Development of High-quality Large-size Synthetic Diamond Crystals

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1 NEW MATERIALS Development of High-qulity Lrge-size Syntheti Dimond Crystls Hitoshi SUMIYA, Nohiro TODA nd Shuihi SATOH High-qulity, lrge-size type dimond rystls up to 8 rts (up to 10 mm in dimeter) hve een suessfully synthesized y using the temperture grdient method t high pressure nd high temperture. The onentrtion of hemil impurities in syntheti dimond rystl, suh s N, B nd Ni, ws redued to the levels lower thn 0.1 ppm y using high-purity ron soure, employing Fe-Co solvent, nd dding Ti to the solvent s nitrogen getter. Inorportion of Cu into the solvent permitted sustntil derese of inlusions in the dimond rystls. In ddition, rystlline qulity ws further improved y using high qulity seed rystl. Furthermore, the optimiztion of metl ompositions nd dditive mounts in the solvent nd the high preision ontrol of growth onditions enled the growth of high-qulity, lrge-size type dimond rystls of 7-8 rts (10 mm in dimeter) t high growth rte of 6-7 mg/hr. The syntheti dimonds show no sorptions due to impurities nd hve very high rystlline qulity. These slient hrteristis of the lrgesize high-qulity syntheti type II dimond permit the pplition to onsiderly wide rnge of industril nd sientifi uses suh s monohromtors for synhrotron X-ry ems, high-pressure nvils, optil prts, semiondutor sustrtes, rdition detetors, nd so on. 1. Introdution Dimond is typil ovlent rystl in whih ron toms re onded together very strongly. This struturl feture leds to the highest hrdness mong ll mterils nd superior therml ondutivity, s well s exellent hemil stility nd trnspreny. Owing to these outstnding properties, dimond hs een employed in wide rnge of uses. Lrge dimond rystls of severl millimeters in size re used for high preision utting tools, wire-drwing dies, het sinks, optil windows, surgil ldes, nd so on. In mny fields of sientifi reserh suh s spe, high pressure nd rdition, lrge dimond rystls re the essentil omponents of reserh equipment, nd will eome more essentil to wider vriety of ommeril nd sientifi purposes in future. It is very diffiult, however, to find lrge nd highqulity rystls mong nturl dimonds. Most nturl dimonds ontin mny nitrogen impurities (type) nd lmost ll nturl dimonds hve numer of defets nd muh strin in the rystls euse of their omplited nd vrying growth proesses tht ourred in the erth's interior. This is true even in high-purity nturl dimonds with impurity less thn 1 ppm (type). On the other hnd, syntheti dimonds, whih re grown under the ontrolled onditions of high pressure nd high temperture in thermodynmilly stle region, hve invrile qulity. In ddition to this, other spets suh s solvent nd growth rte n e ontrolled to high preision, possily leding to the onsistent synthesis of high-qulity dimonds. Among the vrious methods of dimond synthesis, the most effetive one to grow lrge dimond single rystls is the temperture grdient method under stti high pressure nd high temperture (1), (2). In 1985, Sumitomo Eletri developed mss-prodution proess of dimond rystls sed on the temperture-grdient method. This enled the ommeriliztion of type dimonds (ontining dispersed nitrogen impurities of severl tens or hundreds of ppm) of 1-2 rts (or 5-6 mm) in dimeter. Sumitomo Eletri pplied the syntheti dimonds (SUMICRYSTAL) in het sinks, wire drwing dies, utting tools for high-preision mhining, nd other produts (3). After ten yers, Sumitomo Eletri sueeded in onsistently synthesizing highpurity type dimonds whih hd formerly een impossily diffiult (4), (5). The syntheti type dimonds (SUMICRYSTAL TYPE ) were pplied to the optil prts