1、Designation: F996 11Standard Test Method forSeparating an Ionizing Radiation-Induced MOSFETThreshold Voltage Shift Into Components Due to OxideTrapped Holes and Interface States Using the SubthresholdCurrentVoltage Characteristics1This standard is issued under the fixed designation F996; the number
2、immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This
3、 test method covers the use of the subthresholdcharge separation technique for analysis of ionizing radiationdegradation of a gate dielectric in a metal-oxide-semiconductor-field-effect transistor (MOSFET) and an isola-tion dielectric in a parasitic MOSFET.2,3,4The subthresholdtechnique is used to s
4、eparate the ionizing radiation-inducedinversion voltage shift, DVINVinto voltage shifts due to oxidetrapped charge, DVotand interface traps, DVit. This techniqueuses the pre- and post-irradiation drain to source current versusgate voltage characteristics in the MOSFET subthresholdregion.1.2 Procedur
5、es are given for measuring the MOSFET sub-threshold current-voltage characteristics and for the calculationof results.1.3 The application of this test method requires the MOS-FET to have a substrate (body) contact.1.4 Both pre- and post-irradiation MOSFET subthresholdsource or drain curves must foll
6、ow an exponential dependenceon gate voltage for a minimum of two decades of current.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated
7、with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:5E666 Practice for CalculatingAbsorbed Dose From Gammaor X Radiat
8、ionE668 Practice for Application of Thermoluminescence-Dosimetry (TLD) Systems for DeterminingAbsorbed Dosein Radiation-Hardness Testing of Electronic DevicesE1249 Practice for Minimizing Dosimetry Errors in Radia-tion Hardness Testing of Silicon Electronic Devices UsingCo-60 SourcesE1894 Guide for
9、Selecting Dosimetry Systems for Applica-tion in Pulsed X-Ray Sources3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 anneal conditionsthe current and/or voltage biasand temperature of the MOSFET in the time period betweenirradiation and measurement.3.1.2 doping concentrationn-o
10、rp-type doping, is theconcentration of the dopant in the MOSFET channel regionadjacent to the oxide/silicon interface.3.1.3 Fermi levelthis value describes the top of thecollection of electron energy levels at absolute zero tempera-ture.3.1.4 intrinsic Fermi levelthe energy level that the Fermilevel
11、 has in the absence of any doping.3.1.5 inversion current, IINVthe MOSFETchannel currentat a gate-source voltage equal to the inversion voltage.3.1.6 inversion voltage, VINVthe gate-source voltage cor-responding to a surface potential of 2fB.1This test method is under the jurisdiction of ASTM Commit
12、tee F01 onElectronics and is the direct responsibility of Subcommittee F01.11 on Nuclear andSpace Radiation Effects.Current edition approved Jan. 1, 2011. Published January 2011. Originallyapproved in 1991. Last previous edition approved in 2010 as F996 10. DOI:10.1520/F0996-11.2McWhorter, P. J. and
13、 P. S. Winokur, “Simple Technique for Separating theEffects of Interface Traps and Trapped Oxide Charge in MOS Transistors,” AppliedPhysics Letters, Vol 48, 1986, pp. 133135.3DNA-TR-89-157, Subthreshold Technique for Fixed and Interface TrappedCharge Separation in Irradiated MOSFETs, available from
14、National TechnicalInformation Service, 5285 Port Royal Rd., Springfield, VA 22161.4Saks, N. S., and Anacona, M. G., “Generation of Interface States by IonizingRadiation at 80K Measured by Charge Pumping and Subthreshold Slope Tech-niques,” IEEE Transactions on Nuclear Science, Vol NS34 , No. 6, 1987
15、, pp.13481354.5For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor
16、Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.7 irradiation biasesthe biases on the gate, drain,source, and substrate of the MOSFET during irradiation.3.1.8 midgap current, IMGthe MOSFET channel currentat a gate-source voltage equal to the midgap voltage.3.1.9 midgap voltag
17、e, VMGthe gate-source voltage corre-sponding to a surface potential of fB.3.1.10 oxide thickness, toxthe thickness of the oxide of theMOSFET under test.3.1.11 potential, fBthe potential difference between theFermi level and the intrinsic Fermi level.3.1.12 subthreshold swingthe change in the gate-so
18、urcevoltage per change in the log source or drain current of theMOSFET channel current below the inversion current. Thevalue of the subthreshold swing is expressed in V/decade (ofcurrent).3.1.13 surface potential, fsthe potential at the MOSFETsemiconductor surface measured with respect to the intrin
