Abstract:
This method for producing a non-planar microelectronic component, especially a concave component, involves superposing a layer that contains an active flexible circuit above a cavity shaped according to the desired profile of said component, said cavity being formed in substrate; and applying a pressure difference either side of said layer thereby causing slumping of the flexible circuit into the cavity therefore causing the circuit to assume the shape of the cavity. Superposition of the flexible circuit and the cavity is realized by filling the cavity with a material capable of being selectively removed relative to the substrate and the flexible circuit; then fitting or forming the flexible circuit on the cavity thus filled; then forming at least one feedthrough to access the filled cavity; and by selectively etching the material that fills the cavity via at least one feedthrough in order to remove said material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 from French Patent Application No. 0959340 filed on Dec. 22, 2009 in the French Patent Office, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the production of curved, concave microelectronic components. It has particular applications in the field of image sensors. 
     DESCRIPTION OF THE PRIOR ART 
     Electronic imagers usually comprise a planar semiconductor image sensor made of silicon that uses CMOS or CCD technology and optics that form an image of the observed scene captured on the image sensor. 
     However, using a simple convex lens as an optic is unsatisfactory to the extent that the image formed by such a lens is not planar but spherical, a phenomenon that is referred to as “curvature of field”. In fact, the image projected by a focusing lens on a planar sensor is either sharp in the centre but not sharp at its edges or the other way round. This explains, in particular, the manufacture of complex optics formed by sets of lenses that have also undergone special-purpose surface treatments in order to shape the images they produce to match the flatness of the sensor. 
     Nevertheless, even the most complex optics currently available still introduce a certain number of both geometrical and chromatic aberrations which include barrel distortion and pincushion distortion, spherical aberrations (or so-called “diffuse light” aberrations), coma, astigmatism, vignetting, blooming, spurious light (reflection) and even chromatic aberrations. 
     Such aberrations have to be corrected at the time when images are actually formed by using bulky, complex optics and/or subsequently by implementing image processing algorithms which demand considerable computing power. Thus, the planar nature of sensors is the direct cause of aberrations and correcting these requires bulky, expensive lenses and powerful on-board computers in cameras and digital cameras. 
     One effective way of eliminating errors caused by curvature of field is to modify the shape of the image sensor so that it is substantially the same shape as the image formed by the optics. The ability to curve the sensor therefore makes it possible not only to correct aberrations but also to design affordable cameras and compact cameras that do not require significant computing power. 
     The attractiveness of designing curved sensors in the field of imaging is therefore readily apparent. 
       FIGS. 1 and 2  show a method for fabricating an imager with a concave sensor according to the prior art as described, for example, in document U.S. Pat. No. 7,390,687. 
     According to this method, a planar sensor  100  is initially produced on the surface of a substrate  102  in which a readout circuit  104  of said sensor  100  and connections  106  are also realized; the entire device is encapsulated in a package  108  capable of subsequently accommodating optics (not shown). Planar sensor  100  and substrate  102  are made thinner to give them sufficient flexibility to be curved. 
     This assembly is then mounted on a second substrate  110  in which a concave cavity  112  is formed, this cavity has the ultimately sought-after shape of the sensor  100 . A feedthrough  114  is also made from the rear face of substrate  110  to cavity  112 . 
     Cavity  112  is then depressurized. This creates a pressure difference that causes sensor  100  and substrate  102  to slump into cavity  112 . The latter thus assume the same shape as cavity  112  and substrate  102  is attached to cavity  112  thanks to the presence of a film of adhesive previously deposited in cavity  112 . 
     However, substrate  102  and sensor  100  are separately fitted on substrate  110  whereas cavity  112  is already formed. Before they are shaped to match the cavity, substrate  102  and sensor  100  therefore have no support. 
     In fact, it is difficult or even impossible to modify substrate  102  and sensor  100  once they have been fitted on cavity  112 . For example, it is difficult to add new elements such as a protective layer or microlenses to substrate  102  or to sensor  100 . In fact, modifying substrate  102  and sensor  100  necessarily involves applying pressure to them regardless whether such modification is mechanical (polishing, bonding, etc.), chemical (vacuum deposition, plasma etching, etc.) or physical (photolithography). Substrate  102  and sensor  100  therefore curve over cavity  112 , making it awkward to modify them using conventional methods. Furthermore, variation in the pressure difference applied to substrate  102  and sensor  100  causes them to vibrate, still making any modification awkward. 
