Chapter 6

~TRANSIENT-RADIATION EFFECTS ON 1

ELECTRONICS (TREE) PHENOMENA.

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INTRODUCTION _

• This chapter introduces the subject oftransient-radiation effects on electronics andprovides a basic description of the interaction ofnuclear radiation with matter as it applies toelectronic components. The response of elec­tronics to the radiation from a nuclear weaponburst depends not only on the radiation presentat the electronics but also depends on the spe­cific operating state of the electronics at thetime of the radiation exposure and on the spe­cific electronics in the system. A knowledge ofthe individual characteristics of the circuits con­tained in an electronics package, of the exactelectronic components used in the circuits, andof the specific construction techniques and ma­terials used in making the electronic componentsconstitutes the necessary background for deter­mining the radiation response of the electronicsystem. This chapter explains how the radiationinteracts with different materials to produce awide variety of effects. These material effectsare used in the discussions of the component­part responses in Section VIl of Chapter 9. Sec­tion IV of Chapter 14 contains a brief discussionof circuit and electronic-system response supple­mented with discussions of general electricalresponses of classes of systems (radios, radar,etc.).

• The cumbersome name applied to theclass of effects that are the subject of this chap­ter, transient-radiation effects on electronics, isgenerally abbreviated to the acronym TREE. Ingeneral, TREE means those effects occurring in

electronics as a result of the transient radiationfrom a nuclear weapon explosion or as a resultof an environment designed to simulate thatradiation. It should be understood that thetransient-radiation from a nuclear explosion canbe and is simulated by the use of controlledsources of steady-state and transient radiationsince the environment produced can be correlated

. to the actual environment of interest, and therebythe effects on electronics can be studied.

• Although the weapons radiation environ­ment lasts for a very short time, its effect onelectronics can be both short or long term. Foremphasis, it must be stated that the word tran­sient in TREE modifies the word radiation anddoes not modify the word effects: the effectsmay be transient, semipermanent, or permanent.

• The term electronics means anyone orall of the following: electronic component parts,electrol!,lic component parts assembled into a cir­cuit, and circuits assembled into a system. TREEstudies also may include the response of electro­mechanical components connected to the elec­tronics, e.g., gyros, inertial instruments, etc.TREE does, however, specifically exclude othertypes of component parts or systems such ashydraulic cylinders and hydraulic systems, fuellines and fuel systems, etc. This exclusion ismade since electronics as a group or as part of ahybrid system are one of the most radiation­sensitive portions of a system...There are several points to be empha­si~out TREE. The TREE interest is in theenvironment at the electronics produced by the

8

_ ENVIRONMENT.

• In order to specify the hardness requiredof an electronic system, or to formulate an anal­ysis or test program on which its hardness levelwill be established, it is necessary to describe theradiation environment to which the electronicsystems may be exposed. Since the multitude ofcomponents of this environment leads to confu­sion in understanding the variety of mechanismswhereby a nuclear explosion can affect the oper­ation of electronic systems, the primary outputsof a nuclear weapon and the mechanisms where­by secondary radiation are generated will be re­viewed. In quantifying the environment towhich the electronics are exposed, it is firstnecessary to treat the primary output of the nu­clear explosion, then transport this radiationthrough the atmosphere and generate secondaryradiations by interaction with the atmosphere.At this point, the radiation incident at the sys­tem of interest can be quantified. Since most ofthe electronics are enclosed by some structuralmaterial, there is the additional effect of trans­port through such shields, and it becomes neces­sary to describe the radiation field as it interactswith the affected part.

_ This step-wise description of the radia­tIon explains the variety of methods and unitsby which the radiation field is described. Theweapon designer usually describes the outputfrom the nuclear explosion-total energy output,total number of neutrons emitted, total gamma­ray energy, and reaction pulse shape, The personwho formulates specifications for system designusually describes the radiation field as it im­pinges on the system, e.g., energy per unit area,neutron fluence, gamma exposure, pulse shapeas affected by radiation transport through inter­vening material. Finally, the designer of the indi­vidual electronic piece-part or circuit is con­cerned with describing the radiation in units thatare convenient for quantifying the radiation ef-

feet that he has to take into account, e.g., ener-.gy deposition per unit mass or volume, I-MeVequivalent neutron fluence, gamma dose, radia­tion pulse shape at the device.

6-1 Weapon Output.

• The weapon radiation output and itsinteraction with the atmosphere will be summar­ized in succeeding paragraphs, together with theunits in which these radiations are usually de­scribed. Following this summary, the individualradiation effects of concern will be discussed,and the appropriate methods of describing theradiation at the affected parts or circuits will bedescribed for each type of effect._ The majority of the energy releasediJ~ear explosion heats the material ofwhich the nuclear device was composed, to tem­peratures of tens and hundreds of millions ofdegrees. A smaller fraction (0.1 to 10%) escapespromptly in the form of fast neutrons andprompt gamma rays (see Chapters 4 and 5). Theintense thermal source radiates most of its ener­gy in the form of X-rays. If the explosion occursin a vacuum, or, if the weapon is a "hot" X-raydevice (see Section II, Chapter 4) that explodesin relatively thin atmosphere, these X-rays canproduce important TREE effects. Depending up­on the specific temperature of the source, theX-rays have energies varying from a few to a fewhundred kilovolts. One method by which theenergy spectrum from such a source is frequent­ly described is by specifying the temperature ofa black body which would emit a spectrum thatapproximates the observed X-ray spectrum(Chapter 4). It should be noted that even thoughthe temperature is usually specified as a numberin units of keV (kilo-electron-Volts), this unitdescribes a spectrum of photon energies extend-.-.~ ... .. .

