Results and Prospective of CVD Processing with Use of Complicated Elementorganic Compounds

Results and prospective of CVD processing with use of complicated elementorganic compounds in preparation of electronic materials

F.A. Kuznetsov, I.K. Igumenov, T.P. Smirnova, N.I. Fainer, N.V. Gelfond,

M.L. Kosinova, N.B. Morozova, Yu. M. Rumyantsev, L.V. Yakovkina

Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk, Russia

Abstract

Chemical vapour deposition were used to produced high k dielectric films of HfO2 films,low k dielectric films of SiCxNyand metal films of platinum group metals. IR and Raman spectroscopy, AES, XPS, ellipsometry, XRD using the synchrotron radiation, EDS, SEM, AFM, measurements of electrophysical, mechanical characteristics and optical properties were applied to study their physicochemical and functional properties.HfO2 filmswere grown on n-type Si(100) substrates using dipivaloilmethanate hafnium (IV) and cyclopentadienyl hafnium bisdiethylamide as precursors. It was found that the deposited HfO2 films react with the SiO2 layer to be synthesized on silicon substrate and an intermediate amorphous layer was formed. The amorphous layer is composed of hafnium silicate and nonreacted SiO2 layer. It was shown by thermodynamic analysis of the Si-SiO2-HfO2-Hfsystem that a composition Si/HfO2-у is thermodynamically stable into narrow interval of oxygen pressure. As the oxygen partial pressure was increased (which is equivalent to presence of SiO2 layer) the composition Si/HfSiO4/HfO2-уis equilibrium.

SiCxNyfilms were synthesized using siliconorganic compound as single-source precursor. It was shown that low temperature films are low-k dielectrics with the following characteristics:a dielectric constant of 3.0 – 7.0, specific resistance,  = 1013-1016 Omcm, Edielectric breakdown~ 1 MV/cm, surface state density Nss ~ 2.4·1011 cm-2·eV-1and fixed charge density of about 1.6 x 1011 cm-2. The bandgap of the films changes from 5.35 up to ~ 3.30 eV. Microhardness of these films changes from 1.9 up to 2.4GPa, and Young's modulus changes from 12.2up to15.9GPa.

Pt, Ir,Rh, and Pd thick coatings were deposited and this paper exposes the conclusions of our experience in order to give explanation of growth process of platinum group metals structures.

Introduction

Silicon electronics as a result of process of miniaturization has passed by present time in the category of nanotechnologies. Last years leading world electronic companies have started mass production of integrated schemes with topological norms of 65 and 45 nanometers.

Transition to the nanosizes demands solving of a number of problems:

- Universally used dielectric, silicon dioxide-SiO2, has to be replaced by at least two other dielectrics: “high-k” dielectric at the gate and “low-k dielectric” used as insulator at interconnections.

- Aluminum and doped poly silicon have to be replaced by materials with higher conductivity

- Processes used for producing different fragments of the nanoscale structure should provide a possibility to synthesize highly uniform materials with precise dimensions.

The paper presents result of investigation of processes synthesis dielectrics with value of dielectric constant higher and lower then this parameter in SiO2.

HighKDielectricFilms

The reduction of the SiO2 layer thickness below 2 nm for future generation of complementary metal oxide semiconductor (CMOS) devices may not be feasible, due to increase of leakage current by direct tunnelling [1,2]. A number of high-k dielectrics have been proposed for reduction of the oxide equivalent thickness below 2 nm. Among possible candidates HfO2 is considered to be a preferable material [3,4]. Previous reports reveal the existence of intermediate, layer between HfO2 and Si substrate that is composed of silicon oxide, hafnium silicate or silicide. On the one hand, these products from the interfacial reactions suppress the effective dielectric constant. On the other hand, the presence of the defect states in this layer is a ground of charge trapping at the interface of HfO2/Si structure. In this reason a careful characterization of the intermediate layer microstructure and chemistry is necessary.

