Article pubs.acs.org/cm
Cycloheptatrienyl-Cyclopentadienyl Heteroleptic Precursors for Atomic Layer Deposition of Group 4 Oxide Thin Films Jaakko Niinistö,*,† Timo Hatanpaä ,̈ † Maarit Kariniemi,† Miia Man̈ tymak̈ i,† Leila Costelle,§ Kenichiro Mizohata,§ Kaupo Kukli,†,‡ Mikko Ritala,† and Markku Leskela†̈ †
Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland Division of Materials Physics, Department of Physics, University of Helsinki, FI-00014 Helsinki, Finland ‡ Institute of Experimental Physics and Technology, University of Tartu, Tähe 4, EE-51010 Tartu, Estonia §
S Supporting Information *
ABSTRACT: Atomic layer deposition (ALD) processes for the growth of ZrO2 and TiO2 were developed using novel precursors. The novel processes were based on cycloheptatrienyl (CHT, -C7H7) − cyclopentadienyl (Cp, -C5H5) compounds of Zr and Ti, offering improved thermal stability and purity of the deposited oxide films. The CpMeZrCHT/O3 ALD process yielded high growth rate (0.7−0.8 Å/cycle) over a wide growth temperature range (300−450 °C) and diminutive impurity levels in the deposited polycrystalline films. Growth temperatures exceeding 400 °C caused partial decomposition of the precursor. Low capacitance equivalent thickness (0.8 nm) with low leakage current density was achieved. In the case of Ti, the novel precursor, namely CpTiCHT, together with ozone as the oxygen source yielded films with low impurity levels and a strong tendency to form the desired rutile phase upon annealing at rather low temperatures. In addition, the thermal stability of the CpTiCHT precursor is higher compared to the usually applied ALD precursors of Ti. The introduction of this new ALD precursor family offers a basis for further improving the ALD processes of group 4 oxide containing thin films for a wide range of applications. KEYWORDS: atomic layer deposition, ALD, ZrO2, TiO2, high-k dielectrics, cyclopentadienyl, cycloheptatrienyl
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because they lead to denser films with minimal impurities and improved crystallinity. In the case of group 4 metal precursors, especially for Ti, the maximum temperature remains often below 300 °C. During the past decade many precursors and processes have been developed for the growth of group 4 metal oxides by ALD.2 In general, when high deposition temperature is aimed for, TiCl4 offers high reactivity and thermal stability, but the generation of corrosive byproduct as well as chlorine impurities in the film may be of a concern. The widely applied alkylamides and alkoxides suffer from limited thermal stability, and deposition temperatures exceeding 300 °C cause precursor decomposition leading to serious problems in film uniformity and conformality.2 In recent years, true organometallics, such as the cyclopentadienyl (Cp, -C5H5) derivatives of group 4 metals have gained interest.8−10 Their thermal stability, in many cases, can be significantly better than that of the alkylamides,10 for example. However, the growth rates may remain at moderate levels, at 0.5 Å/cycle. Further tailoring of the precursor molecules to form mixed alkylamido-cyclopentadienyl precursors of Zr and Hf resulted in higher thermal stability compared to the alkylamides but with similar high growth rate (0.8−0.9
INTRODUCTION The oxides of group 4 metals (Ti, Zr, Hf), either as binary oxides or mixed with other oxides, have been of a high interest in development of high dielectric constant (high permittivity, high-k) insulators for microelectronics, in particular as dielectrics in complementary metal oxide semiconductor (CMOS) transistors and in dynamic random access memory (DRAM) capacitors. Currently, Zr- and Hf-based oxide materials are in high volume manufacturing enabling so far the target of continuously shrinking semiconductor devices. For producing these oxide thin film materials, atomic layer deposition (ALD) is the method-of-choice.1−4 ALD is based on alternate, saturative surface reactions of precursor vapors.5−7 The self-limiting characteristics of the method enable perfect conformality and thickness control of the deposited film on wide area substrates. Because of the chemical nature of the growth process, the selection of suitable precursors is the key issue in ALD. Among the many requirements for an efficient ALD precursor, sufficient volatility, thermal stability, and reactivity are, of course, important. In general, a large number of precursor families fulfills most of the requirements. However, in many cases thermal decomposition of the metal precursor in the gas phase or at the substrate surface destroys the ALD growth mode and limits the applicable process temperature to lower than desired.6 Higher deposition temperatures are often preferred © 2012 American Chemical Society
