Using FIB for Wafer Lot Acceptance and Design Verification

FIB (focused ion beam) machines were notoriously difficult to maintain in their early years and necessitated a high level of expertise and commitment. The small number of good operators that did exist were highly prized and formed a sort of alchemist brotherhood. Early machines had small chambers, poor imaging capabilities, and rudimentary beam control for cutting and depositing. Physical potentiometers allowed for manual adjustment of the beam’s raster pattern.

The incorporation of tester hardware allowed devices to be really operated upon while executing test vectors under typical working conditions, and voltage-contrast microscopy was made possible with later generations of FIB gear. The device’s physical design database was eventually integrated into the navigation process, making it possible to travel to a specific node by name from a netlist, image it using voltage contrast, and then modify it via FIB cuts or depositions—all in one motion.

The capabilities of contemporary FIB machines completely eclipse the fundamental capabilities admired in the 1980s. FIB makes an excellent diagnostic tool in the modern era of System-on-Chip (SoC) designs with 10 and 11 metal layers, copper metallization, unusual dielectric materials, and the utilization of area pads dispersed across the full die area of a design. Even today, “backside FIB,” which entails milling into the die from the substrate side (as opposed to the top metal/passivation side), is still practicable. By approaching active devices from below, the operator can avoid having to cut through numerous metal layers and intricate, dense routing arrangements.

These skills have been extremely helpful for electronic failure analysis methods as a whole. Using the surgical precision of the ion beam milling method, FA experts can remove only certain layers of overburden to reach and photograph very small features that they think are causing yield problems, infant mortality, or electromigration problems.

Modern FIB hardware has little trouble supporting uncommon technologies like III-V semiconductors or Silicon-on-Insulator. The same is true for cutting-edge three-dimensional technologies like FinFET or GAA (Gate All Around, or nanowire) transistor designs, which modern FIB machines can handle rather well. With technology going down to crucial dimensions on the order of tens of nanometers, imaging and milling deposition resolutions have easily kept up, and the ion beam milling method is particularly compatible with delicate 3D structures. Technology today offers debugging and diagnostic tools that were completely unthinkable even ten years ago.

A robust software-driven user interface has replaced the previous front-panel beam control pots. The operators, despite any potential secret handshakes, are still the masters of debugging, bring-up, and failure analysis in the brave new nanoscale world.  FIB can assist if the object is small, expensive, exotic, or not performing as it should. Also read about Scale Verification.