1 Diamond and Related Material 1 (003) R&D of diamond film in the Frontier Carbon Technology Project and related topic a, a a a b Koji Kobahi *, Yohiki Nihibayahi, Yohihiro Yokota, Yutaka Ando, Takehi Tachibana, b b b c c Nobuyuki Kawakami, Kazuhi Hayahi, Kenichi Inoue, Kiichi Meguro, Hirohi Imai, d d e f f f Hirohi Furuta, Takahi Hirao, Kenjiro Oura, Yauhito Gotoh, Hironori Nakahara, Hirohi Tuji, f g g h h Junzo Ihikawa, Franz A. Koeck, Robert J. Nemanich, Tadahi Sakai, Naohi Sakuma, Hiroaki Yohida i a FCT ProjectyJFCC, Center for Advanced Reearch Project, 6F, Oaka Univerity, -1 Yamada-oka, Suita, Oaka , Japan b FCT ProjectyJFCC, cyo Kobe Steel, Ltd, Electronic Reearch Laboratory, Takatukadai, Nihi-ku, Kobe , Japan c FCT ProjectyJFCC, cyo Sumitomo Electric Indutrie, Ltd, Koya-kita, Itami , Japan d Department of Electrical Engineering, Graduate School of Engineering, Oaka Univerity, -1 Yamada-oka, Suita, Oaka , Japan e Department of Electronic Engineering, Graduate School of Engineering, Oaka Univerity, -1 Yamada-oka, Suita, Oaka , Japan f Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto Univerity, Yohida-Honmachi, Sakyo-ku, Kyoto , Japan g Department of Phyic, North Carolina State Univerity, Raleigh, NC , USA h FCT ProjectyJFCC, cyo Tohiba Corp., Corporate Reearch and Development Center, 1, Komukai Tohiba-cho, Saiwai-ku, Kawaaki 1-858, Japan i FCT ProjectyJFCC, FCT Laboratory, Reearch Center for Advanced Carbon Material, National Intitute of Advanced Indutrial Science and Technology (AIST) Tukuba Central 5, Higahi, Tukuba, Ibaraki , Japan Abtract R&D activitie on diamond chemical vapor depoition (CVD) and field emiion in the Frontier Carbon Technology Project are preented. The topic are (1) morphology control of diamond film grown by a 60-kW, 915-MHz microwave plama CVD reactor, () growth technology of large ingle crytal diamond with a low denity of defect, (3) heteroepitaxial growth technology of diamond film on Pt, (4) fabrication of harp emitter tip on ingle crytal diamond, (5) field emiion tudy from diamond particle, and (6) intene field emiion from ion implanted homoepitaxial diamond layer. Reearch reult of field emiion obtained by Kyoto Univerity and North Carolina State Univerity are alo decribed. 00 Elevier Science B.V. All right reerved. Keyword: Diamond film; Chemical vapor depoition; Etching; Field emiion 1. Introduction Thi article preent elected diamond film reearch in the Frontier Carbon Technology (FCT) Project: (1) diamond film growth by a 60-kW, 915-MHz microwave plama chemical vapor depoition (MPCVD) reactor w1 3x, () growth of large ingle crytal by CVD w4x, (3) heteroepitaxial growth on Pt w5x, (4) fabrication of harp emitter tip w6,7x, and (5) fabrication of a field emiion device with high current w8x. Our activitie mainly aim not at cientific finding but at development *Correponding author. Tel.: q ; fax: q addre: (K. Kobahi). of real product and production technologie for the future. Thu, we tudy on (1) finding CVD condition for oriented growth, () optimization of homoepitaxial growth without noticeable defect, (3) finding optimum condition for heteroepitaxy on Pt(1 1 1), (4) optimization of diamond microfabrication procee including reactive ion etching (RIE), and (5) optimization of fabrication procee for a high current field emiion device. Our emphai on technological apect of diamond film, more than on cientific apect, in thi project i baed upon our recognition that a productiontype large CVD reactor are being developed, application area of diamond film will be rapidly expanding, and new buinee uing diamond film a key com /03/$ - ee front matter 00 Elevier Science B.V. All right reerved. doi: /s (0)0098-4
