MONICHINO, Massimo (Mendrisio, CH-6850, CH)
NEBBIA, Fabio (Mendrisio, CH-6850, CH)
SALMASO, Luca (Mendrisio, CH-6850, CH)
MONICHINO, Massimo (Mendrisio, CH-6850, CH)
NEBBIA, Fabio (Mendrisio, CH-6850, CH)
| CLAIMS 1. A pressure sensor having: - a pressure-sensitive element, comprising a semiconductor-based die (6), particular silicon, defining a blind cavity (6a), the cavity (6a) having an end open at a first face of the die (6), the opposite end of the cavity (6a) being closed by a diaphragm portion (6b) defined by the die (6) at a second face thereof, which is opposed to the first face; - a substrate (2) traversed by a through-opening (4) and having a flat surface (5) at which an 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 corresponding cavity (7a) facing the through-opening (4) of the substrate (2); - an electric circuit (7) to which the die (6) is electrically connected, characterized in that the first face of the die (6) is bonded and sealed onto the flat surface (5) of the substrate (2) by means of at least one layer of a glassy sealing material ( 10, 1 0') extending between the die or the sem iconductor material (6) and the material forming the substrate (2) to obtain a seal therebetween. 2. The sensor according to claim 1 , wherein the electric 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 electric circuit being formed on the flat surface (5) of the substrate (2). 3. The sensor according to claim 1 or claim 2, wherein the substrate (2) is made of electrically insulating material. 4. 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 an aluminum oxide (AL2O3), a polymer. 5. The sensor according to claim 1 or claim 2, wherein the substrate (2) comprises a body made of an electrically conductive material, such as a metal or a conductive polymer, in particular having a flat face partly coated with a film made of insulating material onto which are the electrically conductive tracks (7). 6. The sensor according to any one of the preceding claims, wherein the die (6) is connected to respective conductive tracks (7) of the electric circuit by wire bonding, i.e. through thin connecting wires (8) made 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 35 microns, and being preferably formed of a material or alloy comprising, or based on, a material selected from gold, platinum, silicon, palladium, beryllium, silver, aluminum and copper. 7. The sensor according to claim 3 or claim 5, comprising a support body having structural functions (2), the substrate being formed of said body or by a part thereof also having structural functions. 8. The sensor according to any one of the preceding claims, wherein the first face of the die (6) is bonded to the flat surface (5) of the substrate (2) by means of at least two layers of glassy sealing material ( 10, 10'). 9. The sensor according to any one of the preceding claims, comprising at least one of: - one layer of insulating material, particularly one layer of non-electrically conductive metal oxide, deposited onto the flat surface (5) of the substrate (2); - electrically conductive tracks (7), preferably deposited onto the flat surface (5) of the substrate (2), formed of a material or an alloy comprising a material selected form gold, platinum, silicon, palladium, beryllium, silver and copper; - one or more circuit components formed of a resistive or piezo-resistive material deposited onto the flat surface (5) of the substrate (2) or onto an insulating film that partially coats the flat surface (5) of the substrate (2); - a layer for protecting at least part of the electric circuit, particularly made of a polymeric material; - portions of conductive tracks (7) formed of the same material of micro- wires (8) connecting the die (6) to the electric circuit, or a material compatible thereto. 10. A process for producing a pressure sensor, particularly according to one or more of claims from 1 to 9, comprising the steps of: a) providing a pressure-sensitive element (6), b) providing a substrate (2), c) assembling the sensitive element (6) on the substrate (2), wherein step a) comprises providing a sensitive element of the semiconductor-based die type (6), particularly silicon, defining a blind cavity (6a), having an end open at a first face of the die (6) and an opposed end closed by a diaphragm portion (6b) defined by the die (6) at a second face thereof opposite to the first face; wherein step b) comprises providing the substrate (2) with a through- opening (4) and a substantially flat surface (5), at which an end of the through- opening (4) opens; wherein step c) comprising positioning or securing the die (6) onto the substrate (2) such that the open end of the cavity (6a) del die (6) faces the though- opening (4) of the substrate (2), characterized in that step c) comprises bonding the first face of the die (6) to the flat surface (5) of the support body (2) by means of at least one glassy sealing layer ( 10, 10') to obtain a seal therebetween. 1 1. The process according to claim 10, wherein step c) comprises the operations of: c l ) depositing at least one layer of a glassy paste ( 1 0a; 10a') in a fluid form onto the flat surface (5) of the substrate (2) around the through-opening (4); c2) causing drying of the at least one layer of glassy paste ( 1 0a; 10a') at a first process temperature, particularly comprised between about 60°C and about 1 90°C, preferably between about 80°C and about 1 70°C, to obtain a dried layer ( 10b; 1 0b"); c3) causing vitrifaction of the dried layer ( 10b; 1 0b') at a second process temperature, higher than the first process temperature, particularly comprised between about 1 80°C and about 320°C, preferably between about 200°C and about 300°C, to obtain one said glassy layer ( 10; 10'). 