited on the die, preferably but not necessarily a noble material, such as for example gold or an aluminum-silicon alloy. 1%; Preferred materials or alloys usable for the purpose may include gold, platinum, silicon, palladium, beryllium, silver, aluminum and copper.

As can be seen in figure 3, in the body of semiconductor material a blind cavity 6a is defined, which has an open end in correspondence with the lower face of the die, while the opposite end of the cavity 6a is closed by a portion membrane 6b, defined by die 6 in correspondence of its upper face.

The die 6 is mounted on the flat upper surface 5 of the body or support 2 at the opening of the passage 4, with the open end of the cavity 6a at this opening.

As said, on face 5 there is the aforementioned electric circuit, shown only in part for the sake of greater clarity, which comprises a plurality of connection tracks 7, formed with electrically conductive material, deposited on the upper surface of the body 5, and organized around the mounting region of the die 6. In figure 1 only three tracks 7 are represented by way of example, assuming that the circuit can include more than three tracks, even having a different layout from the one illustrated in figure 1.

The die 6 is connected to respective tracks 7 of the aforementioned circuit by means of thin connection wires, some indicated generically with 8, of electrically conductive material, particularly flexible micro-wires having a thickness or diameter between about 5 and 100 microns, preferably from about 25 to about 35 microns. The aforesaid wires 8 are preferably formed with a material or an alloy comprising a material selected from gold, platinum, silicon, palladium, beryllium, silver, aluminum and copper. The microfiles 8 are connected between the contacts of the die 6 and the tracks 7 and / or the components of interest of the circuit formed on the face 5, using processes of the type known as â € œwire bondingâ €, and particularly of the "wedge-bonding" type or â € œball-bondingâ €, for example through thermo-compression, or ultrasonic welding, or thermo-sonic welding. The micro-wires 8 can have different shapes or sections, such as a circular or quadrangular or substantially flattened shape or section, for example flexible micro-ribbons (mico-ribbons) of conductive material, without an insulating coating.

Some of the conductive tracks 7 of the circuit are in electrical connection with respective terminals of the sensor 1, indicated with 9, for the electrical power supply and / or the transport of signals generated by die 6, possibly treated, conditioned and / or processed for by means of electrical and / or electronic components of the circuit. The circuit provided on the surface of the face 5 can in fact include a plurality of circuit components, of a conception generally known in the field, including active components and / or passive components and / or an integrated circuit, in addition to die 6.

According to a characteristic of the present invention, the lower face of the die 6, such as the face of the silicon body of the die 6 at which the relative cavity 6a opens, is glued and sealed directly to the substrate constituted by the flat surface 5 of the body 2 by means of at least one layer of glassy material.

In the embodiment of figures 1-3, at least one layer of glassy material is provided, indicated with 10. As can be seen, therefore, the die 6 is mounted on the same surface 5 in which the electric circuit is provided.

Figures 4 and 5 refer to a second embodiment of the invention. In these figures the same reference numbers of the previous figures are used, to indicate elements technically equivalent to those already described.

The sensor 1 of Figures 4 and 5 differs from the sensor 1 of Figures 1-3 substantially in the shape of the body 2 which defines the flat face 5 on which the electrical circuit is provided. Also in the embodiment of Figures 4-5 the body 2 is directly formed of electrically insulating material, particularly a ceramic material or material based on one or more oxides. In a preferred embodiment, this body 2 is of alumina, ie aluminum oxide (AL2O3). Similarly to the body 2 of figures 1-3, also the body 2 of figures 4-5 can be obtained by sintering.

The body 2 is crossed by the relative axial passage 4 and, in this embodiment, the die 6 is mounted on the upper face of the body 2, by means of a relative layer of glassy material 10.

It will be appreciated that, also in this embodiment, being the body 2 of electrically insulating material, on its flat surface or upper face 5 it is possible to easily form at least part of the electrical circuit of the sensor 1. In particular, on the upper face 5 of the body 2 can be directly deposited the electrically conductive material necessary for the formation of the tracks 7 and any other circuit components, whether these can be obtained by depositing material, whether they can be associated with the circuit, such as for example resistors.

Also in this embodiment, therefore, the material constituting the body 2 is used directly as a substrate for the circuit of the sensor 1, without the need for a suitable printed circuit board. Said body 2 is preferably substantially cylindrical in shape, provided with suitable references or perimeter seats, in particular for suitable positioning in another body, such as grooves extending between said upper face and a lower face.

