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Written by Aparna Joshi   

Wafer probe data or On-Wafer data


How is this generated? This is a complicated process. Main entities involved are, Prober,  probe card, load board, tester. let's see how they are connected. A Prober is responsible for physical connection with a die on the wafer under testing. Probe card is attached to the prober. It has a structure that looks like a spider. All those wires touch the bond pads on the wafer. Load board acts as a middle agent for a tester and a probe card. It receives test signals from the tester and transmits to the probe card. Information about moving the spider like structure to contact the next die or next set of dies (in case of multi-die testing). Testers are loaded with test programs. These paradigms of instructions are based on the circuit design of the die (product) and what parameters to be tested. Testers send signals to load board according to this programs. They also collect the output resulted from the signals. collection of these results along with other data is called as Wafer probe data. Some people call On-wafer data.

What does it contain? It contains the results of all test parameters at given conditions such as a range of frequencies and/or outputs based on the received input signals.

What is a format? In general, the format of the data is STDF which stands for Standard Test Data Format. This is a binary format. Binary format is the fastest way to log the data. This saves precious time of tester and prober. This time is precious as it contributes directly in the cost of the individual chip. Please refer to another article to see how exactly STDF is created during testing.

What is a usage? The most important for yield improvement. company saves more money if the defects are caught earlier. Yield engineers use this information against the design of the chip. The parameter values must be within the expected range which is also called as specification limits. Engineers look at the trend of the values and may tighten the limits, in other words, reduce the range of values for better reliability. These are called production limits. The dies which have values out of the range, are called bad dies. They are marked later and skipped during final packaging. This data is used for parametric wafermaps. The wafer maps can lead to pinpoint a problem in the process. It can also help diagnose the problems in test programs, probers as well. Wafermaps are used in Signature Analysis. We will discuss this in different article.


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