Abstract:
A method of fabricating a package with a light emitting device includes depositing a first metallization to form a conductive pad on which the light emitting device is to be mounted and to form one or more feed-through interconnections extending through a semiconductor material that supports the conductive pad. Subsequently, a second metallization is deposited to form a reflective surface for reflecting light, emitted by the light emitting device, through a lid of the package. Deposition of the second metallization is de-coupled from deposition of the first metallization.

Description:
TECHNICAL FIELD 
     This disclosure relates to a fabrication process for a package with a light emitting device on a sub-mount. 
     BACKGROUND 
     The design of packages that house a light emitting diode (LED) or other light emitting device is an important factor in optimizing the amount of light output from the package. LEDs often are housed in packages that include multiple components, which occupy an area much larger than the LED chip itself. To increase the amount of light emitted from the package, a reflective material, such as metallization, sometimes is provided on the inner surface of the package. To maximize the amount of light reflected out of the package, it is desirable to provide the reflective material over a significant part of the package interior. 
     In some packages, the LED chip is bonded to a thermally conductive pad. The metallization for the reflective mirror also may serve as part of a thin-film stack for the conductive pad and for plated through-wafer interconnections. Such processes, however, tend to complicate the fabrication process and reduce the amount of the package surface that can be used to reflect light out of the package. 
     SUMMARY 
     In one aspect, a method of fabricating a package with a light emitting device, such as a LED, includes depositing a first metallization to form a conductive pad on which the light emitting device is to be mounted and to form one or more feed-through interconnections extending through a semiconductor material that supports the conductive pad. Subsequently, a second metallization is deposited to form a reflective surface for reflecting light, emitted by the light emitting device, through a lid of the package. Deposition of the second metallization is de-coupled from deposition of the first metallization, which, in some cases, can increase the area covered by the reflective metallization, thereby increasing the amount of light reflected out of the package. 
     In some implementations, the first metallization is deposited so as to form respective overhangs around the top of the conductive pad and each of the feed-through interconnections. The second metallization is deposited over a surface of the semiconductor material such that the overhangs serve as shields to substantially prevent the second metallization from being deposited on areas directly below the overhangs. Such a technique allows the second metallization to form a reflective surface that is electrically disconnected from the conductive pad and the feed-through interconnections. 
     Some implementations include forming a cavity in a first side of a semiconductor wafer and forming one or more through-holes that extend from a bottom of the cavity to a second side of the wafer. The first metallization is deposited to form the conductive pad for mounting the light emitting device and to form feed-through interconnections that extend through the one or more through-holes. The first metallization is deposited so as to form overhangs around the top of the conductive pad and each of the feed-through interconnections. The second metallization is deposited over the first side of the semiconductor wafer including over bottom and side surfaces of the cavity and over top surfaces of the conductive pad and the feed-through interconnections. The overhangs serve as shields to substantially prevent the second metallization from being deposited on areas directly below the overhangs. The second metallization is removed selectively from top surfaces of the conductive pad and the feed-through interconnections, so that the remaining second metallization forms a reflective surface that is electrically disconnected from the conductive pad and the feed-through interconnections. The light emitting device then is mounted on the conductive pad. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the detailed description, the accompanying drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of an example of a package that houses a light emitting device. 
         FIG. 2  is a top view of the base of the package. 
         FIGS. 3 through 7  illustrate fabrication steps for depositing the various metallization layers. 
         FIG. 8  is an enlarged view of overhangs at the top of feed-through and conductive pad metallization regions. 
         FIG. 9  illustrates how the overhangs shield areas adjacent the side edges of the feed-through and conductive pad metallization regions from being covered by subsequently deposited metallization. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in the example of  FIG. 1 , two structures  12 ,  14  are soldered together to provide a hermetically sealed package  10  that encapsulates a light emitting device, such as a LED  16 . Techniques other than soldering may be used as well (e.g., but not limited to, anodic bonding and adhesive bonding). The upper structure  12  serves as a lid and is transparent to the wavelength(s) that are emitted by the LED  16 . 
     In the illustrated example, the LED chip  16  is mounted on a conductive pad  18  on the lower structure  14 , which serves as a base. A solder seal ring  20  is provided on the cavity-side surface of the base structure  14  for hermetically attaching the lid  12  to the base. 
       FIG. 2  is a top view of the base  14  with the LED chip  16  removed. 
     As shown in  FIGS. 1 and 2 , the LED  16  is mounted within a recessed cavity  22  formed in the base  14 , which also includes feed-through metallization  24 . Other circuitry as well as passive components may be mounted in the recessed cavity  22  and encapsulated within the package. The feed-through metallization  24  extends through one or more micro-vias (i.e., through-holes) in the lower section of the base  14 . As illustrated in the example of  FIG. 1 , the feed-through metallization  24  extends along the outer surface of the base  14  and may be electrically connected to solder bumps  26  for printed circuit board assembly. Wire bonds  28  may provide the electrical connections from the LED chip  16  to the feed-through metallization  24 . Alternatively, the LED  16  may be flip-chip connected directly to the feed-through metallization  24 . Metallization  30  also is provided on inner surfaces of the base  14 , including the bottom  32  and sidewalls  34 , and serves as a mirror to reflect additional light, emitted by the LED  16 , through the lid  12 . 
     The base  14  can be formed, for example, from a silicon wafer into which the recessed cavity  22  and through-holes for the feed-through metallization  24  are etched using standard techniques. For example, a double-sided etching technique can be used. 
