Patent Publication Number: US-6342406-B1

Title: Flip chip on glass image sensor package fabrication method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating an image sensor package. 
     2. Description of the Related Art 
     Image sensors are well known to those of skill in the art. An image sensor included an active area, which was responsive to electromagnetic radiation. The image sensor was used to fabricate an image sensor assembly. 
     In one prior art image sensor assembly, the image sensor was located within a housing, which supported a window. Radiation passed through the window and struck the active area of the image sensor, which responded to the radiation. 
     To form the image sensor assembly, the image sensor was mounted to a printed circuit mother board. After the image sensor was mounted, a housing was mounted around the image sensor and to the print circuit mother board. This housing provided a seal around the image sensor, while at the same time, supported a window above the image sensor. 
     As the art moves to smaller and lighter weight electronic devices, it becomes increasingly important that the size of the image sensor assembly used within these electronic devices is small. Disadvantageously, the conventional image sensor assembly described above required a housing to support the window and to seal the image sensor. However, this housing was relatively bulky and extended upwards from the printed circuit mother board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more. As a result, the image sensor assembly was relatively large. 
     In the event that moisture was trapped inside of the housing, defective operation or failure of the image sensor assembly was observed. More particularly, the moisture had a tendency to condense within the housing and on the interior surface of the window. Even if the housing later dried out, a stain was left on the window. In either event, electromagnetic radiation passing through the window was distorted or obstructed by either moisture condensation or stain, which resulted in defective operation or failure of the image sensor assembly. 
     For this reason, an important characteristic was the temperature at which condensation formed within the housing of the image sensor assembly, i.e., the dew point of the image sensor assembly. In particular, it was important to have a low dew point to insure satisfactory performance of the image sensor assembly over a broad range of temperatures. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an image sensor package includes an image sensor having an active area and bond pads on a front surface of the image sensor. A window of the image sensor package has an interior surface and an exterior surface opposite the interior surface. Electrically conductive interior traces are formed on the interior surface of the window. The window is mounted to the image sensor by flip chip bumps formed between the bond pads of the image sensor and the interior traces on the interior surface of window. 
     The window includes a central region aligned with the active area of the image sensor. A peripheral region of the window includes the interior traces, electrically conductive exterior traces formed on the exterior surface of the window, and electrically conductive vias electrically connecting the exterior traces to the interior traces. Electrically conductive pads are formed on the exterior traces. Electrically conductive interconnection balls are formed on the pads. 
     Advantageously, the interior traces, the vias, and the exterior traces are formed within the peripheral region of the window and the central region of the window remains unobstructed. Accordingly, radiation passing through the central region of the window, which strikes the active area of image sensor, is not obstructed or distorted. 
     The window has an area less than an area of the front surface of the image sensor. Advantageously, this allows the image sensor package to be the size of the image sensor, i.e., the image sensor package is chip size. Further, by avoiding the use of the housing of a prior art image sensor assembly, the image sensor package in accordance with the present invention can also be made relatively thin. Accordingly, the image sensor package is extremely well suited for use with miniature lightweight electronic devices, which require small, thin and lightweight image sensor assemblies. 
     The window, a bead between the window and the image sensor, and the image sensor define a sealed cavity in which the active area is located. Advantageously, the volume of the cavity is relatively small. By minimizing the volume of the cavity, the amount of any moisture trapped within the cavity is also minimized. This, in turn, essentially eliminates the possibility of moisture condensation on the interior surface of the window or on the active area of the image sensor. As a result, the image sensor package has a very low or nonexistent dew point. 
     In an alternative embodiment, the cavity is completely filled with a transparent encapsulant. Advantageously, by eliminating the prior art cavity between the active area and the window, the possibility of moisture condensation within the cavity is also eliminated. Accordingly, the image sensor package in accordance with this embodiment has no dew point. 
     Further, by using a transparent encapsulant having a refractive index similar to the refractive index of the window, the amount of reflected radiation is minimized. This improves the sensitivity of the image sensor package compared to prior art image sensor assemblies. 
     Also in accordance to the present invention, a method of forming an image sensor package includes forming an electrically conductive exterior trace on an exterior surface of a window. An electrically conductive interior trace is formed on an interior surface of the window, the interior trace being electrically connected to the exterior trace. The interior trace is aligned with a bond pad on a first surface of an image sensor. An electrically conductive bump is formed between the interior trace and the bond pad thus mounting the window to the image sensor. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of an image sensor package in accordance with the present invention. 
