Abstract:
An electronic module adapted to sense light and configured to minimize the entry of stray light into the module. The module includes a housing having an opening through which light enters the housing, a first substrate coupled to the housing, a second substrate on the first substrate opposite the housing, and a chip on the second substrate. The first substrate defines a window aligned with the housing so that light traveling through the housing also passes through window. The second substrate defines an opening aligned with the window, and the chip is located over the opening in the second substrate so that a light-sensing element on the chip senses light passing through the opening. The module is equipped with features that prevent light from entering the module through the second substrate, the first substrate, and between the chip and second substrate.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to electronic devices. More particularly, this invention relates to reducing the amount of stray light that can enter an optical sensing module. 
   2. Description of the Related Art 
   A variety of applications exist and continue to be developed for electronic devices that operate on the basis of optics. For example, image sensors have been developed for use in automotive applications to sense the presence of vehicle occupants and objects in the vicinity of a vehicle. An example of such a sensor is represented in  FIG. 1 , which depicts a module  10  comprising a lens assembly  12  containing a pair of lenses  14 , a housing  16  in which the lens assembly  12  is installed, a glass substrate  18  to which the housing  16  is affixed, and an imager chip  20  with one or more appropriate sensing elements and circuitry for sensing light that has passed through the lenses  14  and the glass substrate  18 . Photons impinging the imager chip  20  are converted by the sensing elements into charge carriers (i.e. electron/hole pairs), so that light is electrically detected. As known in the art, the sensing elements may be individual detectors or arrays of detectors, as in CMOS or CCD imager arrays. While two lenses  14  are used in the module  10 , it is foreseeable that fewer or more lenses could be employed in optical path, and a lens may not be required for certain imaging applications. 
   The chip  20  is physically attached to a chip carrier  22  with electrically-conductive (e.g., solder or conductive adhesive or Au/Cu stud bump) connections  24 , which also electrically connect the chip circuitry to conductors  26  on the chip carrier  22 . The chip carrier  22  can be a flexible substrate laminated to the glass substrate  18 . As an example, the chip carrier  22  can be a flexible circuit (also referred to as a flex circuit), which as known in the art is a flexible substrate of an electrically insulating material such as a polyimide or polyester film, often in the form of a flat cable, with patterned conductors (signal lines) along its length. The chip  20  is underfilled on the chip carrier  22  with an underfill material  28  to promote the reliability of the connections  24 . The underfill material  28  also fills a gap  30  between the chip  20  and glass substrate  18 . Because light sensed by the sensing elements on the chip  20  must pass through the underfill material  28  and gap  30 , the underfill material  28  is formed of an optically-matching material, preferably an adhesive, that fully fills the gap  30  without voids and allows unimpeded passage of light. 
   For use in its application, the module  10  may be used without any enclosure, or placed in an enclosure that minimizes the amount of light leaking in from the environment. From  FIG. 1 , it can be appreciated that light can enter the module  10  through the glass substrate  18  and the optically-matching underfill material  28 , as well as the chip carrier  22  if formed of conventional flex circuit materials. Infrared light can also enter through the chip  20  if formed of a material such as silicon. For modules ultimately housed in an enclosure, it is advantageous to be able to test the modules as they are produced, prior to assembly into enclosures, since their enclosures may or may not be light-impermeable. Modules used alone without an enclosure must be capable themselves of preventing stray light from being detected by the enclosed imager chip  20 . 
   In view of the above, it can be appreciated that it would be desirable to reduce the amount of light that can enter an optical sensing module at the module level. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to an electronic module adapted to sense light, such as an optical sensing module, that reduces and preferably substantially eliminates the entry of stray light into the module. 
   An electronic module in accordance with the present invention generally includes a housing having an opening through which light enters the housing, a first substrate coupled to the housing, a second substrate on a surface of the first substrate opposite the housing, and an imager chip disposed on the second substrate. Optionally, one or more lenses may be disposed in the housing so that light entering the opening of the housing passes through the lenses. The first substrate defines a window formed of a material that is at least semitransparent to light having a predetermined wavelength or range of wavelengths (e.g., visible light, near-infrared light, infrared light, etc.), and is aligned with the housing so that light passing through the housing also passes through the window. The second substrate comprises electrical conductors in a material that may be semitransparent to light. The second substrate defines an opening therein that is aligned with the second opening of the housing so that light passing through the housing and the window of the first substrate also passes through the opening. The chip is disposed on the second substrate so as to be located over the opening therein, and so that one or more light-sensing elements on the chip senses light passing through the opening in the second substrate. Electrically-conductive connections electrically connect the light-sensing elements to the electrical conductors on the second substrate. Finally, and according to a preferred aspect of the invention, an opaque layer surrounds the opening of the second substrate and coincides with an edge of the opening to prevent stray light from entering the module and being absorbed by the imager chip. In addition, the module is provided with components for preventing light from entering the module through the first substrate and between the chip and second substrate. 
