Recently, the scaling down of SiO₂ thickness has exacerbated problems such as increased leakage current, poly-Si gate depletion, and boron penetration into the channel region. Therefore, the still high-k materials, such as HfO₂, Al2O₃, and HfAlO₃, have been studied as substitutes for SiO₂, as the insulator layer of transistors [1-6]. These high-k films can be used as insulators for metal-insulator-metal (MIM) capacitors, to improve the packing density of integrated circuits, by increasing the dielectric constant of the insulators [7-9]. However, new insulator candidates must satisfy the CMOS processing procedure. In addition, it is very important to use HfAlO₃ as the insulator, in order to achieve a high etch rate and good profile, for the sake of the throughput and reliability of the MIM capacitor.

In this work, we investigated the etch characteristics of the HfAlO₃ thin films, using an inductively coupled plasma (ICP) system. The etching characteristics of the HfAlO₃ thin film were investigated in terms of the selectivity of HfAlO₃ to SiO₂ as a function of added O₂ gas in Cl-based gas chemistry. The chemical states on the surface of the etched HfAlO₃ thin film were investigated by X-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy (AES) was used for elemental analysis on the surfaces of the etched HfAlO₃ thin film. Field emission-scanning electron microscopy (FE-SEM) was used to investigate the etch profile.

The HfAlO₃ thin films were deposited on SiO₂/Si 6-in p-type (100) wafers, by an atomic layer deposition (ALD) system. HfAlO₃ thin film was deposited on the SiO₂(100 nm)/Si(100) by ALD, with a thickness of 200 nm. The dry etching of the HfAlO₃ thin films was performed, using an ICP system [10,11]. A 13.56 MHz power generator was connected to a 3.5 turn copper coil, to generate the ICP. Another 13.56 MHz power generator was attached to the substrate electrode, to control the DC-bias voltage. The substrate temperature was kept at 30℃, with a water-cooling system. The chamber was pumped to 10^-4 Pa. The final pressure was lower than 10^-4 Pa, using a turbo-molecular pump. The etching characteristics of the HfAlO₃ thin films were investigated as functions of the O₂/Cl₂/Ar and the O₂/BCl₃/Ar gas mixing ratio. The etching conditions of the RF power, DC-bias voltage, and process pressure were 600 W, - 150 V, and 1 Pa, respectively. The etch rates were measured, using a surface profiler (Alpha-Step 500, KLA-Tencor). AES analyses were performed with a scanning auger microprobe (Microlab 350, VG Scientific). The composition change on the surface of the etched HfAlO₃ thin film was analyzed by XPS. The XPS spectra were collected using a VG Scientific ESCALAB 250 instrument, equipped with an analytical chamber. The chamber was pumped down to a base pressure of less than 10^-8 Pa with a twin-anode x-ray source, and a spherical sector analyzer with multichannel detectors. The samples were mounted on stainless steel stubs using double-sided adhesive tape, and Al Kα radiation (hv = 1,486.6 eV) was used as an X-ray source. All XPS data were corrected for sample charging during x-ray irradiation, using the adventitious hydrocarbon referencing peak (C 1s at 284.6 eV). The etching profile of the crosssection was characterized using FE-SEM (Sirion 400, FEI). The HfAlO₃ thin films used for measuring the etch rate and etching profile had a PR pattern, with width and thickness of 1.63 ㎛ both.

To evaluate the etch characteristics of the HfAlO₃ thin films in an ICP etching system, the HfAlO₃ thin films were etched by varying the BCl₃/Ar and Cl₂/Ar gas chemistry. The etch characteristics of the film in chlorine plasma need to be studied, when chlorine gas was used for the HfAlO₃ thin film etching process. So, we investigated the etching characteristics of HfAlO₃ thin film in BCl₃- and Cl₂- inductively coupled plasma. The O₂ gas was added for passivation of the etching profile and the etch rate in the BCl₃/Ar and Cl₂/Ar mixing ratio [11]. Figure 2 shows the etch rate of HfAlO₃ thin film, and the selectivity of HfAlO₃ to SiO₂ as a function of O₂ content in the Cl₂/Ar and BCl₃/Ar gas mixture. The other conditions, of the RF power, DC-bias voltage, and process pressure, were kept at 600 W, - 150 V, and 1 Pa, respectively. The etch rate of the HfAlO₃ thin film and the selectivity of the HfAlO₃ to SiO₂ were 36 nm/min and 1.5, and 30.1 nm/min and 0.73, in the O₂/BCl₃/Ar(=3:4:16) and O₂/Cl₂/Ar(=3:4:16) gas mixtures, respectively. As the O₂ concentration increased from 0 to 9 sccm in the BCl₃/Ar (=4:16) gas mixture, the etch rate of the HfAlO₃ thin films and the selectivity of HfAlO₃ to SiO₂ decreased from 53.54 to -50.05 nm/min, respectively. As the O₂ concentration increased from 0 to 9 sccm in the Cl₂/Ar (=4:16) gas mixture, the etch rate of the HfAlO₃ thin film decreased from 33.7 to -9.8 nm/min. In our opinion, the domination of the chemical reactions can be explained by the following factors. A) The melting points of HfCl₄ and AlCl₃ are about 432 and 192.6℃, and the domination of the chemical pathway may be related to its lack of volatility; by contrast, the melting points of the Hf-Cl₄ and Al-Cl₃ are lower than the melting point of Hf-Al-O (=up to 2,000℃). B) The Hf-Cl and Al-Cl chemical bonds (kJ/mol) have a lower Gibb’s free energy. The decrease of chemical bonds was due to the decreasing density of BCl or Cl radicals by reaction with O radicals, because the BCl and Cl atoms formed in the plasma spontaneously reacted with HfAlO₃ [12,13].

