Influence of oxygen on the morphological and structural properties of Ti thin films grown by ion beam-assisted deposition

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1 Ž. Thin Solid Films Influence of oxygen on the morphologicl nd structurl properties of Ti thin films grown by ion bem-ssisted deposition J.M. Lopez b, F.J. Gordillo-Vzquez, M. Fernndez, J.M. Albell, Instituto de Cienci de Mteriles de Mdrid ( ICMM-CSIC ), Cntoblnco, Mdrid, Spin b Uni ersidd Cyetno Heredi, Deprtmento de Fısic, A. Honorio Delgdo 430, Lim 31, Peru Received 16 June 2000; received in revised form 28 September 2000; ccepted 9 October 2000 Abstrct Ti thin films hve been deposited on silicon wfers by electron bem evportion of Ti powder with nd without simultneous ssistnce of rgon ion bombrdment Ž IBAD technique.. We hve investigted the influence of the oxygen content on both the morphologicl nd mechnicl properties of the films. We found tht the oxygen content in the films is minimum when Ti pre-evportion tretment is performed just before the deposition process. Moreover, the concentrtion of oxygen Ž 5 t.%. found in non-ssisted films grown with Ti pre-evportion is very similr to tht mesured in Ti films deposited with the ssistnce of rgon ions with different bombrdment energies. The influence of the ion-bem ccelertion voltge nd the incident ngle on the film properties ws lso investigted. It ws found tht the film oxygen concentrtion is criticl for determining both the mteril film morphologicl performnce Ž best t lower oxygen contents. nd the film hrdness, which is mximum with high nd low oxygen contents in non-ssisted nd ion-ssisted films, respectively Published by Elsevier Science B.V. All rights reserved. Ž. Keywords: Titnium; Oxygen; Hrdness; Ion bem-ssisted deposition IBAD 1. Introduction There is much interest in thin films of both pure Ti nd Ti compounds, like TiN, TiC nd TiCN, due to their outstnding properties, such s high hrdness, melting point, mechnicl stbility nd their electricl conductivity. These fetures mke them very useful, not only for mechnicl pplictions, but lso for the microelectronics, medicl nd erospce industries. On the other hnd, TiN grown by different techniques Žchemicl vpour deposition Ž CVD., sputtering, pulsed lser deposition Ž PLD. nd ion bem-ssisted deposition Ž IBAD. 1. is one of the most widely investi- Corresponding uthor. Tel.: ; Fx.: E-mil ddress: lbell@icmm.csic.es Ž J.M. Albell.. gted nitrides. The use of IBAD is, however, especilly convenient, due to its flexible controllbility nd reproducibility of the structurl nd chemicl properties of the resulting films 5 8. However, in high vcuum systems like IBAD, not only nitrogen Žin the cse of TiN films., but other rective gses such s minly wter vpour nd oxygen re lso present in the rection chmber. Even when present t low concentrtion, they re incorported into the growing films Žminly of Ti. due, to lrge extent, to the extremely high ffinity of titnium towrds oxygen. The presence of contminnts like oxygen cn dversely ffect some properties of the deposited films. For exmple, oxygen contmintion leds to n increse in the TiN film resistivity, which decreses the efficiency of the metllic junctions in microelectronic devices 9. In ddition to this, severl uthors, like Ensinger nd Mnory et l. 11 hve reported results indicting $ - see front mtter 2001 Published by Elsevier Science B.V. All rights reserved. Ž. PII: S

