100 (2001). ACTA PHYSICA POLONICA A. No. 5. Proceedings of the XXX VI Zakopane School of Ph ysics, Zakopane 2001. Combined An alyses of Ion Beam.
V ol . 1 0 0 ( 2001)
A CT A P HY SIC A P O LO N IC A A
No . 5
P ro ceed in gs of t h e XXX V I Z ako p ane Sch oo l o f Ph ysi cs, Z ak opa n e 200 1
C om b i n ed An al y ses of Ion B eam Sy n t h esized L ayers in P oro u s Sili con Ç VÇ A .R . Ramo sa , F. P Ç a szt i c , Z. E. H or v Ç a t h d , E. azson yi d , O. Co nd eb , M.F. a
da Si l va a , M.R . da Sil va a and J.C. So are sa
I TN - In st. Tecnol Ç ogi co e Nucl ear , Estr. Naciona l 10, 26 86-953 SacavÇ em , Po rtug al an d Centro de FÇ ±sica Nucl ear da Uni versi da de de Li sb oa A v. Pro f. G ama Pi nto 2, 1649- 003, Li sboa, Po rtug al b
FCUL- Fac. de Ci ^encias da Uni v. de Li sboa, Dep. FDZsica Ed. C1 , 1749-016, Li sbo a, Portug al c
KFKI- R es. Inst. for Pa rti cle and Nucl ear P h ysi cs P.O. BO X 49, 1525, Buda p est, Hung ary
d
MT A- Res. Inst. f or Techni cal Phys. and Ma teri al s Science P.O. BOX 49, 1525, Buda p est, Hung ary
H igh dose ion i mpl antation w as used to form p olycrysta ll i ne silicid e Ùlms on porous silicon w ith di ˜erent nati ve concentrations of light impuri ties (C and O) . Porous silico n layers several ñ m thic k w ere implanted w ith 170 keV C r + ions to Ûuences of 3 È 1 0 1 7 ions/cm 2 both at ro om temp erature and 4 50 C . Simil ar samples w ere implan ted w ith 100 keV Co + ions to Ûuences of 2 È 10 17 ions/cm 2 at ro om temp erature , 350 C , and 450 C . T he formed sili cid e comp ounds w ere studied by Rutherf ord backscattering spectrometry , elastic recoil detection, glancing incidence X -ray di˜racti on, and four p oint- prob e sheet resistance measurements. Selected C o implan ted samples w ere analysed by cross- section transmissi on electron microscopy . Results show that the light impuriti es were partiall y exp ell ed from the forming silici de layer. C ombining cross- section transmissio n electron microscopy w ith ion b eam metho ds it was possibl e to show that, in the implanted region, the p orous structure collaps es and densiÙes during implantation, but the underlying porous silicon remains intact. T he layer structure, as w ell as the quality and typ e of the formed sili cid e, were found to depend on the original impurity level, implantation temp erature, and annealing . £
£
PACS numb ers: 61. 10. {i, 61.18.Bn, 61. 43.Gt, 68.55. Ln
(7 73)
£
774
A . R. Ram os et al. 1. I n t r o d u ct io n
Po ro us sili con (PS) i s extensi vel y i nvesti gated for i ts el ectro l um i nescence pro perty [1]. Its use coul d al l ow the i nteg ra ti on of opti cal and electro ni c uni ts i nto a sing le devi ce wi th conventi ona l techno l ogy. A ba sic requi rem ent i s the form ati on of electri c conta cts on PS: conducti ve Ùlm s and conta cts a re necessary to pro vi de i nterco nnecti on b etween devi ces a nd the outsi de worl d. G i ven the structura l and com p ositi ona l pro p erti es of PS, i on b eam synthesi zed (IBS) ref racto ry m etal sil i cides are a pri vi l eged choi ce fo r m etal l i zati on. Previ ous experi m ents revealed tha t PS densiÙes under i on b omba rdm ent [2]. If the i m pl anted i ons react wi th sili con, a s the ones ro uti nel y used i n sil i cide form ati on do , e.g. 100{ 200 keV Ti , W , and Co i ons [3], prepa ra ti on of compa ct sil i cide conta cts on PS may be achi eved i n one sing l e step. Cr wa s the Ùrst species chosen: PS usuall y has a hi gh na ti ve concentra ti on o f conta m i nants, mostl y C and O, and i t i s kno wn tha t i m pl an ted Cr can, even i n sil i ca, react wi th sil i con to form sil i ci de [4]. Ho wever, Cr form s a rel ati vel y hi gh resisti vi ty semi conducti ng di sil i cide ( ¤ 1 5 0 0 ñ ¨ cm ). Theref ore, a second batch of sam pl es was i mpl anted wi th Co i n order to study the p ossibi l i ty of form i ng a l ower resisti vi ty metal l i c sil i cide al ready after the i m pl anta ti on, thus avoi di ng the need for a hi gh tem pera ture annea l ing tha t woul d destro y the PS [1].
