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Vacuum 80 (2006) 1362–1366

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Technical note

Numerical study on effects of secondary electrons generatedby TEMP II accelerator

Di Wu�, Ye Gong, Jin Yuan Liu, Xiao Gang Wang, Yue Liu, Teng Cai Ma

State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Physics Department,

Dalian University of Technology, Dalian 116024, China

Abstract

A pulsed voltage of magnetically insulated diode and intense pulsed ion beam (IPIB) models have been built according to the

experimental results. A model of secondary electrons yield while IPIB irradiated metal target has also been established. The temporal law

of secondary electrons induced by IPIB irradiated titanium target according to the energy deposition at surface layer has been calculated

based on the models. The maximum number of secondary electrons reached is 1013/cm2 while the energy density of IPIB is 4 J/cm2. And

near the surface of target, the electric field rises up to 1010 V/m with the same IPIB energy density irradiation. The evaporation ions will

be accelerated near the target, therefore it is important to the formation of films.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: IPIB; Secondary electrons; Electric field; Numerical method

1. Introduction

Intense pulsed ion beam (IPIB) is produced by magneticinsulated diode (MID) of TEMP II type accelerator [1]. TheIPIB with low energy density can be directly used to modifythe physical properties of target surface [2,3], and that ofhigh energy density can be used to heat the surface, meltand even evaporate it to form plasma which can be used togenerate film and nano-size crystal [4,5]. The range ofenergy density of IPIB is from 1 to 50 J/cm2, pulse durationis from 10 to 1000 ns. IPIB has many advantages comparedto the laser beam and the electric beam, such as theirradiation area on target surface is about a 100 cm2 andeven 1000 cm2. There is no energy loss due to the reflectionof the target and the energy usage is higher compared withlaser beam as well as the energy of single particle is largerthan that of electrons. Certainly, the energy transformationis more efficiency [6]. The beam is produced by MID ofaccelerator, so the equipment is small. Especially it needsnot to heat and add negative bias voltage to the substrate[7]. Recently, there has been more attention for theapplication of IPIB. But more experimental studies have

ee front matter r 2006 Elsevier Ltd. All rights reserved.

cuum.2006.01.018

ing author. Fax: +86 0411 84709304.

ess: cn_wudi@yahoo.com.cn (D. Wu).

been done than mechanism ones. Zhang et al. [8] hasdetected the velocity of ejection particles, and Akamatsuet al. [9] has discussed the thermal effects in the targets. Butthe mechanism of formation of film is not clearly discussed.In this paper, the result of Doyle et al. [10] has been used todiscuss the secondary electron effects while the targetirradiated by IPIB generated by TEMP type accelerator.The generation and evolution of electric field has beencalculated. The results showed that the electric field play animportant role in the emission direction and acceleration ofevaporation ions has been obtained.

2. Theoretical models

The MID and the chamber of TEMP II type acceleratorimported from Russia are shown in Fig. 1. The ions lead tothe chamber and impact on target. The substrate surface onwhich film would be formed was parallel to the surface oftarget. The components of IPIB are C+ and H+ ions.While ions impact on the target, the secondary electron willbe generated. During the pulsed time, the number of ionsimpacted on the target increased, consequently thesecondary electrons generated by C+ and H+ are increasedas well. For metal target, when electron escape from

ARTICLE IN PRESS

Transformer

Cathode

Ion beam

Target

Substrate

To pumpAnode

Unipolar Pulse

Fig. 1. Section profile of MID and vacuum chamber of TEMP II

accelerator.

D. Wu et al. / Vacuum 80 (2006) 1362–1366 1363

surface, a hole with positive charge will remain there.During the pulse time, the number of ions impacting ontarget per unit time increases and then decreases. And thegenerated secondary electrons change as function ofimpacting ions. Generally speaking, the secondary elec-trons will escape from the target surface. Certainly, theelectric field will be formed. If the ions density is largeenough, the temperature of the target surface will increaseand if the surface absorbs enough energy, it may melt oreven evaporate to form the ejecting plasma. The electricfield will affect the direction of ejected plasma. So it isimportant to discuss the formation and development ofsecondary electrons.

2.1. Models of voltage and ion beam density of TEMP II

accelerator

At the beginning of a pulse, the voltage of MID is zeroand then it increases to the maximum value and thendecreases, finally it will disappear, that is to say the energyof ions has distribution function. Combining the voltage ofMID and the wave of ion beam density detected near thefocus of MID and Gaussian distribution models have beenused to fit them, they are given as

U ¼ A exp �t2

2s21

� �, (1)

J ¼ B exp �ðtþ DtÞ2

2s22

� �, (2)

where Dt is the delay time of ions arriving at the target, ands1,s2 are the standard deviation for voltage and ion beamdensity.