The HfO2 films obtained with the physical deposition methods and CVD (ALD) method from HfCl4 as precursor are the most extensively studied [5-11]. The HfO2/Si structures obtained from organometallic precursors are less examined.

A goal of this work is to characterize HfO2 films grown on the silicon substrate by CVD method using dipivaloilmethanate hafnium (IV) - Hf(dpm)4. In addition, structure of the films prepared from cyclopentadienyl hafnium bisdiethylamide - (C5H5)2Hf(N(C2H5)2)2 was studied with a view to establish the influence of the precursors design on the film structure.

Experiment

HfO2 films were grown by CVD process with use of (C5H5)2Hf(N(C2H5)2)2 and Hf(dpm)4 as precursors and argon as a carrier gas. The experimental setup and techniques have been described in detail elsewhere [15]. The films were deposited on n-type Si(100) substrates. The Si substrates were precleaned by a sequence of chemical cleaning in CCL4, and acetone. The etching was carried out in the H2SO4+HNO3 = 1:1 mixture, and for final etching the diluted HF (50%) solution was used. The cleaning procedure resulted in removal of the native oxide and surface contaminations. Before the film deposition from Hf(dpm)4, the substrates were oxidized at T = 1173 K in oxygen ambient. As a result, two type of structures, like HfO2/SiO2/Si, with different thickness of the SiO2 layer (d = 20 nm in the sample A; d = 5 nm in the sample B) were formed.

The microstructure and surface morphology of the films were examined by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). XRD patterns of the films were obtained using a DRON-SEIFERT-RM4 diffractometer (Cu Kα radiation) with a graphite monochromator over a 2θ range from 5 to 60° at step intervals of 0.1°. The phases existing in the films were identified based on the ICDD database and original XRD data [16,17]. The crystallinity and structure of HfO2/Si composition were investigated by transmission electron microscopy with a JEOL JEM-2010 operated at 200 kV. Specimens for the TEM cross-sectional examination were prepared by the FIB technique (FEI FIB-200) [18,19]. The sample surface was coated with carbon to avoid the HfO2 layer from damage during the first stage of FIB preparation. The surface morphology of the HfO2 films was examined using a JEOL JSM-6700F scanning electron microscope without any coating for charge reduction. The refractive index and thickness of the films were obtained from ellipsometry measurements. The later were carried out with a single wavelength LEF-3M ellipsometer equipped with a He–Ne laser (λ = 632.8 nm).

The film compositions were determined by X-ray photoelectron spectroscopy with use of VG ESCALAB HP electron spectrometer (Al Kα radiation, hν = 1486.6 eV). All XPS spectra were referenced to the substrate Si 2p binding energy at 99.3 eV. To extract the information on chemical states of the elements, the narrow regions of their core-level spectra have been analyzed, original XPS spectra being decomposed onto separate components. The latter procedure involved Shirley background subtraction and a curve fitting using the symmetric Doniach-Sunjic functions. The quantitative analysis was based on comparison of the XPS peak areas, corrected on the literature atomic sensitivity factors [20]. The depth profiling was carried out by Ar+ sputtering at 3 keV with an argon partial pressure of 3×10-5 mbar.

Results and discussion.

Structure and morphology. The surface morphology of the HfO2 films deposited from both precursors used was studied by scanning electron microscopy. In the both cases, the slightly rough, crack-free surface was observed. A representative plane-view SEM image of the HfO2 film is shown in Fig. 1.

Fig. 1. SEM image of the surface of HfO2 film deposited at Ts=873K from Hf(dpm)4.

One can see a granular surface structure, which indicates that the HfO2 films have a polycrystalline structure. The polycrystalline nature of the films was confirmed by the cross-sectional TEM data. Fig. 2 shows dark-field and bright-field cross-sectional TEM images of the FIB-thinned HfO2/SiO2/Si stack.