     FIG. 2 is a cross-sectional view of the image sensor package along the line II—II of FIG.  1 . 
     FIG. 3 is a cross-sectional view of an image sensor package in accordance with an alternative embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of a window during fabrication in accordance with one embodiment present invention. 
     FIGS. 5,  6  and  7  are cross-sectional views of the window of FIG. 4 at later stages of fabrication. 
     FIG. 8 is a cross-sectional view of a structure during the fabrication of a plurality of image sensor packages in accordance with one embodiment of the present invention. 
     FIGS. 9,  10  and  11  are cross-sectional views of the structure of FIG. 8 at further stages of fabrication. 
    
    
     In the following description, the same or similar elements are labeled with the same or similar reference numbers. 
     DETAILED DESCRIPTION 
     An image sensor package  100  (FIGS. 1,  2 ) includes an image sensor  102 , sometimes called an optical sensor, having an active area  104  and bond pads  106  on a front surface  102 F of image sensor  102 . A window  110  of image sensor package  100  has an interior surface  110 I and an exterior surface  110 E opposite interior surface  110 I. Electrically conductive interior traces  114  are formed on interior surface  110 I of window  110 . Window  110  is mounted to image sensor  102  by flip chip bumps  112  formed between bond pads  106  of image sensor  102  and interior traces  114  on interior surface  110 I of window  110 . 
     Window  110  includes a central region CR aligned with active area  104  of image sensor  102 . A peripheral region PR of window  110  includes interior traces  114 , exterior traces  116  formed on exterior surface  11 OE of window  110 , and vias  118  electrically connecting exterior traces  116  to interior traces  114 . Pads  120  are formed on exterior traces  116 . Interconnection balls  122  are formed on pads  122 . 
     Advantageously, interior traces  114 , vias  118 , exterior traces  116  are formed within peripheral region PR of window  110  and central region CR of window  110  remains unobstructed. Accordingly, radiation passing through central region CR of window  110 , which strikes active area  104  of image sensor  102 , is not obstructed or distorted. 
     Window  110  has an area less than an area of front surface  102 F of image sensor  102 . Advantageously, this allows image sensor package  100  to be the size of image sensor  102 , i.e., image sensor package  100  is chip size. 
     More particularly, FIG. 1 is a top plan view of an image sensor package  100  (hereinafter package  100 ) in accordance with the present invention. FIG. 2 is a cross-sectional view of package  100  along the line II—II of FIG.  1 . 
     Referring to FIGS. 1 and 2 together, package  100  includes an image sensor  102 . Image sensor  102  includes an active area  104  on a front, e.g., first, surface  102 F of image sensor  102 . Image sensor  102  further includes a plurality of bond pads  106  on front surface  102 F of image sensor  102 . 
     Generally, active area  104  is responsive to radiation, e.g., electromagnetic radiation, as is well known to those of skill in the art. For example, active area  104  is responsive to infrared radiation, ultraviolet light, and/or visible light. Illustratively, image sensor  102  is a CMOS image sensor device, a charge coupled device (CCD), a pyroelectric ceramic on CMOS device, or an erasable programmable read-only memory device (EPROM) although other image sensors are used in other embodiments. 
     Package  100  further includes a window  110  above active area  104 . Generally, window  110  is transparent to the radiation of interest, e.g., to the radiation to which active area  104  of image sensor  102  is responsive, as those of skill in the art will understand. Generally, the transmittance of window  110  is sufficient to allow the necessary minimum amount of radiation needed for the proper operation of image sensor  102  to pass through window  110 . 
     Illustratively, window  110  is borosilicate glass, but can be formed of other material depending upon the application. In one particular example, window  110  is fusion formed  1737  glass composition, designated as  1737 F, supplied by Corning Inc. of Corning, N.Y. For a detailed description see: (1) “material information” sheet for “code:  1737 F” issued 1/96, 5 pages total; (2) “product information” sheet for “display grade products”, revised 7/95, 4 pages total; and (3) “product information” sheet for “industrial grade products”, revised 3/97, 2 pages total, which are all by Corning Inc., Advanced Display Products, Corning, N.Y., 14831, which are all herein incorporated by reference in their entireties. Illustratively, window  110  has a thickness of 0.7 mm (0.027 in.) but has other thicknesses in other embodiments. 