Received: October 12, 2011 Revised: April 12, 2012 Published: May 17, 2012 2002
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evaporated under vacuum. Sublimation of the residue (130 °C/0.05 mbar) gave a bright blue solid: (45.9 g); mp 258−262 °C; 1H NMR (C6D6, 300 MHz): 4.91 (s, 5H, CH), 5.43 (s, 7H, CH); 13C{1H} NMR (C6D6, 75 MHz): 86.9 (CH, Cp ring), 97.6 (CH, C7-ring); MS (EI, 70 eV) m/z: 204 [M]+ with right isotopic distribution. Synthesis of (MeC5H4)Zr(C7H7). The synthesis was performed in a fashion similar to that described for CpZrCHT by Tamm et al.21 The method is also similar to that presented for CpTiCHT. CpMeZrCl3 needed in the synthesis was synthesized using the method of Hitchcock et al.22 A Schlenk flask was charged with magnesium turnings (6 g, 247 mmol), catalytic amounts of FeCl3 (0.6 g, 3.7 mmol), cycloheptatriene (15 mL), and THF (50 mL). This reaction mixture was treated dropwise with a solution of CpMeZrCl3 (17.3 g, 62.4 mmol) in THF (150 mL) over a period of 1 h. After the mixture was stirred overnight at room temperature, all volatiles were removed in vacuo. The air-sensitive residue was sublimed at 140 °C/0.05 mbar to obtain 13.0 g (79.6%) of (MeC5H4)Zr(C7H7) as a purple crystalline solid. Mp. 174−176 °C, 1H NMR (C6D6, 300 MHz) 1.81 (s, 3H, CH3), 5.14 (t, 2H, CH)), 5.23 (m, 2H, CH) 5.24 (s, 7H, CH); 13 C{1H} NMR (C6D6, 75 MHz) 14.8 (CH3), 81.6(C7-ring), 101.1 (Cp ring), 103.6 (Cp ring). MS (EI, 70 eV) m/z: 260 [M]+ with right isotopic distribution. Film Growth. ZrO2 and TiO2 thin films were grown in a hot-wall flow-type ALD reactor (F-120 by ASM Microchemistry Ltd., Finland) at pressure of 5−10 mbar. Nitrogen (>99.999%), generated from air by Nitrox UHPN 3000 N2-generator, was used as carrier and purging gas. The precursors, CpMeZrCHT and CpTiCHT, were evaporated from open crucibles kept at 112 and 103 °C, respectively. As the oxygen source, ozone (obtained from 99.999% O2 by Wedeco ozomatic modular HC Lab-generator) with concentration of approximately 100 g/m3, was applied. The growth temperature range was 250−475 °C using a pulsing sequence of 0.6/1.0/1.0/1.5 s (metal precursor pulse/purge/ozone pulse/purge). Self-limited growth of ZrO2 and TiO2 was determined by studying the growth rate as a function of the metal precursor pulse length i.e. by varying x in the pulsing sequence x/x+0.5/1.0/1.5 s. As-received Si(100) measuring 5 × 5 cm2 were used as substrates. For selected samples, also TiN covered Si substrates were used. Film Characterization Methods. For thickness measurements, reflectance spectra of the thicker films (>40 nm) were measured with a Hitachi U-2000 double beam spectrophotometer. Thicknesses were then evaluated by the optical fitting method originally described by Ylilammi and Ranta-aho.23 The thicknesses below 40 nm were evaluated by X-ray reflectometry (XRR) using a Bruker D8 Advance X-ray diffractometer. Crystallinity of the films was studied using a Panalytical X’pert Pro MPD X-ray diffractometer (XRD). Film morphology was studied with an atomic force microscope (Nanoscope V, Veeco Instruments), operated in the tapping mode with a scanning frequency of 1 kHz. Roughness values were calculated as root-meansquare (rms) values. Chemical compositions of the films were measured by time-of-flight elastic recoil detection analysis (TOFERDA),24 where heavy ions are accelerated and projected into the sample. Upon hitting the sample, high energy ions result in elastic collisions and recoils of the sample atoms are detected. Mass separation is calculated by determining the recoil velocity and energy by timing gates and charged particle detectors, respectively. For these TOF-ERDA studies, a 38 MeV 79Br8+ ion beam was used, obtained from a 5 MV tandem accelerator EGP-10-II. The electrical measurements were carried out on Al/Ti/ZrO2/TiN/ p-Si(100)/Al capacitors with top electrodes consisting of 25 nm thick Ti and 100−130 nm thick Al layers with an area of 0.204 mm2, electron-beam evaporated through a shadow mask. Capacitance− voltage (C−V) curves were recorded using a HP4284A precision LCRmeter in a two-element series circuit mode. The stair-sweep voltage step was 0.05 V. The period between voltage steps was 0.5 s. The AC voltage applied to the capacitor was 0.05 V, while the frequency of the AC signal was 1 kHz. The current−voltage (I−V) curves were measured with a Keithley 2400 Source Meter in the stair sweep voltage mode, while the voltage step used was 0.05 V and the top electrode dots were biased negatively in relation to the TiN/Si substrate, i.e.
