Si by the plasma oxidation of sputtered metallic Hf thin films

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Journal of Crystal Growth 268 (2004) 155–162

The structural and interfacial properties of HfO2/Si by the plasma oxidation of sputtered metallic Hf thin films G. Hea,*, Q. Fanga,b, M. Liua, L.Q. Zhua, L.D. Zhanga a

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China b Electronic and electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 31 March 2004; accepted 1 May 2004

Communicated by D.P. Norton

Abstract The thermal stability and interfacial structure characteristics for HfO2 dielectrics formed on Si substrates by plasma oxidation of sputtered metallic Hf films were investigated. The sputtered Hf metallic films were annealed under oxygen and nitrogen atmospheres. The structural characteristics and surface morphology of the Hf/HfO2 layers at varies of postannealing temperatures (from 600
C to 900
C) were examined by X-ray diffraction and scanning electron microscopy. The structure of the formed HfO2 films undergo a transformation of tetragonal to monoclinic phase with increase of the annealing temperature and demonstrated a polycrystalline structure at high temperature annealing. The growth and properties of the interfacial SiO2 layers formed at the HfO2/Si interface were observed by using fourier transform infrared spectroscopy. It has been found that formation of the interfacial layer depends on the postdeposition annealing temperature. N2-annealed HfO2 films will lead to the decomposition of interfacial SiO2 layer and bring about the slight shift of Si–O–Si bonds vibration peak position toward lower wave numbers. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Interface; A1. Surface structure; A2. Natural crystal growth; B1. Oxides; B2. Dielectric materials; B3. Field effect transistors

1. Introduction Continued scaling of device dimensions has led to greater emphasis on such issues, and indeed fundamental limits imposed by gate leakage and *Corresponding author. Key laboratory of Materials Physics, Institute of Solid State Physics, P.O. Box 1129, Hefei 230031, China. Tel.: +86-551-5591476; fax: +86-551-5591434. E-mail address: [email protected] (G. He).

intrinsic reliability are expected to prevent reduction of the thickness of the SiO2-based gate dielectric in MOS below B1.2 nm. The trend in reducing lateral dimensions of devices brings a reduction of the capacitance of the involved MOS structures. Therefore, to keep device areas small and prevent leakage current while maintaining the same gate capacitance equivalent to a thinner (B1.0 nm and below) SiO2 layer, a physically thicker film and a material with a significantly

0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.038

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higher dielectric constant (high-k) is required. By far Y2O3, Ta2O5, Al2O3, TiO2 and SrTiO3 (STO) have received considerable attention as the gate oxide thickness of MOS devices are being scaled down [1–5]. Moderate high-k materials such as Al2O3, has limited effective oxide thickness (Teff ) shrinking beyond a sub-0.1 mm gate dielectric due to medium permittivity of Ko10 [6]. Meanwhile, many of the high-k materials are not thermally stable on silicon. The formation of SiO2 or metal silicates often occurs when there materials are grown on silicon or during subsequent annealing and ultrahigh-k materials such as STO have been predicted to cause poor channel effects because of the fringing field-induced-barrier lowering effect produced due to its relatively high dielectric constant [7]. Recently, HfO2 and ZrO2 have attracted great attention as a replacement for the nitrided-SiO2 gate oxide film, since they are thermodynamically stable in contact with Si. Among them, HfO2 has been recently highlighted [8–10] due to its reasonably high dielectric constant (B25), a relatively large band gap (5.68 eV) [11], high heat of formation (271 kcal/mol) and good thermal stability on Si against reactions with the formation of SiO2-like interface. In addition, excellent metaloxide-semiconductor capacitors with HfO2 have been demonstrated [12]. Therefore, it seem that HfO2 will be a very promising candidates gate dielectric materials. High-quality HfO2 thin films have been deposited by PVD [13,14], CVD [15], Metal-organic chemical-vapor-depositon (MOCVD) [16] and atomic-layer deposition [17]. Among these deposition techniques, one of the serious problems is the interfacial layer growth due to the oxidization of the Si substrate surface, which is brought about in excess oxygen ambient at elevated temperature. CVD processes are also suffering from contaminants that need to be removed by a high temperature deposition or a post deposition annealing. PVD has advantages of simple process, high purity and low cost-of-ownership. In this paper, we report on a plasma oxidation of sputtered metallic hafnium films to form the stoichiometric HfO2 at temperature as low as