     During use, radiation is directed at package  100 . This radiation passes through window  110 , through medium  140  and strikes active area  104 , which responds to the radiation as is well known to those of skill in the art. However, in an alternative embodiment, active area  104  of image sensor  102  transmits radiation such as electromagnetic radiation. For example, image sensor  102  is a light emitting diode (LED) microdisplay. In accordance with this embodiment, radiation transmitted by active area  104  passes through medium  140 , through window  110 , and emanates from package  100 . For simplicity, in the above and following discussions, active area  104  as a receiver of radiation is set forth. However, in light of this disclosure, those of skill in the art will recognize that generally active area  104  can be a receiver of radiation, a transmitter of radiation, or a transceiver, i.e., a transmitter and a receiver, of radiation. 
     Window  110  includes an interior, e.g., first, surface  110 I, which faces towards front surface  102 F of image sensor  102 . Window  110  further includes an exterior, e.g., second, surface  110 E, opposite interior surface  110 I. Exterior surface  110 E is exposed to the ambient environment. Sides  110 S of window  110  extend between interior surface  110 I and exterior surface  110 E. 
     Formed on interior surface  110 I are a plurality of electrically conductive interior traces  114 , which include a first interior trace  114 A. Interior traces  114  are illustrated in dashed lines in FIG. 1 for purposes of clarity. Bond pads  106  are electrically connected to corresponding interior traces  114  by corresponding electrically conductive bumps  112 , sometimes called flip chip bumps  112 . Illustratively, bumps  112  are: (1) stud bumps, i.e., gold balls; (2) electrically conductive epoxy paste; (3) electrically conductive epoxy film; or (4) solder. 
     Formed on exterior surface  110 E are a plurality of electrically conductive exterior traces  116 , which include a first exterior trace  116 A. Exterior traces  116  are electrically connected to corresponding interior traces  114  by corresponding electrically conductive vias  118 . Vias  118  extend through window  110  from interior surface  110 I to exterior surface  110 E. 
     Formed on exterior traces  116  are a plurality of electrically conductive pads  120 , which include a first pad  120 A. Formed on pads  120  are a plurality of electrically conductive interconnection balls  122 , e.g., solder, which include a first interconnection ball  122 A. Interconnection balls  122  are used to connect package  100  to a printed circuit mother board having an aperture aligned with active area  104 . 
     To illustrate, a first bond pad  106 A of the plurality of bond pads  106  is electrically and physically connected to interior trace  114 A by a first bump  112 A of the plurality of bumps  112 . Interior trace  114 A is electrically connected to exterior trace  116 A by a first via  118 A of the plurality of vias  118 . Formed on via  118 A is pad  120 A. Formed on pad  120 A is interconnection ball  122 A. 
     As set forth, an electrically conductive pathway between bond pad  106 A and interconnection ball  122 A is formed by bump  112 A, interior trace  114 A, via  118 A, exterior trace  116 A, and pad  120 A. The other bond pads  106 , interior traces  114 , vias  118 , exterior traces  116 , pads  120 , and interconnection balls  122  are electrically connected to one another in a similar fashion so are not discussed further to avoid detracting from the principals of the invention. 
     Although a particular electrically conductive pathway between bond pad  106 A and interconnection ball  122 A is described above, in light of this disclosure, it is understood that other electrically conductive pathways can be formed. For example, contact metallizations can be formed between the various electrical conductors, e.g., between bond pads  106  and bumps  112 , between bumps  112  and interior traces  114 , between exterior traces  116  and pads  120 , and/or between pads  120  and interconnection balls  122 . Alternatively, pads  120  are not formed and interconnection balls  122  are formed directly on exterior traces  116 . 
     In one embodiment, interior traces  114  are lands aligned horizontally in the view of FIG. 2 with vias  118 , bumps  112  and bond pads  106 . To illustrate, a second interior trace  114 B of the plurality of interior traces  114  is a land. Interior trace  114 B is aligned with a second via  118 B of the plurality of vias  118 , with a second bump  112 B of the plurality of bumps  112  and with a second bond pad  106 B of the plurality of bond pads  106 . 
     Alternatively, interior traces  114  are metallizations which extend along interior surface  110 I of window  110  such that vias  118  are not aligned with bumps  112  and bond pads  106 . To illustrate, interior trace  114 A extends horizontally in the view of FIG. 2 from bump  112 A (and bond pad  106 A) to via  118 A. Stated another way, via  118 A is offset from bump  112 A and interior trace  114 A extends along interior surface  110 I to electrically connect via  118 A to bump  112 A. 
