Surface Science North-Ilokmd 86 (1979) 300.-307 Publishing Cornpan) SPECIAL FEATURES OF THIN COMPOUND FILMS PREPARED BY MAGNETRON SPUTTERING Kiyotaka WASA and Shigeru HAYAKAWA hlatatczrialslicsrard! Manuscript received I.ahoratoric~s Matsushita in final form Novcmbcr Electric Irdustrial Co Isd Radoma, Japrr 1978 Magnetron sputtering is now widely used for depositing thin metal films The m;lpnetron sputlering system, in contrast to the conventional diode sputtering system, is characterized h? high sputtering rate and low worhing pressure The low working pressure causes the impinpemcnt of high speed sputtered atoms on substrates This may induce peculiar propcrtics in the deposited films Some interesting phenomena, e.g nn abnormal crystal growth and low temperature synthesis of a high temperature compound are found when WC deposit thin compound o\idc films Introduction Increasing interest has been slmwn in the use of low pressure sputtering systems of the magnetron type for depositing thin solid films Penning first proposed the use of magnetic fields in sputtering systems [ 11 Gill and Kay proposed a sputtering system with inverted magnetron geometry for depositing high purity thin films [I?] We previously studied magnetron sputtering for depositing elemental or compound thin films [3] and proposed improved systems with cylindrical or planar geometry [41 Recent technological progress in sputtering enables this kind of system to be ot industrial use [S] The system is now an important alternative to electron beam, induction heating or flash evaporation as a method of metallizing semiconductor devices [6] The low working pressure, lo-” to lo-” T err in the magnetron sputtering system affects the nature of the sputtered films and the films frequently show peculiar properties in contrast to the films prepared in a conventional diode sputtering system which operates at gas pressure of 10e2 to 10-l Torr This report describes the fundamental operation of the magnetron sputtering system and the features of the resultant sputtered films K Wasa, S Ilayakawa / Special jkatlrres of compound Operation of magnetron 2.1 Comtnictiorl films 30 I sputtering of the sputtering system Fig shows the construction of the dc magnetron sputtering system used in this experiment The sputtering system is similar to the cold cathode magnetron of a coaxial cylinder type An important aspect of this sputtering system is the application of a strong magnetic field (>lOOO G), enabling the sputtering to operate in the low gas pressure region down to lo-’ Torr [4] 2.2 Sprltteriilg profile In the magnetron sputtering system the working pressure is so low that the scattering of sputtered atoms by gas molecules can be neglected The sputtered atoms arrive at the substrates by a direct line of flight When we place the substrates near the anode and assume that the current distribution is uniform over the cathode surface, the deposition rate R is given by R = (w/pt) (rc/ra), where w is the mass of the material sputtered off unit cathode area, p the density of the deposited films, t the sputtering time, rc and r, the radius of the cathode and anode, respectively For the sputtering in a rare gas such as argon, IV is governed by w = (i+/e)St(A/N), so that R is expressed by R z (i-,/e) SGI/N)~~/PW&J (1) where i, is the ion current density at the cathode, S the sputtering rate, A the atomic weight of sputtered materials, and N Avogadro’s number In dc magnetron sputtering the energy of the impact ion eVi is of the order of the discharge voltage eVs and ranges from 300 to 1500 eV Further, S = kVi, where k is a constant [4] ; 1, 71i,, since the secondary electron emission coefficient is much smaller than one MAGNETIC POLES S N CYLINDRICAL CATt-DDE CVLI NDRlCAL SU BSTR ‘ATE ANODE i vip Construction ofdc m;ignetron sputtering system 6000 calhode diameter 20mm 60mm anode dsameter ;sooo I- ,E o;154000 ii a3000 i= POWER I(J-~ Torr of Ar + Nz (Ar/Nl INPUT v, I$ ( volt~anp.km2 ) = 7/3) for TIN Thus, the deposition rate is proportional tu the discharge power input V, i, In a reactive gas such as oxygen and nitrogeli, thin films of oxides or nitrides are deposited similar to the conventional diode sputtering These oxides or nitrides are chiefly formed at the cathode surface and/or at the substrates [7] The dep~~siti~~~~ rates are usually reduced in the reactive gas Fig shows typical deposition rates measured in the dc magnetron system for various sputtered films Phenomena of some interests 3.1 Abnormal crystal growth P~)lyc~stal~ille ZnO films of a hexagonal structure are prepared on a glass substrate by