Effect of sulfur hexafluoride gas and post-annealing treatment for inductively coupled plasma etched barium titanate thin films
© Wang et al.; licensee Springer. 2014
Received: 11 July 2014
Accepted: 5 September 2014
Published: 15 September 2014
Aerosol deposition- (AD) derived barium titanate (BTO) micropatterns are etched via SF6/O2/Ar plasmas using inductively coupled plasma (ICP) etching technology. The reaction mechanisms of the sulfur hexafluoride on BTO thin films and the effects of annealing treatment are verified through X-ray photoelectron spectroscopy (XPS) analysis, which confirms the accumulation of reaction products on the etched surface due to the low volatility of the reaction products, such as Ba and Ti fluorides, and these residues could be completely removed by the post-annealing treatment. The exact peak positions and chemicals shifts of Ba 3d, Ti 2p, O 1 s, and F 1 s are deduced by fitting the XPS narrow-scan spectra on as-deposited, etched, and post-annealed BTO surfaces. Compared to the as-deposited BTOs, the etched Ba 3d5/2, Ba 3d3/2, Ti 2p3/2, Ti 2p1/2, and O 1 s peaks shift towards higher binding energy regions by amounts of 0.55, 0.45, 0.4, 0.35, and 0.85 eV, respectively. A comparison of the as-deposited film with the post-annealed film after etching revealed that there are no significant differences in the fitted XPS narrow-scan spectra except for the slight chemical shift in the O 1 s peak due to the oxygen vacancy compensation in O2-excessive atmosphere. It is inferred that the electrical properties of the etched BTO film can be restored by post-annealing treatment after the etching process. Moreover, the relative permittivity and loss tangent of the post-annealed BTO thin films are remarkably improved by 232% and 2,695%, respectively.
Today, ferroelectric thin films have been identified as a promising candidate material for capacitors in the next generation ultra-large-scale integrated dynamic random access memories, infrared sensors, electro-optic, RF, and microwave devices [1–3]. Among the numerous ferroelectrics, barium titanate (BTO) thin films are known as one of the leading candidates for use in applications due to their high dielectric constant, low leakage current, lack of fatigue, and low crystallization temperature [4–6]. Currently, a new, green and environmentally friendly approach called aerosol deposition (AD) has attracted great interest to fabricate BTO, which is a low-temperature and low-cost method featuring room-temperature processing with high deposition rate and high density [7–9]. To realize highly integrated microelectronic silicon-based devices involving BTO films, dry etching processes should be developed [10, 11]. However, there is currently no feasible technology known for the etching of BTO films. In addition, the data concerning the etching properties of such films exhibit significant deviations from the results obtained from other studies conducted under similar conditions . These obstacles have led to a misunderstanding of the basic effects of the BTO etching process and have hindered the optimization of this process. Therefore, the majority of the work conducted to understand the etching mechanisms has focused on elucidating the role of both physical and chemical etching effects. However, even after the SF6/O2/Ar-based over-etching process, a large amount of sticky black by-products (BaF2 and TiF4) is still observed on the desired etching area; moreover, BTO thin films exhibit relatively inferior electric properties than those in their bulk ceramic and single crystal counterparts . The above-mentioned factors are influenced by the post-annealing process, which is essential for the cleanup of the by-products during the etching process and the densification of the BTO thin films. High annealing temperature is inevitable because the low-volatility compound BaF2 requires more energy to break the Ba-F bonds . Additionally, the annealing duration is indispensable for obtaining good quality crystalline BTO films [15, 16].