400
C and focus on the effect of the postdeposition annealing on the properties of as-grown films on silicon substrate. The properties of structural and interfacial of high-k HfO2 films prepared are demonstrated.

2. Experimental details The p-type Si (1 0 0) substrates (cut from one piece of as-purchased Si wafer used in chip manufacturing. This is to ensure that each substrate has an identical native oxide layer, surface morphology, and crystalline orientation.) with a resistivity of 2–5 O cm were pre-cleaned by a standard clean process (NH4OH:H2O2:H2O, 2:1:7) at 60
C for 15 min. This cleaning process removes organic matters and other impurity ions adhered to the surface of the substrates, but typically results in a chemical oxide of 0.5 nm thick on the substrate Si prior to Hf metal deposition. Then, the Si surfaces were prepared by etching in diluted HF (1%) solutions to remove native oxide and the silicon dangling bonds passivated with hydrogen atoms. The cleaned substrates were put into the deionized water to remove the impurity ion introduced in the course of etching. Finally the substrates were dried and put into the deposition chamber. A hafnium metal layer was pre-deposited by a DC magnetron sputtering in Ar ambient at room temperature. The sputtering power density for hafnium was 3 W/cm2 and the sputtering target used in this experiment was a high-pure hafnium metal (99.99%). After the Hf metal deposition, the wafers were transferred to a plasma oxidation chamber and the Hf metal layer was oxidized by an Ar/O2 plasma generated in chamber. In order to improve the qualities of the resultant HfO2 films, additional post oxidation annealing was performed at 500–900
C for 5 min in O2 or N2 ambient. The main process flow is briefly listed in Fig. 1. Microstructures of the films were characterized by X-ray diffraction (XRD, Philips, X’Pert-PRO system) using CuKa radiation at 40 kV and 40 mA. The surface and cross-sectional morphology of HfO2 films were observed by using a field–emission scanning electron microscopy (FE-SEM) (JEOL

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standard

cleaning

m

P (100) Si substrate

Plasma oxidation

m

HfO2 high-k layer P (100) Si substrate

Intensity(a.u)

Hf deposition

RTA in O2

m

(HF dip) Hf metal layer P (100) Si substrate

157

m

m (e)900oC

m

m

t

m

(d)800oC

m t (c)600oC

t t

t

(Annealing)

(b)500oC (a) as-deposited

Measurement

Fig. 1. The main experimental process flow.

JSM-6700F). Analysis of the interfacial layer between the HfO2 films and Si substrate was carried out on an as-deposited control and asannealed samples by fourier transform infrared spectroscopy (FTIR) in a Nicolet Magna 750 operating in transmission mode.

3. Results and discussion Fig. 2 shows a typical series of XRD patterns of the as-grown and annealed HfO2 films at various temperatures. In Fig. 2(a), the as-grown HfO2 films show a featureless diffraction and demonstrate an apparent amorphous structure. Spectra (b) in Fig. 2 show only weak diffraction peaks attributed to tetragonal phase of HfO2 films; no diffraction peaks of Si substrate have been found. This result is in agreement with the report from Neumayer, which demonstrated the crystallization of HfO2 was only observed after annealing at 500–1200
C [18]. The spectrum (c) shows some other weak diffraction peaks, which are identified from monoclinic phase of HfO2 in addition to the remanent weak peak characteristic of tetragonal HfO2, this case indicated that a partial transition from the tetragonal phase of HfO2 to the monoclinic phase of HfO2 with increasing thermal budget. A obvious crystalline structure mixed with monoclinic and tetragonal is shown in Fig. 2(d). A