The voltage and the ion beam density are both functionsof time. The energy of ions can be determined according toEq. (1) at certain time. The number of ions interacted withtarget between time t and t+dt can be determined as

dN ¼1

qJðtÞdt, (3)

where q represents charge of an ion.The number and energy of ions interacting with target at

certain time can be obtained by solving Eqs. (1)–(3).

2.2. Model of secondary electrons

The SE emission is normally generated by two differentmechanisms. One is induced by lower energy ions impact-ing on target which is mainly due to the potential emission,and the second by higher energy of ions which is calledkinetic emission. The kinetic emission is mainly discussedamong the energy from several keV to 1MeV. The SE willbe generated while ions interact with target, commonly, itincludes the excitation of electrons and the excitedelectrons transport in the target and finally the electronsescape from the surface of the target if they have enoughenergy to overcome the work function of metal. Thomton[11] researched the interaction between ions and targetexperimentally, and he gave the secondary electron yieldsof aluminum, iron and stainless steel for proton impactingcases; Doyle et al. [10] and Rothord et al. [12] haveanalyzed the relationship between electron yield and energyloss of non-elastic collision of ions with target, the formulais as follow:

g ¼ LðdE=dxÞ, (4)

where L represents coefficient, it is determined by the kindof ions and target materials, dE/dx is the non-elasticcollision energy loss while ions impact target vertically.Stoltz et al. [13] has discussed the cases of ions impacted

non-vertically, and gave the yield formula of secondaryelectrons with relation to the incident angle y (the angle ofthe direction of impacting ions and the surface). And thetheory obtained agrees well with the experiments for highcurrent ions impacting on target. So the model is used toresearch the secondary electron effects when targetirradiated by IPIB.There are H+ and C+ ions in IPIB, the non-elastic

collision energy losses are different for the two kinds ofions and certainly the emission coefficients of secondaryelectrons are different either. To obtain the principle of SEproduced by mixed ions beam, a component model hasbeen built by using the formula of SE produced by singleion, that is

gt ¼ ½LH ðdE=dxÞHieZH þ LCðdE=dxÞCieZC �= cos y, (5)

where ZH and ZC are portion of H+ and C+ in the beam,respectively. (dE/dx)Hie, (dE/dx)Cie represent the energyloss of H+ and C+, respectively, the unit is eV/A, LH and

ARTICLE IN PRESS

5

4

3

2

1

00

Num

ber

of io

ns (

1010

cm

-2 )

0.5 1.5 2.5 3.5321Depth (µµ m)

20ns C+

50ns C+

70ns C+

20ns H+

35ns H+

50ns H+

70ns H+

35ns C+

Fig. 2. Number profile of H+ and C+ shoot in titanium target during a

flash time.

2.5

1.5

0.5

0

1

2

6040

200 3

2

01

Time (ns) Depth (µµm)

Num

ber

of I

ons

(1015

Cm

-2)

Fig. 3. Temporal–spatial evolution of number of ions in titanium target

with the incident current density of 200A/cm2 and energy density of

4 J/cm2.

D. Wu et al. / Vacuum 80 (2006) 1362–13661364

LC are the coefficient for H+ and C+ impacting cases, fortitanium target they are 0.17 and 0.075, respectively [12].

According to the Gaussian model of beam, the numberof secondary electrons yielded during time t and t+dt

should be

dNSE ¼ gtdN. (6)

The temporal evolution profile of SE when targetirradiated by IPIB can be obtained by solving Eqs. (2)–(6).

Electrons, impacting ions and holes form the electricfield near the target. As the time increased the number ofions colliding with target increased quickly as well as thenumber of SE also increased remarkably. The number ofSE was greater than the impacting ions, so we could ignorethe affection of incidental ions. Meantime the area of ionsinteraction with target was as large as several ten squarecentimeters, the surface of the target could be taken asunlimited large panel, and the electric field could be takenas the one which generated by charged unlimited panel, it isevaluated as: EðtÞ ¼ seðtÞ=e0. Here seðtÞ was the density ofholes when electrons escaped from the metal surface, itequaled to the surface density of SE:

seðtÞ ¼eR t

0 gtJðtÞdt

q, (7)

where e represents charge of electron.The temporal evolution profile can be obtained by

solving Eqs. (1)–(3) and (5)–(7).