12. The process according to claim 10 or claim 1 1 , wherein the first face of the die (6) is bonded to the flat surface (5) of the substrate (2) by a plurality of superimposed glassy sealing layers (10, 10'), deposited in sequence, by the operations of: cl . l ) depositing one layer of glassy paste ( 10a) to obtain one first layer ( 10b), cl .2) vitrifying the first layer ( 10b) to obtain one first glassy layer ( 10), c l .3) depositing at least one further layer of glassy paste ( 10a') onto the first vitrified layer ( 10), to obtain at least one second layer ( 10b'); c l .4) causing vitrifaction of the at least one second layer ( 10b'), to obtain a second glassy layer ( 10') on the first glassy layer ( 10), possibly depositing and vitrifying one or more further layers of glassy paste on an underlying glassy layer, and wherein, before vitrification of the last layer of the deposited glassy paste, onto said last layer the die (7) is positioned. 13. The process according to any one of claims 10 to 12, wherein the glassy paste in fluid form ( 10a, 10a') comprises particulate glass and additives or particles dispersed in an organic matrix, the organic matrix being eliminated following upon an operation of drying the corresponding layer of glassy paste (10a, 10a'). 14. The process according to any one of claims 10 to 13, also comprising forming on the flat surface (5) of the substrate (2) an electric circuit, particularly at least in part by a deposition of material. 15. The process according to any one of claims 10 to 14, wherein the glassy paste or material comprises at least one of: - particulate glass, in particular in form of sphere, fiber of flake particles; - one or more additives, such as thallium oxide, vanadium oxide, phosphorous oxide; - an organic matrix, comprising in particular one or more aromatic or aliphatic solvents; - first-type particles and second-type particles, in particular glassy particles of a first dimension and glassy particles of a second dimension. |
DESCRIPTION
Field of the 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 regards a pressure sensor having:
- a pressure-sensitive element, comprising a die made of semiconductor material, particularly silicon, defining a blind cavity having an end open at a first face of the die, the opposite end of the cavity being closed by a diaphragm portion defined by the die at the face thereof opposite to the first face;
- a substrate, traversed by a through opening and having a flat surface at which an end of the through-opening opens, the sensitive element being mounted on the flat surface of the substrate with the open end of the corresponding cavity facing the through opening of the substrate;
- an electric circuit to which the die is electrically connected.
The invention also regards a method for producing such sensor, according to at least the preamble of claim 10.
Prior art
Pressure sensors having the previously indicated structure are generally known and used in devices for detecting the pressure of fluids (liquids and aeriform substances) in various industries, such as the automotive industry, household and household appliances industry, air conditioning industry and hydro-sanitary heating industry in general.
In a first type of solutions the substrate is constituted by a main body of the sensor, having structural functions, for example made of metal material, to which there is generally coupled a closing cover, which protects the die of semiconductor and the electric/electronic circuitry of the sensor, and integrates a connector.
In another type of solution the substrate is instead constituted by a PCB
(Printed Circuit Board) made of fibreglass, which thus supports both the die and the aforementioned circuitry. In these solutions, the substrate constituted by the PCB is mounted within a plastic or metal casing. The silicon die is welded to a corresponding axially hollow glass base through anodic bonding, i.e. a process which provides for the use of electricity for heating and melting between silicon and glass, obtaining a chemical bond. The aforementioned base is sealingly bonded on the corresponding substrate, through a silicon or epoxy resin, at a corresponding through opening.
Regardless of the specific embodiment, and very schematically, the through opening of the substrate is placed in fluid communication with a circuit in which there is the medium whose pressure is to be detected, be it a liquid or an aeriform substance. Thus, also the cavity of the die is in fluid communication with the circuit in which the medium subject of the measurement is found. The pressure of the aforementioned medium generates a more or less marked flexure of the diaphragm portion of the die. The degree of deformation, representative of the pressure, is detected by specifically providing the die, for example by providing a miniaturized resistive bridge at the diaphragm portion thereof.
The second type of solutions indicated revealed to be poorly suitable for using semiconductor pressure sensors combined with fluids that are particularly aggressive from a chemical point of view, due to the risks of deterioration of the PCB. Similar notion shall also apply for the cases where the sensor is used for detecting the pressure of fluids having high temperatures, for example in the order of 150°C, or in the case where the substrate is potentially subjected to high mechanical stresses, like in case of high pressures.
