Abstract:
A photo-sensing device package and the method of packaging such device is provided. The package includes an assembly portion having a substrate formed of a material substantially transparent to light within a predetermined range of wavelengths; a sensing portion including at least one photo-sensing die photo-electronically transducing light within the predetermined range of wavelengths; and, a plurality of first solder joints joining the sensing and assembly portions. The assembly portion is formed with at least a first metal layer disposed on the substrate about a front surface region thereof; and, at least one passivation layer formed to extend over the first metal layer. The passivation layer is patterned to define a plurality of first and second access openings which respectively describe on the first metal layer a plurality of first and second solder bump pads, each of which is interconnected to at least one of the second solder bump pads. The sensing portion&#39;s photo-sensing die is positioned with its photo-sensing area opposing the front surface region of the assembly portion&#39;s substrate, and has formed thereon a plurality of solder bump pads electrically coupled to the photo-sensing area. Each of the first solder joints extends between one of the sensing portion&#39;s solder bump pads and one of the assembly portion&#39;s first solder bump pads.

Description:
This application claims benefit of 60/507,100 filed Oct. 1, 2003. 

   PRIOR ART 
   Photo-sensing semiconductor devices have been typically mounted in ceramic packages.  FIG. 1  shows a schematic cross-section view of a ceramic leadless chip carrier (CLCC), which is the most popular package format for photosensing devices. As shown in the figure, a photo sensing semiconductor die is mounted face-up on a ceramic substrate  4  by using epoxy or the like inside of an enclosure that is covered by a glass lid  6 . Wire bonding  8  is typically used to connect the photo-sensing die  2  to the ceramic substrate  4 . Solderable pads  10  are provided on the bottom of ceramic substrate  4  to connect the package to a circuit board. 
   Perhaps the most critical drawback of this package is that it is very expensive. Another drawback is that the package size is not small enough for some hand-held applications such as cameras in cellular phones where small size and light weight are essential features. Another drawback is that the package&#39;s configuration does not allow very precise placement of the photo-sensing device in relation, for instance, to the focal plane of a lens because, among other things, the photo-sensing die is mounted with epoxy or the like, and the package itself is mounted with solder paste. 
   U.S. Pat. No. 5,302,778, entitled “Semiconductor Insulation for Optical Devices,” teaches the mounting of a photo-sensor on a printed-circuit board by integrating a sensor, a lens, and a molded mount in a molded mount package providing locating pins. The aforementioned Patent provides limited improvements in positioning accuracy for the sensor with respect to its primitive lens system. It also provides at best mediocre accuracy in the placement of the package itself on a mounting board. 
   Another known packaging approach for photo-sensing semiconductor devices is one offered by Shellcase, Inc. Detailed techniques are disclosed by U.S. Pat. Nos. 5,716,759, 6,040,235, and 6,117,707.  FIG. 2  shows a schematic cross-section of a package formed in accordance with those techniques. A patterned metal layer is applied to a photo-sensing semiconductor wafer to extend bonding pads to its dicing area having a narrow width between neighboring dice. A photo-sensing wafer is attached to a glass substrate by using epoxy. After that, the backside of the wafer is ground for to thin out the wafer. The silicon of the dicing area is then removed to expose metal lines. Many more process steps are needed to complete fabrication, but a detailed explanation is omitted, as such is not necessary for a clear understanding in the present invention. 
   The advantage of this package compared to a CLCC package is its smaller size. A number of drawbacks are nonetheless found in this package as well. Perhaps the most critical drawback with this package is the complexity both in structure and fabrication process. This complexity is a significant factor in mass production since complexity tends to increase the processing yield loss. As a result of its complexity and attendant yield loss, the package is expensive to fabricate. 
   Other nontrivial disadvantages of this technique include its need for a wide dicing line, which cuts against the trend in semiconductor manufacturing to decrease the width of dicing lines to achieve more dice per wafer. The current typical dicing line width of about 100 micrometers is not wide enough to support this technique. Consequently, the packaging technique is not compatible with semiconductor wafers having standard dicing line widths, and requires customizing measures to ensure the wider than usual dicing line widths. 
