What is a defect in microelectronic?
Submitted by SDRAM Technology on Sat, 02/27/2010 - 23:41
It is not a seecret that micrtoelectronic chips are fabricated in "clean rooms". The reason for this is very simply: to avoid defects which could compromise the functionality of the chips.
However, what is a defect? That’s a simple question, but very difficult to answer!
A very simple definition of a microelectronic defect can be: A defect is a localized deviation from an ideal pattern defined by the design. For example a particle with the size in the same range or much bigger as the structured paterns on the wafer is a defect (see Figure 1).
Figure 1: Example of a particle-defect
However is important to know that:
- Defects are more than just particles
- Not all defect issues are caused by particles.
- Not all defects are causing electrical fails
- Not every electric fail is caused by a defect
- Majority of the Random Yield Loss is caused by defects
- Minority of the Systematic Yield Loss is caused by defects
In order to find the defects on the wafers special inspection tools are used. In general there are three types of events which are detected with the inspection tools: True events, False events, and Nuisance.
A True event is when the inspection tool detects something and this "something" is also visible in review by an optical or electronic microscope.
A False event is when the inspection tool detects something but nothing is visible in review by an optical or electronic microscope. Due to the fact that the inspection tools are not ideal, each inspection tool detects False events with a certain rate. A realistic and acceptable rate for false events could be for example ~5%.
A Nuisance event is when the inspection tool detects a difference that is visible in review, but the deviation is within the "acceptable" process variations. Due to the fact that not always is known what is an acceptable or not acceptable deviation it is very difficult to judge what is really a nuisance and what a defect.