These drawbacks are reduced in the solutions wherein the glass base of the die is bonded onto a metal substrate, typically formed by the main body of the sensor, having structural functions. However, also these solutions are generally not suitable for use combined with fluids that are particularly aggressive from a chemical point of view, given that in this case the glass base of the die is sealingly bonded through a silicon or epoxy resin, and these resins are poorly resistant to chemical aggressions and/or to high temperatures. The sensor however requires a connection electric circuit, whose support should be positioned in proximity of the die. Thus, the face of the metal body, at which the die and circuit are assembled is typically not flat but machined so as to define two separate parallel support surfaces, at different heights. In particular, the aforementioned face of the metal body is machined so as to define a projecting central portion - traversed by an end portion of the pressure port passage - on whose top surface there is bonded the glass base of the die. The support circuit or PCB is formed with an opening having such dimension to be received passing through the aforementioned projection and rest on the lower surface of the substrate. Thus, the PCB never comes to contact with the fluid subject of measurement and it can extend around the projection, for an easier connection to the die.
In the known aforementioned configurations, with a printed circuit board or PCB added on the substrate, were the substrate in question to be entirely flat, the chip or die bonded thereto would be inserted within the aforementioned opening of the circuit support: hence, in presence of a relatively thick PCB, the top surface of the die— whose body is typically very thin - could be at a level lower than the top surface of the PCB, with the ensuing difficulty or impossibility to provide the electrical connections or a wire bonding between the die and PCB, a technology which typically requires considerable accuracy, in particular for providing an automated process. On the other hand, in case of use of a thin PCB, such as a PCB made of flexible material, there could arise difficulties regarding the certain positioning between the PCB and the die, and in particular between the die and the contacts of the PCB on which the connections are to be provided through the wire bonding technique.
Such drawback is accentuated by the fact that the PCB should be bonded to the substrate, with an ensuing increase of thickness, due to the layer of boding agent. In absence of such bonding between the PCB and the substrate, the electrical connection would not be feasible, in that formed by capillary wires - the aforementioned wire bonding - which would break at the least movement. This risk also arises in presence of bonding between the PCB and substrate, due to the potential movements between the two parts, for example during the bonding step due to subsequent dilatations or deformations of the relative materials in temperature.
This type of solution, with upper face defining two different support planes, complicates the provision of the device, and in particular of the metal body, which should be subjected to mechanical machining to define the two parallel surfaces, the higher one for supporting the base of the die, and the lower one for supporting the circuit support. As mentioned, also these solutions leave the problem regarding the deterioration of the epoxy or silicon resins used for bonding the substrate of the glass base of the die unresolved, given that the resins tend to deteriorate given that they are at contact with aggressive fluids and/or high temperatures.
US-A-5721446 discloses a pressure sensor wherein a semiconductor detection element having a thin wall diaphragm is fixed to a thick film substrate by a die bonding material, such as a silicone resin. A convex member of a glass material is interposed between the thick film substrate and the detection element and fixed by the die bonding material.
JP-A-58056367 discloses a pressure sensor having a body consisting of a stainless disk, in the bottom of which a ring-shaped groove is formed, such that a ring-shaped thin section is defined, between a peripheral ring-shaped thick section and a central cylindrical thick section. A semiconductor detection element is fixed through glass spacers between a localized portion of the peripheral thick section and a localized portion the central thick section, so as to bridge over the thin section. When pressure is applied to the body, flexure of the thin section is detected by the semiconductor detection element.
US-A-4840067 discloses a semiconductor pressure sensor having a cylindrical housing with a cavity having a blind bottom which is exposed to a pressure to be detected, wherein the top wall of said cavity of the cylindrical housing defines a metal diaphragm for receiving the pressure to be detected. A metal oxide layer is formed in a surface of the metal diaphragm, at the opposite side with respect to the cavity, by oxidizing the surface of the metal diaphragm, and a glass layer is formed on the metal oxide layer. A semiconductor chip, on which a strain gauge is formed, is firmly and surely bonded to the metal diaphragm, on the opposite side with respect to the cavity receiving the pressure to be detected, through the glass layer by virtue of the metal oxide layer. A similar solution is also disclosed by US-A-4712082.
Summary and object of the invention
The present invention essentially aims at providing a pressure sensor, which can also be used with aggressive fluids and/or fluids having high temperatures, having an improved structure and reliability with respect to the aforementioned prior art.
These and other objects, to be described hereinafter, are attained 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 disclosure provided in relation to the invention.