2 70 J.M. Lopez et l. Thin Solid Films 384 ( 2001) tht irrdition of titnium with ions diminishes the oxygen content in the films. The results of Mnory et l. 11 even suggest significnt improvement in tribologicl properties, such s dhesion nd wer performnce, of TiN films with low oxygen content. The oxygen content in TiN mterils deposited by IBAD is thus criticl prmeter for their dequte performnce in mny technologicl pplictions 12,13. Although some works hve been published on the role plyed by oxygen on the properties of the deposited TiN thin films, some controversy still persists. Some uthors, like Koniger et l. 14, estblish direct correltions between the nnohrdness of TiN films nd their oxygen content, thus reporting incresing nnohrdness with growing oxygen concentrtion. The nnohrdness vlues obtined re, however, very low: up to mximum of only 3 GP with lmost 25 t.% oxygen content. Other uthors, like Mnory et l. 11, do not find definitive correltion between ion bombrdment Ž leding to low oxygen content. nd film hrdness. Mnory et l. 11 lso found tht, depending Ž on the implnted ions C or Ar., the nnohrdness might significntly reduce or improve, s function of the ion dose, with respect to the unimplnted Žricher in oxygen content. TiN smples. On the other hnd, Ensinger studied how oxygen is incorported into the TiN films deposited by IBAD nd its reltionship with the ion bem energy; however, Ensinger does not nlyse the influence of oxygen on the morphologicl nd tribologicl properties, for exmple, men grin size, roughness, nd hrdness. Therefore, in order to clrify the influence of the oxygen content on the morphologicl nd tribologicl properties of Ti thin films deposited by IBAD, we hve designed two series of model experiments: Ž. i deposition of Ti without rgon ion-bem ssistnce; nd Ž ii. deposition of Ti with rgon ion-bem ssistnce. These two sets of experiments were performed s function of the incident ngle nd the ion ccelertion voltge. In both cses, Ti pre-evportion ws performed Žsee detils below.. 2. Experimentl procedure 2.1. Preprtion of the Ti thin films The deposition of Ti films ws crried out by electron bem evportion Ž APT& T EVM-5. of titnium with nd without simultneous rgon ion bombrdment Ž see Fig. 1.. The ion source ws of the Kufmn type Ž IBS-250. with bem dimeter of 3 in. Depositions without ion ssistnce were crried out with 2-h evcution time, while the non-ssisted films were deposited t evcution times rnging from 0 to 24 h. In order to elucidte the role plyed by oxygen, both sets of experiments Ž with nd without ion ssistnce. in- Fig. 1. Experimentl set-up. cluded smple for which Ti pre-evportion tretment ws performed. The Ti films were deposited with n ccelertion voltge rnging between 0 Žexperiments without ion bem ssistnce. nd 450 V nd n ion current of 15 ma. During the deposition, the pressure in the chmber ws kept constnt t mbr Žwithout. ion ssistnce nd 3 mbr Ž with ion ssistnce.. The direction of the rgon ion bem ws 54, 68 nd 89 off the norml, nd the ngle of the evported toms Ž using n electron bem. ws to the norml of the substrte surfce. Moreover, the distnce between the ion nd electron bem sources nd the substrte surfce ws 20 nd 15 cm, respectively. In our experiments, Ti films were deposited on Ž 0. -oriented silicon substrtes crefully clened, before the deposition process took plce, in n cetone followed by n ethnol bth. In ddition to this, nd prior to film deposition, ll substrtes were subjected to 5-min sputter-clening tretment with Ar ions. The experimentl conditions used in the present study re listed in Tbles 1 nd 2 for experiments performed with nd without ion bem ssistnce, respectively Structurl nlysis nd mechnicl chrcteristion The Ti thin film composition ws determined through Auger electron spectroscopy Ž AES. mesurements performed in n ultr-high vcuum chmber with bse pressure of 5 mbr nd using JEOL-JAMP- S spectrometer with cylindricl mirror nlyser Ž CMA.. The AES dt were cquired in the differentil mode. The kinetic energy of the incident electron bem ws 5 kev, with totl current of 90 na. Moreover, X-ry diffrction Ž XRD. ws lso used in order to corroborte the nlysis of the film composition previously performed with AES techniques.