2 . E x p er i m en t a l Three di ˜erent ki nds of 5{ 10 ñ m thi ck PS l ayers were prepa red by ano di c etchi ng . Sampl e prepa rati on deta i l s can b e found i n R efs. [5, 6]. Chosen sampl es were pre- oxi di zed ri ght after m anuf acturi ng by annea li ng at 3 0 0 C for 60 m i n i n an N 2 atm osphere wi th ¤ 1 % O 2 . Between prepa ra ti on and i mpl anta tio n, the sam pl es were kept in conta i ners at either no rm al , arg on, or p enta ne atm osphere. Arg on wa s found to be e˜ecti ve i n delayi ng the O upta ke, whereas the sampl es kept i n p enta ne oxi dized more qui ckl y. One ba tch of sam pl es wa s im pl an ted wi th 170 keV Cr + i ons, at 5 ñ A / cm 2 current densi ty and 1 5 ti l t, up to a Ûuence of 3 È 1 0 1 7 i ons/ cm 2 at ei ther ro om tem p erature (R T) or 4 5 0 C. The l atter sampl es were anneal ed at 7 0 0 C for 90 m i n and the chosen ones were further annea led at 1 0 0 0 C for 15 mi n. A second ba tch of sam pl es was i m pla nted wi th 100 keV Co i ons, at 3 A/ cm current density and 1 5 ti l t, up to a Ûuence of 2 È 1 0 i ons/ cm at ro om tem p erature, 3 5 0 C, and 4 5 0 C. Al l these sampl eswere annea led at 6 0 0 C for 60 m i n. The 3 5 0 C i mpla nted sam pl es su˜ered an extra anneal at 1 0 0 0 C for 30 mi n. Al l anneal s were do ne i n ¤ 5 È 1 0 m bar vacuum . The i n- depth com positi on of the vari ous sampl es wa s determ i ned by the R utherf ord ba ckscatteri ng spectro metry (R BS) and el asti c recoi l detecti on (ER D ) (R BS at = 1 4 0 and 1 8 0 scatteri ng ang l e, IBM geometry , 1.6 MeV He , 1.5/ 1.73 MeV H or = 1 6 5 , Cornel l geom etry , 3.043/ 3.145/ 3.8 MeV He ; £
£
C ombi ned A nal yses of Ion Beam Synt hesized L ayer s .
..
775
ER D at È = 2 3 , IBM geom etry , 3.043 MeV 4 He + ). 1.73 MeV pro to ns are esp ecial ly sensiti ve to C and 3.145 MeV He i ons to O due to the resona nces at 1.730 MeV and 3.045 MeV. The densiÙcati on e˜ect of i on- im pl an tatio n i n PS was i nv esti gated both by cross-section tra nsm i ssio n electro n mi croscopy (XTEM) and the resona nce m etho d [7]. The l atter pro cedure m easures the poro sity of the sam pl e thro ugh the enhanced wi dth of the 1 6 O resona nce p eak. Io n b eam anal ysi s da ta were eva lua ted wi th RUMP [8], R BX [9], and ND F [10]. G l anci ng i nci dence X- ra y di ˜ra cti on (G IXR D ) m easurem ents were do ne wi th a Si emens D -5000 di ˜ra cto meter (1 or 0 : 5 i nci dence, Cu K ˜ ra di a ti on). Pha se i denti Ùcati on was ba sed on R efs. [11] and [12] (f or Cr 5 Si3 ). Sampl esfor XTEM were prepa red by arg on i on m i ll ing (see deta i l s i n R ef. [7]) and then inv esti gated wi th a Phi l i ps CM 20 typ e electro n m icro scop e at 200 keV e energy. £
£
£
À
3.1. Porosi ty change Po ro sity chang e m easurem ents using the O resonance m etho d were carri ed out on selected vi rg in and i m pla nted sampl es. R esults o bta i ned i n sampl es i m pl an ted wi th 20% of the reg ul ar Ûuence show them to b e al most com pl etel y densiÙed [6]. In hi gher do se sam pl es, al l pro cessescan, theref ore, b e considered to ha ve occurred i n compa ct materi al , li ttl e i nÛuenced by the ori gi nal structure. The onl y structure -rel ated di ˜erence observed was the \ p eeling o˜ " of the p oro us l ayer i n non pre- oxi di zed spong y sampl es when i m pla nted at 4 5 0 C. In al l other sam pl es, tho ug h the surf ace densiÙes, the i n- depth PS reta i ns its i ntegri ty , as shown by the XTEM pi ctures (Fi gs. 1a, c, and d). 