for FT-IR spetrosopy. The syntheti type dimonds re trnsprent over wide rnge of wvelengths from ultrviolet to fr infrred regions, showing no sorptions due to impurities. They lso hve high rystlline qulity with fewer rystl defets, less internl strin, nd less vritions in defets mong rystls thn those of nturl dimonds or onventionl syntheti type dimonds (6). The outstnding hrteristis of syntheti type dimond permit pplition to new industril nd sientifi uses suh s high-pressure nvils, monohromtors, semiondutor sustrtes nd rdition detetors (7). Reently, the uthors mde it possile to grow muh lrge high-qulity type dimonds of 7-8 t (out 10 mm ross) with prolonged high preision temperture ontrol nd dequte seletion of solvent metl nd dditives (8). In this pper, the uthors provide summry of the reserh onerning high-qulity lrge syntheti dimonds. 10 Development of High-qulity Lrge-size Syntheti Dimond Crystls

2 2. Temperture grdient method under high pressure As it is well known, there re vrious methods for dimond synthesis suh s temperture grdient (differene) method or soluility differene method under stti high pressure, dynmi high pressure (shok wve) method, nd hemil vpor deposition method. Among these methods, using the temperture grdient method under stti high pressure is thought to e most effetive to grow lrge, good qulity dimond rystls of severl millimeters. In this method, ron soure is pled t the hotter prt ove the solvent (mde from ferrous metls, suh s Fe, Ni nd Co) nd the seed rystl is positioned t the ooler prt under the solvent in the high-pressure retion ell. The driving fore for rystlliztion rises from the differene in the soluility of dimond in the solvent tht is used y the temperture grdient in the retion ell. In 1971, Generl Eletri Reserh Center reported tht they sueeded in growing dimond rystls up to 1 rt using this method (1), (2). The proess, however, ws onsidered too ostly to e trnslted into ommeril prodution t tht time, euse the growth rte must e restrited to very slow in order to grow good qulity dimond. When ttempts were mde to grow rystls t growth rte fster thn ertin limit (hereinfter referred to s the limit growth rte ), mny solvent metls would e inluded in the grown rystls or polyrystlliztion or skeletoniztion would our. At tht time, it ws reported tht the limited growth rte of type dimond ontining nitrogen impurities of ppm is out 2.5 mg/hr, nd tht of type dimond is less thn mg/hr. In 1985, Sumitomo Eletri enled the ommeriliztion of type dimond rystls of out 1 t, y inresing the limit growth rte to up to 4 mg/hr through the optimiztion of solvent metl nd growth onditions s well s the development of mss-prodution proess using the temperture grdient method. Furthermore, in 1990, lrger-size type dimond rystls weighing 9 t (12 mm ross) were grown t growth rte s high s 15 mg/hr using the lrge seed method (9), (10). Thus, lrge yellow type dimond rystls ould e grown t high growth rte nd produed ommerilly. It hd een impossile, however, to produe high-purity type dimond rystls ommerilly euse of the diffiulty in inresing the limit growth rte. The entrpment of inlusions in rystl tends to e filitted when nitrogen getter is dded to the solvent metl for synthesizing type dimond. To void the ourrene of inlusions, growth rtes hd to e kept very slow suh s 1 mg/hr. 3. Elimintion of impurities nd inrese of limit growth rte In 1994, Sumitomo Eletri sueeded in growing high-qulity type dimond rystls tht ontin impurities less thn 0.1 ppm even t high growth rte of 3-4 mg/hr (4), (5). This suess enled the ommeril prodution of type dimond rystls. The following is n outline of the dimond growth tehnique. Figure 1 shows