In this work, the etching characteristics and mechanisms of BTO films in SF6/O2/Ar plasma and the effect of post-annealing treatment are investigated in detail on the nanoscale structural, physical, chemical, and microwave dielectric properties. The chemical compositions and the binding states on the surface of the BTO films are analyzed by X-ray photoelectron spectroscopy (XPS). The exact peak positions and the chemical changes in the elements of interest are deduced by fitting the XPS narrow-scan spectra of Ba 3d, Ti 2p, O 1 s, and F 1 s for each BTO sample. The experimental results indicate that there are no significant differences between the as-deposited BTO film and the post-annealed film after etching with respect to the fitted narrow-scan spectra. Through post-annealing in an O2-rich environment for 2 h, an improvement in the dielectric properties of BTO thin films is achieved. Based on these results, annealing treatment of the BTO thin film after the etching process is carried out for the room-temperature-deposited AD-based BTO thin films. A satisfactory etching scheme along with appropriate electrical properties are required for high-K dielectric materials to be used in the production of metal-insulator-metal capacitors in integrated passive devices (IPDs) [17, 18] and high-K gate dielectric in high electron mobility transistor (HEMT) applications [19, 20].
The BTO thin films are deposited onto Pt/Ti/SiO2/silicon substrates via an AD process using a commercial 300 nm BTO powder (SBT-03B, Samsung Fine Chemicals Co., Ltd., Ulsan, South Korea) as the starting material. The particles are aerosolized in an aerosol chamber and transported into a deposition chamber using 5 L/min N2 gas. The transported BTO powders are continuously ejected through the nozzle and deposited onto the silicon substrate. The orifice size of the nozzle, the deposition area, the distance between the nozzle and the substrate, the working pressure, and the deposition time are 10 × 0.4 mm2 (10 mm wide, 0.4 mm slit width), 10 × 10 mm2, 5 mm, 3.4 Torr, and 10 min, respectively. The final thickness of each BTO film is approximately 1 μm. A negative photoresist, specifically 3.5-μm thick DNR-L300-40 (Dongjin Semichem Co., Ltd., Seoul, South Korea), is spun at 5,000 rpm for 40 s in the track before being baked for 90 s at 90°C. Afterwards, a photolithographic process is performed with an exposure energy of 120 mJ/cm2, and then a post-exposure bake is performed for 90 s at 100°C. The photoresist is then developed in an AZ300MIF developer (AZ Electronic Materials USA Corp., NJ, USA) for 60 s. Next, a 10/1,490-nm Ti/Cr metal shadow mask is fabricated by e-beam evaporation and used during the BTO etching. After the metallization process, the photoresist is stripped using acetone. The BTO films are etched in an inductively coupled plasma (ICP) etching system (STS Multiplex ICP ASE Etcher, Surface Technology Systems Ltd., Wales, United Kingdom). The flow rates of the SF6, O2, and Ar gases into the operation chamber are controlled by mass flow controllers. The BTO films are etched under the flow rates of SF6/O2/Ar of 75/5/10 sccm. Finally, the Ti/Cr shadow mask is stripped using hydrofluoric acid and a Cr etchant; the etched BTO films are post-annealed at 750°C with the rate of up and down temperature of 3°C/min for 2 h under O2 atmosphere via mini furnace annealing (SJVF-2100, Sungjin-Semitech Co., Siheung, South Korea) to completely remove all of the fluoride formed during the etching process.
Results and discussion
where x and y are unknown variables (Ti has oxidation states = +1, +2, +3, and +4, and the known chemical compounds of F and O are OF2, O2F2, and O3F2), e is an electron, and F* is a fluoride atom, which is highly reactive. The long arrow indicates a highly volatile substance, while the short arrow denotes a somewhat volatile material. We assume that the mechanism of BTO etching involves Ar+-based physical sputtering etching and an F*-assisted chemical reaction; the etching effect is enhanced because O2 acts as a catalyst, and the low-volatility compounds, including BaF2 and TiF x , adhere to the etched surface due to a charging effect.