19、sicFermi level.4. Summary of Test Method4.1 The subthreshold charge separation technique is basedon standard MOSFET subthreshold current-voltage character-istics. The subthreshold drain or source current at a fixed drainto source voltage, VDS, is measured as a function of gatevoltage from the leakag
20、e current (or limiting resolution of themeasurement apparatus) through inversion. The drain currentand gate voltage are related by IDa 10VG.When plotted as log IDversus VG, the linear I-V characteristic can be extrapolated to acalculated midgap current, IMG. By comparing the pre- andpost-irradiation
21、 characteristics, the midgap voltage shift, DVMGcan be determined. The value of DVMGis equal to DVot, whichis the voltage shift due to oxide trapped charge. The differencebetween the inversion voltage shift, DVINV, and DVMGis equalto DVit, which is the voltage shift due to interface traps. Thisproce
22、dure is shown in Fig. 1 for a p-channel MOSFET.5. Significance and Use5.1 The electrical properties of gate and field oxides arealtered by ionizing radiation. The method for determining thedose delivered by the source irradiation is discussed in Prac-tices E666, E668, E1249, and Guide E1894. The tim
23、e depen-dent and dose rate effects of the ionizing radiation can bedetermined by comparing pre- and post-irradiation voltageshifts, DVotand DVit. This test method provides a means forevaluation of the ionizing radiation response of MOSFETs andisolation parasitic MOSFETs.5.2 The measured voltage shif
24、ts, DVotand DVit, can providea measure of the effectiveness of processing variations on theionizing radiation response.5.3 This technique can be used to monitor the total-doseresponse of a process technology.6. Interferences6.1 Temperature EffectsThe subthreshold drain currentvaries as the exponenti
25、al of qfB/kT, and other terms whichvary as a function of temperature. Therefore, the temperatureof the measurement should be controlled to within 6 2C,since the technique requires a comparison of pre- and post-irradiation data. At cryogenic temperatures, this test methodmay give misleading results.4
26、6.2 Floating Body (Kink) EffectsFloating body effectsoccur in MOSFETs without body (substrate) ties. This testmethod should not be applied to a MOSFET without asubstrate or substrate/source contact.6.3 Short Channel EffectsTo minimize drain voltage de-pendence on the subthreshold curve, a small drai
27、n measure-ment voltage is recommended but not necessary.6.4 Leakage CurrentBecause the MOSFET midgap cur-rent is below the capabilities of practical current-voltagemeasurement instrumentation, extrapolation of the subthresh-old swing is required for the determination of a MOSFETmidgap voltage. Extra
28、polation of ideal linear MOSFET sub-threshold current-voltage characteristics is unambiguous, be-cause of the constant subthreshold swing. An example of nearideal subthreshold characteristics is given in Fig. 2, where thesubthreshold current swing is relatively constant between 1011and 106A. Nonidea
29、l subthreshold characteristics, that areaberrations from the theoretical linear subthreshold swing, cancomplicate the subthreshold current swing extrapolation to themidgap voltage. For subthreshold characteristics that havemultiple subthreshold swings, the value of the midgap voltagewould be depende
30、nt on the values of the subthreshold currentFIG. 1 Determination of Radiation Induced Voltage Shift forp-Channel MOSFETFIG. 2 Near Ideal Subthreshold Characteristics from ann-Channel TransistorF996 112from which the extrapolation is made. Nonideal subthresholdcharacteristics are caused by MOSFET lea
31、kage currents thatcan be either independent of, or a function of, gate-sourcevoltage.6.4.1 Junction Leakage CurrentThis leakage current isfrom the drain to the substrate and is independent of gate-source voltage. Junction leakage current masks the actualMOSFET channel subthreshold current below the
32、leakagecurrent level. Junction leakage current is easily distinguishedfrom the channel subthreshold current as is shown in Fig. 2 bythe flat section of the drain current, ID, below 1011A. Thisfigure also shows the advantage of using the source current, IS,for extrapolation. The source current is not
33、 affected by junctionleakage so that a measure of the MOSFET channel current isobtained to the instrumentation noise level. However, if there isnot a separate source and substrate contact (for example, powerMOSFETs), the drain current must be used. Only the part of thesubthreshold curve above the ju
34、nction leakage or instrumenta-tion noise level should be used for extrapolation. A minimumof two decades of source or drain current above the leakage ornoise is required for application of this test method.6.4.2 Gate LeakageGate leakage can be any combinationof leakage from the gate to source, drain
