Low-temperature crystallization of tantalum pentoxide films under elevated pressure

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Low-temperature crystallization of tantalum

pentoxide films under elevated pressure

Chung-Hsin Lu

∗

, Chung-Han Wu

Electronic and Electro-optical Ceramics Laboratory, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan ROC

Received 27 February 2005; received in revised form 28 June 2005; accepted 3 July 2005 Available online 1 September 2005

Abstract

Ta2O5thin films deposited via a metal-organic decomposition method were crystallized via atmospheric pressure annealing and a high-pressure

crystallization (HPC) process. Ta2O5thin films started to become crystallized at 700◦C as subjected to atmospheric pressure annealing. When

the HPC process was adopted and annealing at 16.5 MPa was performed, the crystallization temperature of Ta2O5films was greatly dropped

to as low as 350◦C. The developed HPC process considerably reduced the thermal budget and energy consumption during film processing. The crystallized Ta2O5 phase was found to be homogeneously distributed within the HPC-derived films. With annealing at 700◦C under

atmospheric pressure, the silicon species diffused from the substrates into the Ta2O5layers, thereby leading to reduced dielectric constants.

The HPC process effectively suppressed the interdiffusion between the substrates and dielectric layers by lowering the required heating temperature, and also significantly increased the dielectric constants of Ta2O5thin films. The HPC process was confirmed to effectively lower

the crystallization temperature and improve the dielectric properties of Ta2O5thin films.

Keywords: Ta2O5; Films; X-ray methods

1. Introduction

Recently there is a rising demand for improving the per-formance of dynamic random access memory (DRAM). This kind of improvement can be achieved by increasing the num-ber of interconnected layers in integrated circuits. In light of the trend for making compact devices, the development of new materials having high dielectric constants becomes much more important.1Tantalum pentoxide (Ta2O5) has been

demonstrated to be a promising dielectric material used in dynamic random access memories.2_Ta

2O5films have been

utilized as capacitors,3 oxygen sensors,4 and optical wave guides.5,6These applications are based on the good chemical stability, high dielectric constants, high ionic conductivity, and superior optical properties of Ta2O5films.

∗_{Corresponding author.}

E-mail address: [email protected] (C.-H. Lu).

Ta2O5 thin films have been deposited by various

techniques, such as chemical vapor deposition, ther-mal oxidation,7–9 sputtering,10 and chemical solution deposition.11The crystallization temperature of Ta2O5films

was reported to be around 700◦C.12 The high-temperature heating results in interdiffusion of species between dielec-tric layers and substrates, and the diffusion of silicon leads to a reduction in the effective dielectric constants. In order to suppress the interfacial reactions between dielectric lay-ers and microelectronic devices, the development of novel low-temperature crystallization processes is necessary.

A new high-pressure crystallization (HPC) process was lately developed by our group to reduce the crystallization temperature of Pb(Zr, Ti)O3films to 350◦C.13,14 For

con-ventional hydrothermal methods, the solution containing the constituent ions is filled into an autoclave container, and the crystallized particles will be formed due to the nucleation and growth in high-pressure environment. On the other hand, in the HPC process, only distilled water is filled in the autoclave

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container and the amorphous precursor films are positioned above the water surface. The amorphous films become crys-tallized during the heating processes in a high vapor-pressure environment at elevated temperatures.

In this study, we applied a similar process to crystallize tantalum oxide thin films at low temperatures. The precur-sor films of tantalum oxide were deposited via a metal-organic deposition (MOD) method. The deposited films were annealed via two processes viz. atmospheric pressure anneal-ing and the HPC process. The microstructural and phase evolutions during these two processes were investigated. In addition, the diffusion phenomenon and the dielectric proper-ties of Ta2O5thin films crystallized via both processes were

also studied. The HPC process was confirmed to effectively lower the crystallization temperature and improve the dielec-tric properties of Ta2O5thin films.

2. Experimental

Ta2O5 thin films were prepared via a metal-organic

decomposition method. Tantalum ethoxide [Ta(OC2H5)5]

dissolved in toluene was used as the starting materials. The prepared precursors were spin-coated onto Pt/Ti/SiO2/Si

sub-strates at a spinning rate of 3000–4000 rpm. Silicon wafers were first thermally oxidized to form a SiO2 layer with a

thickness of 200 nm. Then titanium was deposited on the top of this SiO2layer via the electron-gun technique to form a

30 nm thick layer. Platinum was subsequently deposited on the top of the titanium layer via the same technique to form a 200 nm thick layer. The platinum layer was used as the bottom electrode, and the titanium layer served to enhance the adhe-sion of the platinum layer onto the silicon oxide layer. The coated films were baked on a hot plate at 150◦C for 30 min to evaporate the organic solvent, and subsequently pyrolyzed at 350◦C for 30 min. The spinning–baking–pyrolyzing cycle was repeated for three to four times to achieve proper film thickness. The 350◦C pyrolyzed films were amorphous with a thickness of around 0.20␮m.

