Quantification of the H Content in Diamondlike Carbon and Polymeric Thin Films by Reflection Electron Energy Loss Spectroscopy

Quan i ica ion o he H con en in diamondlike ca bon and polyme ic hin ilms by e lec ion elec on ene gy loss spec oscopy F. Yube o,a兲V. J. Rico, J. P. Espinós, J. Co ino, and A. R. González-Elipe ICMSE (CSIC-USE) Amé ico Vespucio s/n, E-41092 Se illa, Spain 共Recei ed 24 Ma ch 2005; accep ed 28 June 2005; published online 15 Augus 2005兲 A nondes uc i e me hod o de e mine he hyd ogen con en a he su ace o diamondlike ca bon and polyme ic hin ilms is p oposed. The me hod elies on he analysis o he elas ic peak p oduced by backsca e ing o elec ons om he hyd ogen a oms p esen a he sample su ace. Quan i a i e analysis o he H con en a he su ace is achie ed h ough use o a phenomenological sensi i i y ac o o elas ic elec on backsca e ing by H a oms wi h espec o o he a oms p esen a he su ace o e e ence polyme ic samples. The alidi y o he me hod is checked wi h elas ic ecoil de ec ion measu emen s and in a ed spec oscopy analysis o he same samples. The accu acy o he me hod in he de e mina ion o H con en a he sample su ace is es ima ed o be ±10%. © 2005 Ame ican Ins i u e o Physics.关DOI: 10.1063/1.2011786兴 Quan i ica ion o he hyd ogen con en in diamondlike ca bon 共DLC兲and polyme ic hin ilms is o he mos impo - ance because hei p ope ies a e igh ly linked o his con en .1Typically he hyd ogen con en in his ype o ma- e ials has been de e mined wi h echniques such as elas ic ecoil de ec ion analysis 共ERDA兲, nuclea eac ion analysis 共NRA兲using he 1H共15N, ␣ 兲12C nuclea eac ion, nuclea magne ic esonance 共NMR兲, he mal hyd ogen e olu ion, o in a ed spec oscopy 共IR兲. Conscious o he g ea demand o al e na i e and simple me hods,1–3 we ha e de eloped a p o- cedu e o quan i y he hyd ogen con en a he su ace o DLC and polyme ic hin ilms. As i will be shown in his pape , he H con en wi hin he su ace egion o hese ype o ma e ials can be de e mined by looking a he elas ically backsca e ed elec ons wi h kine ic ene gies in he o de o 1 o 2 keV as in s anda d e lec ion elec on ene gy-loss spec- oscopy 共REELS兲analysis. The basis o he quan i ica ion is he same as ha o Ru he o d backsca e ing spec oscopy 共RBS兲, wi h he di e ence ha in RBS he p obe pa icles a e usually alpha pa icles and in ou me hod elec ons 共no e he di e ence o mass be ween hese wo p ojec iles兲. In his espec , i mus be men ioned ha e y ecen ly O osz e al.4 ha e epo ed ha H a oms in polye hylene can be de ec ed by his echnique. I is also wo h men ioning ha REELS was used in he pas o de ec H induced chemical shi s in he plasmon exci a ions.5 The p esence o H can be easily de ec ed by s anda d REELS measu emen s wi h 1 o 2 keV elec on beams. The physical basis o he de ec ion o H a oms a he su ace o ma e ials con aining ligh elemen s is he expec ed di e ence in he ecoil ene gy ⌬E 6in he in e ac ion o a as elec on wi h a s a ic nucleus, ei he hyd ogen o ano he ype o a om, which can be es ima ed by he exp ession ⌬E =4me MH E0sin2 ␪ 2 冉 1− 1 A 冊 ,共1兲 whe e me,MH, and Aa e he elec on mass, he a omic weigh o he H a om and he o he nucleus conside ed, e- spec i ely, ␪ is he sca e ing angle, and E0is he p ima y ene gy o he incoming elec ons. This exp ession p edic s ha , o a ypical sca e ing angle o ␪ =120° and a p ima y ene gy o 1500 eV, he di e ence in ecoil ene gies be ween H and o he a oms hea ie han C is ⬃2.1 eV, so ha we migh easily dis inguish be ween H and hese o he a oms p esen in he sample wi h he s anda d ene gy esolu ion o REELS measu emen s 共i.e., ew en hs o eV兲. Figu e 1 shows he REELS spec a, aken wi h a p ima y elec on beam o 1500 eV, o se e al e e ence samples 关polyca bona e 共PC, -C16H14O3-兲, polys y ene 共PS, -C8H8-兲, polyme hyl-me hac yla e 共PMMA, -C5H8O2-兲, and poly e - a luo e hylene 共PTFE, -CF2-兲and highly o ien ed py oli ic g aphi e 共HOPG, C兲兴. The REELS