     Similarly, exterior traces  116  are lands aligned horizontally in the view of FIG. 2 with vias  118 , pads  120  and interconnection balls  122 . To illustrate, exterior trace  116 A is a land. Exterior trace  116 A is aligned with via  118 A, with pad  120 A and with interconnection ball  122 A. 
     Alternatively, exterior traces  116  are metallizations which extend along exterior surface  110 E of window  110  such that vias  118  are not aligned with pads  120  and interconnection balls  122 . To illustrate, a second exterior trace  116 B of the plurality of exterior traces  116  extends horizontally in the view of FIG. 2 from second via  118 B to a second pad  120 B of the plurality of pads  120 . Stated another way, via  118 B is offset from pad  120 B and exterior trace  116 B extends along exterior surface  110 E to electrically connect via  118 B to pad  120 B. A second interconnection ball  122 B of the plurality of interconnection balls  122  is formed on pad  120 B. 
     As yet another alternative, interconnection balls  122  are distributed in an array format to form a ball grid array (BGA) type package. Alternatively, interconnection balls  122  are not formed, e.g., to form a metal land grid array (LGA) type package. Other electrically conductive pathway modifications will be obvious to those of skill in the art. 
     Of importance, window  110  includes a central region CR and a peripheral PR. Central region CR is aligned with and is above active area  104 . During use, radiation striking active area  104  has passed through central region CR. Accordingly, it is important that central region CR is free from defects and contamination, which could obstruct or distort radiation. 
     In this embodiment, peripheral region PR surrounds central region CR and is formed around a periphery of window  110  adjacent sides  110 S. Advantageously, interior traces  114 , vias  118 , and exterior traces  116  are formed within peripheral region PR of window  110 . Accordingly, bumps  112 , interior traces  114 , vias  118 , exterior traces  116 , pads  120 , and interconnection balls  122  do not obstruct or distort radiation striking active area  104 . 
     A bead  130  contacts the periphery of front surface  102 F of image sensor  102  adjacent sides  102 S of image sensor  102 . Bead  130  also contacts peripheral region PR of window  110  thus securing window  110  to image sensor  102 . In this embodiment, bead  130  contacts sides  110 S of window  110  and extends slightly under window  110  between interior surface  110 I of window  110  and front surface  102 F of image sensor  102 . In one embodiment, bead  130  encloses bond pads  106  and bumps  112 . In another embodiment, bead  130  extends slightly over and contacts exterior surface  110 E of window  110  directly adjacent sides  110 S. 
     Bead  130  enhances the reliability of package  100  by preventing the failure of bumps  112  and preventing the associated dismounting of window  110 . For example, bead  130  insures that window  110  does not become dismounted from image sensor  102  as a result of any differential thermal extension between window  110  and image sensor  102 . 
     Further, bead  130  forms a seal between peripheral region PR of window  110  and image sensor  102 . Thus, window  110 , bead  130 , and image sensor  102  define a cavity  132 , which is sealed. In particular, active area  104  is located within cavity  132 , which is sealed to protect active area  104  against external moisture, dust and contamination. 
     Generally, cavity  132  contains a medium  140 , which is transparent to the radiation of interest. In one embodiment, medium  140  is air. 
     Advantageously, the volume of cavity  132  is relatively small. By minimizing the volume of cavity  132 , the amount of any moisture trapped within cavity  132  is also minimized. This, in turn, essentially eliminates the possibility of moisture condensation on interior surface  110 I of window  110  or active area  104  of image sensor  102 . As a result, package  100  has a very low or nonexistent dew point. 
     Of importance, the area of window  110  in a plane parallel to front surface  102 F of image sensor  102  is less than the area of front surface  102 F in this same plane. This allows package  100  to be the size of image sensor  102 , i.e., package  100  is chip size. Bead  130  has outer sides  130 S coplanar with sides  102 S of image sensor  102 . 
     Further, package  100  is relatively thin compared to prior art image sensor assemblies. Package  100  is relatively thin since window  110  is mounted directly to image sensor  102  thus avoiding the use of a housing of the prior art. Illustratively, the thickness of package  100  is within the range of 1.0 mm to 1.5 mm (0.039 in. to 0.059 in.), e.g., is 1.2 mm (0.047 in.), although package  100  can have other thicknesses depending primarily upon the thickness of image sensor  102  and window  110 . Accordingly, package  100  is extremely well suited for use with miniature lightweight electronic devices, which require small, thin and lightweight image sensor assemblies. 