3. 2. C ompositi on pro Ùles Fi gure 2 shows a set of representa ti ve spectra and sim ula ti ons. The as-i m pl anted pro Ùles were found to b e nearl y recta ngul ar b oth for the ro om tem p erature (R T) and hi gh tem p erature i m pl anta ti ons (see, for exampl e, Fi g. 2c). Cr and Co pro Ùles are aro und 150 nm and 60 nm thi ck, respecti vely. These thi ckness values, cal culated using Bra gg' s rul e [13], agree well wi th the ones obta i ned for Co f rom the XTEM pi ctures (see Fi g. 1). A di ˜usi on- l i ke ta i l exi sts as wel l, i ncreasing wi th i mpl anta tio n tem pera ture. The sam e pi ctures al so reveal i ncreased surf ace ro ug hness i n the hi gh tem p erature i m pl anted sampl e. Acco rdi ng to the sim ul ated depth pro Ùles, duri ng i m pla nta ti on, C a nd O were parti al l y expell ed fro m the i m pl anted l ayer, pi l ing up behi nd i t. In the case of Cr, thi s pro cess di d no t occur at RT , i ndi cati ng tha t i t i s rel ated to sil i cide f orm ati on and/ or enhanced ato m m obi l it y at 4 5 0 C. For Co, the opp osite ha pp ens: the expell i ng pro cess i s m ore pro nounced at RT , the concentra ti on o f reta i ned O
776
A . R. Ramos et al.
Fig. 1. X T EM pictures of several Co implanted porous samples: (a) columnar typ e sample of 40% p orosity implanted at 350 C; (b) the same sample after annealin g at £
1000
£
C ; (c) 70% porosity , pre-oxidi zed columnar sample implan ted at ro om temp erature;
(d) the simila r sample implan ted at
450
£
C. T he high oxygen content of the latter sample
did not allow the grow th of large silici de grains as in part (a).
i ncreasing wi th i m pl antati on tem pera ture. Such b ehavi our may i ndi cate oxyg en redi stri buti on i n the i m pl anted l ayer duri ng hi gh tem p erature i m pl an ta tio n, supp osedly rel ated to a hi gher di ˜usi on coe£ cient and/ or sol ubi li ty of O i n Co sil i cide. The di ˜erence b etween Co and Cr i m pla nted sam pl es i ndi cates tha t the expell i ng pro cess i s no t a bal l i sti c e˜ect, but tha t i t i s i ndeed rel ated to sil i cide form atio n: no sil i cide pha ses exist i n RT i m pl anted sam pl es for Cr, whi l e for Co, CoSi, and Co 2 Si are observed by G IXR D . H i s al so rem oved fro m the i m pla nted layer (see Fi g. 2b). W hi l e for mono crysta l l i ne Si the reta i ned io n do se i ncreases wi th i m pla nta ti on tem p erature, for PS i t decreases. In the case of hi gh tem p erature i m pla nta ti ons, the reta i ned do se decreases wi th O content, wi th hi gher val ues i n m onocrysta l l i ne Si and lo wer val ues i n heavi l y conta mina ted PS sam pl es. Conversel y, for the RT i mpl anta tio ns, the reta i ned do se i ncreases wi th the O content. Such b ehavi our m ay i ndi cate an oxyg en enha nced meta l di ˜usi on mechani sm to wards the surf ace duri ng hi gh tem p erature i mpla nta ti on.
C ombined A nal yses of Ion Beam Synt hesized L ayer s .
Fig. 2.
777
..
RBS and ER D spectra for a spongy pre- oxidized sample of 60% porosity , im-
planted with C r at 3. 4043 MeV
4
+
450
He , È
=
£
C . (a) RBS 1.73 MeV H + È 23
£
=
1 80
£
, IBM geometry; (b) ERD
, I BM geometry; (c) RBS 3.8 MeV
4
H e+ , È
=
1 65
£
, C ornell
geometry . Dots are measured spectra, w hile lines corresp ond to simulations.
