Thin Solid Films 409 (2002) 138–146 Review Atomic layer deposition (ALD): from precursors to thin film structures Markku Leskela*,¨ Mikko Ritala Department of Chemistry, P.O. Box 55, FIN-00014, University of Helsinki, Finland Abstract The principles of the atomic layer deposition (ALD) method are presented emphasizing the importance of precursor and surface chemistry. With a proper adjustment of the experimental conditions, i.e. temperatures and pulsing times, the growth proceeds via saturative steps. Selected recent ALD processes developed for films used in microelectronics are described as examples. These include deposition of oxide films for dielectrics, and nitride and metal films for metallizations. The use of a plasma source to form radicals is expanding the selection of ALD films to metals. Plasma-enhanced ALD also facilitates the deposition of nitride films at low temperatures. ᮊ 2002 Elsevier Science B.V. All rights reserved. Keywords: Atomic layer deposition (ALD); Atomic layer epitaxy (ALE); Thin films 1. Introduction II–VI compounds gained much interest w7–9x, but no real breakthrough was achieved in this area due to the The ALD method relies on alternate pulsing of the complicated surface chemistry. Since the mid 1990s, precursor gases and vapors onto the substrate surface rapidly increasing interest towards ALD has originated and subsequent chemisorption or surface reaction of the from the silicon based microelectronics. This increase is precursors w1x. The reactor is purged with an inert gas a consequence of the ever decreasing device dimensions between the precursor pulses. With a proper adjustment and increasing aspect ratios in integrated circuits (IC). of the experimental conditions the process proceeds via In addition, the required thickness of the films has, in saturative steps. Under such conditions the growth is many cases, decreased to the order of a few nanometers stable and the thickness increase is constant in each and therefore the main drawback of ALD, the low deposition cycle. The self-limiting growth mechanism deposition rate, is becoming less important. The proc- facilitates the growth of conformal thin films with essing temperatures should also be kept low (preferably accurate thickness on large areas w2x. The growth of -400 8C). These requirements further increase the w x different multilayer structures is also straightforward 3 . attraction of the ALD method in the IC industry w10x. These advantages make the ALD method attractive for ALD has recently been described in several reviews microelectronics for manufacturing of future generation focusing on different areas; catalysts w11x, nanotechnol- w x integrated circuits 4 . ogy w12x, and electronic and optoelectronic materials ALD was originally developed for fabrication of w13,14x. In this review the principles of the ALD method polycrystalline luminescent ZnS:Mn and amorphous are presented emphasizing the importance of precursor Al23 O insulator films for electroluminescent flat panel and surface chemistry. Based on recent literature, select- displays, and the method itself was termed atomic layer ed ALD processes developed for films used in microe- ( ) w x epitaxy ALE 5 . The early ALE literature dealt with lectronics are described as examples and they include polycrystalline II–VI compounds and amorphous oxide w x deposition of oxide films for dielectrics, and nitride films 6 . Since 1985, epitaxial growth of III–V and diffusion barriers and metals for metallizations. Plasma- enhanced ALD has recently been introduced to materials *Corresponding author. Tel.: q358-9191-50195; fax: q358-9191- 50198. applied in silicon based microelectronics and the first E-mail address: [email protected] (M. Leskela¨). papers are reviewed here. 0040-6090/02/$ - see front matter ᮊ 2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090Ž02.00117-7 M. Leskela,¨ M. Ritala / Thin Solid Films 409 (2002) 138–146 139 2. Precursor chemistry and contained only small amounts of carbon. Recently, cyclopentadienyl compounds were introduced as ALD Precursor chemistry plays a key role in ALD. The precursors and they have been used in deposition of precursors must of course be volatile and thermally both alkaline earth titanate and sulfide films w23,24x. stable but they may be gases, liquids or solids. Precur- The commercial availability of new metalorganic pre- sors must chemisorb on the surface or react rapidly with cursors is often limited. The scale-up of precursor the surface groups and react aggressively with each syntheses and the development of new precursors are other. In that way, it is possible to reach the saturation challenges for the further progress of ALD in industrial stage in a short time (less than 1 s) and thereby ensure scale. a reasonable deposition rate. The aggressive reaction During the last 2 years, a few new precursors have requirement is contrary to traditional CVD precursors. been introduced to ALD and reaction mechanistic stud- The desired ALD reactions should