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Semiconductor Packaging: Die Preparation
Die preparation is the process by which the wafer is singulated into individual dice in preparation for assembly. Die preparation consists of two major steps, namely, wafer mounting and wafer saw.
Wafer mounting is the process of providing support to the wafer to facilitate the processing of the wafer from Wafer Saw through Die Attach. During wafer mounting, the wafer and a wafer frame are simultaneously attached on a wafer or dicing tape. The wafer frame may be made of plastic or metal, but it should be resistant to warping, bending, corrosion, and heat. The dicing tape (also referred to as a wafer film) is just a PVC sheet with synthetic adhesive on one side to hold both the wafer frame and the wafer. Typically measuring 3 mils thick, it should be flexible yet tough and strong, and with low impurity levels as well.
Using a special wafer mounting machine, wafer mounting consists of the following steps: 1) frame loading; 2) wafer loading; 3) application of tape to the wafer and wafer frame; 4) cutting of the excess tape; and 5) unloading of the mounted wafer.
During wafer mounting, the following concerns must be prevented: wafer cracking or breakage, bubble trapping on the adhesive side of the tape, scratches on the active side of the wafer, and non-uniform tape tension which can result in tape wrinkles.
Wafer saw follows wafer mounting and is the step that actually cuts the wafer into individual dice for assembly in IC packages. The wafer saw process consists of the following steps: 1) the frame-mounted wafer is automatically aligned into position for cutting; 2) the wafer is then cut thru its thickness according to the programmed die dimensions using a resin-bonded diamond wheel (see Fig. 3) rotating at a very high rpm; and 3) the wafer goes through a cleaning process using high pressure DI water sprayed on the rotating workpiece and then dried by air-blowing.
Kerf width is defined as the average width of the cutline plus the error attributed to microchipping. The singulated dice may be left attached to the wafer tape for pick-and-place during die attach or may be individually kept in waffle packs for future assembly.
Important parameters for consideration during wafer saw include the following: cut mode (direction and manner of cutting), feed speed (speed at which the wafer is being introduced to the blade), spindle rev (speed of revolution of the cutting wheel), blade height, and cutting water flow. Important parameters for the washing step include the following: wash time, wash rpm, DI water pressure, dry time, dry rpm, temperature and air flowrate.
Common Die
Prep-related
Failure Mechanisms:
Die Lifting - detachment of the die from the die pad or cavity. Wafer backside contamination during die preparation may inhibit good adhesion between the die backside and the die attach material. Other Common Causes of Die Lifting: contamination on the die pad or cavity, excessive die attach voids, incomplete die attach coverage, inadequate die attach curing
Die Cracking - occurrence of fracture anywhere in the die. Incorrect wafer saw and washing parameters can result in microcracks in the wafers, which can propagate into larger die cracks during later stages of the assembly process. Other Common Causes of Die Cracking: excessive die attach voids, die overhang or insufficient die attach coverage, excessive die ejection force on the wafer tape
Die
Scratching - inducement of any
mechanical damage on the die, as when an operator scratches a die
with tweezers due to mishandling. Die scratches can also result from
mishandling of wafers during the die preparation process.
Other Common Causes of Die Scratching:
insufficient operator training, disorderly workplace, use of
improper tools
Die
Metallization Smearing - depression
or deformation of any metal line on the die surface.
Common Causes: dirty or worn-out
die attach pick-and-place tool, wafer mishandling
Die Corrosion
- corrosion of the metallization and other components of the die.
Common Causes:
corrosive contaminants on the wafer tape or rinsing water
Die Contamination
- contamination of the die
surface with silicon dust or foreign material that may be attached
either by electrostatic, mechanical, or chemical means.
Common Causes:
contaminated rinsing water; static charge
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