     In order to overcome these drawbacks, there is a need to monitor the pressure difference to which they are subjected very precisely in order to keep them flat at all times, but this proves to be impossible without accurately knowing the pressure exerted by the methods. 
     Thus, the method according to the prior art described above does not make it possible to modify substrate  102  and sensor  100  once they have been fitted on cavity  112 . The latter have to be produced in their definitive state before they are fitted. In particular, they must be fitted while they are flexible because of their reduced thickness. This therefore imposes considerable constraints during the imager&#39;s fabrication process. 
     What is more, electronic circuits made of silicon are usually only flexible if they are less than 50 μm thick. With such thicknesses, they are awkward to grip and any manipulation error can result in undesirable creasing or twisting. In addition, in the example of the method explained above, substrate  102  and sensor  100  are attached to package  108  in order to facilitate handling them, thus improving ease of handling, but the package is an obstacle to subsequent modification of the imager once substrate  102  and sensor  100  have been shaped to match cavity  112 . 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to solve the above-mentioned problems by proposing an alternative method for producing a non-planar microelectronic component, especially a concave component, by using a subsequently shaped cavity and by applying a pressure difference that substantially reduces the stresses imposed by producing dishing and allows easy handling of the component at all times during its fabrication. 
     To achieve this, the object of the invention is a method for fabricating a non-planar microelectronic component, especially a concave component, that involves superposing a layer that includes an active flexible circuit above a cavity shaped to match the profile that one wishes to give said component, formed in a substrate, and applying a pressure difference either side of said layer, thereby causing the flexible circuit to slump into the cavity and hence assume the shape of the cavity. 
     According to the invention, the flexible circuit is superposed on the cavity by previously filling the cavity with a material capable of being selectively removed relative to the substrate and the flexible circuit; then fitting or forming the flexible circuit on the cavity thus filled; then forming at least one feedthrough to access the filled cavity; and by selectively etching the material that fills the cavity through at least one access feedthrough in order to remove said material. 
     In other words, once the cavity has been filled, the substrate that accommodates it has substantially the same mechanical properties as a solid substrate. Because of this, all the constraints associated with the cavity before the material that fills the cavity is etched disappear. In particular, it is possible to opt to produce the flexible circuit right on the substrate by using conventional methods, to fit the flexible circuit once it has been fabricated or to fit a thick block comprising the flexible circuit and then use classic methods in order to make this block thinner. It is even possible to modify the flexible circuit once it has been superposed on the substrate because it rests on a solid surface. 
     The term “flexible circuit” here is taken to mean a circuit that is capable of being deformed without this causing any alteration in its structure. Typically, this flexibility is a result of the thickness of the layer that contains the circuit and, because silicon is involved, the maximum thickness of this layer is 50 μm. 
     The term “concave” here is taken to mean a dish-shaped space. 
     For example, the flexible circuit consists of a conventional CMOS circuit thinned to a thickness of 2 to 50 μm making it possible to obtain curvature of the thinned substrate with a deflection of 10 to 200 μm at atmospheric pressure. 
     One or more feedthroughs can also be formed once the flexible circuit has been superposed on the cavity or the flexible circuit may comprise openings that allow access to the material that fills the cavity. 
     According to the particular embodiments of the invention, the method comprises one or more of the following aspects. 
     The material that fills the cavity is resin that is capable of being removed by an oxygen plasma. 
     The flexible circuit is bonded on the substrate by molecular bonding. In particular the flexible circuit is covered in a silicon oxide layer; the substrate is covered in a silicon oxide layer once the cavity has been formed and filled; the flexible circuit is fitted on the substrate, with the silicon oxide layer of the flexible circuit being deposited on the silicon oxide layer of the substrate; and the silicon oxide layers are heated. 
     Heating makes it possible, in particular, to strengthen the molecular bonding. 