measu emen s we e pe - o med wi h special ca e o minimize elec on damage a he su ace o he samples. Thus, ypically he measu ing ime was o ⬃1 min. wi h elec on beam cu en s below 1 ␮ A/cm2.7The angle o incidence o he elec on beam wi h espec o he su ace no mal was 60° 共i.e., ␪ =120°兲and he elec on de ec ion was se no mal o he sample su ace. In he elas ic egion o hese spec a 共be ween ⬃1497 and 1502 eV兲 he H signal can be clea ly iden i ied as a weak peak a 2.1 eV lowe kine ic ene gy han he mo e in ense ea u e in he spec a a 1500 eV. This la e peak is due o he elec ons backsca e ed by he a oms o he han H p esen a he su ace o he samples. No e ha he small peak a 2.1 eV is only p esen in he H con aining polyme s 共i.e., PS, PMMA, and PC兲and is no de ec ed in he case o PTFE o HOPG, as expec ed. This peak shi s o 3.0 eV lowe kine ic ene gy han he main peak when he exci a ion ene gy is changed o 2000 eV 共no shown兲. This esul con i ms i s assignmen as elec ons backsca e ed by he H a oms in he samples. A his poin i is wo h men ioning ha using p i- ma y beam ene gies la ge han ⬃1800 eV has he e ec o inc easing he backg ound be ween he wo con ibu ions o he elas ic peak p obably due o mul iple elas ic sca e ing and damage. On he o he hand, he wo con ibu ions o he elas ic peak do no sepa a e enough o acili a e quan i ica- ion when p ima y elec on ene gies smalle han ⬃1200 eV a e conside ed. Besides he small ea u e a ⬃2.1 eV om he main elas ic peak o E0=1500 eV, hese spec a a e cha ac e ized by he ypical ␲ plasmon ea u es a ⬃6eV ene gy loss, co esponding o he benzoic ings in PC and a兲Au ho o whom co espondence should be add essed; elec onic mail: [email p o ec ed] APPLIED PHYSICS LETTERS 87, 084101 共2005兲 0003-6951/2005/87共8兲/084101/3/$22.50 © 2005 Ame ican Ins i u e o Physics87, 084101-1 12 June 2025 14:52:16 PS, and o he sp2cha ac e o g aphi e. Besides, he co e- sponding ␴ + ␲ bulk plasmons a e indica ed o each ma e- ial. All hese e e ence samples we e also analyzed by XPS o con i m he le el o su ace con amina ion. I was ound ha a 5%–10% o O con amina ion was p esen a he su - ace o polyme s ha do no con ain oxygen in hei s uc- u es. A simila de ia ion om he heo e ical oxygen con en was ound in hose ma e ials such as PC o PMMA ha con ain his elemen in hei s uc u es. F om he ela i e in ensi y o he H peak wi h espec o he main ea u e o he spec a, i is possible o es ima e a phenomenological sensi i i y ac o ␴ exp ha , in he o m o a calib a ion cu e, se es o co ec o he di e en c oss sec ions o elas ic elec on backsca e ing by H o o he a - oms 共C and, o a mino ex en , O a oms兲p esen a he su - ace o he polyme s. The inse in Fig. 1 shows he co ela- ion be ween he in ensi y a io IH/Ino_H, and he nominal a io be ween H and o he ype o a oms 共C and o O兲p esen in he polyme s conside ed in his s udy. He e IHis he a ea o he H signal in he back e lec ed elec ons and Ino_His he a ea o he elas ic peak coming om a oms o he han H. I can be obse ed ha he e exis a linea co ela ion wi hin he expe imen al unce ain ies be ween he nominal a omic a io and he expe imen al in ensi y a io. In his analysis, we did no a emp quan i ica ion o high-H con en polyme s such as polyp opylene 共-C3H6-兲o polye hylene 共-CH2-兲be- cause hey a e known o deg ade easily wi h elease o hy- d ogen when hey a e bomba ded wi h low-ene gy elec ons.7By pe o ming a linea eg ession passing h ough he o igin o coo dina es wi h he o he poin s in he g aph, he slope o he i ed line is 37. This alue can be conside ed as a phenomenological ela i e c oss sec ion ␴ exp be ween he wo con ibu ions o he elas ic peak signal. No e ha he alue ob ained o ␴ exp is in easonable ag eemen wi h he p edic ions o he NIST da abase o elas ic sca e ing c oss sec ions8共neglec ing he p esence o oxygen a oms a he su ace and mul iple elas ic sca e ing, ha may be signi i- can in his case9兲. A ough heo e ical es ima ion