Brief description of the invention
Further objects, characteristics and advantages of the present invention will be apparent from the detailed description that follows and from the attached drawings, provided purely by way of non-limiting example, wherein:
- figure 1 is a perspective, partial and schematic view of a pressure sensor according to a first embodiment of the invention;
- figure 2 is a schematic cross-section of the sensor of figure 1 ;
- figure 3 is an enlarged detail of figure 2;
- figure 4 is a perspective, partial and schematic view of a pressure sensor according to a second embodiment of the invention;
- figure 5 is a schematic cross-section of the sensor of figure 4, in larger scale;
- figure 6 is a perspective, partial and schematic view of a pressure sensor according to a third embodiment of the invention;
- figure 7 is a schematic cross-section of the sensor of figure 6, in larger scale;
- figure 8 represents, through partial and schematic sections, the main steps of a bonding and sealing process used for producing pressure sensors according to figures 1 -7;
- figures 9 and 10 are views similar to those of figures 1 and 2, but regarding a pressure sensor according to a fourth embodiment of the invention;
- figures 1 1 and 12 are views similar to those of figures 4 and 5, but regarding 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 regarding a pressure sensor according to a sixth embodiment of the invention;
- figure 15 represents, through partial and schematic sections, an example of sequence of steps of a bonding and sealing process used for producing pressure sensors according to figures 9-14.
Description of preferred embodiments of the invention
Reference to "an embodiment" in this description indicates that a particular configuration, structure or characteristic described regarding the embodiment is included in at least one embodiment. Hence, expressions such as "in an embodiment", possibly present in various parts of this description do not necessarily refer to the same embodiment. Furthermore, particular configurations, structures or characteristics may be combined in any suitable manner into one or more embodiments. References herein are used for the ease of understanding and thus they do not define the scope of protection or the range of the embodiments.
Hereinafter in the present description, terms such as "upper " and "lower " shall be intended as simple spatial reference to facilitate the description of the details illustrated in the figures.
With particular reference to figures 1 , 2 and 3, an example of a pressure sensor according to the present invention is indicated in its entirety with 1. It should be observed that in the various figures the sensor 1 is shown solely regarding the parts directly related to the understanding of the invention, and thus without a corresponding body or cover which - depending on the cases - partially covers or entirely encloses the parts illustrated herein in the figures.
In the exemplified embodiments, the sensor 1 has a main body 2, having structural functions, with a threaded cylindrical lower part 2a, above which there is defined a seat for a sealing ring 3. The part 2a can be used, for example, for directly fixing 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 detected is found.
The body 2 is traversed by an axial passage, indicated with 4 in figures 2 and 3, which provides a pressure port, such passage being preferably, but not necessarily, substantially coaxial to the axis of the body 2. The passage 4 completely traverses the body 2, opening at the upper end or face of the body, indicated with 5, which is flat. An electric circuit is associated to such flat upper face 5.
In an embodiment, the body 2 - or at least the part thereof defining the flat face 5 - thus meets the function of circuit support and it is preferably made of an electrically insulating material, preferably a ceramic material, for example obtained by sintering alumina powder, or a polymer; the part of the body 2 which serves the function of circuit support could also be made of suitable glass.
As observable hereinafter, in a different embodiment, the body 2 or the part thereof defining the face 5 can also be made of electrically conductive material, such as a metal material, for example made of steel or aluminium, or a conductive polymer: in these cases, on the face 5 there is preferably deposited a layer made of insulating material, such as a layer of electrically insulating metal oxide, on which there are further deposited electrical connections and/or tracks, as described hereinafter.
As mentioned, the surface or face 5 is substantially flat and the upper end of the passage 4 opens therein. On the face 5 there is mounted a pressure-sensitive element, comprising a die, i.e. a small block or plate based on semiconductor material, typically silicon, which is die-bonded to the face 5. In the abovementioned die, indicated in its entirety with 6 in the figures, the integrated circuit which generally supervises the general operation of the pressure sensor 1 can be directly obtained in miniaturized form. The die 6 can be configured as a single plate or quadrangular-shaped silicon block; however, such solution shall not be deemed restrictive given that the die 6 can have different shapes with respect to the illustrated one and it can be formed by a plurality of mutually joined silicon parts or layers. The die 6 can be obtained through the technique per se known in the industry for producing semiconductor chips.
Contrary to a common integrated circuit, the die 6 is preferably not provided with its own casing or package, and thus also without relative projecting connection terminals (pin or lead), typically made of relatively rigid metal elements. For such purpose, on the upper face made of semiconductor material there are directly applied contacts, not represented, in form of thin films made of an electrically conductive material deposited on the die, preferably but not necessarily a noble material, such as for example gold or a silicon aluminium alloy at 1 %; preferred materials or alloys which can be used for the purpose comprise gold, platinum, silicon, palladium, beryllium, silver, aluminium and copper.
As observable in figure 3, in the body made of semiconductor material there is defined a blind cavity 6a, which has an end open at the lower face of the die, while the opposite end of the cavity 6a is closed by a diaphragm portion 6b, defined by the die 6 at the upper face thereof.
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 such opening.
