SCALE-UP OF THE BARIUM TITANATE ATOMIC LAYER DEPOSITION PROCESS ONTO 200 MM WAFER

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1 1.1149/ , copyright The Electrochemical Society SCALE-UP OF THE BARIUM TITANATE ATOMIC LAYER DEPOSITION PROCESS ONTO 2 MM WAFER R. Matero, A. Rahtu, S. Haukka, M. Tuominen, M. Vehkamäki 1, T. Hatanpää 1, M. Ritala 1, M. Leskelä 1 ASM Microchemistry Oy, Hämeentie 135 A, FI-56 Helsinki, Finland 1 Laboratory of Inorganic Chemistry, P. O. Box 55, FI-14 Helsinki, Finland BaTiO 3 films were deposited onto 2 mm silicon wafers by Atomic Layer Deposition (ALD) from barium bis(tris(tert-butyl)cyclopentadienyl) Ba( t Bu 3 CpH 2 ) 2, titanium methoxide Ti(OMe) 4 and water H 2 O. The films were characterized for thickness uniformity, stoichiometry and crystallinity. The thickness non-uniformity varied from 2.7 to 6.% with different Ba( t Bu 3 CpH 2 ) 2 H 2 O / Ti(OMe) 4 H 2 O cycle ratios. The most stoichiometric films were obtained with the 5:3 and 2:1 BaO:TiO 2 cycling ratios. The as-deposited films were amorphous but were crystalline after thermal annealing. The study showed that this process can be transferred from small experimental scale to large production scale wafers. INTRODUCTION Atomic Layer Deposition (ALD) of BaTiO 3 has recently been studied for its potential use e.g. in the fabrication of dynamic random access memories (DRAMs) and metalinsulator-metal (MIM) -capacitors [1-3]. ALD deposited films are being considered for these purposes due to their excellent conformality and because they are expected to give good large-area uniformity. In addition, high permittivities of 7 28 (film thickness nm) have been measured for post-deposition annealed ALD grown BaTiO 3 films embedded in a test MIM structure [3]. In this study, the scale-up of a BaTiO 3 ALD process onto 2 mm silicon wafers was studied. The precursors were barium bis(tris(tert-butyl)cyclopentadienyl) Ba( t Bu 3 CpH 2 ) 2, titanium methoxide Ti(OMe) 4 and water H 2 O. Although this process has been optimized earlier in a small-scale reactor [3], the process parameters need to be re-optimized when depositing in a larger scale. The films were deposited by mixing binary oxide cycles (i.e. Ba( t Bu 3 CpH 2 ) 2 H 2 O and Ti(OMe) 4 H 2 O) in various ratios. EXPERIMENTAL BaTiO 3 films were deposited in an F-45 ALCVD TM reactor (ASM Microchemistry Ltd.) on 2 mm silicon wafers (with native oxide top layer). Reaction temperature was 3 ºC. Evaporation temperatures for the metal sources were 155 and 15 ºC for Ba( t Bu 3 CpH 2 ) 2 and Ti(OMe) 4, respectively. H 2 O was held at room temperature (2 ºC). The film thickness was measured with a spectroscopic ellipsometer (SENTECH SE8). 137

2 A Bruker D8 Advance diffractometer was used for the X-ray diffraction measurements. The chemical composition and stoichiometry were examined by time-of-flight elasticrecoil detection analysis (TOF-ERDA) using a 24 MV 127 I 5+ beam obtained from a 5 MV tandem accelerator. RESULTS AND DISCUSSION Film Growth and Uniformity The deposition was accomplished by repeating binary oxide metal source oxygen source cycles; the Ba( t Bu 3 CpH 2 ) 2 H 2 O / Ti(OMe) 4 H 2 O cycle ratios that were used were 1:1, 4:3, 3:2, 5:3 and 2:1 in the order of increasing Ba precursor pulse fraction of total metal precursor pulses. The highest growth rate of.76 Å/cycle was obtained with the 5:3 BaO:TiO 2 cycle ratio (Fig. 1). Growth Rate (Å/cycle) Figure 1. The BaTiO 3 mean growth rate obtained at 3 ºC with different Ba( t Bu 3 CpH 2 ) 2 H 2 O / Ti(OMe) 4 H 2 O cycle ratios. Thickness (nm) Figure 2. The mean (circles), maximum (squares) and minimum (triangles) thicknesses measured on 2 mm Si wafers for the BaTiO 3 films deposited with various BaO/TiO 2 cycle ratios. 138