     At least one feedthrough is blocked by taking care to subject the cavity to a first pressure, for example a vacuum or a reduced pressure, the pressure difference being obtained by then subjecting the assembly to a second pressure that exceeds the initial pressure. 
     According to one particular embodiment of the invention, the slumped flexible circuit is attached to the cavity. In particular, the bottom of the cavity is covered in a first material and the surface of the material that fills the cavity is covered in a second material, said first and second materials been capable of forming a eutectic system. Once the material that fills the cavity has been removed, the flexible circuit is attached to the cavity by bringing the component to the eutectic temperature of the eutectic system. 
     The microelectronic component is an image sensor. However, the invention is applicable to any type of microelectronic component that one wishes to curve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be made more readily understandable by reading the following description which is given merely by way of example and relates to the accompanying drawings in which identical references denote identical or analogous components and in which: 
         FIGS. 1 and 2  are schematic cross-sectional views of a back-side electronic imager during fabrication in accordance with a method according to the prior art. 
         FIGS. 3 to 19  are cross-sectional views of an electronic imager during various stages of its fabrication using the method according to the invention; 
         FIGS. 20 and 21  are, respectively, top views of a substrate in which several cavities are formed and a wafer comprising a plurality of image sensors for simultaneously fabricating several concave imagers; and 
         FIGS. 22 to 24  are cross-sectional views of a so-called grip substrate in one embodiment of a concave cavity. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A method for fabricating a back-side electronic imager having a concave image sensor is described below. 
     The method starts ( FIGS. 3A and 3B ) by forming a thick silicon substrate  12  on which a layer of SiO 2    14  having a thickness of 0.05 to 1 micrometer is deposited. Layer  14  will ultimately be used as a barrier layer when substrate  12  is thinned, as explained below in greater detail. 
     A silicon detection circuit  16  having a thickness of 2 to 5 μm is formed on layer  14  and comprises unitary detection elements  18  (phototransistors or photodiodes for example), or “pixels”, as well as the imager&#39;s electronic analogue and logic processing components. This therefore constitutes the active circuit in the sense of the invention. 
     Metallic interconnections  22  for reading and polarizing the components of circuit  16  are produced on circuit  16 , these interconnections are surrounded by a dielectric such as SiO that forms layer  20  which has a thickness of 1.5 to 4 μm. 
     Finally, a layer of silicon oxide SiO  24  having a thickness of 1 to 5 μm is deposited on dielectric layer  20  which contains the interconnections. Layer  24  is then planarized, for example by mechanical chemical polishing, in order to allow subsequent molecular bonding with another substrate referred to as the “grip substrate”, as explained below in greater detail. 
     For example, detection circuit  16  is a rectangular circuit having dimensions of 1 to 5 mm and comprises an array of pixels having a side dimension of 1 to 10 μm giving a total number of between 300,000 and 12,000,000 pixels. 
     Note that detection circuit  16  and dielectric layer  20  comprising the interconnections are conventional and define the functional layers of the imager&#39;s image sensor. Note that any type of sensor and technology (CMOS, CCD) can be implemented here, the choice being dictated by the intended application. Consequently, detection circuit  16  and dielectric layer  20  comprising the interconnections will not be explained in greater detail below. It should simply be noted that the thickness of elements  16  and  20  makes them flexible. 
     The method also includes forming a silicon substrate, referred to as the “grip” substrate  30 , on which a layer of resin  32  having a thickness of 10 to 100 μm, for example, is deposited ( FIG. 4 ). 
     A concave cavity  34  is then formed in resin  32  by using a convex die, the shape whereof is a “negative” of the desired shape for detection circuit  16  ( FIG. 5 ). The thickness of resin layer  32  is chosen to ensure that its residual thickness e once the cavity has been formed is as thin as possible, for instance around 1 μm. 
     Ion-beam etching is then performed on resin layer  32  with a resin/silicon selectivity substantially equal to 1. The shape of cavity  34  impressed in resin layer  32  is thus transferred to silicon layer  30  in order to produce a concave cavity  36  in layer  30  ( FIG. 6 ). If the etching selectivity equals 1, cavity  34  is precisely transferred to layer  30 . Alternatively, the shape of cavity  34  is transferred and amplified by selecting a selectivity less than 1 or attenuated by selecting a selectivity greater than 1. In such a case where a selectivity other than 1 is chosen, the shape of the die that forms cavity  34  in resin layer  32  and the thickness of this layer are adapted in order to obtain the desired final shape of detection circuit  16  in silicon layer  30 . 