The pyrolyzed films were annealed via two types of annealing processes. The first method was to anneal the pyrolyzed films in a furnace under atmospheric pressure (0.1 MPa) in flowing oxygen at temperatures ranging from 500 to 800◦C at a heating rate of 30◦C/s. The second method (the HPC process) was to anneal the precursor films in a closed bomb under high pressure. A sealed stainless-steel (T316-SS) bomb was used as the autoclave container. The bottom of the bomb was filled with distilled water to produce a high vapor-pressure environment at elevated temperatures. The pyrolyzed films were positioned above the water surface and the pressure in the bomb was determined by the saturated vapor-pressure. This autoclave container was surrounded by a heating system which was controlled by a programmable controller. One thermal couple was inserted into the water for measuring the temperature. The heating rate was set to 10◦C/min. Once it reached the pre-set temperature in the

autoclave, the heating process was continued for 30 min to 1 h. Being sealed in a closed bomb at elevated temperature, the saturated vapor created a high-pressure environment dur-ing the heatdur-ing process. When the temperature was raised to 250–350◦C, the pressure was increased to 4.0–16.5 MPa.

The formed crystalline phases of the heated films were identified via X-ray diffraction (XRD) analysis. The sur-face morphologies were examined via scanning electron microscopy (SEM) and also via an atomic force microscope (AFM) using a tapping mode with amplitude modulation. Secondary ion mass spectroscopy (SIMS) was conducted to determine the depth profiles of the prepared thin films. In order to analyze the distribution of phases in the crystallized thin films, grazing incident X-ray diffraction (GIXRD) was performed. The dielectric properties of tantalum oxide films were measured using an impedance analyzer. The measuring voltage was 0.1 V and the frequency was between 50 Hz and 1 MHz.

3. Results and discussion

3.1. Crystallization of Ta2O5thin ﬁlms annealed under

atmospheric pressure

The as-deposited films pyrolyzed at 350◦C were found to be amorphous according to XRD analysis. Post-annealing was required to crystallize Ta2O5thin films.Fig. 1illustrates

the XRD patterns of Ta2O5thin films annealed under

atmo-spheric pressure at various temperatures. When the annealing temperature was increased up to 650◦C, the heated films remained amorphous. With 700◦C annealing, the crystalliza-tion process began to proceed, and␤-Ta2O5was formed. The

XRD pattern of this formed phase was consistent with the data recorded in ICDD No. 25-922.15With a further rise in the annealing temperature, the crystallinity of␤-Ta2O5was

Fig. 1. X-ray diffraction patterns of (a) as-pyrolyzed Ta2O5thin films and

the films annealed under atmospheric pressure at (b) 600◦C, (c) 650◦C, (d) 700◦C, (e) 750◦C, (f) 800◦C, and (g) 850◦C.

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Fig. 2. X-ray diffraction patterns of (a) as-pyrolyzed Ta2O5thin films and

the HPC-derived Ta2O5 films annealed at (b) 300◦C under 8.6 MPa, (c)

320◦C under 11.3 MPa, (d) 340◦C under 14.5 MPa, and (e) 350◦C under 16.5 MPa.

enhanced. These results indicate that the crystallization tem-perature of Ta2O5thin films needs to be as high as 700◦C,

which is similar to that reported in literature.16

3.2. Crystallization of Ta2O5thin ﬁlms via the HPC

process

In order to lower the crystallization temperature of Ta2O5

thin films, the high-pressure crystallization process was adopted. The XRD patterns of Ta2O5films annealed via the

HPC process are illustrated inFig. 2. No crystallized phase was observed after annealing at 300◦C under 8.6 MPa for 1 h. As the heating temperature and process pressure increased respectively to 320◦C and 11.3 MPa, a small amount of crystallized Ta2O5was formed. Well-crystallized Ta2O5thin

films were obtained as the heating temperature reached 340◦C under a process pressure of 14.5 MPa. The formed phase was confirmed to be␤-Ta2O5. Fully crystallized Ta2O5

thin films were produced after annealing at 350◦C under 16.5 MPa, and the formed phase was confirmed to be ␤-Ta2O5. The above results indicate that the adoption of the

high-pressure process successfully lowered the crystalliza-tion temperature of Ta2O5 thin films from 700 to 350◦C.