20

30

40

50 60 2 Theta(degree)

70

80

Fig. 2. XRD pattern of as-grown and annealed HfO2 film at various temperatures for 5 min in O2 ambient. The letters m and t represent the monoclinic and tetragonal structure of the HfO2 film, respectively.

completely polycrystalline structure transformation of the tetragonal to the monoclinic phase at high temperature annealing over 900
C was demonstrated in Fig. 2(e). In addition, different orientations of crystallites in this film are observed, compared to HfO2 annealed at 800
C. This result clearly indicates that the structure of the HfO2 films is closely dependent on the annealing temperature; in other words, the annealing temperature plays an important role during the process of crystalline structure transformation. It has been reported that the crystallization of very thin HfO2 films depends not on the growth temperature, but also on the film thickness and the type of underlayer [19]. While Cho et al. also have confirmed that the structure stability of HfO2 depends on the film thickness and the annealing temperature [20]. In our case the defects formed at the film growth may provide nucleation sites for crystalline growth, which lead to the increase of the nucleation of other grown directions. In addition, the presence of metastable HfO2 polymorphy at lower annealing temperature may be related to other impurities, deposition rate or the kinetic growth regime of the film, the increase of the stress with the film thickness can contribute to the change in structure. Surface morphology is another important physical property which may affect the electrical

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Fig. 3. The SEM images of HfO2 films fabricated by (a) as-deposited ;(b) annealed at 600
C in O2; (c) annealed at 900
C in O2 ;(d) and (e) show the cross-sectional view of (b) and (c), respectively.

properties of dielectric thin films. Fig. 3 illustrates the microstructural evolution of the films at different annealing temperatures. The average grain size of the HfO2 film presented in Fig. 3(a) is approximately 10 nm after plasma oxidation of sputtered Hf thin film. Meanwhile, there are some voids present. The presence of the small voids within the film structure provide alternative but direct conducts for the access of oxygen molecules flowing inward from the ambient environment. With increasing annealing temperature, some small voids disappear and the grain

sizes of the HfO2 films enlarge, which are shown in Fig. 3(b) and (c). Based on the theory of forming nuclei by Chen and Mackeinize [21], the rate of forming nuclei and the growth rate of crystalline nuclei can be determined as followed: dN=dt ¼ N0 expð  DGN =RTÞ;

ð1Þ

U ¼ U0 expð  DEu=RTÞ;

ð2Þ

where N0 and U0 are constants; N represent the number of crystalline nuclei ; DGN is the Gibbs

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film and the Si substrate. In our work, we investigated SiO2 layers formed at the HfO2/Si interface by using FTIR, which provides additional information. Fig. 4 shows FTIR spectra of HfO2 annealed for 5 min in O2 ambient at various temperatures. In order to ensure that Si–O bonding is associated with the interfacial layer, the infrared absorption spectra were obtained with the backside of all wafers etched in diluted HF solution to remove any native oxide and subtract the absorbance of the sample and a reference from the same wafer with no HfO2 present. As a result, we detected signals from the interfacial SiO2 films. Fig. 4 likely shows that the peak height of Si–O stretching mode near 1050 cm1 due to the interfacial SiO2 increases with the annealing temperature, suggesting the formation of the interfacial Si-oxide layer associates with the annealing temperature. The peak of pure Si–O stretching mode near at 1075 cm1 shift to 1050 cm1 implies the possibility of formation of silicates. It has been generally believed that the interfacial SiO2 is formed by oxidation of Si substrates by oxygen in the annealing atmosphere [22]. Cracium et al. has demonstrated that the growth of the interfacial layer during deposition was connected to the amount of physisorbed oxygen present in film. Meanwhile, their groups have confirmed that the amount of oxygen that is available in the

