With the rapid downsizing of wafer fabrication technology, a combination of laser grooving with blade dicing or laser dicing is adopted in semiconductor manufacturers for optimum die separation quality. Traditionally, monitoring groove and dicing qualification is visually inspected by a microscope operator. The whole process can be time consuming and introduce inevitable human error. Using a Vision PE microscopy system, Clemex has demonstrated a new automated process that reduces the technician time spent analyzing the die, at the same time improving the accuracy of the measurement.
Ever since silicon wafers have been setup to accommodate multiple dies, there has been the problem of the safe and uniform removal of the dies. The traditional technique has been to use an extremely sharp blade to saw through the wafer around the outline, leaving a buffer of material and the die in the center.
There are some problems with blade slicing, mainly that, however sharp the blade, it can still introduce stresses into the die and the wafer as a whole. It is widely recognized that defects are introduced into the wafer as a result of the grinding and shearing mechanism of the saw cutting the wafer. These defects can induce passivation and metal layers peeling, chipping, cracks, and ILD delamination, all of which must be avoided to ensure a stable device.
In an effort to avoid these issues, chip manufacturers have turned to an alternative method of scoring the silicon: laser grooving. Laser grooving is a thermal energy based process and there is no direct tool-to-work piece contact. It uses a focused high-energy laser to transfer thermal energy to the wafer, which is absorbed by the topmost low thermal energy ILD metal layers. These metal layers then heat up and melt into molten and vaporized solids, which can be removed by directional flow of air pressure. There is now a substantial groove in the wafer, which is thinner and much less resistant to the blade dicing process, resulting in cleaner removal of the die.
Laser grooving is therefore now the primary choice for manufacturers looking to improve the quality of wafer dicing. But even this process is not free from drawbacks. The laser itself is a complex system that needs to be positioned and repositioned accurately over the wafer to ensure it grooves along the correct point.
The groove itself must be extremely precise and conform to the shape and tolerance set by the manufacturer. A groove that does not meet this tolerance can result in an improperly die cut, which is not fit for use and must be discarded. A significant amount of time must be spent by an operator using a microscope to analyze the tolerances of the grooves and clear it for dicing.
Using Vision PE microscope system, Clemex has developed a new method of automating the analysis of distance between the groove and the edge of the die.
First the wafer had to be correctly positioned under the lens. This was done by using a reference point and then adjusting the whole wafer so it was correctly orientated. Once this was achieved, each of the four dies that had been applied to the wafer could be analyzed.
After analysis of the dies, the laser groove itself could be analyzed. This was done by using a reference line that had been drawn around the die, and measuring the mean distance from this line to the groove itself. As this distance can vary slightly, a mean distance ΔD was taken.
To get an overall mean distance, the absolute distance of the north edge of the die was subtracted from that of the south edge, to form and overall mean distance: ΔD1. The same procedure was applied to the east and west edges to get ΔD2. If either of ΔD1 or ΔD2 are below the tolerance that is required to remove the die, then it is known that the grooving process hasn’t worked as intended, and the die cannot be removed unscathed.
This process was repeated for all four of the dies on the wafer, all whilst working within a time constraint, with the analysis a success thanks to the extreme customizability afforded by the Vision PE. The entire program made use of “if” statements, automatic centering, moving from target to target, and getting a clear and focused image from large distances. This can be tricky to do when the silicon wafer is over a foot in diameter.
Currently, chip manufacturers are relying on human operators to manually move around the wafer and visually asses the tolerance. This process is time consuming, and prone to human error, but there hasn’t been any alternative until now. The process described in the article, using the Vision PE system’s excellent suite of features, means that for manufacturers are finally able to automate their analysis of laser grooving.
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