dc or rf sputtering from Zn or ZnO target in an oxidizing atmosphere c~~ldit~~ns and the crystallograp~lic structure Table shows the typical s~~~ltterill~ of the ZnO films prepared in the de conventional sputtering system and in the sputtering system where the magnetron sputtering system In the conventional working pressure is lo-’ to 10 -’ Torr, it is seen that the (z-axis is preferentially oriented normal to the film surface, i.e the (001 ) plane is parallel to the film sur- K Wasa, S HaJ’akawa / Special features of comportr~d Table Crystallogaphic orientation of polycrystalline Sputtering sy ctem Gl Sputtering Substrate pressure tetnp Ikposition rate I:ilm thickness ( 10e3 Torr) CC) (!Jlll/hJ o.ll11) 35 b 40 100 200 200 ‘00 300 300 300 0.03 0.15 0.03 0.3 0.03 0.15 0.3 0.1 0.3 0.1 0.3 0.3 0.1 0.3 0.3 40 40 70 150 150 200 270 ‘70 0.03 0.12 0.7 0.1 0.7 0.7 0.07 0.6 0.1 0.36 0.35 0.3 0.3 II.3 0.2 0.3 Cr)nventional tic diode dc magnetron a b ’ d 1’ fZlms 303 ZnO films 0.15 Crystallographic d orientation Cl Cl CJ CL CL CL Cl CJ~ Cl Cl1+ Cl Cll Cl1 Cl1 Cl1 Cll Cll l’urc Zn cathode, 7059 $ISS substrates 02 Ar+02.50% Ar+02.30:: Oz Cl c-axis normal to film surface; Cfl, c-skis in film surface face While, when the ZnO films are prepared at a low working pressure of 10m3 Torr or less by magnetron sputtering, the c-axis is predominantly parallel to the film surface, i.e the (1 IO) or (100) plane is parallel to the film surface [8] The c-axis orientation obtained in conventional sputtering is explained by the fact that the surface mobility of adatoms is high during film growth and the sputtered films obey the empirical law of Bravais The change of crystallographic orientation with the sputering system may be related to the difference in the working pressure In the low working pressure the oxidation of the Zn cathode during sputtering will be not completed Moreover, the low working pressure causes the impingement of high energetic sputtered Zn atoms and/or negative oxygen ions on the substrates This may lead in the nucleation and film growth process to an unusual state, so that the familiar empirical Bravais law becomes inapplicable 3.3 12~1 tmperature synthesis of high terliperatrwe cot~pour~ds Previously, Bickly and Champbell deposited mixed films of PbO and TiOz by reactive sputtering from a composite lead&titanium cathode in an oxidizing atmos- 304 RATIO OF Pb/T, l8CCI l600 l400 l200 :: :: + -200 -400 %AREA 10 15 OF Pb l’ip Dielectric properties of Pb-Ti-0 magnetron sputtering (measured at Ml17, 20 25 100 ON CATHODE films of 3UOO ,4 thick RT) on 7059 glac dcp~~sited h!, phere [9] They used a conventional dc diode sputtering system In this case, the mean permittivity of the resultant films with the nominal chemical composition PbTiOa was 33, which is much smaller than that of bulk PbTi03 The as-grown films scarcely showed any indication of formation of PbTi03 In order to synthesize PbTiOs, the substrate temperature should be more than 600°C Contrary to their experiments, we have found that when dc magnetron sputtering is used, the resultant films exhibit several features of the PbTi03 compound, even at a low substrate temperature of 200°C or less during film deposition [lo] The composite lead-titanium cathode was sputtered at X 10m4 Torr of a mixed gas of Ar and O2 (Ar/O, = 1) The deposition rates and substrate temperatures were 30 to 600 A and 150 to 3OO”C, respectively Experiments with pure lead cathodes [ l] and titanium cathodes suggest that in these sputtering conditions PbO and TiOz are co-deposited onto the substrate and thus mixed films of PbO and TiOz will be formed Fig shows the dielectric properties of the mixed films for various chemical compositions It is seen that the permittivity maximum is observed at the chemical composition of PbTi03 The maximum permittivity is higher than that of PbO or TiOz It is also found that the temperature variation of the permittivity shows a maximum at about 490°C as indicated in fig 4, which is expected in the PbTi03 compound These electrical properties suggest that the PbTi03 is synthesized in the mixed films prepared by magnetron sputtering K Wasa,S Hayakawa/ Special features of compound firms 305 100 ,001 100 200 3cO uKJ 500 600 TEMPERATURE (“C) F’ig Temperature deposited variation of permittivity by magnetron sputtering (measured of PbbTi-0 at MHz) films of 3000 Ii thick on 7059 glass The permittivity of the sputtered PbTi03 films is, however, still lower than that of bulk PbTiOj Thus the PbTi03 films will be composed of a mixture of PbO, TiOz and PbTiO, The contents of PbTi03, XpT, are estimated by