   Accordingly, it is an object of the present invention to provide a lower cost package for photo-sensing devices. Another object of the present invention is to provide a sufficiently compact package for photo-sensing devices to accommodate hand-held applications like cellular phone cameras where the small size is possibly the single most important packaging factor. Yet another object of the present invention is to provide a simple and conveniently fabricated and assembled package in which precise positioning of a focal plane in the horizontal as well as the vertical plane is effected in an effortless manner. 
   SUMMARY OF THE INVENTION 
   Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a structure of an electronic package of photosensing semiconductor devices and the method of fabrication and assembly thereof. 
   In accordance with the present invention, a plurality of solder bumps are provided on a photo-sensing semiconductor wafer via “wafer bumping” or any other suitable technique known in the art. After that, the bumped wafer is diced to separate into individual dice which form the sensing portions. 
   A substrate is fabricated separately. This substrate may be configured, for example, as a circular wafer or a rectangular panel. This substrate will have a plurality of unit substrates in much the same manner as the semiconductor wafer having a plurality of dice. Each unit substrate becomes an assembly portion which, together with the sensing portion, forms the electronic package after fabrication and assembly are complete. The material for the substrate is preferably transparent at the wavelength of light where the photo-sensing devices respond. Borosilicate glass is one example of a material having sufficient transparency for photo-sensing devices in the visible portion of the spectrum. The fabrication of this substrate includes forming at least one patterned metal layer to make solder bump pads and interconnection lines between the solder bump pads by deposition and patterning using any suitable measures known to persons versed in semiconductor fabrication art. These solder bump pads are grouped into at least two sets. The solder bump pads in the first set are relatively smaller and correspond to the solder bumps of the photo-sensing dice for making interconnections to those photo sensing dice. The solder bump pads in the second set are relatively larger and are used for making interconnections of the resulting package itself to external circuits, such as supporting printed circuit board. Also, the substrate includes at least one patterned passivation layer over the patterned metal layer for protecting the interconnection lines formed thereby. This patterned passivation layer has openings at the solder bump pads. Preferably, the substrate also includes a patterned dust-seal layer over the patterned passivation layer around the photo-sensing area for preventing dust particles from entering the area. 
   Solder bumps are mounted on the second set of solder bump pads for connection of the resulting package to an external circuit. Preformed solder spheres can be used for this solder bumping process in much the same manner as they are, for example, in BGA or CSP solder ball mounting. This process generally includes flux application onto the solder bump pads, solder sphere placement onto the fluxed solder bump pads, and heating the substrate up to a characteristic reflow temperature of the solder to cause the solder spheres to melt and wet down onto the solder bump pads. 
   The bumped and diced photo-sensing dice are then mounted onto the substrate using, preferably, a suitable flipchip mounting technique known to persons versed in semiconductor manufacturing art. The process includes serial pick-and-flip-and-place of each photo-sensing dice onto a predetermined location of the substrate until each unit substrate has all the necessary photo-sensing dice placed thereon as designed. This pick-and-flip-and-place process typically includes flux application onto the soldering area, after which the substrate is heated to a characteristic reflow temperature of the solder bumps for making interconnections between the unit substrate and the photo-sensing dice. 
   Dicing of the large substrate follows thereafter to separate each unit substrate. Each substrate unit may then be picked-and-placed into a preferred packing media such as tray, tube, or tape and reel. 
