ASTM F996-98(2003)
(Test Method)Standard Test Method for Separating an Ionizing Radiation-Induced MOSFET Threshold Voltage Shift Into Components Due to Oxide Trapped Holes and Interface States Using the Subthreshold Current-Voltage Characteristics
Standard Test Method for Separating an Ionizing Radiation-Induced MOSFET Threshold Voltage Shift Into Components Due to Oxide Trapped Holes and Interface States Using the Subthreshold Current-Voltage Characteristics
SIGNIFICANCE AND USE
The electrical properties of gate and field oxides are altered by ionizing radiation. The time dependent and dose rate effects of the ionizing radiation can be determined by comparing pre- and post-irradiation voltage shifts, ΔVot andΔ Vit. This test method provides a means for evaluation of the ionizing radiation response of MOSFETs and isolation parasitic MOSFETs.
The measured voltage shifts, ΔVot andΔ Vit, can provide a measure of the effectiveness of processing variations on the ionizing radiation response.
This technique can be used to monitor the total-dose response of a process technology.
SCOPE
1.1 This test method covers the use of the subthreshold charge separation technique for analysis of ionizing radiation degradation of a gate dielectric in a metal-oxide-semiconducter-field-effect transistor (MOSFET) and an isolation dielectic in a parasitic MOSFET. The subthreshold technique is used to separate the ionizing radiation-induced inversion voltage shift, V INV into voltage shifts due to oxide trapped charge, Vot and interface traps, Vit. This technique uses the pre- and post-irradiation drain to source current versus gate voltage characteristics in the MOSFET subthreshold region.
1.2 Procedures are given for measuring the MOSFET subthreshold current-voltage characteristics and for the calculation of results.
1.3 The application of this test method requires the MOSFET to have a substrate (body) contact.
1.4 Both pre- and post-irradiation MOSFET subthreshold source or drain curves must follow an exponential dependence on gate voltage for a minimum of two decades of current.
1.5 The values given in SI units are to be regarded as standard. No other units of measurement are included in this test method.
1.6 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: F996 – 98 (Reapproved 2003)
Standard Test Method for
Separating an Ionizing Radiation-Induced MOSFET
Threshold Voltage Shift Into Components Due to Oxide
Trapped Holes and Interface States Using the Subthreshold
Current–Voltage Characteristics
ThisstandardisissuedunderthefixeddesignationF996;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 This test method covers the use of the subthreshold
bility of regulatory limitations prior to use.
charge separation technique for analysis of ionizing radiation
degradation of a gate dielectric in a metal-oxide-
2. Terminology
semiconducter-field-effect transistor (MOSFET) and an isola-
, ,
2 3 4 2.1 Definitions of Terms Specific to This Standard:
tion dielectic in a parasitic MOSFET. The subthreshold
2.1.1 anneal conditions—the bias and temperature of the
technique is used to separate the ionizing radiation-induced
MOSFET in the time period between irradiation and measure-
inversion voltage shift, DV into voltage shifts due to oxide
INV
ment.
trapped charge,D V and interface traps,D V . This technique
ot it
2.1.2 doping concentration, N—N-or P-type doping, is the
usesthepre-andpost-irradiationdraintosourcecurrentversus
concentration of the MOSFET channel region adjacent to the
gate voltage characteristics in the MOSFET subthreshold
oxide/silicon interface.
region.
2.1.3 inversion current, I —theMOSFETchannelcurrent
INV
1.2 Procedures are given for measuring the MOSFET sub-
at a gate-source voltage equal to the inversion voltage.
thresholdcurrent-voltagecharacteristicsandforthecalculation
2.1.4 inversion voltage, V —the gate-source voltage cor-
INV
of results.
responding to a surface potential of 2f .
B
1.3 The application of this test method requires the MOS-
2.1.5 irradiation biases—the biases on the gate, drain,
FET to have a substrate (body) contact.
source, and substrate of the MOSFET during irradiation.