     The metallization for the pad  18  and feed-through connections  24 , as well as the mirror metallization  30 , subsequently are deposited. As explained in greater detail below, deposition of the mirror metallization  30  is de-coupled from deposition of the metallization for the pad  18  and feed-through connections  24 . 
       FIGS. 3 through 7  illustrate fabrication steps for depositing the various metallization layers. As shown in  FIG. 3 , following etching of the cavity  22  and the through-holes for the feed-through metallization  24 , and depositing or growing a passivation layer, a thin-film metallization stack  40  is deposited over the surfaces of the wafer, including in the cavity and the through-holes. In the illustrated example, the thin-film stack  40  includes layers of aluminum (Al), titanium (Ti), nickel (Ni) and gold (Au). Other implementations may include fewer than all the foregoing materials. Furthermore, additional, or different, materials may be included for the thin-film stack in other implementations. 
     Next, a thin plating mould  42  is provided over the surfaces of the silicon wafer other than the areas where the feed-through metallization  24  and conductive pad  18  are to be deposited. A photoresist mask can be used as the plating mould  42 . Although  FIGS. 3-7  illustrate an example with respect to the feed-through metallization  24 , the same processes are used with respect to the pad metallization  18 . The photoresist mask can be deposited by any of several techniques, including, for example, spin coating, dip coating, spray coating or electro-deposition. 
     After depositing the plating mould  42 , the metallization for the feed-through connections  24  and the pad  18  is deposited using, for example, an electroplating process. The electroplated metallization  18 ,  24  is deposited so that there is an overhang around the top of each conductor line and/or pad. An enlarged example of such an overhang  100  is illustrated in  FIG. 8 . During subsequent fabrication processes, the overhang  100  serves as a shield to prevent the mirror metallization  30  from being deposited too close to the side edges of the metallization for the feed-through connections  24  and the pad  18 . 
     In the illustrated example, gold (Au) or gold-tin is used as the metallization for the feed-through connections  24  and the pad  18 . Once the thickness of deposited gold exceeds the thickness of the plating mould  42 , isotropic growth of the gold layer results in formation of the overhang  100 . In the illustrated example, the thickness of the plating mould  42  is about 7-8 microns (μm), and the thickness of the gold metallization is about 10 μm. The thickness of the overhangs is about 2-3 μm. Similarly, in the illustrated example, the overhangs  100  extend about 2-3 μm beyond the lower portions of the metallization. In other implementations, those values may differ. 
     Next, as illustrated in  FIG. 4 , the plating mould  42  is removed, and the remaining thin-film stack  40  is patterned to form metal structures, such as a solder bond  44  and solder dam  46  on the backside of the silicon wafer. In the illustrated example, the solder bond  44  includes all the layers of the film-stack  40 ; the solder dam  46  includes the Al and Ti layers. 
     Next, as shown in  FIG. 5 , the mirror metallization (e.g., aluminum)  30  is deposited on substantially all the exposed areas of the cavity-side of the base  14 . Evaporation or sputtering techniques can be used to deposit the mirror metallization  30 , which should have a final thickness that is less than the thickness of the lower portion of the metallization for the feed-through connections  24  and the conductive pad  18 . In the illustrated example, the mirror metallization  30  has a thickness of about one hundred nanometers (nm). As illustrated in  FIG. 5 , the mirror metallization is deposited on the top of the overhangs  100  as well as on exposed areas of the cavity-side of the base  14 . As mentioned above, however, and as illustrated more clearly in  FIG. 9 , the overhangs  100  for the feed-through connections  24  and the conductive pad  18  serve as shields and prevent the mirror metallization from being deposited too close to the edges of the feed-through connections  24  and the pad  18 . 
     As is clear from the foregoing description, the process of depositing the mirror metallization  30  is de-coupled from the process of depositing the metallization for the feed-through connections  24  and the conductive pad  18 . That can result in the mirror metallization  30  covering a large percentage of the interior surfaces of the base  14 , while preventing the mirror metallization from contacting the side edges of the feed-through connections  24  and the pad  18 . 
     Next, as shown in  FIG. 6 , the aluminum mirror metallization  30  is removed from the gold plating layers  24 ,  18  (i.e., the aluminum is removed from the top of the feed-through connections  24  and the conductive pad  18 ). That can be achieved, for example, by selectively depositing a photoresist layer  48  (e.g., by an electro-deposition technique) on areas of the aluminum mirror metallization layer  30  other than on those areas where the aluminum mirror metallization layer is to be removed (i.e., other than on the feed-through connections  24  and the conductive pad  18 ). The exposed aluminum metallization on the tops of the gold (or gold-tin) feed-through connections  24  and the conductive pad  18  then can be removed by placing the silicon wafer in an aluminum etchant. 
     After removing the aluminum from the feed-through connections  24  and the conductive pad  18 , the electro-deposited photoresist layer  48  is stripped, as shown in  FIG. 7 . The result is a semiconductor sub-mount for the LED chip with a significant portion of the inner surface covered by a reflective (mirror) metallization to enhance optical output. The mirror metallization is electrically disconnected from the conductor lines (i.e., the feed-through connections  24  and the conductive pad  18 ) as a result of the overhangs  100 . 
     Although the foregoing description focuses on formation of the base  14  for a single package, the process can be performed as a batch process at the wafer level. After the various metallization layers have been deposited, the LED chip  16  is placed on the conductive pad  18 , and the wire-bonds  28  are attached. The cavity  22  can be filled with a silicone gel, and the transparent lid  12 , which can include a plastic or glass lens, is attached to the base  14 . 
     Other implementations are within the scope of the claims.