High Density Plasma Etching of IrRu Thin Films as a New Electrode for FeRAM

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1 Integrated Ferroelectrics, 84: , 2006 Copyright Taylor & Francis Group, LLC ISSN print / online DOI: / High Density Plasma Etching of IrRu Thin Films as a New Electrode for FeRAM Jang Woo Lee, Su Ryun Min, Han Na Cho, and Chee Won Chung Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon , Korea ABSTRACT The etch characteristics of IrRu thin films with TiN hard mask were studied using a high density inductively coupled plasma of Cl 2 /O 2 /Ar gas mixture. The etch rate and etch profile were investigated by varying the gas concentrations of O 2 and Cl 2. As the O 2 concentration in a 30% Cl 2 /O 2 /Ar gas mix increased, the etch selectivity of IrRu to TiN increased, resulting in the vertical etch profile. As the Cl 2 concentration increased, the etch profile became worse due to the low etch selectivity. It was revealed that the etch selectivity of IrRu to TiN was closely related to the etch profile. The residue-free IrRu etching with a high degree of anisotropy was achieved using the Cl 2 /O 2 /Ar mixture at the optimized etching conditions. Keywords: IrRu; inductively coupled plasma; reactive ion etching; FeRAM; Cl 2 /O 2 /Ar 1. INTRODUCTION Platinium (Pt) and iridium (Ir) have been investigated as the electrode materials for ferroelectric (e.g., Pb(Zr X Ti 1 X )O 3,Bi 4 X La X Ti 3 O 12 ) capacitors in ferroelectric random access memory (FeRAM) devices since these materials exhibit improvement in the fatigue properties and leakage current density of ferroelectric capacitors [1]. Among these materials, Ir thin films along with IrO 2 have been used as the electrode materials because of the excellent endurance property and low leakage current density. Despite these advantages of Ir thin films, novel electrode materials need to be developed because Ir materials are very expensive and cause the process issue such as the etching. From the previous study, the etching of iridium thin films using a photoresist mask brought about Received May 31, Corresponding author. cwchung@inha.ac.kr 169

2 170 Jang Woo Lee et al. improper etch characteristics [2]. The etch issues of iridium films using the photoresist mask, which are similar to the platinum etching, are the redeposition along the sidewall, the shallow angle of the etched patterns, and slow etch rate [3 6]. At the initial stage of the development on FeRAM, Ru thin films were employed as the electrode materials. They had relatively good endurance properties but showed high leakage current. However, they had the excellent etch characteristics in dry etching and are inexpensive. Based on both the excellent electrical properties of Ir thin films and good etch characteristics and economical properties of Ru thin films, the use of iridium ruthenium (IrRu) alloy as an electrode material for ferroelectric capacitors has been introduced with the purpose of lessening the etch issues and maintaining the high endurance and low leakage current density. Also, IrRu thin films are very cost-effective relative to Pt or Ir thin films. In this work, the dry etch characteristics of IrRu thin films using a hard mask were studied in a high density inductively coupled plasma (ICP). The etch rate and etch selectivity along with the etch profile were obtained using a Cl 2 /O 2 /Ar gas mixture. The etch mechanism of IrRu films was examined using x-ray photoelectron spectroscopy (XPS) for IrRu thin films etched at different chemistries. 2. EXPERIMENTAL The IrRu thin films of A in thickness were deposited by dc magnetron sputtering system onto Ti/SiO 2 /Si wafers, where the Ti layer was an adhesive layer between the SiO 2 and IrRu films. The etching study of IrRu thin films with a photoresist mask or a TiN hard mask was carried out using a high density inductively coupled plasma reactive ion etching (ICPRIE) system (A-Tech System, Korea). The equipment was configured with two separate MHz RF power supplies and an advanced cooling system. One of the RF supplies was connected to a coil fastened to the lid of the etching chamber, and was used to generate a high density plasma. The second RF supply was capacitively coupled to the substrate susceptor, and was used to provide a dc self-bias for accelerating ions into the substrate. The advanced cooling system provided flows of chilled water to the substrate platen, and helium gas to the backside of the substrate. The wafer was mechanically held down on the substrate platen [7]. Before the etching of IrRu thin films, the TiN films which were the hard mask for IrRu etching were etched by varying Cl 2 gas concentration in Cl 2 /Ar gas. The etch rates of TiN and IrRu thin films, and etch selectivity of IrRu to TiN hard mask were examined by varying Cl 2 or O 2 gas concentration in a Cl 2 /O 2 /Ar mixture. For the etch profiles study of IrRu films, TiN thin films were deposited on top of them. These specimens were then patterned by photolithography and the TiN films were etched by reactive ion etching in a Cl 2 /Ar gas. Finally, the photoresist mask was removed by photoresist stripper, and then IrRu thin