     The residual resin is then removed from layer  30  and thermal oxidation of layer  30  is performed over thickness  38  from 50 nm to 0.5 micrometer, for example, in an oxidation furnace. 
     A metal layer  40  having a thickness of 100 nm to 1 micrometer is then deposited in cavity  36  ( FIG. 7 ). This metal is, for example, deposited over the entire surface area of layer  38  and then the metal deposited outside of cavity  36  is removed in an appropriate way, for instance by using masking followed by selective chemical etching relative to silicon oxide  38  or by mechanical chemical polishing. 
     The metal in layer  40  constitutes one of the components of a eutectic system that will make it possible to subsequently weld layer  24  to the bottom of a concave cavity as explained below in greater detail. The metals that form the eutectic system are selected so that the melting temperature of this system is less than the melting temperature of the fragile materials of the imager, especially the metal of the interconnections in layer  20  and the material of the microlenses that will subsequently be formed. For example, the metal of layer  40  is tin intended to be combined with gold in order to form a eutectic system having a melting temperature less than 240° C. 
     The method then continues ( FIG. 8 ) by depositing a polyimide type resin  42  so to fill cavity  36  followed by removal of the resin deposited outside cavity  36  in order to expose silicon oxide layer  38  over the flat portion of layer  30 . 
     The assembly is then annealed at a temperature at least equal to the maximum temperature to which said assembly will be subjected during the remainder of the method according to the invention in order to preventively degas resin  42  contained in cavity  36 . 
     A metal layer  26  having a thickness of 100 nm to 1 micrometer is deposited in filling resin  42  ( FIG. 9 ). The constituent metal of layer  26 , for the eutectic system in question, complements the metal that constitutes metal layer  40 . For example, if metal layer  26  is made of tin, metal layer  40  consists of gold. The gold/tin eutectic system obtained has a melting temperature of 200° C. to 240° C., depending on the relative proportions of these two materials. 
     A dielectric layer  37 , made of SiO for example, having a thickness of 0.1 to 2 micrometers is then deposited at a temperature that is less than the previous annealing temperature in order to seal cavity  36 . 
     The assembly that forms grip substrate  30  is then planarized, for instance by mechanical chemical polishing. 
     The assembly comprising detection circuit  16  and the assembly forming grip substrate  30  are then fitted one above the other so that silicon oxide layers  24  and  37  line up ( FIGS. 10A and 10B ). 
     Low-temperature molecular bonding of the layers is then performed in order to attach the two assemblies, this bonding is then strengthened by heating both assemblies to a temperature that is below the annealing temperature of resin  42 . A single silicon oxide layer  44  is obtained in this way. 
     Substrate  12  placed underneath detection circuit  16  is then thinned ( FIG. 11 ) in order to make it possible to obtain detection of electromagnetic radiation via the back side  46  of the imager, with SiO 2  layer  14  being used as a barrier layer during thinning of substrate  12 . 
     In the embodiment shown, substrate  12  is removed in its entirety. Detection circuit  16  having a thickness of 1 to 5 μm then captures photons in a manner that is known in itself from the prior art for back-side imagers. 
     Once substrate  12  has been thinned, the assembly is then flipped over ( FIG. 12 ). 
     The method then continues by finalizing the imager depending on the intended application. 
     For example, if detection in the visible domain is required, the method continues by using photoetching to deposit beads of red resin  48 , green resin  50  and blue resin  52  opposite the pixels of detection circuit  16  in order to define RGB detection mastering as is known in itself in the field of color detection. Microlenses  54  made of transparent resin are then made on resin beads  48 ,  50 ,  52  in order to focus the electromagnetic radiation on unitary detection elements  18  ( FIG. 3B ) of circuit  16  ( FIG. 13 ). Microlenses  54  are then hardened by annealing at a temperature of 200° C. to 240° C. 