35、, or substrate.Typically this leakage will be a function of the gate-sourcevoltage. If gate leakage is greater than 1.0 A for anygate-source voltage, the test method should not be applied.Gate leakages less than 1.0 A can still cause nonidealsubthreshold characteristics. The minimum value of the sub
36、-threshold source or drain current used for extrapolation to themidgap voltage must be above any changes in the subthresholdswing that can be attributed to gate leakage. Plotting the log ofthe gate leakage along with log source and drain current on thesame graph, will aid in the determination of gat
37、e leakageeffects on the drain and source subthreshold swing.6.4.3 Edge Leakage CurrentMost microcircuit MOSFETsuse an open geometry layout so that ionizing radiation induceddrain to source leakage can occur in n-channel devices outsideof the intentional MOSFET channel. The effect of this edgeleakage
38、 on the subthreshold swing is dependent on the aspectratios and threshold voltages of the intentional and parasiticMOSFETs. The aspect ratio of the parasitic MOSFET wouldusually be much smaller than a standard width MOSFETlayout. Thus, when the MOSFET channel is in strong inver-sion, the channel cur
39、rent will typically dominate. However, asthe channel current is reduced, edge leakage can go from aminimal fraction to dominating the measured drain or sourcecurrent if the parasitic MOSFET inversion voltage is less thanthe intentional MOSFET. This effect can be observed in themeasured subthreshold
40、characteristics as a deviation from theideal linear subthreshold curve that is a function of thegate-source voltage. Examples of parasitic MOSFET induceddeviations from the ideal linear subthreshold swing are given inFig. 3 and Fig. 4.InFig. 3, the subthreshold swing changesfrom the initial swing ne
41、ar inversion to a much larger mV/decade swing. In Fig. 4, a more pronounced deviation isshown. The section of the subthreshold curve that should beused for extrapolation to the midgap voltage is shown in bothfigures. The upper section of the subthreshold curve above thelower current level deviations
42、 was used. Any lower currentchange in the subthreshold swing from the initial subthresholdswing below strong inversion should be considered a parasiticMOSFET induced deviation. Only the part of the subthresholdcurve above this deviation should be used for extrapolation asis shown in Fig. 3 and Fig.
43、4. Some n-channel MOSFETs mayhave post-irradiation edge leakage sufficiently large to preventany observation of a subthreshold swing. The subthresholdcharge separation technique cannot be applied to thesesamples.Aminimum of two decades of source or drain currentabove any subthreshold swing deviation
44、 is required for appli-cation of this test method. Open and closed (annular) geometrylayouts can be used to separate edge leakage current from theMOSFET channel current.6.4.4 Backchannel and Sidewall Leakage in a SOIMOSFETIn a silicon-on-insulator (SOI) MOSFET, the back-channel leakage arises from a
45、 parasitic MOSFET located at theinterface between the epitaxial silicon and the insulator. Side-wall leakages arise from the parasitic MOSFET formed at theFIG. 3 Example of a Parasitic MOSFET Induced Deviation Fromthe Ideal Linear Subthreshold SwingFIG. 4 Example of a Parasitic MOSFET Induced Deviat
46、ion fromthe Ideal Linear Subthreshold SwingF996 113edges of the intentional MOSFET. These parasitics distort thesubthreshold curve in the same manner as described in 6.4.3.7. Apparatus7.1 To measure the subthreshold current-voltage character-istics of a MOSFET, the instrumentation required consists
47、of,as a minimum, two voltage sources and four ammeters.7.2 The power supplies are used to apply voltage to the gateand drain of the MOSFET. The ammeters are used to measurethe gate, drain, source, and substrate currents.7.3 For MOSFETs that have a common source/substratecontact, only three ammeters
48、are required.7.4 For a typical digital microcircuit MOSFET the voltagesources and ammeters should meet the specification given inTable 1.7.5 For a power, parasitic field oxide, or high voltage linearMOSFET, the maximum voltage requirement for the gate-source power supply can be substantially larger.
49、 Field oxidefield effect transistor (FETs) may have pre-irradiation thresholdvoltages of several hundred volts. The gate-source powersupply is required to be such that the MOSFET drain-to-sourcesubthreshold current can be measured from leakage into stronginversion. The resolution of a gate-source power supply mustbe at least 0.5 % of the maximum gate-source voltage, for theMOSFET subthreshold current-voltage measurement.7.6 The test fixture, containing the MOSFET under test(DUT), and the cabling connecting the test instrumentation,must be designed fo