Fig. 2. Dark-field (a) and bright-field (b) cross-sectional TEM images of the HfO2 film deposed from Hf(dpm)4 at 873 K. The electron diffraction pattern of HfO2 layer is also shown as inset in the dark-field image

HfO2 film were deposed from Hf(dpm)4 at 873 K. The average size of the microcrystalline domains is about 10 nm. Electron diffraction pattern (see inset in Fig. 2a) with well-defined rings formed by discrete points can be observed, which also points to the polycrystalline nature of the HfO2 film. In the bright-field image, one can see that at the intermediate between the Si substrate and the HfO2 film, some amorphous interlayer is present. The contrast between the two layers indicates a distinct change in chemical and crystalline structure between the intermediate layer and bulk HfO2. It can be supposed that the film stack is composed of a polycrystalline HfO2 layer with a thickness of 70 nm and an amorphous intermediate layer (probably Hf1-xSixO2 or SiO2) with a thickness of ~6 nm.

Fig. 3a and 3b show dark-field cross-sectional TEM images of the HfO2 film before and after annealing in oxygen at 1073 K for 1 hour. The HfO2 film was grown at 623 K from (C5H5)2Hf(N(C2H5)2)2.

Fig. 3. Dark-field cross-sectional TEM images of the HfO2 film before (a) and after (b) annealing in oxygen at 1073 K for 1 hour. The film was deposited from (C5H5)2Hf(N(C2H5)2)2 at 623 K.

In both cases, the columnar HfO2 layer with a thickness of about 100 nm as well as thin amorphous interfacial layer is observed. The structure of the films deposited from the other nitrogen containing precursor, that is tetrakis-diethylamido-hafnium (Hf(Net2)4, was studied by TEM method in work of Yoshio Ohshita et al. [14]. They indicated that the films, as it coincide with our findings, show also a columnar structure which is polycrystalline. Therefore, a preliminary conclusion can be made that the formation of the columnar structure is typical for the films deposited from nitrogen containing precursors.

It can be concluded from comparison of Fig. 2 and 3 that the microstructure of HfO2 films depends on the type (chemical structure) of precursors which are used in the MOCVD process. It was shown by XRD analysis, that crystalline component of the filmshas the monoclinic structure. Fig. 4 shows XRD patterns of the HfO2 films deposited from Hf(dpm)4 and from (C5H5)2Hf(N(C2H5)2)2 before and after annealing.

Fig. 4. X-ray diffraction patterns of: 1 - monoclinic HfO2 phase (ICDD, card 34-104); 2 - HfO2 film deposed from Hf(dpm)4 at 873 K; 3 and 4 - HfO2 film deposed from (C5H5)2Hf(N(C2H5)2)2 at 623 K before and after annealing in oxygen at 1073 K for 1 hour, respectively.

The XRD pattern of bulk HfO2 with the monoclinic structure is also shown for comparison [16]. One can see that in the XRD data of the as-deposited films, wide peaks, which correspond to the monoclinic phase, are presented. Annealing at 1073 K notably reduces the width of these reflections, which may be interpreted as an evidence of structural ordering. These results are in agreement with a previous observation that HfO2 films crystallize during MOCVD deposition at temperatures above 800 K (monoclinic phase) independently of the precursors used [21,22].

Chemical composition. In order to determine the chemical composition of the HfO2 films, as well as the origin of the intermediate layers, the XPS depth profiling was performed. The survey XPS spectrum of the film before the Ar+ sputtering is pictured in figure 5.

Fig. 5. The survey XP spectrum of the film before the Ar+ sputtering

The area ratio between O 1s and Hf 4f peaks was found to be near to 2, indicating that the film composition corresponds to HfO2. Figures 6 and 7 show the Hf 4f, Si 2p, and O 1s core-level spectra of the 30-nm HfO2 films deposited from Hf(dpm)4 on SiO2-covered silicon substrates (samples A and B, respectively).

