ASTM F448-99
(Test Method)Test Method for Measuring Steady-State Primary Photocurrent
Test Method for Measuring Steady-State Primary Photocurrent
SCOPE
1.1 This test method covers the measurement of steady-state primary photocurrent, Ipp, generated in semiconductor devices when these devices are exposd to ionizing radiation. These procedures are intended for the measurement of photocurrents greater than 10-9 A-s/Gy(Si or Ge), in cases for which the relaxation time of the device being measured is less than 25% of the pulse width of the ionizing source. The validity of these procedures for ionizing dose rates as great as 108Gy(Si or Ge)/s has been established. The procedures may be used for measurements at dose rates as great as 1010Gy(Si or Ge)/s; however, extra care must be taken. Above 108Gy/s the package response may dominate the device response for technologies such as complementary metal-oxide semiconductor, (CMOS)/silicon-on sapphire (SOS). Additional precautions are also required when measuring photocurrents of 10-9 A-s/Gy(Si or Ge) or lower.
1.2 Setup, calibration, and test circuit evaluation procedures are also included in this test method.
1.3 Because of the variability between device types and in the requirements of different applications, the dose rate range over which any specific test is to be conducted is not given in this test method but must be specified separately.
1.4 The values state in International System of Units (SI) are to be regarded as the standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, 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.
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Designation:F 448–99
Standard Test Method for
Measuring Steady-State Primary Photocurrent
This standard is issued under the fixed designation F 448; 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.
1. Scope F526 Test Method for Measuring Dose for Use in Linear
Accelerator Pulsed Radiation Effects Tests
1.1 Thistestmethodcoversthemeasurementofsteady-state
primary photocurrent, I , generated in semiconductor devices
pp
3. Terminology
when these devices are exposed to ionizing radiation. These
3.1 Definitions:
procedures are intended for the measurement of photocurrents
−9
3.1.1 fall time, n—the time required for a signal pulse to
greater than 10 A·s/Gy(Si or Ge), in cases for which the
drop from 90 to 10% of its steady-state value.
relaxation time of the device being measured is less than 25%
3.1.2 primary photocurrent, n—the flow of excess charge
of the pulse width of the ionizing source. The validity of these
8 carriers across a p-n junction due to ionizing radiation creating
procedures for ionizing dose rates as great as 10 Gy(Si or
electron-hole pairs throughout the device. The charges associ-
Ge)/s has been established. The procedures may be used for
10 ated with this current are only those produced in the junction
measurements at dose rates as great as 10 Gy(Si or Ge)/s;
depletion region and in the bulk semiconductor material
however, extra care must be taken. Above 10 Gy/s the
approximately one diffusion length on either side of the
package response may dominate the device response for
depletion region (or to the end of the semiconductor material,
technologies such as complementary metal-oxide semiconduc-
whichever is shorter).
tor,(CMOS)/silicon-onsapphire(SOS).Additionalprecautions
−9
3.1.3 pulse width, n—the time a pulse-amplitude remains
are also required when measuring photocurrents of 10
above 50% of its maximum value.
A·s/Gy(Si or Ge) or lower.
3.1.4 rise time, n—the time required for a signal pulse to
1.2 Setup,calibration,andtestcircuitevaluationprocedures
rise from 10 to 90% of its steady-state value.
are also included in this test method.
1.3 Because of the variability between device types and in
4. Summary of Test Method
the requirements of different applications, the dose rate range
4.1 In this test method, the test device is irradiated in the
over which any specific test is to be conducted is not given in
primary electron beam of a linear accelerator. Both the irradia-
this test method but must be specified separately.
tion pulse and junction current (Fig. 1) are displayed and
1.4 The values stated in International System of Units (SI)
recorded. Placement of a thin, low atomic number (Z#13)
are to be regarded as standard. No other units of measurement
scattering plate in the beam is recommended to improve beam
are included in this standard.
uniformity; the consequences of the use of a scattering plate
1.5 This standard does not purport to address all of the
relatingtointerferencefromsecondaryelectronsaredescribed.
safety concerns, if any, associated with its use. It is the
The total dose is measured by an auxiliary dosimeter. The
responsibility of the user of this standard to establish appro-
steady-statevaluesofthedoserateandjunctioncurrentandthe
priate safety and health practices and determine the applica-
relaxation time of the junction current are determined from the
bility of regulatory limitations prior to use.
data trace and total dose.
