Controlled growth of HfO2 thin films by atomic layer ... - CiteSeerX

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Controlled growth of HfO2 thin films by atomic layer deposition from cyclopentadienyl-type precursor and water Jaakko Niinisto¨,*a Matti Putkonen,a Lauri Niinisto¨,a Sarah L. Stoll,b Kaupo Kukli,{c Timo Sajavaara,{d Mikko Ritalac and Markku Leskela¨c Received 25th November 2004, Accepted 17th February 2005 First published as an Advance Article on the web 3rd March 2005 DOI: 10.1039/b417866c HfO2 thin films have been deposited onto p-Si(100) substrates by atomic layer deposition (ALD) using Cp2Hf(CH3)2 (Cp 5 cyclopentadienyl) and water as precursors at 300–500 uC. Processing parameters were optimised and the ALD type growth mode corroborated at 350 uC where a ˚ cycle21 was obtained. The crystallinity, morphology and chemical deposition rate of 0.42 A composition of the deposited films were characterised. Films deposited at 300–450 uC were polycrystalline with monoclinic (2111) as the preferred orientation. Impurity levels of the stoichiometric HfO2 films deposited at 350 and 400 uC were very low, or below 0.4 and 0.25 atom% for carbon and hydrogen, respectively. In addition, ultrathin HfO2 films showed good dielectric properties such as low hysteresis and nearly ideal flatband voltage.

Introduction Hafnium oxide is presently considered as one of the most promising candidates for alternative gate oxide insulating layers in complementary metal oxide semiconductor devices because of its expected thermodynamic stability in contact with Si, high permittivity (15–26), and relatively large bandgap.1–3 Other possible applications for HfO2 include DRAM (dynamic random access memory) capacitors and optical coatings.4,5 In order to deposit highly conformal and uniform ultrathin films with accurate thickness control over a large substrate area, and into deep trenches as required in trench capacitor DRAMs, atomic layer deposition (ALD) is considered as a method with extremely high potential for dielectric oxide deposition. ALD, originally known as atomic layer epitaxy, is based on self-limiting surface reactions achieved by alternating surface-saturating precursor doses separated by inert gas purging.6–9 In ALD of HfO2, HfCl4 and water has been by far the most frequently used precursor combination.10–14 Unfortunately, the process has distinct drawbacks such as poor nucleation on H-terminated silicon at temperatures exceeding 300 uC, leading to non-uniform growth.15 A major drawback with this precursor system, however, is chlorine contamination in the films, being more pronounced near the HfO2 film/silicon interfacial region.16 Therefore alternative precursor chemistries have been sought. Table 1 summarises the reported ALD processes of HfO2 thin films. As can be seen, a wide selection of different precursor combinations other than HfCl4/H2O has been used. Changing the oxygen source from water to ozone in the HfCl4 process has a favourable effect on most electrical properties { Also at: University of Tartu, Institute of Experimental Physics and Technology, Ta¨he 4, EE-51010 Tartu, Estonia. { Also at: K. U. Leuven, Instituut voor kern- en Stralingsfysica, Celestijnenlaan 200D, B-3001 Leuven, Belgium. *[email protected]

This journal is ß The Royal Society of Chemistry 2005

but after annealing the interfacial SiO2 thickness increases more than in the H2O-processed films.17 This undesirable interfacial oxide formation increases the overall capacitance equivalent oxide thickness (CET) value. The HfI4/H2O process yields slightly lower halide content in the films than the conventional HfCl4 process when applied at 300 uC but the difference is not significant.13,18,19 Using the iodide precursor with molecular oxygen eliminates hydrogen residues but the process requires a rather high temperature.20 High growth temperatures, on the other hand, are preferred when films free of residual halide ions need to be obtained. At the same time, low process temperatures are required to increase the growth rate and nucleation density while decreasing the degree of polycrystallization. Liquid hafnium alkylamides, namely Hf(NEtMe)4, Hf(NEt2)4 and Hf(NMe2)4 (Et 5 C2H5, Table 1 Published processes for the atomic layer deposition of HfO2 thin films on silicon Metal precursor/oxygen source