3 Tble 1 Experimentl conditions with Ar ion bem ssistnce with n incident ngle of 54 Conditions J.M. Lopez et l. Thin Solid Films 384 ( 2001) 71 Evcution time Ž h Ion bem ccelertion energy Ž kev Ion bem intensity Ž ma e-bem ccelertion energy Ž kev e-bem intensity Ž ma Bckground pressure Ž mbr. Operting pressure Ž mbr. For this ion energy, the mteril properties hve been studied t two dditionl impct ngles: 68 nd 89. The film morphology nd nnostructure were observed by tomic force microscopy Ž AFM. under mbient conditions with homemde microscope, s described by Kolbe et l. 15. Surfce imges were tken in the contct mode using shrpened silicon nitride cntilevers ŽPrk Scientific Instruments nd Olympus.. The root men squre Ž rms. roughness vlues, together with men grin size, were obtined from nlysis of the different AFM imges. Different res of ech smple were exmined to check the uniformity of the films. The hrdness of the Ti films ws mesured using nnoindenttion techniques. For the mesurements presented here, we used Nno-Indenter II ŽNno Instruments Inc.. with Berkovich pyrmid dimond indenter. We hve used the.c. indenttion technique, which ws first introduced by Oliver et l. 16. This technique llows the continuous mesurement of the hrdness of the smple s function of the indenttion depth. The mesurements were crried out t the mximum possible lod in order to determine the effect of the substrte on the mechnicl properties of the film. Finlly, the thickness of the deposited cotings on the silicon substrtes ws mesured using surfce profiling system, Dektk Results nd discussion 3.1. Film composition nd phse nlysis Tbles 3 nd 4 show the elementl composition, obtined by AES, of our Ti films deposited with nd without rgon ion bem ssistnce with different bombrdment energies, respectively. In the cse of non-ssisted films, it is evident tht the oxygen content of such films is clerly dependent, not only on the evcution time, but lso on whether Ti pre-evportion tretment is performed or not before deposition tkes plce. The smple deposited with previous Ti preevportion Ž smple 7. presents the lest oxygen content, due to the very high ffinity of titnium towrds oxygen, thus eliminting much of the oxygen remining in the rection chmber. Also, the mount of oxygen in the Ti films decreses with incresing evcution time, since, obviously, more oxygen is eliminted from the rection chmber Žsee the cses of 24 nd 2 h evcution time in Tble 4.. Anlysis by XRD of the non-ssisted films llows better understnding of the role plyed by oxygen on these films. Results from XRD indicte tht only one phse ŽTi O, growing in the Ž preferentil direction. is detected in films with high oxygen content Ž 30 t.%.. As the mount of oxygen in the films decreses Ž 23 t.%., XRD nlysis confirms the ppernce of second growing phse, specificlly Ti in the Ž 1. preferentil orienttion, while the Ti 2 O 3 phse pek decreses. Therefore, two min phses ŽTi nd Ti O. co-exist in the films for moderte Ž t.%. oxygen content. For lower oxygen content Ž 5 t.%., only one phse Žtht of Ti in the Ž 1. direction. remins. Tble 2 Experimentl conditions without ion bem ssistnce Conditions Evcution time Ž h e-bem ccelertion energy Ž kev e-bem intensity Ž ma Bckground pressure Ž mbr. Operting pressure Ž mbr. Immedite deposition but prior evportion of Ti for two cycles of 2 min ech. The shutter ws kept closed during these two prior Ti evportions, so tht no deposit ws formed.