Figures 6 and 7 refer to a third embodiment of the invention. Also in these figures the same reference numbers of the previous figures are used, to indicate elements that are technically equivalent to those already described. The sensor 1 of figures 6 and 7 differs from the sensor 1 of the previous figures substantially in the shape of the body 2 and / or the presence, on the die 6, of a hollow cover 11, defining a relative blind cavity 11a. The lid 11, such as a glass or silicon lid, is hermetically sealed, by means of a suitable sealant or fixing by anodic bonding, on the upper face of the die 6, so that the membrane portion 6b faces the ™ opening of cavity 11a. In this configuration, sensor 1 is an absolute pressure sensor, with the hermetic chamber defined between the lid 11 and the top face of die 6 providing a pressure reference.

The surface or flat face 5 on which the die 6 is mounted, with or without the cover 11, can belong to a main body 2 of insulating material.

As previously mentioned, however, the substrate on which the die 6 is mounted can also be of electrically conductive material. For this purpose, for example, the body 2 shown schematically and partially in Figures 6 and 7 - on the upper face 5 of which the die 6 is directly mounted - can be made of metal, such as steel or aluminum. In an embodiment of this type, a layer of electrically insulating material, for example an electrically insulating metal oxide, is deposited on the upper face 5, preferably after the assembly of the die 6, with the exception of the assembly area of the die 6, and on this layer of insulating material the material intended to form the connection tracks 7, with any other components of the circuit, is then deposited.

Regardless of the type of substrate or material which constitutes it, also in the embodiment of figures 6 and 7, the die 6 is glued and sealed directly on the flat upper surface 5 of the body 2 by means of at least one layer of glass material 10.

Figure 8 illustrates the main steps of the method of bonding and sealing the die 6 to the flat face 5 of the relative substrate 2.

Basically, on the flat surface 5 of the substrate 2 a layer 10a of vitreous paste in fluid form 10a is deposited in the region surrounding the opening of the passage 4, for example by screen-printing or serigraphy.

In the preferred embodiment of the invention, the paste constituting the layer 10a comprises glass particles and additives dispersed in a suitable organic matrix. The particulate can be made up of first glass particles up to 20 microns in size, well above the typical roughness of the surfaces to be bonded. The preferred additives provided, chosen to lower the vitrification point, can comprise for example one or more of thallium oxide, vanadium oxide, phosphorus oxide. The organic matrix can comprise, for example, one or more aromatic and / or aliphatic solvents.

After the deposition of the layer of paste 10a, the die 6 is directly positioned on it, as visible in part A of figure 8, for example by means of the die attach technique and / or using suitable positioning templates.

Subsequently, the semi-finished product is heated in an oven, to subject the layer 10a to a drying step. The process temperature is preferably between about 60 ° C and about 190 ° C, preferably between about 80 ° C and about 170 ° C. During this phase, the organic matrix of the vitreous paste is dissolved thanks to a burn out process, in order to obtain a dried layer 10b including only the glass and additives, as shown in schematic form in part B of figure 8.

The semi-finished product is then subjected, again in the oven, to a treatment phase at a temperature higher than the drying temperature, to determine the remelting and / or the final vitrification of the residual material constituting the dried layer 10b, or to determine a tight fastening. of die 6 on the substrate 2. The process temperature is preferably between about 180 ° C and about 320 ° C, preferably between about 200 ° C and about 300 ° C, to obtain the glass layer 10. With this second phase the bonding and sealing is determined between the die 6 and the relative substrate 2, by means of the layer 10.

In essence, therefore, the glass-based paste 10a is subjected to a thermal profile, preferably in two phases, in the first of which the organic matrix is burnt and completely removed, leaving only the glass structure and any additives, and in the second of which, at a higher temperature, the glass melts. However, the process could also comprise a single thermal phase, for example without the intermediate drying phase, or by carrying out the removal of the organic matrix directly during the recasting and vitrification phase.

With this process of vitrification the paste is transformed into a block of amorphous material, which allows to establish a bond with the surfaces to be glued, creating a single package.

The bonds involved in the adhesion between the surface of the substrate 2 and the die through the glass paste are preferably of a mechanical and / or chemical and / or physical nature.

From a mechanical point of view, the adhesion between the parts is favored by the penetration of the molten glass paste into the micro-roughness of the surface of the substrate; this effect is maximized in the case of a substrate 2 of ceramic material, particularly sintered, due to the surface roughness of the substrate itself.

From a chemical point of view, the vitrification phase gives rise to oxidative and diffusion processes, which create a transition zone between the vitreous paste and the surfaces to be bonded.

