Significance and Use
Electronic circuits used in many space, military and nuclear power systems may be exposed to various levels of ionizing radiation dose. It is essential for the design and fabrication of such circuits that test methods be available that can determine the vulnerability or hardness (measure of nonvulnerability) of components to be used in such systems.
Manufacturers are currently selling semiconductor parts with guaranteed hardness ratings, and the military specification system is being expanded to cover hardness specification for parts. Therefore test methods and guides are required to standardize qualification testing.
Use of low energy (≈10 keV) X-ray sources has been examined as an alternative to cobalt-60 for the ionizing radiation effects testing of microelectronic devices (3, 4, 5, 6). The goal of this guide is to provide background information and guidance for such use where appropriate.
Note 3—Cobalt-60The most commonly used source of ionizing radiation for ionizing radiation (“total dose”) testing is cobalt-60. Gamma rays with energies of 1.17 and 1.33 MeV are the primary ionizing radiation emitted by cobalt-60. In exposures using cobalt-60 sources, test specimens must be enclosed in a lead-aluminum container to minimize dose-enhancement effects caused by low-energy scattered radiation (unless it has been demonstrated that these effects are negligible). For this lead-aluminum container, a minimum of 1.5 mm of lead surrounding an inner shield of 0.7 to 1.0 mm of aluminum is required. (See 18.104.22.168 and Practice E1249.)
The X-ray tester has proven to be a useful ionizing radiation effects testing tool because:
It offers a relatively high dose rate, in comparison to most cobalt-60 sources, thus offering reduced testing time.
The radiation is of sufficiently low energy that it can be readily collimated. As a result, it is possible to irradiate a single device on a wafer.
Radiation safety issues are more easily managed with an X-ray irradiator than with a cobalt-60 source. This is due both to the relatively low energy of the photons and due to the fact that the X-ray source can easily be turned off.
X-ray facilities are frequently less costly than comparable cobalt-60 facilities.
The principal radiation-induced effects discussed in this guide (energy deposition, absorbed-dose enhancement, electron-hole recombination) (see Appendix X1) will remain approximately the same when process changes are made to improve the performance of ionizing radiation hardness of a part that is being produced. This is the case as long as the thicknesses and compositions of the device layers are substantially unchanged. As a result of this insensitivity to process variables, a 10-keV X-ray tester is expected to be an excellent apparatus for process improvement and control.
Several published reports have indicated success in intercomparing X-ray and cobalt-60 gamma irradiations using corrections for dose enhancement and for electron-hole recombination. Other reports have indicated that the present understanding of the physical effects is not adequate to explain experimental results. As a result, it is not fully certain that the differences between the effects of X-ray and cobalt-60 gamma irradiation are adequately understood at this time. (See 8.2.1 and Appendix X2.) Because of this possible failure of understanding of the photon energy dependence of radiation effects, if a 10-keV X-ray tester is to be used for qualification testing or lot acceptance testing, it is recommended that such tests should be supported by cross checking with cobalt-60 gamma irradiations. For additional information on such comparison, see X2.2.4.
Because of the limited penetration of 10-keV photons, ionizing radiation effects testing must normally be performed on unpackaged devices (for example, at wafer level) or on unlidded devices.
ASTM F1467 - 11 Standard guide for use of an X-ray tester
Significance and Use