Effects of post-metallization annealing of high-k dielectric thin films grown by MOMBE

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1 Microelectronic Engineering 77 (2005) Effects of post-metallization annealing of high-k dielectric thin films grown by MOMBE Minseong Yun a, Myoung-Seok Kim a, Young-Don Ko a, Tae-Hyoung Moon b, Jang-Hyuk Hong b, Jae-Min Myoung b, Ilgu Yun a, * a Department of Electrical and Electronic Engineering, CITY-Center for Information Technology of Yonsei University, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul , Korea b Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul , Korea Received 19 January 2004; received in revised form 9 April 2004; accepted 25 August 2004 Available online 5 October 2004 Abstract In this paper, the effect of post-metallization annealing (PMA) of high-k (HfO 2 ) thin films grown by MOMBE method was investigated. Au/HfO 2 /p-si MOS capacitor structures were fabricated. In turn, the current voltage (I V) and high frequency (HF) capacitance voltage (C V) characteristics were measured to analyze the electrical characteristics of dielectric layers. As the result of PMA at 250 C, it was found that the interface state density decreased after PMA. It was also observed that as the annealing time increased, the leakage current at operating voltage increased while the breakdown field was reduced. Through the experiments, it is found that PMA is a critical factor in determining the quality of gate dielectric layer. Ó 2004 Elsevier B.V. All rights reserved. Keywords: High-K dielectric; HfO 2 ; MOMBE; Post-metallization annealing 1. Introduction Recently, the demand of high performance semiconductor devices leads to researches about high speed and highly integrated fabrication of devices. * Corresponding author. Tel.: ; fax: address: iyun@yonsei.ac.kr (I. Yun). In general, reduction of the thickness of gate oxide is a pivotal factor in CMOS scaling. However, reduction of the thickness results in the exponential increase in leakage current. To overcome this problem, high-k dielectric constant material is required to replace the conventional SiO 2 [1]. Among the candidates predicted to be thermodynamically stable in contact with Si, HfO 2 have risen as promising insulator materials because of /$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi: /j.mee

2 M. Yun et al. / Microelectronic Engineering 77 (2005) high dielectric constant, large band-gap energy and high breakdown field. The deposition method is also an important factor to determine the property of gate dielectric layer. In our experiments, we pursued the appropriate experimental condition and characteristics of HfO 2 films using the metalorganic molecular beam epitaxy (MOMBE) system. MOMBE is one of the powerful techniques obtaining abrupt interface and controlled thickness of films, mainly due to source evaporation at a controlled rate under ultra high vacuum condition [2]. It was previously reported that HfO 2 /silicon oxide interface has a high interface state density for their high oxygen diffusivity [3]. However, thermal annealing can help to reduce the interface state density [4]. This paper focuses on the effect of post-metallization annealing (PMA) on the electrical properties of the HfO 2 gate dielectric films grown by MOMBE. The high-frequency (HF) capacitance voltage (C V) measurements, current voltage (I V) measurements, X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) were used to analyze the properties of HfO 2 films and annealing effect. 2. Experiments HfO 2 thin film was grown on a p-type Si (100) substrate, of which the native oxide was chemically eliminated by (50:1) H 2 O:HF solution prior to growth by MOMBE. Hafnium-tetra-butoxide [Hf(O Æ t-c 4 H 9 ) 4 ] was chosen as the MO precursor because it has an appropriate vapor pressure and relatively low decomposition temperature. Highpurity (99.999%) oxygen gas was used as the oxidant. Hf-t-butoxide was introduced into the main chamber using Ar as a carrier gas through a bubbling cylinder. The bubbler was maintained at a constant temperature to supply the constant vapor pressure of Hf-source. The apparatus of the system is schematically shown in Fig. 1. High-purity Ar Fig. 1. Schematic illustration of MOMBE system.