In the ideal case, during the cooking / vitrification process, one or more layers of oxides are formed both on the paste side and on the substrate side, which share part of the atoms. The types of bonds are based on the affinity of the materials: based on glass (therefore Si and O) towards the die (Si), and based on inorganic oxides towards the substrate. For example, in the case of alumina substrate (Al2O3), the oxygen atoms are shared with the SiO2 ones of the glass of the vitreous paste. The bond is very intimate and such as to no longer allow the atom to be attributed to one or the other molecule. This allows for a very high bond strength and surface sealing.

Similarly, in the case of a metal substrate, the oxidative phenomena that occur in the cooking phase determine a sharing of oxygen atoms between those of the pasta and the metal oxides. Finally, from a physical point of view, weak intermolecular interaction forces (Van der Vaal forces) act between the vitrified paste and the substrate, whose contribution to the bond is in any case lower than the previous ones.

Figures 9 to 10 illustrate a sensor solution structurally similar to that of Figures 1-3, in which at least two glass layers 10 and 10â € ™ are provided between the die 6 and the face 5 of the support or substrate 2. The two layers 10 and 10â € ™ are formed in succession starting from respective layers of vitreous paste of the type previously indicated. These layers of vitreous paste can also be more than two and / or have different thicknesses, for example, with a layer 10â € ™ having a greater thickness than the underlying layer 10. In order to obtain at least one glassy layer of greater thickness, the vitreous paste can advantageously be added or loaded with second particles, for example in the form of spheres, fibers or flakes, preferably glassy, which help to maintain a certain consistency and thickness. of the paste during at least a first deposition phase.

The second particles can have dimensions greater than the first particles constituting the particulate of the glass paste, or have dimensions greater than 20 microns, preferably at least 50 microns. Preferably the paste, loaded with the first particles and the second particles, is such as to allow the deposition of a layer with a thickness greater than 0.1 mm, such as a thickness of dough between 0.1 mm and 1 mm.

According to a preferential version, the second particles are at least capable of defining a structure that determines the thickness of the paste being deposited and / or the distance between the chip and the substrate, with the first particles being at least capable of re-melting to determine the bonding or fixing the die to the substrate.

Similarly, figures 11-12 and 13-14 illustrate solutions similar to those of figures 4-5 and 6-7, respectively, but characterized by at least two layers 10 and 10â € ™ of gluing and sealing between the die 6 and the relative body 2 which forms the substrate.

Figure 15 illustrates the subsequent typical steps of a bonding and sealing process using two glass layers 10 and 10â € ™. The sequence of phases indicated is provided by way of example, some of these phases being able to be partly different and / or repeated in the case of deposition of several glass layers.

In general terms, the process provides for the formation of the die 6 and the relative substrate 2, according to methods known per se.

Preferably, at least the part of the upper surface 5 of the substrate 2 in which the passage 4 opens is first cleaned by means of a solvent, for example acetone or isopropyl alcohol (part A of Figure 15).

Subsequently, on the upper surface 5 of the substrate 2, a layer of a glass paste 10a of the type indicated above is deposited, so as to circumscribe the region of the upper surface 5 of the substrate 2 in which the passage 4 opens (part B of figure 15).

The drying of the layer 10a in an oven is then caused, as previously explained, at a first temperature, to obtain the dried layer 10b (part C of figure 15). Subsequently, the dried layer 10b is vitrified, at the second temperature, to obtain the glassy layer 10 (part D of Figure 15). Subsequently, on the vitrified layer 10 a new layer 10aâ € ™ of vitreous paste is deposited (part E of figure 15), then dried to obtain a second dried layer 10bâ € ™ (part F of figure 15).

In the event that more than two glassy layers are to be provided between the die and the relative substrate, the second dried layer 10bâ € ™ is vitrified and a new layer of vitreous paste is deposited on it, then dried and vitrified, with similar operations to those already described above.

In any case, before the vitrification of the last expected dried layer (layer 10bâ € ™, in the case of figure 15), die 6 is placed on it (part G of figure 15) and the aforementioned last dried layer is then vitrified (part H of figure 15).

In an alternative procedure, after the deposition of the second layer 10aâ € ™, die 6 can be placed on it, after which it is dried and then vitrified.

