Abstract:
A lid for a wafer-scale package includes a body having a bond area around a cavity defined by the body, an oxide layer atop the bond area and the cavity, and a reflective layer atop the oxide layer. The cavity has an angled sidewall where a portion of the reflective layer over the angled sidewall forms a mirror for reflecting a light. The lid further includes a solder layer atop another portion of the reflective layer over the bond area, and a barrier layer atop the mirror. The barrier layer is solder non-wettable so it prevents the solder layer from wicking into the cavity and interfering with the mirror. The barrier layer is also transparent to the light and has a thickness that either does not affect the light reflection or improves the light reflection.

Description:
FIELD OF INVENTION 
     This invention relates to a method for creating a wafer of lids for wafer-scale optoelectronic packages. 
     DESCRIPTION OF RELATED ART 
     Optoelectronic (OE) devices are generally packaged as individual die. This means of assembly is often slow and labor intensive, resulting in higher product cost. Thus, what is needed is a method to improve the packaging of OE devices. 
     SUMMARY 
     In one embodiment of the invention, a lid for a wafer-scale package includes a body having a bond area around a cavity defined by the body, an oxide layer atop the bond area and the cavity, and a reflective layer atop the oxide layer. The cavity has an angled sidewall where a portion of the reflective layer over the angled sidewall forms a mirror for reflecting a light. The lid further includes a solder layer atop another portion of the reflective layer over the bond area, and a barrier layer atop the mirror. The barrier layer is solder non-wettable so it prevents the solder layer from wicking into the cavity and interfering with the mirror. The barrier layer is also transparent to the light and has a thickness that either does not affect the light reflection or improves the light reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are cross-sections of a wafer-scale optoelectronic package in one embodiment of the invention. 
         FIG. 3  is a top view of a sub-mount of the optoelectronic package of  FIGS. 1 and 2  in one embodiment of the invention. 
         FIG. 4  is a flowchart of a method for making a lid for the wafer-scale optoelectronic package of  FIGS. 1 ,  2 , and  3  in one embodiment of the invention. 
         FIGS. 5 ,  6 ,  7 ,  8 ,  9 A,  9 B,  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16  are the structures formed by the method of  FIG. 4  in one embodiment of the invention. 
         FIG. 17  is a mask used in the method of  FIG. 1  in one embodiment of the invention. 
         FIG. 18  is a flowchart of a method for making a lid for the wafer-scale optoelectronic package of  FIGS. 1 ,  2 , and  3  in another embodiment of the invention. 
         FIGS. 19 and 20  are the structures formed by the method of  FIG. 18  in one embodiment of the invention. 
     
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. The cross-sectional figures are not drawn to scale and are only for illustrative purposes. 
     DETAILED DESCRIPTION 
       FIGS. 1 ,  2 , and  3  illustrate a wafer-scale optoelectronic package  150  including a sub-mount  80  and a lid  130  in one embodiment of the invention. Sub-mount  80  includes an optical lens  52  formed atop a substrate  54  and covered by an oxide layer  56 . Buried traces  90 ,  92 ,  98 , and  100  are formed atop oxide layer  56  and covered by a dielectric layer  64 . Contact pads  82 ,  84 ,  86 , and  88  (all shown in  FIG. 3 ) are connected by plugs to buried traces  90 ,  92 ,  98 , and  100 , which are themselves connected by plugs to contact pads  94 ,  96 ,  102  and  104  (shown in  FIG. 3 ) located outside of a seal ring  106 . A laser die  122  is bonded atop contact pad  82  and wire bonded to contact pad  84 , and a monitor photodiode die  124  is bonded atop contact pad  86  and wire bonded to contact pad  88 . Seal ring  106  is connected to contact pads  108  and  110  for grounding purposes. 
     Lid  130  includes a body  133  that defines a lid cavity  131  having a surface  132  covered by a reflective material  134 . Lid cavity  131  provides the necessary space to accommodate the dies that are mounted on sub-mount  80 . Reflective material  134  on surface  132  forms a 45 degree mirror  135  that reflect a light from laser die  122  to lens  52 . A seal ring  136  is formed on the bond area along the edge of lid  130  around lid cavity  131 . Reflective material  134  over lid cavity  131  also serves as an EMI shield when it is grounded through seal ring  136  and contact pads  108  and  110 . In one embodiment, a barrier  322  is formed over reflective material  134  to define where seal ring  136  is to be formed. Barrier  322  confines seal ring  136  so the seal ring material (e.g., a solder) does not wick into cavity  131  and interfering with mirror  135 . 
     In one embodiment, lid  130  has a (100) crystallographic plane oriented at a 9.74 degree offset from a major surface  138 . Lid  130  is anisotropically etched so that surface  132  forms along a (111) crystallographic plane. As the (100) plane of lid  130  is oriented at a 9.74 degree offset from major surface  138 , the (111) plane and mirror  135  are oriented at a 45 degree offset from major surface  138 . 