2 34 K. Kobahi et al. / Diamond and Related Material 1 (003) Fig kW; 915-MHz MPCVD reactor. ponent are expected to be created in the near future. Regarding cientific apect, however, we are trongly upported by the member of Oaka Univerity and Prof. Ihikawa group at Kyoto Univerity on field emiion. Alo, Prof. Nemanich group at North Carolina State Univerity undertake field emiion meaurement of our pecimen uing photoemiion electron microcopy (PEEM) and field emiion electron microcopy (FEEM). It hould be noted that in the FCT Project, diamond film reearch i alo done by Material Reearch Laboratory (former NIRIM, n-type doping), Dr Okuhi group (depoition of high quality homoepitaxial layer and tudy of electronic propertie), Prof. Kawarada group (microfabrication), and Mitubihi Material (large area depoition by DC plama). Our group i cloely aociated with thee group. In thi paper, however, we have to leave them out due to pace limitation, but the reader can conult their pat and coming reference to know the detail. i normally operated under condition of (P 60 kw, m P100 Torr, T C). It might be aumed that given c and T, a diamond film with a imilar morphology i grown in both cae. It wa, however, found that thi i not really the cae. Fig. how the reult of a parameter and direction of uniaxial growth obtained by Koidl group w9x uing a quartz-tube-type reactor. Thi diagram ha the following feature: (i) the diamond film i N110M-textured for the mall c and high T region; (ii) a the a-value change from 1.5 to 3.0, the uniaxial growth orientation change from N110M to N100M; and (iii) in the region below a3.0, the diamond film become microcrytalline. In thi work, the direction of uniaxial growth were the ame a the direction of fatet growth derived from. Diamond film growth on large area The purpoe of thi reearch i to control diamond film morphologie, uing a 60-kW, 915-MHz MPCVD reactor (ASTeXySeki Technotron), a hown in Fig. 1, by changing the CH concentration c and the ubtrate 4 temperature T that depend on the ga preure P and the microwave power P. Our major interet i to know m if there are any difference in a parameter and direction of uniaxial growth between the conventional,.45- GHz, quartz-tube-type mall reactor that i uually operated under condition of (P W, P30 m Torr, T C) and the preent large reactor that Fig.. c T diagram obtained by a.45-ghz quartz-tube-type reactor w9x.
3 K. Kobahi et al. / Diamond and Related Material 1 (003) Fig. 3. c T diagram for the large reactor uing CH w3x. The N1 1 1M and N1 0 0M-oriented domain were determined by X-ray diffraction and the 4 film morphologie were oberved by SEM. The rhombu and quare point indicate the experimental point. the hape of CVD diamond crytal, or equivalently a parameter, and thi ha been conidered to be a baic rule for a and the direction of uniaxial growth. In the preent work, however, we found w3x that the c T diagram for the large reactor wa ignificantly different from Fig.. The c T diagram, when CH4y H wa ued a the ource ga, i hown in Fig. 3, where we evaluated a and determined uniaxial growth direction by X-ray diffraction. One can ee the following difference between Fig. and 3: in Fig. 3, (i) the a1.5 curve i hifted to the higher T ide by a much a approximately 00 8C; (ii) imilarly, the a3 curve i hifted to the higher T ide; (iii) diamond grain with well-defined facet were grown even in the region below the curve of a3, but only microcrytalline diamond film were grown in Fig. ; (iv) the diamond film in the region below the a3 curve have an uniaxial growth direction of N111M, while thoe in the region immediately above the a3 curve have an uniaxial direction of N100M. Thi reult eem to how that the direction of uniaxial growth are not directly related with the direction of fatet growth derived from a parameter, or diamond crytal hape. If thi i the cae, it i in contradiction with the baic rule mentioned above. We have repeated growth experiment many time and reproduced the c T diagram of Fig. 3. We do not know yet why uch phenomena a (iii) and (iv) occur in the preent cae, nor do we undertand what govern the phenomena. We can only tell at preent that either the high ga preure or the longer microwave length influence the film morphology. It hould be remarkable that in the N1 1 1M-oriented film (the bottom photo of Fig. 3), the majority of the diamond face i (111) which tend to be directed upward, and there are many