1.4 Both pre- and post-irradiation MOSFET subthreshold
2.1.6 midgap current, I —the MOSFET channel current
MG
source or drain curves must follow an exponential dependence
at a gate-source voltage equal to the midgap voltage.
on gate voltage for a minimum of two decades of current.
2.1.7 midgap voltage, V —the gate-source voltage corre-
MG
1.5 The values given in SI units are to be regarded as
sponding to a surface potential of f .
B
standard. No other units of measurement are included in this
2.1.8 oxide thickness, t —the thickness of the oxide of the
ox
test method.
MOSFET under test.
1.6 This standard does not purport to address all of the
2.1.9 potential, f —the potential difference between the
B
safety concerns, if any, associated with its use. It is the
Fermi level and the intrinsic Fermi level.
2.1.10 subthreshold swing—the change in the gate-source
This test method is under the jurisdiction of ASTM Committee F01 on
voltage per change in the log source or drain current of the
Electronics and is the direct responsibility of Subcommittee F01.11 on Nuclear and
MOSFET channel current below the inversion current. The
Space Radiation Effects.
Current edition approved Dec. 1, 2003. Published December 2003. Originally value of the subthreshold swing is expressed in V/decade (of
approved in 1991. Last previous edition approved in 1998 as F996–98. DOI:
current).
10.1520/F0996-98R03.
2.1.11 surface potential, f —the potential at the MOSFET
s
ForformulationofsubthresholdchargeseparationtechniqueseeMcWhorter,P.
semiconductor surface measured with respect to the intrinsic
J.andP.S.Winokur,“SimpleTechniqueforSeparatingtheEffectsofInterfaceTraps
and Trapped Oxide Charge in MOS Transistors,” Applied Physics Letters, Vol 48,
Fermi level.
1986, pp. 133–135.
DNA-TR-89-157, Subthreshold Technique for Fixed and Interface Trapped
3. Summary of Test Method
Charge Separation in Irradiated MOSFETs, available from National Technical
3.1 The subthreshold charge separation technique is based
Information Service, 5285 Port Royal Rd., Springfield, VA 22161.
Saks, N. S., and Anacona, M. G., “Generation of Interface States by Ionizing
on standard MOSFET subthreshold current-voltage character-
Radiation at 80K Measured by Charge Pumping and Subthreshold Slope Tech-
istics.The subthreshold drain or source current at a fixed drain
niques,” IEEE Transactions on Nuclear Science, Vol NS–34, No. 6, 1987, pp.
to source voltage, V , is measured as a function of gate
1348–1354. DS
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959, United States.
F996 – 98 (2003)
voltage from the leakage current (or limiting resolution of the 5.4 Leakage Current—Because the MOSFET midgap cur-
measurement apparatus) through inversion. The drain current rent is below the capabilities of practical current-voltage
V
G
and gate voltage are related by I a 10 . When plotted as log measurement instrumentation, extrapolation of the subthresh-
D
I versus V , the linear I-V characteristic can be extrapolated old swing is required for the determination of a MOSFET
D G
toacalculatedmidgapcurrent, I .Bycomparingthepre-and midgap voltage. Extrapolation of ideal linear MOSFET sub-
MG
post-irradiationcharacteristics,themidgapvoltageshift,D V threshold current-voltage characteristics is unambiguous, be-
MG
can be determined.The value ofD V is equal to DV , which cause of the constant subthreshold swing.An example of near
MG ot
is the voltage shift due to oxide trapped charge.The difference ideal subthreshold characteristics is given in Fig. 2, where the
−11
between the inversion voltage shift, DV , andD V is equal subthreshold swing is relatively constant between 10 and
INV MG
−6
to DV , which is the voltage shift due to interface traps. This 10 A. Nonideal subthreshold characteristics, that are aberra-
it
procedure is shown in Fig. 1 for a p-channel MOSFET. tions from the theoretical linear subthreshold swing, can
complicate the subthreshold swing extrapolation to the midgap
4. Significance and Use
voltage. For subthreshold characteristics that have multiple
4.1 The electrical properties of gate and field oxides are
subthresholdswings,thevalueofthemidgapvoltagewouldbe
alteredbyionizingradiation.Thetimedependentanddoserate dependent on the values of the subthreshold current from
effects of the ionizing radiation can be determined by compar- which the extrapolation is made. Nonideal subthreshold char-
ing pre- and post-irradiation voltage shifts, DV andD V .This acteristicsarecausedbyMOSFETleakagecurrentsthatcanbe
ot it
test method provides a means for evaluation of the ionizing either independent of, or a function of, gate-source voltage.
