Abstract:
A process for manufacturing encapsulated optical sensors, including the steps of: forming a plurality of mutually spaced optical sensors in a wafer of semiconductor material; bonding a plate of transparent material to the wafer so as to seal the optical sensors; and dividing the wafer into a plurality of dies, each comprising an optical sensor and a respective portion of the plate.

Description:
PRIORITY CLAIM  
         [0001]    This application claims priority from European patent application No. 02425334.6, filed May 27, 2002, which is incorporated herein by reference.  
         TECHNICAL FIELD  
         [0002]    The present invention relates generally to a process for manufacturing an encapsulated optical sensor, and an encapsulated optical sensor manufactured using this process.  
         BACKGROUND OF THE INVENTION  
         [0003]    As is known in today&#39;s electronics industry, there is an increasingly marked trend towards integrating a number of devices in a single multifunctional system. For example, the field of mobile telephones is undergoing an extremely rapid evolution, and one of the most important lines of development envisages the integration of a miniaturized videocamera in a cellular phone.  
           [0004]    For this purpose, it is necessary to design optical sensors which, on the one hand, present overall dimensions that are as small as possible and are simple and inexpensive to manufacture, and which, on the other hand, will have optical characteristics that will not degrade the quality of the images that are detected. For this reason, the optical sensors must also be protected against contamination by external agents, such as dust and humidity, which could lead to irreparable damage. In particular, optical sensors are normally sealed inside hollow encapsulation structures. For greater clarity of exposition, we shall refer to FIG. 1, where a sensor  1  is illustrated, for example, an array CMOS sensor, formed in a die of semiconductor material obtained by cutting a wafer (not shown herein). The sensor  1  is encapsulated in a protective structure  2 , which comprises a base  3 , for example, formed by a lamina of pre-set thickness, and a supporting frame  4  of ceramic or plastic material, formed on the base  3  and having a depth greater than that of the sensor  1 . In greater detail, the supporting frame  4  and the base  3  define a cavity  5 , in which the sensor  1  is housed. In addition, the cavity  5  is sealed by means of a plate  6  of transparent material, preferably glass, which is bonded to the supporting frame  4 , at a distance from the sensor  1 . The plate  6  protects the sensor  1  from contaminating agents, without altering the optical properties of the incident light beams. Connection lines  7  enable the contacts of the sensor  1  to be brought outside the protective structure  2 , through the base  3 , said contacts being formed on the face of the sensor  1  facing the plate  6 .  
           [0005]    Known processes for encapsulating optical sensors present, however, various drawbacks.  
           [0006]    In the first place, the overall dimensions of the hollow protective structures are considerable as compared to the size of the optical sensor and cannot be reduced beyond a certain limit. In fact, the supporting frame  4  must be quite thick in order to enable bonding of the plate  6 . Furthermore, between the supporting frame  4  and the sensor  1 , it is necessary to provide a region of free-space, in which the connection lines  7  are formed. In practice, the protective structure  2  has a width and a length that may be even twice those of the sensor  1 . It is, therefore, evident that the degree of miniaturization of devices incorporating sensors encapsulated in hollow protective structures is accordingly limited.  
           [0007]    In the second place, it is very likely for impurities to penetrate inside the cavity  5  when the sensor  1  is being manufactured and mounted in the protective structure  2 . In fact, the sensor  1  can be encapsulated only after the initial wafer has been divided into individual dies. On the other hand, it is known that, during the steps of cutting of the wafers, a considerable amount of dust is created. Furthermore, even in the most well-controlled production areas, there is inevitably present some dust which, during the steps of conveying, handling, and assembly, may deposit on top of the sensor  1 . Given that, in this way, the quality of the images that can be detected may easily be impaired, many sensors have to be eliminated and, in practice, the overall output of the manufacturing process is other than optimal.  
