The design of an electrostatic variable energy positron beam for studies of defects in ceramic coatings and polymer films

The design o an elec os a ic a iable ene gy posi on beam o s udies o de ec s in ce amic coa ings and polyme ilms E. Abadjie a * , H. Schu , A. Alba Ga cı ´a, R. Escoba Galindo, A. an Veen, S.M. Pimblo 1 In e acul y Reac o Ins i u e, Del Uni e si y o Technology, Mekelweg 15, NL-2629 JB Del , The Ne he lands Abs ac An elec os a ic a iable ene gy posi on beam o s udying de ec s in ce amic coa ings, polyme ilms and MOS-de ices is de eloped o ope a ion in Dopple -b oadening (DB) and posi on annihila ion li e ime (PAL) modes. In DB mode he implan a ion ene gy can be a ied be ween 3 and 30 keV wi h a beam diame e on he a ge anging om 0.4 o 0.5 mm FWHM. In PAL mode he s a -signal is gi en by seconda y elec on emission om a 25 nm hin ca bon oil placed in on o he a ge . A e passing he oil he posi on beam is ocused on he a ge wi h a spo size o 2 mm FWHM a 1 keV down o 0.6 mm FWHM a 30 keV. The a ge chambe is equipped wi h an in si u ou -poin bending de ice o s udying de ec s in oduced by ensile and comp essi e s esses. #2002 Published by Else ie Science B.V. Keywo ds: Elec os a ic posi on beam; Dopple -b oadening; Posi on li e ime 1. In oduc ion The posi on annihila ion Dopple -b oadening (DB) and posi on annihila ion li e ime (PAL) echniques a e known as sensi i e me hods o he cha ac e iza- ion o de ec s in a wide a ie y o solids. Combined wi h a beam o a iable ene gy posi ons hey o e he possibili y o cha ac e ize no only he bulk, bu also o s udy he p ope ies o su aces, in e ace egions and hin ilms. An o e iew o he acili ies and echniques employed is gi en by, e.g. Coleman [1] and Schul z and Lynn [2]. In o de o s udy ce amic and polyme coa ings, and MOS-de ices we designed a a iable ene gy posi on beam acili y, which ope a es in bo h DB and PAL modes. Bo h echniques can be pe - o med subsequen ly using he same beam-line. The posi ion o he a ge emains ixed and he op ical sys em, based on a se o elec os a ic lenses, can be uned acco ding o he espec i e expe imen . 2. Desc ip ion o he acili y 2.1. Dopple -b oadening mode The op ical design o he acili y is shown in Fig. 1. Posi ons om a 22Na sou ce a e mode a ed in a 3.5 mm hick annealed polyc ys alline ungs en oil. A modi ied Soa gun and Einzel-lens, and addi ional de lec ion pla es o ine alignmen , ocus he beam a he en ance o a cylind ical mi o , which bends he beam o e 908[3]. In DB mode he beam is Applied Su ace Science 194 (2002) 47–51 * Co esponding au ho . Tel.: þ31-152781961; ax: þ31-152786422. E-mail add ess: [email p o ec ed] (E. Abadjie a). 1 Pe manen add ess: Radia ion Labo a o y, Uni e si y o No e Dame, No e Dam, IN 6556-0579, USA. 0169-4332/02/$ – see on ma e #2002 Published by Else ie Science B.V. PII: S 0169-4332(02)00087-9 anspo ed and ocused on a a ge , which is kep a g ound po en ial. The anspo a ion and ocusing a e achie ed by ou symme ic Einzel-lenses (lenses 1, 2, 3 and 6). Lenses 4, 5 and 7 a e a g ound po en ial. The implan a ion ene gy is se by changing he po en- ial di e ence be ween he sou ce and he a ge and can be a ied be ween 3 and 30 keV. Lenses 1 and 2 ope a e a a ixed ol age a io. Thei diame e (D) is 26 mm, he a io g/D¼0:15, whe e gis he gap be ween he elec odes, and he a io A/D¼1, whe e Ais he sum o he middle elec ode leng h plus he gap dis ance g. Lenses 3 and 6 ha e a i- able po en ial a ios. Thei diame e s a e 38 and 44 mm, espec i ely. They ha e a ios o : g/D¼ 0:1, espec i ely 0.06 and A/D¼0:73, espec i ely 1.8. Behind he a ge , which is 35 mm away om he las lens, a Ge-de ec o is placed ou side he acuum chambe . 