3.3. Phase for mat i on The X- ra y di ˜ra cti on data i ndi cate tha t several sil i cide pha ses form , dep endi ng on tem pera ture and oxyg en content (see Fi g. 3). CoSi2 or CrSi 2 gro w pref erabl y i n low oxyg en, hi gh tem pera ture i m pla nted sampl es. The i nteg ra ted i ntensi ty of the di ˜ra cti on li nes and the a verage gra i n size o f the preci pi tates decrease as the O content gro ws. For Co, at ab out 0.22 O/ Si ra ti o ano ther sil i cide pha se app ears | CoSi | whi ch di sapp ears at even hi gher O contents. For such sam pl es, the i ncreasing of the i m pl anta tio n tem p erature fro m 3 5 0 C to 4 5 0 C do es no t pro m ote Co Si form ati on (see Fi g. 3). CrSi form ati on i n hi g h tem p erature i mpl anted sam pl es conti nuousl y decreases unti l 0 2 5 O/ Si ra ti o, when i t di sapp ears. Weak Cr Si peaks app ear i n heavi ly conta m ina ted sam pl es. For the RT i m pla nted sam pl es, CrSi di sapp ears alrea dy at 0 0 3 O/ Si ra ti o and no other sil i cide pha ses are vi sibl e. W eak Cr and Si oxi de l i nes al so app ear i n hi ghl y conta m ina ted sam pl es. No Co oxi des or meta ll i c Cr or Co peaks are di scerni bl e. Sheet resista nce da ta i ndi cate tha t i n m ost sam pl es a l ow resisti vi ty l ayer form s i n spite of the i mpuri ti es. In the hi gh tem p erature Cr i mpl anted sam pl es, the resista nce coi nci des wi thi n erro r to tha t of Cr i m pl anted m ono crysta l l ine Si; for the RT sampl es it i s al ways hi gher. T aki ng the thi ckness m easured by RBS, the obta i ned resisti vi ty i s 1300 ¨ cm , i n good agreement wi th other val ues found i n l itera ture fo r IBS CrSi [14]. The Co i m pl anted l ayers are m ore sensiti ve £
778
Fig. 3.
A . R. Ramos et al.
Glancin g incidence X -ray di˜raction
planted w ith C o at di˜erent ature implanted
spectra of several columnar
samples im-
temp eratures. C oSi 2 forms in low O/Si ratio, high temp er-
samples (a). H igher O/Si ratios result in CoSi forming instead (b). For
these samples, increasin g the temp erature do es not e£ciently
promote C oSi 2 formation
(c). Room temp erature implan ted samples show practically
no silici de phase but w eak
C o 2 Si + C oSi formation (d).
to the ori gina l O content: the sheet resista nce increases wi th the O/ Si ra ti o i n the i m pla nted l ayer, but di ˜erentl y fo r the vari ous tem pera tures. For l ower ra ti os, the hi g h tem p erature i m pla nted sam pl es ha ve l ower resisti vi ty; abo ve 0.22 O/ Si ra ti o, the sheet resista nce i ncreases faster fo r the hi gh tem p erature sam pl es tha n for the RT ones. Thi s may b e the resul t of coarser SiO 2 preci pi ta tes i n the hi gh tem p erature sam pl es, tho ug h thei r presence i s not easil y di sti ng ui shed i n the X- ra y di ˜ra cti on spectra . Best values are around 4 5 ñ ¨ cm ( ¤ 3 ti m es hi gher tha n for l ayers f orm ed on mono crysta l l i ne Si). 3. 4. A nneal i ng resul ts X- ra y di ˜ra cti on m easurem ents reveal ed tha t, up to 0.70 O/ Si, annea l ing pro motes the tra nsform ati on of CoSi i nto CoSi2 , i f the Ùrst was present, or the gro wth of the CoSi 2 . For even hi gher O/ Si ra ti os, CoSi al so sta rts gro wi ng . In m ore conta m ina ted sampl es, it i s the onl y pha se present. Sim il arly , anneal i ng enhances the CrSi 2 content and/ or gra i n size. As the O content i ncreases, other Cr ri ch pha ses b egi n to form , na m ely Cr 5 Si 3 and even Cr 3 Si . As exp ected, CrSi i s no t observed [15]. The sheet resista nce sli ghtl y i ncreases duri ng the 7 0 0 C annea l ing of the Cr im pl an ted sam pl es, f oll owing an i ncrease i n O content i n the i m pla nted l ayer. In the case of hi gh tem p erature Co i m pla nted sampl es, anneal i ng at 6 0 0 C resul ts i n l ower resisti vi ty . Conversel y, f or RT im pla n tati ons, the same annea li ng results i n a resisti vi ty i ncrease for sam pl es of 0 2 5 O/ Si. The addi ti onal 1 0 0 0 C anneal i ng i nduces a dra mati c structure m odi Ùcati on as shown i n Fi g. 1b: £
C ombined A nal yses of Ion Beam Synt hesized L ayer s .