radiation response of MOSFETs and isolation parasitic MOS- 5.4.1 Junction Leakage Current—This leakage current is
FETs. from the drain to the substrate and is independent of gate-
4.2 Themeasuredvoltageshifts, DV andD V ,canprovide source voltage. Junction leakage current masks the actual
ot it
a measure of the effectiveness of processing variations on the MOSFET channel subthreshold current below the leakage
ionizing radiation response. current level. Junction leakage current is easily distinguished
4.3 This technique can be used to monitor the total-dose from the channel subthreshold current as is shown in Fig. 2 by
−11
response of a process technology. the flat section of the drain current, I , below 10 A. This
D
figurealsoshowstheadvantageofusingthesourcecurrent, I ,
S
5. Interferences
forextrapolation.Thesourcecurrentisnotaffectedbyjunction
5.1 Temperature Effects—The subthreshold drain current leakage so that a measure of the MOSFET channel current is
varies as the exponential of qf /kT, and other terms which
obtainedtotheinstrumentationnoiselevel.However,ifthereis
B
vary as a function of temperature. Therefore, the temperature notaseparatesourceandsubstratecontact(forexample,power
ofthemeasurementshouldbecontrolledtowithin 62°C,since
MOSFETs),thedraincurrentmustbeused.Onlythepartofthe
the technique requires a comparison of pre- and post- subthreshold curve above the junction leakage or instrumenta-
irradiation data. At cryogenic temperatures, this test method
tion noise level should be used for extrapolation. A minimum
may give misleading results. of two decades of source or drain current above the leakage or
5.2 Floating Body (Kink) Effects—Floating body effects noise is required for application of this test method.
occur in MOSFETs without body (substrate) ties. This test 5.4.2 Gate Leakage—Gate leakage can be any combination
method should not be applied to a MOSFET without a of leakage from the gate to source, drain, or substrate.
substrate or substrate/source contact.
Typically this leakage will be a function of the gate-source
5.3 Short Channel Effects—To minimize drain voltage de- voltage. If gate leakage is greater than 1.0 µA for any
pendence on the subthreshold curve, a small drain measure-
gate-source voltage, the test method should not be applied.
ment voltage is recommended but not necessary. Gate leakages less than 1.0 µA can still cause nonideal
FIG. 1 Determination of Radiation Induced Voltage Shift for FIG. 2 Near Ideal Subthreshold Characteristics from an
p-Channel MOSFET n-Channel Transistor
F996 – 98 (2003)
subthreshold characteristics. The minimum value of the sub-
threshold source or drain current used for extrapolation to the
midgapvoltagemustbeaboveanychangesinthesubthreshold
swing that can be attributed to gate leakage. Plotting the log of
thegateleakagealongwithlogsourceanddraincurrentonthe
same graph, will aid in the determination of gate leakage
effects on the drain and source subthreshold swing.
5.4.3 Edge Leakage Current—MostmicrocircuitMOSFETs
useanopengeometrylayoutsothationizingradiationinduced
drain to source leakage can occur in n-channel devices outside
of the intentional MOSFET channel. The effect of this edge
leakage on the subthreshold swing is dependent on the aspect
ratios and threshold voltages of the intentional and parasitic
MOSFETs. The aspect ratio of the parasitic MOSFET would
usually be much smaller than a standard width MOSFET FIG. 4 Example of a Parasitic MOSFET Induced Deviation from
the Ideal Linear Subthreshold Swing
layout. Thus, when the MOSFET channel is in strong inver-
sion, the channel current will typically dominate. However, as
the channel current is reduced, edge leakage can go from a
above any subthreshold swing deviation is required for appli-
minimal fraction to dominating the measured drain or source
cationofthistestmethod.Openandclosed(annular)geometry
current if the parasitic MOSFET inversion voltage is less than
layouts can be used to separate edge leakage current from the
the intentional MOSFET. This effect can be observed in the
MOSFET channel current.