4 72 J.M. Lopez et l. Thin Solid Films 384 ( 2001) Tble 3 Film composition, thickness, roughness nd men grin size for the experimentl conditions of Tble 1 Mteril property Smple 1 Smple 2 Smple 3 Evcution time Ž h Ion bem energy Ž kev Ti Ž t.% O Ž t.% C Ž t.% Thickness Ž nm Roughness Ž nm Hrdness Ž GP Grin size Ž nm For this ion energy, the film roughness nd grin size hve been studied t two dditionl incident ngles: 68 nd 89. On the other hnd, the oxygen content of the Ti films grown with rgon ion ssistnce is much lower Ž see Tble 3. thn tht found in non-ssisted Ti films grown under the sme conditions Ž 2 h evcution time.; this result is in greement with tht previously reported by Mnory et l. 11 on TiN films grown with ion ssistnce. Moreover, it is evident in Tble 3 tht, while the oxygen concentrtion t nd kev is prcticlly the sme, there is trnsition energy Ž kev. t which rgon ions stop llowing the mximum incorportion of oxygen nd begin to etch oxygen from the growing Ti film, thus lowering its oxygen content t higher ion energies Ž see Fig. 2,b.. A cler vrition in the roughness nd men grin size s function of the ion bombrdment energy cn be observed in Fig. 2, nd in the AFM imges shown in Fig. 3c,b,d, respectively. As reveled by the X-ry diffrction nlysis Žnot shown here., the deposited Ti films ssisted with rgon ions exhibit the presence of different phses s function of the ion impct energy: t kev no oxide phse is detected lthough we found three preferentil growing directions for Ti: Ž 0., Ž 002. nd Ž 1.. The sme preferred orienttions of Ti were found t kev, but in this cse smll pek lso ppers, indicting the presence of the oxide phse TiO2 growing in the preferentil direction Ž At kev, only TiŽ 002. nd the oxide phse Ti O in the Ž growth preferentil direction were found. When no ion ssistnce Ž 0 kev. ws supplied, only Ti O Ž ws found Microstructure nd mechnicl properties The AFM imges of the film surfces tken t incresing deposition times Žsee Tble 2 for non-ssisted films. revel tht the films grow homogeneously with the deposition time. The influence of the oxygen content in the morphology of the film is evluted by mesuring the men grin size nd the root men squre Ž rms. roughness of the smples. Tble 4 shows the men grin size nd surfce roughness vlues obtined from AFM imges Ž see Fig. 4. of the nonssisted Ti films. Our results indicte tht the higher the oxygen content, the higher the film roughness nd the lower the men grin size, lthough the ltter is hrdly visible in high oxygen content films, since the films re hrdly textured. Moreover, s is evident in Fig. 5b, both the roughness nd hrdness of non-ssisted films increse with growing oxygen content. This suggests tht, with high oxygen contents, the dominnt growing phse is Ti 2O 3. Since Ti 2O3 hs ionic covlent bonds 17 Ž much more ionic thn covlent., which re stronger thn the metllic titnium bonds 18, its presence would explin the observed film hrdness increse with incresing oxygen concentrtions in the non-ssisted films. On the other hnd, nlysis of the rgon ion-ssisted Ti films t different ion energies shows tht the film thickness clerly decreses Ž see Fig. 2b. with incresing ion energy while, s is observed in Fig. 2, the surfce roughness decreses up to kev, but then increses for incresing ion energies Ž0.450 kev nd bove. due to the effect of sputtering. This film smoothing effect with incresing ion bombrdment energy cn be better seen in the sequence of AFM imges shown in Fig. 3. Tble 4 Film composition, thickness, roughness nd men grin size for the experimentl conditions of Tble 2 Mteril property Smple 4 Smple 5 Smple 6 Smple 7 Evcution time Ž h Ti Ž t.% O Ž t.% C Ž t.% Thickness Ž nm Roughness Ž nm Hrdness Ž GP Grin size Ž nm Immedite deposition but prior evportion of Ti for two cycles of 2 min ech. The shutter ws kept closed during these two prior Ti evportions, so tht no deposit ws formed.

J.M. Lopez et l. Thin Solid Films 384 ( 2001) 73 Fig. 2. Ž. Surfce roughness Ž. nd grin sizes Ž.; nd Ž. b film thickness Ž. nd oxygen content Ž. s function of the ion energy.