o ␴ exp is 42, ob ained as he a io o he di e en ial elas ic sca e ing c oss sec ion wi h espec o he solid angle be ween C and H o 120° sca e ing angle and 1500 eV elec on ene gy.8In he ollowing we will used ou phenomenological ␴ exp=37 o quan i y he H con en in DLC samples. Thus, he H quan i- ica ion a he su ace o unknown samples by REELS can be pe o med acco ding o he exp ession %H= 100 IH ␴ exp IH ␴ exp +Ino_H ,共2兲 whe e he meaning o he di e en magni udes has been in- oduced p e iously. To check he alidi y o his p ocedu e, a i s compa a- i e s udy was ca ied ou by ERDA/RBS and REELS o a se ies o DLC samples p epa ed wi h ixed bias ol age and di e en con en o H in he plasma gas. The DLC hin ilms we e p epa ed by plasma Enhanced Chemical Vapo Deposi- ion in a plasma eac o wi h a pla e con igu a ion. Mix- u es o C2H2, A , and H2we e used as he plasma gas, while a sel -induced bias ol age was applied o he pla e elec ode con aining he subs a e o he samples. I was ound ha he pe cen age o H a oms in he DLC ilms measu ed by bo h echniques 共no shown兲was he same wi hin 10%, so ha , wi h he unce ain y limi s o hese echniques, hey yield simila alues o he H con en o he ilm. This p o es ha REELS can be an al e na i e echnique o he de e mina ion o H con en a he su ace in DLC ilms. To check u he he alidi y o he me hod, he hyd o- gen con en has been also de e mined o a se ies o DLC hin ilms p epa ed wi h a s anda d plasma composi ion by a ying he bias ol age applied o he subs a e pla e. Bias ol age is ecognized as a c i ical pa ame e o he con ol o he ha dness and o he p ope ies o DLC hin ilms.10 Fo his se ies o samples, he pe cen age o hyd ogen in he ilms has been plo ed in Fig. 2 agains he bias ol age. Fo compa ison, his igu e also includes he e olu ion o he in ensi y o he 2750–3050 cm−1 IR band, co esponding o he s e ching C-H ib a ions, no malized o he ilm hick- ness as de e mined by x- ay luo escence spec oscopy in a scanning elec on mic oscope. I is in e es ing ha , wi hin he e o ba s in bo h p epa a ion and quan i ica ion p o o- FIG. 1. REELS spec a measu ed wi h 1500 eV kine ic ene gy o PTFE, g aphi e 共G兲, PC, PS, and PMMA. Inse : Co ela ion be ween he heo e ical H con en in he e e ence polyme samples and he a io IH/Ino_Has de e - mined by REELS measu emen s. FIG. 2. Co ela ion be ween he no malized in ensi y o he in a ed C-H s e ching modes, accele a ion ol age du ing p epa a ion and he a omic pe cen age o H in he ilm as de e mined by quan i ica ion o he elas ic signal in REELS da a. Full ci cles: In a ed quan i ica ion; open squa es: REELS quan i ica ion. 084101-2 Yube o e al. Appl. Phys. Le . 87, 084101 共2005兲 12 June 2025 14:52:16 cols, hese wo magni udes ollow simila endencies. This e olu ion is cha ac e ized by a p og essi e dec ease in he hyd ogen con en as he bias ol age inc eases. This co ela- ion shows ha , despi e he su ace cha ac e o REELS, i s quan i ica ion da a o hyd ogen can be aken as oughly ep- esen a i e o bulk composi ion. To conclude, he p esen esul s ha e shown ha REELS may p o ide a way o quan i ying he amoun o hyd ogen p esen a he su ace o DLC o polyme ic ma e ials whe e C and H a e he majo i y cons i uen elemen s. The su ace cha ac e o REELS is con olled by he mean ee pa h o elec ons ha a el h ough he analyzed ma e ials ha , o he usual elec on ene gies u ilized in his echnique, is ⬃3 nm. We wan o emphasize ha his analysis p ocedu e measu es di ec ly he amoun o hyd ogen by a su ace sen- si i e echnique. I could hus be use ul o combine REELS wi h XPS o su ace analysis since XPS does no di ec ly de ec H. We hank he Spanish Minis y o Science and Educa ion 共MAT2004-01558兲 o inancial suppo . 1J. Robe son, Ma e . Sci. Eng., R. 37, 129 共2002兲. 2W. Jacob and M. Unge , Appl. Phys. Le . 68, 475 共1996兲. 3J. Ris ein, R. T. S ie , L. Ley, W. Beye , J. Appl. 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