     FIG. 3 is a cross-sectional view of an image sensor package  300  (hereinafter package  300 ) in accordance with an alternative embodiment of the present invention. Package  300  of FIG. 3 is similar to package  100  of FIG.  2  and only the significant differences are discussed below. 
     Referring now to FIG. 3, in this embodiment, medium  140 A is a transparent encapsulant, not air. In one embodiment, the refractive index of medium  140 A is similar to the refractive index of window  110 . In this manner, the sensitivity of package  300  is improved compared to the prior art. 
     Recall that in the prior art, a housing was mounted around the image sensor and to the print circuit mother board. This housing supported a window above the image sensor. However, located between the window and the image sensor was air. Disadvantageously, air has a relatively low refractive index compared to the window. As those skilled in the art understand, as visible light or other electromagnetic radiation passes from a material having a high refractive index to a material having a low refractive index and vice versa, a significant percentage of the electromagnetic radiation is reflected. 
     To illustrate, for a window having a refractive index of 1.52, at each window/air interface, approximately 4 percent of the electromagnetic radiation is reflected. Since the electromagnetic radiation had to pass from air, through the window, and back through air to reach the active area of the image sensor in the prior art, a significant percentage of the electromagnetic radiation was reflected. This resulted in an overall loss of sensitivity of prior art image sensor assemblies. 
     In contrast, window  110  and medium  140 A of package  300  have a similar refractive index. Illustratively, the difference between the refractive index of window  110  and the refractive index of medium  140 A is such that the amount of radiation reflected at the interface of window  110  and medium  140 A is one percent or less. As an example, window  110  has a refractive index of 1.52 and medium  140 A has a refractive index of 1.40. Accordingly, the amount of reflected radiation is reduced compared to the prior art. This improves the sensitivity of package  300  compared to prior art image sensor assemblies. In one embodiment, package  300  is 13% more sensitive to electromagnetic radiation than prior art image sensor assemblies. 
     Further, instead of having air between the window and the active area of the image sensor as in the prior art, medium  140 A completely fills the region between window  110  and active area  104  and, more generally, fills cavity  132 . In other words, package  300  is a cavityless package, i.e., package  300  does not have a cavity between window  110  and active area  104 . Advantageously, by eliminating the prior art cavity between the active area and the window, the possibility of moisture condensation within the cavity is also eliminated. Accordingly, package  300  has no dew point. 
     In contrast, prior art image sensor assemblies had a dew point, i.e., a temperature at which condensation formed within the housing, which enclosed the image sensor and supported the window. In general, moisture had a tendency to condense within the housing and on the interior surface of the window. To avoid this condensation, it was important to avoid subjecting the image sensor assembly to extreme low temperatures. Disadvantageously, this limited the temperature range over which the image sensor assembly would satisfactorily perform. 
     Since package  300  does not have a dew point, package  300  operates satisfactorily over a broader range of temperatures and, more particularly, at lower temperatures than image sensor assemblies of the prior art. Further, since package  300  is a cavityless package, there is no possibility that moisture will leak into package  300 . Accordingly, the reliability of package  300  is greater than that of the prior art. 
     As shown in FIG. 3, interior trace  114 B and exterior trace  116 A are lands aligned with a via  118 C of the plurality of vias  118 . More particularly, interior trace  114 B and exterior trace  116 A are aligned horizontally in the view of FIG. 3 with via  118 C, bump  112 A, bond pad  106 A, pad  120 A and interconnection ball  122 A. 
     In contrast, interior trace  114 A and exterior trace  116 B are metallizations which extend along interior surface  110 I and exterior surface  110 E of window  110 , respectively, such that a via  118 D of the plurality of vias  118  is not aligned with either bump  112 B or pad  120 B. 
     FIG. 4 is a cross-sectional view of window  110  during fabrication in accordance with one embodiment of the present invention. As shown in FIG. 4, a mask  402 , e.g., photoresist, is formed on central region CR of exterior surface  110 E to cover and protect central region CR of exterior surface  110 E from subsequent processing described below. Similarly, a mask  404 , e.g., photoresist, is formed on central region CR of interior surface  110 I to cover and protect central region CR of interior surface  110 I from subsequent processing described below. 
