ASTM E1249-15(2021)

4.1?Division of the Co-60 Hardness Testing into Five Parts:?

4.2?Low-Energy Components in the SpectrumSome of the primary Co-60 gamma rays (1.17 and 1.33 MeV) produce lower energy photons by Compton scattering within the Co-60 source structure, within materials that lie between the source and the device under test, and within materials that lie beyond the device but contribute to backscattering. As a result of the complexity of these effects, the photon energy spectrum striking the device usually is not well known. This point is further discussed in Section 5 and Appendix X1. The presence of low-energy photons in the incident spectrum can result in dosimetry errors. This practice defines test procedures that should minimize dosimetry errors without the need to know the spectrum. These recommended procedures are discussed in 4.5, 4.6, Section 7, and Appendix X5.

4.3?Conversion to Equilibrium Absorbed Dose in the Device MaterialThe conversion from the measured absorbed dose in the material of the dosimeter (such as the CaF₂ of a TLD) to the equivalent absorbed dose in the material of interest (such as the SiO_{4.4?Absorbed-Dose Enhancement EffectsIf a higher atomic number material lies adjacent to a lower atomic number material, the energy deposition in the region adjacent to the interface is a complex function of the incident photon energy spectrum, the material composition, and the spatial arrangement of the source and absorbers. The absorbed dose near such an interface cannot be adequately determined using the procedure outlined in 4.3. Errors incurred by failure to account for these effects may, in unusual cases, exceed a factor of five. Because microelectronic devices characteristically contain layers of dissimilar materials with thicknesses of tens of nanometres, absorbed-dose enhancement effects are a characteristic problem for irradiation of such devices (see 6.1 and Appendix X2).}

4.5?Minimizing Absorbed-Dose Enhancement EffectsUnder some circumstances, absorbed-dose enhancement effects can be minimized by hardening the spectrum. Hardening is accomplished by the use of high atomic number absorbers to remove low energy components of the spectrum, and by minimizing the amount and proximity of low atomic number material to reduce softening of the spectrum by Compton scattering (see Sections 6 and 7).

4.6?Limits of the Dosimetry ErrorsTo correct for absorbed-dose enhancement by calculational methods would require a knowledge of the incident photon energy spectrum and the detailed structure of the device under test. To measure absorbed-dose enhancement would require methods for simulating the irradiation conditions and device geometry. Such corrections are impractical for routine hardness testing. However, if the methods specified in Section 7 are used to minimize absorbed-dose enhancement effects, errors due to the absence of a correction for these effects can be kept within bounds that may be acceptable for many users. An estimate of these error bounds for representative cases is given in Section 7 and Appendix X5.

4.7?Application to Non-Silicon DevicesThe material of this practice is primarily directed toward silicon based solid state electronic devices. The application of the material and recommendations presented here should be applied to gallium arsenide and other types of devices only with caution.

1.1?This practice covers recommended procedures for the use of dosimeters, such as thermoluminescent dosimeters (TLD's), to determine the absorbed dose in a region of interest within an electronic device irradiated using a Co-60 source. Co-60 sources are commonly used for the absorbed dose testing of silicon electronic devices.