Patent Publication Number: US-2023136491-A1

Title: Apparatus and method for curved-surface image sensor

Description:
BACKGROUND 
     Light from a distant scene focused through a simple single-element lens forms a best-focus, undistorted, image surface that is curved, not flat, typically with a concave radius of curvature approximating a focal length of the lens. Use of such simple refractive lenses to image distant scenes on a flat surface, such as traditional flat film or flat image sensors, results in peripheral distortion and peripheral loss of focus. Similarly, reflecting telescopes may produce an image having an effective convex radius of curvature. 
     While the human eye has a concave curved retina surface that helps overcome this distortion, curving surfaces of film or image sensors to overcome this distortion has rarely been done. Since the 19th century, correction of this distortion has typically required complex, multi-element, lens systems. 
     Integrated circuits, including image-sensor integrated circuits, are typically fabricated as multiple integrated-circuit die on flat-surfaced wafers because of the way semiconductor fabrication equipment is designed. These wafers are typically fairly thick to avoid warpage during high-temperature processing. 
     Some wafers, and the integrated circuits on them, are thinned after high temperature processing to form transistors and other circuit elements on their “front-side” surfaces. Among thinned wafers are those bearing backside-illuminated image sensors and single-crystal solar cells for bendable solar panels. Thinning greatly increases flexibility of circuits. 
     A thinned CMV20000 image sensor (Austria Mikro Systeme International Aktiengesellschaft m.b.H., Premstätten, Austria) has previously been studied, see Curved detectors for astronomical applications: characterization results on different samples, by Simona Lombardo, et al., Applied Optics Vol. 58, No. 9/20 Mar. 2019 (Lombardo). The Lombardo article describes image sensors that have been thinned and deformed with both concave and convex curvature with  150  mm radius of curvature. 
     Similarly, U.S. Pat. No. 9,998,643 describes a method of forming a curved image sensor involving positioning a thinned image sensor over an adhesive-coated, curved-surface, substrate and using compressed gas or a mechanical plunger to force the image sensor onto the curved-surface substrate. During positioning and curving of the image sensor, it is possible to chip an edge of the image sensor particularly if the image sensor is clamped to the curved-surface substrate. It is also possible for air to reach the non-illuminated side of the image sensor causing the process to fail, or for an air bubble to remain under the image sensor circuit after air pressure is used to force the image sensor into the cavity of the curved-surface substrate. 
     SUMMARY 
     In an embodiment, a curved-surface image sensor assembly has a porous carrier having a concave surface with a thinned image sensor bonded by an adhesive to its concave surface of the porous carrier; the porous carrier is mounted into a water-resistant package. 
     In an embodiment, the sensor assembly is made by fabricating a thinned, flexible, image-sensor integrated circuit (IC) and applying adhesive to a non-illuminated side of the IC; positioning the IC over a concave surface of a porous carrier; applying vacuum through the porous carrier to suck the IC onto the concave surface of the porous carrier; and curing the adhesive to bond the IC to the concave surface of the porous carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross sectional view of a thinned image sensor integrated circuit with a layer of adhesive applied and positioned over a concave porous substrate. 
         FIG.  2    is a cross sectional view of a thinned image sensor integrated circuit with a layer of adhesive fixing the thinned image sensor integrated circuit to the concave porous substrate as achieved following application of vacuum to the concave porous substrate, the concave porous substrate shown positioned on a vacuum distribution fixture. 
         FIG.  3    is a cross sectional view of a thinned image sensor integrated circuit with a layer of adhesive fixing it to the concave porous substrate, and with the concave porous substrate packaged in a non-porous, waterproof, package ready for assembly into a camera. 
         FIG.  4    is a flowchart of a method of assembly of a curved-surface image sensor. 
         FIG.  5    is a cross sectional view of a thinned image sensor integrated circuit of an alternative embodiment with a ring of adhesive fixing the thinned image sensor integrated circuit to the concave porous substrate as achieved following application of vacuum to the concave porous substrate. 
         FIG.  6    is a cross sectional view of a thinned image sensor integrated circuit of an alternative embodiment where the non-porous package beneath the concave porous substrate has vacuum distribution channels and serves as the vacuum distribution fixture, illustrated with the thinned image sensor integrated circuit is positioned over the concave porous substrate but before vacuum is applied. 
         FIG.  7    is a cross sectional view of the embodiment of  FIG.  6    after application of vacuum, wirebonding, and attachment of a cover glass to the package. 