   As with previous electronic devices that operate on the basis of optics (e.g., the module  10  of  FIG. 1 ), the electronic module of the present invention preferably is capable of capturing available photons and converting the photons into charge carriers so that light is electrically detected. As such, a preferred aspect of this invention is that, to the extent possible, the module does not allow light (visible, infrared, etc.) that enters the module to escape and does not allow stray light (visible, infrared, etc.) within the surrounding environment to enter the module and be detected, i.e., converted to charge carriers. According to the invention as described above, possible sources of stray light are identified and eliminated to a significant degree, such that the only path of significance for photons of any detectable wavelength to arrive at and be detected is through the path through the opening in the housing, the first substrate, and the opening in the second substrate. 
   It should be understood that the term “light” as used herein refers to photons of any wavelength that can be detected by the sensing elements contained in the module. As such, the module of this invention can be adapted for detecting visible light (wavelengths of about 400 to 700 nm), near-infrared light (wavelengths of greater than 700 nm up to about 1100 nm), or light of even greater wavelengths. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  represents an electronic optical sensing module in accordance with the prior art. 
       FIGS. 2 and 3  show electronic optical sensing modules in accordance with different embodiments of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  represent optical sensing modules  110  and  210  of the type suitable for use in a variety of applications. Notable examples include automotive applications for sensing the presence of vehicle occupants and objects in the vicinity of a vehicle. For this purpose, each module  110  and  210  is equipped with light-sensing elements (not shown) carried on a chip  120 . The particular configuration and operation of these elements is not the subject matter of the present invention, such that any type of light-sensing element can be chosen and used according to the particular application for the module  110  and  210 . For example, the sensing elements may be CMOS sensors or common CCD sensors, as well as non-silicon based sensors. Furthermore, while light sensing is of primary interest, the teachings of the invention can be extended to the use of emissive elements (light sources), for example, inorganic or organic light emitting diodes (LEDs) or arrays of LEDs. Finally, though the term “light” is used in the sense of radiation within and near the visible spectrum, most notably visible light and infrared light, it is foreseeable that radiation well outside the visible spectrum may be of interest, and the benefits of this invention could be extended to such applications. 
   Similar to the module  10  of  FIG. 1 , the module  110  is represented in  FIG. 2  as comprising a lens assembly  112  containing a pair of lenses  114 , a housing  116  in which the lens assembly  112  is installed, a substrate  118  to which the housing  116  is bonded, and an imager chip  120  with appropriate elements and circuitry for sensing light that has passed through the lenses  114  and substrate  118 . Suitable materials for the substrate  118  are general glass materials, such as Corning Code 1737F, which are essentially transparent to light in the visible and near-infrared wavelengths. In this manner, the substrate  118  defines a window that is aligned with the housing  116  so that light traveling through the housing  116  and lenses  114  also passes through the window. The housing  116  is opaque to the light wavelengths of interest, but otherwise can be of any suitable construction, including molded or machined plastic or metal. The housing  116  has a generally annular or tubular-shape by which an internal passage and oppositely-disposed openings  132  and  134  are defined. The lens assembly  112  can be secured within the internal passage of the housing  116  by any suitable means, such as complementary threads on the lens assembly  112  and housing  116 . The type, design and number of the lenses  114  will depend on the particular application, but generally function to focus light passing through the interior of the housing  116  between its openings  132  and  134 . As with the housing  116 , the lens assembly  112  can be formed of any suitable material, preferably with a coefficient of thermal expansion compatible with that of the housing  116  to avoid or minimize changes in the focal length of the optical system, which would result in blurring of data or images in response to changes in ambient temperature. 
   In  FIG. 2 , a chip carrier  122  in the form of a flexible substrate is laminated to a surface of the glass substrate  118  opposite the housing  116 . The portion of the housing  116  defining the opening  134  is represented as abutting and bonded to a surface of the chip carrier  122 . The chip  120  is physically attached to an opposite surface of the chip carrier  122  with electrically-conductive connections  124 , which also electrically connect the chip circuitry to conductors  126  on the chip carrier  122 . Connection to the conductors  126  can be through bond pads formed on the carrier  122  or portions of the conductors  126  defined by a cover layer or photoimageable mask on the carrier  122 . According to conventional flip-chip technology, the connections  124  are preferably reflowed solder bump joints spaced along the perimeter of the chip  120 , individually interconnecting the conductors  126  with bond pads on the chip  120 . It is envisioned that other contact technologies, including, for example gold stud bumps, may be similarly employed. As a flexible substrate (flex circuit), the chip carrier  122  comprises an electrically-insulating polymer material such as a polyimide or polyester film, such as in the form of a flat cable, with the conductors  126  serving as circuitry traces for the chip  120  as well as any other surface-mount components (not shown) on the chip carrier  122 . While a flexible substrate is a preferred configuration for the chip carrier  122 , other suitable substrates could be used. Polymer materials suitable for the carrier  122  are generally semitransparent, such that light is able to pass through the carrier  122  to a degree that the sensitivity of the module  110  would be impaired. As such, the chip  120  is mounted over an opening  136  in the chip carrier  122 , so that light passing through the housing  116 , lenses  114  and glass substrate  118  is able to freely pass through the carrier  122  and impinge the light-sensing elements on the chip  120 . 
