Significance and Use
4.1 Division of the Co-60 Hardness Testing into Five Parts:
4.1.1 The equilibrium absorbed dose shall be measured with a dosimeter, such as a TLD, located adjacent to the device under test. Alternatively, a dosimeter may be irradiated in the position of the device before or after irradiation of the device.
4.1.2 This absorbed dose measured by the dosimeter shall be converted to the equilibrium absorbed dose in the material of interest within the critical region within the device under test, for example the SiO2 gate oxide of an MOS device.
4.1.3 A correction for absorbed-dose enhancement effects shall be considered. This correction is dependent upon the photon energy that strikes the device under test.
4.1.4 A correlation should be made between the absorbed dose in the critical region (for example, the gate oxide mentioned in 4.1.2) and some electrically important effect (such as charge trapped at the Si/SiO2 interface as manifested by a shift in threshold voltage).
4.1.5 An extrapolation should then be made from the results of the test to the results that would be expected for the device under test under actual operating conditions.
Note 5: The parts of a test discussed in 4.1.2 and 4.1.3 are the subject of this practice. The subject of 4.1.1 is covered and referenced in other standards such as Practice E668 and ICRU Report 14. The parts of a test discussed in 4.1.4 and 4.1.5 are outside the scope of this practice.
4.2 Low-Energy Components in the Spectrum—Some 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 Material—The 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 SiO2 of the gate oxide of a device) is dependent on the incident photon energy spectrum. However, if the simplifying assumption is made that all incident photons have the energies of the primary Co-60 gamma rays, then the conversion from absorbed dose in the dosimeter to that in the device under test can be made using tabulated values for the energy absorption coefficients for the dosimeter and device materials. Where this simplification is appropriate, the error incurred by its use to determine equilibrium absorbed dose is usually less than 5 % (see 6.3).
4.4 Absorbed-Dose Enhancement Effects—If 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 Effects—Under 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 Errors—To 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 Devices—The 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.
Scope
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.
Note 1: This absorbed-dose testing is sometimes called “total dose testing” to distinguish it from “dose rate testing.” Note 2: The effects of ionizing radiation on some types of electronic devices may depend on both the absorbed dose and the absorbed dose rate; that is, the effects may be different if the device is irradiated to the same absorbed-dose level at different absorbed-dose rates. Absorbed-dose rate effects are not covered in this practice but should be considered in radiation hardness testing.
1.2 The principal potential error for the measurement of absorbed dose in electronic devices arises from non-equilibrium energy deposition effects in the vicinity of material interfaces.
1.3 Information is given about absorbed-dose enhancement effects in the vicinity of material interfaces. The sensitivity of such effects to low energy components in the Co-60 photon energy spectrum is emphasized.
1.4 A brief description is given of typical Co-60 sources with special emphasis on the presence of low energy components in the photon energy spectrum output from such sources.
1.5 Procedures are given for minimizing the low energy components of the photon energy spectrum from Co-60 sources, using filtration. The use of a filter box to achieve such filtration is recommended.
1.6 Information is given on absorbed-dose enhancement effects that are dependent on the device orientation with respect to the Co-60 source.
1.7 The use of spectrum filtration and appropriate device orientation provides a radiation environment whereby the absorbed dose in the sensitive region of an electronic device can be calculated within defined error limits without detailed knowledge of either the device structure or of the photon energy spectrum of the source, and hence, without knowing the details of the absorbed-dose enhancement effects.
1.8 The recommendations of this practice are primarily applicable to piece-part testing of electronic devices. Electronic circuit board and electronic system testing may introduce problems that are not adequately treated by the methods recommended here.
1.9 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