measured subthreshold characteristics as a deviation from the
5.4.4 Backchannel and Sidewall Leakage in a SOI
ideal linear subthreshold curve that is a function of the
MOSFET—Backchannel leakage arises from a parasitic MOS-
gate-source voltage. Examples of parasitic MOSFET induced
FET located at the interface between the epitaxial silicon and
deviationsfromtheideallinearsubthresholdswingaregivenin
the insulator. Sidewall leakages arise from the parasitic MOS-
Fig. 3 and Fig. 4.In Fig. 3, the subthreshold swing changes
FET formed at the edges of the intentional MOSFET. These
from the initial swing near inversion to a much larger mV/
parasitics distort the subthreshold curve in the same manner as
decade swing. In Fig. 4, a more pronounced deviation is
described in 5.4.3.
shown. The section of the subthreshold curve that should be
used for extrapolation to the midgap voltage is shown in both 6. Apparatus
figures. The upper section of the subthreshold curve above the
6.1 To measure the subthreshold current-voltage character-
lower current level deviations was used. Any lower current
istics of a MOSFET, the instrumentation required consists of,
change in the subthreshold swing from the initial subthreshold
as a minimum, two voltage sources and four ammeters.
swing below strong inversion should be considered a parasitic
6.2 Thepowersuppliesareusedtoapplyvoltagetothegate
MOSFETinduced deviation. Only the part of the subthreshold
and drain of the MOSFET. The ammeters are used to measure
curve above this deviation should be used for extrapolation as
the gate, drain, source, and substrate currents.
is shown in Fig. 3 and Fig. 4. Some n-channel MOSFETs may
6.3 For MOSFETs that have a common source/substrate
have post-irradiation edge leakage sufficiently large to prevent
contact, only three ammeters are required.
any observation of a subthreshold swing. The subthreshold
6.4 For a typical digital microcircuit MOSFET the voltage
charge separation technique cannot be applied to these
sources and ammeters should meet the specification given in
samples.Aminimumoftwodecadesofsourceordraincurrent
Table 1.
6.5 For a power, parasitic field oxide, or high voltage linear
MOSFET, the maximum voltage requirement for the gate-
source power supply can be substantially larger. Field oxide
fieldeffecttransistor(FET)smayhavepre-irradiationthreshold
voltages of several hundred volts. The gate-source power
supplyisrequiredtobesuchthattheMOSFETdrain-to-source
subthresholdcurrentcanbemeasuredfromleakageintostrong
inversion. The resolution of a gate-source power supply must
be at least 0.5% of the maximum gate-source voltage, for the
MOSFET subthreshold current-voltage measurement.
6.6 The test fixture, containing the MOSFET under test
(DUT), and the cabling connecting the test instrumentation,
TABLE 1 Minimal Instrumentation Specifications
Drain-source power supply 610 V, 0.01 V resolution
FIG. 3 Example of a Parasitic MOSFET Induced Deviation From
Gate-source power supply 610 V, 0.005 V resolution
the Ideal Linear Subthreshold Swing Ammeters 610 mA, 10 pAresolution
F996 – 98 (2003)
must be designed for low current measurements specified in gate-source voltage step size is adjusted so that a minimum of
Table 1. Probe stations can be used for MOSFETs in wafer twoorthreedrainandsourcecurrentsaremeasuredperdecade
form as long as a current resolution of 100 pA can be of subthreshold current.
maintained. 9.3.3 The source and substrate are grounded.
6.7 The test fixture may be the same or different from the 9.4 Measure the DUT subthreshold current-voltage charac-
fixture(s) use
...
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