Figure 3 shows the etch rate of HfAlO₃ thin film as a function of the process pressure, in the O₂/BCl₃/Ar(=3:4:16 sccm) plasma. As the process pressures increased from 0.5 to 2.0 Pa, the etch rates of HfAlO₃ thin film decreased from 45.9 to 23.74 nm/min. The decrease of the process pressure enhances the density of the chemically neutral active species, which accelerates the chemical reaction [23]. However, since the mean free path and energy of the ions decrease with decreasing process pressure, the ion stimulated desorption of the reaction products is decreased, and the fraction of free surface available for the chemical reaction is increased. As a result of the pressure effect on the etch rate of the HfAlO₃ thin film, the etch process is limited by the supply of the chemical source; and thus, the etch rate of HfAlO₃ thin film decreases with decreasing process pressure [16].

The samples were investigated by XPS and AES, to analyze the chemical states on the surface of the etched HfAlO₃ thin films exposed to the O₂/Cl₂/Ar(=3:4:16 sccm) and O₂/BCl₃/Ar(=3:4:16 sccm) plasmas. The input plasma parameters were set to an RF power of 600 W, DC-bias voltage of - 150 V, process pressure of 1 Pa, and substrate temperature of 30℃. No boron peaks were detected. This means that the boron and chlorine compounds are highly volatile ones, such as boron-oxy-chloride (BOCl) or trichloro-boroxin (BOCl)₃ [24].

Figure 4 shows the AES surface scan spectra of the HfAlO₃ thin film for (a) the as-deposited films and the surface of the etched HfAlO₃ thin films in (b) the O₂/Cl₂/Ar, and (c) O₂/BCl₃/Ar plasma, respectively. The etching time of the HfAlO₃ thin film was 10 sec. Figure 4 (a) shows an AES surface scan of the as-deposited HfAlO₃ thin film, in which O peaks are detected. The origin of carbon elements is the contamination in the etching chamber or the air. After etching, no B or Cl-related peaks were observed. With the addition of O₂ gas, the intensity of Hf, Al and O peaks decreased, compared to that of the etched sample in BCl₃/Ar and Cl₂/Ar plasmas. This result means that by-products, such as Hf- Ox and Al_x-O_y, increased on the etched surface in O₂/BCl₃/Ar and O₂/Cl₂/Ar plasmas. Therefore, the AES data clearly show that the surface of the etched HfAlO₃ thin films in O₂/BCl₃/Ar and O₂/Cl₂/ Ar plasmas are formed with nonvolatile etch products, such as Hf-O_x, Al_x-O_y, Hf-Cl_x, and Al-Cl_x .

Figure 5 shows the deconvoluted peaks of the Hf 4f XPS narrow scan spectra for (a) the as-deposited films and the surface of the etched HfAlO₃ thin films in (b) the O₂/Cl₂/Ar, and (c) O₂/ BCl₃/Ar plasma, respectively. Figure 4(a) shows the Hf 4f peak from the as-deposited HfAlO₃ thin film, which must originate from Hf-O or Hf-Al-O bonds. It can be seen that the Hf 4f peak can be deconvoluted into two peaks, viz. Hf-O or Hf- Al-O (18.7, 17.1 eV). As shown in Figs. 5(b) and (c), it can be seen that when the HfAlO₃ thin films were exposed to both the O₂/Cl₂/Ar (3:4:16 sccm) plasma and O₂/BCl₃/Ar (3:4:16 sccm) plasma, new peaks of Hf-Cl bonds appear at 17.2 eV and 16.5 eV (in O₂/Cl₂/Ar plasma), and 18.2 eV and 16.5 eV (in O₂/BCl₃/ Ar plasma), which were higher binding energy by about ±0.1 eV. When adding O₂ to Cl₂/Ar plasma and BCl₃/Ar plasma, the intensity of Hf-O or Hf-Al-O, and Hf-Cl bonds decreased, but the intensity of Hf-O or Hf-Al-O bonds O₂/BCl₃/Ar plasma is higher than in O₂/Cl₂/Ar plasma [22]. This result means that by-products, such as Hf-O_x and Al_x-O_y, in O₂/BCl₃/Ar plasma increased on the etched surface, rather than in the O₂/Cl₂/Ar plasma. The XPS data give clear confirmation that etching of the HfAlO₃ thin film in the O₂/Cl₂/Ar and O₂/BCl₃/Ar plasmas results in the formation of Hf-Cl_x bonds on its surface. Thus, the etch rate of HfAlO₃ thin films increased, at the O₂ flow rate of 3 sccm [17,18]

