Abstract:
An electronic MEMS device is formed by a chip having with a main face and bonded to a support via an adhesive layer. A cavity extends inside the chip from its main face and is closed by a flexible film covering the main face of the chip at least in the area of the cavity. The support has a depressed portion facing the cavity and delimited by a protruding portion facing the main face of the chip. Inside the depressed portion, the adhesive layer has a greater thickness than the projecting portion so as to be able to absorb any swelling of the flexible film as a result of the expansion of the gas contained inside the cavity during thermal processes.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electronic micro-electro-mechanical system (MEMS) device comprising a chip bonded to a substrate and having cavities and to the manufacturing process thereof. In particular, the present disclosure applies to chemical sensors for detecting odorous matters. 
     2. Description of the Related Art 
     As is known, in devices for recognizing odorous matters (such as the one described, for example, in U.S. patent application Ser. No. 12/648,996, published as U.S. Application Publication No. 2010/0170324, and incorporated herein by reference in its entirety) the semiconductor material chip accommodates one or more cavities delimiting respective membranes carrying, i.a., respective adsorbent layers. Each membrane and the respective adsorbent layer form part of an oscillating circuit having electrical characteristics that vary with the weight of the ensemble including the membrane, the adsorbent layer, and any possible adsorbed material, enabling detection and possibly the amount of the adsorbed material. 
     Membranes may be manufactured using various techniques, some of which envisage formation of cavities extending on one side of the chip. In this case, when the chip is bonded on a substrate, the cavities are to be kept clean without being contaminated with glue or any other material. 
       FIGS. 1-3  show, for example, a chip  1  fixed to a substrate  10 . The chip  1  has a plurality of cavities  2 , each delimiting at the top an own membrane  4  formed in the same monolithic semiconductor material as the chip  1 . When the chip  1  forms a sensor for detecting odorous matters or in general a device for detecting chemical matters, each cavity may have, for example, a width of 250-300 μm and a depth of 500 μm, and the cavities may be made arranged at a mutual distance of 150-400 μm. In this case, the area of the membranes forms a sensitive region  5 , and electrodes and other sensitive layers, such as adsorbent layers (not shown), are formed on top of the membranes  3 , whereas a circuitry area  6 , shown only schematically in  FIG. 1  and including electronic components, extends on the side of the sensitive area  5 , for example, according to U.S. patent application Ser. No. 12/648,996. 
     An adhesive layer or film  7 , for example a die-attach film, is applied to the rear side of the chip  1  in order to close the cavities  2  at the bottom and prevent contamination thereof. The adhesive film  7  may be laminated on the back and cured using a thermal process so as to seal and protect the cavities. 
     A glue layer  8  fixes the chip  2 , through the adhesive film  7 , to the substrate  10 . The substrate  10  may be of any type; for example, it may be formed by a printed-circuit board, comprising a core region  11  overlaid by at least one conductive layer  12 , typically a copper layer, covered by a dielectric layer  13 , typically a solder-mask layer. The conductive layer  12  is shaped so as to form conductive regions and connections, as shown, for example, in the top plan view of  FIG. 2 . Here, the conductive layer comprises a quadrangular region  12   a  of an area slightly greater than that of the chip, and connection regions  12   b.    
     This type of attachment entails, however, problems. In fact, during bonding, when the structure undergoes thermal treatments, for example during the polymerization of the glue (carried out typically at 100-190° C.), since the air in the cavity  2  cannot exit and increases in volume, it exerts a pressure on the adhesive film  7  and the glue layer  8 , which are yielding. Swellings are thus created that tend to raise the chip and, in particular in presence of lack of uniformity, can cause tilting of the chip  1  and delaminating of the glue layer. 
     The consequence thereof is that the yield of the assembly process is low, even lower than 50%. 
     The same problem applies to other types of MEMS devices, having cavities closed by sealing layers of compliant material and/or bonded on areas of compliant material. 
     BRIEF SUMMARY 
     Some embodiments of the present disclosure provide a device and a method that overcome the drawbacks of the prior art. 
     According to some embodiments of the present disclosure, there are provided an electronic device and the manufacturing process thereof, as defined in claims  1  and  12 , respectively. 
