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Semiconductors used in electronic systems are susceptible to damage by ionizing radiation. The purpose of the material on radiation issues in SD-18 is to introduce the reader to the basic concepts of how various forms of radiation degrade semiconductor performance, the radiation environments that systems are typically exposed to, and the evaluation methods used to assure that a component will survive its specified radiation environments. In general, this guide will focus on the issues at the electronic component level, not the complex issues of system design for a radiation environment.

Basic Concepts

The source of radiation is either a particle or a photon. Particles include alpha particles (helium nuclei), beta particles (electrons), protons and neutrons. Photons are either x-rays or gamma rays, with the difference being the source of the photon, where gamma rays are defined as the result of a nuclear process, while x-rays are extra-nuclear. Particles may or may not carry an electrical charge, and have mass, while photons carry no charge or mass. To first order, the energy of the particles is kinetic (½mv²) while the energy of the photon is hn, where h is Planck's constant and n is the frequency of the photon. The two primary interactions of radiation with electronic components are ionization and displacement.

In ionization, a electron is ejected from an atom due to the collision, by one of three processes. The photoelectric process dominates at low energy. In the photoelectric process, a photon is absorbed completely in a collision with an electron, and the electron is ejected from the atom. This can happen whenever the energy of the photon is greater than the binding energy of the electron. The kinetic energy of the freed electron is given by T = hn-B, where hn is the photon energy, and B is the binding energy of the electron.  For silicon, the binding energy of an electron is 3.2 eV.  Compton scattering and pair-production are the other mechanisms, and are described in references [xx].  In semiconductors, ionization creates a trail of free electron-hole pairs that did not exist before the radiation event. While some fraction recombine quickly, many are either (a) swept away by the existing field (behaving like a short-duration current source at the circuit node), (b) trapped in non-conducting oxides or (c) drift within the semiconductor, where it either recombines eventually, or is rendered immobile by an interface trap. If the mobile charges large, then these photo-currents directly upset or damage the transistors, as high current densities are obtained, a phenomena called dose rate upset or burnout. When the photo-current is generated by a single energetic particle, and only effects one circuit node, this is considered a single event upset or burnout. While the photo-currents are a transient event, their residual effects can be permanent. When the mobile charges are trapped in the semiconductor, the effects of the trapped charge modify the expected characteristics of the transistor (e.g., a N-channel MOSFET is biased on by trapped holes) and thus degrade the performance of the circuit with total dose effects. While trapped charge can and does anneal over time and thermal cycling, to first order it can be considered as permanent damage.

Displacement damage occurs when a particle impacts the nucleus with sufficient energy to remove or displace it from its location in a crystal lattice. The classic example is neutron damage. Neutrons are uncharged particles and hence can penetrate the electron cloud of an atom and interact with the nucleus. With sufficient energy, this elastic scattering displaces the nucleus from its lattice location, and causes most of the electrical, chemical and physical changes observed. Since this is a degradation of the bulk characteristics of the silicon, bulk effects, such as the gain of a bipolar transistor or the resistance of a diffused resistor, are affected first by displacement damage. Displacement damage is also considered to be permanent damage.

Radiation Environments

There are three primary radiation environments of concern to electronic components. First are nuclear weapon effects. While primarily a concern for military systems, some effects such as electromagnetic pulse (EMP) are so widespread that commercial infrastructure has concerns as well. The second environment is outer space, primarily the natural environment, which can be quite harsh in certain orbits, but also the enhanced radiation environment if nuclear weapons are detonated in space. Finally, nuclear reactors and certain high energy accelerators create a radiation environment that requires care in implementing electronic systems. For the purposes of this introduction, reactors include not only nuclear power stations, but also experimental reactors, high energy physics experiments and medical instruments used in most hospitals as well.

The classic military concern since WWII has been the effects of nuclear weapons. When a nuclear weapon is detonated in the atmosphere (endo-atmospheric) a large burst of fission X-rays and gamma rays and fission and fusion neutrons are generated. While a fraction of this energy survives to directly impact the exposed system, much of the emitted radiation reacts with the surrounding atmosphere to produce a broad spectrum of X-rays, gamma rays and energetic particles and thermal energy. These prompt effects occur within milliseconds of the burst (depending on distance from the burst), delayed radiation, consisting of fission-product neutrons and gamma rays, and activation gamma rays occur at a relatively high rate for the first minute. Finally, residual radiation from fallout, consisting of several sources of gamma rays, lasts for years and can accumulate a significant dose over time. If the weapon is detonated outside the atmosphere (exo-atmospheric), the results are quite different. Since there is little or no atmosphere to interact with the radiation, there is no attenuation of the initial x-ray, gamma ray and neutron yield, except as a function of distance from the burst (1/R²). The activated debris field expands radially from the burst. Emitted electrons and protons are captured by the earth's magnetic field lines and held for long periods of time (pumping the belts), creating a high dose environment for a long period of time. The effects of the Starfish atmospheric test were detectable for several years after the 1962 test, destroying several satellites in the process.