     In one embodiment, an alignment post  140  is bonded to the backside of sub-mount  80 . Alignment post  140  allows package  150  to be aligned with an optical fiber in a ferrule. 
       FIG. 4  illustrates a method  200  for forming a wafer-scale lid  130  in one embodiment of the invention. 
     In step  202 , as shown in  FIG. 5 , nitride layers  302  and  304  are formed on the top and the bottom surfaces of a substrate  306 , respectively. In one embodiment, substrate  306  is silicon having a thickness of about 675 microns, and nitride layers  302  and  304  are silicon nitride (SiN 4 ) formed by low pressure chemical vapor deposition (LPCVD) and have a thickness of about 1000 to 2000 angstroms. In one embodiment, if adhesion of nitride layers  302  and  304  to a silicon substrate  306  becomes problematic, nitride layers  302  and  304  can be made low stress by modifying the gas ratio (dichlorosilante to ammonia) and the amount of gas flow. In one embodiment, if denser nitride layers  302  and  304  are needed to withstand a KOH etch, nitride layers  302  and  304  can be made silicon rich in order to become denser. 
     In step  204 , as shown in  FIG. 6 , a photoresist  308  is next spun, exposed, and developed on nitride layer  302 .  FIG. 17  illustrate a mask  412  used in this lithographic process in one embodiment. Mask  412  includes lid cavity patterns  414  that define the shape of lid cavity  314 B in  FIGS. 9 to 16 . In one embodiment, lid cavity patterns  414  are trapezoidal so that the sidewalls formed by the nonparallel sides are flat instead of stepped. Mask  412  also includes scribe line patterns  416  that define the separation cavities  314 A and  314 C in  FIGS. 9A and 10  to  16 . Scribe line patterns  416  are oriented along a direction on wafer  306  that provides a symmetric etch angle. Note that  FIGS. 6 to 9A  and  10  to  16  show the cross-section of the resulting structure formed by method  200  along lines AA′ while  FIG. 9B  shows the cross-section of the resulting structure formed by method  200  along lines BB′. 
     In step  206 , as shown in  FIG. 7 , areas of nitride layer  302  exposed by windows  310 A,  310 B, and  310 C in photoresist  308  are etched down to substrate  306 . In one embodiment, nitride layer  302  is etched using a reactive ion etching (RIE) process. The remaining portions of nitride layer  302  serve as a mask for an anisotropic etch. 
     In step  208 , as shown in  FIG. 8 , resist  308  is stripped. As can be seen, windows  312 A,  312 B, and  312 C are formed in nitride layer  302 . The dimensions of these windows and the space between them are application dependent. 
     In step  210 , as shown in  FIG. 9A  along line AA′ and in  FIG. 9B  along line BB′, areas of substrate  306  exposed by windows  312 A to  312 C in nitride layer  302  are etched to form separation cavities  314 A and  314 C, and lid cavity  314 B. As can be seen in  FIG. 9B , lid cavity  314 B has a 45 degree wall  315  (which corresponds to surface  132  in  FIG. 1 ) and a 64.48 degree wall  317 . In one embodiment, silicon substrate  306  is anisotropically etched using a KOH solution having a (100) to (111) plane selectivity of 400 to 1. In one embodiment, each cavity is etched to 375 microns deep, which results in an undercut of 1 micron in nitride layer  302  due to the selectivity of the etchant. 
     In step  214 , as shown in  FIG. 10 , nitride layers  302  and  304  are removed. In one embodiment, nitride layers  302  and  304  are removed using a hot phosphoric wet etch. 
     In step  216 , as shown in  FIG. 11 , an oxide layer  316  is formed over cavities  314 A,  314 B, and  314 C, and on the top surface of substrate  306 . In one embodiment, oxide layer  316  is silicon dioxide that is thermally grown from silicon substrate  306  and has a thickness of about 1000 angstroms. 
     In step  218 , as shown in  FIG. 12 , a reflective layer  320  is formed over oxide layer  316 . In one embodiment, reflective layer  320  is a metal stack of a titanium-platinum-gold (TiPtAu) sequence deposited by e-beam evaporation or sputtering. In one embodiment, the titanium layer has a thickness of about 500 angstroms, the platinum player atop the titanium layer has a thickness of about 1000 angstroms, and the gold layer atop the titanium has a thickness of about 1500 angstroms. Metal stack  320  is the reflective material  134  ( FIG. 1 ) that forms mirror  135  ( FIG. 1 ) on the (111) plane surface  132  ( FIG. 1 ). 