5 J.M. Lopez et l. Thin Solid Films 384 ( 2001) 73 Fig. 2. Ž. Surfce roughness Ž. nd grin sizes Ž.; nd Ž. b film thickness Ž. nd oxygen content Ž. s function of the ion energy. All depositions were mde t mbr nd 3 sccm of Ar flow. It is lso worth mentioning tht the observed trend in Fig. 2 for the men grin size with incresing ion energy follows very closely tht of the film oxygen content. It is evident in Fig. 5, together with Tble 3, tht the hrdness of the ssisted films increses with decresing oxygen content; however, in this cse the film oxygen content is not key prmeter controlling the hrdness of the ssisted films, since other effects Žre- lted with the ion bombrdment. must be considered: the cretion of defects, nd the vrition of both the men grin size nd the preferentil growing directions relted to the ion bombrdment of the films. The opposite trend is found in Fig. 5b, where it is evident tht the non-ssisted film hrdness increses with incresing oxygen concentrtion in the films. The influence of vrying the ion incident ngle Žt constnt ion energy of kev. on the roughness nd men grin size of the film surfce hs lso been investigted. We found tht both the surfce roughness nd men grin size decrese with growing incident Fig. 3. AFM imges of rgon ion-ssisted Ti films deposited t mbr with 3 sccm of Ar, nd different ion energies: Ž. 0; Ž. b 0.225; Ž. c 0.300; Ž. d 0.375; nd Ž. e kev. ngles rnging from 50 to 70 ; however, t higher ngles Ž up to 90., the mgnitude of both strts to Fig. 4. AFM imges of non-ssisted Ti films deposited t mbr Ž. Ž. Ž. with n oxygen content of: 30; b 23; nd c 6 t.%.

74 J.M. Lopez et l. Thin Solid Films 384 ( 2001) increse, though the roughness growth is fster thn tht of the men grin size Ž see Fig. 2.. The grnulr structure of the ion-ssisted films t 0.

6 74 J.M. Lopez et l. Thin Solid Films 384 ( 2001) increse, though the roughness growth is fster thn tht of the men grin size Ž see Fig. 2.. The grnulr structure of the ion-ssisted films t kev is shown in Fig. 6 for different ion impct ngles. It cn be clerly seen tht the smoothest nd most homogeneous film is tht corresponding to n ion impct ngle of 68 Ž Fig. 6b.. It is worth mentioning tht t kev, the lowest roughness nd men grin size re chieved t 54 Ž see Fig. 2.. However, it is highly probble tht even lower roughness nd grin size vlues could be obtined t 68 working t kev Žthough this experiment ws not ctully performed in this work.. Therefore, in terms of morphologicl qulity Žof gret interest for pplictions in microelectronics., the optimum processing ngle would be 68 t kev Žt kev, the mount of oxygen in the films is minimum.. Nnoindenttion studies demonstrted tht, keeping the ion energy constnt t kev, the Ti film hrdness increses slightly by incresing the impct ion ngle from 54 Ž 5.23 GP., to 68 Ž 4.7 GP. nd 89 Ž5.6 GP.. Fig. 6. AFM imges of keV rgon ion-ssisted Ti films deposited t three different ion incident ngles: Ž. 54 ; Ž. b 68 ; nd Ž. c 89. According to the previous results, n ion energy of kev nd n ion impct ngle of 68 should be used where ion-ssisted films with optimum morphologicl fetures Ž relted to minimum oxygen content. re desired. In the cse of non-ssisted films, the best morphologicl chrcteristics re chieved for the 0-h evcution time smple Ž see Tble 4., for which Ti pre-evportion tretment is performed before