As mentioned, on the face 5 there is provided the aforementioned electric circuit, shown solely partly for the sake clarity, which comprises a plurality of connection tracks 7, made of electrically conductive material, deposited on the upper surface of the body 5, and organised around the assembly region of the die 6. In figure 1 there are represented - by way of example - only three tracks 7, assuming that the circuit can comprise more than three tracks, even having a layout different from the one exemplified in figure 1.
The die 6 is connected to respective tracks 7 of the abovementioned circuit through thin connecting wires, some of which are generally indicated with 8, made of an 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. The aforementioned wires 8 are preferably made of a material or an alloy comprising a material selected from gold, platinum, silicon, palladium, beryllium, silver, aluminium and copper. The micro-wires 8 are connected between 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 wire bonding, and particularly of the wedge-bonding or ball- bonding type, for example through thermocompression, ultrasonic or thermosonic welding. The micro-wires 8 can have different shapes or cross-sections, such as circular or quadrangular shapes or sections or substantially flat shapes or sections, for example flexible micro-ribbons made of conductive material, without insulation coating.
Some of the conductive tracks 7 of the circuit are electrically connected with respective terminals of the sensor 1 , indicated with 9, for power supplying and/or conveying signals generated by the die 6, possibly processed through electrical and/or electronic components of the circuit. The circuit provided for on the surface of the face 5 can include a plurality of circuit components, of the type generally known in the art, also among which active and/or passive components and/or an integrated circuit besides the 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 corresponding cavity 6a opens, is bonded and sealed directly to the substrate formed by the flat surface 5 of the body 2 through at least one layer of glassy sealing material.
In the embodiment of figures 1 -3 there is provided for at least one layer of glassy material, indicated with 10. Thus, as observable the die 6 is mounted on the same surface 5 in which the electric circuit is provided for.
Figures 4 and 5 refer to a second embodiment of the invention. In such figures the same reference numbers of the preceding figures are used for indicating elements technically equivalent to those described previously.
The sensor 1 of figures 4 and 5 differs from the sensor 1 of figures 1 -3 substantially due to the shape of the body 2 which defines the flat face 5 on which the electric circuit is provided for. Also in the embodiment of figures 4-5 the body 2 is directly made of electrically insulating material, particularly a ceramic material or based on one or more oxides. In a preferred embodiment, this body 2 is made aluminium, i.e. alumina oxide (AL2O3). Similarly to the body 2 of figures 1 -3, also the body 2 of figures 4-5 can be obtained through sintering. The body 2 is traversed by the corresponding axial passage 4 and, in this embodiment, on the upper face of the body 2 there is mounted the die 6, through a corresponding layer of glassy material 10.
It shall also be observed that in this embodiment, given that the body 2 is made of electrically insulating material, the electric circuit of the sensor 1 can be easily formed at least partly on the surface or flat upper face 5. In particular, on the upper face 5 of the body 2 there can be directly deposited the electrically conductive material required for the formation of the tracks 7 and other possible circuit components, whether obtainable through deposition of material or associable to the circuit, such as for example resistors.
Thus, also in this embodiment the material forming the body 2 is directly exploited as a substrate for the circuit of the sensor 1 , without requiring a special printed circuit board. Such body 2 is preferably substantially cylindrical-shaped, provided with suitable references or perimeter seats, in particular for a suitable positioning in another body, such as grooves which extend between the abovementioned upper face and a lower face.
Figures 6 and 7 refer to a third embodiment of the invention. Also in such figures the same reference numbers of the preceding figures are used for indicating elements technically equivalent to the ones described previously. The sensor 1 of figures 6 and 7 differs from the sensor 1 of the previous figures substantially due to the shape of the body 2 and/or the presence, on the die 6, of a hollow cover 1 1 , defining a corresponding blind cavity 1 la. The cover 1 1 , such as a cover made of glass or silicon, is mounted sealed hermetically, through a suitable sealing agent or a fixing through anodic bonding, on the upper face of the die 6, so that the diaphragm portion 6b faces the opening of the cavity 1 l a. In this configuration, the sensor 1 is an absolute pressure sensor, with the airtight chamber defined between the cover 1 1 and the upper face of the die 6 which provides a pressure reference.
The flat surface or face 5 on which there is mounted the die 6, with or without the cover 1 1 , may belong to a main body 2 made of insulating material.
As mentioned previously, the substrate on which there is mounted the die 6 can also be made of an electrically conductive material. For such purpose, for example, the body 2 represented schematically and partly in figures 6 and 7 - on the upper face 5 of which there is mounted directly the die 6 - can be made of metal, such as steel or aluminium. In an embodiment of this type, on the upper face 5, preferably subsequently to mounting the die 6, there is deposited a layer made of electrically insulating material, for example an electrically insulating oxide metal, except for the area for mounting the die 6, and on such layer made of insulating material there is then deposited the material intended to form the connection tracks 7, with the possible other components of the circuit.
