Wayne and Layne

Picosecond imaging circuit analysis

Picosecond imaging circuit analysis (PICA) is a very cool method for measuring the activity of transistors in a CMOS integrated circuit. See [1] for details, but the basic idea is this: When a CMOS transistor switches state, it has a small probability of emitting an infrared (IR) photon. Since silicon is transparent to light with a wavelength longer than 1 micron, the IR photons can pass right through the silicon base of the chip. Researchers at IBM and elsewhere are setting up high-speed IR photodetectors to record these photons over many clock cycles, and produce very cool, visual records of which transistors are firing, and when in the clock cycle they are firing. As noted before, generation of an IR photon is not guaranteed, so the following movie was created from the records of a large number of clock cycles.

In this movie, the chip being tested has a distributed and hierarchical clock distribution network. This means that the initial clock buffered and amplified by a tree of buffers. As you can see in the video linked below, there is an initial pair of flashes near the top, followed by a large flash near the center, then a handful of distributed flashes, after which the chip lights up as all of the individual gates switch. Very cool stuff.

Here is a screenshot from the original source video at [2]:

1. Picosecond Imaging Circuit Analysis, J. C. Tsang, J. A. Kash, and D. P. Vallett, IBM Research
2. PICA Video, IBM Research