the relation XPT = 1og(EM/XTi02XPbo) (2) 10dEPT/XTi02XPb0)’ if we assume that Lichteneker’s empirical logarithmic mixing rule will be established between fpbo, ETiOZ and EpT, where EpbO, ETiOZ, epT and EM are the permittivity of PbO, TiOz, PbTi03 and the sputtered mixed fdms, respectively, and XpbO, XTiOZ and XpT are the proportions by volume of PbO, TiOz and PbTi03, respectively, such that Xpbo + XTiOz + XpT = Putting EM 120, EpT ‘” 200, EpbO ‘c 25 and ETiOZ = 60, we have XpT * 0.7 This estimation suggests that 70% of the sputtered films are composed of PbTi03 compound 3.3 Low temperature dophg of foreign atoms into semiconducting films Co-sputtering of foreign atoms seems to be useful for controlling electrical conductivity of semiconducting films during sputtering deposition Table shows typical experiments for polycrystalline ZnO thin films in various sputtering systems In the experiments Al or Cu auxiliary cathodes were co-sputtered with the Zn main cathode in an oxidizing atmosphere [8] It is seen that in the conventional sputtering system the co-sputtering of Al or Cu scarcely affects the conductivity of the resultant films While, in the magnetron sputtering system the co-sputtering of Al or Cu strongly affects the conductivity: Al increases the conductivity by over three orders of magnitude and Cu decreases it Sputtcrin; pressure ( IO-BTorr) Sputtcrinp s~stcrll H C‘ontentc \)I foreign ni~t3ls iat ‘Y) ~ fit-l [’ ~~~n~crltion~l1 dc diode 1u dc rnagnctron I) ! (;\I) Cf.:! (C’li) 1.3 (Al) .- a Pure Zn catIwde 0.5 (Crr) -. -. ~._l Suhstratc temp CC) Depwiticin rate Film thickness ~pm/h) _- -. - lhrk c(~n[i~lcti~it~ (f2-1 Lx-t (pm) _ ._“_“ . 3N~ r10 300 0.25 0.075 0.1 0.5 (I.1 iI _7 I.6 Y 10-6 % 1P 1.9 x 10-6 2ttc1 701) 0.7 I.? II.3 0.b x 10-2 200 0.9 _-. _ O.JS _ _ x 10-R * 1K4 _ 71IS9 glass substr;ttes, b Ar + 02, 30:: 02 c Ar + o-J,swr 02 2 2.4 2.6 103/T Fig Temperature variation of dark c(~nd~ictivity foreign metals prepared by magwtron sputtering 2.0 (Ok-’ 30 3.2 3L ) for ZnO films with and without co-sputtered _ K Wasa, S Ha.vakawa /Special features of compoundfilms 307 in approximately the same ratio This suggests that in magnetron sputtering Al is probably introduced as donor and Cu is introduced as acceptor or deep trap In conventional sputtering the co-sputtered atoms chiefly stay at crystal boundary in the sputtered films and are scarcely incorporated into the crystal lattice Fig shows the typical temperature variation of the dark conductivity of ZnO films prepared by magnetron sputtering The conductivity is controlled from 10-l to IO-’ R-r cm-’ by doping the foreign atoms in the co-sputtering process Optical absorption measurements suggest that the width of the forbidden gap of these films is 3.3 eV and the acceptor or deep trap level due to Cu is 2.5 eV below the conduction band at room temperature The temperature dependence of the carrier concentration suggests that the donor level to to Al is 0.08 eV below the conduction band The doping by foreign atoms in the co-sputtering process is possibly caused by the impingement of high energetic sputtered atoms during film growth The highly conductive Al-doped ZnO films are applicable for making ZnO/Si heterojunction photo diodes [ 121 and switching diodes [ 131 Conclusions The magnetron sputtering system is now believed to be available in industry for the metallization of semiconductor devices due to its high deposition rate However, the system will offer much more attractive features when we deposite films of compounds such as oxides, nitrides and carbides [ 141 Further study will bring succes in the formation of exotic materials References [l] I:.M Penn&, US Patent 2146025, 1939 [2] W.D Gill and E Kay, Rev Sci In&r., 36 (1965) 277 [3] K Wasa and S Hayakawa, IEEE Trans Parts, Mater Packaging PMP-3 (1967) K Wasa and S Hayakawa, Proc IEEE 55 (1967) 2179 [4] K Wasa and S Hayakawa, Japan Patent 642012,1972 K Wasa and S Hayakawa, Rev Sci Instr 40 (1969) 693 K Wasa and S Hayakawa, US Patent 3528902,197O [5] J.S Chapin, Res Develop 25 (1974) 37 [6] R.W Wilson and L.E Terry, J Vacuum Sci Technol 13 (1967) 157 [7] K Wasa and S Hayakawa, Microelectron ReIiab (1967) 213 [8] T Hada, K Wasa and S Hayakawa, Thin Solid Films (1971) 135 [9] W.P BickIey and D.S Champbell, Vide 99 (1962) 214 [lo] K Kusao, K Wasa and S Hayakawa, Japan J Appl Phys (1968) 437 [ 11) K Wasa and S Hayakawa, Japan J Appl Phys (1969) 276 [12] K Wasa and S Hayakawa, Japan J Appl Phys 10 (1971) 1732 [13] T Hada, K Wasa and S Hayakawa, Japan J Appl Phys 10 (1971) 521 [14] K Wasa and S Hayakawa, Thin Solid Films 10 (1972) 367 70