Regardless of the type of substrate or material it is formed with, also in the embodiment of figures 6 and 7, the die 6 is bonded and sealed directly on the flat upper surface 5 of the body 2 through at least one layer of glassy sealing material 10.
Figure 8 illustrates the main steps of the process for bonding and sealing the die 6 to the flat face 5 of the corresponding substrate 2.
Thus, on the flat surface 5 of the substrate 2 there is deposited a layer 10a of glassy paste in fluid form 10a, in the region that surrounds the opening of the passage 4, for example through silk screen-printing.
In the preferred embodiment of the invention, the paste constituting the layer 10a comprises particulate glass and additives dispersed in a suitable organic matrix. The particulate can be constituted by first glass particles measuring up to 20 microns, well above the typical roughness of the surfaces to be bonded. The preferred additives, selected for lowering the vitrification point, may for example comprise one or more of thallium oxide, vanadium oxide, phosphorous oxide. The organic matrix may for example comprise one or more aromatic or aliphatic solvents.
After depositing the paste layer 10a, the die 6 is directly positioned thereonto, as observable in the part A of figure 8, for example through the "die attach" technique and/or using suitable positioning templates.
Then, the semi-finished product is heated in a furnace, to subject the layer 10a to a drying step. The process temperature is preferably comprised between about 60°C and about 190°C, preferably between about 80°C and about 1 70°C. During this step, the organic matrix of the glassy paste is dissolved due to a burn out process, so as to obtain a dried layer 10b including the glass alone and the additives, as schematically shown in the part B of figure 8.
The semi-finished product is then subjected, still in the furnace, to a treatment step at a temperature exceeding the drying one, to determine the final remelting and/or vitrification of the residue material forming the dried layer 10b, i.e. for determining a sealing fixing of the die 6 on the substrate 2. The process temperature is preferably comprised between about 180°C and about 320°C, preferably between about 200°C and about 300°C, to obtain the glassy sealing layer 10. The bonding and sealing between the die 6 and the corresponding substrate 2 is determined with this second step through the layer 10.
Thus, concretely the glass-based paste 10a is subjected to a thermal profile, preferably in two steps, in the first of which the organic matrix is burnt and removed completely, solely leaving the glassy structure and the possible additives, and in the second of which, at a higher temperature, the glass melts. The process could however comprise even only one thermal step, for example without the intermediate drying step, i.e. removing the organic matrix directly during the remelting and vitrification step.
Through such vitrificatioin step, the paste is transformed into a block of amorphous material, which allows establishing a bond with the surfaces to be bonded, thus obtaining a single packet.
The bonds involved in the adhesion between the surface of the substrate 2 and the die through the glassy paste are preferably mechanical and/or chemical and/or physical.
From a mechanical point of view, adhesion between the parts is facilitated by the penetration of the glassy paste molten in the micro-projections of the surface of the substrate; such effect is maximised in the case of a substrate 2 made of ceramic material, particularly sintered, due to the surface roughness of the substrate.
From a chemical point of view, the vitrification step leads to oxidative and diffusion processes, which create a transition area between the glassy paste and the surfaces to be bonded.
In the ideal case, during the baking /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 (hence Si and O) towards the die (Si), and based on inorganic oxides towards the substrate. For example, in the case of alumina substrate (AI2O3), the oxygen atoms are shared with the SiO? ones of the glass of the glassy paste. The bond is very intimate and such to no longer allow attributing the atom to one or the other molecule. This allows an extremely high adhesive force and the sealing of the surface.
Similarly, in case of a metal substrate, the oxidative phenomena which intervene during the baking step determine the sharing of oxygen atoms between those of the paste and of the metal oxides. Lastly, from a physical point of view, weak intermolecular interaction forces (Van der Vaal forces), whose contribution to the bond is however lower than the previous ones, intervene between the vitrified paste and the substrate.
Figures 9 and 10 illustrate a sensor solution structurally similar to that of figures 1-3, in which between the die 6 and the face 5 of the support or substrate 2 there are provided at least two glassy layers 10 and 10 " . The two layers 10 and 10' are formed in sequence starting from respective layers of glassy paste of the previously indicated type. The layers of glassy paste can be more than two and/or have different thicknesses with respect to each other, for example, with a layer 10' having a greater thickness with respect to the underlying layer 10. In order to obtain at least one glassy sealing layer with greater thickness, the glassy paste can advantageously be added or filled with second particles, for example in form of spheres, fibres or flake particles, preferably glassy ones, which contribute to maintain a given consistency and thickness of the paste during at least a first deposition step.