Figure 6 shows the Al 2p peaks of the XPS narrow scan spectra of the as-deposited and the etched HfAlO₃ thin films in the O₂/ Cl₂/Ar and O₂/BCl₃/Ar plasmas. The Al 2p peaks were not deconvoluted, because their intensity was too low. The Al 2p peaks of binding energy were not shifted, but the intensity of increased peaks in O₂/BCl₃/Ar and in O₂/Cl₂/Ar plasma is higher, than in as-deposited film. This result means that the Al atoms of the HfAlO₃ react with the Cl and O radicals. We supposed that the fluorine compounds, such as Al-Cl_x, remained on the surface of the HfAlO₃ thin film.

Figure 7 shows the deconvoluted peaks of the O 1s XPS narrow scan spectra for (a) the as-deposited films and the surface of the etched HfAlO₃ thin films, in (b) the O₂/Cl₂/Ar, and (c) the O₂/BCl₃/Ar plasmas, respectively. For the O 1s peak, the peaks at 530.1, 529, and 527.7 eV correspond to those of O-O, O-Hf, and O-Al, which originate from the interface of HfAlO₃ thin film. As shown in Figs. 7(b) and (c), the shoulder peaks of O-O, O-Hf, and O-Al decreased significantly in the O₂/Cl₂/Ar and the O₂/BCl₃/Ar plasmas, as a result of the preferential removal of Hf, Al, and O. However, when the HfAlO₃ thin films were etched in the O₂/Cl₂/Ar and the O₂/BCl₃/Ar plasmas, the O 1s peak intensity at 530.1, 529, and 527.7 eV decreased, due to the formation of Cl-O bonds [20]. The intensity of the O 1s peak for the surface of the etched HfAlO₃ thin film in O₂/BCl₃/Ar plasma was higher than that of the etched HfAlO₃ thin film in the O₂/Cl₂/Ar plasma. This is due to the dramatic increase in the number of Cl-O bonds, resulting from the effective dissociation of the Hf- Al-O bonds by ion bombardment. Based on the XPS result, it was revealed that Hf, Al, and O were removed by the chemical reactions with the Cl radicals, and the physical bombardment of the Ar ions [20-22].

Figure 8 shows cross-sectional SEM images of the HfAlO₃ thin film for (a) the as-deposited films and the surface of the etched HfAlO₃ thin films, in (b) the O₂/BCl₃/Ar plasmas, at a process pressure of 1 Pa, RF power of 600 W, DC-bias voltage of - 150 V, and substrate temperature of 30℃. As shown in Figs. 8(b) and (c), a clean surface and side wall on the PR and HfAlO₃ thin film were obtained with the PR mask erosion. It may be possible, in order to obtain a high etch rate and a vertical etching profile, to also change the etching conditions to be higher than the existing experimental. So, PR mask erosion was caused by the physical etch pathway. But, the experimental results showed that the vertical etching profile of the HfAlO₃ thin film was obtained by the RIE using Cl radicals, due to the high etch rate of the HfAlO₃ thin film.

The etching characteristics of the HfAlO₃ thin film were investigated in the O₂/Cl₂/Ar and the O₂/BCl₃/Ar plasmas. The etch rate of the HfAlO₃ thin film were 30.1 nm/min, 36 nm/min in the O₂/Cl₂/Ar(=3:4:16 sccm) and the O₂/BCl₃/Ar(=3:4:16) gas mixture. As the O₂ fraction in the Cl₂/Ar and BCl₃/Ar plasmas was increased from 0 to 9 sccm, the etch rate of HfAlO₃ thin film decreased. In order to the O₂/BCl₃/Ar(=3:4:16) gas mixture, the etch rate of the HfAlO₃ thin film in the O₂/BCl₃/Ar plasma was higher, than in O₂/Cl₂/Ar plasma. Also, passivation layer formation occurred on sidewall, because of the small amount of O₂. However, the etch rate of the HfAlO₃ thin film increased, with the further addition of O₂ gas. XPS was used to investigate the chemical states of the etched HfAlO₃ thin films. The etching mechanism of the HfAlO₃ thin film can be explained as follows: Ta interacted with the Cl radicals in the BCl₃- and Cl₂ - containing plasmas, but remained at the surface, due to the low volatility of H_f-Cl_x and Al_x-Cl_y, which could be effectively removed with the help of ion bombardment.