   As seen in  FIG. 2 , the chip  120  is underfilled on the chip carrier  122  with a material  128  that also completely fills a gap  130  between the chip  120  and the glass substrate  118 . As a result, the material  128  must be capable of providing optical matching as well as have flow properties that ensure complete underfilling of the chip  120  and filling of the gap  130 . Suitable compositions for this purpose include unfilled epoxy. To promote the reliability of the solder connections  124  in accordance with conventional flip-chip technology, the fill material  128  completely envelopes the solder connections  124  to fill gaps between the connections  124 , and forms a fillet that extends onto the sides of the chip  122 . Because it fills the gap  130  through which light must pass to reach the chip  120 , the fill material  128  is transparent to the light wavelengths to be sensed by the chip  120 . 
   Because of their translucency/transparency, the glass substrate  118 , chip carrier  122 , and fill material  128  would permit stray light to enter the module  110 , reducing its sensitivity. Furthermore, infrared light can enter through the chip  120  if formed of a conventional semiconductor material such as silicon. As such, the module  110  of this invention is adapted to inhibit light within the surrounding environment from entering the module  110  through unintended paths. In particular, the module  110  is configured to inhibit stray light from entering the module  110  through the glass substrate  118 , chip carrier  122 , fillet of the fill material  128 , and chip  120 . To obstruct light passing through its semitransparent material, the chip carrier  122  is equipped with a layer that serves as an opaque shielding layer  138 . A preferred shielding layer  138  is a metallic, preferably copper, conductor underlayer deposited or laminated on the polymer substrate of the chip carrier  122 . Alternatively, the shielding layer  138  can be formed of an electrically-insulating opaque material, e.g., a coating of a dark organic ink, and/or can be applied using a variety of deposition methods. The shielding layer  138  is separate and in addition to the conductors  126  that provide the electrical signal lines through the carrier  122  for the chip  120 . In addition to its light-blocking role, the shielding layer  138  in the form of a conductor underlayer can also serve as a ground layer for the chip carrier  122  as a result of being electrically conductive. As seen in  FIG. 2 , the shielding layer  138  surrounds the opening  136  in the carrier  122  and coincides with the edge of the opening  136  to obstruct light that would otherwise enter the module  110  through the chip carrier  122 . 
   To block light from entering the module  110  through the fill material  128  surrounding the perimeter of the chip  120  as an underfill, an opaque body  140  is shown surrounding the chip  120  to obstruct light paths that include gaps between the connections  124 , chip  120  and carrier  122 , and the fill material  128 . The body  140  is also shown as being deposited over the chip  120  to prevent the entry of infrared radiation through the chip  120 . Suitable materials for the body  140  include polymeric adhesives used to overmold electronic devices, such as those known in the art as glob top encapsulants. The body  140  preferably contains black pigment to block light from passing through it, and may contain additional fillers (e.g., silica and alumina) to adjust its mechanical properties and further enhance its light-blocking function. 
   Finally,  FIG. 2  shows the housing  116  as being configured so that the glass substrate  118  is entirely accommodated within its opening  134 , such that the portion of the housing  116  defining the opening  134  blocks light that would otherwise enter through the edge of the substrate  118 . As seen in  FIG. 2 , the housing  116  abuts the shielding layer  138 , thereby forming an interface that is substantially impermeable to light. If additional light obstruction is necessary, an opaque coating (not shown) can be applied to the surfaces of the substrate  118  along and adjacent its peripheral edges to block light from entering via the edge. 
   The embodiment of  FIG. 3  differs from that of  FIG. 2  primarily by packaging the optical sensing module  210  in a BGA (ball grid array) format. For convenience, components of the module  210  are identified in  FIG. 3  by the same reference numbers used to identify functionally equivalent components of  FIG. 2 . With this approach, the module  210  is mounted to a motherboard  142  (e.g., a printed circuit board or other suitable substrate) with a plurality of reflowed solder connections  144  that provide electrical connection between conductors (not shown) on the motherboard  142  and the conductors  126  of the chip carrier  122 , such as through openings or plated through-holes (not shown) on the carrier  122 . The body  140  is dispensed after the module  210  is mounted to the motherboard  142 , and in this manner can also serve as an underfill to improve the mechanical reliability of the solder connections  144  and the assembly as a whole. The body  140  is also shown as being between the chip  120  and motherboard  142  to prevent the entry of infrared radiation through the chip  120 . Under some circumstances in which the motherboard  142  is opaque to the wavelengths of interest, the motherboard  142  may sufficiently obstruct light that would otherwise pass through the gaps between the connections  124  and  144 , the chip  120  and carrier  122 , and the fill material  128 , such that the body  140  can be eliminated. 
   While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the modules  110  and  210  could be configured differently from the embodiments shown, and materials could be used other than those noted. Furthermore, individual or arrays of light emissive elements (e.g., LEDs) could be enclosed in accordance with the invention, using functionally-identical components and light-blocking features as described herein. Accordingly, the scope of the invention is to be limited only by the following claims.