The assembly of the die by means of the layer 10, or the plurality of layers 10, 10â € ™, is preferably carried out before the deposition on the support 2 of the material necessary for the formation of the relative conductive tracks and of any circuit components. In essence, therefore, the conductive tracks 7 are formed on the upper surface 5 of the substrate as in part C of figure 8 or part H of figure 15, by depositing - preferably with a screen printing technique - of a conductive material or ink, such as for example a silver-palladium alloy; relative drying and firing phases follow, which are preferably carried out at lower process temperatures than those used for vitrification of layer 10 or layers 10 and 10â € ™.

Also on the upper surface of the substrate can be deposited also a material destined to realize any other components of the circuit, such as resistors. The material in question can be, for example, an ink or a resistive or piezo-resistive paste, preferably deposited by screen printing; obviously this resistive or piezo-resistive material is deposited so that the resistances obtained are electrically in contact with relative conductive tracks 7, preferably slightly overlapping each other. Also in this case, after the material has been deposited, the semi-finished product is dried and fired, in particular with process temperatures lower than those used for vitrification of layer 10 or layers 10 and 10â € ™.

On the ends of the tracks 7 which must be connected to the die 6 by means of the micro-threads 8, a thin layer of the same material constituting the micro-threads 8, or compatible with the material constituting the micro-threads, is deposited, preferably by screen printing. Also in this case, the deposition of the material is followed by relative drying and firing phases at temperatures lower than those used for vitrification of layer 10 or layers 10 and 10â € ™.

Subsequently on the upper face 5 of the substrate 2 can be deposited, for example again by screen printing, a polymeric material intended to form a protective layer, with subsequent drying and firing. This protective layer is deposited in such a way as to leave exposed the areas in which any additional circuit components must be connected to the conductive tracks 7, such as diodes, capacitors, an integrated circuit other than die 6, etc. Obviously the protective layer is not deposited in the area where die 6 is mounted; the ends of the tracks to which the micro-wires 8 must be connected are also kept exposed in this area.

The above possible additional components of the circuit are then mounted on the upper part of the substrate 2, preferably by means of the SMD technique. In this phase, the connection terminals of the components in question are electrically connected to the relative tracks. In the electrical connection or wire bonding phase of the die 6, the microfiles 8 are connected between the relative upper contacts of the die 6 and the ends of the conductive tracks 7 of interest of the body or substrate 2, according to a known technique.

Finally or previously, the connection terminals 9 can be assembled and welded to the conductive tracks 7 of interest of the circuit, necessary for the connection of the sensor 1 for its use.

In the event that the sensor 1 must be of the absolute type, as mentioned, the cover 11 of figures 7 or 14 can be applied on the upper face of the die 6.

The same operations indicated above are also substantially carried out in the case in which the mounting substrate 2 of the die 6 is electrically conductive. In such an application, preferably after the assembly of the die 6, the upper surface 5 of the substrate 2 is covered at least in part with the layer of electrically insulating material, on which the material necessary for the formation of the tracks 7 and any possible circuit components obtainable by deposition, as previously described. The insulating layer, which can be deposited by screen printing technique, is preferably dried and / or baked in an oven at process temperatures lower than those required for the vitrification of layer 10 or layers 10 and 10â € ™.

Alternatively, however, a layer of insulation could be first deposited and fired, and eventually the tracks 7 deposited and the components fixed, taking care to leave the substrate area free on which then to fix the die 6 by means of glass paste.

As we have seen, the bond between the silicon constituting the die 6 with the relative support body 2, be it in insulating material (for example a ceramic material, glass or an insulating polymer), or in metal (for example a metal or a conductive polymer), takes place through one or more glassy sealing / gluing layers, by means of a deposition process during which a partial dissolution of the surface of the components to be welded takes place.

The solution envisaged according to the invention, with the relative manufacturing process, allows to obtain:

- an intimate bond between the parts, such as to give a high airtightness to the sealed area (up to 10-9 atm-cc / sec.);

- a tenacious bond between the parts, such as to give a high adhesion force to the glued surfaces, which in fact makes the burst pressure dependent only on the membrane 6b of the die 6, and no longer on the bonding material ;

- greater reliability than silicone and / or epoxy resins traditionally used for bonding silicon dies; these known resins are subject to greater degradation as a result of mechanical stress over time (pressure blows) and / or exposure to high temperatures (greater than 140 ° C);

- a complete insensitivity to the medium being measured, which therefore can be an aggressive gas or fluid (for example high temperature gas), contrary to the aforementioned traditional bonding resins, which undergo heavy degradation in the presence of aggressive media;

- great stability at the junction (stable up to about 200 ° C), as the glass transition temperature is about 215 ° C;

- a great reduction of the differential deformations in temperature that induce variations in the sensor reading, since the coefficient of thermal expansion of the glass paste used is about 7 ppm / ° C, and is therefore similar to that of silicon (2, 5-3 ppm / ° C), in contrast to traditional epoxy and silicone resins which instead reach much higher values (between 40 and 200 ppm / ° C)

- a precise and stable positioning of the die with respect to the circuit, deposited directly on the substrate on which the die is glued, in particular in order to avoid incorrect or anomalous micro-connections (wire bonding) and / or damage due to mutual micro-movements.