Range of Tgrowth/uC

HfCl4/H2O HfCl4/O3 HfCl4/Hf(NO3)4 HfI4/H2O or H2O2 HfI4/O2 Hf(NEtMe)4/H2O Hf(NEtMe)4/O3 Hf(NMe2)4/H2O Hf(NMe2)4/O3 Hf(NEt2)4/H2O Hf(OtBu)4/O2 Hf(OtBu)4/O3 Hf(OtBu)2(mmp)2/H2Oa Hf(mmp)4/H2O Hf(ONEt2)4/H2O Hf(NO3)4/H2O Cp2HfMe2/H2O Cp2Hf(CH3)2/H2O

160–940 300 150–190 225–500 400–755 200–350 100–400 50–500 160–420 50–500 350–480 250 275–400 275–425 250–350 160–190 300–500

a

Preferred or most frequently used Tgrowth/uC

Ref.

300 300 150–190 300 570–755 250 250–300 ,350 200–320 ,450 350–480 250 360 360 300 180 350–400 350

10–17 17 18 13, 19 20 21–23 25 22, 26 24 22 27 25 28 29 30 31,32 This study This study

mmp 5 1-methoxy-2-methyl-2-propanolate (OCMe2CH2OMe)

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Me 5 CH3), are interesting alternatives together with water leading to films with less than 1 atom% carbon and 0.25 atom% nitrogen as impurities.21–24 On the other hand, when applying ozone together with Hf(NEtMe)4, considerably higher carbon impurity values were reported.25 In addition, at least in the case of the Hf(NEtMe)4/H2O process, hydrogen impurity levels of 2–5 atom% were detected.21 The suitable ALD growth temperature range for the Hf(NEtMe)4/H2O process is 200– 350 uC but slight decomposition of the precursor was probably affecting the growth rate at temperatures above 300 uC.21 Using the Hf(NMe2)4/H2O process, it was recently proposed that during the early stages of HfO2 growth on H-terminated silicon a SiNx interfacial layer instead of SiO2 was formed, thus yielding a very promising CET of 1.8 nm with low leakage current density.26 Another ALD-precursor family, viz. the hafnium alkoxide complexes,25,27–30 exhibit rather poor thermal stability and thus true self-limiting ALD growth might only be achievable below 275–300 uC yielding a rather low deposition rate around ˚ cycle21. Due to the decomposition of the precursor, the 0.2 A carbon and hydrogen impurities remain high, e.g. the ALD process of hafnium tetrakis(1-methoxy-2-methyl-2-propanolate) with H2O at 360 uC resulted in films with approximately 12 and 6 atom% of C and H, respectively.29 Anhydrous volatile Hf(NO3)4 has also recently been applied as an ALD precursor for the deposition of oxygen-rich hafnium oxide films.31,32 A promising CET value of 2.1 for a 5.7 nm film deposited directly on H-terminated Si was reported but the precursor decomposition limits its use to only very low temperatures of 180 uC and slightly below.31 Still another group of potential ALD precursors is formed by the organometallic cyclopentadienyl (Cp, g-C5H5) compounds which are generally volatile and reactive towards water at reasonable temperatures. For ALD of binary oxides, the Cp compounds have been so far used for MgO,33 Sc2O3,34 Y2O335 and ZrO2.36 It was recently shown that the cyclopentadienyl precursor of Zr, namely Cp2Zr(CH3)2, together with water yields stoichiometric ZrO2 films with extremely low impurity contents (below 0.1 atom% for C and H) and promising electrical properties.36,37 These results have motivated us to exploit cyclopentadienyl compounds also in the ALD of HfO2. In the present study, HfO2 films were deposited by ALD from Cp2Hf(CH3)2 with water as an oxygen source.

Experimental HfO2 film deposition Cp2Hf(CH3)2 was used as the metal precursor and distilled water as oxygen source. Cp2Hf(CH3)2 was synthesised by the method described by Samuel and Rausch.38 Hafnium oxide films were deposited at 2–3 mbar pressure in a flow-type hot-wall ALD reactor MC-120 manufactured by ASM Microchemistry Ltd (Finland). Cp2Hf(CH3)2 was evaporated from an open crucible kept at 70 uC. Nitrogen (.99.999%, Schmidlin UHPN 3000 N2 generator) was used as carrier and purging gas. For substrates, p-type Si(100) (Okmetic, Finland) was used. The substrate area was 10 6 5 cm2. Si substrates covered by the native oxide were ultrasonically cleaned in ethanol and distilled water. The deposition rate for HfO2 2272 | J. Mater. Chem., 2005, 15, 2271–2275

