Noise in SPADs

Today I'm going to continue my post from two weeks ago and talk about noise in SPADs.

As I discussed in my previous post, a SPAD consists of a fast, positive feedback system in conjunction with a slow, negative feedback system. The positive feedback system detects generated electrons, whether they're created by tunneling electrons, traps, thermal electrons, or photons. Since we're focusing on photons, the other types of generated electrons are noise.

If you don't know much about silicon, you probably don't know what a trap is. Standard silicon consists of a repeating pattern of atoms. We need to implant ions into this repeating pattern so we create the properties we need - raise the voltage here and it goes down over there, etc. However, implanting these ions can damage silicon's structure so that the atoms no longer form a neat pattern. When atoms don't form a neat pattern, electrons interact differently with the damaged part of the lattice. Sometimes electrons get stuck in these damaged parts for a bit of time, hence the term "trap". When the electrons finally leave the trap, they can cause an avalanche.

The most prevalent location of traps is at the interface between the silicon and the insulating silicon oxide. Every modern SPAD I've seen buries the region in which the avalanche occurs, the multiplication region. Burying the multiplication region keeps it away from most of the traps and "trap-generated" electrons.

Another source of noise is tunneling. Tunneling is a phenomenon that requires knowledge of quantum mechanics, and I don't think I can explain it well in a paragraph or two. A good one sentence description of tunneling might be, "The laws of nature do not stop electrons from jumping over barriers of any size, although the odds decrease with different barriers." The main way to control tunneling is to change the properties of the barriers between different regions of the chip, specifically the doping of the different implants we add into the silicon.

There is another source of noise I didn't mention called crosstalk. Crosstalk is ostensibly named because improperly-wired phone circuits can pick up pieces of other phone conversations, or leak portions of your phone conversations. SPAD crosstalk is the phenomenon of one detector's firing causing the unwanted firing of another detector. Crosstalk can be standard electrical crosstalk, which is uncommon in SPADs, or optical. Optical crosstalk is pretty cool - it turns out that stray electrons can create photons much as photons create electrons. During the middle of an avalanche a SPAD gives off a lot of photons, and these photons might accidentally cause another detector's firing. Optical crosstalk isn't a severe issue in most SPADs, but it varies from environment to environment.

Though optical crosstalk can increase the noise from SPADs, the created photons do have one advantage. Since an avalanching SPAD gives off photons in the multiplication region, taking a long exposure picture of a SPAD allows us to see whether the multiplication region has the shape we expect. If our silicon implants go poorly, we won't see a uniform multiplication region.

I hope you enjoyed a discussion of the source of noise in SPADs. If this seems boring, keep in mind that we're detecting single photons here! Over the next few weeks I'll start to discuss some additional properties of SPADs, and eventually I'll get to the point where I start comparing them to CCD or CMOS image sensors.

PS Blogspot's dictionary claims that avalanching isn't a word, but it doesn't contain truthiness either. Very suspect.