the growth method of the developed high-qulity dimond rystl. It is well known tht N, B nd Ni re esily inorported into the syntheti dimond s hemil impurities. This method ws designed to eliminte the impurities. A high-purity ron mteril ontining less thn 1 ppm of oron is used s the ron soure. High-purity Fe-Co lloy is used s the solvent metl, nd Ti is dded into the solvent s the nitrogen getter. Here, when Ti is dded, lrge mount of TiC is formed in the solvent. The formtion of TiC inhiits the trnsport nd diffusion of ron, thus reduing the mount of ron supplied to the rystl growth surfe nd delying the lterl growth on rystl surfe. This leds to the trpping of metl inlusions. TiC prtiles themselves re lso trpped into the grown rystl s fine inlusions. It is known tht TiC deomposes in the molten group metls (Cu, Ag, Au, et.) (11). Bsed on this ft, esides dding Ti s nitrogen getter, ddition of Cu to the solvent ws lso onsidered to suppress the formtion of ride (TiC). Tle 1 shows the experimentl results of dimond growth with vrying mounts of Ti nd Cu. Nitrogen impurities re found to e redued to less thn 0.1 ppm Cride die Pressure medium Grphite heter Cride piston High-purity ron soure Nitrogen getter Solvent metl (Fe-Co +Ti, Cu) Seed rystl High-qulity dimond (100), up to 0.5 mm TiC formtion inhiitor Grphite heter (High temperture region) Temperture differene (20-50 C) (Low temperture region) Grown rystl Insultor Fig. 1. Growth method of high-qulity type dimond rystls (Temperture grdient method under high pressure) Tle 1. Qulity of type dimonds with different mounts of Ti nd Cu dded to the solvent metl. Added mount(wt%) Ti Cu Qulity of grown dimonds Nitrogen (ppm) + Inlusions (*) Metl + + Fine *Degree of inlusions :, non; +, few;, mny. SEI TECHNICAL REVIEW NUMBER 60 JUNE

3 y dding Ti of more thn wt%. The experimentl results lso indite tht, y the ddition of Cu, the formtion of TiC prtiles is suppressed. This is ompnied y the dereses in metl nd fine inlusions, so good-qulity type dimond rystls n e synthesized. However, lrge exess of Cu (3 wt%) uses metl inlusions to our. The optimum dded mount of Cu ppers to e 1-2 wt%. Figure 2 shows the onditions of dimond formtion for different Co ontents of Fe-Co solvent. Good qulity dimond rystls re otined in Co wt %. When the Co ontent is either too low or too high, highqulity dimond rystls nnot e otined euse of the formtion of inlusions or skeleton rystls. Even with suitle Fe-Co omposition, the vlid growth temperture region is very nrrow. The width of synthesis temperture region for high-qulity dimond rystl is less thn 10 C. Figures 3 nd 4 show the pressure-temperture regions for growing high-qulity type nd type Temperture (ûc) Co ontent in Fe-Co solvent (wt%) Pressure: 5.5 GP Growth rte: 2 mg/hr Fig. 2. Conditions of type dimond formtion with different Co ontents of Fe-Co solvent: ( ) high qulity dimond, () dimond with mny inlusions, () skeleton rystl, ( ) no dimond formtion. dimond rystls, respetively. Figures 3 shows the se where lrge (100) rystls re used s seeds (lrge seed method). High-qulity type dimond rystls n e otined in region A. When the lrge seed method is used, the region eomes nrrower (region B) euse the rystl must e grown in low temperture growth ({100} dominnt) region to prevent the ourrene of inlusions (9), (10). As for type dimond rystls (Fig. 4), the region is onsiderly nrrower (region C). The width of the llowle temperture for A, B nd C t 5.5 GP is out 40 C, 20 C nd 10 C, respetively. When the temperture ondition is higher thn these vlid regions, mny inlusions will e ontined in the grown rystls. At lower temperture, well-formed dimond rystls nnot e otined euse