Fitted XPS narrow-scan spectra
The fitted Ti 2p narrow-scan spectra of the as-deposited BTO film are demonstrated in Figure 4b (1), which is composed of two wide peaks of Ti 2p3/2 and Ti 2p1/2 attributed to Ti-O bonds with binding energies of 458.4 and 464.1 eV, respectively. The Δ value of the Ti 2p doublet is equal to 5.7 eV, which is comparable to the theoretical value (ΔTi 2p) of Ti for Ti oxide. After etching, the intensity of the Ti-O peaks decreases due to the volatility of TiFx, which is partly removed from the surface during the thermal desorption process. The suppressed peak shifts to higher binding energy regions by 0.4 and 0.35 eV for of Ti 2p3/2 and Ti 2p1/2, respectively, as shown in Figure 4b (2). This result can be explained by a bond shift compensation scheme between TiFx and the etched BTO, in which the Ti4+ cations are partially reduced to create Tix+ (x = 1,2,3) cations in the presence of adequate oxygen vacancies . The peak intensities of Ti 2p3/2 (458.5 eV) and Ti 2p1/2 (464.15 eV) are strengthened when compared to the etched counterparts, as shown in Figure 4b (3), and return identically to their original binding energies found in the case before the etching process.
A broad O 1 s peak (529.15 eV) of the as-deposited BTO film contains three sub-peaks located at 529.1, 530.8, and 531.5 eV, as revealed in Figure 4c (1). Because the BTO film consists of two components (BaO and TiO2) in a BTO solid solution and C-O bonds due to surface contamination, the sub-peaks are mainly ascribed to Ba-(O 1 s) (780 eV), Ti-(O 1 s)2 (529 eV), and C-(O 1 s) (532.3 eV) bonds [25, 28, 29]. The significant shoulder is attributed to O vacancies and surface species, such as H2O and CO2, adsorbed from the air during the AD process. In Figure 4c (2), the C-O bands disappear after the etching process. This result indicates that the formation of the C-O bands is limited to a near-surface area of the film. The other O 1 s spectra display a chemical shift to a higher binding energy area, which can be attributed to the disconnection of some of the Ba-(O 1 s) and Ti-(O 1 s)2 bonds due to the physical sputtering of Ar+ ions and the chemical reaction with F* during the etching process. In Figure 4c (3), there is a small chemical shift in comparison with the as-deposited O 1 s because there is an oxygen vacancy compensation on the etched surface during the post-annealing process in O2-excessive environment; three sub-peaks, as in the as-deposited BTO thin film, were observed.
The fitted F 1 s narrow-scan spectra of the as-deposited BTO film are exhibited in Figure 4d (1). As expected, the spectra do not show signals from a fluorine-containing compound. Adding SF6/O2/Ar as the etching reaction gas is accompanied by the appearance of a F 1 s peak with a binding energy of 683.1 eV, as demonstrated in Figure 4d (2). The sub-peaks are situated at 682.95 and 685.1 eV, which are assigned to the product of the etching reaction of Ba-(F 1 s)2 (684.5 eV) and a residue of Ti-(F 1 s)4 (684.9 eV), respectively [28, 30]. In the view of the narrow-scan F 1 s spectrum of the post-annealed BTO thin film in Figure 4d (3), there are no detected signals of fluorides from the etched BTO surface because all of the fluorides are vaporized after the post-annealing treatment.
AD-derived BTO thin films are etched by sulfur hexafluoride ICP technology. The AFM images show that the roughness of the etched surface becomes rugged in comparison with the as-deposited surface because of the generation of the fluoride. After the post-annealing treatment, cross grain boundary diffusion becomes significant and the crystallite size grows, which caused a poor surface smoothness when compared with that of the other samples. The chemical compositions and bonding states for each BTO sample were studied by XPS. The low-volatility compound BaF2 was observed after the etching process and is due to a chemical reaction with F* after the destruction of O bonds via ion bombardment. Thus, the Ti-O bonds are destroyed by chemical reaction and the products of TiF4 are formed, and all of these residues could be removed by post-annealing process. The XRD studies indicated that the BTO thin films are well crystallized and have a preferred orientation of (100) after post-annealing process, which help to improve the dielectric properties. The post-annealed BTO thin films exhibit higher dielectric constant and lower dielectric loss compared to the as-deposited ones, which is important for practical device applications. This result leads to the conclusion that the post-annealing process is a cost-effective and appropriate method for both effectively removing etching by-products and obtaining nanocrystalline films, which plays an important role on the disposal of etching residue and dielectric characteristics of BTO thin films.
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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