Comparative analysis of barium titanate thin films dry etching using inductively coupled plasmas by different fluorine-based mixture gas
© Li et al.; licensee Springer. 2014
Received: 17 July 2014
Accepted: 20 September 2014
Published: 26 September 2014
In this work, the inductively coupled plasma etching technique was applied to etch the barium titanate thin film. A comparative study of etch characteristics of the barium titanate thin film has been investigated in fluorine-based (CF4/O2, C4F8/O2 and SF6/O2) plasmas. The etch rates were measured using focused ion beam in order to ensure the accuracy of measurement. The surface morphology of etched barium titanate thin film was characterized by atomic force microscope. The chemical state of the etched surfaces was investigated by X-ray photoelectron spectroscopy. According to the experimental result, we monitored that a higher barium titanate thin film etch rate was achieved with SF6/O2 due to minimum amount of necessary ion energy and its higher volatility of etching byproducts as compared with CF4/O2 and C4F8/O2. Low-volatile C-F compound etching byproducts from C4F8/O2 were observed on the etched surface and resulted in the reduction of etch rate. As a result, the barium titanate films can be effectively etched by the plasma with the composition of SF6/O2, which has an etch rate of over than 46.7 nm/min at RF power/inductively coupled plasma (ICP) power of 150/1,000 W under gas pressure of 7.5 mTorr with a better surface morphology.
Recently, gate insulator materials of downscaling MOSFET devices and insulator materials for metal-insulator-metal (MIM) capacitor have become key issues in semiconductor memory application field. The existence of gate dielectric suffers from increased gate leakage , and the insulator of MIM also cannot meet the requirement of high capacitance density and low leakage current [2–4]. To solve these challenges, high-k materials are needed for gate insulator and insulator of MIM capacitor. Until now, high-k materials including TiO2, TiN, HfAlO3, BaSeTiO3 and BaTiO3 have been widely studied [5–9]. Among these materials, BaTiO3 is emerging as a promising material due to the merits of high dielectric constant, low leakage current and excellent piezoelectric and ferroelectric properties [10–12]. Using BaTiO3 thin film as the gate insulator and insulator of MIM capacitor can greatly improve the performance and the density of integrated circuit. So far, although a great deal of researchers devoted to researching the characteristics of BaTiO3 thin film for using different applications, there has been little study on micropatterning properties of BaTiO3. A research presents an investigation of the chemical mechanical polishing (CMP) process . However, this CMP method has a significant limitation and complicated fabrication process. With regard to the etching technology, only in , a study on characterization of dry etching process is presented, but the authors just give a simple presentation about the relationship between plasma etch rate and applied RF power and mixture gas mixing ratio; there is no deep and systematic characterization for etching mechanism. To date, there is no feasible technology known for the etching of BaTiO3 thin film. These obstacles hinder understanding the properties of the BaTiO3 thin film etching process and further impede the related optimization of process. Therefore, it is necessary to study on how obtain a high etch rate and a good etch profile for dry etching mechanism of BaTiO3 thin film.
In this research, BaTiO3 thin films were etched using inductively coupled plasma (ICP) system with different fluorine-based plasmas. The etch rates of BaTiO3 thin films etched in different fluorine-based (CF4/O2, C4F8/O2 and SF6/O2) plasmas were compared. A comparative study of etch characteristics of the BaTiO3 thin films in these plasmas was conducted. The surface morphology of BaTiO3 thin films was examined by atomic force microscopy (AFM). Also, the chemical compositions and the binding states of the corresponding elements on the surface for each etched films were analysed by X-ray photoelectron spectroscopy (XPS).
Results and discussion
Etching rate and surface morphology
Before analysing the etching rate of the BaTiO3 thin films using fluorine-based plasmas, the basic etching behaviour characterizations have to be presented firstly. Actually, the mechanism of the ICP process uses both chemical reaction and physical sputtering. In the CF4/O2 and C4F8/O2 mixing gas experiment, F− ions from the fluorocarbon (CF4 or C4F8) has strong chemical reactivity. It reacts with BaTiO3 thin film to form the low volatile reaction byproducts which include BaF x and C x F y . Because of the charging effect, these byproducts are adhered to the etched surface. Meanwhile, the various detached CF m + ions originating from plasma sputter the reaction product from the surface and keep fluoride free to make further chemical reaction . Under the SF6/O2 plasma environment, F− ions from sulphur fluoride react with BaTiO3 thin film. The reacted byproducts such as BaF x passivate the surface. In this case, SF n + ions sputter the reaction product to stimulate the chemical reaction. During etching process, lots of volatile carbonmonoxide, carbondioxide and gaseous sulphur were pumped off by vacuum pumps. In this research, the introduced O2 played a role of catalyst, which can enhance the etch rate effectively.