     After masks  402 ,  404  are formed, apertures  410  are formed in peripheral region PR of window  110 . Illustratively, apertures  410  are formed, e.g., by mechanical or laser drilling, through window  110 . Apertures  410  extend from interior surface  110 I to exterior surface  110 E. 
     FIG. 5 is a cross-sectional view of window  110  of FIG. 4 at a later stage of fabrication. Referring now to FIG. 5, an electrically conductive layer  502 , hereinafter referred to as metal layer  502 , is formed above exterior surface  11 oE. More particularly, metal layer  502  is formed directly on peripheral region PR of exterior surface  110 E of window  110  and is also formed directly on mask  402 . Advantageously, mask  402  prevents metal layer  502  from directly contacting central region CR of exterior surface  110 E of window  110 . 
     Similarly, an electrically conductive layer  504 , hereinafter referred to as metal layer  504 , is formed above interior surface  110 I. More particularly, metal layer  504  is formed directly on peripheral region PR of interior surface  110 I of window  110  and is also formed directly on mask  404 . Advantageously, mask  404  prevents metal layer  504  from directly contacting central region CR of interior surface  110 I of window  110 . 
     Illustratively, metal layers  502 ,  504  are formed by evaporation or plating of an electrically conductive material such indium, nickel, e.g., electroless nickel, or gold, e.g., electroless gold. Although not illustrated in FIG. 5, in one embodiment, metal layer  502  and/or metal layer  504  are also formed on sides  110 S of window  110 . In another embodiment, metal layer  502  and metal layer  504  are formed simultaneously, i.e., metal layer  502  and metal layer  504  are a single metal layer, which encloses window  110  and masks  402 ,  404 . 
     Metal layer  502  and/or metal layer  504  fill apertures  410  (FIG. 4) forming electrically conductive vias  118 . 
     FIG. 6 is a cross-sectional view of window  110  of FIG. 5 at a later stage of fabrication. Referring now to FIG. 6, a mask  602 , e.g., photoresist, is formed on metal layer  502 . Mask  602  is formed to cover and protect a protected, e.g., first, region  502 M of metal layer  502 , which corresponds to exterior traces  116 . Mask  602  also covers and protects vias  118  at the interface of exterior surface  110 E, i.e., vias  118  terminate at exterior surface  110 E within protected region  502 M of metal layer  502 . An unprotected, e.g., second, region  502 E of metal layer  502  is not covered by mask  602 , and is therefore exposed and unprotected. 
     Similarly, a mask  604 , e.g., photoresist, is formed on metal layer  504 . Mask  604  is formed to cover and protect a protected, e.g., first, region  504 M of metal layer  504 , which corresponds to interior traces  114 . Mask  604  also covers and protects vias  118  at the interface of interior surface  110 I, i.e., vias  118  terminate at interior surface  110 I within protected region  504 M of metal layer  504 . An unprotected, e.g., second, region  504 E of metal layer  504  is not covered by mask  604 , and is therefore exposed and unprotected. 
     FIG. 7 is a cross-sectional view of window  110  of FIG. 6 at a later stage of fabrication. Referring now to FIGS. 6 and 7 together, unprotected regions  502 E,  504 E of metal layers  502 ,  504 , respectively, are removed, e.g., by etching. The remaining protected regions  502 M,  504 M of metal layers  502 ,  504  form exterior traces  116  and interior traces  114  on exterior surface  110 E and interior surface  110 I of window  110 , respectively. Of importance, central regions CR of exterior surface  110 E and interior surface  110 I of window  110  are protected by masks  402 ,  404  during etching of metal layers  502 ,  504 , respectively. After etching of metal layers  502 ,  504 , masks  402 ,  404 ,  602 ,  604 , are removed. 
     Referring again to FIG. 2, in one embodiment, pads  120  are formed on exterior traces  116 . Illustratively, to form pads  120 , window  110  is masked to only expose the portions of exterior traces  116  upon which pads  120  are to be formed. An electrically conductive material is then formed, e.g., by plating or evaporation, on these exposed portions of exterior traces  116  to form pads  120 . 
     Although fabrication of a single window  110  is illustrated in FIGS. 4,  5 ,  6 , and  7  and discussed above, in an alternative embodiment, a plurality of windows  110  are formed from a single sheet, e.g., of borosilicate glass, in a manner similar to that described above and this single sheet is singulated to form windows  110 . 