The natural space environment is a concern for all satellites. There are three primary sources: electrons and protons trapped in the van Allen radiation belts, the solar proton flux (varies with the 11-year sunspot cycle), and galactic cosmic rays. The trapped radiation is highly dependent on altitude and latitude. Low earth orbit (LEO) (less than about 480 km) is where manned missions including the shuttle and ISS occur, and are mostly below the belts. This is the most benign environment. From about 500 km to 6,000 km are the proton belts, which are hard to shield and significantly damaging, making certain orbits such as mid-earth orbits (e.g., half geo-synchronous) a harsh environment. The electron belts extend from about 6,000 km to 60,000 km, and while easier to shield than protons, still create significant system problems. The more energetic electrons cause accumulated dose problems as well. Latitude causes problems due to the shape of the earth's magnetic field. Since the field lines flow from north pole to south pole, and are toroidal in shape, maximum protection is found near the equator, and minimal near the poles. Thus, the inclination of the satellite’s orbit also has a bearing on the average dose rate. In addition, many satellites fly in elliptical orbits, thus the dose rate varies considerably during the orbit. The solar proton flux is protons released from the sun as part of the fusion process. While there is a constant flux, during solar storms (sunspots) this flux can increase by orders of magnitude, affecting even LEO satellites. The pressure of this flux modifies the shape of the magnetosphere, further enhancing dose. Finally galactic cosmic rays are very energetic ions of all elements (though there is a dramatic falloff in number above iron) that originate from outside the solar system. These energetic ions primarily cause single event effects wherein the function of a circuit is modified by a single particle passing through a sensitive volume (e.g., a digital latch switches state). More permanent effects such as latch-up and burn-out are also possible.

Nuclear power systems primarily emit neutrons, which activate surrounding materials, creating gamma rays. To the extent possible, sensitive systems are protected by massive shields, but some elements must be in substantial radiation fields. High energy physics accelerators often have harsh radiation environments in various areas. For example, the Large Hadron Collider currently under construction at CERN in Switzerland will subject the experiments to massive doses of both atomic and sub-atomic particles, over an extended period of time. The electronics required in the experimental area must withstand very high radiation levels.

Effects & Environment
Quality System
Selection Guidance
Application Practice
Element Design
System Guidance

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The various quality systems, for example QML, Class M, MIL-STD-883 compliant, Automotive Quality Systems and commercial grade have unique implications for the selection and acquisition of pieceparts for radiation hardness assured applications that should be understood. The primary concern is that for the acquisition of a device that must be radiation hardness assured, the entire cost of ownership (i.e., purchase price, cost of additional supplier data, radiation characterization costs, radiation test and screening costs, subsequent radiation acceptance test costs, increased radiation hardness assurance costs, maintenance and surveillance costs, increased system design costs, etc.) must be considered. 

RHA Provisions 

The QML system has extensive provisions to qualify a line as capable of meeting a set of radiation requirements. A sub-set of QML manufacturers have qualified RHA products. The radiation limits set by the vendor are guaranteed as an ongoing part of the SPC system and expressed as a RHACL for a technology or product. If available in appropriate RHA levels, RHA-QML parts often provide the most cost-effective solution due to reduced part test requirements. Any claims of RHA are controlled by the vendor. To assure consistent RHA product in Class M devices will require  investigation of each vendor's method of certifying and maintaining RHA levels. 

RHA in other quality systems 

The three commercial quality systems do not incorporate radiation requirements. However, these parts can be radiation tested to define their capability limits (up-screening). The limitation of this method is that no process control can be assumed concerning radiation response since suppliers frequently change and update their process. This requires an ongoing radiation test program for each new lot of devices, or proof of continuity from the vendor. This has a severe impact on total cost of the parts.