of the formtion of skeleton rystls. This mens tht high preision temperture ontrol tehnique is required for growing high-qulity type dimond rystls. The reson is tht the lower limittion of growth temperture eomes C higher y dding the nitrogen getter suh s Ti to the solvent. Figure 5 shows the photos of () high-qulity type dimond rystls otined t region C, () skeleton rystls grown t low temperture region, nd () rystls ontining metl inlusions grown t high temperture region. On the other hnd, well-formed type dimond rystl nnot e otined t {100} ui morphology (low temperture) region, euse of the formtion of skeleton rystls t lower tempertures. This indites tht unlike the se of type dimond, the lrge seed method is not pplile for growing type dimond rystl on the (100) seed. Thus, the llowle temperture region for growing good-qulity type dimond is onsiderly nrrow. Therefore, high preision temperture ontrol tehnique is required for growing high-qulity type dimond. Bsed on these experimentl results, the uthors tried to grow severl type dimond rystls of 1-2 rts using Fe-40Co with dding wt% Ti nd wt% Cu t C for 70 hours. The growth rtes were Pressure (GP) GP B (Skeleton) A {100} {100} + {111} Temperture ( C) {111} (Inlusion) Dimond/grphite equilirium line Solvent/ron euteti line Fig. 3. Growth region of high-qulity type dimond rystl: (A) smll seed (0.5 mm), (B) lrge seed (5 mm). Pressure (GP) GP (Skeleton) {100} C Temperture ( C) {100} + {111} {111} (Inlusion) Dimond/grphite equilirium line Solvent/ron euteti line Fig. 4. Growth region of high-qulity type dimond rystl (C). 12 Development of High-qulity Lrge-size Syntheti Dimond Crystls

4 () Good-qulity dimond rystls over wide rnge from the ultrviolet region to the fr infrred region. In ddition, the dimond hs high rystlline qulity. Tle 2 shows the evlution results of rystlline perfetion of dimonds (6). The results show tht the syntheti type dimond hs exellent rystlline qulity with onsiderly less residul strin nd fewer rystl defets thn nturl dimonds or onventionl syntheti dimonds. It ws lso demonstrted tht the syntheti type dimond hs exellent mehnil properties euse the dimond hs few rystl defets nd little internl strins whih my e the origins of destrution or plsti deformtion (12). Tle 2. Crystlline qulity of vrious dimonds. Syntheti Syntheti Nturl Nturl () Skeleton dimond rystls (grown t low temperture region) Nturl undne (%) Nitrogen ontent (ppm) Internl strin oserved muh none or little y polrizing mirosopy Defets oserved y x-ry topogrphy FWHM of 004 roking urve (CuKα1, rse) FWHM of Rmn spetr (m -1 ) x 1-2 up to 98 up to 100 <1 (dispersed) some (line, plne) muh (rdil, stripe) mny (line, stripe) (ttmi) very mny (ttmi) up to 1000 (ggregted) muh (rdil, stripe) mny (line, stripe) () Dimond rystls ontining inlusions (grown t high temperture region) Fig. 5. Syntheti type dimond rystls. vried y hnging the temperture differene. The results of the growth test showed tht good-qulity type dimond rystls of 1-2 rts ould e otined even t growth rtes s high s 3 mg/hr s shown in Fig. 6. The tehnil development mentioned ove hs enled the ommeril prodution of syntheti type dimond rystl of out 1 rt. This syntheti type dimond ontins very few impurities, eing trnsprent Metl inlusion ontent (wt%) 5.5 GP, C, up to 70hr (Fe-40Co) 4 1wt%Al 3.5 1wt%Ti 2wt%Ti 3 wt%ti+wt%cu Growth rte (mg/hr) Fig. 6. Metl inlusion ontent in syntheti dimond rystls plotted ginst growth rte. The dotted line indites the limit growth rte for good-qulity dimond. 