Å/cycle).11,12 It seems that Cp-ligands, also with alkyl substituents, improve the thermal stability, and finding the optimum structure can significantly enhance the applicability of ALD. For titanium precursors a very recent example is the introduction of Cp*Ti(OMe)3, (Cp*=C5(CH3)5) where thermal stability could be increased to over 300 °C.13,14 In addition, heteroleptic Ti(OiPr)2(thd)2 (thd=2,2,6,6-tetramethyl-3,5-heptanedione) has showed high thermal stability.15 It should be noted that in the case of Ti, finding thermally stable precursors could improve also the ALD process for SrTiO3, a very high-k material with strong potential achieving demanding requirements as a capacitor dielectric in the future DRAM technology nodes. In the case of SrTiO3, crystallization during the deposition itself seems to be the key in achieving the required electrical characteristics, thus it may be beneficial to grow the film at temperatures exceeding 300 °C.3 In addition, rutile structured TiO2,16 also when doped with Al, is of high interest,17 and also in that case high thermal stability Tiprecursor may lead to improved results. As using Cp-ligands in group 4 metal precursors showed to be beneficial it is important to explore other heteroleptic cyclopentadienyl complexes as novel precursors for ALD of group 4 oxides. Our current approach is to apply cycloheptatrienyl (CHT, -C7H7) ligands in such compounds. In this paper, we introduce a new family of ALD precursors, namely CpRMCHT (M = Ti, Zr, R = H, Me) compounds. The applicability of these Cp-CHT precursors is exemplified by reporting ozone-based ALD processes of ZrO2 and TiO2 using η7-cycloheptatrienyl-η5-methylcyclopentadienylzirconium (CpMeZrCHT) and η7-cycloheptatrienyl-η5-cyclopentadienyltitanium (CpTiCHT), respectively.
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EXPERIMENTAL SECTION
Synthesis and Analysis of the Precursors. All complex preparations were done under rigorous exclusion of air and moisture using standard Schlenk and glovebox techniques. Toluene and xylene were dried and stored over 4 Å molecular sieves. THF was freshly distilled from sodium benzophenone ketyl. Anhydrous zirconium(IV) chloride (Aldrich 99.999%), titanium(IV) chloride (Fluka >99.0%), bis(cyclopentadienyl)titanium(IV) dichloride (Aldrich 97%), iron(III) chloride (Riedel-de Haën), magnesium turnings, and cycloheptariene (Aldrich 90%) were used as received. Dicyclopentadiene and methylcyclopentadiene dimer were cracked to corresponding monomer just before usage. 1H and 13C NMR spectra were recorded with a Varian Mercury Plus 300 MHz instrument at ambient temperature. Chemical shifts were referenced to SiMe4 and are given in ppm. Thermogravimetric analyses (TGA) were carried out on a Mettler Toledo Stare system equipped with a TGA 850 thermobalance using a flowing nitrogen atmosphere at 1 atm. The heating rate was 10 °C/ min, and the weights of the samples prepared to 70 μL pans were 10− 11 mg. Melting points were taken from the scanning differential thermal analysis (SDTA) data measured by the thermobalance. Mass spectra were recorded with a JEOL JMS-SX102 operating in an electron impact mode (70 eV) using a direct insertion probe and sublimation temperature range of 50−300 °C. Synthesis of (C5H5)Ti(C7H7). The synthesis was done using the method of Demerseman et al.18 Prior the synthesis CpTiCl3 had to be synthesized, and two different methods were employed synthesizing different batches. At first the method of Sloan et al.19 was employed, but then the method of Hitchcock et al.20 was taken into use as Cp2TiCl2 is readily available. To a 1-L flask containing 20 g of magnesium chips were added 2 g of anhydrous FeC13, 50 mL of cycloheptatriene, and 50 mL of THF. A solution of 57.45 g (0.26 mol) of CpTiCl3 in 400 mL of THF was added over a 3-h period in order to allow the warming of the stirred reaction mixture. The mixture was stirred at room temperature overnight, and the volatile products were 2003
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electrons were injected from the top electrode. All measurements were performed at room temperature without postdeposition annealing.