     FIG. 8 is a cross-sectional view of a structure  800  during the fabrication of a plurality of packages  100  in accordance with one embodiment of the present invention. Referring to FIG. 8, structure  800  includes an image sensor substrate  802  such as a silicon wafer. Image sensor substrate  802  includes a plurality of image sensors  102  integrally connected together. Image sensors  102  include active areas  104  formed on an upper, e.g., first, surface  802 U of image sensor substrate  602 . Image sensors  102  further include bond pads  106  on upper surface  802 U of image sensor substrate  802 . 
     To illustrate, a first image sensor  102 A of the plurality of image sensors  102  includes a first active area  104 A of the plurality of active areas  104 . Image sensor  102 A also includes a first bond pad  106 A. The other image sensors  102  include active areas  104  and bond pads  106  in a similar manner. 
     Image sensors  102  are integrally connected together in an array, e.g., a 2=2, 3=3 . . . or nxm array. Each of image sensors  102  is delineated by a singulation street  804 , which is located between adjacent image sensors  102 . For example, a first singulation street  804 A of the plurality of singulation streets  804  delineates first image sensor  102 A from a second image sensor  102 B of the plurality of image sensors  102 . The other image sensors  102  are similarly delineated from adjacent image sensors  102  by corresponding singulation streets  804 . 
     FIG. 9 is a cross-sectional view of structure  800  of FIG. 8 at a further stage of fabrication. Referring now to FIG. 9, windows  110  are mounted to image sensors  102  and above active areas  104  by bumps  112 . Stated another way, windows  110  are flip chip mounted to image sensors  102 . 
     To mount windows  110 , each window  110  is aligned with image sensor substrate  802  using any one of a number of alignment techniques, e.g., windows  110  are optically or mechanically aligned, and attached to image sensor substrate  802 . More particularly, interior traces  114  on interior surfaces  110 I of windows  110  are aligned with corresponding bond pads  106 . Bumps  112  are formed between interior traces  114  and bond pads  106  thus mounting windows  110  to corresponding image sensors  102 . 
     To illustrate, a first interior trace  114 A is formed on interior surface  110 I of a first window  110 A of the plurality of windows  110 . Interior trace  114 A is aligned with bond pad  106 A. Bump  112 A is formed between interior trace  114 A and bond pad  106 A. Bump  112 A physically and electrically connects interior trace  114 A to bond pad  106 A thus mounting window  110 A to image sensor  102 A. The other windows  110  are mounted to the other image sensors  102  in a similar manner. 
     Windows  110  are attached to image sensors  102  by bumps  112  using any one of a number of techniques. For example, solder bumps are formed on bond pads  106  of image sensors  102  or interior traces  114 , and these solder bumps are reflowed to form bumps  112  and to attach bond pads  106  to interior traces  114 . Alternatively, bond pads  106  of image sensors  102  are attached to interior traces  114  by bumps  112  formed by applying an electrically conductive epoxy paste or film to bond pads  106  or interior traces  114  and thermally or optically curing this electrically conductive epoxy paste or film. As a further alternative, bond pads  106  of image sensors  102  are attached to interior traces  114  by bumps  112  formed by thermal or thermosonic bonding of gold bumps formed on bond pads  106  or interior traces  114 . In light of this disclosure, those of skill in the art will understand that other methods of attaching windows  110  to image sensor substrate  802  can be used. 
     FIG. 10 is a cross-sectional view of structure  800  of FIG. 9 at a further stage of fabrication. Referring now to FIG. 10, a sealer  1010  is applied above singulation streets  804  and between windows  110 . Illustratively, sealer  1010  is a limited flow encapsulant such as Hysol  4323 ,  4450 , or  4451 . Sealer  1010  forms a seal between windows  110  and front surfaces  102 F of image sensors  102 . In one embodiment, sealer  1010  is drawn between interior surfaces  110 I of windows  110  and front surfaces  102 F of image sensors  102  due to capillary action. However, sealer  1010  is only drawn slightly towards active areas  104  such that sealer  1010  does not contact active areas  104 . 
     To illustrate, sealer  1010  is applied above singulation street  804 A and between window  110 A and a second window  110 B of the plurality windows  110 . Sealer  1010  forms a seal between image sensors  102 A,  102 B and windows l 1 A,  110 B, respectively. Sealer  1010  is applied between the other windows  110  in a similar manner. 