4. Improvement of rystlline qulity As mentioned ove, the syntheti dimond hs higher rystlline qulity thn nturl dimonds or onventionl syntheti dimond. However, the X-ry topogrphy experiments revel tht the syntheti dimond hs mny line defets tht pper to rdite outwrd from the seed rystl s shown in Fig. 7(). Suh defets re normlly present in onventionl syntheti dimonds. The origins of the line defets oserved in syntheti dimond rystls re the rystl imperfetions present in the seed rystls. The syntheti dimond rystls desried ove were grown on the dimond grits synthesized y the phsedifferene method. It is known tht dimond grits hve mny rystl defets, nd the mounts of inlusions nd strins in syntheti dimond grits re lso muh lrger thn those in the syntheti dimond rystls grown y the temperture-grdient method. From the investigtion, it ws found out tht the FWHM of dimond grits (25-85 rseonds under MoKα1 rdition) is muh wider thn tht of the syntheti dimond rystls grown y the temperture-grdient method (2-6 rseonds). Then, the uthors ttempted to synthesize dimond rystls with muh fewer defets y using the dimond seeds of high rystlline qulity. High-qulity seeds ( mm) were ut from the lrge syntheti rystls grown y the method mentioned ove, nd some new syntheti dimonds of out 1 rt were grown on the seeds s shown in Fig. 7. An X- SEI TECHNICAL REVIEW NUMBER 60 JUNE

5 ry topogrphi imge of n s-grown syntheti dimond rystl grown on the high-qulity seed is shown in Fig. 7(). It is pprent tht there re very few line defets in the rystls. These results indite tht the line defets in grown rystls n e redued nd rystlline qulity n e improved y using high-qulity dimond s the seed rystl. Figures 8 nd 9 show high-resolution roking urves nd topogrphi imges of the syntheti type dimond rystls grown on highqulity seed nd on dimond grit seed, respetively (13). It is lerly seen in Fig. 8 tht the rodening of the roking urve of the syntheti dimond is minly due to line defets. The width of the roking urve of the syntheti type dimond rystl grown on high-qulity seed (FWHM = 1.2 rseonds, Fig. 9) is lose to tht of the lulted one (1.0 rseonds for the intrinsi width). This indites tht the syntheti dimond rystl is very lose to perfet rystl. INTENSITY (.u.) FWHM : 1.2 rse d SR Bem. λ=0.76å ANGLE (rse) Line defets (dislotion undles) Dimond grit seed (synthesized y phse-differene method) 1mm () Syntheti type dimond grown on dimond grit seed Seed Fig. 9. X-ry roking urve nd topogrphs of syntheti type dimond grown on high-qulity seed for (004) refletion under synhrotron rdition. d High-qulity dimond seed (ut-out from lrge rystl grown y TGM) INTENSITY (.u.) SR Bem. λ=0.76å FWHM : 4.4 rse d 2000 e Seed ANGLE (rse) Fig. 8. X-ry roking urve nd topogrphs of syntheti type dimond grown on dimond grit seed for (004) refletion under synhrotron rdition. 1mm () Syntheti type dimond grown on high-qulity dimond seed Fig. 7. X-ry topogrphs of s-grown syntheti type dimond rystls. (220 refletions) d 5. Growth of lrge-size (1 m lss) high-qulity dimond Further growth tests of muh lrger-size high-qulity type dimond rystls (up to 8 t, or up to 10 mm ross) were rried out using Fe-40Co+Ti+Cu solvent nd high-qulity seed rystl nd onduting preision temperture ontrol t C over prolonged time (up to 200 hours). The volume of the solvent were mde lrger nd the temperture grdient ws djusted y hnging the heter shpe to grow the lrger-size rystls. Figure 10 shows the weight of the rystls grown t vrious growth rtes nd growth times. In the erly stge (growth time less thn hours), the limit growth rte (mximum stle growth rte) for high qulity dimond without inlusions is 3-4 mg/hr s mentioned ove. When the growth time is over 100 hours, highqulity dimond rystls n e otined even t high growth rte of 6-7 mg/hr. Osionlly, slight hnge or vrition in growth temperture leds to the formtion of inlusions. It is pprent tht the limit growth rte inreses s the rystl grows. This tendeny is lso oserved when growing type dimond rystl. While the deposition re (surfe re) of grown rystl enlrges s the growth time inreses, the deposition rte per unit re (mm/hr) whih influenes the qulity of rystl is unhnged. This seems to e the reson why the limit growth rte per rystl (mg/hr) inreses s 14 Development of High-qulity Lrge-size Syntheti Dimond Crystls

