Abstract:
A method for separating dies on a wafer includes etching channels around the dies on a first side of the wafer, mounting the first side of the wafer to a quartz plate with an UV adhesive, and grinding a second side of the wafer until the channels are exposed on the second side of the wafer. At this point, the dies are separated but held together by the UV adhesive on the quartz plate. The method further includes mounting a second side of the wafer to a tack tape, exposing UV radiation through the quartz plate to the UV adhesive. At this point, the UV adhesive looses its adhesion so the dies are held together by the tack tape. The method further includes dismounting the quartz plate from the first side of the wafer and picking up the individual dies from the tack tape.

Description:
FIELD OF INVENTION 
     This invention relates to a method for separating dies on a wafer. 
     DESCRIPTION OF RELATED ART 
     Currently wafers are diced using a scribe and break technique or a semiconductor-dedicated saw. Scribe and break technique uses a diamond scribe to create scribe marks in the alleys (i.e., scribe streets) between dies on a wafer. A special “breaking tool”—typically an anvil above a doctor blade—snaps the wafer into discrete dies. 
     Dicing using a saw requires the wafer to be placed on a tape that is stretched taut across a round hoop. The hoop and the wafer are loaded into the saw and the circular spinning blade is moved back and forth to cut the alleys between the dies. 
     The width of the alley necessary to accommodate the scribe or the saw is about 100 microns. If the active area on a die is 160,000 square microns (400 microns on a side), then the total area of the die including the 50 micron alley around each die is 250,000 square microns, or a 57% increase over the total active area. If a 10 micron alley is added around the die, then the total die area becomes 168,100 square microns, or a 5.1% increase in area. The difference between a 100 micron alley and a 10 micron alley gives approximately 33% saving in area. Thus 33% more die can be produced on the same wafer. 
     Thus, what is needed is a method to separate dies on a wafer using smaller alleys. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the invention, a method for separating dies on a wafer structure includes forming channels around the dies on a first side of the wafer structure, mounting the first side of the wafer structure to a plate having a first adhesive, and removing material from a second side of the wafer structure until the channels are exposed on the second side of the wafer structure. At this point, the dies are separated but held together by the first adhesive on the plate. The method further includes mounting a second side of the wafer structure to a second adhesive and dismounting the plate from the first side of the wafer structure. In one embodiment, the plate is dismounted by exposing UV radiation through the plate to the first adhesive so the first adhesive looses its adhesion to the wafer structure. At this point, the dies are held together by the second adhesive. The method further includes picking up the individual dies from the second adhesive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a method to separate dies on a wafer structure in one embodiment of the invention. 
     FIGS. 2A,  2 B,  2 C,  2 D,  2 E, and  2 F illustrate cross-sections of a wafer structure being singulated by the method of FIG.  1 . 
     FIG. 3 illustrates a method to separate microcap wafer-level packages on a wafer structure in one embodiment of the invention. 
     FIGS. 4A,  4 B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H,  4 I, and  4 J illustrate cross-sections of a wafer structure being singulated by the method of FIG.  3 . 
     FIG. 5 illustrates a method to separate microcap wafer-level packages on a wafer structure in another embodiment of the invention. 
     FIGS. 6A,  6 B,  6 C,  6 D,  6 E, and  6 F illustrate cross-sections of a wafer structure being singulated by the method of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     In embodiments of the invention, a deep silicon etch is performed on a first side of a wafer to form channels around dies on the wafer. The wafer is then mounted on the first side to a UV transparent plate with an UV adhesive or tape. A second side of the wafer is grinded to a depth that exposes the channels and thereby singulating (i.e., separating) the dies. The wafer is then mounted on the second side to a tack tape. UV radiation is shone through the transparent plate to cause the UV adhesive to release the wafer from the transparent plate. The dies are then picked up from the tack tape. 
     FIG. 1 illustrates a method  100  to separate dies on a wafer structure in one embodiment of the invention. Method  100  starts with a wafer structure  202  shown partially in FIG.  2 A. Wafer structure  202  is a semiconductor wafer to be divided into dies having devices such as an FBAR (film bulk acoustic resonators) device  203  (only one shown). 
