Preparation of boron carbide thin film by pulsed KrF excimer laser deposition process

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1 Thin Solid Film 07 (00) Preparation of boron carbide thin film by puled KrF excimer laer depoition proce a, b b b c Shin-ichi Aoqui *, Hiatomo Miyata, Tamiko Ohhima, Tomoaki Ikegami, Kenji Ebihara a Department of Electrical Engineering, Royal Intitute of Technology, KTH, Electrum 9, S-16 0 Kita, Sweden b Graduate School of Science and Technology, Kumamoto Univerity, Kurokami -39-1, Kumamoto, , Japan c Department of Electrical and Computer Engineering, Faculty of Engineering, Kumamoto Univerity, Kurokami -39-1, Kumamoto , Japan Abtract Puled laer depoition (PLD) technique ha been widely ued in thin film preparation becaue of it wonderful and excellent propertie. Boron carbide (BC) thin film are recognized to have potential for application like hard coating and electron field emiion device. BC i the third hardet material after diamond and cbn, with a highet hardne of HV ;5000. Furthermore, BC i intereting from the point of view of wear reitance and tability at high temperature. We have depoited BC thin film by KrF excimer laer (l8 nm) ablation of a toichiometric BC target in high vacuum. In thi paper, we have prepared the BC thin film on Si(100) ubtrate under variou condition, uch a ubtrate negative DC bia voltage from between 0 to y 800 V, ubtrate temperature from room temperature up to 550 8C, and laer fluence between and 8 Jycm on the target. The typical aborption of B C tretching bond wa detected from the infrared aborption meaurement by FTIR of the depoited BC thin film. We concluded that the hardne of the BC thin film increae a the boronycarbon (ByC) ratio increae. The ByC ratio depend on the laer fluence. We obtained the maximum ByC ratio of 3. at 5 Jycm. The hardne of that ample reached 5.8 time of that of the Si(100) ubtrate. 00 Elevier Science B.V. All right reerved. Keyword: Boron carbide; Fourier tranform infrared pectrocopy (FTIR); Hardne; Laer ablation; Surface morphology; X-Ray diffraction 1. Introduction Puled laer depoition (PLD) technique ha been ued to produce thin film of a variety of material uch a diamond-like carbon (DLC), boron carbide (B C), cubic boron nitride (cbn), carbon nitride (C N) and 3 boron carbide nitride (B C N) w1 7x. Thee covalently bonded olid coniting of boron, carbon and nitrogen are hard material, and of the material that have been prepared uccefully, BC i the third hardet material after diamond and cbn. BC alo ha other excellent propertie uch a high temperature melting point and high reitance to chemical reagent w8x. BC melt at 3 0 8C. Since it denity i.5 gycm, it i one of the extreme lightweight ceramic. The thermal expanion coefficient amount to.3=10 K and the y6 y1 y1 y1 thermal conductivity i 0. Wm K ) w9x. Moreover, BC exhibit many attractive propertie for thin film application. Becaue of it high temperature tability, *Correponding author. Tel.: q ; fax q addre: aoqui@pe.ojo-u.ac.jp (S.-i. Aoqui). BC i conidered to be a good candidate material for high temperature thermoelectric energy converter, emiconductor that can operate at high temperature and high intenity electron emitting device. In addition, it low pecific weight and high impact reitance can be ued a the firt wall in fuion reactor a well a control rod in fiion reactor. BC thin film have been prepared by chemical vapor depoition (CVD), magnetron puttering, the plama pray method and intene puled ion beam evaporation (IBE) w10 13x. It ha been reported that film depoited by CVD and puttering method were amorphou, and crytallization of BC film required ubtrate heating w1x. On the other hand, the thin film depoited by IBE wa crytallized BC at room temperature. A to the depoition of BC thin film, few PLD method have been reported w15 17x. PLD i rapidly proving to be an effective method for preparation of variou thin film. We have invetigated and prepared BC thin film uing KrF excimer laer (l8 nm) depoition under high vacuum condition. In thi paper, we invetigated the influence of laer /0/$ - ee front matter 00 Elevier Science B.V. All right reerved. PII: S Ž

S.-i. Aoqui et al. / Thin Solid Film 07 (00) 16 131 17 Fig. 1. Schematic diagram of the PLD ytem. Fig.. EF-SEM image of a BC film (a depoited, 5 Jycm, T R.T., Vy600 V).

e BC film were depoited in the temperature range from room temperature (R.T.) to 550 8C, DC bia voltage ranging from 0 to y800 V and laer fluence ranging from to 5 Jycm.