3. Results and discussion

IPIB impacting on Ti target has been simulated byTRIM code. Graphite was used as anode of MID, 70% ofthe beam was C+ and 30% of it was H+ [1]. While the ionbeam density was 200A/cm2, and the energy density was4 J/cm2, the number of C+ and H+ ions impacting on Titarget changing with time during the pulsed time as shownin Fig. 2. The number of C+ deposited in the targetreached maximum value 20 ns later when a pulse started, itwas about 1010 ions/cm2, it decreased in the target; thenumber of C+ increased rapidly with time. It can be seenthat H+ impacted into the deep layer of the target in Fig. 3.At the end of a pulse the number of ions in the surface layerreaches maximum value. The spatial and temporal evolu-tion of IPIB impacted on Ti target with the same energydensity is shown in Fig. 3. And the energy depositionprofile is shown in Fig. 4. The ions deposited more energyin the surface. If the beam only consists of H+ , the rangewhich the energy deposited in is about 3 mm, if only C+, therange is about 0.5 mm. Because there are more C+ ions inIPIB, the energy mainly deposited in the range of 0.5 mm.The number of emission SE is directly proportional to theenergy deposition near the surface. The temporal evolutionof the number of SE is shown in Fig. 5. The impacting ionsare function of time and they are also drawn in the samefigure. It can be seen from the figure that per nanosecondthe impacting ions number reach maximum at the time of

35 ns from the beginning of a pulse, meanwhile the numberof generated SE also reach maximum value 1013 cm�2. Thetemporal evolution of SE generated only by H+ or C+

with the same energy density are also shown in the samefigure. The number of SE generated by H+ increasedslowly, but the number generated by C+ increase severely.This is the result that the energy of H+ deposited in deeplayer of target and C+ in the near surface layer. Thenumber of SE changed as function of the energy density ofimpacting ions, it is shown in Fig. 6, the energy density are1.0, 2.0, 4.1 and 6.2 J cm�2, respectively, and the targetwas Ti.The electric field was built up as the ions impacting on

the target. It is shown in Fig. 7, at the end of a pulse, theelectric field reached 1010Vm�1.

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5

4

3

2

1

00

Dep

osit

ed e

nerg

y (1

015 eV

/A)

0.1 1

Depth (µµ m)

70%C+ + 30%H

+

C+

H+

Fig. 4. Energy deposition profile of H+, C+ and IPIB in titanium target

with the same energy density of 4 J/cm2.

15

10

5

00 20 40 60 80

Time (ns)

Num

ber

of p

arti

cles

(10

12 cm

-2)

SEproducedby IPIB

SE produced by H+

SE produced by C+

Impacting ions

35ns

Fig. 5. Temporal evolution of secondary electron numbers induced by

H+, C+ and IPIB irradiating titanium target per nanosecond.

3.5

6.2J/cm2

4.1J/cm2

2.0J/cm2

1.0J/cm22.5

1.5

0.5

00 20 40 60 80

1

2

3

Time (ns)

Num

ber

of s

econ

dary

ele

ctro

ns (

1014

cm

-2 )

Fig. 6. Temporal evolution profile of secondary electron numbers induced

by IPIB with different energy density irradiated target.

0

6

4

2

010 20 30 40 50 60 70

15

10

5

0

Time (ns)

Num

ber

of s

econ

dary

ele

ctro

ns (

1012

cm

-2 )

Ele

ctri

c fi

eld

(1010

V/m

)

SE

Fig. 7. Profile of electric field near the surface of target as function of

time.

D. Wu et al. / Vacuum 80 (2006) 1362–1366 1365

If the energy density of IPIB increases to a proper value,the surface of target can absorb enough energy and it willevaporate, in this case the electric field is greater than theone obtained. Generally speaking, the ablation scar takesthe shape of parabola surface, and the direction of theemission plasma is divergent. But according to the CCDphotograph of the experiment, it is known that the plasmaalmost ejected in the same direction. The direction ofelectric field near the target surface is normal to the surfaceof target. The evaporation ions have positive charges.Certainly the electric field affects the moving direction of

evaporation ions. That is to say the electric field formed bySE has directional effect to the evaporation ions. Andmeanwhile it will accelerate the evaporation ions too. Theevaporation ions may obtain energy from the electric field,and speed up to the substrate, so it need neither to heat norto add bias voltage to substrate to form the film. This is notlike the other method of film formation [14].

4. Conclusions

According to the results of calculation, the followingconclusions have been obtained:

(1)

The energy of C+ deposited near surface region, butH+ in deep layer.

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(2)

The number of secondary electrons generated by IPIBimpacting on target is a function of time, during thepulsed time, the number of secondary electronsincreases first and then decreases.

(3)

The number of secondary electrons generated by C+

are larger than which generated by H+, the formerincreases as time evolving faster than last one.

(4)

The stronger the IPIB density, the more the number ofgenerated SE.

(5)

The electric field near the target surface increases asfunction of time during the pulsed time, it may reach upto 1010V/m, and it plays an important role to thedirection and acceleration of evaporation ions.

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