         FIG.  8    is a cross sectional view of an alternative embodiment where plated-through holes in the concave porous substrate and solder bumps on the image sensor integrated circuit replace wirebonding. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A curved-surface image-sensor assembly, adapted to be included within telescopes or cameras, is manufactured according to method  400  ( FIG.  4   ). With reference to  FIGS.  1  and  4   , in an embodiment, an image sensor integrated circuit (IC) is fabricated  402  as known in the art of CMOS and CCD image sensors. The image sensor integrated circuit (hereinafter image sensor)  102  is thinned  404  to make it flexible in a manner similar to thinning of backside-illuminated image sensors. Any necessary backside processing, such as addition of color filters and/or microlenses for backside illuminated image sensors, and addition of substrate metallization for frontside illuminated image sensors, is then performed  406 . 
     For purposes of this document, the side of the image sensor  102  onto which image light is expected to be focused is known as the illuminated side and the side of the image sensor  102  opposite the illuminated side is known as the non-illuminated side. 
     The image sensor is assembled into a curved-surface image sensor assembly  700 ,  800  following the method of  FIG.  4    with intermediate steps illustrated in  FIGS.  1 - 3   , and  5 - 6 . In an embodiment, a coating, or thin layer, of adhesive  104  is applied to the non-illuminated side of the image sensor  102 ; in some embodiments this non-illuminated side is the backside of frontside-illuminated image sensors, and in other embodiments this non-illuminated side is the frontside of backside-illuminated image sensors. The image sensor  102  is then placed  410  over a concave cavity  106  of a porous carrier  108 . 
     In some embodiments, adhesive  104  is a thin layer of a hot-melt adhesive. In other embodiments adhesive  104  is a thin layer of a thermosetting epoxy. 
     In some embodiments, porous carrier  108  is a plastic or ceramic carrier that has had a pattern of microholes laser-drilled through it so air can pass from its concave surface  110  to its bottom, flat, surface  112 . In some other embodiments, carrier  108  is formed of a porous ceramic. In other embodiments, carrier  108  is formed by powder metallurgy where metal powders are formed to shape, then fired at a temperature high enough that the metal particles are sintered together but low enough that the metal particles do not melt, thus leaving the carrier porous; aluminum, bronze, brass, copper, gold, silver, or steel particles may be sintered to form a porous carrier. Carriers formed by powder metallurgy have higher thermal conductivity than carriers formed of laser-drilled plastic and may be preferred for systems, like some infrared image sensors, that require active cooling. 
     In an embodiment, the porous or perforated carrier  108  and image sensor  102  are placed  410  on a vacuum chuck  202  ( FIG.  2   ) having vacuum distribution passages  204  that are coupled to a vacuum pump (not shown). Vacuum is applied  412  through the vacuum chuck  202  and porous carrier  108  to suck the image sensor  102  downward, deforming the image sensor  102  such that the image sensor  102 , adhesive  104 , and concave surface  110  are in contact. The adhesive is then cured  414 , in some embodiments by heating vacuum chuck  202 , to bond the image sensor  102  to the carrier  108 . 
     In embodiments, the carrier  108  with attached image sensor  102  is mounted  416  onto a flat surface in the interior of a non-porous water-resistant package  302  ( FIG.  3   ) having feedthroughs  304 . The image sensor  102  is then bonded  418 , in embodiments, by wirebonding bondpads of the image sensor  102  to feedthroughs  304  of the water-resistant package, and a transparent lid  306  may be sealed  420  over the image sensor. In an alternative embodiment, bonding of the image sensor is achieved by ball bonding and in another alternative embodiment bonding of the image sensor is performed with an anisotropic conductive polymer film. 
     In an alternative embodiment ( FIG.  5   ), instead of a full layer of adhesive  104 , one or more circular grooves  502  is formed on the concave surface  110 A of the porous carrier  108 A, the groove or grooves being slightly overfilled with adhesive. When the carrier  108 A and image sensor  102  are placed on the vacuum chuck  202 , vacuum is applied  412 , and adhesive melted or cured  414 , typically by heat, the image sensor  102  is thus bonded to the carrier  108 A. Remaining steps are as described with reference to  FIG.  4   . 
     In an alternative embodiment, after placing image sensor  102  with adhesive  104  on carrier  108 , carrier  108  is mounted in a water-resistant package  602  having formed within it vacuum passages  604 , passages  604  being accessible through a hole  606 . The vacuum is applied  412  through hole  606 , adhesive melted or cured  414 , typically by heat, bonded  418 , and sealed  420 . In this embodiment, hole  606  is sealed with a plug  610  ( FIG.  7   ). 
     In another alternative embodiment, solder bumps (not shown) are formed on bondpads of the non-illuminated surface of image sensor  102 C. Electrical conductors  802  are formed through porous carrier  108 C, in this embodiment carrier  108 C is formed of a non-conductive plastic or ceramic. After vacuum is applied  412  and adhesive melted or cured  414 , instead of wirebonding, the solder bumps are melted to bond the bondpads of the image sensor  102 C to electrical conductors  802  of the water-resistant package  302 C. 
     Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.