3 The thickness variation on the 2 mm wafer with different BaO/TiO 2 cycle ratios is presented in Figure 2. The best film non-uniformity was obtained with the 5:3 cycle ratio, Uf 1σ 2.7%. Film composition The Ba:Ti stoichiometry of the films obtained with the different BaO/TiO 2 cycle ratios is shown in Figure 3. The 1:1 cycle ratio resulted in a film with only 3% of Ba. The cycle ratios 5:3 and 2:1 produced almost stoichiometric BaTiO 3. The composition was assumed uniform over the entire 2 mm wafer on basis of the good refractive index uniformity. In this case, the refractive index is a very sensitive indicator of composition due to the large difference between the single oxides (TiO 2 : 2.5; BaO ~1.6). The refractive index for the 5:3 cycle ratio was 1.8 and the Uf 1σ.69%. After annealing for 3 minutes at 6 C under oxygen flow, the refractive index of this film was increased to 2.2. Ba/(Ba+Ti) % Figure 3. The Ba/(Ba+Ti) % in the BaTiO 3 films as determined by TOF-ERDA. The cycle ratio is the number of repeated BaO and TiO 2 reaction cycles. The films with the highest Ba content contained also the largest amount of hydrogen as an impurity (5-8 at.%). This hydrogen may be present due to the tendency of Ba to form Ba hydroxide. However, much lower hydrogen contents were measured for the other compositions, and part of the hydrogen and carbon impurities presented in Table 1 are situated on the film surface and at the film/substrate interface. The hydrogen amount can be further reduced by thermal annealing under O 2 flow. 139

4 Table 1. Hydrogen and carbon impurity content of BaTiO 3 films as determined by TOF-ERDA. H (at.%) C (at.%) 1:1.5 ±.2.2 ±.2 4:3 1.4 ±.3.3 ±.2 3:2 1.5 ±.3.4 ±.2 5:3 8.3 ±1..4 ±.2 2:1 4.8 ± ±.2 Film Crystallinity All the as-deposited films were amorphous regardless of the BaO:TiO 2 cycling ratio. After thermal annealing for 3 minutes under oxygen flow at 6 C, the films with the largest Ba fraction, i.e. BaO:TiO 2 cycling ratios 4:3, 5:3 and 2:1, crystallized (Figure 4). The (11) reflection has the highest intensity, but reflections (1) and (111) were also detected. Counts (a.u.) :1 5:3 4:3 3:2 1:1 (1) (11) (111) theta Figure 4. XRD patterns of thermally annealed BaTiO 3 films. The data has been shifted vertically for clarity. CONCLUSIONS This study shows that even a multi-component ALD process can without difficulty be scaled-up from small research size to manufacturing size substrates when the properties of the precursors are suitable for ALD. We obtained good film uniformity and nearly stoichiometric BaTiO 3 on 2 mm wafers. 14

5 ACKNOWLEDGEMENTS Dr. Kai Arstila is thanked for the TOF-ERD analysis. REFERENCES [1] Vehkamäki, M., Hatanpää, T., Hänninen, T., Ritala, M. and Leskelä, M., Electrochem. Solid-State Lett. 2, 54 (1999) [2] Hatanpää, T., Vehkamäki, M., Mutikainen, I., Kansikas, J., Ritala, M. and Leskelä, M., Dalton Trans (24) [3] Vehkamäki, M., Hatanpää, T., Ritala, M. and Leskelä, M., submitted to Chem. Vap. Deposition 141