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Fig. 6. The Hf 4f (a), Si 2p (b) and O 1s (c) core-level spectra of the HfO2 films deposed on SiO2-covered Si substrates (samples A, d SiO2 = 20 nm): 1 – before, 2-6 – after Ar+ sputtering during 1, 4, 7, 10, and 20 min, respectively.

Fig. 7. The Hf 4f (a), Si 2p (b) and O 1s (c) core-level spectra of the HfO2 films deposed on SiO2-covered Si substrates (samples A, d SiO2 = 5 nm): 1 – before, 2-6 – after Ar+ sputtering during 1, 4, 7, 10, and 20 min, respectively.

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The spectra 2-6 were recorded as the films were sputtered with Ar+ ions for 1, 4, 7, 10 and 20 minutes respectively. As can be seen from figures 6a and 7a the Hf 4f spectra of the films before the Ar+ sputtering (lines 1) may be fitted with 4f7/2 - 4f5/2 doublet peak with the Hf 4f7/2 binding energy of 16.6 eV. This value is in good agreement with 16.3-17.1 eV [23- 25].In the O 1s spectra, two peaks can be resolved. The main peak at 529.9 eV is attributed to O2- lattice ions. Another peak at 531.5 eV can be attributed to defects like Oδ- or OH groups at the surface [23]. After Ar+ sputtering during 1 minute, an additional doublet with the Hf 4f7/2 binding energy of 15 eV is observed. Close binding energies were reported for hafnium suboxide [26]. Recently, Nieveen et al. have shown that the Ar+sputtering process can induce the changes of chemical state within the HfO2 films resulting in formation of hafnium suboxide [27]. The difference in the Si 2p spectra of the A and B samples was detected after 10 min of Ar+ sputtering (Fig. 6,7). Thus, in the Si 2p spectra of the sample A, three peaks at 99.3, 100.7 and 103.7 eV are observed (Fig. 6b). According toXPS data [4,6,28], we attributed these peaks to silicon, hafnium silicate, and silicon dioxide respectively. A doublet with the Hf 4f7/2 binding energy in a range 17.7-18.7 eV was ascribed to hafnium silicate(Fig. 6a). It should be noted that oxygen in HfSiO4 and in SiO2 is characterized by close values of the O 1s binding energy. Therefore, the wide O 1s peak at 532.7 eV can be attributed to both species. Finally, after Ar+ sputtering for 20 minutes, a full removal of the HfO2 films was occurred and a sharp Si 2p peak at 99.3 eV was observed in spectra 6 (Fig. 6b and 7b).

At the same time, in the Hf 4f spectra (Fig’s 6a and 7a), a weak doublet with the Hf 4f7/2 binding energy at 14.7 eV have been observed. This feature may be attributed to hafnium silicide [23]. As it follows from thermodynamic consideration (see next paragraph) the formation of silicides may occursat the oxygen deficiency. As the films synthesis have been performed in a system with the oxygen excess the silicide formation during the film synthesis is unlikely. On the other hand, Wilk et al.[28] mentioned the formation of the silicide bonding under e-beam excitation during of hafnium silicate sputtering. In turn, Fang et al. [23] observed Hf-Si bonds formation in the intermediate layer due to diffusion of Hf into Si substrate during the HfO2 film sputtering by Ar+ ions. Hence, occurrence in the Hf 4f spectra (Fig’s 6a and 7a) a weak doublet with the Hf 4f7/2 binding energy at 14.7 eV is a result of Ar+ ions interaction with hafnium silicate located in the intermediate layer.

In contrast, in the Si 2p spectra of the sample B, with thickness of the SiO2 layer equal to 5 nm a sharp peak at 99.3 eV dominates (Fig. 7b). Weak Si 2p features at 100.3 and 102.7 eV appeared in the spectrum after 7 minutes of the film sputtering with Ar+ ions. It means that the intermediate layer, composed of hafnium silicates and SiO2-x, has an insignificant thickness. Therefore, in the Hf 4f spectra recorded after the film sputtering for 7, 10 and 20 minutes, a doublet with the Hf 4f7/2 binding energy at 14.6 eV is clear resolved.