pair of (1 1 1) face that are mutually twinned. To invetigate the poibility of growing diamond film with a-1.5, we undertook CVD experiment uing CH4qCOyH a a ource ga, and the reult i hown in Fig. 4. Since both CH4 and CO can be carbon ource for diamond growth and oxygen generated by decompoition of CO can etch carbon, including diamond, we defined t in uch a way a t100 (c(ch 4 )y c(co ))y(c(ch 4)yc(CO )qc(h )), where c(x) indicate the concentration of X in the ource ga. The number aociated with each region in Fig. 4 i the a parameter in the region. By comparing Fig. 4 with Fig. 3, one can ee the following: (i) the curve for a1.5 and 3 are ignificantly hifted toward the low temperature ide; (ii) diamond film growth with a-1.5 i now poible, and indeed, cubic diamond particle with a;1 were grown, a een in the canning electron micrograph (SEM) at the top of Fig. 4; and (iii) no diamond wa formed for t(0 becaue of etching due to oxygen. Thu, we now conclude that the control of diamond film morphology i poible uing CH4yH orch4qcoy H a the ource ga in the full range of 1(a(3 by the large reactor. Thi achievement i of ignificance becaue one can make both highly oriented diamond (HOD) film formed on Si ubtrate and pontaneouly coaleced diamond (SCD) film formed on Pt(111) thicker in a relatively hort time to enhance a coalecence between diamond grain at the film urface, which will lead to ingle crytal diamond film. 3. Large-area ingle crytal diamond ynthei by the moaic method It i generally recognized that practical application of diamond, particularly electronic application, have been hampered by the fact that ingle crytal with large
4 36 K. Kobahi et al. / Diamond and Related Material 1 (003) Fig. 4. c T diagram for the large reactor uing CH qco. The number aociated with each region i the a parameter. See text for the definition 4 of t. urface area are not commercially available. One of the method to olve thi problem i to homoepitaxially grow diamond by CVD on a diamond moaic or tile bae. Such tudie have been done before w10,11x uing up to 7 piece of diamond. In the preent tudy, we ued 16 piece of (100)-cut Ib diamond of 3=3 and 4=4 mm in ize, making the total area 1.44 and.56 cm, repectively. The mot difficult iue of thi ubject i to depoit diamond by minimizing defect, particularly econdary nucleation due to twin formation. To thi end, a 915-MHz, 60-kW CVD reactor wa newly deigned and contructed with an aid of computer imulation on ga flow and electromagnetic field a well a plama emiion meaurement to obtain uniform plama and temperature ditribution acro the diamond moaic. In the latet experiment, uing P 40 kw, P100 Torr, m and 5 vol.% CH yh, a growth rate of 10 mmyh wa 4 achieved. Fig. 5a how a CVD diamond layer with approximately 8 mm in diameter, which wa depoited on a type-ib diamond (1 0 0) urface of approximately 7 mm in diameter. One can notice that there i no abnormal growth. Fig. 5b how a moaic pecimen, where a CVD diamond layer of approximately 1 mm thickne wa depoited. Although (1 1 1) face are expoed at the boundarie between adjacent diamond plate, the gap between the adjacent diamond plate were totally filled up by CVD diamond. It i expected that a flat urface can be obtained by further optimization of the growth condition and the baal moaic arrangement. 4. Heteroepitaxial growth of diamond film on Pt(111) The purpoe of thi ubject i to depoit a grainboundary-le diamond film on an area of 10 mm in diameter. Specifically, we are developing a heteroepitaxial growth technology of SCD film on Pt(111) whoe diameter and thickne are 11 and mm, repectively. Thi i baed on the finding by Shintani w1x that a coaleced diamond film can be depoited on Pt(1 1 1), and we try to achieve a full coalecence over the entire urface of the ubtrate. Unlike HOD film on Si or SiC, the bia-enhanced nucleation i not neceary in thi proce. The tandard polihing method i ued to increae the nucleation denity. Important factor for thi Fig. 5. (a) 8 mm ingle crytal of CVD diamond; and (b) 1=1 mm ize moaic diamond of 1 mm thickne.
