For those of you who have been reading my blogs for some time you probably know that I worked at Analog Devices supporting high speed ADCs for many years and moved over to our space products group within the last two years. While brainstorming on topics to write about this month it occurred to me to combine the two areas in which I’ve worked and talk a bit about a few types of radiation effects on high speed ADCs. Before we can dive into some specifics on how radiation affects high speed ADCs, we must first understand a bit more about radiation effects in general. This will be a multipart series where, over the course of a few blogs, we will look at what types of effects exist and we will subsequently look at how several of these effects specifically impact a high speed ADC.
When a device is placed into the harsh environment of space it can expect to see radiation that can cause different types of undesired behavior. This is why we use facilities such as the Cyclotron at Texas A&M University among others in the US and around the world. These facilities allow us to expose devices to radiation to gauge the performance before the device is placed into an application on a satellite for example. Unlike terrestrial applications where a failed device can be replaced, once a device is launched into space there is not an way to easily replace it. At the very least it is a very expensive venture to consider.
I think many of us can recall the early issues with the Hubble Telescope. I am sure those are painful memories for all those involved. While the issue there was not with radiation in space, but a design flaw, the difficulty of repair once launched was still an issue. By radiation testing devices here on Earth, we can give ourselves confidence that they can survive in the harsh, radiation-rich environment in space. Now that we understand a little about why radiation testing is performed, let’s look at an overview of the different effects we might encounter.
In general, there are two types of effects that are observed, cumulative effects and single event effects (SEE). Cumulative effects occur over extended periods of time after a device has been repeatedly exposed to radiation and the device performance begins to shift in some manner. In the case of cumulative effects, a reset or power cycle of the device does not return the device to its nominal state of operation. These cumulative effects result in a ‘semi-permanent’ to permanent shift in device performance. I use the term semi-permanent because the radiation induced effects in this case are not removed by a device reset or power cycle, but may possibly be annealed out over time or via high temperature exposure. I won’t get into the details of this annealing process here. For the purposes of this blog we’ll assume the cumulative effects remain in the device.
Radiation Effects – Cumulative and Single Event Effects
In terms of cumulative effects there are two main categories, total ionizing dose (TID) and displacement damage. When considering TID effects, generally these occur over long period of time during the device lifetime. When testing for TID effects a device is exposed to radiation until a certain dosage is reached. The dosage defines the type of TID testing that is performed. There are two types, low dose rate (LDR) and high dose rate (HDR). In general, exposure less than or equal to 30mrad/s is considered LDR and exposure in the range of 50 – 300 rad/s is considered HDR. A total ionizing dose in the range of 30 kRad to 100kRad is fairly common. The intention is to expose the device to a large amount of radiation to gauge its lifetime performance in a space application.
Typically, the device would be tested prior to the radiation exposure to establish a baseline performance. It would then be exposed to a particular dose rate of radiation (either LDR or HDR) for a period of time to reach the desired total ionizing dose. After exposure to radiation the device would then be retested to determine any shifts in performance. During the radiation exposure, the device would be biased into normal operating mode to emulate the same conditions the device would see in its application in space.
Cumulative Effects – TID and Displacement Damage
Displacement damage occurs where the ions from the radiation are striking the device and, as a result, displace atoms from the material that makes up the device. This displacement can result in lattice vacancies or interstitials. These atoms can subsequently either recombine or form stable defects. For the purposes of this blog series more emphasis will be place on TID effects and not as much on the displacement damage. There is a fair amount of heavy physics involved when attempting to understand the intricate details of displacement damage which is a bit beyond the scope of this blog.
There are many journal papers and technical articles that go into much more detail on TID and displacement damage. The goal here is to give a high level view of these cumulative effects, with a bit more focus on TID. This will enable us to understand these cumulative effects and see how they can affect a semiconductor integrated circuit, specifically when dealing with high speed ADCs. In the next installment we will look at the single event effects (SEE) caused by radiation.