2 S.-i. Aoqui et al. / Thin Solid Film 07 (00) Fig. 1. Schematic diagram of the PLD ytem. Fig.. EF-SEM image of a BC film (a depoited, 5 Jycm, T R.T., Vy600 V). fluence, ubtrate temperature and ubtrate negative DC bia voltage on crytallization and characteritic of the BC thin film. The BC film were depoited in the temperature range from room temperature (R.T.) to 550 8C, DC bia voltage ranging from 0 to y800 V and laer fluence ranging from to 5 Jycm. The depoited film were characterized by an electron probe urface roughne analyzer and a field-emiion canning econdary electron microcope (FE-SEM), a Fourier tranform infrared (FTIR) pectrophotometer, an electron probe microanalyzer (EPMA) equipped with an energy diperive pectrometer (EDS) and an X-ray diffractometer (XRD). The hardne of the thin film wa meaured uing a nanoindentation teter.. Experiment Fig. 1 how a chematic diagram of the PLD ytem ued in thi tudy. After the chamber (f300 mm) wa exhauted by a turbo-molecular pump to a bae preure y of 1.3=10 Pa; in the chamber a intered BC target (Koujyundo kagaku, purity 99.5%) of 30 mm in diameter and 5 mm thickne wa mounted on the rocking holder. A KrF puled excimer laer beam (Lambda Phyik, Model LPX305icc, l8 nm) wa introduced into the chamber through lene and a quartz window. The laer beam impinge on the target at an incident angle of 58. In order to prepare BC thin film, n-type Si(100) wafer, which were treated in 10% HF and ethanol and deionized water, were poitioned 0 mm apart from the BC target. The ubtrate were heated with an IR lamp, which wa mounted on the exterior of the vacuum chamber, from room temperature to 550 8C. Subtrate temperature (T ) wa meaured by uing a thermocouple. An external negative DC bia voltage (V ) wa applied to the ubtrate holder. The urface morphology of the thin film wa oberved with an electron probe urface roughne analyzer (ELIONIX, Model ERA-8800) and a FE-SEM (JEOL, JSM-6301F). The ByC ratio of the BC thin film wa meaured by an EPMA equipped with energy diperive pectrum (EDS) (ERA-8800 equipped with EDAX CDU Leap detector). IR aborption wa invetigated uing a FTIR pectrophotometer (BIO-RAD, Excalibur FTS 3000) and the nanoindentation hardne of the BC thin film wa meaured uing a nanoinden- tation teter (ELIONIX, Model ENT-1100a). The crytal tructure of the film wa characterized by glancing angle X-ray diffraction (XRD) (Rigaku, RINT 100y PC) uing Cu-Ka radiation where the angle of incidence wa kept at Table 1 how the experimental parameter for preparation of the BC thin film on the Si(100) ubtrate. Table 1 Experimental condition Ablation laer KrF excimer laer l8 nm Repetition rate10 Hz No. of laer hot9000 Laer fluence, 5 Jycm Target Sintered boron carbide (purity 99.5%) Ambient condition y 1.3=10 Pa Subtrate n-type Si(100) Subtrate temperature Room temperature (R.T.), 350 8C, 550 8C Subtrate DC bia voltage 0;y800 V

18 S.-i. Aoqui et al. / Thin Solid Film 07 (00) 16 131 Fig. 3. Surface roughne analyzer image of a BC film. (After rined in acetone, 5 Jycm, T550 8C, Vy800 V). (a) Regular SEM image.

Many micrometer cale particle were oberved on the film urface. The particle depoited on the film depended on depoition condition, uch a laer fluence and target ubtrate ditance (d TyS).