The second particles can have greater dimensions than the first particles constituting the particulate of the glassy paste, i.e. having dimensions larger than 20 microns, preferably of at least 50 microns. Preferably the paste, filled with the first particles and the second particles is such to allow depositing a layer with thickness greater than 0.1 mm, such as a paste thickness comprised between 0.1 mm and 1 mm.
According to a preferred version, the second particles are at least adapted to define a structure that determines the thickness of the paste during deposition and/or the distance between the chip and the substrate, with the first particles being at least adapted to be remelted to determine the bond and fixing of the die to the substrate.
Analogously, figures 1 1 -12 and 13-14 illustrate solutions similar to those of figures 4-5 and 6-7, respectively, but distinguished by at least two layers 10 and 10' for bonding and sealing between the die 6 and the corresponding body 2 which provides the substrate.
Figure 15 illustrates the subsequent steps typical of a bonding and sealing process through two glassy layers 10 and 10'. The indicated sequence of steps is provided by way of example, some of these steps being partly different and/or repeated in case of deposition of several glassy layers.
Generally, the process provides for the formation of the die 6 and the corresponding substrate 2, according to per se known methods.
Preferably, at least the part of the upper surface 5 of the substrate 2 in which the passage 4 opens is first cleaned using a solvent, for example acetone or isopropyl alcohol (part A of figure 15).
Hereinafter, on the upper surface 5 of the substrate 2, there is deposited a layer of a glassy paste 10a of the previously indicated type, so as to circumscribe the area of the upper surface 5 of the substrate 2 in which the passage 4 opens (part B of figure 15).
The layer 10a is subsequently dried in an oven, as outlined previously, 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). Hereinafter, on the vitrified layer 10 there is deposited a new layer 10a' of glassy paste (part E of figure 15), then dried to obtain a second dried layer 10b' (part F of figure 15).
In cases where it is intended to provide more than two glassy layers between the die and the corresponding substrate, the second dried layer 10b " is vitrified and a new layer of glassy paste is deposited thereonto, then dried and vitrified, through operations similar to the previously described ones.
In any case, before the vitrification of the last dried layer (the layer 10b', in the case of figure 15), the die 6 is positioned thereon (part G of figure 15) and the aforementioned last dried layer is then vitrified (part H of figure 15).
In an alternative process, after depositing the second layer 10a', the die 6 can be positioned thereon, then there follows the drying and then the vitrification.
The mounting of the die through the layer 10, or the plurality of layers 10, 10', is preferably carried out before deposition on the support 2 of the material required for the formation of the corresponding conductive tracks and possible circuit components. Thus, concretely, on the upper surface 5 of the substrate like in part C of figure 8 or part H of figure 15 there are formed the conductive tracks 7, by depositing - preferably by screen-printing - a conductive material or ink, such as for example a silver-palladium alloy; there follows the corresponding drying and baking steps, which are preferably carried out at lower process temperatures with respect to those used for vitrifying the layer 10 or layers 10 and 10'. Still on the upper surface of the substrate there can then also be deposited a material intended to provide possible other components of the circuit, such as resistors. The material in question can for example be an ink or a resistive or piezo-resistive paste, preferably deposited through silk screen printing; obviously, such resistive or piezo-resistive material is deposited so that the obtained resistors are electrically at contact with corresponding conductive tracks 7, preferably slightly superimposed with respect to each other. Also in this case, after depositing the material there follows the drying and baking of the semi-finished product, in particular with process temperatures below those used for vitrifying the layer 10 or layers 10 and 10'.
On the ends of the tracks 7 to be connected to the die 6 through the micro- wires 8 there is deposited, preferably through silk screen printing, a thin layer made of the same material forming the micro-wires 8, or compatible with the material forming the micro-wires. Also in this case, the deposition of the material is followed by drying and baking steps corresponding to temperatures lower than those used for vitrifying the layer 10 or layers 10 and 10'.
Then, on the upper face 5 of the substrate 2 there can be deposited, for example still through silk screen printing, a polymer material intended to provide a protective layer, with subsequent drying and baking. This protective layer is deposited so as to leave the areas - in which possible additional components of the circuit such as diodes, capacitors, an integrated circuit different from the die 6 etcetera are to be connected to the conductive tracks 7 - exposed. Obviously the protective layer is not deposited in the area in which the die 6 is mounted; also the tracks to which the micro-wires 8 should be connected are left exposed at such areas.
Thus, the aforementioned possible additional components of the circuit are mounted on the upper part of the substrate 2, preferably through SMD technique. In such step, the connection ends of the components in question are electrically connected to the corresponding tracks. In the step of electrical or wire bonding of the die 6 the micro- wires 8 are connected between the corresponding 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 technique known per se.