3 High Density Plasma Etching of IrRu Thin Films 171 films masked with TiN films were etched by ICP system in a Cl 2 /O 2 /Ar gas mixture. The etch rates of TiN and IrRu thin films were measured using a Dektak surface profilometer and etch profiles were observed using field emission scanning electron microscopy (FESEM). In addition, x-ray photoelectron spectroscopy (XPS) was utilized to examine the existence of possible etch products or redeposited materials as well as the etch mechanism. 3. RESULTS AND DISCUSSION The dry etching of TiN thin films was examined using a photoresist in a Cl 2 /Ar gas. Figure 1 shows the variation in etch rate for the variation of Cl 2 gas concentration. The etch condition was coil rf power of 700 W, dc-bias voltage of 300 V and gas pressure of 5 mtorr. As the Cl 2 concentration increased, the etch rate of TiN gradually increased. The etch rates were relatively fast and clean etch profiles were achieved. This result indicates that the dry etching of TiN thin films obeys the reactive ion etching mechanism. The etch profiles of TiN films etched at different Cl 2 concentrations were given in Fig. 2. Figure 2(a) shows the photoresist profile before etching. When pure Ar was used as an etching gas, the severe redeposition along the sidewall took place, as shown in Fig. 2(b). As the Cl 2 gas was added into Ar, the redeposited materials were not formed but the degree of anisotropy became worse. The sidewall slope of the etched patterns at 20% Cl 2 was approximately 80 and that at 40% Cl 2 was 60. The etch pattern obtained at 20% Cl 2 showed better etch profile without the redeposition or etch residue than that obtained at 40% Cl Coil rf power 700 W DC bias voltage 300 V Gas pressure 5 mtorr Cl 2 Etch rate (nm/min) %Cl 2 in Cl 2 /Ar Figure 1. Etch rate of TiN thin films by varying Cl 2 concentration in Cl 2 /Ar gas.

4 172 Jang Woo Lee et al. Figure 2. Etch profiles of TiN films etched at different Cl 2 concentrations in a Cl 2 /Ar. (a) Resist pattern before etching, (b) pure Ar, (c) 20% Cl 2 and (d) 40% Cl 2. Finally the TiN thin films were etched at the optimized etch conditions which were 900 W coil rf power, 400 V dc-bias voltage and 2 mtorr gas pressure. Figure 3 shows the variation in etch rates of TiN hard mask and IrRu films by varying O 2 concentration. The Cl 2 concentration was fixed at 30% in a Figure 3. Etch rates and etch selectivities of TiN hard mask and IrRu films by varying O 2 concentration in a O 2 /30% Cl 2 /Ar.

5 High Density Plasma Etching of IrRu Thin Films 173 Figure 4. Etch profiles of IrRu films etched at different O 2 concentrations in a O 2 /30% Cl 2 /Ar. (a) etched TiN film, (b) 10% O 2, (c) 20% O 2 and (d) 30% O 2. O 2 /Cl 2 /Ar gas mixture. The etch condition was 900 W coil rf power, 300 V dc-bias voltage and 5 mtorr gas pressure. As the O 2 gas increased, the etch rate of TiN hard mask rapidly decreased but that of IrRu film slightly decreased. As a result, the etch selectivity of IrRu film to TiN mask greatly increased at more than 15% O 2 concentrations. The high etch selectivities of were obtained. The addition of O 2 gasincl 2 /Ar gas mix had great effect on the etch selectivity of IrRu to TiN films. The etch profiles of IrRu thin films with TiN hard masks were shown in Fig. 4. Figure 4(a) showed the etch profile of TiN hard mask before etching. The vertical etch slope of 90 was obtained. With increasing O 2 concentration in O 2 /30% Cl 2 /Ar gas mixture, the etch slopes were enhanced without any etch residues and redeposition. At 10% O 2 concentration, the etched IrRu film showed shallow etch slope due to the low selectivity of IrRu/TiN films. As the O 2 concentration increased, a degree of anisotropy of etched IrRu films was enhanced. This result is attributed to the increase of etch selectivity of IrRu/TiN. At the 30% O 2 concentration, the highly anisotropic etching of 80 was achieved. Figure 5 shows the etch rate variation of IrRu films and TiN hard masks as a function of Cl 2 concentration at 20% O 2 /Cl 2 /Ar gas mixture. The etch