     In practice, in the support, underneath the area of the chip where the cavities are formed, there is a depressed portion containing part of the glue. During the thermal treatments, the glue, being compliant, allows for expansion of the air in the cavities within the depressed area, thus preventing raising and/or tilting of the chip with respect to the substrate. Typically, the depressed portion comprises a recess formed by the surface layers of the support. In addition, spacer elements may extend within the glue layer so as to ensure the thickness of the swelling to be always smaller than the distance between the chip and the plane underlying the chip itself and to ensure planarity of the chip. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present disclosure, preferred embodiments are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  is a cross-section through a chip having cavities and bonded to a substrate; 
         FIG. 2  is a top plan view of the substrate of  FIG. 1 ; 
         FIG. 3  shows the chip of  FIG. 1  in case of tilting and delamination; 
         FIG. 4  shows a cross-section of the present device; 
         FIG. 5  is a top plan view of the device of  FIG. 4 ; 
         FIG. 6  shows a cross-section, taken along section line VI-VI of  FIG. 5 ; and 
         FIG. 7  shows an integrated chemical sensor for detecting odorous matters, incorporating the present device. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 4-6  show an electronic device  20  including a chip  21  bonded to a substrate  22  by a glue layer  40 . The chip  21  has cavities  23  extending from the rear surface of the chip  21  and delimiting membranes  24  at the bottom. If the electronic device  20  forms part of a chemical sensor, adsorbent layers (not shown) may be formed on the membranes  24  and may be of a material able to bind with the chemical substance to be detected, as described in detail in aforementioned patent application U.S. patent application Ser. No. 12/648,996. For example, the adsorbent layers can contain metal-porphyrins having an affinity with the chemical matters to be detected and form, together with the membranes  24 , a sensitive area  26 . For the rest, the chip  21  may comprise a circuitry area  25  including electronic components (shown schematically) so as to form, with the sensitive area  26 , a device for detecting chemical matters, for example, odorous chemical matters. 
     An adhesive film  30  covers the rear surface  21   a  of the chip  21  and closes one or more cavities  23  at the bottom. The adhesive film  30  may be a die-attach film, for example, of epoxy material having a thickness of 10-50 μm, laminated on the rear surface  21   a.    
     The substrate  22  comprises a base layer  33  having a surface  33   a  covered by a conductive layer  34  and a protective layer  35 , for instance of insulating material, arranged on top of the conductive layer. For example, the substrate  22  may be formed by a printed-circuit board, and in this case the base layer  33  may be a core layer, the conductive layer  34  may be a metal material layer, such as copper, and the protective layer  35  may be a solder-mask layer. The core layer  33  is of an organic material, for example BT (bismaleimide triazine), epoxy resin, FR-4 (Flame Retardant 4), LCP (Liquid Crystal Polymer), or polyimide. 
     In the example shown, part of the conductive layer  34  and part of the protective layer  35  are removed so as to form a recess  45  extending at least underneath the area of the cavity or cavities  23 , as shown in  FIG. 4 , and having an area smaller than the area of the chip  21 , in top plan view. In practice, here the conductive layer  34  is shaped so as to form, i.e., an ring-shaped region  34   a  and its width d e  (distance between the inner and the outer edges of the ring) is such that the lateral surface of the chip  21  falls, in top plan view, inside the ring-shaped region  34   a . In other words, the external edge of the ring-shaped region  34   a  delimits a greater area than the area of the main face of the chip, in top plan view. 
     In the embodiment of  FIG. 4 , moreover, the glue layer  40  incorporates spacer elements  41 , for instance balls of insulating or conductive material. For example, the spacer elements  41  may be of a polymeric material, such as polytetrafluoroethylene (PTFE), or glass, metal material, such as silver, and the like, and the glue may be of a polymeric material, such as an epoxy resin or a silicone material, or in any case of a softer material than the spacer elements  41 , the support  22 , and the chip  21 . 
     The spacer elements  41  preferably all have substantially the same preset thickness so as to ensure a plane rest for the chip  21 . 
     In particular, by designating by A the depth of the recess  45  (sum of the thicknesses of the conductive layer  34  and of the protective layer  35 ), by S the diameter or thickness of the spacer elements  41 , by C the expected thickness of the swelling that forms underneath the cavities  23  because of thermal effect, and by B the distance between the bottom surface  21   a  of the chip  21  and the surface  33   a  of the base layer  33  (neglecting the thickness of the adhesive film  30 ), the dimensions are chosen so that:
 
 B=A+S&gt;C   (1)
 
     Generally, with current manufacturing techniques, C is approximately 10% of the depth of the cavities  23 . With cavities  23  having a depth of 500 μm, C is thus normally comprised between 40 and 60 μm, and on average is approximately 50 μm. 
     In case of a printed-circuit board, with a thickness of the copper conductive layer  34  of 15-20 μm and a thickness of the protective layer  35  of 30-50 μm, the thickness A is typically comprised between 45 and 70 μm, for example 50 μm. 
     To satisfy Eq. (1), it is thus sufficient to use spacer elements  41  having a diameter of 25 μm. According to the application, materials, and dimensions of the various parts, the spacer elements  41  may, however, have typical thicknesses of 10, 15, 20, 25, 30, 50 μm or even 100 μm. 