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RESULTS AND DISCUSSION Two precursor compounds new for ALD, namely cyclopentadienyl cycloheptatrienyl titanium and methylcyclopentadienyl cycloheptatrienyl zirconium (Figure 1) were synthesized
Figure 2. TGA curves of CpTiCHT and CpMeZrCHT.
(endotherm in SDTA) at 258−262 °C. The residue after the measurement was 5.5%, and the evaporation step was complete at 315 °C. For CpMeZrCHT SDTA reveals three endothermic changes. One is due to the evaporation of the compound (peak at 270 °C). Other endotherms are at 67−73 °C and 176−179 °C. The first one of these is due to some change in solid phase and the second one due to the melting of the compound. It is likely that the lower melting temperature of CpMeZrCHT is due to the methyl substituent in the cyclopentadienyl ligand which decreases the symmetry of the compound. Previous reports of these compounds do not mention the melting point temperatures which are quite high. The deposition rates were studied with both precursors as a function of the growth temperature (Figures 3 and 4). Ozone
Figure 1. Schematic pictures of the precursors applied in this study: a) η7-cycloheptatrienyl-η5-cyclopentadienyltitanium (CpTiCHT) and b) η7-cycloheptatrienyl-η5-methylcyclopentadienylzirconium (CpMeZrCHT).
and used in ALD of titanium and zirconium oxides. The compounds were synthesized using literature methods,18 and the yields of the synthesis relative to the metal source compounds were good (79−89%). Both compounds are known and well characterized including the crystal structures.18,25,26 However, we also did solve the crystal structure of CpMeZrCHT which was found to be in agreement with that already published.27 Only minor differences were observed.28 In both compounds the metal center is surrounded by η5coordinated cyclopentadienyl ligand and η7-coordinated cycloheptatrienyl ligand. Our NMR results are in agreement with those reported earlier18,27 with the exception that in 13C spectrum measured for CpMeZrCHT we could not observe the peak for the ipso carbon of the methylcyclopentadienyl ligand. The compounds are stable against H2O but sensitive to air. At first sight it seems that the metal centers are formally in oxidation state +II in these compounds. However, the oxidation state has been debated in many articles, and there seems to be evidence supporting both oxidation states +II and +IV.18,26,27,29 There is quite a lot covalent character in bonding and the oxidation state +IV seems more likely, as common to the group 4 elements. Thermal properties of the compounds were studied with TGA/SDTA (Figure 2). TG curves of both compounds are very similar and show only single almost overlapping major step of weight losses which can be attributed to evaporation of the compounds. The compounds start clearly to lose weight at 150 °C. T1/2 is 245 °C, and evaporation processes are complete at 275 °C. Residues at 400 °C are 3.3 and 4.4% for CpTiCHT and CpMeZrCHT, respectively. The residues are believed to be due to the air sensitivity of the compounds, not due to thermal decomposition. Samples were loaded into TGA in ambient air. CpTiCHT does not melt before sublimation, and thus in SDTA only one endotherm is seen due to the sublimation. TGA/SDTA measured with using a crucible with a lid that has only a small orifice through which the evaporation takes place shows evaporation at higher temperature and melting
Figure 3. The growth rate of ZrO2 vs deposition temperature using CpMeZrCHT and ozone as precursors. In the inset the growth rate as a function of the pulse length at 350 °C is depicted.
was used as the oxygen source in both cases as no growth with water was obtained. In the case of ZrO2 the growth rate was relatively constant at 0.7−0.8 Å/cycle at the growth temperature range from 300 to 400 °C (Figure 3). Such a wide process window with constant growth rate is often observed in ozonebased ALD processes. This so-called ALD-window6 is not a necessary requirement for a well-behaving ALD process or a solid proof for saturative growth mode but rather implies good reproducibility characteristics. However, the inset shows the growth rate as a function of the CpMeZrCHT pulse length at 350 °C clearly verifying the ALD-type saturative growth mode. 2004
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type growth mode was verified at 300 °C as the growth rate remained constant at 0.34 Å/cycle when the CpTiCHT pulse length was increased (Figure 5). At 325 °C some scattering of
Figure 4. The growth rate of TiO2 vs deposition temperature using CpTiCHT and ozone as precursors. Pulsing times were 1 s for both CpTiCHT and ozone.
Figure 5. The growth rate of TiO2 as a function of the CpTiCHT pulse length at 300, 325, and 350 °C.