           [0008]    A further drawback is due to the wide tolerances that are to be envisaged in the steps of manufacturing the protective structure  2  and encapsulating the sensor  1 . Since these tolerances render the process far from easily repeatable, successive processing steps must be optimized for each individual piece. In particular, in the manufacture of miniaturized video cameras, an optical assembly is coupled to the encapsulated sensor  1 . More specifically, the optical assembly is bonded to the plate  6 . On account of the intrinsic imprecision of the protective structures  2 , it is not sufficient to use exclusively mechanical centering references prearranged during the manufacture of the sensor  1  and of the protective structure  2 . Instead, it is necessary to carry out a laborious process for aligning the optical assembly with respect to the surface of the sensor  1 , so that the optical axis will be orthogonal to said surface, and, subsequently, for focusing the optical assembly. As mentioned above, this process must be carried out for each individual piece manufactured and has a marked incidence on the overall cost of manufacture.  
         SUMMARY OF THE INVENTION  
         [0009]    A purpose of an embodiment of the present invention is to provide an encapsulated optical device and a corresponding manufacturing process that will be free from the drawbacks described above.  
           [0010]    According to this embodiment of the present invention, a process for manufacturing encapsulated optical sensors is provided, as well as an encapsulated optical sensor manufactured using this process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a better understanding of the invention, there will now be described an embodiment provided purely by way of non-limiting example and with reference to the annexed drawings, in which:  
         [0012]    [0012]FIG. 1 is a cross section of a known encapsulated optical sensor;  
         [0013]    [0013]FIG. 2 is a perspective cross section through a wafer of semiconductor material in an initial step of the manufacturing process according to an embodiment of the present invention;  
         [0014]    [0014]FIGS. 3 and 4 are perspective cross sections, at an enlarged scale, of the wafer of FIG. 2 in successive manufacturing steps according to an embodiment of the invention;  
         [0015]    [0015]FIG. 5 is a cross section through the wafer of FIG. 4, in a subsequent manufacturing step according to an embodiment of the invention;  
         [0016]    [0016]FIG. 6 is a top plan view of the wafer of FIG. 4 according to an embodiment of the invention;  
         [0017]    [0017]FIG. 7 is a cross section through the wafer of FIG. 5, in a subsequent manufacturing step according to an embodiment of the invention;  
         [0018]    [0018]FIG. 8 is a top plan view of a transparent plate used in the present process according to an embodiment of the invention;  
         [0019]    [0019]FIG. 9 is a front view of the plate of FIG. 8, sectioned along a plane of trace IX-IX according to an embodiment of the invention;  
         [0020]    FIGS.  10 - 12  are cross sections through a composite wafer formed starting from the wafer of FIG. 7 and from the plate of FIG. 9, in successive manufacturing steps according to an embodiment of the invention;  
         [0021]    [0021]FIG. 13 is a cross section through a die obtained from the composite wafer of FIG. 12 according to an embodiment of the invention; and  
         [0022]    [0022]FIG. 14 is a top plan view of the die of FIG. 13 according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]    With reference to FIG. 2, a wafer  10  of semiconductor material, for instance, monocrystalline silicon, comprises a substrate  11 , preferably with a low doping level; in particular, the substrate  11  is suitable for manufacturing integrated circuits and/or sensors. Using a mask  12 , the substrate  11  is etched, and closed trenches  13  are dug, preferably having an annular shape. The trenches  13  delimit internally respective, low-conductivity, cylindrical regions or plugs  14  extending perpendicular to a front face  15  of the substrate  11  down to a pre-set depth, preferably of between 50 μm and 100 μm. In addition, the trenches  13  are distributed in pairs of parallel rows, along the edges of active areas  16  of pre-set width.  
         [0024]    Then (FIG. 3), the substrate  11  is doped in an oven using the mask  12  to form highly-doped, highly-conductive regions  17  around the trenches  13 . In greater detail, the highly conductive regions comprise inner annular portions  17   a , outer annular portions  17   b , and bottom portions  17   c . The trenches  13  are then filled with a dielectric material, for example silicon dioxide, so as to obtain insulating structures  19  (FIG. 4) arranged between respective inner annular regions  17   a  and outer annular regions  17   b . In practice, the plugs  14  have respective inner portions, which have a low doping level and are weakly conductive, and peripheral portions, which have a high doping level and are strongly conductive (the inner annular portions  17   a ). In addition, the plugs  14 , which are insulated at the sides from the rest of the substrate  11 , are designed to form through interconnections  20 , as is clarified further below. The mask  12  is then removed, and the wafer  10  is planarized.  