Figure 6b shows the photoelectron peaks of Ti 2p from the as-deposited and etched BaTiO3 films surface. In Figure 6b (1), the unetched Ti 2p consists of two wide peaks of Ti 2p 3/2 (457.8 eV) and Ti 2p 1/2 (463.57 eV) due to Ti-O bonds. After etching in CF4/O2, C4F8/O2 and SF6/O2 plasma, the peaks of Ti 2p 3/2 and Ti 2p 1/2 shift towards higher binding energy regions by 0.05 and 0.23, 1.35 and 0.98, and 0.2 and 0.43 eV, respectively, which is shown in Figure 6b (2, 3, 4). When BaTiO3 film is etched in C4F8/O2 plasma, the intensity of the Ti 2p 3/2 and Ti 2p 1/2 peaks decreased obviously because of the higher volatility of byproduct TiF x . The byproduct TiF x can be partly removed from the film surface as the thermal desorption process. The reason why the chemical shifts towards higher binding energy can be explained by the theory of bond shift compensation scheme between TiF x and the etched BaTiO3 film .
The fitted O 1 s narrow scan spectra of each BaTiO3 sample is shown Figure 6c. An O 1 s (531.24 eV) peak of the as-deposited BaTiO3 film which consists of three sub-peaks located at 529.65, 531.2 and 532.4 eV is shown in Figure 6c (1). The three sub-peaks are mainly affected by Ba-(O 1 s) (780 eV), Ti-(O 1 s) (529 eV) and C-(O 1 s) (532.3 eV) bonds . The two oxides of Ba are made up of BaO and TiO2 in the BaTiO3 film, the surface contamination introduced the C-O bonds. The shoulder located at 532.4 eV is ascribed to the surface water vapour and carbon dioxide. In this research, the BaTiO3 film was deposited by AD method, the surface phase was formed with water vapour and carbon dioxide inevitably. After etching in each fluorine-based plasma, the etched film shows a chemical shift towards higher binding energy region, which is demonstrated in Figure 6c. It is revealed that the disconnection between Ba-O and Ti-O and re-connection between Ba-F and Ti-F happened through the physical sputtering of CF m + and SF n + ions and chemical reactions with reactive fluorides. A phenomenon can be observed that the sub-peaks at 532.4 eV is disappeared after etching in different fluorine-based plasmas. The reason of the decrease of sub-peaks in Figure 6c (2, 3, 4) compared with Figure 6c (1) is that the physical bump of ions removed the surface contamination (carbon dioxide) and the etching process is in the vacuum conditions, which would not introduce secondary contamination. Therefore, the sub-peaks at 532.4 eV in Figure 6c (2, 3, 4) cannot be found anymore.
Figure 6d shows the F 1 s narrow-scan spectra of the as-deposited and each etched BaTiO3 film surface. As shown in Figure 6d (1), there is no signal from a fluorine-contained compound. While adding the etching reaction CF4/O2 and SF6/O2 plasma for each sample, F 1 s appear at the binding energy of 684.1 and 684.02 eV, as revealed in Figure 6d (2 and 4). The sub-peaks are situated at 684.1/686.1 and 684.02/686.2 eV, respectively, which are assigned to the product of the etching reaction of Ba-F and a residue of Ti-F . After etching in C4F8/O2 plasma, the F 1 s signal emerged and consisted of three sub-peaks (683.86, 686.01 and 688.15 eV). Unlike the CF4/O2 and SF6/O2 plasma, the main contributions of these three sub-peaks result from Ba-F, Ti-F and a residue of C-F compounds .
In this present work, an investigation of dry etching mechanisms for BaTiO3 thin films in ICP system using different fluorine-based plasmas was carried out. Experimental results indicate that a higher BaTiO3 thin film etch rates were achieved with SF6/O2 plasmas. The etch rate of SF6/O2 plasmas is over than 46.7 nm/min at RF power/ICP power of 150/1,000 W under gas pressure of 7.5 mTorr. The result of AFM reveals that the roughness of all etched surfaces by fluorine-based plasmas ameliorated in comparison with the as-deposited surface. Moreover, a better etched surface morphology can be achieved using SF6/O2 plasmas. Chemical compositions and bonding states on as-deposited and each etched BaTiO3 thin films were investigated by XPS. The XPS analysis indicated the accumulation of reaction products. According to the comprehensive analysis and comparison, SF6-based plasmas showed higher etch rates and excellent surface morphology. In addition, in terms of recent severe environment, SF6 gas is not a potent greenhouse gas compared with other two greenhouse effect gas CF4 and C4F8. SF6-based plasmas can be recommended to be an ideal candidate gas for BaTiO3 dry etching.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) no. 2013-067321 and a grant supported from the Korean government (MEST) no. 2012R1A1A2004366 and (MSIP) no. 2014R1A1A1005901. Also, we would like to thank Mr. Ho-Kun Sung from Korea Advanced Nano Fab Center (KANC) for his technical support with the materials and circuit fabrications during this work. This work was also supported by a Research Grant of Kwangwoon University in 2014.
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