6-3

'DIJ/'.~-)(;

•initial nuclear radiation (i.e., that radiation emit- though the radiation persists only for a shortted within I minute following the burst) which,11IM.e.in turn, consists of both prompt and delayed Examples of system responses toradiation. The weapon-burst radiations of inter- RE an their consequences are given below.est are neutrons, gamma rays, X-rays, and to a These examples represent only a small samplemuch less extent, electrons. The effects of inter- and are not necessarily representative of presentest are both temporary and permanent even day problems.

Effect in System Due to TemporaryDisturbances of the Electronics

Change in logic state in missile-borne guidancecomputer

Spurious (ill-timed) fuzing signal

Excess currents in transistors and capacitors inservo control loop

Excess currents in memory write circuits

Microcircuit latchup following ionization pulse

Effect in System Due to PermanentDegradation of the Electronics

Neutron-induced loss of gain in lower-frequencytransistors

Delamination of semiconductor wire bonds dueto thermomechanical shock

Neutron-induced loss of gain in higher-frequencytransistor structures

Metallization burnout due to excess ionization­induced currents

Typical Consequence

Program jump or disturbance of key data causingmission failure

Warhead dudding or premature detonation

Excessive steering maneuvers causing structuralinstability

Writing erroneous data in memory, usually caus­ing mission failure

Functional disabling of microcircuit until poweris cycled off and on

Typical Consequence

Loss of power supply regulation; de ';reased servo­loop gain

Functional failure of the affected device, usuallyleading to mission failure

Decreased fan-out capability in computer logic

Functional failure of affected device, usuallycausing mission failure

".

promp ra Ia Ions have beenem1 from the nuclear explosion, a residue ofhot and radioactive debris remains. Some of.itsenergy continues to be irradiated as thermalenergy in the ultra violet and visual regions ofthe spectrum. Subsequent radioactive decay ofthe debris produces lower intensity gamma raysas well as emitting high energy electrons. Theseelectrons are particularly important for high alti­tude nuclear detonations, which may inject theelectrons into orbits trapped by the earth's mag­netic field. These electrons are of particular con­cern to space vehicles whose orbits intercept theearth's radiation belts, because continued expo­sure to the electrons can cause significan t perma-

Wegradation.If the explosion occurs within or near

. t e atmosphere, the prompt radiations interactwith the constituents of the atmosphere, pro­ducing secondary effects. The X-rays are absorb­ed most strongly and, depending upon altitude,the air is heated by the interaction to producean intense thermal source and a blast wave in theair (Chapters I through 4). The gamma raysinteract to produce secondary electrons. In con­cert with the earth's magnetic field and/or in­homogeneities, such as the surface of the earth,these electrons create a radiating electromag­netic pulse (EMP) (Chapter 7). The neutronsinteract with the atmosphere to produce second­ary gamma rays (Chapter 5).

6-2 Time Considerations.

~ The general time frame for the arrival oft~rious radiation components at the elec­tronics package is sometimes important. In gen­eral, for the purposes of electronic vulnerability,the prompt gamma radiation can be consideredas a sin·gle pulse. Since the flight time for X-raysand gamma ray's of all energies equals the speed

6-4

of light, unscatteredphotons arrive at the equip­ment with the same time distribution they hadat the source, with a time delay of 3.33 Msec/kmfrom the weapon burst. The arrival of the gam­ma pulse approximately coincides with the un­scattered prompt X-rays._ The neutrons arrive after the initiationo~ gamma- and X-radiation. Their arrivaltime depends on the neutron energy and therange to the receiver. The first neutrons to arrivewill be the unscattered 14-MeV neutrons. Theirtime of flight is 19.3 Msec/km, therefore, theywill arrive 16 psec/km after photon arrival. Thephotons resulting from neutron inelastic­scattering (Chapter 5) will also begin to arrive atabout this time. Within 32.8 psec/km (the arrivaltime after photon arrival for 4-MeV neutrons),the photons resulting from neutron inelastic­scattering will be completed, since the inelastic­scatter contribution is negligible for lower ener­gy neutrons. Within 69.0 psec/km, the bulk ofthe unscattered I-MeV neutrons will have beendeposited. The photons resulting from neutroncapture (or thermalization) typically will peakshortly after the arrival of the 1~MeV neutronsdepending on the system configuration beingstudied. Both the thermalization time and thecapture time following thermalization dependstrongly on the interacting materials and thesystem configuration. The radiation of interestto TREE is complete in less than 1 minute.