In summary of this part it should be emphasized that the use of the structures, like HfO2/SiO2/Si which was designed specifically, permits to show that the formation of the amorphous intermediate layer occurs during the films synthesis.

We have also performed a thermodynamical analysis of Hf-Si-O system to understand what phases can be formed at the HfO2-Si interface. Details of the analysis will be published separately. As it follows from the thermodynamic consideration the chemical composition of the intermediate layer depends on the partial pressure of oxygen and temperature. The hafnium silicate has to be appeared in the Si/HfO2-y composition as a stable component at large oxygen pressure. A mixture of Si and stoichiometric HfO2 is unstable in this case. An existence of thin SiO2 film on the silicon surface is interchangeably with the local rise of oxygen pressure. At small oxygen pressure the inclusions of silicides have to be present in the Si/HfO2-y composition.

Low K Dielectric Films

Among various candidates for low-k materials with a dielectric constant of 2.0-3.0, Si–C–N films are very promising because of their low dielectric constant and high hardness, and other excellent functional properties, such as superplasticity, a high strength, enhanced oxidation, corrosion resistance and Cu diffusion protection.

Currently, silicon carbonitride films have been produced with ion sputtering deposition of carbon and silicon in nitrogen atmosphere, N+ implantation into SiC surface, laser vapor phase reaction of hexamethyldisilazane (HMDS) Si2NH(CH3)6 with ammonia, chemical vapor deposition (CVD) and plasma enhanced CVD using Si(CH3)4-NH3-H2, SiH4-NH3(N2)-CH4 (or N2H4)-H2(Ar), SiCl4+NH3+C3H8+H2, as initial atmospheres [39-42]. HMDS is the most important single-source precursor for preparation low-k dielectrics on base of SiCxNy due to the molecules of HMDS contain ready fragments with less polarizable bonds such as Si-C, C-C, Si-CH3, C-H.

The goal of our research is to develop low temperature method of obtaining of SiCxNy films with low permittivity, to study their physicochemical, and electrophysical properties, and to determine the relationship between properties, chemical composition and chemical bonding.

Experimental

Thelow temperature synthesis of SiCxNyfilms with the wide interval of x and y was carried out by remote plasma enhanced decomposition of HMDSas single-source precursor using gaseous mixtures:(HMDS+He), (HMDS+N2)in the temperature range of 373-623 K, in r. f. power range of 15 - 40 Watt and total pressure in reactor of (4-6)10-2 Torr [43-44]. Wafers of Si(100) and fused silica were used as substrates. The effect of the growth temperature, chemical composition of the initial gas phase, r.f. plasma power, total pressure in the reactor on the certain electrophysical characteristics, microstructure, chemical and phase composition of the films was studied. The influence of chemical composition on the physical and chemical properties of the silicon carbonitride films was investigated using a complex of the following modern methods: FTIR and Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), microhardness and Young modulus determination by nano-indenter, electrophysical (I-V and C-V) measurements, optical measurements, ellipsometry. X-ray diffraction using synchrotron radiation (XRD – SR) measurements were carried out at the station «Anomalous scattering» of the SiberianCenter of Synchrotron Radiation (BINP SB RAS).

Thickness and refractive index of the all films were measured by means ellipsometer model LEF-3 at the wavelength of 632.8 nm. The measurements were carried out at seven angles. FTIR absorption spectra of the films were recorded in a transmission mode on FTIR SCIMITAR FTS 2000 spectrometer with range 300 - 4000 cm-1. 32 scans and the aperture equal 4 at achievable resolution 2 cm-1 were used during the measurements. Raman measurements were carried out by PHILIPS PU – 95 and Triplemate, Spex spectrophotometer.