Vacuum 62 (2001) 237}240

The surface modi"cation of dielectrics by a CW electron beam

Sergey Korenev*, Jerry Kriebel

STERIS Corporation, STERIS Isomedix Services, 2500 Commerce Drive, Libertyville, IL 60048, USA

Abstract

The e!ect of surface modi"cation of dielectric materials on a sample of Te#on and dielectric with low dielectricconstant by scanning a parallel electron beam is considered in this report. The irradiation of samples has been performedon the CW (Continuous Wave) electron accelerator `Rhodotrona, with the following main parameters: kinetic energy5MeV and a current of 1}16mA in air conditions. The nature of physical processes in dielectric materials duringirradiation is di!erent in comparison with conducting materials. The distribution of absorbed doses and electrons in thedielectric material is discussed. The e!ect of surface modi"cation of dielectrics consists in the treatment of its surface bysurface electrical discharge from charging the dielectric by the electron beam. The e!ect of the in#uence of magnitude ofinternal electric charge on the surface discharge is discussed. � 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Electron beam; Irradiation; Surface treatment; Surface discharge; Dielectrics

1. Introduction

The question of surface modi"cation of dielec-trics with low dielectric constant by high energyelectron beams is very important for understandingthe physics and chemistry of these materials. Te#onis a simple material for investigation [1], is stable tocorrosion e!ects from ozone in the area of irradia-tion and is considered in this report.

2. Experimental

2.1. Electron accelerators

The experiments were conducted on the elec-tron accelerator `Rhodotrona [2,3]. The main

*Corresponding author. Tel.: #1-847-573-3223; fax: #1-847-247-0882.E-mail address: sergey}korenev@steris.com (S. Korenev).

parameters of the electron beams are:

� kinetic energy, 5.0MeV;� beam current, 1}16mA;� frequency of operating RF accelerating structure,

107.5MHz;� full diameter of electron beam, 8 cm;� repetition of scanning for beam, 100Hz;� The trajectories of scanning electron beams are

parallel.

The accumulated absorbed doses in the irradiationof samples were of the order of 10}200 kGy. Ab-sorbed dose D is de"ned as the mean energy im-parted, d�, to an incremental quantity of matter,divided by the mass of that matter, dm [4]:

D"d�/dm"Gy. (1)

2.2. Samples

The irradiation was made in air static condi-tions.

0042-207X/01/$ - see front matter � 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 4 2 2 - X

Fig. 1. The distribution of absorbed doses in Te#on for electron with kinetic energy 5MeV.

The dielectric samples from Te#on (dielectricconstant �&2), glass (�&6), polyethylene (�&2)and BaTiO

�(�&1000}3000) are presented as

plates with thickness 1.5mm and are "xed in theunit with a total thickness of 15mm.The methods of investigations of samples in-

cluded SEM methods and the electrical measure-ments of surface resistance and plasma resistanceon the surface of dielectrics.

3. Computer simulation

The computer simulations include the simula-tion of the propagation of electrons in the di-electrics and the e!ect of back scattering. Theresults of computer modelling for dielectricswith low dielectric constant have more homo-geneous distribution of absorbed doses in com-parison with the same distribution for metals(see Fig. 1). The electron trajectories in Te#onand polyethylene have the same form. As anexample, electron trajectories for Te#on are shownin Fig. 2.

4. Experimental results

The SEM photographs of surface for Te#on be-fore irradiation and after electron beam irradiationare shown in Fig. 3a and b. The analysis of surfacesof other dielectrics shows the same type of picture.The measurements of surface resistance of thesedielectric samples indicated a weak dependence ofsurface resistance on absorbed doses in the range10}200kGy. However, the resistance of surfaceplasma on Te#on varied. The resistance was 14 k�for an electron beam current 1mA and 100k� for16mA. The irradiation of a dielectric with a lowdielectric constant allows the formation of stablesurface plasma. The irradiation of BaTiO

�with

a high dielectric constant leads to an electricaldischarge inside the dielectric. The surface hasa multipoint character of surface discharge [5].

5. Discussion

The question of surface modi"cation of dielec-trics having di!erent dielectric constants is not

238 S. Korenev, J. Kriebel / Vacuum 62 (2001) 237}240

Fig. 2. The distribution of 5MeV electron trajectories in Te#on.

Fig. 3a and b. SEM photographs of surface before and after electron beam irradiation.

understood at present. The use of a dielectric withhigh dielectric constant causes internal electricaldischarges [5,6]. For low dielectric constant mater-ials these e!ects must be reduced to make correctmeasurements of the surface discharge. The form-ing plasma allows observation of e!ects of ionetching of the surface of dielectrics. The SEM pho-

tographs con"rm this. Te#on has high resistance tocorrosion. The ozone in the area of irradiation isvery active and other dielectrics have fast oxida-tion.The processes of dissipation of energy in the

irradiated product have di!erent characters for di-electrics when compared to metals. Fig. 1 con"rms

S. Korenev, J. Kriebel / Vacuum 62 (2001) 237}240 239

this. For metals, we have the e!ect of thermalprocesses and we have a classical distribution ofabsorbed doses on the level of 20% for averageabsorbed doses. The e!ect of accumulation ofcharge inside a dielectric leads to an in#uence onelectrical "eld from space charge, accumulated dur-ing irradiation.

6. Conclusion

The following conclusions can be made from thisstudy:

� the irradiation of a dielectric with low dielectricconstant leads to more homogeneous distribu-tion of absorbed doses in these dielectrics incomparison with metals;

� etching of the surface of a dielectric by the sur-face plasma was observed;

� dielectrics with low dielectric constant allow theformation of a homogeneous surface discharge.

References

[1] Korenev S, Mase"eld J, Kriebel J, Johnson S. Proceedingsof the Tenth International Seminar of Laboratory of Sur-face Science, Rutgers University, 1999. p. 23.

[2] Pottier J. Nucl Instr and Meth 1989; B40/41:p. 943.[3] Jongen Y. Manufacturing of electron accelerators. Proceed-

ings of the European Particle Accelerator Conference(EPAC '96), Barcelona, Spain, 1996. p. 260.

[4] McLaughlinWL, Boyd AW, ChadwickKH,McDonald JC,Miller A. Dosimetry for radiation processing. London:TaylorFrancis, 1989.

[5] Gromov VV. Electrical discharge in the irradiated mater-ials. Moscow: Energoizdat, 1982.

[6] Korenev SA. Determination of admittance irradiation loadsby electron beam on The BaTiO

�. Proceedings of the all

Soviet Union Conference on Physics of Dielectrics, Tomsk,vol. IV. 1988. p. 103.

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