grown from Cp2Hf(CH3)2 was studied as a function of the deposition temperature at 300–500 uC. The standard pulsing sequence (ALD growth cycle) was the following: 1.0 s pulse of Cp2Hf(CH3)2, followed by 1 s nitrogen purge, 1.5 s of water, and finally 1.5 s nitrogen purge. Characterisation methods Reflectance spectra were measured in a Hitachi U-2000 double beam spectrophotometer. The film thicknesses of the deposited films were determined by measuring the optical reflectance spectrum between the wavelengths of 190 and 1100 nm and fitting a theoretical spectrum to the measured one.39 The thickness of the films thinner than 50 nm was evaluated by X-ray reflectometry (XRR) using a Bruker D8 Advance X-ray diffractometer. Crystallite orientations and crystallinity of the deposited films were determined by X-ray diffraction with CuKa radiation in a Philips MPD 1880 diffractometer. Surface morphology was studied by using a Nanoscope III atomic force microscope (Digital Instruments) operated in tapping mode. Samples were measured with a scanning frequency of 1 Hz. Several large scan area images (10 6 10 mm2) were recorded from different parts of the samples in order to check the uniformity of the sample. Final images were measured from a scanning area of 2 6 2 mm2. Roughness values were calculated as root mean square values (rms). Film composition was measured by time-of-flight elastic recoil detection analysis (TOF-ERDA) at the Accelerator Laboratory of the University of Helsinki. In this method,40,41 heavy ions are projected into the sample and the resulting signal consists of forward recoiling sample atoms ejected by the ion beam. Both velocity and energy for recoiled atoms are determined using timing gates and a charged particle detector, which enables the differentiation of the masses. With known stopping power and scattering cross sections, elemental depth distributions can also be calculated. For these TOF-ERDA studies, a 53 MeV 127I10+ ion beam was used, obtained from a 5 MV tandem accelerator EGP-10-II. For heavy recoil, energy spectra were obtained from the TOF signals while hydrogen spectra were obtained from the charged particle detector. In TOF-ERD analysis the uncertainties of the impurity contents are due to statistical and possible systematic errors in the stopping power values. For the electrical characterisation, HfO2 from CpHf(CH3)2/ H2O was deposited onto p-type Si(100)-substrates at 350 uC. Prior to the depositions the substrates had a native SiO2 layer which was not removed. The HfO2 layer thicknesses were 2.2 or 4.9 nm, depending on the sample. Aluminium gate electrodes with an effective area of 0.204 mm2 were e-beam-evaporated onto the HfO2 film surface through a shadow mask. The backsides of the Si substrates were HF-etched before evaporating the 100 nm thick aluminium electrodes to create ohmic contacts. Thus, the capacitance–voltage (C–V) and current–voltage (I–V) measurements were carried out on Al/HfO2/native SiO2/p-Si(100)/Al capacitor structures. C–V characteristics were measured with a HP 4284A precision LCR-meter. The voltage step was 0.05 V and the frequency of the ac signal was 500 kHz. The I–V curves were measured with a Keithley 2400 Source Meter with a voltage step of 0.05 V. All This journal is ß The Royal Society of Chemistry 2005

measurements were performed at room temperature without post-deposition annealing.

Results and discussion ALD depositions The growth rate of HfO2 films deposited by the Cp2Hf(CH3)2/ H2O process as a function of the growth temperature is plotted in Fig. 1. At the deposition temperature of 300 uC, the growth rate remains very low and true self-limiting growth is not achieved due to the insufficient reactivity of the precursor. When the deposition temperature is increased to 350 uC, a ˚ cycle21 is achieved, similarly as in the growth rate of 0.42 A case of ZrO2 deposition by the Cp2Zr(CH3)2/H2O process.36 Up to 400 uC, the growth rate slightly depends on the deposition temperature increase. A further increase in the deposition temperature to above 450 uC destroys the selflimiting growth mode and the decomposition of the precursor molecule leads to non-uniform films. It is noteworthy that at the deposition temperatures of 450 uC and below all films were uniform over the whole substrate area of 10 6 5 cm2. In order to confirm the self-limiting ALD growth, pulse times of the Cp2Hf(CH3)2 precursor were varied from 0.6 to 2 s (Fig. 1, inset). Surface saturation was achieved for the 1 s pulse and doubling the pulse time did not alter the growth rate, thus confirming the ALD-type growth. Furthermore, the film thickness shows a linear dependence on the number of reaction cycles applied at 350 uC (Fig. 2). HfO2 film characteristics Results of TOF-ERD analysis for the HfO2 film composition show generally low impurity contents (Table 2). In addition, the films deposited at 300, 350 and 400 uC were stoichiometric. The film deposited at 300 uC, i.e. at a temperature just below the surface controlled temperature regime, contained 1.5 atom% of carbon and 2.2 atom% of hydrogen as impurities. However, increasing the deposition temperature

