Wednesday, 14 October 2015

VIA Concept

In a IC, a via is used to connect a metal track of one metal layer with a metal track of another metal layer. Generally, for signal lines other than the power lines, one via is provided for each connection point. Such via is called a "single-cut via".
With decreasing technology nodes in IC industry, the metal width gets reduced, and also the cross-sectional area of a via will decrease. Accordingly, in the manufacturing process, it has become difficult to form a via of a desired pattern. In a worst case scenario, an open failure occurs in the single-cut via formation part and thus the desired device operation cannot be implemented, resulting in a lower yield.

Further, as the cross-sectional area of a via decreases, the delay time in signal lines increases, and disconnection rate also increases due to electro-migration. This causes lowering of device reliability.

As a way out of these issues, multiple vias may be provided in parallel for each connection point. Such via is called a "multi-cut via". If two vias are provided for each connection point, such via is called a "double-cut via" . After layout, single-cut vias are replaced with multi-cut vias as many as possible so that the device reliability improves.

In nutshell the importance of using multiple Via's are as follows

1effective connection resistance reduces because of parallel connection.
2probability of yield reduction due to spot defect or improper manufacturing greatly gets reduced. deep sub micron technologies increased cap may be noticed.

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