6-3 Description of Radiation Fields II.. A complete description of the radiationfi~oduced at the system should include thetime dependence, angular distribution, and ener­gy spectrum of each of the components. Ofcourse, this should be done for a wide variety ofconditions to cover all possible interaction sce­narios. In practice, it is not necessary to be sothorough, and the following approximations areusually made:

• A worst case radiation environment is spe-

• cified and the objective is to have the sys-tem tolerate all combinations of environ­ments that are less stringent than the spec­ification.

• The incident radiation is assumed to beunidirectional. This is usually the worstcase angular distribution, and it representsa reasonable approximation to reality.

• Where possible, the individual radiationfields are specified in units that facilitateconversion to those units that are conve­nient for describing the effect.

.. For example, it is possible to specify theti~ependent neutron energy spectrum inci­dent on the system. Since the time dependenceof arrival of unscattered neutrons is correlatedwith neutron energy, as discussed above, specify­ing the neutron energy spectrum, and the rangeof. distances between nuclear explosion and theirradiated system, also is equivalent to specifyingthe time dependence. Finally, if the only effectof interest is permanent neutron-induced dis­placement effects, the spectrum can be replacedby a single quantity such as the I-MeV damageequivalent neutron fluence. The meaning of theseunits and the method of calculating them willbe discussed in fqllowing paragraphs. It must beemphasized, however, that such convei1ient unitsare intended for simplification only. They areuseful only if the assumptions underlying theiruse are valid. In the foregoing example, specifyingonly the I-MeV damage equivalent neutron flu­ence is useless for determining the magnitude ofneutron-induced ionization effects quantitatively.-...The degree to which such simplifica­~e used depends in large measure uponthe simplicity of the interaction of the incidentradiation with the target materials. A particu­larly simple case is the description of thegamma-ray environment. High energy gammarays suffer negligible attenuation in passingthrough significant quantities of material, andthey interact with matter to produce approxi-

mately the same energy deposition, independeritof atomic composition of the target. Therefore,it has been possible to describe the gamma rayenvironment fairly simply and this has frequent­ly led to carelessness. A more complicated situa­tion, in which carelessness cannot be tolerated,is represented by the X-ray interactions. X-raysof energies of tens to a few hundred kilovolts areattenuated significantly even by thin missileskins and electronic subsystem boxes, and theenergy deposition produced by such X-rays is astrong function of the atomic number of thetarget material. For this reason, it is invariablynecessary to specify not only the total X-rayenergy fluence incident on the system (usuallygiven in calories/cm 2 ), its pulse width (usuallygiven in nanoseconds), but also a range of pos­sible energy spectra (sometimes given as explicitspectra, and at other times specified by a rangeof characteristic black body temperatures). Therelating of such an exposure to the intensity ofthe radiation present at the affected area, suchas the junction region of a transistor, requiresdetailed and specific calculations of the trans­port of the spectrum through the interveningmaterial and the resultant energy deposition inthe affected volume.

• The transport of the various radiation:components to the site of the equipment of in­terest is discussed in Chapters 4 and 5. It is ofvalue, however, to note that typical electronicpackaging materials will not produce significantattenuation of the neutron and gamma ray com­ponents of the environment. They do representsignificant shields of X-rays, particularly if high­Z materials are used for electronic envelopes.Only the higher energy photons penetrate to theelectronics of interest. Since the lower energyphotons produce the highest energy depositionper unit volume when they interact, such shield­ing is especially useful because it removes prefer­entially that portion of the photon energy spec­trum that would be most damaging if it were

Change 16-5

•allowed to penetrate to the sensitive devices.This fact reinforces the observation that is neces­sary to specify both the X-ray energy fluenceand its spectrum outside and inside the shield.Indicating only the energy-fluence attenuationfactor of a shield would ignore the fact that re­maining photons are less effective in producingdamage.

• INTERACTIONS BASIC TO TREE II6-4 Ionization.

• Ionization is that process by which elec­trons are freed from their parent atoms in a ma­terial. A free electron carries a negative charge.After losing an electron, the atom (then calledan ion) carries a net positive charge. Thus, theprocess of ionization results in the formation ofelectron-ion pairs in a material. If none of theelectrons or ions leave the material, the materialremains electrically neutral, since the positiveand negative charges balance one another. Never­theless, characteristics of the material may bealtered considerably by ionization. The numberof electron-ion pairs formed and their subse­quent behavior are of prime interest in determin­ing the effects of ionization.