3 18 S.-i. Aoqui et al. / Thin Solid Film 07 (00) Fig. 3. Surface roughne analyzer image of a BC film. (After rined in acetone, 5 Jycm, T550 8C, Vy800 V). (a) Regular SEM image. (b) Concavo-convex image. 3. Reult and dicuion Fig. how a FE-SEM image of the a-depoited BC thin film prepared at 5 Jycm, dtys0 mm, T R.T., Vy600 V. Many micrometer cale particle were oberved on the film urface. The particle depoited on the film depended on depoition condition, uch a laer fluence and target ubtrate ditance (d TyS). The number denity of the particle increaed with increae of the laer fluence. Moreover, with dtys0 mm or larger, there wa no formation of BC thin film, the depoition reulting only in particle adhering on the ubtrate. Converely, in the cae of Jycm of laer fluence and dtys0 mm of ditance, few particle were oberved. The urface of the thin film were rined off by acetone before further analye. A regular SEM image and the concavo-convex image obtained from the urface roughne analyzer are hown in Fig. 3a,b. The urface roughne analyzer i equipped with four econdary electron detector, and urface unevenne can be oberved. Unevenne i detectable with the addition and ubtraction of ignal from the four detector. The film prepared at 5 Jycm, T550 8C, Vy800 V conit of many circular grain with a ize of approximately 0.5;5 mm. The concavo-convex image how that all circular grain are convex. After thi, three-dimenional analyi (3-DA) of the film wa carried out. The reult of the 3-DA i hown in Fig.. The field of view i 60=5 mm. The urface wa mooth with an average roughne of 0.19 nm though the highet point of a grain wa.6 mm above the flat urface. Fig. 5 how the FTIR aborption pectra of the film depoited at variou laer fluence, V and T. Fig. 5a,b how a difference of T and V with a laer fluence of 5Jycm. The typical aborption of the B C tretching y1 bond (1085 cm ) wa detected. Though in the cae of T350 8C, the aborption of the larget B C tretching wa the ame a the one hown in Fig. 5a (Vy00 V, T550 8C). The aborption of the B C tretching at T 0 V and y800 V wa larger than for V y00 V a in Fig. 5b. In the cae of a laer fluence of Jycm, the effective aborption wa not detected a in Fig. 5c. We concluded that a more detailed examination of the relation between T and V i neceary. Fig. 6 how the ByC atomic ratio of the BC film. In the EDS meaurement, a intered BC bulk target wa ued a a tandard ample with a ZAF correction becaue an epitaxial BC crytal wa not available. An accelerating voltage of 5 kv wa ued to preclude any effect of the Si (100) ubtrate by the EDS meaurement. No pectral peak due to ilicon wa detected in the meaured EDS except a few ample film which howed very mall peak but, with EDS, the meaured value for light atomic element uch a boron, carbon and nitrogen have relative error that commonly tend to become large. Therefore, we alo ued another EDS, (OXFORD EDS Model 685) equipped with an FE- SEM (JEOL model JSM-6301F), to confirm the EDS data. The ByC ratio increaed from approximately.8 to 3. with an increae in the laer fluence from Jy cm to 5 Jycm. However, the ByC ratio of all the film were maller than for the BC bulk target. The By C ratio depend only a little on V and T. The oxygen atom concentration in the film that were prepared with 5Jycm ranged from 1.1 to 3.0 at.%, but in the cae of Jycm, it ranged from.0 to 5.1 at.%. The hardne of the BC film wa meaured by a nanoindentation teter with a load of 00 mgf. A Berkovich indenter wa ued in thi ytem. A Berkovich indenter i a three-ided triangular-baed, pyramidal diamond, with a nominal angle of between the ide face, and the normal to the bae at apex, an angle of between the edge and the normal, and with a radiu of the tip of le than 0.01 mm. Under the mall indentation load (approx. le than 00 mgf), the elaticity of material become prominent and the elatic hardne become much greater than when meaured with a large load. Firt, we prepared a BC film ample (approx. 3 mm) depoited with TR.T., Vy800 V Fig.. Three-dimenional roughne analyi of the BC film of Fig. 3.

4 S.-i. Aoqui et al. / Thin Solid Film 07 (00) Fig. 5. The FTIR pectra of BC thin film depoited under different condition. (a) T350 8C and5jycm a a function of ubtrate DC bia voltage (V). (b) T550 8Cand 5 Jycm a a function of ubtrate DC bia voltage (V ). (c) Jycm a a function of ubtrate DC bia voltage (V) and ubtrate temperature (T ). and laer pule, and we invetigated the indentation load dependence. Fig. 7 how the indentation load dependence of the hardne of the BC film and ilicon ubtrate. The hardne hown here i the averaged value from ix point in the film. We ued 100 m of load tep interval time and 1 maximum load retention time. The hardne of Si(100) i at an almot contant value of 157 kgfymm at a load ranging from 10 to 1000 mgf, while it ha trong load dependence at load le than 100 mgf. On the other hand, the hardne of the thick BC film changed at around 000 mgf and abruptly increaed at a mall indentation load, jut like for Si. We conidered the change of the hardne value a an effect of the ilicon ubtrate. Therefore, if the BC film i thicker, a hardne value a hown by the dotted line, would be expected. When the load wa 00 mgf, the hardne of the Si(100) and the BC film were 05 kgfymm and 8903 kgfymm, repectively. Fig. 8 how typical nanoindentation loadingyunloading curve of the film meaured at a load le than 00 mgf. The indentation depth wa approximately 80 nm at 00 mgf indentation load, including the elatic and platic deformation, while the thickne of the BC thin film wa approximately 1.5 mm. The indentation depth i mall enough to avoid the effect of hardne of the ilicon ubtrate. The T dependency of hardne for the BC thin film prepared at 5 Jycm and Vy800 V at 00 mgf indentation load i hown in Fig. 9. Hardne of the film increaed from 8903 to kgfymm with increaing T from R.T. to 550 8C. At higher T, greater hardne of the BC film can be expected.

