SAWING

There are many variables involved in sawing the wafers. Wafer thickness, the width of the saw streets, the metal composition of the wafer, and die size all have to be considered when formulating the correct saw parameters. Failure to perform the process correctly can cause mechanical damage to the die.
Wafers are designed to hold as many die as possible, because there is a fixed cost to produce a wafer. The more die that can be fit onto a wafer, the lower the die production costs. Each die is segregated by a narrow "street" that is the cut line for singulating the die. Some wafers also have extra monitoring circuits built onto the wafers that take the place of a few die. These circuits are used to measure process controls during wafer fab.

Saw
Wafers are first mounted on a wafer cutting ring. A piece of filter paper is centered on the mounting chuck. Then the wafer is positioned face down on the filter paper. The wafer tape is stretched over the back of the wafer. A roller evenly distributes the tape on the wafer. Excess tape is cut away from the ring. Wafers are now ready for the wafer dicing process.
During the wafer dicing process, the wafer is divided into single die. The wafer is mounted on a chuck that rotates to align the X and Y axes of the wafer. A diamond stud blade is mounted on a spindle perpendicular to the wafer. It spins at a high speed and travels along the street, cutting the die apart. Usually the blade cuts 100% through the wafer and 1 to 3 mils into the wafer tape. The blade can travel either forward or reverse across the wafer. Once all the parallel streets are cut, then the chuck rotates 90 degrees and all of the other streets are cut.
Chip Supply uses two K&S automatic saws with high volume capability including 20-wafer carousels. We are well-equipped to respond quickly to new demands.
Once the wafer is fully sawn, the individual die are removed from the wafer tape. There are 2 methods used to accomplish this process. There is the manual pick and load method by which the die are peeled from the tape. The operator then transfers the die to the appropriate carrier utilizing a vacuum pencil. The vacuum pencil is positioned over the center of the die. The vacuum is applied and the die is lifted. The tool is moved to the carrier and the die is lowered, the vacuum is released, and the die falls into the carrier cavity. The other method uses pick and place machines. The wafer is loaded onto the pick and place. As it is clamped into place, the tape is stretched to increase the space between each die. The carriers (usually 4) are loaded onto the discharge fixture of the machine. Utilizing the XY travel knobs, the operator centers the die in the cross hairs on the monitor. Eject pins push up through the wafer tape to elevate the die and loosen it from the tape. The vacuum driven pickup arm retrieves the die and deposits the die in the carrier.
Chip Supply employs 3 shifts for manual pick and place and the pick and place machines. One is outfitted to pick from up to 6" wafers, and the other handles up to 8" wafers. In addition to the vacuum tool, the 8" pick and place has an edge-pick tool that can pick up die that are especially fragile on the surface, thereby eliminating any contact whatsoever with the active surface.

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CARRIERS

Carriers
Sometimes die are delivered to a customer's production line still mounted on the wafer tape and the customer's automatic die placement equipment picks the die off and places it on the circuit. Many times, the die are placed into a carrier prior to delivery, The most common form of carrier is the tray, commonly called "waffle pack" because of their appearance. Each tray has square or rectangular cavities in a matrix There are endless sizes because the cavity has to be just slightly larger than the die in each direction, including the depth of the cavity. The idea is to hold the die as motionless as possible and maintain its alignment in the cavity for ease of removal; if it rattles around, bits of silicon will break off and can damage the sensitive top side of the die. It also has to be large enough that the die can be easily removed.
Other types of carriers are Gel Pak® and various forms of tape and reel. Gel Paks® hold the die by surface tension, so there is no possibility of die movement. While there is no cavity size to worry about, Gel Paks® should be selected with various sizes of mesh and surface tackiness depending on die size. Gel Paks® require the use of an inexpensive vacuum release tool to remove the die.
Both trays and Gel Paks® are available in 2"" or 4" square sizes.
Tape and reel, of increasing interest to our customers, is available in a range of formats.
The final operation in wafer processing is visual inspection. Inspection can be done on unsawn wafer, singulated wafers, or die in carriers. Die on tape and reel is inspected prior to loading the tape.

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INSPECTION

Occasionally visual inspection is waived, if the die is from a particularly mature fabrication process and the inspection operation itself costs more than the value of its results. Most often, the value of inspection is well worth the cost associated with it.
Inspection can be done manually by an operator using a microscope, or it can be performed by sophisticated equipment that utilizes vision recognition (VR) software.
Inspection looks for defects that would either prevent the device from operating correctly or pose a reliability concern for long term operation. Defects can be divided into two major categories: fabrication and mechanically-induced (by mishandling) defects. The extent to which any defect may be acceptable would be defined by the appropriate criteria selected to fit the application environment. [see publication]
There are many different inspection criteria, but the most common one is MIL-STD-883, Method 2010. The test method provides two levels of inspection targeted for the applications environment, one high reliability and the other for space/ultra-high reliability. The test method defines the equipment to be used for the inspection, magnification ranges, what defects to look for and quantifies the amount of any particular effect that is acceptable.
In the early 90's, we at Chip Supply observed that the MIL test method was not meeting the needs of customers in a couple of significant ways. Having been written when state of the art lithography was greater than one micron, the magnification required by the test method was often not adequate to identify or classify an anomaly.
Furthermore, new on-die technologies such as fuses, laser trimming and redundant cells, which improve performance or correct for defective locations on the die, can appear to be visually rejectable. These enhancements to the devices leave visual anomalies that would be considered rejectable to an inspector without product knowledge, so we developed a high reliability commercial visual inspection specification that defines a more appropriate inspection and links product knowledge to the criteria.
Chip Supply employs a highly qualified staff of inspectors who receive an initial 80 hours of training before being certified to inspect. We also have three grades of inspectors, depending on their skill level and re-certify all of them on a 6-month cycle.
In addition to our manual capability, Chip Supply uses an August Technologies NSX-80 automatic vision machine.
This machine compares each die to a perfect die and then presents detected anomalies to an inspector for final determination.

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CLEAN ROOM / STOCK ROOM

Clean Room
It should be noted that all of these operations are conducted in a Class 10,000 clean building, at Class 100 workstations. The number identifies how many particles are present in one cubic foot of air. Particles are measured in microns. As a point of reference, a home usually has in excess of 100,000 particles per cubic foot!
In addition to cleanliness, humidity and temperature must be controlled within tight parameters. At Chip Supply, we set up environmental monitors around our clean areas and wrote software to continually monitor these three critical parameters. An audible and visual alarm would sound if any one of the criteria exceeds the control limits.

Stockroom
Chip Supply has a stockroom that currently holds over 6 million die of thousands of geometries. We have programs to stock certain die for particular customers in order to meet agreed-on turn times. We also collaborate on obsolescence programs and provide kitting and customer-owned die storage/disbursement.

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