have a large negative ies have been carried out for several older precursors. DG value, but unfortunately thermodynamic data are Although halides have been known for a long time as available only for a limited number of precursors. It good ALD precursors, not much attention has earlier seems that in ALD, not only the properties of a single been given to fluorides, bromides and iodides. Recently, precursor molecule are important, but the combination fluorides and iodides have gained more attention, but of the precursors also matters. also new chloride precursors have been taken into use. The number of precursors and reactions used in ALD BCl3 is a new precursor, which has been introduced is high as can be seen from the recent reviews made by very recently. BCl3 reacts with ammonia and forms BN the authors w15,16x. The two cases: non-metal precur- film in ALD fashion w25x. Shimogaki et al. w26x reported sors; and metal precursors are very different and show about the first chloride adduct precursor in their flow different features. The non-metal precursors are usually modulation CVD process. They used tantalum chloride ( ) hydrides H22 O, H S, NH 3 and AsH 3without problems thioether adduct and ammonia in deposition of TaN in volatility and thermal stability. Reactivity at reasona- films. The important benefit of this adduct is that, unlike (- 8 ) ble temperatures 500 C and formation of suitable TaCl5 itself, it is a liquid. Tungsten hexafluoride has surface species for the metal precursor to be anchored, been examined as a precursor for tungsten metal and are properties of H22 O and H S as shown in many ALD tungsten nitride films. With smart chemistry WF6 can w x papers, but NH33 and AsH are examples of precursors be transformed to W with disilane 27 . The particular with more limited reactivity. In the deposition of oxide features of the WF626 –Si H process are: it involves an films, the importance of surface –OH groups has been exchange reaction during both reaction steps; the growth reported several times w17,18x. rate is high, 2.5 A˚ ycycle; and the process window is Metal halides, especially chlorides, have been widely wide, 150–330 8C. The films were, however, amorphous applied in ALD for deposition of oxide, sulfide and and rather resistive. WF6 is also a suitable precursor in nitride films w16x. Most of the halides are solids, but in combination with ammonia for nitride films w28,29x. ALD the solid precursors are not a problem because it The stoichiometry of the films may vary but the for- is just crucial that the precursor dose is large enough to mation of polycrystalline W2 N has been reported. Vol- saturate the surface, but the flux must not necessarily atile, in situ prepared tungsten oxofluorides are w x be constant or homogeneous. The other important group precursors for tungsten oxide, WO3 30 . Polycrystalline ( ) 8 of compounds is metal alkyls. In many cases Al, Zn , films can be grown even at 200 C. WF6 could not be the alkyl compounds behave almost ideally in making used as a precursor for oxide films because of etching oxide and sulfide films, but in some cases (Ga, In) the effects. results have not been very promising due to the complex The use of iodides as metal precursor has been studied and unfavorable surface chemistry. Metal alkoxides form in deposition of titanium nitride w31x, and oxides of the third important group of precursors and they have titanium w32,33x, zirconium w34,35x and tantalum w36x. been successfully employed in several oxide processes The oxide formation reaction is enhanced if H22 O is w16x. Electropositive metals (alkaline earth metals, rare used instead of water. The reaction is self-limiting at earth metals) form a challenge for chemical thin film lower temperatures, but at higher temperatures metal depositions because they do not have many volatile iodides slightly decompose. Reaction mechanism studies w x b compounds 19 . -Diketonates are the most studied show that in the case of TiO2 , the situation is compli- compounds of these metals and good results have been cated because of the structural change (anatase-rutile) obtained in growth of alkaline earth sulfide films w20x, of the oxide, which changes the growth mechanism w33x. but the growth of oxide films using water as oxygen The TiI422 –H O process has even facilitated epitaxial source does not work. b-Diketonates can be used as growth at low temperature (-400 8C) on single crystal precursors in deposition of, for example, rare earth oxide sapphire and MgO substrates w37x. The oxide films but then ozone has been employed as an oxygen source grown from the iodides are iodine-free and the properties w21,22x. The resulting films have been polycrystalline equal to the films deposited with chloride-water ALD 140 M. Leskela,¨ M. Ritala / Thin Solid Films 409 (2002) 138–146 processes. In the TaI522 –H O process, etching takes place composition and contained approximately 15 at.% of 8 8 above 350 C, which is 50 C higher than in the TaCl5 – carbon impurity.