   While a CLCC package requires a ceramic substrate for interconnection and a glass lid for light transmission, a package formed in accordance with the present invention requires only a glass substrate having both functions. Also, the present invention employs batch processes such as wafer bumping for photo-sensing device wafer. It similarly employs batch processes for the fabrication and assembly of the substrate having a plurality of unit substrates. As a result of this simplification and batch processing, the present invention decreases packaging costs significantly. Unlike certain packaging techniques heretofore known, for instance, the present invention uses flipchip mounting which is a simple and field proven technique instead of very complicated processes that require extending bonding pads onto dicing areas, thinning of the wafer, and removing of silicon to expose the metal on dicing areas. Also, such features as flipchip mounting of photo-sensing dice onto a substrate and the use and arrangement of solder bumps for the disclosed package provide consistent and precise, guided positioning of a focal plane in the horizontal and vertical directions in an effortless manner. Self-alignment effectively occurs during the solder reflow process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a prior art CLCC package for a photo-sensor device; 
       FIG. 2  is a schematic cross-sectional view of another prior art photosensing package; 
       FIG. 3A  is schematic cross-sectional view of an undiced photo-sensing semiconductor wafer in one exemplary embodiment of the present invention, before processing; 
       FIG. 3B  is schematic cross-sectional view of an undiced photo-sensing semiconductor wafer in one exemplary embodiment of the present invention, after a patterned metal layer is applied; 
       FIG. 3C  is schematic cross-sectional view of an undiced photo-sensing semiconductor wafer in one exemplary embodiment of the present invention, after solder bumping; 
       FIG. 4  is an explanatory schematic diagram illustrating the air-to-glass transmission and reflection of impinging light at transition from a low index of refraction medium to a high index of refraction medium; 
       FIG. 5  is an explanatory diagram graphically illustrating an example of photo-sensing response modification due to forming an optical filter on a substrate; 
       FIG. 6A  is schematic cross-sectional view of a diced unit substrate at a certain stage of fabrication in one exemplary embodiment of the present invention; 
       FIG. 6B  is schematic cross-sectional view of a diced unit substrate at a certain stage of fabrication in an alternate exemplary embodiment of the present invention, utilizing multiple metal layers; 
       FIG. 7  is a series of schematic cross-sectional views illustrating an undiced substrate at certain stages of fabrication in one exemplary embodiment of the present invention; 
       FIG. 8  is a series of schematic cross-sectional views illustrating an undiced substrate at certain further stages of fabrication in one exemplary embodiment of the present invention; 
       FIG. 9  is a series of schematic cross-sectional views illustrating a flipchip assembly of photo-sensing dice onto an undiced substrate in one exemplary embodiment of the present invention; 
       FIG. 10  is schematic cross-sectional view of an electronic package formed in accordance with an exemplary embodiment of the present invention, shown illustratively mounted onto a printed circuit board; and, 
       FIG. 11  is a block diagram illustrating the fabrication and assembly steps for an electronic package formed in accordance with one exemplary embodiment of the present invention; and, 
       FIG. 12  is a block diagram illustrating the fabrication and assembly steps for an electronic package formed in accordance with an alternate exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A photo-sensing semiconductor wafer has a plurality of dice, and each die has integrated circuitry formed on a front surface of the wafer, much as in other semiconductor wafers. Each die has a plurality of bonding pads. The wafer has a patterned passivation layer over the front surface for protecting the integrated circuitry underneath. The passivation layer has openings on said bonding pads. Each such photo-sensing die has at least one photo-sensing area on a front surface. 
   Wafer bumping is a well-known technique that has been widely used since its initial teaching, as reflected in U.S. Pat. No. 3,292,240 entitled “Method of Fabricating Microminiature Functional Components,” assigned to IBM. A typical wafer bumping process includes at least one patterned metal layer for making solder bump pads connected to bonding pads on the wafer. Metallurgy used for solder bump pads is commonly referred to as under bump metallurgy (UBM) and typically utilizes a multilayered structure to provide multiple functions, such as good adhesion to bonding pad, good diffusion barrier against solder, and good wettability to solder (and oxidation protection if necessary). Various techniques for depositing UBM are available, including sputtering, electroplating, electroless plating, and the like. 