6 Crystl weight (mg) mg/hr mg/hr Growth time (hr) Fig. 10. Growth rte nd degree of inlusions of lrge-size syntheti type dimond rystls. The dotted line shows the limit growth rte for voiding metl inlusions. the rystl grows. Thus, lrge high-qulity type dimond rystls of 7-8 t (out 10 mm ross) n e grown t high growth rte of 6-7 mg/hr through the high preision temperture ontrol over prolonged time with n dequte seletion of solvent metl nd dditives. The growth rte is onsiderly higher thn those reported previously (-1.8 mg/hr (2), (14) ). The high-qulity lrgesize type dimond rystls of 7-8 t hve the sme properties s those of 1-2 t mentioned in the previous setion, whih mens the lrge-size type dimond rystls ontin very few impurities nd hve exellent rystlline qulity. Figure 11 shows the lrge dimond pltes ( mm 3 ) prepred from the 8 t type dimond rystls otined in this study. The dimond pltes re lredy used for the monohromtors of synhrotron rdition ems (15). 6. Conlusions The uthors hve sueeded in synthesizing highqulity, lrge-size type dimond rystls up to 8 rts (up to 10 mm ross) t high growth rte of 6-7 mg/hr y the temperture grdient method under high pressure nd high temperture. The syntheti dimonds show no sorptions due to impurities nd hve very high rystlline qulity. These slient hrteristis of lrge-size high-qulity syntheti type dimond permit the pplition to onsiderly wide rnge of industril nd sientifi uses suh s monohromtors for synhrotron X-ry em, high-pressure nvils, optil prts, semiondutor sustrtes, rdition detetors, nd so on. Referenes (1) R. H. Wentorf, Jr., J. Phys. Chem., 75, 1833 (1971). (2) H. M. Strong nd R. M. Chrenko, J. Phys. Chem., 75, 1838 (1971). (3) A. Hr, Seimitsukiki, 51, 1497 (1985) [in Jpnese]. (4) H. Sumiy, S. Stoh, Y. Nishiyshi nd Y. God, Sumitomo Eletri Teh. Rev., 39, 69 (1995). (5) H. Sumiy, S. Stoh, Dimond nd Relted Mterils, 5, 1359 (1996). (6) H. Sumiy, N. Tod, Y. Nishiyshi, S. Stoh, J. Crystl growth, 178, 485 (1997). (7) H. Sumiy, S. Stoh, S. Yzu, Rev. High Pressure Si. Tehnol., 7, 960 (1998). (8) H. Sumiy, N. Tod, S. Stoh, J. Crystl Growth, , 1281 (2002). (9) H. Sumiy, S. Stoh, K. Tsuji nd S. Yzu, 31st High Pressure Conf. Jpn, progrmme nd strts, pp (1990) [in Jpnese]. (10) S. Stoh nd H. Sumiy, The Review of High Pressure Siene nd Tehnology, 2, 315 (1993) [in Jpnese]. (11) M. Wktsuki, Jpn. J. Appl. Phys., 5, 337 (1966). (12) H. Sumiy, N. Tod, S. Stoh, Dimond nd Relted Mterils, 6, 1841 (1997). (13) H. Ymok, K. Otomo, D. Hirt nd T. Ishikw, SR Si. nd Tehnol. Inform., 5, 6 (1995). (14) R. C. Burns, J. O. Hnsen, R. A. Spits, M. Sind, C. M. Welourn, D. L. Welh, Dimond nd Relted Mterils 8, 1433 (1999). (15) M. Ymmoto, T. Kumsk, T. Ishikw, New Dimond, 58, 16 (2000) [in Jpnese]. Fig. 11. Lrge-size syntheti dimond pltes prepred from 7-8 t type dimond rystls. SEI TECHNICAL REVIEW NUMBER 60 JUNE

7 Contriutors H. SUMIYA Dr. Eng., Assistnt Generl Mnger, Advned Mterils R&D Deprtment, Eletronis & Mterils R&D Lortories N. TODA Mnger, Advned Mterils R & D Deprtment, Eletronis & Mterils R&D Lortories S. SATOH Dr. Eng., Senior Assistnt Generl Mnger, Fine Cermis Reserh Assoition 16 Development of High-qulity Lrge-size Syntheti Dimond Crystls