     In step  220 , as shown in  FIG. 12 , a barrier layer  322  is formed over reflective layer  320 . In one embodiment, barrier layer  322  is a metal oxide formed over reflective layer  320 . For example, barrier layer  322  is a titanium dioxide (TiO 2 ) layer that is thermally deposited upon the TiPtAu metal stack  320  and has a thickness about 500 angstroms. Alternatively, barrier layer  322  can be a nitride, a boride, a fluoride, a fluorocarbon, a polyimide, or any other material that can withstand the soldering temperatures without adhering to the solder. Furthermore, barrier layer  322  can be formed by other processes, including sputtering, reactive sputtering, chemical vapor deposition, and plasma enhanced chemical vapor deposition. 
     In step  222 , as shown in  FIG. 13 , a photoresist  324  is next deposited on (e.g., spun on or sprayed on) barrier layer  322 . 
     In step  224 , as shown in  FIG. 14 , photoresist  324  is exposed and developed to form windows  326 A,  326 B,  326 C, and  326 D. Areas of barrier layer  322  exposed by windows  326 A to  326 D are etched down to reflective layer  320 . In one embodiment, a titanium dioxide barrier layer  322  is etched using a solution of diluted HF (1000:1) and nitric acid (100:1). 
     In step  226 , as shown in  FIG. 15 , a solder is plated through windows  326 A to  326 D onto reflective layer  320 . The solder forms seal ring  136  ( FIGS. 1 and 2 ) on the bond area around lid cavity  314 B (also shown as lid cavity  131  in  FIG. 1 ). In one embodiment, the solder is a gold-tin (AuSn) solder including a gold layer  328  having a thickness of 18,500 angstroms, and a tin layer  330  having a thickness of 18,500 angstroms on top of gold layer  328 . In one embodiment, photoresist  324  is stripped, reapplied, and patterned again to form windows  326 A to  326 D prior to plating the solder. This is because the gold plating (on the bottom) may mushroom over the top of the initial resist for gold plating. Therefore, in order to get somewhat vertical edges, it may be necessary to remove the original resist and reapply a thicker resist that will provide a form for the solder plating. 
     In step  228 , as shown in  FIG. 16 , photoresist  324  is stripped and lid  130  can now be singulated from adjacent lids  130  (shown partially) along imaginary lines  332 . 
       FIG. 18  illustrates a method  400  for forming a wafer-scale lid  130  in another embodiment of the invention. As can be seen, method  400  is similar to method  200  except that steps  426  and  428  have replaced steps  226  and  228 . 
     In step  426 , as shown in  FIG. 19 , photoresist  324  is stripped. This leaves barrier layer  322  as the mask during the solder plating. 
     In step  428 , as shown in  FIG. 20 , a solder including gold layer  328  and tin layer  330  are plated through windows  326 A to  326 D (defined now by barrier layer  322 ) onto reflective layer  320 . Again, lid  130  can be singulated from adjacent lids  130  (shown partially) along imaginary lines  332 . 
     In method  200 , photoresist  324  is left on as a mask during the solder plating. In method  400 , photoresist  324  is stripped and barrier layer  322  is used as the mask during the solder plating. The advantage of method  400  is that photoresist  324  does not have to be a thick resist. In addition, the uniformity of photoresist coverage is unimportant. Note that the solder and the resulting seal ring  136  will experience a small amount of mushrooming because the solder grows vertically by about the same amount that it grows laterally. In one embodiment, the total plating thickness is about 3 microns so the lateral growth is not problematic. 
     As described above, TiO 2  may be used as the barrier layer. TiO 2  makes a particularly good barrier layer in the present application for many reasons. First, the AuSn solder will not adhere to it. Second, it adheres well to gold in the metal stack while not many materials do. Third, although it has a high refractive index, which can alter the reflective of the gold, it is possible to deposit a very thin layer (e.g., much less than a quarter wavelength). At this thickness, there should be little effect on light transmission through the lid. Another advantage is that the methods described require only one mask after the cavity etch. This provides a great cost advantage over other methods that often require up to three masks after the cavity etch. 
     Although TiO 2  has been disclosed as a material for the barrier layer, other materials having the following characteristics can also be used: (1) good adherence to the mirror (i.e., the reflective layer); (2) non-wetable to solder; (3) transparent to light; and (4) non-soluble in the plating solution. 
     Furthermore, the barrier layer does not have to be thin (e.g., less than a quarter wavelength). In some applications, it is advantageous to have a thick barrier layer. As the barrier layer gets to a geometric thickness (angle dependent) near a quarter wave length, substantial changes in reflectance will become evident. These can either be more or less reflective. If the laser is collimated, these interference effects can be exploited to improve the reflectivity of the mirror. However, if the laser is not collimated, the wide range of angles of the light will cause a variable reflectance across the mirror depending on the local angel, resulting in variable intensity of the beam when it leaves the mirror. 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.