   A predetermined amount of solder material is applied onto the solder bump pads. There are numerous ways to apply solder—electroplating, solder paste printing, etc. There are several solder materials often used in wafer bumping—eutectic tin-lead, high lead (tin-lead solder having more than 80% of lead in weight scale), and lead-free (typically tin based solder such as pure tin, tin-silver, tin-copper, tin-silver-copper, etc). 
   The wafer bumping process also includes heating the wafer to a characteristic reflow temperature of the solder for causing connection of solder to the solder bump pads. Wafer bumping could optionally include at least one patterned passivation layer under the patterned metal layer, which is so-called “re-passivation.” Also, wafer bumping could optionally include interconnection metal traces between the bonding pads and the solder bump pads, which is so-called “redistribution.” This redistribution typically necessitates another patterned passivation layer for protecting the interconnection metal traces. Such various structures for wafer bumping are well known to persons versed in wafer bumping art. 
   In accordance with the present invention, a bumped wafer is preferably used for making an electronic package for photo-sensing devices, but is not limited to any specific structure, bumping technique, or solder material used in such wafer bumping. In a preferred embodiment, the solder bump height of the photo-sensing wafer after wafer bumping is preferably less than 100 micrometers.  FIGS. 3A-3C  respectively illustrate schematic cross-sections of a photo-sensing semiconductor wafer  100  before processing, after applying a patterned metal layer to form a plurality of solder bump pads  102 , and after solder bumping to form a plurality of solder bumps  104 . 
   Before, during, or after wafer bumping, the wafer  100  is thinned out to a certain thickness, if necessary, by mechanical grinding using any suitable means known in the art of semiconductor manufacturing. The object of this thinning process will be explained later in this section. Preferably, the thickness of the wafer  100  after thinning is approximately 250-350 micrometers, and where sufficient resources are available, approximately 150-350 micrometers. The photo-sensing wafer is thereafter diced to separate each die  101  along dicing lines  103 , again using any suitable means known in the art of semiconductor manufacturing. 
   A substrate is fabricated separately. This substrate is preferably in wafer or panel form having a large area sufficient to form a plurality of unit substrates in a batch process in a manner similar to that of a semiconductor wafer being made to have a plurality of dice. In general, the primary requirements for the substrate material include: transparency, mechanical rigidity, and chemical stability. The substrate material is selected to be one which is transparent to a certain wavelength or range of wavelengths, so as to transmit such light onto the photo-sensing device. Suitable substrate materials include, but are not limited to, glass, quartz, sapphire, silicon or other such infrared transparent materials. The choice of the substrate material depends on the range of wavelengths of interest, such that photo sensing devices operating at wavelengths in any of the ultraviolet, visible, or infrared spectra, for instance, may benefit from the present invention. Chemical resistance and mechanical stability are required to endure the temperature and various processing steps during fabrication, as well as to resist the environment during the resulting devices&#39; expected life. A typical substrate material for photo-sensing devices operating in the visible range of wavelengths is borosilicate glass. It is a preferred material because its chemical and temperature stability may be obtained at reasonable cost, and because it is available from many sources. 
   The substrate may be coated with at least one thin film layer on both surfaces to enhance light transmission therethrough. For example, an antireflection coating (ARC) or other suitable coating known to persons versed in the optical art may be applied. One purpose of this coating is to minimize the reflection loss of light over the entire spectrum of light of interest.  FIG. 4  illustrates the refection of light by the substrate. 
   Similarly, the substrate may be coated with at least one thin film layer on just one of its surfaces to enhance or reduce light transmission at a specific range of wavelengths. Such “optical filtering” may be effected using any suitable technique known in the art of optics like those documented in books such as “Optical Properties of Thin Solid Films” by O. S. Heavens published by Butterworth in 1955, “Thin-Film Optical Filters” by H. A. Macleod published by American Elsevier in 1969, “Optics of Thin Films, an Optical Multilayer Theory” by Z. Knittl published by John Wiley in 1976 or “Optical Thin Film&#39;s User&#39;s Handbook” by J. D. Rancourt published by MacMillan in 1987. 
