Role of HCl in Atomic Layer Deposition of TiO 2 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 4 1195 http://dx.doi.org/10.5012/bkcs.2014.35.4.1195 Role of HCl in Atomic Layer Deposition of TiO 2 Thin Films from Titanium Tetrachloride and Water Jina Leem, a Inhye Park, a Yinshi Li, Wenhao Zhou, Zhenyu Jin, Seokhee Shin, and Yo-Sep Min * Department of Chemical Engineering, Konkuk University, Seoul 143-701, Korea. * E-mail: ysmin@konkuk.ac.kr Received January 28 2014, Accepted February 5, 2014 Atomic layer deposition (ALD) of TiO 2 thin film from TiCl 4 and H 2 O has been intensively studied since the invention of ALD method to grow thin films via chemical adsorptions of two precursors. However the role of HCl which is a gaseous byproduct in ALD chemistry for TiO 2 growth is still intriguing in terms of the growth mechanism. In order to investigate the role of HCl in TiO 2 ALD, HCl pulse and its purging steps are inserted in a typical sequence of TiCl 4 pulse-purge-H 2 O pulse-purge. When they are inserted after the first-half reaction (chemisorption of TiCl 4 ), the grown thickness of TiO 2 becomes thinner or thicker at lower or higher growth temperatures than 300 o C, respectively. However the insertion after the second-half reaction (chemisorption of H 2 O) results in severely reduced thicknesses in all growth temperatures. By using the result, we explain the growth mechanism and the role of HCl in TiO 2 ALD. Key Words : TiO 2 , Atomic layer deposition, HCl, TiCl 4 , H 2 O Introduction Atomic layer deposition (ALD) is a special modification of chemical vapor deposition (CVD) to grow various thin films via self-limiting chemisorption. 1-3 While in CVD an appropriate precursor vapor and a reaction gas are simult- aneously supplied to a substrate, in ALD they are alternately exposed onto the substrate of which temperature is main- tained to be low enough in order to avoid thermal decom- position of the precursor. The ALD reactor is purged with an inert gas between the exposures of the precursor vapor and the reaction gas. Each cycle of ALD process generally con- sists of precursor exposure - purge - reaction gas exposure - purge. Therefore, the film grows through chemisorption bet- ween the gaseous molecules (i.e., precursor vapor or reactant gas) and reactive functional groups on the surface (i.e., hydr- oxyl groups or chemisorbed organometallic groups). Once vacant adsorption sites are saturated by adsorbate molecules to form one monolayer (practically sub-monolayer is formed due to the bulkiness of adsorbate molecules), the precursor or reactant in excess do not chemically adsorb on the mono- layer. Consequently the film grows via the self-limiting mechanism. For an ideal ALD process, several characteristics are required in ALD chemistry: (1) the chemisorption should be highly exothermic with a high activation of desorption (E d ) in order to guarantee 100% sticking coefficient in the chemi- sorption, (2) byproducts should not chemically re-adsorb on the vacant sites after the chemisorption, and (3) the by- product should not etch the growing film through a series of reverse reactions of the growth. In terms of these require- ments, the chemistry for ALD of Al 2 O 3 from trimethyl- aluminum (TMA) and water is nearly ideal. It shows ex- tremely high reaction enthalpies of chemisorption with high E d values for both the first- and second-half reactions. For the first half-reaction between Al-OH and TMA, the enthalpy change is around -1.70 eV (E d = 1.61 eV), and for the second-half reaction between Al-(CH 3 ) 2 and H 2 O, it is around -1.48 eV (E d = 1.61 eV). 4 Furthermore methane (the byproduct in the first- and second-half reactions) does not chemically re-adsorb on the surface and is not able to etch the growing film. However many precursors for ALD rather show deviation from the ideal ALD chemistry. TiO 2 ALD from titanium tetrachloride (TiCl 4 ) and water is an example for the non-ideal ALD. Several groups had theoretically or experimentally studied the chemistry of TiO 2 ALD. 5-13 In the generally accepted mechanism of TiO 2 growth, TiCl 4 mainly reacts with the surface OH groups releasing HCl in the first-half reaction: n(-OH)(s) + TiCl 4 (g) → (-O-) n TiCl 4-n (s) + nHCl(g) (1) where (s) denotes surface. The adsorbed chlorotitanium (TiCl x ) species may react with water releasing HCl again in the second-half reaction, which results in the recovery of the surface OH groups for the next cycle: (-O-) n TiCl 4-n (s) + (4-n)H 2 O(g) → (-O-) n Ti(OH) 4-n (s) + (4-n)HCl(g) (2) Because the growth of TiO 2 is mainly contributed from the surface exchange reactions (Eqs. 1 and 2), the surface OH groups play a key role in the ALD sequence. There are two kinds of OH groups on the surface of oxides: isolated OH and adjacent hydrogen-bonded OH (H-boned OH) groups. The H-bonded OH groups are dominant at low temperatures, but they are condensed at high temperatures by dehydrox- ylation to form oxygen bridges liberating H 2 O: a These authors contributed equally to this work.