At temperatures exceeding 400 °C signs of thermal decomposition of the precursor were observed: the thickness uniformity was weakened, and the thickness profile along the gas flow direction across the substrate was observed. The growth characteristics of the CpMeZrCHT/O3 process resemble the (CpMe)2Zr(OMe)Me/O3 process.9 However, here the growth rate is slightly higher, actually close to the alkylamidobased processes at lower temperatures and also the growth rate slightly decreases rather than increases as the decomposition begins to have an effect on the growth mode. It seems that the smaller size of the CpMeZrCHT molecule decreases the steric hindrance, and thus the growth rate is somewhat higher than in the case of the (CpMe)2Zr(OMe)Me/O3 process.9 In addition, the bond strength between Zr and the CHT ligand is strong and ozone is needed in order to remove the ligands, whereas with (CpMe)2Zr(OMe)Me also water can be applied.30 One might expect that the protective nature of the ligands against ZrO2 formation could lead to higher amounts of impurities, i.e. that some of the ligands are not completely removed by the ozone. However, according to TOF-ERDA the films deposited at 300 and 350 °C were stoichiometric and pure: the Zr:O ratio was 0.5, and diminutive amounts of carbon and hydrogen were detected. At 350 °C the C and H impurity levels were less than 0.1 and 0.5 at.%, respectively. At 300 °C the amount of carbon was slightly higher, 0.7 at.%, but the amount of hydrogen was at the same level as at 350 °C. In general, ZrO2 (and HfO2) films grown by ALD from Cpprecursors exhibit high purity, and this seems to be valid also when CHT ligands are applied. Even though an analogous Hf-precursor of the Cp-CHT family is not studied in this paper, we strongly believe that similar process characteristics could be found for the growth of HfO2 films. It has been shown frequently that these chemically related elements, Zr and Hf, exhibit similar ALD growth characteristics when analogous precursors are applied. However, Hf-compounds usually show somewhat higher thermal stability than the analogous Zr-precursors.30,31 Thus, we believe that Cp-CHT precursors can be used effectively also for ALD of high-quality HfO2 films at relatively high deposition temperatures. ALD-window is not observed when applying CpTiCHT and ozone for the growth of TiO2 thin films (Figure 4). No film growth with water was observed. The growth rate steadily increases as the deposition temperature is increased. The ALD-
the results was observed, but at 400 °C doubling the pulse length from 1 to 2 s caused the growth rate to increase from 0.75 to 0.95 Å/cycle. This can be interpreted as a sign of thermal decomposition of the CpTiCHT precursor but the increase of the growth rate is not rapid, thus suggesting that the thermal stability of the precursor is rather good compared to other available Ti-precursors, e.g. alkoxides32 and alkylamides.33 According to our results, the upper limit for the ALD-type selflimiting growth mode in the CpTiCHT/O3 process is definitively higher than 300 °C, most likely close to 350 °C, as in the Cp*Ti(OMe)3/O3 ALD process.14 Further improvement in the thermal stability of the precursor may be achieved if e.g. methyl substituted Cp-ligands are used as in the case of Zr, described above. Another option may be to introduce alkyl groups into the CHT-ring. This new precursor group with CHT ligands opens wide possibilities to develop improved ALD precursors, also for titanium. According to TOF-ERD analysis, the impurity levels were low in the deposited TiO2 films, less than 1 at.% for carbon and hydrogen (Figure 6) in a film grown at 300 °C. The metal to oxygen ratio was almost stoichiometric, 0.46. When ozone is used as the oxygen source, a slight excess of oxygen has often been detected in metal oxide films.9,12,14
Figure 6. TOF-ERDA profiles of atoms/elements in a 35 nm TiO2 film deposited on Si at 300 °C. Depth scale was calculated using a value of 4.0 g/cm3 for the density of the film. 2005
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The ZrO2 films deposited on Si were strongly polycrystalline, regardless of the growth temperature (300−400 °C) for about 50−60 nm thick films. Usually, when different Cp-precursors are applied to deposit ZrO2 films with similar thickness range the XRD patterns mainly depict reflections which can be attributed to monoclinic ZrO2 (JCPDS 36-420) and in addition the (111) reflection of the cubic or tetragonal phase (JCPDS 27-997 and 14-534). Also in the current case, the strongest peak in the films deposited at 350 and 400 °C is the monoclinic (−111) reflection at 28.5°. However, at a lower deposition temperature of 300 °C the cubic/tetragonal (111) reflection has the highest intensity similar to the thinner films discussed below. Thin films (
