ASTM E1249-00

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Designation: E 1249 – 00
Standard Practice for
Minimizing Dosimetry Errors in Radiation Hardness Testing
of Silicon Electronic Devices Using Co-60 Sources
This standard is issued under the fixed designation E1249; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope absorbed dose in the sensitive region of an electronic device
can be calculated within defined error limits without detailed
1.1 This practice covers recommended procedures for the
knowledge of either the device structure or of the photon
use of dosimeters, such as thermoluminescent dosimeters
energyspectrumofthesource,andhence,withoutknowingthe
(TLD’s), to determine the absorbed dose in a region of interest
details of the absorbed-dose enhancement effects.
within an electronic device irradiated using a Co-60 source.
1.8 The recommendations of this practice are primarily
Co-60 sources are commonly used for the absorbed dose
applicable to piece-part testing of electronic devices. Elec-
testing of silicon electronic devices.
tronic circuit board and electronic system testing may intro-
NOTE 1—This absorbed-dose testing is sometimes called “total dose
duce problems that are not adequately treated by the methods
testing” to distinguish it from “dose rate testing.”
recommended here.
NOTE 2—The effects of ionizing radiation on some types of electronic
1.9 This standard does not purport to address all of the
devicesmaydependonboththeabsorbeddoseandtheabsorbeddoserate;
safety problems, if any, associated with its use. It is the
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 responsibility of the user of this standard to establish appro-
effects are not covered in this practice but should be considered in
priate safety and health practices and determine the applica-
radiation hardness testing.
bility of regulatory limitations prior to use.
1.2 The principal potential error for the measurement of
2. Referenced Documents
absorbed dose in electronic devices arises from non-
2.1 ASTM Standards:
equilibriumenergydepositioneffectsinthevicinityofmaterial
E170 Terminology Relating to Radiation Measurements
interfaces.
and Dosimetry
1.3 Information is given about absorbed-dose enhancement
E 666 Practice for Calculating Absorbed Dose From
effects in the vicinity of material interfaces. The sensitivity of
Gamma or X-Radiation
such effects to low energy components in the Co-60 photon
E668 Practice for Application of Thermoluminescence-
energy spectrum is emphasized.
Dosimetry(TLD)SystemsforDeterminingAbsorbedDose
1.4 A brief description is given of typical Co-60 sources
in Radiation-Hardness Testing of Electronic Devices
with special emphasis on the presence of low energy compo-
E1250 Test Method for Application of Ionization Cham-
nents in the photon energy spectrum output from such sources.
bers to Assess the Low Energy Gamma Component of
1.5 Procedures are given for minimizing the low energy
Cobalt-60 Irradiators Used in Radiation-Hardness Testing
components of the photon energy spectrum from Co-60
of Silicon Electronic Devices
sources, using filtration.The use of a filter box to achieve such
2.2 International Commission on Radiation Units and
filtration is recommended.
Measurements Reports:
1.6 Information is given on absorbed-dose enhancement
ICRUReport14 RadiationDosimetry:X-RaysandGamma
effectsthataredependentonthedeviceorientationwithrespect
Rays With Maximum Photon Energies Between 0.6 and
to the Co-60 source.
50 MeV
1.7 The use of spectrum filtration and appropriate device
ICRU Report 18 Specification of High Activity Gamma-
orientation provides a radiation environment whereby the
Ray Sources
This practice is under the jurisdiction of ASTM Committee E10 on Nuclear
Technology and Applications and is the direct responsibility of Subcommittee
E10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices. Annual Book of ASTM Standards, Vol 12.02.
Current edition approved June 10, 2000. Published July 2000. Originally Available from International Commission on Radiation Units, 7910Woodmont
published as E1249–88. Last previous edition E1249–93. Ave., Washington, DC 20014.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1249
3. Terminology 4.1.1 Theequilibriumabsorbeddoseshallbemeasuredwith
a dosimeter, such as a TLD, located adjacent to the device
3.1 absorber—material that reduces the photon fluence rate
under test. Alternatively, a dosimeter may be irradiated in the
from a Co-60 source by any interaction mechanism.
position of the device before or after irradiation of the device.