• Gamma rays interact with matter in three /ways. The first is called the Compton effect. Inthis type of interaction, a gamma ray (primaryphoton) collides with an electron, and some ofits energy is transferred to the electron (see Fig­ure 6-1 a). A secondary photon, with less energy,is created and departs in a direction at an angleto the direction of motion of the primary pho­ton. The second type of interaction of gammarays with matter is the photoelectric effect (seeFigure 6-1 b). A gamma ray, with energy some­what greater than the binding energy of an elec­tron in an atom, transfers all its energy to theelectron, which is consequently ejected from theatom. Since the photon involved in the photo­electric effect transfers all of its energy, it ceases

Change 16-6

to exist and is said to be absorbed. The thirdtype of interaction is pair production (see Fig­ure 6-lc). When a gamma ray photon with en­ergy in excess of 1.02 MeV passes near the nu­cleus of an atom, the photon may be convertedinto matter with the formation of a pair of elec­trons, equally but oppositely charged. The posi­tive electron soon annihilates with a negativeelectron to form two photons, each having anenergy of at least 0.51 MeV. In some cases, ifthe interaction takes place near the nucleus ofa heavy atom, only one photon of about 1.02MeV energy may be created. . ~

~ Any photon (e.g., an X-ray or a gammara~an produce ionization in a material bythese processes of creating secondary electronsthat deposit their kinetic energy by ionizing themedium in which they are created. The relativeimportance or frequency with which each pro­cess occurs depends upon the photon energy andthe characteristics of the material. The Comptonprocess is the dominant ionization mechanismfor most gamma rays of interest, particularly inelectronic materials such as silicon, of whichm.nsolid-state devices are fabricated.

Fast neutrons can produce ionizationin lrectly. As neutrons undergo inelastic scatter­ing and capture in a material (see Section I,Chapter 5), gamma rays that are emitted cancause ionization. In addition, collision of a neu­tron with an atom may impart sufficient energyto the atom for it to cause ionization. Only high­energy neutrons (E > 1 MeV) contribute signifi­cantly to ionization. The 14-MeV neutrons aris­ing from fusion reactions in a weapon are partic­ularly important.

• The types of radiation that cause ioniza­tion In materials - namely, gamma rays, elec­trons, X-rays, and, to a lesser extent, fast neu­trons - are known collectively as ionizing radia­tion ... Once created by the ionization processes(e~ primary or secondary), the charged par-

I ,J\ \ I Electron with I

\ \ / increased energy;\

/1 " / " /

~ Electron .-lr&// ...... ........ _-_.-/e-'' ....... -8-.....-/Gamma ray ~ommoraY

of lowerenergy

a. Compton Effect

I I I I\ \ I I\ Electron bound \ / /\ to an atom \. / /

" /' "- /1'......... /

+ '-8-/ ~e-// ......._--~-Gamma ray

Free electron

b. Photoelect ric Effect

Nucleus

Gamma ray(E> 1.02 M

(±)~

Positive 8"and negative electron

Lower

~~I'.. rays

tc. Pair Product ion

Figure 6-1.• Gamma Ray Interaction with Matter.

Change 1 6-7

•ticles (electrons or ions) are free to move in amaterial, scattering frequently and following arandom-walk pattern. If the concentration ofelectric charge carriers throughout the materialis not uniform, and if no externally applied elec­tric field is present, the carriers will move fromregions of high concentration to regions of lowconcentration. This movement is known as dif­fusion and it would be superimposed on the nor­mal random-movement. If an electric field is pres­ent (e.g., as a result of an intentionally appliedvoltage), the carriers drift in the electric fieldwhile they undergo a predominantly randomscattering. If impurities are present in the ma­terial (as they always are in solid-state devices,such as transistors and diodes), carriers may becaptured (trapped) and immobilized by impurityatoms (traps). Eventually, the trapped carrierswill be annihilated by their mates (oppositely­charged carriers) in a process called recombina­tion. The net result of these processes is that thecarriers diffuse and/or drift until they are trap­ped and usually recombined.

6-5 DisPlacement.II As described above, ionization involves

the movement of electrically charged electronsand ions in a material. Displacement involves themovement of atoms (which are electrically neu­tr.

• Any material may be described as beingeither crystalline or amorphous. The atoms of acrystalline material (a crystal) are arranged in adefinite, repeated, three-dimensional patterncalled a lattice: the atoms of an amorphous ma­terial have no definite arrangement. Displace­ment is an important phenomenon in crystallinematerials, and it is a very important phenome­non in TREE because many electronic devices(e.g., transistors, diodes, integrated circuits) areconstructed from crystalline semiconductors ­primarily silicon and germanium. Lattice defectsresult from the displacement of atoms from their

6-8 Change 1

usual sites in crystal lattices. The simplest latticedefects are extra atoms inserted between latticepositions (interstitials) and unoccupied latticepositions (vacancies). At least part of the resul­tant damage to the material is stable and accountsfor permanent property changes of irradiatedcrystalline materials._The production of displacement damage

in a crystalline solid is a complex process. Anabbreviated history of this process follows.

(1) Radiation of an appropriate form entersthe material, interacts with a lattice atom,and imparts to it a certain energy.

(2) The target (recoil) atom leaves its latticesite, thus creating a vacancy, and collideswith other lattice atoms (see Figure 6-2).

(3) Other atoms are displaced from theirsites, creating more vacancies.

(4) Eventually, most recoil atoms come torest in interstitial positions, while a fewfall into vacancies. Some of the intersti­tials and vacancies may be isolated, butmost of them will be associated \vithother defects in cluster formations.

(5) The simple defects and defect clustersmigrate through the crystal.

(6) Eventually, the mobile defects are annihi­la ted by recombination of vacancy­interstitial pairs, are immobilized by theformation of stable defect clusters withother impurities of lattice defects (eitherpresent in the original material or createdby the irradiation), or escape to a freesurface.