deposition. If, on the other hnd, optimistion of the film hrdness is desired, the film oxygen content should be incresed with dequte deposition conditions: this is the cse of smple 4 in Tble 4 Ž non-ssisted films. nd smple 3 in Tble 3 Ž ion-ssisted films.. In the ltter cse, the ion energy determines the film hrdness Ž higher bombrding energies led to hrder films., while, for non-ssisted films, the oxygen content is the min prmeter controlling the hrdness of the films Ž higher oxygen levels led to hrder films.. 4. Conclusions Fig. 5. Film hrdness nd surfce roughness s function of: Ž. the ion energy for ion bem-ssisted films; nd Ž b. oxygen in non-ssisted films. Titnium films hve been deposited by evportion of titnium with nd without simultneous bombrdment with rgon ions. It ws found tht, in generl, the ion-ssisted films exhibit lower oxygen content thn those films grown by simply using Ti evportion. However, it ws observed tht Ti pre-evportion tretment before deposition produces remrkble reduction of the film oxygen concentrtion. A cler connection is found between oxygen content nd the non-ssisted Ti film hrdness: high oxygen concentrtion leds to hrder films. It ws found tht, in generl,

7 J.M. Lopez et l. Thin Solid Films 384 ( 2001) 75 the non-ssisted grown films were hrder thn the ion-ssisted ones. On the other hnd, optimum morphologicl fetures, such s film roughness nd men grin size, re connected with low oxygen concentrtion in the films. Moreover, the influence of different ion impct ngles on the morphologicl nd mechnicl properties of the films hs lso been studied. It ws found tht the lowest film roughness nd min grin size were obtined by using n impct ngle of 68 nd n ion bombrdment energy of kev. However, t constnt bombrdment energy of kev, the hrdest films re deposited when working with rgon ions impcting the substrte with ngles rnging between 68 nd 89. Acknowledgements This work ws prtilly funded by CICYT Ž Spin. nd the Europen Union Ž E.U.. under Project Nos. MAT CO3 nd BRPR-CT , respectively. The uthors would like to thnk Dr O. Bohme for helpful comments nd Dr L. Vzquez for AFM observtions. Thnks re lso due to D. Cceres for his ssistnce with nnoindenttion mesurements. One of the uthors, J.M.L., grtefully cknowledges finncil support from the Spnish Agency for Interntionl Co-opertion. One of us, F.J.G.V., cknowledges contrct from the Spnish Ministry of Eduction nd Culture Ž MEC.. References 1 F.A. Smidt, Int. Mter. Rev. 35 Ž.Ž G.K. Wolf, Surf. Cot. Technol. 434 Ž J.P. Hirvonen, Mter. Sci. Rep. 6 Ž W. Ensinger, Surf. Cot. Technol. 80 Ž W. Ensinger, A. Schroer, G.K. Wolf, Surf. Cot. Technol. 51 Ž G.K. Wolf, Nucl. Instrum. Methods B 65 Ž J.S. Colligon, Mter. Sci. Eng. A139 Ž W. Tin, W. Ci, J. Li, R. Wu, Mter. Sci. Eng. A 116 Ž M. Wittmer, J. Vc. Sci. Technol. A 3 Ž W. Ensinger, Surf. Cot. Technol. 99 Ž R.R. Mnory, L.J. Liu, D.K. Sood, Z.M. Sho, C. Kylner, M. Brun, Surf. Cot. Technol. 70 Ž D. Muller, Y.R. Cho, E. Fromm, Thin Solid Films 236 Ž D. Muller, E. Fromm, Thin Solid Films 270 Ž A. Koniger, J.W. Gerlch, H. Wengenmir, C. Hmmerl, J. Hrtmnn, B. Ruschenbch, Surf. Cot. Technol. 84 Ž W.F. Kolbe, D.F. Ogletree, M. Slmeron, Ultrmicroscopy 424 Ž W.C. Oliver, G.M. Phrr, J. Mter. Res. 7 Ž H. Holleck, Surf. Cot. Technol. 434 Ž R. Riedel, Adv. Mter. 4 Ž

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