     FIG. 5  shows an example of a modified photo-response obtained with a filter placed at a front of the photo-sensing device. In this particular example, the filter is designed to have the photo-sensing device mimic the sensitivity of the human eye and cut-off the intrinsic response of silicon in the ultra-violet (UV) and the infrared regions. In the preferred embodiment, a large wafer or panel of borosilicate glass is used for the substrate, preferably having approximately a 400-800 micrometer thickness, and even a 250-800 micrometer thickness where the available resources permit. Also in the preferred embodiment, the substrate is coated with at least one layer of thin film on either or both its front and rear surfaces to minimize reflection loss, or to enhance or reduce light transmission in the range of wavelengths of interest. 
   Referring to  FIGS. 6A-8 , at least one patterned metal layer  202  is applied on a front surface  204  of the substrate  200  for making solder bump pads  206   a ,  206   b  and interconnection lines  208  connecting such solder bump pads  206   a ,  206   b . Then, at least one patterned passivation layer  210  is applied on the patterned metal layer  202  for protecting the interconnection lines  208  formed thereby. The solder bump pads  206   a ,  206   b  are grouped into two sets. The solder bump pads  206   a  in the first set are relatively small for making interconnections to a photo-sensing semiconductor die  101 . The solder bump pads  206   b  in the second set are relatively large for making interconnections of the resulting electronic package itself to an external circuit or device, such as a printed circuit board. 
   There are two widely used approaches to making solder bump pads and interconnection lines, and passivation. The first approach shown in  FIG. 6A  is to use one patterned metal layer  202  to provide both the solder bump pads  206   a ,  206   b  and interconnection lines  208 , with one patterned passivation layer  210  formed over appropriate portions of the metal layer to protect the interconnecting lines  208 . The second approach shown in  FIG. 6B  is to apply one patterned metal layer  212  to form interconnection lines  214 , with one patterned passivation layer  216  for protecting the interconnecting metal line portions  214 , then apply another patterned metal layer having portions  218   a ,  218   b  to form with the underlying portions of the metal layer  212  the solder bump pads  220   a ,  220   b . In the latter case, interconnection between the first and the second metal layers  212  and  218   a ,  218   b  are made by openings in the passivation layer  216 . In both options, the metal layer for solder bump pads typically has, itself, a multilayered structure including an adhesive layer for providing good adhesion to adjacent material underneath, a good diffusion barrier for solder, and a layer having good wettability for solder material (and oxidation protection if necessary), much as with the UBM used in forming solder bumping pads for the photo-sensing wafer as explained earlier. 
   The first option is economical because it uses only one metal layer  202 ; but, essentially the interconnecting metal layer includes a layer of diffusion barrier material which is not only unnecessary for interconnection metal lines but also causes high film stress. This film stress is detrimental in terms of reliability because it causes stress migration and also even delamination of the film. The second option is better in terms of reliability because it does not have the diffusion barrier layer in the interconnection metal lines; however, the option incurs increased fabrication costs by requiring multiple patterned metal layers  212  and  218   a ,  218   b . Therefore, the choice among such options usually depends on two primary requirements, cost and reliability, which will vary depending on the intended application. 
   In a preferred embodiment illustrating the first option, an approximate 1-2 micrometer layer of aluminum is used as an adhesion layer, an approximately 200-500 nanometer layer of Ni—V is used as a diffusion barrier, and an approximate 500-1000 nanometer layer of copper is used as a solder wettable layer. These layers are sequentially deposited onto the substrate to collectively form the metal layer  202  at the solder burn pads  206   a ,  206   b , preferably by sputtering without vacuum break between each layer of deposition. 