Fig. 1 Growth rate of HfO2 films deposited from Cp2Hf(CH3)2 and H2O as a function of the deposition temperature. The inset shows the growth rate at 350 uC as a function of the metal precursor pulse duration.


Fig. 2 Thickness of HfO2 films deposited by the Cp2Hf(CH3)2/H2O process as a function of the number of deposition cycles applied. The deposition temperature was 350 uC. The metal pulse duration was 1 or 2 s depending on the sample. Table 2 Compositions of as-deposited HfO2 films grown on silicon, as measured by TOF-ERDA Tgrowth/uC

O (atom%)

Hf (atom%)

H (atom%)

C (atom%)

300 350 400

64 ¡ 1 65 ¡ 2 66 ¡ 2

32 ¡ 0.5 34 ¡ 1 33 ¡ 1

2.2 ¡ 0.3 0.4 ¡ 0.2 0.3 ¡ 0.2

1.5 ¡ 0.3 0.19 ¡ 0.05 0.22 ¡ 0.05

to 350 uC yielded HfO2 films with only 0.4 and 0.2 atom% hydrogen and carbon, respectively. In the films deposited at 400 uC the impurities were found also to be at the same low level. Since the C and H impurities retained in the HfO2 films are low at the deposition temperatures of 350 and 400 uC, it is believed that only the metal–carbon bond is broken as a result of the exchange reaction between the surface OH groups and the metal precursor. The following H2O pulse then liberates the remaining ligands and thus carbon containing fragments are not left in the film. In addition, small amounts (,0.3 atom%) of zirconium, a common impurity in hafnium precursors, was found in all HfO2 films. Also, 0.1 atom% chlorine was detected in the film deposited at 300 uC. Chlorine probably originates from the precursor synthesis involving Cp2HfCl2.38 In films deposited at higher temperatures (350 and 400 uC) chlorine was not detected. As mentioned earlier, hafnium alkylamide-based processes resulted in films with less than 1 atom% of carbon but hydrogen impurity levels were higher due to the lower deposition temperature used.21,22 In water-based processes at low temperatures some OH groups are always remaining in the films. For comparison, films deposited by the HfCl4/H2O process at 300 uC typically contain about 0.4 atom% of Cl and 1 atom% of H.13 Crystallinity and crystallite orientation in the as-deposited films were determined by X-ray diffraction (XRD). In Fig. 3, typical XRD pattern of an 85 nm thick HfO2 film deposited at 350 uC is shown. The monoclinic phase is the preferred one with (2111) as the most intense reflection. The intensity of the (2111) reflection increased as a function of the temperature (Fig. 3, inset); this reflection is by far the most intense one in the whole temperature range studied. Several other low intensity reflections belonging to the monoclinic phase were J. Mater. Chem., 2005, 15, 2271–2275 | 2273

Fig. 3 XRD pattern of the HfO2 film deposited using the Cp2Hf(CH3)2/H2O process at 350 uC. The film thickness was 85 nm. The inset shows the effect of the deposition temperature on the intensity of the monoclinic (2111) reflection. The film thicknesses were in the range of 85–100 nm. Diffraction peaks were identified and indexed according to the JCPDS card 34-104.

also observed (Fig. 3). However, the often reported weak reflections10,21 from metastable tetragonal or orthorhombic phases were not detected. Atomic force microscopy (AFM) was used to study the effect of deposition temperature on the surface morphology of the HfO2 films. The roughness, calculated as a rms value, of a 100 nm thick film deposited at 350 uC was 4.3 nm (Fig. 4a) but decreased to about 3 nm when the deposition temperature was