     In action  102 , channels  208  are formed around the dies on topside  204  of wafer  202  as shown in FIG.  2 B. In one embodiment, channels  208  are defined by photoresist  207  and then formed by deep reactive ion etching (DRIE) using the “Bosch process.” The width and depth of channels  208  can be varied to suit the application. In one embodiment, the width of a channel  208  between adjacent dies ranges from 10 to 20 microns. After channels  208  are etched, photoresist  207  is removed. 
     In action  104 , topside  204  of wafer  202  is mounted to a transparent plate  210  by an adhesive  212  as shown in FIG.  2 C. Plate  210  is used to handle and support wafer  202  for further processing. In one embodiment, plate  210  is a quartz plate and adhesive  212  is a conventional UV-releasable adhesive or UV-releasable tape such as “SP-589M-130” from Furukawa Electronic, Co., Ltd. of Japan. 
     In action  106 , the material from backside  206  of wafer  202  is removed to expose channels  208  on backside  206  as shown in FIG.  2 D. Once channels  208  are exposed on backside  206 , dies  214  are separated but held together by adhesive  212  on plate  210 . In one embodiment, conventional mechanical grinding is used to remove material from backside  206  to a depth that exposes channels  208 . Plate  210  and adhesive  212  provide the proper support during the grinding process. 
     In action  108 , backside  206  of wafer  202  is mounted to a tape  216  as shown in FIG.  2 E. In one embodiment, tape  216  is a conventional tack tape such as “Blue Low Tack” or “Blue Medium Tack” made by Semiconductor Equipment Corp. of Moorpark, Calif. 
     In action  110 , topside  204  of wafer  202  is exposed to UV radiation  218  as shown in FIG.  2 E. UV radiation  218  passes through plate  210  and causes adhesive  212  to lose its adhesion to topside  204 . As shown in FIG. 2F, dies  214  become released from plate  210  but are held together by tape  216 . 
     In action  112 , dies  214  are picked up from tape  216 . In one embodiment, tape  216  is stretched to increase the space between dies  214 . Eject pins push up through tape  216  to elevate and loosen dies  214  from tape  216 . A vacuum driven pickup device then retrieves dies  214  and deposits them in a carrier. 
     The method described above can be modified for application to microcap wafer-level packages formed with bonded wafers. 
     FIG. 3 illustrates a method  300  to separate dies, such as microcap wafer-level packages, on a wafer structure in one embodiment of the invention. Method  300  starts with a microcap wafer  402 A shown partially in FIG. 4A, and a device wafer  402 B shown partially in FIG.  4 C. Microcap wafer  402 A consists of dies to be divided into the microcaps of the microcap wafer-level package (hereafter “microcap dies”). Device wafer  402 B consists of dies with FBAR devices  403  (only one shown) to be divided into the bases of the microcap wafer-level package (hereafter “base dies”). 
     In action  302 , channels  408 A are formed around the microcap dies on underside  406 A of microcap wafer  402 A as shown in FIG.  4 B. In one embodiment, channels  408 A are defined by photoresist  407 A and then formed by DRIE using the “Bosch process.” The width and depth of channels  408 A can be varied to suit the application. In one embodiment, the width of a channel  408 A between adjacent dies ranges from 10 to 20 microns. After channels  408 A are etched, photoresist  407 A is removed. 
     In action  304 , channels  408 B are formed around the base dies on topside  404 B of device wafer  402 B as shown in FIG.  4 D. In one embodiment, channels  408 B are defined by photoresist  407 B and then formed by DRIE. The width and depth of channels  408 B can be varied to suit the application. In one embodiment, the width of a channel  408 B between adjacent dies ranges from 10 to 20 microns. After channels  408 B are etched, photoresist  407 B is removed. 
     In action  306 , microcap wafer  402 A and device wafer  402 B are bonded to form a wafer structure  402  shown partially in FIG.  4 E. Specifically, gasket  30  on microcap wafer  402 A are bonded with peripheral pads  16  on device wafer  402 B. 
     In action  308 , the material from topside  404 A of microcap wafer  402 A is removed to expose channels  408 A on topside  404 A as shown in FIG.  4 F. Once channels  408 A are exposed on topside  408 A, microcap dies are separated but held together by device wafer  402 B. In one embodiment, conventional mechanical grinding is used to remove material from topside  404 A to a depth that exposes channels  408 A. Device wafer  402 B provides the proper support during the grinding process. 