   In a preferred embodiment illustrated for the second option, an approximate 1-2 micrometer layer of aluminum is deposited onto the substrate for making interconnection metal lines, preferably by sputtering. The solder bump pads  220   a ,  220   b  in this second option are formed by sequentially depositing an approximate 200-2000 nanometer layer of aluminum as an adhesion layer to the interconnecting metal lines, an approximate 200-500 nanometer layer of Ni—V as a diffusion barrier, and an approximate 500-1000 nanometer layer of copper as a solder wettable layer. Each layer is deposited, preferably, by sputtering without vacuum break between layers of deposition. 
   In both options, a polymer layer is preferably applied as the passivation layer  210 ,  216 . An approximate 4-20 micrometer thickness is preferred for the polymer passivation layer, which may be formed by any suitable means known to persons versed in semiconductor fabrication art. 
   Referring to  FIGS. 7-8 , there are shown schematic cross sectional views of an undiced substrate  200  (in the form of a glass wafer) at different stages of fabrication. The substrate  200  is divided by dicing lines  203  into a plurality of unit substrates  201  having patterned metal  202 , passivation  210 , and dust-seal layers  222  formed thereon to define solder bumps  206   a ,  206   b  and a transmission region  223 . As illustrated in  FIG. 8 , at least one patterned dust-seal layer  222  may be applied onto the substrate  200  after the metal and passivation layers  202 ,  210  are formed, for preventing particles of dust from reaching and obscuring the photo-sensing area. This patterned layer  222  is configured to form a dust sealing structure around the photo-sensing area at the transmission region  223  of the resulting package. Any dust obstructing light will cause errors in the light sensing. Hence, the necessity for this dust-seal layer depends on the required restriction of dust particles in the applications of interest. In the preferred embodiment illustrated, the thickness of the dust-seal layer  222  is preferably less than 80 micrometers, and a polymer material is preferred for this dust-seal layer  222 . 
   There are several ways to create this patterned dust-seal layer  222 . The most common approach is to dispense an epoxy material or the like. Another approach is to apply a blanket polymer layer and pattern it by using a photolithography process. These or any other suitable means known to persons versed in semiconductor fabrication and packaging arts may be employed in accordance with the present invention. 
   Next, solder bumps  224  are mounted on the second set of solder bump pads  206   b  formed over the substrate  200 . In the preferred embodiment illustrated, solder flux is preferably applied on each appropriate solder bump pad  206   b , preferably by screen printing, then preformed solder spheres  224  are placed onto each solder bump pad  206   b  in the second set having the solder flux applied thereon. The resulting substrate structure is heated to a characteristic reflow temperature of the solder material for melting the placed solder spheres  224  and causing them to wet down onto the solder bump pads. The solder bump  224  height is preferably, but not necessarily, more than 250 micrometers. 
   There are numerous solder materials that may be used in the present invention. Eutectic tin-lead solder is a common material. Lead-free solders such as pure tin, tin-silver, tin-copper, and tin-silver-copper is likely to be more widely used in the future because of generally tightening regulations to eliminate lead in the semiconductor industry. High lead solders having more than 80% of lead in weight scale is a common solder material for high temperature applications because it has higher melting temperature and less consumption of diffusion barrier in solder bump pads. The present invention is not limited to any specific solder materials. 
   Once the substrate  200  is fabricated to form a plurality of unit substrates, or assembly portions,  201  as described above, the photo sensing dice  101  forming the sensing portions are mounted on the assembly portions  201  of the substrate  200 , preferably by use of a suitable flipchip assembly process known in the art. As  FIG. 9  schematically illustrates, this flipchip assembly process includes pick-and-flip-and-place of each photo sensing die  101  having solder bumps  104  onto pre-determined locations of each unit substrate  201  of the substrate  200  until all appropriate unit substrates  201  are populated with their necessary photo-sensing semiconductor dice  101 . Multiple photo-sensing dice  101 , of same or different kinds, may be mounted on one unit substrate  201 . Other non-photo-sensing active and/or passive dice (not shown) may also be mounted on a unit substrate  201  to form a multi-chip module. 