3 50 M. Yun et al. / Microelectronic Engineering 77 (2005) carrier gas passed through the bubbler containing the Hf-source. The gas line from the bubbler to the nozzle was heated to the same temperature. The mixture of Ar and metal-organic gases heated at the tip of the nozzle flows into the main chamber. The introduced Hf-source decomposed into Hf and ligand parts when it reached a substrate maintained at high temperature and the Hf ion was combined with O 2 gas supplied from another nozzle. The base pressure and working pressure were 10 9 and 10 7 Torr, respectively. The HfO 2 films grown by MOMBE were annealed at 700 C for 2 min. in N 2 ambient. Detailed experimental conditions are listed in Table 1. After metal deposition was performed, the post-metallization annealing (PMA) at 250 C was carried out in O 2 ambient. In turn, I V and HF (1 MHz) C V measurements were executed to analyze the electrical characteristics of dielectric layers. To carry out the I V characteristics, Keithley 236 source measure unit (SMU) was used. The C V measurements were also performed with the Keithley 590 C V analyzer at room temperature. Table 1 Growth condition of HfO 2 films grown by MOMBE Process variables Range Substrate temperature ( C) 450, 550 Nozzle temperature ( C) 270 Bubbler temperature ( C) 120 Base pressure (Torr) 10 9 Working pressure (Torr) 10 7 Growth time (min) 30 Ar gas flow (sccm) 3 O 2 gas flow (sccm) 3, 6 PMA temperature ( C) Results and discussion The metal oxide semiconductor (MOS) capacitor structures Au/HfO 2 /p-si were fabricated to measure their electrical properties. The high frequency (1 MHz) C V curve and I V curve of HfO 2 films grown by MOMBE are shown in Fig. 2. From measured capacitance and physical thickness, the dielectric constant and the equivalent oxide thickness can be obtained. Actually, total capacitance must be calculated as series connection of HfO 2 layer capacitance and SiO 2 layer capacitance because SiO 2 layer between dielectric layers and p-si shown in Fig. 3 affects the total capacitance strongly [5]. The thickness of SiO 2 layers was measured by HRTEM and had a value of Å. A large portion of the HfO 2 film was crystallized and some of the grain boundaries were observed. Ignoring the depletion region effect, the dielectric constant of HfO 2 (19 21) can be calculated. This relative low dielectric constant compared with the bulk HfO 2 dielectric constant (k = 25 30) may be due to the polycrystalline nature of the grown HfO 2 films. Leakage current densities of HfO 2 films at 1.5 V are approximately 10 9 A/cm 2. Leakage current density of our samples is a relatively low level compared with that of the conventional SiO 2 and other high-k materials. The I V curves of the HfO 2 film before and after PMA are shown in Fig. 4. It was observed that as the annealing time increased, leakage current at the same voltage was increased. It can be explained by the creation of new leaky paths in HfO 2 films during PMA process resulting from Capacitance (pf) Cacc positive flatband voltage shift T ox =30nm stretched -out Gate voltage (V) Current density (A/cm 2 ) 1E-5 1E-6 1E-7 1E-8 1E-9 T ox =30nm Gate voltage (V) Fig. 2. HF (1 MHz) C V curve and I V curve of HfO 2 film of 30 nm grown by MOMBE.

4 M. Yun et al. / Microelectronic Engineering 77 (2005) Fig. 3. HRTEM image of HfO 2 film grown by MOMBE. 1E-7 Current density (A/cm ) 2 1E-8 1E-9 1E-10 before PMA 5hour PMA 17hour PMA 34hour PMA 1E Gate Voltage (V) Fig. 4. Leakage current density of HfO 2 film before and after PMA process. Fig. 5. XRD spectra of HfO 2 film before and after PMA process. HfO 2 films can attribute to the creation of new leaky paths. The oxide breakdown of the dielectric layer, was also examined by I V measurements. As shown in Fig. 6, the breakdown is indicated by the sudden increase of the gate leakage currents the excess thermal energy. To examine the above assumption, XRD spectra of HfO 2 films before and after PMA are shown in Fig. 5. HfO 2 films have been found to exist in monoclinic phase, tetragonal phase, cubic phase, and amorphous structure. This crystal structure depends on the growth method and experimental condition of HfO 2 films. It was found that HfO 2 film grown by MOMBE in our experiments had a monoclinic (m) dominant phase. Through XRD spectra, it was identified that PMA process enhanced the degree of crystallization and created new crystal direction. These changes of crystal structure in Current density (A/cm 2 ) Temp. of substrate=450 o C 1.0x10-4 O 2 /Ar= x x x x Gate voltage (V) before 17h PMA 34h PMA Fig. 6. Breakdown characteristics of HfO 2 film before and after PMA process.