6 174 Jang Woo Lee et al. Figure 5. Etch rates of TiN hard mask and IrRu films and etch selectivity of IrRu to TiN by varying Cl 2 concentration in a Cl 2 /20% O 2 /Ar. condition was the same as the one in Fig. 3. As Cl 2 gas was added into 20% O 2 /Ar, the etch rate of TiN hard mask rapidly increased, showing the reactive ion etching mechanism. On the other hand, the etch rate of IrRu thin films initially increased and decreased at more than 30% Cl 2. Therefore, as the Cl 2 concentration increased, the etch selectivity of IrRu to TiN greatly decreased from 19 to 1. The selected etch profiles of IrRu films masked with TiN were shown in Fig. 6. Figure 6(a) is the etch profile of TiN hard mask before etching. It is evident that the etch slope of IrRu films became shallow with increasing Cl 2 concentration. The IrRu thin films etched at 30% Cl 2 showed the vertical etch slope of approximately 75, as shown in Fig. 6(b). It is observed that TiN hard mask on the etched pattern at 60% Cl 2 was disappeared by the erosion of Cl 2 gas due to low etch selectivity of IrRu films to TiN hard mask (Fig. 6(d)). In order to elucidate the etch mechanism, the XPS analysis was carried out for IrRu surfaces etched at different etch chemistry such as Cl 2 /Ar and O 2 /Ar gas mix. Figure 7 shows the full spectrum for the etched IrRu surfaces. The careful observation of XPS spectra reveals that Ru thin films were etched faster than Ir thin films. The relative atomic concentrations for IrRu thin films etched at Cl 2 /Ar and O 2 /Ar gas mixture was given in Fig. 8. The relative atomic concentrations of Ir and Ru elements in Fig. 8 were determined from XPS spectra of Fig. 7. The sensitivity factors of each element for XPS analysis were calibrated with the film composition obtained by the ICP analysis of as-deposited IrRu films. Compared to the relative atomic concentrations of Ir and Ru before etching, the relative atomic concentration of Ir after etching at Cl 2 /Ar and O 2 /Ar gas mix increased while that of Ru decreased. It means that Ru has better reactivity with Cl 2 /Ar and O 2 /Ar than Ir.

7 High Density Plasma Etching of IrRu Thin Films 175 Figure 6. Etch profiles of IrRu films etched at different Cl 2 concentrations in a Cl 2 /20% O 2 /Ar. (a) etched TiN film, (b) 30% Cl 2, (c) 40% Cl 2 and (d) 60% Cl 2. O1s Ir4d Ir4f Intensity (arb. units) O 2 /Ar Cl 2 /Ar Ir4p 1/2 Ir4p 3/2 Ru3p Ru3d Before etching Binding Energy (ev) Figure 7. XPS spectra of IrRu films etched at different etch gases. The etching condition: coil rf power of 900 W, dc-bias voltage of 300 V, and gas pressure of 5 mtorr.

8 176 Jang Woo Lee et al. 100 Atomic concentration (%) Ruthenium Iridium 0 Before etching Cl 2 /Ar O 2 /Ar Figure 8. The relative atomic concentration of Ir and Ru after etching of IrRu films at Cl 2 /Ar and O 2 /Ar gas mixture. 4. CONCLUSIONS The inductively coupled plasma reactive ion etching of TiN hard mask and IrRu thin film have been performed in a Cl 2 /O 2 /Ar gas mixture. The gas concentration of Cl 2 and O 2 was changed to examine the etch rate of TiN and IrRu films and etch selectivity of IrRu to TiN. The etch profiles also were observed for the selected etch conditions. For the TiN hard mask etching, as the Cl 2 concentration increased, the etch rate gradually increased. The good etch profile was obtained at 20% Cl 2. For the IrRu etching, as the O 2 gas increased in O 2 /30% Cl 2 /Ar gas mixture, the etch rate of TiN hard mask rapidly decreased but that of IrRu film slightly decreased. Therefore, the etch selectivity of IrRu to TiN greatly increased. With increasing O 2 concentration, the etch profiles were enhanced without the redeposition. As the Cl 2 gas increased in Cl 2 /20% O 2 /Ar gas mixture, the etch rate of TiN increased while IrRu etch rate initially increased but decreased with further increase of Cl 2 gas. With increasing Cl 2 concentration, the etch profiles became worse because of low etch selectivity of IrRu to TiN. It is clear that the etch profile of IrRu films has close relationship with the etch selectivity. The IrRu etching with a high degree of anisotropy was achieved using the Cl 2 /O 2 /Ar mixture at the optimized etching conditions. ACKNOWLEDGMENT This work was supported by an INHA UNIVERSITY Research Grant.

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