Time-depending yield degradation at microchip manufacturing
Silicon wafers are processed to create hundreds of dies. During manufacturing, transistors and interconnections are made. At the end of the processing, the upper layers and bumps are fabricated outer connections of a die.
A typical processed wafer is shown below:
Bumps made of the Cooper are located on the very top of each die. Typical bumps are shown below:
The bumps are used for the final test of all dies. The final test is an electrical test that is performed using prob-cards equipped with micro pins. Micro pins touch the bumps that allow them to perform electrical tests: send signals and receive results. Only good dies are taken for the packaging. All dies that fail the test will be scrapped. The process yield depends on the amount of "good" and "bad" dies.
It was revealed that the time between the end of the process and the final test impacts the yield. The longer the dwelling the more dies fail the final test. If the dwelling exceeds hundreds of hours the amount of failed dies becomes dramatically high, which results in the scrapping of the whole wafer.
Time-dependence yield degradation is illustrated in the picture below. Failed dies are marked by blue an red colors.
The end of the process is shown in the picture below:
The bumps are built from Cu on the top of the dielectric layer. Ti thin layer is used to improve the adhesion of the Cu to the dielectric material. The purpose of the operations shown above is to disconnect the bumps.
The process integrates both wet etch and plasma etch operations.
Chem 1 - is the mixture of H2SO4 + H2O2 - classical solution to remove Cu. Chem 1 does not interact with Ti.
Dry etch of Ti is performed with fluorine-containing gas that removes Ti and does not remove Cu.
Chem 2 - is the HF-diluted solution that dissolves Ti and does not interact with Cu and the dielectric material.
The time depending dies degradation is observed.
TRADITIONAL APPROACH
The main reason is an oxidation of the Cu - Proposed solution - to keep the wafers in an inert atmosphere.
This is the wrong concept: even oxidized Cu remains conductive, the experiments with an inner atmosphere did not show any improvements.
The managerial problem solution is to avoid dwelling between the end of the process and the final test: plan processing and perform the final test just after processing.
Applying such a solution increases production costs and is not always viable.
The project aims to analyze the phenomenon and find a creative solution.
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