5 K. Kobahi et al. / Diamond and Related Material 1 (003) Fig. 6. (a) SCD film grown on Pt(111) w5x; (b) bunched tep on the peripheral region; (c) SEM micrograph of B-doped film urface; and (d) a magnified view of (c). technology are: (i) the crytal quality of Pt mut be high, (ii) the orientation of nuclei mut be bet aligned by optimizing the initial growth condition, and (iii) the lateral growth condition in the final tage mut be optimized. For experimental detail, ee Ref. w5x. Fig. 6a how a urface morphology of SCD film grown under tandard condition. Fig. 6b how the urface morphology in the peripheral area of the film depoited for 100 h. It i een that the urface i covered with bunched tep, and no grain boundarie are een. It wa only 5 30% of the ubtrate urface that wa covered by the morphology, and otherwie the film morphology wa like the one hown in Fig. 6a. It ha been known that (111) face of diamond tend to appear if the diamond film i doped with boron (B). Thu, we depoited B-doped diamond on Pt(111) by adding BH 6 in the ource ga. A a reult of 100-h growth, a well-coaleced, N111M-oriented diamond film wa grown, demontrating that the coalecence wa accelerated between adjacent diamond (111) face, a een in Fig. 6c and d. Experiment are ongoing to further optimize variou experimental condition to achieve flatter film urface. 5. Microfabrication of diamond urface w6,7x Fabrication technologie of a harp tip and it array at (1 0 0) urface of ingle crytal diamond i being developed uing a combination of RIE, aniotropic growth of diamond by MPCVD, and microwave etching uing CO qh. Such tructure are expected to be ued, for intance, for field emiion tip, enor and device uing vacuum microelectronic, actuator, and optical component. For uch application, a ue of ingle crytal diamond i advantageou over polycrytalline diamond film, becaue array of harp tip with a regular hape can be fabricated uniformly, and the technique of aniotropic growth can be utilized. In the RIE technique, a MHz radio frequency (RF) plama generation ytem wa ued. Uually, O i ued for diamond etching, but we found that an addition of 1 vol.% CF to O give marked effect 4 on etching. The total ga preure ued wa 5 Pa, and the input RF power wa 00 W. For microfabrication, circular Al mak were photolithographically fabricated on (100) diamond urface prior to the RIE. Conequently, an array of cylindrical column were made, a hown in Fig. 7a. The length of the diamond column wa 9.5 mm, and it diameter wa 1. mm, i.e. the apect ratio i 8. The etching rate wa remarkably high, 9.5 mmyh. In certain cae, very thin columnar tructure with a diameter of 0. mm and a length of 7.5 mm were formed among the cylindrical array, preumably becaue reidual Al particle became micromak. It i remarkable that the baal etched urface wa very mooth: indeed, it roughne wa maller than the initial diamond urface. We infer that the formation of both the cylindrical tructure with high apect ratio and the mooth baal urface arie from a depoition of fluorinated carbon film that protect irregular etching. Alo, we etablihed a method to harpen the top of each diamond cylinder of Fig. 7a uing Al mak with nonuniform thickne in which the central region i thicker than the peripheral area. A the plama etching proceed, the Al mak alo are gradually etched from the peripheral to the central area, and hence a harp needle hape i formed. A a reult, a needle with a - nm top radiu wa fabricated. Two other method of fabricating protuberance on diamond urface have been developed. The firt method i to tart with a cylindrical column and undertake diamond CVD under proper condition. Thi reult i hown in Fig. 7b. The econd method alo tart with cylindrical column of diamond hown in Fig. 7a, but followed by microwave etching uing CO qh. A a conequence, we could fabricate a variety of hape, from column to protruded crytal a well a their combination at (100), (110) and (111) urface of ingle crytal diamond. Thi technique will be ueful for fabricating diamond electron emitter, for intance, and currently emiion meaurement are ongoing.