   The engagement of the solder bumps  104  of each semiconductor die  101  and the solder bump pads  206   a  correspondingly positioned on each unit substrate  201  ensures convenient yet consistently precise relative positioning of the photosensing die  101  and unit substrate  201 . The mated engagement of preformed solder bumps and their receiving solder bump pads serves a self-guiding function as the photo-sensing die  101  is placed onto a unit substrate  201 . This pick-and-flip-and-place operation includes flux application onto the solder bumps  104  of the photo-sensing dice  10 , preferably by a suitable “dipping” process known in the flipchip assembly art. A rosin based water soluble flux or other suitable material may be used in this application. A so-called “no clean” flux which is organic based may also be used. The substrate is then heated to a characteristic reflow temperature of the solder material for melting the solder  112  and making solder joints  104  between the first set of solder bump pads  206   a  of the substrate  200  and each photo-sensing die  101 . In the preferred embodiment illustrated, the height of the solder joints  104  connecting the substrate  200  and semiconductor dice  101  is preferably less than 80 micrometers. 
   Finally, the substrate  200  is diced along dicing lines  203  to separate the unit substrates  201 . Each electronic package  300  (having at least one unit substrate structure  201 ) which then results is picked-and-placed onto preferred packing media such as tray, tube, or tape and reel for packing and packaging. 
   Referring to  FIG. 10 , assembly of an electronic package  300  formed in accordance with the present invention onto a PCB board  400  may then be carried out by employing ball grid array (BGA) package techniques, as each unit substrate structure  201  is formed much like a typical BGA package having solder bumps  224  at peripheral portions of the package  300 . This process typically includes application of solder paste onto solder bump pads  402  at the opposing PCB portions followed by a process, whereby the package  300  is inverted and mounted onto the board  400 . Solder bumps  224  are thus placed onto corresponding pads  402  having solder paste applied thereon. 
     FIG. 11  illustratively shows in block diagram form the fabrication and assembly steps for the preferred embodiment of the package  300  discussed in preceding paragraphs.  FIG. 12  illustratively shows in block diagram form similar fabrication and assembly steps for an exemplary alternative embodiment of the package utilizing a plurality of metal layers in forming the solder bumps of each unit substrate. 
   In the preferred embodiment illustrated, the height of each big solder joint formed by a solder bump  224  connecting the resulting package  300  with the PCB board  400  is preferably greater than the collective height of the small solder joint formed by a solder bump  104  (connecting a photo-sensing die  101  to a unit substrate  201 ) and the thickness of the photo-sensing die  101 . This ensures that a gap between the photo-sensing die  101  and the PCB  400  is maintained. In the preferred embodiment illustrated, the photo-sensing semiconductor wafer  100  from which the photo-sensing dice  101  are formed is thinned down to preferably about 250-350 micrometers (150-350 micrometers where the available resources permit); the height of the small solder joints  104  connecting the photosensing die  101  and the unit substrate  201  is set to be less than approximately 80 micrometers; and, the height of big solder joints  224  connecting the resulting package  300  to the PCB  400  is set to be greater than 250 micrometers, as mentioned earlier. 
   This new package in the present invention is applicable to all types of photo sensors or photo detectors fabricated with various types of technologies such as CCD or CMOS. The present invention is applicable wherever area image sensors are used, such as in camcorders, digital still cameras, PC cameras, mobile phone cameras, PDA and handheld cameras, security cameras, toys, automotive, biometrics, and the like. The present invention is also applicable to linear array image sensors such as those used in fax machines, scanners, bar code readers and scanners, digital copiers, and the like. It is equally applicable in packaging non-imaging photo-sensors such as single diode or four-quadrant diodes used in motion detectors, light level sensors, positional or tracking systems, and the like. 
   Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modification other than those discussed above may be resorted to without departing form the spirit or scope of the invention. For example, equivalent elements may be substituted for those specifically shown or described, certain features may be used independently of other features, and in certain cases, particular combinations of fabrication or assembly steps may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.