         [0025]    By means of known manufacturing steps, active and/or passive components are then formed inside the active areas  16 . In particular, CMOS photodetector arrays  21 , represented schematically in FIG. 5, are formed. Then, metallization lines  23  are formed for connecting the photodetector arrays  21  to respective through interconnections  20 . In FIG. 5, the metallization lines  23  are only partially illustrated for sake of simplicity. In practice, at this point, optical sensors  24  are formed in the wafer  10 , each of which comprises a photodetector array  21  housed in a respective active area  16 , and the through interconnections  20  are also formed adjacent to the active region  16  (see FIG. 6). In addition, adjacent optical sensors  24  are separated by cutting regions  25 .  
         [0026]    Then (FIG. 7), a transparent insulating layer  27 , for example, made of silicon nitride, silicon oxynitride, or silicon dioxide, is deposited on the face  12  of the substrate  11  and the metallization lines  23 . Then, on the insulating layer  27 , and more precisely above the optical sensors  24 , there are formed resin micro-lenses  28 , each of which overlies a respective photodetector  21 .  
         [0027]    As is illustrated in FIGS. 8 and 9, a transparent plate  30 , preferably made of glass and having a thickness of between 200 μm and 1 mm, is then prearranged for being bonded on the wafer  10 . In greater detail, a layer of bonding material, for example, dry resist, is deposited on a first face  30   a  of the plate  30  and is subsequently defined by means of a photolithographic process so as to form a bonding matrix  31 . The said bonding matrix  31  has a thickness of, for example, between 10 μm and 30 μm and is substantially shaped so as to overlap the separation regions  25  of the wafer  10 . A second face  30   b  of the plate  30 , opposite to the first face  30   a , is coated by an anti-reflecting layer  32 . The plate  30  is then turned upside down, aligned, and bonded to the wafer  10  so as to form a composite wafer and to seal the optical sensors  24 , as is illustrated in FIG. 10.  
         [0028]    Alternatively, the bonding matrix  31  can be formed directly on the wafer  10 , in particular, above the insulating layer  27 , by using a pantograph.  
         [0029]    Thanks to the thickness of the bonding matrix  31 , gaps  33  separate the plate  30  from the optical sensors  24 , so favoring proper transmission of the incident light. In addition, the anti-reflecting layer  32  is arranged at an optimal distance from the front face  15  of the wafer  10 , where the photodetectors  21  are located. This distance is substantially equal to the sum of the thicknesses of the bonding matrix  31  and the plate  30  and can reach up to approximately 2 mm.  
         [0030]    Then (FIG. 11), the wafer  10  is thinned out by grinding until the ends of the through interconnections  20  are uncovered. In this step, in particular, the bottom portions  17   c  of the highly conductive regions  17  are removed, and then the plugs  14  are electrically insulated from the rest of the substrate  11 . In greater detail, by means of the insulating structures  19 , the inner annular portions  17   a  are separated from the respective outer annular portions  17   b , embedded in the substrate  11 . Furthermore, the plugs  14  and, in particular, the inner annular portions  17   a  extend between the front face  15  and an opposite rear face  34  of the wafer  10 . The through interconnections  20  thus made have a very low resistance value, corresponding, for example, to 100 mΩ.  
         [0031]    Then, the rear face  34  of the wafer  10  is processed to prepare the individual optical sensors  24  for subsequent standard steps of assembly on respective cards (these steps will be performed after cutting of the wafer  10  and are not described here). In particular (FIG. 12), metal pads  37  are formed, which contact the through interconnections  20  and bumps  38  for electrical connection of the optical sensors  24  to the respective cards. Then, a protective resist layer  35  (solder resist) is deposited on the rear face  34  and defined by means of a photolithographic process, so as to uncover the pads  37  and bumps  38 . The protective layer  35  enables subsequent bonding on a card (not illustrated) and, at the same time, prevents any accidental contact of the substrate  11  with conductive paths made on the card.  