Streching Absorption(a.u)

free energy change; DEu is the activation energy of the growth of crystalline nuclei. For the samples prepared by plasma oxidation of sputtered Hf thin film, thermodynamically, heat energy supplied by outside is too low to satisfy the change of Gibbs free energy, which gives rise to the production of amorphous structure. With the increasing temperature, many factors contribute to the growth of crystalline nuclei, including the voids, oxygen defects, and atom vacancies in the films. The defects formed at the growth may provide nucleation sites for crystalline growth. At low annealing temperature, some defects can only centralize in some regions, which become preferential of crystalline nuclei to separate out. At high annealing temperature, the rate of forming nuclei will follow (2) and the growth rate of crystalline nuclei increases, which brings about the polycrystalline structure mixed with monoclinic and tetragonal phases. Fig. 3(d) and (e) show the cross-sectional views of the Fig. 3(b) and (c) HfO2 films, respectively. It is obvious that higher temperature annealing demonstrates more denser and less porous structure in the film. During the process of high temperature annealing, higher temperature provided energy of surface atoms to enhance the surface diffusion. When these atoms transfer to where there exists steps and defects and stop here, abundant atoms will focus together to form clusters and result in finer uniform grains and smoother surface, which will give rise to the production of polycrystalline structure ultimately. One of the serious problems is the formation of an interfacial SiO2-like layer due to the oxidation of the Si substrate surface in excess oxygen ambient at elevated temperatures. Since SiO2 has a lower dielectric constant, an underlying SiO2 layer can reduce the effective capacitance of the film. In addition, the amorphous SiO2 on silicon leaves dangling bonds that may results in electronic defects disrupting translational symmetry at the interface. The framework of first-principle calculations indicates that the formation of an interfacial layer cannot be completely avoided since the total capacitance of a multiplayer stack is dominated by the material with the lowest-k: It is desirable to minimize the thickness of any low-k SiO2 layer between the high-k gate dielectric

159

____

900oC

__ _

800oC

........

600oC as-deposited

1000

1100 Wave Numbers(cm-1)

Fig. 4. The FTIR absorption peak due to stretching mode of Si–O–Si bonds formed at the HfO2/Si interface after postannealing.

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physisorbed state is limited. The significant growth of the interface layer requires an additional oxygen source from the ambient diffusing through the growth sample, migrating to the interface, and reacting within the interfacial region [23]. But Haruhiko Ono held contrary standpoint that the SiO2 layer is formed not a diffusion of the oxygen from annealing atmosphere, but by a reaction between the high-k film and the Si substrate [22]. However, our work shows that the interfacial layer can be attributed to the process of plasma oxidation and postannealing. The Hf metal layer is oxidized from the surface and high-energy oxygen radicals (ions and molecule ions) generated in the process of plasma oxidation easily react with the Hf atoms to form stoichiometric HfO2. Meanwhile, partial oxygen ions with high-energy can penetrate the formed HfO2 layer into the Si substrate, and formed the SiO2 interfacial layer. In the oxygen annealing, on the other hand, oxygen molecules might also diffuse into the Si substrate without reacting with the residual metallic Hf, which lead to the increase in intensity of Si–O–Si bonds in the interfacial region. The FTIR data provide the evidence for the annealing-induced interaction of the Si substrate with oxygen from annealing ambient resulting in the production of increased numbers of Si–O–Si bonds. Fig. 5 shows the absorption intensity of the Si– O–Si bonds plotted against the annealing tem-