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Radiation Hardness Assurance (RHA)

RHA is required for all devices that must operate in a radiation environment. Three distinct situations are possible and each must be addressed separately as follows:

(1)       QML Vendor Technology: Pieceparts are accepted as qualified for the specified RHA level with no additional testing required when die are procured from a qualified QML vendor and application parameter limits lie within the die specification (a QML qualified die fabrication technology). The parts used in the equipment must pass all TCI/QCI test for the specified RHACL of the QML fabrication technology.  

When the specified RHA levels and parameter limits for the qualified die and planned circuit application do not match, additional specification controls are needed. These may best be accomplished with a selected item drawing (SID). 

(2)       Vendor RHA Product Qualification: When pieceparts are procured from a vendor where the supplier maintains die fabrication technology change control, the parts shall be qualified to the RHA level for the required RHA environment. The qualification test requirements shall be based on the requirements of Mil-PRF-38535, Appendix A, for Class B or Class S devices as appropriate for the application. Group "C" steady state life test shall be performed on a sample of each lot of die to establish parameter deltas. Post-radiation temperature and end-of-life deltas shall be established and documented. Qualification shall be reperformed as a result of a major change of the die vendors die fabrication technology. 

(3)       Commercial Vendor Lot Qualification: When pieceparts are procured from a commercial semiconductor supplier who does not guarantee change control of the fabrication process, the equipment supplier shall develop and document a plan to assure fabrication lot uniformity (i.e., same wafer lot, homogeneous lot process, etc.) The plan shall identify a lot sample plan and qualification test for each lot based on the RHA requirements for the equipment. The qualification test requirements shall be based on the requirements of Mil-PRF-38535, Appendix A, type of requirement for Class B or Class S devices as appropriate for the application. Group "C" steady state life test shall be performed on a sample of each lot to establish parameter deltas. Post radiation temperature and end-of-life deltas shall be established and documented. Any fabrication lot exceeding initial established deltas shall be scrapped. 

Parts Control Procurement Plan (PCPP) for RHA Devices 

Since radiation requirements impose additional requirements on the parts control process the basic PCPP will have to be augmented to reflect these requirements. In addition to the normal controls, additional controls are required to establish both the radiation hardness of the device, and the maintenance of this level through the duration of the program. Assurance that the initial hardness level has not changed is a larger effort than establishing the initial level, particularly for the commercial quality system devices.  Additional information is required for RHA devices and applications requiring equipment to operate in a radiation environment.

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General 

The correct application of microcircuits is essential to overall performance, hardening and survivability for nuclear and space environments, reliability and cost/availability of systems. This section provides a discussion of the issues that must be addressed to ensure correct part selection for systems with a radiation hardening requirement.

Radiation Hardness Assurance 

The proper operation of microelectronics in a radiation environment requires knowledge and understanding of the radiation environment (e.g., nuclear weapons engendered, earth's magnetosphere, space, man-made commercial, etc.), the performance required of the microelectronics in the environment (e.g., operate-through, etc.), the equipment configuration (e.g., shielding, shadowing, circumvention, etc.), the response of the actual device and the device response in the intended circuit application. The last point is especially important in a nuclear weapons environment (NWE) due to dose-rate and internal electromagnetic pulse (IEMP) effects.

Qualification Requirements 

For the case of the QML supplier the RHACL shall be used to determine the margin between the actual radiation levels and the device capability. For those suppliers who are not QML certified but maintain stringent SPC of the critical design and process parameters, report all design and process changes and have characterization data concerning device, the application of devices use shall be based on radiation test results.

A proposed method to derive the required data base would consist of radiation testing using MIL-PRF-38535, Appendix A, Class Q or Class V requirements (as appropriate to the application), performing steady state life tests on a sample of each lot of die to establish parameter deltas and finally establishing post-radiation temperature and end-of-life deltas. 

In addition, this process should be reperformed following any major change of the vendor’s fabrication methodology. 

For those vendors who provide commercial parts without any guarantee of change control, a lot sample plan or equivalent would have to be developed by the equipment manufacturer to assure fabrication lot uniformity (i.e., same wafer lot, homogeneous lot process, etc.). The plan must identify a lot sample plan and qualification testing procedures. A more detailed description of these requirements is provided in the Selection Guidance section.

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Introduction 

This section provides guidance concerning RHA, RHM, and RHS on system development issues.

These activities should be initiated as early as possible in the system development cycle to minimize cost and effort. Moreover, these efforts should be integrated to the maximum extent practicable in the system’s testability requirements. 