Connection terminals 9 can be finally or previously mounted and welded to the conductive tracks 7 of interest of the circuit, required for the connection of the sensor 1 with the aim of use thereof. Should the sensor 1 be of the absolute type, as mentioned, the cover 1 1 of figures 7 or 14 can be applied on the upper face of the die 6.
The same operations indicated above are substantially carried out even in cases where the substrate 2 for mounting the die 6 is electrically conductive. In such an application, preferably after mounting the die 6, the upper surface 5 of the substrate 2 is covered at least partly with the layer made of electrically insulating material, on which there is then deposited the material required for the formation of tracks 7 and possible circuit components which can be obtained by deposition, according to what has been described above. The insulating layer, which can be deposited through the silk screen printing technique, is preferably dried and/or baked in a furnace at process temperatures lower than those required for vitrifying the layer 10 or layers 10 and 10\
Alternatively, firstly an insulating layer could be deposited and baked and then the tracks 7 could be possibly deposited and the components fixed taking care to leave the area of substrate on which the die 6 is to be fixed by means of a glassy paste clear.
As observed, the bond between the silicon constituting the die 6 with the corresponding support body 2, whether made of insulating material (for example a ceramic material, glass or an insulating polymer) or whether made of metal (for example a metal or a conductive polymer), occurs through one or more sealing/bonding glassy layers, through a deposition process during which there occurs a partial dissolution of the surface of the components to be welded.
The solution provided for according to the invention, with the corresponding machining process, allows obtaining:
- an intimate bond between the parts, such to confer high air-tightness to the sealed area (up to 10-9 atm-cc/sec);
- a tenacious bond between the parts, so as to confer a high adhesion force of the bonded surfaces, which actually makes the burst pressure solely dependent on the diaphragm 6b of the die 6, and no longer on the bonding material;
- a greater reliability with respect to the silicon and/or epoxy resins traditionally used for bonding the silicon die; these known resins are subjected to greater degradation due to mechanical stresses over time (pressure rams) and/or exposure to high temperatures (greater than 140°C);
- a complete non-sensitiveness to the medium subject of measurement, which can thus be an aggressive gas or fluid (for example a high temperature gas), contrary to the mentioned conventional bonding resins, which are subjected to serious deterioration in presence of aggressive media;
- a great stability to the joint (stable up to about 200°C), in that the glassy transition temperature is of about 21 5°C;
- a great reduction of the differential deformations in temperature which cause reading variations of the sensor, in that the coefficient of thermal expansion of the used glassy paste is of about 7 ppm/°C, and it is thus similar to that of silicon (2.5-3 ppm/°C), contrary to the conventional epoxy and silicon resins which instead settle on much higher values (comprised between 40 and the 200 ppm/°C)
- an accurate and stable positioning of the die with respect to the circuit, deposited directly on the substrate on which the die is bonded, in particular so as to avoid erroneous or unusual micro-connections (wire bonding) and/or damage due to mutual micro-movements.
Furthermore, it shall be observed that the proposed solution allows reducing the number of pieces required to produce the sensor, in particular without requiring providing the die 6 with a specific glass base which should be suitably formed and then sealingly attached to the die.
The obtainment of the substrate with upper surface substantially entirely flat is simpler with respect to the known solutions which provide for a specific projecting portion for positioning the die.
The obtainment of a circuit directly on the substrate, with flat upper surface, also allows generally improving the reliability of the product and/or facilitating the production process thereof.
Depositing several glassy layers, when provided for, allows obtaining high dimensional accuracy, better uniformity of thickness and a better adhesion to the substrate (for example, thin thicknesses allow preventing high tensions during the baking processes) and/or a high final thickness of the glassy sealing layer. A greater thickness of the glassy layer, revealing to be capable of also serving stress and/or dilatation compensation functions between the substrate and/or circuit on one hand, and silicon die on the other hand, is preferably obtained through a glassy paste loaded with particles, fibres or flake particles of relatively high thickness.
Due to the aforementioned characteristics, the pressure sensor according to the invention is thus suitable to be used in environments with aggressive fluids, at high temperatures, with high burst pressure, with greater accuracy in wide thermal ranges and, finally, with greater reliability.
It is clear that the sensor described for exemplification purposes may be subjected - by a man skilled in the art - to numerous variants, without departing from the scope of the invention as defined in the attached claims.
The materials fixed on the surface of the substrate (such as the glassy paste, the material for the conductive tracks and the possible deposited circuit components, the possible insulation material and the possible protection) can be deposited on the substrate of the die through techniques different from the one indicated above, for example through lithography, photo-lithography, spraying.
At least some of the characteristics indicated in the various embodiments could be mutually combined even differently, to obtain pressure sensors different from the ones described and represented. Such characteristics, described by way of preferred example with reference to a silicon die could be at least partly associated to a different pressure sensor die or chip.
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