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Written by Aparna Joshi   

Let's discuss PCM (Process Control Monitoring) data....

What is a source of PCM data? This data is generated after a wafer is completely fabricated or manufactured. It is collected using test structures which are either dropped in place of a die or placed on the scribe line. Scribe lines are the lines separating a die from its all neighbouring dice, in short, they are grid lines. The test structures are also called as PCM structures. They are distributed on a wafer equally. Sometimes, there is one PCM structure per reticle. One reticle contains many dice and a wafer has many reticles. These reticles are created to reduce the processing time. Refer another article on reticles for more information.

What does it contain? The locations of the PCM structures are called PCM sites. The data is collected only from PCM sites and not from any individual die.

What is a format of this data? Generally, because of its simplicity and small size, most of the foundries generate this data in ASCII, a comma separated values format (CSV format). This is a common format and readable in any spreadsheet application such as Microsoft Excel.

What is a main usage of this data? This data is used for Wafer Acceptance Test. This data tells engineers whether a wafer is worth for further processing or not. If PCM paramter values are not as per process specifications then most of the dice are going to fail or may not function consistently. It is always cost-efficient to catch failures as early as possible. Fabless semiconductor companies use foundries to manufacture their product. In this case, how to make sure that wafers are correctly processed? There has to be some criteria to say that wafer is manufactured/fabricated correctly.

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