4.2 In special cases, these parameters may be measured at a
2. Referenced Documents
single dose rate under one bias condition if the test is designed
2.1 ASTM Standards:
to generate information for such a narrow application. The
E668 Practice for theApplication ofThermoluminescence-
preferred approach, described in this test method, is to char-
Dosimetry(TLD)SystemsforDeterminingAbsorbedDose
2 acterize the radiation response of a device in a way that is
in Radiation-Hardness Testing of Electronic Devices
useful to many different applications. For this purpose, the
response to pulses at a number of different dose rates is
This test method is under the jurisdiction of ASTM Committee F-1 on required. Because of the bias dependence of the depletion
Electronics and is the direct responsibility of Subcommittee F01.11 on Quality and
Hardness Assurance.
Current edition approved Jan. 10, 1999. Published March 1999. Originally
published as F448–75T. Last previous edition F448M–94.
2 3
Annual Book of ASTM Standards, Vol 12.02. Annual Book of ASTM Standards, Vol 10.04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 448
FIG. 1 Ionization Radiation Pulse and Typical Primary Photocurrent Response
volume, it is possible that more than one bias level will be checked by measuring the current while irradiating the test
required during the photocurrent measurements. fixture in the absence of a test device.Air ionization contribu-
tions to the observed signal are proportional to applied field,
5. Significance and Use
whilethoseduetosecondaryemissioneffects(see6.2)arenot.
5.1 The steady-state photocurrent of a simple p-n junction
The effects of air ionization external to the device may be
diode is a directly measurable quantity that can be directly minimized by coating exposed leads with a thick layer of
related to device response over a wide range of ionizing
paraffin, silicone rubber, or nonconductive enamel or by
radiation. For more complex devices the junction photocurrent making the measurement in vacuum.
may not be directly related to device response.
6.2 Secondary Emission —Another spurious component
5.2 Zener Diode— In this device, the effect of the photo- of the measured current can result from charge emission from,
current on the Zener voltage rather than the photocurrent itself
or charge injection into, the test device and test circuit. This
is usually most important. The device is most appropriately may be minimized by shielding the surrounding circuitry and
testedwhilebiasedintheZenerregion.IntestingZenerdiodes
irradiating only the minimum area necessary to ensure irradia-
or precision voltage regulators, extra precaution must be taken tion of the test device. Reasonable estimates of the magnitude
tomakecertainthephotocurrentgeneratedinthedeviceduring
to be expected of current resulting from secondary-emission
irradiations does not cause the voltage across the device to effects can be made based on the area of metallic target
−11
change during the test.
materialsirradiated.Valuesgenerallyrangebetween10 and
−9 2
5.3 Bipolar Transistor—As device geometries dictate that 10 A·s/cm ·Gy, but the use of a scatter plate with an intense
photocurrent from the base-collector junction be much greater
beam may increase this current.
than current from the base-emitter junction, measurements are 6.3 Orientation— The effective dose to a semiconductor
usually made only on the collector-base junction with emitter
junction can be altered by changing the orientation of the test
open; however, sometimes, to obtain data for computer-aided unit with respect to the irradiating electron beam. Most
circuit analysis, the emitter-base junction photocurrent is also transistors and diodes may be considered “thin samples’’ (in
measured.
terms of the range of the irradiating electrons). However,
5.4 Junction Field-Effect Device—A proper photocurrent high-power devices may have mounting studs or thick-walled
measurement requires that the source be shorted (d-c) to the
casesthatcanacttoscattertheincidentbeam,therebyreducing
drain during measurement of the gate-channel photocurrent. In the dose received by the semiconductor chip. Care must be
tetrode-connected devices, the two gate-channel junctions
taken in the mounting of such devices.
should be monitored separately. 6.4 Bias—As the effective volume for the generation of
5.5 Insulated Gate Field-Effect Device—In this type of
photocurrent in p-n junction devices includes the space-charge
device, the true photocurrent is between the substrate and the region, I may be dependent on applied voltage. As applied
pp
channel, source, and drain regions. A current which can
voltagesapproachthebreakdownvoltage, I increasessharply
pp
generate voltage that will turn on the device may be measured due to avalanche multiplication. If the application of the test
by the technique used here, but it is due to induced conduc-
device is known, actual bias values should be used in the test.
tivity in the gate insulator and thus is not a junction photocur- If the application is not known, follow the methods for
rent.
checking the bias dependence given in Section 10.