2.2. Posi on li e ime mode In o de o pe o m posi on li e ime measu emen s a 25 nm hin ca bon oil can be in oduced which in e cep s he beam 60 mm behind he lens 3 (Fig. 1). The oil is moun ed on a mo able holde and is il ed o e 458wi h espec o he beam axis. The backwa ds emi ed seconda y elec ons gene a ed by he inciden posi ons a e collec ed by a mic o-channelpla e (MCP) wi h anode assembly posi ioned no mal o he oil su ace, on he side whe e he posi ons en e he oil. The ou pu pulses o he MCP a e ed in o a ime- o-ampli ude con e e (TAC) gi ing he s a - signal o he li e ime measu emen . The elec os a ic lens sys em (lenses 4–7) ocuses he ansmi ed beam on he a ge . The e he annihila ion ga e de ec ed by a BaF scin illa ion de ec o p o iding he s op-signal o he TAC. Mon e Ca lo simula ions we e pe o med o a se o selec ed ene gies o he incoming beam o calcula e he ac ion, ene gy and angula dis ibu ions o he ansmi ed posi ons esul ing om he inelas ic and elas ic posi on sca e ing p ocesses in he oil. The used me hodology is desc ibed in [4].Fig. 2 shows he calcula ed angula and ene gy dis ibu ions o posi- ons wi h an ini ial ene gy o 7 keV, a which he beam ansmission p o ile is in op imum. A his ene gy 90% o he beam is ansmi ed wi hin a solid angle o 158. The a e age ene gy loss is calcula ed o be 3% o he ini ial alue. These da a de e mine he op ical design, shown in Sec ion 3.2. In addi ion, he beam p ope ies can be moni o ed by a channel on a he posi ion o he ca bon oil o he a ge . The acili y is equipped wi h a PC and so wa e o au oma ed se ings and con ol o he lens po en ials and da a accumula ion in bo h DB and PAL mode. A ou -poin bending de ice can be ins alled in he sample chambe o s udying he de ec gene a ion and adhesion p ope ies o polyme and ce amic coa ings unde comp essi e and ensile s ess condi ions. Fig. 1. Op ical design o an elec os a ic a iable ene gy posi on beam. 48 E. Abadjie a e al. / Applied Su ace Science 194 (2002) 47–51 3. T ajec o y calcula ions 3.1. Dopple -b oadening mode The elec ic ield dis ibu ions along he beam-line and ay- acing a e pe o med using he simula ion p og am Simion 7 [5]. The emission angles o he mode a ed posi ons om he ungs en oil su ace a e assumed o ha e a Gaussian dis ibu ion wi h an angula HWHM o 58[2]. The wo k- unc ion is 2.9 eV. The emi ing a ea o he mode a o oil has a adius o 3 mm. We ha e simula ed he ajec- o ies o posi ons wi h 3 eV ini ial ene gy and assum- ing a uni o m angula dis ibu ion wi hin 108 Fig. 2. Calcula ed angula and ene gy dis ibu ions o posi ons wi h an ini ial ene gy o 7 keV ansmi ed h ough a 25 nm hin ca bon oil. Fig. 3. Lens po en ial diag am in DB mode o 25 keV posi ons and in PAL mode o 3 and 25 keV posi ons, espec i ely. E. Abadjie a e al. / Applied Su ace Science 194 (2002) 47–51 49 emission angles, he eby aking in o accoun he pos- sible oughness o he mode a o su ace. Posi ons a e gene