Indeed, if an aspect of an overall system development activity entails the development and demonstration of a "new" technology, this development effort should also extend to qualification, RHA, RHM, and RHS tasks as appropriate. 

One example of such a situation would be the need to develop a radiation hardened cryogenic microelectronics technology to support a system development. Since the areas of radiation hardening and testing, reliability testing, process qualification, etc. are ill-defined for this type of technology, it would be highly desirable and cost effective to initiate technology qualification, reliability characterization testing, and RHA efforts in conjunction with the basic development tasks. 

Clearly such a proactive approach is also appropriate for devices without radiation requirements. However, the imposition of this additional set of constraints greatly exacerbates the situation. 

Radiation Hardness Assurance and System Radiation Hardening Considerations 

RHA program - The microcircuit RHA program must include an allocation of radiation design margin in the part acceptance specification limits which can be combined with other parameter degradation stresses, such as time and temperature, to assure each system relevant parameter has tolerable end-of-line (EOL) limits. 

As previously stated, the selection of devices for a particular application requires knowledge of the radiation response of that device, a description of the environment, an understanding of the specific device application, and a description of the system/subsystem where the device will be used. 

Electronic pieceparts are normally obtained for a system through the implementation of a parts control plan (see the Selection Guidance section) and an integral part of such a plan is the radiation hardness assurance (RHA) program. The RHA program refers to all of the methods and procedures which control the acquisition to specified radiation performance levels. Specific RHA requirements for various classes of semiconductor suppliers are also discussed in the Selection Guidance section. 

RHA activities are most apparent during the production phase of a program. However, RHA considerations (e.g., parts selection, parts control, etc.) should begin during the initial stage of a program (i.e., concept definition) and pervade all phases of a program. Such an approach should preclude the need to retrofit radiation hardening into a system which can be extremely costly. If radiation hardening is addressed during the initial stages of a systems development the cost of hardening can be less than 5% of the entire satellite cost. 

In addition to RHA, hardness maintenance and surveillance programs are required to ensure that the robustness of a system is not compromised during its operational phase due to incorrect maintenance. 

For suppliers that provide radiation hardened parts, in general all RHA SMDs require devices to be characterized to indicate device capability (not to system survivability) using the following MIL-STD-883 Test Methods 1017, 1019, 1020, and 1021; and ASTM Test Method 1192. 

RHA designators have been developed to allow for the categorization of total ionizing dose capability levels, as follows:

M = 3 X 10³ rad(Si)                         F = 3 X 10⁵ rad(Si)

D = 1 X 10⁴ rad(Si)                         G = 5 X 10⁵ rad(Si)

P = 3 X 10⁴  rad(Si)                         R = 1 X 10⁵ rad(Si)

L = 5 X 10⁴ rad(Si)                         H = 1 X 10⁶ rad(Si) 

For example, if a part is characterized to 5 X 10⁴ rads(Si) the part would be listed as a D level part, but if that same part from a different manufacturer shows a capability to 5 X 10⁵ rads(Si), the part would be listed as an R level on the same SMD. 

The other test methods are handled within the MIL-PRF-38535, Group E paragraphs in each detailed specification as required by design or by the purchase order. The Mil-PRF-38535, Group E Table designates the test method, sample size, identify specific technology types that allow certain tests to be eliminated or retained and contain a variety of caveats concerning radiation testing in general. 

It should be noted that the utmost care must be exercised before a specific test is eliminated. This warning is important since some technologies contain parasitic structures sensitive to radiation effects that don't affect the primary structure but are capable of affecting the overall circuit performance. An example of such a situation would be a combined MOS digital circuit and CCD device. In general, an MOS digital structure is insensitive to neutron irradiation, but neutrons can dramatically degrade the operation of a CCD. Thus, the deletion of neutron testing, which is normally allowed for MOS technology, would be inappropriate for this case. 

By providing a fully characterized detailed device specification the user knows the device capability and can make a better judgment on which part best suits his particular application. However, for the situation where a device without an RHA specification is used in a situation where radiation hardness is required, as is often the case, a complete characterization of the device is required for those applicable environments (e.g., total ionizing dose, SEE, etc.) at the anticipated radiation levels. Also, any decisions concerning the appropriateness of the device must include the statistical variations associated with the device response, anticipated/statistical variations in the operating environment (e.g., solar max, solar min, solar flares, etc.) and the actual system parameters (e.g., shielding, shadowing, end-of-life performance needs, allowable number of upsets, etc.).