This temperature was lower than the results reported by Lin et al.,17who deposited Ta2O5thin films on Ru-film coated

sub-strates and annealed the films in H2atmosphere to reduce the

crystallization temperature of Ta2O5thin films to 400◦C. The

developed HPC process in this study considerably reduced the thermal budget and energy consumption during film pro-cessing.

Fig. 3shows the SEM micrographs of Ta2O5thin films

crystallized via the HPC process. After annealing at 300◦C

under 8.6 MPa as shown in Fig. 3(a), the films were amor-phous and no specific feature was observed. Once the films began to become crystallized after annealing at 320◦C under 11.3 MPa, certain special clusters with irregular shapes were formed as demonstrated in Fig. 3(b). These clusters were nuclei of crystallized Ta2O5. With 340◦C annealing under

14.5 MPa, the surface of Ta2O5thin film was covered with

the clustered grains with a size of 0.1–0.2␮m. When the tem-perature and pressure were increased respectively to 350◦C and 16.5 MPa, the number of the clustered grains was fur-ther increased. By comparing the XRD data inFig. 2 with the SEM photographs inFig. 3, the crystallization of Ta2O5

was confirmed to take place after annealing at 320◦C under 11.3 MPa.

The surface morphology of Ta2O5 thin films prepared

via the two processes was examined via AFM. Fig. 4(a) shows the AFM image of the 350◦C pyrolyzed films. The pyrolyzed film was rather smooth without any specific fea-ture. The microstructures of Ta2O5prepared via atmospheric

pressure annealing at 700◦C and the HPC process at 350◦C under 16.5 MPa are shown inFig. 4(b) and (c), respectively. Both films exhibited a particulate feature. The magnitudes of the roughness of the film prepared via atmospheric pres-sure annealing and the HPC process were 4.5 and 10.1 nm, respectively. The greater roughness in the films prepared via the HPC process is considered to result from the formation of clustered grains during the crystallization process.

3.3. Effects of pressure and annealing time on the formation of crystallized Ta2O5during the HPC process

For investigating the influence of pressure on the forma-tion of crystallized Ta2O5, various pressures were applied

during the HPC process. To adjust the pressure during the HPC process, various amounts of distilled water were filled into the bomb. Saturated vapor-pressure was generated when provided with sufficient quantity of water. If the water amount was insufficient, vapor-pressure was reduced. As illustrated in Fig. 5, the films remained amorphous after heating at 340◦C under 10.3 MPa. When the pressure was increased to 11.0 MPa, a small amount of crystallized Ta2O5was formed.

Once the vapor-pressure was raised to 14.5 MPa and reached the saturated status at 340◦C, well-crystallized Ta2O5 thin

films were formed. These results indicate that the crystalliza-tion process is dependent on the pressure during the HPC process.

For elucidating the effects of annealing time on the for-mation of crystallized Ta2O5, various heating durations were

performed during the HPC process.Fig. 6(a)–(c) illustrate the XRD patterns of Ta2O5thin films heated at 350◦C under

16.5 MPa for 15, 30, and 60 min, respectively. After heat-ing for 15 min, a small amount of crystallized Ta2O5 was

produced. With an increase in the heating time, the crys-tallization degree of Ta2O5 thin films was increased. After

heating for 60 min, the crystallization of Ta2O5thin films was

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Fig. 3. Scanning electron micrographs of the HPC-derived Ta2O5films annealed at (a) 300◦C under 8.6 MPa, (b) 320◦C under 11.3 MPa, (c) 340◦C under

14.5 MPa, and (d) 350◦C under 16.5 MPa.

development of crystallized Ta2O5. The formation of

crys-tallized phases from amorphous films usually involves both the nucleation and growth stages. According to the classi-cal nucleation theory, it is known that the criticlassi-cal nucleation energy for forming stable nuclei depends on the pressure.18,19 The high-pressure annealing process might lead to a reduc-tion in the critical free energy required for the formareduc-tion of stable nuclei, and promotes the nucleation process at low temperatures. A possible alternative mechanism is that the high vapor-pressure developed during the process probably forms a water coating on the film surface, thereby facilitating a dissolution-precipitation process to produce the crystallized nuclei at low temperatures.20

In order to examine the distribution of crystallized Ta2O5

within the films, grazing incidence X-ray diffraction analysis was performed.Fig. 7shows the GIXRD patterns of Ta2O5

thin films prepared via the two different processes at inci-dence angles␣ = 0.5◦, 1◦, and 1.5◦. Based on the calculation equation of penetration depth,21the depths that X-ray

pene-trated into the thin films were calculated to be 0.07, 0.14, and 0.21␮m for the above three incidence angles. As illustrated inFig. 7(a), the crystallinity of Ta2O5thin films annealed at

