Matching: Myths and Misconceptions

Over the years, a number of rules have accumulated around analog design, especially concerning matching devices. For example, most designers believe that matching devices should be intermingled and as close together as possible, because the diffusions or implants have gradients, meaning they vary gradually in depth or concentration over the area of the chip.

A few years ago, I had an opportunity to examine this. I measured the matching of adjacent devices and compared it with devices which were farther apart. To my surprise, I found no statistically valid difference in matching for a distance of up to 2 mm.

Perhaps diffusion gradients were present in the early days. But with better furnaces—and especially with ion implantation—it seems they might have diminished to the point where they simply no longer play a role.

To be sure, there are thermal gradients. These are created by devices that heat one area of a chip more than another. For this reason alone, it's wise to intermingle devices and place them close together (and as far from the heat source as possible).

A second belief divides matching devices into as many small pieces as possible, so that they benefit from the statistical effect (large groups of devices match better than two single ones).

This has proved to be only marginally true. As you decrease the size of the layout features, the percentage variation becomes greater. Because of this, matching actually becomes worse for the same overall area as you approach minimum geometry.

The third belief holds that you should add dummy devices at the periphery. There appear to be two different explanations to justify this practice:

Shadows or reflections during exposure act differently on the remote edges than on devices in close proximity.
The etch rate for wide spaces is different from narrow ones.

I found no difference between groups of resistors with and without dummy devices at the periphery. It appears that you might be better off using the extra space to make the devices larger.