It will also be appreciated that the proposed solution allows to reduce the number of pieces necessary for the production of the sensor, in particular without necessarily having to equip the die 6 with a specific glass base which must be suitably formed and then sealed to the die itself.

Obtaining the substrate with a substantially completely flat upper surface is simpler than with known solutions which provide a specific raised portion for positioning the die.

The obtaining of a circuit directly on the substrate, with a flat upper surface, also allows to improve the overall reliability of the product and / or facilitate the production process.

The deposition of several glass layers, when foreseen, allows to obtain a high dimensional accuracy, a better thickness uniformity and a better adhesion to the substrate (for example, with thin thicknesses, high tensions are avoided during the firing processes) and / or a high final thickness of the glass layer. A greater thickness of the glass layer, which is shown to also perform compensation functions for stress and / or for dilatations between the substrate and / or circuit, on the one hand, and the silicon die on the other side, is preferably obtained by a glassy paste loaded with particles, fibers or flakes of relatively high thickness.

Thanks to the aforementioned characteristics, the pressure sensor according to the invention is therefore suitable for use in environments with aggressive fluids, at high temperatures, with high burst pressure, with greater accuracy in extended thermal ranges and, ultimately, with superior reliability. .

It is clear that numerous variants are possible for the person skilled in the art to the sensor described as an example, without thereby departing from the scope of the invention as defined in the attached claims.

The materials affixed to the surface of the substrate (such as the glass paste, the material for the conductive tracks and any circuit components deposited, any insulating material and any protective material) can be deposited on the die substrate with techniques other than those indicated above, for example by lithography, photo-lithography, spraying.

At least some of the characteristics indicated in the various embodiment examples could also be combined with each other in a different way, to realize pressure sensors different from those described and depicted. These characteristics, preferably described with reference to a silicon die, could be at least partially associated with a different pressure sensor die or chip.

Claims (15)

