What is the effect of radiation falling from outer space on personal equipment?

Recent advances in technology have led to higher integration and miniaturization of semiconductors, leading to the miniaturization of the electronic devices that support our surroundings. On the other hand, however, it has been confirmed that a small amount of radiation falling from space can cause the device to malfunction. In this series titled "Evaluation and Countermeasures for Soft Errors Caused by Cosmic Rays and Neutrons Revisited Now", I would like to introduce what kind of malfunctions caused by radiation, their causes, and specific countermeasures. This first article focuses on the effects of radiation falling from outer space on the devices around us.

Malfunctions caused by ``advancement of technology''

Outer space that extends beyond the atmosphere. Radiation pouring into the earth from there has a great impact on the semiconductors used in all electronic devices, including personal computers and mobile phones.

A long time ago, electronic circuits using discrete components had a large semiconductor chip area compared to today's ones, and the effects of radiation from space were negligible. However, in highly integrated LSIs, the effects of radiation cannot be ignored due to the effects of lower operating voltages. In particular, in the age of SRAM memory and FPGA, where semiconductors are becoming more highly integrated and miniaturized due to technological advances, the incidence of malfunctions tends to increase unless this effect is taken into consideration in advance. . Since the early 2010s, many countries around the world have been discussing the effects of radiation. In Europe, RADECS (RADiation and its Effects on Components and Systems); It is the subject of discussion at international conferences such as IRPS (International Reliability Physics Symposium).

In the past, the subject of discussion at these international conferences was centered on the aerospace field. This is because the aerospace field operates on the premise that the atmosphere is thin or that it is exposed to strong radiation in outer space. Recently, however, high performance computing, so-called high-performance calculation processing fields, industrial equipment, and automotive fields are also targeted.

As for the automotive field, some readers may ask "Why?", but in fact, as we aim to realize self-driving cars, we are aiming to control the car body such as braking and steering operation while grasping the surroundings of the car body. In doing so, semiconductors with high arithmetic processing capabilities have become indispensable. When considering the body control of self-driving cars, if a malfunction occurs due to the effects of radiation, it will directly lead to a serious incident involving human life, so it is being discussed as a hot topic at international conferences. We are no longer able to ignore the effects of radiation.

What kind of malfunctions are there due to radiation?-The types and countermeasures

What are the specific types of malfunctions due to radiation? Here, I would like to introduce the malfunction that occurs when radiation enters a semiconductor.

First, take a look at the diagram above. Malfunctions can be broadly divided into "hard errors" and "soft errors." A hard error physically damages an electronic circuit and becomes, so to speak, a "permanent failure." TID (total dose effect) and DD (extrusion damage effect) correspond to this.

Examples of countermeasures against hard errors include preparing a redundant circuit to secure time until the entire system stops working, and taking physical radiation shielding measures in advance. , or not to be used in an environment where a large amount of radiation is incident in the first place. But if it's not for a satellite or an X-ray machine/nuclear reactor system, it probably doesn't matter as it is.

On the other hand, soft errors are the subject of much discussion. In contrast to hard errors, which are "permanent failures," they are "temporary failures," in which radiation is incident once and the circuit malfunctions. SEL (Single Event Latch), in which current flows in an unintended path, may eventually lead to circuit damage and is classified as a hard error, but a soft error caused by a single radiation event SBU (Single-bit Upset) and MCU (Multiple Cell Upset) are extremely transient. Both are intrusions of high-energy particles that invert the internal data, collectively called SEU (Single Event Upset).

The most recognized countermeasure against such soft errors is the built-in error detection/correction function by Parity or ECC. adoption, selection of a circuit configuration that is resistant to soft errors, or use of a structure that is resistant to soft errors (for example, FD-SOI). Although this is not the main subject of this paper, even if such a circuit or structure is adopted, it is necessary to confirm "whether it really correctly handles soft errors". In other words, it is necessary to understand that the task of evaluating soft errors is a process that must be included in device development.

Radiation that causes soft errors - What are the types? There are various types of radiation emitted. They are roughly classified as follows.

Alpha rays: from alpha particles. The ionization effect is strong, but the range in the atmosphere is only a few centimeters, and shielding is easy. However, semiconductor package materials sometimes contain minute amounts of radioactive isotopes, which may generate alpha particles through nuclear decay, so countermeasures are necessary.

Beta rays: High-speed electrons emitted during nuclear decay. Compared to alpha rays, they have a longer range, but they can be shielded by a few millimeters of aluminum, and they are less likely to be exposed to beta rays in normal environments.

Heavy particle beam: nuclei of atoms heavier than helium. In addition to ionizing itself, nuclear reactions can produce secondary particles that can cause ionization. Shielding is relatively easy like alpha rays, but countermeasures are necessary for semiconductors used in outer space.

Neutron beams: High-speed neutrons. Since neutrons themselves have no electric charge, they do not directly cause soft errors, but they collide with silicon nuclei and excite the nuclei to generate heavy particle beams. This is the cause of the SBU and MCU mentioned above. Neutron beams are extremely difficult to shield.

According to the above, alpha and neutron beam tests are generally used when evaluating soft errors in semiconductors.

Also, I would like to explain how to evaluate this. For example, if the equipment is placed by the window of the development room, alpha rays and neutron rays will not enter easily, and the intended result will not be obtained. Therefore, it is usually necessary to prepare a radiation source separately, but among these, α rays can be obtained relatively easily. However, as for neutron beams, it is difficult to obtain them, and the shielding equipment is also large-scale, so it is common to use radiation equipment such as universities and research institutes for measurement.

In this paper, I explained the effects of radiation falling from outer space on the equipment around us. In the next article, I will introduce how to evaluate soft errors using the "CloudTesting(TM) Service", a service that provides a measurement environment for semiconductor devices.

[PR] Courtesy of Cloud Testing Service