(7) Meanwhile, the physical properties of thematerial are changed by the presence of

11the defects.Fast neutrons are very effective in pro­

ducing displacement damage. Energetic electronscan also produce displacement damage: however,their displacement effects are negligible com­pared to those of fast neutrons.

Vacancy

Path of fast neutron-~

Recoil atom

Recoil atom comes torest in interstitial position

Environmental Parameter:Fast-neutron flux:Fast-neutron Fluence:

Figure 6-2. _ Displacement Damage in a Crystalline Solid 11

• The absolute magnitude of damage in amaterial caused by neutrons is difficult to pre­dict because some of the defects that are pro­duced will effectively -disappear at room tem­perature, that is, anneal. However, much usefulinformation can be gained from the relativevalue of the concentration of defects producedbefore annealing. Assuming the same fractionanneals, the relative concentration of stable de­fects would be in the same ratio. It has beenfound that the number of unannealed defectsthat are generated depends on the energy of theimpinging neutron. The number of unannealeddefects generated by a 14-MeV neutron (onefrom a fusion weapon) is about 2.5 times thenumber of unannealed defects generated by aI-MeV neutron (roughly the average energy for aneutron from a fission weapon). As will be dis­cussed below, the number of defects generatedin semiconductor materials is directly related totileange in semiconductor device parameters.

Not all of the defects produced in thedisp acement process are stable. Some defectsare annihilated by recombination of vacancy­interstitial pairs, some combine with pre-existinglattice defects, and some eventually escape to a

free surface of the material. The stable defectscontribute to the permanent damage of the ma­terial. The unstable defects are said to disappear,or anneal, with time. In practice, this means thatthe degree of displacement damage in a crystal­line semiconductor varies with time, reaching apeak rapidly and then partially annealing withtime. The temperature of the material exerts aconsiderable influence on the amount of anneal­ing that takes place. More annealing is observedat~ated temperatures.

_ Annealing may be divided roughly intotwo time frames. Rapid, or short term, annealingoccurs in times of the order of hundredths of asecond. Long term annealing continues at 'aslower rate for times of the order of tens ofseconds (see Figure 6-3). If the temperature re­mains constant, annealing will be essentiallycomplete after one-half hour. The ratio of thedamage observed at early times (number of de­fects present) to the damage after a very longtime is called the annealing factor, which is afunction of the time of measurement and otherparameters. The maximum damage created atshort times following the fast-neutron burst fre­quently is important to electron-system perfor-

6-9

Figure 6-3.• Annealing Due to Vacancy­

Interstitial Recombination and Escape of

Defects from Semiconductor •

Imperfect heatdissipation tosurround ings[)X-rays

---'VV-

~It£:.~. -~nv;ronmenl Po"'mete".li> ~ X-ray dose rate~ X-ray dose~.. Thermal- energy flux

Time Thermal- energy f luence

Figure 6-4. 11 Heating 111

X-ray dose. Predictions of the X-ray environ­ment from nuclear weapon bursts and the X-rayabsorption mechanisms are discussed in Chapter4. Responses of electronic components to heat­ing are discussed in Section VII, Chapter 9.

MANIF~TIONS OF TREEIN MATERIALS ..

6-7 Ionization Effects •

• The important manifestations of ioniza­tion mclude (I) charge transfers, (2) bulk­conductivity increases, (3) excess minority­carrier generation, (4) charge trapping, and (5)C.ical effects.

Charge transfer results from the escapeof some electrons produced during ionizationfrom the surface of the material being ionized. Ifthe net flow of electrons is out of the material,the material will be left with a net positivecharge. If these electrons are stopped in an adja­cent material, a transfer of charge will have

. occurred from one material to the other, and adifference of potential (or voltage) will exist be­tween the two materials.

Time

anneal

Long-term annealStable defects(permanent damage)

----'---

Environmental Parameters:Fast-neutron flux:

Fast-neutron Fluence:

VI

U<I>­<I>o

6-6 Heating •

• Whenever a material absorbs energyfrom its surroundings and cannot instantaneous­ly dissipate that energy, the temperature of thematerial will rise, Le., the material will be heat­ed. The temperature will return to ambient at arate determined by the efficiency with whichthe material can dissipate heat to its surround­ings. If the energy deposition is great, and themechanisms for heat dissipation are inadequate,the temperature rise will be significant and willp!lr' for a considerable time (see Figure 6-4).

X-rays are the primary contributors tohea mg. Therefore, the relevant environmentalparameters are the X-ray dose rate, and the

•mance. Therefore, the maximum annealing fac-tor is an important quantity, because it indicatesthe peak damage that must be tolerated abovethe permanent damage in the steady state.Values of the annealing factor depend on thetemperature and on the electrical condition ofthe material. An annealing factor of three iscommonly used for room temperature at -10msec. Larger factors have been observed at shortertimes, or low injection conditions (e.g., cut-offtransistors).