CLAIMS 1. A pressure sensor having: - a pressure sensitive element, including a die based on semiconductor material (6), particularly silicon, defining a blind cavity (6a), the cavity (6a) having an open end in correspondence with a first face of the die ( 6), the opposite end of the cavity (6a) being closed by a membrane portion (6b) defined by the die (6) in correspondence with its second face, which is opposite to the first face; - a substrate (2), crossed by a through opening (4) and having a flat surface (5) at which one end of the through opening (4) opens, the die (6) being mounted on the flat surface (5) of the substrate (2) with the open end of the relative cavity (7a) facing the through opening (4) of the substrate (2); - an electric circuit (7) to which the die (6) is electrically connected, characterized by the fact that the first face of the die (6) is glued and sealed on the flat surface (5) of the substrate (2) by means of at least one layer of glassy material (10, 10â € ™) which extends between the die or semiconductor material (6) and the material constituting the substrate (2). The sensor according to claim 1, wherein the electrical circuit comprises a plurality of electrically conductive tracks (7), to at least some of which the die (6) is electrically connected, at least part of the electrical circuit being formed on the flat surface (5) of the substrate (2). The sensor according to claim 1 or claim 2, wherein the substrate (2) is of electrically insulating material. The sensor according to claim 3, wherein the electrically insulating material comprises one of a ceramic material, glass, a material based on one or more oxides, such as an aluminum oxide (AL2O3), a polymer. The sensor according to claim 1 or claim 2, wherein the substrate (2) comprises a body of electrically conductive material, such as a conductive metal or polymer, in particular having a flat face partially coated with a film of insulating material on which the electrically conductive tracks (7) are located. The sensor according to any one of the preceding claims, wherein the die (6) is connected to respective conductive tracks (7) of the electrical circuit by wire bonding, i.e. by means of thin connecting wires (8) of electrically conductive material, particularly flexible micro-wires having a thickness or diameter comprised between about 5 and 100 microns, preferably from about 25 to about 35 microns, and being preferably formed with a material or an alloy comprising or based on a material selected from gold, platinum, silicon, palladium , beryllium, silver, aluminum and copper. The sensor according to claim 3 or claim 5, comprising a support body having structural functions (2), the substrate being made from said body or from a part of said body also having structural functions. The sensor according to any one of the preceding claims, in which the first face of the die (6) is glued to the flat surface (5) of the substrate (2) by means of at least two layers of glassy material (10, 10â € ™). The sensor according to any one of the preceding claims, comprising at least one of: - a layer of insulating material, particularly a layer of electrically non-conductive metal oxide, deposited on the flat surface (5) of the substrate (2); - electrically conductive tracks (7), preferably deposited on the flat surface (5) of the substrate (2), formed with a material or an alloy comprising a material selected from gold, platinum, silicon, palladium, beryllium, silver, aluminum and copper; - one or more circuit components formed with a resistive or piezoresistive material deposited on the flat surface (5) of the substrate (2) or on an insulating film which partially covers the flat surface (5) of the substrate (2); - a protective layer of at least part of the electric circuit, particularly of polymeric material; - portions of conductive tracks (7) formed with the same material, or a compatible material, of micro-wires (8) which connect the die (6) to the electrical circuit. 10. A process for producing a pressure sensor, particularly according to one or more of claims 1 to 9, comprising the steps of: a) provide a pressure sensitive element (6), b) providing a substrate (2), c) mount the sensitive element (6) on the substrate (2), wherein step a) comprises providing a sensitive element of the die type based on semiconductor material (6), particularly silicon, defining a blind cavity (6a), having an open end at a first face of the die ( 6) and an opposite end closed by a membrane portion (6b) defined by the die (6) in correspondence with its second face opposite the first face; in which step b) comprises providing the substrate (2) with a through opening (4) and a substantially flat surface (5), at which one end of the through opening (4) opens ; in which step c) comprises positioning or securing the die (6) to the substrate (2) in such a way that the open end of the cavity (6a) of the die (6) faces the through opening (4 ) of the substrate (2); characterized in that step c) comprises gluing the first face of the die (6) to the flat surface (5) of the support body (2) by means of at least one glass layer (10, 10â € ™). 11. The process according to claim 10, in which step c) comprises the operations of: c1) depositing at least one layer of a glassy paste (10a; 10aâ € ™) in fluid form on the flat surface (5) of the substrate (2) around the through opening (4); c2) cause the drying of at least one layer of glass paste (10a; 10aâ € ™) at a first process temperature, particularly between about 60 ° C and about 190 ° C, preferably between about 80 ° C and about 170 ° C, to obtain a dried layer (10b; 10bâ € ™); c3) cause the vitrification of the dried layer (10b; 10bâ € ™) at a second process temperature, higher than the first process temperature, particularly between about 180 ° C and about 320 ° C, preferably between about 200 ° C and about 300 ° C, to obtain a said glassy layer (10, 10â € ™). 12. The process according to claim 10 or claim 11, wherein the first face of the die (6) is glued to the flat surface (5) of the substrate (2) by means of a plurality of superimposed glass layers (10, 10â € ™ ), deposited in succession, through the operations of: c1.1) deposit a layer of glass paste (10a) to obtain a first layer (10b), c1.2) vitrify the first layer (10b) to obtain a first vitreous layer (10), c1.3) deposit at least one further layer of vitreous paste (10aâ € ™) on the first vitrified layer (10) to obtain at least a second layer (10bâ € ™); c1.4) cause the vitrification of at least a second layer (10bâ € ™), to obtain a second glassy layer (10â € ™) on the first glassy layer (10), eventually deposit and vitrify one or more further layers of vitreous paste on an underlying vitreous layer, and in which, before the vitrification of the last layer of vitreous paste deposited, the die (7) is positioned on this last layer. 13. The process according to one of claims 10 to 12, wherein the glassy paste in fluid form (10a, 10aâ € ™) comprises glass particles and additives or particles dispersed in an organic matrix, the organic matrix being eliminated as a result of a drying operation of the relative layer of vitreous paste (10a, 10aâ € ™). The method according to any one of claims 10 to 13, further comprising forming on the flat surface (5) of the substrate (2) an electrical circuit, particularly at least in part by depositing material. The process according to one or more of claims 10 to 14, wherein the paste or glassy material comprises at least one of: - glass particles, particularly in the form of spheres, fibers or flakes; - one or more additives, such as thallium oxide, vanadium oxide, phosphorus oxide; - an organic matrix, comprising in particular one or more aromatic and / or aliphatic solvents; - particles of the first type and particles of the second type, in particular glassy particles of a first size and glassy particles of a second size.