Single bit failures are the dominant failure mode for SRAM 6T bit cell memory devices. The analysis of failing single bits is aided by the fact that the mechanism is localized to the failing 6T bit cell. After electrically analyzing numerous failing bits, it was observed that failing bit cells were consistently producing specific electrical signatures (current-voltage curves). To help identify subtle bit cell failure mechanisms, this paper discusses an MCSpice program which was needed to simulate a 6T SRAM bit cell and the electrical analysis. It presents four case studies that show how MCSpice modeling of defective 6T SRAM bit cells was successfully used to identify subtle defect types (opens or shorts) and locations within the failing cell. The use of an MCSpice simulation and the appropriate physical analysis of defective bit cells resulted in a >90% success rate for finding failure mechanisms on yield and process certification programs.
The physical analysis of sub-100nm device technologies in many cases requires the total or partial removal of the multiple layers of metallization that route electrical signals and power through the device. This paper presents a simple and quick polishing technique that will remove the entire metallization stack above metal 1 for a 55nm technology device, which results in significantly reducing the time needed to reach the transistor level of the device and also greatly improving uniform planarity across the device. This method is intended for those cases in which gaining access to the transistor layer is required for electrical characterization and physical analysis. The improved speed of this polishing technique to reach the transistor layer has greatly reduced cycle time. The results for the polishing method have been relatively reliable with over a 95% success rate.
Atomic force probing (AFP) uses very sharp tungsten tips (100nm in radius) which wear out rather quickly, even with the greater durability of tungsten as compared to silicon. This paper demonstrates how worn tips that no longer image and probe properly can be reconditioned using the focus ion beam (FIB) tool. The method works best for tips that are under approx. 750nm in diameter and are not bent. It works well for freshly manufactured tips that do not work properly due to mishandling or improper storage which allowed particulates/oxide to build up on the tip. The method also works well for fresh tips that have been worn down (or slightly bent) after several hours of scanning and probing. This method is straightforward and requires a minimal amount of time. Typically, four probe tips can be reconditioned in about 30 minutes on the FIB.
Passivation damage, a common failure mode in microelectronics circuitry, can be easily identified by optical inspection in the form of a local 'discoloration' after exposing the die to a chemical that would penetrate through the crack and attacks metal lines. Unfortunately, this process destroys evidence of what damaged the passivation, since it attacks the damaged region. As a result, in many cases, the mechanism by which the passivation damage occurred is unclear. This problem is addressed in this paper by a procedure to examine passivation damage by transmission electron microscopy (TEM) of a cross-section sample prepared from the backside and without exposing the die from the top side. The backside approach was successfully used to assign the root cause of the passivation damage to packaging process. A topside approach to characterize the passivation damaged region can result in destruction of evidence at the defect location.
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