Optical probing of flip-chip-packaged microprocessors

With the growing number of IC interconnect layers and flip chip packaging, node access for signal probing from the frontside of the chip becomes practically impossible. Optical probing make it possible to directly probe p-n junctions that form the drains of MOS transistors in a CMOS IC, from the silicon side of the chip. The probe consists of a focused infrared laser that reaches the diffusions by propagating through the silicon substrate. The reflected beam acts as an optical carrier whose amplitude and phase is modulated by the waveform on the node being probed. Although the optical modulation is small, it can be recovered with a combination of low noise signal recovery and stroboscopic techniques. From the chip backside, there is no metallization in the way to obscure the transistor drains. Hence, every signal in the chip can be probed.