May

29

Poisson’s or Murphy’s yield models?

Filed Under Semiconductor Industry, Video Gallery | 1 Comment

A yield prediction model is used to estimate the yield for a given die area. Traditionally, Poisson’s model is the most dominant model used by most fabs. Recently, I came across some fabs exactly used Murphy’s model instead of Poisson’s model. What is Murphy’s yield model? I have not heard of this yield model before. After googling on the internet, I found that Murphy’s yield model was actually a general yield methodology and it was studied in detail way back in the 80’s and 90’s by a group of IBM’s researchers [Ref].

By assuming a certain defect clustering distribution, it is possible to work out an analytical yield model based on Murphy’s yield methodology. If we assume a uniform defect distribution across the wafer, we get the traditional Poisson’ yield model. If we assume a triangular defect profile, then we got the Murphy’s yield model [Ref]. A comparison of the 4 major yield models is shown in the figure. The question is “What is the most accurate yield model should the fab use?” It is difficult to tell, it depends on the defect distribution. From the figure, poisson’s yield model shows the most pessimistic yield prediction for larger die area . If we want to quote the worst case, then we should use Poisson’s yield model. However, some fabs may want a better yield number for marketing purpose, and they might prefer to other models :)

It seems that IBM has done a lot of fundamental research way back in the 80’s. It is amazing. Take a look at the IBM 12″ Fab in the following video.

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May

26

Carbon nanotube is cancerous!!

Filed Under Related Industry, Video Gallery | 1 Comment

The next time you touch carbon nanotube, beware!! Recent scientific studies showed that carbon nanotube can be cancerous!! Carbon nanotube has generated a lot of hype in recent years. For example last year, UC Berkeley scientists have demonstrated the world’s smallest raido using single carbon nanotubes.

A group of scientists led by Professor Kenneth Donaldson at the University of Edinburgh in the UK has carried out an experiment on the effect of prolonged carbon nanotube exposure on mice. These mice showed inflammatory response after long exposure to carbon nanotubes, indicating a sign of mesothelioma which is a form of cancer that is almost always caused by previous exposure to asbestos (Ref). The scientists found out that carbon nanotubes could release asbestos into the air if it was handled inappropriately or incorrectly (Ref).

The result was also confirmed by another group led by Jun Kanno and Akihiko Hirose of Japan’s National Institute of Health Sciences. They observed the formation of cancerous lesions consistent with mesothelioma when they injected Multi-walled carbon nanotubes into the abdominal cavity of mice that were genetically predisposed to develop cancer quickly (Ref).

How ironic is this world. We are blessed with the 21st century most wonderful material - carbon nanotube, yet we have to be extremely carefully when dealing with it.

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