     The recess  45  may be coupled to trenches  46  extending peripherally from the recess  45  throughout the width of the ring  34 . In practice, as may be seen in  FIG. 6 , in the area of the trenches  46  the conductive layer  34  is removed. For example, the trenches  46  may have a width of 100-200 μm and be arranged at a distance from one another of 300-500 μm. 
     In this way, during fabrication, after application of the glue on the chip  21  or on the substrate  20 , also because the glue  40  is soft, it is possible to cause any possible air trapped underneath the chip  21  to exit, thus eliminating a source of disturbance during bonding. In practice, the trenches  46  form venting channels. 
     The recess  45  and the trenches  46  may be formed while defining the conductive layer  34  and the protective layer  35 , with a standard photolithographic technique, in a simple and inexpensive way. Alternatively, they may be formed at the end of the manufacturing of the substrate  22 , prior to bonding, using a specific milling operation. In this latter case, it is possible to remove just part of the conductive layer  34  or, if the protective layer  35  is sufficiently deep, to remove only the protective layer  35 . 
     The device described herein has numerous advantages. 
     By virtue of a depressed area or recess underneath the chip  21 , at least in the area of the cavities  23 , the air can expand during the thermal cycles and form swellings without causing raising of the chip from the nominal position. In fact, the chip always rests on a surface of constant thickness, guaranteed by the spacer elements  41 . Any air possibly trapped therein can exit from the trenches  46 . 
     In this way, any tilting of the pads and delamination of the glue layer or of the film  30  are prevented, and a yield of the assembly process higher than 80% is obtained. 
     Moreover, the recess  45  forms a side delimitation of the area subject to deformation, ensuring a good structural and conformational stability of the finished device. 
     Manufacturing and bonding may be carried out without additional costs as compared to the device of  FIGS. 1-3  if the recess is formed while manufacturing the substrate  22 , by simply modifying the design of the structures, without requiring specific operations during production of the substrate or in the assembly process. 
     The performances of the device are not affected by the assembly. 
       FIG. 7  shows an integrated chemical sensor  50  for detecting odorous matters that may incorporate the device  20 . The chemical sensor  50  is formed, for example, as described in U.S. patent application Ser. No. 13/016,086 filed on Jan. 28, 2011, published as U.S. Application Publication No. 2011/0209524, and incorporated herein by reference in its entirety. The chemical sensor  50  comprises a casing formed by a base  51  and by a lid  52 , enclosing the printed-circuit board that forms the support  22 . 
     The lid  52  (represented in ghost view) has an input port  53  for introduction of gases to be analyzed and defines a channel  54  extending from the input port  53  to a suction fan  55 , in turn connected to an output port  56 . 
     The channel  54  extends on top of the chip  21 , bonded on the side of the support  22  facing the lid  52  so that the gases entering the channel  54  for the suction of the fan  55  lap the chip  21 , and the odorous matters to be recognized are captured in the sensitive area  26 . 
     The support  22 , on the side opposite the chip  21 , carries other components, such as, for example, a fan-control device  60 , coupled via conductors (not shown) to the fan  55 , and an auxiliary chip  61 , for example a controller with memory, a signal-processing circuit, or the like. In turn, the auxiliary chip  61  may be coupled to an external data-processing apparatus (not shown). 
     The base  51 , which is to couple with the lid  52  so as to enclose the support  22  in between, has an input  63  and an output  64  for coolant air. 
     Finally, it is clear that modifications and variations may be made to the device and the manufacturing process described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     In particular, the depressed area may be formed in a surface layer of the substrate, creating a local recess or depressed area where a glue layer can accumulate and which has a greater thickness than the neighboring areas so as to contain expansion and swelling of the air in the cavities. 
     The spacer elements  41  may have a shape other than the spherical shape; they may, for example, be spheroidal, even irregular, with projections, for example shaped as flakes, but in any case preferably able to ensure a constant distance from the underlying layer of the chip, for example as a result of a preferential lie position that may be achieved during application of the glue layer or by compression during application of the chip. 
     The conductive region  34  underneath the chip  21  may have a shape different than the annular shape shown; for example, it may be formed by portions of any shape set alongside one another, or else delimit a plurality of recesses, one for each cavity  23  of the chip  21  or may even be absent. 
     Finally, the chemical sensor may be made differently, for example for analysis of matters in a liquid, as described in U.S. patent application Ser. No. 13/170,058, which published as U.S. Application Publication No. 2011/0318840, is incorporated herein by reference in its entirety, and discloses a support that is not completely contained in the casing of the chemical sensor. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.