perature corresponding to Fig. 4 .The annealing temperatures vary from 500
C to 900
C for 5 min in O2 ambient. The peak intensity of Si–O–Si bonds in interfacial layer increases with the annealing temperature, which implies that the SiO2 layer increases. Fig. 2 above has confirmed that the as-deposited HfO2 films show amorphous structure and begin to crystallize over 500
C, but the interfacial layer SiO2 exists between HfO2/Si. It is likely that the formation of interfacial SiO2 layer depends only on the annealing temperature and not whether it is crystalline or amorphous. Fig. 6 shows Infrared absorption for the HfO2 thin films annealed at 600–900
C in N2 ambient for 5 min.The band at 1000-1200 cm1 observed in the as-annealed samples was due to the Si–O bond vibration mode in the interfacial layer. It is noted that there is a slight shift toward lower wavenumber with the increase of annealing temperature. Furthermore, the peak intensity also increases with the increase of annealing temperature. According to our result, we guess there are two mechanisms attributed to it: (1) Plasma oxidation at low temperature led to Hf-HfO2, but there exists some residual Hf component in the HfO2 films due to not full oxidation. The oxygen in status of plasma cannot diffuse to react with underlying substrate. So the interstitial oxygen (instead of

(a)

peak intensity(a.u)

Absorbance(a.u)

as-deposited

(as-deposited) 400

500 600 700 800 annealing temperature(oC)

(b) 600oC (c) 800oC

600

900

Fig. 5. Si–O–Si absorbance peak intensity vs annealing temperature for the samples HfO2 deposited on a Si substrate and annealed in O2 ambient for 5 min.

(d)

900oC

800 1000 Wave number(cm-1)

1200

Fig. 6. Annealing temperature dependence of absorption peak position of Si–O–Si bonds obtained by FTIR for HfO2 films in nitrogen. (a) as-deposited; (b) annealed HfO2 at 600
C; (c) annealed HfO2 at 800
C; (d) annealed HfO2 at 900
C.

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outside oxygen) enters Si substrate, which lead to the production of peaks at 1105 cm1. (2) Annealing under high-purity N2 ambient ensures an oxygen deficient condition, the inset oxygen releases and forms SiO2 at high temperature. Meanwhile, an oxygen deficient HfO2 film has a higher free energy than an oxygen stoichiometric film, which provides the driving force for the farther reaction. The free energy of the oxygen deficient film can be lowered by oxidation. The full oxidation of Hf oxide can be attribute to the incorporation of oxygen atoms from SiO2 layer formed in the plasma oxidation process of Hf metal layer, while the SiO2 is decomposed to SiOxo2. Therefore, we think the SiO2 layer on the silicon substrate can be an oxygen source for the oxygen deficient Hf oxide. This process can be described by the following chemical reaction: SiO2 þHf þ Ox ðinsetÞ ¼ HfO2 þSiOx :

161

(mixed with tetragonal and monoclinic phases) and result in smoother face and finer uniform grain. The present FTIR analyses have confirmed that the presence of the interfacial SiO2 layers and the growth of the interfacial layer for O2-annealed samples under high temperature annealing due to the oxygen diffusion mechanism from annealed atmosphere. N2-annealed samples have demonstrated the decomposition of SiO2 layer and attributed to the slight shift of Si–O–Si bonds vibration peak position toward lower wave numbers.

Acknowledgements We are grateful to National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No.19994506) and Hundred Talent Program from Chinese Academy of Science (Grant No.B20010404).

ð3Þ

Copel et al. reported HfO2/SiO2 structure is unstable during growth at limited oxygen pressure [24]. The some of the instability is not silicate formation, but rather SiO2 decomposition, probably due to migration of oxygen vacancies from the growth front to the interfacial SiO2. The decomposition of interfacial SiO2 will lead to the presence of Si suboxides near the Si/SiO2 interface and attribute to the shifting of Si–O bonding vibrational peak position. So that we can see peak shift into lower wavenumber.

4. Conclusions In summary, by subjecting HfO2 films prepared by the plasma oxidation of sputtered metallic Hf film on the substrate to an oxygen/nitrogen postdeposition anneal (PDA), the structural and interfacial properties in relation to annealing temperature were investigated. Based on the Xray diffraction and SEM analysis of the asdeposited and as-annealed HfO2 samples in O2/ N2 ambient, we have shown that higher temperature annealing will bring about the transformation of amorphous structure to crystalline structure

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