     In one embodiment, after sealer  1010  is applied, and cured if necessary, image sensor substrate  802  is singulated along singulation streets  804 . During this singulation of image sensor substrate  802 , sealer  1010  is also singulated into beads  130  (FIG.  2 ). Alternatively, image sensor substrate  802  is singulated after interconnection balls  122  are formed on corresponding pads  120  as discussed in greater detail below with reference to FIG.  11 . 
     FIG. 11 is a cross-sectional view of structure  800  of FIG. 10 at a further stage of fabrication. Referring now to FIG. 11, interconnection balls  122  are formed on corresponding pads  120 . To illustrate, interconnection ball  122 A is formed on pad  120 A. The other interconnection balls  122  are formed on the other pads  120  in a similar manner. 
     After formation of interconnection balls  122 , image sensor substrate  802  and sealer  1010  are singulated along singulation streets  804  resulting in a plurality of packages  100  (FIGS.  1  and  2 ). Alternatively, interconnection balls  122  are formed after image sensor substrate  802  and sealer  1010  are singulated along singulation streets  804 . 
     As described above in reference to FIGS. 8,  9 ,  10  and  11 , a plurality of packages  100  are fabricated simultaneously. However, in light of this disclosure, those of skill in the art will recognize that packages  100  can also be manufactured on an individual basis, if desired. 
     In accordance with an alternative embodiment, a plurality of packages  300  (FIG. 3) are simultaneously fabricated. Packages  300  are fabricated in a manner similar to the fabrication of packages  100  described above and illustrated in FIGS. 8,  9 ,  10  and  11  and only the significant differences are discussed below. 
     Referring again to FIG. 8, in accordance with this embodiment, before windows  110  are mounted, drops  806 , e.g., of a transparent liquid encapsulant, are applied to each active area  104  such that drops  806  are on active areas  104 . To illustrate, a first drop  806 A of the plurality of drops  806  is applied to, and is on, active area  104 A. In one embodiment, drop  806 A is applied by pin transfer of an appropriate bonding material, such as adhesive. More particularly, a pin is dipped in a bath of the bonding material, the pin is removed from the bath such that the tip of the pin is coated with the bonding material, and the tip of the pin is moved adjacent to active area  104 A. The bonding material is transferred from the tip of the pin to active area  104 A to form drop  806 A. However, in light of this disclosure, those of skill in the art will recognize that other techniques can be used to apply drop  806 A to active area  104 A. For example, drop  806 A is formed using a syringe and/or screen printing techniques. The other drops  806  are formed in a similar manner simultaneously or, alternatively, one at a time. 
     Of importance, drop  806 A has an apex  808 A near, or at, the horizontal center of drop  806 A. The other drops  806  have corresponding apexes  808  near the corresponding horizontal centers in a similar manner. 
     Referring now to FIGS. 8 and 9 together, windows  110  are pressed into corresponding drops  806  (drops  806  are illustrated in dashed lines in FIG. 9) during flip chip mounted of windows  110  to image sensors  102 . 
     To illustrate, first window  110 A is pressed into drop  806 A during flip chip mounted of window  110 A to image sensor  102 A. Of importance, since drop  806 A is formed to have an apex  808 A, window  110 A initially contact apex  808 A. As window  110 A is pressed into drop  806 A, drop  806 A is squeezed by window  110 A downwards, e.g., in a first direction, towards active area  104 A and outwards from apex  808 A. Squeezing drop  806 A in this manner avoids bubble formation, i.e., avoids entrapment of air under window  110 A. These bubbles would otherwise distort radiation striking active area  104 . 
     Further, drop  806 A has a volume sufficient to entirely cover active area  104 A of image sensor  102 A after window  110 A is pressed into place. In one embodiment, drop  806 A is formed within active area  104 A and is squeezed to entirely covers active area  104 A after window  110 A is pressed into place. If necessary, drop  806 A is cured. Drop  806 A forms medium  140 A (FIG.  3 ). Fabrication then proceeds in a manner similar to that illustrate in FIGS. 10 and 11 and described above resulting in the fabrication of a plurality of packages  300  (FIG.  3 ). 
     This application is related to Glenn et al., co-filed and commonly assigned U.S. patent application Ser. No. 09/713,848 entitled “FLIP CHIP ON GLASS IMAGE SENSOR PACKAGE,” which is herein incorporated by reference in its entirety. 
     The drawings and the forgoing description gave examples of the present invention. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of the invention is at least as broad as given by the following claims.