6 38 K. Kobahi et al. / Diamond and Related Material 1 (003) We are tudying temperature dependence of field emiion from diamond with H- and O-terminated urface to ee if there are any difference between them. In the preent tudy, B-doped diamond layer wa depoited on two type-ib diamond with (100) urface of 3 mm. For the diamond CVD, 0.5% CH4yHq6.5 ppm BH 6wa ued a the ource ga, and the film thickne wa etimated to be approximately 1 mm. After CVD, both pecimen were cleaned uing chromic acid and aqua regia, and one pecimen wa treated by hydrogen plama at 60 Torr for 5 min. The filed emiion meaurement were carried out uing a ytem decribed elewhere w13x. The anode i a gold ball with a diameter of mm, movable by a piezoelectric device, and the ditance between the anode and the pecimen wa 1.5 mm. The reult of meaurement are hown in Fig. 8a and b for H- and O-terminated pecimen, repectively. It i een that the temperature dependence are ignificantly different between the two. For the O-terminated pecimen (Fig. 8b), the field emiion current firt decreaed when the temperature T wa increaed from 0 to 70 8C, then increaed when T wa further raied to 170 8C. By contrat, for the H-terminated pecimen (Fig. 8a), the field emiion current increaed monotonically a T wa raied from 0 to 170 8C. Thi demontrate that the field emiion current i trongly dependent on how diamond urface i terminated. It hould, however, be noted that in a eparate erie of experiment uing B-doped polycrytalline diamond film, in which the BH 6 concentration were 1 and 10 ppm, the field emiion current of H-terminated diamond film decreaed with increaed T. We thu conclude that the field emiion current i influenced by both orientation of the crytal facet and the B-doping concentration in addition to the urface termination. 6.. High field emiion current from ion implanted homoepitaxial diamond It appear to be widely accepted o far that the field emiion from diamond i mot efficient if the diamond Fig. 7. (a) Array of cylindrical column fabricated at ingle crytal diamond (1 0 0) urface; and (b) array of crytal fabricated by growing diamond on cylindrical column. 6. Field emiion from diamond 6.1. Temperature dependence Fig. 8. Temperature dependence of field emiion. (a) Fowler Nordheim (F N) plot for H-terminated ingle crytal diamond (1 0 0); and (b) F N plot for O-terminated pecimen.
7 K. Kobahi et al. / Diamond and Related Material 1 (003) denity of 10 ycm in diamond at the maximum. Fig. 9b how a SEM of the pecimen urface (after the field emiion meaurement) implanted with P. It i een that dark pot, which are actually vertical line, were formed by ion implantation. For the meaurement of field emiion current, a rod of W Re alloy with a diameter of 300 mm wa placed 8 mm above the center of the pecimen. The applied voltage wa 4 kv at the maximum. It wa found that the emiion current i correlated with the conductivity of the implanted diamond layer, and a maximum current of 1.5 Aycm wa oberved for a P-implanted pecimen. It i inferred that electron are upplied from the ohmic contact at the corner, travel along the implanted layer, and come up to the urface through the vertical channel formed by ion implantation. Since diamond i o hard and ha uch a high thermal conductivity, the pecimen wa able to reit the heat generated by the channel conduction of electron, and utain uch a high current denity. Fig. 9. (a) Device tructure uing ingle crytal diamond; and (b) P- implanted urface oberved by SEM after electron field emiion. film i microcrytalline, containing a high denity of grain boundarie and graphitic component. In the preent tudy, we depoited a high quality homoepitaxial diamond layer on a type-ib diamond (100) urface, ion implanted P, Bor S, and meaured the field emiion from the pecimen thu made. A a reult, an emiion current denity of approximately 1.5 Aycm wa achieved from the area of approximately 0.07 mm that had been P implanted w8x. Fig. 9a how a chematic tructure of the pecimen. An undoped diamond layer with a thickne of 6 mm wa depoited by MPCVD uing 1 vol.% CH yh under 4 condition of (Pm4 kw, P Torr, and T C). The growth rate wa approximately 3 mmyh. A cathodoluminecence meaurement of thi pecimen exhibited an intene free exciton band at 35 nm at room temperature, indicating that the film quality wa very high. To fabricate ohmic electrode in the four 16 corner of the pecimen, Ar ion wa implanted to 10 y cm to make the urface graphitic, which wa then followed by a depoition of AuyPtyTi electrode by electron beam depoition. A a reult, a contact reitance of 10 V cm wa achieved. Finally, multiple ion y7 implantation of P, Bor S wa done to achieve an atomic Fig. 10. FEEM and PEEM image of polycrytalline diamond film with different Bconcentration. The number in each figure tand for ByC, the atomic Bconcentration againt the C concentration in the ource ga, for CVD. The window i 50 mm in diameter.