3.2 absorbed-dose enhancement—increase (or decrease) in
the absorbed dose (as compared to the equilibrium absorbed 4.1.2 This absorbed dose measured by the dosimeter shall
be converted to the equilibrium absorbed dose in the material
dose) at a point in a material of interest. This can be expected
to occur near an interface with a material of higher or lower of interest within the critical region within the device under
test, for example the SiO gate oxide of an MOS device.
atomic number.
3.3 absorbed-dose enhancement factor— ratio of the ab-
4.1.3 A correction for absorbed-dose enhancement effects
sorbed dose at a point in a material of interest to the
shall be considered. This correction is dependent upon the
equilibrium absorbed dose in that same material.
photon energy that strikes the device under test.
3.4 average absorbed dose—mass weighted mean of the
4.1.4 A correlation should be made between the absorbed
absorbed dose over a region of interest.
dose in the critical region (for example, the gate oxide
3.5 average absorbed-dose enhancement factor—ratio of
mentioned in 4.1.2) and some electrically important effect
the average absorbed dose in a region of interest to the
(such as charge trapped at the Si/SiO interface as manifested
equilibrium absorbed dose (1).
by a shift in threshold voltage).
NOTE 3—For a description of the necessary conditions for measuring
4.1.5 Anextrapolationshouldthenbemadefromtheresults
equilibrium absorbed dose, see 6.3.1 and the term charged particle
of the test to the results that would be expected for the device
equilibrium in Terminology E170, which provides definitions and de-
under test under actual operating conditions.
scriptions of other applicable terms of this practice.
NOTE 5—The parts of a test discussed in 4.1.2 and 4.1.3 are the subject
3.6 beam trap—absorber that is designed to remove the
of this practice. The subject of 4.1.1 is covered and referenced in other
beam that has been transmitted through the device under test.
standards such as Practice E668 and ICRU Report 14. The parts of a test
Its purpose is to eliminate the scattering of the transmitted
discussed in 4.1.4 and 4.1.5 are outside the scope of this practice.
beam back into the device under test.
4.2 Low-Energy Components in the Spectrum—Someofthe
3.7 clean spectrum—one that is relatively free of low
primary Co-60 gamma rays (1.17 and 1.33 MeV) produce
energy components in the photon energy spectrum. For ex-
lower energy photons by Compton scattering within the Co-60
ample, for a Co-60 source an ideally clean spectrum would
source structure, within materials that lie between the source
contain only the primary 1.17 and 1.33 MeVphotons of Co-60
and the device under test, and within materials that lie beyond
decay.
the device but contribute to backscattering. As a result of the
3.8 equilibrium absorbed dose—absorbed dose at some
complexity of these effects, the photon energy spectrum
incremental volume within the material in which the condition
striking the device usually is not well known. This point is
of electron equilibrium (the energies, number, and direction of
further discussed in Section 5 andAppendix X1. The presence
charged particles induced by the radiation are constant
of low-energy photons in the incident spectrum can result in
throughout the volume) exists (see Terminology E170).
dosimetry errors. This practice defines test procedures that
NOTE 4—For practical purposes the equilibrium absorbed dose is the
shouldminimizedosimetryerrorswithouttheneedtoknowthe
absorbed dose value that exists in a material at a distance from any
spectrum. These recommended procedures are discussed in
interface with another material, greater than the range of the maximum
4.5, 4.6, Section 7, and Appendix X5.
energy secondary electrons generated by the incident photons.
4.3 Conversion to EquilibriumAbsorbed Dose in the Device
3.9 filter box—container, made of one or more layers of
Material—Theconversionfromthemeasuredabsorbeddosein
different materials, surrounding a device under test or a
thematerialofthedosimeter(suchastheCaF ofaTLD)tothe
dosimeter, or both, for the purpose of minimizing low energy
equivalentabsorbeddoseinthematerialofinterest(suchasthe
components of the incident photon energy spectrum.
SiO ofthegateoxideofadevice)isdependentontheincident
3.10 spectrum filter—materiallayerinterceptingphotonson
photon energy spectrum. However, if the simplifying assump-
their path between the Co-60 source and the device under test.
tion is made that all incident photons have the energies of the
Thepurposeofthefilteristoreducelowenergycomponentsof
primaryCo-60gammarays,thentheconversionfromabsorbed
the photon energy spectrum.
dose in the dosimeter to that in the device under test can be
3.11 spectrum hardening—process by which the fraction of
made using tabulated values for the energy absorption coeffi-
low energy components of the photon energy spectrum is
cients for the dosimeter and device materials. Where this
reduced.
simplification is appropriate, the error incurred by its use to
3.12 spectrum softening—process by which the fraction of
determine equilibrium absorbed dose is usually less than 5%
low energy components of the photon energy spectrum is
(see 6.3).
increased.