     In action  310 , topside  404 A of microcap wafer  402 A is mounted to a transparent plate  410  by an adhesive  412  as shown in FIG.  4 G. Plate  410  is used to handle and support packages  414  for further processing. In one embodiment, plate  410  is a quartz plate and adhesive  412  is a conventional UV-releasable adhesive or UV-releasable tape. 
     In action  312 , the material from backside  406 B of wafer  402 B is removed to expose channels  408 B on backside  406 B as shown in FIG.  4 H. Once channels  408 B are exposed on backside  406 B, packages  414  are separated but held together by adhesive  412  on plate  410 . In one embodiment, conventional mechanical grinding is used to remove material from backside  406 B to a depth that exposes channels  408 B. Plate  410  and adhesive  412  provide the proper support during the grinding process. 
     In action  314 , backside  406 B of wafer  402 B is mounted to a tape  416  as shown in FIG.  4 I. In one embodiment, tape  416  is a conventional tack tape. 
     In action  316 , topside  404 A of microcap wafer  402 A is exposed to UV radiation  418  as shown in FIG.  4 I. UV radiation  418  passes through plate  410  and causes adhesive  412  to loose its adhesion to topside  404 A of microcap wafer  402 A. As shown in FIG. 4J, packages  414  become released from plate  410  but are held together by tape  416 . 
     In action  318 , packages  414  are picked up from tape  416 . In one embodiment, tape  416  is stretched to increase the space between packages  414 . Eject pins push up through tape  416  to elevate and loosen packages  414  from tape  416 . A vacuum driven pickup device then retrieves packages  414  and deposits them in a carrier. 
     FIG. 5 illustrates a method  500  to separate dies, such as microcap wafer-level packages, on a wafer structure in another embodiment of the invention. Method  500  starts with a wafer structure  602  consisting of microcap wafer  602 A and device wafer  602 B bonded together as shown partially in FIG.  6 A. Specifically, gasket  30  on microcap wafer  602 A are bonded with peripheral pads  16  on device wafer  602 B. 
     In action  502 , channels  608  are formed around the packages in wafer structure  602  as shown in FIG.  6 B. Specifically, channels  608  are formed by etching completely through microcap wafer  602 A and etching partially through device wafer  602 B. In one embodiment, channels  608  are defined by photoresist  607  and then formed by DRIE using the “Bosch process.” The width and depth of channels  608  can be varied to suit the application. In one embodiment, the width of a channel  608  between adjacent dies ranges from 10 to 20 microns. After channels  608  are etched, photoresist  607  is removed. 
     In action  504 , topside  604 A of microcap wafer  602 A is mounted to a UV transparent plate  610  by an adhesive  612  as shown in FIG.  6 C. Plate  610  is used to handle and support packages  614  for further processing. In one embodiment, plate  610  is a quartz plate and adhesive  612  is a conventional UV-releasable adhesive or UV-releasable tape. 
     In action  506 , the material from backside  606 B of device wafer  602 B is removed to expose channels  608  on backside  606 B as shown in FIG.  6 D. Once channels  608  are exposed on backside  606 B, packages  614  are separated but held together by adhesive  612  on plate  610 . In one embodiment, conventional mechanical grinding is used to remove material from backside  606 B to a depth that exposes channels  608 . Plate  610  and adhesive  612  provide the proper support during the grinding process. 
     In action  508 , backside  606 B of wafer  602 B is mounted to a tape  616  as shown in FIG.  6 E. In one embodiment, tape  616  is a conventional tack tape. 
     In action  510 , topside  604 A of microcap wafer  602 A is exposed to UV radiation  618  as shown in FIG.  6 E. UV radiation  618  passes through plate  610  and causes adhesive  612  to loose its adhesion to topside  604 A of microcap wafer  602 A. As shown in FIG. 6F, packages  614  become released from plate  610  but are held together by tape  616 . 
     In action  512 , packages  614  are picked up from tape  616 . In one embodiment, tape  616  is stretched to increase the space between packages  614 . Eject pins push up through tape  616  to elevate and loosen packages  614  from tape  616 . A vacuum driven pickup device then retrieves packages  614  and deposits them in a carrier. 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. For example, in the embodiments of method  300  and  500 , the top and bottom arrangement of microcap and device wafers can be reversed. Furthermore, in embodiments of method  100 , the wafer can be mounted to a tack tape instead of a UV tape in action  104  and the dies can be directly picked up from the tack tape after the backside of the wafer is grinded in action  106 . Numerous embodiments are encompassed by the following claims.