5 130 S.-i. Aoqui et al. / Thin Solid Film 07 (00) Fig. 6. ByC ratio of depoited BC film a a function of the ubtrate DC bia voltage (V ). Fig. 9. Nanoindentation hardne of the depoited BC film a a function of the ubtrate temperature (T ). Fig. 7. Indentation load dependence of the hardne of the BC film and Si(100) bulk. Fig. 8. Nanoindentation loadingyunloading curve for the BC film meaured at a light load. Fig. 10. Relative nanoindentation hardne (BCySi) of the depoited BC film a a function of the ubtrate DC bia voltage (V ). The dependence of film hardne on laer fluence, T and V, i hown in Fig. 10. To clear the BC thin film effect on hardne, relative hardne of Si (BCy Si) meaured at an indentation load of 00 mgf are hown. The film prepared with Jycm howed low hardne in comparion with the film prepared with 5 Jycm. We obtained a maximum relative hardne of 5.8 at 5 Jycm and Vy00 V. The hardne value barely depended on V but on T. The increae in hardne of BC film wa caued by the increae in laer fluence, and at the ame time the ByC ratio alo increaed with laer fluence. The above reult mean that the film hardne relate to ByC ratio of the film. Therefore, we conider that increaing the ByC ratio to the toichiometric value i important for depoition of hard BC film. The crytallization of the film wa tudied by XRD meaurement. Fig. 11 how the typical X-ray diffraction pattern of the BC thin film prepared by 5 Jycm, V y00 V and T550 8C. Crytalline property wa not

6 S.-i. Aoqui et al. / Thin Solid Film 07 (00) Acknowledgment The author gratefully acknowledge the contribution of Mr T. Uematu and Mr T. Ito (ELIONIX Inc.) for the hardne meaurement by the nanoindentation ytem, and EDS and urface roughne meaurement. Reference Fig. 11. A typical XRD pattern of a BC film (5 Jycm, T550 8C, Vy00 V). good, although two BC peak, (10) and (01), were detected.. Concluion In thi tudy, BC thin film were depoited uing KrF excimer laer depoition at high vacuum condition, and the following reult were obtained: 1. BC film depoited at 5 Jycm obtained a mooth urface and an average roughne of 0.19 nm though the highet point of the grain wa.6 mm above the flat urface.. The ByC ratio increaed from approximately.8 to 3. with increaing of the laer fluence from to 5 Jycm. However, the ByC ratio of all the film were maller than for the BC bulk target. 3. The film hardne depend on the ubtrate temperature and laer fluence, but hardly depended on the ubtrate negative DC bia voltage.. The maximum hardne of the BC thin film wa approximately ix time that of the Si(100) urface. w1x F. Kokai, Jpn. J. Appl. Phy. 36 (1997) 350. wx K. Kobayahi, N. Mutukura, Y. Machi, J. Appl. Phy. 59 (3) (1986) 910. w3x S. Aoqui, T. Ikegami, K. Ebihara, Tran. IEEE of Japan, Vol. 119-A, No.6, 796 (1999). wx S. Aoqui, T. Ikegami, K. Ebihara, Tran. IEEE of Japan, Vol. 119-A, No.7, 1059 (1999). w5x S. Aoqui, T. Ikegami, K. Ebihara, Proceeding of 1th International Sympoium on Plama Chemitry, Vol. 3, pp (1999). w6x Y. Suda, T. Nakazono, K. Ebihara, K. Baba, S. Aoqui, Carbon 36 (5-6) (1998) w7x S. Aoqui, K. Ebihara, Y. Yamagata, T. Ikegami, Compoite Part B: Engineering 30 (1999) w8x T. Hu, L. Stehl, W. Rafaniello, et al., Thin Solid Film 33 (1998) 80. w9x J.F. Lynch, C.G. Rueder, W.H. Duckworth, Eng. Prop. of Selected Ceramic Material, American Ceramic Society, Columbu, OH (1996). w10x J.C. Oliveira, M.N. Oliveira, O. Conde, Surf. Coating Technol. 80 (1996) 100. w11x O. Knotek, E. Lugcheider, C.W. Siry, Surf. Coating Technol. 91 (1997) 167. w1x Y. Zeng, J. Feng, C. Ding, J. Mater. Sci. Technol. 16 (000) 63. w13x K. Kitajima, T. Suzuki, W. Jiang, K. Yatui, Jpn. J. Appl. Phy. 0 (001) w1x K. Ploog, E. Amberger, J. Le-Common Met. 3 (1971) 33. w15x M.S. Donley, J.S. Zabinki, W.J. Seeler, V.J. Dyhoue, S.D. Walck, N.T. McDevitt, Proceeding of Mat. Re. Symp., pp. 61 (199) w16x W. Kautek, S. Pentzien, A. Conradi, J. Kruger, K. Brzezinka, Appl.Surf. Sci. Vol. 106 (1996) 158. w17x F. Kokai, M. Taniwaki, K. Takahahi, et al., Diamond Relat. Mater. 10 (001) 11.

The research of simplified method of calculating wind and rain loads and its validation

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