6-10

• The transfer of charge from one materialto another can have a number of effects. Themost obvious one is that a current will flowthrough any electrical circuit connecting the twoma terials to restore charge neutrality. Thecharge will produce electric and magnetic fieldsduring transit. If there is matter in the gap be­tween the materials, the charge transfer also willproduce ionization and conduction in responseto local electrical fields. Finally, if the chargeeither originates or embeds itself in an insulator,a long-lived local space charge may result.Charge transfer, therefore, may result in either ate.orary or a semipermanent effect.

The free carriers produced during ioniza­tion respond to an applied electric field by pro­ducing a net drift current. This is precisely themechanism by which a material conducts elec­tricity. Therefore, ionization induces a transientincrease in conductivity. An example of a detri­mental effect to the operation of electronicequipment resulting from an increase in bulkconductivity occurs in a capacitor exposed to aweapon burst environment. The ability of a ca­pacitor to retain, or restore, electrical charge isdependent upon the low conductivity of the di­electric, or insulating material, that is containedwithin the capacitor. In an ionizing environmentthe increase in bulk conductivity results in a de­crease of stored charge in the capacitor.II ~he ion~zation effect ~f excess min?rityearners IS a pnme concern In many semIcon­ductors and is usually the most important mani­festation of TREE. Semiconductor devices, suchas transistors and diodes, employ both positiveand negative charge carriers, either of which maybe in the minority with respect to concentra­tion. The characteristics of many such devicesdepend strongly upon the instantaneous concen­tration of minority carriers in various regions ofthe device. Since ionizing radiation creates large(and equal) numbers of positive and negativecharge carriers, the concentration of minority

carriers in the device is temporarily enhanced bya large percentage, and the electrical operationof the device may be affected adversely. Themost familiar example of this effect is the cur­rent flow across a reverse-biased PN junction,such as those that are found in a diode or thebase-collector junction of a transistor (see Sec­tion VII, Chapter 9).

• When free carriers are created in insulat­ing materials, and are trapped at impurity sites,many may not undergo recombination withtheir mates, which may be trapped elsewhere. Inthese cases, the material properties may be alter­ed semipermanently, even though there is no netcharge in the material. This ionization effect isknown as charge trapping.II Trapped charge can change the opticalproperties of materials (e.g., F centers in alkalihalides, coloration of glasses). The trapped car­riers may be released thermally, either at theirradiation temperature or by elevating the tem­perature. In either case the resultant creation ofsome free carriers is manifested by an increase inconductivity and sometimes by the emission of

Iiilt.The chemical effects of ionization occur

dunng the processes of trapping and recombina­tion when sufficient energy is available to dis­rupt chemical bonds. At the completion of theionization cycle (Le., after recombination iscomplete), the material may return to electricalinactivity, but its chemical composition may bealtered permanently. The resulting chemicalchanges may be manifested as permanentchanges in physical and/or electrical propertiesof the material. The radiation dose required tocause such effects is larger than normally will beencountered; therefore, the effect will not bediscussed further.

• Since ionization effects do not occuran~r recover in the same time period, the timedomain for their occurrence and recovery mustbe considered. There are three categories of time

••dilendence - prompt, delayed, and long tenn.Prompt effects are those in which the

wi of the ionization pulse is longer than thetimes required for atoms or electrons within thematerial being exposed to make a specifiedamount of recovery. The magnitude of the ef­fect is a function of the density of the positiveand negative particles created during the ioniza­tion, which in turn is a function of the dose rate.Examples of prompt effects are charge transferand prompt bulk-conductivity increases in insu­lating and semiconductor materials.

•Delayed effects are those in which the

wi t of the ionization pulse is shorter than thetimes required for atoms or electrons within thematerial being exposed to make a specifiedamount of recovery. The initial response of thematerial or device is a function of dose, and itspersistence is determined by the length of thespecified recovery time. An example of this ef­fect is delayed bulk conductivity of insulatormaterials. In this p~rticular example the recov­ery times for the prompt and delayed effects arebased on different mechanisms of carrier genera­tion (ionization induced or thermal trapping)and, hence, they have very different timeperiods... Long-term effects are those which per­sis~ periods longer than minutes. These ef­fects can be, but are not necessarily, permanent.Recovery may be so slow that it takes days,months, or years for apparent complete recov­ery. Examples of long-term effects are somecases of trapped charge and chemical effects.... An important point, emphasized here, isth~general class of materials or devices (plas­tics, transistors, etc.) could be both dose anddose-rate sensitive with respect to the ionizatione!Jiiis observed.WI It has been established that, with the ex­

ceptIOn of charge transfer, the magnitude of theionization effects is primarily a function of thetotal concentration of thermalized charge car-

riers (electrons and holes). These carriers aregenerated at a rate proportional to the instan­taneous ionization energy deposition and inde­pendent of the na'ture of the radiation producingthat energy deposition. Therefore, it is appropri­ate to use units that quantify the energy deposi­tion in the material of interest when describingthe radiation field at the responding device.Such units include ergs/gram (material), rads(material), and calories/gram (material). Sincethese are descriptions of energy deposition in agiven material, they are called units of dose. Thetime dependence of energy deposition can bespecified by these same units per second, and arecalled dose rate...Unfortunately, the magnitude of chargetr~r depends not only on the energy deposi­tion but the spectrum of the secondary elec­trons. Therefore, quantitative evaluations ofsystems in which charge transfer represents a sig­nificant vulnerability mode must use a morecomplex characterization of the spectrum of theincident photons, together with a calculation ofthe photon interactions, to produce the spec­trum of secondary electrons.