400 uC (Fig. 4b). The roughness was further reduced to 1.7 nm for a 130 nm film deposited at 450 uC. The reduction of roughness may be due to a change in growth mechanism, increase in nucleation density, or increase in surface mobility. Similar roughness reduction, when the deposition temperature is increased from around 300–350 to 450 uC, has been reported also with cyclopentadienyl-based ALD processes of Sc2O334 and ZrO2.36 Dielectric behaviour of the as-deposited films was evaluated from Al/HfO2/native SiO2/p-Si MOS structures where the HfO2 film thickness was 2.2 and 4.9 nm. As seen in Fig. 5, the hysteresis of the capacitance–voltage curves was narrow indicating a low effect of rechargeable traps inside the oxide dielectrics. For the Al/insulator/p-Si(100) structure the flatband voltage, VFB, should be around 21 V.42 In the present case, the VFB shift is diminutive indicating a small amount of fixed charge in the oxide as well as relatively low significance of changes in the interface trap density. Accumulation capacitance decreases with increasing insulator thickness yielding CET values of 2.7 and 3.4 nm for the structures with 2.2 and 4.9 nm thick HfO2 films, respectively. It should be noted that the native oxide was not removed, thus strongly increasing the CET values. When calculating the permittivity of the HfO2 layer only, using the series capacitance model1 and estimating the native SiO2 layer thickness to be 2.0 nm, values of 12 and 13.5 for HfO2 layers with thicknesses of 2.2 and 4.9 nm were obtained. Fig. 6 depicts the leakage current density vs. applied voltage curves for the above-mentioned capacitor structures. The leakage current density at 1 V was below 1 6 1025 A cm21 for the 4.9 nm thick film but was slightly increased for the thinner 2.2 nm film. The breakdown voltage exceeded 2 V in both cases.

Conclusions Stoichiometric HfO2 films were deposited onto Si(100) substrates by ALD from Cp2Hf(CH3)2 and water as oxygen source in the temperature range of 300–500 uC. The self˚ cycle21 limiting ALD growth mode with growth rate of 0.42 A

Fig. 4 AFM images of 100 nm thick HfO2 films deposited at 350 uC (a) and 400 uC (b). The rms roughness values were 4.3 (a) and 2.9 nm (b). Image size: 2 6 2 mm2. Depth scale: 50 nm from black to white.

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Fig. 5 Capacitance–voltage curves of Al/HfO2/native SiO2/p-Si capacitor structures with HfO2 film grown at 350 uC. Labels indicate the HfO2 thickness. Native SiO2 thickness was 1.8 ¡ 0.3 nm in both structures.


Fig. 6 Leakage current density–applied voltage curves for Al/HfO2/ native SiO2/p-Si structures with HfO2 film grown at 350 uC. Labels indicate the HfO2 thickness.

was confirmed at 350 uC yielding uniform films of the monoclinic phase with (2111) as the preferred orientation and very low impurity contents of 0.4 and 0.2 atom% hydrogen and carbon, respectively. The impurity levels remained low also in the films deposited at 400 uC but films deposited at 300 uC contained 2.2 and 1.5 atom% C and H, respectively. The insufficient reactivity of the precursor at 300 uC resulted also in a rather low growth rate. Preliminary studies on the dielectric properties revealed a promising CET value of 2.7 nm obtained from a 2.2 nm HfO2 layer on native oxide covered silicon. This novel ALD precursor system can thus be considered as a promising alternative for producing highquality HfO2 films to replace the hafnium halide, alkoxide or alkyl amide processes.

Acknowledgements The authors thank Prof. P. Hautoja¨rvi, Laboratory of Physics, Helsinki University of Technology, for providing facilities for the AFM measurements. Jaakko Niinisto¨,*a Matti Putkonen,a Lauri Niinisto¨,a Sarah L. Stoll,b Kaupo Kukli,{c Timo Sajavaara,{d Mikko Ritalac and Markku Leskela¨c a Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P. O. Box 6100, FIN-02015 Espoo, Finland. E-mail: [email protected] b Department of Chemistry, Georgetown University, 37th and O Streets, N. W., 20057 Washington DC, USA c Department of Chemistry, University of Helsinki, P. O. Box 55, FIN-00014 University of Helsinki, Finland d Accelerator Laboratory, University of Helsinki, P. O. Box 43, FIN-00014 University of Helsinki, Finland

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