6.5 Nonlinearity— Nonlinearities in photocurrent response
6. Interferences
result from saturation effects, injection level effects on life-
6.1 Air Ionization— A spurious component of the current
times, and, in the case of bipolar transistors, a lateral biasing
measuredduringaphotocurrenttestcanresultfromconduction
throughairionizedbytheirradiationpulse.Althoughthisisnot
Sawyer, J.A., and van Lint, V.A. J., “Calculations of High-Energy Secondary
likely to be a serious problem for photocurrents greater than
Electron Emission,” Journal of Applied Physics, JAPIA, Vol 35, No 6, June 1964,
−9
10 A·s/Gy(Si or Ge), the spurious contribution can easily be pp. 1706–1711.
F 448
effect which introduces a component of secondary photocur- 7.4 Cabling, to complete adequately the connection of the
rent into the primary photocurrent measurement. For these test circuit in the exposure area with the power supply and
reasons, photocurrent measurements must generally be made oscilloscopes in the data area.Any type of ungrounded wiring
over a wide range of dose rates. may be used to connect the power supply to the bias points of
6.6 Electrical Noise— Since linear accelerator facilities are the test circuit; however, coaxial cables properly terminated at
inherent sources of r-f electrical noise, good noise-minimizing the oscilloscope input are required for the signal leads.
techniques such as single-point ground, filtered d-c supply 7.5 Test Circuits— One of the following test circuits:
lines, etc., must be used in photocurrent measurements. 7.5.1 Resistor-Sampling Circuit (Fig. 2)—For most tests,
6.7 Temperature— Device characteristics are dependent on the configuration of Fig. 2(a) is appropriate. The resistors R
junction temperature; hence, the temperature of the test should serve as high-frequency isolation and must be at least 20 V.
be controlled. Unless otherwise agreed upon by the parties to The capacitor C supplies the charge during the current tran-
the test, measurements will be made at room temperature (23 sient; its value must be large enough that the decrease in
6 5°C). voltage during a current pulse is less than 10%. Capacitor C
6.8 Beam Homogeneity and Pulse-to-Pulse Repeatability— should be paralleled by a small (approximately 0.01 µF)
The intensity of a beam from a linear accelerator is likely to low-inductance capacitor to ensure that possible inductive
vary across its cross section. Since the pulse-shape monitor is
effects of the large capacitor are offset. The resistor R is to
placed at a different location from the device under test, the provide the proper termination (within 62%) for the coaxial
measured dose rate may be different from the dose rate to
cable used for the signal lead. When the photocurrents are
which the device was exposed. The spatial distribution and large, it is necessary to use a small-value resistor, R , in the
intensity of the beam may also vary from pulse to pulse. The
configuration of Fig. 2(b) to keep the signal small so as to
beam homogeneity and pulse-to-pulse repeatability associated maintain the bias across the junction within 10% of its
with a particular linear accelerator should be established by a
nominal value during the test. The response characteristics of
thoroughcharacterizationofitselectronbeampriortoperform- thiscircuitmustbeadequatetoensurethatthecurrentsignalis
ing a photocurrent measurement. accurately displayed (see 9.4).
6.9 Ionizing Dose— Each pulse of the linear accelerator 7.5.2 Current Transformer Circuit (Fig. 3)—In this circuit,
impartsadoseofradiationtoboththedeviceundertestandthe
R and Chavethesamesignificanceasintheresistor-sampling
device used for dosimetry. The ionizing dose deposited in a circuit, but it may be required that the signal cable monitoring
semiconductor device can change its operating characteristics. thecurrenttransformerbematchedtothecharacteristicimped-
As a result, the photocurrent that is measured after several ance of the transformer, in which case R would have this
pulses may be different from the photocurrent that is charac- impedance (within6 2%), which is specified by the manufac-
teristic of an unirradiated device. Care should be exercised to turer of the current transformer. The current transformer must
ensure that the ionizing dose delivered to the device under test have a bandwidth sufficient to ensure that the current signal is
is as low as possible consistent with the requirements for a accurately displayed. Rise time must be less than 10% of the
given dose rate and steady-state conditions. Generally, this is pulse width of the radiation pulse being used. The low
done by minimizing the number of pulses the device receives.
frequency cutoff of some commercial current transformers is
The dose must not exceed 10% of the failure dose for the such that significant droop may occur for pulse widths greater
device.
than 1 µs. Do not use a transformer for which this droop is
6.10 The test must be considered destructive if the photo- greater than 5% for the radiation pulse width used. When
current exceeds the manufacturer’s absolute limit.
monitoring large photocurrents, care must be taken that the
ampere-microsecond saturation of the current transformer is
7. Apparatus
not exceeded.
7.1 Regulated d-c Power Supply, with floating output to
7.6 Irradiation Pulse-Shape Monitor—Oneofthefollowing
produce the voltages required to bias the junction.
todevelopasignalproportionaltothedoseratedeliveredtothe
7.2 Oscilloscopes— Either a single dual-beam, or two
test device:
single-beam oscilloscopes that have adequate bandwidth capa-
7.6.1 Fast Signal-Diode, in the circuit configuration of Fig.
bility of both main frames and plug-ins to ensure that radiation
2 (a) as described in 7.5.1. The response of the diode must be
responseandpeaksteady-s
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