8 40 K. Kobahi et al. / Diamond and Related Material 1 (003) PEEM and FEEM meaurement w14,15x Both PEEM and FEEM are very ueful tool to identify the location of electron emiion. Such meaurement have been done at North Carolina State Univerity for a variety of pecimen that the FCT Project had made. One example i PEEM and FEEM meaurement of B- doped diamond film grown on p-si(1 0 0) ubtrate. The film were yntheized uing a quartz-tube-type MPCVD reactor for 1 h for atomic ByC ratio in the ource ga below 500 ppm. The oberved reult are hown in Fig. 10. We had anticipated that the field emiion intenity change monotonouly with B-doping concentration. Unlike uch an aumption, the oberved reult howed that for FEEM, the intenity wa maximum when ByC1000 ppm, a een in Fig. 10. Unlike the reult of FEEM, PEEM data howed no ditinction between pecimen with different Bconcentration. One may notice that PEEM image omewhat reproduce the original film morphology, a electron tend to be emitted from the top and edge area of each grain more than lower region uch a grain boundarie. It ha not yet been undertood why the 1000-ppm B-doped pecimen emit electron more than other, but it i inferred that the pecimen ha an optimized film reitivity and urface roughne. 7. Concluion For practical application of diamond film, (i) development of production cale reactor with precie proce control and (ii) large ingle crytal ynthei by homoand heteroepitaxy are mot important at the preent tage along with feaibility tudie of device and other application. It i our expectation that the technologie and international collaboration etablihed during the FCT Project will give ueful contribution for diamond reearch in the coming generation. Acknowledgment Our activitie have been trongly upported by Prof. M. Yohikawa, Dr H. Okuhi, Prof. H. Kawarada, Dr S. Fujiwara, and Dr Y. Koga a well a FCT taff member. We would like to thank all of them for continuou encouragement. Thi work wa upported by Japan Fine Ceramic Center (JFCC) that wa conigned by New Energy and Indutrial Technology Development Organization (NEDO). Reference w1x E. Sevillano, in: B. Dichler, C. Wild (Ed.), Low Preure Synthetic Diamond, Springer, 1998, p. 11. wx T. Tachibana, Y. Ando, A. Watanabe, et al., Diam. Relat. Mater. 10 (001) w3x Y. Ando, Y. Yokota, T. Tachibana, et al., Diam. Relat. Mater. 11 (00) 596. w4x K. Meguro, T. Matuura, T. Imai, Diam. Relat. Mater., ubmitted for publication. w5x T. Tachibana, Y. Yokota, K. Miyata, et al., Phy. Rev. B56 (1997) w6x Y. Nihibayahi, H. Saito, T. Imai, N. Fujimori, Diam. Relat. Mater. 9 (000) 90. w7x Y. Nihibayahi, Y. Ando, H. Saito, T. Imai, T. Hirao, K. Oura, Diam. Relat. Mater. 10 (001) 173. w8x T. Sakai, H. Sakuma, M. Suzuki, T. Ono, H. Yohida, Proceeding of the Fifteenth New Diamond Sympoium, New Diamond Forum, pp. 00 (in Japanee). w9x C. Wild, R. Kohl, N. Herre, W. Muller-Sebert, P. Koidl, Diam. Relat. Mater. 3 (1994) 373. w10x G. Janen, L.J. Giling, Diam. Relat. Mater. 4 (1995) 105. w11x C. Findeling-Dufour, A. Gicquel, Thin Solid Film (1997) 178. w1x Y. Shintani, J. Mater. Re. 11 (1996) 955. w13x Y. Gotoh, T. Kondo, M. Nagao, et al., J. Vac. Sci. Technol. B 18 (000) w14x F.A.M. Kock, J.M. Garguilo, B. Brown, R.J. Nemanich, Mater. Re. Soc. Symp. Proc. 61 (000) R651. w15x F.A.M. Kock, J.M. Garguilo, R.J. Nemanich, Diam. Relat. Mater. 10 (001) 1714.