• In some materials, particularly insu­lators, the ionization effects are also a weakfunction of the microscopic concentration ofthe ionization around individual particle tracks.For example, neutron-induced ionization cre­ated by intensively ionizing recoil atom may beless effective in producing conductivity in an in­sulator than the same dose or dose rate impartedby lightly ionizing gamma rays. Units for de­scribing this process include specific ionization(ratios of ionization to the minimum level ofionization at high velocities of a singly chargedparticle), and linear energy transfer (MeV/cm orMeV cm2 /gram). Fortunately, these effects areof second order importance for most TREE ap­plications. A specification of dose rate and dose~n the majority of applications.~The gamma-ray field incident on a

system is frequently described by its exposuremeasure in roentgens. The roentgen is deter­mined by specifying the energy deposition in astandard material (dry air under standard tem­perature and pressure conditions). For high ener­gy gamma rays that interact primarily via theCompton process, exposure of almost any ma­terial to one roentgen of gamma rays produce(within 20%) approximately one rad of energydeposition in any material. This factor has en­abled the users of these units to become some­what careless, without serious consequence, solong as only high-energy photons are of interest.However, these relations do not carry over intophotons of lower energies (200 kilovolts or lessin medium atomic-number materials) and carefultreatment of the units is required. One way ofminimizing the chances of misinterpretation isto use units of caloriesjcm2 with a defined spec­trum for external exposure and units of cal/gram(material) for dose.

6-8 Displacement Effects •

• The displacement effects of prime con­cern to TREE are those generated in semicon­ductor materials. The lattice damage resultingfrom displacement degrades the electrical char­acteristics of semiconductor devices by increas­ing the number of trapping, scattering, andrecombination centers. The effect of displace­ment in semiconductors is, therefore, threefold:(l) the trapping centers remove charge carriersfrom the electrical conduction process (reduceselectrical current flow), (2) the additional scat­tering centers reduce the capability of the chargecarriers to move through the semiconductormaterial (reduces charge-carrier mobility), and(3) the recombination centers reduce the timethat the minority charge carriers are available forelectrical conduction (reduces the lifetime of theminority-charge carriers). This last effect is mostimportant for prediction of semiconductor de­vice performance in radiation environments thatcause displacement. The decrease in minority-

carrier lifetime (r) is predicted according to therelationship

I I-=-+ Kr.p,r I{J "'0

where

r I{J =minority-carrier lifetime at fluencer.p in seconds,

r = initial minority-carrier lifetime in secondso

K =lifetime damage constant, cm2 /(neutron •second)

r.p =total fast-neutron fluence, neutrons/cm2 .

• Typical values for T in device materialsof interest are 10-8 to 10-.f seconds. The valueof the lifetime damage constant, K, is dependenton the type of material, the type and amount ofimpurities in the material, the operating voltageapplied to the material, the temperature, theenergy spectrum of incident neutrons, and, be­cause of defect annealing, the time after thenuclear radiation is incident on the material. Ageneral value for silicon, the prime material usedin transistors, diodes, and integrated circuits, isK::::: 1 x 10-6 cm2 /(n . sec).II To illustrate the magnitude of displace­

ment effects that occur in semiconductors com­pared to those that occur in other materials, acomparison of neutron fluences that will causesignificant effects is made in the following para­graph. Interest is focused only on those propertychanges that affect the normal use of the ma­terials being compared.

_ Semiconductor lifetime can begin toshow significant effects in devices at a fluence of1011 n/cm2 (Pu, fission)* and by 1016 n/cm2

*Accurate neutron dosimetry requires that the foil used for mak­ing the neutron measurement and the energy spectrum of theneutrons be specified with the value measured. Therefore, in theexample presented here, lOll neutrons per· square centimeter .were detected with a plutonium foil and the energy spectrum ofthe neutrons was a nominal fission spectrum.

6-13

•(Pu, fission) the lifetime in most semiconductordevices is so short that the device is no longeruseful. Metals such as nickel and copper start toshow effects in material strength at a fluence of1018 n/cm2 (Pu, fission). As a general rule, theelectrical properties will not start to change instructural materials until the material propertieschange. Glasses and ceramics are much less sus­ceptible to neutron damage than semiconduc­tors, but they are more susceptible than metals.The point that is emphasized is that semiconduc­tors are among the devices that are most sus­Celiible to displacement effects.