a ed andomly om a uni o m spa ial dis ibu- ion om su ace wi h adius 3 mm. The lens po en ial diag am o achie ing a beam o 25 keV posi ons is p esen ed in Fig. 3. The esul ing beam diame e a he a ge is 0.4 mm FWHM. F om 3 o 30 keV he beam diame e changes be ween 0.4 and 0.5 mm FWHM. In his ene gy ange he longi udinal ene gy sp ead is calcula ed o be less han 0.1 keV. 3.2. Posi on li e ime mode In PAL mode he sou ce is kep ixed a 5 keV. Lenses 1, 2 and 3 a e se a 3.3, 2.8, and 3.8 kV, espec i ely. The posi on beam is hus ocused o a beam diame e less han 1 mm on he ca bon oil, which is kep a 2 kV. A 2D cu o he op ical sys em be ween he ca bon oil and he a ge is shown in Fig. 4. Lenses 4 and 5 se e o minimize he beam di e gence. Lenses 6 and 7 a e used in combina ion o ocus he beam on o he a ge . The inal implan a ion ene gy can be a ied be ween 1 and 30 keV by chan- ging he po en ial on he a ge . In he simula ions he ini ial coo dina es o he 8000 posi ons a e gene a ed andom om a spo o 1 mm diame e , angles and ene gies co esponding o he dis ibu ions shown in Fig. 2. The beam diame e a he a ge a ies om 2 mm FWHM a 1 keV o 0.6 mm FWHM a 30 keV. Fo he ene gy ange be ween 1 and 5 keV he op ical sys em wo ks in decele a ing mode and om 5 o 30 keV in accele a ing mode. Examples o de i ed op imum lens po en ials o bo h cases a e gi en in Fig. 3. The implan a ion ene gy sp ead coming om educ ion o he longi udinal elo- ci y is calcula ed o be 50 eV a 1 keV and 0.5 keV a 15 keV. This only causes small shi s in he li e ime dep h-p o iling da a. Examples o he ime-o - ligh dis ibu ions o he elec ons (gi ing he s a -signal) and he posi ons o h ee implan a ion ene gies 1, 5, and 15 keVa e shown in Fig. 5. F om his da a we es ima e ha he op ical design con ibu ion o he ime esolu ion is, app oxi- ma ely 300 ps. The elec onics con ibu ion is in o de o 250 ps. Fig. 4. Two-dimensional cu o he op ical sys em be ween he ca bon oil and he a ge . Fig. 5. Calcula ed ime-o - ligh dis ibu ions o elec ons and posi ons a elling om he ca bon oil su ace o he MCP and o he a ge , espec i ely. 50 E. Abadjie a e al. / Applied Su ace Science 194 (2002) 47–51 4. Conclusion An elec os a ic a iable ene gy posi on beam is designed o ope a e in wo modes. In DB mode he beam ene gy can be a ied om 3 o 30 keV wi h an ene gy sp ead o less han 0.1 keV and beam diame e o 0.4–0.5 mm FWHM. In PAL mode he beam ene gy can be a ied be ween 1 and 30 keV. The beam diame e a ies om 2 o 0.6 mm FWHM. The posi- on implan a ion ene gy sp ead is less han 5%. The o al ime esolu ion o he acili y is de e mined by he ime-sp ead due o he op ical design (300 ps) and he con ibu ion o he elec onics (250 ps). Re e ences [1] P. Coleman, Posi on Beams and hei Applica ions, Wo ld Science Publ. Co. P e. L d., Singapo e, 2000. [2] P. Schul z, K.G. Lynn, In e ac ion o posi on beams wi h su aces, hin ilms, in e aces, Re . Mode n Phys. 60 (1988) 701. [3] L.V. Jo gensen, A. an Veen, H. Schu , A. Winkelman, P. K ui , Nucl. Ins . Me h. A427 (1999) 131. [4] S.M. Pimblo , J.A. LaVe ne, A. Alba Ga cı ´a, L.D.A. Siebbeles, J. Phys. Chem. B 104 (2000) 9607. [5] D.A. Dahl, Simion 3D, Ve sion 7, Use s Manual, Idaho Na . Eng. and En . Lab., Idaho Falls, 2000. E. Abadjie a e al. / Applied Su ace Science 194 (2002) 47–51 51