700◦C under atmospheric pressure was almost the same in different depths of thin films. FromFig. 7(b) and (c), similar behaviors were also observed in Ta2O5thin films prepared

via the HPC process. In spite of the difference in crystalliza-tion temperature, the crystallinity of Ta2O5 thin films was

independent of the distinct position of depth. The GIXRD results corroborate that crystallized Ta2O5phase was

homo-geneously distributed in the thin films prepared via the both processes.

The depth profiles of constituent species measured via SIMS for Ta2O5thin films prepared via both processes are

illustrated inFig. 8. As shown inFig. 8(a), the distribution of silicon and titanium expended through platinum to Ta2O5

layer as the thin film was annealed at 700◦C under atmo-spheric pressure. The SIMS analysis indicated that the high-temperature annealing inevitably caused extensive outward

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Fig. 4. Atomic force micrographs of (a) as-pyrolyzed Ta2O5films, (b) Ta2O5films annealed at 700◦C under atmospheric pressure, and (c) Ta2O5films annealed

at 350◦C under 16.5 MPa.

diffusion of silicon and titanium species from the substrates and bottom electrodes into the dielectric layers. On the other hand, when the films were annealed via the HPC process at 350◦C as illustrated inFig. 8(b), the diffusion of titanium and silicon species was limited. The SIMS results suggest that the interdiffusion between the substrates and dielectric layers was effectively suppressed by employing the HPC process due to lowering of the required crystallization temperatures.

The dielectric properties of Ta2O5thin films crystallized

via atmospheric pressure annealing and the HPC process were determined. The dielectric constant of Ta2O5thin films

annealed at 700◦C under atmospheric pressure was 27.5. After annealing at 850◦C, the constant was reduced to 11. The decrease in the dielectric constant of Ta2O5thin films

was considered to relate to the diffusion of silicon species. The diffused silicon species formed SiO2 at the interface

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Fig. 5. X-ray diffraction patterns of Ta2O5thin films annealed at 340◦C

under (a) 10.3 MPa, (b) 11.0 MPa, and (c) 14.5 MPa.

between Ta2O5and Pt layers and resulted in reduced

dielec-tric constant of dielecdielec-tric layers.22 On the other hand, the dielectric constant of Ta2O5thin films crystallized at 350◦C

via the HPC process was 30.0. The greater dielectric con-stant of Ta2O5thin films crystallized via the HPC process was

ascribed to the low crystallization temperature that effectively suppressed the diffusion of silicon species. The HPC process was confirmed to not only lower the crystallization temper-ature of Ta2O5 thin films, but also improve their dielectric

properties.

Fig. 6. X-ray diffraction analysis for Ta2O5thin films crystallized via HPC

process at 350◦C for (a) 15 min, (b) 30 min, and (c) 60 min.

Fig. 7. GIXRD diffraction patterns of Ta2O5thin films annealed at (a) 700◦C

under atmospheric pressure, (b) 350◦C under 16.5 MPa, and (c) 320◦C under 11.3 MPa.

Fig. 8. Secondary ion mass spectroscopic profiles of Ta2O5thin films on

Pt/Ti/SiO2/Si substrates. The films were annealed at (a) 700◦C under

atmo-spheric pressure and (b) 350◦C under 16.5 MPa.

4. Conclusions

The crystallization process of Ta2O5thin films deposited

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in this study. When Ta2O5films were annealed under

atmo-spheric pressure, the crystallization of the films started to take place at 700◦C. As the HPC process was adopted and anneal-ing at 16.5 MPa was performed, the crystallization tempera-ture of Ta2O5films was substantially lowered to 350◦C. The

development of crystallized Ta2O5during the HPC process

was related to the applied pressure and heating durations. According to SIMS analysis, crystallized Ta2O5phase was

found to homogeneously distribute within the films prepared via the HPC process. During high-temperature annealing under atmospheric pressure, the silicon species diffused from the substrates into Ta2O5layers, and the dielectric constants

of the prepared films were decreased. On the other hand, the interdiffusion between the substrates and dielectric layers was suppressed during the HPC process by lowering the heating temperatures, and the dielectric constants of Ta2O5thin films

were increased. The HPC process was confirmed to not only effectively reduce the thermal budget and energy consump-tion during film processing, but also improve the dielectric properties of Ta2O5thin films.

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