Post-silicon debug of flip-chip parts is sometimes done by shining a laser on the chip at a very specific spot and detecting backscatter from the laser spot to

Post-silicon debug of flip-chip parts is sometimes done by shining a laser on the chip at a very specific spot and detecting backscatter from the laser spot to determine if a transistor is on or not. If you turn the laser power up, you can switch specific transistors open or closed. Semiconductors in general are pretty sensitive to light, which is why they are usually packaged in a way that keeps them dark during operation.