     Microlenses  54  are then protected by depositing unhardened resin  56  over the entire surface area  46  ( FIG. 14 ), followed by removal of said resin away from the area of microlenses  54  in order to expose surfaces in which shafts will be formed as explained below. Deposition of silicon oxide at a low temperature below 150° C. is then performed in order to protect the resin deposited on microlenses  54 . 
     Note here that it is the solid nature of grip substrate  30  (cavity  36  being filled) that makes it possible to use conventional techniques to make substrate  12  thinner, to produce resin beads  48 ,  50 ,  52  by photolithography and to produce microlenses  54 . 
     Once the detection and optical part  57  of the imager has been finalized, the method continues by curving this part  57 . 
     More especially, drain shafts  58  are formed by photoetching through layer  14  of circuit  16 , layer  20  and layer  44  as far as cavity  36  and, more especially, resin  42  that fills it. Shafts  58  are formed around the periphery of the pixels of circuit  16  and have a diameter selected to allow subsequent stopping up without difficulty. For example, the diameter of shafts  58  is 400 nm to 2 micrometers. 
     Once shafts  58  are formed, selective etching of resin  42  is performed, especially using an oxygen plasma fed as far as said resin by shafts  58 . This oxygen plasma is known to interact with organic compounds such as resin or polyamide, thereby creating volatile compounds that escape via shafts  58 . Resin  42  that fills cavity  36  is then entirely removed from the cavity ( FIG. 16 ). 
     Once cavity  36  has emptied, the pressure inside it is identical to the external pressure thanks to shafts  58 . At this stage there is therefore no pressure difference between the cavity and the external environment regardless of the pressure of the latter. 
     The method then continues by depressurizing the assembly and depositing silicon oxide  60  in a vacuum so as to stop up shafts  58  ( FIGS. 17A and 17B ). 
     Once shafts  58  have been stopped up, the vacuum to which the assembly was subjected is broken, for example by re-establishing atmospheric pressure. This then causes slumping of the layers positioned above cavity  36  and causes them to assume the shape of cavity  36 , especially that of detection circuit  16  ( FIGS. 18A and 18B ) because of the pressure difference between the vacuum inside cavity  36  when emptied of resin  42  and the atmospheric pressure exerted on surface  46 . 
     The method then continues by removing the protection deposited on microlenses  54  (SiO 2  and resin layer) by using photolithography. The resin is selectively removed from microlenses  54  because the latter have been annealed and therefore hardened at a temperature of at least 200° C. whereas the protective resin has been annealed at a temperature of 150° C. ( FIG. 19 ). 
     The assembly is then raised to the melting temperature of the tin/gold eutectic system formed by layers  26  and  40 , so that oxide layer  24 , and consequently all the layers formed on top of the latter, are attached to the bottom of the cavity. 
     The formation of a concave sensor is described above. A plurality of concave sensors can, however, be produced simultaneously. For example, the grip substrate comprises a plurality of cavities, preferably circular cavities that are as close as possible to the desired spherical concave shape of the image sensors formed in the manner described above, as illustrated in  FIG. 20 . Accordingly, several detection circuit and their associated connections can be produced in a single element, as is shown in  FIG. 21 . 
       FIGS. 22 to 24  illustrate an alternative way of fabricating cavity  36 . In this variant, a hard mask  70  having a thickness of 100 nm to 1 micrometer is formed on the surface of substrate  30  ( FIG. 22 ). The hardness of mask  70  here is defined in terms of subsequent polishing which will not attack or only slightly attack said mask. 
     Mask  70  is then photoetched at the location where the cavity will be formed in substrate  30  ( FIG. 23 ), then mechanical chemical polishing is performed. This type of polishing naturally produces a concave surface similar to the desired shape of the cavity when it is applied over a large surface area ( FIG. 24 ). 
     The present invention is described in the context of fabricating an imager equipped with a concave sensor in the preceding text. It is readily apparent that the present invention can be applied to any type of electronic components that make it necessary to have a concave surface. Cavity  36 , for example is flat and slopes at a predetermined angle relative to the plane of the grip substrate so that a planar circuit that slopes relative to the substrate is obtained. It is also possible to obtain micromirrors that are tilted relative to a reference surface.