4.4 Absorbed-Dose Enhancement Effects— If a higher
4. Significance and Use
atomicnumbermaterialliesadjacenttoaloweratomicnumber
4.1 Division of the Co-60 Hardness Testing into Five Parts:
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 arrange-
The boldface numbers in parentheses refer to the list of references appended to
this practice. mentofthesourceandabsorbers.Theabsorbeddosenearsuch
E 1249
an interface cannot be adequately determined using the proce- 5.3 The following Co-60 source types are described briefly
dure outlined in 4.3. Errors incurred by failure to account for andlistedintheorderofdecreasingrelativespectrumhardness
these effects may, in unusual cases, exceed a factor of five. under the most favorable conditions of irradiation.
Becausemicroelectronicdevicescharacteristicallycontainlay-
NOTE 8—Diagrams of typical sources, a nominal photon energy spec-
ers of dissimilar materials with thicknesses of tens of nanome-
trum for each, and references are given in Appendix X1.
tres, absorbed-dose enhancement effects are a characteristic
5.3.1 A teletherapy source is a completely shielded source
problem for irradiation of such devices (see 6.1 andAppendix
from which the photon output is confined to a beam that is
X2).
usually collimated.The source output is normally directed into
4.5 Minimizing Absorbed-Dose Enhancement Effects—
a shielded room, but a shielded container, or box, is used in
Under some circumstances, absorbed-dose enhancement ef-
some cases.
fects can be minimized by hardening the spectrum. Hardening
5.3.2 Aroom sourceisasourcecontainedinashieldedwell
isaccomplishedbytheuseofhighatomicnumberabsorbersto
fromwhichitismovedintoashieldedroombyremotecontrol.
remove low energy components of the spectrum, and by
Its position in the room relative to walls, floor, and ceiling and
minimizing the amount and proximity of low atomic number
other scattering material determines the relative hardness of its
material to reduce softening of the spectrum by Compton
scattering (see Sections 6 and 7). effective photon energy spectrum. As a result, the photon
energy spectrum obtained in a room source can be relatively
4.6 Limits of the Dosimetry Errors— To correct for
hard or relatively soft as compared with other Co-60 sources.
absorbed-dose enhancement by calculational methods would
require a knowledge of the incident photon energy spectrum 5.3.3 Awater well sourceisacompletelyshieldedsourceat
and the detailed structure of the device under test. To measure a certain depth in a pool of water to which access for
absorbed-dose enhancement would require methods for simu- irradiations is by means of a water-tight container, or can. A
lating the irradiation conditions and device geometry. Such cylindrical array of sealed stainless-steel pencils containing
corrections are impractical for routine hardness testing. How- Co-60 pellets is the normal source geometry. The photon
ever,ifthemethodsspecifiedinSection7areusedtominimize energy spectrum depends on whether irradiations are made
absorbed-dose enhancement effects, errors due to the absence insideoroutsidethearray,withtheformerarrangementhaving
of a correction for these effects can be kept within bounds that the hardest spectrum.
may be acceptable for many users. An estimate of these error
5.3.4 A shielded-cavity irradiator is a self-contained
bounds for representative cases is given in Section 7 and
shielded source that is usually contained in steel and lead
Appendix X5. surrounding a cavity in which irradiations can be carried out.
4.7 Application to Non-Silicon Devices— The material of
Self-absorption and scattering affect the photon energy spec-
this practice is primarily directed toward silicon based solid trum.
state electronic devices. The application of the material and
recommendations presented here should be applied to gallium
6. Factors Affecting Absorbed Dose Measurement
arsenide and other types of devices only with caution.
6.1 Absorbed-dose Enhancement Near Material Interfaces:
6.1.1 For illustration, most semiconductor devices can be
5. Description of Co-60 Sources
represented as one-dimensional planar layers of active and
5.1 Cobalt-60principallydecaysbyemittinggammaraysof
structural materials. The energy deposition by second
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