779
..
l arg e i sol ate sil i cide preci pi ta tes g row and the underl yi ng p oro us sili con i s severely da m aged, whi ch results i n i ncreased resisti vi ty for al l sam pl es. 4 . Co n cl u si o n s In spi te of the hi gh i mpuri t y l evel of PS, we were abl e to form a good qual i ty p ol ycrysta l l i ne sil i cide Ùlm i n most sam pl es. Best results a re o bta i ned by hi gh tem p erature i mpla nta ti ons, where the i m puri ti es are pa rti al ly exp ell ed fro m the form i ng sil i cide, pi li ng up b ehind i t. V ery hi gh oxyg en contents i nhi bi t sil i cide form ati on. Under the exp eri menta l condi ti ons used, i n p oro us sil i con wi th a n O/ Si ra ti o < 0 : 2 2 (f or Co) or < 0 : 2 5 (f or Cr), i t i s p ossibl e to form a goo d qual i ty di sil i cide l ayer i mm ediatel y duri ng i m pla nta ti on. In more conta m i nated substra tes, other m ore m etal ri ch pha ses app ear. Ab o ve 0.7 0 O/ Si ra ti o no sil i cide can b e detected by X- ra y di ˜ra cti on. Lo wer resisti vi ty values are obta i ned for Co , wi th the 3 5 0 C i m pl anted sam pl es, tho ug h thei r resisti vi ty i s sti l l ¤ 3 ti mes hi gher tha n tha t of thei r mono crysta l l i ne counterpa rt (1 5 vs. 45 ñ ¨ cm ). The 1 0 0 0 C annea l ing resul ts i n the destructi o n of the underl yi ng p oro us sil i con and the gro wth of l arge i sol ated sil i cide preci pi ta tes. £
£
Ac kn owl ed gm en t s Thi s work was supp orted by the Hung ari an{ Po rtug uese bi l atera l i nterg overnm ental coop erati on TÇeT P- 14/ 97 and ICCTI 423/ OMFB. A. R.R. woul d l i ke to ackno wledge gra nt BD P/ 3 557/ 2000 f rom FCT , Portug al . R ef er en ces [1] Porous Si li con Sc i ence and T echn ology , Eds. J.C . V ial, J . Derrien, Springer- Verlag, Berlin 1995. Ç VÇ [2] F. P Ç aszti, A . Manuaba, E. SzilÇ agyi, E. azsonyi, Z. V Ç artesy , N ucl . Instru m. Me thods Ph ys R es. B 11 7, 253 (1996). [3 ] S.P. Murark a, [4 ] E. Cattaruzza,
, A cademic Press, N ew Y ork 1985. G. Mattei, P. Mazzoldi, R. Berto cello, G. Battaglin, 2884 (1995).
[5] A .R. Ramos, O. C onde, F. PÇ aszti, G. Battistig, M. F. da Silv a, J .C. Soares, (2000).
Ç VÇ E. azsonyi, M. R. da Silv a, 926
Ç VÇ [6] E. azsonyi, M. K oÇ os, G. J alsovszky , I. PÇ ocsik, [7] F. PÇ aszti, E. SzilÇagyi, E. V Ç azsonyi, [8 ] L. R. Doolittle , [9] E. K Ç otai,
L. Mirenghi,
121 (1993).
Zs. H orvÇath, A . Manuaba,
G. Battistig, Z. H aj nal, 533 (1998). 344 (1985).
588 (1994).
780
A . R. Ramos et al.
[10] N .P. Barradas, C . Jeynes, R. P. W ebb, A ppl . Ph ys. Lett. 71, 291 (1997). [ 11] Joint committee on Pow der Di˜ractio A STM, Philadel ph ia 1992.
n Standards,
Pow der Di˜ractio
n File,
[12] E. Mazzega, M. Michelini , F. N ava, J. Ph y s. F 17, 1135 (1987). [13] H an dbook of Mod ern Ion Bea m Ma teri als A n alysi s, Eds. J .R. T esmer, M. N astasi, MRS, Pittsburgh 1995. [14] Chr. Dudda, S. Mantl, C h. Dieker, M. Dolle, H . L uth, N ucl . I nstru m. Me thods Ph ys. R es. B 80 -81, 764 (1993). [15] R. Pretorius,
Th i n Sol i d Fi l ms 2 90- 291, 477 (1996).