5 52 M. Yun et al. / Microelectronic Engineering 77 (2005) Capacitance (pf) before PMA 1hour PMA 2hour PMA 3hour PMA 4hour PMA V FB Temp. of substrate=550 o C O 2 /Ar= Gate Voltage (V) Fig. 7. HF C V characteristic of HfO 2 film before and after PMA process. in the positive voltage region. It was found that increase of the annealing time causes reduction of dielectric breakdown field. Fig. 7 shows the HF C V characteristics of HfO 2 film before and after annealing treatment. It was observed that the capacitance in accumulation region varied with PMA time. Variation of accumulation capacitance can be interpreted as the change of dielectric constant [6], the creation of Hf-silicate layer, or the variation in film thickness [7]. However, in our PMA experiment the temperature is fixed at 250 C, which is relatively low temperature to change the dielectric constant of a bulk HfO 2. Therefore, it can be assumed that the change in accumulation capacitance indicates the reduction of the oxide layer thickness and the creation of Hf-silicate layer. As shown in other previous work [8], the annealing process may give rise to the reduction of film thickness, which is identified by TEM. It shows that thermal energy in the annealing process enhances the crystalline nature of the films and makes the films denser. In addition, as indicated in [9], the variation in accumulation capacitance can be due to the formation of Hf-silicate (HfSi X O Y ), which has relatively low dielectric constant compared with HfO 2 films. A positively directed flat-band voltage shift (DV FB ) of 3.5 V was also observed in this figure. Although there are several possible reasons for a flat-band voltage shift, DV FB is generally interpreted as oxide charges within the film [2,4]. Therefore, PMA treatment made it possible to reduce oxide charges (Q ox ) in the oxide layer. Using the HF C V data, interface state density (D it ), which attribute to the interface trap density, can be calculated at flat-band voltage (flat-band condition) from LehovecÕs method [10], which gives D it ¼ ðc 0 C FB ÞC FB C 2 0 ; 3ðdC=dV Þ FB qkta ðc 0 C FB ÞAq 2 ð1þ where C 0 is the capacitance of accumulation region, C FB is the flat-band capacitance, A is the area of electrode, k is the BoltzmannÕs constant, T is the absolute temperature, q is the electronic charge, and (dc/dv) FB is the slope at flat-band. Also, C FB is calculated by the following formula [11], ½ðC o C FB Þ=ðC o C 1 ÞŠ ¼ ½C o =ðc o þ 6C 1 ÞŠ; ð2þ where C 1 is the capacitance of inversion region. The interface state density of HfO 2 dielectric layer was plotted in Fig. 8 as a function of the annealing time. Before annealing treatment, D it is approximately and ev 1 cm 2 with experimental condition, respectively. The decrease of interface state density was observed with increasing the annealing time, which implies that the quality of the interfacial layer of HfO 2 seems to be improved by the annealing. It is attributed to decrease of the number of oxygen vacancies as the annealing time is increasing. Since oxygen vacancies often act as shallow donors in oxide layer, annealing in O 2 ambient removes oxygen vacancies in the layer [8]. Fig. 9 shows the surface morphology of HfO 2 film before and after the PMA process. As surface roughness might be an integration issue for various device fabrications, it is important that one could control the surface roughness and obtain smooth HfO 2 films. HfO 2 film grown by MOMBE had r.m.s. roughness of 38.5 Å before PMA process. However, after 4 h PMA, r.m.s. roughness of HfO 2 film was 8.1 Å. Through this result, it was identified that surface morphology of HfO 2 film could be improved by PMA process.

6 M. Yun et al. / Microelectronic Engineering 77 (2005) Dit (ev -1 cm -2 ) C PMA Temp. of substrate = 450 C O 2 /Ar = Annealing time (h) C PMA Temp. of substrate = 550 C O 2 /Ar = Annealing time (h) Fig. 8. Interface state density versus the PMA process time of HfO 2 film. Fig. 9. Surface morphology of HfO 2 film before and after PMA process. 4. Conclusions Post-metallization annealing (PMA) effects of HfO 2 films grown by MOMBE have been investigated. The leakage current and the interface state density were analyzed from our experiments using the HF C V and I V measurements. It is found that the reduction of interface state density was obtained with the annealing treatment. It was also observed that oxide charges were reduced in HfO 2 film and surface morphology was improved as the annealing time increased. However, the increment of the gate leakage current and reduction of breakdown field were observed. Therefore, it is concluded that PMA can be impacted on the characteristics of high-k dielectrics and the optimization of the annealing process can be crucial for determining the quality of the high-k dielectrics. Acknowledgement This work was supported by the Brain Korea 21 Project in References [1] H. Iwai, S. Ohmi, Fourth IEEE Int. Conf. Device, Circuits Syst. (2002) D [2] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy : Theory and Practice, second ed., Academic Press, San Diego, USA, [3] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) [4] D. Han, J. Kang, C. Lin, R. Han, Microelectron. Eng. 66 (2003) 643. [5] E.H. Nicollian, J.R. Brews, MOS (metal oxide semiconductor) Physics and Technology, Wiley/Interscience, New York, 1981.

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