It is frequently desirable to express neu­tron e fects data observed by different experi­menters using different neutron-energy spectrain terms of an equivalence fluence unit in addi­tion to the measured flux and fluenee units. Thispermits easy comparison of damage levels ob­tained from different neutron test facilities.Since silicon is the material of most interest todisplacement effects, the equivalence is usuallybased upon damage in silicon. The energy spec­trum typically used as a standard for comparisonis a hypothetical I-MeV monoenergetic neutronsource. The damage caused by the neutrons (ofsome known spectrum) is compared to the dam­age done by the neutron spectrum which is usedas a standard. The neutron fluence of the stan­dard which would cause the same damage in thegiven material as observed for the known spec­trum is then specified as the damage equivalentfluence for that material. An example of thisequivalence unit for I-MeV neutrons standardspectrum and silicon material is: 1013 n/cm2

(I-MeV damage equivalence in silicon). The pro­cedures for obtaining a I-MeV equivalent flu­ence for any known neutron spectrum are speci­fied in the TREE Handbook (see bibliography).

6-9 Heating Effects •

-The radiation environment, espe­~-rays, produced by nuclear weapons

can deposit considerable energy in electronicmaterials. The energy deposited is sufficient toheat some materials to such an extent as to causepartial melting, and it is sufficient to change elec­trical properties in other materials. These temper­ature transients may last from fractions of a sec­ond to minutes. If the energy is deposited in thematerial in a very short time (deposition time istypically 10-8 seconds for a nuclear weapon) anadditional effect is observed; the material isheated very rapidly but does not have time toexpand. The result is the instantaneous creationof a shock wave, or pressure pulse, which tendsto compress the material. This compress,ion wavestarts at the point of energy absorption, typical­ly close to the material's front surface uponwhich the incident energy impinges, and quicklypropagates to the back surface. When it reachesthe back surface, it is reflected, becomes a ten­sion wave, and propagates toward the front sur­face. For energy deposition of sufficient magni­tude delivered in a sufficiently short interval, thetension wave can be intense enough· to exceedthe strength of the material. Several consequencesare possible.

( I) Portions of the material may be removedfrom the back surface - spallation (seeFigure 6-5).

(2) Fragments of the material may separatefrom the front surface - blowoff.

(3) The material may separate between frontand back surfaces - delamination.

(4) Agglomerates or layers of dissimilar ma­terials bonded together will tend to sep­arate at the interfaces - also called de­lamination.

(5) In brittle crystalline materials (e.g., Si orGe), crystal fracture can occur.

Generally, effect~ of this nature are referred to_c.A-..~~1

as thermo"fHH~:}e:u shock effects. Obviously, theconsequences of these effects can be catastroph­ic. An example important to transistors and inte-

Hot

coc;;~

'" >e 0a.~

E cBlow-off 8 ~

~",Cal al

'., g:; Hot X-rays not absorbed~I' £ g --.f\ f\ available to produce

X-rays .. ; -~- V v--- additional thermomechanical~ · K shock and ionization

Spallation

Delamination

Environmental Parameters:X-ray dose rate:X-ray dose:

Figu" 6-5. __ Th"mom""'ooi,,' Shook Eft."" •

grated circuits is the delamination of the elec­trical contacts to the semiconductor chip, result­ing in complete loss of the device function. Thethermomechanical-effect damage threshold isdifficult to determine. It depends not only onthe energy deposition and energy spectrum but

also on the thickness of the materials, the com­

pressibility, the expansion coefficient, and the

dynamic strength of the materials. Thermo­

mechanical effects on materials are discussed in

more detail in Section VII of Chapter 9.

• BIBLIOGRAPHY •

DASA EMP (Electromagnetic Pulse) Handbook, DASA 2114-1, DASIAC, Santa Barbara,California, September 1968, (to be replaced by DNA 2114H-l duringcalendar year 1972).

DASA EMP (Electromagnetic Pulse) Handbook, Classified SupplementDASIAC, Santa Barbara, California, September 1968,(to be replaced by DNA 2114H-2, 2114H-3, and D1972).

Gwyn, C. W., D. L. Scharfetter, and J. L. Wirth, The Analysis ofRadiation Effects in Semi­conductor Junction Devices, SC-R-67-1158, Sandia Corporation, Albuquerque, NewMexico, July 1967

J. J., A Management Guide to Transient-Radiation Effects on ElectronicsDNA 2051 H, Battelle Memorial Institute, Columbus, Ohio, February 1972

Larin, F., Radiation Effects in Semiconductor Devices, John Wiley and Sons, Inc., NewYork, 1968

TREE (Transient-Radiation Effects on Electronics) Handbook. Vol. 1 Edition No.3,DNA 1420H-l, Battelle Memorial Institute, Columbus, Ohi~ecember 1971 •

TREE (Transient-Radiation Effects on Electronics) Handbook II(Classified SupplementJ,Edition 2, Revision 2, DASA 1420-1, Battelle Memorial Institute, Columbus, Ohio, Sep­tember 1970, (to be replaced by DNA 1420H-2 duringcalendar year 1

TREE Preferred Procedures (Selected Electronic Parts), DASA 2028, Defense Atomic Sup-port Agency, Washington, D.C., May 1968, (to be replaced by DNA2028H during calendar year 1972).

Vook, F. L., Editor, Radiation Effects in Semiconductors, Proceedings of the Santa FeConference on Radiation Effects in Semiconductors, Plenum Press, New York, 1968