For Wafer Probe, the effect of reprobing dice on production run rates can be significant. Recently we’ve developed tools for wafer probe floors for analysing this, including computing wafer test times, recovery rates (i.e. how effective reprobe is to recover dice), including per bin, and UPH (units per hour), taking advantage of our automated system for processing datalogs into a highly scalable private cloud database. By having such information available to engineers on an on-going basis, any probe manufacturing floor should be able to increase UPH over time for all probed products. Remember, you can only improve what you measure!
How dice are reprobed
There are two principal methods used when reprobing dice:
- In-line reprobe: where as soon as a die fails it is immediately reprobed
- Separate reprobe: where if a die fails it is not immediately retested. It is only retested after the wafer is finished and the overall yield typically determines whether retest of failed dice should happen
Sometimes the decision to reprobe in either of the above methods depends on the fail bin for the die in question.
Effect of Reprobing on UPH
For UPH, the higher the better. If dice are reprobed, this figure suffers. If the program is optimised well then the highest failing tests typically will be early on in the program, which reduces the effect of reprobe on UPH. If the recovery rates per bin are available it should be possible to switch off reprobe for certain tests and bins. Below are two views of what can be analysed.
The system which reports on UPH and recovery rate also enables thorough yield, bin and parametric analysis. The system needs to take into account reprobes for any analysis to maintain integrity. For example, when viewing a histogram of a test across 1,000 wafers of STDF-based data, there should only be one instance per die for the test even though many die may have been re-probed.
- UPH and recovery rate can be tracked automatically and having them available for engineers will lead to efficient reprobe and UPH improvement
- Recovery rate and UPH analysis must also ensure data integrity taking reprobe into account when producing charts and analysis for yield, bin and parametric analysis