         [0032]    Using a diamond wheel, the wafer  10  and the plate  30  are then cut along the separation regions  25  (indicated by dashed lines in FIG. 12) and are divided into dies  40 . As illustrated in FIGS. 13 and 14, each die  40  comprises a respective portion  11 ′ of the substrate  11 , an optical sensor  24 , provided with respective micro-lenses  28  and, in addition, respective through interconnections  20  (i.e., the ones adjacent to the active area  16  in which the photodetector array  21  is made). Furthermore, the die  40  is equipped with a chip  41  and with a bonding frame  42 , obtained by cutting the plate  30  and the bonding matrix  31 , respectively. The chip  41  and the bonding frame  42  seal the optical sensor  24 , thus preventing impurities from depositing on the micro-lenses  28 . In addition, the portion of substrate  11 ′ housing the optical sensor  24  and the chip  41  have equal width L1 and equal length L2.  
         [0033]    From what has been illustrated above, it emerges clearly that the described embodiment of the invention presents numerous advantages.  
         [0034]    In the first place, the optical sensors according to the embodiment of the invention are already provided with respective protection structures and do not need to be further encapsulated within hollow protective structures. Consequently, on the one hand, a lower number of manufacturing and assembly steps is necessary and, on the other hand, the overall dimensions are minimized. In fact, the chip  41 , which seals the optical sensor  24 , has substantially the same area as the sensor itself and, in practice, occupies the minimum space necessary.  
         [0035]    The use of through interconnections, instead of the traditional connection lines, in turn enables the production of optical sensors which are compact and have small overall dimensions. Furthermore, the through interconnections can be formed in a simple and reliable way. In particular, the process described enables highly conductive through interconnections to be obtained in a low doping substrate, in which it is, therefore, possible to integrate other electronic components.  
         [0036]    Furthermore, the optical sensors are sealed directly during manufacture and, in particular, before cutting of the wafer in which they are formed. In this way, the possibility of contaminating agents damaging the sensors is drastically reduced. As a result, the number of faulty pieces is reduced, and the overall output of the manufacturing process is considerably improved.  
         [0037]    A further advantage is provided by the very high precision that can be achieved. In particular, the manufacturing steps used for the formation of optical sensors according to the described embodiment of the invention have extremely low tolerances, and hence the dimensional deviations between different sensors can be substantially neglected. Consequently, in the steps of mounting the optical assemblies, the sensors can be positioned in an optimal way, using only mechanical references. In other words, the precision with which each die can be manufactured is such that the use of mechanical references is sufficient for aligning and focusing the optical assembly with respect to the sensor. Hence, the need for carrying out purposely designed aligning and focusing steps for each individual device is overcome, thus simplifying considerably the manufacturing process and accordingly reducing the costs involved.  
         [0038]    It is to be noted that the plate  30  advantageously functions as a support for the wafer  10  during the grinding step, as well as during the subsequent steps. Without the above support, in fact, the wafer  10  might be too thin for it to be able to undergo further machining processes.  
         [0039]    Finally, the thickness of the bonding matrix  31  can be easily controlled in order to position the plate  30  in such a way that the anti-reflecting layer  32  is at an optimal distance from the optical sensor  24 .  
         [0040]    Furthermore, the dies  40  (FIGS. 13 and 14) can be formed into an integrated circuit, which can be incorporated into an electronic system such as a digital camera or other image-capture system.  
         [0041]    Finally, it emerges clearly that numerous modifications and variations can be made to the sensor device described herein, without thereby departing from the spirit and scope of the present invention.  
         [0042]    In particular, the through interconnections could be obtained following a process different from the one described herein. Furthermore, the bonding matrix can be formed by applying the silk-screen printing process, using materials different from dry resist, such as vitreous pastes or epoxy resins.